Thesis (Ph.D.)--University of Nebraska-Lincoln, 2008. / Title from title screen (site viewed Mar. 10, 2009). PDF text: xv, 182 p. : ill. (some col.) ; 6 Mb. UMI publication number: AAT 3331444. Includes bibliographical references. Also available in microfilm and microfiche formats.

Surface plasmons have been attracted extensive interests in recent decades due to the novel properties in nanometer sized dimensions. My work focused on the novel optical properties of metal nanostructures based on surface plasmons using theoretical simulation methods. In the first part, we investigated metal nanofilms and nanorods and demonstrated that extremely low scattering efficiency, high absorption efficiency and propagation with long distance could be obtained by different metal nanostructures. With a perforated silver film, we demonstrated that an extremely low scattering cross section with an efficiency of less than 1% can be achieved at tunable wavelengths with tunable widths. The resonance wavelength, width, and intensity are influenced by the shape, size and arrangement pattern of the holes, as well as the distance separating the holes along the polarization direction. The extremely low scattering could be used to obtain high absorption efficiency of a two-layer silver nanofilm. Using the discrete dipole approximation method, we achieved enhanced absorption efficiencies, which are close to 100%, at tunable wavelengths in a two-layer silver thin film. The film is composed of a 100 nm thick perforated layer facing the incident light and a 100 nm thick solid layer. Resonance wavelengths are determined by the distances between perforated holes in the first layer as well as the separation between two layers. The resonance wavelengths shift to red with increasing separation distance between two layers or the periodic distance of the hole arrays. Geometries of conical frustum shaped holes in the first layer are critical for the improved absorption efficiencies. When the hole bottom diameter equals the periodic distance and the upper diameter iv is about one-third of the bottom diameter, close to unit absorption efficiency can be obtained. We examined the electromagnetic wave propagation along a hollow silver nanorod with subwavelength dimensions. The calculations show that light may propagate along the hollow nanorod with growing intensities. The influences of the shape, dimension, and length of the rod on the resonance wavelength and the enhanced local electric field, |E|2 , along the rod were investigated. In the second part, a generalized electrodynamics model is proposed to describe the enhancement and quenching of fluorescence signal of a dye molecule placed near a metal nanoparticle (NP). Both the size of the Au NPs and quantum yield of the dye molecule are crucial in determining the emission intensity of the molecule. Changing the size of the metal NP will alter the ratio of the scattering and absorption efficiencies of the metal NP and consequently result in different enhancement or quenching effect to the dye molecule. A dye molecule with a reduced quantum yield indicates that the non-radiative channel is dominant in the decay of the excited dye molecules and the amplification of the radiative decay rate will be easier. In general, the emission intensity will be quenched when the size of metal NP is small and the quantum yield of dye molecule is about unity. A significant enhancement factor will be obtained when the quantum yield of the molecule is small and the particle size is large. When the quantum yield of the dye molecule is less than 10-5 , the model is simplified to the surface enhanced Raman scattering equation

Carbon nanostructure based functional hybrid molecules hold promise in solarenergy harvesting. Research presented in this dissertation systematically investigates building of various donor-acceptor nanohybrid systems utilizing enriched single walled carbon nanotube and graphene with redox and photoactive molecules such as fullerene, porphyrin, and phthalocyanine. Design, synthesis, and characterization of the donor-acceptor hybrid systems have been carefully performed via supramolecular binding strategies. Various spectroscopic studies have provided ample information in terms of establishment of the formation of donor-acceptor hybrids and their extent of interaction in solution and eventual rate of photoinduced electron and/or energy transfer. Electrochemical studies enabled construction of energy level diagram revealing energetic details of the possible different photochemical events supported by computational studies carried out to establish the HOMO-LUMO levels in the donor acceptor systems. Transient absorption studies confirmed formation of charge separated species in the donor-acceptor systems which have been supported by electron mediation experiments. Based on the photoelectrochemical studies, IPCE of 8% was reported for enriched SWCNT(7,6)-ZnP donor-acceptor systems. In summary, the present investigation on the various nanocarbon sensitized donor-acceptor hybrids substantiates tremendous prospect, that could very well become the next generation of materials in building efficient solar energy harvesting devices andphotocatalyst.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

No description available.

Plasmonic structures with nanometer scale gaps provide localized field enhancement and allow for engineering of the optical response, which is well described by conventional classical models. For subnanometer scale gaps, quantum effects and nonlocal effects become important and classical electromagnetics fail to describe the plasmonic coupling response. Coupled plasmonic system of gold nanoparticles on top of thin gold film separated with self-assembled monolayers (SAMs) provides a convenient geometry to experimentally explore plasmonic features in subnanometer scale gaps. However, the surface roughness of the thin metal film can significantly influence the plasmonic coupling properties. In this dissertation, I suggest modifying the coupled nanoparticles-film structures by using ultraflat thin metal films. Using these structures, I investigated the far-field optical response for gap size variations by dark field scattering measurements. A red-shift of the plasmon resonance wavelength was observed by reducing the gap width. However, I did not observe the previously reported saturation trend of the resonance shift for subnanometer scale gaps. I attribute the difference to surface roughness effects in past works since as they were not present in my studies with ultraflat films. To study the near-field enhancement in subnanometer scale gaps, I used third harmonic generation as a method that is highly sensitive (as the third power) to the local field intensity. The onset of the quantum tunneling regime was determined for gap thicknesses of 0.51 nm, where there was a sudden drop in the third harmonic when the gap width decreases from 0.69 nm to 0.51 nm. The experimental observations were consistent with analytical calculations that applied the quantum-corrected model for SAM separating two gold regions. In comparison to the gap without SAMs in which the onset of the tunneling regime was reported at 0.31 nm, the onset of tunneling across the gap with SAM occurred for larger gaps. This was an expected outcome because the material in the gap reduced the barrier height to tunneling. Furthermore, I investigated the wavelength dependence of the third harmonic generation for the gold plasmonic system to determine the role of the interband transitions in the nonlinear response of gold. Past works reported a strong wavelength dependence of the nonlinear response of gold for the fundamental wavelength at about 550 nm, attributed to the interband transitions between the 5d to 6s-6p bands. However, the roles of the interband transitions and wavelength-dependent field enhancement in the nonlinear response of gold was not investigated. In this dissertation, results showed the third harmonic generation enhanced by an order of magnitude by the interband transition (as compared to the non-resonant case). In my research I also used an analytic model for the dielectric function of gold in which contributions of the interband transitions were considered. This model was also consistent with the experimental observations. / Graduate / 0752 / 0544 / Ghazal.hajisalem@gmail.com

