Porous silicon / noble metal nanocomposites for catalytic applications

Today, bulk silicon is one of the best studied semiconductors. However, in its different nano-modifications, e.g. as porous silicon, totally new properties are exhibited. Despite the fact, that porous silicon is widely known and has been extensively studied since the 1990s, many unique features of this material are still unexplored. In this work, specific functionalities of porous silicon prepared, utilising both solid (via electrochemical or stain etching processes) and gas phase (from silane) syntheses, were investigated. Since this study was in-part industry oriented, the emphasis has been placed upon the investigation of porous silicon nanostructures, made from low cost metallurgical grade polycrystalline silicon powder. It has been previously demonstrated that porous silicon exhibits a very large, hydrogenated internal surface area (up to 500 m2 g−1). It is verified in this work, that morphological properties of this material result in a high reductive potential of its internal surface due to hydrogen passivation. Therefore, in this thesis, we would like to show that porous silicon-based reactive templates are promising for their applications in nanometal-supported catalysis. We used salts of platinum, gold, palladium, silver and their mixtures, which were reduced on the silicon nanocrystalline internal surface, resulting in formation of metal nanoparticles embedded into porous silicon matrix. Various experimental techniques were used to evaluate the morphology, size and composition of metal nanoparticles, as well as their growth rates. Hydrogen effusion experiments proved the crucial difference between porous silicon and other chemically inert supporting templates for the process of metal nanoparticles formation. The catalytic activity of the synthesised materials was evaluated in gas phase conversion of CO to CO2. Furthermore, the new porous silicon-based catalysts were tested in gas/liquid phase reactions as well, using hydrogenation, oxidation, dehalogenation and C-C coupling class reactions. Following the trends of “state of the art” current Si technology, we present the design of the developed flow microreactor, based on patterned Si wafer, which can be implemented in future work to catalyse selected reactions. Results obtained in this work suggest that porous silicon matrices are promising supports for metal nanoparticle based catalysis.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Periodic orbit resonances in layered metals in tilted magnetic fields

The frequency dependence of the interlayer conductivity of a layered Fermi liquid in a magnetic field which is tilted away from the normal to the layers is considered. For both quasi-one- and quasi-two-dimensional systems resonances occur when the frequency is a harmonic of the frequency at which the magnetic field causes the electrons to oscillate on the Fermi surface within the layers. The intensity of the different harmonic resonances varies significantly with the direction of the field. The resonances occur for both coherent and weakly incoherent interlayer transport and so their observation does not imply the existence of a three-dimensional Fermi surface.Comment: 4 pages, RevTeX + epsf, 2 figures. Discussion of other work revised. To appear in Phys. Rev. B, Rapid Commun., October 1

Soft X-ray induced oxidation on acrylic acid grafted luminescent silicon quantum dots in ultrahigh vacuum

Water soluble acrylic acid grafted luminescent silicon quantum dots (Si-QDs) were prepared by a simplified method. The resulting Si-QDs dissolved in water and showed stable strong luminescence with peaks at 436 and 604?nm. X-ray photoelectron spectroscopy (XPS) was employed to examine the surface electronic states after the synthesis. The co-existence of the Si2p and C1s core levels infers that the acrylic acid has been successfully grafted on the surface of silicon quantum dots. To fit the Si2p spectrum, four components were needed at 99.45, 100.28, 102.21 and 103.24?eV. The first component at 99.45?eV (I) was assigned to Si–Si within the silicon core of the Si-QDs. The second component at 100.28?eV (II) was from Si–C. The third at 102.21?eV (III) was a sub-oxide state and the fourth at 103.24?eV (IV) was from SiO2 at Si-QDs surface. With an increase in exposure to soft X-ray photons, the intensity ratio of the two peaks within the Si2p region A and B increased from 0.5 to 1.4 while the peak A intensity decreased, and eventually a steady state was reached. This observation is explained in terms of photon-induced oxidation taking place within the surface dangling bonds. As the PL profile for Si-QDs is influenced by the degree of oxidation within the nanocrystal structure, the inducement of oxidation by soft X-rays will play a role in the range of potential applications where such materials could be used – especially within biomedical labelling

Polarization properties of the luminescence from silicon nanocrystals

Polarization dependent photoluminescence (PL), time-resolved PL and PL excitation experiments are performed in order to clarify the origin of the linear polarization of the PL of porous silicon excited by linear polarized light. It is shown that this effect, when PL is excited significantly above the detection energy, is not related to a coherent exciton alignment or selective optical excitation of those nanocrystals whose transition dipole moments are oriented parallel to the polarization vector of the exciting light. The experimental results are interpreted in the framework of a dielectric model assuming aspheric nanocrystals

Polarization properties of the luminescence from silicon nanocrystals

Polarization dependent photoluminescence (PL), time-resolved PL and PL excitation experiments are performed in order to clarify the origin of the linear polarization of the PL of porous silicon excited by linear polarized light. It is shown that this effect, when PL is excited significantly above the detection energy, is not related to a coherent exciton alignment or selective optical excitation of those nanocrystals whose transition dipole moments are oriented parallel to the polarization vector of the exciting light. The experimental results are interpreted in the framework of a dielectric model assuming aspheric nanocrystals

Cyclotron resonance in ultra-low-hole-density narrow p-type GaAs/(Al,Ga)As quantum wells

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