Thesis advisor: Kian L. Tan / Thesis advisor: Dunwei Wang / Silicon nanowires are utilized to harvest the energy from visible light. The introduction of a nickel pre-catalyst, 1, allows for this energy to be stored in chemical bonds, which are subsequently used in the carboxylation of 4-octyne. / Thesis (MS) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Hong Liang = 用化学气相淀积方法生长氮化物纳米材料 / 洪亮. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / Hong Liang = Yong hua xue qi xiang dian ji fang fa sheng chang dan hua wu na mi cai liao / Hong Liang. / Acknowledgements --- p.ii / Abstract --- p.iii / Contents --- p.v / Chapter Chapter 1 --- Introduction / Chapter 1.1 --- Background --- p.1 / Chapter 1.2 --- Motivation --- p.2 / Chapter 1.2.1 --- A1N and AlGaN nanowires --- p.2 / Chapter 1.2.2 --- CVD --- p.3 / Chapter 1.3 --- Our work --- p.3 / Chapter Chapter 2 --- Experiment --- p.7 / Chapter 2.1 --- CVD system --- p.7 / Chapter 2.2 --- Sources and Substrates --- p.7 / Chapter 2.3 --- Growth of A1N nanowires --- p.8 / Chapter 2.4 --- Growth of AlGaN nanowires --- p.9 / Chapter Chapter 3 --- Characterization --- p.11 / Chapter 3.1 --- Scanning Electron Microscopy --- p.11 / Chapter 3.1.1 --- Topographic images by secondary electrons --- p.11 / Chapter 3.1.2 --- Elemental Analysis by Energy Dispersive X-ray --- p.12 / Chapter 3.2 --- Transmission Electron Microscopy --- p.12 / Chapter 3.3 --- X-Ray Diffraction --- p.14 / Chapter 3.4 --- Micro-Raman --- p.15 / Chapter Chapter 4 --- Results and Discussion --- p.18 / Chapter 4.1 --- A1N nano-structures --- p.18 / Chapter 4.1.1 --- A1N nano-leaves grown on silicon substrates --- p.18 / Chapter 4.1.2 --- A1N nanowires grown on silicon substrates --- p.19 / Chapter 4.1.3 --- SiNx nanowires grown on silicon substrates --- p.22 / Chapter 4.1.4 --- A1N nanowires grown on sapphire substrates --- p.26 / Chapter 4.1.5 --- Comparison with the results of other research groups --- p.31 / Chapter 4.2 --- AlGaN nano-structures --- p.33 / Chapter 4.2.1 --- AlGaN nanowires grown on silicon substrates --- p.33 / Chapter 4.2.2 --- Temperature dependence --- p.38 / Chapter 4.2.3 --- The influence of the mass ratio (Ga/Al) in the precursor metal sources --- p.43 / Chapter 4.2.4 --- Substrate effect --- p.46 / Chapter Chapter 5 --- Suggestion of the growth mechanism --- p.51 / Chapter 5.1 --- Growth mechanisms: an introduction --- p.51 / Chapter 5.2 --- The growth mechanisms for our produced samples --- p.57 / Chapter 5.2.1 --- Growth mechanism for A1N nanowires --- p.58 / Chapter 5.2.2 --- Growth mechanism for AlGaN nanowires --- p.61 / Chapter 5.2.3 --- Substrate effect --- p.65 / Chapter Chapter 6 --- Conclusions --- p.71 / Appendix --- p.73

Lead halide perovskites are a class of soft ionic semiconductors characterized by strong excitonic absorption and long carrier lifetimes. Recent studies suggest that electrons and holes in these materials interact with longitudinal optical phonons to form large polarons on subpicosecond time-scales. The same interaction is responsible for hot electron cooling via phonon emission and is thought to be screened by large polaron formation resulting in the long-lived hot electrons observed in methylammonium lead iodide perovskite. Time-resolved two-photon photoemission is used to follow the initial hot electron cooling and large polaron formation dynamics in single-crystal cesium lead bromide perovskite at 80 K and 300 K. The initial relaxation rates are found to be weakly temperature-dependent and are attributed to the cooling of unscreened hot electrons by the emission of longitudinal optical phonons. The large polaron formation times, however, are inferred to be approximately three times faster at 300 K. The decrease in polaron formation time with temperature is correlated with the broadening in phonon linewidths, suggesting that disorder can assist large polaron formation. In addition, the initial electron relaxation is faster than large polaron formation explaining the absence of long-lived hot electrons in cesium lead bromide perovskite as opposed to