Thesis (Ph. D.)--University of Oregon, 2000. / Includes reprints of articles previously published by the author. Typescript. Includes vita and abstract. Includes bibliographical references (leaves 184-190). Also available for download via the World Wide Web; free to University of Oregon users.

In this thesis the interaction of ultrashort laser pulses with metal nanostructures is investigated via two different phenomena: coherent acoustic oscillations of nanoparticles and generation of THz pulses on metal surfaces. Both of these effects rely on the collective oscillations of free conduction electrons in metal surfaces, plasmons. The field of plasmonics gained a great interest in the last twenty years due to the unique properties of these surface modes. It is the effects of the resonant response of plasmonic structures to incident electromagnetic wave, in particular, in visible and infrared bands and the concentration of the electromagnetic field in small subwavelength regions with significant enhancement of the incident field that make plasmonics so attractive for various applications, such as biochemical sensing, enhanced fluorescence, surface-enhanced Raman scattering, and second harmonic generation, amongst others. Investigation of the coherent particle vibrations is performed using the pump-probe technique which allows measurement of the transient transmission signals. The expansion and subsequent contraction of the nanoparticle following the ultrashort laser pulse excitation lead to a shift of the plasmon band which can be traced by transient spectroscopy. We have investigated the effect of the particle thickness on the frequency of the fundamental vibrational mode. In addition, we measured the vibrational particle response during the particle shape deformation, both symmetrical and asymmetrical. Exploration of the THz generation phenomena on plasmonic structures was performed using THz time-domain spectroscopy, the method which allows tracing of the generated THz field in the time-domain. We were able for the first time to measure the THz pulses generated from arrays of metal nanoparticles. Our observations verify the role of the particle plasmon mode in the generation of THz pulses. In addition, by exploring the dependence of the THz emission on the femtosecond pulse intensity we showed a high nonlinearity in the THz generation mechanism. The experimental results were assessed in the context of a recently proposed model where the THz radiation is generated via the acceleration of the ejected electrons by ponderomotive forces. To reveal another proposed mechanism of the THz generation from plasmonic structures, namely optical rectification, we investigated the THz generation and electron emission from the arrays of nanoparticles and nanoholes. Our results suggest that both mechanisms may contribute to generation of THz pulses from the same sample under different illumination conditions. In addition to periodic arrays of nanoparticles and nanoholes, THz generation from random metal-dielectric films was investigated. The microstructuring of such films allowed selective THz frequency generation which was explained by a model of dipole THz emitters. In addition, the effects of low temperature and pressure on the THz generation efficiency were investigated.

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The power of parallel supercomputing resources has progressed to the point where first-principles calculations involving systems up to 10³ atoms are feasible, allowing ab initio exploration of increasingly complex systems such as amorphous networks, nanostructures, and large defect clusters. Expansion of our fundamental understanding of modified Si-based materials is paramount, as these materials will likely flourish in the foreseeable cost-driven future in diverse micro- and nanotechnologies. Here, density-functional theory calculations within the generalized gradient approximation are applied to refine configurations of Si-based materials generated from Metropolis Monte Carlo simulations and study their resultant structural properties. Particular emphasis is given to the contributions of strain and disorder on the mechanical, optical, and electronic properties of modified Si-based materials in which aspects of compositional variation, phase, strain scheme, morphology, native defect incorporation, and quantum confinement are considered. The simulation strategies discussed are easily extendable to other semiconductor systems. / text

Nanostructures often possess unique properties, which may lead to the development of new microelectronic and optoelectronic devices. They also provide an opportunity to test fundamental quantum mechanical concepts such as the role of quantum confinement. Considerable effort has been made to understand the electronic and structural properties of nanostructures, but many fundamental issues remain. In this work, the electronic and structural properties of nanostructures are examined using several new computational methods. The effect of dimensional confinement on quantum levels is investigated for hydrogenated Ge <110> using the plane-wave density-functional-theory pseudopotential method. We present a real-space pseudopotential method for calculating the electronic structure of one-dimensional periodic systems such as nanowires. As an application of this method, we examine H-passivated Si nanowires. The band structure and heat of formation of the Si nanowires are presented and compared to plane wave methods. Our method is able to offer the same accuracy as the traditional plane wave methods, but offers a number of computational advantages such as the ability to handle large systems and a better ease of implementation for highly parallel platforms. Doping is important to many potential applications of nano-regime semiconductors. A series of first-principles studies are conducted on the P-doped Si <110> nanowires by the real-space pseudopotential methods. Nanowires of varied sizes and different doping positions are investigated. We calculate the binding energies of P atoms, band gaps of the wires, energetics of P atoms in different doping positions and core-level shift of P atoms. Defect wave functions of P atoms are also analyzed. In addition, we study the electronic properties of phosphorus-doped silicon <111> nanofilms using the real-space pseudopotential method. Nanofilms with varied sizes and different doping positions are investigated. We calculate the binding energies of P atoms, band gaps of the films, and energetics of P atoms in different doping positions. Quantum confinement effects are compared with P-doped Si nanocrystals and as well as nanowires. We simulate the nanofilm STM images with P defects in varied film depths, and make a comparison with the experimental measurement. / text

