A systematic investigation into the growth of one-dimensional nanostructures of select II-VI compounds with the wurtzite crystal structure. Two process parameters are systematically altered to observe how each affects deposition. The results of which give a further understanding into the formation of one nanostructure over another, as well as experimental parameters for optimizing the growth of particular CdSe nanomaterials. A statistical analysis is conducted on the experimental data to quantitatively determine the variability and robustness of the experimental setup and process. The information complied from this extensive study will yield a more complete understanding of the experimental setup and how improvements can be made to reduce variability, increase yield, and gain insight into the mechanisms controlling this class of materials.

Motivated by a desire to understand the basic concepts of one-dimensional nanostructure growth, the research described in this thesis aims at understanding the basic mechanisms controlling the synthesis and formation of a specific group of II-VI semiconducting nanostructures. In particular, this thesis examines one-dimensional nanostructures (such as nanobelts and nanowires) and different morphologies of ZnS that result from the interesting properties that the materials have at the nanoscale. In order to understand how to tune these properties in the nanostructure, it is necessary to have an understanding of the growth mechanism that dictates the morphology, structure, and rate of growth of the nanomaterial. It is necessary to understand what impact changes to the macroscopic setup in the experiment have on the nanoscopic scale of the nanomaterials. Having a larger understanding and exerting more precise control over the growth of nanomaterials will allow a higher level of selectivity, more control over dimensionality and the type of morphology, easier manipulation, and the simpler incorporation of these structures into a nanotechnological device. The main focus of the research was on CdSe and ZnS, with the bulk of the research being conducted on ZnS nanostructures. These materials were chosen for their potential for extensive research, their possible applications in optoelectronics, their potential to form the wurtzite crystal structure, and the potential generalization of results to other nanomaterials. The framework for the research is given first. Then a description of the experimental setup and a model for the growth of nanostructures is discussed. A brief overview of the synthesis of CdSe nanostructures is given and then a detailed analysis of the synthesis of specific ZnS one-dimensional morphologies is presented.

ZnO is one of the most important materials for electronics, optoelectronics, piezoelectricity and optics. With a wide band gap of 3.37eV and an exiton binding energy of 60meV, ZnO 1D nanostructures exhibit promising properties in a lot of optical device applications. It is also an important piezoelectric material and has applications in a new category of nanodevices, nano-piezotronics. Demonstrated prototype of devices includes nanogenerators, piezoelectric-FET, and a series of evolutive devices based on the concept of nanogenerator. This is based on working principle of a semiconductor and piezoelectric coupled property. This thesis is about the growth, characterization and device fabrication of ZnO nanowires and nanobelts for sensors and UV detectors. First, the fundamental synthesis of ZnO nanostructurs is investigated, particularly polar surface dominated nanostructues, to illustrate the unique growth configurations of ZnO. Detail study in this part includes nanobelts, nanorings, nanocombs, nanonetworks, and nanodiskettes synthesis. Important factors in driving the nanostructure synthesis mechanism are analyzed, such as the chemical activities of different surface of ZnO and the polar surface dominated effects. Then, the devices fabricated methods using individual nanowires/nanobelts and their electrical transport properties were carefully characterized. In this part, dominant factors which are critical for nanobelt device performance are investigated, such as the contact properties, interface effects, and durability testing. Also, a metal doping method is studied to explore the controlling and modification of nanowire electric and optical properties. Further more, I will present the surface functionalization of nanobelt for largely improving its electrical, optoelectronic and chemical performance. Surface functionalization of nanobelts is proven to be an effective method in enhancing the semiconductor and metal contact. Piezoelectric field-effect transistors will be demonstrated as a powerful approach as chemical sensors. Finally, a technique is illustrated for functionalizing the surfaces of ZnO nanobelts for enhancing its UV sensitivity by over five orders of magnitude. This demonstrates an effective approach for fabricatiing ultrasensitive UV detectors. The research results presented in this thesis have made great contribution to the growth, device fabrication and novel applications of ZnO nanostructures for photonics, optoelectronics and sensors.

