M A Vial Professeur

Utt -Hdr Président

CHAIR PROFESSOR M Y Cho

Rapporteur M V Consonni

Charge De Recherche

Rapporteur Mme

Y Leprince -Wang

Professeure Des

Universites Examinatrice

M A Lusson

Charge De

Examinateur Recherche Cnrs

M A Schwartzberg Staff

Scientist Examinateur

M G Lérondel Professeur

Des Universites

M P Miska

Maitre De Conference -Hdr

M K Nomenyo Docteur

Ingenieur De Recherche

Prof Yoon-Hoon Cho

Dr Vincent Consonni

Prof Leprince Yamin

Dr Alain Wang

Dr Adam Lusson

Prof Schwartzberg

Alexandre Vial

Rémi Vincent

Rafael Salas-Montriel

Anne-Laure Baudrion-Béal

Aurélien Serguei Bruyant

Jérémie Kostcheev

Christoph Kastl

Christopher Chen

Shaul Aloni

Lorenzo Maserati

Tev Kuykendall

Tracy Mattox

Alex Weber-Bargioni

Miquel Salmeron

Takuya Uehara

Dongsheng Dong Li

Feng, Yubing, Qingxian Tzu Shijie

Heng

JURY

Optical materials, including metals and semiconductors, are nowadays widely fabricated into thin films, which are essential for optoelectronic devices such as sensors, lighting devices and solar cells.

Thanks to tremendous progress in nanotechnologies, the structure of the thin films can be more and more engineered at different scales to enhance the optical performance of the materials, which is attributed to the enhancement of the interaction between the light waves and these nanostructured thin films.

This thesis intended to be devoted to two main luminescent semiconductors, namely, zinc oxide (ZnO) and transition metal dichalcogenides monolayers (TMDMs). ZnO is a wide bandgap material with excellent optical properties. TMDs monolayers like WS2 and MoS2, are new types of direct band gap materials with atomic thickness. The optical properties of ZnO have already been widely investigated.

ZnO has high transparency in the visible and strong absorption in the UV range. Typical photoluminescence of ZnO contains two main contributions: near band edge transition inducing ultraviolet emission and a relatively wide visible emission. The goal of this thesis was to achieve nanostructured and microstructured ZnO as well as ZnO based hybrid structures with enhanced optical properties.

In this thesis, ZnO nanowires thin films were first synthesized by the chemical bath deposition method, which was found here to enable low-temperature wafer-scale production of homogeneous well-aligned nanowires arrays. To study the optical properties of the nanowires samples, a phenomenological model combining effective medium theory and Mie scattering has been developed and discussed by using two representative ZnO nanowires samples. As for the active optical properties, we report on an efficient technique namely desulfurization to increase the green light emission from ZnO nanowires. We also observed a strong emission enhancement with tunable wavelength from the ZnO microrods after thermal treatment. Finally, as a perspective work, high crystalline WS2 and MoS2 monolayers were synthesized by atomic layer deposition on SiO2 coated Silicon wafer.

Overview ix

Overview

This thesis is in line with two previous projects that helped to launch at UTT the UV photonics activity based on multifunctional materials and especially ZnO. The first national project, ULTRAFLU, lay in the development of a new concept called GWET for gain wave guiding energy Transfer [1]. GWET relies on active substrates used to efficiently pump ultrathin polymer films for the detection of explosive traces by fluorescence quenching. The second project called MATISSE aimed at developing photonic crystals doubly resonant in both the UV and visible range. Based on the hybridization with quantum dots emitting in the visible, two applications were targeted trace detection and lighting.

Besides the experimental validation, these two projects have allowed the group to acquire a set of skills in the field of nanofabrication, synthesis and modelling of active materials. The initial aim of this thesis was to develop one dimension nanostructured ZnO thin film as an active substrate and to extend the GWET concept to a type of luminescent materials, transition metal dichalcogenide monolayers (TMDMs), typically MoS2 and WS2 monolayer.

The integration of TMDMs on ZnO could be realized in two ways: Wet chemical transfer and Atomic layer deposition (ALD) based Chemical vapor deposition (CVD). For the transferred TMDMs, the photoluminescence exhibits large enhancement when the TMDMs are suspended on the ZnO nanowires with limited contact surface as compared to the case of the SiO2 substrate [2]. The second technique, ALD based CVD, involves two steps: the deposition of atomically thin oxides and the sulfurization of the ALD deposited oxides in a CVD quartz tube. By this method, we aimed at depositing in conformal way the TMDMs on wafer scale ZnO nanowires samples.

Along with this thesis, the synthesis and optical properties of chemical bath deposition (CBD) ZnO nanowires thin films were first studied. During the deposition of TMDMs, the photoluminescence of ZnO nanowires was found to be dramatically modified, especially for the defect emission. Further engineering of the defect emission was afterwards achieved by external treatments, including desulfurization of the nanowires, high temperature thermal annealing on ZnO microrods, which are byproducts from the chemical solution. Low temperature photoluminescence has been conducted to have an in-depth analysis of the excitonic behavior of the material. As for the TMDMs integration, the first attempt and management of the synthesis will be introduced in this report as well.

Overview x

The thesis is composed of the following parts:

The first Chapter begins with a literature survey on the possibility of enhancing the optical properties, from two perspectives of views: absorption enhancement and emission enhancement. A new concept based on gain assisted waveguiding energy transfer is also presented.

The second chapter introduces the synthesis of the ZnO nanowires and microrods by chemical bath deposition method. The structural and optical properties of the CBD grown ZnO are introduced. The growth mechanism of the ZnO nanowires has been systematically studied by exploring the following two factors: (a) The influence of the seeds layer, by varying the number if spin coating repetitions, the annealing temperature, the size of the substrate and the substrate type. (b) The growth time, by monitoring the saturation time for different kinds of the substrate size and type. By the end of this thesis, we are now able to homogeneously deposit ZnO nanowires at a wafer scale up to 4 inch.

The third Chapter presents the modelling works on calculating the optical properties of nanowires arrays. A phenomenological 1D multilayer model has been developed and discussed to explain light interaction within the nanowires. The functions of modelling include: 1. Analytically simulate light transmission through randomly distributed nanowires arrays and thus access the extinction part (light absorption and scattering). 2. Calculate the thickness of the thin film (i.e. nanowires length) by fitting the interference pattern; 3. Simulate the density of nanowires thin film through effective medium theory.

The fourth chapter is devoted to the investigation on the green light emission enhancement of the ZnO nanowires through sulfurization and desulfurization process. This work was a preliminary study aiming at observing the sulfurization effect on the ZnO prior to the integration of WS2 and MoS2 on ZnO by the sulfurization technique. It was found that a large amount of defects had been efficiently generated through this procedure. The chemical origin of the enhanced defect emission has been investigated by various techniques including XPS, Low temperature EPR and Oxygen plasma treatment.

Low temperature photoluminescence has been conducted as well to monitor the excitonic behavior as a function of temperature. Internal quantum efficiency (IQE) was calculated to illustrate the efficiency of the treatment in terms of emission enhancement.

The fifth chapter is about the defect emission engineering on ZnO microrods, which are the byproducts of CBD ZnO. The emission enhancement was achieved by rapid thermal annealing at a high temperature. A broad visible emission has been observed. By annealing together with a piece of ZnS, the Finally, the manuscript offer a general conclusion with some perspective works which have been initiated. Perspectives involve experiments on the tests of the ligand assisted CBD growth, the further measurements on the enhanced defection emission and the fabrication of WS2 and MoS2 monolayers as well as the anticipation and perspective of conformal deposition of TMDs on ZnO nanowires thin film and realize a hybrid structure based on the GWET concept.

Optical properties enhancement

In this chapter, an overview of the different ways of enhancing the optical properties in nanoscience, more specifically nanophotonics are presented. For optics, light always comes from electron transitions in a quantum object. The transition rate between the quantum states involves the carrier generation and radiative recombination processes, which are reflected respectively by the absorption and emission properties of the materials. Basically, modern strategies to enhance the optical performance of the materials in nanophotonics can be divided into two: Absorption engineering and emission engineering. The absorption enhancement involves the augmentation of light interaction within the materials, in order to increase the photon excited exciton density. The emission engineering is based on the improvement of the spontaneous emission rate which is governed by the Fermi's Golden rule:

𝑤 = ( 2𝜋 ħ )|𝑉| 2 𝜌(𝐸) (1)
where the V involves the electronic density of states and 𝜌(𝐸) is concerning the optical density of states.

It is worth here noting that in confined system like OD, 1D or 2D quantum structures the electronic density of states is strongly modified. A change in the electronic density of state can simply be obtained intentionally or unintentionally by doping a material (example of the phosphor materials).

I-1 Absorption enhancement I-1.1 Optical path lengthening

The Optical path length (OPL) is the product of the geometrical length of the light path and the refractive index n(s) of the medium in which the light passes through:

  C dC s n OPL ) ( (1)
where 𝑛(𝑠) is the refractive index, which is as a function of the distance 𝑠, and 𝐶 is the path light follows. OPL is a concept that is used to calculate the optical path length differences which determines the phase of the wave and governs the interferences and the light diffraction as it propagates. In thin film science, the OPL also determines the length of interaction between light and the optical thin film.

According to the Beer-Lambert law, a longer OPL will results in the enhancement of the absorption of light in the thin films. Optical path lengthening can be achieved by increasing the refractive index (imaginary part) or the geometrical light path. To realize the optical path lengthening in a thin film, two approaches are usually used: the surface texturing with random structures and the periodical nanostructure design.

(1) Random surface texturing Surface texture results from the local deviations of height from a perfect flat surface. Surface texturing refers to a modification of the surface, which can increase the incoming light interaction with the material via scattering into the thin film. This approach is even more efficient if waveguiding is possible i.e. if the thin film has a refractive index larger than the substrates. Many theories and numerical approaches have been developed to describe the scattering effect of a rough surface [1]. The small-perturbation method is used for surfaces with small roughness. The Kirchhoff approximation method is used for long correlation length surfaces. There are also four-wave method analysis developed by Bahar [2], surface-field phase-perturbation technique by Ishmaru and Maradulin [3,4]. The simulation we did in the roughness Model (Chapter III) was based on the analysis by Lérondel and Romestain [5].

The surface roughening can be realized by many methods, such as magnetron-sputtered and texture-etched method. Gray [6] proposed photo-lithography using the speckle of a scattered laser beam, which can tune the statistical properties of the generated surface by simply modifying the exposure parameters.

Black silicon is a typical case that has remarkable anti-reflection properties and is highly absorbing in both the visible range and the near infrared below the band gap of silicon. Mazur et al. [7] have fabricated black silicon with quasi-periodical conical structures of height and a period of a few microns (cf. Figure I-1), by sending a femtosecond UV laser pulse in a SF6 gas atmosphere on a silicon surface.

Sulphur is embedded into the silicon surface, which is shown as the irregular nanometer structures on the top of the cones. Note here that the structure obtained strongly scattered the light. (2) Nanostructure design Another way to realize optical path lengthening is to do nanostructure design on periodical structure. In thin film physics, it is well known that low energy evaporation condensation techniques based on e-beam gun or heated crucible give columnar thin films for most of the materials [8].

There are two physical principles accounting for the absorption enhancement in these nanostructured thin films. First, a lower refractive index of the thin films results in smaller reflection coefficient in the air. Compared to the refractive index of the bulk thin film, the refractive index of the nanostructured thin films is generally smaller. This is because the nanostructured thin film can be seen as a mixture of two materials, air or water embedded in the material by which the thin film is made of.

Following the effective medium theories, the permittivity  of the nanostructured thin film is the weighted mean value of the dielectric constants, Secondly, the absorption enhancement is also attributed to the attenuation of the refractive index away from the surface. The reflection is suppressed in the direction the light is passing through the thin film.

Zhu et al. [9] have fabricated nanowires and nanocones of a-Si:H on ITO-coated glass substrates, as shown in Figure I-2, the absorption is enhanced over a large range of wavelengths and incident angles. [9].

I-1.2 Light (field) confinement (1) Surface Plasmons

Surface plasmon resonances (SPR) are the oscillation of electrons between negative and positive permittivity media. The SPR occur when the frequency of the incident light matches the natural frequency of the surface electrons oscillating against the force of nuclei. Then the light is confined on the boundaries of the negative permittivity media. When the dimension of metallic nanoparticles is around a few tens of nanometer, localized surface plasmon resonance can be excited on the nanoparticles surface [10]. There are the two key main interests of SPR: 1) The ability to confine the light at the sub-wavelength scale. 2) The strong field confinement thanks to the surface plasmon resonances. This can be applied to increase the absorption of the surrounding materials or to enhance fluorescence and Raman scattering. It is also worth mentioning here that localized surface plasmon resonances can also be used to efficiently scatter light as in stained glass.

(2) Nano slot Many researches have been conducted on the absorption enhancement in the nanometer wide low refractive-index material. The optical field in the low index region is largely enhanced due to the discontinuity of the electric field at high-index contrast interfaces. [11].

Xu et al. [11] have fabricated a SOI-based slot waveguide (cf. [11].

The simulated results (cf. Figure I-4 a) show that the transverse electric field of the quasi-TE mode is strongly enhanced in the slot. Then the researchers compared to the simulated curve without the nano slot and the measured curve with the slot structure. When a nano slot is introduced, the effective index for the quasi-TE mode largely decreases, which demonstrates that the light is confined in the low index region.

(3) Optical Cavity

An optical cavity is an optical resonator that is formed by an arrangement of mirrors. Certain light frequencies can be confined in the cavity and reflected multiple times. The typical optical cavities are Fabry-Pérot interferometers and spherical cavities, both of them exhibit very high quality factors. A few researches based on cavities are introduced in the microdisk cavity section (section I-2).

(4) Hybrid structure

Many researches have been conducted to combine the metallic and dielectric nanostructures to achieve the enhancement of absorption, since both of the materials have highly tunable resonance.

Mann et al. [12] established a core-shell model, with the metal nanowires wrapped by a thin semiconductor films. They demonstrated that the two key principles to achieve extremely high light absorption are: 1. Using a critical coupling to maximize the absorption of each individual resonance. 2.

Increasing the total number of degenerate resonances. Similarly, Haus et al. [13] created a metal-coated nanoparticles model with a realistic distribution of coating thicknesses to achieve enhanced absorption.

(5) Optical coating

Interference coating is a type of optical coating that suppresses reflection by interference effect.

Quarter-wave coating is a typical interference coating for which the thickness of the layer is precisely controlled. Then for a normal incident light, the optical path difference of the incident beam and reflected beam is half its own wavelength, which leads to a destructive interference. The unsatisfactory aspect of this method is that the anti-reflection properties have a strong wavelength and incident angle dependence.

1D multilayer film is another kind of optical coating that can exhibit omnidirectional reflection.

Winn et al. [14] demonstrate that the multilayer coating to create a one-dimensional photonic crystal structures is able to achieve complete reflection of radiation in a given frequency range for all incident angles and polarization. By chirping the structure, one can also increase the rejection bandwidth [15].

I-2 Emission enhancement I-2.1 Photonic crystal

A photonic crystal is an optical structure of dielectric materials with periodical refractive indexes that creates a band gap which restricts the motion of the photons. Interference coating can be seen as one-dimension photonic crystal as the refractive index varies in one direction. According to the dimension number of the periodic dielectric constants, the photonic crystal can be divided into three categories: One-dimension, two-dimensions and three-dimensions photonic crystals. Note here that alumina has a refractive index of almost 3, higher than ZnO (2.4 in the UV). The idea behind a photonic crystal is to suppress the spontaneous emission by creating band gap which matches the emission wavelength of the dielectric material. This is obtained by adjusting the parameter of the crystals, because of the inhibition of propagation, the emission properties are enhanced.

I-2.2 Microdisk cavity

Optical cavities can not only increase the absorption but also enhance the emission properties of particles or thin films.

Many Researches have conducted studies on Microdisk cavities based on ZnO. Nomenyo et al. [17] have fabricated ZnO microdisk thin films by a top-down method to demonstrate that the stimulated emission can be enhanced. Chen et al. [18] studied the lasing thresholds of hexagonal-shaped ZnO micro-disks with different diagonals of the disks. Results show that the lasing threshold decreases as the diagonal of the ZnO micro-disk increases.

Besides microcavities, numerous researches have been recently undertaken for nanoscale optical cavity and more specifically for single photon source. As one of the most representative examples, Thanks to local field enhancement at the top of a tip like integrated metal pyramid, two photon excitation of a single quantum dot has been demonstrated leading to 5000 enhancement factor of the photoluminescence [19].

I-3 Gain-assisted Waveguiding Energy Transfer

Gain-assisted Waveguiding Energy Transfer (GWET) is a new concept of active substrate (cf. Figure I-6) that was introduced by Roy et al. [20]. The concept was first introduced using a fluorescent ultrathin nanolayer that was deposited on a planar ZnO thin film. The first idea is that in the case of a nanostructrured thin film thanks to the large interaction surface area of the substrate, the absorption of light is enhanced. The second idea is that in the case of very thin films the main part of the excitation is not directly absorbed. If now the ultrathin film is deposited on a luminescent waveguiding substrate, the substrate can absorb the remaining excitation and reemit photons that can be subsequently reabsorbed by the ultrathin film. Because of the waveguiding effect, the light emitted by the substrate is propagating in the plane and therefore optical path lengthening is achieved. Amplified emission is here necessary to ensure a high conversion efficiency between the absorbed photons and reemitted photons.

Eventually, the re-emitted light from the substrate is absorbed by the fluorescent ultrathin nanolayer, with very high radiative energy transfer efficiency.

I-4 Chapter conclusion

This chapter intended to present the different possible approaches to achieve enhanced optical properties. The enhancement of absorption and transmission were introduced separately. Enhanced optical properties can be achieved by mainly two approaches:

Optical Path Lengthening (OPL). OPL refers to the increase the propagation length of the light in a thin film that can increase the absorption of light according to the Beer-Lambert law. Random structures can be designed to scatter light in different direction; Periodical structured thin films with low effective refractive index and attenuation of the index along the height of thin films can suppress reflections of light.

Strong Field Confinement (SFC).

SFC is an approach that enhances the electromagnetic field in the nanostructures. SFC can be achieved in many nanostructured thin films. For example, surface plasmonic resonances in the metallic thin films, nano slot structure with discontinuity of the refractive index, Optical resonators, Hybrid structures like the wrapped metallic nanowire.

