Thèse Pour

Obtenir Le Grade De

Docteur De L'université De Montpellier

David Muñoz-Rojas

Para Mi

Mamá Pili

Cyril Oberlin

Lucie Ries

Marie-Jo Valero

Damien Alligier

Oleksandra Ganzenko

Anne-Sophie Bonnet-Lebrun

Alexis Lemaire

Rémi Patin

Laure Olazcuaga

Linda Slaim

Francisco Zuloaga

Rose Joubert

Jen- Nifer Sok

Victor Cazalis

Thomas Aubier

Francisco Rivero

Octavio Graniel Tamayo

-

Atomic layer deposition (ALD) has emerged as an essential vapor deposition technique of thin films for countless applications. The rising demand for electronic components and nanostructured materials has established ALD as one of the key fabrication processes in the nanotechnology market.

In this work, novel nanostructured materials that can be used as transducers in biosensor devices are presented. These materials have been prepared by a combination of ALD with top-down and bottom-up techniques such as nanosphere lithography (NSL), physical vapor deposition (PVD), metal-assisted chemical etching (MACE), and electrodeposition.

As a first promising candidate, silicon nanowires (SiNWs) covered with ZnO by ALD were fabricated. These 3D structures are quite attractive for optical biosensing applications thanks to their intense photoluminescence (PL) activity at room temperature. These core/shell nanostructures were fully characterized and tested as possible sensors for the detection of hydrogen peroxide, which is a common reaction product of several oxidoreductases.

In addition, Au-covered hollow urchin-like ZnO nanostructures were prepared with controlled size by combining NSL, ALD, electrodeposition, and electron beam (E-beam) evaporation. The influence of the Au film thickness on the surface-enhanced Raman scattering (SERS) capabilities of the substrates was investigated. The optimized structures were used to detect thiophenol molecules with a limit of detection (LOD) of 10 -8 m. Additionally, adenine can be detected with a concentration as low as 10 -6 m. The excellent uniformity and batch-tobatch repeatability of the substrates makes them excellent candidates for reliable SERS sensing and biosensing.

Moreover, a miscellaneous group of novel materials with enticing features that can be readily applied in sensing, catalysis, and plasmonics is presented. Bimetallic Pd/Au nanoparticles supported on SiNWs with ALD and galvanic replacement were fabricated. Furthermore, hollow urchin-like ZnO structures with ZIF-8 via electrodeposition were fabricated for possible SERS applications.

iii

Résumé

Le dépôt de couche atomique (ALD) est devenu une technique essentielle de dépôt en phase vapeur de couches minces pour de nombreuses applications. La demande croissante de composants électroniques et de matériaux nanostructurés a fait de ALD l'un des processus de fabrication clés du marché des nanotechnologies.

Dans ce travail, nous présentons de nouveaux matériaux nanostructurés pouvant être utilisés comme transducteurs dans des dispositifs à biocapteurs. Ces matériaux ont été préparés en combinant ALD avec des techniques "topdown" et "bottom-up" telles que la lithographie par nanosphère (NSL), le dépôt physique en phase vapeur (PVD), la gravure chimie assistée par des métaux (MACE) et électrodéposition.

En tant que premier candidat prometteur, des nanofils de silicium (SiNWs) recouverts de ZnO par ALD ont été fabriqués. Ces structures 3D sont très attractives pour les applications de biocapteurs optiques en raison de leur activité intense de photoluminescence (PL) à température ambiante. Dans une première approche, ces nanostructures coeur/coquille ont été entièrement caractérisées et testées en tant que capteurs possibles pour la détection du peroxyde d'hydrogène, qui est un produit de réaction courant de plusieurs oxydoréductases.

De plus, des nanostructures creuses en ZnO semblables à des oursins recouvertes de Au ont été préparées avec une taille contrôlée en combinant NSL, ALD, électrodéposition et évaporation par faisceau d'électrons (E-beam). L'influence de l'épaisseur du film Au sur les capacités de diffusion Raman (SERS) améliorées en surface des substrats a été étudiée. Les structures optimisées ont été utilisées pour détecter des molécules de thiophénol avec une limite de détection (LOD) de 10 -8 m. De plus, l'adénine peut être détectée avec une concentration aussi basse que 10 -6 m. L'excellente uniformité et la répétabilité lot par lot des substrats en font d'excellents candidats pour une détection et une biocapture SERS fiables.

En outre, un groupe diversifié de nouveaux matériaux présentant des caractéristiques attrayantes qui peuvent être facilement appliqués à la détection, à la catalyse et à la plasmonique est présenté. Des nanoparticules bimétalliques de Pd/Au supportées sur SiNWs avec ALD et un remplacement galvanique ont été fabriquées. De plus, des structures ZnO creuses de type urchin avec ZIF-8 par électrodéposition ont été fabriquées pour de possibles applications SERS.

v First of all, I want to thank Mikhael Bechelany for answering that very first e-mail that put me on the road to pursuing a PhD. His mentoring and expertise in materials science, the countless constructive discussions, and the reassuring support throughout these last three years are some of the reasons why I can present this PhD work today. Thank you for giving me the chance to travel around Europe (and one country in Asia) to present our work and build a strong network of researchers with many different backgrounds.

I want to thank as well Philippe Miele and Sébastien Balme for giving me the chance of working at the Institut Européen des Membranes (IEM), the continuous guidance, and their sincere suggestions to carry out my experiments. I want to acknowledge the immense help of Jean Marc Janot with photoluminescence and the long discussions about stepping motors, lasers, computer programming, and optics. I want to thank Sophie Tingry and Yaovi Holade for answering all of the questions I had about electrochemistry and for letting me use their lab freely. I want to give a special mention to Martin Dobrek for his support on the annealing of most of my samples and for making sure that I followed the safety rules when handling dangerous materials (e.g., HF). I want to thank also Vincent Rouessac for sharing his vast knowledge on ellipsometry and for kindly showing me how to perform reactive ion etching. Regarding ALD, I want to mention the great help of Bruno Navarra when it came to fixing the ALD system.

The CanBioSe MSCA RISE Horizon 2020 project allowed me to meet some extraordinary people that contributed enormously to this work. I want to thank Igor Iatsunskyi and Emerson Coy for the TEM images and Raman measurements, Roman Viter for some photoluminescence studies, Martin Järvekülg, Jaan Aarik, and Aarne Kasikov for their help with Au sputtering.

For this dissertation, I want to thank my reading committee members David Muñoz-Rojas and Grégory Barbillon for their time, valuable comments, and interest to read this thesis. I want to thank as well my oral defense committee members Frédéric Favier and Mathilde Lepoitevin for their time and insightful remarks.

During these three years at the IEM, I had the honor of meeting some of the most incredible and kindest people. I want to give a special mention to all of the students that I met in the lab of Mika who helped me in some way or another: Viktoriia Fedorenko for the guiding with metal-assisted chemical etching (MACE), Sakthivel Nagarajan and Socrates Radhakrishnan for keeping me company late at night just before the lab closed, Chantal Karam for teaching me how to "float" polystyrene microspheres, Amr Nada and Heba El-Maghrabi

Introduction

"The coming nanometer age can, therefore, also be called the age of interdisciplinarity [1]." Heinrich Rohrer, considered one of the fathers of nanotechnology, encouraged scientists from different academic fields to combine their knowledge for developing new ways to interact with matter at the nanoscale. We have learned over the years that, long before we set out to manipulate single atoms or detect single molecules, nature had already perfected the art of biology at the nanoscale: DNA, one of the backbones of living organisms, has a diameter of roughly 2 nm. However, our ability to mimic nature has not been entirely successful. When comparing nature's elegant and effortless methods to our rather cumbersome fabrication techniques, we can unequivocally see that there is yet a wide gap waiting to be closed. Nevertheless, a great deal of progress has been made, as we are finally approaching the limits of Moore's law, which states that the number of transistors in an integrated circuit doubles every two years, and 7 nm nodes are a reality and are available in the market [2]. Part of this progressive miniaturization comes from the combination of bottom-up and top-down approaches that combine lithographic techniques and a peculiar vapor deposition technique: atomic layer deposition (ALD).

It comes as no surprise then that almost every chip manufacturing company uses a form of this technique and that recognition to its inventor, Tuomo Suntola, has been paid by receiving numerous awards (including the 2018 Millenium Technology Award). However, the full potential of ALD has just started to be explored. A plethora of applications that touch almost every imaginable field has been demonstrated and keeps growing. As Tuomo Suntola puts it: "Let's first think what to do, and then do it [3]." We have thus set ourselves to the task of thinking what else can we do with ALD and how we can achieve it.

Atomic layer deposition

ALD is a vapor deposition technique enabling the preparation of thin film materials with high conformality and excellent control over the thickness [4][5][6]. Its origins can be traced back to two different places [7]. ALD was first developed by Aleskovskii during the 1960s in Russia and was referred to as molecular layering (ML) [8]. Later on, at the beginning of the 1970s in Finland, Suntola developed the atomic layer epitaxy (ALE) technique to deposit ZnS for electroluminescent displays [7,9]. In the next decades and up to today, the route was then referred to as ALD.

ALD is based on self-limiting reactions between a gas precursor and a solid surface that allows the deposition of thin films in a layer-by-layer fashion. The 1. Introduction ability to deposit conformal films on high aspect ratio structures, with high uniformity over large areas, at (relatively) low temperatures, has made ALD a technique of choice for the preparation of ultrathin films and a key enabling technology [4,10,11]. ALD allows the deposition of a wide range of materials such as oxides [4,12], nitrides [13,14], sulfides [15,16], and pure elements [17][18][19][20]. These features have made ALD a relevant technique for many applications such as fuel cells [21], metal oxide semiconductor field effect transistors (MOSFETs) [22], water splitting [23] and purification [24], encapsulation [25], membranes [14], solar cells [26,27], and batteries [28]. Also, the possibility of controlling the composition of the deposited layer by making nanolaminate or alloyed structures allows the tailoring of the chemical and physical properties of the ALD prepared materials [29,30]. ). In the first step, a precursor is introduced into the reactor chamber and is left enough time to react with the surface groups of the substrate. Next, the unreacted precursor molecules and the by-products are removed by purging/pumping the system with an inert gas (usually N 2 or Ar). The third step involves the introduction of a second precursor (the co-reactant) to react with the adsorbed molecules. The final step consists in purging/pumping again the system to remove unreacted precursor and by-products molecules. As a result, one (sub) monolayer of the desired material is deposited on the substrate surface. This cycle is then repeated until the desired thickness is obtained.

The use of ALD in the fabrication of nanomaterials for biological and medical applications has proliferated and is now widely spread [31][32][33][34][35]. The compatibility of ALD with the nanoscale of the components found in biomedical devices, the biocompatibility of the materials that can be deposited, and the tuning of the chemical reactivity are some of the reasons why there is a keen interest for using Biosensors ALD for biomedical applications. Among these applications, biosensing has recently benefited from ALD as a tool for the fabrication of biosensors.

Biosensors

Generally speaking, biosensors are devices that allow the selective detection of a target molecule [36], making them useful for many applications such as clinical diagnosis [37], food safety [38], environmental monitoring [39], security and bioterrorism [40,41]. A typical biosensor configuration is presented in Fig. 1.2. Biosensors have two main components: a biorecognition layer and a physical transducer. When the analyte of interest (e.g., low molecular compound, [bio] macromolecules, protein, virus, cell) is captured by the biorecognition element, a biochemical signal is produced, and the transducer element transforms it into a signal that can be measured and correlated (in some cases) to the concentration of it. Biosensors are selective thanks to the bioselective layer that interacts only with the analyte of interest. However, the robustness, sensitivity, and the detection limit of a biosensor are strongly influenced by the physical, chemical, optical, and mechanical properties of the transducer material, which can be finetuned by choosing a suitable material with unique architectures. Nanomaterials have become more attractive than conventional materials in the micro/macroscale due to their high surface area and their ability to produce a stronger signal upon interaction with analytes due to their enhanced physical properties. Furthermore, the reduced size of nanomaterials can be exploited to create portable devices, reduce the volume of the sample, improve the speed of detection, and reduce the overall cost of production [42]. At the same time, as the activity of the biorecognition element depends on its structure upon attachment to the transducer layer, several immobilization strategies have been developed. For 1. Introduction instance, enzymes can be immobilized through covalent bonding, entrapment, and cross-linking [43,44]. Each one of these approaches presents advantages and drawbacks that are determined by the type of biorecognition element, the transducer, and the final biosensing application.

In general, biosensors can be classified either by the type of biorecognition element (e.g., enzyme, antibodies, cell, nucleic acids) or by the nature of transducer signal (i.e., electrochemical, optical, electrical, mass-sensitive, magnetic). As one of the most reported types of biosensors, optical biosensors offer numerous advantages such as direct, real-time, and label-free detection of both biological and chemical analytes. The detection in this type of biosensors is based on the interaction between the optical field and the biorecognition element [45]. Optical biosensors can be divided into label-free and label-based. In label-free mode, the signal comes directly from changes in the properties of the transducer material (e.g., photoluminescence [PL], surface plasmon resonance [SPR], ellipsometry, and surface-enhanced Raman spectroscopy [SERS]). On the other hand, in labelbased biosensors, the measured optical signal comes from the attached tag (e,g., fluorescence, colorimetric, and luminescence) [46].

Motivation and outline

The primary intention of this thesis is to apply ALD in the fabrication of nanostructured transducer materials for biosensors. To this end, the combination of ALD with other nanofabrication techniques is of utmost importance and represents the core of this thesis. As briefly described in the introduction, transducer materials need imperatively to be biocompatible and, preferably, nanostructured. For this reason, ALD is one of the few techniques that allow depositing biocompatible materials with a precise thickness control, and that can be applied in different stages of the fabrication process of biosensors. Besides, ALD is a high-throughput technique that can be easily scaled up for the industrial production of biosensors, which represents a solution to one of the most significant drawbacks in the biosensing technology: the mass production of affordable and robust devices.

As one of the most mature ALD processes, the deposition of ZnO is suggested as a candidate for the fabrication of transducers. The attractive physicochemical and optical properties of ZnO, its biocompatibility with biorecognition elements, and the recent advances in its crystal growth technology are fully exploited. Also, the flexibility of ALD to either coat a pre-fabricated nanostructure to produce core-shell hybrids or to deposit a "seed" layer for the growth of defined nanostructures is explored. To illustrate this, a combination of colloidal lithography and ALD is proposed for the fabrication of core/shell structures. Likewise, a different method that combines colloidal lithography, ALD, and electrodeposition is offered for the growth of ZnO nanostructures.

Taking into consideration that the primary goal of biosensing is to obtain a precise and rapid detection of an analyte, it is safe to say that optical biosensors offer a possibility to tackle these needs. Thus, a variety of detection techniques Motivation and outline have been developed, and their proof of concept has been demonstrated. On one hand, conventional optical methods such as absorption, reflection, or scattering provide information from changes in the incident beam of light after its interaction with a sample. On the other hand, there is another type of optical sensing that is based on the emission of light at a different wavelength than the incident one. In particular, this work focuses on photoluminescence and surface-enhanced Raman spectroscopies, which are two of the most promising optical sensing approaches. For instance, PL can be employed for sensing by exciting a sample and looking at changes in the PL spectrum upon interaction with an analyte. However, in the case of SERS, the optical signal comes directly from the inelastically scattered photons that interact with the analyte.

The motivation of this thesis is thus to provide a general overview of the flexibility and versatility of ALD for the fabrication of biosensors. Several fabrication techniques are used in combination with ALD to produce nanostructured transducers. Even though the transducer materials showed in this thesis are not fully introduced in a real biosensor device, this work envisions their application and availability to a broad range of scientific fields.

The rest of the thesis is organized as follows:

Chapter 2 presents a short survey of the various ways ALD has been used for biosensing technology. These involve nanofabrication steps, protection of the biosensor, and enhancement of the optical, chemical, and catalytical properties of the transducer material.

Chapter 3 follows the literature review by describing the fabrication and characterization techniques used for the production of nanostructured transducers. Furthermore, the experimental details for the fabrication of the two nanostructures used in this work are addressed.

Chapter 4 displays the results obtained from the fabrication of hybrid ZnO/Si core/shell structures and their possible application for PL biosensing.

Chapter 5 emphasizes on another type of high-aspect-ratio hybrid nanostructures based on ZnO and Au and their application as SERS substrates.

Chapter 6 features a set of hybrid materials that were produced by combining galvanic replacement and electrodeposition and the nanostructures from Chapter 4 and Chapter 5. Although no biosensing application is achieved, these materials have potential feasibility to do so. Chapter 2

ALD for biosensing applications *

ALD has proven to be a versatile technique to fabricate nanostructures [1][2][3][4]. The precise thickness control and excellent conformality allowed by the technique enables to fabricate complex nanostructures with a high surface area that can be used for the design of biosensors. Some of the films deposited by ALD can function as sacrificial layers that can be removed later by chemical or physical etching to produce the desired nanostructure. Other films can be used to fabricate the transducer element of a biosensor, which requires biocompatible materials that can interact with the bio-recognition elements. Additionally, they can be used to protect biosensors and improve their stability towards harsh environments. In this chapter, ALD films deposited by ALD used in combination with other techniques for nanofabrication will be discussed.