A variety of nanostructures of tin (IV) oxide (TO) are synthesized using two fabrication methods: a solution spin-coating method followed by post-annealing in an oxygen flow and a newly developed catalyst-assisted pulsed laser deposition (PLD) technique. The spin-coating method is used to fabricate granular TO films with monodisperse, stable, ultra-small nanocrystallites (4-5 nm in size), the size of which is found to increase exponentially with post-anneal above 500??C. These nanocrystalline films are conductive and highly transparent, and their bandgap shows broadening due to a high carrier concentration. Their resistivity behavior as a function of temperature in the 50-280 K range can be explained by a two-medium transport model, i.e. transport through the crystalline grains and across the grain boundaries, and through the charge-depletion layer, where a potential barrier is found for transport across the grain boundaries. Electronic transport in these films follows a 3D-variable range hopping model, which reveals an increase in the localization length of carriers with increasing the TAnneal above the onset of exponential growth at TAnneal= 500??C. By homogenously doping Eu3+ in these nanocrystalline films up to a high doping level of ~ 8%, optical luminescence and magnetic orderings can be introduced into these nanocrystalline TO films. Both characteristic Eu3+ emission and defect-related TO emissions are observed in the otherwise transparent TO films upon UV-excitation. In spite of the non-magnetic nature of Eu3+ ions, magnetic orderings appear in the highly doped TO films below 50 K upon the emergence of Eu2Sn2O7 phase. In the second part of this work, we employ a layer of gold nanoislands with controlled sizes (10-50 nm) as catalysts for pulsed laser deposition of TO nanostructures. Highly crystalline TO nanobricks, cuboid nanoparticles, nanowires and nanobelts are obtained for the first time through vapour-solid or vapour-liquid-solid (VLS) mechanisms. Of particular interest are the micron long one-dimensional (1D) nanowires and nanobelts, with the smallest square and rectangular cross-sections, respectively, ever reported. These single-crystalline nanostructures are obtained at relatively low temperatures of 600??C, for nanowires, and 500??C, for nanobelts, and their cross-sectional sizes can be easily controlled by the size of the gold nanoislands. The nanobelts are found to grow along the [100] and [101] axes, while the nanowires appear to grow along the [100] axis. The growth evolution of the nanobelts are also investigated in detail revealing their VLS growth mode and their single-crystalline structure throughout the growth, which opens the prospect of controlling their growth axis and consequently their side-surface planes by pinning the base to the substrate at the desired crystalline orientation. Together, the two fabrication methods developed in the present work offer facile approaches to growing two scientifically and technologically important classes of TO nanostructures, i.e., nanocrystalline film and 1D nanostructures. Thorough characterization of the resulted nanostructured materials using advanced microscopic, spectroscopic and other techniques, including Helium Ion Microscopy, has been provided. Modification of structure, morphology and physical properties of these functional nanostructured materials are also illustrated by controlling the growth parameters and by (Eu-)doping, which pave the way for introducing new properties for applications in chemical sensing, (opto)electronics and displays.

