Ternary indium aluminum nitride (InXAl1-XN) semiconductor is an attractive material with a wide-range bandgap energy varied from ultraviolet (Eg(AlN): 6.2 eV) to near infrared (Eg(InN): 0.7 eV). With tuning composition, it can be widely used to many optoelectronic device applications. In this thesis, I have studied InXAl1-XN film deposited on Si (111) substrate using natural and isotopically enriched nitrogen as reactive gas by reactive magnetron sputter epitaxy (MSE). Four series of experiments were performed, which are I. InAlN presputtering, II. InAlN sputter deposition, III. InAlN direct deposition, and IV. InAlN direct deposition using isotopically enriched nitrogen. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX). The θ-2θ XRD scan confirms that the designed composition x = 0.17 of InXAl1-XN film was obtained. TEM images shows that an amorphous interlayer with a thickness ranging from 1.2 nm to 1.5 nm was formed between Si substrate and InXAl1-XN film. However, high-resolution TEM shows that the interlayer actually contains partial crystalline structures. EDX line profile indicates that the chemical composition of the amorphous interlayer is silicon nitride (SiNX). By comparing d-spacing measurement of partial crystalline structures with EDX line profile, it reveals that partial SiNX crystal is formed in the interlayer. Nonetheless, the samples (IAD01, IAD02, IAD03, IAD04), grown without presputtering procedure, contain both crystalline SiNX and InXAl1-XN embedded in the amorphous interlayer. It means that SiNX and InXAl1-XN film can be directly grown on the substrate in the beginning of deposition. Moreover, the samples (IAD01, IAD03), quenched directly after deposition, have less crystalline structures in the interlayer then the samples (IAD02, IAD04), maintained at 800℃ for 20 min.

Transmission electron microscopy (TEM) is a powerful tool for studying solidstate crystalline systems. With the advances in aberration correction, monochromation, and in situ capabilities, these microscopes are now more useful for addressing fundamental materials chemistry problems than ever before. This dissertation will illustrate the ways in which I have been using high-resolution imaging and in situ heating in the TEM during my Ph.D. research to investigate unique solid state chemistry questions. This dissertation will focus on four unique crystal systems: thermoelectric skutterudite crystals, vapor-liquid-solid (VLS) grown nanowires, and hafnium dioxide nanorods. Although these systems are very different from one another, high resolution and/or in situ heating in TEM is an integral part of each study. Through these techniques, we gain insight and knowledge of these systems that may have gone unknown through different analysis techniques. The experiments I will describe in some cases provide surprising and unexpected results that arise from the nanoscale nature of the materials and would be difficult to observe through bulk analytical methods. The work presented here helps to demonstrate the strength and versatility of TEM to address solid state chemistry questions.

Ultrasound contrast agents (CAs) are typically used in clinic for perfusion studies (blood flow through a specific region) and border delineating (differentiate borders between tissue structures) during cardiac imaging. The CAs used during ultrasound imaging usually consist of gas filled microbubbles (MBs) (diameter 1-5 μm) that are injected intravenously into the circulatory system. This thesis partially involves a novel polymer-shelled ultrasound CA that consists of air filled MBs stabilized by a polyvinyl alcohol (PVA) shell. These MBs could be coupled with superparamagnetic iron oxide nanoparticles (SPIONs) in order to serve as a combined CA for ultrasound and magnetic resonance imaging. The first three papers (Paper A-C) in this thesis investigate the structural characteristic and the elimination process of the CA. In Paper A, two types (PVA Type A and PVA Type B) of the novel CA were analyzed using transmission electron microscopy (TEM) images of thin sectioned MBs. The images demonstrated that the SPIONs were either attached to the PVA shell surface (PVA Type A) or embedded in the shell (PVA Type B). The average shell thickness of the MBs was determined in Paper B by introducing a model that calculated the shell thickness from TEM images of cross-sectioned MBs. The shell thickness of PVA Type A was determined to 651 nm, whereas the shell thickness of PVA Type B was calculated to 637 nm. In Paper C, a prolonged blood elimination time was obtained for PVA-shelled MBs compared to the lipid-shelled CA SonoVue used in clinic. In addition, TEM analyzed tissue sections showed that the PVA-shelled MBs were recognized by the macrophage system. However, structurally intact MBs were still found in the circulation 24 h post injection. These studies illustrate that the PVA-shelled MBs are stable and offer large chemical variability, which make them suitable as CA for multimodal imaging. This thesis also involves studies (Paper D-E) of the molecular chaperones (Hsp21 and DNAJB6). The small heat shock protein Hsp21 effectively protects other proteins from unfolding and aggregation during stress. This chaperone ability requires oligomerization of the protein. In Paper D, cryo-electron microscopy together with complementary structural methods, obtained a structure model which showed that the Hsp21 dodecamer (12-mer) is kept together by paired C-terminal interactions.The human protein DNAJB6 functions as a very efficient suppressor of polyglutamine (polyQ) and amyloid-β42 (Aβ42) aggregation. Aggregation of these peptides are associated with development of Huntington’s (polyQ) and Alzheimer’s (Aβ42) disease. In Paper E, a reconstructed map of this highly dynamic protein is presented, showing an oligomer with two-fold symmetry, indicating that the oligomers are assembled by two subunits. / <p>QC 20160527</p>

