This study was undertaken to investigate the possibility of synthesis of nitride based semiconductors. To this end hydrogenated amorphous silicon nitride (a-SiNx:Hy) has been deposited as the starting material using PECVD (plasma enhanced chemical vapour deposition). Then the effects of implanting gallium into the a-SiNx:H target material have been studied with the aim of forming GaN compounds. Should this technique work, it opens the possibility of carrying out similar synthesis of Al, hi and other nitride based compounds. PECVD hydrogenated amorphous silicon nitride thin films are studied as a function of the ammonia/silane gas ratio. The power coupled to the plasma, pressure and the substrate temperature were held constant, while the gas flow ratio of NH3/SiH4 was varied. IV measurements on the metal- nitride-metal structures indicated that the conduction mechanism might be explained by Poole-Frenkel conduction. The composition of the a-SiNx:Hy films were analysed using Rutherford backscattering spectroscopy (RBS) and Elastic recoil detection analysis (ERDA). The Si content decreased in a logarithmic manner for 0 < NH3/SiH4 < 4 and saturated for higher NH3/SiH4, ratios at 27 at.%. The N content mirrored this trend and saturated to a maximum asymptotic value of 49 at.% for NH3/SiH4 > 4. Stress, refractive index and optical absorption studies were also conducted. A turning point for most of the properties was observed at a NH3/SiH4 ratio of 4. This corresponds to a N/Si ratio of about 1.4 with a hydrogen content of 22 at.% for the deposited films. Below this ratio, a-SiNx:H films have high growth rates, a refractive index between 1.9 and 2.7, a N/Si ratio between 0.5 and 1.5 and moderate values of compressive stress (~ 0.7 GPa). While, above this pivotal ratio, growth rates become significantly lower, the refractive index minimises to 1.8, N and Si concentrations in the films saturate and the compressive stress rapidly increases. Evidence has been found for Ga-N bonds by implanting gallium into amorphous silicon nitride films. The a-SiNx:H films grown at high gas ratios (R > 70) are highly stressed and the nitrogen content is saturated. They are, therefore, ideally suited to forming GaN bonds under the high energy conditions of Ga ion implantation, a phase that is not thermodynamically favourable. X-ray photoelectron spectroscopy (XPS), FTIR and RBS have been used to examine the bond structure, composition and the depth profile of the synthesised material. It has been found that the implanted Ga bonds with the N from the NH2, NH and SiN bonds and the released Si and H from these bonds combine to form additional SiH. From the RBS and XPS data, annealing at 200&deg;C was shown to increase the thickness of the a-GaN and transform more of the target. The Ga profile moves deeper into the material and the stoichiometric phase of SiN that is thermodynamically stable, recovers. Annealing at a higher temperature (500&deg;C) shows a significant reduction in the amount of H from the amorphous network (ERD), mainly from the NH bonds (FTIR), thus leaving the free N available to bond with the unbonded Ga in the material. Up to ~ 22 at.% Ga is present in the material and can be converted to GaN on annealing. Electron diffraction through the material shows no evidence of any crystallites in the synthesised material.

The aims of this thesis were two-fold: A) To use high internal phase emulsion (HIPE) templated materials to produce a chromatographic stationary phase for the size separation of engendered nanoparticles (NPs). B) To produce well characterised lanthanide doped polymer NPs with a potential use as analytical standards. Initially, silica materials were prepared from oil-in-water HIPEs by a two stage acid/base catalysed sol gel process. As well as presenting the expected macroporosity typical of HIPE templated materials, it was also found that micro- and meso-porosity could be influenced by surfactant choice and reaction with iron (III) chloride or copper (I) chloride which had been included in the HIPE. However, the resulting silica materials were deemed inappropriate for the desired chromatography. Monolithic columns were prepared from HIPE templated polymers (polyHIPEs) and incorporated into a HPLC system. Poly(styrene-co-divinylbenzene) and poly(ethylene glycol dimethacrylate) polyHIPE columns were able to separate sub-micron polystyrene latexes, detected by UV absorption, and dysprosium doped polystyrene latex particles and gold nanoparticles detected by inductively coupled plasma mass spectrometry (ICP-MS).Dysprosium, gadolinium and neodymium doped polystyrene NPs were prepared by micro-emulsion polymerisation. Particle size was controlled (over a 40 – 160 nm range) by tailoring of surfactant and initiator concentrations. Particles were characterised by dynamic light scattering, differential centrifugal sedimentation, transition electron microscopy and hydrodynamic chromatography (HDC)-ICP-MS. Also, particle surface change, lanthanide content and solids content were analysed. The latter two appear related to particle size. As far as the author is aware there are no cases of the use of polyHIPE columns size separation in the literature. Nor are there any cases of encapsulation of metals within polymer nanoparticles by micro-emulsion polymerisation reported.