II-2 Synthesis: Chemical bath deposition

Chemical bath deposition (CBD), or chemical solution deposition, has been developed to grow high quality semiconductor thin film and nanomaterials from an aqueous solution at relatively low temperature (< 90°). The typical CBD semiconductors are metal chalcogenides and metal oxides. The earliest report on CBD dates back to 1869 and was on the synthesis of Thiourea (organosulfur compound) [21]. In 1884, CBD PbS film was reported by Emerson-Reynolds, which was later on identified to have good infrared photoconductivity [START_REF] Emerson-Reynolds | XXV.-On the synthesis of galena by means of thiocarbamide, and the deposition of lead sulphide as a specular film[END_REF]. During the 50s, CBD method was largely applied in the industry to fabricate infrared detectors [START_REF] Kutzscher | Chemical solution deposition of semiconductiong and non-metallic films: proceedings of the international symposium[END_REF][START_REF] Milner | Lead Selenide Photoconductive Cells[END_REF]. In 1960s, The CBD CdS thin film was reported to have good crystal quality comparable to the sample synthesized by vacuum vapor deposition methods [25]. Moreover, the good functionality of CBD thin film can be obtained without a post annealing process. After that, CBD of chalcogenides becomes a rapidly growing field. CBD were widely used to synthetize CdS based solar cells, with the efficiency gradually rising from 10.6% [START_REF] Rockett | CuInSe2 for photovoltaic applications[END_REF] to 14.5% achieved in the year 1991 [START_REF] Mauch | IEEE Photovoltaï c Solar Energy Conference[END_REF]. Nowadays, CBD is the most universally used method to deposit CdS layer for CuIn(Ga)Se2 thin film PV cells at the industrial level and on large areas up to 1 m 2 [START_REF] Chu | Solution-grown cadmium sulfide films for photovoltaic devices[END_REF]. As for CBD metal Oxide, the early studies can trace their roots back to the works on the controlled precipitations via hydrolysis reaction of metal ions and/or complexes in chemical solution during the 30s. The formation of various shapes and sizes metal oxide, such as high aspect ratio nanowires and nanoparticles with quantum size effect, has attracted many interests in late 80s and beginning of 1990s [START_REF] Chittofrati | Uniform particles of zinc oxide of different morphologies[END_REF][START_REF] Vogel | Sensitization of highly porous, polycrystalline TiO2 electrodes by quantum sized CdS[END_REF]. Although the CBD method is quite easy to be carried out, the precipitation procedure in the solution is yet not fully understood, the theory behind the CBD method is still continuously investigated, with the addition of complexing ligands, pH value effect, time dependent growth study as well as the choice of different substrates [START_REF] Boyle | Novel low temperature solution deposition of perpendicularly orientated rods of ZnO: substrate effects and evidence of the importance of counter-ions in the control of crystallite growth[END_REF][START_REF] Govender | Understanding the factors that govern the deposition and morphology of thin films of ZnO from aqueous solution[END_REF][START_REF] Cao | From ZnO Nanorods to Nanoplates: Chemical Bath Deposition Growth and Surface-Related Emissions[END_REF].

In general, the deposition of semiconductors by the CBD method comes from the decomposition of the unstable precipitations or/and complexes. As for the ZnO, zinc hydroxide and zinc ammonium are

II-2.1 Seed layer preparation

The seed layer, which is composed of ZnO nanoparticles, acts as nucleation center for the nanowire growth. Typical preparation methods include sputtering deposition, physical vapor deposition, atomic layer deposition and thermal decomposition of Zinc acetate. In our experiment, we adopted what may be considered as the simplest way. We spin coat the Zinc acetate solution followed by an annealing on a hot plate.

II-2.2 Growth in the nutrient bath

For the nutrient solution preparation, the Zn(Ac)2 is first dissolved in certain volume (typically 250 mL, 500 mL, 1.5L) of deionized water, the concentrations of Zn(Ac)2 that we usually used are 0.025 M and 0.035 M. The solution is then stirred for 1 min. The next step is to add certain amount of ammonium, typically 0.3 mL for 250 mL of water, to the solution during the stirring. The solution is heated up to 87 °C, the seeded substrate is then kept in the solution for a certain period of time, varying from 10 min to several hours. As we chose ammonium and Zn(Ac)2 as the precursors, the chemical reactions can be summarized in the following equations [START_REF] Izaki | Room-Temperature Deposition of Defect-Free Magnetite Film by Chemical Reaction from an Aqueous Solution[END_REF]:

Decomposition reaction: As we have observed during the solution preparation, once the ammonium is added to the Zn(AC)2 solution, the solution becomes immediately milky. That is because a lot of OH -is produced in a short time, the solution is supersaturated and precipitated out quickly due to high pH environment. After the substrate was inserted into the solution, the nanowires began to grow by heterogeneous nucleation on the seeds. The proportion of Zn 2+ that is converted to nanowires are very small with a value less than 1% (details are presented in the section II-4.3). The Figure II-5 shows that the growth of ZnO nanowires occurs at around a pH of 5.5, which is similar to the growth at near-neutral pH condition reported by McPeak et al [START_REF] Mcpeak | Chemical Bath Deposition of ZnO Nanowires at Near-Neutral pH Conditions without Hexamethylenetetramine (HMTA): Understanding the Role of HMTA in ZnO Nanowire Growth[END_REF]. Figure II-8 shows typical PL spectra of nanowires, with different thicknesses (600 nm, 900 nm and 1.9 µm). The PL measurements were conducted at room temperature with a 325 nm continuous He-Nd laser. A typical PL spectrum has two kinds of emission: near bandgap induced UV emission which is centered at around 380 nm;and defect emission in the visible range between 450 -800 nm. A higher peak intensity for the UV emission as to compare with the defect emission is usually considered to be an indication of good crystal quality of ZnO nanowires [START_REF] Cao | Ultraviolet-light-emitting ZnO nanosheets prepared by a chemical bath deposition method[END_REF]. One should nevertheless be careful that the defect emission is not saturated. For longer nanowires thin film, we can usually see fringes in the visible part, which come from the interferences within the top and bottom interfaces of the thin film i.e.

Zn (CH 3 COO) 2 ↔ Zn 2+ + 2CH

II-2.3 Structure and photoluminescence

air/ZnO and ZnO/substrate interfaces respectively. This in turn, indicates that one can consider the nanowire arrays as a thin homogenous film with a well defined thickness (nanorods length). This will be further discussed in chapter 3. As for the amplitude of the fringes, the contrast is larger in the longer wavelength range as expected from the absorption and scattering losses both being less important for longer wavelength. nanowires sample with a thickness of 1.9 µm, we can note that the reflection and luminescence spectrum fringes match rather well.

This in turn, supports the interference explanation as described. It is worth mentioning here that this is the transmission and the luminescent spectrum that should match. Nevertheless, a perfect matching is hard to obtain as the angle of collection and the incident angle for transmission measurement may not be the same. As shown in the inset picture, the ratio of the integrated UV emission between the spectrum taken at 300 K and that at 2 K in the range of 2.9 eV and 3.5 eV is equal to 5.1%, which indicates that the energy relaxation for the near band gap transition of the photoexcited excitons is very low as compared to the absorbed energy. As for the defect emission, the intensity is much lower than the UV emission and the intensity decreases slowly when the temperature increases.

II-3 Growth mechanism study

The nanowire growth on the substrates was found to depend on the seeds layer and the supersaturation level of the aqueous solutions. Many parameters have been previously explored and discussed to manipulate/control the morphology of the CBD synthesized ZnO nanowires, including the pH [START_REF] Cao | From ZnO Nanorods to Nanoplates: Chemical Bath Deposition Growth and Surface-Related Emissions[END_REF], the addition of auxiliary agents such as organic ligands [START_REF] Mcpeak | In Situ X-ray Absorption Near-Edge Structure Spectroscopy of ZnO Nanowire Growth During Chemical Bath Deposition[END_REF][START_REF] Parize | Effects of Polyethylenimine and Its Molecular Weight on the Chemical Bath Deposition of ZnO Nanowires[END_REF] and metal-ion impurities [START_REF] Joo | Face-selective electrostatic control of hydrothermal zinc oxide nanowire synthesis[END_REF][START_REF] Verrier | Tunable Morphology and Doping of ZnO Nanowires by Chemical Bath Deposition Using Aluminum Nitrate[END_REF]. Generally, the crystal growth along the c-axis of ZnO is faster than the lateral growth because of higher surface energy [START_REF] Kong | Polar-surface dominated ZnO nanobelts and the electrostatic energy induced nanohelixes, nanosprings, and nanospirals[END_REF]. As we can see in the figure II-6 (c), the ZnO microrods obtained from the nucleation coming directly in the solution, the length is larger than the diameter, which confirms the difference in growth speed between the c-axis direction and the lateral directions. As for the nucleation on the substrate, the growth speed along the c-axis and the lateral growth speed is highly dependent on the parameters of the seed layer, including the size, the density [START_REF] Guillemin | Critical Nucleation Effects on the Structural Relationship Between ZnO Seed Layer and Nanowires[END_REF], the shape as well as the polarity [START_REF] Consonni | Selective Area Growth of Well-Ordered ZnO Nanowire Arrays with Controllable Polarity[END_REF][START_REF] Cossuet | Polarity-Dependent Growth Rates of Selective Area Grown ZnO Nanorods by Chemical Bath Deposition[END_REF][START_REF] Guillemin | Quantitative and simultaneous analysis of the polarity of polycrystalline List of reference for Chapter III[END_REF]. As a consequence, the aspect ratio (length/diameter) of ZnO nanowires is also expected to highly depend on the seed layer. As for the density of the rods, it depends on several parameters, including the density of the seeds, the growth time and the orientation of the (002) plan relative to the substrate surface as well. On can suppose that if the nanowires are growing at an oblique angle, a certain number of wires stops growing when they come in contact with other wires.

In order to well control the growth of the ZnO nanowires on spin coated seed layers, the growth mechanism study has been further investigated, with a particular emphasis on the influence of the seed layer including the size, the density and the amount of seed grains. The parameters were varied by changing the deposition conditions including the repetition of spin coating procedure, annealing temperature, the size and the type of the substrate. The time dependent growth will also be presented in the following sections. For comparison, the following parameters of the nanowires after growth have been statistically accounted: Average length measured from the cross-sectional view of the SEM images;

Average diameter calculated from 40 randomly chosen nanowires; Aspect ratio i.e. the ratio between the average length and the average diameter; The number of nanowires per unit of surface area counted from the top view SEM images. Finally, we introduce the wafer scale (4-inch) deposition of ZnO nanowires by the CBD method. The homogeneity of the growth was assessed by measuring the thickness.

II-3.1 Number of spin coating repetition

The first method to change the size and density of the seed grains was conducted by changing the number of time the spin coating was repeated. We repeated the spin coating and annealing two times and six times respectively on the same silicon substrate size, the annealing temperature and time at each time was 400 °C and 4 min respectively. The growths were then conducted in the chemical solution with a concentration of Zn(Ac)2 equals to 0.025M for one hour. As seen from the SEM images, the average grain size of the seed layer obtained when the deposition is repeated 6 times in a row is much larger than the one deposited two times in a row, the number of seed grains per surface area is less. Table II-1 lists the average length, average diameter, aspect ratio, and the accounted number of the nanowires corresponding to the two different samples. After growth, we see obvious difference for the two samples. The nanowires are longer and bigger for the 6 times repeated seed layer sample, while the aspect ratio is smaller, which indicates that there exits more suppression of lateral growth within the nanowires during the growth. As for the counted number of the rods, the 2 times repeated seed layer sample has more rods than the other one, which is due to the fact that the number of seed grains is larger and the growth of nanowires is more vertical, as we can see from the cross-sectional view of the SEM images. By studying these two samples, we concluded that by changing the number of times the spin coating process was repeated, we can vary the size and aspect ratio of the rods.

II-3.2 Annealing temperature

Annealing temperature was then varied to change the size and density of the seed grains as well. By increasing the annealing temperature, the size of the seed grains is expected to increase, while the density of the grains decreases. The growth on the three pieces of seeded substrates were conducted in the same solution by using the 1L flask with a concentration of Zn(Ac)2 equals to 0.035M for one hour.

Figure shows the SEM image of the seeds, the top view and the cross-sectional view of the grown nanowires. Each column corresponds to one temperature. As seen from the SEM images, the seeds prepared by the annealing at higher temperature, the size of the seeds grains are bigger while a spacing between the grains larger. Obviously the number of grains decreases when the annealing temperature increases.

Table II-2 lists the average length, average diameter, aspect ratio and the accounted number of nanowires for the different samples. After growth, both the length and the diameter increase as a function of the annealing temperature. This is due to the number of seeds that decreases, thus the number of Zn 2+ ion available for each wire increases, which results in bigger and longer rods. The similar aspect ratios for the three samples indicate that the suppression effect is similar among the samples. The number of rods for the 200°C annealed sample is the largest, which is mainly due to the number of seed grains. The number of rods for 400°C annealing and 550°C annealing sample are the same, which indicates that a large number of the seed grains not necessarily results in more wires since the wires may come from the combined growth on several seed grains.

By studying these samples, we have observed that seed grain can be formed on the substrate at 200°C, which is essential for obtaining a seed layer on flexible polymer substrate (i.e. PET). By increasing the annealing temperature, the number of grain get slightly smaller, the sizes and length of the nanowires get slightly consequently increased.

II-3.3 Influence of the size of the substrate

The size of the substrate, which in turn controls the total amount of the seed grains, has also been investigated. Following the previous study, with larger amount of seed grains, the grown nanowires are expected to be smaller in diameter and shorter in length. In this study, the nanowires were deposited respectively by keeping the substrates in the 500 mL nutrient solution with a concentration of Zn(Ac)2 equals to 0.035M for one hour. to the different samples sizes. After growth, both the length and the diameter decreased as a function of the surface increases. This is due to the larger number of seed grains, thus the number of Zn 2+ ion shared by each wire decreased, which results in smaller and shorter rods. The similar aspect ratios observed for these samples indicate that the growth is not influenced by the substrate size, and more related to the state of the seed layer.

The volume of the ZnO nanowires thin film has been calculated by multiplying the surface of the substrate with the average length of the nanowires. From Figure II-12, we can see that the volume increases first almost linearly with the surface and begins to saturate when the surface is larger than 700 mm 2 . The linearly increase shows that the growth speeds in the vertical direction is almost the same for surfaces below 700 mm 2 within one-hour growth. We may conclude there that the growth is not limited by the Zn 2+ ion concentration. This also indicates that the decrease speed of the amount of Zn 2+ ion is almost constant. As for the surface area larger than 700 mm 2 , the growth speed after one hour is smaller than the growth speed with smaller substrate, which is due to the limitation in the amount of Zn 2+ ion. We fitted the experimental data with a saturation curve given by y = y s (1 -exp ( -x τ )), where y represents the volume, x represents the surface, y s the saturation value and τ a surface constant. The surface constant τ and the saturation volume y s , were found here to be equal respectively to 1125 mm 2 and 1030000 mm 2 × nm or roughly 1 mm 3 . These values will be further discussed in the section II-4 where we present the growth on even larger surface. From this study, we can here conclude that for the larger seeded surfaces we achieved the saturation for the growth time that was used here (1 hour). Note that the saturation value will be growth time dependent.

II-3.4 Influence of the substrate type

In our experiment, we use both silicon and quartz as a substrate. We repeated twice the spin coating of the seed layer on the two kinds of substrates (quarter 3-inch size) keeping their size equal. The seeded substrates were then put in the same solution using the 1L flask with a concentration of Zn(Ac)2 equals to 0.035M. The time that the samples were kept in the solution is one hour.

Figure II-13 shows the AFM images of the seed layer, the SEM top and cross-sectional view of the grown nanowires. Each column corresponds to the substrate type. As seen from the AFM images, the seed layer on the quartz substrate exhibit smaller grain size. It has to be noted that we have two typical feature sizes one is the grain size the other one may be due to the roughness. This will need to be further investigate. AFM here were preferred because of the poor conductivity of quartz, which makes the observation of the seed layer by SEM difficult.

Table II-4 lists the average length, average diameter and aspect ratio corresponding to the different substrate. After growth, the length, the diameter the nanowires grown on quartz substrate are larger than the ones grown on silicon substrate. According to the previous investigations and considering the grain size it is obvious that there is no direct correspondence here between the grain size and nanowire size. Indeed, the quartz substrate on which seed grain size is smaller is more suitable to grow bigger rods with smaller aspect ratio. These results are consistent with the work reported by S. Guillemin et al [START_REF] Consonni | Selective Area Growth of Well-Ordered ZnO Nanowire Arrays with Controllable Polarity[END_REF], the ZnO nanowires diameter has been found to increase as the seed grains decreases. Apart from the grain size on should also consider the roughness and the density of the seed layer as well. Further analysis is currently undertaken to link the rod size with roughness feature size.

Remark: On quartz, more nanowires quantity according to SEM images that means more Zn 2+ consumed.

II-3.5 Influence of growth time

After getting a general idea on the influence of the seeds layers, we conducted a time dependent studies on different type of the substrate: A quarter of three-inch size wafers, full three-inch size silicon wafer and a quarter of three-inch size quartz wafer. The seeds are prepared by the same method, with the repeated twice spin coating and 400°C annealing after each spin coating. The time dependent growths were conducted in the solution with a volume of 500 mL and a concentration of Zn(Ac)2 equals to 0.035 M. As we have concluded from the study on the influence of the size of the substrate, with larger amount of seeds, we expected that the saturation of the growth would be quicker. And also, for the smaller density of seeds, the nanowires are expected to have smaller aspect ratio with the same growth time. The study on the quarter 3-inch wafers is first discussed here. diameter, the number of nanowires and the aspect ratio of the nanowires grown on a quarter of 3-inch wafer. We fitted them with the same exponential (saturation) function used in II-3.3. As we can see, the saturation time for the length (28 min) is longer than that of the diameter (14 min), which is mainly because of the fact that the lateral growth speed is more suppressed by the neighbor seeds that the suppressions on the c-axis direction when the nanowires get larger. According to the fittings, the saturation length is about 1132 nm and the saturation diameter is 76 nm. The aspect ratio keeps increasing within one hour, which is due to the lateral growth stops after a while when the rods diameter becomes large enough to touch each other. Indeed, the number of rods is found to decrease when the time increases indicating that a few tilted nanowires stop growing when touching the neighbor rods. This effect can be seen from the cross section view of the SEM images as well (cf. For the growth on the quarter 3-inch wafer quartz substrate, the aspect ratio of the nanowires decreases as the growth time increases (cf. Table II-5), which is very different from the growth conducted on the silicon substrate. Our interpretation is that the lateral growth is favored or more exactly merging occurs that is to say, the big nanowires are the results of multiple growth on clusters of seed grains. The question will be now to determine what limit the spacing between the wires. It could be related to the roughness feature size. Further analysis is needed at this stage.

II-4 Wafer scale growth

The possibility of large scale deposition of nanowires or thin film by the CBD method is one of the main advantages of the method. Wafer scale (4-inch) deposition of ZnO nanowires has been reported in Yang's group [46]. In this work, we have also updated the volume of the growth flask, in order to further increase the deposition surface per growth. to the liquid/air interface. These stripes may be due to the temperature gradient at the top part of the flask. This effect needs to be further investigated but as it will be shown later the color difference correspond to a 10% to 20% percent length variation. In order to further investigate the growth on large scale substrates, we did a time dependent growth study and further assess the homogeneity of the samples.

