With the rapid progress of nanotechnologies, there are two developing trends for the next generation of sensors: miniaturization and multi-functionality. Device miniaturization requires less power consumption, or even self-powered system. Multi-functional devices are usually based on multi-property coupling effects. Piezoelectric semiconductors have been considered to be potential candidates for self-powered/multi-functional devices due to their piezotronic coupling effect. In this dissertation, ZnO and CdSe nanowire arrays have been synthesized as the piezoelectric semiconductor materials to develop the following self-powered/multi-functional sensors: (1) self-powered gas sensors of ZnO/SnO2, ZnO/In2O3, ZnO/WO3 and CdSe nanowire arrays have been assembled. All these gas sensors are capable of detecting oxidizing gas and reducing gas without any external power supply owing to piezotronic effect which can convert mechanical energies to electrical energy to power the sensors; (2) a self-powered ZnO/ZnSe core/shell nanowire array photodetector has been fabricated. This photodetector is able to detect the entire range of the visible spectrum as well as UV light because of its type II heterostructure. The absolute sensitivity and the percentage change in responsivity of the photodetector were significantly enhanced resulting from the piezo-phototronic effect. The photodetector also exhibited self-powered photodetection behavior; (3) three dimensional nanowire arrays, such as ZnO and ZnO/Co3O4, have been synthesized to investigate piezo-magnetotronic and piezo-photo-magnetotronic effects. Under magnetic field, the magnetic-induced current of ZnO nanowire array decreased as magnetic field increased, and the current difference was magnified by one order of magnitude caused by piezo-magnetotronic effect through applying a stress. In contrast, under UV light illumination, the current response increased with an increment of magnetic field. The current difference was enhanced by at least two orders of magnitude attributed to piezo-photo-magnetotronic effect. Furthermore, ZnO/Co3O4 core/shell structure was employed to further improve the magnetic-induced current difference. This phenomenon projects a potential for multi-functional piezo-magnetotronic and piezo-photo-magnetotronic device development.

La détection de faibles concentrations d’acides nucléiques est essentielle pour certaines applications comme la biologie médicale, où elle permet le diagnostic d’une multitude de pathologies par l’identification de biomarqueurs spécifiques. Par rapport aux techniques traditionnelles de détection par voie biochimique, la détection électrique par effet de champ présente l’avantage d’être une mesure directe, sans marquage, et à réponse rapide. Les transistors à nanofils semiconducteurs sont des dispositifs prometteurs qui permettent potentiellement d’atteindre des limites de détection très basses et une sensibilité élevée, grâce à leur grand rapport surface/volume et leurs propriétés électroniques uniques. Le carbure de silicium (SiC) est un matériau semiconducteur dont les qualités le rendent particulièrement adapté aux applications visées, telles que sa très grande stabilité physico-chimique et biocompatibilité.Dans cette thèse, des transistors à effet de champ à base de nanofils de Si et SiC ont été conçus dans une approche descendante pour être fabriqués par photolithographie. Un procédé de fabrication basé sur la filière silicium a été développé et optimisé afin de réaliser des dispositifs à nanofils et à nanorubans de Si de manière reproductible. Une étude détaillée a permis de démontrer la stabilité chimique supérieure des nanofils de SiC par rapport aux nanofils de Si en conditions physiologiques. Fort de ce résultat, nous avons exploré deux approches pour l’élaboration d’une couche mince de SiC autour de ces nanostructures de Si, pour leur conférer cette résistance chimique en milieu liquide. Ces dispositifs cœur-coquille Si/SiC reproductibles ont finalement été fonctionnalisés et intégrés dans un système microfluidique complet afin de réaliser des premières mesures novatrices de détection de pH et d’ADN en temps réel et en milieu liquide. / Sensing of low concentrations of nucleic acids is essential to a variety of applications such as bio-medical analysis, in which case it allows the diagnosis of pathologies by identifying specific biomarkers. Compared to traditional sensing techniques based on biochemistry, the advantage of electrical field-effect detection is that it relies on a direct, label-free, and fast-response measurement. Transistors based on semiconducting nanowires are promising devices that theoretically enable very low detection limits and a high sensitivity, thanks to their high surface-to-volume ratio and unique electronic properties. Silicon carbide (SiC) is a semiconductor material with qualities such as very high physical and chemical stability and high biocompatibility, which make it particularly suited for aforementioned applications.In this thesis, field-effect transistors based on Si and SiC nanowires were designed with a top-down approach to be fabricated using photolithography techniques. The Si-based process was developed and optimized in order to fabricate reproducible devices made of nanowires and nanoribbons. A detailed study was conducted to demonstrate the superior chemical stability of SiC nanowires over Si nanowires under physiological conditions. Based on these results, we investigated two ways of elaborating a thin SiC layer around these Si nanostructures to provide them with its chemical resistance in liquid medium. These reproducible core-shell Si/SiC devices were eventually functionalized and integrated into a microfluidic system in order to achieve novel measurements of DNA detection in real time and in liquid media.