ALD to produce nanostructured biosensors

Recently, the interest in fabricating biosensors with nano-sized features has grown because their dimensions are in the same range as the biological components of the bio-recognition layer. A selection of nanostructured biosensors fabricated using ALD is presented in ALD to be used as a sacrificial layer for the fabrication of a vertical nanogap in a label-free electrical biosensor for the detection of streptavidin [5]. Al 2 O 3 films of 5 nm, 10 nm and 15 nm were deposited to explore the effect on sensitivity from different gap sizes. Fig. 2.1 shows the vertical nanogap fabricated with 5 nm of Al 2 O 3 and 2 nm of Ti. By comparing the ratio of the current before and after binding of streptavidin to biotin, they found out that the 7 nm gap is too small for the binding to take place (no change in current). However, they observed significant current increases for the 12 nm and 17 nm gaps. Similarly, Im et al. combined nanosphere lithography (NSL) and ALD for the fabrication of a periodic array of ring-shaped nanocavities with 10 nm gap size to be used as a SERS substrate [6]. Fig. 2.2 presents a schematic representation of the fabrication process. The number of ALD cycles controls the thickness of the Al 2 O 3 layer and this, in turn, controls the resulting gap size. Adenine was used as an analyte to test the film-over-nanospheres (FON) substrate "FON-gap." SERS measurements revealed a strong 731 cm -1 purine stretch coming from adenine that was used to test the detection limits of the SERS substrate. A 76 nm detection limit was obtained, which makes the FON-gap an excellent bioanalytical platform for SERS biosensing. In order to fabricate materials with low dimensions, ALD has been used to fill high aspect ratio nanoholes of porous materials [11] such as anodic aluminum oxide (AAO) [12,13]. Thanks to its controllable pore diameter, periodicity, and density distribution [14], AAO has been incorporated successfully as a template for biosensors fabrication. As an example, Tarish et al. obtained highly ordered ZnO/ZnS core/shell nanotube arrays by using ALD of ZnO and rapid thermal deposition with AAO as a template [15]. ALD has also been used as a tool for tuning single solid-state nanopore sensors. Such sensors, inspired by the pioneering work of Kasianowicz et al. on α-hemolysin [16], are nanometer-sized apertures fabricated in thin films that allow label-free detection of single molecules [17][18][19]. Thanks to its conformal deposition, ALD has been used to coat nanopores with high aspect ratio [20]. For example, Cabello-Aguilar et al. deposited Al 2 O 3 /ZnO nanolaminates to reduce the size of a hydrophobic nanopore [21]. The diameter of the nanopore was fine-tuned for α-hemolysin insertions. Al 2 O 3 /ZnO nanolaminates were chosen due to their low surface roughness, which prevents the collonar growth of layers that can clog the nanopore [22]. The ability of ALD to homogeneously coat the internal surface of a nanopore allows an excellent surface functionalization [23]. This capability limits the non-controlled adsorption of proteins outside the track-etched nanopore and is useful for biosensing [9]. ALD has also been used to reduce electrical noise in SiN [24] and PET [25] nanopores by coating them with a thin layer of Al 2 O 3 .

ALD for biosensing applications

These examples show the relevance of ALD for the fabrication of nanostructured biosensors and its versatility to be used in combination with other fabrication techniques. The advantages of using ALD to deposit thin films can be seen in the nanometer scale features achieved in the different types of biosensors and the various morphologies that can be coated.

ALD for the fabrication of transducers

In order to produce a signal that can be measured and correlated to the presence and concentration (in some cases) of a specific analyte, the transducer element of a biosensor requires well-defined morphologies with optimal optical, electrical, chemical, mechanical, and structural properties [26][27][28]. It is crucial that the interaction between the analyte and the biorecognition element on the transducer material produces a change in one or more of its physicochemical properties (e.g., mass change, photoluminescence change, pH change, photocurrent change). Also, the biocompatibility of the transducer element is necessary to facilitate the immobilization of the biorecognition elements [29][30][31]. The capabilities offered by ALD can effectively meet these challenges and examples of oxide transducers fabricated with ALD are presented in Table 2.2.

Recently, Tereshchenko et al. used a ZnO film deposited by ALD as an optical biosensor platform to detect Grapevine virus A-type (GVA) proteins (GVA-antigens) for the first time [34]. ZnO was chosen due to its high isoelectric point (∼9.5), biocompatibility for the immobilization of GVA-antibodies, and ability to change its photoluminescence (PL) emission when interacting with ALD for the fabrication of transducers Sensitivity in the range from 0.001 nm/ml to 10 nm/ml [34] biomolecules. In their work, they deposited a 110 nm thick ZnO layer on a Si substrate at a low deposition temperature of 100 • C. By looking at the PL signal of the biosensor while adding different concentrations of GVA-antigen, they detected the protein with sensitivity in the range from 0.001 nm/ml to 10 nm/ml. Kim et al. grew ZnO nanorods using a hydrothermal method and a seed layer deposited by ALD [33]. A 20 nm ZnO was deposited and used to promote the growth of high aspect ratio ZnO nanorods in a layer-by-layer fashion. Glucose oxidase (GO x ) was immobilized on the ZnO nanorods to detect glucose over a range of different concentrations by cyclic voltammetry. The biosensor showed a sensitivity of 69.8 nm/(µm cm 2 ) for the ZnO nanorods with the highest surface area.

Transducer materials can also be functionalized chemically to immobilize the biorecognition element [35,36]. The chemical grafting of biomolecules shows an increase of the surface coverage when compared to physical adsorption and improves the general stability and performance of the biosensor [37]. TiO x was deposited on a long-period grating (LPG) induced in an optical fiber as a transducer for the recognition of biotin-avidin interactions [32]. The transducer layer was functionalized with amine groups to form a peptide bond with the carboxyl group of biotin. X-ray photoelectron spectroscopy (XPS) was used to confirm the successful functionalization with 3-aminopropyltriethoxy silane (APTES) as well as biotin on the TiO x layer. A 13.2 nm shift in the resonance wavelength was obtained when binding of avidin to biotin took place, proving the biosensor's ability to detect this event.

The possibility of fabricating the transducer element in biosensors with ALD has been demonstrated in the examples mentioned above. With the growing need to fabricate biosensors with smaller dimensions, faster response rates, and higher sensitivities, the use of transducers prepared by ALD will become more common and will play a key role in the development of these devices.

ALD for biosensing applications

ALD layers to protect biosensors from their environment

ALD deposited films have been used as protective coatings, for example, to limit corrosion processes [38][39][40]. The ability to coat large areas with substantial uniformity and excellent conformality are some of the advantages of ALD over other thin film deposition techniques. Also, ALD provides dense, pinhole-free films with outstanding adhesion properties [41]. These benefits have made ALD an excellent choice to protect the surface of biosensors from aqueous environments and render them stable and, in some cases, reusable. To illustrate this, a list of biosensors with protective coatings deposited by ALD is presented in Table 2.3. Cha et al. used ALD of SiO 2 to protect the surface of a photonic crystal (PC) band-edge laser (BEL) structure [44]. These In-P based structures are subject to chemical attack, and their surface is full of defects that can cause rapid carrier annihilation. By depositing a 5 nm SiO 2 film, they successfully protected the nanolaser device from harsh chemicals and used it as a layer for biotin functionalization. Fig. 2.3 shows the overall functionalization process schematically. Regions with PC pattern show a stronger fluorescence thanks to their higher surface to volume ratio when compared to planar regions. Also, they detected streptavidin with a figure of merit of ∼800, which showed the high sensibility of the BEL biosensor.

The sensitivity of a biosensor can be lowered when a protective layer is applied. The properties of the material to be deposited, as well as its thickness, must be optimized to reduce the possible detrimental effects on the performance of the biosensor. Kim et al. used an ALD Al 2 O 3 film as a passivation layer to protect the ZnO piezoelectric layer from bacterial growth media or animal serum of a surface acoustic wave (SAW) biofilm sensor [43]. They calculated the normalized theoretical sensitivity of the SAW sensor after applying a protective layer of different materials (Al 2 O 3 , Si 3 N 4 , SiO 2 , and Teflon) and observed that Al 2 O 3 was the one that provided the lowest degradation in sensitivity. A 45 nm Al 2 O 3 film was deposited by E-beam evaporation, radio frequency (RF) sputtering and ALD to assess the performance of each of the deposition techniques. After two days in Lysogeny broth (LB) media bacterial suspension, the films prepared by E-beam evaporation and RF sputtering led to some damage and did not protect the ZnO layer successfully as compared to the ALD film (Fig. 2.4). E. coli was cultured to test the SAW sensor for biofilm growth monitoring, and a detection limit of 5.3 pm was achieved.

Zhang et al. deposited an ultrathin Al 2 O 3 layer on a AgFON substrate for SERS detection [42]. The Al 2 O 3 layer was used to protect Ag against oxidation and allows preserving an excellent sensitivity. Also, the adsorption affinity of the anthrax biomarker detected in this study for the Al 2 O 3 is five times stronger than that for the AgFON alone. The SERS substrate was later used for bacillus spores detection, and a limit of detection (LOD) of ∼1.4 × 10 3 spores was attained. The SERS substrate remained functional over a period of 9 months and opens up the possibility to be used for biomedical, homeland security, and environmental applications. These examples illustrate the feasibility of ALD to deposit highly dense and conformal films for the protection of biosensors. The improved long-term stability of ALD coated devices allows them to be reused without having a detrimental impact on their sensing performance and opens up the possibility for biosensors to be used for in vivo monitoring.

Materials deposited by ALD

The properties of materials at the nanoscale strongly depend on their size, shape, chemical composition, and surface area [45,46]. Thus, ALD has been extensively used to modify and enhance different electrical, optical, catalytical, and structural properties of biosensor materials [24,[47][48][49]. In order to be used in biosensing applications, the materials need to be biocompatible with the biological components of the biosensor and should not compromise the sensitivity and overall performance of these devices. Due to the popularity of FET type biosensors [50][51][52], most of the applications of ALD in biosensing have been focused so far on depositing high dielectric materials. Nonetheless, materials with high refractive index and high catalytic activity have also been reported. In this section, the materials deposited by ALD to fabricate biosensors have been classified by their relevant property and are showcased with a few examples.

Materials with high dielectric constant

ALD has been used to deposit thin films of materials with high dielectric constant to reduce leakage current, passivate surfaces and, in some cases, serve as gate dielectrics [53,54]). Examples of high dielectric constant oxides films deposited by ALD for the fabrication of biosensors are presented in Table 2.4. One of the most used high k oxides is Al 2 O 3 [62,63]. Due to its high breakdown field and high thermal stability, Al 2 O 3 deposited by ALD can successfully reduce surface leakage current [64]. Another positive aspect of this material for biosensing is the fact that Al 2 O 3 films by ALD present good biocompatibility [65]. Chen et al. deposited a 10 nm film of Al 2 O 3 to prevent current leakage between silicon nanowires and analyte solution in a FET biosensor for urea detection [55]. The Al 2 O 3 film was later treated with oxygen plasma (to clean it and render its surface hydrophilic) before functionalization with APTES and urease enzyme. Krishna et al. developed a giant magnetoresistance (GMR) biosensor based on magnetic nanoparticles for detection of influenza A virus [56].

Materials deposited by ALD

In their work, they deposited an 18 nm thick Al 2 O 3 film onto a GMR chip to prevent current leakage.

Vello et al. deposited a 3.3 nm Al 2 O 3 insulating film on nickel interdigitated electrodes to prevent leakage current and enable them to operate in an aqueous buffered medium [66]. To obtain this, they immersed the interdigitated electrodes coated with Al 2 O 3 in a phosphate buffered saline (PBS) solution while applying 1 V to the pads of the device. The Al 2 O 3 coated electrodes were functionalized with tripeptide reduced glutathione (GSH) to detect the target enzyme glutathione S-transferase (GST) by evaluating variations on the overall capacitance values. The biosensor could detect GST at concentrations as low as 200 pmol/L (the lowest value reported according to references) and could be easily regenerated to be used several times. Yoo et al. deposited an Al 2 O 3 layer on an SU-8 organic layer to fabricate a hybrid gate dielectric in a MoS 2 FET biosensor [57]. The 30 nm Al 2 O 3 layer provided high insulating properties and improved the adhesion between the organic SU-8 layer and the MoS 2 /source-drain electrodes multilayer. Fig. 2.5 shows the different layers of the epidermal skin-type MoS 2 biosensor.

The flexible biochip was used for real-time detection of prostate-specific antigen (PSA) with concentrations as low as 1 pg/ml, which is much lower than the value needed for clinical trials (∼4 ng/ml). In addition, they introduced a commercial light-emitting diode (LED) to provide a direct diagnostic result, which opens up the possibility to use this type of biosensors in point-of-care (POC) and forensic applications.

Another high-κ material that has been deposited by ALD for biosensing applications is HfO 2 . HfO 2 is known for its thermodynamic stability when deposited on Si, which effectively reduces leakage current and makes it suitable as a gate material for metal-oxide-semiconductor (MOS) devices [67][68][69]. Also, HfO 2 has an isoelectric point around pH 7 that makes it uncharged in many biological solutions and can be functionalized with biomolecules [58].

Lee et al. developed a biosensor based on HfO 2 for human interleukin-10 (IL-10) detection by electrochemical impedance spectroscopy (EIS) [70]. They deposited a 10.7 nm film of HfO 2 on a silicon wafer which was later functionalized with self-assembled monolayers (SAMs) of an aldehyde-silane (11-(triethoxysilyl) undecanal (TESUD)) and anti-human IL-10 mAb. By following the EIS of the modified HfO 2 with increasing human-IL-10 concentrations, they demonstrated that the biosensor had a working linear range of 0.1 pg/ml to 20 pg/ml and a sensitivity of 49 ng/ml. Wang et al. reported a label-free MoS 2 nanosheet-based FET biosensor covered with a 7 nm to 8 nm layer of HfO 2 [59]. Fig. 2.6 shows the home-built microfluidic channel system where sample solutions were flowed to be detected by the active area. Figure 6c shows the different layers that make up the FET biosensor. They used HfO 2 to serve as the gate dielectric layer, to protect the metal electrodes from a fluidic environment, and to serve as a starting layer for functionalization with silanes. The FET biosensor showed good sensitivity (down to the femtomolar level) to cancer marker protein PSA and high selectivity by not responding to BSA protein.

Besides Al 2 O 3 and HfO 2 , ALD of ZrO 2 has also been reported for biosensors fabrication. Thanks to its large energy bandgap and thermodynamic stability, ZrO 2 has been considered as an alternative gate dielectric material [71][72][73]. Barik et al. deposited ZrO 2 by ALD as gate insulator on an enzyme FET biosensor [60]. They deposited ZrO 2 on the channel region of the biosensor to increase its capacitance and thus gain sensitivity. Sticker et al. deposited a 15 nm thick ZrO 2 insulation layer on interdigitated microelectrodes for nanotoxicological cell analysis [61]. By running computational simulations, they showed that insulated interdigitated electrode structures had an improved electrical current distribution when compared to bare electrodes. They successfully evaluated the toxicity of silica nanoparticles (with and without protein coatings) on H441 cells by monitoring the impedance signal over time. The numerous examples of biosensors that employ high-κ oxides films prepared by ALD highlight the importance of this technique for the fabrication of these devices. Moreover, the conformal and pinhole free thin films of metal oxides successfully passivate the surface of the biosensors without loss of sensitivity and make them promising for miniaturized POC diagnostics.

Materials with high refractive index

Materials deposited by ALD with high refractive index have been used in optical applications [74,75] and microelectromechanical systems [76]. Owing to the smoothness of the films and the ability to coat complex-shaped substrates with a precise thickness control, ALD has been preferred over other thin film deposition techniques like dip coating and self-assembly [77,78]. Examples of high refractive index films deposited by ALD for the fabrication of biosensors are presented in Table 2.5.

Recently, Oubaha et al. deposited a Ta 2 O 5 layer by ALD to increase the sensing properties of a multianalyte biosensor [79]. Ta 2 O 5 was chosen due to its high refractive index, which increases the intensity of the evanescent field by 440 times when compared to the system without the high refractive index layer (HRIL). The thickness of the HRIL chosen for this work was 32 nm because it provides the maximum evanescent wave enhancement. Cy-5-labeled mouse immunoglobulin G (IgG) antibody was detected through fluorescence detection, and a limit of detection of 0.25 µg/ml0.25 µg/ml was achieved. Smietana et al. deposited an ALD layer of TiO 2 on LPGs for a label-free optical biosensor [80]. TiO 2 was used for improving the refractive index sensitivity of the biosensor and as a biocompatible material for endotoxin binding protein (adhesin) functionalization. The deposition of a 70 nm thick TiO 2 layer on the LPG increased the refractive index sensitivity 2.8 times when compared to the bare LPG. Binding of E. coli B lipopolysaccharide (LPS) to bacteria adhesion was confirmed thanks Materials deposited by ALD to an increase in the spectral separation of resonances.

Materials with catalytic activity

ALD has been used to improve the catalytic activity of materials thanks to its ability to precisely control the catalyst particles properties and uniform dispersion on the surface of the support [81][82][83]. Examples of materials with catalytic activity deposited by ALD for the fabrication of biosensors are presented in Table 2.6. Recently, Choi et al. reported the decoration of carbon nanotubes (CNTs) with Ni nanoparticles by ALD for non-enzymatic glucose sensing [85]. The presence of Ni nanoparticles on CNTs was confirmed by TEM, high-angle annular dark-field (HAADF), and energy-dispersive X-ray spectroscopy (EDX). The ∼8 nm Ni nanoparticles were uniformly distributed on the walls of the CNTs. The selectivity of the sensor was confirmed by showing minimum changes in the oxidation current when adding ascorbic acid or uric acid, whereas, in the case of glucose, the value of the oxidation current was far more important. The sensor presented a detection limit of 2 µm and a linear range of 0.005 mm to 2 mm. Furthermore, it showed a rapid response and repeatability for non-enzymatic detection of glucose.

At the nanoscale, both mass transport and electron transport play a crucial role in obtaining excellent electrocatalytic properties. Nanoporous metals decorated with transition-metal oxides have been proposed as structures that can serve as electrochemical biosensors. Zhang et al. deposited a CoO layer on nanoporous gold (NPG) for the detection of glucose and H 2 O 2 [84]. The Au nanopores with a size ranging between 40 nm to 100 nm were covered with 100 to 800 cycles of CoO. When less than 200 cycles were used, the amorphous CoO formed a discontinuous layer. On the other hand, a continuous layer was observed at higher cycles. The NPG/CoO heterostructure showed an excellent electrocatalytic activity of glucose oxidation owing to the interconnected Au skeletons and the synergistic effect between Au and CoO. Furthermore, the electrochemical biosensor could detect concentrations of hydrogen peroxide (H 2 O 2 ) as low as 0.1 mm and its sensitivity was comparable to other types of composite electrodes based on graphene sheets, noble metals, and metal oxide nanoparticles.