II-4.1 Time dependent growth

We report here on the time dependent growth which was conducted on 3-inch size wafers. The nanowires were grown in the solutions with a volume of 1.5 L and a concentration of Zn(Ac)2 equals to 0.035 M. Figure II-17 shows the diameter and length of the nanowires after growth. It has to be mentioned that, the parameters are obtained from the central area of the wafer. We fitted the time evolution of the diameter and length by exponential (saturation) function as used previously. The fitting curves allows the saturation time and saturation value to be obtained. By the comparison of the saturation times between the growth on the full 3-inch wafers and the ones on the quarter 3-inch size wafer, we can see that the growth saturates earlier for the larger substrate size sample. We also observed that the aspect ratio of the nanowires (cf. Table II- As it has been mentioned previously in the introduction part, the proportion of Zn 2+ ions which contributes to the ZnO nanowire growth is very small (<1%). We have weighed the sample before and after the growth. The loading efficiency of Zn 2+ ions can be calculated by dividing the mass of the deposited ZnO by the mass of the initially added Zn 2+ ion. Figure shows the values of the loading efficiencies of the samples obtained from the time dependent growth studies on the full 3-inch substrates and one quarter size substrates. Using an exponential (saturation) fitting curve, we obtained that the saturation time for the growth on 3-inch wafers is equal to 14.5 min. The saturation time for the growth on quarter size wafers is longer and equal to 20 min. Figure shows the ratio of the two fitted curves. Interestingly, we noted that the initial ratio is around 4, which is equal to the ratio of the substrate surfaces (full size vs one quarter size). This correspondence indicates that the initial growing speed is the same for both size substrates. This in turn, confirms that at the beginning the concentration of Zn 2+ ion is not an issue but after some time it becomes the main factor which limits the growth of the nanowires. After one-hour growth, the ratio of the loading efficiency has been reduced to Since we were able to grow the ZnO nanowires on wafer scale surface, we could continue exploring the influence of the size of the substrate on the growth. As it has been presented in the section II-3.3, the volume was obtained by multiplying the average length of the nanowires by the surface of the substrate.

According to the fittings in the The seeds coated wafer is afterwards put in a nutrient solution with a concentration of 0.035M for a certain time.

II-5 Chapter conclusion

In this chapter, a brief history of the CBD method has been introduced. The chemical reaction and the typical structure and optical properties of the CBD ZnO nanowires have been presented.

The influence of the seeds layer has been investigated by statistically studying the length, diameter, aspect ratio and the number of nanowires. It has been found that: 1. The number of time the spin coating was repeated, the annealing temperature, the size of the substrate and the type of the substrate influence the seed layer. 2. Generally speaking, there is no direct correlation between the grain size and the wire diameter. Small size and dense grains leads to bigger nanowires as observed on quartz.

According to our observations this could be due to the merging of small size nanowires and space limitation. Density may also affect this merging effect. 3. When the density of the seed grains or the size of the substrate is larger, the length and diameter of the nanowires are reduced and the growth of the nanowires begins to saturate earlier. The later observation can be explained by the limited amount of the available ion.

The time dependence growth has been conducted as well on the same size substrate. For a quarter 3-inch wafer substrate, the aspect ratio increases as the growth time. This is due to the increase of the nanowire length but also due to the fact that lateral growth stops when nanowires are touching each other. Another interesting finding is that the number of rods counted from the SEM images is found to decrease. This was interpreted as a merging of tilted rods with neighbor rods to finally give rise to bigger and better aligned wires. By the exponential fitting of the loading efficiencies, we found that the saturation of the grown on one quarter 3-inch size wafer is 20 min and for the case of a full 3-inch wafer is 14.5 min. For the growth on the quartz substrate, the big nanowires were obtained due the merging of small size rods. Finally, the growth on 4-inch wafer was demonstrated and homogeneity of the seeds layer and the length of the nanowires within a range of 10 to 15% on the 3-inch wafer confirmed.

Besides the optimization of the grown, the idea was also here to find a way to simply characterize the nanowires samples.

Chapter III

Modelling of optical properties

III-1 Background

Many theoretical works that have aimed at calculating the transmission of nanowire thin films are based on numerical studies using the finite-difference time-domain (FDTD) method [1][2][3][4][5][6]. FDTD appears as a straightforward method to simulate the light interaction with nanowires, however, this approach is very time consuming and requires to know the exact geometry of the sample. As the solution grown nanowires are randomly distributed and usually tilted on the substrate, the exact geometry is almost impossible to assess. In this context, phenomenological modelling appears more appropriate to describe the optical properties of the arrays. As an example, similar approach has already been applied to simulate the optical properties of Cu nanowires [7]. In this paper, we report on the phenomenological modelling using parameters coming from the fitting of the direct transmission of a ZnO nanowire array sample. A rather simple analytical model is developed to describe the optical response of the sample.

III-2 Theory and calculation

III-2.1 Transfer matrix method and rough Fresnel coefficients

The transmission of a multilayer system is readily calculated by the transfer matrix method [8].

In the matrix, 𝐴 𝑖 ↓ is the amplitude of the electromagnetic wave propagating in the positive direction, which is the same direction as the incident light. 𝐴 𝑖 ↑ is the amplitude of the electromagnetic wave propagating in the negative direction. The Light amplitude in air 𝐴 0 ↓ and 𝐴 0 ↑ can be calculated in the following form by:

[ 𝐴 0 ↓ 𝐴 0 ↑ ] = 𝑀 1 𝐿 1 𝑀 2 𝐿 2 ⋯ 𝑀 𝑖 [ 𝐴 𝑖 ↓ 𝐴 𝑖 ↑ ] (1)
where 𝑀 𝑖 is the transmission matrix at the flat interface between two medium layers 𝑖 -1 and 𝑖

given by:

𝑀 𝑖 = 1 𝑡 𝑖-1,𝑖 [ 1 𝑟 𝑖-1,𝑖 𝑟 𝑖-1,𝑖 1 ] (2)
where 𝑡 𝑖-1,𝑖 and 𝑟 𝑖-1,𝑖 are respectively the Fresnel transmission and reflection coefficients of the interface between layers 𝑖 -1 and 𝑖. The propagation matrix 𝐿 𝑖 , which relates the field amplitude in the top and bottom sides of the layer 𝑖, is given by:

𝐿 𝑖 = [ 𝑒 -𝑗𝛽 𝑖 0 𝑂 𝑒 𝑗𝛽 𝑖 ] (3)
where 𝛽 𝑖 is given by:

𝛽 𝑖 = 2𝜋 𝜆 𝑑 𝑖 𝑛 𝑖 cos 𝜃 𝑖 (4)
𝜆 is the wavelength of the incident field, 𝑑 𝑖 is the thickness of the 𝑖-th layer and 𝜃 𝑖 is the incident angle at the interface between the layers 𝑖 -1 and 𝑖.

If we assume the field amplitudes in the bottom layer to be 𝐴 𝑖 ↓ = 1 and 𝐴 𝑖 ↑ =0, then the transmission of the multilayers structure can be expressed as:

𝑇 = 𝑛 𝑖 𝑛 0 ( 1 𝐴 0 ↓ ) 2 (5)
To calculate the surface scattering between the interfaces of the rough layers, the rough Fresnel coefficients equations were applied. G. Lérondel and R. Romestain [9] have derived a second-order approximation solution, and the simplified reflection coefficients 𝑟 𝑖-1,𝑖 and transmission coefficients 𝑡 𝑖-1,𝑖 of a rough layer have been given in the special case of small scattering angles:

𝑟 𝑖-1,𝑖 = 𝑘 𝑖-1 -𝑘 𝑖 𝑘 𝑖-1 + 𝑘 𝑖 {1 -4𝑘 𝑖-1 2 𝜉 𝑖 2 } (6) 𝑡 𝑖-1,𝑖 = 2𝑘 𝑖-1 𝑘 𝑖-1 + 𝑘 𝑖 [1 -(𝑘 𝑖-1 -𝑘 𝑖 ) 2 𝜉 𝑖 2 ] (7)
where 𝑘 𝑖-1 and 𝑘 𝑖 are the wave factors of the incident wave and transmitted wave at the rough surface of the layer 𝑖 and 𝜉 𝑖 is the amplitude of the sine profile roughness.

The transfer coefficients for the amplitude of forward and backward waves 𝐴 𝑖-1 and 𝐴 𝑖-1 ′ in the medium 𝑖 -1 as a function of the amplitude 𝐴 𝑖 and 𝐴 𝑖 ′ in the medium 𝑖 are obtained as follow:

[ 𝐴 𝑖-1 ↓ 𝐴 𝑖-1 ↑ ] = 𝑀 i [ 𝐴 𝑖 ↓ 𝐴 𝑖 ↑ ] (8)
Then the transmission matrix 𝑀 i that links the amplitude of the waves on the two sides of the rough interfaces is given by:

𝑀 𝑖 = [ 𝑎 𝑖 𝑏 𝑖 𝑐 𝑖 𝑑 𝑖 ] (9)
with:

𝑎 𝑖 = 𝑘 𝑖-1 + 𝑘 𝑖 2𝑘 i-1 [1 + (𝑘 𝑖-1 -𝑘 𝑖 ) 2 𝜉 𝑖 2 ] ( 10
)
𝑏 𝑖 = 𝑘 𝑖-1 -𝑘 𝑖 2𝑘 i-1 {1 + (𝑘 𝑖-1 -𝑘 𝑖 ) 2 -4𝑘 𝑖-1 2 𝜉 𝑖 2 } ( 11
)
𝑐 𝑖 = 𝑘 𝑖-1 -𝑘 𝑖 2𝑘 i-1 [1 + [(𝑘 𝑖-1 -𝑘 𝑖 ) 2 -4𝑘 𝑖-1 2 ]𝜉 𝑖 2 ] ( 12
)
𝑑 𝑖 = 𝑘 𝑖-1 + 𝑘 𝑖 2𝑘 i-1 {1 + 3(𝑘 𝑖-1 -𝑘 𝑖 ) 2 𝜉 𝑖 2 } (13)

III-2.2 Effective medium theory (EMT)

EMTs have been developed to calculate the effective permittivity of a composite material system. In our modelling, we selected Maxwell-Garnett model [10] . To simulate the porous properties of an effective layer composed of Mie scattering nanosphere and air, the effective permittivity 𝜀 1 of this layer can be derived from the following equation:

𝜀 1 -𝜀 𝑎𝑖𝑟 𝜀 1 + 2𝜀 𝑎𝑖𝑟 = 𝑓 1 𝜀 𝑠𝑝 -𝜀 𝑎𝑖𝑟 𝜀 𝑠𝑝 + 2𝜀 𝑎𝑖𝑟 (14)
Where 𝑓 1 is the density of the nanospheres in the effective layer, 𝜀 𝑎𝑖𝑟 is the air permittivity and 𝜀 𝑠𝑝 is the permittivity of the nanospheres, which will be introduced in the following section.

As for another effective layer, which are composed of nonspherical material and air, we selected the Landau Lifshitz/Looyenga model [11] [12] as the effective medium model. The effective permittivity 𝜀 2 can be deduced from the following equation:

√𝜀 2 3 = 𝑓 2 √𝜀 𝑛 3 + (1 -𝑓 2 )√𝜀 𝑎𝑖𝑟 3 (15)
While 𝑓 2 and 𝜀 𝑛 are the volume fraction and permittivity of the component n.

III-2.3 Effective extinction coefficient of a homogenous nanosphere slab

Mie theory was applied to calculate effective extinction coefficient of a homogenous nanospheres slab [13]. The volume attenuation coefficient is given by:

𝛼 = 𝐶 𝑒𝑥𝑡 𝑣 (16)
While 𝐶 𝑒𝑥𝑡 is the extinction cross-section of a nanosphere and 𝑣 is the volume. From equation (16), one can easily retrieve the imaginary part of the refractive index that accounts for the extinction (absorption and scattering):

𝑘 = 𝐶 𝑒𝑥𝑡 𝜆 4𝑣𝜋 (17)
The refractive index of a homogenous nanospheres slab with Mie contribution can be expressed as:

𝑛 ̃= 𝑛 + 𝐶 𝑒𝑥𝑡 𝜆 4𝑣𝜋 𝑖 (18)
we consider here that the real part is not affected as we are dealing with a homogenous slab.

III-2.4 The combined model

In By applying Eq. ( 1) we can calculate the field amplitude in air (see the appendix 2 for the matrix configuration):

[ 𝐴 0 ↓ 𝐴 0 ↑ ] = 𝑀 1 𝐿 1 𝑀 2 𝐿 2 𝑀 3 𝐿 3 𝑀 4 … 𝑀 𝑛 𝐿 𝑛 𝑀 𝑠 [ 𝐴 𝑠 ↓ 𝐴 𝑠 ↑ ] (19)
while 𝑀 1 , 𝑀 3 are transmission matrices at the two rough interface 1 and 3, and 𝑀 4 , 𝑀 n and 𝑀 s the transmission matrix at the interface 4, n and s.Then the Eq. ( 19) can be given by:

[ 𝐴 0 ↓ 𝐴 0 ↑ ] = [ 𝑎 1 𝑏 1 𝑐 1 𝑑 1 ] [ 𝑒 -𝑗𝛽 1 0 𝑂 𝑒 𝑗𝛽 1 ] 1 𝑡 12 [ 1 𝑟 12 𝑟 12 1 ] [ 𝑒 -𝑗𝛽 2 0 𝑂 𝑒 𝑗𝛽 2 ] [ 𝑎 3 𝑏 3 𝑐 3 𝑑 3 ] [ 𝑒 -𝑗𝛽 3 0 𝑂 𝑒 𝑗𝛽 3 ] … 𝑀 𝑠 [ 𝐴 𝑠 ↓ 𝐴 𝑠 ↑ ] (20)
The transmission is given by:

𝑇 𝑚 = 𝑛 𝑠 𝑛 0 ( 1
𝐴 0 ↓ ) 2 (21)
The multilayers transmission is modified by the bottom reflection of the glass, which is simply the Fresnel reflection between glass and air, and then the transmission is given by:

𝑇 𝑠 = 𝑇 𝑚 (1 -𝑅 𝑠 ) (22)
As for the thickness and diameter fluctuations called here thickness and diameter waviness respectively we took a simple weighting function by considering the fraction as 1:2:5:2:1, which come from the Gaussian-like distribution of the thickness. This method has already been used in the case of porous silicon film [14] and Gaussian distribution of diameters has been observed in the CBD growth nanowires as well [15]. Then the final transmission of the combined model is expressed as:

𝑇 = ∑ 𝐶 𝑤 ∑ 𝐶 𝑣 ∑ 𝐶 𝑢 𝑇 𝑠 (𝑑 𝑢 + 𝑢𝜎 𝑢 , 𝑑 𝑣 + 𝑣𝜎 𝑣 , 𝑑 𝑤 + 𝑤𝜎 𝑤 ) 2 𝑢=-2 2 𝑣=-2 2 𝑤=-2 (23)
where 𝑢, 𝑣, 𝑤 represent the coating layer of the substrate, the effective layers and the nanospheres, 𝑑 is the thickness (diameter for the nanospheres), 𝜎 is the waviness (standard deviation of the nanosphere diameter), 𝐶 is the weighting value:

𝐶 -2 = 1 11 , 𝐶 -1 = 2 11
, 𝐶 0 = 5 11 , 𝐶 1 = 2 11

, 𝐶 2 = 1 11

.

The reflection of the combined model is calculated in the same way:

𝑅 𝑠 = ( 𝐴 0 ↑ 𝐴 0 ↓ ) 2 (24)
The final reflection of the combined model is given by:

𝑅 = ∑ 𝐶 𝑤 ∑ 𝐶 𝑣 ∑ 𝐶 𝑢 𝑅 𝑠 (𝑑 𝑢 + 𝑢𝜎 𝑢 , 𝑑 𝑣 + 𝑣𝜎 𝑣 , 𝑑 𝑤 + 𝑤𝜎 𝑤 ) 2 𝑢=-2 2 𝑣=-2 2 𝑤=-2 (25)
The signal attenuation (absorption and scattering) then is expressed as:

𝐴 = 1 -𝑇 -𝑅 (26)

III-3 Experimental details

The first ZnO nanowires thin film sample studied in this paper was prepared by the CBD method.

III-4 Results

III-4.1 Morphology and Transmission curves

The SEM image (inset of

III-4.2 The rough layer model

The applicability of the rough layer model was first verified by respectively modelling the optical transmission curves of the sample 1 and of the FTO substrate used for the sample 2. optimization method, which is comparable to the refractive index for the FTO thin film in the literature [18]. We used the same optimization method to optimize the thickness, the waviness parameter, the roughness amplitude and the refractive index of FTO at the same time. The optimized values are as follows: the thickness of the rough FTO layer 𝑑 𝐹𝑇𝑂 = 1010 nm, the roughness amplitude 𝜉 𝐹𝑇𝑂 = 21 nm and the thickness waviness parameter, 𝜎 𝐹𝑇𝑂 equals to 20 nm.

III-4.3 The combined model

To achieve the best fitting between the calculation from the combined model and measured curve of sample 2, we first assess the contributions of the different parameters. Referring to the transmission curve of the FTO substrate, this calculated curve exhibits a larger number of interference fringes because of the larger thickness. There are obvious disturbances of the fringes from 420 nm to 500 nm, which is due to the difference in the effective refractive index of the two layers.

The dotted curve of which is around 20% of the total thickness. This optimized curve is the one that agrees the most with the measurement.

III-5 Discussion

The transmission of the ZnO nanowires is phenomenologically modelled by the combined model. In As for the phenomenological description, it is worth comparing the parameters of the combined model to the parameters of the nanowires in the real case. In the combined model, the combined structure including the slab of nanospheres, the rough and wavy ZnO layer is aiming at modelling the light interaction in the nanowires arrays. As shown in Table III-1 comparing the measured and retrieved parameters, we find that the thickness of the combined structure (the layer of ZnO nanospheres plus the ZnO layer) is comparable to the length of the nanowires and that the diameter of the nanospheres (about 220 nm) matches the diameter of the nanowires as well.

This very good agreement between measured and retrieved parameters confirms the veracity of the combined model proposed here. As the model i.e. generic model relies on three opto-geometrical parameters, the length and the diameter of the nanowires and their refractive index. The difference in height is accounted by the roughness amplitude whereas the potential light scattering induced by the structures themselves is accounted by the Mie-theory. This phenomenological model can be easily applied to any other material based nanowire films, and allows for a simple physical description of light interaction within the nanowire arrays based thin films.

While spheres could be replaced by ellipsoids, the current description does not account for anisotropic diffractive structures. This will only be a limitation to describe the light transmission through an array of nanowires laying on a substrate. Instead this model first intends to describe nanowire based thin films with the wires mainly oriented normal to the substrate. This is the case for the most current used transfert (etching) methods or chemical and physical growth techniques.