abstract: In this work, I worked on the synthesis and characterization of nanowires and belts, grown using different materials, in Chemical Vapor Deposition (CVD) system with catalytic growth method. Through this thesis, I utilized the Photoluminescence (PL), Secondary Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD) analyses to find out the properties of Erbium Chloride Silicate (ECS) and two segment CdS-CdSe samples. In the first part of my research, growth of very new material, Erbium Chloride Silicate (ECS), in form of core/shell Si/ECS and pure ECS nanowires, was demonstrated. This new material has very fascinating properties for new Si based photonic devices. The Erbium density in those nanowires is which is very high value compared to the other Erbium doped materials. It was shown that the luminescence peaks of ECS nanowires are very sharp and stronger than their counterparts. Furthermore, both PL and XRD peaks get sharper and stronger as growth temperature increases and this shows that crystalline quality of ECS nanowires gets better with higher temperature. In the second part, I did a very detail research for growing two segment axial nanowires or radial belts and report that the structure type mostly depends on the growth temperature. Since our final step is to create white light LEDs using single axial nanowires which have three different regions grown with distinct materials and give red, green and blue colors simultaneously, we worked on growing CdS-CdSe nanowires or belts for the first step of our aim. Those products were successfully grown and they gave two luminescence peaks with maximum 160 nm wavelength separation depending on the growth conditions. It was observed that products become more likely belt once the substrate temperature increases. Also, dominance between VLS and VS is very critical to determine the shape of the products and the substitution of CdS by CdSe is very effective; hence, CdSe growth time should be chosen accordingly. However, it was shown two segmented products can be synthesized by picking the right conditions and with very careful analyses. We also demonstrated that simultaneous two colors lasing from a single segmented belt structures is possible with strong enough-pumping-power. / Dissertation/Thesis / M.S. Electrical Engineering 2012

Orientador: Prof. Dr. Alexandre José de Castro Lanfredi / Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Nanociências e Materiais Avançados, 2016. / This work discusses the study of growth mechanisms of oxide nanobelts by chemical vapor deposition. Initially, were synthesized tin oxide (SnO2) nanobelts and from images obtained by Scanning Electron Microscopy (SEM) was verified that the growth of nanobelts is a combination of two main mechanisms: vapor-liquid-solid and vapor-solid. Thus, similar thermodynamics conditions were used to synthesize SnO2 germanium (Ge), silicon (Si) and zinc (Zn) doped. Crystalline structure of the samples was determined by X-ray Powder Diffraction (XRD). The chemical composition and doping was verified by Energy Dispersive X-ray Spectroscopy (EDS). In addition, Ge doped SnO2 nanobelts were characterized by High Resolution Electron Microscopy (HRTEM) and Raman spectroscopy. The results suggest that the synthesis method used in this work allows to obtain monocrystalline materials and also the presence of doping elements in SnO2 structure. It was also observed that the doping elements do not form core-shell structures. We also study electronics transport mechanisms in a single nanobelt. First, we studied the electronic transport properties and electron resistance as a function of temperature (R(T)). The results suggest that there is a similar behavior in the samples: there is an interface between metallic (T > 240 °C) and semiconductor (T < 240 °C) behavior. In the region of semiconductor behavior, the Arrhenius model, Efros-Shklovskii and variable range hopping were adjusted and the results showed that the conduction mechanisms in this range occurs through variable range hopping. The model also allows to obtain the parameters of average distance hopping and the values obtained agree with the dimensionality of the eletronic system of the samples. Then, studies were carried out to verify the influence of ultraviolet light on the electronics properties. The photoconduction behavior was adjusted by the Bloch Gr¨uneisen model, from these results adjustments n value and Debye temperature was obtained and the results indicates that electronic transport is strongly dependent on the electron-phonon scattering. Finally, a photoconduction study was carried out as a function of time, the results obtained indicated that oxygen atoms and vacancies influence the conduction of the materials. / Este trabalho envolve o estudo dos mecanismos de crescimento de nanofitas de oxidos pela deposição química em fase vapor (CVD). Inicialmente, foram sintetizadas nanofitas de oxido de estanho (SnO2) e, a partir de imagens obtidas por Microscopia Eletronica de Varredura (MEV), verificou-se que o crescimento das nanofitas ocorre a partir da mistura de dois mecanismos principais: vapor-líquido-solido (VLS) e vapor¿solido (VS). Desse modo, condições termodinamicas semelhantes foram utilizadas para sintetizar nanofitas de SnO2 dopadas com germanio (Ge), silício (Si) e zinco (Zn). As amostras foram caracterizadas por Difraçao de raios X (DRX) para investigar a estrutura cristalina e fases presentes nas amostras de SnO2 pura e dopadas. Espectroscopia de raios X por Dispersao de Energia (EDS) foi utilizada para analisar a razão da composição o química de nanofitas e verificar a efetividade da dopagem. Al'em disso, as nanofitas de SnO2 dopadas com Ge foram caracterizadas por Microscopia Eletronica de Transmissão de Alta Resolução (HRTEM) e pela técnica de Espectroscopia Raman. Os resultados sugerem que a partir do metodo de síntese utilizado neste trabalho foram obtidos materiais monocristalinos, indicando a presen¸ca dos elementos dopantes na estrutura do SnO2 e que estes não formaram estruturas do tipo core-shell. Foi realizado tamb'em um estudo dos mecanismos de transporte eletronico em uma 'unica nanofita. Primeiramente, estas propriedades foram estudas a partir de medidas de resist¿encia el'etrica como função da temperatura (R(T)). Os resultados mostraram que o comportamento de R(T) 'e semelhante em todas as amostras: ha uma interface entre o comportamento met alico (para T > 240°C) e semicondutor (para T < 240 °C). Na regiao com comportamento semicondutor, as curvas foram ajustadas pelo modelo de Arrhenius, Efros-Shklovskii e hopping de alcance variavel e os resultados sugerem que o mecanismo de conduçao nessa faixa de temperatura ocorre por meio de hopping de alcance vari'avel. Alem disso, esse modelo permitiu calcular os parametros de distancia media de hopping e os valores obtidos estao de acordo com a dimensionalidade do sistema eletronico das amostras. Em seguida, foram realizados estudos para verificar a influ¿encia da luz ultravioleta nas propriedades eletronicas. O comportamento de fotocondução foi ajustado pelo modelo de Bloch Gruneisen, a partir destes ajustes obteve-se valores de n e a temperatura de Debye e os resultados indicam que a condução é fortemente dependente do espalhamento eletron-fonon. Finalmente, foi realizado um estudo de fotocondução como função do tempo e os resultados obtidos indicaram que atomos de oxigenio e vacancias influenciam a condutividade eletrica do material.