Conclusions and future perspectives

ALD has rapidly become a valuable technique for the fabrication of biosensors thanks to its ability to deposit a wide range of materials with precise thickness control and excellent conformality. In this minireview, the crucial role of ALD for the fabrication of different types of nanostructured biosensors and its capability to improve their sensing properties has been shown. Immobilization of biorecognition elements has also been demonstrated on layers deposited by ALD thanks to their biocompatibility and controlled chemical composition. The continuous miniaturization trend of biosensor devices and the complexity of the structures to coat demand high conformality, uniformity, and film quality that conventional thin film deposition techniques such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) cannot achieve [87]. However, there are still some challenges to overcome. Conventional ALD remains a slow technique and requires mosf ot the time vacuum conditions that make the processes somewhat expensive and difficult to scale up [88]. For this reason, different approaches such as spatial atmospheric ALD [89][START_REF] Hoye | Research Update: Atmospheric pressure spatial atomic layer deposition of ZnO thin films: Reactors, doping, and devices[END_REF][START_REF] Poodt | Spatial atomic layer deposition: A route towards further industrialization of atomic layer deposition[END_REF], roll-to-roll systems [START_REF] Ali | High rate roll-to-roll atmospheric atomic layer deposition of Al 2 O 3 thin films towards gas diffusion barriers on polymers[END_REF][START_REF] Maydannik | Roll-to-roll atomic layer deposition process for flexible electronics encapsulation applications[END_REF], and rotating reactors [START_REF] Longrie | Thermal and Plasma-Enhanced Atomic Layer Deposition of TiN Using TDMAT and NH3 on Particles Agitated in a Rotary Reactor[END_REF][START_REF] Sharma | Spatial atomic layer deposition on flexible substrates using a modular rotating cylinder reactor[END_REF] are being developed to make the process (even) more scalable and compatible with industrial requirements. Additionally, the design and synthesis of new and affordable precursors are needed to further increase the availability of materials that can be deposited. This is a difficult task given the constraints that the ALD reaction imposes on the precursors. The chemicals must be volatile at room temperature (or by being slightly heated) and thermally stable. Also, they must react quickly with the substrate to allow surface saturation and provide a fast growth [START_REF] Johnson | A brief review of atomic layer deposition: From fundamentals to applications[END_REF][START_REF] Leskelä | Atomic layer deposition (ALD): From precursors to thin film structures[END_REF][START_REF] Marichy | Atomic layer deposition of nanostructured materials for energy and environmental applications[END_REF]. Furthermore, while some biosensors have passed the testing phase and have become available in healthcare applications as handheld devices or portable units [START_REF] Ronkainen | Electrochemical biosensors[END_REF], their design must still be improved so they can diversify their applications in fields such as environmental monitoring, security and bioterrorism, and food safety. Finally, it is safe to say that ALD (in combination with other fabrication techniques) will push biosensors past the limitations they currently face by providing robust, sensible, and selective platforms that will become part of our everyday lives.

Chapter 3

Materials and methods

Following the ideas reflected in Chapter 2, this chapter is dedicated to introducing some of the underlying principles of the techniques used for the fabrication and characterization of nanostructures. By combining high-throughput methods such as colloidal lithography, physical vapor deposition (PVD), and electrodeposition, with ALD, we attain nanometer-sized structures with different compositions. Thus, the fabrication of silicon nanowires (SiNWs) and urchin-like structures are investigated and optimized to function as reliable biosensors.

Fabrication techniques

Atomic layer deposition

The deposition of ZnO and Pd were carried out in a home-built ALD system depicted in Fig. 3.1. The ALD reactor chamber has a circular (diameter of 83 mm) sample stage that can fit samples with a maximum height of 10 mm. This chamber is connected to an Ar line, precursor vessels, and a rotary pump that can achieve a base pressure of ∼10 -2 mbar

ZnO

Zinc oxide (ZnO) is a direct wide band gap (E g ∼3.3 eV) semiconductor with an intense near-band-edge excitonic emission at room temperature and high 3. Materials and methods isoelectric point. ZnO has been grown by pulsed laser deposition (PLD) [1], molecular beam epitaxy (MBE) [2], metal organic chemical vapor deposition (MOCVD) [3], and sputtering [4]. The rising interest for using ZnO in microelectronic applications that require a though control over the growth of the film and high conformality, has paved the way for ALD to be one of the preferred techniques to deposit it. As one of the first materials to be deposited by ALD, ZnO has been prepared with different precursors, but by far the most common is diethylzinc (DEZ). DEZ reacts with water following the reaction (where * denotes surface species):

Zn(C 2 H 5 ) * 2 + H 2 O --→ ZnO * + 2 C 2 H 6 {1}
This reaction is highly exothermic and can be used to deposit ZnO from ambient temperature up to 600 • C [5]. The reactant and co-reactant tubes connected to the ALD chamber were heated at 100 • C to avoid condensation of the precursors. The deposition was carried out at either 80 • C or 100 • C depending on the substrate to cover. Table 3.1 contains the process parameters used in this work.

Pd

The long-standing attractiveness of palladium (Pd) for catalytic applications [6] and its hydrogen storage capability [7] have been spotted by the ALD community and different approaches have been developed for its deposition [8,9]. To date, the most used precursor for the deposition of Pd is Pd(II) hexafluoroacetylacetonate Pd(hfac) 2 . In order to use this precursor, different co-reactants such as H 2 [10] and formalin (CH 2 O) [11] have been used to reduce Pd 2+ to Pd 0 . Furthermore, the use of H 2 and O 2 plasmas to provide additional surface reactivity and lower working temperatures than purely thermal processes has also been explored [12]. For this work, Pd was deposited from Pd(hfac) 2 as reactant and formalin as co-reactant following the reaction pathway proposed by Elam et al. [11]:

Pd-H * x + Pd(hfac) 2 --→ Pd-Pd(hfac) * 2-x + x Hhfac {2} Pd-Pd(hfac) * 2-x + HCOH --→ Pd-Pd-H * x + 2 -x Hhfac + CO {3}
In reaction {2}, x Hfac molecules are liberated as a result of the Pd -H reaction with one Pd(hfac) 2 . Next, the Pd(hfac) surface reacts with HCOH as displayed on reaction {3}, where the HCOH is decomposed into 2H and CO. Finally, the surface is regenerated by the reaction of H with Pd -hfac.

The Pd(hfac) 2 was kept in a stainless steel bubbler heated at 70 • C. The deposition of Pd was carried out at 220 • C and the tubes connecting the precursor and co-reactant were heated at 110 • C to avoid condensation. The parameters used for the deposition of Pd are displayed in Table 3

Colloidal lithography

Since the first description of two-dimensional colloid crystallization by Pieranski [13], the first reported lithographic mask by Fischer [14], and the first fabrication of metallic nanostructures by R.P. van Duyne [15], the use of colloidal crystals for the fabrication of nanostructures remains a powerful and dynamic tool. A plethora of complex shapes such as nanotriangles [16], nanorings [17], nanobowls [18], nanoneedles [19], and nanowires [20] has become possible by using colloidal masks and the applications continue to grow. Colloidal lithography, more commonly known as nanosphere lithography (NSL), is based on the deposition of spheres (polystyrene, SiO 2 , PDMS, etc.) via direct assembly on a solid substrate or by liquid interface-mediated processes and their subsequent use as a lithographic mask. Each one of the two approaches to deposit the spheres presents its advantages and limitations. For instance, direct assembly on a solid substrate methods (e.g., drop casting, convective assembly, spin coating, and electrophoretic deposition) are experimentally uncomplicated and do not require fancy setups. Moreover, they can be used to prepare different colloid arrangements with substrates that are already structured. On the contrary, when liquid interface-mediated processes are used, the monolayer is not able to adapt to nanostructured substrates due to its inherent low-energy state, which prevents it from disassembling into different arrangements.

Overall, interface-mediated techniques do not suffer from the formation of undesired multilayers as the colloidal particles are formed in a liquid interface where they can assemble with the lowest free energy possible. Moreover, the composition of the subphase can be tuned to (i.e., changing the pH or adding electrolytes) promote the formation of macroscopic single crystal domains. Additionally, different types of substrates can be used as the monolayer is formed at the interface and only needs to be transferred to the surface of the substrate. However, the majority of direct assembly methods are constrained by the choice of substrate as they usually require a particular surface charge or hydrophilicity. Indeed, the Langmuir-Blodgett technique is the most well-known interfacemediated process for colloidal crystallization [21]. This method was first conceived for the assembly of molecular monolayers and involved the controlled compression of molecular monolayers formed by spreading amphiphilic compounds at the air-water interface. Once assembled, the monolayers adhere to a hydrophilic substrate that is drawn in an upward fashion, and the process can be repeated several times to produce multiple layers. Nevertheless, this technique requires an extremely clean environment, and it is time-consuming.

To avoid the drawbacks of the Langmuir-Blodgett technique, Vogel et al. developed a method that involves the direct assembly of colloidal crystals at the air-water interface (Fig. 3.2). In this method, an ethanolic solution of polystyrene spheres is deposited on the surface of a glass slide that is immersed in a crystallization dish filled with water. As soon as the polystyrene spheres touch the water, they assemble into a monolayer that can be later transferred to a substrate immersed in the subphase. ,d,e) monolayer transfer by immersion of the transfer substrate and subsequent elevation under a shallow angle, f) photograph of the deposition of 1 µm colloids onto the air/water interface; Individual, crystalline monolayer patches floating at the interface can be seen, g) monolayer after drying on a Si wafer. Reprinted with permission from [22]. Copyright 2011 John Wiley and Sons.

Non-close packed monolayers

The interspacing of colloidal crystals can be modified to obtain sophisticated lithographic masks that can be used in etching steps or angular evaporation to fabricate different structures from those obtained with the classical NSL method. There are two main approaches to obtain non-close packed monolayers. The first one involves the modification of a close-packed monolayer by using an etching technique or by separating the spheres with a mechanical stretching (using an elastomeric substrate). The second one uses the spin coating technique to deposit solutions of nanospheres and monomers that can be later polymerized.

By far, the size reduction of close-packed 2D colloidal crystals by etching methods such as isotropic plasma etching, anisotropic reactive ion etching (RIE), and electron beam irradiation, is the most used approach in the literature owing to its simplicity and precision [23]. Since its first introduction by Haginoya et al. [24], the thinning of polystyrene spheres by RIE remains a powerful tool for the fabrication of non-close packed lithographic masks [25]. Fig. 3.3 shows a schematic diagram of the RIE chamber for the etching of polystyrene spheres. By carefully choosing the size of the polystyrene spheres and the etching parameters, the final size of the polystyrene spheres and their spacing can be modified for the desired application. In general, the plasma etching process is performed at low pressures (below 10 mbar) and the plasma can be started by either by a RF source or as an inductively coupled plasma (ICP) through a coil [26]. The RIE process has four main steps: 1) generation of chemically reactive species as a result of the interaction of the generated plasma and the gas of interest; 2) chemisorption of the reactive species on the surface of the polystyrene spheres; 3) formation of the volatile reaction product; and 4) desorption of the reaction product and removal from the reaction chamber [27].

Materials and methods

Metal-assisted chemical etching

The fabrication of silicon (Si) nanostructures by using top-down and bottom-up approaches remains one of the most active areas in the semiconductor industry [28]. Si nanostructures have been fabricated by different methods such as RIE, vapor-liquid-solid (VLS) growth, electrochemical etching, laser ablation, molecular beam epitaxy, and metal-assisted chemical etching (MACE). First introduced by Li and Bohn [29], the MACE technique allows the fabrication of Si nanostructures with precise control on the size, length, crystallographic orientation, doping type, and doping concentration [30]. In contrast, the fabrication of large arrays of vertically-aligned Si nanostructures by using the VLS method poses a challenge due to catalyst contamination and difficult orientation control. Furthermore, the VLS method and the RIE etching introduce many crystallographic defects that affect the overall quality of the Si nanostructures.

In a traditional MACE process, a Si substrate covered by a noble metal thin film is immersed in an etching solution of hydrogen fluoride (HF) and H 2 O 2 ). Consequently, the Si underneath the noble metal is etched faster than the one not covered by the noble metal and, depending on the initial morphology of the noble metal, Si nanopores or SiNWs are created. For example, Huang et al. developed a method to fabricate SiNWs by using a patterned Ag mask with defined size-pores created with NSL [31]. This NSL-based method allows controlling the final diameter of the SiNWs (by changing the parent sphere size) as well as their length (by adjusting the etching time).

Different reported models describe the chemical and electrochemical reactions that occur during the MACE process [32]. It is widely accepted that the reactions occur preferentially at the metal/etching solution interface. For simplicity, we will consider the model proposed by Huang et al. [28] involving a Si substrate covered by a Au mask fabricated with NSL. Fig. 3.4 displays the different process taking place during MACE. First, the reduction of the H 2 O 2 occurs at the metal (cathode reaction):

H 2 O 2 + 2 H + + 2 e ---→ 2 H 2 O {4}
The e -in reaction {4} are taken from Au being dissolved:

Au 0 --→ Au 3+ + 3 e - {5}
Second, owing to the more electronegative character of Au when compared to Si, the Au ions are reduced back to Au 0 by taking e -from the Si substrate. The generated holes are locally injected underneath the Au layer and they oxidize Si into SiO 2 . Third, HF dissolves SiO 2 :

SiO 2 + 6 HF --→ H 2 SiF 6 + 2 H 2 O {6}
Fourth, the Si in contact with Au is etched at a faster rate than the bare Si substrate. Finally, the holes generated underneath the Si in contact with the noble metal are diffused to the walls of the pore.

Electrodeposition

The electrodeposition technique to deposit metals (and alloys) from a solution containing ionic species has been widely used at the industrial scale for several decades and is well documented in the literature [33,34]. In particular, this fabrication technique has been used to increase the corrosion resistance of materials, increase the conductivity in electronic components, and even for decorative purposes to imbue materials with an attractive appearance [35]. Simultaneously, the electrodeposition of semiconductors as thin films and nanostructures has found many practical applications in the field of photovoltaic solar panels and large-area display devices [36,37].

ZnO

As an important semiconducting oxide material, ZnO has been electrodeposited from aqueous and non-aqueous media to form thin films, nanorods [38], flat disklike-deposits [39], and more recently, urchin-like structures [40]. To this day, the two main routes to electrodeposit ZnO are via an oxygen saturated solution [41] or a nitrate solution [42]. In general, the electrodeposition of ZnO described by Peulon and Lincot requires an oxygen-saturated solution containing the adequate concentration of ZnCl 2 and three electrodes, as shown in Fig. 3.5.

A cathodic potential is applied and hydroxide ions are generated at the surface of the working electrode:

O 2 + 2 H 2 O + 4 e ---→ 4 OH - {7}
The generated hydroxide ions react with Zn ions and ZnO is precipitated:

Zn 2+ + 2 OH ---→ ZnO + H 2 O {8}
By changing the electrochemical conditions, ZnO can be deposited with different morphologies depending on the desired application. Likewise, Elias et al. [43] developed a method to fabricate urchin-like nanostructures by combining NSL, ALD, and electrodeposition. In this technique, the length and diameter

ZIF-8

The research interest in the electrochemical synthesis of metal organic frameworks (MOFs) has risen tremendously in the past few years. These nanoporous materials whose framework is based on metallic nodes held together by bridging organic linkers have found potential applications in gas separation [44], energy storage [45], sensing [46], and microelectronics [47]. While this might seem exciting, the majority of these applications are based on MOFs in bulk form, which hinders their use in membranes, sensors, and integration with microelectronics that require MOF crystals as thin films or coatings [48]. Recently, MOF films have been produced by means of electrochemical synthesis methods, which opens up the possibility for their large-scale production in industrial processes. The electrochemical deposition of MOFs can proceed anodically or cathodically. In the anodic approach, metal ions are introduced to a solution containing an organic ligand by electrochemically oxidizing a metal support, which produces a thin MOF film on the anodic surface [49]. In cathodic deposition, the reduction of ions (e.g., nitrates) at the cathode induces the deprotonation of organic linkers that can react with metal ions, which triggers the formation of MOFs [50]. Although attractive enough, the cathodic approach is usually accompanied by co-deposition of the corresponding metal, which is unwanted in many cases. Recently, the first cathodic deposition of a zeolitic imidazolate framework material (ZIF) was achieved by Zhang et al. [51]. In their method, they perform a ZIF-8 synthesis by saturating with oxygen the precursors solution of ZnCl 2 and imidazole organic ligands in methanol (Fig. 3.6). This oxygen-triggered approach allows depositing large-area uniform ZIF-8 films while avoiding the plating of Zn metal. The proposed reaction mechanism involves the reduction of O 2 to O 2 -on the electrode surface (reaction {9}), the subsequent formation of a superoxo-Zn II complex (reaction {10}), and the reaction between Zn II -imidazole complexes and superoxo-Zn II intermediates (reaction {11}), which finally evolve to ZIF-8 via ligand-exchange, nucleation, and crystal growth [52].

O 2 + e ---→ O 2 - {9} Zn 2+ + O 2---→ (Zn 2+ )(O 2 -) {10} 2 (Zn 2+ )(O 2 -) + 2 [Zn(HmiM) m L n ] --→ {11} 2 [Zn(miM)(HmiM) m-1 L n ] + 2 Zn 2+ + H 2 O 2 + O 2

Galvanic replacement

Bimetallic nanostructures exhibit unique catalytic, electronic, optical, and magnetic properties that set them apart from their monometallic analogs [53]. Several methods such as precipitation [54], sol-gel [55], hydrothermal [56], galvanic replacement [57], and ALD [58] have been developed to precisely control their shape, size, and composition. Among these, the galvanic replacement approach represents one of the most versatile and simple methods that enable the synthesis of bimetallic and hollow materials with a high degree of control in their composition, surface morphology, and size [59]. The galvanic replacement reaction is a spontaneous redox reaction that occurs between a metal and ions of another metal in solution. The reaction is driven by the difference in standard electrode potentials of the metals, which results in dissolution (oxidation) of one of the metals and the plating of the second metal (reduction). Recently, the galvanic replacement reaction has been exploited to produce bimetallic nanostructures based on Au and Pd for applications in catalysis [60], sensing [61], and plasmonics [62]. Given the difference in reduction potential between Au 3+ /Au (1.4 eV) and Pd 2+ /Pd (0.195 eV) pairs, the replacement reaction can proceed as follows:

3 Pd 0 + 2 Au 3+ --→ 3 Pd 2+ + 2 Au {12}
When a Pd nanoparticle is in contact with a Au 3+ ion, the Pd atoms from the surface are quickly oxidized and dissolved to produce Pd 2+ ions into the suspension reaction. At the same time, Au 3+ are reduced and deposited on the surface of the nanoparticle. The composition of the resulting alloy can be changed by adjusting the amount of Au 3+ ions relative to the amount of Pd metal [63].

