Alain ) Clémence

Marie Linh

Antoine Feng

Adrien Khaled

Arnaud, Guillaume, Yiting, Philippe Fatima Lucile

Jean-No Merci

Léonard, Gader Merci À Hocine

Cora

Jubba

Lorenzo

Alexandre Henry

Matthieu Carolina

Juan Ouafi

Viviane Riccardo Gilles

Au Picm

Merci À Loïc

Leandro

Fatima Chiara

Anna Sanghyuk D'avoir, Entre Autre

Yvan, Jérôme, Léo, Laurence, Cyril À Nada

Frédérics Jean-Luc Bérengère

Anne Léa

Sacha Caro

Arthur Je Remercie Chaudement

Lise

Matchilde, Leslie... Mais je remercie Adrien Jean

Mathieu, Camille Si- Mon Moly

Alex Colas

Ce travail de thèse a été effectué au sein d'une collaboration entre trois laboratoires: le Laboratoire de Chimie Inorganique (LCI) à l'Université Paris-Sud, le Laboratoire d'Etude des Microsctructures (LEM) à l'ONERA, et le Laboratoire de Physique des Interfaces et des Couches Minces (LPICM) à l'Ecole Polytechnique. Ainsi, je souhaite tout d'abord remercier les directeurs de ces trois laboratoires: Talal Mallah, Alphonse Finel, et Pere Roca i Cabarrocas, de m'avoir permis d'y travailler pendant ces trois années.

Je tiens à remercier les membres de mon jury de thèse, pour avoir accepté de juger ce travail avec bienveillance. Mes deux rapporteurs, Sofie Cambré et Shigeo Maruyama, ainsi que Vincent Jourdain, examinateur de ce travail, et Pierre Mialane, qui a présidé mon jury.

Ce travail a été encadré par Vincent Huc et Annick Loiseau. Je vous remercie tous les deux pour m'avoir permis d'explorer ce sujet avec beaucoup de liberté, de présenter mes travaux, et de participer à des formations, sans aucune restriction. Annick, merci pour ta présence pendant la rédaction de ce manuscrit, nos longues conversations auront largement contribué à le rendre plus intéressant et m'ont beaucoup appris! Merci à vous deux pour votre soutien pendant les dernières répétitions de ma soutenance! Le travail présenté dans ce manuscrit étant le fruit d'une collaboration entre plusieurs équipes de recherche, j'aimerais aussi remercier les personnes qui ont contribué, de près ou de loin, à l'encadrement de cette thèse. Merci à Costel, d'avoir construit des manips qui peuvent tout faire, et pour ta bienveillance. Un grand merci à Laure, dont je regrette de n'avoir sollicité l'aide que très tardivement. Ton implication m'a permis d'explorer d'autres pistes et de ne pas perdre le moral quand les ABP me rendaient la vie dure! Je souhaite aussi exprimer toute ma gratitude envers les capitaines de la team "solubilité", Hakim et Christophe. Merci pour toutes les conversations scientifiques enrichissantes qui ont pu donner un peu de sens à certains de mes résultats! Je souhaite aussi exprimer toute ma reconnaissance envers Maoshuai He, qui m'a beaucoup appris sur la littérature, et la croissance en général.

Je n'aurais pas pu présenter tous ces résultats sans l'aide expérimentale précieuse de nombreuses personnes. Je souhaite d'abord remercier Frédéric Fossard, qui m'a appris à me servir d'un TEM, à tenter de dompter (un peu) le Zeiss, et pour les nombreuses manips qu'il a effectuées sur le Titan de Centrale, ou l'ARM à Paris 7. Merci aussi pour ton aide dans la préparation de ma soutenance! Merci à Amandine d'avoir développé une super méthode de transfert qui aura débloqué beaucoup de choses! Merci à Ahmed pour les tubes triés, ainsi qu'à Jean Sébastien-Lauret et ses étudiants Lucile et Géraud pour l'aide sur ces manips. Pour les diverses caractérisations présentées dans ce manuscrit: un grand merci à François Brisset pour l'EDX, à Ileana pour de magnifiques images TEM au PICM. Merci à Diana Dragoe pour l'XPS, pour sa disponibilité pour les manips comme pour les discussions sur l'interprétation des résultats. Je souhaite enfin remercier chaleureusement Sandra Mazerat, qui a effectué toutes les images AFM qui sont présentées ici. Merci pour ta patience, et d'avoir contribué à calmer ma détresse en passant des échantillons quand tu n'en avais pas vraiment le temps.

Je tiens à remercier tout particulièrement ma camarade de thèse, Salomé. Notre entraide pendant ces trois années m'a été indispensable, que ce soit pour les manips, les articles, les questions existentielles sur notre sujet partagé, et le soutien moral pendant les temps sombres des échantillons contaminés. Merci aussi pour tout le reste, les conférences-voyage, tout ce fromage, la bonne ambiance généralisée tout simplement! Travailler dans trois laboratoires permet de rencontrer beaucoup de gens! Ainsi j'aimerais remercier les membres du LCI, du LEM et du PICM. Au LCI, merci à tous les étudiants croisés, et notamment Virgile, Irene, Charlotte, Benjamin (ou

Introduction

Materials of low dimensions have seen a surge of research interest during the past decades, due to the curiosity raised by their size-dependent properties. The allotropic family of carbon which was only considered to be composed of diamond, graphite, and amorphous carbon for a very long time soon welcomed new members. From the 1980's to the early 2000's, three additional forms of carbon were discovered: fullerenes, carbon nanotubes, and graphene, sparking scientific enthusiasm around the world. It was now possible to study low dimension carbon allotropes (2D with graphene, 1D with CNTs, and 0D with fullerenes). Single-walled carbon nanotubes (SWCNTs) were widely studied right after their first observation in 1993. Their outstanding optical and electronic properties were shown to strongly depend on their atomic structure. The fact that SWCNTs can either be semiconducting or metallic, depending solely on their diameter and chiral angle, which dictates the angle at which a graphene sheet is hypothetically rolled to form the nanotube, is a telling example.

The ambition to exploit the structurally-dependent properties of SWCNTs made it indispensable to have access to single-structure samples, or at least samples with narrow diameter and chiral angle distributions. The usual synthesis routes of SWCNTs (arc discharge, laser ablation, chemical vapor deposition(CVD)) usually lead to random chirality samples. Therefore, efforts for either structurally selective growth techniques, or effective sorting methods, were of crucial importance. The work presented in this thesis focuses on the first option.

Attaining the goal of structurally selective SWCNT growth requires a fine understanding of the mechanisms at play during growth. The complexity of these mechanisms and the difficulty to observe them at the atomic scale make this understanding a true scientific challenge. Over the past twenty years, theoretical works together with experimental trial and error have provided pieces of answers to a few of the many questions surrounding the SWCNT growth mechanisms.

INTRODUCTION

Experimentally, CVD became the most used synthesis technique, since it allows the fine tuning of a wide range of parameters (temperature, pressure, carbon source, ambient composition, growth time. . . ). Though a large number of factors have an influence of SWNCT growth, and its potential selectivity, the crucial importance of the catalyst nanoparticle was underlined by experimental studies, as well as theoretical calculations.

For instance, most advanced atomistic simulations of the nucleation and growth have shown that the carbon content in the catalyst nanoparticle can drive the adhesion of the SWCNT wall to its surface, as well as the growth mode. Simultaneously, experimentally, the use of bimetallic catalyst systems has been a widespread approach for selective growth. While some studies put forward the argument of a synergistic effect between the two used metals, some other results are claimed to rely on the formation of highly refractory alloys, their solid and crystalline state being assumed to be able to drive a selective growth. In a majority of cases, the phenomena responsible for the observed selectivity with regard to the bimetallic catalyst system are not very well understood. The idea that the carbon content in the nanoparticle -driven in certain conditions by its carbon solubility limit -can have an influence on the growth may be linked to the various experimental results showing either selective growth using bimetallic nanoparticles, or variations in the grown SWCNT populations depending on the nature of the catalyst.

INTRODUCTION 3 preparation of alloyed bimetallic catalyst nanoparticles with various bimetallic combinations in similar experimental conditions. The bimetallic particles we were primarily interested in, are nanoalloys with tunable carbon solubility.

To reach this objective, we have chosen to work with chemically synthesized catalyst precursors. These precursors are synthesized as stable nanoparticle dispersions, and can be deposited on a substrate and subsequently reduced into metallic nanoparticles. The idea behind this strategy is twofold: first, the precursor nanoparticles are directly synthesized with the metal atoms mixed within the structure, potentially facilitating the formation of alloyed nanoparticles; second, the precursor type is chosen because of its versatility: using the same methodology, precursors with a wide variety of bimetallic combinations could be obtained and used for SWCNT growth in the same conditions. This could enable the study of SWCNT growth with nanoparticles with tuned carbon solubilities in identical conditions, allowing a study of the phenomena at play.

Another crucial point in SWCNT growth study is their characterization. SWC-NTs are often observed indirectly, relying on resonance phenomena (Raman spectroscopy), or optical transitions (optical absorption and photoluminescence excitation spectroscopy). Although many research groups have focused on the various characterization techniques to observe SWCNTs and evaluate the selectivity of a growth, there are no reliable standardized characterization methodologies. Therefore, selectivity assessment is usually performed using the most practical, available, or less time consuming technique, with little concern for accuracy. Another aspect of the present work is therefore to try to evaluate the methods used for determining the diameter distribution of a SWCNT sample, using two different characterization techniques. This manuscript will be divided in five chapters. The first chapter is an overview of the literature on SWCNTs, their synthesis, and characterization. The focus will be set on selective growth of SWCNTs using CVD, and selectivity evaluation. The second chapter will present the different chemically synthesized catalyst precursors that were used in the present work, and how they will be integrated in the SWCNT growth process. The two characterization techniques used for selectivity evaluation (transmission electron microscopy (TEM), and Raman spectroscopy) will then be presented in detail. The results of the present work will be articulated in three chapters. In the first chapter (Chapter 3), a study employing Prussian blue (PB), and Prussian blue analog (PBA) nanoparticles as catalyst precursors for SWCNT growth will be discussed. The INTRODUCTION second (Chapter 4) will consist of a comparative study of TEM and Raman spectroscopy for the determination of the diameter distribution of a SWCNT growth sample. Finally, preliminary results on two other chemically synthesized catalyst precursors will be given in Chapter 5.

Chapter 1

Introduction and state of the art

Since their discovery, the growth of SWCNTs has been a tremendously active research field. The ultimate goal being the structural control of SWCNTs during growth, so as to exploit their outstanding physical properties to their full potential. Over the years, this task has proven to be a huge challenge, and has underlined that the understanding of the complex mechanisms behind SWCNT growth were of crucial importance. The goal of selective SWCNT growth has also put forward the importance of accurate, reliable, and standardized characterization techniques for selectivity assessment. This chapter is divided into three main sections. The first is a general introduction to SWCNTs, where their atomic structure and physical properties will be discussed, as well as the various ways in which they can be synthesized. The second section will focus on the vast subject of selective SWCNT growth by CVD. The current knowledge on the effects of growth conditions, and the influence of the catalyst nanoparticles on the resulting SWCNT sample will be first presented. Then, we will present the various characterization techniques that can be used for the assessment of growth selectivity, and their use in the literature will be discussed. In the third and final section, a few theories regarding SWCNT growth mechanisms, stemming either from theoretical or experimental works (and both) will be introduced, along with the resulting potential strategies for controlled growth. Finally, we will present the general objectives of this thesis' work.

An introduction to SWCNTs

Historically, the first known forms of carbon were graphite, diamond, and amorphous carbon. The existence of these carbon based materials was mentioned by Le Chatelier in 1908 [START_REF] Colas | Leçons sur le carbone[END_REF]. Other forms of carbon were discovered much later in the XX th century, starting with the discovery of fullerenes in 1985 [START_REF] Kroto | C60: Buckminsterfullerene[END_REF], and of carbon nanotubes in the early 1990's. Though tubular carbon structures had been observed as early as 1953 [START_REF] Davis | An unusual form of carbon[END_REF], the discovery of CNTs is attributed assembled in bundles, in order to minimize their energy through Van der Waals interactions [START_REF] Thess | Crystalline ropes of metallic carbon nanotubes[END_REF], or individualized. MWCNTs are concentric SWCNTs with an inter-wall distance of 3.4 Å, which corresponds to the distance between two graphene sheets, interacting via Van der Waals forces.

Electronic and optical properties

Because of their structural similarities, the electronic structure of SWCNTs can easily be derived from that of graphene. Before going into the electronic structure of SWCNTs, we should therefore give some details on the structure of graphene. As explained above, the unit cell of the honeycomb lattice of graphene is defined in the direct space by the ( -→ a 1 , -→ a 2 ) basis. In the reciprocal space, the lattice is defined by (

- → b 1 , - → b 2 )
, expressed in an orthogonal base as:

- → b 1 = ( 2π √ 3a , 2π a ), - → b 2 = ( 2π √ 3a , - 2π a ) (1.6)
The first 2D Brillouin zone is a hexagon, comprising three notable high symmetry points: Γ, K, and M, which are respectively located at the center of the hexagon, the corners, and the centers of the edges (see Figure 1.4.b). The band structure of graphene was first calculated using the tight-binding (TB) approach in 1947 [START_REF] Wallace | The band theory of graphite[END_REF]. In the graphene band structure, the π and π * bands join at the six K points at a zero energy. Close to the K points, the dispersion relation is linear, forming Dirac cones, as shown in Figure 1.4.a. Calculations of the density of sates (DOS) for graphene, which corresponds to the number of allowed states in a given dE energy interval, show that it reaches zero at the K points. Moreover, the Fermi energy is exactly at this energy since the π subband contains two electrons (taking the spin into account). This makes graphene a so-called zero-gap semiconductor.

When forming a SWCNT from a graphene sheet, we essentially add periodic boundary conditions in the circumferential direction of the nanotube, i.e. the -→ C h direction:

- → k • -→ C h = p2π, p ∈ Z (1.7)
This leads to the quantization of the wave vectors in this direction. Along the tube axis, a continuum of -→ k are allowed (for an infinite tube), but only discrete values are allowed in the direction perpendicular to it. This defines lines of allowed wave vectors "cutting" the 2D graphene Brillouin zone. The resulting nanotube band structure is the intersection between the planes defined by the Chapter 1. Introduction and state of the art Further calculations and experimental studies were therefore conducted in order to obtain a more accurate Kataura plot, which is crucial for SWCNT sample characterization. This will be discussed in Chapter 2.

Synthesis methods

After the publication of Sumio Iijima's work in 1991, various methods for the synthesis of carbon nanotubes were developed. Those methods are divided into two categories, depending on the temperature range at which they operate: high temperature, and medium temperature. There are two CNT growth methods at high temperature (laser ablation, and arc discharge or electric arc), and the medium temperature methods consist of variations of the catalytic chemical vapor deposition (CCVD, or CVD for short) method.

High temperature methods

There are two high temperature routes, which both rely on the sublimation of graphite under an inert atmosphere at temperatures higher than 3000-4000 • C (close to the sublimation temperature of graphite), and the condensation of the obtained evaporated carbon under a high temperature gradient. The difference between those synthesis methods, known as arc discharge and laser ablation, lies in the way the graphite is sublimated.

The arc discharge method is directly derived from the method developed by Krätschmer and Huffman in 1990 [START_REF] Krätschmer | Solid c60: A new form of carbon[END_REF] for the mass production of fullerenes. In this process, a voltage difference is applied between two graphite electrodes placed in a chamber under argon or helium partial pressure (around 600 mbar). The electrodes, usually cooled with water, are then progressively brought closer to each other, until the creation of an electric arc, increasing the temperature to about 6000 • C. The graphite from the anode then sublimates, forming a plasma, and the carbon atoms move towards colder regions in the chamber. A nanotube deposit is formed on the cathode, as a result of this rapid condensation within a high temperature gradient.

When using pure graphite electrodes, the product of this synthesis contains various carbon based materials, including amorphous carbon, fullerenes, and MWC-NTs. SWCNTs were obtained with this technique by co-evaporating graphite and metals [START_REF] Iijima | Single-shell carbon nanotubes of 1-nm diameter[END_REF], [START_REF] Bethune | Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls[END_REF](often Fe, Ni, or Co). The yield in SWCNTs was still very low, until Journet et al. achieved gram-scale production of SWCNTs by adding small amounts of yttrium in the Ni catalyst [START_REF] Journet | Large-scale production of single-walled carbon nanotubes by the electric-arc technique[END_REF].

In the case of the laser ablation technique, the graphite target is sublimated using a focused laser beam (continuous [START_REF] Maser | Production of high-density singlewalled nanotube material by a simple laser-ablation method[END_REF] or pulsed [START_REF] Guo | Self-assembly of tubular fullerenes[END_REF]). The graphite target is placed in a quartz tube, that is heated to 1200 • C under an argon or helium flow. The vaporized carbon moves through the furnace with the gas flow, the nanotubes form in the gas phase, and condense on a water-cooled copper collector. As for the electric arc technique, SWCNTs are obtained when including metals in the graphite target.

For both of these techniques, in the case of SWCNT growth, the product only contains a portion, as high as it may be, of SWCNTs usually assembled in bundles at the surface of the catalyst particles [START_REF] Gavillet | Root-growth mechanism for single-wall carbon nanotubes[END_REF]. Consequently, the SWCNT diameter is not correlated to the catalyst nanoparticle size. Other forms of carbon are found, as well as metal catalyst nanoparticles. In order to rid the SWCNTs of these impurities, purification steps are needed. Moreover, the lack of control over the many parameters that have an influence on SWCNT growth has pushed researchers to focus on other techniques for selective growth of SWCNTs.

Medium temperature methods: CVD

In CVD methods, nanotubes are formed by the catalytical decomposition of a carbon-containing gas at the surface of a metal catalyst nanoparticle, in a furnace at temperatures below 1200 • C. CVD had been used for a long time to grow carbon nanofibers [START_REF] Rodriguez | A review of catalytically grown carbon nanofibers[END_REF], and was then adapted to grow MWCNTs [START_REF] José-Yacamán | Catalytic growth of carbon microtubules with fullerene structure[END_REF] and SWCNTs [START_REF] Dai | Single-wall nanotubes produced by metal-catalyzed disproportionation of carbon monoxide[END_REF]. CVD can either be operated with catalyst nanoparticles formed in situ in the gaseous phase, or with supported catalysts. The general principle of CVD is to heat a catalytic material inside a furnace, and flowing a carboncontaining gas through the furnace over a certain time period. The carbon precursor molecules decompose catalytically at the surface of the catalysts, and the carbon is dissolved, and diffuses through the catalyst nanoparticle. A solid sp 2 tubular carbon structure is formed when the carbon content in the particle reaches a saturation point and precipitates at the surface of the nanoparticle. Carbon precursors can be hydrocarbons C x H y (methane (CH 4 ) being the most widely used), or oxygen-containing molecules, such as carbon monoxide (CO), or ethanol. In the first case, the growth is conducted in reductive conditions (dihydrogren (H 2 ) is a product of C x H y decomposition), and in the second, the growth takes place in oxidative conditions. Here, contrary to the case of the which are finally activated (formation of effective catalyst), usually by a reduction step, or spontaneously during growth by introduction of the carbon precursor. Various methods can be used for the first step of this process, the most widely used are impregnation [START_REF] Hafner | Catalytic growth of single-wall carbon nanotubes from metal particles[END_REF]- [START_REF] Bachilo | Narrow (n, m)-distribution of single-walled carbon nanotubes grown using a solid supported catalyst[END_REF], precipitation [START_REF] Loebick | Effect of chromium addition to the co-mcm-41 catalyst in the synthesis of single wall carbon nanotubes[END_REF], [START_REF] Loebick | Effect of manganese addition to the co-mcm-41 catalyst in the selective synthesis of single wall carbon nanotubes[END_REF], and atomic layer deposition (ALD) [START_REF] He | Growth mechanism of single-walled carbon nanotubes on iron-copper catalyst and chirality studies by electron diffraction[END_REF], [START_REF] He | Selective growth of swnts on partially reduced monometallic cobalt catalyst[END_REF]. Impregnation and precipitation are similar methods: a metal complex is put in contact with a porous, or mesoporous support; the resulting solid can be calcinated, then reduced. Co-impregnation, and co-precipitation are used for the formation of bimetallic catalysts. ALD is a thin film deposition technique that relies on the adsorption and subsequent thermolysis of gaseous metallic compounds.

In the case of flat substrates, many different methods of catalyst formation can be used. They can be directly deposited on the substrate before being introduced in the furnace [START_REF] Cheung | Diameter-controlled synthesis of carbon nanotubes[END_REF], or formed in situ prior to or at the same time as the introduction of the carbon precursor. The in situ formation on CVD catalyst nanoparticles can be done in many different ways: dewetting of a continuous thin film [START_REF] Takagi | Singlewalled carbon nanotube growth from highly activated metal nanoparticles[END_REF], calcination and/or reduction of pre-deposited precursors [START_REF] Yang | Chirality-specific growth of single-walled carbon nanotubes on solid alloy catalysts[END_REF], usually deposited by dip-coating, spin-coating, or just drying drops of solutions containing the precursors at the surface of the substrate. Thus, in most cases, the catalyst precursor comes in the form of a thin film.

There are three main variations of the classical CVD process, where the decomposition of the carbon precursor is enhanced in order to increase the yield, or reduce the growth temperature. The plasma-enhanced CVD (PECVD) process uses a plasma to decompose the carbon precursor into reactive intermediates like radicals [START_REF] Li | Preferential growth of semiconducting single-walled carbon nanotubes by a plasma enhanced cvd method[END_REF]. Hot filaments can also be used, at the gas inlets, to enhance the decomposition of the carbon-containing gas molecules (HFCVD) [START_REF] Makris | Carbon nanotubes growth by hfcvd: Effect of the process parameters and catalyst preparation[END_REF]. The photothermic effect of a focused laser spot can also be used to locally heat the substrate during growth (LCVD) [START_REF] Bondi | Laser assisted chemical vapor deposition synthesis of carbon nanotubes and their characterization[END_REF].

In the past three decades, many research groups have worked on optimizing CVD for SWCNT growth with various goals (yield, defect-free SWCNTs, length, absence of byproducts such as amorphous carbon and carbon onions...) and some success. There have been some groundbreaking steps in SWCNT synthesis research. The "supergrowth" (SG) process, for example, that uses water stimulation of catalytic activity to obtain dense vertically aligned SWCNT forests in short growth times (2.5 mm high forest in 10 minutes) [START_REF] Hata | Water-assisted highly efficient synthesis of impurity-free single-walled carbon nanotubes[END_REF], and that is now industrially implemented for mass production of SWCNTs, was a major Chapter 1. Introduction and state of the art advancement. The presence of water decreases the formation of amorphous carbon at the surface of the catalyst nanoparticles, preventing their deactivation. This process produces SWNCTs with random chiralities over a wide range of diameters going from 1 to 6 nm. The development of the HiPco process [START_REF] Nikolaev | Gas-phase catalytic growth of singlewalled carbon nanotubes from carbon monoxide[END_REF], which uses a flowing catalyst CVD setup at high pressure, was also a major step for commercialization of SWCNTs. The use of CO as a carbon source, instead of a hydrocarbon, allowed to obtain sub-nanometer diameter SWCNTs. Lastly, the sale of chirality-enriched SWCNT samples has been made possible after the development of the CoMoCAT process in the early 2000's [START_REF] Resasco | A scalable process for production of single-walled carbon nanotubes (swnts) by catalytic disproportionation of co on a solid catalyst[END_REF], where a smart engineering of the catalyst coupled with growth at low temperatures is very probably the key to this claimed selectivity. The catalyst consists of a mix of molybdenum and cobalt in small proportions, during growth, the formation of a molybdenum carbide leads to the segregation of very small Co nanoparticles with a well-controlled size distribution. Now that yield and the presence of defects is not a concern for CVD growth, the ultimate goal for researchers has naturally shifted towards structural control during growth. When it comes to controlling the structure of the SWC-NTs, interesting results have been published, and though certain groups claim to be very close to the goal of perfect single-chirality growth, a lot of effort and progress remain to be made.

Selective growth of SWCNTs by CVD

SWCNT applications and related challenges

This section's aim is not to go into details about all possible applications of SWCNTs, but rather to show the wideness of potentially affected fields, and the challenges they bring to the field of SWCNT growth. We can refer the reader to book chapters, or reviews that go into great detail on this subject [START_REF] Endo | Potential applications of carbon nanotubes[END_REF]- [START_REF] Baughman | Carbon nanotubesthe route toward applications[END_REF].

With their outstanding electronic properties competing with currently used materials, both metallic and semiconductor SWCNTs are great candidates for integration in electronic devices. Metallic SWCNTs have been used to produce highly conductive flexible and transparent films [START_REF] Mustonen | Uncovering the ultimate performance of single-walled carbon nanotube films as transparent conductors[END_REF], [START_REF] Kaskela | Highly individual swcnts for high performance thin film electronics[END_REF], and semiconducting SWCNTs for the fabrication of high performance field effect transistors (FETs) [START_REF] Avouris | Carbon-based electronics[END_REF]- [START_REF] Qiu | Scaling carbon nanotube complementary transistors to 5-nm gate lengths[END_REF]. Coupled with their optical properties, SWCNTs have been successfully integrated in optoelectronic devices such as solar cells [START_REF] Cui | Air-stable highefficiency solar cells with dry-transferred single-walled carbon nanotube films[END_REF], and light-emitting diodes as the emitting material [START_REF] Wang | High-performance carbon nanotube lightemitting diodes with asymmetric contacts[END_REF]. The fluorescence [START_REF] Cherukuri | Mammalian pharmacokinetics of carbon nanotubes using intrinsic near-infrared fluorescence[END_REF]- [START_REF] Yomogida | Industrial-scale separation of high-purity single-chirality singlewall carbon nanotubes for biological imaging[END_REF], or the strong Raman scattering [START_REF] Cottenye | Raman tags derived from dyes encapsulated inside carbon nanotubes for raman imaging of biological samples[END_REF] of SWCNTs has also been exploited in bio-imaging applications. Moreover, SWCNTs have also been widely studied for gas sensing applications [START_REF] Qi | Toward large arrays of multiplex functionalized carbon nanotube sensors for highly sensitive and selective molecular detection[END_REF]- [START_REF] Bondavalli | Gas fingerprinting using carbon nanotubes transistor arrays[END_REF].

In order to exploit the properties of SWCNTs within these ever-evolving potential applications, the availability of SWCNT samples with a controlled and specific chirality is an absolute necessity. In the case of nanoelectronic devices, pure semiconducting or metallic SWCNT samples can also be interesting. In any case, obtaining those SWCNT samples implies one of two technical challenges: they either require effective post-synthesis sorting methods, or direct selective growth. The following sections will review the progress and achievements in selective CVD growth of SWCNTs.

Defining growth selectivity

Since the ultimate goal to synthesize SWCNTs with pre-defined (n, m) is extremely challenging, selective growth can be broken down into different categories depending on the structural parameter, or property that the synthesized SWCNTs have in common.

As explained in the previous section, CVD allows the synthesis of nanotubes with a very wide range of structures. Two ways to narrow down the pool of chiralities are attempting to control the diameter, and attempting to control the chiral angle of synthesized SWCNTs. Controlling just one of these two structural parameters, meaning succeeding to narrow down the diameter or chiral angle distribution of the sample, leads to either diameter-selective (or diameterspecific when the selectivity is high) growth, or chiral angle-selective growth. When both are controlled, the growth is, of course, considered chirality-or (n, m)-selective, or specific.

Potential applications of SWCNTs often exploit their electronic properties. For these applications, it is very interesting to have samples of either semiconducting or metallic SWCNTs. A lot of recent research focuses on trying to obtain semiconducting-, or metallic-enriched SWCNT samples, by optimizing the semiconducting to metallic (SC/M) ratio.

Influence of CVD parameters on selectivity

Since the first SWCNT growth by CVD, research groups have conducted countless studies on the influence of all possibly controllable parameters on the resulting SWCNTs, and growth selectivity. In CVD, the original widely spread hypothesis concerning the control of SWCNT diameter was that is was dominated by catalyst nanoparticle size. The control of chirality distribution, or SC/M ratio of a sample is more of an empirical search, since no hypotheses were initially set on the matter. This section gives a non-exhaustive list of the CVD parameters that have been shown to affect selectivity.

Growth temperature

It is generally observed that increasing the temperature tends to lead to the growth of large-diameter SWCNTs [START_REF] He | Low temperature growth of swnts on a nickel catalyst by thermal chemical vapor deposition[END_REF]- [START_REF] Li | Diameter tuning of single-walled carbon nanotubes with reaction temperature using a co monometallic catalyst[END_REF]. The most widely used argument to explain this, is a thermally-facilitated coarsening of the catalyst nanoparticles, as the nanoparticle diameter is assumed to determine the SWCNT diameter. However, a contradicting experimental study by Yao et al. [START_REF] Yao | Temperaturemediated growth of single-walled carbon-nanotube intramolecular junctions[END_REF] showed that the diameter of an individual SWCNT could be tuned through growth temperature changes, with a decrease in diameter when the temperature was increased.

Choice of carbon feedstock and feeding rate

There are very contradicting results regarding the effect of carbon feeding rate on the diameter distribution of grown SWCNTs. This might also be highly dependent on the carbon source. While certain groups have claimed that reducing the carbon feeding rate led to smaller diameter nanotubes, others have shown the opposite. Saito et al. [START_REF] Saito | Selective diameter control of single-walled carbon nanotubes in the gasphase synthesis[END_REF] showed, using a floating catalyst reactor, that increasing the flow rate of ethylene (the carbon source), yields in shifting the diameter distribution towards smaller diameters. On the contrary, Geohegan et al. [START_REF] Geohegan | Flux-dependent growth kinetics and diameter selectivity in single-wall carbon nanotube arrays[END_REF] performed experiments in which increasing acetylene (C 2 H 2 ) flow rate in a pulsed-CVD apparatus led to a shift of the diameter distribution towards higher diameters, over an overall very broad range of diameters (sub-nanometer, to 6 nm).

When it comes to the choice of carbon feedstock, the carbon source used for CVD growth can have a very significant impact on the resulting SWCNT population. He et al. have conducted studies on the comparison between CO and CH 4 as carbon sources in the same experimental conditions and using the same Fe catalyst where they found that CO had a tendency to favor the growth of small diameter (sub-nanometer to around 1.6 nm) SWCNTs with large chiral angles, while CH 4 led to growth of SWCNTs with diameters ranging from 1.0 to 4.5 nm [START_REF] He | Chiral-selective growth of single-walled carbon nanotubes on stainless steel wires[END_REF], [START_REF] He | Diameter and chiral angle distribution dependencies on the carbon precursors in surfacegrown single-walled carbon nanotubes[END_REF]. Wang et al. have compared the use of four different gas sources (CO, ethanol, methanol, and C 2 H 2 ) for SWCNT growth in the same experimental conditions and noticed strong variations in the chirality distributions of the obtained samples [START_REF] Wang | n, m) selectivity of single-walled carbon nanotubes by different carbon precursors on co-mo catalysts[END_REF].

The choice of carbon feedstock has also proven to have an effect on the SC/M ratio of the SWCNT sample [START_REF] Parker | Increasing the semiconducting fraction in ensembles of single-walled carbon nanotubes[END_REF], [START_REF] Wang | Direct enrichment of metallic single-walled carbon nanotubes induced by the different molecular composition of monohydroxy alcohol homologues[END_REF]. Parker et al. showed SC percentages going from less than 20 % to between 50 -70 % when switching from CH 4 to isopropanol (from estimations based on FET devices) [START_REF] Parker | Increasing the semiconducting fraction in ensembles of single-walled carbon nanotubes[END_REF]. The general observation is that the percentage of semiconducting tubes is increased by an increase in oxygen content in the carbon precursor molecules. The explanation proposed for this phenomenon is an in situ selective etching of metallic SWCNT by the oxygen present in the reactor, which is a known method for selective growth [START_REF] Liu | Controlled growth of semiconducting and metallic single-wall carbon nanotubes[END_REF].

CVD ambient and selective etching

The fact that metallic SWCNTs are more chemically reactive to oxidation [START_REF] Liu | Chirality-dependent reactivity of individual single-walled carbon nanotubes[END_REF], has led some research groups to try to optimize the CVD ambient for selective etching of metallic SWCNTs [START_REF] Yu | Selective removal of metallic single-walled carbon nanotubes by combined in situ and post-synthesis oxidation[END_REF]. Introducing the suitable amount of oxygen (through carbon-containing molecules such as ethanol [START_REF] Ding | Selective growth of well-aligned semiconducting singlewalled carbon nanotubes[END_REF], water [START_REF] Che | Selective synthesis and device applications of semiconducting single-walled carbon nanotubes using isopropyl alcohol as feedstock[END_REF], [START_REF] Zhou | General rules for selective growth of enriched semiconducting single walled carbon nanotubes with water vapor as in situ etchant[END_REF]), or hydrogen (etching through formation of hydrocarbons instead of carbon oxides) [START_REF] Li | High-quality, highly concentrated semiconducting single-wall carbon nanotubes for use in field effect transistors and biosensors[END_REF], can substantially increase the semiconducting SWCNT content of a given sample (up to an estimated 93 % of SC SWCNTs for [START_REF] Li | High-quality, highly concentrated semiconducting single-wall carbon nanotubes for use in field effect transistors and biosensors[END_REF], for instance). UV irradiation has also been used to successfully etch metallic SWCNTs during growth [START_REF] Hong | Direct growth of semiconducting single-walled carbon nanotube array[END_REF].