No description available.

In this thesis, a systematic study has been carried out on the synthesis, characterization and device fabrication of piezoelectric ZnO nanstructures. The achieved results are composed of the following four parts. Firstly, through a systematic investigation on the Sn-catalyzed ZnO nanostructure, an improved understanding of the chemical and physical process occurring during the growth of hierarchical nanostructures has been achieved. Decomposed Sn from SnO2 has been successfully demonstrated and proved to be an effective catalyst guiding the growth of not only aligned ZnO nanowires, but also the hierarchical nanowire-nanoribbon junction arrays and nanopropeller arrays. During the vapor-liquid-solid (VLS) catalyzing growth process at high temperature, Sn in the liquid state has been proved to be able to guide the growth of nanowires and nanoribbons in terms of growth directions, side facets, and crystallographic interfaces between Sn and ZnO nanostructures. Secondly, using pure ZnO as the only source material, by precisely tuning and controlling the growth kinetics, a variety of hierarchical polar surface dominated nanostructures have been achieved, such as single crystal nanorings, nanobows, nanosprings and superlattice nanohelices. High yield synthesis of ZnO nanosprings over 50% has been successfully obtained by mainly controlling the pre-pumping level associated with the partial pressure of residual oxygen during the vapor-solid growth process. The rigid superlattice nanohelices of ZnO have been discovered, which is a result of minimization of the electrostatic energy induced by polar surfaces. The formation process of the nanohelix has been systematically characterized. Thirdly, two new strategies have been successfully developed for fabricating ZnO quantum dots and synthesis of ZnO nanodiskettes and nanotubes. The formation process is based on a common concept of self-assembly. Finally, a series of devices and applications studies based on several piezoelectric ZnO nanostructures, such as nanobelts, nanopropellers and nanohelices, have been carried out utilizing the electro-mechanical resonance, bio-surface functionalization, devices fabrication and electrical characterization. Individual nanobelt and nanohelix based nanodevices have been successfully fabricated for applications in chemical and biological sensing. The study opens a few new areas in oxide nanostructures and applications.