III-6 Chapter conclusion

In summary, we have proposed a 1D matrix based phenomenological model to describe the transmission of randomly distributed nanowires arrays. The model relies on Mie scattering, interface roughness, and waviness. The simulations on the CBD grown nanowires sample and the FTO substrate firstly verified the applicability of the rough layer model. Then using the combined model, we have modelled light transmission of randomly distributed ECD growth ZnO nanowire array samples.

Parameters determined from the fit are in a very good agreement with the experimental ones. Such a systematic study could potentially be used to predict the optical properties of any kind of nanowire samples and thus allowing for optimizing the structure features by controlling the growth conditions on specific substrates.

Chapter IV

Visible emission enhancement: ZnO nanowires

In this chapter, we report on an efficient technique namely desulfurization to increase defect emission of ZnO nanowires synthesized by a solution-based method. Prior to the desulfurization, ZnO nanowires were exposed to hydrogen sulfide (H2S) environment, which convert the sample into a ZnO-ZnS core-shell like structure with bandgap redshift and enhanced defect emission as compared with the as-grown sample. In the case of the desulfurized sample, the integrated intensity of the photoluminescence (PL) spectra in visible light range is increased 5 times as to compare with the sulfurized one and more than 40 times as to compare with the as-grown sample. X-ray photoelectron spectroscopy (XPS) shows that the desulfurized ZnO is under oxygen-deficient condition. Low temperature Electron paramagnetic resonance shows a strong resonant which corresponds to oxygen vacancy. After oxygen plasma treatment on the surface, the PL emission of the desulfurized ZnO is decreased by nearly 2 times. Our data suggests that the underlying mechanism of the green light emission enhancement in desulfurized ZnO can be interpreted in terms of creation of oxygen vacancies.

The evolution of the PL spectra as a function of the temperature reveals that the dominant 3.24 eV line at low temperature from 2K to 70K is related to phonon replica of the 3.31 eV bound, which is enhanced by the presence of large amount of defects. We also provide evidence that the negative thermal quenching of the green light emission originates from energy transfer between the 3.31 eV band to the defect emission. The internal quantum efficiency (IQE) of the desulfurized ZnO is significantly higher than the as grown sample. As a conclusion, the desulfurization technique proves to be an efficient method to enhance the green light emission and this work provides the direct relation between the green light emission and the oxygen Vacancy.

IV-1 Introduction

The optical properties of the wide band gap semiconductor ZnO has been widely investigated in details for decades [1][2][3][4]. Typical photoluminescence (PL) of ZnO contains two parts of emission: near bandgap transition induced ultraviolet (UV) emission and a relatively wide visible emission ranging from green to red, which is closely related to the concentration of the point and structural defects in the material. While the green luminescence has been reported to be associated with oxygen vacancies Vo [5,6], it has also been reported by several groups that the photoexcited phosphor ZnO:Zn with oxygen vacancies and a strong broad green/white emission band could be synthesized by vacuum annealing at about 900°C [7]. The extensive research of ZnO:Zn has indicated the possibility of rare-earth free white light phosphor based on ZnO powders [8]. Additionally, in a recent report, Jay G. Simmons et al. [9] reported that the sulfurized ZnO doubled the intensity of the green defect emission, which has been observed in our work as well.

In this chapter, we describe essentially a simple route to prepare a nanostructured ZnO thin film by using vapor reaction process composed of sulfurization and desulfurization in sequence. From PL spectroscopy, the desulfurized ZnO thin film exhibits giant green emission enhancement as compared with the as grown ZnO nanowires sample. We applied a number of complementary techniques such as X-ray photoelectron spectroscopy (XPS), Low temperature electron paramagnetic resonance (EPR) and oxygen plasma treatment to further understand the mechanism of the green emission enhancement.

Our data show a strong correlation between the oxygen vacancy and green emission. Temperature dependent PL spectra show that the desulfurized sample has a strong excitonic line at around 3.24 eV at low temperature, which is related to phonon replica of both defect related transition and the free exciton. The green light emission at around 2.5 eV presents negative thermal quenching (NTQ) with thermal activation energy of about 5.3 meV which shows a good agreement to the ambient thermal energy at 70 K while the 3.24 eV line become less obvious. Based on these evidences, we elucidate the mechanism of the green light emission enhancement which originates from the energy transfer from the 3.31 eV band to the emission related to the oxygen vacancy introduced through the desulfurization process.

IV-2 Experimental details IV-2.1 Preparations

ZnO nanowires were prepared by chemical bath deposition on quartz and silicon substrates,

IV-2.2 Characterization

The crystal structure was measured by X-ray diffraction (INEL EQUINOX100) with Cu Kα1 radiation (𝝀 = 1.5406 Å, at 30 kV and 0.9 mA). HITACHI S-3400N scanning electron microscope was used to study the morphology the ZnO samples. The PL spectra were recorded by using a He-Cd laser (325nm) as an excitation source, and a ANDOR SR500i spectrometer equipped with a CCD camera was used. The optical transmission curves of the samples were measured using a VIS-NIR spectrophotometer (Cary 100), with a spot size of about 2mm×5mm. Low temperature photoluminescence spectrum was conducted in a cryostat cooled down by liquid helium. The X-ray photoelectron spectroscopy was measured by a high resolution X-ray photoelectron spectrometer (ULVAC-PHI XPS: PHI Quantera SXM). Electron paramagnetic resonance was conducted at liquid helium temperature at around 1.7 K. The oxygen plasma treatment was carried out by Reactive Ion Etching (RIE), PLASSYS MU400, the desulfurized sample was exposed to an oxygen flow of 10 sccm with the pressure of 20 mTorr for 5 min.

IV.2.3. Theoretical calculations

To estimate the volume fraction of ZnS and ZnO in the sulfurized sample, light transmission through the as grown sample and the sulfurized one were simulated based on the transfer matrix method (cf. chapter 3 for further details). A rough porous thin film model was applied to fit the transmission curves. Based on the assumption of a small angle scattering, rough Fresnel coefficient was applied in the calculation matrix. As for the modeling on the porous properties of the thin film, we selected the Landau Lifshitz/Looyenga model [11] to simulate the effective dielectric properties of the effective medium that is composed of nanowires and air. The effective permittivity can be deduced from following equation:

√ 𝜀 𝑒𝑓𝑓 3 = 𝑉 1 √𝜀 1 3 + 𝑉 2 √𝜀 2 3 + ⋯ + 𝑉 𝑛 √𝜀 𝑛 3 (1)
While V 𝑛 and ε 𝑛 are the volume fraction and permittivity of the component 𝑛. The porous thin film model was applied to estimate the volume proportion of each component in the as grown and sulfurized samples. For the as grown ZnO nanowires thin films, the simulated curve in The other characteristic is that the green light emission which is centered at about 510 nm is largely enhanced as compared to the one of the as grown sample, while the UV emission is decreased. The ratio between defect and UV emissions is enhanced by about 40 times, which is bigger than the enhancement value reported by G. Shen et al. [14]. The mechanism of this enhancement has been proposed in the report of Jay G. Simmons [9]. After sulfurization, the ZnS shell is formed around the ZnO core, which traps the photon-excited holes from the valence band, more electrons in the conduction become available for deep level transition, which results in the defect emission enhancement. We have taken again the PL spectra to characterize the radiative defects in the desulfurized ZnO. To investigate the origin of the green emission, we have examined the atom ratio for the desulfurized ZnO. As shown in the figure IV-7, the zinc to oxygen ratio as measured by the XPS is about 6:4 for the desulfurized sample, which indicates that this sample is under oxygen deficiency condition. Low temperature EPR measurements were conducted for the three samples as well. Only the spectra of the desulfurized ZnO has exhibited a resonance at the position which corresponds to the oxygen vacancy according to the literature [15] (cf. Figure IV-8).

IV-3 Sulfurization

IV-3.1 Structure

IV-3.2 Bandgap and defect emission

IV-4 Desulfurization

IV-4.1 Structure and room temperature PL

We have also explored the possibility of passivating the green emission by an oxygen plasma

IV-4.2 Low temperature PL

To further understand the excitonic behavior of the desulfurized ZnO, the low temperature PL spectra have been taken at temperature rising from 2K to 300K. Figure IV-10 is the PL spectra of the as grown and desulfurized ZnO nanowires taken at 2 K. The as grown sample shows a single peak at around 3.361 eV, which can be attributed to the neutral donor exciton transition (D°X), as reported by Reynolds et al. for bulk ZnO [16] and He et al. for nanorods [17]. For the desulfurized ZnO, it shows an intense defect emission peak. The defect emission to UV emission ratio is much bigger as compared with the as grown one, which indicated the existence of large amount of defects. This could explain why more excitons are trapped by the defects than neutral donor or free exciton (FX) in the desulfurized ZnO. ). As we note that the left three peaks have equal distance of 72 meV, which is equal to the longitudinal-optical (LO) phonon energy of ZnO. This leads us to think that the emission at the 3.31 eV band (often called A band as well) can be the main emission peak, while the other two are its phonon replicas. Several interpretations about the 3.31 eV band has been discussed in the literatures, such as first LO phonon replica of free exciton (1LO-FX replica) [18], stacking fault bound exciton [19], surface exciton [20]. For the reports, which assigned the 3.31 eV to the 1LO-FX replica, it is explained that the electron-phonon coupling in ZnO mainly comes from Frohlich interaction, and such interactions are very small in a perfect crystal due to parity conservation.

However, the 1LO-FX replica can be very strong due the presence of surface defects. In a recent work, Tainoff et al. have reported that both surface defect states hypothesis and the exciton-phonon interaction are valid to explain the origin of the 3.31 eV band. In our work, we note that the dominant peak corresponds to the position of the second phonon replica of FX and the first phonon replica of the 3.31 eV band. Following the discussion in the above reports, we think the 3.31 eV band has two accumulated contributions: defect-related transition and 1LO-FX replica. As the desulfurized ZnO is under oxygen deficiency condition, we can assume that the excitons can be easily trapped by the defects, which will result in a very intense defect-related UV emission. Thus the dominance of 3.31 eV band-1LO peak is due to both the 1LO of defect-related transition and the 2LO of the free exciton. We also note that the 3.31 eV band-1LO peak maintains dominant until the temperature rises to 70 K. At 100 K, 3.31 eV band becomes dominant. At this stage, we assign here the 3.31 eV band to 1LO-FX replica excluding the contribution of defect-related transition since its phonon replica can no longer be tracked as the temperature rising from 100 K to 300 K. shows that the defect emission begins to increase while the temperature rises from 70 K to 150 K, while the UV emission continues to decrease. The NTQ of the green emission has thermal activation energy of 5.3 meV, which is equal to the thermal energy of the environmental temperature at 63.5 K. This temperature is very close to the beginning temperature of NTQ at around 70K, which indicates that the negative thermal quenching is coming from the defect emission being activated as the temperature increases. From the view of exciton, the thermally activated exciton can be re-trapped by the deep level defects at the surface. The diffusion length of exciton in ZnO nanowire is relatively large (150 -200 nm) at low temperature [21].

The internal Quantum efficiency (IQE) of a material is the ratio between the number of emitted photon d the number of absorbed photon. From the low temperature PL, the IQE can be derived through two steps: 1. Assuming the IQE at cryogenic temperature (2K) to be 1. 2. Calculating the ratio of the area under the emission peak at certain measurement temperature (e.g. 300K) vs. that at 2K. The IQE of the green light emission has been calculated by doing the ratio between the emission at certain temperature and the total emission at 2 K. Figure shows that due to negative thermal quenching, the maximum IQE of the green light emission is 109%. And the IQE at room temperature is as high as 33%, which is significantly higher than the as grown sample which is below 1%. By combining the analysis on the thermal evolution of excitonic peaks and the defect emission, we present the energy level diagrams in Figure IV-14, which summarizes the various mechanisms to explain the dominance of 3.31 eV band -1LO peak and the NTQ of the green emission. At 70 K, the dominated excitonic line is the 3.31 eV band -1LO peak, which is due to the contributions of the 1LO of defect bound exciton and 2LO of the free exciton peak which were enhanced by the presence of defects generated by the desulfurization process. On the other hands, the photon-excited electrons decay also to the oxygen vacancy level via non-radiative channels (dotted arrow), eventually emitting light at 510 nm. When the temperature arrives at 100 K, the defect bound excitons have been thermally activated, which leads to an additional non-radiative channel represented by the red dotted arrow. This channel leads to an increase in the green emitting defect level transition, which correspond to the NTQ phenomenon. This diagram strongly supported and well aligns with the various results that the 3.31eV band -1LO is much weaker at 100 K as compared to 70 K, while the NTQ of the green light occurs with the thermal activation energy corresponds to the ambient thermal energy at around 70K.

IV-5 Chapter conclusion

In summary, desulfurized samples were prepared by the chemical deposition method followed by sulfurization and desulfurization process at relatively low temperature. The sulfurized ZnO sample has more than 55% of ZnS. The bandgap emission of ZnO redshift and the defect emission in visible range is enhanced. The desulfurization treatment has further enhanced the defect emission and the integrated defect emission has been enhanced by 42 times as to compared to the emission of the as grown sample.

The XPS analysis, EPR spectra and the oxygen plasma treatment suggest that the enhancement in green emission comes from the generation of a large amount of oxygen vacancies during the removal of the sulfur atoms from the sulfurized ZnO. Low temperature PL study on the desulfurized ZnO sample shows that the phonon-assisted excitonic emission at 3.31 eV -1LO position can be enhanced by the presence of defects.

The NTQ was clearly observed from 70 K to 150 K. We propose that by the presence of large amount of defects, the interaction of excitons with the defects have been enhanced, which induces the dominance of defect bound related emission in the excitonic emission and the strong defect level related transition. The defects can re-trap excitons at higher temperature and the radiative energy relaxation is further enhanced by the re-trapping effect.

As for application, this desulfurization technique has a good potential method served as the technique to synthesize ZnO based green light emitters.

Chapter V Visible light enhancement: ZnO microrods

ZnO microrods are the ZnO crystals formed by the nucleation in the CBD solution. The ZnO microrods can be collected by horizontally introducing a bare substrate in the solution. The microrods were observed to be formed abundantly in the solution when the Zinc acetate concentration was above 0.056M. X-Ray diffraction studies on these microrods revealed that they have a hexagonal wurtzite structure and are perfectly crystalline. Raman studies showed that the thermal annealing of as-grown samples helped to evacuate the zinc hydroxide defects trapped inside the ZnO microrods during their growth process and also increases the crystallinity of the samples. Thermal annealing, however, is also accompanied by the formation of nanopores in the microrods possibly due to the loss of zinc hydroxide.

In depth low temperature PL spectra study indicates the existence of extended structural and surface defects. Similarly, to the phenomenon observed in the previous chapter, the evolution of 3.31eV line became dominant as temperature rises from 70K to 200K, which is due to the exciton being trapped by deep level defects. In the mean while the negative thermal quenching of green light emission was observed in this temperature range. The comparison of 300K to 2K integrated PL ratio and the internal quantum efficiency (IQE) measured using an integrating sphere shows that the light extraction rate of the microrods is about 10%. The internal quantum efficiency has been improved from 1.3% to 5% through thermal annealing, it is due to the improvement of crystal quality and creation of optically active defects. By doping the ZnO with sulfur (S), the ratio of integrated green light emission between spectra taken at 300K and 5K is around 30% at room temperature. These highly luminescent microrods have good potential to be used for rare-earth elements free phosphors with tunable colors.

V-1 Introduction

The synthesis of the ZnO microrods prepared by hydrothermal technique has been reported before [1][2][3]. The articles have discussed on the growth mechanisms of these twinned structures.

They demonstrated that the formation of twin morphologies of ZnO crystallites is either a result of the differences between symmetrical and energetic most favorable structural arrangements or due to the consequence of oriented intergrowths.

In this chapter, the synthesis of highly crystalline, hexagonal shaped ZnO microds by a simple, low temperature, low-cost chemical bath deposition technique together with an in-depth structural and optical analysis will be presented. These microrods were observed to freely grow in the solution and do not require the presence of a seed layer. Room-temperature photoluminescence (PL) show a slight increase in the ZnO excitonic UV emission of the annealed ZnO microrods, which is however, accompanied by a more important increase in the ZnO defect emission in the visible as compared to the as-grown samples. Temperature-dependent PL measurements reveal an increase of defects of the annealed ZnO microrods, as compared to the as-grown samples, which is marked by the appearance of shallow defect bound exciton and negative thermal quenching (NTQ) in the PL spectra of the annealed samples at low-temperatures. In the previous desulfurized study, we have observed that, the exciton dissociated from the D°X can be retrapped by the defects center at higher temperature (around 50 K). The re-trapping results in both domination of shallow defect bound excitonic peak and the increase of deep level defects related transitions. In a consequence, the quantum efficiency has been improved by the existence of defects. This phenomenon is further discussed in this chapter by studying the high temperature annealed microrods sample and a S doped ZnO microrods sample which shows strong phonon electron coupling at the origin of the enhanced green light emission spectra.

V-2 Experimental details

In a typical series of experiments, the precursor solution used consisted of Zinc acetate dihydrate (Zn(CH3COO)2• 2H2O, ACS reagent, ≥ 98%), ammonium hydroxide (NH3 basis, ACS reagent, 28-30%) and deionised water. Silicon and quartz substrates are used to collect the microrods in the solution. For the chemical solution, 0.056 M zinc acetate was dissolved in 250 ml of water. 1 ml of ammonium hydroxide was added to the solution and stirred at room temperature. Synthesis is conducted in the absence of metal catalysts or additives. This mixed solution is heated at 87°C in a three neck round bottom flask. The cleaned substrates are immersed in this solution for two hours. Thereafter, the sample is washed with water and dried in air.

The microrods sample was cut into three pieces. Two of them were annealed. The first annealed sample was treated at 900°C, in nitrogen atmosphere in a rapid thermal annealing system for 10 mins. The second one was annealed together with a small piece of sulfurized ZnO, which has been presented in the previous chapter.

The ZnO microrods and their surface morphology were examined using a scanning electron microscope (SEM, HITACHI S-3400N, 30kV). Phase identification of the microrods was done with a Philips powder X-Ray diffractometer equipped with a Cu 𝑲 ∝ line.