Physical vapor deposition

Physical vapor deposition processes are based on the production of a condensable vapor from a material (in liquid or solid form) and its subsequent condensation in a substrate to form a thin film [64]. In general, PVD processes are used to deposit films of elements and alloys with thicknesses that go from just a few nanometers to several microns [65]. Also, the technique presents exceptional flexibility in terms of the size (which can go as large as 10 ft × 12 ft) and the geometrical complexity of substrates. Depending on the way the vapor is produced, PVD processes can be classified into vacuum deposition (evaporation), sputter deposition, arc vapor deposition, and ion plating.

Electron beam evaporation

Vacuum evaporation by electron beam (E-beam) is a PVD process where intense beams of high-energy electrons are used to vaporize materials [66]. Fig. 3.7 presents a schematic representation of an E-beam evaporation system. E-beam evaporation allows the deposition of most pure metals (even those with high melting points) and is especially suitable for refractory materials such as most ceramics (oxides and nitrides), glasses, carbon, and refractory metals [67]. Typically, the electron beams used in this technique are accelerated to high voltages (10 kV to 15 kV) and are focused by electric or magnetic fields to bombard a material contained in a crucible. To avoid the interaction between the vaporized material and the gas molecules contaminating the system, the deposition usually takes place in the range of 10 -5 -10 -9 mbar. Additionally, deposition rates as high as 50 µm s -1 can be achieved.

Sputter deposition

The sputtering phenomenon is based on the bombardment of a solid surface by high-energy ions and the resulting physical (not thermal) vaporization of atoms from the surface by the momentum transfer [68]. The sputter deposition technique can be used to deposit films either by sputtering from an elemental (or an alloy) target or by sputtering from a compound target (where, in most cases, there is a loss of the more volatile material). Depending on the condition under which the sputtering is done, the process can be classified into direct current (DC), RF, magnetron, and ion beam sputtering [69]. Among these, DC sputtering presents the simplest configuration [70]. Fig. 3.8 shows a typical configuration of a DC sputtering system schematically. Generally, the DC configuration comprises a cathode (target), an anode (where the substrate to cover is placed), and a DC power source (supplying a potential between 0.5 keV to 5 keV) that are placed in a vacuum chamber at either low pressure (<5 × 10 -3 mbar) or high pressure (5 × 10 -3 mbar to 3 × 10 -2 mbar). The sputtering chamber is filled with Ar, and a plasma is created (and sustained) by applying a DC voltage between the electrodes. The Ar ions generated by the glow discharge are accelerated to the negatively biased target, and sputtering of the surface of the target occurs. The sputtered species proceed in the direction of the substrate, and a thin film is formed.

Conclusions

In the first half of this chapter, we described nano and micro fabrication processes that combine both top-down and bottom-up approaches. These techniques, which are currently used in many well-established and emerging applications in the industry, will continue to be key players in the ongoing industrial revolution. In the next section, the underlying principle of the characterization techniques used to study the physicochemical properties of the SiNWs and urchin-like structures will be discussed.

Characterization methods

It is well known that, once you obtain a new material, the first thing to do is to study its physical, chemical, optical, and mechanical properties. In particular, when a novel nanomaterial is produced, the characterization and measurement of its physicochemical properties represents an interesting challenge. Similarly, when the fabrication process of a material involves several steps, careful attention must be paid between each of the stages to make sure that no problems have occurred. In the second half of this chapter, the elemental aspects of the characterization techniques that were used to study the materials fabricated in this work are introduced.

Scanning electron microscopy

Scanning electron microscopy (SEM) is an electron probe characterization method that uses a high-energy electron beam for imaging and chemical analysis of a material. Fig. 3.9 shows the principal components of an SEM instrument. The electron gun at the top of the column produces a beam that is focused through a series of magnetic lenses and is rastered in a squared area of the surface of a sample. The variety of signals produced by the interaction of the electrons with the sample (e.g., secondary electrons, backscattered electrons, x-rays) are detected to provide information about the topography and composition of the sample (with the aid of a solid state x-ray detector for EDX). In general, there are no complicated steps to prepare samples, and the only requirement is for the samples to be sufficiently conductive (non-conductive samples are usually coated with a thin film of Pt).

The morphology of all the samples was characterized by a Hitachi S4800 SEM.

Transmission electron microscopy

Transmission electron microscopy (TEM) is based on the same principles as SEM except for the detector (direct imaging on a fluorescent screen or PC screen with a charge coupled device or CCD) that is capable of capturing images generated by transmitted electrons. Fig. 3.10 shows the principal components of a TEM apparatus. Moreover, TEM uses even higher voltages than SEM (60 keV to 300 keV compared to 1 keV to 50 keV) and can achieve a spatial resolution of up to ∼50 pm (∼0.5 nm for SEM). Due to the requirement of transmitted electrons, the samples must be very thin (<150 nm for regular TEM and <30 nm for highresolution TEM). This thinning usually involves tedious and complex preparation techniques that require a trained and experienced user. TEM experiments were performed in an HR-TEM JEOL ARM-200F, working at 200 kV, equipped with an EDX detector.

Atomic force microscopy

Atomic force microscopy (AFM) is another powerful imaging technique that, by scanning in a grid-like fashion an atomically sharp probe tip over a surface, can produce topographical images and also, with some modifications of the tip, allows to measure the forces of adhesion between the surface of the sample and the tip [72]. A generic AFM system is shown in Fig. 3.11. The sample is positioned on a piezoelectric stage that can move in the x, y, and z direction. The cantilever tip is positioned close to the surface of the sample. The stage is moved laterally to scan the desired area of the sample. As the cantilever moves over the sample, it becomes deflected due to the topography of the surface. The degree of deflection is measured by following changes in the reflected laser beam that moves on the photodetector. The distance between the cantilever and the sample is held constant by an electronic feedback control that adjusts the z-position of the sample. There are two main configurations for an AFM system. In contact mode, the tip of the cantilever remains in intimate contact with the sample while the stage is moving. This mode is appropriate for hard materials as it does not damage their surface. For weakly-bound specimens or softer materials, the tapping mode is used. In this mode, the tip of the cantilever has intermittent contact with the sample. This mode prevents damage to the sample and allows to image the amplitude and phase difference of the resonance vibration. The affinity of the tip for the surface of the sample can be used to create a chemical map based on the differences between the materials [73]. AFM was performed with a NANOMAN 5 device from Veeco, and it was controlled with the Nanoscope V software in tapping mode.

X-ray diffraction

X-ray diffraction (XRD) is one of the most noteworthy analytical methods to analyze the crystalline structure of materials. X-rays get diffracted because the wavelength of x-rays is in the same order of magnitude of the spacing between atoms and planes of atoms of a crystalline compound. The diffracted x-rays provide a pattern that can be used to determine the structural orientation of the atoms in a given compound. One of the keystones of XRD is the Bragg equation:

nλ = 2d sin θ (3.1)
In this equation, n is an integer (1,2,3,...n), λ is the wavelength, d is the interplanar distance between atomic planes, and θ is the angle of incidence of the x-ray beam with respect to these planes. Crystalline materials show long-range periodic structures that are displayed as Bragg diffraction peaks. In the simplest case, an XRD measurement consists of a group of intensities and the angles at which they are observed [74]. The resulting pattern represents the fingerprint of the material and can be later used for chemical identification by comparing it to diffraction patterns found in databases such as the International Centre for Diffraction Data (ICDD). For this work, XRD was performed with a PANalytical X'pert-PRO diffractometer equipped with an X'celerator detector using Ni-filtered Cu Kα radiation).

X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is technique that allows to study the surface chemistry of a material [75]. The technique involves irradiating a material in a high vacuum (∼1 × 10 -8 mbar) with monochromatic or polychromatic soft x-rays and analyzing the kinetic energy of the detected electrons coming from the top 1-10 nm of the material being analyzed. The emitted electrons have measured kinetic energies given by:

Materials and methods

KE

= hν -BE -φ s (3.2)
were hν is the energy of the photon, BE is the binding energy of the atomic orbital from which the electron originates, and φ s is the spectrometer work function. By counting these emitted electrons over a range of kinetic energies, a photoelectron spectrum is obtained. The position and intensity of the peaks in this spectrum enable the proper identification and quantification of the elements in the material. Changes in the chemical state of an element can be determined by shifts in the BE due to changes in the measured KE. For instance, the BE of the core electrons will increase if the valence electrons are slightly more distant to the core due to the formation of bonds with other elements [76]. XPS measurements were performed with a VG ESCALAB 250 spectrometer (ThermoFisher Scientific, Waltham, MA, USA).

Photoluminescence spectroscopy

Photoluminescence (PL) spectroscopy is a nondestructive characterization technique used to study the electronic structure of materials, especially for semiconductors. In some ways, the emission of photons from semiconductors is the reverse of the absorption of photons. However, emission only occurs in a narrow band of empty states that produces a spectrum much thinner than that of the absorption (which can involve all states in a semiconductor) [77]. Fig. 3.12 shows a schematic representation of the PL process. In a PL spectroscopy setup, a laser beam (with energy much larger than the bandgap of the semiconductor) is directed toward a sample to generate free electrons and holes in the conduction and valence bands, respectively. These excited carriers can later relax and recombine by emitting light with the energy of the bandgap [78]. The emitted photons are dispersed with a monochromator and detected with a CCD or a photomultiplier tube. Fig. 3.13 shows a schematic representation of the experimental setup for PL spectroscopy. While it is possible to study almost all semiconductor materials with PL, only a weak signal can be measured from indirect bandgap semiconductors. Thus, for the case of direct bandgap semiconductors (e.g., ZnO) an abundant amount of information regarding their localized defects and impurities (and their concentration), optical emission efficiencies, and composition of the material (i.e., alloy composition) can be extracted by analyzing the PL spectrum.

Raman spectroscopy

Raman effect

The interaction of light with matter can result in either absorption, scattering, or no-interaction with the material (photons pass through it). The inelastic scattering of light was first described theoretically by Smekal in 1923 [80] and observed experimentally for the first time in 1928 by Raman and Krishnan [81]. In Raman spectroscopy, two types of scattering are involved. The first one is called Rayleigh scattering, and it occurs when the emitted photon has the same energy as the incident photon (elastic scattering). In the second case, the incident photon can transfer energy to the material (or vice versa), and it will be scattered with different energy (inelastic scattering). Light scattered with less energy than the incident light is called Stokes, while the one with higher energy is called anti-Stokes. Fig. 3.14 displays the elastic and inelastic light scattering processes. In the Stokes process, the material goes from a base level E 0 to virtual state with higher energy. However, in the anti-Stokes process, if the material is in an excited state E vib , the scattered photon will have higher energy and the material will return to the base level E 0 . At room temperature, the number of molecules in an excited state is minimal and, for this reason, the anti-Stokes process is less intense. In general, Raman spectroscopy involves only the detection of the Stokes signal [82].

Surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a technique that is used to amplify by several orders of magnitude (reaching 8-9 orders of magnitude per single molecule) the Raman signals coming from a material [83]. The resulting peaks in a Raman spectrum correspond to vibrational modes of the analyte.

The effect was first observed by Fleischman et al. [84] and further explained firstly by Jeanmaire and Van Duyne [85] and secondly by Albrecht and Creighton [86]. The amplification effect of the Raman signal comes from two main mechanisms: electromagnetic (EM) and chemical (CE). The first involves the electromagnetic interaction of light with metals through excitations known as plasmon resonances. The second mechanism is related to changes in the polarizability of the probe due to its adsorption on the metal surface. Although both mechanisms are multiplicative, it is widely accepted that the main contribution to the enhancement comes from the EM mechanism [87]. To perform a SERS experiment, care must be taken while choosing the metallic substrate (usually Ag or Au), the characteristics of the laser excitation (e.g., wavelength), and the adsorption properties of the analyte. In general, Au and Ag are the two Materials most used metals for the fabrication of SERS substrates [88] due to their optical properties (their plasmon resonances are in the visible/infrared) and their ease of manipulation for preparing nanostructures.

In this work, Raman measurements were performed with an InVia Raman spectrometer equipped with three different lasers (532 nm, 633 nm, and 785 nm) and an optical microscope with various objectives (50 × -0.50 NA and 100 × -0.9 NA) in backscattering geometry. Fig. 3.15 shows a schematic representation of the Raman spectrometer used in this work.

Conclusions

The structural, morphological, chemical, and optical properties of nanomaterials and nanostructures can be achieved by using a combination of techniques such as SEM, TEM, AFM, XRD, XPS, PL, and Raman spectroscopy. The necessary information to successfully fabricate and test a nanostructure/nanomaterial can be extracted by using the aforementioned techniques.

Materials

Polystyrene spheres (aqueous dispersion, 10 % w/w, size 1 and 0.5 µm), sodium dodecyl sulfate (SDS, CAS: 151-21-3, purity > 99 %), hydrofluoric acid (HF, CAS: 47590, 48 % w/w), hydrogen peroxide (H 2 O 2 , CAS: 31642, 30 % w/w in H 2 O), nitric acid (HNO 3 , CAS: 30709, ≥65 % w/w), and hydrochloric acid (HCL, CAS: 258148, 37 % w/w), and 2-Methylimidazole (HmiM, CAS: 693-98-1, > 99 %) were purchased from Sigma-Aldrich. Carboxylate-modified polystyrene spheres (aqueous dispersion, 4 % w/w, size 5 µm) were purchased from Thermo Scientific. ZnCl 2 (CAS: 7646-85-7, purity > 98 %) and potassium chloride (KCL, CAS: 7447-40-7, purity > 99 %) were purchased from Fluka. Thiophenol (C 6 H 5 SH, CAS: 108-98-5, purity > 99 %) and hydrogen tetrachloroaurate (III) trihydrate (HAuCl 4 • 3 H 2 O, CAS: 16961-25-4, purity > 99 %) were purchased from Alfa Aesar. Diethylzinc ((C 2 H 5 ) 2 Zn, DEZ, CAS: 557-20-0, purity > 95 %) was purchased from Strem Chemicals. Boron-doped (8-25 Ω cm) p-type (100) crystal orientation Si wafers were purchased from LG Siltron Inc. Korea. Indium tin oxide (ITO) deposited on quartz was purchased from Präzisions Glas & Optik.

Experimental details for silicon nanowires covered with

ZnO by ALD

Fabrication of silicon nanowires covered by ALD

Fig. 3.16 presents a schematic view of the overall process for the synthesis of SiNWs. A Si wafer was cut into 1 cm × 1 cm pieces and were cleaned sequentially with deionized (DI) water(18.2 MΩ cm), ethanol and acetone by ultrasonication for 15 min in each solvent. Then the substrates were treated by oxygen plasma to have a hydrophilic surface. The floating-transferring technique was used to deposit polystyrene spheres on the Si substrates. Briefly, 40 µL of polystyrene spheres diluted by an equal amount of ethanol, was applied onto the modified substrates. After holding the substrate stationary for a while to disperse the solution, the wafer was then slowly immersed into the glass vessel filled with DI water, and polystyrene spheres started to form a monolayer on the water surface. After, 5 µL of an SDS solution (10 % w/w) were added to the water interface to change the surface tension and consolidate the particles. As a result, a large monolayer with highly ordered areas was obtained. Then, this monolayer of polystyrene spheres was transferred to the target substrate. The quality of the polystyrene spheres monolayer mask can be assessed right away by looking at the uniformity of the color throughout the whole area.

The reflected color of the pattern varies with the size of the spheres and the quality of the arrangement. After drying the sample in air at room temperature, the spheres were self-assembled into a close-packed, two-dimensional ordered lattice via attractive capillary forces.

After the consolidation of the polystyrene spheres monolayer, the diameter of the polystyrene spheres was decreased by oxygen plasma etching for 5 min to expose a part of the surface of the wafer for metal deposition. In order to stick the polystyrene spheres to the Si surface, a heat treatment at 100 • C for 30 min was performed. Next, a 30 nm Au film was deposited by sputter deposition. The sputtering was carried out at a discharge of 25 mA in a vacuum with a pressure Experimental details for silicon nanowires covered with ZnO by ALD

Characterization of ZnO deposited by ALD on silicon nanowires

The XRD patterns were measured in the 2θ angular region between 10°and 60°w ith a scan speed of 2 °min -1 and a step rate of roughly 0.02 °s-1 . From the XRD patterns, the grain size was calculated using the Debye-Scherer equation. SiNWs covered by ALD of ZnO were also investigated by TEM. The cross-sections and lamellas for TEM investigations were prepared by Focused Ion Beam (FIB).

The FIB milling was carried out with a JEOL, JIB-4000.

Raman scattering was measured in backscattering geometry with a spectral resolution of 1 cm -1 . The incident light was not polarized, and also the light detector contained no polarization filters. Raman spectroscopy was performed with a 488 nm laser. The beam was focused on the samples with a 50 × microscope objective with a NA = 0.4. The incident optical power was changed by using neutral density filters in the beam path. PL spectroscopy was studied in the spectral range of 350 nm to 800 nm with an excitation wavelength of 280 nm.

To test the capability of ZnO for detecting H 2 O 2 , a home-made PL setup was used. The PL was studied in the spectra range of 360 nm to 600 nm with an excitation wavelength of 330 nm. The samples were introduced into a black 53 3. Materials and methods anodized aluminum chamber filled with a solution of 0.01 m phosphate buffered saline (PBS) and varying concentrations of H 2 O 2 .

