Synthesis And Characterization Of Zero, One And Two Dimensional Metallic And Ceramic Nanostructures

Nanostructured materials are of great interest because the properties of a material at the nanoscale may differ significantly from the properties of the same material in the bulk form. This has led to a lot of new applications for nanomaterials owing to their unique physical, chemical, electrical, optical and magnetic properties. The present work reports on the synthesis and characterization of zero, one, and two dimensional nanostructured materials. Nanostructured materials in the present study were all grown using a pulsed laser deposition technique. Gold (Au) nanodots (zero-dimensional nanostructure) were grown on silicon (Si) substrates and subsequently used in the growth of titanium nitride (TiN) nanowires (one-dimensional nanostructure). TiN nanowires were grown under different conditions; energy entering the chamber (70 mJ, 80 mJ and 90 mJ) and deposition temperature (600 °C, 700 °C and 800 °C) leading to nanowires of varying length (50 nm – 200 nm), diameter (25 nm-50 nm) and spatial density. Corrosion tests run on TiN nanowires, thin films and magnesium (Mg) bulk showed that TiN nanowires degraded faster than TiN thin films but were still better than Mg bulk. The thesis work has also focused on growing nickel (Ni) thin films (two-dimensional nanostructure) sandwiched between an alumina (Al2O3) substrate and thin film. The nickel films were deposited at different substrate temperatures (liquid nitrogen, room temperature and high temperature) keeping all other deposition parameters the same. Magnetic moment versus magnetic field measurements showed that Ni thin film samples deposited at room temperature and liquid nitrogen temperature had almost the same remanent magnetization; however, samples deposited at liquid nitrogen had the highest saturation magnetization and coercivity. The coercivity values at 10K for Ni thin film samples grown at liquid nitrogen, room temperature, and high temparature were found to be 58.92 Oe and 255.15 Oe respectively

Growth of GaN Nanowires: A Study Using In Situ Transmission Electron Microscopy

abstract: Owing to their special characteristics, group III-Nitride semiconductors have attracted special attention for their application in a wide range of optoelectronic devices. Of particular interest are their direct and wide band gaps that span from ultraviolet to the infrared wavelengths. In addition, their stronger bonds relative to the other compound semiconductors makes them thermally more stable, which provides devices with longer life time. However, the lattice mismatch between these semiconductors and their substrates cause the as-grown films to have high dislocation densities, reducing the life time of devices that contain these materials. One possible solution for this problem is to substitute single crystal semiconductor nanowires for epitaxial films. Due to their dimensionality, semiconductor nanowires typically have stress-free surfaces and better physical properties. In order to employ semiconductor nanowires as building blocks for nanoscale devices, a precise control of the nanowires' crystallinity, morphology, and chemistry is necessary. This control can be achieved by first developing a deeper understanding of the processes involved in the synthesis of nanowires, and then by determining the effects of temperature and pressure on their growth. This dissertation focuses on understanding of the growth processes involved in the formation of GaN nanowires. Nucleation and growth events were observed in situ and controlled in real-time using an environmental transmission electron microscope. These observations provide a satisfactory elucidation of the underlying growth mechanism during the formation of GaN nanowires. Nucleation of these nanowires appears to follow the vapor-liquid-solid mechanism. However, nanowire growth is found to follow both the vapor-liquid-solid and vapor-solid-solid mechanisms. Direct evidence of the effects of III/V ratio on nanowire growth is also reported, which provides important information for tailoring the synthesis of GaN nanowires. These findings suggest in situ electron microscopy is a powerful tool to understand the growth of GaN nanowires and also that these experimental approach can be extended to study other binary semiconductor compound such as GaP, GaAs, and InP, or even ternary compounds such as InGaN. However, further experimental work is required to fully elucidate the kinetic effects on the growth process. A better control of the growth parameters is also recommended.Dissertation/ThesisPh.D. Materials Science and Engineering 201

The development of micropillars and two-dimensional nanocavities that incorporate an organic semiconductor thin film

Photonic crystals (PC) are periodic optical structures containing low and high refractive index layers that influence the propagation of electromagnetic waves. Photonic cavities can be created by inserting defects into a photonic crystal. Such structures have received significant attention due to their potential of confining light inside volumes (V) smaller than a cubic wavelength of light (λ/n)3 which can be used to enhance light-matter interaction. Cavity quality factor (Q) is useful for many applications that depend on the control of spontaneous emission from an emitter such quantum optical communication and low-threshold lasing. High Q/V values can also result in an enhancement of the radiative rates of an emitter placed on the surface of the cavity by means of the Purcell effect. This thesis concerns the fabrication and study of two types of optical cavity containing an organic-semiconductor material. The cavities explored are; (1) one-dimensional micropillar microcavities based on multilayer films of dielectric and organic materials, and (2) two-dimensional nanocavities defined into a photonic crystal slab. Firstly, light emission from a series of optical micropillar microcavities containing a thin fluorescent, red-emitting conjugated polymer film is investigated. The photoluminescence emission from the cavities is characterized using a Fourier imaging technique and it is shown that emission is quantised into a mode-structure resulting from both vertical and lateral optical confinement within the pillar. We show that optical-confinement effects result in a blue-shift of the fundamental mode as the pillar-diameter is reduced, with a model applied to describe the energy and distribution of the confined optical modes. Secondly, simulation, design, and analysis of two dimensional photonic crystal L3 nanocavities photonic crystal are presented. Nanocavities were then prepared from silicon nitride (SiN) as the cavity medium with the luminescence emitted from an organic material at red wavelengths that was coated on the cavity surface. To improve the quality factor of such structures, hole size, lattice constant and hole shift are systematically varied with their effect as cavity properties determined. Finite Difference Time Domain (FDTD) modelling is used to support the experimental work and predict the optimum design for such photonic crystal nanocavity devices. It is found that by fine-tuning the nearest neighbour air-holes close to the cavity edges, the cavity Q factor can be increased. As a result, we have obtained a single cavity mode having a Q-factor 938 at a wavelength of 652 nm. Here, the cavity Q factor then increases to 1100 at a wavelength of 687 nm as a result of coating a red-emitting conjugated polymer film onto the top surface of the nanocavity. We propose that this layer planarizes the dielectric surface and helps reduce optical losses as a result of scattering