methylammonium lead iodide perovskite where the two processes are competitive. The second part of this thesis focuses on the strong light-matter interaction in nanowire waveguide geometries of single-crystal lead halide perovskites which are well known for their emission tunability and low lasing thresholds under pulsed optical excitation. Using fluorescence microscopy, it is found that the luminescence from single-crystal cesium lead bromide perovskite nanowires is dominated by sub-bandgap modes called exciton-polaritons, i.e. hybridized exciton-photon states. A one-dimensional exciton-polariton model reproduces the observed modes at the bottleneck of the lower polariton branch with a Rabi splitting of about 200 milli-electron volts. As the power density increases under continuous excitation, the exciton-polaritons undergo Bose-stimulated scattering and a super-linear increase in mode intensity is observed. This is the first demonstration of continuous-wave lasing in lead halide perovskite nanowires and reveals an inherently strong light-matter interaction in lead halide perovskites that can be used for continuous-wave optoelectronic applications. These findings corroborate the role of dynamic screening in unifying these two regimes of carrier-carrier interactions responsible for the strong absorption and subsequent carrier protection. We also demonstrate the viability of lead halide perovskite nanowires for future optoelectronics.

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Low-dimensional systems are an important and intensely studied area of condensed matter physics. When a material is forced to adopt a low-dimensional structure, its behaviour is often dramatically different to that of the bulk phase. It is vital to predict the structures of low-dimensional systems in order to reliably predict their properties. To this end, the ab initio random structure searching (AIRSS) method, which has previously been used to identify the structures of bulk materials, has been extended to deal with the case of nanowires encapsulated inside carbon nanotubes. Such systems are a rapidly developing area of research with important nanotechnological applications, including information storage, energy storage and chemical sensing. The extended AIRSS method for encapsulated nanowires (ENWs) was implemented and used to identify the structures formed by germanium telluride, silver chloride, and molybdenum diselenide ENWs. In each of these cases, a number of novel nanowire structures were identified, and a phase diagram predicting the ground state nanowire structure as a function of the radius of the encapsulating nanotube was calculated. In the case of germanium telluride, which is a technologically important phase-change material, the potential use of GeTe ENWs as switchable nanoscale memory devices was investigated. The vibrational properties of silver chloride ENWs were also considered, and a novel scheme was developed to predict the Raman spectra of systems which can be decomposed into multiple weakly interacting subsystems. This scheme was used to obtain a close approximation to the Raman spectra of AgCl ENWs at a fraction of the computational cost that would otherwise be necessary. The encapsulation of AgCl was shown to produce substantial shifts in the Raman spectra of nanotubes, providing an important link with experiment. A method was developed to predict the stress-strain response of an ENW based on a polygonal representation of its surface, and was used to investigate the elastic response of molybdenum diselenide ENWs. This was used to predict stress-radius phase diagrams for MoSe_2 ENWs, and hence to investigate stress-induced phase change within such systems. The X-ray diffraction of ENWs was also considered. A program was written to simulate X-ray diffraction in low-dimensional systems, and was used to predict the diffraction patterns of some of the encapsulated GeTe nanowire structures predicted by AIRSS. By modelling the interactions within a bundle of nanotubes, diffraction patterns for bundles of ENWs were obtained.