Union between top-down and bottom-up assembly is inevitable when scaling down physical, chemical, and biological sensors and probes. Current sensor/probe-based technologies are firmly founded on top-down manufacturing, with limitations in cost of production, manufacturing methods, and material constraints. As an alternative to such limitations, contemporary synthesis techniques for one-dimensional nanostructures have been combined with established methods of micro-fabrication for the development of novel tools and techniques for nanotechnology. More specifically, this dissertation is a systematic study of the synthesis and characterization of ZnO nanostructures for piezoelectric applications. Within this study the following goals have been achieved: 1) rational design and control of a diversity of novel ZnO nanostructures, 2) improved understanding of polar-surface-dominated (PSD) phenomena among Wurtzite crystal structures, 3) confirmation of Taskers Rule via the synthesis, characterization, and modeling of polar-surface-dominated nanostructures, 4) measurement of the surface-charge density for real polar surfaces of ZnO, 5) confirmation of the electrostatic polar-charge model used to describe polar-surface-dominated phenomena, 6) dispersion of ZnO nanobelts onto the selective layers of surface acoustic wave (SAW) devices for gas sensing applications, 7) manipulation of ZnO nanostructures using an atomic force microscope (AFM) for the development of piezoelectric devices, 8) fabrication of bulk acoustic resonator (BAR) and film bulk acoustic resonator (FBAR) devices based on the integrity of individual ZnO belts, 9) electrical characterization of a ZnO belt BAR device, 10) prediction and confirmation of the electrical response from a BAR device using a one-dimensional Krimholt-Leedom-Matthaei (KLM) model, and 11) development of a finite element model (FEM) to accurately predict the electrical response from ZnO belt BAR and FBAR devices in 3D.

Miniaturisation of electronic and optoelectronic devices have enabled the realization of system-on-a-chip technology in modern image sensors, where the photo sensor arrays and the corresponding signal processing circuitry are monolithically integrated in a single chip. Apart from intrinsic advantages, the drive towards miniaturisation has been further fuelled by the exotic properties exhibited by semiconductor materials at the nano scale. As the dimension of the material is gradually reduced from the bulk, interesting physical and chemical properties begin to emerge owing to the increased confinement of charge carriers in different spatial dimensions. Solution-processed optoelectronics have revolutionised the field of device physics over recent years due to the superior performance, ease of processing, substrate flexibility, cost-effective production of large-area devices and other advantages associated with the technique. In the present work, solution-processed photo detectors have been fabricated on SiO2/Si substrate facilitating the ease of integration with conventional silicon CMOS technology. The present thesis deals with the successful exploitation of most common point defects in semiconductor nanostructures to reduce the overlap of hole wave function with the envelop wave function of the ground state electron to improve photoconduction. As a result of the investigation process, successful strategies have been devised for the improvement of photoconduction by engineering the defect states. In the first study, the intrinsic copper vacancies and the capping agent thiol have been employed to trap photo holes in photo detectors based on copper indium selenide nanoparticles, thereby allowing the photoelectrons to transit the device. In the second study, the optical excitation of charge carriers into the defect-related band originating from oxygen vacancies further raises the photoconductivity of molybdenum trioxide nanobelts based photodetectors. In the third study, the absence of photoconductivity in zinc selenide based quantum dots has been attributed to the radiative recombination of photogenerated carriers at the donor-acceptor states caused by the self-compensation of point defects in the dots. In the final study, the crucial role of the energy depth of trap states in determining the carrier relaxation dynamics (temporal response) of the photodetector based on SnO2 nanowires has been discussed in detail. .