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

No description available.

No description available.

Semiconductor nanostructures have attracted great interest owing to their unique physical properties and potential applications in nanoscale functional devices. The enhancement of the physical properties of semiconductor nanostructures and their performance in devices requires a deeper understanding of their fundamental microstructural properties. Thus this thesis is focused on the experimental and theoretical studies of the microstructural properties of two important semiconductor nanostructures: axial heterostructured silicon nanowires with varying doping and indium nitride colloidal nanoparticles. In this thesis, axial heterostructured silicon nanowires with varying doping were synthesized on an oxide-removed Si{111} substrate using a vapour-liquid-solid approach. Their fundamental microstructural properties, including the crystalline structure, wire growth direction and morphologies, were studied using various characterization techniques. It is found that a very small fraction of the silicon nanowires crystallize in a hexagonal (wurtzite) phase, which is thermodynamically unstable in bulk silicon under ambient conditions, while a large majority of the synthesized silicon nanowires exhibit the expected diamond cubic crystalline structure. About 75% of the diamond cubic silicon nanowires synthesized grow in a single <111> direction, while the rest contain growth-related kinks, where the nanowire switches to another direction during the growth. The ~109° silicon nanowire kinks are the most commonly observed, and the growth direction before and after such ~109° kink are both <111>. The sidewalls of silicon nanowires do not change abruptly at the ~109° kink, but exhibit an elbow-shaped structure. It is also found that the nanowire sidewalls exhibit periodic nanofaceting, which is strongly doping-dependent. The nanofaceting is found to occur during the enhanced sidewall growth that arises when the diborane dopant gas is introduced. A thermodynamic model predicting the dependence of nanofacet period on the wire diameter is developed. Another semiconductor nanostructure studied in this thesis is indium nitride colloidal nanoparticles, which were grown using a solution-phase chemical method. The formation of such indium nitride colloidal nanoparticles is confirmed by studying their compositions, crystalline structures and shape using various electron microscopy techniques. The size of the indium nitride colloidal nanoparticles was controlled by varying the time of solution-phase reactions. The most probable size of the colloidal nanoparticles increases and the size distribution broadens with the increase of reaction time. The crystalline structures of the indium nitride colloidal nanoparticles are found to be particle size dependent. The observed dependence of the band gap blueshift of the indium nitride colloidal nanoparticles on the reaction time (hence the particle size) is explained by the quantum-size effect.

The interwall spacing of multi-walled carbon nanotubes has an effect on their physical and chemical properties. Tubes with larger interwall spacing - compared to the spacing where the carbon atoms are in their natural distance to each other - are for instance expected to be mechanically less stable. Considering the MWCNT interwall spacing’s dependence on the tube size, three interesting previous studies with slightly different conclusions can be found. All of them conclude an increase of the interwall spacing with a decreasing tube size. We describe their analysis procedure, compare them to each other and to our own measured data. In the beginning of our analyses, we determine the expected inaccuracy for measured distances out of TEM images being up to 10 % and we show the impacts of the TEM’s defocus, a powerful setting in TEM imaging. Finally, we suppose that the interwall spacings are not as strongly varying as one previous study concludes, but our analyses are relatively in harmony with the two other studies. The interwall spacings from tubes with an inner diameter larger than 5 nm are relatively constant within the whole tube. Furthermore, it appears that the middle spacings (excluding the outer- and innermost ones) show values that are most consistent with the interlayer spacings of turbostratic graphite. In underfocused images, the outer- and innermost spacings tend to have values being slightly smaller than the middle ones from the same tube.