Experimental details for Au-covered hollow urchin-like

ZnO structures

Fabrication of Au-covered hollow urchin-like ZnO structures

Fig. 3.17 presents a schematic view of the overall process for the synthesis of Au-covered ZnO urchin-like structures. First, ITO substrates 1 cm × 2 cm were cleaned by sonication for 15 min in acetone, ethanol, and isopropanol sequentially and finally dried under air. After, the PS spheres were deposited on the ITO substrates using a previously reported method with slight modifications [22].

Briefly, a 1:2 solution of polystyrene (PS) spheres and anhydrous ethanol was added dropwise with a micropipette onto the surface of a tilted glass slide (previously treated with oxygen plasma) that was resting on the wall of a Petri dish filled with DI water. Afterward, 5 µL of an SDS solution (10 % w/w) were added to the water surface to change the surface tension and pack together the polystyrene spheres. The self-assembled PS spheres were then transferred to the ITO substrate (previously exposed to a 4 W, 254 nm UV lamp for 15 min to render it hydrophilic) by carefully introducing the substrate into the Petri dish and removing it from the solution at an angle of 45°. Once dry, the substrates were heated in an oven at 100 • C for 30 min to promote the adhesion of the PS spheres to the substrate. Next, the size of the spheres was reduced to produce a non-close-packed arrangement by reactive ion etching in O 2 plasma (0.6 mbar, 50W). The size of the polystyrene spheres was controlled by adjusting the o 2 plasma exposure time. As an example, after being etched for 5 min, the diameter of the PS spheres was reduced to 460 nm, leaving an interparticle distance of 60 nm (Fig.

B.1).

Afterward, a 20 nm-thick ZnO layer was deposited on the PS spheres-covered substrates by ALD. The ZnO films were grown at 80 • C with the parameters showed on Table 3.1. Subsequently, electrodeposition of ZnO was performed using a previously reported procedure [40]. A three electrode configuration was used with the ITO/polystyrene spheres/ZnO ensemble as a working electrode, a Pt plate as the counter electrode, and Ag/AgCl as the reference electrode. An aqueous solution of 0.05 mm ZnCl 2 (zinc precursor) and 0.1 m KCl (supporting electrolyte) was used as the electrolyte and was continuously bubbled with O 2 during the whole electrodeposition process. The electrodeposition was carried out at 80 • C with a constant electric potential of -1 V during 15 min in a VersaSTAT 3 potentiostat. Once the electrodeposition was completed, the PS spheres were burned off in air at 600 • C for 2 h. Finally, a Au layer (10, 30 and 50 nm) was deposited by E-beam evaporation on top of the substrates.

Experimental details for Au-covered hollow urchin-like ZnO structures

Characterization of Au-covered hollow urchin-like structures

The morphology of the as-prepared Au-covered urchin-like ZnO samples was characterized by SEM (Fig. 3.18).

In addition, samples were prepared for TEM experiments by performing angular cuts using FIB using a gallium source working at 10 kV, with 60°, 40°and 30°incident angle geometries over a large area (50 µm). In this way, several particles were cut at different axis and geometry. TEM experiments were performed at 200 kV. Samples were sonicated in ethanol after ion milling and drop cast on commercially available Cu grids and vacuum dried overnight in a desiccator. Fig. 3.18 displays TEM and EDX measurements for the Au-covered ZnO urchin-like structures.

The XRD patterns were measured in the 2θ angular region between 20°and 70°with a scan speed of 2 °min -1 and a step rate of roughly 0.02 °s-1 . Fig. 3.18 shows the XRD patterns of ITO, urchin-like ZnO, and Au-covered urchin-like ZnO structures for comparison. SERS measurements were performed with a Raman microscope with a 785 nm laser and a 100 × objective lens with a NA = 0.9 in a backscattering geometry. The samples were irradiated with laser powers of 0.48 mW (633 nm) and 0.55 mW (785 nm) with acquisition times between 1 s and 10 s. For comparison, all the collected spectra were normalized by laser power and acquisition time. The average SERS intensities reported and relative standard deviations (RSD) were calculated with 25 SERS spectra. in DI water with a final volume of 2 ml was prepared. The HAuCl 4 concentration was chosen according to the Pd loading in our sample. The HAuCl 4 solution was poured into a small vial and was heated in a silicon oil bath at 70 • C. The SiNWs/Pd sample was introduced into the previously heated solution and was left to react for 16 h. Finally, the SiNWs/Pd/Au sample was thoroughly washed with DI water and left to dry at ambient temperature.

The morphology and composition of the SiNWs/Pd/Au samples was investigated with SEM and EDX. The composition was also studied by XPS with an excitation source of Al Kα (1486.6 eV). The analyzed surface had a diameter of 500 µm. The BEs of the obtained peaks were calibrated with the C1s peak for -c-c -bonds at 284.8 eV.

Experimental details for hybrid Pd/Au and ZnO/Au/ZIF-8 materials

Fabrication and characterization of ZnO/Au/ZIF-8 hollow urchin-like structures

Au decorated hollow ZnO urchin-like structures were prepared following the procedure displayed in Section 3.5.1 with slight modifications. Briefly, 5 µL of an SDS solution (10 % w/w) were added to the surface of a Petri dish filled with DI water. After, a 1:2 solution of 5 µm carboxylate-modified polystyrene spheres and ethanol was added dropwise to the water surface of the Petri dish. After the polystyrene spheres had formed a monolayer, the O 2 plasma treated ITO substrate was carefully introduced into the Petri dish and removed from the ) with a potential range of -1 V to 0 V was performed during 4 h. After deposition, the samples were washed with copious amounts of methanol and dried at ambient temperature. The sample morphology was characterized with SEM, while the chemical composition was studied with XRD.

Conclusions

Judging from the various techniques used in combination with ALD, we can say with confidence that it is possible to fabricate nanostructures with precise control of the size, morphology, and composition. For instance, SEM, TEM, and AFM were used to confirm the high aspect ratio of the SiNWs and urchin-like structures. Specifically, by performing SEM after each of the fabrication steps, the quality of the resulting nanostructures could be assessed. In the case of TEM, the crystalline quality of ZnO in both structures could be studied, and a better understanding of the individual features and details could be achieved. Equally important, techniques like XRD, XPS, and Raman are crucial for the correct identification of each of the chemical elements found on the produced nanostructures. As an illustration, XRD was used to evaluate the crystallinity of the ZnO covering the SiNWs. In a similar way, XPS was used to determine the oxidation state of Pd and Au from the Pd/Au decorated SiNWs structures. Equally important, the photoluminescence and Raman spectroscopies were used to test the feasibility of the as-prepared samples to be used as transducers for sensing.

In the next three chapters, the morphological, chemical, and optical characteristics of the prepared nanostructures will be discussed in detail. In the case of SiNWs/ZnO structures, a preliminary application for H 2 O 2 sensing will be shown. Similarly, the application of the ZnO/Au urchin-like structures as potential SERS substrates will be demonstrated by detecting two different analytes. Finally, the potential applications of SiNWs decorated with Pd/Au nanoparticles and urchin like ZnO/Au/ZIF-8 structures will be discussed.

Chapter 4

Si nanowires covered with ZnO by atomic layer deposition for photoluminescence sensing * Silicon (Si) continues to be the most widely used semiconductor due to its high stability and non-toxicity, quantum confinement effects, high carrier mobility, and well-established nanofabrication techniques [1,2]. Recently, one dimensional (1D) silicon nanostructures (e.g., SiNWs and Si nanopillars) have received considerable attention thanks to their abilities to scatter and trap incident light, large surface to volume ratio, and other unique electronic and optical properties that make them attractive for a wide range of applications such as electronic devices [3][4][5], energy storage devices [6,7], thermoelectrics [8][9][10], and biosensors [11][12][13][14].

The first preparation of Si whiskers with <111> orientation was reported in 1957 by Treuting and Arnold [15]. In 1964, Wagner and Ellis performed an illuminating work and established the vapor-liquid-solid (VLS) mechanism for the growth of Si whiskers [16]. These pioneering studies opened up exciting possibilities for the fabrication and investigation of SiNWs. In 2002, Peng et al. introduced an HF-etching-assisted nanoelectrochemical strategy to synthesize wafer-scale aligned SiNWs [17]. To date, SiNWs can be fabricated by different methods such as CVD [18], laser ablation [19], molecular beam epitaxy [20], chemical etching [21], and solution growth [22]. Among these preparation methods, the metal-assisted chemical etching (MACE) of silicon substrates in combination with NSL has emerged as a promising method to fabricate large areas of ordered SiNWs [7,23]. MACE and NSL techniques have experience an increasing attention, mainly because MACE is an inexpensive and straightforward process that allows controlling various parameters of the etched nanostructures such as shape, height, diameter, and crystallographic orientation [24][25][26]. The main advantages of NSL are its short preparation time, high level of hexagonal structure orientation, and the possibility of applying it directly onto different types of surfaces [27].

ZnO has attracted intensive research effort for its unique properties such as thermal and chemical stability, optical transparency, and piezoelectricity [28]. The addition of ZnO as shells around SiNWs (as cores) can have substantial and beneficial impact on the stability, as well as on the mechanical, photoelectrochemical, and sensing properties when compared to bare ZnO nanowires fabricated * Parts of this chapter have been published as: O. Graniel, V. Fedorenko, R. Viter, I. Iatsunskyi, G. Nowaczyk, M. Weber, K. Załęski, S. Jurga, V. Smyntyna, P. Miele, A. Ramanavicius, S. Balme, M. Bechelany, Optical properties of ZnO deposited by atomic layer deposition (ALD) on Si nanowires, Materials Science and Engineering: B, 236-237 (2018) 139-146. doi:10.1016/j.mseb.2018.11.007.

Si nanowires covered with ZnO by atomic layer deposition for photoluminescence sensing

with other techniques [29][30][31]. Similarly, Si is an attractive substrate thanks to its excellent thermal conductivity, superior crystalline quality, low cost, and size availability with different types of doping.

ZnO thin films can be prepared using various techniques, but the conformality of the deposit required for its preparation as a shell around NWs requires the use of ALD [32][33][34][35]. To date, ALD is one of the most promising deposition methods for growing uniform thin films, especially in the cases where precise film thickness control, high reproducibility, thickness uniformity, and excellent conformity are required [36,37].

In the present work, SiNWs produced by MACE in combination with NSL followed by an ALD deposition of ZnO are investigated. The structural and optical properties of the obtained nanostructures are evaluated. Finally, the SiNWs/ZnO hybrid structures are tested as possible transducers for H 2 O 2 sensing by photoluminescence spectroscopy.

Results and discussion

The final diameter of the SiNWs depends on the O 2 plasma etching time as seen in Fig. 4.1. The etching of polystyrene spheres from 1 to 10 minutes allows decreasing the polystyrene spheres diameter from (960 ± 10) nm to (610 ± 10) nm. Fig. 4.1a shows a SEM image of PS spheres deposited on a Si wafer, which confirms the possibility to obtain a relatively large area, close-packed, hexagonal polystyrene monolayer, produced by the floating-transferring technique. Fig. 4.1f shows a photograph of a monolayer pattern of polystyrene spheres prepared by the mentioned technique on a 2 cm × 2 cm silicon wafer. The substrate presents a characteristic color that is caused by the diffraction of light from the polystyrene sphere array. Fig. 4.2 shows the final structure of SiNWs (etching time is 5 min) after depositing 50 nm ZnO film by ALD. The SEM images display the conformal ZnO coating on the SiNWs by ALD. Fig. 4.3 (a) 2D and (b) three dimensional (3D) images of the SiNWs covered with a 50 nm ZnO layer by ALD. The array of SiNWs exhibit hexagonal packing over a large area of 15 µm × 15 µm as previously shown by SEM. Due to the high aspect ratio of the SiNWs, the AFM probe cannot reach the bottom part of the sample and degrades quickly. For this reason, the total length of the SiNWs cannot be seen by AFM and would require the use of exotic FIB milled tips and carbon nanotube/fiber tips [38].

XRD patterns of SiNWs (etched for 7 min) covered with 20 and 50 nm of ZnO by ALD are shown in Fig. 4.4. Three prominent peaks appear at 2θ = 31.78°, 34.35°, 36.25°and 56.69°, which correspond to the (100), (002), (101), and (110) reflections of the hexagonal wurtzite phase of ZnO, respectively. This indicates that both films are polycrystalline, as commonly reported for ZnO films deposited by ALD [34,39,40]. A peak with low intensity at 2θ = 47.46°a ppears for the 50 nm thick sample, which corresponds to the (102) reflection of ZnO. The grain sizes D of the deposited films are estimated using the following formula [41]:

D = 0.9λ β cos θ (4.1)
where λ is the wavelength of X-ray used (λ = 0.154 nm), β is the full width at half maximum intensity in radians, and θ is the Bragg angle. The average value of grain size is found to be (7.50 ± 0.45) nm, and (14.0 ± 6.5) nm for samples etched for 7 minutes and covered with 20 and 50 nm of ZnO, respectively. Fig. 4.5 shows the high-resolution TEM images of ZnO/SiNWs with different ZnO layer thicknesses. SiNWs coated with 20 nm of ZnO by ALD are presented in Fig. 4.5a. The TEM images show that the 20 nm ZnO layer covers conformally and homogeneously the SiNWs ZnO. The distance between the nearest wires is reduced from 150 to 300 nm. Also, we noticed that the diameter of the wires is about 450 nm on the top and 850 nm at the bottom. Thus, the wires have the shape of a truncated trapezoid. This shape could be due to the longer exposure time of the top part with the etching solution according to Dawood et al. [42]. Macroporous structure of the wires could also be observed (Fig. 4.5a). This can be explained by the lateral transport of the charge carriers [23].

Fig. 4.5b shows a TEM image of silicon nanowires covered by a 50 nm ZnO layer. This layer has a maximum thickness of 55 nm at the top of the SiNWs. For both samples, the ZnO layers have a polycrystalline phase. The size of nanocrystallites was estimated by doing an elliptical shape fit, and the longer axis was used as the nanocrystallite size. The average grain size for the 50 and 20 nm ZnO layers was almost the same (11.8 ± 2.5) nm and (10.0 ± 2.5) nm, respectively), which confirmed the results obtained by XRD. The TEM images demonstrate the ability to produce highly uniform layers of ZnO on SiNWs by using ALD. high]) of Si is observed at 520 cm -1 due to light penetrating through the thin ZnO films [43]. However, by increasing the number of accumulations, a Raman peak at (432 ± 2) cm -1 (E 2 [high]) that corresponds to the wurtzite phase of ZnO, was observed (inset of Fig. 4.6). This mode is associated with the motion of oxygen photoluminescence sensing atoms [43][44][45][46]. The E 2 (high) mode peak (full width at half maximum [FWHM] of ∼20 cm -1 ) is broader than the one for bulk ZnO (FWHM is less than 10 cm -1 ) and is shifted to higher frequencies or blue shifts. The broadening of the peak and the blue shift are attributed to phonon confinement effects [47]. ). The spectra deconvolution showed peaks centered at 376-379, 411-415, 434-437, 447-480, 490-540, 570-640 nm, and 660-740 nm related to free exciton, Zn interstitials, Zn vacancies, neutral oxygen vacancies, single charged oxygen vacancies, double charged oxygen vacancies, and surface defects, respectively [38,56,68,69] [34, 40, 48, 49]. The domination of defect emission bands points to deviations in the stoichiometry of the prepared 3D ZnO nanostructures. The decrease of PL intensity for ZnO samples deposited on 7 min etched SiNWs could be related to higher light scattering caused by their length. It is suggested that both the high surface area and the anti-reflective properties of the nanostructures might have an impact on the overall PL emission [50].

As a first approach for biosensing, the SiNWs/ZnO structures etched for 5 min and covered with a 50 nm layer of ZnO were tested for the detection of hydrogen peroxide H 2 O 2 , which is catalytically produced by many oxido-reductase enzymes when acting on a substrate. The samples etched for 5 min and covered with 50 nm of ZnO were selected because they displayed the strongest PL emission that allows improving the overall signal-to-noise ratio. This concept has been reported for the indirect detection of glucose by Sodzel et al. by using ZnO The PL intensity decreases in the presence of H 2 O 2 and continues to do so as bigger amounts are added into the solution. The nature of this PL quenching can be explained by a collisional mechanism presented schematically in Fig. 4.9. After exciting ZnO, its charge carriers are separated into electrons and holes that go to the conduction and valence bands, respectively.

The radiative recombination of these carriers is followed by the emission of a photon with energy close to the ZnO bandgap (near band edge emission or NBE) or by relaxation through deep level effects (deep level defect emission or DLE) [51]. At the same time, H 2 O 2 decomposes catalytically into H 2 O and O 2 on the surface of ZnO by acting as an electron acceptor. This acceptance of electrons from the conduction band of ZnO prevents its radiative recombination and quenching of the PL occurs. In addition, several reports show that H 2 O 2 can reduce deep level defects by filling O or Zn vacancies to form ZnO, which could account for the quenching of the DLE (but not for the NBE) [52][53][54].

Besides quenching of the PL emission, we observed a shift of the main DLE peak (∼418 nm as we increased the concentration of H 2 O 2 . Table 2.1 shows the position of the four main peaks after Gaussian fitting with Origin software (see Appendix Fig. A.2). The violet emission centered at ∼418 nm has been attributed to zinc vacancies [55], interstitial zinc [56], and interface traps [57], which reflects the poor consensus among the different sources in the literature. As we increase the H 2 O 2 concentration, a red shift is observed, reaching ∼431 nm for the highest H 2 O 2 concentration. This blue emission has been attributed to oxygen vacancies at the surface of ZnO [58]. However, as previously mentioned, H 2 O 2 can fill O or Zn vacancies or form ZnO, which could decrease the number of oxygen vacancies and would contradict the reports on the origin of the blue emission.