This in situ etching has also been used to grow metallic-enriched SWCNT samples. Hou et al. optimized CVD conditions in order to simultaneously grow small-diameter semiconducting SWCNTs and large-diameter metallic SWCNTs, after which they introduced hydrogen in the reactor to selectively etch the semiconducting tubes [START_REF] Hou | Preparation of metallic single-wall carbon nanotubes by selective etching[END_REF] (estimated 88 % metallic SWCNT content), leading to the direct preparation of conducting SWCNT films. This study also shows the limitations of the selective etching methods: since the etching is also diameterdependent, it can only be fully efficient if the growth is highly diameter-selective.

Chapter 1. Introduction and state of the art

Choice of substrate

In the case of supported CVD growth, the catalyst nanoparticles are in contact, and therefore interact, with a substrate. The effect of the substrate on catalytic activity of nanoparticles has been demonstrated in the literature. For instance, the catalytic activity of gold nanoparticles for CO oxidation was demonstrated to be much lower when supported on Al 2 O 3 than on other chemically inert metal oxides [START_REF] Haruta | Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxide[END_REF]. The observed lattice-guided alignment of SWCNTs grown on single-crystal substrates such as quartz [START_REF] Yuan | Horizontally aligned single-walled carbon nanotube on quartz from a large variety of metal catalysts[END_REF] and sapphire [START_REF] Han | Template-free directional growth of singlewalled carbon nanotubes on a-and r-plane sapphire[END_REF] is also indicative of an influence of the substrate on SWCNT growth. The strength and extent of the anchoring of the catalyst nanoparticles onto the substrate varies, leading to different effects. Some influence of the substrate on growth selectivity has also been demonstrated. Ishigami et al. showed preferential growth of near zigzag nanotubes on A-plane sapphire, and near armchair on R-plane sapphire [START_REF] Ishigami | Crystal plane dependent growth of aligned single-walled carbon nanotubes on sapphire[END_REF], hinting to the importance of substrate choice.

Influence of the catalyst on selectivity

While a significant amount of studies emphasize on the control of CVD parameters and ambient for selective SWCNT growth, the choice of catalyst has also been put forward as one of the key parameters for achieving selectivity. Obviously so, the catalyst nanoparticle plays a key role in the growth mechanism, and has therefore to be strongly taken into account.

Size and morphology

The size and morphology of catalyst nanoparticles seem to play important roles in the selectivity of SWCNT growth. Harutyunyan et al. showed that by modifying CVD ambient, they could alter the morphology of Fe catalyst nanoparticles, with an impact on the electronic properties of the grown SWCNTs [START_REF] Harutyunyan | Preferential growth of single-walled carbon nanotubes with metallic conductivity[END_REF]. Many papers in the literature focus on the effect of the size of the catalyst particles and SWCNT growth selectivity.

There is experimental evidence of the control of SWCNT diameter through catalyst size control [START_REF] Kim | Heat-driven size manipulation of fe catalytic nanoparticles for precise control of single-walled carbon nanotube diameter[END_REF]- [START_REF] Song | Synthesis of bandgapcontrolled semiconducting single-walled carbon nanotubes[END_REF]. In these studies, the proposed explanation for this was that by controlling the diameter of the catalyst nanoparticles, and assuming a matching between nanoparticle diameter and SWCNT diameter, the SWCNT diameter distribution would inevitably be controlled. However, Fiawoo et al. [START_REF] Fiawoo | Evidence of correlation between catalyst particles and the single-wall carbon nanotube diameter: A first step towards chirality control[END_REF] have shown experimental proof of the existence of two growth modes. By defining the d SW CN T /d N P aspect ratio, they were able to discriminate between the tangential mode (d SW CN T /d N P 0.75), and the perpendicular mode (d SW CN T /d N P < 0.75). This implies that controlling SWCNT diameter through nanoparticle size control can only be done in the tangential growth mode. Moreover, in situ HRTEM experiments have shown that catalyst nanoparticles tend to continuously rearrange during CVD growth, modifying the d SW CN T /d N P ratio. This indicates that controlling SWCNT diameter may be done by controlling other growth parameters.

Composition

The interaction between carbon and the catalyst nanoparticle is at the heart of the SWCNT growth process. Since this interaction strongly depends on the nature of the catalyst, and its composition, an influence of these factors on growth selectivity is expected. Regarding the nature of the catalyst, meaning the element used as a catalyst, many results in the literature attest to the differences between metals when it comes to selectivity. For example, Chen et al. conducted a comparative study between monometallic Co-MCM-41 (MCM-41 is a commercially available mesoporous silica substrate) and Ni-MCM-41 [START_REF] Chen | Single-wall carbon nanotube synthesis by co disproportionation on nickel-incorporated mcm-41[END_REF]. It was found that the Co catalyst led to the growth of small diameter SWC-NTs with a narrow diameter distribution, whereas the Ni catalyst, in the same conditions, did not exhibit any selectivity. This was explained by the higher nucleation rate of Ni nanoparticles under CO. This experimental proof of the effect of the nature of the catalyst metal shows that tuning catalyst nanoparticle properties may be an interesting approach to selective SWCNT growth.

Using bimetallic catalysts is a good way to tune the properties of the catalyst nanoparticles, and it gives access to more possibilities. The use of bimetallic nanoparticles as catalysts for the growth of SWCNTs has been a popular option since the development of the CoMoCAT process [START_REF] Resasco | A scalable process for production of single-walled carbon nanotubes (swnts) by catalytic disproportionation of co on a solid catalyst[END_REF]. It was shown to produce SWCNT samples enriched in (6, 5) and (7, 5) chiralities (representing 38% of the SWCNT sample considering that metallic SWCNTs represented 1/3 of all nanotubes) [START_REF] Bachilo | Narrow (n, m)-distribution of single-walled carbon nanotubes grown using a solid supported catalyst[END_REF], and its selectivity was explained by a synergistic effect of Co and Mo [START_REF] Alvarez | Synergism of co and mo in the catalytic production of single-wall carbon nanotubes by decomposition of co[END_REF], sparking interest towards bimetallic catalysts. Since then, a significant amount of studies reporting selective growth from bimetallic catalysts has been published, using a wide variety of bimetallic combinations. A number of papers have reported near-armchair chiral selectivity, using, to name Chapter 1. Introduction and state of the art a few, FeCo [START_REF] Miyauchi | Fluorescence spectroscopy of single-walled carbon nanotubes synthesized from alcohol[END_REF], FeCu [START_REF] He | Predominant (6, 5) single-walled carbon nanotube growth on a copper-promoted iron catalyst[END_REF], FeRu [START_REF] Li | Selective synthesis combined with chemical separation of single-walled carbon nanotubes for chirality selection[END_REF], CoMo [START_REF] Wang | n, m) selectivity of single-walled carbon nanotubes by different carbon precursors on co-mo catalysts[END_REF], CoPt [START_REF] Liu | High temperature selective growth of single-walled carbon nanotubes with a narrow chirality distribution from a copt bimetallic catalyst[END_REF], FePt [START_REF] He | Environmental transmission electron microscopy investigations of pt-fe 2 o 3 nanoparticles for nucleating carbon nanotubes[END_REF], FeTi [START_REF] He | Fe ti o based catalyst for large-chiral-angle single-walled carbon nanotube growth[END_REF]. Some diameter control was also shown using CoRh [START_REF] Thurakitseree | Diameter controlled chemical vapor deposition synthesis of single-walled carbon nanotubes[END_REF], CoCr [START_REF] Loebick | Effect of chromium addition to the co-mcm-41 catalyst in the synthesis of single wall carbon nanotubes[END_REF], CoMn [START_REF] Loebick | Effect of manganese addition to the co-mcm-41 catalyst in the selective synthesis of single wall carbon nanotubes[END_REF], CoCu [START_REF] Cui | Synthesis of subnanometer-diameter vertically aligned single-walled carbon nanotubes with copper-anchored cobalt catalysts[END_REF] bimetallic systems.

Pioneering work by Chiang et al. demonstrated the influence of the composition of a Ni x Fe 1-x alloy on the selectivity of SWCNT growth. Using a microplasma reactor, the authors were able to synthesize monometallic Fe and Ni nanoparticles, as well as bimetallic alloy nanoparticles with various compositions, with similar sizes. The changes in composition of the Ni x Fe 1-x alloys led to changes in the chirality distribution of the resulting SWCNT sample [START_REF] Chiang | Linking catalyst composition to chirality distributions of as-grown single-walled carbon nanotubes by tuning ni x fe 1-x nanoparticles[END_REF]. Using a Ni 0.27 Fe 0.73 catalyst led to a narrower (n, m) distribution, with a single dominating [START_REF] Novoselov | Electric field effect in atomically thin carbon films[END_REF][START_REF] Iijima | Helical microtubules of graphitic carbon[END_REF] population, while the Ni 0.5 Fe 0.5 catalyst led to less selective growth. Semiconductor enriched samples were obtained using a specific alloy composition [START_REF] Chiang | Nanoengineering ni x fe1-x catalysts for gas-phase, selective synthesis of semiconducting single-walled carbon nanotubes[END_REF], highlighting the crucial role of catalyst composition in growth selectivity.

All of these studies attest to the significant interest directed towards catalyst composition optimization for selective SWCNT growth. However, the term "bimetallic" usually indicates that two metals were used for the preparation of the catalyst, or that the catalyst precursor contains two metals. For a long time, and aside from a few exceptions, the so-called bimetallic catalyst nanoparticles were rarely characterized, whether it is before or after SWCNT synthesis. When they are characterized, we can distinguish three types of bimetallic catalysts (see examples extracted from the literature in Figure 1.9). On rare occasions, the catalyst nanoparticles are proven to be nanoalloys (Figure 1.9.c), where the two metals involved in the catalyst synthesis form homogeneously mixed bimetallic nanoparticles [START_REF] Liu | High temperature selective growth of single-walled carbon nanotubes with a narrow chirality distribution from a copt bimetallic catalyst[END_REF], [START_REF] Chiang | Linking catalyst composition to chirality distributions of as-grown single-walled carbon nanotubes by tuning ni x fe 1-x nanoparticles[END_REF]. Here, the composition of the catalyst is thought to play an important role in the selectivity. In most cases, however, the bimetallic catalyst displays phase-segregation due to its fabrication process, and can take one of two general forms. The first one, analogous to the CoMoCAT catalyst, consists of small monometallic clusters anchored onto a metallic, or metal oxide matrix [START_REF] Loebick | Effect of chromium addition to the co-mcm-41 catalyst in the synthesis of single wall carbon nanotubes[END_REF], [START_REF] Loebick | Effect of manganese addition to the co-mcm-41 catalyst in the selective synthesis of single wall carbon nanotubes[END_REF], [START_REF] He | Predominant (6, 5) single-walled carbon nanotube growth on a copper-promoted iron catalyst[END_REF], [START_REF] He | Fe ti o based catalyst for large-chiral-angle single-walled carbon nanotube growth[END_REF] (Figure 1.9.b). In this particular case, the selectivity of the growth is owed to the size control of monometallic catalyst nanoparticles by the matrix. The second one consists of phase-segregated nanoparticles, with a "support" side made of an alloy or solid solution of the two metals, or simply one of the metals, and an effective catalyst side made of the other metal [START_REF] Cui | Synthesis of subnanometer-diameter vertically aligned single-walled carbon nanotubes with copper-anchored cobalt catalysts[END_REF], or a metal carbide [START_REF] He | Environmental transmission electron microscopy investigations of pt-fe 2 o 3 nanoparticles for nucleating carbon nanotubes[END_REF] (Figure 1.9.a).

Table 1.1 -Non-exhaustive list of bimetallic catalysts used for selective SWCNT growth, and experimental details. In most cases, the given catalyst preparation refers to the first step in the preparation process (impregnation, precipitation...), which is usually followed by a calcination step, and reduction. [START_REF] He | Precise determination of the threshold diameter for a single-walled carbon nanotube to collapse[END_REF][START_REF] Bethune | Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls[END_REF] [START_REF] Yang | Chirality-specific growth of single-walled carbon nanotubes on solid alloy catalysts[END_REF] or (16,0) [START_REF] Yang | Growing zigzag (16, 0) carbon nanotubes with structuredefined catalysts[END_REF] chirality-specific growths, depending on growth temperature (1030 • C or 1050 • C) using a cobalttungsten catalyst (Co-W). The highly refractory property of tungsten was supposed to be responsible for the high stability of the catalyst nanoparticles under growth conditions. The reason for the selectivity of the growth according to experimental observations and density functional theory (DFT) calculations (at 0 K without considering the presence of carbon during the growth process) by Y. Li et al. is based on a matching between the nanotube wall and a specific plane of the solid catalyst nanoparticle. However, there is little proof that the catalyst nanoparticles remain solid under CVD conditions. The actual structure of the catalyst nanoparticles during SWCNT growth is also still debated [START_REF] An | Chirality specific and spatially uniform synthesis of single-walled carbon nanotubes from a sputtered co-w bimetallic catalyst[END_REF]. Interestingly, the two studies cited here [START_REF] Yang | Chirality-specific growth of single-walled carbon nanotubes on solid alloy catalysts[END_REF], [START_REF] An | Chirality specific and spatially uniform synthesis of single-walled carbon nanotubes from a sputtered co-w bimetallic catalyst[END_REF] showed selectivity toward the same chirality population in two very different experimental setups, indicating that the Co-W bimetallic combination, whatever its structure during CVD growth, can be of great interest for understanding the role of the catalyst in growth selectivity.

We should note here that the use of the word "epitaxy" may not be adapted to the growth of SWCNTs. Epitaxy is a term used for the growth of an atomic layer on top of a flat crystalline surface, the atomic layer forms with the same orientation and lattice as the crystalline surface. To transpose this physical concept to SWCNT growth is probably not exactly accurate.

Growth selectivity characterization

Since their discovery, a number of techniques have been used for SWCNT characterization. The aim of this section is not to make an exhaustive presentation of these techniques, but rather to present the ones that are the most commonly used for SWCNT growth sample characterization, and selectivity assessment. The discussion will focus more on their potential biases in selectivity assessment than technical details of the techniques themselves.

Resonant Raman spectroscopy

Raman spectroscopy is one the most used characterization techniques in the field of SWCNT growth. It is based on the inelastic scattering of light by matter. The technique will be presented in more detail in Chapter 2. SWCNTs show several Raman-active modes, that provide information on their structure. The most widely used SWCNT Raman mode for selectivity assessment is the radial breathing mode (RBM), which corresponds to a vibration of the carbon atoms in the direction perpendicular to the tube axis [START_REF] Dresselhaus | Raman spectroscopy of carbon nanotubes[END_REF].

The RBM frequency (ω RBM ) is inversely proportional to the SWCNT diameter, and it can therefore be extracted from the Raman spectrum, using an empirical law of general formula ω RBM = A/d t + B, where A and B are constants determined experimentally or by calculations. Since the RBM is a resonant mode, it only appears on the Raman spectrum if the energy of the laser used to Chapter 1. Introduction and state of the art probe the sample is equal to, or within a resonance window around an optical transition energy of the SWCNT. It is therefore possible, using the Kataura plot, to determine whether the probed SWCNT is metallic or semiconducting. With careful calibration of the Kataura plot, an adapted ω RBM = f (d t ) law, and fine tuning of resonant conditions by adjusting the excitation energy, it is also possible to unambiguously determine the (n, m) indices of the SWCNT.

The resonant aspect of Raman spectroscopy implies that using one excitation wavelength is not sufficient for probing all SWCNT populations, leading to potential blind spots when characterizing a growth sample. The Raman crosssection is also known to be chirality dependent [START_REF] Doorn | Resonant raman excitation profiles of individually dispersed single walled carbon nanotubes in solution[END_REF], which can have a strong impact on quantitative selectivity data.

Transmission electron microscopy

Historically, TEM is the first technique used to identify and characterize nanotubes, as it was through TEM that they were first discovered. As Raman spectroscopy, this technique will be presented in more detail in the next chapter. The basic principle of TEM is to shine an electron beam on a thin sample, and collect the beam transmitted through the sample. TEM imaging is one of the most robust SWCNT characterization methods. The many techniques related to TEM allow the unambiguous structural characterization of SWCNT samples. A routine TEM with limited resolution can give basic information on nanotubes: diameter, number of walls, length. With aberration-corrected microscopes, it is also possible to determine the chirality of SWCNTs in atomically resolved imaging mode. With the adapted experimental techniques, it is possible to acquire an electron diffraction (ED) pattern from an isolated SWCNT, from which the chirality of the SWCNT can be extracted [START_REF] Lambin | Structural analysis by elastic scattering techniques[END_REF]. The advantage of TEM is that it allows the direct observation of all SWCNTs, regardless of their electronic or optical properties. TEM is also a very powerful tool to make joint observations of the SWCNT and its catalyst nanoparticle, whether it be post-synthesis, or in situ observation.

Optical absorption spectroscopy

Optical absorption spectroscopy (OAS) is a very common technique for the characterization of a wide variety of materials. The sample is exposed to a monochromatic light beam, and the intensity of the radiation at the exit of the sample is compared to the incident intensity. This is done over a range of wavelengths, to form an absorption spectrum. For SWCNTs, an absorption peak Chapter 1. Introduction and state of the art This broadening is due to the heterogeneity of the sample's chirality distribution, and to environmental effects. When the sample contains a range of chiralities, absorption occurs for all allowed transitions over a range of energies, corresponding to several unresolved (n, m) specific peaks. Bundling effects, or the interaction with other compounds also contribute to peak broadening. The absorption cross-section is also chirality-dependent [START_REF] Vialla | Chirality dependence of the absorption cross section of carbon nanotubes[END_REF], making it difficult to quantitatively determine the chirality or diameter distribution, or the SC/M ratio. Moreover, enven though absorption experiments have been conducted on isolated SWCNTs, a large amount of SWCNTs are necessary to obtain measurable absorption peaks, meaning that this characterization technique is not well adapted to all sample types. Nonetheless, a non-negligible advantage of this technique is that in principle, it enables the observation of all SWCNTs, regardless of chirality, diameter, or SC/M character.

Photoluminescence excitation spectroscopy

The process through which light is absorbed by matter, leading to the formation of an excited state, which is relaxed to a ground state by emission of light with a lower energy, is called photoluminescence (PL). PL in SWCNTs was first reported in 2002 [START_REF] O'connell | Band gap fluorescence from individual single-walled carbon nanotubes[END_REF]. In the specific case of SWCNTs, the PL process can be described as follows: a photon is absorbed, creating an electron-hole pair, that is annihilated by emission of a lower energy photon. The absorption occurs at an allowed optical transition energy (E ii ), corresponding to the distance between a pair of vHSs, and the emission at a lower transition energy, mostly E SC 11 . The time between absorption and emission is a few nanoseconds, implying that "fluorescence" is a more precise term define the process, but the more general term PL is mostly used [START_REF] Lefebvre | Photoluminescence: Science and applications[END_REF].

In PL excitation (PLE) spectroscopy, a map of PL emission intensity (emission energy along the x-axis) as a function of excitation energy is plotted. An intense spot on a PLE map corresponds to an intense emission, as a result of an excitation at a higher transition energy. For each (n, m) population present in the sample, PLE provides a measurement of the corresponding (E SC 11 , E SC 22 ) pair. Those (E SC 11 , E SC 22 ) spots are present in a certain energy domain on the PLE map called the "fingerprint region". The chirality distribution of the sample can be extracted from this region (see Figure 1.10).

The main drawback of PLE for the assessment of growth selectivity is that since the PL phenomenon is specific to semiconducting SWCNTs, only those can be detected. Bundled nanotubes do not luminesce, making this technique suitable for isolated SWCNTs only. More extensive work on chirality-dependent PL intensity is also needed for accurate quantitative analysis of the chirality distribution [START_REF] Tsyboulski | Structure-dependent fluorescence efficiencies of individual singlewalled carbon nanotubes[END_REF]. Moreover, it has been shown that the luminescence depends strongly on the environment.

Atomic force microscopy

Atomic force microscopy (AFM) is often used for the measurement of SWCNT diameters supported on flat substrates. It is a common characterization technique for evaluating the topography of a sample. The principle of AFM resides in the detection of forces involved when a surface is approached and scanned by a tip [START_REF] Binnig | Atomic force microscope[END_REF]. The force field between the extremity of this tip and the atoms beneath it is measured. Since the force field depends on the distance between the tip and the surface, their measurement gives insights into the topography of the sample.

When used for SWCNT characterization, AFM is used for length and diameter measurement. The length of a supported straight SWCNT is easily extracted from a topographic image of the substrate, and the diameter is determined by measuring the height of the nanotube laying flat on the substrate surface. AFM cannot give access to the chirality, or semiconducting or metallic character of the SWCNT. Contrary to TEM, the number of walls cannot be evaluated.

SC/M ratio evaluation through device fabrication

In a significant amount of studies, the performance of electronic devices fabricated using as-grown SWCNTs is used as a way to evaluate the SC/M ratio of a given sample. As mentioned previously, an application of interest of SWCNTs is their use in FETs, the fabrication of which relies on the obtention of high quality semiconducting SWCNTs. A FET is an electronic device composed of a semiconducting channel connected on either side to a source, and a drain electrode. A voltage is applied between the source and a third electrode, called the gate, which is separated from the channel by a dielectric material. This voltage controls the current through the channel. A way to measure the performance of a FET is to calculate the on-current to off-current ratio (I on /I of f ): a high I on /I of f (above 10 4 at minimum) means a high on-current, combined with a low leakage current, which is associated with high performance. Usual (I on /I of f ) ratios for SWCNT-based FETs are in the 10 5 -10 7 range [START_REF] Avouris | Carbon-based electronics[END_REF].

Chapter 1. Introduction and state of the art

In a SWCNT-based FET, the channel material is either an array of semiconducting SWCNTs, or an individual semiconducting SWCNT. The direct integration of synthesized SWCNTs in these electronic devices can therefore be a way to indirectly measure the semiconductor content of a sample. The general idea behind this measurement is that the presence of metallic SWCNTs will have a tendency to degrade the performance of the devices, leading to low I on /I of f ratios.

Use of characterization techniques for selectivity evaluation

As we have established, a few characterization techniques can be used to determine whether a SWCNT growth is selective. Those techniques give access to different types of information, with different sources of bias, and there is no perfectly accurate way to evaluate the diameter distribution, chirality distribution, or SC/M ratio of a SWCNT growth sample. In order to have a better understanding of the use of these techniques for selectivity assessment by research groups, we looked closely at more than 50 papers on the CVD growth of SWCNTs, published by a dozen of research groups from the early 2000's to 2016, with a focus on selective growth. We will quickly review the methodologies for selectivity assessment affiliated with the previously presented techniques, the use of cross-characterization in general, and finally the occurrence of TEM and Raman cross-characterization in those papers.

Selectivity evaluation methods related to characterization techniques

As explained previously, Raman is used in selectivity assessment to determine the diameter distribution, SC/M ratio, and sometimes (n, m) distribution of samples. In most cases, Raman is simply employed as a qualitative evaluation technique to assess the evolution of diameter range [START_REF] Li | Diameter tuning of single-walled carbon nanotubes with reaction temperature using a co monometallic catalyst[END_REF] or SC/M ratio [START_REF] Ding | Selective growth of well-aligned semiconducting singlewalled carbon nanotubes[END_REF] with growth condition changes. One can easily assess a shift in diameter distribution towards higher diameters through an increase in growth temperature by observing a global shift of RBM signals towards lower frequencies, for instance [START_REF] Miyauchi | Fluorescence spectroscopy of single-walled carbon nanotubes synthesized from alcohol[END_REF], [START_REF] Wang | Low temperature growth of single-walled carbon nanotubes: Small diameters with narrow distribution[END_REF].

Raman is also used as a quantitative selectivity assessment method. Depending on the sample type, different methodologies are applicable. Here, we will distinguish two types of samples: diluted SWCNT mats on flat substrates, or "bulk" SWCNT samples, obtained from powder supported catalysts, or by floating catalyst CVD, or dispersed in solution. If the sample is supported on a flat substrate, it is necessary to acquire multiple spectra (mappings) to obtain a global view of the sample. In the case of supported catalysts in the form of powders, or unsupported catalysts (so-called "bulk" samples), the amount of SWCNTs probed under one laser spot is much higher, and one spectrum is sufficient to get an overview of the sample.

In the latter case, quantitative selectivity evaluation is usually performed by looking at RBM peak intensities, relying on the hypothesis that peak intensity is directly related to the abundance of a certain species. This can only be accurate if the chirality dependence of Raman cross-sections is taken into account (see Chapter 2.3.2.3). When it comes to using Raman for SC/M ratio evaluation, various methods have been implemented over the years. Some groups relied on a comparative study of RBM intensities attributed to metallic or semiconducting SWCNTs [START_REF] Harutyunyan | Preferential growth of single-walled carbon nanotubes with metallic conductivity[END_REF], [START_REF] Chiang | Nanoengineering ni x fe1-x catalysts for gas-phase, selective synthesis of semiconducting single-walled carbon nanotubes[END_REF] (see Figure 1.11). In other cases, the SC/M ratio is calculated on global spectra of a "bulk" sample, by counting the number (n, m) indexed RBM peaks corresponding to semiconducting tubes, and those corresponding to metallic tubes, without taking into account multiple SWCNTs were actually contributing to the peak intensity [START_REF] Zoican Loebick | Selective synthesis of subnanometer diameter semiconducting single-walled carbon nanotubes[END_REF].

In the case of flat substrates, when acquiring Raman mappings on a less abundant sample, the usual method is to acquire multiple Raman spectra, and count each RBM peak as representative of one resonating SWCNT, not taking into account their relative intensities. The hypothesis that one RBM peak counts as one SWCNT cannot be systematically verified, and is used for simplification. This methodology has been widely used in the past few years for the determination of chirality distribution using four [START_REF] Yang | Growing zigzag (16, 0) carbon nanotubes with structuredefined catalysts[END_REF], [START_REF] An | Chirality specific and spatially uniform synthesis of single-walled carbon nanotubes from a sputtered co-w bimetallic catalyst[END_REF], or three excitation wavelengths [START_REF] Zhao | Chemical vapor deposition synthesis of near-zigzag single-walled carbon nanotubes with stable tube-catalyst interface[END_REF], diameter distribution [START_REF] He | Diameter and chiral angle distribution dependencies on the carbon precursors in surfacegrown single-walled carbon nanotubes[END_REF], or SC/M ratio using four [START_REF] Liu | Controlled growth of semiconducting and metallic single-wall carbon nanotubes[END_REF], or two [START_REF] Zhang | Growth of semiconducting single-wall carbon nanotubes with a narrow band-gap distribution[END_REF], [START_REF] Qin | Growth of semiconducting single-walled carbon nanotubes by using ceria as catalyst supports[END_REF] excitation wavelengths. The value(s) of the excitation wavelength(s), as well as their number, varies from one study to another. Figure 1.11 shows an example of SC/M ratio determination from a "bulk" sample (left), using relative intensities of RBM peaks, and from SWCNTs on a flat substrate (right), where each RBM peak counts as one SWCNT in the statistical SC/M ratio evaluation.

TEM is mostly used as a way to attest that SWCNTs are grown, and to quickly verify the diameter range of a sample without statistical study [START_REF] Zoican Loebick | Selective synthesis of subnanometer diameter semiconducting single-walled carbon nanotubes[END_REF], [START_REF] Ciuparu | Uniformdiameter single-walled carbon nanotubes catalytically grown in cobaltincorporated mcm-41[END_REF]- [START_REF] Li | Selective synthesis of large diameter, highly conductive and high density single-walled carbon nanotubes by a thiophene-assisted chemical vapor deposition method on transparent substrates[END_REF]. Some groups determine the diameter distribution of a sample from TEM images [START_REF] Zhang | Growth of semiconducting single-wall carbon nanotubes with a narrow band-gap distribution[END_REF], [START_REF] Liu | Diameter-selective growth of single-walled carbon nanotubes with high quality by floating catalyst method[END_REF]. Even though measuring a SWCNT diameter from a TEM image is not trivial [START_REF] Fleurier | Transmission electron microscopy and uv-vis-ir spectroscopy analysis of the diameter sorting of carbon nanotubes by gradient density ultracentrifugation[END_REF], no information is given on the method used. In very of the amount of a certain SWCNT chirality [START_REF] Wang | Selective synthesis of (9, 8) single walled carbon nanotubes on cobalt incorporated tud-1 catalysts[END_REF], or the intensities are "corrected" by taking into account (n, m)-dependent quantum efficiency [START_REF] Wang | Pressureinduced single-walled carbon nanotube (n, m) selectivity on co-mo catalysts[END_REF].

On the rare occasions where AFM is featured in SWCNT growth papers, the technique is systematically used to determine the diameter distribution of a sample, through height measurement [START_REF] Ding | Selective growth of well-aligned semiconducting singlewalled carbon nanotubes[END_REF], [START_REF] Kim | Size engineering of metal nanoparticles to diameter-specified growth of single-walled carbon nanotubes with horizontal alignment on quartz[END_REF].

Finally, there is no pre-defined procedure to determine the SC/M ratio from electronic devices. In a few papers, the fabrication of FETs is only used to prove the capacities of the grown SWCNTs as a channel material. Quantitative determination of the SC/M ratio is measured in several different ways. For multiple-SWCNT devices with over 500 tubes between the source and the drain, Ding et al. suggested that the I on /I of f directly represented the average SC/M ratio [START_REF] Ding | Selective growth of well-aligned semiconducting singlewalled carbon nanotubes[END_REF]. For individual-SWCNT devices, it is possible to count the SC devices, using a somewhat arbitrary threshold I on /I of f value. Certain groups considered a device to be semiconducting if I on /I of f was above 10 [START_REF] Zhang | Diameter-specific growth of semiconducting swnt arrays using uniform mo2c solid catalyst[END_REF], [START_REF] Li | Importance of diameter control on selective synthesis of semiconducting single-walled carbon nanotubes[END_REF]. On the other hand, Li et al. considered a device to be metallic when I on /I of f was below 100 [START_REF] Li | Selective synthesis of large diameter, highly conductive and high density single-walled carbon nanotubes by a thiophene-assisted chemical vapor deposition method on transparent substrates[END_REF].

For each presented technique, aside from AFM and ED, the methodology for statistical, or quantitative selectivity assessment is never quite clear. Different groups tend to use different selectivity evaluation methodologies when using the same characterization technique, inevitably leading to different results. Whether or not the shift from one result to another is significant is difficult to establish. In summary, absolute SC/M ratio values, chirality enrichment percentages, or mean diameters are to be compared from one study to another with a bit of caution, even if the characterization technique is the same.

SWCNT sample cross-characterization in the literature

Considering that there is no absolute characterization technique, cross-characterization is indispensable for selectivity assessment. In almost all of the selected studies, more than one technique is used for the characterization of SWCNT samples. For a wide majority, more than two techniques are used, and including device fabrication, up to six characterization techniques can be used. When it comes to selectivity assessment, more than one technique are used in a very large majority of cases. However, studies comparing quantitative, or statistical data coming from more than one characterization technique only represent a quarter of the studies with selectivity claims. In most cases, quantitative evaluation is done using a technique of choice, and another (or a few others) are relied upon for consistency. For instance, TEM images showing one or two SWCNTs can be used to ensure that the diameter range determined by a statistical diameter analysis conducted through AFM is reliable [START_REF] Zhang | Diameter-specific growth of semiconducting swnt arrays using uniform mo2c solid catalyst[END_REF], or absorption can be used to qualitatively confirm a quantified chirality enrichment measured by PLE [START_REF] Liu | High temperature selective growth of single-walled carbon nanotubes with a narrow chirality distribution from a copt bimetallic catalyst[END_REF]. This makes it difficult to estimate and compare the accuracy of quantitative methods for selectivity assessment.

Focus on TEM and Raman

In an effort to try to evaluate the relative accuracy of available characterization techniques for the quantitative measurement of growth selectivity, a part of the work presented in this thesis will focus on the comparison between conventional TEM, and multi-wavelength resonant Raman spectroscopy for the determination of the diameter distribution of a SWCNT growth sample.

When looking at the number of publications with growth selectivity claims in the literature, Raman and TEM (conventional TEM, and electron diffraction combined) seem to be, by far, the two most used characterization techniques, with more than two thirds of the selected papers featuring TEM, and more than three quarters featuring Raman (the next most used characterization technique is OAS, used in less than half of the selected papers). It is striking, however, to see that quantitative data are extracted from TEM and Raman in only a fraction of these publications. Diameter distributions are extracted from conventional TEM data in only 30% of the time, and quantitative selectivity evaluation is performed in only 45% of papers featuring Raman spectroscopy (SC/M ratio calculation, diameter distribution determination, and (n, m) indexing combined). This is even more surprising considering that electronic devices, AFM, and PLE are used for quantitative evaluation in a wide majority of cases.

Though cross-characterization between TEM and Raman is often performed, comparison of statistical data from the two methods is rarely seen. We found two papers featuring statistical data for both TEM and Raman regarding SWCNT sample diameter distributions. Song et al. determined the diameter distributions of several SWCNT samples by measuring the diameter of 50 tubes by TEM, and counting RBM peaks from several Raman spectra using one excitation wavelength [START_REF] Song | Synthesis of bandgapcontrolled semiconducting single-walled carbon nanotubes[END_REF]. The two distributions for each sample (mean diameter ranging from 0.9 nm to 1.5 nm) appear to be close to identical, with mean [START_REF] He | Diameter and chiral angle distribution dependencies on the carbon precursors in surfacegrown single-walled carbon nanotubes[END_REF]. The two distributions are similar, but their shape differ and there is a slight shift in mean diameter (0.2 nm). This is explained by the fact that the Raman characterization is done using only one excitation wavelength, potentially setting aside multiple SWCNT populations.