Temperature-dependent photoluminescence (PL) experiments were conducted using a He-Cd laser (325 nm) as an excitation source, and a CCD camera as a detector. An unfocused laser beam was used for the PL studies, in order to not saturate the defect emission in visible. Liquid helium was used for cooling the sample down to 2K. An 80 cm focal length spectrometer with a 3200 lines/mm grating was used for high resolution PL spectroscopy. Micro-Raman measurements were conducted at room temperature using a laser wavelength of 514 nm. The general chemical reactions occurring in the precursor solution are the same as the listed equation in chapter II. In our synthesis process the source of OH -ions is ammonium hydroxide. More and more OH -ions will combine with Zn 2+ ions to form a complex of Zn(OH)4 2-. This complex acts as an intermediate growth unit. The important prerequisite for the formation of these microrods in the solution is that the solution should be supersaturated. When the solution is supersaturated, nucleation initiates which leads to the formation of ZnO microrods in the solution as reported for nanocrystals by Cao et al. [4]. The different morphologies of microstructures such as trirods and tetrarods observed are generated by direct ZnO precipitation. Direct ZnO precipitation from an aqueous solution helps in twinning of structures, growth of multiple lattices from a common junction with individual crystals growing along their c-axes tetrahedral to each other and leads to generation of trirod and tetrarod microstructures. The crystalline quality of the two samples was further investigated using Raman spectroscopy measurements (cf. Figure V-3). For the two samples, the spectra exhibited various modes, namely, E2(L), 2E2(L), E2(H)-E2(L), A1(TO), E1(TO) and E2(H) modes at 99, 203, 333, 378, 410 and 438 cm -1 , respectively. The E2(L) frequency mode relates with the vibration of heavy Zn atoms while E2(H) frequency mode reflects the vibrations of oxygen atoms in the ZnO crystal structure. Standard peak positions reported for E2(L) and E2(H) modes in the case of bulk ZnO are 101 and 439 cm -1 , respectively [5]. From Figure V-3 one can clearly observe that the E2(H) mode has considerably increased in intensity after annealing. Furthermore, the line width of the E2(H) mode is ~ 10 cm -1 for the as-grown and it drops to 7 cm -1 for the annealed sample. This further confirms that the microrods have an improved crystal quality after annealing. This is in good agreement with a general improvement of the crystalline quality, as determined by XRD method too. We also performed a slow scan in the 3000 to 4000 cm -1 range for detecting the presence of OH groups or Zn(OH)2 groups. We can see a strong, broad structured band in the case of the as-grown sample in this range. This corresponds to the O-H stretching mode. However, in the case of annealed microrods sample, there is a complete absence of this structured band.

V-3 Structural study

V-3 Annealed ZnO: White light emission

The room-temperature PL spectra of the as-grown and 900°C annealed microrods are shown in Figure V-4 shows. Both as-grown and annealed samples have UV emission peak around 387 nm (~3.2 eV) and a broad emission peak in the visible part extending from 450 nm till 900 nm, due to ZnO defect emission [6][7][8][9]. In the annealed sample, a small peak at 375nm (3.31eV) is observed on the left shoulder of the dominant UV emission peak. This peak can be assigned to the free excitons (FX), which is consistent with the FX exciton peak at room temperature observed by X. Guo et al. [10]. The existence of FX emission peak is another proof of better crystal quality after the thermal treatment. As compared to the as grown sample, the dominant UV peak red shifted about 7 nm. The red shift can be attributed to the lattice expansion, creation of Zn interstitials [11] and the enhancement of phonon assisted excitonic emission induced by the high temperature thermal annealing. In term of emission intensity, the annealed ZnO microrods show a strong increase in the ZnO defect (i.e. visible) emission and a slight increase in the ZnO NBE (i.e. UV) emission as compared to the as-grown ZnO microrods.

To investigate further the excitonic behavior of the annealed ZnO microrods, we conducted low temperature dependent PL studies. Another peak with a line width of about 40meV appears after annealing. The maximum energy position of this peak lies at around 3.33eV, which has been assigned to the Y line by Meyer et al. [13] and Wagner et al. [14] with a relatively low thermal activation energy of 10meV±2meV despite large localization energy, which is around 40meV. Based on the conclusion drawn by Meyer, the Y line is most likely related to the structural defect bound excitons. The structural defects can be generated by high temperature annealing which has also been reported by Jiang et al. [11]. As reported by Varshni [15], the LO-phonon replicas occur with a separation of 72 meV. By referring to the peak position distance from the zero phonon peaks, we ascribe the peaks at 3.28 eV, 3.21eV, 3.26 eV and 3.19 eV to the D°X-1LO, D°X-2LO, Y line-1LO and Y line-2LO phonon replicas, respectively. To confirm the above assignments, the evolution of PL as a function of temperature was also monitored for both the samples. For the annealed sample, as seen from Figure V-6 (a), the D°X and Y line quenched gradually with increasing temperature. At 70K, a peak starts to emerge at 3.31 eV. As described previously in chapter 4, several interpretations about this peak have been discussed in the literature, such as first LO phonon replica of FX, stacking fault bound exciton, surface exciton. In a recent article, Guo et al. have assigned the 3.31eV band of the microrods to the LO phonon replica for the spectra recorded at temperatures from 15K to 150K. However, Tainoff et al. has reported that the origin of the 3.31 eV band can be explained both by the surface defect states hypothesis as well as the exciton-phonon interactions. In our work, we observed that the 3.31eV peak first appears at temperature higher than 70 K and it even becomes dominant with the temperature increases from 70K to 200K as shown in Figure V-6 (a). This peak does not exhibit the characteristic asymmetric shape of phonon replica. We thus consider surface defects as the dominant origin of this peak in this temperature range. Since the Y line and D°X have a small dissociation energy, the delocalized exciton from the structural defect or neutral donor can be re-captured by the surface defects. The observation of NTQ in this temperature range, lead us to visualize that the 3.31 eV band is closely related to the excitons that bind to the surface defect which are generated during annealing.

Based on the observations mentioned above, the peak positions are shown in In Figure V-7, showing the defect emission as a function of the temperature, we observed that the green light (GL) emission shows a negative thermal quenching (NTQ) [16,17] behavior at temperatures rising from 70K to 200K, while the emission in the orange light range keep decreasing. This phenomenon indicates that in annealed ZnO microrods, the green light emitting defect acts as trapping centers but not the orange light emitting centers.

In General, the internal quantum efficiency (IQE) and external quantum efficiency (EQE) are used to evaluate the efficiency of the PL. As it has been mentioned in the previous chapter, we have derived the IQE by doing the ratio between the emission at certain temperature and that at 2K. For the annealed ZnO microrods, we took the total emission at 2K as the denominator to calculate the IQEs of UV emission and defect emission at room temperature (RT). For the as grown sample, the IQE of the UV emission was calculated by taking the UV emission at 2 K as the denominator as the defect emission is negligible as compared to the UV. The EQE, which denotes the ratio between the extracted photon and the absorbed photon, can be approached by using an integrating sphere (IS), since IS enables the collection of the total extracted photon. Following the above description, the light extraction factor can be calculated by doing the ratio, between the IQE and EQE.

Table V-1:

The ratio of the integrated PL intensity at 300K to the intensity at 2 K and the internal quantum efficiency measured in the integrating sphere.

The quantum efficiency values derived by the above two methods are listed in the Table V-1. For the as grown sample, the measured EQE is a very low value around 0.1%, which is comparable to the absolute values reported by Hausser et al. The extraction factor is about 10%. For the annealed sample, the IQE of UV emission drops to 0.38% while the IQE of defect emission has a value of 4.9% bigger than the UV IQE of 1.3% of the as grown sample. This indicates that the annealing treatment has improved radiative relaxation rate of the photon excited exciton. In the mean while the extraction factor of the annealed sample is about 10%, which is close to the value of the as grown sample. This indicated that the microrods have waveguiding effect. Indeed, the extraction efficiency is very closed to the expected extraction value considering total internal refection in a material of refractive index of 2.4 i.e. 8,7%

(1/2n 2 ).

V-4 Annealed ZnO: Green light emission Enhancement

From the previous study, we have concluded that negative thermal quenching is due to the trapping of exciton by the green light emitting defect center. The following study provides further supporting information. For this sample, we annealed the microrods with the same condition, but together with a small piece of ZnS. The annealing process has significantly changed the optical properties of the ZnO microrods. As we can see from the inset of Figure V-8, as compared to the as grown sample, the room temperature PL spectra of the annealed sample at 300K shows lower UV emission but higher defect emission. The peak position of the defect emission shifts from 2.15 eV to 2.45 eV, which indicate that the green light emitting defects become dominant after the annealing. The integrated emission in the wavelength ranging from 1.6 eV to 3 eV increases by 23 times, after annealing together with a small piece of ZnS, which illustrates that the amount of green light emitting defect has largely increased. As for the PL spectra at 5K, the emission from the annealed sample has the characteristic structure reported by Dingle and peaks are centrered at 2.45 eV [18]. A series of sharp peaks on the high-energy side of the defect emission are well resolved. The peaks are two sets of equally separated fine structures with the same periodicity of around 72 meV. The defect emission peak centered at 2.45 eV has been assigned to a localized excitation of an isolated Cu 2+ ion [19,20]. The strong electron-LO-phonon coupling is coming from two electronic transitions strongly coupled to lattice vibrations of ZnO. But in our case, there was no Cu being introduced during the treatment. We propose here that this deep level transitions may also be related to intrinsic defect states of ZnO or dopant of Sulfur atom that were created during the annealing.

As shown in Figure V-9, the fine structures fade out as the temperature increase from 5K to 90K and eventually the defect emission becomes smooth at higher temperature. The evolution of UV emission peaks is very similar to the annealed microrods sample, at 50 K the A line and FX-2LO are well resolved, as the temperature increases, the A line becomes dominant from 70K to 170K. Figure V-9 (c) shows the evolution of integrated defect emission and UV emission as the temperature arises from 5 K to 300 K. The UV emission decrease as the temperature increases, while the intensity of the defect emission is almost constant in the temperature range of 5 K to 50 K. As we have concluded from the previous low temperature PL study on the desulfurized ZnO, the green light emitting defects can be activated by thermal energy depending on the temperature, NTQ can be observed due to the thermal activation. However, for this S doped sample, the NTQ is not that obvious and need to be further investigated. Figure V-9 (d) shows that this sample has an IQE as high as 30% at room temperature.

V-5 Chapter conclusions

We have deposited ZnO microrods on silicon and quartz in the CBD solution. XRD patterns showed that these microrods have a wurtzite hexagonal structure and are highly crystalline. The room temperature of the as-grown ZnO microrods shows a strong UV excitonic peak with a low defect emission. The ZnO microrods were further treated by 900°C annealing. The annealed ZnO shows an enhanced defect emission and a well-resolved free exciton emission peak, which indicates that the crystal quality of the ZnO has been improved after annealing. Low temperature PL reveals that the annealing has increased the amount of defects in the microrods. By monitoring the evolution of defect bound excitonic emission and the green light emission, we confirmed that the negative thermal quenching is related to the thermal activation of green light emitting center at higher temperature. The IQE of the microrods has been improved by about a factor 5 by the annealing. For the S doped ZnO. The defect emission is enhanced 23 times, while the UV emission is almost zero. The green light emitting defects acts as efficient trapping centers, since the IQE keeps very high as the temperature rises form 5 K to 300 K.

General Conclusion

From the point of view of the Fermi golden rule, the enhanced optical properties can be achieved by modifying respectively the electronic states and optical density of states. Finally, this thesis was more about the former factor as the defect related emission engineering within the nano and microstructured ZnO induces the modification of the deep level bands and influences the inter-bands transitions of photon-excited excitons.

The nanostructured/Microstructured ZnO were synthesized by the chemical bath deposition method, which has been widely adopted in the industry to synthesize nanomaterials on a large scale.

The influence of the seeds layer has been investigated in this thesis by statistically studying the length, diameter, aspect ratio and the number of nanowires. The density of the nanoparticles in the seeds layer has a big influence on the nanowires after growth. We verified that via the number of spin coating repetition, the annealing temperature and the coating substrate it was possible to adjust the aspect ratio of the nanowires. Generally, when the seed grains are dense, the lateral growth of the nanowires can be suppressed, which results in smaller aspect ratio nanowires after growth. Secondly, with the size of the seeded substrate, the length and diameter of the nanowires decreases. Time dependent study indicates that the growth of the nanowires began to saturate earlier on bigger size substrate. By increasing the size of the flask, we were able to grow nanowires homogenously on 3-inch and even 4-inch wafers.

A model to study the passive properties of ZnO nanowires was proposed in the chapter II. Noting that the exact geometry of the randomly distributed nanowires array is difficult to approach, we have established a 1D matrix based phenomenological model to describe the transmission of these kind of sample. The model relies on Mie (volume) scattering, interface roughness and waviness. The simulations on the CBD grown nanowires sample and the FTO substrate firstly verified the applicability of the rough layer model. Then using the combined model, we have modelled the direct transmission of light through randomly distributed ECD growth ZnO nanowire array samples. Parameters determined from the fit are in a very good agreement with the experimental ones. Such a systematic study could potentially be used to predict the optical properties of any kind of vertically oriented nanowire based thin films and thus allowing for optimizing the structure features, by controlling the growth conditions on specific substrates.

The photoluminescence of ZnO has been continuously studied along the thesis. The PL of the CBD ZnO nanowires indicates that the as grown sample has a good crystal quality, since the defect to UV emission ratio is relatively low and comparable to the vapor method synthesized ZnO. The emission properties of ZnO nanowires were found to be greatly modified during the sulfurization and desulfurization treatment. The sulfurized ZnO sample contains more than 55% of ZnS forming on hybrid material. The bandgap emission of ZnO was found to redshift and the defect emission to be enhanced. The desulfurization treatment was found to further enhance the defect emission. The integrated defect emission has been increase by 42 times as compared to the emission of the as grown sample. The XPS analysis, EPR spectra and the oxygen plasma treatment suggest that the enhancement of the green emission comes from the generation of large amount of oxygen vacancies during the removal of sulfur atoms from the sulfurized ZnO. Low temperature PL study on the first desulfurized ZnO sample shows that the phonon-assisted excitonic emission at FX-2LO position can be enhanced by the presence of defects. A negative thermal quenching was clearly observed when the temperature rises from 50 K to 150 K. Noticeably, the integrated PL intensity at 150K is even higher than the total emission at 2 K. The internal quantum efficiency of the defect emission is as high as 33% at room temperature. For the second desulfurized sample, the evolution of the well resolved 3.31 eV shows that the defect bound excitons have a dominant contribution to this line. NTQ was found to start close to the temperature at which the 3.31 eV becomes dominant, which indicates that the defects can re-trap the excitons at higher temperature and the radiative energy relaxation is enhanced by the re-trapping effect.

Further investigations were conducted on the study of the optical properties of ZnO microrods, which are the byproducts of the CBD growth of ZnO. XRD patterns showed that these microrods have a wurtzite hexagonal structure and are highly crystalline. The ZnO microrods were further treated by a 900°C annealing. The annealed ZnO shows an enhanced defect emission and a well-resolved free exciton emission peak, which indicates that the crystal quality of the ZnO has been improved after annealing. Low temperature PL reveals that the annealing has increase the amount of defects in the microrods. By monitoring the evolution of defect bound excitonic emission and the green light emission, we confirmed that the negative thermal quenching is related to the thermal activation of green light emitting center at higher temperature. The IQE of the microrods has been improved about 5 times by the annealing. For the S doped ZnO, the defect emission is enhanced by 23 times, while the UV emission is almost zero. The green light emitting defects acts as efficient trapping centers and the IQE has been further improved.

According to many recent reports, the addition of the organic ligands helps adjust the pH value, slow down the release of OH -ion and passivate the lateral grown of the nanowires. During this thesis, Hexamethylenetetramine (HMTA) has been tested to improve the aspect ratio of the ZnO nanowires. Both of them are grown in a 0.025 M Zn(Ac)2 solution. As it can be noted, for the sample grown with adding HMTA, it took only 35 min to obtain 940 nm long nanowires, while without using HMTA it took one hour to grow 960 nm. The diameter for the former sample is even smaller. We can see that HMTA has passivated the lateral growth and improve the growth speed along the vertical direction. As we can see from the table P-1.1, the aspect ratio has been improved by the presence of HMTA. We have tried afterwards to grow for 6h. Nanowires can grow up to 2300 nm, with an aspect ratio about 27 (cf. The ligand (HMTA) assisted growth appears to be a good way to achieve long nanorods on a large scale. Further experiments need to be conducted to monitor the growth within the 6 h time. While wires longer than 2 microns have been obtained it seems that the growth is still limited. This aspect will need to be further investigated to understand what really limits the length of the nanowires. Most probably this has to do with the efficiency of the overall chemical reactions.

Towards a better understanding of the defect emission mechanism and related devices

For the desulfurized ZnO, we have confirmed that the green light emission is related to the oxygen vacancy efficiently generated through the desulfurization treatment. In order to directly characterize the oxygen vacancy and potentially additional point/structural defect types, high resolution transmission electron microscope should be conducted. FIB may be needed in order to slice the sample.

In terms of further optical characterization, cathodoluminesence should be useful to check the distribution of defect in the desulfurized ZnO. PLE may also help to see the details of the specific transitions of the excitons. To further understand the exciton trapping effect, lifetime of each assigned lines, including the D°X, defect bound exciton peak, green light emission peak, need to be measured. For the S doped ZnO Microrods, the existence of sulfur in the ZnO has not been identified. The mechanism of efficient green light emission should be further investigated by the characterization tools mentioned above. ZnO based green/white light phosphor can be synthesized by CBD method. Further control of the desulfurization, thermal annealing and doping may allow the amount of defects in ZnO to be controlled.

As an example of applications, Figure P-2.1 shows a picture of white light emission from a hybrid device realized using a UV LED (275nm) to excite the annealed ZnO microrods. A rather pure white emission is observed.

Integration of TMDMs on ZnO

The Graphene-like Transition metal dichalcogenides monolayers (TMDMs) have recently generated an entirely new field of study. When the bulk TMDs are thinned down to monolayers, numerous unique properties emerge, including direct bandgap, high fluorescence quantum yield, large binding energy, efficient heterojunction charge transfer, and access to spin polarized valleys, which indicate that the TMDMs have good potentials to be applied in a number of emerging areas such as superconductors, room temperature excitonic Bose-Einstein condensate, photovoltaics, light emitting diodes, transistors, and solid state memory [1]. Since the TMDMs are atomically thin, the influences of the substrate have been found to be particularly crucial [2]. In this thesis work, we have tried to do the conformal deposition of TMDMs on CBD ZnO nanowires by the method developed in the Molecular Foundry. The method consists in sulfurizing a transition metal oxide film deposited by atomic layer deposition (ALD).

The experimental details are presented in the appendix 4 [3,4].

Prior to the use of the ALD based technique, TMDMs have been transferred on the ZnO nanowires thin films by PMMA assisted thermal annealing transfer method in collaboration with the CINAP (SKKU -SK). The TMDMs were found to free stand on the ZnO nanowires. For the transferred TMDMs, the photoluminescence was strongly enhanced as compared to the same TMDMs deposited on SiO2 substrate [5]. The PL enhancement factor for MoS2 on the nanowires is as high as 150 and 50 for WS2.