The current work is part of the EPSRC MUZIC project, which established the collaboration among several universities to carry out a multidiscipline study on the breakaway oxidation of zirconium alloys. The overall goal of the project is to further understand the mechanisms of the oxidation and breakaway process of zirconium alloys. This thesis describes the nano/micro-structural study and nano-analysis of the corroded zirconium alloys using up-to-date TEM and 3D focused ion beam (FIB) slicing and reconstruction techniques. The work mainly focused on the characterization of ZIRLO. The oxide morphology in general comprises an inner columnar layer and an outer equiaxed layer, except for a post-second transition oxide grown on a Zr-Nb-Ti test alloy with a very poor corrosion resistance, which exhibits generally only equiaxed grains throughout the whole oxide scale. Detailed investigation reveals oxides in a slower oxidation stage exhibit better developed columnar grain structure. All the oxides, independent of different corrosion stages and alloy types, contain predominantly monoclinic oxide and a small amount of tetragonal oxide. Defects at different length scales were examined. In stead of a sudden burst of crack nucleation at the kinetic transition, a gradual introduction of cracks parallel to the metal/oxide interface throughout the pre-transition stage is found, suggesting no direction correlation between the formation of cracks and the transition. Besides cracks, the oxide also contains different forms of nano-porosity: isolated pores of 1-3 nm or interconnected pores at grain boundaries. The density of interconnected porosity, especially those along the oxide growth direction, increases towards the oxide surface, evolving over time. It is suggested that the kinetic transition is related to the development of an interconnected porosity down to the metal/oxide interface, providing easy pathways for the transportation of oxidation species. The metal-oxide interface has a wavy morphology both in the micrometer and nanometer scale. The roughness develops to a maximum just before the first kinetic transition. An intermediate suboxide layer with complex 3D morphology between the bulk oxide and the metal substrate is found. Quantitative EELS analysis shows the composition of this layer to be 40-50 at. % oxygen. The suboxide appears to develop in thickness with increasing oxidation time for the pre-transition oxides, while is very thin or absent in the post-, and post-second transition oxides. In the suboxide region, multiple phases including &alpha;-Zr, &omega;-Zr, tetragonal oxide and a phase with an unidentified structure were found, suggesting different structures can coexist in the suboxide layer. Second-phase particles (SSPs) of &beta;-Nb and hexagonal Zr(Fe,Nb)₂ types were found in ZIRLO samples and FCC Zr(Fe,Cr)₂ was the predominant type in Zircaloy-4. The SPPs showed delayed oxidation compared to surrounding Zr. In ZIRLO, those containing high Fe contents were found to be oxidized and transform into an amorphous state much earlier than &beta;-Nb. Hydrides of different types (&gamma;, &sigma; and &epsilon;) were observed in the metal and metal/oxide region for both Zircaloy-4 and ZIRLO samples. A higher density of hydrides was seen in post-transition oxides of ZIRLO than in pre-transition oxides.

This thesis considers the theory and calculations of image formation mechanisms for various modes of three-dimensional imaging in aberration-corrected scanning transmission electron microscopy. Discrete tomography is used to determine and refine the three-dimensional structure of molecular nanowire bundles. The structure determination is expedited by the use of annular dark-field imaging, an incoherent imaging mode which provides directly interpretable images. The development of spherical aberration correctors and the subsequent reduction in probe sizes, including the depth of field, has made optical depth sectioning a feasible technique. The localisation in three dimensions of substitutional impurity atoms in zone-axis imaging is discussed. Both the channelling of the probe and the pre-focussing effect of the atomic column play an important role in determining the depth response of the impurity atom. Interband scattering within a sample is shown to be influential in imaging crystals containing dislocations and optical depth sectioning is explored as a possible option for overcoming surface relaxation effects in the imaging of screw dislocations end-on. The possibility of extending the optical depth sectioning approach using aberration-corrected scanning confocal electron microscopy is discussed. The coherent and incoherent imaging modes, involving elastically and inelastically scattered electrons respectively, are investigated.