From the conclusions of these two publications, it is unclear whether TEM and Raman can coincide for diameter distribution assessment. In both cases, close results are obtained, but not satisfyingly enough in one of the papers. Moreover, the Raman characterization in both cases is conducted with only one excitation wavelength, inevitably making the probing of the SWCNT samples incomplete. It would therefore be interesting to conduct a comparative study between TEM and Raman for diameter distribution assessment, using multiple wavelengths, and types of samples.

Theoretical and experimental insights into SWCNT nucleation and growth

Since nanotubes were first synthesized, researchers have proposed various theories for their growth mechanism. Throughout the years, models have been both confirmed and refuted by experimental results, highlighting the complexity of the matter. But the aim of chirality selective growth has kept this motivation to understand the CVD growth process very much alive. Unfortunately, though the understanding of the mechanisms driving SWCNT nucleation and growth is the theoretical key to chirality selective growth, many questions on the subject are yet to be answered [START_REF] Amara | Modeling the growth of single-wall carbon nanotubes[END_REF].

In this section, we will first give a chronological overview of the proposed growth mechanisms for nanotubes. The latest theoretical insights on growth mechanisms will then be discussed, and the resulting suggestions for achieving selective growth will be presented.

Chapter 1. Introduction and state of the art

The VLS growth model

In the early 1970's, when Baker et al. [START_REF] Baker | Formation of filamentous carbon from iron, cobalt and chromium catalyzed decomposition of acetylene[END_REF] grew carbon nanofibers from the catalytic decomposition of C 2 H 2 on metallic particles, they developed a model to understand their growth mechanism. They made the hypothesis that the vaporliquid-solid (VLS) model proposed to explain the growth of Si whiskers [START_REF] Wagner | Vapor-liquid-solid mechanism of single crystal growth[END_REF] was adaptable to the case of carbon nanofibers (CNF). This growth mechanism can be decomposed into three steps:

1. Adsorption and dissociation of the carbon-containing molecules at the surface of the catalyst nanoparticle, to form elementary carbon 2. Dissolution of the carbon in the bulk of the particle, leading to the formation of a liquid metal-carbide, and diffusion of the carbon in the bulk of the particle 3. When the catalyst particle becomes supersaturated with carbon, precipitation of solid carbon at the surface of the particle

The continuous supply of carbon keeps the fiber growth going until deactivation of the catalyst nanoparticle. According to Baker et al., the driving force of the diffusion of carbon through the bulk of the particle was a temperature gradient caused by the exothermicity of carbon precursor dissociation at the surface of the nanoparticle, and the endothermicity of the precipitation of solid carbon. This interpretation, however, could not explain CNF growth using carbon precursors whose decomposition is endothermic, which is the case for CH 4 [START_REF] Evans | Growth of filamentary carbon on metallic surfaces during the pyrolysis of methane and acetone[END_REF] for instance. It was later demonstrated that the driving force of the mechanism was in fact a chemical potential gradient [START_REF] Tibbetts | Why are carbon filaments tubular?[END_REF] between the carbon atoms brought to the surface of the nanoparticle by the decomposition of the carbon feedstock molecules, and the carbon sp 2 wall [START_REF] Snoeck | Filamentous carbon formation and gasification: Thermodynamics, driving force, nucleation, and steady-state growth[END_REF].

In some ways, SWCNT growth is similar to CNF growth, and the VLS mechanism still serves as a foundation to explain SWCNT growth today. The considered objects here are different, essentially much smaller, so we can expect different behaviors. Over the years, a few variations of this model were proposed to explain SWCNT growth. HRTEM evidence [START_REF] Gavillet | Root-growth mechanism for single-wall carbon nanotubes[END_REF]. In this case, the mechanism (root-growth) was applied to both CVD and high temperature synthesis of SWCNTs, and an emphasis was put on the competition between growth and deactivation of the catalyst by encapsulation by a graphitic shell. Today, the experimental and theoretical study of the growth mechanism is far from over, and many questions remain debated, or simply unanswered. Here is a non-exhaustive list of these questions:

• Does the carbon diffuse through the bulk or the surface of the catalyst?

• What is the physical state of the particle?

• What is the chemical state of the particle?

• What allows the nucleation, and growth of a SWCNT?

• How is the growth stopped?

From the VLS to the VSS model

As mentionned above, one of the big debates concerns the physical state of the catalsyt nanoparticle during the growth. According to the VLS model, the nanoparticle should be a liquid droplet of metal supersaturated with carbon. However, starting with the first in-situ HRTEM experiments during growth, various groups have claimed that catalyst nanoparticles could be solid during the growth process for MWCNTs [START_REF] Helveg | Atomicscale imaging of carbon nanofibre growth[END_REF], and later SWCNTs [START_REF] Hofmann | In situ observations of catalyst dynamics during surface-bound carbon nanotube nucleation[END_REF], [START_REF] Yoshida | Atomicscale in-situ observation of carbon nanotube growth from solid state iron carbide nanoparticles[END_REF]. There have also been experiments and theoretical arguments supporting surface diffusion of carbon. This goes against the VLS model, which implies bulk diffusion of carbon through catalyst nanoparticles. This gave rise to a new model to explain SWCNT growth: the vapor-solid-solid (VSS) model (first proposed for the growth of GaAs nanofibers [START_REF] Persson | Solid-phase diffusion mechanism for gaas nanowire growth[END_REF]), where the catalyst nanoparticle is crystalline, and the carbon diffuses through the surface of the catalyst only.

This model encouraged the development of a strategy for chirality specific growth based on so-called epitaxy [START_REF] Reich | Control the chirality of carbon nanotubes by epitaxial growth[END_REF]. DFT simulations have been used to support this strategy, by calculating the interaction energy between nanotube caps with various chiralities and a flat metal surface on which the cap is lying (implying a perpendicular growth mode), and looking for lattice matching [START_REF] Reich | Epitaxial growth of carbon caps on ni for chiral selectivity[END_REF]. This kind of calculation is performed at T = 0 K, and is therefore not appropriate to model the state of a crystalline metal surface at typical growth temperatures.

Chapter 1. Introduction and state of the art Furthermore, this strategy relies on the idea that the cap formed during nucleation will necessarily determine the chirality of the resulting SWCNT. This idea is realistic because the breaking and reforming of a C-C bond to heal or make a defect that would modify the tube structure is quite energy consuming. However, in situ studies have shown that short nanotubes at the very beginning of growth tend to be unstable [START_REF] Hofmann | In situ observations of catalyst dynamics during surface-bound carbon nanotube nucleation[END_REF]. Statistical TEM studies on SWCNT lengths conducted after growth evidenced the existence of a length threshold of about 5 mn after which durable growth could occur [START_REF] Fiawoo | Evidence of correlation between catalyst particles and the single-wall carbon nanotube diameter: A first step towards chirality control[END_REF]. Page et al. [START_REF] Page | Mechanisms of singlewalled carbon nanotube nucleation, growth, and healing determined using qm/md methods[END_REF] showed that the number of pentagons, hexagons, or heptagons formed strongly depended on carbon feedstock decomposition rate. A fast decomposition leads to a high growth rate, which occurs with the formation of a high density of defects (pentagons and heptagons), eventually leading to structure modification of the nanotube during growth. A slower decomposition rate will give more time for healing defects during growth, leading to a better chance of conserving the cap chirality through the growth process.

More recently, with a similar objective of so-called expitaxial-like growth, studies aiming at finding adequate lattice matching between certain SWCNT chiralities and crystalline planes on tungsten-based catalysts have been conducted [START_REF] Yang | Chirality-specific growth of single-walled carbon nanotubes on solid alloy catalysts[END_REF], [START_REF] Zhang | Arrays of horizontal carbon nanotubes of controlled chirality grown using designed catalysts[END_REF]. However, there is no direct way to prove that the specific lattice planes matching with certain tube chiralities actually exist in the CVD growth conditions. During CVD, high temperature, the presence of carbon-containing molecules (and possibly other compounds) in the system, and the presence of carbon dissolved in the particle strongly affect its properties. We should also note that the fact that in situ TEM experiments have allowed the observation of crystalline catalysts during growth does not mean that it is the case in "real" CVD conditions (higher pressure and feedstock flow rate, possibly different substrate...).

To summarize, if the catalyst is crystalline during growth (which is possible and clearly depends on the catalyst size and composition, as well as CVD parameters), and if the cap formation determines SWCNT chirality (by somehow promoting a slow growth rate), then so-called epitaxial (which does not constitute an actual epitaxy, but rather an orientation relation between tube and particle at most) growth could be a viable strategy. Aside from these conclusions solely concerning cap structure engineering, computer simulations on the nucleation of SWCNTs can give very insightful information on the role of the [START_REF] Amara | Understanding the nucleation mechanisms of carbon nanotubes in catalytic chemical vapor deposition[END_REF]. The method allows to take into account the carbon chemical potential (µ C ) gradient, which is the thermodynamic driving force of the nucleation, which is not the case for in molecular dynamics studies.

The carbon chemical potential is experimentally fixed by the thermochemistry driving the carbon precursor decomposition at the surface of the catalyst, which can essentially be controlled by CVD parameter tuning. The calculation of adsorption isotherms on Ni nanoparticles revealed the existence of an optimal µ C window for possible SWCNT growth. It was also shown that sp 2 carbon segregation was initiated when C content in the particle attained a solubility limit. The findings were in good agreement with the criterium experimentally determined according to which a metal can be a good catalyst for SWCNT growth if it has a non-zero but low carbon solubility [START_REF] Deck | Prediction of carbon nanotube growth success by the analysis of carbon-catalyst binary phase diagrams[END_REF]. In 2012, Diarra et al. used the same model to simulate the very early stages of SWCNT growth [START_REF] Diarra | Importance of carbon solubility and wetting properties of nickel nanoparticles for single wall nanotube growth[END_REF]. They proceeded by fixing short nanotube sections to a small Ni nanoparticle (around 80 C atoms and 80 Ni atoms total), with varying chiralities within a diameter range of 0.7-0.8 nm, heating them, and letting the systems relax to their equilibrium state. Those systems were then used as starting points for GCMC simulations, where a wide range of parameter (temperature, µ C ) combinations were tested, only to keep the 10 where growth was successful. These calculations allowed the atomic-level observation of a tangential growth mechanism. Since it had been previously demonstrated that carbon solubility played an important role in the nucleation process, they tried to understand how this solubility was affected by size or temperature. This paper also discussed the wetting properties of sp 2 carbon on a Ni nanoparticle as a function of its carbon content. As demonstrated much earlier for the wetting properties of bulk metal droplets on graphite [START_REF] Naidich | Wetting of graphite by nickel as affected by the liquid-phase dissolution process of carbon[END_REF], dewetting was favored with increasing C content.

In light of the simulations of SWCNT growth, the growth phenomenon was interpreted as a balance between two antagonist phenomena: dewetting of the team shows a strong dependence of d SW CN T /d N P on carbon content inside the particle. It is interesting to note that these two growth modes result in two different tube-catalyst interaction schemes, tangential growth implying surface interaction, and perpendicular growth only a line interaction. These two types of interactions mean two ways of trying to influence growth by controlling the tube-nanoparticle interaction.

We can extract strategies for controlling the growth of SWCNTs from these findings. A possibly interesting way to move towards chirality-specific growth would be twofold: first a good control of the nanoparticle diameter distribution, and second a control of the growth mode [START_REF] Amara | Modeling the growth of single-wall carbon nanotubes[END_REF]. The control of catalyst nanoparticle size has to be done through control of the nanoparticle synthesis and optimization of their deposition on the substrate, or of CVD parameters, in order to prevent coalescence. The control of the growth mode, is achievable in two ways: first through CVD parameters, and second through fine tuning of carbon solubility in the catalyst nanoparticles. For the latter, of course, modifying the catalyst nanoparticle's properties through its composition could be a promising route towards growth selectivity through growth mode control. Aside from selective growth considerations, tuning the catalyst's interaction with carbon through its composition could be an interesting experimental way to better understand the role of carbon solubility in SWCNT growth in general.

Objectives of this work

In this chapter, we have seen that the research on selective growth in the last decades has been extremely abundant and fruitful. However, many questions remain regarding what exactly is responsible for growth selectivity, and the different effects growth parameters and catalyst nature have on growth selectivity.

We have established that it is sometimes difficult to extract trends from the literature, for a few reasons. First, there are clearly contradicting results regarding the effect of some growth parameters. This can be explained by the second reason for which the study of literature on SWCNT growth is a quite complex manner: growth conditions obviously vary drastically from one study to another. Each laboratory has its own CVD reactor, employs different carbon sources, at varying flowrates, in varying temperature and pressure conditions.

It is clear, when analyzing the literature, that the use of bimetallic catalysts for Chapter 1. Introduction and state of the art selective growth may be an interesting route. This, judging both from promising experimental results, and theoretical calculations. Modulating the catalyst properties and its interactions with carbon atoms through nanoalloying has indeed been shown to be a potentially interesting strategy. We have shown in this chapter that the comparison of different bimetallic combinations and their performances for selective growth is rendered very impractical by the fact that growths are conducted in different conditions, the catalyst nature is not always clear, the catalyst nanoparticles are not always alloys, and their preparation methods vary drastically from one study to another.

In order to have experimental insights into the effect of catalyst composition on SWCNT growth selectivity, it is necessary to elaborate a SWCNT growth method enabling the use of a wide variety of bimetallic combinations as catalysts with similar sizes and morphologies in the same conditions, without changing the catalyst preparation method depending on the chosen combination. We have evidenced that in a majority of cases, when catalyst nanoparticles are fabricated through physical routes (Table 1.1), the bimetallic catalyst nanoparticles exhibit phase segregation, meaning that the effective catalyst, in so-called bimetallic catalysts, is in fact monometallic. It is therefore important to make sure that the preparation method leads to nanoalloys. To achieve this, we have chosen to focus on chemically-synthesized catalyst precursor nanoparticles with the two metals already mixed within the structure. This route is original, because it allows for the deposition of preformed catalyst precursors with controlled sizes and composition.

The first objective of this work is therefore the elaboration of this SWCNT synthesis route, using various chemically-synthesized catalyst precursors. We have chosen, in a first attempt, to work using Prussian blue (PB), and its analogs as catalyst precursors. Three catalyst systems have been selected: a pure iron catalyst, obtained by the reduction of PB, and two bimetallic systems obtained by the reduction of PB analogs (PBAs). To test the ability to produce nanoalloys with the developed method, we selected a first system whose bulk binary phase diagram predicts the formation of a solid solution in the case of a 1:1 stoichiometry, and a system for which a phase-segregation is expected at a 1:1 stoichiometry. The first system is the Fe-Ni bimetallic system [START_REF] Swartzendruber | The fe-ni (iron-nickel) system[END_REF], and the second is the Cr-Ni bimetallic system [START_REF] Turchi | Modeling of ni-cr-mo based alloys: Part i-phase stability[END_REF] (see Appendix B for binary phase diagrams). In the case of the Ni-Fe system, we expect the formation of a nanoalloy with a lower carbon solubility limit than pure iron, which will enable a comparison based on carbon solubility. Two other chemically synthesized precursor types ("cyanosols", and polyoxometalates (POMs)) were also selected for their potential to form other nanoalloys of interest for SWCNT growth. An Fe-Pd system, with a even lower carbon solubility than Fe-Ni will be studied through the "cyanosol" (which will be defined in Chapter 2) route, and a Co-W system will be studied using POM precursors.

The second objective will be to evaluate the catalysts' abilities to grow SWCNT with any kind of selectivity, and to try to optimize the synthesis conditions with selectivity aims. The comparison of the different prepared catalyst systems with regard to SWCNT growth will be attempted.

Our review of the literature has also led to the realization that there is a strong need for a standardized characterization methodology for selectivity assessment of CVD-grown SWCNT samples. The third objective of this work will be to try to evaluate the accuracy of the two available characterization techniques (TEM and Raman spectroscopy). This will be done through a comparative study on various SWCNT samples, focusing on the determination of the diameter distributions of those samples.

Chapter 2

Experimental methods

The previous chapter ended by a brief presentation of the objectives of this work. With the goal to obtain alloyed bimetallic catalyst nanoparticles, we chose to work with chemically synthesized catalyst precursors with defined sizes, structures, and compositions. The first section of this chapter will give a brief description of the catalyst precursor "families" that have been chosen for this study, along with an explanation on how they will be synthesized and/or used as SWCNT growth catalyst precursors. The first presented family (Prussian blue and its analogs) is the one that was the most extensively studied. The other two families were studied as preliminary work. The second section of the chapter is a presentation of the two characterization techniques that have been used throughout this work for SWCNT sample characterization. The characterization methodology for selectivity assessment, and more precisely diameter distribution measurement of SWCNT samples, will also be discussed.

Chemically synthesized nanoalloy catalyst precursors

This section will focus on the three different types of catalyst precursors that have been chosen for this thesis. The first type, Prussian blue (PB) and its analogs (PBA), is the one that has been the main focus of this work. The second is, in a way, a derivative of the PBA family, known as the "cyanosol" family. The third type of catalyst precursors, polyoxometalates (POM), are studied here because a specific compound from this family has been proven to lead to some selectivity in SWCNT growth.

Chapter 2. Experimental methods ) and a ferric (Fe(III)) salt (chloride or nitrate, for instance) in aqueous solution. The origin of its color and its crystallographic structure were uncovered much later. Though structures were proposed with the emergence of X-ray diffraction experiments in the 30's [START_REF] Keggin | Structures and formulae of the prussian blues and related compounds[END_REF], the structure was only accurately resolved in 1977 by Buser et al. [START_REF] Buser | The crystal structure of prussian blue: Fe4 [fe (cn) 6] 3. xh2o[END_REF]. They also showed that PB had the same structure as the Turnbull blue, obtained from the coprecipitation of K 3 [Fe III (CN) 6 ] and a Fe(II) salt in water.

Nanoparticles of Prussian blue and its analogs

PB crystallizes in a face centered cubic (FCC) lattice (generally considered as Fm3m, but Pm3m space group), with a 10.17 Å lattice parameter, where the motif consists of alternating Fe III and Fe II linked by cyanide bridges, K atoms are present in variable amounts in tetrahedral interstitial sites. The fact that Fe III and Fe II are linked by cyanide bridges implies a substantial spacing between the metallic atoms. For this reason, zeolithic water molecules can be present in the interstitial tetrahedral sites of the structure. The Fe II (CN) 6 content in the structure varies from 3/4 ("insoluble" form) to 1 ("soluble" form), depending on the amount of potassium in the interstitial sites (ranging from 0 to 1). In the occurrence of Fe II (CN) 6 vacancies, Fe(III) ions complete their coordination sphere with water molecules. Figure 2.1 shows a hypothetical PB structure without any vacancies that may be close to the "soluble" form, and the real structure containing vacancies and zeolithic water molecules.

The color of PB stems from a metal-to-metal intervalence charge transfer from low spin Fe(II) to high spin Fe(III) atoms, leading to a strong absorption band at 700 nm. Aside from its use as a pigment, PB has been widely studied for its interesting electrochemical properties that may be exploited in biosensors and electrochromic devices [START_REF] Itaya | Electrochemistry of polynuclear transition metal cyanides: Prussian blue and its analogues[END_REF]. PB serves as a generic model for a much bigger family of compounds called PBAs.

precursors for SWCNT growth. The two former require the use of organic solvents, and stabilizing surfactants or polymers cover the surface of the particle. The latter leads to the formation of particles embedded in a solid template, and additional steps are needed to retrieve them. In all cases, processing the obtained objects as bare nanoparticles involves additional purification steps that may lead to their alteration, and coalescence.

Since 2005, a method has been developed in our laboratory by Catala, Mallah et al. to synthesize self-standing charged PBA particles in aqueous solution. This method is obviously advantageous because it does not utilize stabilizing agents such as surfactants. It is also extremely simple to operate and allows the synthesis of nanoparticles of many different PBA systems under similar experimental conditions. The stability of the nanoparticle dispersions ranges from several weeks to a few months, depending on the PBA system. The obtained particles are individualized in solution because of repulsive Coulomb interactions due to their negative charge. This stability relies on the fine tuning of transition metal salt concentrations.

Published studies during the PhD of Daniela Brinzei present the principle of the synthesis for Ni 2+ -based PBA particles (CsNiFe [START_REF] Brinzei | Magnetic behaviour of negatively charged nickel (ii) hexacyanoferrate (iii) coordination nanoparticles[END_REF], and CsNiCr [START_REF] Brinzei | Spontaneous stabilization and isolation of dispersible bimetallic coordination nanoparticles of cs x ni [cr (cn) 6][END_REF]). The particles exhibit a hydrodynamic diameter of 5-7 nm depending on system and experimental conditions. A chemical analysis of the particles post-retrieving using cetyltrimethylammonium cations (CTA + ) shows that their general formula is Cs x Ni II [M' III (CN) 6 ] y , with x = 0.4 -0.5, and y = 0.95 -1. Their overall negative charge is therefore due to both the inherent negative charge of the core of the particle according to the alkali content in the structure and the very low amount of hexacyanometallate vacancies, as well as to the negative surface charges of the coordinated hexacyanometallates.

Since then, the method has been applied and optimized to a number of other PBA systems (KFeFe, CsNiCo, KFeRu, etc.) with equal success for obtaining individualized, stable particles with a controlled size and overall 1:1 -1:0.95 metallic ratio. The synthesis method is extremely simple: an aqueous solution of the hexa-aquo metal salt (M) (concentration ranging from 0.5 mM to 4 mM) is quickly added to an aqueous solution of equal concentration of the hexacyanometallate (M') containing the alkali cation in excess (when its presence is necessary for obtaining negatively charged particles) under vigorous stirring.

The PBA particles are obtained dispersed and stable in water. Depending on the system, the synthesis is done either at ambient temperature or at 2 • C. Methods have also been developed in the lab to tune the size of the particles by adding shell layers to a pre-existing PBA particle seed, with the same or different PBA system, leading to either simply bigger particles with unchanged composition [START_REF] Prado | Tailored coordination nanoparticles: Assessing the magnetic single-domain critical size[END_REF], or core-shell PBA heterostructures [START_REF] Prado | Magnetic anisotropy of cyanidebridged core and core-shell coordination nanoparticles probed by x-ray magnetic circular dichroism[END_REF].

In summary, a simple method is an efficient synthesis solution to a wide range of PBA nanoparticles with controlled sizes. It is applicable to monometallic, such as PB, or bimetallic systems, like NiFe and NiCr, with a 1:1 stoichiometry. Further control over the size and composition can also be considered by adding shells to a pre-existing PBA structure, leading to the synthesis of potential trimetallic PBAs, the only liability here would be the increasing size of the nanoparticles. One could also consider the potential tuning of the nanoparticle compositions by playing with the metal salts concentrations, or specifically by trying to tune the content of the alkaline cation in the PBA nanoparticle structure, which is partly responsible for the vacancy content in the structure. However, this tuning is quite unfortunately limited with this synthesis method. This is why the work will focus on 1:1 stoichiometry compounds in the first instance, while future works could focus on metal stoichiometry tuning.

Use as catalyst precursors

It has been established in the first chapter of this manuscript that a good catalyst precursor for exploring potentially selective SWCNT growth should be small in size, with a narrow size distribution and controlled composition, and give access to many bimetallic combinations in the same conditions. So as to conserve their small size, the particles need to be protected from coalescence, which starts by synthesizing non-aggregated particles. All of these conditions seem to be fulfilled by PBA nanoparticles synthesized by the co-precipitation method developed by Catala et al [START_REF] Brinzei | Magnetic behaviour of negatively charged nickel (ii) hexacyanoferrate (iii) coordination nanoparticles[END_REF], [START_REF] Brinzei | Spontaneous stabilization and isolation of dispersible bimetallic coordination nanoparticles of cs x ni [cr (cn) 6][END_REF]. They are also free of stabilizing agents, preventing potential contamination and deactivation of the catalysts without involving purification steps. The open question that remains is the possibility of reducing these particles into bimetallic alloys while keeping a good control of the size distribution.

As explained previously, manufacturing nanoalloy nanoparticles for the growth of SWCNTs is challenging. One of the main challenges is to avoid phase segregation during the catalyst fabrication process. Using precursor nanoparticles where the two metals are already mixed within the structure is obviously advantageous. Provided that the desired solid solution or alloy is thermodynamically allowed to form at the nanoscale for the considered metallic combination. PBAs have already been successfuly used for that purpose. Yamada et al. have grown Pt-Co nanoparticles 10-15 nm in diameter with various metallic Pt:Co ratios by thermal reduction under H 2 of Pt IV /Co tetracyanoplatinate PBA nanoparticles synthesized themselves by the reverse micelle method [START_REF] Yamada | Novel synthetic approach to creating ptco alloy nanoparticles by reduction of metal coordination nano-polymers[END_REF]. Another example in the literature reports the formation of supported FeNi bimetallic nanoparticles from PBAs embedded in mesoporous silica [START_REF] Folch | A coordination polymer precursor approach to the synthesis of nife bimetallic nanoparticles within hybrid mesoporous silica[END_REF]. The formed bimetallic nanoparticles are, in those cases, too large to serve as seeds for the growth of SWCNTs, and it is not entirely clear whether the formed particles are truly alloyed. However, their synthesis demonstrates the feasibility of reducing PBAs into metallic compounds. The capabilities of a reductive treatment of small bimetallic PBA nanoparticles synthesized by the co-precipitation method in water after deposition on a substrate to lead to the formation of nanoalloy catalyst is one of the questions the work presented here attempts to answer.

Method

In the present work, the PB and PBA nanoparticles were synthesized using the spontaneous stabilization method in water. Three systems were tested: one monometallic (PB, ie Fe), and two bimetallic (NiFe and NiCr PBAs). A process for the deposition of the as-obtained nanoparticles was developed in order to obtain a dense monolayer of the PB/PBA nanoparticles on a SiO 2 /Si wafer. This process, given in more detail in Chapter 3 of this manuscript, relies on the anchoring of the PB/PBA nanoparticles onto a silane molecule comprising a terminal pyridine group previously attached to the wafer through a self-assembling process.

Then, the aim is to form nanoalloy nanoparticles in situ using a reductive pretreatment (H 2 atmosphere), directly followed by the CVD growth of SWCNTs.

The fact that the effective catalyst nanoparticles are formed directly inside the CVD chamber ensures that they do not undergo any possible chemical transformation between their formation and their use as catalysts nanoparticles.

"Cyanosol" nanoparticles

General presentation

In the 1990's, a research group in Princeton University used sol-gel chemistry to synthesize amorphous cyanide-bridged transition metal polymers they named "cyanogels" [START_REF] Heibel | Use of sol-gel chemistry for the preparation of cyanogels as ceramic and alloy precursors[END_REF], which are essentially an amorphous form of PBAs. They are prepared by the reaction of a chlorometalate [M II Cl 4 ] 2-and a hexacyanometalate [M' II/III CN 6 ] 4-/3-in a sol-gel process. The chloride ligands of M are partially substituted by cyanide ligands, bridging M and M' in a PBA-like structure.

First, the formation of a sol takes place: small clusters of coordination networks nucleate (cyanosol), then the clusters collide and bond with each other, so as to gradually form a bulk 3D amorphous and porous negatively-charged network called a gel (cyanogel).

Therefore, for forming small size catalyst nanoparticles, the first step is the synthesis of the so-called cyanosols which consists in stopping the sol-gel process in its very early stage, so as to freeze the reactional medium as a colloidal nanoparticle suspension. This was achieved by Bocarsly et al. in 2009 [START_REF] Burgess | Stabilizing cyanosols: Amorphous cyanide bridged transition metal polymer nanoparticles[END_REF].

Being a transition state before the formation of the cyanogel, cyanosol nanoparticles are considered to be very unstable and tend to aggregate. The method employed by Bocarsly et al. was to prevent the strong interactions between the nanoparticles by replacing their counter ions by CTA + . Figure 2.3.a shows a schematic representation of the synthesis process. The obtained nanoparticles are soluble in organic solvents. This cyanosol synthesis method has been studied for a few bimetallic systems: M = Pd, Pt, and M' = Fe, Co, Ru, leading to the preparation of nanoparticles with sizes ranging from 4 to 5 nm (see Figure 2.3). These studies showed that the time at which precipitation was performed was crucial to obtain isolated particles.

Very recent work has been conducted on cyanosol nanoparticles in our laboratory with the aim of synthesizing cyanosol nanoparticles with a method similar to the PBA particle co-precipitation synthesis. Small (2-4 nm) free-standing PdFe and PtFe cyanosol nanoparticles were obtained in water, with stabilities ranging from a few days to several months, that had not been reported. These nanoparticles constitute an interesting candidate as catalyst precursors for SWCNT growth.

characterized with HRTEM-EDX showing that the obtained 5±1 nm nanoparticles were crystalline, and that both metals were present in the particles. Though the size of the obtained nanoparticles is bigger than the sizes needed for selective tangential growth, and their composition homogeneity is not entirely unambiguously demonstrated, these results are encouraging, and could be optimized for other metal combinations in order to obtain monodisperse nanoalloy catalyst nanoparticles for SWCNT growth.

Another potential advantage of cyanosol compounds as catalyst precursors for SWCNT growth is their enhanced composition flexibility. In comparison with PBAs, whose composition ranges are limited by the imposed FCC crystalline structure (depending on the vacancies and presence of alkali cations, the two extreme stoichiometries (M:M') are 1.0 and 1.5) the amorphous cyanosols open a wider range of metallic ratios. The synthesis of 2:1 Pd:Co cyanosol nanoparticles is an example illustrating this flexibility [START_REF] Burgess | Stabilizing cyanosols: Amorphous cyanide bridged transition metal polymer nanoparticles[END_REF]. This offers the possibility to tune the catalyst nanoparticles' composition over a broader stoichiometry range.

Method

Since the cyanosol systems are chemically similar to the PBA systems, our intent here is to mimic the methodology applied for the PBA systems. The feasibility of using the same deposition process on a SiO 2 /Si wafer, as well as for the formation of nanoalloy catalyst nanoparticles and consequent SWCNT growth will be tested. The cyanosol bimetallic system used in the present work is FePd. The POM used in this approach and described in [START_REF] Yao | Two hexa-tm-containing (tm= co2+ and ni2+) (p 2w12)-based trimeric tungstophosphates[END_REF], is made of three hexavacant Dawson-type phosphotungstate P 2 W 12 sub-units linked by WO(H 2 O) entities, forming a P 6 W 39 shell that contains six vacant sites to incorporate Co 2+ atoms. A solution containing the clusters is simply dropped onto a SiO 2 wafer, and the Co-W catalyst nanoparticles are formed by first calcinating the POM layer in air, and second reducing it at about 1000 • C under hydrogen, leading to the formation of Co-W bimetallic nanoparticles. Figure 2.5 shows the structure of the POM used in this approach, and the schematic of the SWCNT implemented growth process. In order to test the feasibility of nanoalloy catalyst formation, and subsequent SWCNT growth from this type of POM, we will first attempt to deposit the POMs in a homogeneous layer on top of a SiO 2 /Si wafer. Since the POMs used here are clusters with smaller sizes than the nanoparticles of PBAs or cyanosols, the feasibility of the SWCNT growth will have to be tested.

Polyoxometalates (POMs)

Method

Synthesis of SWCNTs from precursors

The SWCNT CVD growths presented in this manuscript have been performed in a HFCDV reactor, at the LPICM in Ecole Polytechnique. Figure 2.7 presents a schematic of the reactor. The reactor is equipped with two gas inlets, each equipped with their own filament. While the first inlet is used for the introduction of the carbon precursor -methane (CH 4 ) in the case of this work-the second is used for the introduction of hydrogen (H 2 ). The filaments are at a fixed distance from the sample, and favor the local decomposition of the carbon precursor and the formation of atomic hydrogen before their contact with the sample. The power of the filaments can be independently controlled, leaving more room for optimization, and allowing the control of the amount of activated hydrogen and carbon radicals. The role of activated H 2 is to perform a reductive pretreatment of the catalyst precursor nanoparticles before the growth, as well as to continuously etch the amorphous carbon formed on the reactor sidewall during growth [START_REF] Kim | The role of catalytic nanoparticle pretreatment on the growth of vertically aligned carbon nanotubes by hot-filament chemical vapor deposition[END_REF]. The reactor is composed of a cylindrical quartz tube enclosed in an 80 mm wide tubular heater with an approximately 250 mm length uniformly heated. The sample can be placed in two positions: in the cold zone, or in the hot zone (blue part on the left, and orange part of the tube on Figure 2.7, respectively). The filaments are made of 0.4 mm thick tungsten wire. The residual base pressure of the reactor is 10 -6 mbar, preventing the presence of adsorbed water molecules on the walls.

First, the sample is placed in the cold zone, and the reactor is pumped to its residual pressure. The reactor is heated at the desired growth temperature, while the sample is kept in the cold zone. H 2 is then introduced at a 100 sccm (standard cubic centimeters per minute) flow rate, and the corresponding filament power of 160 W is turned on. Once the pressure of the chamber is stabilized at 90 mbar, the sample is moved to the hot zone. At this point, the chamber only contains H 2 , and the reductive pretreatment takes place. At the end of the desired pretreatment time, we introduce the carbon precursor at a 20 sccm flow rate, with a filament power of 120 W, and increase the pressure to 100 mbar. The growh of SWCNTs takes place, for usually 30 minutes. At the end of the growth, the sample is placed back in the cold zone, the gas inlets are closed, the chamber is pumped, the filaments and the heater turned off. When the chamber has cooled down to a temperature below 150 • C, the sample can be taken out of the chamber for characterization.