Conclusions

In summary, we have demonstrated a simple method for the fabrication of ordered aligned SiNWs/ZnO core shell nanostructures. The method combines colloidal lithography with MACE. The conformal deposition of ZnO on the SiNWs was achieved by using ALD. The ordered SiNWs produced by this method may find many applications in array devices such as field-effect transistors, sensors, electrodes, and two-dimensional photonic crystals. The detailed study of structural and optical properties of the core-shell SiNWs/ZnO heterostructures was presented. The X-ray diffraction analysis revealed that all samples have a hexagonal wurtzite structure. The grain sizes, as measured using XRD data, were found to be in the range of 7 -14 nm and were confirmed by TEM. The TEM and SEM images demonstrated the ability to produce highly uniform layers of ZnO covered silicon nanowires by the ALD technique. The study of PL spectra of SiNWs/ZnO showed the domination of defect emission bands, which points to deviations in the stoichiometry of the prepared 3D ZnO nanostructures. We also observed the reduction of the PL intensity of SiNWs/ZnO etched for 7 min that could be due to the higher light scattering caused by increasing the length of the nanowires. Finally, we tested the SiNWs/ZnO structures as transducers for the detection of H 2 O 2 . Quenching of the PL signal was observed, and a mechanism for this effect was proposed. These results open new perspectives for the preparation of optical and sensing devices.

Chapter 5

Au-covered hollow urchin-like ZnO nanostructures for surface-enhanced Raman scattering sensing * Surface-enhanced Raman spectroscopy (SERS) has emerged as one of the most powerful analytical techniques that offer single-molecule detection [1]. Its outstanding sensitivity and non-destructive character have found many applications in many fields such as chemistry [2], medicine [3], biology [4], and environmental sciences [5]. SERS substrates are mainly fabricated with noble metals (Ag, Au, Cu) due to their strong interaction with electromagnetic waves and the excitation of the localized surface plasmon resonance (LSPR) that can be tuned in the ultraviolet-visible-near infrared (UV-Vis-NIR) region [5,6]. These noble metals are usually combined with nanostructures such as nanospheres [7], nanowires [8], nanogaps [9], nanotrees [10], and nanorods [11] that provide roughened metal surfaces and plasmonically active "hot spots" that enhance the Raman signal by several orders of magnitude [12]. Additionally, thanks to their inherent SERS activity [13], semiconductors have been used to fabricate metal/semiconductor hybrid nanostructures that exhibit both electromagnetic enhancement and charge-transfer effects [14]. Furthermore, these highly-efficient SERS hybrids present additional properties that make them attractive for applications such as photocatalysis [15], water splitting [16], and solar energy conversion [17]. As a semiconductor material with a wide direct bandgap (3.37 eV), biocompatibility [18], and promising optical properties [19], ZnO has been employed for the construction of high-surface-area SERS substrates with a wide range of tunable morphologies [20]. However, the fabrication of reproducible and stable SERS substrates remains a significant challenge [21]. Therefore, much attention has been paid to find facile, repeatable, and high-throughput fabrication methods for large-area nanostructured substrates. Among these methods, NSL has proven to be a reproducible and inexpensive technique that offers well-ordered arrays of nanostructures with wafer-scale throughput [22,23]. Thus, NSL has been used in combination with other techniques to fabricate different types of SERS substrates such as silver film over nanosphere (AgFON) [24], gold semishells [25], nanoring cavities [26], nanopyramids [27], and optrodes [28]. Recently, high-surface-area urchin-like structures decorated with Ag have been successfully used as SERS substrates [29][30][31]. Nevertheless, even though Ag is preferred over Au due to its higher * Parts of this chapter will be submitted to the Journal of Materials Chemistry C 5. Au-covered hollow urchin-like ZnO nanostructures for surface-enhanced Raman scattering sensing SERS activity, Ag is prone to oxidation in air or water environments [32]. Consequently, decreasing its SERS enhancement factor (EF) as a result of changes in its chemical and plasmonic properties [33]. Herein, we report a high-surface-area reproducible SERS substrate consisting of Au-covered ZnO urchin-like hollow structures for the detection of thiophenol. This substrate was fabricated by using scalable techniques that include NSL, ALD [34], electrodeposition, and electron beam (E-beam) evaporation. NSL was used to obtain a template of well-organized spheres that were later covered with a seed layer of ZnO deposited by ALD. Afterward, ZnO nanowires (NWs) were grown via a hydrothermal method to obtain the urchin-like structures which were finally subjected to E-beam evaporation of Au.

Results and discussion

Among the different approaches to fabricate ordered arrays of PS spheres, the direct assembly at the air-water interface has been demonstrated to be a simple and low-cost method that offers high-quality monolayers over large areas. Fig. 3.18a shows a SEM micrograph of a large area monolayer of PS spheres with a high degree of order. In order to produce a non-closed-packed monolayer, the PS spheres are exposed to oxygen plasma (Fig. 3.18b). This treatment allows controlling the size of the urchins and space between them. After reducing the size of the PS spheres, a thin 20 nm ZnO layer was deposited by ALD as a seed layer to promote the growth of ZnO NWs (Fig. 3.18b). Owing to its high conformality and homogeneity, the ALD method promotes the uniform generation of ZnO NWs on the surface of the PS spheres, unlike pure hydrothermal methods or sputtering were the thickness control of the seed layer becomes a challenge [35]. In addition, the ALD layer anchors the PS spheres to the ITO substrate and renders their surface electrically conductive for the electrodeposition process. Fig. 3.18c shows the morphology of the urchin-like ZnO structures. Thanks to the homogeneous ALD seed layer, the electrodeposited ZnO NWs are evenly distributed on the surface of the PS spheres. The size and length of the ZnO NWs of the urchin-like structures deposited by electrodeposition can be tuned by changing the electrochemical conditions (i.e., zinc precursor concentration and charge density) [36]. Finally, the urchins are homogeneously covered by a Au film via E-beam evaporation with thicknesses of 10 nm, 30 nm and 50 nm. The morphology and crystallinity of the urchin-like structures were studied by TEM. Fig. 3.19a clearly shows the Au film that covers the top of the urchin-like structures and the 20 nm ZnO seed film deposited by ALD. Notably, the TEM images display a cavity with a reduced contrast and a shell with a uniform dark gray color (Fig. B.3), clearly showing the hollow character of the structures. The high-resolution (HR) TEM image from Fig. 3b shows the lattice fringes that correspond to ZnO and Au. Likewise, the fast Fourier transform (FFT) of the HRTEM image shows the (002) plane of ZnO. Additionally, to further confirm that the PS core of the urchin structures is removed after annealing, an EDX mapping was performed. The EDX elemental mapping (Fig. 3.19d) shows that Zn and O are located in the core of the urchin, while Au is covering the outer shell of the structure.

XRD measurements were carried out to study the cristallinity of the samples. The XRD patterns of ITO, urchin-like ZnO, and Au-covered urchin-like ZnO structures are displayed in Fig. 3.20. The diffraction peaks at 31.8°, 34.4°and 36.4°can be assigned to the hexagonal wurtzite planes of (100), (002), and (101) of ZnO [37,38], while the peaks at 38.2°, 44.5°and 64.6°can be assigned to the face-centered-cubic (fcc) planes of (111), (200), and (220) of Au [39,40]. These results show the crystalline nature of the samples and the successful fabrication of a ZnO/Au composite. The remaining peaks correspond to the ITO film deposited on quartz that is used as a substrate [41].

The SERS activity of the substrates was optimized by depositing different thicknesses of Au. Fig. 5.1 shows the Raman spectra of thiophenol molecules grafted on the ZnO urchin-like structures covered with 10 nm, 30 nm and 50 nm of Au deposited by e-gun evaporation. Thiophenol was selected due to its ability to form SAMs via S -Au bonds [42] and its non-resonance behavior near the selected excitation wavelengths (633 nm and 785 nm) [43].

The characteristic vibrational modes of thiophenol molecules (10 -3 m) are well-observed (see refs [44][45][46]) like those at 999 cm -1 corresponding to the ring out-of-plane deformation and the C -H out-of-plane bending (noted, respectively: r-o-d and γ(CH)), 1022 cm -1 corresponding to the C -C symmetric stretching and the ring in-plane deformation (noted, respectively: ν(CC) and r-i-d), 1072 cm -1 corresponding to the C -C symmetric stretching and C -S stretching (noted, respectively: ν(CC) and ν(CS)), and at 1572 cm -1 the C -C symmetric stretching (noted: ν(CC)). For comparison, we studied the intensity of the peak at 999 cm -1 . It is noticeable that the Raman peaks intensities become larger as we increase the Au layer thickness for the two excitation wavelengths. This is in accordance with literature reports where it is shown that the thickness of the metallic layer plays an important role on the overall SERS enhancement [47][48][49]. For our complete study, we chose the 30 nm-thickness with the excitation wavelength of 785 nm to obtain a good compromise between the SERS signal enhancement and a low overall cost of production of these SERS substrates. Fig. 5.2a shows the thiophenol spectra obtained from different structures for the purpose of comparison. When ZnO nanowires covered with Au (30 nm) of a size comparable from those on the ZnO urchin-like structures were used, the characteristic Raman peaks of thiophenol could be barely distinguished. Similarly, for the bare Au film (30 nm), the thiophenol peaks could be hardly identified. This result shows that the urchin-like structures present the highest SERS activity from these three types of substrate configurations.

Given the complex 3D morphology of the urchins, it is difficult to know the exact number of molecules that are excited in the SERS measurements [50,51]. For this reason, the SERS gain (G SERS ) for the 30 nm Au-covered ZnO urchins SERS substrates was calculated by using the formula: Thus, the highest G SERS value for the Raman peak at 1572 cm -1 was 5.3 × 10 3 )(Table 5.1). This outstanding G SERS can be attributed to the combination of different effects such as efficient light trapping [30] and the generation of hot spots from almost-touching NWs from adjacent urchins [52]. Additionally, the size of the gaps between the Au-covered urchin-like structures is in the same range as the excitation wavelength, which could contribute to the enhancement of localized electromagnetic fields and effective absorption of light by the LSPR [9,31]. SERS measurements with a range of concentrations from 10 -5 m to 10 -8 m were performed in order to assess the limit of detection (LOD) of the substrates . Fig. 5.2b shows the Raman spectra of the substrates grafted with different thiophenol concentrations. The intensities of the Raman peaks remain strong up to a concentration of 1 × 10 -7 m and then decrease considerably for the concentration of 10 -8 m where only the peak at 999 cm -1 remains visible. This LOD is comparable with values reported in the literature for thiophenol and similar Au-based structures [45,48].

G SERS ≈ I SERS I Raman ( 5
To demonstrate the large-area uniformity of the Au-covered hollow urchin-like structures, we constructed a Raman intensity map of the 999 cm -1 peak over a 20 µm × 20 µm area with a step size of 1 µm (Fig. 5.3a). The 400 point map shows an almost continuous bright-colored area with a few darker spots where the intensity is lower, which demonstrates the reasonable uniformity of the substrate.

Additionally, the substrate-to-substrate repeatability was investigated by measuring the intensity of the 999 cm -1 peak from three different batches. Fig. 5.3b shows the intensity distribution for three different samples. The average RSD value was found to be <10 % which indicates decent repeatability of the SERS substrates and shows their excellent performance.

Finally, to demonstrate the biomolecule sensing capability of these substrates, we chose the biomolecule adenine which can be used in the detection of DNA or RNA analytes [9,26]. Fig. 5.4 shows Raman spectra of adenine at three different concentrations. The characteristic purine stretch at 736 cm -1 is noticeable up to a concentration of 1 µm and the spectra show a low signal-to-noise ratio, which is beneficial for future biosensing applications and is comparable to results published in the literature [9,26].

Conclusions

In summary, high-surface-area Au-covered hollow ZnO urchin-like structures were prepared by combining NSL, ALD, electrodeposition, and E-beam evaporation. These high-throughput methods are readily scalable and allow the precise control of the size of the urchins and the interparticle distance. The amplification of the SERS signal of thiophenol was investigated by depositing different thicknesses of Au and the optimal thickness was selected. The LOD of the optimized substrates was tested by detecting thiophenol down to 10 -8 m. Additionally, the batch-tobatch repeatability was demonstrated and an average RSD <10 % was obtained. Finally, adenine was detected up to a concentration of 10 -6 m to demonstrate the biosensing capability of our substrate. These results suggest that the Au-covered hollow urchin-like ZnO structures are viable candidates for ultrasensitive and repeatable SERS sensing and biosensing. coming from Ag, Au, and Cu (10 6 ).

Thus, one of the strategies that have been proposed to improve the SERS activity of Pd has been to combine it with SERS-active materials like Ag and Au. This approach takes advantage of the strong electromagnetic field created by the Ag or Au SERS-active substrate when excited with the right excitation line. For instance, Hu et al. prepared core-shell Au@Pd structures by chemical deposition of Pd on Au seeds with different sizes [6]. The Au@Pd nanoparticles were deposited on a glassy carbon electrode for in-situ electrochemical SERS measurements of carbon monoxide (CO) to confirm that the Pd shell on the Au core was uniform. Furthermore, the Au@Pd nanoparticles displayed a higher SERS enhancement of more than two orders of magnitude when compared with bare roughened Pd electrodes. Similarly, Zhang et al. developed a seed-mediated growth method to produce highly branched concave Au/Pd nanocrystals with fivefold symmetric features [7]. Owing to their sharp multitips and edges, the Au/Pd nanostructures exhibited efficient SERS-activity and superior electrocatalytic activity. Crystal violet dye was detected at concentrations as low as 10 -12 m.

Results and discussion for silicon nanowires decorated

with Pd/Au nanoparticles Fig. 6.1 shows the SEM images of SiNWs decorated with Pd and Pd/Au nanostructures by combining ALD and galvanic replacement. After optimization of the deposition process, it was found that 100 ALD cycles produced an homogeneous coating of highly dispersed Pd nanoparticles on the surface of SiNWs (Fig. 6.1a,b). Next, Au was introduced in the Pd nanoparticles by performing a galvanic replacement reaction. Fig. 6.1d shows a top view image of the SiNWs decorated with Pd/Au nanostructures and confirms the preservation of the SiNWs architecture. Additionally, EDX was performed to confirm the presence of Pd and Au on the SiNWs as shown in Fig. 6.2. Considering that the depth of penetration of the EDX measurement is ∼500 nm, the majority of the signal comes from the SiNWs (Si, O) and only a small portion is due to the Pd/Au nanoparticles. [8].

The Pd/Au decorated SiNWs were further analyzed by XPS to determine the oxidation states of Pd and Au. The two peaks with binding energies of 84.5 eV and 88.2 eV correspond to the Au 4f7 ⁄2 and Au 4f5 ⁄2 , respectively, which indicate that Au is in its metallic form (Au 0 ) [9]. Furthermore, the 3.7 eV binding energy difference of the Au 4f7 ⁄2 and the Au 4f5 ⁄2 components is attributed to the Au 4f ionization process, which is in agreement with the values found in the literature [10,11]. Likewise, the presence of metallic Pd can be confirmed by looking at the Pd 3d5 ⁄2 and Pd 3d3 ⁄2 at binding energies of 335.4 eV and 340.5 eV, respectively [12]. Also, no Au 3+ or chlorine signals were detected in their corresponding photoelectron region, which tells us that the washing procedure was effective for the removal of Au 3+ salt residues.

To have an insight into the distribution of Au and Pd species on the SiNWs TEM and EDX-element mapping should be performed. Moreover, different Au to control the Au:Pd ratio. Finally, Raman measurements should be conducted to test the SERS activity of the substrates. In the next section, a hybrid 3D nanostructure based on ZIF-8 and ZnO will be discussed.

ZIF-8

Metal organic frameworks (MOFs) constitute a novel class of crystalline materials that are formed through the self-assembly of metal cations (nodes) and organic ligands (linkers). Their diverse structural topologies, unprecedented porosity, and versatile chemical functionalities (host-guest interactions) have made them attractive for applications such as gas storage and separation [13], fabrication of luminescent materials [14], drug delivery [15], and catalysis [16]. The size and aperture of the pores can be fine-tuned by carefully selecting the appropriate metal ion clusters and organic linker sizes and shapes, controlling the catenation (i.e., interpenetration or interweaving of identical frameworks), and taking into account the directional mobility of the linkers.

Recently, the potential of MOFs to improve the detection capabilities of SERS substrates has been explored. For instance, He et al. developed a facile one-pot method to prepare AU@MOF-5 nanoparticles wherein a single Au nanoparticle is coated with a uniform MOF-5 shell [17]. The core-shell Au@MOF-5 nanoparticles with a shell thickness of ∼3 nm were highly selective towards CO 2 in a gas mixture of N 2 , O 2 , and CO. For comparison purposes, bare Au nanoparticles and MOF-5 spheres where exposed to CO 2 and no Raman signals were observed, which demonstrated the critical role of the combination of Au and MOF-5 in the specific core-shell nanostructure. Impressively enough, the influence of aggregation of the nanoparticles towards the repeatability of the measurements can be seen as a drawback. Recently, Kreno et al. deposited a thin film of ZIF-8 to detect a series of volatile organic compounds (VOCs) that Chapter 7

Summary and outlook

The primary motivation of the different projects presented in this thesis was to develop different nanostructured transducers by using ALD in combination with other fabrication techniques. This motivation was addressed by developing two distinct approaches. The first one was based on the combination of ALD and lithographic etching processes to produce high aspect ratio structures that could be used as optical transducers. The second approach involved the elaboration of complex architectures based on ZnO by combining ALD and electrodeposition to introduce structural features that can benefit future biosensing applications.

In the following, a summary of the specific achievements is given:

• SiNWs/ZnO were produced with a simple method that combines MACE, NSL, and ALD. The versatility of the method to change the spacing between the SiNWs and their height is demonstrated by modifying the RIE treatment and etching times. The ability of ALD to conformally coat these 3D SiNWs nanostructures with ZnO was confirmed by TEM. Also, XRD and Raman spectroscopy were employed to determine the crystallite size of ZnO and the implications on its optical properties. Photoluminescence measurements of the structures with different etching times and ZnO thicknesses were performed and it was found that the nanostructures obtained with a 5 min etch showed the strongest PL emission. Finally, the 5 min etched SiNWs/ZnO were exposed to varying concentrations of H 2 O 2 while recording their PL emission spectra as a first approach for sensing. Quenching of the PL emission was observed with the first addition of H 2 O 2 , and a mechanism for this observation was proposed.