In the pursuit of economical and rapid fabrication solutions on the micro and nano scale, polymer replication has proven itself to be a formidable technique, which despite zealous development by the research community, remains full of promise. This thesis explores the potential of elastomers in what is a distinctly multidisciplinary field. The focus is on developing innovative fabrication solutions for planar photonic devices and for nanoscale devices in general. Innovations are derived from treatments of master structures, imprintable substrates and device applications. Major contributions made by this work include fully replicated planar integrated optical devices, nanoscale applications for photolithographic standing wave corrugations (SWC), and a biologically templated, optical fiber based, surface-enhanced Raman scattering (SERS) sensor. The planar devices take the form of dielectric rib waveguides which for the first time, have been integrated with long-period gratings by replication. The heretofore unemployed SWC is used to demonstrate two innovations. The first is a novel demonstration of elastomeric sidewall photolithographic mask, which exploits the capacity of elastomers to cast undercut structures. The second demonstrates that the corrugations themselves in the absence of elastomers, can be employed as shadow masks in a directional flux to produce vertical stacks of straight lines and circles of nanowires and nanoribbons. The thesis then closes by conceptually combining the preceding demonstrations of waveguides and nanostructures. An optical fiber endface is em ployed for the first time as a substrate for patterning by replication, wherein the pattern is a nanostructure derived from a biological template. This replicated nanostructure is used to impart a SERS capability to the optical fiber, demonstrating an ultra-sensitive, integrated photonic device realized at great economy of both time and money, with very real potential for mass fabrication.

A highly self-ordered hexagonal array of cylindrical pores has been fabricated by anodizing a thin film of Al on substrate and subsequent growth of GaN and InGaN in these nanoholes has been performed. This AAO template-based synthesis method provides a low cost process to fabricate GaN-based nanomaterials fabrication. / Singapore-MIT Alliance (SMA)

In this work we investigated fabrication of semiconductor nanomaterials and evaluated their potential for photo-chemical and photovoltaic applications. We investigated different II-VI semiconductor nanomaterial systems; (i) ZnO oriented nanowire arrays non-epitaxially grown from a substrate; and (ii) colloidal CdE (E=Te,Se,S) quantum structures synthesized by solution-based thermal decomposition of organo-metallic precursors. We have studied the synthesis of vertically aligned ZnO nanowire arrays (NWA), by a wet chemical process on various substrates. We have extended this method wherein nanofibers of poly-L-lactide act as a substrate for the radially oriented growth of ZnO nanowires. By combining the large surface area and the flexibility of the PLLA-ZnO hierarchical nanostructure we have shown the proof-of-principle demonstration of a ‘continuous-flow’ water treatment system to decompose known organic pollutants in water, as well as render common waterborne bacteria non-viable. We have studied synthesis of colloidal quantum dots (QD), and show size, morphology and composition tailored nanocrystals for CdE (E=S, Se, Te) compositions. We have studied the influence of crystal growth habits of the nanocrtsyals on the final morphology. Furthermore we have synthesized core-shell, CdSe-CdS QDs with spherical and tetrahedral morphologies by varying the reaction conditions. We show that these core-shell quantum dots show quasi-type II characteristics, and demonstrate with I-V measurements, the spatial localization of the charge carriers in these hetero-nanocrystals. For this purpose, we developed hybrid materials consisting of the core-shell quantum dots with electron acceptors (ZnO nanowires) and hole acceptors (polymeric P3HT nanofibers). In addition we have also compared the synthesis reaction when carried out with conventional heating and microwave-mediated heating. We find that the reaction is enhanced, and the yield is qualitatively better when using microwave induced heating. / QC 20120525