Characterization of SWCNT samples

The various characterization methods that are used for SWCNTs have been presented in brief in the previous chapter. Here, we will discuss in more detail the two methods that were available and used for this work which are TEM and Raman spectroscopy, along with the developed methodology used for these two methods regarding growth selectivity assessment.

Transmission electron microscopy

Historically, the invention of electron microscopes stemmed from the limitations in the resolution of optical microscopes. The wave-like characteristic of electrons was first theorized by de Broglie in 1925. Electron diffraction experiments conducted in 1927 confirmed their wave nature. Considering electrons like optical photons with a smaller wavelength was then possible, and their use as the "seeing" radiation for microscopes with the aim of decreasing the resolution was conceivable. Ruska and Knoll were the first to put forth this idea in 1932, and the first TEMs were commercially available a few years later [START_REF] Williams | Transmission electron microscopy: a textbook for materials science[END_REF].

Why use electrons?

In optical microscopy, the resolution is defined by the shortest distance between two points that can be distinguished as separate entities. The Rayleigh criterion defining resolution in classical optical microscopy gives δ, this smallest resolved distance by the following equation:

δ = 0.61λ µ sin β (2.1)
Where λ is the wavelength of the radiation, µ the refractive index of the medium, and β the semi-angle of the magnifying lens collection (µ sin β is the numerical aperture(NA)). The wavelength of the radiation is clearly a limitation for resolution: in the case of visible light, with roughly 400nm < λ < 700nm, and taking NA equal to 1, the resolution cannot go below a few hundreds of nanometers.

De Broglie's work on the wave characteristic of electrons showed that they could be used to overcome this intrinsic limitation of optical microscopy. Considering an electron (mass m 0 , charge e, and particle momentum p), it can be associated to a wave with a wavelength λ = h/p. The relativistic wavelength of an electron accelerated through a potential difference V is given by the following:

λ = h 2m 0 eV (1 + eV 2m 0 c 2 ) (2.2)
A first information is given by this relation, which is that increasing accelerating voltage leads to a decrease in electron wavelength, which means higher resolution. Secondly, considering an accelerating voltage of 100 kV, a theoretical resolution of about 4 pm could be reached, going below the atomic radius. This theoretical resolution has not yet been reached, because of our inability to fabricate ideal electron lenses. Decades of development have allowed for the TEM resolution to decrease significantly, and the recent emergence of aberrationcorrected microscopes has brought it down to 50 pm.

In a TEM, a high energy electron beam is formed, then the electrons interact with the specimen, some of the electrons will be transmitted through the specimen, and an image of the specimen is projected onto a screen. The interaction between electrons and matter is many-fold, and is at the heart of the versatility of TEM as a characterization technique. These interactions are detailed in the next section.

Electron-matter interactions

Figure 2.8 depicts the various signals that can be generated from the interaction between an electron beam and a thin specimen. When an electron interacts with a specimen, it can be either transmitted, or scattered. The transmitted beam is parallel to the incident beam. The scattering processes can be subdivided into two categories: elastic scattering, when the electron's energy is unchanged through the process, and inelastic scattering, when the electron looses a measurable amount of energy.

Secondary

Elastic scattering

The most important part of elastic scattering (and of scattering as a whole) is electron diffraction: when interacting with a crystalline specimen, the electron Chapter 2. Experimental methods beam is scattered and the scattered waves interfere to form diffraction patterns with high intensities in well-determined directions. Those diffraction patterns can be acquired and studied in a TEM, and carry structural information on the specimen. The interference between diffracted beams and the transmitted beam give rise to high resolution imaging, which will be explained later.

The second, less significant, type of elastic scattering is the large-angle deviation elastic scattering. The interactions between the incident beam and the specimen are Coulombic, and it interacts with the electron cloud as well as the nucleus. When the incident electrons interact with atom nuclei, large-angle deviation elastic scattering (backscattering when the deviation angle reaches 180 • ) occurs. Certain microscopes are equipped to detect these electrons for imaging purposes.

Inelastic scattering

When colliding with electrons from the specimen, the incident electrons give away a part of their kinetic energy, which constitutes an inelastic interaction and results in the generation of various signals. This electron energy loss can be measured by collecting the inelastically scattered electrons and dispersing them depending on their kinetic energy, this is known as electron energy loss spectroscopy (EELS). The lower energy loss interactions correspond to valence to conduction band transitions, then surface and volume plasmons. Energy losses at higher energies correspond to the ejection of core electrons, which are characteristic of the atom and make an elemental analysis of the sample possible.

In this latter case, the vacancy created is filled by an electron from an outer-shell of the electron cloud. This relaxation is either accompanied by the emission of an X photon, or the ejection of a weakly bound electron called an Auger electron. The energies of the X-ray photons generated by these inelastic interactions are characteristic of the atom, and their measurement through X-ray dispersive spectroscopy (EDX) can therefore give quantitative chemical information on the sample. The low mean free path of Auger electrons implies that they only leave the specimen if they are generated at its surface, and are not used for characterization in TEM. The electrons ejected by the collisions with incident electrons are called secondary electrons, and are generally, as the Auger electrons, not used for characterization.

In the case of a semiconducting specimen, the energy obtained from the incident electron can generate an electron-hole pair: an electron from the valence band is promoted to the conduction band. The hole-electron recombination is accompanied by the emission of a photon. This process is known as cathodoluminescence (CL), and gives insights into the optical and electronic structure of the material. It is mostly studied in SEMs, which are operated at much lower voltages than TEMs.

Basic operation principles of TEM

Drawing an analogy with optical microscopy is very useful for understanding how a TEM works. In optical microscopy, an incident light beam passes through (and interacts with) a thin object, and an image of this object is formed through a simple optical lens system. In TEM, the basic concept is exactly the same, aside from the fact that electrons are used instead of photons, and electromagnetic lenses drive the electrons through a column under high vacuum. Electromagnetic lenses consist of copper coils that create a magnetic field between two polepieces. The intensity of the current in the coils allows the control of the electron's trajectory, and the modulation of the focalization of the beam. They can be considered as convex optical lenses with controllable focal length.

A TEM can therefore be understood in a first approach, by analogy with the optical microscope, by using a geometric optics formalism. Figure 2.9 shows the schematic representations of a TEM, using geometric optics to depict image formation. The process of obtaining the image of an object with a TEM can be divided into four steps:

1. Electron beam formation 2. Electron-sample interaction 3. Image formation

Image acquisition

The interaction between the electron beam and the specimen has already been discussed, we therefore only focus on the other three steps in the following.

Forming the electron beam

Depending on the microscope, different electron sources can be used. The two main kinds of electron sources are thermoionic and field-emission sources. The first generates electrons when heated, and the latter when a strong electric potential is applied between the source and an anode.

When heating any material and if the energy thus provided to its electrons is higher than their workfunction, the material is turned into an electron source. However, if the material is heated too much, it can melt or evaporate. A good thermoionic source is therefore either a high-melting point material, or a material with a very low workfunction. In the first microscopes, a tungsten filament was used as a thermoionic source because of its refractory characteristic; today the microscopes that still use thermoionic sources use lanthanum hexaboride (LaB 6 ) crystals because of its low workfunction. The main disadvantage of thermoionic sources is their "high" energy dispersion (∆E = 1.5 eV), which limits the resolution of the microscope because of chromatic aberrations. The lifetime of a LaB 6 source is also quite low and it has to be changed often.

Field-emission sources are commonly referred to as FEGs (for field-emission guns). They are based on the fact that an electric field is increased at sharp points. By applying a several kV potential difference between a 100 nm tip (tungsten single crystal) and an anode, an electrostatic field as high as 10 10 V/m is obtained. This induces the generation of an electron current through a tunneling effect. Impurities and oxidation at the surface of the tip may hinder the field-emission effect. Operating in ultra-high vacuum conditions (P< 10 -9 Pa) while keeping the tip at room temperature can be a solution, this is the principle of the cold-FEG. Another solution is to heat the tip, which actually leads to a hybrid thermoionic and field emission process. Such FEGs are known as Schottky, and their tip is a tungsten single crystal coated with zirconium oxide (ZrO 2 ) which enhances electron emission (low worfunction) and helps preserving the tip. FEGs provide the TEM with a much more temporally coherent electron source (∆E = 0.7 eV for a Schottky FEG, and ∆E = 0.3 eV for a cold FEG) with a much higher current density, and they last longer.

The electrons are then progressively accelerated by a series of anodes, with the chosen accelerating voltage which is typically between 80 and 300 kV. Whatever the type of electron source, we can consider it to be punctual, meaning that the electron beam is a spherical wave before entering the condenser. The condenser is a series of electromagnetic lenses that control the size and convergence angle of the beam illuminating the sample. In conventional TEM, the condenser allows for parallel illumination of the specimen (planar wave).

Image formation

When the planar wave of incident electrons interacts with the specimen through the electrostatic atomic potential V( -→ r ), the wave of electrons at the exit of the specimen Ψ t ( -→ r ) is the solution of the Schrödinger equation. When considering a perfect crystal, the solution is written as a sum of planar waves in the reciprocal space.

After the electron beam has traversed the specimen, the objective lens will first form a magnified image of the object. The objective lens forms the Fourier transform (FT) of Ψ t ( -→ r ) in its back focal plane, and the magnified image is formed by inverse Fourier transformation. The image is then projected onto the screen (or detector) by the projection system.

An area-selection diaphragm can be placed in the diffraction plane (back focal plane of objective lens) for different imaging modes. Selecting the transmitted beam leads to bright field (BF) imaging, the diffracting parts of the specimen Chapter 2. Experimental methods are dark. Selecting the diffraction spot from a specific crystallographic plane family is known as the dark field (DF) mode.

Acquisition system

In order to obtain an image from the observed object, electron intensity needs to be converted into light intensity. This is done by two different technologies on traditional TEMs. The image can be directly projected and viewed onto a fluorescent screen, or digitalized and observed on a computer screen via a CCD (for charge-coupled device) camera. The fluorescent screen is located at below the projection system, and is coated with a material emits photons (in the 450 to 550 nm wavelength range) when excited with electrons.

As shown in the previous section, the signals generated by the interaction between the electron beam and the sample are not limited to images or diffraction patterns. In order to collect those various signals, TEMs are equipped with other adapted detectors such as X-ray detectors, and high-angle annular dark field (HAADF) detectors, which will be mentioned later in this manuscript.

HRTEM

In conventional TEM, a certain part of the electron beam is selected after it has passed through the specimen. In so-called high resolution TEM (HRTEM) mode, the image is constructed through the interference of diffracted and transmitted electrons. A high resolution image is an interferential image, whose contrast is determined by the phase shift between the interfering waves.

In the case of a very thin specimen, of ∆z thickness, the exit electron wavefunction only undergoes a phase shift Φ( -→ r ) defined as σV p ( -→ r )∆z, where σ is the relativistic interaction constant, and V p ( -→ r ) is the projected atomic potential responsible for the scattering of the electron beam. The transmitted wavefuncion can therefore be written as:

Ψ t ( - → r ) = Ψ 0 ( - → r ) exp iσV p ( - → r )∆z (2.3)
Within the thin and weakly diffracting object consideration, the exponential can be approximated using a Taylor expansion, which leads to:

Ψ t ( - → r ) = Ψ 0 ( - → r ).[1 + iσV p ( - → r )∆z] = Ψ 0 ( - → r ) + iΨ 0 ( - → r )σV p ( - → r )∆z) (2.4)
The transmitted wavefunction is therefore the superposition of two planar waves, one being the incident wave propagated through ∆z, and the other a wave having undergone a diffusion of small amplitude, and with a π/2 phase shift. This defines the so-called weak phase approximation which applies in the case of SWCNT observation, because they are very thin objects (two atomic layers), with very light atoms, therefore diffracting the electron beam weakly.

The observed image in TEM does not directly correspond to the exit wavefunction, because it is affected by the imperfections of the electron source and of the lenses of the microscope, the most important for the HRTEM mode being the objective lens, called aberrations. The phase of the electron waves is affected by the spherical aberration, defocus (∆f ), and astigmatism of this lens, while the chromatic aberrations only affect their amplitude. We only consider the phase shift induced by aberrations, and the most important components are the spherical aberration (C s ), and the defocus. The function that represents this effect of the microscope on the phase of the electron wave is called the coherent transfer function and is defined as follows:

T (ν) = exp[-iπλ(C s λ 2 ν 4 + ∆f ν 2 )] (2.5)
Where ν is the spatial frequency, λ the wavelength, and C s is the spherical aberrations coefficient. In the reciprocal space, the transfer of the microscope is represented by the multiplication of the FT of the exit wavefunction by the transfer function. Within the weak phase approximation, and considering a normalized incident wavefunction, this leads, in the real space, to the following image wavefunction:

Ψ i ( - → r ) = F T -1 [Ψ( - → r )] = 1 + iF T -1 [exp(iχ(ν)).F T (Φ( - → r ))] (2.6)
Where Ψ( -→ r ) is the wavefunction in the object focal plane of the objective lens.

Without taking into account magnification, we then define the image intensity by the square of the amplitude of Ψ i ( -→ r ), which leads to:

I( - → r ) = 1 -2F T -1 [sin(χ(ν)).F T -1 (Φ( - → r ))] (2.7)
The contrast is defined by Γ( -→ r ) = I( -→ r ) -1. The contrast is optimized in a HRTEM image when the phase shift induced by the microscope is the same for all spatial frequencies ν used for image formation. This is achieved at the so-called Scherzer defocus:

∆f = -1.2(C s λ) 1/2
(2.8)

TEM applied to SWCNT characterization

Depending on the resolution of the microscope, the information that can be extracted from the TEM image of a SWCNT varies. Using conventional TEM, it is possible to measure the diameter of the nanotubes with significant precision.

Using HRTEM on a microscope with a high enough resolution (better than the C-C distance), the chirality of the nanotube can be unambiguously determined.

Image of a SWCNT and diameter determination

A SWCNT, being made of a rolled-up atomic sheet of graphene, the weak phase object approximation applies, if the nanotube is observed in a longitudinal projection. The mean projected potential is higher at the edges of the nanotube. This defines two parallel lines of highest atomic density. When the TEM has a "low" resolution, only these lines contribute to the contrast image, and the diameter of the SWCNT can be extracted from this image.

A study of the contrast images of SWCNTs by Loiseau et al. using image simulations [START_REF] Loiseau | Understanding carbon nanotubes[END_REF], [START_REF] Stadelmann | Ems-a software package for electron diffraction analysis and hrem image simulation in materials science[END_REF] has shown that this fringe image varies substantially with defocus. Around the Scherzer defocus, only one dark fringe (called Fresnel fringe) is observed on each side of the SWCNT. When going away from the Scherzer defocus, the fringes tend to widen until other sets of fringes start to appear. The highest contrast is obtained for the Scherzer defocus, and this is where the diameter should be measured.

Fleurier et al. [START_REF] Fleurier | Transmission electron microscopy and uv-vis-ir spectroscopy analysis of the diameter sorting of carbon nanotubes by gradient density ultracentrifugation[END_REF] have established a method for the measurement of nanotube diameter from a TEM image. An image is taken of the nanotube at (or very close to) the Scherzer defocus, and an intensity profile is extracted from the image, perpendicularly to the nanotube axis. The diameter is then defined as the distance between the two inflection points of the derivative of the contrast intensity, meaning at the midpoint between the minimum of the dark fringe, and the maximum of the subsequent light fringe. Figure 2.10 shows at TEM image of SWCNTs, and an extracted intensity contrast with a diameter measurement. The method for diameter measurement was validated by TEM image simulations, and it is estimated to give an accurate value of the diameter with an error of 0.5Å.

Chapter 2. Experimental methods

Selectivity assessment methodology

In this work, the main use of TEM for characterization of SWCNT growth samples was the measurement of diameters. For a given sample, the diameter distribution was systematically determined using the same method, so as to accurately compare samples with each other.

For each considered sample, we took images of SWCNTs on three different squares of the TEM grid when possible. In some occasions, the grid is badly damaged from the transfer process, and only a few squares remain to be observed, and in other cases, when the growth yield is too low, SWCNTs are present on only a very limited number of squares. On each square, various locations containing nanotubes were randomly selected, and we recorded images containing all the present SWCNTs. All the images were recorded at the same magnification (160 k).

The diameters of all the suspended SWCNTs on theses images with a large enough clean section were measured. At least one hundred diameters were measured for each sample, and the diameter distribution histogram was plotted with a 0.1 nm binning.

TEM was also a way to simply ensure that the growth product consisted only of SWCNTs, or that SWCNTs were the great majority of products. It is difficult or impossible to determine the percentage of MWCNTs with the other available characterization techniques (SEM and Raman).

STEM-EDX for catalyst characterization

Though the subject of this section is to present TEM for SWCNT characterization, it seemed important to mention a TEM-derived technique that was used for catalyst characterization in this work, which is scanning transmission electron microscopy (STEM) coupled with EDX. The electron beam can be focused on the specimen, and used as a probe that scans the specimen (similar to SEM). As mentioned previously, EDX consists in the measurement of the X-ray photons generated by the inelastic interactions between the electron beam and the specimen, which have an energy that is characteristic of the energy difference between the two electron shells involved in the transition (as explained previously, the ionized atom returns to its ground state by filling the hole in its inner shell). This energy difference is characteristic of the atom. Phonons are collective vibrational modes of atoms in a solid. They have a strong impact on thermal, and mechanical properties, as well as electron transport. Their study provides valuable information on the probed material. One of the two main techniques available for phonon study is Raman spectroscopy. It consists in shining a monochromatic light using a laser onto a sample. The scattered light is then collected, the Rayleigh component is filtered out, and a grating is used to disperse the scattered light, which is then detected with a CCD detector. The intensity of the scattered light is plotted versus the Raman shift ν 1 .

When the excitation wavelength is equal to, or close, to an optical transition energy, the Raman signal is enhanced through electron-phonon coupling. This is known as the resonant Raman effect, which is indispensable to SWCNT characterization and can enhance the signal by a factor of about 10 3 [START_REF] Dresselhaus | Raman spectroscopy of carbon nanotubes[END_REF]. In the case of SWCNTs, both the resonant process and the existence of vHSs in the DOS contribute to the strong enhancement of the Raman intensity. This explains how a single resonant SWCNT can give rise to a measurable Raman signal, which contains information on its phonon structure and electronic properties. The resonant aspect of Raman spectroscopy for SWCNTs has very important implications in the viability of their characterization.

Raman spectroscopy for SWCNT characterization

The phonon structure of SWCNTs can be theoretically derived from the graphene phonon structure, as for the electronic structure, by the zone-folding approximation. Graphene has a two atom unit cell, and therefore has six phonon branches (three acoustic branches and three optical branches). In the case of SWCNTs, the number of phonon modes depends on the chirality of the nanotube: since the unit cell defined by -→ C h and -→ T contains much more atoms, the number of phonon branches can go up to 150 for an achiral SWCNT, and even more for a chiral tube. The number of Raman-active modes, however, is somewhat independent of diameter and chiral angle. There are 15 Raman-active modes in SWCNTs [START_REF] Saito | Physical properties of carbon nanotubes[END_REF].

Characteristic Raman modes of CNTs

The most characteristic Raman signals of SWCNTs, and easiest to measure are the RBM mode, the tangential mode (TM, or the G band), and the D band. Figure 2.16 shows a typical SWCNT Raman spectrum displaying these dispersion, or interaction with a substrate and/or catalyst nanoparticles. The second relation, proposed later by Araujo et al. [START_REF] Araujo | Nature of the constant factor in the relation between radial breathing mode frequency and tube diameter for single-wall carbon nanotubes[END_REF] is:

ω RBM = 227 d t 1 + C e d 2 t
(2.11)

Where ω RBM = 227/d t corresponds to a pristine isolated nanotube grown from water-assisted CVD with negligeable environmental effect, and all other ω RBM values are an upshift from this pristine value, with C e an adjustable constant representing the environmental effect. This relation is considered to be more accurate than the previous law, but the latter is still widely used for SWCNT sample selectivity assessment.

Since the RBM is a resonant feature, having determined the diameter using this equation, and knowing the excitation wavelength can hypothetically give even more information on the probed SWCNT. For an RBM signal to appear, we need one of its E ii to be equal to the excitation energy E L , or within the resonant window around E L . Reporting to the Kataura plot, the chirality of the resonant nanotube can hypothetically be determined. This (n,m) indexation process however requires precise calibration based on Raman characterization of isolated SWCNTs of known chirality, for instance. The semiconducting or metallic character of the probed nanotube can nonetheless be quite accurately determined with this method.

The G band corresponds to vibrations in the direction parallel to the nanotube surface (see Figure 2.16.c). This vibration mode is also present in 2D graphite, but its multiband structure for SWCNTs is unique. The G band is located on the Raman spectrum between 1585 cm -1 and 1595 cm -1 , the peak observed at the highest Raman shift is called the G + band, and the others are called G -. In the case of metallic nanotubes, the G -band has a broad and asymmetrical profile called the Breit-Wigner-Fano lineshape [START_REF] Brown | Origin of the breit-wigner-fano lineshape of the tangential g-band feature of metallic carbon nanotubes[END_REF]. The G band is useful in SWCNT characterization to control sample heating by laser irradiation. It is known to shift towards lower frequencies when the sample is heated [START_REF] Chiashi | Cold wall cvd generation of single-walled carbon nanotubes and in situ raman scattering measurements of the growth stage[END_REF]. Monitoring the frequency of the G band allows for proper setting of the laser power during acquisition.

At lower Raman shifts, at about 1350 cm -1 is the D band. Its frequency depends on the excitation wavelength. It is theoretically not Raman-active, and its presence on the Raman spectrum stems from a double-resonance process where electrons are scattered by defects in the sp 2 carbon structure. For this reason, its intensity is indicative of the quantity of defects in the probed SWC-NTs. It can also be indicative of the presence of other carbonated structures, like amorphous carbon. The D band is quite useful to have insights into the crystalline quality of nanotubes. A commonly used factor for this is the ratio of the intensity of the D band, over that of the G band (I D /I G ), it can be used to compare samples, but does not have quantitative value.

Selectivity assessment by Raman spectroscopy

Diameter distribution assessment

As explained in the previous section, the RBM is a specific spectroscopic signature of a SWCNT. Being able to determine the diameter of the probed nanotube directly means that by accumulation of RBM frequencies from SWCNTs from a given sample gives access to the diameter distribution of the sample. The diameter selectivity can therefore be assessed through RBM acquisition.

The RBM frequency is very sensitive to the environment surrounding the SWCNT.

For a SWCNT within a bundle, the RBM frequency can be increased by up to 20 cm -1 [START_REF] Venkateswaran | Probing the single-wall carbon nanotube bundle: Raman scattering under high pressure[END_REF], a study comparing two different surfactants as a dispersion medium for SWCNTs in solution shows substantial shifts in RBM frequencies [START_REF] Maultzsch | Radial breathing mode of single-walled carbon nanotubes: Optical transition energies and chiralindex assignment[END_REF], and shifts of up to 12 cm -1 have been observed between suspended SWCNTs and SWCNTs supported on SiO 2 [START_REF] Zhang | Substrate-induced raman frequency variation for single-walled carbon nanotubes[END_REF].

Since ω RBM is sensitive to the environment, the law used to determine the diameter of the SWCNT has to reflect the environment representative of the sample. Table 2.2 gives a non-exhaustive overview of the different w RBM = A/d t + B found in the literature, testifying to the wide range of possible choices.

The resonant aspect of Raman spectroscopy implies that only the SWCNTs with an E ii within the resonance window around the excitation energy E L may be detected. Using only one laserline would drastically restrict the pool of detectable SWCNTs within the actual sample total population. To prevent this restriction and have an assessment that is as accurate as possible, using several laserlines is crucial. [START_REF] Araujo | Resonance raman spectroscopy of the radial breathing modes in carbon nanotubes[END_REF].

SWCNT sample type A (cm -1 nm) B (cm -1 )

Supergrowth CVD SWCNTs [START_REF] Araujo | Nature of the constant factor in the relation between radial breathing mode frequency and tube diameter for single-wall carbon nanotubes[END_REF] 227 0 Alcohol CVD SWCNTs on quartz [START_REF] Araujo | Third and fourth optical transitions in semiconducting carbon nanotubes[END_REF] 217 15 CVD SWCNTs on SiO 2 (Fe catalyst) [START_REF] Jorio | Structural (n, m) determination of isolated single-wall carbon nanotubes by resonant raman scattering[END_REF] 248 0 Free-hanging isolated SWCNT [START_REF] Paillet | Raman active phonons of identified semiconducting singlewalled carbon nanotubes[END_REF] 204 27 HiPco SDS-dispersed SWCNTs [START_REF] Bachilo | Structure-assigned optical spectra of single-walled carbon nanotubes[END_REF] 223.5 12.5 HiPco SDS-dispersed SWCNTs [START_REF] Jorio | Resonance raman spectroscopy (n, m)-dependent effects in small-diameter single-wall carbon nanotubes[END_REF] 227 17.3/11.8 CVD SWCNTs on SiO 2 235.9 5.5 (Fe, Ni, Co-W catalyst) [START_REF] Zhang | n, m) assignments and quantification for single-walled carbon nanotubes on sio 2/si substrates by resonant raman spectroscopy[END_REF] Semiconducting to metallic ratio assessment Having determined the diameter of a SWCNT from its measured RBM frequency, and knowing the E L , makes it possible, by reporting back to the Kataura plot, to discriminate a semiconducting nanotube from a metallic one. Using the same statistical approach as for diameter assessment, the selectivity on electronic properties can also be assessed. And as for the RBM frequencies, the E ii of SWCNTs are affected by their environment. The E ii are for instance observed to increase for an isolated SWCNT, as opposed to a bunddled SWCNT [START_REF] Fantini | Optical transition energies for carbon nanotubes from resonant raman spectroscopy: Environment and temperature effects[END_REF], and were seen to vary when using different surfactants for solution dispersion [START_REF] Maultzsch | Radial breathing mode of single-walled carbon nanotubes: Optical transition energies and chiralindex assignment[END_REF]. Using the Kataura plot adapted to the SWCNT sample is therefore important.

As mentioned in Chapter 1, the original version of the Kataura plot [START_REF] Kataura | Optical properties of single-wall carbon nanotubes[END_REF] is based on a simple nearest-neighbor tight-binding model, within the zone folding approximation. As revealed by experimental results very early on, the importance of self-energy and excitonic effects result in an overall blue-shift of the E ii . As a result, calculations taking the many-body effects into account were performed. Some groups used a so-called extended tight-binding model (ETB) [START_REF] Jiang | Chirality dependence of exciton effects in singlewall carbon nanotubes: Tight-binding model[END_REF], [START_REF] Popov | Comparative study of the optical properties of single-walled carbon nanotubes within orthogonal and nonorthogonal tight-binding models[END_REF], including a large number of neighbors, and σπ hybridization, while others used ab initio calculations, directly describing excitonic effects [START_REF] Spataru | Excitonic effects and optical spectra of single-walled carbon nanotubes[END_REF]. Kataura plots were also elaborated by fitting experimental data. Araujo et al. used resonant Raman spectroscopy with a tunable laser to measure the E ii of as-grown alcohol-assisted and SG SWCNTs, and fitted the obtained data with an analytic equation, leading to the construction of an experiment-based Kataura plot [START_REF] Araujo | Resonance raman spectroscopy of the radial breathing modes in carbon nanotubes[END_REF], [START_REF] Araujo | Third and fourth optical transitions in semiconducting carbon nanotubes[END_REF], [START_REF] Araujo | The role of environmental effects on the optical transition energies and radial breathing mode frequency of single wall carbon nanotubes[END_REF]. Liu energies [START_REF] Liu | An atlas of carbon nanotube optical transitions[END_REF]. Michel et al. plotted a "normalized" Kataura plot by comparing E ii calculated by a non-orthohonal TB model, and coupled ED and Raman spectroscopy of individual free-standing SWCNTs [START_REF] Michel | Indexing of individual single-walled carbon nanotubes from raman spectroscopy[END_REF]. Aside from relative shifts of optical transition energies due to environmental effects, these plots were in overall agreement. In this study, we chose to determine the SC/M ratio of our samples using an easily accessible calculated ETB Kataura plot from [START_REF] Sato | Discontinuity in the family pattern of single-wall carbon nanotubes[END_REF], shown in Figure 2.17. One should keep in mind that this SC/M ratio evaluation is only meant to compare samples, and not to establish any absolute values, which would require to construct a Kataura plot adapted to our samples.

Assessing the resonance window for RBMs is also crucial for proper delimitation of SC and M diameter ranges around a given excitation energy. The resonant window is affected by the environment, as well as nanotube chirality.

Fantini et al. have shown that the resonance window was 112 meV wide in the case of bundled SWCNTs, and only 65 meV in the case of SWCNTs dispersed in solution using SDS [START_REF] Fantini | Optical transition energies for carbon nanotubes from resonant raman spectroscopy: Environment and temperature effects[END_REF]. Calculations by Park et al. [START_REF] Park | Raman resonance window of single-wall carbon nanotubes[END_REF] have shown that the resonance window is always several tens of meV, and increases with diameter. Souza Filho et al. gave a much higher values [START_REF] Souza Filho | Anomalous twopeak g-band raman effect in one isolated single-wall carbon nanotube[END_REF] of about 200 meV for CVD Chapter 2. Experimental methods grown SWCNTs on SiO 2 . The resonance window is defined as the full width at half maximum energy range over which a given SWCNT is in resonance, meaning that there can be a Raman signal outside of this resonance window. The experiments conducted to determine this resonance window for a given SWCNT in a given environment are performed using a tunable laser over a wide energy range. In our experiments, an RBM peak could be counted outside of the resonance window. For this reason, and because the SiO 2 substrate may have a strong impact on this value, we chose a wider "resonance" width around the excitation energy of ± 100 meV.

Blind spots and uncertainties

Since resonant Raman spectroscopy is not a characterization technique that provides a direct view of the SWCNT's diameter, or chirality, as opposed to HRTEM or ED, the results obtained from its analysis have to be considered with great caution. Several factors can have a non-negligible impact on the end result when using Raman spectroscopy for growth selectivity assessment for instance.

When it comes to determining the diameter of a SWCNTs by measuring its RBM frequency, some uncertainty is expected. Aside from the uncertainty in the measurement of the frequency, which is related to the spectrometer and the grating that is used during the experiment, an additional uncertainty is due to the ω RBM → d t relation that is used to calculate the diameter based on the RBM frequency. As explained previously, this relation depends on the specific type of SWCNT sample that is being probed. The error linked to the relation that is used will also obviously have an impact when trying to determined the metallic or semiconducting properties of the probed SWCNT, and even more so when trying to determine its (n,m) indices.

When characterizing not an individual SWCNT, but an ensemble of SWCNTs with a distribution of diameters or (n,m) indices, even more care should be taken for the analysis of the Raman characterization. First, the pool of SWC-NTs that can be probed is determined by the excitation wavelength(s) used, as only SWCNTs in resonance or quasi-resonance with the excitation energy will be detected. Moreover, determining the diameter distribution of the sample may be rendered difficult due to the differences in Raman cross-sections from one SWCNT to another.

Further, attempting to evaluate the SC/M ratio, or the (n,m) distribution of a SWCNT sample is an even more challenging task. Indeed, the E ii of a SWCNT, which determine its resonance conditions, depend on the environment. In order to discriminate between a semiconducting or a metallic SWCNT when knowing the RBM frequency and the excitation wavelength for instance, requires very good knowledge of the sample. The means of bias of resonant Raman spectroscopy for the evaluation of the diameter distribution of an as-grown SWCNT sample will be further discussed in Chapter 4.

Methodology

All Raman spectroscopy characterizations were performed using a HORIBA LabRam ARAMIS spectrometer, using a x100 objective, with four excitation wavelengths (473 nm, 532 nm, 633 nm, and 785 nm). The spectra were acquired for 1s using a 1800 groove per mm grating, giving a spectral resolution of 0.7 cm -1 .

In order to obtain reliable statistical data for diameter distribution assessment, we proceeded systematically with the same method. Raman mappings were recorded on the surface of the characterized wafer. The dimensions of the mappings were 50 µm by 50 µm, with a 5 µm step in both directions, which amounted to a total of 121 recorded spectra for each mapping. We used a large enough step to avoid detecting the same SWCNT twice in a row, but small enough not to avoid going over a too large area in one mapping, and possibly losing focus conditions (see Figure 2.18). The mappings were done on random locations on the substrate with the four excitation wavelengths. Depending on the yield of a given CVD growth, the number of mappings varied from one sample to the other, aiming at counting at least one hundred, to several hundreds of RBM peaks (when possible) total. The same number of mappings was of course recorded with each laser. The laser power and acquisition time were carefully adjusted in order to have significant RBM signal in a short enough time without heating the sample. The spectrometer was calibrated for a Si signal at 520.7 cm -1 .