Rao, Yangyan = 氧化鋅的納米結構以及准一維氧化鋅稀磁半導體的合成及研究 / 饒洋燕. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Abstracts in English and Chinese. / Rao, Yangyan = Yang hua xin de na mi jie gou yi ji zhun yi wei yang hua xin xi ci ban dao ti de he cheng ji yan jiu / Rao Yangyan. / Abstract --- p.1 / 摘要 --- p.3 / Acknowledgments --- p.4 / Table of contents --- p.5 / Chapter Chapter 1 --- Introduction --- p.7 / Chapter 1.1 --- Zinc oxide --- p.7 / Chapter 1.2 --- Mn doped ZnO diluted magnetic semiconductors --- p.7 / Chapter 1.3 --- Motivations --- p.9 / Chapter 1.4 --- Our Work --- p.10 / Chapter 1.5 --- Overview of the thesis --- p.11 / References --- p.11 / Chapter Chapter 2 --- Experimental set-up and conditions --- p.15 / Chapter 2.1 --- Chemical Vapor Deposition --- p.15 / Chapter 2.1.1 --- Key Steps in Chemical Vapor Deposition --- p.15 / Chapter 2.2 --- Experiments on the synthesis of ZnO nanostructures --- p.16 / Chapter 2.2.1 --- Set-up --- p.17 / Chapter 2.2.2 --- Growth mechanism --- p.18 / Chapter 2.2.3 --- Experimental conditions --- p.20 / References --- p.23 / Chapter Chapter 3 --- Characterization of pure ZnO --- p.25 / Chapter 3.1 --- Morphology of ZnO nanowires --- p.25 / Chapter 3.2 --- Lattice structure --- p.30 / Chapter 3.3 --- Photoluminescence property (PL) --- p.32 / Chapter 3.4 --- Lattice dynamics´ؤRaman spectra --- p.33 / References --- p.35 / Chapter Chapter 4 --- Properties of Mn doped ZnO nanowires --- p.37 / Chapter 4.1 --- Morphology and composition --- p.37 / Chapter 4.2 --- Lattice structure of Mn-doped ZnO --- p.40 / Chapter 4.3 --- Lattice dynamics´ؤRaman study of Mn-doped ZnO nanowires --- p.46 / References --- p.48 / Chapter Chapter 5 --- Magnetic properties of Mn doped ZnO nanowires --- p.50 / Chapter 5.1 --- Theory of DMS --- p.50 / Chapter 5.2 --- Magnetic results of Mn doped ZnO nanowires --- p.52 / Chapter 5.2.1 --- Paramagnetism of Mn doped ZnO --- p.52 / Chapter 5.2.2 --- Ferromagnetism of Mn doped ZnO --- p.58 / References --- p.65 / Chapter Chapter 6 --- Conclusions --- p.67

Scanning electron microscope, X-ray diffraction and photoluminescence measurements were conducted on Be(x)Zn(1−x)O nanowires prepared by electrochemical and hydrothermal deposition to study their morphology, structure and optical properties. The bowing parameter for nanowires prepared by electrochemical and hydrothermal deposition was found to be 4.8 eV and 3.6 eV, respectively. It was observed that for electrochemical deposition, it is more difficult for Be(2+) to incorporate in the crystal lattice than Zn(2+). The electrochemical deposited samples exhibited stronger deep level emissions, indicating a higher density of deep level states. A home-made Optically Stimulated Luminescence (OSL) system was successfully constructed and calibrated with Al2O3:C and BeO. The OSL lifetime measurements on electrochemical deposited samples showed there were measurable OSL signals even on 500 nm long Be(x)Zn(1−x)O nanowires. The lifetimes of these OSL signals were found to decrease with increasing Be concentration. Be(x)Zn(1−x)O nanowires show considerable promise as new OSL materials.