• Though there exist numerous approaches to produce nanostructures based on ZnO, their reliable and controlled generation is still not trivial. One of the most powerful techniques to create ZnO-based structures is ALD. In this thesis, a combination of NSL, ALD, electrodeposition, and e-beam evaporation was used to produce highly-ordered ZnO/Au urchin-like structures.

The SERS capabilities of these substrates was investigated by grafting them with thiophenol. After testing three different Au layer thicknesses, it was found that the both 30 nm and 50 nm thicknesses presented the highest enhancements. The LOD of the optimized substrates was determined to be 10 -8 m. Furthermore, the batch-to-batch repeatability was tested and a RSD of <10 % was obtained. Adenine was detected with concentrations as low as 10 -6 m, which demonstrates the capability of the substrates to biomolecules sensing. These results suggest that the urchin-like ZnO/Au structures can be successfully applied as SERS substrates.

film. The thickness of the ZIF-8 film could be changed in order to study the influence on the morphology and optical properties of the hybrid structures. Extinction and SERS measurements could be conducted with a suitable probe that cannot be adsorbed on a bare Au surface (e.g., benzene) to study the capability of the ZIF-8 film for trapping these type of molecules.

In the last decade, the vast amount of theoretical and experimental work on transducer elements for biosensing has expanded the collection of available materials and nanostructures. The influencing demand of fabricating reliable and robust biosensors has stimulated the development of novel approaches that can deliver reproducible nanostructured substrates with a tough control of their size, composition, and morphology. As one of the few established methods that allows depositing thin films with high conformality over complex 3D structures, ALD has revolutionized the semiconductors industry by becoming a key technology for microelectronic devices fabrication. Moreover, the ALD technology has had a direct impact in the development of novel types of biosensors platforms by being combined with other fabrication techniques such as NSL, electrodeposition, and electrospinning. However, a cost-effectiveness analysis should be performed before these approaches can be implemented into large scale production lines. For instance, ALD is a slower (up to two orders of magnitude) technique than CVD and usually requires vacuum conditions, thus making it hard and expensive to scale up. Additionally, when lithographic techniques based on nanospheres are used to produce wafer-scale nanostructures, the resulting substrate is usually accompanied by point defects that can have a negative effect on their overall performance. Thus, to overcome these limitations, novel approaches such as spatial ALD and nanoimprint lithography (NIL) have been developed and represent cost-effective tools that can be implemented to produce nanostructured transducer materials for biosensors.

Going forward, we believe that the SiNWs and urchin like structures presented in this work will serve as the basis for future transducer elements by targeting portable systems that are non-invasive and that can be used at home. Moreover, their tailorability and potential surface functionalization with bio-recognition elements derived from semi-synthetic ligands (e.g., aptamers, affibodies, peptide arrays, and molecularly imprinted polymers) will further expand their use as transducers for sensitive, selective, and stable biosensors. Finally, we envision that the nanostructures and techniques presented in this work will continue to expand their presence in real-world applications.

Figure 1 . 1 :

 11 Figure 1.1: Schematic representation of the ALD process

Figure 1 . 2 :

 12 Figure 1.2: Typical biosensor configuration

Figure 2 . 1 :

 21 Figure 2.1: (a) Cross-sectional transmission electron microscopy (TEM) images of a 7 nm vertical gold nanogap. Vertical line is an interfacial boundary of Al 2 O 3 and air, which is filled with Pt for TEM analysis and (b) close-up view of circle in (a). Reprinted with permission from [5]. Copyright 2007 American Vacuum Society.

Figure 2 . 2 :

 22 Figure 2.2: (a) A schematic representation of the fabrication process for plasmonic nanoring cavities based biosensors, using NSL and ALD. (b) Cross-sectional schematics of the fabrication process. (c) Scanning electron micrograph (SEM) of the nanoring cavities on the FON substrates. Scale bar: 200 nm. (d) SEM image of the nanoring cavity array formed over a 16 µm × 10 µm area. (e) Photograph of the nanoring cavity (FON-gap) sample. On the standard glass slide, the FON-gap structures are made in a 2 cm-wide circular area. Reprinted with permission from [6]. Copyright 2013 John Wiley and Sons.

Figure 2 . 3 :

 23 Figure 2.3: (a) Schematic representation of the surface functionalization steps for biosensing test. From top-left: 2D PC BEL fabrication; conformal deposition of ALD-SiO 2 layer; biotinylation of silica-terminated surface; and chemical interaction between biotin and streptavidin molecules tagged with streptavidinphycoerythrin conjugate (SAPE). (b) Fluorescence microscopy image taken after the dye-conjugated streptavidin was bound onto the biotin-functionalized PC surface. Reprinted with permission from [44]. Copyright 2015 Royal Society of Chemistry.

Figure 2 . 4 :

 24 Figure 2.4: Optical images showing the surface of the SAW sensor passivated with a 45 nm Al2O3 film using (a) E-beam evaporation, (b) RF sputtering (dark area is ZnO) and (c) ALD (no ZnO damage) in LB media with the bacterial solution after two days. Reprinted with permission from [43]. Copyright 2012 Elsevier.

Figure 2 . 5 :

 25 Figure 2.5: Illustration of the platform of an epidermal skin-type MoS 2 . (a) Schematic of the PSA binding with the PSA antibody functionalized on the MoS 2 surface. (b) and (c) schematic layout of the 2D multilayer MoS 2 FETs. Photograph (d) of an epidermal skin-type MoS 2 biosensor system consisting of the biosensor, read-out circuits, and an LED as an indicator. (e) Optical image and (f) atomic force microscopy (AFM) image of a flexible MoS 2 device. Reprinted with permission from [57]. Copyright 2017 Springer Nature.

Figure 2 . 6 :

 26 Figure 2.6: Schematic (a) and photo (b) of the biosensor configuration. (c) Schematic of the biofunctionalization layers on the device surface (S [source]; D [drain]). Reprinted with permission from [59]. Copyright 2014 John Wiley and Sons.

Figure 3 . 1 :

 31 Figure 3.1: Schematic diagram of the home-built ALD system used to deposit ZnO and Pd in this work.

Figure 3 . 2 :

 32 Figure 3.2: Schematic illustration and photographs of the monolayer fabrication process. a,b) Addition of colloids to the interface via a tilted glass slide, c) formation of a close-packed monolayer, d,e) monolayer transfer by immersion of the transfer substrate and subsequent elevation under a shallow angle, f) photograph of the deposition of 1 µm colloids onto the air/water interface; Individual, crystalline monolayer patches floating at the interface can be seen, g) monolayer after drying on a Si wafer. Reprinted with permission from [22]. Copyright 2011 John Wiley and Sons.

Figure 3 . 3 :

 33 Figure 3.3: Schematic illustration of the RIE process to create a non-close packed monolayer.

Figure 3 . 4 :

 34 Figure 3.4: Schematic illustration of the different processes involved in MACE. The numbers indicate the steps introduced in this subsection. Reprinted with permission from [28]. Copyright 2011 John Wiley and Sons.

Figure 3 . 5 :

 35 Figure 3.5: Schematic illustration of the electrochemical cell used in this work for the electrodeposition of ZnO.

Figure 3 . 6 :

 36 Figure 3.6: Schematic illustration of the oxygen mediated cathodic deposition of ZIF-8. Reprinted with permission from [51]. Copyright 2019 John Wiley and Sons.

Figure 3 . 7 :

 37 Figure 3.7: Schematic illustration of an E-beam evaporation system. Reprinted from Evaporation materials, Neyco. Retrieved June 06 2019 from https://www. neyco.fr/en/page/evaporation-materials. Copyright 2019 Neyco.

Figure 3 . 8 :

 38 Figure 3.8: Schematic illustration of a DC sputtering evaporation system. Reprinted from What is DC sputtering?, Semicore. Retrieved June 06 2019 from http://www.semicore.com/news/94-what-is-dc-sputtering. Copyright 2019 Neyco.

Figure 3 . 9 :

 39 Figure 3.9: Schematic illustration of the main components of a scanning electron microscope. Reprinted with permission from [71]. Copyright 2008 Taylor & Francis.

Figure 3 . 10 :

 310 Figure 3.10: Schematic diagram of the internal structure of a transmission electron microscope. Reprinted from Basic principles of TEM operation, Microscopy Australia. Retrieved June 06 2019 from https://myscope.training/legacy/tem/ introduction/. Copyright 2019 Microscopy Australia.

Figure 3 . 11 :

 311 Figure 3.11: Schematic illustration of an atomic force microscope. Reprinted from Nanoscale Informal Science Education Network. Retrieved June 06 2019 from https://myscope.training/legacy/tem/introduction/. Copyright 2019 NISE network.

Figure 3 . 12 :

 312 Figure 3.12: Schematic illustration of the PL process in a direct bandgap semiconductor.

Figure 3 . 13 :

 313 Figure 3.13: Schematic diagram of a standard PL spectroscopy setup. Reprinted with permission from [79]. Copyright 2008 Springer Nature.

Figure 3 . 14 :

 314 Figure 3.14: Schematic diagram of a Raman spectrum showing vibrational Raman effects. The Stokes process produces scattered light with lower energy than the incident light, while anti-Stokes produces scattered light with a higher energy than the incident light. Retrieved June 06 2019 from http://www.ltbberlin.de/en/technologies/raman/. Copyright 2019 NISE network.

Figure 3 . 15 :

 315 Figure 3.15: Schematic illustration of a Renishaw InVia InVia Raman spectrometer. Reprinted with permission from [89]. Copyright 2011 John Wiley and Sons.

Figure 3 . 16 :

 316 Figure 3.16: Schematic illustration of the different processes involved in MACE. The numbers indicate the steps introduced in this subsection.

Figure 3 . 17 :

 317 Figure 3.17: Schematic illustration of the different processes involved in the fabrication of Au-covered hollow urchin-like structures.

Figure 3 .

 3 Figure 3.18: SEM micrographs of (a) an array of well-organized PS spheres, (b) PS spheres after been subjected to an oxygen plasma treatment, (c) ordered hollow urchin-like ZnO structures, and (d) urchin-like ZnO structures covered by a 30 nm-thick Au layer deposited by e-gun evaporation. The insets show higher magnifications of the corresponding SEM images.

3. 6 materials 3 . 6 . 1

 6361 Experimental details for hybrid Pd/Au and ZnO/Au/ZIF-8 Fabrication and characterization of silicon nanowires decorated with Pd/Au nanoparticles Silicon nanowires were prepared following the same procedure shown in Section 3.4.1. For the galvanic replacement reaction, a 15.66 mm solution of HAuCl 4

Figure 3 .

 3 Figure 3.19: a) TEM image of a Au-covered ZnO urchin-like structure and b) high-resolution image of the selected area in image a). The inset in b) shows the FFT operation of the image. c) TEM and (d-g) EDX elemental mapping images of a Au-covered hollow ZnO urchin-like structure. Blue, red, and green illustrate Zn, O, and Au, respectively.

Figure 3 . 20 :

 320 Figure 3.20: XRD patterns of ITO, urchin-like ZnO, and Au-covered urchin-like ZnO structures in the 20°to 70°2θ

Figure 4 . 1 :

 41 Figure 4.1: Schematic illustration of the RIE process to create a non-close packed monolayer.

Figure 4 . 2 :

 42 Figure 4.2: SEM images of SiNWs etched for 5 min and covered with a 50 nm ZnO layer deposited by ALD.

Figure 4 .

 4 Figure 4.3: a) 2D and b) 3D AFM images SiNWs covered with a 50 nm ZnO layer by ALD.

Figure 4 . 4 :

 44 Figure 4.4: XRD patterns of SiNWs (etched for 7 min) covered with 20 and 50 nm of ZnO by ALD.

Figure 4 . 5 :

 45 Figure 4.5: Cross-sectional TEM images of SiNWs with (a) 20 and (b) 50 nm of ZnO by ALD.

  [

Figure 4 . 6 :

 46 Figure 4.6: Raman spectra of bare SiNWs (etched for 7 min) and after the deposition of ZnO (20 and 50 nm) by ALD.

Figure 4 . 7 :

 47 Figure 4.7: PL spectra of SiNWs (etched for 2, 5, and 7 min) covered with (a) 20 and (b) 50 nm of ZnO by ALD.

Figure 4 . 8 :

 48 Figure 4.8: Dependence of the PL signal on the H 2 O 2 concentration.

Figure 4 . 9 :

 49 Figure 4.9: Schematic representation of the PL quenching mechanism of ZnO upon exposure to H 2 O 2 .

Fig. 3 .

 3 18d shows the as-synthesized Au-covered urchin-like structures with a Au layer thickness of 30 nm (10 nm and 50 nm are shown on Fig. B.2).

Figure 5 . 1 :Figure 5 . 2 :

 5152 Figure 5.1: SERS spectra of thiophenol (10 -3 m) from different Au film thicknesses (10, 30, and 50 nm) for the two excitation wavelengths used in our experiments: (a) 633 nm and (b) 785 nm.

. 1 )[ 9 ]

 19 were I Raman represents the Raman intensity of a planar 30 nm Au thin film functionalized with thiophenol and I SERS represents the Raman intensity of of a thiophenol solution in ethanol (1 m; see Fig. B.4).

Figure 5 . 3 :

 53 Figure 5.3: (a) Raman map image of the 999 cm -1 peak from a randomly selected area (20 µm × 20 µm) on the Au-covered hollow urchin-like structures at the 785 nm excitation wavelength. (b) Intensity distribution of the 999 cm -1 peak of three different samples with an average RSD value <10 %.

Figure 5 . 4 :

 54 Figure 5.4: SERS detection of adenine with a LOD of 1 µm. An offset was applied to improve the data visualization.

3 +Figure 6 . 1 :

 361 Fig.6.1 shows the SEM images of SiNWs decorated with Pd and Pd/Au nanostructures by combining ALD and galvanic replacement. After optimization of the deposition process, it was found that 100 ALD cycles produced an homogeneous coating of highly dispersed Pd nanoparticles on the surface of SiNWs (Fig.6.1a,b). Next, Au was introduced in the Pd nanoparticles by performing a galvanic replacement reaction. Fig.6.1d shows a top view image of the SiNWs decorated with Pd/Au nanostructures and confirms the preservation of the SiNWs architecture.Additionally, EDX was performed to confirm the presence of Pd and Au on the SiNWs as shown in Fig.6.2. Considering that the depth of penetration of the EDX measurement is ∼500 nm, the majority of the signal comes from the SiNWs (Si, O) and only a small portion is due to the Pd/Au nanoparticles.[8].The Pd/Au decorated SiNWs were further analyzed by XPS to determine the oxidation states of Pd and Au. The two peaks with binding energies of 84.5 eV and 88.2 eV correspond to the Au 4f7 ⁄2 and Au 4f5 ⁄2 , respectively, which indicate that Au is in its metallic form (Au 0 )[9]. Furthermore, the 3.7 eV binding energy difference of the Au 4f7 ⁄2 and the Au 4f5 ⁄2 components is attributed to the Au 4f ionization process, which is in agreement with the values found in the literature[10, 11]. Likewise, the presence of metallic Pd can be confirmed by looking at the Pd 3d5 ⁄2 and Pd 3d3 ⁄2 at binding energies of 335.4 eV and 340.5 eV, respectively[12]. Also, no Au 3+ or chlorine signals were detected in their corresponding photoelectron region, which tells us that the washing procedure was effective for the removal of Au 3+ salt residues.To have an insight into the distribution of Au and Pd species on the SiNWs TEM and EDX-element mapping should be performed. Moreover, different Au 3 + concentrations and galvanic replacement reaction times should be tested in order

Figure 6 . 2 :

 62 Figure 6.2: EDX spectrum of the as-synthesized SiNWs/Pd/Au structures.

Figure 6 . 3 :

 63 Figure 6.3: XPS spectra of the Pd/Au system showing (a) Au 4f and (b) Pd 3d core level spectra.

Figure 6 . 5 :

 65 Figure 6.5: XRD patterns of the urchin-like ZnO/Au structures and the urchinlike ZnO/Au/ZIF-8 structures. The simulated XRD pattern of ZIF-8 is given under the corresponding measured pattern. The peaks of ITO come from the underlying substrate.