The obtained data were analyzed as follows: for every spectrum of every mapping, each RBM peak (with an intensity higher than three times the background intensity) was counted as one SWCNT. They were fitted with a Lorentzian profile, and their frequency was measured. We consider the uncertainty on the RBM frequency to be of 1 cm -1 . The diameters of all the counted SWCNTs Chapter 3

Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors

It has been established in the first chapter of this manuscript that catalyst engineering is a key point for selective growth of SWCNTs using CVD. Here, we present a new general synthesis method with the aim of producing homogeneous bimetallic alloy nanoparticles, with a controlled size and morphology, able to catalyze the growth of SWCNTs. The basics of this catalyst synthesis is to use preformed stoichiometric PBA nanoparticles. Catalyst nanoparticles are then prepared in situ in a hot filament CVD reactor with subsequent high temperature treatment in reducing atmosphere prior to SWCNT growth. With this method, we hope to enable the comparison between catalyst nanoparticles with different behaviors in their catalytic activity, and potential role in growth selectivity. For this reason, multiple catalyst systems are tested. A monometallic iron catalyst system, derived from the original PB structure, as well as two bimetallic systems derived from easily synthesized PBA systems (NiFe, and NiCr) were tested in the following study. The reduction of the NiFe PBA nanoparticles is expected to yield alloyed nanoparticles, having a lower carbon solubility than pure Fe, and the reduction of the NiCr PBA nanoparticles is expected to lead to a phase segregation. These assumptions are based on the bulk binary phase diagrams of both systems (see Appendix B.1).

The first aim of this study is to successfully synthesize and characterize several PB and PBA pre-catalyst systems, which will be presented in the first section of this chapter. Then, a crucial objective was to ensure the possibility of forming well-defined catalyst nanoparticles, with a controlled size, and in the case of the bimetallic NiFe system, as nanoalloys, with a controlled composition. This is discussed in the second section of the chapter. The abilities of 3.2. Synthesis and characterization of catalyst precursors: PB and PBA nanoparticles 91 the wafer, forming a homogeneous monolayer. The catalyst precursor-coated wafer is then placed in a CVD reactor, where the catalyst nanoparticles will be formed by reduction under a H 2 atmosphere (at growth temperature) of the PB/PBA nanoparticles into metallic nanoparticles. The carbon precursor in directly introduced after 5 minutes of reductive pretreatment, and SWCNT growth takes place. After growth, the samples can be taken out of the reactor for characterization.

In the next sections, we will discuss each step of this process, along with the characterizations that help to understand them, starting from the synthesis of the catalyst precursors, up to the growth of SWCNTs. As explained in Chapter 2, the synthesis method employed here has been developed in our laboratory for the past decade. The three precursors are synthesized in very similar conditions, according to the method presented in [START_REF] Catala | Nanoparticles of prussian blue analogs and related coordination polymers: From information storage to biomedical applications[END_REF]:

• K 4 F e II (CN ) 6 + F e(N O 3 ) 3 → K x F e III [F e II (CN ) 6 ] y • K 3 F e III (CN ) 6 + N iCl 2 + CsCl → Cs x N i II [F e III (CN ) 6 ] y • K 3 Cr III (CN ) 6 + N iCl 2 + CsCl → Cs x N i II [Cr III (CN ) 6 ] y
The first system is the monometallic PB system, and the other two are bimetallic PBA systems. In all three cases, an alcaline cation is present in the structures' tetrahedral interstitial sites with variable stoichiometry: K + in the case of PB, and Cs + in the cases of the two PBA systems (A cation mentioned in Chater 2.1.1). In order to simplify the notations in this chapter, the three systems will be referred to using the two metal ions on each side of the CN bridges in the PB/PBA structure: FeFe for PB, NiFe, and NiCr for the two PBA systems. Further in the chapter, this notation will also be used for referring to the catalyst systems for SWCNT growth, keeping a clear distinction between the effective catalyst, meaning the nanoparticles obtained from the reduction of the PB/PBA nanoparticles, and the catalyst precursor, meaning PB/PBA system.

Chapter 3. Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors

For the synthesis of PB nanoparticles, an aqueous solution of F e(N O 3 ) 3 at 0.5 mM is poured into a F e II (CN ) 6 solution at 0.5 mM under high stirring, after having been cooled down to 2 For all studied systems, the as-obtained PB/PBA nanoparticles are stable in water for at least several weeks without any stabilizing agent, which leaves their surface available to get coordinated to an organic ligand.

Characterization of the obtained nanoparticles

The as-obtained PB/PBA nanoparticles were fully characterized to measure their sizes, and confirm their structure, and composition. For TEM investigation, a few drops of the solution containing the as-synthesized nanoparticles are deposited on a Cu TEM grid coated with a full carbon membrane. For size measurements using dynamic light scattering (DLS), the measurements were performed directly using the as-obtained solution. For the X-ray powder diffraction (XRPD) and infrared (IR) spectroscopy, the nanoparticles were precipitated by adding an excess of calcium chloride (CaCl 2 ) to the aqueous dispersion. The solution is centrifuged, the resulting powder is rinsed using DI water, and dried under vacuum. A quick presentation, along with experimental details, on all characterization techniques used in this chapter are given in Appendix A.

Size

DLS allows the measurement of the mean hydrodynamic size of the nanoparticles dispersed in the solution. This measurement is performed as a routine check of our PB/PBA nanoparticle syntheses, it gives a general idea of the sizes and dispersion state of the synthesized objects.

3.2. Synthesis and characterization of catalyst precursors: PB and PBA nanoparticles 95 PB nanoparticles are slightly bigger than the NiFe and NiCr PBA nanoparticles. According to TEM analysis, however, NiFe and NiCr PBA nanoparticles are synthesized with the same mean sizes, whereas DLS gives a slightly lower hydrodynamic size for the NiCr system. Since TEM is a more accurate way to determine the size distributions of the particles, we assume that the difference between the DLS measurements between the NiFe and NiCr systems is due to the enhanced error margin when using DLS for the characterization of low concentration dispersions.

The size analysis by DLS and TEM imaging demonstrates our ability to synthesize PB/PBA nanoparticles for the three mono-and bimetallic systems presented in this study. The particles are obtained individualized as stable dispersions in water, with similar, and well-controlled sizes. The obtained sizes are ideal for reduction into metallic nanoparticles, when considering the substantial decrease in lattice parameter between PB/PBA and the zero oxidation number corresponding compounds. In order to confirm that the nanoparticles detected by DLS, and observed by TEM are PB/PBA nanoparticles, the crystallographic structure was studied.

Crystallographic structure

The crystallographic structures of the PB and PBA nanoparticles were verified using XRPD. The XRPD patterns obtained for the PB and PBA precipitated and dried nanoparticles are presented in Figure 3.4. The PB/PBA FCC structure was confirmed for all three systems, and the lattice planes are indexed on the patterns. The lattice constants were extracted from the patterns, and are given in Table 3.1, along with the calculated correlation domains using the Scherrer relation [212]:

τ = Kλ βcos(θ) (3.1)
Where τ is the mean crystalline domain size, K is the shape factor (taken as 0.89), λ is the wavelength of the X-ray beam, β is the full width at half maximum (FWHM) of the diffraction peak, and θ is the Bragg angle.

The calculated lattice constants are 10.19 Å, 10.31 Å, and 10.51 Å for FeFe, NiFe and NiCr, respectively. They are all in good agreement with values obtained in the literature [START_REF] Buser | The crystal structure of prussian blue: Fe4 [fe (cn) 6] 3. xh2o[END_REF], [START_REF] Brinzei | Spontaneous stabilization and isolation of dispersible bimetallic coordination nanoparticles of cs x ni [cr (cn) 6][END_REF]. The Scherrer domain sizes are calculated as an average on the three most intense peaks for the NiFe and NiCr PBA systems, and directly evaluated from the most intense peak for the FeFe system. The domain sizes are overestimated compared to the sizes determined by TEM, but this is to be expected as the bigger nanoparticles constitute a higher contribution to the intensity of the peaks in the XRD pattern [START_REF] Brinzei | Spontaneous stabilization and isolation of dispersible bimetallic coordination nanoparticles of cs x ni [cr (cn) 6][END_REF]. Moreover, the uncertainty associated with this relation increases when measuring sub-10 nm objects. This error is probably even more marked in the case of the FeFe system, were the calculation was done using only one peak, since the too small amount of powder (directly related to the low concentration of the aqueous dispersion of nanoparticles) derived from the nanoparticle synthesis led to low intensity and a high signal-to-noise ratio on the XRPD pattern.

Chemical analysis

IR spectroscopy was performed on all systems in order to further confirm the PB/PBA network formation. The spectra are shown in Figure 3.5, in the vincinity of the CN stretching spectral range (2300 -1800 cm -1 ). The spectra obtained for the PB/PBA powders are superimposed with the corresponding hexacyanometallate complex spectrum. In each case, the CN band in the PB/PBA system is blue-shifted in comparison to the CN band of the complex, attesting to the formation of the PB/PBA coordination network by cyanide bridging. In the case of the FeFe system, a broad band is observed around 2072 cm -1 , in good agreement with values seen in the literature [START_REF] Ghosh | Infrared spectra of the prussian blue analogs[END_REF]. For the NiCr PBA, the bridging CN ligands (Ni II -CN-Cr III ) stretching mode can be observed at 2171 cm -1 , and a shoulder corresponding to terminal CN ligands, present at the surface of the nanoparticles at 2130 cm -1 . In the case of the NiFe system, two intense peaks are observed for the bridging CN ligands. This has been previously observed [START_REF] Heurtaux | Nanoparticules de réseaux de coordination à ponts cyanures: Systèmes superparamagnétiques, photomagnétiques et multi-composants[END_REF]. The band at 2164 cm -1 , blue shifted compared to the free CN stretching mode observed at 2118 cm -1 for the hexacyanometalate K 3 Fe III (CN) 6 , corresponds to the CN in Fe III -CN-Ni II . The second band, which is red shifted compared to the free CN in the hexacyanometalate, corresponds to the CN ligands in Fe II -CN-Ni II . We can also observe a shoulder on this last stretching band at 2060 cm -1 , corresponding to the presence of terminal CN ligands (Fe II -CN). In summary, the NiFe system displays a partial reduction of Fe III to Fe II . All the values for the CN vibration bands are given in Table 3.2. For the two PBA systems, SEM-EDX analysis was performed on the powders after precipitation and drying of the nanoparticles. The results of the quantitative analysis of this EDX data are presented in Table 3.3. The obtained atomic fractions roughly correspond to a 1:1 metallic ratio for both systems (Ni:Fe, and Ni:Cr), and a 0.6:1 Cs:M ratio. This characterization indicates that the bimetallic catalyst precursor nanoparticles obtained with this synthesis method are, globally, stoichiometric.

as evidenced by DLS. TEM analysis showed narrow size distributions, and individual nanoparticles, with sizes well-suited for use as catalyst precursors for tangential growth of SWCNTs after reduction. The PB/PBA structure was confirmed for all systems through IR spectroscopy and XRPD. The M:M' ratios were determined by a SEM-EDX study, confirming the 1:1 stoichiometry for both bimetallic systems. In the next sections, these FeFe, NiFe, and NiCr PB/PBA nanoparticles are integrated in our process, and all steps are subsequently studied. The next section therefore focuses on wafer preparation, and the PB/PBA nanoparticle deposition step (steps 1 and 2 in Figure 3.1).

Sample preparation and PB/PBA precursor deposition

Wafer preparation

All wafers used in this study are 300 nm thermal SiO 2 coated silicon (SiO 2 /Si) wafers purchased from SiMat. Each sample was cleaned by ultrasonication in dichloromethane (CH 2 Cl 2 ) for 15 minutes, followed by 10 minutes of Ar-O plasma treatment in order to activate the SiO 2 surface. The cleaned SiO 2 /Si wafers were functionalized, according to the synthesis protocol described in [START_REF] Bouanis | Direct synthesis and integration of individual, diameter-controlled single-walled nanotubes (swnts)[END_REF], with a self-assembled monolayer (SAM) of a silane molecule having a terminal pyridine group able to form a coordination bond with the PBA nanoparticles (see Appendix B.2 for details on synthesis). The wafers were immersed in a 10 -3 M solution of the silane molecule in distilled toluene (C 7 H 8 ) for 12 hours. The samples were subsequently rinsed with toluene and dichloromethane followed by annealing at 100 • C for two hours in ambient air.

PB/PBA nanoparticle deposition

In order to graft the PB/PBA nanoparticles, the silanized wafers were immersed in the as-obtained colloidal PB/PBA solutions for one hour. The excess of nanoparticles was rinsed off with DI water, and the wafers were left to dry under air.

The surfaces of the obtained PB/PBA-coated wafers were examined through AFM. Typical AFM images for the three systems are shown in Figure 3.6, for each system, a height line profile is extracted to give an indication of the roughness of the surface. The large 2 µm by 2 µm images clearly show that the PB/PBA nanoparticles are deposited as a homogeneous layer. The observed surfaces appear to be fully covered by nanoparticles, without unwanted deposition of oversized objects, and no zones showing excess thickness are observed. High density of the PB/PBA mat is evidenced by the zoomed-in images also presented in Figure 3.6. The line height profiles show peaks going from, roughly, 1 to 6 nm, in good agreement with the TEM measurements presented in the previous section. We should note that the particle layer seems to be denser for the bimetallic NiFe and NiCr PBA systems, than for the monometallic FeFe PB system. This can be attributed to the difference in concentrations between the mono-and bimetallic PB/PBA nanoparticle dispersions. As it has been said in the previous section, the concentration of the hexa-aquo and the hexacyanometalate complex solutions are four times lower (0.5 mM) in the case of the PB system, than for the PBA systems (2.0 mM), resulting in an overall lower concentration of the particle suspension. Since the deposition rate depends on the concentration, we can assume that a longer deposition time is needed for full coverage for the FeFe system. This could be prevented by a longer deposition time for the PB system, or extracting the nanoparticles, and re-dispersing them with a higher concentration, prior to deposition, but both solutions are time consuming. The density of the nanoparticle layer obtained with a 1 hour deposition time is nonetheless satisfying, as it can help prevent coalescence in the CVD reactor.

A control experiment was performed to demonstrate the importance of the anchoring SAM in this process, in the case of the FeFe PB system. A SiO 2 /Si wafer was sonicated for five minutes in a series of solvents (acetone, IPA, CH 2 Cl 2 ), dried, and immersed in the aqueous solution containing the FeFe nanoparticles. After 24 hours, the wafer was taken out of the solution, successively rinsed using DI water and methanol, and dried under vacuum. AFM imaging was performed on the dried wafer in order to evaluate the deposition efficiency. Virtually no nanoparticles were detected (see Appendix B.4), whereas sufficient coverage of the wafer surface is obtained after one hour of immersion time when the wafer is coated with the silane molecule. This confirms the necessity of the SAM, especially in the case of this system, which is obtained as a suspension at a much lower concentration.

The presence of the deposited metals was also confirmed by X-ray photoelectron spectroscopy (XPS) for the two bimetallic PBA systems. The global Ni:Fe and Ni:Cr ratios were quantified, so as to confirm that the metallic ratios on Chapter 3. Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors the surface of the wafers were in agreement with the stoichiometric ratios observed on the PBA powders by EDX. This is an important point. Indeed, since the deposition is directly made from the as-obtained nanoparticle-containing solution, the process presents a risk of anchoring metal cations that have not reacted, which could lead to an effective metallic ratio on the surface that does not correspond to the wanted 1:1 ratio. The XPS-extracted atomic percentages of the present elements, as well as the atomic fractions of the elements of interest in the Cs, M, M' mixture are reported in Table 3.4. In the case of the NiCr system, the 1:1 M:M' ratio was preserved after deposition on the surface. Moreover, the Cs content is left unchanged.

However, in the case of the NiFe system, a slight increase in the Ni content (Ni:Fe ratio of 1:0.7), and decrease in the Cs content are observed. First of all, a slight difference can be expected between the EDX and XPS quantifications, owing to the increased error margin of the XPS quantification which can go up to 15 %. However, these changes are significant and though they may be toneddown by the error margin increase, they cannot be fully ignored. The significant decrease in Cs content could be explained by the fact that the EDX characterization was performed on the powder retrieved from the as-synthesized particle suspension, whereas the XPS characterization was performed on the wafer after PBA deposition and thorough rinsing with DI water. It is not excluded that part of the Cs atoms are removed during the rinsing step. When it comes to the increase in Ni content, this may be due to residual adsorption of Ni 2+ cations on the surface. We should also keep in mind that XPS analysis gives a global idea of the chemical composition at the surface of the wafer, and not the composition of individual nanoparticles.

Conclusion

In summary, the analysis of the surfaces of the wafers after SAM and PB/PBA nanoparticle depositions shows that our process allows for the formation of a homogeneous catalyst precursor nanoparticle layer, with controlled size and composition in the case of the bimetallic systems. In the specific case of the NiFe PBA system, a slight change in the surface composition is observed, as compared to EDX results, which can only be partly attributed to the lower accuracy of XPS when it comes to quantification. The next section focuses on the reductive pre-treatment step that leads to the formation of the effective catalyst nanoparticles, with a focus on the two bimetallic systems.

Catalyst nanoparticle formation and characterization

After the catalyst precursors have been deposited on the samples, they have to undergo a reductive pretreatment under activated H 2 atmosphere at growth temperature (step 3 in Figure 3.1). The wafers coated by the PB/PBA nanoparticles were placed in the cold zone of the CVD chamber, the chamber was pumped to a 10 -6 mbar pressure, and the temperature was set to 800 • C in the hot zone. After stabilization of the temperature, H 2 was introduced at 100 sccm (standard cubic centimeters per minute) flow rate, and the corresponding filament power of 160 W turned on. Once the pressure in the chamber was stabilized at 90 mbar, the wafers were moved into the hot zone and maintained in the reductive atmosphere for 5 minutes. This treatment is immediately followed by the SWCNT synthesis, by directly introducing CH 4 (without modifying H 2 flow rate) at 20 sccm flow rate and for a corresponding hot filament power of 120 W, while the overall pressure was increased to 100 mbar. In order to characterize the effective catalyst nanoparticles however, the samples were taken out of the CVD chamber right after the 5-minute pretreatment step.

Wafer surface analysis after reductive pretreatment

The surfaces of the wafers taken out of the CVD reactor before performing SWCNT growth were studied using AFM. In such a way, one can observe the are cooled to ambient temperature and exposed to air, the studied nanoparticles may not be in the exact chemical state as the effective catalyst nanoparticles that remain in the CVD chamber until the end of the SWCNT growth step, and are therefore, not exposed to air and oxidation.

Typical AFM images of the catalyst nanoparticle-coated wafers are presented in Figure 3.7. The results are presented in the same manner as the AFM analyses previously shown on the PB/PBA nanoparticle-covered wafers. We can observe that the homogeneity of the nanoparticle layer is preserved after the reductive pretreatment. For all systems, the observed surfaces are fully covered by nanoparticles, evidenced again by the zoomed-in images. Compared to the line height profiles for the PB/PBA deposition step, we can see that, globally, the sizes of the nanoparticles on the surface of the wafers have decreased significantly. This was to be expected since the PB/PBA structures have a much larger lattice constant than the corresponding metals or alloys. After reduction, the catalyst nanoparticles have sizes compatible with tangential growth of SWCNTs, which was the desired result.

We should note here that for the NiCr system, the AFM image as well as the featured height profile, reveal that two populations of nanoparticles can be distinguished. Nanoparticles with sizes around roughly 1 -3 nm on one hand, and larger nanoparticles with sizes around 5 -10 nm on the other hand. This is not the case for the FeFe and NiFe systems, which exhibit much more homogeneous sizes.

TEM characterization of the catalyst nanoparticles

For all catalyst systems, the wafers that were taken out of the CVD chamber before undergoing SWCNT growth were used for TEM characterization. In order to study the effective catalyst nanoparticles, they were transferred onto Cu TEM grids coated with a carbon membrane, using the method previously described in Chapter 2.3.1.5. In this specific case, preventing contamination during TEM observation by residual PMMA and organic solvent impurities was attempted by a more thorough rinsing than for observing SWCNTs (where a PMMA residue can be helpful for maintaining SWCNTs in place over holes in the membrane), and a systemic rinsing in IPA before the acetone evaporated.

Chapter 3. Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors

Monometallic FeFe catalyst

A rapid TEM characterization of the transferred FeFe catalyst nanoparticles was performed, so as to confirm the narrow size distribution of the catalyst nanoparticles after the reductive pretreatment, as well as the size reduction in comparison to the precursor nanoparticles. The two size distributions (FeFe PB, and FeFe effective catalyst) are displayed in Figure 3.8. The mean nanoparticle size for the effective catalyst nanoparticles is 1.8 ± 0.6 nm, which corresponds to about half of the precursor nanoparticle mean size (3.7 ± 0.9 nm). Since the density of nanoparticles on the TEM grid is extremely low, the sizes of only about 50 nanoparticles were counted (compared to about 200 for the PB nanoparticles). The results are nonetheless consistent with the values extracted from AFM height profile measurements.

Bimetallic catalysts

A typical TEM image of catalyst nanoparticles after transfer is shown in Figure 3.9, along with the size distribution histograms obtained by statistical analysis of particle sizes for both bimetallic systems (roughly 150 particles for each distribution). Since a thorough rinsing was required for satisfactory observation of the nanoparticles, the surface density found on the TEM grids is extremely low, compared to the aspect of the surface of the wafers observed by AFM. This implies that a lot of material is lost during the transfer process.

Chapter 3. Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors dark field (HAADF) STEM image, elemental maps for the two metallic components, and the relative map obtained by superimposing the two elemental maps, attest that the alloying is effective. Elemental maps were extracted at energies of 6.30-6.50 keV (Fe Kα), and 7.37-7.58 keV (Ni Kα). According to the bulk Ni-Fe binary phase diagram, a solid solution can form at 800 • C for a 1:1 ratio [START_REF] Swartzendruber | The fe-ni (iron-nickel) system[END_REF]. The STEM-EDX data displayed in Figure 3.11.a is in agreement with this, even though arguments based on bulk phase diagrams should be taken with a grain of salt, as they are known to change at the nanoscale [START_REF] Vallee | Size and segregation effects on the phase diagrams of nanoparticles of binary systems[END_REF]. This indicates that our method enables the formation of FeNi alloyed nanoparticles.

However, a majority of the observed nanoparticles display a phase segregation, with the appearance of a Ni-rich phase, and a Fe-rich phase. An example of this situation is shown in Figure 3.11.b., where a very clear boundary can be seen between the two phases, like in a so-called Janus configuration. This observed demixion, and the fact that it is not systematic, as some observed nanoparticles are alloyed, raises a few questions.

The first question is whether the phase segregation appears upon cooling, and/or exposure to ambient air, or during the reductive pretreatment. In other words, can we suppose that the observed nanoparticles correspond to the actual effective catalyst nanoparticles from which SWCNTs grow in the next step of the process? Since some of the nanoparticles appear to be alloyed and others are not, it is difficult to conclude on whether or not SWCNTs would be grown from alloyed or segregated nanoparticles.

Second, we can question the potential role of the size of the nanoparticle in the phase segregation. The study was conducted on nanoparticles whose sizes are not representative of the actual diameter distribution obtained from the analysis of TEM images discussed previously. Figure 3.12 shows a comparison of the nanoparticle size distribution obtained from measurements of around 150 NiFe nanoparticles (showed in Figure 3.9), and the size distribution of the nanoparticles observed in our STEM-EDX study. It clearly shows that the nanoparticles studied by STEM-EDX have sizes that are, for the most part, larger than the mean diameter of the TEM-extracted diameter distribution. It is interesting to note that the three observed "alloyed" nanoparticles have diameters of 2.8 nm, 2.4 nm, and 2.2 nm. They are some of the smallest nanoparticles observed by STEM-EDX. For these small objects, a size-dependent behavior could be expected. We could assume that the behavior of the NiFe nanoparticles that were not observed by STEM-EDX (sizes ranging from 1 to 2 nm) is closer to that of the alloyed nanoparticles (smallest observed nanoparticles) than the Janus nanoparticles. It could also be argued that the nanoparticle displayed in Figure 3.11.a could be a Janus nanoparticle rotated by a 90 • angle (as compared to the particles observed in Figure 3.11.b), whose projection would give the impression of an alloyed system. Regardless, we still have good reason to assume that the nanoparticles are alloyed in CVD conditions. As explained above, the phase segregation could be size-related, as indicated by the measured sizes of the seemingly alloyed nanoparticles. The bulk binary phase diagram predicts a solid solution in growth conditions, and even in the phase-segregated nanoparticles, the Fe (Ni) content in the Ni-rich (Fe-rich) phase is non-zero. This cannot be affirmed in all certainty, but the more probable scenario is that the effective catalyst is a Fe-Ni solid solution with a composition that cannot be predicted. We can therefore expect the behavior of this catalyst with regard to SWCNT growth to differ from that of the FeFe catalyst.

We should note that the STEM-EDX analysis allowed to confirm the absence of Cs in the nanoparticles (see spectra in Appendix B.6). The Cs atoms initially present in the PBA structure are expected to sublimate during the pretreatment step, as Cs has a boiling point of 670.8 • C at atmospheric pressure. Moreover, sulfur is found in the nanoparticles. Since the silane molecule used to functionalize the wafer surface contains one atom of sulfur, it is possible that it is the source of the sulfur observed in the nanoparticles. However, the sulfur content the formation of alloyed nanoparticles when the alloy is not thermodynamically favored in the bulk phase. We should also note that again, we were able to confirm the absence of Cs in the nanoparticles, meaning that it will not play any role in the SWCNT growth process further on. Judging from this STEM-EDX study, we can safely assume that SWCNT nucleation and growth from this NiCr catalyst probably arise from the Ni-rich (or pure Ni) phase of the segregated nanoparticle. Indeed, nickel is well known for its ability to catalyze the growth of SWCNTs [START_REF] He | Low temperature growth of swnts on a nickel catalyst by thermal chemical vapor deposition[END_REF], [START_REF] Chen | Single-wall carbon nanotube synthesis by co disproportionation on nickel-incorporated mcm-41[END_REF], [START_REF] Chiang | Linking catalyst composition to chirality distributions of as-grown single-walled carbon nanotubes by tuning ni x fe 1-x nanoparticles[END_REF]. Moreover, the growth of SWCNTs using pure chromium nanoparticles as catalysts has not, to our knowledge, been demonstrated in the literature. A study by Deck et al. even reported a failed attempted CNT growth using a Cr-based catalyst [START_REF] Deck | Prediction of carbon nanotube growth success by the analysis of carbon-catalyst binary phase diagrams[END_REF]. We may therefore consider the effective catalysts to be Ni nanoparticles. This catalyst system could be compared to studies in the literature showing SWCNT growth from segregated nanoparticles, where only one of the metals catalyzes the growth. In a study by He et al. previously mentioned in this manuscript, in situ HRTEM experiments showed that the SWCNT growth stemmed from the Fe-pure shell of a complex Fe-Pt bimetallic catalyst system [START_REF] He | Environmental transmission electron microscopy investigations of pt-fe 2 o 3 nanoparticles for nucleating carbon nanotubes[END_REF]. A study by Cui et al. reported the formation of a phase-segregated Co-Cu bimetallic catalyst system by STEM-EDX investigation, where small Co nanoparticles were anchored onto Cu nanoparticles, and assumed a growth stemming from the Co nanoparticles [START_REF] Cui | Synthesis of subnanometer-diameter vertically aligned single-walled carbon nanotubes with copper-anchored cobalt catalysts[END_REF].

Conclusion

In this section, we have showed that our process allows the efficient fabrication of three different catalyst systems. In all cases, the reductive pretreatment led to the formation of a dense layer of catalyst nanoparticles with sizes well-adapted to tangential SWCNT growth. We observed a systematic size reduction in agreement with the difference in lattice parameters between a PB/PBA structure and the corresponding metallic structures. We also confirmed that coalescence was under control both after the reductive pretreatment, and after SWCNT growth.

Three catalyst systems with different behaviors have been successfully prepared: a monometallic Fe catalyst with controlled size, a bimetallic NiFe catalyst that can be assumed to be alloyed under growth conditions, and a bimetallic NiCr system that displays phase segregation, by which we can assume a growth stemming from a small monometallic Ni effective catalyst. All of these systems are prepared in identical conditions, and have different behaviors with regard to their interaction with carbon, or evolution with temperature.

Chapter 3. Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors

SWCNT growth

In this section, we present the results obtained for the growth of SWCNTs (fourth and final step of our process in Figure 3.1) from the three presented catalytic systems elaborated from PB/PBA precursors. We will first focus on SWCNT growth from the FeFe system, which serves as a monometallic reference, and then we will discuss SWCNT growth from the two bimetallic systems.

SWCNT growth from the monometallic PB catalyst precursor

PB nanoparticles were successfully synthesized, with a controlled size, and their crystallographic structure has been unambiguously confirmed through XRPD and IR characterizations. We have shown that, using a SAM of a silane molecule bearing a pyridine group, we were able to coat SiO 2 /Si wafers with a homogeneous layer of those nanoparticles, as confirmed by AFM measurements. Our AFM and TEM study of the FeFe catalyst nanoparticles after reductive pretreatment of the BP nanoparticles has shown that we obtain a monolayer of catalyst nanoparticles with a controlled size, logically smaller than that of the catalyst precursors. The AFM also shows no sign of coalescence. We have therefore validated our process for the formation of FeFe catalyst nanoparticles.

We will now discuss their performance as SWCNT growth catalysts.

SWCNT growth feasibility

SWCNT growths were performed using the HFCVD reactor presented in Chapter 2.2. We should underline that after the formation of the catalyst nanoparticles by a reductive pretreatment under activated H 2 at growth temperature, the carbon source is directly introduced in the CVD chamber to pursue SWCNT growth. The samples are not in contact with air at any point between their introduction in the chamber (wafer coated with SAM and catalyst precursor) and their removal after the end of the growth process and cooling. The generic conditions of the growth are as follows: H 2 flow rate is maintained at 100 sccm (and the filament power maintained at 160 W), CH 4 is introduced with a flow rate of 20 sccm, the pressure is increased to 100 mbar, the filament at the CH 4 insert is set to 120 W, and the growth lasts 30 minutes.

The first growth trial was performed at 800 • C (meaning the pretreatment was also performed at 800 • C). SWCNTs were successfully grown using our Chapter 3. Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors

of SWCNTs present on this surface unit, but we found it was a practical way to compare our samples with regard to yield. The mean number of SWCNTs detected under one laser spot for this sample is 4.2.

The good quality of the synthesized SWCNTs is demonstrated by the Raman mapping in Figure 3.14.d recorded in the D and G bands spectral range using a 532 nm laser. The intensity of the D band (around 1350 cm -1) is visibly low, and it rarely exceeds 10 % of the intensity of the G band (around 1590 cm -1).

The HRTEM image displayed in Figure 3.14.b, showing the lattice structure of the SWCNTs, also attests to the good crystallinity of the grown SWCNTs.

We can also note that the HRTEM image clearly shows a SWCNT linked to the catalyst nanoparticle it grew from (upper right), with a tube diameter to particle size close to 1, indicating a tangential growth mode in this particular case. Right below this, a nanoparticle and a carbon cap, that could have continued as a tangential growth can be observed as well. According study comparing the use of CO and CH 4 for SWCNT growth and their respective effect on growth modes, we could expect tangential growth in the present growth conditions [START_REF] Amara | Modeling the growth of single-wall carbon nanotubes[END_REF]. Unfortunately, observing these occurrences of a tube-particle relation on our transferred samples is rare, and we should note that the lack of statistical data on this specific matter prevents us from drawing any clear conclusions. However, no perpendicular growth tube-particle relations were observed, justifying the assumption that the experimental conditions used in the present study favor the tangential growth mode.

We have therefore shown that our growth process does enable the growth of SWCNTs from PB nanoparticles as catalyst precursors. We have obtained high quality SWCNTs with a high yield, and shown the occurrence of a tangential growth mode.

Raman study of growth temperature effect

SWCNT growths from the FeFe catalyst were then attempted at various temperatures ranging from 700 • C to 1000 • C, with the aim of studying the effect of increasing growth temperature on the resulting SWCNTs. The obtained samples were characterized using SEM and Raman spectroscopy, for the evaluation of relative yield, diameter distribution, SC/M ratio, and SWCNT approximate length. This is a classical phenomenon that has been widely reported in the literature, as discussed in Chapter 1. In most cases, it is explained by an Oswald ripening effect, favored at high temperature because of an increased metal mobility. Iron being a metal with a rather low melting point, this explanation could very well be valid here. Assuming that the SWCNTs grow in tangential mode, a catalyst coarsening phenomenon could explain why the diameter distribution is significantly shifted towards larger diameters at 900 • C, by a global increase in catalyst nanoparticle size. In the present case, however, metal diffusion should be prevented by the presence of hydrogen in the reactor during growth, keeping Oswald ripening and coalescence under control [START_REF] Jeong | Atomic hydrogendriven size control of catalytic nanoparticles for single-walled carbon nanotube growth[END_REF]. Moreover, the size of the catalyst nanoparticles after SWCNT growth at 800 • C were measured by TEM, and the mean diameter was found to be 2.0 ± 0.9 nm (see section 3.4.2.3). The mean diameter is very close to the value obtained from nanoparticles before growth (2 Å shift). As explained previously, some coalescence is observed, but it involves nanoparticles with sizes above 3.0 nm, which are not involved in the tangential growth mode. The predominant population of catalyst nanoparticles does not evolve after 30 minutes at 800 • C under a CH 4 /H 2 ambient, indicating an almost non-existent coalescence, and Oswald ripening.