The emission enhancements are mainly due to the strain relaxation while the monolayers have only point contacts with the nanowires. The ZnO nanowires act here only as a supporting substrate to preserve the optical properties of the TMDMs. In a recent report, Kim MS et al. [6] deposited TMDMs on ZnO films. When the 1 L-TMD/ZnO vertical heterostructure was excited by a 355nm laser, the TMDMs showed up to 17-fold increases as compared with the TMDMs on quartz. The underlying mechanism of this enhancement is completely different from the previous study and was interpreted as a charge transfer from the TMDMs to ZnO, which helped converting trions to neutral exciton in MoS2.

Whatever the substrates, a radiative energy transfer has not yet been observed. Our initial goal was to extent the GWET concept to the TMDMs and for this reason conformal deposition was preferred.

While this goal has not yet been achieved and sulfurization and desulfurization processes have been found to be very interesting, a few attempts for conformal deposition have been undertaken. and blue shifts as the excitation moving from area 1 to area 3. In the meanwhile, the Raman signal decreases and the E2g phonon mode shits. These results indicate that the transition metal oxide was indeed transformed during the chalcogenization process. This may be due to the humidity which in turns was too high in the CVD furnace. The Si contaminate acts as the accumulation center for the transition precursor. As it has been reported by C. Kastl et al. [3], the humidity should be carefully controlled, since the gas phase water content plays an important role in the volatilization and transport of metal oxide precursor in 2D TMDs growth. Most likely monolayers are expected to be found far away from the contaminant i.e. in the area 3.

Similar distribution of WS2 after conversion was observed while using the ZnO nanowires as the substrate (cf.

Méchanisme de croissance

Dans cette partie, on a étudié les mécanismes de croissance par la méthode de CBD en insistant sur l'effet de la couche de germe dont, la variation de taille de grain, de densité et d'épaisseur Cela peut s'expliquer à partir des degrés de liberté, la croissance latérale étant limitée dans l'espace (couches denses).

Modélisation

Après avoir optimisées les conditions de croissance des nanofils, on a cherché une méthode pour simplifier la caractérisation des couches minces de nanofils. La transmission optique qui reste une des techniques d'analyse les plus simples a été privilégiée. Dans cette partie, est présentée un modèle phénoménologique pour la modélisation des couches minces de nanofils aléatoirement distribués sur un substrat. Le Tableau 3-1, résume les paramètres utilisés qui de nouveau ont été déterminés à partir de la géométrie réelle des nanofils.

Cette simulation montre ici l'importance de la diffusion de volume qui traduit par une baisse constante du signal mais également par un effet de résonance observable vers 500 nm qui induit un creux dans la transmission.

Emission de défauts : Désulfurisation

Dans la partie précédente, on a étudié les propriétés optiques passives des nanofils de ZnO. Cette partie et la partie suivante, sont consacrées aux propriétés actives. Les études ont porté sur les spectres de photoluminescence (PL), qui a pu être renforcée par différents traitements après la croissance en solution. Pour mieux comprendre la nature excitonique de l'émission, on a étudié l'évolution de la photoluminescence en fonction de la température. La On voit que l'émission est constante quand on passe de 2 K à 50 K. En revanche on voit que lorsque l'on passe de 50 K à 150 K, l'intensité d'émission sur une échelle logarithmique augmente linéairement. Ce phénomène a été reporté dans la littérature, l'augmentation d'émission de défaut (Ou negative thermal quenching, NTQ) vient de la capture des excitons par des défauts. On voit que l'énergie d'activation de Ecole Doctorale "Sciences pour l'Ingénieur" Thèse réalisée en partenariat entre :

  Overviewxi green light has been largely enhanced. Low temperature photoluminescence study was performed on these two types of annealed microrods. External quantum efficiency (EQE) and IQE were compared to obtain the light extraction factors.

Figure I- 1 :

 1 Figure I-1: Low resolution SEM of the black silicon surface [7].

1  and 2 

 12 of the two materials or more precisely of the polarizability of the different elements. The weights are the volume proportions of each material.

Figure I- 2 :

 2 Figure I-2: Absorption curves of a-Si:H on ITO for different wavelengths (a) and angles (b) [9].

Figure II- 3 :

 3 Figure II-3: Nano slot, drawing (a) and SEM image (b)[11].

  Figure I-3) to demonstrate the guiding and confinement light in the nano slot structure.

Figure II- 4 :

 4 Figure II-4: Nano slot waveguide: (a) Three-dimensional profile of the field amplitude. (b) Two-dimensional contours of the logarithm of the normalized field amplitude [11].

  photonic band structures for Alumina rods arranged in a square or hexagonal lattices[16]. The yellow region in the Figure I-5 (b) represents the range of light frequencies where light propagation is inhibited whatever the polarization in the photonic crystal but this is possible only for the hexagonal structure. The square pattern only allows for the TM modes bandgap. This can be simply explained by the compacity (compactness) of the structure.

Figure I- 5 :

 5 Figure I-5: Energy band structures of alumina photonic crystal for square a) and hexagonal lattices b) [16].

Figure I- 6 :

 6 Figure I-6: Principle of Gain-assisted waveguiding energy transfer to enhance the absorption in the case of ultrathin luminescent films. [Patent 106080/FR (2009)].

  • Photonics crystal • Self-formed cavity Gain-Assisted Waveguiding Energy Transfer (GWET) Zinc oxide (ZnO) is a II-VI binary compound semiconductor. The crystal structures shared by ZnO are rocksalt, zinc blende and wurtzite [1]. Under ambient condition, the wurtzite structure of ZnO (cf. Figure II-1), formed by hexagonal close-packed Zn atoms interlaced with hexagonal close-packed O atoms, is the most thermally stable structure. The ZnO lattice has two parameters 𝑎 and 𝑐 in the ratio of c/a=1.633. The wurtzite structure of ZnO has a large direct band gap (~3.37eV) at room temperature and a large exciton binding energy (~60 meV). The excellent optical and electronic properties of ZnO has attracted many research attentions in the past decades, including many applications in optoelectronic devices, such as light emitting diode, ultraviolet lasers, chemical sensors and solar cells [3-20].

Figure II- 1 :

 1 Figure II-1: ZnO wurtzite structure with two lattice parameters a and c. Gray and yellow spheres denote O and Zn atoms, respectively [2].

Figure II- 2

 2 Figure II-2 shows a typical room temperature absorption spectrum (obtained from the transmission spectrum) and photoluminescence (PL) spectrum of a ZnO thin film sample grown by Chemical bath deposition (the details of this technique is presented in the next part II-2). The absorption of the sample increases rapidly when the incident light energy increases in the near bandgap range (~3.3 eV). The band to exciton absorption is clearly shown in the spectrum as well, which indicates that the exciton of ZnO is stable due to its large binding energy. The passive optical properties will be further discussed in chapter 3. The PL spectrum has a sharp near bandgap emission in the UV light range and a low defect related emission in the visible light range. The defect emission mainly comes from point defects, such as oxygen vacancy and zinc interstitials. More details about the defect emission will be discussed in Chapter 4 and Chapter 5.

Figure II- 2 :

 2 Figure II-2: Typical absorption and photoluminescence of a chemical bath grown ZnO nanowires thin film. The arrow indicates the position of the excitonic absorption edge.

  believed to get firstly formed in the solutions. The process for vertically aligned ZnO nanowires grown on substrate by chemical bath deposition is as follow: (a) Deposition of the seed layer of ZnO nanoparticles on the substrate; (b) Preparation of the precursor solution by mixing the ammonium with Zn(Ac)2 dissolved in deionized water. (c) Heating the solution up to a certain temperature. (d) Keeping the seeded substrate in the nutrient solution for a certain period of time. (e) Wash the sample after growth and dry.

   shows the procedure of the seed layer preparation. The seed layer solution is prepared by mixing 0.1M Zinc acetate (Zn(CH3COO)2• 2H2O) powder in ethanol. After stirring the solution for 10h, we filter it with a 200 nm filter. The seed layer is then spin coated on the target substrate (silicon wafer or quartz substrate). The spin coated substrate is afterwards annealed at a certain temperature for a few minutes. To obtain a thicker seed layer, we can repeat the spin coating and annealing procedures several times.

Figure II- 3 :

 3 Figure II-3: Seeding: (a) Schemes of the seed layer preparation procedure from left to right: Droplet of the seed layer solution, spin coating and annealing on a hot plat. (b) Representative AFM image of the spin coated seed layer after annealing at T = 400 °C for 4 min.

Figure II- 3

 3 Figure shows the typical topography images of a spin coated seed layer taken by AFM. The sizes of the ZnO nanoparticles called nanograins later are in the range of 5 nm to 50 nm. The average size depends on the annealing temperature, the number of time the spin coating is repeated, as well as the type of substrate, which will be discussed in the following sections III-3.

  Figure II-4 shows the typical setup of the CBD growth. The three necks flask is used to have the thermometer and sample holder in the solution. The seeded substrate is vertically placed in the solution.

Figure II- 4 :

 4 Figure II-4: Scheme and picture of the growth flask with the volume of 1 L. The volume of the flask has been upgraded to 2L later during the thesis work, to enable ZnO deposition on up to 4-inch wafers.

  Figure II-5 shows a plot of the pH value as a function of time for a 500 mL solution mixture of 0.035 M of Zn(Ac)2 and 0.6 mL of Ammonium. The initial change of value is due to the heating of the reactor and the precipitation. The pre-seeded substrate is introduced when the temperature reaches 87 °C. As indicated by the linear curves, the pH values show a first linear evolution due to the heating. A second linear evolution is then observed at constant temperature after the introduction of the substrate. This evolution is most probably due to the chemical reaction.

Figure II- 5 :

 5 Figure II-5: pH evolution as a function of time for a 500 mL of 0.035 M Zn(Ac)2 and 0.6 mL ammonium solution. Initial change of pH occurs during the first 20 min mainly because of heating the solution. Lines indicate the linear evolution of the pH before and after the introduction of the seeded substrate.

Figure II- 6

 6 Figure II-6 shows typical SEM images of the ZnO, namely nanowires and microrods, obtained from the CBD solution.Figure II-6 (a) and (b) shows the cross sectional and top views of typical, well-aligned

  Figure II-6 (a) and (b) shows the cross sectional and top views of typical, well-aligned nanowires samples. The ZnO microrods, which come from the nucleation in the solution, could be collected by horizontally placing a bare substrate. SEM observations (Figure II-6 (c)) showed that the ZnO microrods have varying number of arms and exhibited a regular hexagonal shape with smooth facets.

Figure II- 6 :

 6 Figure II-6: Typical SEM images of the ZnO materials deposited by the CBD method on a seeded substrate (a) and (b), and on a bare substrate(c). One obtains either nano (a and b) or microrods (c).

Figure II- 7

 7 Figure II-7 illustrates the XRD patterns of as grown ZnO nanowires sample on a pre-seeded silicon substrate, which revealed that the dominant diffraction plan measured at the grazing incidence angle is

Figure II- 7 :

 7 Figure II-7: Typical XRD pattern of the as grown ZnO nanowires measured at the grazing incidence angle.

  Figure II-8 (c) shows the reflection spectrum and the PL spectrum of one ZnO

Figure II- 8 :

 8 Figure II-8: Typical PL spectrum of the as grown CBD ZnO nanowires with different length: (a) ~600 nm. (b) ~900 nm. (c) ~1.9 µm.

Figure II- 9 :

 9 Figure II-9: Temperature dependent photoluminescence of the ZnO nanowires (from 2 K to 150 K): The near-band edge excitonic emission decreases much faster than the defect related emission as the temperature rises. Inset: UV emission of ZnO nanowires at T=2k (black curve) and T= 300K (red curve).

Figure II- 10

 10 Figure II-10 shows the SEM image of the seeds, the top view and the cross-sectional view of the grown nanowires. Images of the samples with different coating repetition number are displayed in two columns. As seen from the SEM images, the average grain size of the seed layer obtained when the

Figure II- 10 :

 10 Figure II-10: Seed layer dependence on the number of spin coating deposition, a) spin coating repeated twice and b) 6 times. SEM images taken from the top and cross-sectional view of the nanowires after the growth are displayed below the corresponding seed layer.

Figure II- 11 :

 11 Figure II-11: Variation of the seed layer annealing temperature: Layer annealed at 200°C (a), 400°C (b) and 550° (c). The cross-sectional and top view SEM images of the nanowires after the growth in each column corresponds to a given annealing temperature.

Figure II- 12 :

 12 Figure II-12: Volume of the ZnO deposited (i.e. quantity of ZnO nanowires deposited) as a function of the size of the substrate for a growth time of 1 hour. As evidenced by the blue line a linear increase is observed before the curve start to saturate. The dotted line is the result of a fitting using a saturation curve.

Figure II- 13 :

 13 Figure II-13: Comparison of CBD growth on silicon (a) and quartz (b). AFM images of the seed layer are displayed above the top and cross-sectional SEM views of the nanowires.

Figure II- 14 .

 14 Figure II-14. Time dependent growth evolution on quarter of 3-inch silicon wafers: Photos of the sample after growth, SEM images taken from the cross-sectional and top views of the nanowires after the growth are respectively displayed below the corresponding growth time.

Figure II- 15

 15 Figure II-15 shows the time dependent evolutions of the parameters, including the length, the

Figure II- 14

 14 ). The saturation time for the counted number and the aspect ratio is 15 min and 17 min, which are very close. The saturation number is 167 and the saturation value for the aspect ratio is 14.

Figure II- 15 :

 15 Figure II-15: Time dependent growth for the silicon substrates with the size of a quarter of 3-inch wafer: (a) Length and diameter of the nanowire. Blue and black dash lines denote the exponential (saturation) corresponding fitting curves. Inset table: fitted 𝜏 and fitted 𝑦 𝑠 (b) Plot of the counted number of nanowires and aspect ratio of the nanowires. Blue and black dash lines denote the exponential (saturation) corresponding fitting curves.

  Figure II-16, the substrate was vertically disposed in the solution, the part held by the sample holder was close to the liquid/air interface and the other side of the substrate was close to the bottom of the growth flask.

Figure II- 16

 16 Figure shows that the sample has green and red colors, which come from the interferences within the top and bottom interfaces of the thin film i.e. air/ZnO and ZnO/substrate interfaces as explained in the previous section. The color strips are denser on the upper part of the sample i.e. closer

Figure II- 16

 16 Figure II-16: 4-inch wafer scale CBD growth: (Left) CBD grown nanowires on a 4-inch silicon wafer. The defect strip close to the ruler is the area hold by the sample holder. (center and Right) up-grated setup parts including the sample holder and the balloon heater.

6 )

 6 increases from 10 min to 20 min as the nanowires get larger.

Figure II- 17 .

 17 Figure II-17. Time dependence growth study on silicon substrates with the size of a full 3-inch wafer: Length and diameter of the nanowire. Blue and black dash lines denote the exponential (saturation) curve corresponding fitting curves. Inset table: fitted 𝜏 and 𝑦 𝑠 values.

1. 5

 5 as shown on Figure II-8 b).

Figure II- 18 :

 18 Figure II-18: Time dependence evolution of the loading efficiency of the Zn 2+ ion, which is obtained by dividing the mass of the ZnO nanowires by the initially added Zn 2+ ion mass: (a) Evolution of the Loading efficiencies for growth conducted on full 3-inch substrates and one quarter 3-inch size substrates. Blue and black dash lines denote the exponential fitting curves for the smaller size and bigger size substrates respectively. The Ratio between the black and blue fitting curves is shown in (b).

  Figure II-18 (a), the loading efficiency of the deposited ZnO on 3-inch wafer is only 1.5 times more than in the case of the quarter size substrate for one hour growth. Then if we assume that the density of the ZnO nanowire thin films is a constant value, we can easily estimate the volume of the deposited ZnO on the 3-inch wafer after one-hour growth knowing the volume of the deposited ZnO on a quarter size substrate. Figure II-19 shows the updated figure of the volume to surface plot, the circled point corresponds to the ZnO volume on the 3-inch wafer with the growth time of one hour. We extended the exponential fitting obtained in Figure II-12 and found the deposited volume on a full 3-inch wafer was already very close to the saturation volume, which indicated that we could not expect to deposit much more ZnO nanowires by further increasing the size of the substrate nor by further increasing the growth time.

Figure II- 19 :

 19 Figure II-19: Volume of the deposited ZnO (i.e. quantity of ZnO nanowires deposited) as a function of the size of the substrate. The circled point was obtained from the study on 3-inch size substrate.

Figure II- 20 :

 20 Figure II-20: Homogeneity of the seeds, left photography of the seeded coated substrate and right and bottom, top view SEM images of the seeds layer and corresponding site number. Scale bar is 100 nm.

  Figure II-21 shows the pictures and thicknesses of the ZnO nanowires samples for different growth times. The 40 min growth sample was found to be the most homogeneous in terms of surface color uniformity. Careful reflectivity analysis (cf. chapter 3) reveals that the thickness deviation is in the range of 25 nm to 50 nm for an initial layer thickness of 600 nm. One could explain the better uniformity of the 40 minutes growth sample by the nanowires size dispersion. Because of the merging of neighbor nanowires the deviation in length decreases as a function of the growth.

Figure II- 21 :

 21 Figure II-21: Homogeneity of the growth on the 3-inch silicon wafer for 4 different growth times (10, 20, 30 and 40 minutes). Photography of the wafers and evolution of the thickness as retrieved by reflectivity analysis. Each point corresponds to one reflection spectra analysis (cf. appendix for further details on the analysis).

A

  phenomenological model has been developed and discussed. The model is able to describe the experimentally measured light transmission of nanowire arrays. Two representative samples were studied in this report. A well aligned nanowire sample was first modelled by an effective layer model with surface roughness added. Then a slightly tilted nanowires sample with round top was phenomenologically modelled by a two layers model with a first top layer of an effective medium which includes a volume scattering contribution (Mie theory). Roughness and waviness are added to account for the nanowires small and large scale fluctuation in height. This phenomenological description was proven to be feasible by fitting the experimental data. As a conclusion, light transmitted through randomly distributed nanowires can be explained by the combination of volume and surface scatterings using respectively Mie theory and rough Fresnel coefficients at the interfaces.

  this work, we have developed a combined model to phenomenologically model randomly distributed nanowire thin film. Figure III-1 shows the sketches of the sample (left) and the model (right). We modeled the nanowire by two effective medium layers with the first top layer that includes Mie contribution accounting for the light scattering by the top of the nanowires. Roughness and waviness are added to account for the nanowires small and large scale fluctuations. Parameters have been chosen to be directly linked to the real geometry of the sample. In the phenomenological model, the average diameter of the nanowires d corresponds to the diameter of the spheres, the average height of nanowires e corresponds to the sum of the first and second layer thicknesses, em + d, the small scale height fluctuations ξ corresponds to the roughness amplitude ξm and finally the large scale height fluctuations w corresponding to the waviness parameter 𝜎 (see Eq. (23)).

Figure III- 1 .

 1 Figure III-1. Phenomenological modelling of nanowires thin film: Sketches of the sample (left) and the used model (right). Volume scattering is accounted by the effective sphere layer whereas the large scale and small scale (wavelength scale) nanowire length fluctuations are accounted respectively by the waviness and roughness effect.