The increasing miniaturization of electric and mechanical components makes the synthesis and assembly of nanoscale structures an important step in modern technology. Functional materials, such as the ferroelectric perovskites, are vital to the integration and utility value of nanotechnology in the future. In the present work, chemical methods to synthesize one-dimensional (1D) nanostructures of ferroelectric perovskites have been studied. To successfully and controllably make 1D nanostructures by chemical methods it is very important to understand the growth mechanism of these nanostructures, in order to design the structures for use in various applications. For the integration of 1D nanostructures into devices it is also very important to be able to make arrays and large-area designed structures from the building blocks that single nanostructures constitute. As functional materials, it is of course also vital to study the properties of the nanostructures. The characterization of properties of single nanostructures is challenging, but essential to the use of such structures. The aim of this work has been to synthesize high quality single-crystalline 1D nanostructures of ferroelectric perovskites with emphasis on PbTiO3 , to make arrays or hierarchical nanostructures of 1D nanostructures on substrates, to understand the growth mechanisms of the 1D nanostructures, and to investigate the ferroelectric and piezoelectric properties of the 1D nanostructures. In Paper I, a molten salt synthesis route, previously reported to yield BaTiO3 , PbTiO3 and Na2Ti6O13 nanorods, was re-examined in order to elucidate the role of volatile chlorides. A precursor mixture containing barium (or lead) and titaniumwas annealed in the presence of NaCl at 760 °C or 820 °C. The main products were respectively isometric nanocrystalline BaTiO3 and PbTiO3. Nanorods were also detected, but electron diffraction revealed that the composition of the nanorods was respectively BaTi2O5/BaTi5O11 and Na2Ti6O13 for the two different systems, in contradiction to the previous studies. It was shown that NaCl reacted with BaO(PbO) resulting in loss of volatile BaCl2 (PbCl2 ) and formation and preferential growth of titanium oxide-rich nanorods instead of the target phase BaTiO3 (or PbTiO3 ). The molten salt synthesis route may therefore not necessarily yield nanorods of the target ternary oxide as reported previously. In addition, the importance of NaCl(g) for the growth of nanorods below the melting point of NaCl was demonstrated in a special experimental setup, where NaCl and the precursors were physically separated. In Paper II and III, a hydrothermal synthesis method to grow arrays and hierarchical nanostructures of PbTiO3 nanorods and platelets on substrates is presented. Hydrothermal treatment of an amorphous PbTiO3 precursor in the presence of a surfactant and PbTiO3 or SrTiO3 substrates resulted in the growth of PbTiO3 nanorods and platelets aligned in the crystallographic <100> orientations of the SrTiO3 substrates. PbTiO3 nanorods oriented perpendicular to the substrate surface could also be grown directly on the substrate by a modified synthesis method. The hydrothermal method described in Paper II and III was developed on the basis of the method described in Appendices I and II. In Paper IV, a template-assisted method to make PbTiO3 nanotubes is presented. An equimolar Pb-Ti sol was dropped onto porous alumina membranes and penetrated into the channels of the template. Single-phase PbTiO3 perovskite nanotubes were obtained by annealing at 700 °C for 6 h. The nanotubes haddiameters of 200 - 400 nm with a wall thickness of approximately 20 nm. Excess PbO or annealing in a Pb-containing atmosphere was not necessary in order to achieve single phase PbTiO3 nanotubes. The influence of the heating procedure and the sol concentration is discussed. In Paper V, a piezoresponse force microscopy study of single PbTiO3 nanorods is presented. The piezoelectric properties were studied in both vertical and lateral mode. Piezoelectric activity and polarization switching was observed in the vertical mode, demonstrating the ferroelectric nature of the nanorods. The nanorods decomposed after repeated cycling of the dc bias at one spot on the nanorod, which resulted in parts of the nanorod disappearing and/or accumulation of particles on the surface of the nanorod. In Paper VI, a method to contact single nanorods by electron beam induced deposition of platinum is presented. An organometallic compound, (trimethyl)-methylcyclopentadienylplatinum(IV), was used as precursor. A home-made apparatus was constructed for the purpose and was mounted onto a scanning electron microscope. Calculations based on apparatus geometry and molecular flow were used to estimate the deposition time and the height of the deposits. The location and height of the deposits were controlled so that single nanorods could be successfully contacted at the ends of the nanorods. Fabrication of a sample device for piezoresponse force microscopy studies of single nanorods using an axial dc bias setup is described in Appendix IV. A proposed experimental setup for such studies is also presented.

Narrow silicon nanowires host a rich set of physical phenomena. Understanding these phenomena will open new opportunities for applications of silicon nanowires in optoelectronic devices and adds more functionality to silicon especially in those realms that bulk silicon may not operate remarkably. Compatibility of silicon nanowires with the mainstream fabrication technology is also advantageous. The main theme of this thesis is finding the possibility of using silicon nanowires in light sources; laser and light emitting diodes. Using Tight Binding (TB) and ab-initio Density Functional Theory (DFT) methods it was shown that axial strain can induce significant changes in the effective mass, density of states and bandgap of silicon nanowires. Generality of the observed effects was proven by investigating nanowires of different crystallography, diameter and material (e.g. germanium nanowires). The observed direct to indirect bandgap conversion suggests that strain is able to modulate the light emission properties of silicon nanowires. To investigate this possibility, spontaneous emission time was formulated using perturbation theory including Longitudinal Optical (LO) and Acoustic (LA) phonons. It was observed that corresponding to bandgap conversion, the spontaneous emission time can be modulated by more than one order of magnitude. This emanates from bandgap conversion and symmetry change of wave function in response to strain. A mechanism for population inversion was proposed in the thesis which is based on the Ensemble Monte Carlo (EMC) study of carrier statistics in direct and indirect conduction sub bands. By calculating all possible electron-phonon scattering mechanisms which may deplete the already populated indirect subband, it was shown that at different temperatures and under different electric fields there is a factor of 10 difference between the population of indirect and direct sub bands. This suggests that population inversion can be achieved by biasing an already strained nanowire in its indirect bandgap state. The light emission is possible then by releasing or inverting the strain direction. A few ideas of implementing this experiment were proposed as a patent application. Furthermore the photo absorption of silicon nanowires was calculated using TB method and the role of diameter, optical anisotropy and strain were investigated on band-edge absorption.