published_or_final_version / abstract / Physics / Master / Master of Philosophy

Colloidal semiconductor nanoparticles (NPs) contain hundreds to thousands of atoms in a roughly spherical shape with diameters in the range of 1-10 nm. The extremely small particle size confines electron transitions and creates size tunable bandgaps, giving rise to the name quantum dots (QDs). The unique optoelectronic properties of QDs enable a broad range of applications in optical and biological sensors, solar cells, and light emitting diodes. The most common compound semiconductor combination is chalcogenide II-VI materials, such as ZnSe, CdSe, and CdTe. But III-V and group IV as well as more complicated ternary materials have been demonstrated. Coordinating organic ligands are used to cap the NP surface during the synthesis, as a mean of protecting, confining, and separating individual particles. This study investigated the impact of the ligand on particle growth and self-assembly into hierarchical structures. ZnSe QDs were synthesized using an aqueous route with four different thiol ligands, including 3-mercaptopropionic acid (MPA), thioglycolic acid (TGA), methyl thioglycolate (MTG), and thiolactic acid (TLA). The particle growth was monitored as a function of reaction time by converting the band gaps measured using UV-vis spectroscopy into particle sizes. A kinetic model based on a diffusion-reaction mechanism was developed to simulate the growth process. The growth data were fit to this model, yielding the binding strength in the order TLA < MTG ≈ TGA < MPA. This result showed the relationship between the QD growth rates and the chemical structures of the ligands. Ligands containing electron-withdrawing groups closer to the anchoring S atom and branching promoted growth, whereas longer, possibly bidendate, ligands retarded it. Removing TGA ligands from the surface of CdTe QDs in a controlled manner yielded new superstructures that were composed of either intact or fused particles. Purifying as-synthesized QDs by precipitating them using an anti-solvent removed most of the free ligand in solution. Aging this purified QD suspension for a week caused self-assembly of QDs into nanoribbons. The long time needed for self-assembly was due to the slow equilibrium between the ligands on QD surface and in solution. Accelerating the approach to equilibrium by diluting purifed CdTe QDs with organic solvents triggered rapid self-assembly of superstructures within a day, forming various nanostructures from nanoribbons to nanoflowers. The type of nanostructures that formed was determined by the solvation of TGA in the trigger solvent. Extracting the smallest portion of TGA in methanol promoted vectorial growth into ribbons consistent with dipole-dipole attractive and charge-charge repulsive interactions. Removing more of the TGA layer in IPA caused the dots to fuse into webs containing clustered ribbons and branches, and the directional nature of the superstructure was lost. Completely deprotecting the surface in acetone promoted photochemical etching and dissolved the QDs, yielding ower-like structures composed of CdS. Nanocrystal (NC) growth mediated by a ligand was also studied in the organic synthesis of FeS₂ nanocubes. Oleylamine was used not only as the ligand but also the solvent and reductant during the reaction. A one hour reaction between iron (II) chloride and elemental sulfur in oleylamine at 200 ℃ and a S to Fe ratio of 6 yielded phase pure pyrite cubes with dimensions of 87.9±14.1 nm. X-ray diffraction (XRD) spectra and Raman peaks for pyrite at 340, 375, and 426 cm⁻¹ confirmed phase purity. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) results showed that the oleylamine remained on the FeS₂ surface as a ligand. The reaction mechanism includes the production of pyrrhotite Fe₁₋ᵪS (0≤x<0.5) via reduction of S⁰ to S²⁻ by oleylamine and the oxidation of pyrrhotite to pyrite with remaining S⁰.

In this thesis, I present an excitonic approach to treating the coherent dynamics of optically generated charge carriers in semiconductor nanostructures. The main feature of this approach is that it includes exchange interactions and phase space filling effects, which have generally been omitted in previous excitonic treatments of coherent dynamics, so that it can go beyond the low excitation limit. In contrast to the well-known semiconductor Bloch equations, this approach treats intraband correlations without factorization. The excitonic formalism and the obtained excitonic equations are shown to be particularly advantageous in systems where bound excitons dominate the optical response and where intraband correlations play a central role. To demonstrate the application of the excitonic approach, we simulate the coherent carrier dynamics of an optically-excited, updoped AlGaAs superlattice in the presence of a terahertz pulse, where 1s excitonic states as well as higher in-plane excited states are included. We find that gain coefficients greater than 20/cm can be achieved over a tuning range of 3-11THz and that due to the coherent cascading of the carriers down the excitonic Wannier-Stark ladder, the gain coefficients have much higher gain saturation fields than comparable two-level systems. To investigate the effects of phase space filling and exchange interaction on exciton dynamics, we then apply the excitonic formalism to a simple model of a quantum ring as well as a realistic model of a quantum well. For the quantum ring, we have obtained numerical results regarding exciton population and interband polarization. We also compared our excitonic approach to the semiconductor Bloch equations in detail using this simple model. For the quantum well, in addition to the investigation of exciton dynamics, we propose and examine several approximations that can make our excitonic dynamic equations very efficient. The excitonic formalism presented in this thesis is an efficient approach that can be applied in a wide range of systems, which makes it a potential alternative to the standard miconductor Bloch equations for many systems where the intraband correlations are crucial. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2008-12-01 18:21:17.181