0.

  035M zinc acetate was dissolved in 750 mL of deionized water. 9ml of ammonium hydroxide was added to this solution. The ZnO seed layer pre-coated quartz substrate was immersed in this solution for one hour at 87℃ (details have been presented in Chapter II). The second ZnO nanowire sample was obtained by electrochemical deposition (ECD) method, in the following condition: C[Zn(ClO4)] = 0.2 mM, Q = 23.81 C.cm -2 and molecular oxygen bubbling (details of the deposition technique and conditions can be found in the literature[16, 17]), The sample was grown on a FTO thin film coated glass substrate, the thickness of the FTO layer was about 1 µm.The morphologies of the samples were characterized by field-emission scanning electron microscope (FEG-SEM Hitachi SU8030 at 10 kV) and Atomic Force Microscope (AFM 5100 Agilent). The optical transmission curves of the ZnO nanowires samples were measured using a UV-VIS-NIR spectrophotometer (Cary 100) with a spot size of 2 mm × 5 mm, which is much bigger than the size of the nanowires and the distance between them. In this regard, the studied nanostructure samples in this work have large scale uniformity compared to the spot size. Moreover large scale homogeneity has been assessed by measuring the transmission on different spots of the sample.

  Figure III-2 (a)) shows that the nanowires of sample 1 were relatively well aligned and perpendicular to the substrate. They were closely compact, with a diameter and length in the order of 50 nm and 810 nm respectively.Figure III-2 (a) displays the transmission curve of sample 1. This curve shows optical fringes in the visible range and a sharp decrease of the transmission at shorter wavelengths, which is attributable to near band edge absorption. Those features are typical of the transmission of a ZnO thin film.

Figure III- 2 :

 2 Figure III-2: Optical transmission of two representative typical nanowires sample: vertical aligned nanowires with flat top surfaces and tilted nanowires with round top. (a) Transmission curve of the CBD grown ZnO nanowires prepared on a quartz substrate. (b) Transmission curves of the FTO coated glass substrate and the ECD grown ZnO nanowires array sample. The inserted images are respectively SEM images of the corresponding samples with repective scale bar.

Figure III- 3 :

 3 Figure III-3: Experimental and corresponding theoretical transmission curves of the sample 1 using the rough surface model (cf. inset) including waviness represented by the dotted line.

Figure III- 4

 4 Figure III-4 (a) shows the calculated transmission curves of a flat substrate and a rough substrate. The transmission of the rough substrate is lower and shows less contrast in the fringes, which is due to the scattering by the surface roughness. The simulated curve of the rough substrate fits better with the experimental data.

Figure III- 4 :

 4 Figure III-4: Parameters assessment: (a) Calculated optical transmission curves of the flat substrate model, the rough substrate model and the experimental data. The inset represents the refractive index (real and imaginary) of FTO, the x axis is the same as the one for the transmission curve. (b) SEM image of the FTO. (c) AFM image of the FTO layer surface. (d) Transmission curves of the rough surface model (cf. inset) with thickness waviness of the FTO thin films as to compare with the experimental data.

   shows the calculated optical transmission curves for three simple models. The first curve is calculated from the FTO coated substrate plus two effective ZnO layers. For the top layer, the thickness 𝑑 = 220 nm and density 𝑓 1 =58%, and for the second layer, the thickness is 𝑒 𝑚 = 480 nm and the density is 𝑓 1 =70%.

  Figure III-5 (a) is the transmission of the substrate plus two ZnO effective layer with roughness amplitude ξm=48 nm in the top layer and total waviness 𝜎= 120nm. Compared to the case without roughness and waviness, this curve has lower transmission and the contrast of the fringes is much less. Both of the two curves have no transmission in UV ranges because of the absorption of the ZnO layers. The third curve is the transmission calculated for the substrate plus an effective layer including Mie contribution which has the equal thickness and diameter of 𝑑 = 220 nm, density of sphere 𝑓 1 =58% estimated from the ratio between ZnO and air from the SEM image (inset of Figure III-2 (b)), dispersion of the diameter 𝜎 𝑠𝑝 =7nm. This curve shows that a dip at around 500 nm can be explained by Mie resonance that will induce a drop in the transmitted intensity as shown in the measured transmission of sample 2.

Figure III- 5 :

 5 Figure III-5: Combined model fitting: (a) Calculated optical transmission curves for the three models: FTO coated substrate plus two effective ZnO layers, FTO layer plus two ZnO effective medium layers with roughness and waviness applied, FTO layer plus a nanosphere layer. The arrows indicate the drop of transmission in the UV and visible range which respectively the ZnO absorption and light scattering by the nanowires top part. (b) Transmission curve calculated by the combined model (inset image) and corresponding experimental data.

  our model, nanospheres are used to simplify the description of the top part of the nanowires, while the different nanowire lengths are accounted via the surface roughness. The volume scattering and surface scattering effects are considered as the main factors for light scattering in the nanowires array. In addition, we add a ZnO thin film layer on the FTO layer to absorb the transmitted UV light through the first top layer. Furthermore, the waviness, thicknesses, and diameters that are applied in the model lead to a remarkable fit for the fringes contrast in the transmission curves. The goal behind the use of the simplified model is to explain that light interaction within the rather complicated sample like nanowire arrays can be simplified and considered as the combination of Mie and rough surface scatterings.

  Scheme IV-1: Desulfurization treatment procedure in chemical vapor deposition reactor and the sample schemes after each step. The proportions of the materials for the as grown sample and sulfurized sample are 70% ZnO + 30% Air and 56 % ZnS + 19 % ZnO + 25 % Air respectively, which have been deduced from quantitative transmission analysis. During desulfurization, the ZnS are removed from the desulfurized ZnO, which leaves many defects in the desulfurized ZnO.

Figure IV- 1

 1 Figure IV-1 (a) illustrates the XRD patterns of as grown and sulfurized ZnO sample, which reveals both samples have (002) and (103) planes of the ZnO at around 34.1° and 62.4°. Apart from the ZnO diffraction peaks, the sulfurized sample have two extra diffraction peaks at 28.52° and 51.82°, which closely match with the (002) and (103) crystalline planes of the hexagonal structure of ZnS in the Crystallography open database CIF1101051. The similar crystal structure of ZnS as compared to ZnO is an indication of the replacement of oxygen with sulfur atom during the sulfurization process. For hexagonal ZnS, the lattice constant along c axis is 6.62 Å, which is larger than that of hexagonal ZnO (c=5.2 Å).Figure IV-1 (a) and (b) show the top view of the as grown and sulfurized ZnO nanowires. Due

  Figure IV-1 (a) and (b) show the top view of the as grown and sulfurized ZnO nanowires. Dueto ZnS has larger lattice constant, the sulfurized sample nanorods have larger average diameter and thus less space between the rods is available, as compared to the as grown sample.

Figure IV- 1 :

 1 Figure IV-1: Sulfuration characterization: (a) XRD patterns of the as grown ZnO and sulfurized ZnO. The point symbols denote the (002) and (103) crystalline planes of the hexagonal structure of ZnO and the star symbols for that of ZnS. (b) and (c) are the SEM image of as grown ZnO and sulfurized ZnO respectively.

Figure IV- 2 :

 2 Figure IV-2: Porosity and proportion assessment: Transmission analysis of the as grown sample (a) and the sulfurized sample (b). For both images, black dots and red line correspond to the experimental and stimulated curves respectively.

Figure IV- 2

 2 Figure IV-2 (a) is calculated based on the following parameters: the thickness of the ZnO is 450 nm, the roughness amplitude is 25 nm, and the volume fraction ZnO is 70%. This optimized curve fit well with the measured curve, while the thickness is known from SEM image and we found that the volume fraction is fairly comparable to the density of ZnO nanowires. As for the sulfurized ZnO, by optimizing the fitting (cf. Figure IV-2 (b)), we found that the volume fraction of ZnS, ZnO and Air is about 56%, 19% and 25% respectively. The thickness is about 660 nm, which is comparable to the measured value as well. This calculation indicates that ZnO is quiet sensitive to the H2S gas, after sulfurization, ZnS occupies big proportion in the ZnS/ZnO core-shell structure.

Figure IV- 3 :

 3 Figure IV-3: PL spectra and bandgap of the sample before and after the sulfurization: (a) Room temperature PL spectra of the as grown and sulfurized sample. The inset represents the UV emission of the two samples, the y axis is the same as the one for the PL spectera. (b) Tauc plot of the as grown and sulfurized samples. Clearly two slope changes can be observed in the sulfurized sample, which induced by the existence of two phases composed of ZnO and ZnS.

  Room temperature PL was conducted to observe the optical properties of the sulfurized ZnO. As shown in the Figure IV-3 (a), the UV emission of the sulfurized sample largely decreased and there is an obvious redshift as shown in the inset figure, which is similar to that reported by Y. Yoo et al. [12] [13] caused by the size difference of the S and O atoms, the bandgap was bowing as O atoms were substituted with S atoms. The redshift is further evaluated by the Tauc plot (inset of Figure IV-3 (b)), while the sulfurized ZnO has another obvious bandgap at 3.4 eV which corresponds to the bandgap transition of ZnS.

Figure IV- 4

 4 Figure IV-4 (a) shows the XRD patterns of the desulfurized sample, the diffraction pattern of the as-grown (S1) and the sulfurized sample (S2) are plotted for comparison. The desulfurized ZnO (S3) does not exhibit the specific peaks of ZnS, which indicates that the ZnS is removed from the sample after being treated in the Air at 550°C. As shown in the Figure IV-4 (b), from the top view, the desulfurized ZnO and sulfurized ZnO have very similar porosity, and the desulfurized ZnO is denser as compared to the as grown one. Since the sulfurization has expanded the lattice of the ZnO, the desulfurization may have larger volume than the as grown ZnO, which suggests that many defects are present inside the desulfurized ZnO. The Figure IV-4 (c) shows the cross section view, the desulfurization process split the nanowires which results in even larger surface to volume ratio, hence more surface defects.

Figure IV- 4 :

 4 Figure IV-4: Desulfurization characterization: (a) XRD patterns of the as grown ZnO, sulfurized ZnO and the desulfurized sample. The point and star symbols denotes the (002) and (103) crystalline planes of the hexagonal structure of ZnO and ZnS. (b) SEM image of the desulfurized sample. (c) Cross-sectional view.

Figure IV- 5

 5 Figure IV-5 shows the PL emission of the desulfurized ZnO. The green light emission which is centered at about 510 nm is largely enhanced as compared to the as grown one, while the UV emissin decreases a lot. The inset figure shows that the UV emission is at the same position as the one of the as grown sample, which may be due to the elimination of the bowing effect by the desulfurization. The integrated defect emission from 410 nm to 829 nm is enhanced by 5 times compared to the emission of the as grown sample, while the emission is enhanced by 42 times compared to the emission of the as grown sample. To our knowledge, this ratio is the highest that have ever been reported, which indicates very efficient energy transfer from UV toward visible emission.

Figure IV- 5 .

 5 Figure IV-5. Room temperature PL spectra of the as grown, sulfurized and desulfurized ZnO. The inserted figure is the PL spectra represented in log scale, the x axis is the same as the one used for the linear plot.

Figure IV- 6 :

 6 Figure IV-6: Visible white light emission observable at room temperature in ambient air and the chromaticity value of the emission.

Figure IV- 7 :

 7 Figure IV-7: XPS profile analysis as a function of the etching time, the etching speed was 8.3 nm/min.

Figure IV- 8 :

 8 Figure IV-8: Low temperature EPR of the silicon substrate, as grown ZnO and desulfurized ZnO.

  treatment. The desulfurized ZnO has been exposed to the oxygen plasma for 5 min. The SEM images of the samples before and after the treatments are shown in Figure IV-9. No significant morphology changes occur after oxygen plasma treatment. From the PL spectra measured before and after the treatment (cf. Figure IV-9 (c)) we can observe that the green emission is largely decreased, which is another indication of the correlation between the green emission and the oxygen deficiency.

Figure IV- 9 :

 9 Figure IV-9: Oxygen plasma treatment effect: SEM images of the sulfurized ZnO before (a) and after (b) being exposed to the oxygen Plasma. (c). PL before and after the oxygen plasma treatment.

Figure IV- 10 :

 10 Figure IV-10: PL spectra of the as grown and desulfurized ZnO nanowires at 2K.

Figure IV- 11 :

 11 Figure IV-11: UV emission spectra taken from 2 K to 50 K and 70 K to 300 K are plotted respectively and vertically displaced for clarity. D°X and A line denote for the donor bound exciton peak and 3.31 eV line respectively. LO is for the longitudinal-optical phonon energy, which is around 72 meV for ZnO.

Figure IV- 12 :

 12 Figure IV-12: The evolution of integrated defect emission and the UV emission as a function of reciprocal temperature.

Figure IV- 12

 12 Figure IV-12 shows another interesting phenomenon of the PL emission, the NTQ, which describes that the emission increases as the temperature increases. The evolution at each measured temperature

Figure IV- 13 :

 13 Figure IV-13: The internal quantum efficiency of defect emission as a function of reciprocal temperature.

Figure IV- 14 .

 14 Figure IV-14. Energy level diagrams summarizing the various mechanisms to explain the dominant 3.31 eV band -1LO line at 70K and the negative thermal quenching observed for green emission by proposing the diagram at 100 K. The dotted arrows represent for the nonradiative channels, while the red dotted arrow represents the re-trapping effect of thermally activated exciton by the oxygen vacancies.

Figure V- 1

 1 Figure V-1 shows the morphologies of the as-grown ZnO microrods using SEM imaging. Silicon substrate serves here only as a support for these microrods. The average length of the microrods is observed to be 10-15 µm and 1-5 µm in diameter. One can clearly observe microrods with two (birods), four (tetrarods), six arms (hexarods) and cross-linked structures. Inset in the upper left hand side corner in Figure V-1 is an enlarged image of a few microrods. The microrods are observed to be perfectly hexagonal in structure and with very smooth sidewalls. However, after annealing at 900°C, formation of pores is seen to occur on these arms, as seen in the inset in the lower right hand corner.

Figure V- 1 :

 1 Figure V-1: SEM image of ZnO microrods consisting of two, four or six arms. Inset in the upper corner shows a zoom in image of the perfectly hexagonal microrods before annealing and in the lower corner after annealing where nanopores (holes) can be seen. (This figure is from Dr. Anisha Gokarna)

Figure V- 2 :

 2 Figure V-2: XRD spectra of as grown microrods on silicon and 900°C annealed microrods. (This figure is from Dr. Anisha Gokarna)

Figure V- 3 :

 3 Figure V-3: Entire Raman spectra of the microrods before and after annealing.

Figure V- 4 :

 4 Figure V-4: Room-temperature PL spectra of the (a) as-grown and (b) 900°C annealed microrods.

  Figure V-5 presents a comparison between the PL spectra of the as-grown and annealed sample taken at 2K. As seen in the Figure V-5, the UV emission of the as-grown and annealed ZnO microrods at 2K is dominated by a strong emission peak at ~3.36 eV, which can be attributed to neutral donor exciton transition (D°X), as was earlier reported by Reynolds et al. [12].

Figure V- 5 :

 5 Figure V-5: PL spectra of as grown and annealed ZnO microrods at 2K. The spectra are plotted in logarithm scale and vertically displaced for clarity. The inset figure shows the two spectra in linear scale.

Figure V- 6 :

 6 Figure V-6: (a) Evolution of PL spectra of the annealed microrods as a function of temperature. The spectra at temperature from 2 to 70 K are plotted in logarithm scale and vertically displaced for clarity. PL spectra from 90 K to 300K are plotted in linear scale and vertically displaced for clarity. (b) Evolution of the different peak positions as the function of the temperature.

  Figure V-6 (b) as a function of the temperature. The evolution of the main excitonic peaks (D°X, Y line, FX and 3.31eV band) positions follow the temperature dependence evolution described by Varshni's equation. At temperature between 2K and 70K, the neutral donor bound exciton and structural defects bound exciton dominate the UV emission. At T>70K, the structural defects bound exciton and donor bound exciton are thermally dissociated and get trapped by the surface defects, resulting in the dominance of surface defect bound excitons.

Figure V- 7 :

 7 Figure V-7: (a) Defect emission from 70 K to 200 K (40 K, 50 K, 70 K, 90 K, 120 K, 150 K, 175 K, 200 K, 250 K, 300 K ).

Figure V- 8 :

 8 Figure V-8:PL spectrum of sulfur doped ZnO microrods taken at 5 K. Inset: Room temperature PL spectrum of as grown and sulfur doped ZnO microrods.

Figure V- 8

 8 Figure V-8 shows the PL spectra of anther annealed ZnO microrods at 5K and room temperature (c.f. inset figure). For this sample, we annealed the microrods with the same condition, but together

Figure V- 9 :

 9 Figure V-9: Temperature dependent PL Spectra: (a) whole spectra. (b) Detail of the UV emission peaks. (c) Integrated intensity of defect and UV emission as a function of reciprocal temperature. (d). IQE of defect emission as a function of reciprocal temperature.

Figure P- 1 .

 1 Figure P-1.1 shows the two ZnO nanowires samples grown on two quarters of 3-inch silicon wafers.

  Figure P-1.3).

Figure P- 1 . 3 :

 13 Figure P-1.3: High aspect ratio obtained by the use of HMTA as the ligand.

Figure P- 2 . 1 :

 21 Figure P-2.1: White light emission from a hybrid device realized using a UV LED (275nm) as a source to excite the annealed ZnO microrods

Figure P- 3 . 1 :

 31 Figure P-3.1: WS2 deposited on SiO2 substrate: (a) SEM image around a Si contaminant and the corresponding area as observed by an optical microscope. SEM Images of WS2 flaks and crystals in the area 1 (c), area 2 (c) and area 3 (d). The scale bars are 200 nm.

  Figure P-3.1 (b) (c) (d) shows that area 1 corresponds to the largest WS2 flaks with clear edges and typical triangle shape. WS2 flaks in area 2 are the smallest, but there exist many WS2 crystals, which indicated that more material accumulates in this area. The area 3 shows middle size flasks.

Figure P- 3 . 2 :

 32 Figure P-3.2: PL (a) and Raman spectrum (b) taken from the area 1, area 2, and area 3 respectively (cf. figure P-3.1). Both PL and Raman signal are normalized to the Raman signal of silicon.

Figure P- 3

 3 Figure P-3.2 shows the PL spectra and Raman spectrum taken from these three areas, by using the 532 nm green laser as the excitation light and the spot size was around 2 μm. The strongest PL and Raman intensities have been observed in the area 1, which indicates that the WS2 flasks in this area have the best crystal quality and thus the most intense emission intensity. The PL emission decreases

Figure P- 3 . 3 )

 33 . The PL emission peaks are located at 1.87 eV, corresponding to the peak position of the signal taken in the area 2 of the previous sample around the Si contaminant. This indicates that the conversion on ZnO nanowires has formed more WS2 bulk crystal and multilayers than the WS2 monolayers. The broad emission measured from the point 2 may be due to the defect emission from the sulfurized ZnO, which need to be further identified. The Raman signals shows the typical phonon mode coming from the bulk WS2.