The first part of my PhD work is about the lattice vibrations in silicon nanowires. First-principles calculations based on the linear response are performed to investigate the quantum confinement effect in lattice vibrations of silicon nanowires (SiNW). The radial breathing modes (RBM) are found in our calculations, which have a different size-dependent frequency shift compared with the optical modes. They are well explained by the elastic model. Finally, the relative activity of the Raman scattering in the smallest SiNW is calculated. The RBM can be clearly identified in the Raman spectrum, which can be used to estimate the size of nanowires in experiment. In the second part of my PhD work, we focus on the electron-hole pair (exciton) in semiconductor nanowires and its influence on the optical absorption spectra. First-principles calculations are performed for a hydrogen-passivated silicon nanowire with a diameter of 1.2 nm. Using plane wave and pseudopotentials, the quasiparticle states are calculated within the so-called GW approximation, and the electron-hole interaction is evaluated with the Bethe-Salpeter Equation (BSE). The enhanced excitonic effect is found in the absorption spectrum. The third part of my work is about the electronic structure in Si/Ge core-shell nanowires. The electronic band structure is studied with first-principles methods. Individual conduction and valence bands are found in the core part and the shell part, respectively. The band offsets are determined, which give rise to the spatial separation of electron and hole charge carriers in different regions of the nanowires. This allows for a novel-doping scheme that supplies the carriers into a separate region in order to avoid the scattering problem. This is the key factor to create high-speed devices. With the confinement effect, our results show important correction in the band offset compared with the bulk heterostructure. Finally, an optimum doping strategy is proposed based on our band-offset data.

This thesis aims to investigate structural, optical and photoelectrochemical behavior of CdS nanowires and their heterojunctions with CdTe and polypyrrole nanowires. In the first part, CdS nanowires have been synthesized via electrochemical template-based route. It has been observed that synthesis conditions, such as bias voltage and deposition time, affect the morphology, optical and photoelectrochemical characteristics of CdS nanowires. Depending on the deposition time, length of the CdS nanowires changed from 100-200 nm to 3-4 m. Also the diameter of the nanowires increased with increasing the deposition time. Structure of the CdS nanowires has been confirmed by X-ray diffraction spectrometry and EDX analysis. Phototelectrochemical performances of the CdS nanowires have been changed dramatically with bias voltage and deposition time.In the second part of this thesis, CdTe nanostructures have been deposited on CdS nanowires. Change in optical and photoelectrochemical behavior of CdS nanowires after CdTe deposition has been investigated. Organic semiconductors and their composites with inorganic materials have been gaining attention due to tunable optical, electrical and magnetic properties. Also, ease of fabrication techniques, and therefore, low cost made these materials attractive for lots of applications including photovoltaic devices and flexible electronics. In the last part of this thesis, heterojunctions of CdS and Polypyrrole (Ppy) nanowires have been synthesized. Like CdS/CdTe heteronanostructures, first the CdS nanowires have been electrochemically deposited in anodized alumina template and then Ppy has been successfully deposited on CdS nanowires. In order to investigate the effects of polypyrrole synthesis conditions on CdS/Ppy heteronanostructures, CdS nanowire synthesis conditions have been kept constant. It has been observed that morphology and photoelectrochemical behavior of the Ppy nanowires has been affected from Ppy synthesis conditions. The photoelectrochemical performance changes of CdS/Ppy heteronanostructures have been also investigated in this part.

Core-shell nanowire LEDs are light emitting devices which, due to a high aspect ratio, have low substrate sensitivity, allowing the possibility of low defect density GaN light emitting diodes. Current growth techniques and physical non-idealities make the production of high conductivity p-type GaN for the shell region of these devices difficult. Due to the structure of core-shell nanowires and the difference in conductivity between ntype and p-type GaN, the full junction area of a core-shell nanowire is not used efficiently. To address this problem, a series of possible doping profiles are applied to the core of a simulated device to determine effects on current crowding and overall device efficiency. With a simplified model it is shown that current crowding has a possible dependence on the doping in the core in regions other than those directly in contact with the shell. The device efficiency is found to be improved through the use of non-constant doping profiles in the core region with particularly large efficiency increases related to profiles which modify portions of the core not in contact with the shell