Regarding nanoparticle coarsening phenomena with increasing temperatures (900, and 1000 • C), TEM analyses should be performed to confirm their absence. Since transfers from wafer to grid after SWCNT growth were optimized for high yield SWCNT growths, transferred grids are not available at 900 and 1000 • C. A comparison between catalyst nanoparticle sizes after SWCNT growth at 800

• C and at 1000 • C was performed for a FeRu PBA system in the exact same conditions, and showed no evolution of the catalyst nanoparticle size distribution. This can lead to another possible explanation for the observed evolution in the Raman-extracted diameter distributions.

Chapter 3. Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors

As discussed in Chapter 1, Monte Carlo simulations published in the last few years have put carbon content, and solubility in the catalyst nanoparticle at the heart of nucleation and growth mechanisms. A successful nucleation and growth was demonstrated to be the result of a fine balance between sufficient dewetting of the sp 2 carbon wall from the catalyst nanoparticle to ensure cap lift-off, while maintaining a certain adhesion in order for the growth to continue by preventing a detachment of the SWCNT from the catalyst nanoparticle from which it grew [START_REF] Aguiar-Hualde | Probing the role of carbon solubility in transition metal catalyzing single-walled carbon nanotubes growth[END_REF]. It was shown that the carbon content in the catalyst nanoparticle controlled this phenomenon: a low carbon content in the nanoparticle favors wetting, whereas a high carbon content leads to dewetting. In our growth conditions a rather high carbon content is used, and a long growth time, we can consider that the carbon content in the catalyst nanoparticles easily reaches the solubility limit. This implies that we can control the carbon content in the catalyst nanoparticles by tuning the carbon solubility in the nanoparticle. Here, at a fixed nanoparticle composition, two key parameters control the carbon solubility in the catalyst nanoparticles: temperature and size.

An increased temperature is known to increase the carbon solubility in metals, and a decrease in the size of nanoparticles also leads to an increase in carbon solubility [START_REF] Magnin | Size dependent phase diagrams of nickel-carbon nanoparticles[END_REF]. To explain the evolution of the Raman-extracted diameter distribution with growth temperature, we can assume the following scenario (see Figure 3.17). Considering that the pool of potentially active catalyst nanoparticles for tangential growth lies within the 0.5 -3.0 nm range: at low temperatures (here 700 • C), small nanoparticles are activated for SWCNT growth, and large nanoparticles are poisoned by carbon encapsulation due to their low carbon solubility and therefore too high adhesion to the carbon wall. At 800 • C, the carbon solubility is increased, and larger catalyst nanoparticles are activated, leading to a slight shift in the diameter distribution towards larger diameters (here: mean Raman-extracted diameter going from 1.11 nm to 1.30 nm). At 900 • C, the carbon solubility in small nanoparticles starts to be high enough to favor tube detachment instead of growth, and even larger nanoparticles are now activated for SWCNT growth. This theory may also explain the absence of SWCNTs at 1000 • C: at this temperature, all nanoparticles have reached the dewetting-favoring regime, precluding any sustainable SWCNT growth. Similar experiments performed by another PhD student using FeRu PBA catalyst precursors show successful growths at 1000 • C of larger diameter SWCNTs with Chapter 3. Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors information on our samples. Here, Raman characterization is used as a mean to compare samples that were characterized with the exact same method.

Effect on SC/M ratio

The percentage of semiconducting SWCNTs was extracted from the Raman characterization using the method described in Chapter 2.3.2.3. The results are presented in Table 3.6. First, the results do not differ substantially from one temperature to another. Moreover, all values are close to a 2/3 semiconducting content, corresponding to a random chirality distribution. One can note, however, that the value obtained at 900 • C is above 70 %, we could assume that in a growth were possible in the same conditions at higher temperatures, higher semiconducting contents could be obtained in our samples. These considerations are to be taken with great caution, as Raman spectroscopy may not be suitable to measure the SC/M ratio of a given growth sample, which will be discussed in the next chapter.

In summary, our process allowed for the growth of SWCNTs from iron catalyst nanoparticles formed by reduction of PB nanoparticles, demonstrating the effectiveness of this new and original process. We have shown a clear effect of growth temperature on the SWCNTs, by Raman and SEM characterization. An optimum yield is found at 800 • C, and the yield is lower both at lower and higher temperatures, with a zero Raman-extracted "yield" at 1000 • C. Assuming a dominating tangential growth mode, judging both from our growth conditions, and HRTEM experiments, the shift in the diameter distribution can be explained by activation of catalyst nanoparticles of different sizes, linked to the changes in carbon solubility driven by the changes in temperature. The following section aims at testing the feasibility of SWCNT growth with our process from bimetallic PBA precursors, and the study of potential composition effects.

SWCNT growth from the bimetallic PB catalyst precursors

SWCNT growths were performed in the same generic conditions in the HFCVD reactor, using wafers coated in NiFe and NiCr PBA precursors. According to the STEM-EDX study presented in the previous section, we can assume that the growth from the NiCr catalyst is led by monometallic Ni nanoparticles anchored onto Cr nanoparticles. In the case of the NiFe system, though the exact chemical nature of the effective catalyst is unclear, we can expect a growth from alloyed Chapter 3. Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors from the NiFe catalyst, after being transferred onto a TEM grid.

The two bimetallic catalyst systems led to successful SWCNT growth in usual conditions. As for the monometallic catalyst, growths at various temperatures were attempted, in order to study the influence of the temperature on SWCNT growth from catalysts with different compositions.

Raman and SEM study of growth temperature effect

As for the FeFe catalyst system, growths were conducted for both the NiFe and NiCr catalyst systems at various temperatures ranging from 700 • C to 1000 • C. Figure 3.19 displays SEM images of the growth samples for the two systems at various temperatures. Despite several trials, the growths at 1000 • C were not successful. As for the FeFe catalyst, we can see a very clear effect of temperature on growth yield, with an optimal growth temperature of 800 • C for both systems. In the case of the NiCr system, the SEM image at 900 • C features only one rather short nanotube, indicating an extremely low density and therefore growth yield. Several Raman spectroscopy mappings on random locations of the sample revealed no detected RBM peaks, i.e. a zero Raman "yield". The calculated Raman-extracted "yields" for all samples will be discussed in the next section.

The Raman characterization was performed for all of the successful growth samples, and the resulting diameter distributions are presented in Figure 3.20, along with the diameter distributions obtained for the FeFe catalyst. Looking at the NiFe case, there seems to be little-to-no evolution of the Raman-extracted diameter distribution with increasing temperature. In the NiCr case, the distribution is similar to the ones obtained using the FeFe and NiFe catalysts at 700 • C. At 800 • C, the population of SWCNTs with diameters around 0.8 -1.2 nm decreases drastically, while a population of SWCNTs with larger diameters appears, leading to an overall increase in the mean diameter (which goes from 1.28 nm at 700 • C to 1.44 nm at • C). An overview of the Raman-extracted data for all the presented SWCNT growths is given in Table 3.7.

Discussion on the effect of catalyst composition

In this section, the three different catalyst systems derived from PB/PBA precursors will be compared, with regard to yield, reproducibility, and growth temperature limit. At 800 • C, we can suppose that the system is in an intermediate temperature condition, where small and large nanopaticles are activated for growth (zone 2 in Figure 3.17): within the pool of potentially active catalyst nanoparticles (size range 0.5 -3.0 nm), a wide range of sizes are active, leading to a higher yield. In that sense, the yield difference between the FeFe and NiFe systems at this temperature also implies a difference in catalyst behavior that could stem from an alloying effect. Another interesting feature here is that the growth temperature limit seems to depend on the catalyst nature.

Reproducibility of the Raman spectroscopy results

Figure 3.22 shows the superimposition of the Raman-extracted diameter distributions for several different batches of SWCNT growths conducted in the same conditions (at 800 • C), in the cases of the FeFe and NiFe catalyst systems. In the case of the FeFe system, we clearly see that the results are very similar from one growth to another. The same SWCNT growth was performed at least five times with close to identical Raman spectroscopy results. When looking at the NiFe case, we can see that in two cases, the Raman diameter distributions also overlap, but the third is completely different. Similar results have been obtained using the NiCr system.

In order to comment on the evolution of the Raman-extracted diameter distribution for the NiFe and NiCr systems, additional experiments should be performed. Indeed, the evolution of the Raman spectroscopy results with increasing temperature would have to be confirmed through growth and characterization

❵ ❵ ❵ ❵ ❵ ❵ ❵ ❵ ❵ ❵ ❵ ❵ ❵ ❵ ❵ System Temperature 700 • C 800 • C 900 • C 1000 • C FeFe XX NiFe XX NiCr XX XX
the catalyst nanoparticles barely evolve during the 30-minute SWCNT growth process, as evidenced by TEM measurements. This allowed us to favor the carbon solubility reasoning in comparison to the usually employed Oswald ripening argument to explain the evolution of SWCNT diameter distributions with increasing temperature. The size of the catalyst nanoparticles were also measured after growth at 800 • C for the NiFe and NiCr systems. The mean diameters after growth are 2.1 ± 0.7 nm, and 2.4 ± 1.1 nm, for the NiFe, and NiCr systems, respectively. Here also, the size distributions after growth indicate the absence of catalyst coarsening, and the limited occurrence of coalescence. Moreover, when looking at the size distributions of the effective NiFe (mean size 2.4 nm) and FeFe (mean size 1.8 nm) catalysts at 800 • C, a strict reasoning based on a SWCNT diameter distribution driven by catalyst size would predict a significant difference between the two obtained SWCNT diameter distributions. This is not the case, since the two Raman extracted diameter distributions (see Figure 3.20.a and b) are very similar. For these reasons, we will use the same carbon solubility argument put forward previously in the case of the FeFe catalyst to try to explain these growth temperature limits.

In the case of the NiFe system, assuming that the SWCNTs grow from a Ni-Fe alloy of unknown composition, the carbon solubility in the catalyst nanoparticles should be lower, as the carbon solubility limit is lower for Ni than for Fe, than in the FeFe case. A lower carbon solubility within the catalyst nanoparticle would mean a lower carbon content (assuming the solubility limit is always reached) than for the FeFe system at equal temperatures. We could therefore assume that a growth at 1000 • C could be observed, because the higher adherence of the sp 2 carbon to the catalyst nanoparticle could enable to sustain growth at a higher temperature. However, this is not observed, which can imply that the difference in carbon solubility between the FeFe and NiFe catalyst is not sufficient.

Chapter 3. Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors

Within this carbon solubility reasoning, the lower growth temperature limit observed for the NiCr system is also surprising. Considering that the SWCNTs grow from a pure, or close to pure, Ni effective catalyst nanoparticle in this specific case, the decreased carbon solubility as compared to the FeFe or NiFe system should delay the detachment of the sp 2 carbon wall from the catalyst nanoparticle (in terms of temperature), which is the opposite of what is observed. Here, the growth temperature limit is decreased. However, this would be considering that the effective catalyst nanoparticles are the same size in all cases. Here, it could be argued that the phase-segregation results in a smaller than expected effective catalyst size. Since a decrease in nanoparticle size has been shown to increase carbon solubility, it could explain the earlier detachment observed in the case of the NiCr system.

General conclusion

In this chapter, we have shown the synthesis and extensive characterization of nanoparticles of three PB/PBA systems. Using a surface functionalization method, these nanoparticles were successfully deposited on SiO 2 /Si substrates, as evidenced by AFM and XPS characterization. The reduction of these PB/PBA precursor nanoparticles into metallic nanoparticles was then studied. We showed that our new and original process allows for the formation of a homogeneous catalyst nanoparticle layer, with overall controlled sizes for each considered system. The original strategy of using coordination and surface chemistry to anchor pre-formed catalyst precursors with a controlled size, coupled with a reductive thermal pretreatment, is an efficient way to finely control the size of the effective catalyst nanoparticles.

The STEM-EDX study of the to bimetallic catalyst systems (NiFe and NiCr) demonstrated the possibility of forming seemingly alloyed nanoparticles when the formation of a solid solution was expected according to the corresponding bulk binary phase diagram. On the contrary, the nanoparticles observed for the NiCr system showed clear phase segregation. Our process allowed for the preparation of three catalyst systems, each showing a different behavior. The formation of catalyst nanoparticle fabrication with three different compositions in the same conditions has rarely been studied with this characterization technique.

SWCNT growth was successfully conducted for all catalyst systems at various 3.6. General conclusion 131 temperatures. We observed a clear evolution of the Raman-extracted diameter distribution with growth temperature in the case of the FeFe monometallic catalyst, which could be explained simply by an Oswald ripening effect, but also by considering the temperature as a way to control carbon solubility inside the catalyst nanoparticles, leading to activation of different nanoparticle size populations inducing changes in the SWCNT diameter distribution. Additional experiments regarding the sizes of the catalyst nanoparticles at various temperatures are needed to conclude on the matter. This is further supported by the absence of SWCNT growth at 1000 • C, which could mean that at this temperature, the carbon solubility is such that there is detachment of caps from all nanoparticles. The SC/M ratio did not appear to be strongly modified by growth temperature.

The difficulty to determine the actual chemical state of the catalyst nanoparticles during growth, and a lack of strong reproducibility, rendered the interpretation of the Raman spectroscopy results for the two bimetallic catalyst systems quite challenging. However, the STEM-EDX study led to the assumption that SWCNTs grow from a Ni-Fe alloy of unknown composition for the NiFe system, and small pure Ni nanoparticles for the NiCr systems. This led to potential interpretations of the observed growth temperature limits, also based on carbon solubility considerations. The different behaviors observed through Raman characterization for the three catalyst systems show the clear influence of the catalyst composition.

Though additional experiments could still enlighten our findings, as well as lead to optimization of growth selectivity, this chapter demonstrated the effectiveness of our process for growing SWCNTs from a variety of catalyst systems, prepared in close to identical conditions. This particular technique for preparing catalyst nanoparticles is simple to implement, efficient, and offers significant improvements compared to usually used physical preparation routes such as co-impregnation coupled with calcination: the composition of each precursor nanoparticle is known and controlled by the inherent structure of the PBA coordination network, the sizes of the precursor nanoparticles are finely controlled, and their deposition and reduction significantly prevent coalescence phenomena. This method can be applied to other PBA systems (bimetallic, or even trimetallic) with more promising compositions. The application of this process, or an adaptation of it, to other families of chemically synthesized catalyst precursors will be presented in Chapter 5.

Chapter 3. Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors

In the present chapter, the SWCNTs were only characterized using Raman characterization. The Raman-extracted diameter distributions that were shown in Figure 3.16.b raise questions concerning the potential distortions of results due to the resonant character of the technique. By optimizing a transfer process, we were able to characterize the SWCNTs of the samples with highest yields by TEM, and compared the results to Raman characterization. The results are shown, and extensively discussed in the next chapter.

Chapter 4

Comparing TEM and resonant Raman spectroscopy for diameter distribution assessment of a SWCNT sample

It was shown in the first chapter of this manuscript that statistical data extracted from more than one characterization technique are rarely compared in publications showing SWCNT growth selectivity. In this chapter, we will explore the comparison between TEM imaging and Raman spectroscopy for determining the diameter distribution of a CVD growth sample, in an effort to validate, or invalidate, our currently used methodology. In the first section, comparisons between TEM-and Raman-extracted diameter distributions on typical CVD growth samples will be shown, and a study on diameter-sorted SWCNTs will then be presented in the next sections, in an attempt to understand the differences observed between TEM-and Raman-extracted diameter distributions on our typical growth samples.

Comparing TEM and Raman spectroscopy on typical CVD growth samples

The first characterization technique used for our growth sample was Raman spectroscopy because it was a fast way to have an idea of the selectivity depending on growth conditions. It is well adapted to the type of samples that are produced with our technique. TEM characterization was performed later: first by looking at SWCNTs grown through the same process as the usual samples, but on holey SiO 2 membranes deposited on molybdenum TEM grids, and The first sample for which this comparison was performed was a SWCNT growth at 800 • C, in the usual ambient conditions, using PB nanoparticles as a catalyst precursor, but submitting them to a pretreatment under argon. The diameter distributions extracted from the Raman characterization on the wafer on one hand, and the TEM characterization on the other, are presented in Figure 4.1.

The distribution extracted from Raman spectra is centered at 1.34 nm, with a standard deviation of 0.26 nm, with more than a quarter of the detected SWC-NTs having a diameter between 1.2 nm and 1.3 nm.

Unfortunately, it cannot relevantly be compared with the TEM-extracted diameter distribution, which does not contain enough data to be statistically representative of the sample (only 23 counted SWCNTs). This is due to two main factors. First, it seems that the yield of the growth on the SiO 2 membrane is much lower than the growth on a wafer, in our growth conditions. The difficulty to detect the grown SWCNTs may also be related to the thickness of the SiO 2 membrane, and the fact that it is composed of heavier atoms than a regular carbon membrane, which prevents the detection of the SWCNTs on top of the membrane. Secondly, in order to accurately measure the diameter of a SWCNT, it has to be suspended, and only a few of the observed SWCNTs were crossing the holes in the membrane. This strategy was thus not used further for TEM and Raman comparison.

However, looking at this distribution, we can still see that it is shifted towards larger diameters, as compared to the distribution determined by Raman spectroscopy. The mean diameter is 1.81 nm, with a high standard deviation of 0.95 nm. Though it is not statistically representative, this shift is quite surprising, and needs to be examined further.

For this, the sample characterized by Raman was transferred onto a TEM grid (using the method described in Chapter 2.3.1.5), with the hope that the higher yield of the growth, and the weaker contrast of the carbon membrane would lead to a much easier detection of the SWCNTs through TEM, and therefore to measurements from a larger number of tubes. TEM images of SWCNTs from this transferred sample, and the corresponding diameter distribution histogram are displayed in Figure 4.2. The transfer method clearly enabled the observation of a significantly larger amount of SWCNTs (45 SWCNTs). The number of measured diameters is of course a lot lower than in the case of the 4.1. Comparing TEM and Raman spectroscopy on typical CVD growth samples 137 locations on the TEM grid, and the fact that the transfer method was not, at the time, fully optimized, leading to significant losses in SWCNTs. After optimization refining, we were able to characterize more samples by TEM, with more statistically representative data.

Other samples

After more practice with the transfer process, and in particular more careful handling of the PMMA rinsing step, we were able to obtain transferred SWCNT growth samples with less damaged membranes, and thus more available SWC-NTs for diameter measurement. We could therefore compare Raman and TEM characterizations with a better accuracy. This comparison was conducted on two different samples, that were chosen for their significant growth yield, so as to obtain usable TEM grids after transfer. The first sample is a typical growth using the FeFe PB catalyst precursor, at 800 • C in the usual ambient conditions and after undergoing the usual reductive pretreatment. The second is the same CVD growth conducted using the NiFe PBA catalyst precursor. The Raman-extracted diameter distribution is systematically centered at a lower diameter, and is systematically narrower than the TEM diameter distributions. Since Raman spectroscopy does not allow the detection of SWCNTs with large diameters, we also calculated the mean diameters and standard deviations of the two TEM-extracted distributions without taking into account the SWCNTs that could not have been probed with Raman spectroscopy. We considered the lower limit of the RBM spectral range to be 100 cm -1 , which corresponds, using the relation between RBM frequency and SWCNT diameter ω RBM = 248/d t , to an upper diameter limit of 2.5 nm. Aside from these SWCNTs, we distinguished three diameter domains in the distributions:

• Domain 1: below 1.3 nm, SWCNT populations seem to be overestimated by Raman spectroscopy and/or underestimated by TEM

• Domain 2: between 1.3 nm and 1.7 nm, similar results are given by both characterization techniques 4.1. Comparing TEM and Raman spectroscopy on typical CVD growth samples 139 with a mean difference (TEM-extracted count -Raman-extracted count) of 4.1 % and 4.9 % for the FeFe, and NiFe samples, respectively. 4.1 shows the mean diameters and standard deviations for both samples, as determined by TEM (with and without taking into account the SWCNTs undetected by Raman), and by Raman. In both cases, excluding the SWCNTs with diameters above 2.5 nm in the TEM distribution leads to an evident decrease in the mean diameter, making them a bit closer to the Raman-extracted mean diameters. This modification is however not sufficient to make the mean diameters of the two TEM distributions overlap with their Raman counterparts, as they are still shifted towards higher diameters by about 0.4 nm. In the case of the FeFe sample, the broadness of the distribution is decreased, whereas it is unchanged for the NiFe sample. In both cases, the standard deviations obtained from TEM for SWCNTs with diameters below 2.5 nm are close to the standard deviations obtained from Raman spectroscopy.

This indicates that the shift in the diameter distribution is not just attributed to the intrinsic limitation of Raman spectroscopy when it comes to detecting SWCNTs using their RBM mode. In the wide range of diameters where both Raman spectroscopy and TEM imaging enable the detection of SWCNTs, different populations are represented in larger or lower numbers depending on the characterization technique used. In Table 4.2, we show the calculated percentages of the total detected SWCNTs for each characterization technique present in each of the three diameter domains defined previously. In the case of the TEM characterization for the two samples, less than 20 % of the total counted SWCNTs are in domain 1, while around 50 % of the SWCNTs have diameters in domain 3. On the contrary, when considering the SWCNTs detected by Raman spectroscopy, more than 50 % of the SWCNTs are in domain 1, while a little more than 10 % of the detected tubes have diameters higher than 1.7 nm Chapter 4. Comparing TEM and resonant Raman spectroscopy for diameter distribution assessment of a SWCNT sample (domain 3). There are a few possible explanations for this shift, which is, if

Potential reasons for observed shift

There are potential explanations, involving both TEM and Raman characterizations, to this shift in diameter distributions. We will first list the reasons related with a possible bias in our TEM characterization method (both inherent to TEM, or to the sample preparation process), and will then address the reasons related with Raman spectroscopy. In this section, an approximate dichotomy is made between "small-diameter" and "large-diameter" SWCNTs. These expressions mainly refer to SWCNTs with diameters in the Raman-dominant range (domain 1), and TEM-dominant range (domain 3), respectively.

TEM-related

Selective SWCNT degradation during transfer process A few studies have given accounts of structural dependence of the reactivity of SWCNTs. Liu et al. showed the dependence of oxidation reactivity of SWCNTs on diameter, chirality, and metallicity [START_REF] Liu | Chirality-dependent reactivity of individual single-walled carbon nanotubes[END_REF]. In a study on covalent functionalization, Strano et al. showed its dependence on metallicity, and the reactivity of SWCNTs was shown to be inversely proportional to their diameter [START_REF] Strano | Electronic structure control of single-walled carbon nanotube functionalization[END_REF]. The latter is explained by curvature-induced strain that is present in small-diameter SWCNTs [START_REF] Niyogi | Chemistry of single-walled carbon nanotubes[END_REF]. The role of metallicity on the reactivity was explained by their electronic band structure [START_REF] Joselevich | Electronic structure and chemical reactivity of carbon nanotubes a chemists view[END_REF]. It therefore seems legitimate to question whether the transfer process can chemically preferably degrade certain SWCNT populations in the sample. The strain in small-diameter SWCNTs make them 4.1. Comparing TEM and Raman spectroscopy on typical CVD growth samples 143

while the SWCNT with a 1.7 nm diameter is intact. This indicates that the latter was indeed more resistant to electron beam degradation.

Because of the high sensitivity of SWCNTs to electron beam degradation, all the TEM observations were performed at 80 kV, and using a condenser aperture in order to protect the rest of the sample from beam degradation when focusing on a particular zone. However, it seems that this precaution is not sufficient to prevent all degradation, especially in the case of small-diameter SWCNTs. For this reason, it is possible that some of those SWCNTs are not counted in the diameter distribution determined by TEM, leading to a potential shift of the distribution towards larger diameters.

Operator bias: attraction to bigger objects

It is well known among experimentators working with TEM that there is a general tendency to be attracted to bigger objects. When operating the TEM at a lower magnitude in order to spot zones on the grid with SWCNTs, a largediameter SWCNT, or a very long SWCNT, can obviously be more easily seen, whereas sub-nanometer SWCNTs require a higher magnification in order to be detected. This can lead to a potential rise in the number of measured largediameter SWCNTs, and a decrease in the number of measured small-diameter SWCNTs, even though the resolution of the microscope allows for the observation of such nanotubes.

As a global conclusion, we can say that all of the potential biases related to TEM characterization for diameter distribution assessment, whether they are intrinsic to TEM or due to our characterization process, may render the characterization of small-diameter SWCNTs difficult, possibly leading to a shift of the diameter distribution towards larger diameters.

Raman-related

As mentioned in Chapter 2, the results of a selectivity assessments based solely on Raman spectroscopy are to be interpreted with caution. This is true for conclusions drawn from every Raman mode (RBM, G, and D bands), but let us focus on the conclusions drawn from the RBM mode with respect to diameter distribution and SC/M ratio. We can distinguish four means of bias and/or uncertainty: Chapter 4. Comparing TEM and resonant Raman spectroscopy for diameter distribution assessment of a SWCNT sample 1. The detectable SWCNT populations are determined by the chosen/available excitation wavelength(s)

2. The Raman cross-section of a given SWCNT is multi-factor-dependent 3. Upper-diameter detection limit (usually no RBM peaks detected for SWC-NTs with diameters above 2.5 nm)

4. The choice of ω RBM → d t law impacts the result

Resonance window

As explained in Chapter 2, only the SWCNTs in resonance with the excitation wavelength will give rise to measurable Raman signals. In order to attempt to have statistical data that are truly representative of the sample, we need to increase the number of probed SWCNTs as much as possible by using more than one laser line. Without a continuous excitation energy range, however, one has to be aware of the inevitable bias this casts on Raman characterization.

The four lasers used in this study cover a very wide range of diameters in both metallic and semiconducting populations. Nevertheless, not all SWCNT populations are detected, which could lead to underestimation and overestimation of certain SWCNT populations through Raman characterization, possibly leading to distortions in the diameter distribution.

Intrinsic Raman intensity, or Raman cross-section

Even when the pool of detectable SWCNTs is substantial enough to have trustworthy characterization results, the dependence of cross-section on various parameters has a strong impact. In an experimental study on RBM intensities, Jungen et al. [START_REF] Jungen | Raman intensity mapping of single-walled carbon nanotubes[END_REF] model the measured Raman intensity I = φ(αI ii ), by the product of the intrinsic Raman intensity I ii of a given SWCNT, attenuated by α(∆E ii ) that takes into account the distance of the optical transition energy of the SWCNT to the excitation energy ((∆E ii ) = E ii -E L ), and a factor φ = φ(∆Θ, ∆r, j) which is experiment dependent. φ is a function of ∆Θ, the angle between the tube axis and the incident light polarization direction, ∆r is the distance between the scatterrer and the center of the laser spot, and j is the number of identical scatterrers under the laser spot. This gives a general idea of the parameters having an influence on the intensity of a specific RBM peak.

We can first note that, using our methodology, j is always considered to be 4.1. Comparing TEM and Raman spectroscopy on typical CVD growth samples 145 equal to 1, which is an approximation which may have a non-negligible impact on the results. Regarding the orientation of the SWCNT relative to the polarization direction, we can consider that it is randomized, due to the disposition of the SWCNTs on the sample. The distance of the optical transition energy of the resonating SWCNT to the excitation energy will come into play later in this chapter.

The important factor in this model for the Raman intensity is definitely the intrinsic Raman intensity of the given SWCNT, I ii . Several experimental and theoretical studies on the (n,m)-dependency of I ii (associated with the Raman cross-section), as well as its dependency, for a given SWCNT, on the optical transition, have been performed in the past. They show tremendous influence of the diameter, chiral angle, and optical transition, on the Raman intensity. The study cited above conducted Raman mappings on an as-grown SWCNT sample on SiO 2 with a 532 nm laser, and confronted the plotted Raman intensity as a function of diameter with calculated E ii using a non-orthogonal tight-binding model (excitons and self-energy not taken into account). The results, shown in Figure 4.6.a give striking evidence of this: many SWCNTs in quasi-resonance conditions do not contribute significantly to the Raman signal, while a few high-intensity SWCNTs determine the shape of the Raman intensity profile. Other experimental studies have been conducted in order to determine the diameter distribution [START_REF] Pesce | Calibrating the single-wall carbon nanotube resonance raman intensity by high resolution transmission electron microscopy for a spectroscopy-based diameter distribution determination[END_REF], or the chirality distribution [START_REF] Jorio | Quantifying carbon-nanotube species with resonance raman scattering[END_REF] of a SWCNT sample by correcting RBM intensities with respect to calculated (n,m)-specific Raman RBM intensities, also evidencing the importance of (n,m)-dependence of Raman cross-section.

More insight into the specifics of the (n,m)-dependence of the Raman intensity has been gained with experimental studies. Doorn et al. reported, by performing Raman spectroscopy on HiPCo SWCNTs dispersed with SDS, that type II semiconducting SWCNTs ((2n+m)mod3 = 2) had much lower Raman cross-sections than type I semiconducting SWCNTs with similar diameters, by at least an order of magnitude [START_REF] Doorn | Resonant raman excitation profiles of individually dispersed single walled carbon nanotubes in solution[END_REF]. Theoretical calculations using the extended tight-binding model also showed diameter and chiral angle-dependence of the Raman intensity, based on electron-phonon matrix element calculations. They observed a general trend of decreasing Raman cross-section with increasing diameter, while in the same family (2n+m = constant) branch, SWCNTs with lower chiral angles (end of branch) tend to have higher Raman intensities than SWCNTs with high chiral angles [START_REF] Sato | Excitonic effects on radial breathing mode intensity of single wall carbon nanotubes[END_REF] (see all SWCNTs within a 0.6 -2.4 nm diameter range, using the non-orthogonal tight-binding model.

All of these factors influencing the intrinsic Raman intensity, or Raman crosssection of a given nanotube inevitably lead to distortions in the diameter distribution. The shape of the distribution, the mean diameter and standard deviation can potentially be greatly affected.

Upper-diameter detection limit

Within our experimental setup, the Rayleigh scattering signal (or the filtering of this signal), makes it difficult to detect RBM peaks below 100 cm -1 . This is also something to be kept in mind. It means that above a diameter of about 2.5 nm, the SWCNTs in the sample are not accounted for in the distribution, which inevitably contributes to a shift of the distribution towards lower diameters, when compared to TEM, in the case of a sample containing large-diameter SWCNTs.

ω RBM -to-diameter relation

The laws that are used to determine the diameter from the RBM frequency are not necessarily applicable in the full diameter range of the SWCNT sample.

As shown by Araujo et al. [START_REF] Araujo | Resonance raman spectroscopy of the radial breathing modes in carbon nanotubes[END_REF] most of the data used to determine those laws relates to SWCNTs within the diameter range of usually 1 to 2 nm. Most of them are in relative agreement within that range, but their extrapolations diverge for lower and higher diameters without real experimental validation. This might lead to uncertainties regarding small and large diameter SWCNTs.

Hypotheses to be tested

It is clear that there are potential biases and/or blind spots when using both TEM and Raman spectroscopy for the characterization of a SWCNT growth sample. When it comes to the assessment of the diameter distribution of a growth sample, both characterization techniques have their own sources of bias.

Using the above discussion, we have established a list of hypotheses that can all partially explain the shift, and shape differences observed in our diameter distributions:

1. TEM is not adapted to the characterization of small-diameter SWCNTs (typically below 1. The first hypothesis regarding TEM combines the easy degradation of smalldiameter SWCNTs by the electron beam, and the fact that it is easier to detect bigger objects when using TEM. Hypothesis 2 takes into account the possible degradation and loss of SWCNTs during the transfer process. Hypothesis 3 combines prevention of the detection of large-diameter SWCNTs by Raman spectroscopy by Rayleigh scattering, and the diameter-dependent scattering cross sections. The fourth hypothesis hints both at the fact that Raman cross-section is (n,m)-and optical transition-dependent, and that the detected populations are dictated by the choice and availability of excitation lasers. The last possible explanation could be that the methodology applied to our samples for Raman spectroscopy assessment of the diameter distribution is not truly adapted. In order to test these various hypotheses, which mainly rely on a small-diameter SWCNT versus large-diameter SWCNT dichotomy, it seemed necessary to conduct the same study on diameter-sorted SWCNT samples.

TEM characterization of diameter-sorted SWC-NTs

A general observation when looking at the diameter distributions of the various samples (determined both by TEM and Raman spectroscopy), is that they are a bit broad. The presence of SWCNTs in the sub-nanometer range, as well as over 1.5 nm makes the differences in the two characterization techniques stand out. It is difficult, from these samples, to truly understand where the shift is coming from. In order to better understand the source of the shift, as well as the shape differences between the diameter distributions assessed by TEM and Raman spectroscopy, we wanted to look at SWCNT samples with a narrower diameter distribution in the diameter range of main interest: between 0.8 nm and 1.3 Chapter 4. Comparing TEM and resonant Raman spectroscopy for diameter distribution assessment of a SWCNT sample SWCNT sorting [START_REF] Arnold | Sorting carbon nanotubes by electronic structure using density differentiation[END_REF]), was adapted by Fleurier et al. to DWSNT samples [START_REF] Fleurier | Sorting and transmission electron microscopy analysis of single or double wall carbon nanotubes[END_REF], and further developed by A. Ghedjatti during his PhD thesis [START_REF] Ghedjatti | Etude structurale des nanotubes de carbone double parois[END_REF] (see Appendix C.1 for method). Since the DGU method enables first the sorting of CNTs according to their number of walls, there will be layers containing a very large majority of SWCNTs. Depending on the chosen layer, the center of the diameter distribution of those SWCNTs (as well as the content of DWCNTs) will be shifted.