No description available.

In this work, ordered all-binary short-period strained InAs/GaAs superlattice quantum wells were studied as an alternative to strained ternary alloy InGaAs/GaAs quantum wells. InGaAs quantum wells QWs have been of great interest in recent years due to the great potential applications of these materials in future generations of electronic and optoelectronic devices. The all binary structures are expected to have all the advantages of their ternary counterparts, plus several additional benefits related to growth, to the elimination of alloy disorder scattering and to the presence of a higher average indium content.

Recent experiments have shown that Ta@Si16+ is a very stable cation from which it should be possible to create Si-based cluster assembled materials. In this paper we have studied, by means of first-principles spin-dependent generalized gradient approximation calculations, the structural and electronic properties of the following systems: (i) Ta@Sin+ clusters in the range n = 14–18; (ii) (Ta@Si16F)m aggregates with sizes m = 1–8 formed by Ta@Si16F molecules; (iii) infinite wires formed by stacking triangular (Ta@Si16F)3 aggregates twisted 60◦ to each other along the vertical axis; and (iv) the fcc phase of bulk Ta@Si16F. The minimum-energy Ta@Si16+ cluster shows C3v symmetry, having 40 meV smaller total energy than a fullerenelike D4d isomer. However, the molecule Ta@Si16F formed with that D4d isomer is 40 meV more stable than that formed with the C3v one. We have optimized several [Ta@Si16F]n aggregates (n = 1–8) which contain the Ta@Si16 unit with D4d symmetry. The more bound (Ta@Si16F)6 aggregate is formed by stacking vertically two triangular (Ta@Si16F)3 aggregates which are twisted 60◦ to each other. The infinite wire formed with that (Ta@Si16F)6 aggregate as the unit cell has a cohesive energy 1.88 eV and a small highest occupied molecular orbital–lowest occupied molecular orbital gap. We have optimized also a metastable fcc bulk phase having the Ta@Si16F supermolecule as the unit cell. A Birch-Murnaghan fit to that phase produces a cohesive energy 0.84 eV at lattice constant 12.27 A, with bulk modulus 7.55 GPa and a phase stability to isotropic compression smaller than 0.75 GPa. That phase is nonmagnetic and shows a band gap of 0.20 eV. Using the values of hardness of Ta@Si16F molecules, we estimated a correction enhancement factor ∼3 to that small band gap. For that metastable solid we performed a 13.5-ps run of first-principles molecular dynamics annealing at 300 K and constant volume, and we found that the Ta@Si16F supermolecule in the fcc cell becomes severely distorted after the first 5 ps.

Developing chemical bonding models in clusters is one of the most challenging tasks of modern theoretical chemistry. There are two reasons for this. The first one is that clusters are relatively new objects in chemistry and have been extensively studied since the middle of the 20th century. The second reason is that clusters require high-level quantum-chemical calculations; while for many classical molecules their geometry and properties can be reasonably predicted by simpler methods. The aim of this dissertation was to study doped clusters and explain their chemical bonding. The research was focused on three classes of compounds: aluminum clusters doped with one nitrogen atom, planar compounds with hypercoordinate central atom, partially mixed carbon-boron clusters, and transition metal clusters. The geometry of the two latter classes of compounds was explained using the concept of aromaticity, previously developed in our group.

No description available.

This study reports on a variety of experimental and theoretical studies conducted in ZnSe, CdTe, and in semiconductor-doped glasses. The transient picosecond degenerate four-wave mixing (DFWM) experiments performed in these II-VI direct-gap semiconductors are part of our efforts to understand the picosecond dynamics of the free-carriers generated via two and three-photon absorption.

No description available.

<p> Ni K extended X-ray absorption fine structure spectroscopy recorded with synchrotron radiation at 77 K and 300 K, has been used to investigate various annealed Pd40Ni40P20 metallic glass samples. The structural parameters (radial distance, coordination number, Debye Waller factor) for the Fourier filtered first coordination shell were obtained via curve analysis using MFIT (multifitting analysis) and plotted as a function of annealing temperature. Similar trends between 77 K and 300 K data were not observed in all cases. These results lead one to believe that the curve fitting procedure is not yet optimized and makes one sceptical of attributing any meaning to the results.</p> <p> X-ray absorption near edge spectroscopy via electron yield detection has also been used to study the B K edge in boron doped silicon. It was found that a strong peak at 195 eV is directly related to trigonally coordinated boron. Non trigonally coordinated boron did not seem to produce this spike. Results show that B 1s spectroscopy is a very sensitive probe of the local structure of boron.</p> / Thesis / Master of Science (MSc)