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Ce travail s'inscrit dans le cadre des études du transfert de spin mesuré sur un domaine ferromagnétique unique. Le monodomaine ferromagnétique est un nanofil unique de Ni ou de Co électrodéposé dans une matrice nanoporeuse. Cette étude cherche à mettre en évidence les propriétés de relaxation des spins des électrons de conduction à l'interface entre le domaine ferromagnétique et le contact normal, en termes d'interaction entre les spins des électrons de conduction et le paramètre d'ordre ferromagnétique. Il s'agit en dernière analyse de mettre en évidence un effet de transfert de spin de type diffusif qui serait une conséquence de cette relaxation. Dans une première approche, une accumulation de spin anisotrope, mettant en jeu l'interaction $s-d$ (ou interaction spin-orbite) est étudiée par le biais du pouvoir thermoélectrique TEP. Un effet TEP dépendant du spin est en effet mis en évidence, qui est restreint à l'interface et qui est anisotrope. Cette contribution est interprétée comme étant l'effet de l'accumulation de spin produit par une relaxation $s-d$. Dans un deuxième temps, l'effet de transfert de spin est étudié par le biais de la susceptibilité thermique de la magnétorésistance d'anisotropie $dR/dT$. On montre que la susceptibilité est due à la susceptibilité de l'aimantation $dM/dT$. Les fluctuations de l'aimantation générées par l'injection de courant fort sont mesurées en termes de variation d'entropie. Une variation de $60\%$ est mesurée pour un courant de l'ordre de $10^{7} A/cm^{2}$. Ces résultats confortent une approche diffusive du transfert de spin.

Silicon as a mono-crystalline bulk semiconductor is today the predominant material in many integrated electronic and photovoltaic applications. This has not been the case in lighting technology, since due to its indirect bandgap nature bulk silicon is an inherently poor light emitter.With the discovery of efficient light emission from silicon nanostructures, great new interest arose and research in this area increased dramatically.However, despite more than two decades of research on silicon nanocrystals and nanowires, not all aspects of their light emission mechanisms and optical properties are well understood, yet.There is great potential for a range of applications, such as light conversion (phosphor substitute), emission (LEDs) and harvesting (solar cells), but for efficient implementation the underlying mechanisms have to be unveiled and understood.Investigation of single quantum emitters enable proper understanding and modeling of the nature and correlation of different optical, electrical and geometric properties.In large numbers, such sets of experiments ensure statistical significance. These two objectives can best be met when a large number of luminescing nanostructures are placed in a pattern that can easily be navigated with different measurement methods.This thesis presents a method for the (optional) simultaneous fabrication of luminescent zero- and one-dimensional silicon nanostructuresand deals with their structural and optical characterization.Nanometer-sized silicon walls are defined by electron beam lithography and plasma etching. Subsequent oxidation in the self-limiting regime reduces the size of the silicon core unevenly and passivates it with a thermal oxide layer.Depending on the oxidation time, nanowires, quantum dots or a mixture of both types of structures can be created.While electron microscopy yields structural information, different photoluminescence measurements, such as time-integrated and time-resolved imaging, spectral imaging, lifetime measurements and absorption and emission polarization measurements, are used to gain knowledge about optical properties and light emission mechanisms in single silicon nanocrystals.The fabrication method used in this thesis yields a large number of spatially separated luminescing quantum dots randomly distributed along a line, or a slightly smaller number that can be placed at well-defined coordinates. Single dot measurements can be performed even with an optical microscope and the pattern, in which the nanostructures are arranged, enables the experimenter to easily find the same individual dot in different measurements.Spectral measurements on the single dot level reveal information about processes that are involved in the photoluminescence of silicon nanoparticles and yield proof for the atomic-like quantized nature of energy levels in the conduction and valence band, as evidenced by narrow luminescence lines (~500 µeV) at low temperature. Analysis of the blinking sheds light on the charging mechanisms of oxide-capped Si-QDs and, by exposing exponential on- and off-time distributions instead of the frequently observed power law distributions, argues in favor of the absence of statistical aging. Experiments probing the emission intensity as a function of excitation power suggest that saturation is not achieved. Both absorption and emission of silicon nanocrystals contained in a one-dimensional silicon dioxide matrix are polarized to a high degree. Many of the results obtained in this work seem to strengthen the arguments that oxide-capped silicon quantum dots have universal properties, independently of the fabrication method, and that the greatest differences between individual nanocrystals are indeed caused by individual factors like local environment, shape and size (among others). / <p>QC 20120920</p>