Figure 4.7 shows the results of the sorting on this DWCNT-containing sample performed by Fleurier et al. [START_REF] Fleurier | Sorting and transmission electron microscopy analysis of single or double wall carbon nanotubes[END_REF], displaying the evolution of the absorption spectra, and diameter distributions of both SWCNTs and DWCNTs in different layers of the sorted sample. Some of the first layers contain no DWCNTs, and the SWCNT diameter distribution is quite narrow, and centered around small diameters, making them ideal samples for our study. Ahmed Ghedjatti performed this DGU sorting method on nanotubes from CIRIMAT, with the aim of obtaining, through several sorting rounds, pure DWCNT samples with an enriched semiconducting population. We used the first layer of his first round of sorting, containing small-diameter SWCNTs.

Sample characterization scheme

In order to test the hypotheses presented in the previous section, we prepared three samples with the diameter-sorted SWCNT solution. Figure 4.8 presents a schematic representation of the preparation process of these samples. First, to determine the diameter distribution of the SWCNTs in the solution using TEM, the solution was directly dropped on a copper TEM grid covered with a holey carbon membrane (sample 1). Second, in order to conduct the Raman characterization in the same conditions as our growth samples, a drop of the diameter-sorted SWCNT solution was deposited onto a SiO 2 /Si wafer, and the Raman spectroscopy characterization was conducted directly on the wafer (sample 3). Before looking into the Raman characterization results, we wanted to make sure that the transfer process was not responsible for the differences observed in the diameter distributions of our growth samples. A piece of the wafer coated with diameter-sorted SWCNTs was used for a transfer onto a TEM grid (sample 2). The TEM-extracted diameter distribution of the transferred diameter-sorted SWCNTs was compared to the diameter distribution extracted from the direct TEM characterization. We then compared the results of the Raman characterization conducted on the SiO 2 /Si wafer to the direct TEM measurements. The numbers of the samples are given by the order in which their respective characterizations will be presented in the next sections.

Direct TEM characterization

In order to analyze the differences observed between the statistical analysis of TEM and Raman characterizations of our typical SWCNT growth samples, a study was conducted on diameter-sorted SWCNTs.

TEM grid preparation

In order to determine the diameter distribution of our sample by TEM imaging, a grid was prepared using the solution constituting the first layer of the sorted nanotube sample. The grid was prepared as follows: a copper TEM grid covered with a holey carbon membrane was placed on an absorbing paper, with the carbon membrane (shiny side) facing up. A drop of solution was then dropped on the grid, and it was left to dry in air, covered by a watch glass for at 152 Chapter 4. Comparing TEM and resonant Raman spectroscopy for diameter distribution assessment of a SWCNT sample least two hours. Since the nanotubes are covered by surfactant molecules, the nanotubes are not directly observable. Therefore, the grid has to be rinsed-free of surfactant which may cause contamination in the TEM.

The dried grid was placed, with the carbon film facing up, onto a filter paper with 0.45 µL pores within a Büchner funnel mounted with a glass column, plugged to a pump. The glass column was then filled with DI water, and the extraction was regulated so that the water passed very slowly through the Büchner funnel. This is important because a high water flow rate would inevitably lead to a degradation of the thin carbon membrane, preventing the nanotubes to remain on the TEM grid. A final rinsing was then performed using a small volume of ethanol, so as to speed-up the drying process. The rinsed grid was left to dry for a few hours in air.

Direct determination of diameter distribution

The TEM characterization of the as-obtained nanotube sample was conducted as explained in Chapter 2. Figure 4.9 shows a typical TEM image obtained from the observation of the prepared grid. Out of over 70 images taken from multiple locations on the grid showing hundreds of nanotubes, only four DWCNT were seen, the rest were SWCNTs. Since they were not suspended, they were not counted in the statistical data. It is clear that the percentage of DWCNTs is extremely low in this sample, therefore, their presence cannot induce a significant distortion in the statistical data, either in TEM or Raman characterization.

We can clearly observe the presence of remaining organic materials on the SWC-NTs, and that a large quantity of the SWCNTs are still covered in surfactant molecules or other contaminants. One could argue that the fact that some of the SWCNTs are not bare and cannot be measured could also lead to distortions in the statistical data. However, by looking closely at the partly covered SWCNTs in the TEM images, there does not seem to be any sort of diameter-selectivity for a more effective rinsing, meaning that the suspended and bare SWCNTs are representative of the sample.

We measured the diameter of 121 SWCNTs from the TEM images of this sample, the diameter distribution histogram is presented in Figure 4.10. The mean diameter value is 1.35 nm, with a standard deviation of 0.63 nm. When setting aside the very few large-diameter SWCNTs (above 2.5 nm, i.e. not detected by Raman), the distribution is quite narrow, with the mean diameter going to 1.15 Chapter 4. Comparing TEM and resonant Raman spectroscopy for diameter distribution assessment of a SWCNT sample our growth sample, the Raman-determined diameter distribution systematically shows the predominance of SWCNTs with diameters between 1.0 nm and 1.3 nm. If this population of SWCNTs were truly predominant in the sample, the present experiments prove that they would have been predominant in our TEM distribution as well.

Validation of transfer method

An important factor to take into consideration in our comparisons between TEM and Raman characterizations for diameter distribution assessment is that the TEM characterization was always conducted on transferred SWCNTs. This means that our TEM characterization is not a direct observation of the assynthesized sample per se, because the SWCNTs have undergone a long process involving heating, rinsing steps, exposure to various organic solvents and NaOH, etc. The various steps of this process may very well alter the composition of our synthesized SWCNT sample, as explained previously. In order to make sure that the transfer process was a robust and non-altering solution for the TEM observation of our SWCNT growth samples, the following experiment was conducted.

Sample preparation

We wanted to compare the TEM characterization before and after the transfer process using the diameter sorted SWCNTs, meaning comparing the diameter distribution obtained in the previous section (direct measurement), and the diameter distribution we would obtain from the same sample if it had been transferred from a SiO 2 /Si wafer following the same process as our growth sample.

To this end, we had to deposit the SWCNTs from the solution onto a SiO 2 /Si wafer, in a sufficiently homogeneous and diluted mat, so that the Raman characterization methodology for the diameter distribution assessment of our growth samples could be used. This was done using the following procedure: a SiO 2 /Si wafer was cleaned by sonication in dichloromethane for 10 minutes, and was placed for 10 minutes in a plasma cleaner. This is necessary for the surface of the wafer to be hydrophilic for optimal wetting of the solution, but also to ensure the adherence of the SWCNTs to the SiO 2 surface. A drop of solution of the diameter-sorted SWCNTs was dropped onto the wafer, and the drop was spread across the surface by nitrogen blowing. The wafer was dried on a hot plate at about 40 • C for a few minutes. The wafer was then very thoroughly rinsed using DI water, and dried again on a hot plate for a few minutes. A SEM characterization of the sample was conducted so as to verify the state of the SWCNT layer. We obtain a rather dense mat of short SWCNTs (about 0.5 -

Comparison with direct measurements

The comparison of the diameter distribution obtained from TEM characterization of the diameter-sorted SWCNTs after the transfer process, with the directly-measured diameter distribution is shown in Figure 4.13. For the two samples, the mean diameters and standard deviations are really close, with and without taking into account the SWCNTs with diameters above 2.5 nm, with similar distribution shapes. We should note that the two diameter distributions were obtained from a number of SWCNTs in the same order of magnitude. We can conclude that the transfer process does not appear to modify the diameter distribution of the SWCNT sample. There is no evident decrease in the sub-nanometer population as a result of exposure to various solutions and heating treatments, proving that our transfer process is reliable when it comes to preserving the content of a SWCNT sample. Hypothesis 2 can therefore be overthrown.

Raman characterization of sorted SWCNTs

For Raman characterization, it is crucial to rinse the sample properly, to prevent the presence of surfactant molecules on the surface of the SWCNTs, which have a significant impact on their RBM frequencies. The same methodology applied to our growth samples was applied here for the Raman spectroscopy characterization. with a shift towards higher diameters of only 0.15 nm for the TEM measurements as compared to the Raman-extracted data. This is much lower than the surprisingly high difference of 0.40 nm seen in our growth samples. When considering the diameter range of the SWCNT sample, we can say that TEM and Raman are in fairly good agreement. However, there seems to be a global shift of the diameter distribution histogram of TEM measurements of 0.1 nm, compared to the Raman-extracted distribution. This is acceptable considering the 0.5 Å error margin associated with the TEM diameter measurement [START_REF] Fleurier | Transmission electron microscopy and uv-vis-ir spectroscopy analysis of the diameter sorting of carbon nanotubes by gradient density ultracentrifugation[END_REF], which could lead to a slight overestimation of diameters.

Results by laserline: confrontation with the Kataura plot

In order to better understand the diameter distribution obtained by Raman spectroscopy, and which SWCNT populations were responsible for the Raman signal in this sample, we looked at the data collected for each separate laser line. We plotted separate Raman-extracted diameter distributions for each excitation wavelength. The first important observation is that, for this particular sample, a much larger number of RBM peaks were counted for the mappings performed at 532 nm and 633 nm excitation wavelengths than at 473 nm and 785 nm excitation wavelengths. For all excitation wavelengths, the acquisitions were recorded at similar laser powers (between 4 and 10 mW). The RBM peaks counted at 633 nm and 532 nm account for 42.6 % and 47.6 % of the total 437 RBM peaks counted, respectively. On the other hand, the RBM peaks counted at 473 nm and 785 nm account for only 8.2 % and 1.6 % of the total counted RBM peaks, respectively. It therefore appears that much more SWCNTs in our sample are in resonance with the red and green laser energies, or that the SWCNTs that should be in resonance with the other two laser lines within the sample's diameter range are not present, or less present in the sample.

Since most of the RBM peaks are detected by the red and green lasers, the distorted shape of the global diameter distribution appears here: in the case of the red laser, two main populations are detected, one below 1.0 nm corresponding to SC-SWCNTs, and another at 1.2 nm corresponding to M-SWCNTs; and in the case of the green laser, one dominating population is detected below 1.0 nm corresponding to M-SWCNTs. This explains the bimodal shape of the global distribution. There is a "void" between 1.0 and 1.1 nm that has to be explained, by something other than the simple absence of this SWCNT population.

We seemed to be more adapted to our sample, it served as our main reference, the Kataura plot (1) served as a backup.

Assuming a random (n,m) distribution in the sample, the drastic differences in the numbers of counted RBM peaks for each laser line cannot be explained by an absence of resonance conditions for the blue laser, as compared to the red laser, for instance. Also, if we compare the results obtained for the 633 nm and 785 nm lasers, it is surprising to see that a very large number of RBM peaks were detected using the first laser between 1.1 and 1.3 nm whereas only a few were detected for the latter between 0.9 and 1.1 nm. In both cases, SWCNTs with E ii within the same distance of laser line are present. This could be explained by large differences in the RBM cross-sections for the SWCNTs in those two cases.

Let us look separately at each laser line, starting with the red laser at 633 nm. In this case, the observed diameter distribution is easily understood: two major features are present within the sample's diameter range, roughly corresponding to the two optical transition energy branches crossing the laser line (E SC 22 , and E M 11 ). The intensity of the feature corresponding to the metallic branch can be explained by the presence of the end of lower branches of two (2n+m) = constant families in the resonance window in the same narrow diameter range. As explained previously, it has been shown that the Raman intensity for SWCNTs in the lower E M 11 L branch is larger than for the upper E M 11 H branch.

In the case of the green laser at 532 nm, the most intense bar in the histogram also corresponds to the end of a lower branch of a (2n+m) = constant family within the resonance window between 0.8 nm and 0.9 nm. The count decreases after 0.9 nm even though a large portion of the sample's SWCNTs have diameters of around 1.0 nm. This could be explained by the fact that the only SWCNTs in resonance with the laser line are on the upper branch of E M 11 . The peak between 1.2 -1.3 nm corresponds to the appearance of the E SC 33 branch. However, it is surprising that a large majority of the detected SWCNTs seem to stem from the E M 11 branch around 0.8 -1.0 nm, whereas only a few stem from the E SC 33 branch at higher diameters (1.2 -1.4 nm).

Judging from the Kataura plot used here, the lack of Raman signal detected using the blue laser (473 nm) may stem from the fact that, within the diameter range of the sample, only a few SWCNTs are in resonant conditions. In the case of E SC 33 , the SWCNTs may have a too large energy distance to the excitation energy. It is interesting to see that there are barely any SWCNTs detected between 0.9-1.1 nm, which corresponds to the presence of upper branch E M 11 optical transitions, which have much lower Raman cross-sections.

The case of the 785 nm laser is quite surprising. A semiconducting SWCNT population around 0.9 -1.2 nm should be in good resonance conditions (similar to the E M 11 branch in the case of the 633 nm laser, with respect to position in the resonance window), but only a few RBM peaks were counted. This is surprising when considering a random chirality distribution in the sample. This hypothesis should be tested using another characterization technique, like ED or HRTEM.

In summary, some of the observed Raman data can be partly explained by differences in Raman cross-sections for various SWCNT populations within the sample's diameter range, and supposing a random (n,m) distribution. In general, it appears that most of the populations of SWCNTs that are not detected Chapter 4. Comparing TEM and resonant Raman spectroscopy for diameter distribution assessment of a SWCNT sample by our Raman methodology are semiconducting nanotubes: the E SC 33 of SWC-NTs with diameters between 1.0 and 1.3 nm should be detected in larger numbers using the 473 nm laser, since they fall in the diameter range the most predominant in the sample according to TEM characterization. The same remark can be made for the E SC 22 branch between roughly 0.8 and 1.1 nm using the 785 nm laser: only 7 RBM peaks were counted in total. Some of our data is still difficult to explain, and would require additional characterizations. However, there is little doubt as to whether the choice of excitation wavelengths, and intrinsic Raman intensity have an impact on our resulting distribution.

Conclusion and perspectives

In this chapter, we have first shown that there is a systematic shift between the diameter distributions assessed by Raman spectroscopy and TEM imaging in our typical CVD grown SWCNT samples. This shift can be as high as 4 Å. Diameter ranges corresponding to Raman dominant, and TEM dominant populations have been identified, on each side of a small range of TEM and Raman partial agreement. To explain the observed shift, we have put forward six hypotheses linked to potential intrinsic biases of both TEM and Raman spectroscopy as characterization techniques, or to potential deficiencies in our methodology.

In order to test these hypotheses and gain more insight into the differences between TEM and Raman spectroscopy for diameter distribution assessment, especially in the Raman dominant diameter range, we have conducted a study on diameter-sorted SWCNTs, with a distribution centered around 1.0 -1.2 nm. This study has enabled us to show that TEM is a technique that is perfectly adapted to the characterization of small-diameter SWCNTs, and that the transfer process from SiO 2 /Si wafers used for SWCNT growth is not responsible for the difference in abundance of SWCNTs within the diameter range of 0.5 -1.3 nm between our growth results as measured by TEM and Raman spectroscopy. The differences in the shapes of the distributions hint at potential distortions due to the (n,m) and optical transition dependency of Raman scattering crosssections, but this needs to be verified with complementary characterization techniques. A comparative study using various relations between the RBM frequency and tube diameter has shown that an error of choice of the relation cannot be responsible for the observed shifts.

The relative accordance on the diameter range between TEM and Raman spectroscopy for the diameter distribution assessment of the diameter-sorted SWCNT sample, and the large discrepancy observed in the case of our wide diameter distribution growth samples, shows the importance of a cross characterization methodology. It appears that the diameter range can be successfully assessed by Raman spectroscopy when the diameter distribution is narrow, but that in the case of a broad distribution, especially including large-diameter SWCNTs, Raman spectroscopy can give an illusion of selectivity. The distortions observed in the shapes of the diameter distribution histograms also calls for extreme caution when determining the (n,m) distribution of a given sample using only Raman spectroscopy with the present methodology. This accordance obtained on the diameter-sorted sample in this chapter serves as a partial validation for our characterization methodology for the diameter range which is the most delicate to assess with TEM. However, the observed differences in shapes also sheds doubt on its validity. Unpublished work conducted in Esko Kauppinen's group at Aalto University gives similar warnings by comparing Raman spectroscopy and electron diffraction for the evaluation of the SC/M ratio of various SWCNT samples.

More experiments would be very insightful additions to this work. First, complementary studies on diameter-sorted SWCNTs with larger diameters could help to verify our third hypothesis regarding the efficiency of Raman spectroscopy for the detection of large-diameter SWCNTs. HRTEM, ED, or PLE studies on the presented diameter-sorted sample would give insights into the potential chirality enrichments of the sample that could explain the seeming over and underestimation of certain populations with Raman spectroscopy that lead to the distortion in the shape of the diameter distribution.

Chapter 5

Exploration of two other families of SWCNT catalyst precursors: Cyanosols, and POMs

In Chapter 3, the growth of SWCNTs using PB/PBA nanoparticles as catalyst precursors was studied. Here, we present preliminary results regarding, first, the attempt to apply the same process using cyanosol nanoparticles instead of PBAs, and second, the use of POM compounds as inspired by recent publications about Co-W-based catalysts for SWCNT growth. In the first section, we will describe the synthesis and characterization of Fe-Pd-based cyanosol nanoparticles, their deposition on a substrate and subsequent reduction, as well as trials of SWCNT growths from this catalyst precursor system. In the second section, SWCNT growth trials using Co-W-based POM clusters will be shown and discussed.

FePd cyanosol nanoparticles as SWCNT growth catalyst precursors

Introduction

As explained in Chapter 3, the three PB/PBA systems that were studied allowed to conduct SWCNT growths using catalysts with variable carbon solubility. We observed that reducing the carbon solubility of the pure iron catalyst by adding nickel led to different results, but no notable improvements of the catalyst performance. In Chapter 2, we presented the cyanosol family as a practical alternative to PBA synthesis for systems where the simple co-precipitation method did not succeed. This is the case, for instance of the FePd system. As Chapter 5. Exploration of two other families of SWCNT catalyst precursors:

Cyanosols, and POMs for the FeNi system, a solid solution is expected to form at typical growth temperatures (800 -1000 • C) according to the Fe-Pd bulk binary phase diagram [START_REF] Franke | Binary systems. part 3: Binary systems from cs-k to mg-zr[END_REF]. We therefore decided to attempt to synthesize and use as catalyst precursors FePd cyanosols nanoparticles. Ideally, a bimetallic nanoalloy FePd catalyst could be formed, with a substantially lower carbon solubility than pure iron, or a NiFe alloy.

Synthesis of Fe

Characterization of the obtained nanoparticles

Size

As for the PB/PBA systems, the as-obtained nanoparticle dispersion was characterized by DLS, in order to measure the hydrodynamic size of the cyanosol nanoparticles, the results are shown in Figure 5.1. Only one peak is observed in the hydrodynamic size distribution by number, and it is centered around 2.8 nm. The synthesis method allows to obtain small nanoparticles, that are individualized in solution. We should note here, that in comparison with the PB/PBA systems presented in Chapter 3, the sizes of the obtained cyanosol nanoparticles are smaller (less than half of the hydrodynamic size of 6.3 nm obtained for the PB nanoparticles for instance).

A few drops of the as-obtained cyanosol nanoparticle dispersion were dropped onto a TEM grid for analysis. A SEM-EDX analysis of this same powder was performed, so as to gain insight into the composition of the cyanosol nanoparticles. The atomic fraction of Fe, Pd, and Cl were measured. Measuring the atomic fraction of Cl enables 5.1. FePd cyanosol nanoparticles as SWCNT growth catalyst precursors 173 the quantification of the substitution of the Cl ligands by CN ligands in the Pd coordination sphere. The obtained Fe:Pd ratio is 1:0.9, meaning a close to stoichiometric composition, with a slight excess of Fe. This is in agreement with the presence of terminal ligands in the structure, as evidenced above by IR spectroscopy. The Pd:Cl ratio is 1:0.1, which implies that most of the Cl ligands have beeen substituted in the structure. Considering the reactant has four Cl ligands per Pd atom, this ratio indicates that 97.5 % of Cl ligands are substituted, which is compatible with the close to 1:1 metallic ratio (see Appendix B.3 for corresponding EDX spectrum).

Conclusion

The method developed in our lab leads to the synthesis of FePd cyanosol nanoparticles, with very small controlled sizes, and close to 1:1 metallic ratio. The nanoparticles are stable in water. In the next section, we present the deposition of these particles on a SiO 2 /Si wafer.

Sample preparation and cyanosol precursor deposition

Wafer preparation

The silanization process used in Chapter 3 in the case of PB/PBA precursors is useful but time consuming. Moreover, it adds steps to the global growth process, which may increase the chances of contamination. Here, the synthesis of the cyanosol precursors is performed at a much higher concentration (twenty times higher than for PB, and five times higher than for both PBA systems). Hence, we first tried to deposit the precursor nanoparticles without using the silane SAM step.

Before the deposition, the bare SiO 2 /Si wafer were simply sonicated subsequently for five minutes in acetone, IPA, and dichloromethane, and were left to dry in air. The particle deposition was performed right after this cleaning process.

Particle deposition

The deposition process was conducted as follows: after the bare wafers were cleaned and dried, they were immersed in the as-obtained cyanosol dispersion in DI water, where they remained for three days. After which they were taken 5.1. FePd cyanosol nanoparticles as SWCNT growth catalyst precursors 175

The sample was analyzed by XPS. The presence of Fe and Pd on the surface of the wafer is confirmed, as well as the preservation of the roughly 1:1 metallic ratio determined using EDX in the previous section. Measurements were performed on three different random spots of the wafer for quantitative analysis, and the obtained mean atomic percentages of Fe and Pd were 49.5 %, and 50.5 %, respectively. Since the uncertainty of the XPS is a few percents, we can affirm that the results are in good agreement with the SEM-EDX analysis of the dried cyanosol nanoparticles. The 1:1 metallic ratio is preserved globally, on the surface of the wafer. The XPS analysis also confirms the absence of contamination that could occur during the various steps of the process. The global survey acquired on several points is shown in Appendix B.5.

Study of the reduction step

As for the PB/PBA systems, we studied the reduction pretreatment of the cyanosol nanoparticles by taking the samples out of the CVD chamber after a 5-minute pretreatment step under H 2 at 800 • C in usual conditions. The wafers were studied by AFM and XPS, and the nanoparticles were transferred onto TEM grids following the process described in Chapter 2 in order to conduct conventional TEM analysis of the nanoparticles, as well as a STEM-EDX study.

Wafer surface analysis

.5 shows typical AFM images of a SiO 2 /Si wafer coated with the FePd cyanosol nanoparticles after undergoing the reductive pretreatment step. The nanoparticle layer is dense, with small sizes (between 1 and 2 nm roughly). We should note that the image shows very little signs of coalescence. Coalescence seems to be kept under control even without the use of an anchoring SAM, which is very encouraging. However, the drastic size reduction observed in the case of the PB/PBA nanoparticles does not appear here. A more extensive statistical study using TEM imaging should be performed to have more insights into this difference.

Here also, XPS analysis was performed to confirm the presence of the two metals, and the Fe:Pd ratio. The quantitative analysis was performed on two random points of the wafer, and the mean Fe and Pd atomic percentage values are 46.8 %, and 53.2 %, respectively. Here, there seems to be a slight excess of Pd on the surface of the wafer, but taking into consideration the uncertainty

Discussion, and perspective

In this section, we have shown the successful synthesis of FePd cyanosol nanoparticles, with a controlled size and composition. These nanoparticles were then deposited in a homogeneous monolayer on a SiO 2 /Si wafer, without needing to functionnalize the wafer prior to deposition, making the process simpler and less time consuming. The global metallic ratio on the surface of the wafer was analyzed using XPS, and the 1:1 metallic ratio was confirmed.

The FePd nanoparticle layer was then reduced in H 2 at 800 • C, and subsequently studied by AFM, XPS, and STEM-EDX after transfer of the nanoparticles onto TEM grids. The aspect of the nanoparticle surface shown by AFM demonstrated the apparent lack of coalescence between the particles, and that the layer remained homogeneous after the pretreatment. The XPS analysis confirmed the 1:1 global metallic ratio on the surface. Unfortunately, STEM-EDX study revealed that a portion of the nanoparticles were pure Pd, while others appeared to be a Pd@Fe core-shells. Since the binary phase diagram predicts a 1:1 solid solution at 800 • C, and the STEM-EDX study is performed after cooling and exposition to ambient air, it is difficult to truly understand the chemical state of the nanoparticles in the CVD chamber. However, core-shell catalysts have proven to be interesting systems for SWCNT growth study [START_REF] He | Environmental transmission electron microscopy investigations of pt-fe 2 o 3 nanoparticles for nucleating carbon nanotubes[END_REF].

An optimization of the process, coupled with more extensive characterization proving the systematic formation of these core-shell nanoparticles could lead to insightful studies.

Finally, SWCNT growth was attempted using the deposited FePd cyanosol nanoparticles. Initial successful SWCNT growth results conducted at 800 • C in usual conditions could not be reproduced. Growth attempts at various temperatures ranging from 800 to 1000 • C, with varying pretreatment times were also performed, with no success. More investigation is necessary to understand these results, since they are surprising. First, our pure FeFe catalyst presented in Chapter 3 was a very effective catalyst, and it is a surprise that a growth in the same conditions using the FePd cyanosol system yields no SWCNTs. Additional tests using FeFe cyanosol nanoparticles could be performed to elucidate whether the precursor type, and its deposition process could be responsible for these results. Moreover, additional optimization of the CVD parameters adapted to this specific catalyst system could also be envisioned as a potential solution. The small size of the cyanosol nanoparticle may also be to blame for the lack of SWCNT growth. Since the potential bimetallic catalyst systems that can be obtained through this method are promising, additional work of the optimization of cyanosol nanoparticle synthesis, with the aim of obtaining particle sizes closer to those of the PB/PBA nanoparticles studied in Chapter 3 are undergoing in our laboratory.

Co-W POM clusters as SWCNT growth catalyst precursors

The work presented in this section consists of preliminary results on the feasibility of SWCNT growth from POM precursors, using our experimental setup. The studies by Yang et al. already discussed in Chapters 1 and 2 of this manuscript feature SWCNT growth by ethanol CVD at atmospheric pressure [START_REF] Yang | Chirality-specific growth of single-walled carbon nanotubes on solid alloy catalysts[END_REF], [START_REF] Yang | Growing zigzag (16, 0) carbon nanotubes with structuredefined catalysts[END_REF].

The chirality specific claims, based on Raman spectroscopy, and the use of a chemically synthesized precursor seemed interesting. We therefore attempted to use similar catalyst precursors to study them with a CH 4 HFCVD. As explained in Chapter 2.1.3.3, the POM precursor used here has an atomically defined W:Co ratio of 34:7 [START_REF] Lisnard | Effect of cyanato, azido, carboxylato, and carbonato ligands on the formation of cobalt (ii) polyoxometalates: Characterization, magnetic, and electrochemical studies of multinuclear cobalt clusters[END_REF], which is different from, but close to the value of the POM precursors used by Yang et al. (36:6). The main difference in composition between the two clusters is that the one used by Yang et al. contains phosphorous (6 atoms per cluster), and the one used here contains silicium (4 atoms per cluster). The structures of the two POMs are quite different, but are similar in sizes.

Deposition

Since the clusters are negatively charged in aqueous solution, we can assume that they can attach by an attractive Coulomb interaction with pyridines at the surface of a silanized wafer that have been protonated. The deposition of the POM clusters was therefore attempted on silanized wafers, though the conditions are not ideal, since only a small portion of the pyridine groups on the surface are protonated.

A 1 mM solution of the POM in DI water was prepared. The silanized wafers were immersed in the solution for 2 hours, before being rinsed with DI water and dried under air at room temperature. AFM and XPS measurements were performed on the surface of the wafer to confirm the deposition of the POMs.

the roughness of the silanized SiO 2 . The height measured here is in good agreements with estimates of the POM cluster sizes (2.5 x 1.5 nm). XPS analysis confirmed the presence of both cobalt and tungsten on several random spots of the wafer, further demonstrating the successful deposition of the clusters on the wafer. Some heterogeneity was noticed on the wafer surface, further optimization of the deposition process could improve this. A quaternization of the pyridine groups could ensure a full coverage of positively charged groups on the surface, leading to a better anchoring of the POMs, for instance.

AFM analysis was also performed on the POM-coated wafer after undergoing a reductive pretreatment under H 2 at 800 • C (see Figure 5.7.b). The images show that a few clusters are present on the surface, and the height profile shows peaks with heights roughly between 0.8 and 1.4 nm. Since the density of the layer is really low, and the observed objects have sizes approaching the substrate roughness, an AFM image of the silanized wafer surface, along with a height profile, are displayed in Figure 5.7.c. It is clear, when comparing the two images, that there is no ambiguity regarding the deposition. Globally, the size of the resulting nanoparticles seem to be slightly smaller than the sizes of the clusters initially deposited on the surface. More investigations by TEM should be done in order to have more information on the nature of those nanoparticles.

SWCNT growth

SWCNT growth was attempted in usual conditions at 800 • C and 1000 • C. Raman spectroscopy mappings were performed, confirming the feasibility of the SWCNT growth from this catalyst precursor using our usual process and CVD setup. For both growths, however, the yield was very low. The calculated Raman "yield" (defined in Chapter 3) values are 0.12 for both growth conditions. In the case of the PB/PBA precursors, we observed an optimal yield temperature at 800 • C, and a decrease in yield with increasing temperature, going to zero at 1000 • C. Here, the Raman-extracted yield does not evolve, and interestingly, a growth is possible at 1000 • C, which was not the case for the PB/PBA precursors.

The Raman characterization was performed, as for all samples in the present work, using four excitation wavelengths. The diameter distribution histograms extracted from the Raman characterization are presented in Figure 5.8. The mean diameters are 1.30 ± 0.19 nm, and 1.80 ± 0.31 nm for growths at 800

Conclusion

In summary, CoW POM clusters were successfully used as catalyst precursors for SWCNT growth at various temperatures using our HFCVD setup. A quick Raman characterization shows an evolution of the diameter distribution with temperature (mean diameter increasing from 1.3 nm to 1.8 nm when increasing growth temperature from 800 • C to 1000 • C), and drastic modification in SC/M ratio. These results are promising in simply demonstrating the feasibility of this specific growth process using a methane CVD reactor, and further studies will certainly lead to more exploration of bimetallic combinations and compositions as catalyst systems, and a better understanding of the phenomena at play in the process.

Discussion and perspectives

In this brief chapter, we have reported studies on two very different chemically synthesized catalyst precursor systems. First, the cyanosol family was explored as a SWCNT growth catalyst precursor with the FePd system. The synthesis of stiochiometric FePd cyanosol nanoparticles with a controlled composition, and small size, was successfully conducted. The precursor nanoparticles were characterized by DLS, IR, and TEM, and EDX. Their deposition on bare SiO 2 /Si wafers was then performed and analyzed with AFM and XPS, evidencing the successful deposition of a dense layer of nanoparticles with sizes ranging roughly from 1 to 2 nm, and an overall Fe:Pd ratio close to 1:1 on the surface. AFM and XPS were also performed on the wafers after the reductive pretreatment. The 1:1 ratio was kept, and the deposit still consisted of a dense monolayer of small nanoparticles. After reduction in the CVD chamber under H 2 , the "effective" catalyst nanoparticles were studied by STEM-EDX, revealing two nanoparticle types: pure Pd nanoparticles, and Pd@Fe core-shell nanoparticles. Unfortunately, the growth of SWCNTs was not reproducible using this precursor in our usual growth conditions at various temperatures, and with varying reduction times. The fact that this catalyst system did not lead to successful SWCNT growth is still surprising, and further optimization of the growth process and CVD conditions could lead to interesting results.

The second catalyst system, derived from Co-W-containing POM clusters, successfully led to the growth of SWCNTs. Here also, further optimization of both the catalyst precursor deposition, and CVD growth are needed to enhance Chapter 5. Exploration of two other families of SWCNT catalyst precursors: Cyanosols, and POMs growth yield. TEM characterization of the catalyst nanoparticles are, of course, a priority in future works, as it is crucial to understand the growth mechanism from this catalyst. Nonetheless, the few results shown here, demonstrating the feasibility of SWCNT growth from these precursors, show a promise in exploiting the richness of the extremely wide and versatile POM family.

The experiments presented in this chapter constitute preliminary results. This work will be continued by a PhD student in the coming years. We have already shown the relevance of using chemically synthesized well-defined coordination networks for SWCNT growth and growth mechanism understanding in Chapter 3. With this chapter, we have shown that this relevance goes beyond the PB/PBA family. Our approach, if optimized for each specific compound family, could lead to very promising research works.

Conclusion and perspectives

The first objective of this work was to explore SWCNT growth selectivity through catalyst design, by developing a new process to prepare bimetallic catalysts with tuned carbon solubility. In this manuscript, we have proposed a new synthesis route to grow SWCNTs by HFCVD using PB/PBA nanoparticles as catalyst precursors. The precursor nanoparticles were synthesized as stable water dispersions, and extensively characterized, confirming their crystallographic structure, composition, and adapted and well controlled sizes. They were successfully used as catalyst precursors for SWCNT growth. Their controlled deposition by coupling surface and coordination chemistry, and controlled reduction under H 2 atmosphere led to the formation of dense layers of catalyst nanoparticles, without signs of coalescence.