Figure P- 3 . 3 : 2 . 2 . 1

 33221 Figure P-3.3: WS2 deposited on ZnO nanowires: (a) SEM image around a contaminant and corresponding area as observed by the optical microscope. PL (b) and Raman spectra (c) taken from the area 1, area 2, and area 3 respectively as identified on the SEM image. Both PL and Raman signal are normalized to the Raman signal of silicon.

Figure 2 . 1 :

 21 Figure 2.1 : Dépôt par bain chimique (CBD) : (a) procédure d'enduction (spin coating), (b) ballon de croissance d'un volume de 1 litre. La taille du ballon a été ensuite augmentée à 2 litres pour effectuer des dépôts sur substrat de 4 pouces.

Figure 2 . 4 .

 24 1 (a)). 2. Préparation de la solution de précurseur en mélangeant l'ammonium avec de l'acétate de Zinc, Zn(Ac)2 dissout dans de l'eau désionisée. 3. Chauffer la solution à une certaine température (inférieure à la température d'ébullition). Conserver le substrat dans la solution mère pendant un certain temps (c.f Figure 2.1 (b)). 4. Laver l'échantillon et le laisser sécher. La figure 2.2 montre un exemple de couche de nanofils obtenus par croissance sur germes déposés sur silicium. La figure d) montre une sédimentation sur substrats de microstructures obtenues par nucléation en solution. En plus de la taille, la différence est notable grâce à la forme des structures, la croissance des microrods se faisant par le centre des structures. Les diamètres (contour hexagonal) typiques des nanorods et microrods sont respectivement entre 20 et 150 nm et 1 à 2 microns.

Figure 2 . 2 :

 22 Figure 2.2: Images MEB de nanofils et micro-batonnets de ZnO obtenus par CBD, (a) couche de germes de nanoparticules de ZnO deposée par la méthode d'enduction, (b) et (c) nanofils de ZnO obtenus par CBD, (d) micro-bâtonnets de ZnO venant de la nucléation en solution.

Figure 2 . 3 :

 23 Figure 2.3 : Spectres de photoluminescence de nanofils de ZnO en fonction de la température. Encart : Emission UV mesurée à 2 K (courbe noire) et 300 K (courbe rouge).

2. 3

 3 Croissance sur substrat de 3 pouces.La figure 2.3 montre un exemple de wafer 3 pouces entrièrement recouvert de nanofils et confirme la possibilité d'une croissance sur une telle surface. On observe des effets de couleur liés à une variation de l'épaisseur optique du matériau (franges d'interférence). A noter que l'oeil est très sensible à ces effets. L'analyse optique des couches minces sera détaillée au chapitre 3. Une étude systématique de la hauteur des nanofils par ajsutement de courbes de réflectivité mesurées localement permet de montrer que la variation d'épaisseur est faible hormi les effets de bord bien connus qui sont liés principalement au dépôt par enduction. Une épaisseur constante sur une distance de 3 cm est observée.

Figure 2 . 2 :

 22 Figure 2.2 : Croissance CBD à l'échelle d'un substrat de 3 pouces. Gauche) Photographie d'un substrat de 3 pouces après croissance dans le réacteur de 2l. Droite) longueur des nanofils en fonction de la distance du bord (longueur mesurée par microscopie électronique à Balayage) .

Figure 2 - 3 .

 23 Figure 2-3. : Evolution de la longueur et du diamètre des nanofils sur substrat de silicium de 3 pouces en fonction du temps. Les lignes bleu et noire en pointillés correspondent à des ajustements exponentiels (courbe de saturation).

Figure 3 - 1 :

 31 Figure 3-1: Modélisation phenoménologique d'une couche mince de nanofils: Dessin de l'échantillon (gauche) et modèle utilisé (droite). Cf. le texte pour le descriptif des paramètres.

Figure 3 - 2 :

 32 Figure 3-2: Transmission mesurée et courbe de transmission calculé du premier échantillon utilisant un modèle simple incluant simplement des fluctuations d'épaisseur (roughness et waviness) identifiées par les lignes en trait plein et pointillé (cf. l'encart). Une vue en coupe MEB de l'échantillon est montrée à droite du graphe.

Figure 3 - 3 :

 33 Figure 3-3: Transmission mesuré e et courbe de transmission calculé e pour le deuxiè me é chantillon utilisant la modèle combinée (cf. l'encart). Une vue en coupe MEB de l'échantillon est montrée à droite du graphe.

Figure 4 - 1 :

 41 Figure 4-1: Procédé de désulfurisation dans un réacteur et représentation schématique des échantillons après chaque étape.

Figure 4 - 2 .

 42 Figure 4-2. Etude de l'effet des procédés de sulfurisation et désulfurisation sur la photoluminescence à température ambiante pour deux échantillons représentatifs.

Figure 4 - 3 :

 43 Figure 4-3 : Emission en fonction de la température et de l'inverse de la température pour le premier échantillon désulfurisé, a) comparaison des spectres pris à T= 2 K et 150 K et b) évolution de l'intensité intégrée de l'émission visible (courbe verte) et UV (courbe violet).

Figure 4 -

 4 photoluminescence en fonction de la température. La Figure 4-3 (a) montre que l'émission de défaut dans le cas du ZnO sulfurisé est plus forte à T=150 K par rapport à T=2 K. L'évolution en fonction de la température de l'intensité d'émission des défauts et de l'émission UV est montrée sur la Figure 4-3 (b).

Figure 5 - 1 .

 51 Figure 5-1. Micro-bâtonnet de ZnO : (a) Image MEB avec dans l'encart gauche un zoom de la face avant d'un bâtonnet et dans l'encart en bas à droite un zoom sur des micro-bâtonnets traités par recuit. (b) Spectres DRX de l'échantillon directement après croissance et traité thermiquement. Les deux courbes ont été déplacées par souci de clarté.

Table I

 I

-1 summaries of the different approaches to achieve enhanced optical properties.

Table I - 1 :

 I1 Summary of the different approaches to achieve enhanced optical properties.

Table II -

 II 2: Length, diameter, aspect ratio and the number of rods and corresponding annealing temperature of the seed layer ((The arrear bar of length and diameter is estimated to be 30 nm and 15 nm respectively).

	Annealing temperature	Length
	(°C)	(nm)	Diameter (nm)	Aspect ratio (L/D) Density (/2 µm 2 )
	200	283	38	7.11	383
	400	312	40	7.8	285
	500	360	44	8.1	285

Table II -

 II

	Number	Surface (mm 2 )	Length (nm)	Diameter (nm)	Aspect ratio (L/D)
	1	270.8	705	55	12.8
	2	398.1	690	52	13.2
	3	705.8	655	48	13.6
	4	943.8	600	46	13.0
	5	1634.9	510	45	11.3

3:

The difference in the size of the substrate: The length, diameter, and aspect ratio for the corresponding size of the silicon substrate. (The arrear bar of length and diameter is estimated to be 40 nm and 15 nm respectively.).

Table II -

 II 3 lists the average length, average diameter, aspect ratio of the nanowires corresponding

Table II -

 II

	Substrate	Length (nm)	Diameter (nm)	Aspect ratio (L/D)
	Silicon	1031	82	12.5
	Quartz	1200	121	9.8

4. The length, diameter, and aspect ratio corresponding to the types of substrate (The arrear bar of length and diameter is estimated to be 50 nm and 25 nm respectively).

Table II -

 II 5: Time dependent growth of the quartz substrates with the size of a quarter 3-inch size wafer: The length, diameter, aspect ratio and corresponding growth time (The arrear bars are estimated).

	Growth time (min)	Length (nm)	Diameter (nm)	Aspect ratio (L/D)
	30	610 ± 40	52 ± 15	11.8
	60	1200 ± 50	121 ± 30	9.8
	150	1900 ± 60	233 ± 40	8.1

Table II -

 II

	Growth time (min)	Length (nm)	Diameter (nm)	Aspect ratio (L/D)
	10	333	38	8.7
	20	702	42	16.4
	30	658	47	13.4
	40	640	44	14.9
	80	710	53	13.3

6: Time dependent growth of the silicon substrates with the size of a full 3-inch wafer: The length, diameter and aspect ratio and corresponding growth time. (The arrear bar of length and diameter is estimated to be aournd 40 nm and 15 nm respectively).

  46]. L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P. Yang,

	ZnO seed layers and related nanowires grown by wet chemical deposition, Nanotechnology, 28
	(2017) 095704.
	Low-Temperature Wafer-Scale Production of ZnO Nanowire Arrays, Angewandte Chemie
	International Edition, 42 (2003) 3031-3034.

For that we tried the easiest optical characterization technique i.e. transmission measurement. Results are the subject of the next chapter.

[

Table III -

 III

	Measured parameters	Corresponding retrieved parameters
	Thickness of FTO layer: About 1 µm		Thickness of FTO layer: 1.01 µm
	Roughness of FTO layer surface:		Roughness of FTO layer surface:
	29.5 nm (root mean square)		21 nm (root mean square)
	Average diameter of the nanowires: about 200 nm	Diameter of the nanospheres : 220 nm
			Thickness of effective layer 1: 220 nm
	Average length of the nanowires: about 700 nm
			Thickness of effective layer 2: 480 nm

1: Comparison between the measured parameters and the retrieved parameters from the combined model.

Table P - 1 :

 P1 Influence of HMTon the length, diameter, aspect ratio and the number of ZnO nanowires

	Ligand	Growth time (min)	Length (nm)	Diameter (nm)	Aspect ratio (L/D)
	HMTA	35	940	55	17.1
	Withou HMTA	60	960	78	12.3

  . La dépendance en temps de la croissance est également présentée. Pour comparer entre ces différentes conditions, une étude statistique paramétrique a été réalisée à partir des images obtenues au microscope électronique à balayage en privilégiant les paramètres suivants : Longueur moyenne; Diamètre moyen calculé à partir d'un échantillon de 40 nanofils selectionnés au hasard; Raport d'aspect, ratio entre la longueur moyenne et le diamètre moyen; Nombre de nanofils par unité de surface.

	Dans le Tableau 2-1, on montre que si on répéte l'enduction plus d'une fois, on peut obtenir des
	nanofils plus longs, de plus gros diamètre, mais le ratio devient plus petit et le nombre de nanofils est
	moindre.

Table 2 -

 2 1 : Effet de la répétition des étapes d'enduction : Longueur, diamètre, rapport d'aspect et nombre de fils (/µm2) en fonction du nombre d'enduction.

	Nombre
	d'enduction	Longueur (nm) diamètre (nm)	Rapport d'aspect (L/D)	Nombre (/µm 2 )
	2 fois	455	35	18.2	144
	6 fois	583	45	12.9	81
	Dans la

Table 2 -

 2 2, on montre que si on augmente la température de recuit de la couche de germes, on peut obtenir des nanofils plus longs, de diamètre plus élevé, mais la densité de nanofils est réduite.

Table 2 -

 2 2 : Effet de la température de recuit sur la longueur, diamètre, rapport d'aspect et nombre de fils pour différentes températures de recuit de la couche de germes.Après l'effet de la couche de germes, on a étudié la croissance de nanofils en fonction du temps de croissance. On voit dans les Figure 2.1 (a) et Figure 2.1 (b) que la longueur, le diamètre et le rapport d'aspect augmentent en fonction de temps. Le nombre de nanofils par contre diminue. Cela peut s'expliquer par un effet de coalescence des nanofils.

				Raport d'aspect	Nombre
	Température de recuit (°C) Longueur (nm)	diamètre (nm)	(L/D)	(/µm 2 )
	200	283	38	7.11	383
	400	312	40	7.8	285
	500	360	44	8.1	285
	Dans la table 2-3, on montre que si on augmente la taille du substrat sur lequel est déposée une
	couche de germes, les nanofils obtenus sont moins longs, moins gros et le rapport d'aspect augmente
	légèrement.

Table 3 -

 3 1: Correspondance entre les paramètres mesurés et paramètres utilisés pour la simulation.En résumé, a été proposé un modèle phenoménologique pour simplifier la description d'échantillons de nanofils ici synthét isés par voies humides mais le modèle s'appliquerait à tout type de croissance. Pour des nanofils bien alignés et relativement dense, la description se limite à une couche mince poreuse avec des pertes faibles liées à la diffusion de surface. Pour des couches de nanofils moins dense et de taille plus importante (quelques centaines de nanomètres) distribués au hasard sur un substrat, on doit ajouter une couche supplémentaire pour expliquer la diffusion venant de l'extrémité des nanofils. Ce travail peut être utilisé pour estimer les propriétés optiques des couches de nanofils et surtout vice versa pour estimer la géométrie des nanofils par mesure directe de la transmission optique.

		Paramètres mesurés	Paramètres utilisés pour l'ajustement
	Epaisseur couche FTO	environ 1µm	1.01 µm
	Rugosité couche FTO	29.5 nm (root mean square)	29.5 nm
	Diamètre nanofils de ZnO environ 200 nm	220 nm (Nanosphère)
			Epaisseur 1 ère couche effective (sphère) :
	Longueur nanofils de Zno environ 700 nm	220 nm
			Epaisseur 2 ème couche effective : 480 nm
			Total : 700 nm

Cette thèse a été réalisée dans l'équipe Lumière, Nanomatériaux, Nanotechnologie (L2n) au sein de l'Université de Technologie de Troyes, en collaboration avec la Molecular Foundry. Elle a porté à la fois sur la synthèse, la modélisation optique et l'ingénierie d'émission sur deux principaux semi-conducteurs luminescents : l'oxyde de zinc (ZnO) et les monocouches de dichalcogénures de métaux de transition (TMDMs). Le ZnO est un matériau à large bande interdite. Les TMDMs comme le disulfure de tungstène WS2 et disulfure de molybdène, MoS2 en monocouche (atomique) deviennent des matériaux à gap direct.Typiquement, la photoluminescence du ZnO contient deux contributions : une émission de bord de bande dans l'UV et une émission visible. Dans cette thèse, on s'est d'abord intéressé à la synthèse en solution de couches minces de nanofils de ZnO dans le but d'obtenir des couches homogènes à l'échelle d'un substrat. Les propriétés optiques passives de ces couches de nanofils ont ensuite été étudiées. Un modèle phénoménologique combinant la théorie des milieux effectifs et la diffusion de Mie a été développé pour expliquer la transmission optique des couches. Concernant les propriétés actives, nous nous sommes intéressés à l'optimisation de l'émission dans le vert en développant une technique originale de désulfuration. Cette technique a développée fortuitement lors d'une pre-étude visant à déposer de manière conforme des TMDMs sur les nanofils de ZnO, idée initiale de la thèse. Après traitement thermique, a pu également être observée une forte augmentation de l'émission venant de micro-bâtonnets de ZnO. Enfin de manière plus prospective, des monocouches de WS2 et MoS2 ont été synthétisées par dépôt de couche atomique sur SiO2 et nanofils de ZnO.

Perspectives

The perspectives of this work could be of three kinds related to the three main chapters. The first one is related to the synthesis i.e. CBD growth, the second one will be more about the characterization and mechanism involved in the emission. The third one is about process again and the integration of TMDs.

Ligand-assisted growth: towards longer ZnO nanowires?

The usage of organic ligand in the chemical solution has been considered as a big progress in the CBD method. The synthesis of TMDMs uses transition metal oxide and H2S as the precursors. Prior to the sulfurization process, an atomic thin oxide layer need to be firstly deposited by plasma enhanced ALD in an Oxford Instruments FlexAl. In the case of tungsten oxide, the process was as follows: The tungsten precursor (bis(tert-butylimido)-bis-(dimethylamido) tungsten) was dosed into the chamber for 3 seconds at 20 mTorr. The chamber was isolated from the turbo pump and held at 30 mTorr for 1 second to compensate for the low reactivity of the precursor. The chamber and dosage line were then purged with 100 sccom of argon and pumped to 15 mTorr and 5 seconds (10 seconds @ 40 °C ). Oxygen plasma was generated via remote inductively coupled plasma at 30 mTorr with 300 W power and maintained for 2 seconds then purged with argon at 15 mTorr for 5 seconds (10 seconds @ 40 °C). This process was repeated as necessary to produce the required thickness at approximately 1 Å/cycle. As for the molybdenum oxide, the process was as follow: The molybdenum precursor (molybdenum hexacarbonyl) was dosed into the chamber for 4 seconds at 180 mTorr. The chamber and dosage line Appendix 115 were then purged with 200 sccm of argon and pumped to 15 mTorr for 6 seconds (10 seconds @ 40°C).

Oxygen plasma was generated via remote inductively coupled plasma at 15 mTorr with 300 W power and maintained for 3 seconds then purged with argon at 40 mTorr for 4 seconds (10 seconds @ 40°C). This process was repeated as necessary to produce the required thickness, at approximately 1 Å/cycle.

After the deposition of the transition metal oxide, the substrate was introudce in a quartz tube. The CVD process is as following: 1. The CVD furnace was pump-purged with ultra-high purity (99.999%) argon before initiating the high temperature growth (500-800 °C. 2. Once the furnace is heated up to the designed temperature, H2S is introduced as a chalcogenization agent by using argon as a carrier gas during the growth. List of references for the Appendix: [1]. C. Kastl, C.T. Chen, T. Kuykendall, B. Shevitski, T.P. Darlington, N.J. Borys, A. Krayev, P.J. Schuck, S. Aloni, A.M. Schwartzberg, The important role of water in growth of monolayer transition metal dichalcogenides, 2D Materials, 4 (2017) 021024. Comme expliqué dans la deuxième partie, l'émission de ZnO se décompose en deux parties : l'émission UV et l'émission visible. L'émission visible vient principalement des transitions excitoniques qui sont liées aux bandes de défauts insérés dans la bande interdite du ZnO. Dans cette partie, on va présenter une technique efficace pour augmenter l'émission de défaut surtout dans le vert (émission centrée vers 2.45 eV).

Traitement utilisé

Le traitement qui a été utilisé, est appelé désulfurisation et est schématisé sur la figure 4-1. La procédure est la suivante : 1. Les nanofils de ZnO sont sulfurisés par H2S à 500°C pendant 10 min. 2. On garde ensuite l'échantillon sulfurisé dans l'air à 550°C pendant 60 min. Ces micro-bâtonnets sont particulièrement intéressants en tant que matière première pour l'émission de lumière. Après recuit, on observe une amélioration de la qualité cristalline des échantillons mais simultanément, on a créé des défauts à minima en surface liés à la désorption des hydroxydes de Zinc.

On note par contre une augmentation de l'émission des excitons liés aux défauts après activation, ce processus renforce l'émission de défauts après désactivation. Le recuit en présence de soufre montre qu'il est possible d'augmenter encore plus l'émission de lumière verte par dopage.