Figure 6 . 6 :

 66 Figure 6.6: EDX spectrum of the as-synthesized ZIF-8/Au/ZnO urchin-like hybrid structures.

  3.2.2 Transmission electron microscopy . . . . . . . . . 3.2.3 Atomic force microscopy . . . . . . . . . . . . . . 3.2.4 X-ray diffraction . . . . . . . . . . . . . . . . . . 3.2.5 X-ray photoelectron spectroscopy . . . . . . . . . 3.2.6 Photoluminescence spectroscopy . . . . . . . . . 3.2.7 Raman spectroscopy . . . . . . . . . . . . . . . . 3.2.8 Conclusions . . . . . . . . . . . . . . . . . . . . . 3.3 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Experimental details for silicon nanowires covered with ZnO by ALD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Fabrication of silicon nanowires covered by ALD 3.4.2 Characterization of ZnO deposited by ALD on silicon nanowires . . . . . . . . . . . . . . . . . . 3.5 Experimental details for Au-covered hollow urchin-like ZnO structures . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

		List of acronyms
	GST RIE	Glutathione S-transferase Reactive ion etching
	GVA RSD	Grapevine virus A-type Relative standard deviation
	H441 SAM	Human adenocarcinoma epithelial Self-assembled monolayer
	HAADF SAPE	High-angle annular dark-field Streptavidin, phycoerythrin conjugated
	HmiM SAW	2-Methylimidazole Surface acoustic wave
	HR SDS	High-resolution Sodium dodecyl sulfate
	HRIL SEM	High refractive index layer Scanning electron microscopy
	ICDD SERS	International Centre for Diffraction Data Surface-enhanced Raman spectroscopy
	ICP SiNWs	Inductively coupled plasma Silicon nanowires
	IgG SPR	Immunoglobulin G Surface plasmon resonance
	IL-10 SWV	Interleukin-10 Square wave voltammetry
	ITO TCID50	Indium tin oxide Tissue culture infective dose
	LB TEM	Lysogeny broth Transmission electron microscopy
	LED TESUD	Light-emitting diode 11-(triethoxysilyl) undecanal
	LOD	Limit of detection
	LPG	Long-period grating
	LPS	Lipopolysaccharide
	LSPR	Localized surface plasmon resonance
	MACE	Metal-assisted chemical etching
	MBE	Molecular beam epitaxy
	MEF	Metal-enhanced fluorescence
	ML	Molecular layering
	MOCVD	Metal organic chemical vapor deposition
	MOF	Metal organic framework
	MOS	metal-oxide-semiconductor
	MOSFET Metal oxide semiconductor field effect transistor
	MQW	Multiple quantum well
	NBE	Near band edge emission
	NPG	Nanoporous gold
	NSL	Nanosphere lithography
	NW	Nanowire
	PBS	Phosphate buffered saline
	PC	Photonic crystal
	PDMS	Polydimethylsiloxane
	PET	Poly(ethylene terephtalate)
	PL	Photoluminescence
	PLD	Pulsed laser deposition
	POC	Point-of-care
	PSA	Prostate-specific antigen
	PVD	Physical vapor deposition
	RF	Radio frequency
	xvi

3.5.1 Fabrication of Au-covered hollow urchin-like ZnO structures . . . . . . . . . . . . . . . . . . . . . . 3.5.2 Characterization of Au-covered hollow urchin-like structures . . . . . . . . . . . . . . . . . . . . . . 3.6 Experimental details for hybrid Pd/Au and ZnO/Au/ZIF-8 materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1 Fabrication and characterization of silicon nanowires decorated with Pd/Au nanoparticles . . . . 3.6.2 Fabrication and characterization of ZnO/Au/ZIF-8 hollow urchin-like structures . . . . . . . . . . . 3.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . .

  Chaaya, A. A. et al. "Tuning optical properties of Al 2 O 3 /ZnO nanolaminates synthesized by atomic layer deposition." The Journal of Physical Chemistry C vol. 118, no. 7 (2014), pp. 3811-3819. [30] Gu, Y.-Z. et al. "Optical and microstructural properties of ZnO/TiO 2

	1. Introduction
	[29] nanolaminates prepared by atomic layer deposition." Nanoscale research
	letters vol. 8, no. 1 (2013), p. 107.
	[31] Guo, D. J. et al. "GaN nanowire functionalized with atomic layer deposition
	techniques for enhanced immobilization of biomolecules." Langmuir vol. 26,
	no. 23 (2010), pp. 18382-18391.
	[32] Marichy, C., Bechelany, M., and Pinna, N. "Atomic layer deposition of
	nanostructured materials for energy and environmental applications." Ad-
	vanced Materials vol. 24, no. 8 (2012), pp. 1017-1032.

[33]

Pessoa, R. et al. "TiO 2 coatings via atomic layer deposition on polyurethane and polydimethylsiloxane substrates: Properties and effects on C. albicans growth and inactivation process." Applied Surface Science vol. 422 (2017), pp. 73-84. [34] Vähä-Nissi, M. et al. "Antibacterial and barrier properties of oriented polymer films with ZnO thin films applied with atomic layer deposition at low temperatures." Thin Solid Films vol. 562 (2014), pp. 331-337.

Table 2

 2

.1. Jang et al. deposited a film of Al 2 O 3 by

Table 2 .

 2 1: Examples of films deposited by ALD for the fabrication of nanostructured biosensors

	Ref.	[5]			[6]			[7]					[8]					[9]					[10]
	Detection range	Detection down to	1.5 nm		Limit of detection	76 nm		Linear dynamic	range of detection	of 0.01 µg/ml to	1 µg/ml Limit of	detection 2 ng/ml	Detection limit of	1 pg/ml				Avidin and	streptavidin	discrimination/	BSA, IgG and	anti-BSA detection	Detection down to	the parts-per-	quadrillion	range
	Detection Method	Electrochemical	(current)		Optical (SERS)			Electrochemical	(differential pulse	voltammetry [DPV] or	square wave	voltammetry [SWV])	Optical (fluorescence)					Electrochemical	(current)				Electrochemical (FET)
	Analyte	Streptavidin			Adenine			Cholera toxin					17-β-estradiol					Avidin,	streptavidin,	bovine serum	albumin (BSA),	IgG, anti-BSA	Explosive	chemical	species
	Substrate	Gold nanogap/Biotin			Plasmonic nanoring	cavities		Nanoscale coaxial	electrodes/anti-cholera	toxin antibody			Microarray platform	on Ag film/split DNA	aptamer			poly(ethylene	terephthalate) (PET)	nanochannel/Al2O3/	ZnO/biotin		SiNWs FETs (field	effect	transistor)/chemical	receptors
	Application	Sacrificial layer	for nanogap	fabrication	Layer for	nanoring	fabrication	Layer for coax	fabrication				Nano-spacer	for metal-	enhanced	fluorescence	(MEF)	Nanopore size	modification,	enhancement of	sensing	capabilities	Decoration of	silicon	nanowires	(SiNWs) with	ZnO	nanoparticles
	Material	Al2O3			Al2O3			Al2O3					Al2O3					Al2O3/	ZnO				ZnO

Table 2 .

 2 2: Examples of films deposited by ALD for the fabrication of nanostructured biosensors

	Material	Application	Substrate	Analyte	Detection	Detection	Ref.
					Method	range
	TiOx	Transducer	LPG/	Avidin	Optical	Detection of	[32]
		layer	TiOx/		(refractive	avidin-biotin
			Biotin		index)	complex
	ZnO	Transducer	ZnO	Glucose	Electro-	Sensitivity of	[33]
		layer	nanorods/		chemical	69.8 nm µm -1
			GOx		(current)	cm -2
	ZnO	Transducer	ZnO	GVA	Optical
		layer	film/GVA	antigen	(photolu-
			antibody		mines-
					cence)

Table 2 .

 2 3: Examples of films deposited by ALD for biosensor protection purposes

	Material	Application	Substrate	Analyte	Detection	Detection	Ref.
					Method	range
	Al2O3	Protection	Silver	Anthrax	Optical	Limit of	[42]
		against	film-over-	biomar-	(SERS)	detection
		oxidation of	nanosphere	ker		1.4 × 10 3
		Ag	(AgFON)	calcium		spores
				dipicoli-
				nate
				(CaDPA)
	Al2O3	Protection of	ZnO	Biofilm	Mass-	Limit of	[43]
		ZnO	passivated		sensitive	detection
			with Al2O3		(SAW)	5.3 pg
	SiO2	Protection of	InGaAsP	SAPE	Optical	Detection of	[44]
		nanolaser	multiple		(air-bridge-	streptavidin-
		structure	quantum		type	biotin
			well		Γ-BEL	reaction
			(MQW)/		lasing
			biotin		wavelength
					shift)

Table 2 .

 2 4: Examples of materials with high dielectric constant deposited by ALD for the fabrication of

biosensors Material Application Substrate Analyte Detection Method Detection range Ref.

	[55]		[56]							[57]				[58]		[59]				[60]		[61]
	0.1 mm to 0.68 mm		Detection range of	1.5 × 10 2 50 %	tissue culture	infective dose	(TCID50)/ml up to	1 × 10 5	TCID50/ml	Down to 1 pg/ml				Biotin	functionalization	Detection down to	375 fm			Linear range of	0.5 mm to 20 mm	Sensitivity of	) 400 µA/(mm mm 2	Successful cell	analysis for	nanotoxicological	studies
	Electrochemical (FET)		Magnetoelectric	(GMR)						Electrochemical (FET)				Electrochemical (FET)		Electrochemical (FET)				Electrochemical (FET)		Electrochemical	(impedance)
	Urea		Influenza A viruses							PSA				Biotin		Cancer marker protein	PSA			Cholesterol		Normal human dermal	fibroblast and human lung	adenocarcinoma epithelial	(H441)
	Silicon wire	FET/urease	Magnetic	nanoparticles/	Influenza A capture	antibody				MoS2/anti-PSA	antibody			Bio-FET		MoS2 nanosheet	FET/monoclonal PSA			Potassium-doped	polypyrrole/carbon	nanotube/cholesterol	oxidase (ChOx)	Polydimethylsiloxane	(PDMS)	microwells/gold	electrodes
	Current leakage	prevention	Current leakage	prevention						Insulating layer,	gate dielectric,	adhesion	improvement	Gate dielectric	material	Gate dielectric	layer, protection of	electrodes from	liquid environment	Gate insulator		Insulating layer
	Al2O3		Al2O3							Al2O3				HfO2		HfO2				ZrO2		ZrO2

Table 2 .

 2 5: Examples of materials with high refractive index deposited by ALD for the fabrication of biosensors

	Material	Application	Substrate	Analyte	Detection	Detection	Ref.
					method	range
	Ta2O5	Evanescent	Waveguide-	Mouse	Optical	Limit of	[79]
		field	based	IgG	(fluores-	detection of
		enhancement	photonic	antigen	cence)	0.25 µg/ml
			platform/
			Cy5-
			labelled
			anti-mouse
			IgG
			antibody
	TiO2	Improvement	LPG/	E. coli B	Optical	Positive test	[80]
		of refractive	TiO2/E.	LPS	(refractive	for
		index	coli B bac-		index)	adhesin-LPS
		sensitivity	teriophage			binding
			g37 adhesin

Table 2 .

 2 6: Examples of materials with catalytic activity deposited by ALD for the fabrication of biosensors

	Material	Application	Substrate	Analyte	Detection	Detection	Ref.
					Method	range
	CoO	Electrode	NPG/CoO	H2O2	Electro-	Linear range	[84]
		body,			chemical	of detection
		catalytic			(current)	0.1 mm to
		activity				92.9 mm
	Ni	Electrocata-	CNT-Ni	Glucose	Electro-	Linear range	[85]
		lyst for	nanocom-		chemical	of detection
		glucose	posites		(current)	of 0.005 mm
		oxidation				to 2 mm
						Detection
						limit of 2 µm
	NiO	Electrocata-	NiO/SiC	Glucose	Electro-	Linear range	[86]
		lyst for	nanocom-		chemical	of detection
		glucose	posite		(current)	of 0.004 mm
		oxidation				to 7.5 mm
						Detection
						limit of
						0.32 µm

Table 3 .

 3 1: ALD process parameters for the deposition of ZnO.

	Pulse (s) Exposure (s) Ar purge (s)
	DEZ 0.3	30	40
	H 2 O 2	30	40

Table 3 .

 3 .2. 2: ALD process parameters for the deposition of Pd.

		Pulse (s) Exposure (s) Ar purge (s)
	Pd(hfac) 2 5	15	10
	CH 2 O	1	15	60

Table 4 .

 4 1: Position of the four main PL peaks before and after interaction with H 2 O 2 .

	H 2 O 2 (mM) Peak 1 Peak 2 Peak 3 Peak 4
	0	384	418	486	529
	10	384	420	481	513
	20	384	427	485	530
	30	384	431	487	530

Table 5 .

 5 1: Calculated SERS gains G SERS for four Raman peaks (1 mm of thiophenol) at the excitation wavelength of 785 nm for ZnO urchin-like structures covered with a 30 nm Au film. × 10 2 3.8 × 10 2 3.1 × 10 2 5.3 × 10 3

	Raman shift (cm -1 ) 999	1022	1072	1572
	I SERS	15066	12199	16926	9539
	I Raman	79	32	55	18
	G SERS	1.9

(a) (b)

Acknowledgements

Chapter 6

Synthesis of hybrid Pd/Au and ZnO/Au/ZIF-8 materials Nanostructured materials have significantly changed materials science as we know it. Their fabrication methods have provided a new generation of hybrid structures with tunable physical, mechanical, chemical, and optical properties. Nanomaterials have gained attention due to their enhanced performance when compared to their bulk counterparts, making them attractive for many industrial applications. The design of innovative nanostructured materials requires a strict control of their size, shape, and composition. Furthermore, these nanoengineered materials must be cost-effective and compatible with large scale production lines. As one of the few techniques that allows to deposit thin films with control down to the angstrom level, ALD has become an essential tool for the assembly of nanostructured materials. In addition, its compatibility with current manufacturing methods makes it a promising candidate for the high-throughput processing of nanostructured components.

In this chapter, we propose two novel materials based on the SiNWs and urchin-like ZnO structures displayed in Chapter 4 and Chapter 5, respectively. First, to overcome the difficulties of depositing bimetallic Pd and Au nanoparticles directly on the SiNWs, a combination of ALD and galvanic replacement reaction are proposed. Second, a novel electrochemically-based approach to deposit a thin film of ZIF-8 MOF is introduced. Third, the fabrication processes, physical properties, and chemical composition will be discussed. Finally, as these are only preliminary results, general perspectives and examples of how they could be applied for SERS sensing will be suggested.

Pd/Au nanoparticles

Bimetallic nanostructures have attracted a great deal of attention in the last decade not only for their distinct electronic, optical, and catalytic properties but also for their potential applications in photocatalysis [1], drug delivery [2], water remediation [3], biosensing [4], and energy conversion in low-temperature fuel cells [5]. Unfortunately, for surface-enhanced Raman spectroscopy (SERS) applications, only substrates made from silver (Ag), gold (Au), and copper (Cu) have been used, which limits their range of applications. For this reason, considerable effort has been put for developing SERS-active substrates based on other transition metals. In particular, Palladium (Pd) has been proposed as an alternative SERS-active material due to its stability and catalytical applications in electrochemistry and surface science. Although present, the enhancement factors that can be obtained with Pd (10 1 to 10 3 ) are much lower than the ones ZIF-8 do not absorb on the surface of Ag "films-over-nanospheres" (FON) substrates [18]. However, the adsorbed molecules were much larger than the 3.4 Å pore apertures, suggesting that the adsorption was preferably at grain boundaries in the film and thus no molecular sieving effect could be envisioned. So far, the presence of ZIF-8 was confirmed by XRD and EDX measurements. A further analysis with TEM should be conducted to have an insight into the distribution of ZIF-8 on the Au/ZnO urchin-like structures. Also, considering that the electromagnetic enhancement suffers an exponential attenuation with distance [21] (within just a few nanometers), different thicknesses of ZIF-8 should be produced to test the influence on the overall SERS enhancement factor.

Results and discussion

Conclusions

Conclusions

Pd/Au nanoparticles supported on SiNWs were synthesized by a combination of ALD and the galvanic replacement reaction. Their general morphology before and after the galvanic replacement reaction was studied. The composition and chemical state of the Pd/Au nanoparticles was confirmed with EDX and XPS. Further characterization techniques like HRTEM would be of great interest to elucidate the exact morphology and crystallinity of the nanoparticles, which could give us an insight into the replacement mechanism and segregation of Au into the Pd nanoparticles. We believe that these bimetallic nanoparticles could be used as SERS substrates for sensing of relevant biomolecules [6,7,22].

ZnO/Au/ZIF-8 hollow urchin-like structures were synthesized by an oxygenassisted cathodic electrodeposition strategy. The overall morphology was studied by SEM and the chemical composition was confirmed by XRD. The microstructures should be further analyzed with TEM and EDX to study the ZIF-8 film thickness and to confirm the ZnO/Au/ZIF-8 core-shell structure. These hybrid structures could be used as potential SERS substrates for the sensing of volatile organic compounds [17,18,23].

• The SiNWs and urchin-like structures described above were used as a basis to prepare Pd/Au nanoparticles and ZnO/Au/ZIF-8 films, respectively. The formation of Pd nanoparticles was achieved by using ALD. After, Au was introduced to the SiNWs/Pd structures by the galvanic replacement reaction. EDX and XPS techniques were used to confirm the chemical composition of these sructures and it was found that Au had been replace successfully by looking at its oxidation state. The deposition of ZIF-8 on ZnO/Au urchin-like structures was performed by using a novel oxygenassisted cathodic synthesis strategy. The morphology of these structures was studied with SEM, which showed an homogeneous film of ZIF-8 covering the 3D structures. Additionally, the composition of the films was studied with XRD, which confirmed the presence of ZIF-8 on the surface of the urchins.

A short outlook for the possible research directions of the projects introduced in this thesis is described as follows:

• The selection of an adequate functionalization method of the SiNWs/ZnO structures for the immobilization of bio-recognition elements should be studied. Furthermore, the selection of a relevant enzyme/substrate system for the biosensing proof-of-concept should be addressed. Also, the stability of the ZnO layer towards aqueous environments should be tested as many biosensing applications involve the introduction of the substrates into complex media (e.g., blood). Because POC applications demand the use of portable devices, the photoluminescence properties of the nanostructures should be tested with adequate compact UV lightsources.

• To have a deeper understanding of the exact Raman enhancement mechanism of the Au/ZnO urchin-like structures, finite-difference time-domain (FDTD) simulations could be performed. In addition, the influence of the size and geometry of the urchins on the localization of the LSPR could be investigated. Similarly, the influence on the optical properties and SERS enhancement of the substrates after annealing (transformation of the Au film into nanoparticles) could be studied. To validate the biosensing capability of the substrates, DNA detection could be envisioned by immobilizing a capture probe DNA on the surface of the Au/ZnO urchin-like structures, which could detect a target DNA carrying a reporter probe labeled with a dye, leading to a detectable SERS signal.

• The morphology of the SiNWs/Pd/Au could be investigated by TEM. Also, the influence on the Pd loading and the ratio of Au replacement could be performed, which could give a broader tunability of the composition and morphology of these bimetallic nanostructures. Moreover, these hybrid structures could be tested for SERS activity by choosing an adequate probe.

Regarding the ZnO/Au/ZIF-8 urchin-like structures, TEM measurements could be performed to have a better idea of the morphology of the ZIF-8

List of publications and conferences

Publications

• O. Graniel