The STEM-EDX study of the bimetallic systems led to very interesting results. In the case of the NiCr system, for which the bulk binary phase diagram does not forecast the occurrence of a 1:1 solid solution in our growth conditions, we observed a very clear phase segregation in the nanoparticles, and assumed the SWCNT growth stems from a pure Ni effective catalyst. In the case of the NiFe system, for which the bulk binary phase diagram predicts the formation of a 1:1 solid solution in our growth conditions, the catalyst nanoparticles -after cooling and exposure to air -were observed to be either alloy-like, or Janus type nanoparticles. Unlike the phase segregation observed in the NiCr catalyst, the Janus nanoparticles observed for the NiFe system displayed two seeming Ni-Fe solid solutions: a Ni-rich phase on one side, and a Fe-rich on the other.

We were therefore able to make the simple assumption that SWCNT growth in this specific case stemmed from a NiFe nanoalloy of unknown composition. Thus, our process allowed us to study three distinct catalyst systems: a pure Fe catalyst, a bimetallic phase-segregated catalyst, and an "alloyed" catalyst.

Using the FeFe monometallic reference system, we observed an evolution of the diameter distribution towards larger diameters with increasing growth temperature. Based on a tangential growth mode, we considered two possible explanations for this evolution: a typical Oswald ripening (and coalescence) argument, or a reasoning based on carbon solubility evolution with temperature. In the first scenario, Oswald ripening shifts the global nanoparticle size distribution towards larger diameters, leading to the growth of similarly larger-diameter SWCNTs. In the second scenario, the nanoparticle size distribution is fixed, and temperature has an influence on carbon solubility in the nanoparticle, and therefore the carbon content within the nanoparticle. This leads to the activation, and deactivation (either by poisoning or detachment) of different nanoparticle populations based on their sizes, at different temperatures. The very little evolution of the size distribution of the catalyst nanoparticles before and after CVD growth suggests that, aside from a minor coalescence effect, Oswald ripening does not seem to have a significant impact on the catalyst size distribution, indicating that carbon solubility may be the most probable parameter driving SWCNT growth in this case. Further AFM and TEM characterization in order to elucidate the evolution of catalyst nanoparticle size distributions with temperature are needed to confirm this.

The effect of catalyst type and composition was also studied. The reproducibility issues encountered with both bimetallic catalyst systems regarding Raman spectroscopy characterization made a discussion on the evolution of the Ramanextracted diameter distributions, or SC/M ratios difficult. But we noticed differences in growth temperature limits based on the catalyst. Using the carbon solubility argument, we were able to propose an explanation for this. Again, further characterization of catalyst nanoparticle sizes at different temperatures are needed to conclude.

The comparison between direct SWCNT diameter measurement by TEM imaging, and our Raman spectroscopy methodology for diameter distribution assessment on our growth samples sparked a few questions on the reliability of both of these techniques for selectivity assessment. The methodology that we applied for Raman spectroscopy characterization is a quite widespread methodology among research groups dealing with selective SWCNT growth, but it is not the methodology proposed by Raman specialists for selectivity assessment. The comparison between TEM-and Raman-extracted diameter distributions systematically showed shifts of about 4 Å towards larger diameter when using TEM on our growth samples. This TEM characterization revealed a quite broad diameter distribution, with a significant portion of SWCNTs with large diameters. We distinguished three diameter ranges based on the relative agreement between TEM and Raman characterizations. By doing so, we established that the broadness of the actual diameter distribution of the sample, and the apparent diameter-dependent adaptability of each technique (Raman spectroscopy seeming more adapted to small diameter SWCNT detection, and the opposite was observed for TEM) were responsible for the observed shift.

For a better understanding, a similar comparison was performed using diametersorted SWCNTs with diameters ranging from 0.8 nm to 1.5 nm. The crosscharacterization by TEM and Raman spectroscopy showed a good agreement between the two techniques regarding the diameter range of the distribution. However, distortions in the shape of the distribution were observed in the case of Raman spectroscopy (bimodal), the TEM-extracted distribution having a classical Gaussian shape. By confronting the Raman results to an adapted Kataura plot, we evidenced the absence of observed resonance from certain SWCNT populations within the resonance windows for each excitation wavelength. We assume these discrepancies to be the result of chirality-dependent Raman crosssections, but not all results can be explained by this. More insights could be gained by performing a HRTEM characterization of the diameter-sorted sample, leading to a detailed chirality distribution of the sample. Moreover, additional experiments on diameter-sorted samples with larger diameters could be very insightful, and make this study more complete.

In the final chapter, we explored two other chemically prepared catalyst precursor families for SWCNT growth. We first showed the synthesis and characterization of FePd cyanosol nanoparticles. Their deposition and subsequent reduction under H 2 led to the formation of a dense layer of nanoparticles with a controlled diameter, showing no signs of coalescence. Through a STEM-EDX characterization of the nanoparticles after cooling and exposure to air, we evidenced two populations of nanoparticles: pure Pd, and Pd@Fe core-shell nanoparticles. Unfortunately, the first results of SWCNT growth using this precursor were not reproduced. Though the initial goal to form alloy nanoparticles did not seem to be reached, preparing core-shell nanoparticles could be of great interest. Further optimization is needed in this case. Moreover, additional CVD tests should be conducted, and this method could also be applied to a variety of other bimetallic systems (FePt, NiPt, CoMo..).

The last precursor system that was studied are W 34 Co 7 O x POM clusters. They were successfully deposited on silanized wafers, and resulted in SWCNT growth. Though the yield was low, interesting temperature effects were observed using Raman spectroscopy. The versatility of the POM family is such that these preliminary results are very promising for application to a wide range of other cluster structures. More optimization of the deposition and CVD parameters could also lead to very promising results.

As a general conclusion, though not all goals were attained, the process developed applied to three PB/PBA systems, and its adaptation applied to a cyanosol system, and a POM system, allowed the preparation of five different catalysts of interest. Their comparison in identical experimental conditions led to potential understanding of phenomena at play during SWCNT growth. This method can be applied to many other systems, as attested by studies conducted on three additional Ru-based PBA systems. Our process opens the way to future investigations of the effect of catalyst composition on growth selectivity. In a more general sense, the promise offered by coordination chemistry, and inorganic synthesis for fabrication of SWCNT growth catalyst systems has been demonstrated through this work.

Finally, the methodology that is becoming a standard for Raman spectroscopy evaluation of growth selectivity has to be used with great caution, as attested by our comparative study. Cross-characterization seems indispensable when looking at the presented results, especially considering that this methodology is used for chirality distribution assessment. This study shows the necessity for standardized selectivity evaluation methodologies that are cost-effective, practical and not too time-consuming, but above all, reliable, accurate, and reproducible. Appendix D. Résumé de la thèse des particules de catalyseurs, dépendant de leur taille, leur composition, et de la température, est proposée pour expliquer ces deux phénomènes.

Dans un second temps, une étude comparée de caractérisation par microscopie électronique en transmission (MET) et spectroscopie Raman multi-longueur d'onde, a été menée dans le but d'évaluer les techniques de caractérisation disponibles dans leurs capacités respectives à permettre une mesure fiable de la distribution de diamètres d'un échantillon de croissance de SWCNTs. Suite à l'observation de différences significatives systématiques entre les résultats obtenus par les deux techniques de caractérisation, plusieurs hypothèses ont été formulées pour tenter d'expliquer ces différences. Une étude complémentaire a été menée sur des SWCNTs triés en diamètres, permettant de tester ces hypothèses. Nous avons pu montrer l'importance du caractère résonant de la caractérisation par spectroscopie Raman dans les différences observés. Cette étude souligne l'importance et la nécessité du développement de méthodes de caractérisation standardisées pour l'évaluation de la sélectivité des croissances de SWCNTs.

Enfin, des études préliminaires sur l'utilisation deux autres familles de composés synthétisés par voie chimique comme précurseurs de catalyseurs de croissance de SWCNTs sont présentées dans ce manuscrit. Un système bimétallique FePd de « cyanosol », famille de composés structurellement proche des ABP, a d'abord été étudié. L'adaptabilité de notre procédé à une nouvelle famille de composés a été confirmée. Le second précurseur étudié est un polyoxometallate (POM) avec un ratio atomique Co-W bien défini. L'étude préliminaire par spectroscopie Raman de croissances de SWCNTs à différentes température utilisant ce précurseur a pu confirmer l'intérêt du système bimétallique Co-W d'un part, et de l'utilisation de POMs d'autre part. Cette étude préliminaire confirme l'intérêt de l'utilisation de précurseurs de catalyseurs synthétisés par voie chimique pour la croissance de SWCNTs, et pour l'étude des mécanismes complexes intervenant dans cette dernière.

Figure 2 . 6 -

 26 Figure 2.6 -Schematic polyhedral structure of the POM used in this study, from [180]. The orange centers correspond to Co atoms, which are surrounded by six oxygen atoms (red) to form CoO 6 octahedra, the blue octahedra are WO 6 , the green tetrahedra are SiO 4 .

Figure 2 . 7 -

 27 Figure 2.7 -Schematic representation of the HFCVD reactor used the growth of SWCNTs.

Figure 2 . 8 -

 28 Figure 2.8 -Schematic depiction of electron-matter interaction and the resulting signals. All straight arrows refer to electrons, the elastically scattered electrons are in dark blue, and inelastically scattered electrons and engendered signals (photons, secondary electrons, Auger electrons) are in purple.

Figure 2 . 9 -

 29 Figure 2.9 -Schematic representation of the basic principle of a TEM.

Figure 2 . 14 -

 214 Figure 2.14 -Schematic representation of the Zeiss Libra 200 MC microscope.

  et al. established an empirical Kataura plot based on electron diffraction and Rayleigh scattering measurements of optical transition

Figure 2 . 17 -

 217 Figure2.17 -Kataura plot from[START_REF] Sato | Discontinuity in the family pattern of single-wall carbon nanotubes[END_REF], based on ETB model calculations. The black circles correspond to metallic SWCNTs, and the blue circles are semiconducting SWCNTs.

3. 2

 2 Synthesis and characterization of catalyst precursors: PB and PBA nanoparticles 3.2.1 Synthesis of three different PB/PBA nanoparticle systems: FeFe, NiFe, and NiCr

Figure 3 . 4 -

 34 Figure 3.4 -XRPD patterns obtained for the three PB/PBA systems, after precipitation of the nanoparticles by CaCl 2 and centrifugation. The peaks marked with a * are attributed to the aluminum sample holder.

Figure 3 . 8 -

 38 Figure 3.8 -Size distributions of the FeFe PB catalyst precursor nanoparticles (light blue), and of the FeFe effective catalyst nanoparticles (dark blue), evidencing the size decrease of the nanoparticles upon reduction.

Figure 3 . 12 -

 312 Figure 3.12 -Comparison between the NiFe effective catalyst nanoparticle size distribution extracted from TEM imaging (orange), and the size distribution of the NiFe nanoparticles studied by STEM-EDX (yellow).

Figure 3 . 21 -

 321 Figure 3.21 -Raman-extracted yields for all catalyst systems at various temperatures. When Raman data was available for several samples, the error bars correspond to the standard deviation of the "yield".

4. 1 .

 1 Comparing TEM and Raman spectroscopy on typical CVD growth samples 135

Figure 4 .

 4 Figure 4.3 shows the superimposed Raman and TEM diameter distribution histograms for the two samples. For the FeFe (NiFe) sample, the TEM distribution was determined from the measurement of 112 (110) SWCNT diameters, and the Raman distribution was determined by counting 512 (194) RBM peaks.

Figure 4 .

 4 6.b). In summary, 4.1. Comparing TEM and Raman spectroscopy on typical CVD growth samples 147

Figure 4 . 8 -

 48 Figure 4.8 -Schematic representation of the three samples prepared using the diameter-sorted SWCNT solution. The samples are numbered and give the overall progression of this section: we will first have a direct look at the composition of the SWCNT sample by direct TEM characterization (1), then the potential effect of the transfer process will be evaluated by characterization of sample (2), and the TEM characterization will be compared to Raman by looking at sample (3).

Figure 4 . 10 -

 410 Figure 4.10 -Diameter distribution histogram obtained from measurements from 121 suspended and bare SWCNTs.

Figure 4 . 11 -

 411 Figure 4.11 -Typical SEM image of diameter-sorted SWCNTs deposited on a SiO 2 /Si wafer.

Figure 4 . 13 -

 413 Figure 4.13 -Overlap of the diameter distribution histograms for the diametersorted SWCNT sample by direct TEM characterization (black), and after the transfer process (green).

Figure 4 . 15 -

 415 Figure 4.15 -Superimposition of the diameter distribution histograms obtained by direct TEM characterization (black), and Raman spectroscopy characterization (blue) of the diameter-sorted SWCNT sample.

Figure 5 .

 5 2 shows a typical TEM image of the Chapter 5. Exploration of two other families of SWCNT catalyst precursors: Cyanosols, and POMs5.1.3.2 Structural and chemical analysisIn order to confirm the formation of the CN-linked polymer, IR spectroscopy was performed on the FePd cyanosol nanoparticles (see Figure5.3 for spectrum in the vincinity of the CN stretching mode). The particles were retrieved using dextran. We can distinguish three different signals: a first band at 2175 cm -1 , and a second at 2118 cm -1 , with a shoulder at 2079 cm -1 . The first band corresponds to the bridging CN ligands (Fe III -CN -Pd II )[START_REF] Pfennig | Synthesis of a novel hydrogel based on a coordinate covalent polymer network[END_REF]. The second signal can be attributed to Fe II -CN -Pd II bridging CN ligands, and the shoulder to terminal Fe III . This indicates a partial reduction of the Fe III to Fe II within the structure. The two bands attributed to bridging CN stretching confirm the formation of the coordination polymer. The frequency of the stretching mode of the terminal CN ligands is in good agreement with the value obtained for the K 4 Fe II (CN) 6 , pure compound, and the band corresponding to the bridging ligands is blue-shifted in comparison to the terminal ligand band. We should note that the relative intensities of the bridging and terminal CN ligands show the abundance of terminal ligands in the structure, in agreement with a high surface to volume aspect ratio.

Figure 5 . 3 -

 53 Figure 5.3 -IR spectrum of the precipitated and dried FePd cyanosol nanoparticles retrieved with 25 equivalents of dextran.

Figure B. 10 -

 10 Figure B.10 -XPS survey of the FePd cyanosol nanoparticle layer after deposition on a bare wafer. The presence of Fe, Pd, and K is confirmed, as well as nitrogen, stemming from the CN bridges.

Figure B. 11 -

 11 Figure B.11 -XPS survey of the Co-W POM layer after deposition on a silanized wafer. The presence of Co, and W is confirmed.

Figure B. 13 -

 13 Figure B.13 -EDX spectra extracted from three regions of the map (image on the left, 5 nm scale bar) for the segregated NiCr nanoparticles. The topmost spectrum is the mean for the entire circled nanoparticle, the presence of Cr (5.3-3.5 keV) and Ni (7.4-7.6 keV) is confirmed. The second spectrum corresponds to a spot within the Cr-rich region of the nanoparticle: no Ni is detected. The third spectrum corresponds to a spot within the Ni-rich phase of the nanoparticle: no Cr is detected. Further, no signal stemming from Cs is observed, evidencing its absence from the nanoparticle. The signal stemming from copper comes from the Cu-TEM grid.

Figure B. 14 -

 14 Figure B.14 -EDX spectra extracted from the two Fe-Pd nanoparticles shown in Chapter 5 (image on the left, 3 nm scale bar). Top: core-shell system. The first spectrum is the global spectrum of the entire map, and the second corresponds to the restricted region around the nanoparticles. Fe and Pd are both present within the nanoparticle. Bottom: "pure" Pd nanoparticle. The top spectrum corresponds to the global map, and the bottom one corresponds to the region around the nanoparticle circled in green. A small signal is observed in the Fe-Kα region, but it is very low, and could be attributed to residual noise. In both nanoparticles, the presence of Ti stems from the TEM grid.

  iii 2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Growth of SWCNTs using Prussian Blue and its analogs as catalyst precursors 3.1 From PB/PBA to SWCNT: general process . . . . . . . . . . . 3.2 Synthesis and characterization of catalyst precursors: PB and PBA nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Synthesis of three different PB/PBA nanoparticle systems: FeFe, NiFe, and NiCr . . . . . . . . . . . . . . . . 3.2.2 Characterization of the obtained nanoparticles . . . . . . 3.2.2.1 Size . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2.2 Crystallographic structure . . . . . . . . . . . . 3.2.2.3 Chemical analysis . . . . . . . . . . . . . . . . . 3.2.2.4 Conclusion . . . . . . . . . . . . . . . . . . . . 3.3 Sample preparation and PB/PBA precursor deposition . . . . . .4.3 STEM-EDX study of bimetallic catalyst nanoparticles . . 3.4.3.1 STEM-EDX study of the NiFe catalyst . . . . . 3.4.3.2 STEM-EDX study of the NiCr catalyst . . . . . 3.4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 SWCNT growth . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 SWCNT growth from the monometallic PB catalyst precursor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1.1 SWCNT growth feasibility . . . . . . . . . . . . 3.5.1.2 Raman study of growth temperature effect . . . 3.5.2 SWCNT growth from the bimetallic PB catalyst precursors122

3.3.1 Wafer preparation . . . . . . . . . . . . . . . . . . . . . . 3.3.2 PB/PBA nanoparticle deposition . . . . . . . . . . . . . 3.3.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Catalyst nanoparticle formation and characterization . . . . . . 3.4.1 Wafer surface analysis after reductive pretreatment . . . 3.4.2 TEM characterization of the catalyst nanoparticles . . . 3.4.2.1 Monometallic FeFe catalyst . . . . . . . . . . . 3.4.2.2 Bimetallic catalysts . . . . . . . . . . . . . . . . 3.4.2.3 Size evolution of catalyst nanoparicles after SWCNT growth . . . . . . . . . . . . . . . . . . . . . . . 33.5.2.1 SWCNT growth feasibility . . . . . . . . . . . . 3.5.2.2 Raman and SEM study of growth temperature effect . . . . . . . . . . . . . . . . . . . . . . .

  1.3. Theoretical and experimental insights into SWCNT nucleation and growth 41 catalyst, and the parameters impacting the phenomenon. More realistic calculations, taking into account carbon presence and temperature for instance, have lead to interesting discoveries on nucleation mechanisms, and the role of the catalyst nanoparticle, leading to other, more general strategies for successful and selective SWCNT growth. Amara et al. developed a TB model for Ni-C interactions coupled with grand canonical Monte Carlo (GCMC) simulations to study the nucleation of carbon caps onto Ni nanoparticles

1.3.3 Towards a more general in-between?

  2.1.1.1 Prussian blue: structure and properties PB has been known since 1704 and used as a pigment because of its strong characteristic blue color. It is a 3D coordination polymer of general formula K x Fe III [Fe II (CN) 6 ] (3+x)/4 .15H 2 O, and is considered to be the first synthetic coordination compound. It is the resulting compound of the coprecipitation reaction between potassium hexacyanoferrate(II) (K 4 [Fe II (CN) 6 ]

  POMs are polynuclear molecular clusters made from the assembly of [MO y ] n- polyhedra where M is a metal, with an almost unlimited range of structures. In most cases, M is a group 5 or 6 transition metal in a high oxydation state (Mo VI/V , W VI/V , or V VI/V ). They are synthesized by the condensation of [MO 4 ] n-monomers in an acidic medium. They can be divided into two main compound families: the isopolyoxometalates ([M

2.1.3.1 Definition, synthesis and architecture x O y ] n-), that are formed by the condensation of same-metallic element polyhedra, and heteropolyoxometalates (HPA) ([X z M x O y ] n-), where the condensation occurs around a heteroatom

Table 2 .

 2 1 -Main characteristics of the two TEMs used for SWCNT characterization in the presented work, adapted from[START_REF] Ghedjatti | Etude structurale des nanotubes de carbone double parois[END_REF].

	Microscope	Zeiss Libra 200 MC JEOL ARM 200F FEI TITAN 3 G2
	Electron source	Schottky FEG	Cold FEG	X-FEG
		energy-filtered
	Accelerating	60, 80-200	80-200	60-300
	voltage (keV)
	Corrector	Monochromator	C s -corrector objective	C s -corrector image
	Energy spread	0.2	0.4
	(eV)
	Defocus spread	7.7	5.5
	(nm)
	Scherzer defocus	-64	-3
	(nm)

Table 2 .

 2 2 -A non-exhaustive list of the A and B constant values in the w RBM = A/d t + B law available in the literature, adapted from

  • C in ice baths. The mixture immediately turns blue, which is a sign of the formation of the PB network, and it is left under high stirring for 30 minutes.

	The conditions are slightly different for preparing the NiFe and NiCr PBA
	nanoparticles: the concentration of each solution is multiplied by 4 (2 mM)
	in comparison to the concentration for the synthesis of PB, and the reaction
	is performed at room temperature. Moreover, cesium chloride (CsCl) salt is
	added in stoichiometric proportions to the solution containing the potassium
	hexacyanometallate [M (CN )

6 ] x+ .

Table 3 .

 3 1 -Lattice constants and Scherrer domain sizes extracted from the XRPD patterns for all three PB/PBA systems.

	System	FeFe NiFe NiCr
	Lattice constant 10.19 10.31 10.51
	( Å)
	Scherrer domain	7.8	5.9	5.7
	size (nm)

Table 3 .

 3

	System	FeFe	NiFe	NiCr
	Bridging CN (cm -1 ) 2072 2164; 2098 2171
	Free CN (cm -1 )		2060	2130

2 -Frequencies of the CN vibration characteristic band (bridging and non-bridging) for all PB/PBA systems.

Table 3 .

 3 4 -Atomic percentages of bimetallic PBA nanoparticle layer deposited on SiO 2 /Si, extracted from XPS analysis, and atomic fractions elements of interest in the M, M', Cs mixture.

		NiFe PBA			NiCr PBA
	Element Global % At. fraction Element Global % At. fraction
	Si 2p	22.9	-	Si 2p	24.6	-
	O 1s	33.9	-	O 1s	37.1	-
	N 1s	5.6	-	N 1s	4.2	-
	C 1s	35.5	-	C 1s	30.9	-
	Ni 2p	1.2	0.52	Ni 2p	1.2	0.38
	Fe 2p	0.8	0.35	Fe 2p	-	-
	Cr 2p	-	-	Cr 2p	1.3	0.41
	Cs 3d	0.3	0.13	Cs 3d	0.7	0.23

Table 3 .

 3 6 -Diameter distributions and SC percentages extracted from Raman spectroscopy characterization of SWCNTs grown from the FeFe catalyst at various temperatures.

	Temperature Mean diameter Standard deviation %SC
	( • C)	(nm)	(nm)
	700	1.11	0.28	65
	800	1.30	0.30	61
	900	1.82	0.40	73

Table 3 .

 3 8 -Report of obtained SWCNT growths for the three catalytic systems at various temperatures. Checkmarks indicate that SWCNT growth is successful, XX indicates a zero Raman "yield".

Table 4 .

 4 1 -Compared mean diameters and standard deviations of the diameter distributions obtained by TEM and Raman spectroscopy on two typical growth samples.

	Sample	Mean TEM d t (nm)	Mean TEM d t < 2.5nm (nm)	Mean Raman d t (nm)
	FeFe 1.73 ± 0.48 FeNi 1.74 ± 0.35	1.67 ± 0.36 1.73 ± 0.35	1.31 ± 0.30 1.35 ± 0.29
	Table

Table 4 .

 4 2 -Comparison of the percentages of the total detected SWCNTs present in each of the delimited 1, 2, and 3 diameter domains defined as d(1) < 1.3 nm, 1.3 ≤ d(2)< 1.7 nm, and 1.7 ≤ d(3) < 2.5 nm, for the two presented samples for TEM and Raman characterizations.

		FeFe		NiFe
	Domain TEM % Raman % TEM % Raman %
	1	19.6	58.0	10.9	51.5
	2	32.1	29.7	37.3	36.6
	3	48.2	12.3	51.8	11.9
	not expected, easily understandable. The next section goes into more detail on
	what explanations can be brought to understand this observed shift.

  3 nm) Chapter 4. Comparing TEM and resonant Raman spectroscopy for diameter distribution assessment of a SWCNT sample

	2. The transfer process modifies the sample by degrading small-diameter
	SWCNTs (below 1.3 nm)
	3. Raman spectroscopy is not adapted to the characterization of large-diameter
	SWCNTs (typically above 2.5 nm)
	4. Raman spectroscopy over/under-estimates the presence of certain SWCNT
	populations
	5. ω RBM → d t relationship is not adapted to the sample
	6. Our methodology is not adapted to our growth samples

Table 4 .

 4 3 -Mean diameters and standard deviations of the distributions obtained for the diameter-sorted SWCNT sample by Raman spectroscopy, direct TEM measurement, and TEM measurement after transfer. In the case of TEM measurements, the values are given without consideration of the SWCNTs with diameters above 2.5 nm.Further, the general shape of the histograms differ quite significantly from one Chapter 4. Comparing TEM and resonant Raman spectroscopy for diameter distribution assessment of a SWCNT sample the diameters of the SWCNTs detected by Raman spectroscopy, the bimodal shape of the distribution is preserved. The shape of the distribution being the most important difference between the TEM-and Raman-extracted distributions for the diameter-sorted SWCNT sample, this is an important point. It leads us to consider with more certainty that the distortion in the shape of the distribution may be due to the resonant character of the Raman method, and to multifactor-dependent Raman intensities.

	Characterization	Mean d t	Standard deviation
	technique	(nm)	(nm)
	Raman spectroscopy	1.00	0.23
	Direct TEM	1.15	0.25
	Transferred TEM	1.17	0.33

  plotted the separate excitation wavelength-specific diameter distribution Chapter 4. Comparing TEM and resonant Raman spectroscopy for diameter distribution assessment of a SWCNT sample (Kataura plot (2)) is adapted from the Raman spectroscopy studies on "supergrowth" SWCNTs by Araujo et al.[START_REF] Araujo | Resonance raman spectroscopy of the radial breathing modes in carbon nanotubes[END_REF]. Using the methodology developed by Zhang et al. for chirality assignment of SWCNTs based on multi-laser Raman spectroscopy characterization of as-grown SWCNTs on SiO 2 substrates, all E ii were redshifted by 80 meV for semiconducting SWCNTs, and 60 meV for metallic SWCNTs to take into account the environmental effects on the E ii that are considered to be virtually non-existent for as-grown supergrowth SWCNTs[START_REF] Zhang | n, m) assignments and quantification for single-walled carbon nanotubes on sio 2/si substrates by resonant raman spectroscopy[END_REF]. These values are average values derived from comparing the E ii values obtained in a study by Hsieh et al. on specific (n,m) SWCNTs on a SiO 2 substrate[START_REF] Hsieh | Chiral angle dependence of resonance window widths in (2 n+ m) families of single-walled carbon nanotubes[END_REF] to the corresponding values for supergrowth SWCNTs obtained using the equation from[START_REF] Araujo | Resonance raman spectroscopy of the radial breathing modes in carbon nanotubes[END_REF].If we consider the global diameter range of the sample to be roughly delimited by the TEM-extracted diameter distribution, we should detect SWCNTs between 0.8 nm and 1.5 nm. Looking at the four portions of the Kataura plot displayed in Figure 4.18, there are SWCNTs with E ii within a ± 100 meV window around the excitation energy (see Chapter 2.3.2.3) for each excitation energy within the global diameter range of the sample. The equivalent of Figure 4.18 for Kataura plot (1) is shown in Appendix C. Since the Kataura plot (2)

  III Pd II cyanosol nanoparticles A synthesis route to cyanosol nanoparticles was developed in our lab during the internship of Adrien Barroux in 2016. The objective of this work was to obtain nanoparticles with controlled size and composition, with a method close to the co-precipitation method developed by Catala et al. for the PB/PBA systems. In the case of the Fe-Pd bimetallic system, the Fe III Pd II cyanosol nanoparticles can successfully be synthesized; the nanoparticles are stable in water for a few days up to a few weeks without the introduction of any stabilizing agents.For this synthesis, a 10 mM solution of K 2 Pd II Cl 4 in DI water is poured into a 10 mM solution of K 3 Fe III (CN) 6 under high stirring. The solution turns a light orange, and it is left under stirring for 30 minutes at room temperature. The as-obtained nanoparticle dispersion is directly used for characterization.

Acknowledgements

The two characterization techniques used for SWCNT sample selectivity assessment were then presented in detail. We established a clear methodology for the evaluation of the diameter distribution of SWCNT growth samples using both Raman spectroscopy, and TEM.

Appendix A

Characterization techniques, and experimental details

The various characterization techniques used in the presented work will be shortly introduced here (in order of appearance in the text), along with experimental details concerning each of them.

A.1 Dynamic light scattering (DLS)

DLS is a characterization technique that allows the measurement of the hydrodynamic size of objects dispersed in solution. The hydrodynamic size includes intrinsic size of the object and its solvation shell. The measured sizes are therefore larger than the actual size of the objects in solution. This technique, which also gives an idea of the state of aggregation of the objects in solution, is simple and fast, and can be used routinely directly using the as-obtained nanoparticle dispersion. DLS measurements were performed on a Malvern Zetasizer.

A.2 TEM: additional experimental details

TEM imaging for the PB/PBA nanoparticles was performed using an image corrected FEI TITAN TEM operating at 300 kV.

A.3 X-ray powder diffraction (XRPD)

When X-rays interact with a crystal, they are scattered. The scattered rays interfere constructively in specific directions corresponding to hkl planes, given by Bragg's law:

Where d is the spacing between two diffracting planes from the same family, θ is the incident angle, λ is the wavelength of the monochromatic X-ray beam, and n is an integer. In XRPD, a diffraction pattern giving the diffracted intensity as a function of 2θ. Each diffraction peak corresponds to a hkl plane family, and can be indexed, to determine the crystallographic structure of the material. XRPD analyses were performed on powder deposited on an aluminum plate and recorded on a Phillips Panalytical X'Pert Pro MPD powder diffractor at CuKα (1.5404 Å) radiation equipped with a fast detector.

A.4 Infrared spectroscopy

IR spectroscopy relies on the fact that chemical compounds absorb IR waves at resonance frequencies characteristic of their structure. It was used in this work to confirm the formation of PB/PBA networks, as well as cyanosol polymers, by looking at the shifts in the frequencies of CN ligand stretching bands. To perform IR spectroscopy, 1 mg of PBA powder is mixed with 99 mg of potassium bromide (KBr), and formed into a pellet. KBr is used because it is transparent in the IR range. The PBA nanoparticles were characterized by transmission IR spectroscopy, performed on a Perkin Elmer Spectrum 100 spectrometer and by reflexion IR performed on a Brüker VERTEX 70.

A.5 Scanning electron microscopy (SEM), and SEM-EDX

A SEM is a microscope that allows the observation of samples using a focused electron beam that scans their surface. The accelerating voltage is much lower than in the case of a TEM (a few kV, up to about 20 kV), since the electrons do not have to be transmitted through the sample, leading to lower potential resolutions. Various signals are produced through the interaction between the electron beam and the sample. In the most common configuration, SEM is performed by collecting secondary electrons, which will give an image of the topography of the sample. This technique was used for the observation of SWC-NTs grown on SiO 2 /Si substrates. SEM observations of SWCNTs were carried out on HITACHI S 4800 microscope at 1 kV (or 0.5 kV) and 10 µA.

The general principle of EDX was presented in Chapter 2 of this manuscript, in the case of its use in a TEM. X-ray photons resulting of the interaction of A.6. Atomic force microscopy (AFM)

the electron beam with the sample can also be collected in a SEM, allowing chemical analysis of the sample. This technique was used for determining the Cs:M:M' ratio in PBA compounds.

A.6 Atomic force microscopy (AFM)

AFM allows the obtention of high resolution images of the topography of samples. AFM was used in the present work to observe the evolution of the topography of SiO 2 /Si substrates after deposition of catalyst precursor nanoparticles or clusters. The basic principle of an AFM is to scan the sample with a tip that is fixed at the end of a cantilever, attached to a piezoelectric scanner that can precisely adjust its height. The cantilever is imposed a frequency close to its resonance frequency (control frequency), and a line by line scan over the sample surface is performed. The attractive and repulsive interactions between the tip and the surface modify the vibration amplitude of the tip. This modification is measured using a laser beam reflected on the cantilever, whose direction is tracked by four photodiodes. The piezeoelectric scanner adjusts the height of the tip with respect to the detected interactions with the surface, so that it returns to its original amplitude. The height is recorded, enabling the formation of topographic images.

AFM measurements (tapping mode topography) were performed using di Innova AFM Bruker with NanoDrive v8.02 software. Images were acquired using silicon tips from nanosensors (PPP-NCSTR) with a resonance frequency ranging between 76 and 263 kHz. Images were processed using WsXM software, freely available on internet.

A.7 X-ray photoelectron spectroscopy (XPS)

XPS is a highly sensitive surface characterization technique, it was used to confirm the presence of certain elements on the surface of SiO 2 /Si substrates after deposition of catalyst precursors and their reduction. Quantitative analysis was also performed after deposition to confirm the metallic ratios obtained by SEM-EDX on powders. XPS is based on the photoelectric effect: when incident X photons hit the surface of the sample, inner shell electrons from the atoms at the surface can be ejected. The sample is irradiated with a monochromatic Xray beam of known energy, the ejected electrons are collected, and their number and kinetic energy are measured. The binding energy of the electrons can be extracted from the measurement of their kinetic energy, by a simple energy conservation equation. An XPS spectrum gives the number of detected electrons as a function of binding energy. The binding energies being characteristic of a specific element, chemical analysis can be performed. This analysis is made quantitative by taking into account the photoionization cross-sections of the different electronic levels.

B.5 XPS surveys of catalyst precursors on SiO