Photonic platform and the impact of optical nonlinearity on communication devices

It is important to understand properties of different materials and the impact they have on devices used in communication networks. This paper is an overview of optical nonlinearities in Silicon and Gallium Nitride and how these nonlinearities can be used in the realization of optical ultra-fast devices targeting the next generation integrated optics. Research results related to optical lasing, optical switching, data modulation, optical signal amplification and photo-detection using Gallium Nitride devices based on waveguides are examined. Attention is also paid to hybrid and monolithic integration approaches towards the development of advanced photonic chips

Mechanistic study of self-assembled tungsten nanogratings on solids induced by femtosecond laser beam

This dissertation describes a mechanistic study on the spontaneous formation of tungsten nanograting induced by a 400 nm femtosecond laser beam on solids observed in laser induced chemical vapor deposition configuration. The formed tungsten nanograting has a periodicity less than half the laser wavelength, and the orientation of the nanograting is parallel to the laser polarization direction, given that the laser is linearly polarized. By translating the substrate with respecting to the fixed laser beam, long-range ordered transverse or longitudinal nanograting is produced depending on the angle between the translating and laser polarization directions. Systematic experimental studies on the effect of laser power, scanning speed of substrate, laser polarization, wavelength and substrate were carried out in detail. The formation of tungsten nanograting is nearly a universal phenomenon observed on a wide range of substrates. Appearance of tungsten nanograting requires a threshold laser power which varies for different substrates. The grating period can be tuned simply by managing writing parameters. Scaling to large area grating pattern and feasibility of growing nanograting on curved surfaces were also demonstrated. Evidence shows that laser heating and local field enhancement are involved in the formation of tungsten nanograting. A crude conjectured theoretical model was proposed to explain the mechanism of tungsten nanograting formation. Due to the novel nature of self-assembled tungsten nanograting, this study is expected to advance the knowledge on laser-material interaction field

Contribution to the Development of Electronic Devices Based on Zn3N2 Thin Films, and ZnO and GaAs Nanowires

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Aplicada. Fecha de lectura: 16-07-201

Optimisation of high-efficiency UV and visible light sources utilising lateral localisation in InAlN and InGaN based nano-structure devices

III-nitride semiconductor materials (including GaN, InN and AlN and their alloys), have the capability to emit light at wavelengths spanning from the near IR to the deep UV. However, understanding these materials is challenging due to the presence of strong polarisation fields and large difference in optimum growth temperature between binary compounds are two such examples. InAlN is perhaps the least well understood III-N alloy. It has potential be applied for optoelectronic devices operating in the UV spectral range. However, the variation of band-gap with alloy composition, particularly in the low In content regime, is not understood. In this work, a strongly composition dependent bowing parameter has been observed for ~100 nm thick InxAl1−xN epitaxial layers with 0 ≤ x ≤ 0.224, grown by metalorganic vapour phase epitaxy (MOVPE), prepared on AlN/Al2O3-templates. Also a double absorption edge was observed for InAlN with x < 0.01, attributed to crystal-field splitting of the highest valence band states. These results indicate that the ordering of the valence bands is changed at much lower In contents than linear interpolation of the valence band parameters would predict. Coupling our results with the published literature data the band-gap and bowing parameter of InAlN across the full composition range were determined. Additionally, applying the InAlN band-gap data with those for other alloys the refractive index of III-N alloys is predicted using an Adachi model resulting in a very good agreement with previous experimental data where available. For InAlN/AlGaN multi-quantum-wells (MQWs) excited by photoluminescence (PL) and emitting between 300-350 nm, high apparent internal quantum efficiencies (IQE) (IPL(300 K)/IPL(T)max) of up to 70% were obtained. This is attributed to the exceptionally strong carrier localisation in this material, which is also manifested by a high Stokes shift (0.52 eV) of the luminescence. A non-monotonic dependence of luminescence efficiency on indium content with a maximum at about 18% In was explained as a trade-off between a strain relaxation for higher indium contents and a type I to type II band line-up conversion for low In content alloys. Nanoscale materials have attracted a lot of attention due to their ability to decrease dislocations as well as build-in field reduction. In the second part of this thesis, GaN nanostructures, were used as templates for InGaN MQW growth targeting nano-LED structures. Two nano-structuring methods were examined; using GaN nano-columns (NCs) following an etch regrowth methodology, and selective area aperture growth (SAG). In the former case we determined the optimal etch conditions for the GaN columns and conditions for overgrowth InGaN QWS. The rod tops formed semipolar facets. InGaN QWs grown on these pyramids were found to be extremely thin leading to difficulties in obtaining PL in our case. Using the SAG approach, nano-pyramids were formed in nano-apertures, with good uniformity. InGaN QWs exhibited blue PL, which cathodoluminescence (CL) showed to be made up of two spectral features, attributed to the pyramid nano-facets and pyramid apex tips, respectively