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<p>A non-contact on-site surface roughness measurement method was investigated in experimental and simulation approaches. The resolution of the vertical surface roughness was obtained at 20 nm by using self-interference theory. Various surface roughness measurement techniques, such as mechanical stylus, AFM and Michelson interferometer, were employed for different roughness samples. The novelty of this study was to measure the surface roughness on a rotating sample. For each sample with different step height, corresponding intensity distribution data was obtained and analyzed. The fringe visibility ratio resulted in a curve that is related to the step height, which represents the roughness. The results from simulations for all samples were compared with experimental data. Good agreements were obtained for the studied conditions.</p> / Master of Applied Science (MASc)

<p>The aim of third-generation photovoltaics (PV) is ultimately to achieve low-cost, high-efficiency devices. This work focused on a third-generation PV concept known as down-shifting, which is the conversion of high-energy photons into low-energy photons which are more useful for a typical solar cell. Silicon nanocrystals (Si-NCs) fabricated using electron-cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD) were studied as a down-shifting material for single-junction silicon cells. A calibration was done to determine optimal deposition parameters for Si-NC formation. An experiment was then done to determine the effect of film thickness on emission, optical properties, and photoluminescence quantum efficiencies.</p> <p>Photoluminescence (PL) peaks varied depending on the stoichiometry of the films, ranging from approximately 790 nm to 850 nm. Variable-angle spectroscopic ellipsometry was used to determine the optical constants of the Si-NC films. The extinction coefficients indicated strong absorption below 500 nm, ideal for a down-shifting material. Transmission Electron Microscopy (TEM) was used to determine the size, density, and distribution of Si-NCs in two of the films. Si-NCs were seen to have an average diameter of approximately 4 nm, with larger nanocrystals more common near the surface of the film. A density of approximately 10⁵ nanocrystals per cubic micron was approximated from one of the TEM samples.</p> <p>The design and implementation of a PL quantum efficiency measurement system was achieved, using an integrating sphere to measure the absolute efficiency of Si-NC emission. Internal quantum efficiencies (IQE) as high as 1.84% and external quantum efficiencies (EQE) of up to 0.19% were measured. The EQE was found to increase with thicker films due to more intense photoluminescence; however the IQE remained relatively independent of film thickness.</p> / Master of Applied Science (MASc)

<p>InP nanowires with short InAs segments were grown on InP (111)B substrates by Au assisted vapour-liquid-solid growth in a gas source molecular beam epitaxy system. Nanowire crystal structure and morphology were investigated by transmission electron microscopy as a function of temperature, growth rate, and V/III flux ratio. At 370C predominantly kinked nanowires with random morphology and low areal density were observed with a rough parasitic 2D film. At 440C, nanowire density was also reduced but the 2D film growth was smoother and nanowires grew straight without kinking. An optimum temperature of 400C maximized areal density with uniform nanowire morphology. At the optimum temperature of 400C, an increase in V/III flux ratio changed the nanowire morphology from rod-shaped to pencil like indicating increased radial growth. Growth rate did not affect the crystal structure of InP nanowires. For InAs nanowires, changing the growth rate from 1 to 0.5 μm/hr reduced the presence of stacking faults to as low as one per nanowire. Short InAs segments in InP nanowires were found to grow through two mechanisms for nanowires of length L and diameter D. The first mechanism described the supply of In to the growth front via purging of In from the Au droplet where L was proportional to D. The second mechanism involved direct deposition of adatoms on the nanowire sidewall and subsequent diffusion to the growth front where L was proportional to 1/D. For intermediate growth durations, a transition between these two mechanisms was observed. For InP and InAs nanowires, the growth mode was varied from axial to radial through the inclusion of Al to form a core shell structure. Al_xIn_1-xAs(P) shells were grown on InAs cores with Al alloy fractions between 0.53 and 0.2. These nanowires were examined by transmission electron microscopy and it was found, for all values of x in InAs-Al_xIn_1-xP structures, that relaxation had occurred through the introduction of dislocations. For InAs-Al_xIn_1-xAs structures, all values except x=0.2 had relaxed through dislocation formation. A critical thickness model was developed to determine the core-shell coherency limits which confirmed the experimental observation of strain relaxation. The effects of passivation on the electronic transport and the optical properties were examined as a function of structural core-shell passivation and chemical passivation. The mechanisms for the observed improvement in mobility for core-shell versus bare InAs nanowires was due to the reduction in ionized impurity scattering from surface states. Similarly an increase in photoluminescence intensity after ammonium sulfide passivation was explained by the reduction of donor type surface states.</p> / Doctor of Philosophy (PhD)

<p>The solid state dewetting characteristics of thin gold films sputtered onto (111) MgAl₂O₄ substrates were investigated. Prior research done on this system reported discovering the self-assembly of intricately shaped nanostructures, consisting of a faceted sphere lying above a truncated triangular pyramid, formed by applying a specific two stage heating profile. The current work was done to provide deeper insight on the odd self-assembly observed in this system. The results indicate that the intricate structures are not purely gold self-assemblies, but in fact consist of three distinct materials: a single crystal or polycrystalline gold faceted sphere, separated by an interfacial boundary layer, from above a crystalline MgAl₂O₄ necking structure that spontaneously developed from the initially flat substrate. The boundary separating these two assemblies is confined within a thin, sharp region of a third material consisting of Au and O elements. The composition and crystalline nature of the individual nanostructures were studied using high angle annular dark-field imaging, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy employed by a high resolution transmission electron microscope. Two-dimensional X-ray diffraction texture analysis revealed that the gold nanoparticles are crystalline, with majority of the maximum intensity signal corresponding to the epitaxial alignment of the gold nanoparticles with the substrate. The MgAl₂O₄ necking structures were found to be sensitive to both the annealing profile, and the thickness of the gold film, which influenced the distinguishable presence, size, and footprint of the MgAl₂O₄ nanostructures. Atomic force microscopy and scanning electron microscopy results were consistent with the gold overlayer playing an essential role in the self-assembly of MgAl₂O₄ nanostructures. While the fundamental mechanisms that govern this phenomenon are not entirely clear, the presented results do provide insight into the role of interfaces in heteroepitaxial systems, especially the self-assembly of crystalline nanostructures from a previously stable substrate.</p> / Master of Applied Science (MASc)