Scanning electron microscope, X-ray diffraction and photoluminescence measurements were conducted on Be(x)Zn(1−x)O nanowires prepared by electrochemical and hydrothermal deposition to study their morphology, structure and optical properties. The bowing parameter for nanowires prepared by electrochemical and hydrothermal deposition was found to be 4.8 eV and 3.6 eV, respectively. It was observed that for electrochemical deposition, it is more difficult for Be(2+) to incorporate in the crystal lattice than Zn(2+). The electrochemical deposited samples exhibited stronger deep level emissions, indicating a higher density of deep level states. A home-made Optically Stimulated Luminescence (OSL) system was successfully constructed and calibrated with Al2O3:C and BeO. The OSL lifetime measurements on electrochemical deposited samples showed there were measurable OSL signals even on 500 nm long Be(x)Zn(1−x)O nanowires. The lifetimes of these OSL signals were found to decrease with increasing Be concentration. Be(x)Zn(1−x)O nanowires show considerable promise as new OSL materials.

Nanohilos de silicio obtenidos mediante el mecanismo de vapor-liquido-solido (VLS) ofrecen extraordinarias propiedades para aplicaciones en dispositivos nanomecánicos. Su calidad estructural (baja densidad de defectos, superficie lisa) y sus propiedades mecánicas únicas (auto-ensamblado robusto, alta rigidez y piezoresistencia gigante) junto con, recientes progresos en el control del crecimiento, prometen permitir un funcionamiento sin precedentes para una gran variedad de sistemas. Sin embargo, la fabricación generalmente está limitada a prototipos y más esfuerzos para conseguir un control simultáneo de las propiedades de los nanohilos y la posición son necesarios. Esta tesis ha sido centrada en el desarrollo de tecnologías de fabricación con alto rendimiento/ a gran escala de dispositivos basados en nanohilos de silicio que exploten sus propiedades excepcionales. Tecnologías de fabricación para el crecimiento selectivo de matrices de nanohilos de silicio y de nanohilos individuales en dispositivos funcionales han sido desarrolladas y posteriormente adaptadas para la fabricación de diversos dispositivos basados en nanohilos. En particular, el diseño, la fabricación y la caracterización de un cantilever piezoresistivo en el que el elemento de sensado está compuesto por una matriz de nanohilos ha sido demostrado. Los coeficientes piezoresistivos gigantes característicos de los nanohilos de Silicio se trasladan en un incremento en la sensibilidad mecánica comparada con dispositivos basados en silicio volumétrico. Por otro lado, se ha realizado la fabricación de resonadores nanomecánicos basados en nanohilos individuales. La caracterización de estos dispositivos demostró que los nanhilos individuales son excepcionales plataformas para el desarrollo de sensores de masa ultra sensibles y para el estudio de propiedades fundamentales de estructuras nanomecánicas. / Silicon nanowires obtained via vapor-liquid-solid (VLS) mechanism offer many extraordinary properties for applications in nanomechanical devices. Their structural quality (low defect density, surface flatness) and unique mechanical properties (robust self-assembly, high stiffness, giant piezoresistance) together with, recent advances in growth control, promise to allow unprecedented performance of wide variety of systems. However, device fabrication is generally limited to prototype fabrication and more efforts to achieve simultaneous control of nanowire properties and location are needed. This thesis has been focused towards the development of high yield/ large scale fabrication technologies based on catalyst grown Si nanowire to realize devices that exploit their exceptional properties. Fabrication technologies for the selective growth of silicon nanowire arrays and single nanowire on functional devices have been developed and posteriorly adapted for the fabrication of several nanowire based devices. In particular, the design, fabrication and characterization of a piezoresistive cantilever in which the active sensor is composed of an horizontal Si nanowire array has been demonstrated. Giant piezoresistance coefficients characteristics of Si nanowires are translated into an increment in the cantilever mechanical sensitivity compared with similar bulk devices. On the other hand, the fabrication of nanomechanical resonators based on single nanowires for mass sensing applications with different transduction mechanims has been performed. The characterization of these devices proved that single nanowires are exceptional platforms to develop ultra-high sensitive mass sensors and to study fundamental properties of nanomechanical structures.