<p>In this thesis, a modern transmission electron microscope is used to perform high-resolution electron energy loss studies of metallic nanostructures and carbon nanotubes.</p> <p>The remarkable optical properties of metallic nanostructures arise from the excita- tion of surface plasmons. With improved instrumentation, surface plasmon resonances are imaged in a variety of nanostructures, enabling a greater understanding of their behaviour in nanoscale systems. It is shown that surface plasmons set up multiple high order resonances in silver nanorods, and they freely propagate around sharp corners in silver nanowires. It is also demonstrated that silver nanorice structures resonate in a similar manner to nanorods, despite the high density of stacking faults in the structure. Finally, a complementary structural pair is found to resonate in a complementary fashion, in agreement with Babinet’s principle.</p> <p>Carbon nanotubes exhibit unique physicochemical properties that have led to their use in a variety of novel materials science applications. Despite rapid progress in the theoretical and experimental investigation of carbon nanotubes, techniques capable of studying the structural and electronic properties of individual tubes are limited. Here, it is demonstrated that the spectral signature of carbon can be used to identify the electronic character of individual single-walled carbon nanotubes. In addition, a new technique is used to map bonding anisotropy in a multi-walled carbon nanotube.</p> <p>Also presented in this thesis is the design and construction of a unique laser-TEM system. Early results from the system include in-situ measurements of laser-induced structural and electronic distortions in individual carbon nanotubes.</p> / Doctor of Philosophy (PhD)

This thesis focuses on two of the major challenges of carbon nanotube (CNT) research: understanding the growth mechanism of nanotubes by chemical vapour deposition (CVD) and the positioning of nanotubes on surfaces. The mechanism of growth of single–walled nanotubes (SWNTs) has been studied in two ways. Firstly, a novel iron nanoparticle catalyst for the production of single–walled nanotubes was developed. CVD conditions were established that produced high quality tubes. These optimised CVD conditions were then used as the basis of several comparative CVD experiments showing that the quality of nanotubes and the yield of carbon depended on the availability of carbon to react. The availability could be controlled by the varying concentration of methane in the gas phase or the residence time of the methane over the catalyst. Evidence is presented that the diameters of the tubes produced were affected by the availability of methane. A second mechanistic investigation was carried out to study the validity of the previously proposed ring addition mechanism for the growth of carbon nanotubes from camphor. In this mechanism, the formation of tubes is thought to occur through the addition of preformed carbon rings: so it would be expected that there would be a relationship between the molecular structure of the precursor and the resulting SWNTs. To explore this relationship, comparative CVDs were carried out to produce SWNTs with several different cyclic and acyclic compounds similar in structure to camphor. The vapour pressure and the chemical stability of the precursor were found to be important to the formation of nanotubes, while the compound’s structure was not related to the quality of tubes produced. The lack of a relationship between the structure of the precursor and the production of SWNTs suggests that preformed rings are not vital to the production of SWNTs. Differences in the growth of SWNT from benzene and methane were related to the stability of each compound. In particular, differences in the distributions of the diameters of the tubes formed from methane and benzene have been observed. These differences have been explained in terms of the relative kinetic stabilities of these molecules, and in terms of a competition between end–cap and sidewall growth. Positioning of nanotubes on surfaces has been explored using two approaches. In the first approach, commercially obtained SWNTs were functionalized by a sulfur plasma so that the products would form bonds with gold surfaces. The nanotubes were found to selectively deposit themselves onto gold surfaces from ethanolic dispersions of the functionalized samples. This selective deposition of the nanotubes allowed the production of prototype carbon nanotube field–effect transistors with higher device yields than were obtained with unfunctionalized tubes. In a second approach to positioning of carbon nanotubes, the growth of tubes on surfaces by CVD was explored. Iron nitrate and different magnesium compounds were dip–coated onto SiO2 surfaces so that MgO supported–Fe catalysts would be formed by calcination. SWNTs were grown on the surfaces by CVD. Surface area measurements of the equivalent powdered catalysts showed that a high surface area was vital to produce dense growth of high quality SWNTs. The morphology of the surface was also found to play a key role in the growth of the tubes. Patterned growth of carbon nanotubes was accomplished using soft lithography techniques to control the localization of catalyst deposition onto a surface. A long calcination step (10 h, 950 °C) before CVD, was found to improve the quality of nanotubes grown. Catalysts that had been calcined for 10 hours were also found to produce smaller diameter nanotubes than uncalcined samples. The formation of smaller diameter tubes was explained in terms of the formation of MgFe2O4 alloys, consistent with results reported previously in the literature. In addition, Raman spectroscopy of the calcined catalysts with 3% w/w loadings of Fe was used to confirm directly the presence of MgFe2O4.

M.Sc. (Nanoscience) / Please refer to full text to view abstract