A self-consistent, full-band, electrothermal ensemble Monte Carlo device simulation tool has been developed. It is used to study charge transport in bulk GaN, and to design, analyze, and improve the performance of AlGaN/GaN high electron mobility transistors (HEMTs) and avalanche photodiodes (APDs). Studies of electron transport in bulk GaN show that both peak electron velocity and saturated electron velocity are higher for transport in the basal plane than along the c-axis. Study of the transient electron velocity also shows a clear transit-time advantage for electron devices exploiting charge transport perpendicular to the c-axis. The Monte Carlo simulator also enables unique studies of transport under the influence of high free carrier densities but with low doping density, which is the mode of transport in AlGaN/GaN HEMTs. Studies of isothermal charge transport in AlGaN/GaN HEMTs operating at high gate bias show a drain current droop with increasing drain-source bias. The cause of the droop is investigated and a design utilizing source- or gate-connected field plate is demonstrated to eliminate the drain current droop. Electrothermal aspects of charge transport in AlGaN/GaN HEMTs are also investigated, and the influence of non-equilibrium acoustic and optical phonons is quantified. The calculated spatial distribution of non-equilibrium phonon population reveals a hot spot in the channel that is localized at low drain-source bias, but expands towards the drain at higher bias, significantly degrading channel mobility. Next, Geiger mode operation of wurtzite GaN-based homojunction APDs is investigated. The influences of dopant profile, active region thickness, and optical absorption profile on single photon detection efficiency (SPDE) are quantified. Simulations of linear mode gain as a function of multiplication region thickness and doping profile reveal that weakly n-type active regions may be exploited to achieve higher avalanche gain, without penalty to either applied bias or active region thickness. A separate absorption and multiplication APD (SAM-APD) utilizing a AlGaN/GaN heterojunction is also investigated. The presence of strong piezo-electric and spontaneous polarization charges at the heterojunction enables favorable electric field profile in the device to reduce dark current, improve excess noise factor, improve quantum efficiency, and improve breakdown probability. To maximize SPDE, a new device structure with a buried absorber is proposed and improved SPDE is demonstrated. Lastly, a new approach for the direct generation of self-sustaining millimeter-wave oscillations is proposed. In contrast to Gunn diodes, which exploit a bulk-like active region, periodic oscillation is achieved in the proposed structures through the creation, propagation and collection of traveling dipole domains supported by fixed polarization charge and the associated two-dimensional electron gas along the plane of a polar heterojunction. Numerical simulation of induced oscillations in a simple triode structure commonly used for AlGaN/GaN HEMTs reveals two distinct modes of self-sustaining millimeter-wave oscillation.

Polythiophenes have great potential as semiconductors for use in organic field effect transistors and light emitting diodes. Recent research has been focused on the design, synthesis and characterization of fluorinated polythiophenes and oligothiophenes. Various fluoroalkyl side chains have been introduced to induce polymer self-assembly, to control the electronic properties of the conjugated backbone, and to modify the solubility of the polymer in supercritical CO2. This work led to the preparation of poly(3-(perfluorooctyl)thiophene), which is one of only a few examples of n-dopable polythiophenes, and is the first supercritical CO2-soluble conducting polymer. An alternating copolymer consisting of 3-perfluoroakyl and 3-alkylthiophene units has been synthesized. This polymer, with alternating electron-donating and withdrawing substituents, has a high quantum yield for fluorescence in solution relative to the two homopolymers, and strong fluorescence in solid state. Based on the study on its nanocrystals, the unusual photophyiscs may be due to the formation of the supramolecular structure with hexagonal packing. A novel thiophene monomer, 3-(1,1-difluorooctyl)thiophene, was prepared to further tune the electronic structure of polythiophenes by changing the fluorination pattern of side chains, while retaining solubility in organic solvents by virtue of the hydrocarbon side chain. a-Hexyl-w-perfluorohexylsexithiophene was synthesized to make a novel amphiphilic material for use in TFTs. Models for interchain charge transfer in doped conducting polymers were also developed. Stacked and unstacked conjugated oligomers have been synthesized as models for conducting polymers. The bis(radical cation) form and the dication-neutral form of compounds in which conjugated oligomers are held in a stacked arrangement are shown to coexist and in equilibrium with each other. The coexistence of these two forms further suggests that both may serve as charge carriers. Interconversion between these forms by disproportionation mimics a possible mechanism for charge migration in doped conjugated polymers.

Structural analyses of DNA oligonucleotides indicate the presence of bound water molecules in the major and minor grooves of DNA. These water molecules participate in DNA charge transfer by their reaction with guanosine radical cation to form 7,8-dihydro-8-oxo-guanine (8-oxoG), which when treated with a base leads to DNA strand cleavage. We probed the reaction of guanosine radical cation with water with series of alkyl substituted cytidines and thymidines by incorporating the modified nucleosides into anthraquinone linked DNA duplexes and irradiating them with UV light at 350 nm. The incorporation of these hydrophobic substituents disrupt the DNA spine of hydration, and we have observed that these modifications in the major and minor groove do not effect the trapping or long distance hopping of radical cations in DNA. The second part of the work reported herein examines the role of charged nucleosides in long range charge transfer in duplex DNA. DNA methylation is a naturally occurring process mediated by enzymes responsible for such functions in biological systems. Hypermethylation of DNA can also occur as a result of environmental alkylating agents leading to mutation of the affected cells. Methylation of the ring nitrogen of a purine base can introduce a positive charge in the ring resulting in the cleavage of the glycosidic bond of the nucleoside. To understand the role of a charged nucleoside on charge transfer in DNA, we designed and synthesized cationic nucleoside mimics, which were incorporated into anthraquinone-linked DNA strands and irradiated at 350 nm. The presence of the cationic bases on the duplexes inhibits the migrating hole from hopping along the DNA strand, and induces a prominent local structural distortion of the DNA as a result of the charged nucleobase.

Two novel distyryl-boradiazaindacene dyes with dimethylaminophenyl and pyridyl appendages, display opposite spectral shifts on protonation with trifluoroacetic acid in organic solvents. This bidirectional switching of the dyes can be shown to be directly related to ICT donor and acceptor characteristics of the dyes. Thus, it has been demonstrated that the switching behavior of the red to NIR emitting dyes can be altered by simple structural modifications.

Mixed-valence compounds are of interest as model systems for the study of electron transfer reactions. The intramolecular electron transfer processes and patterns of charge delocalization in such compounds depend on the interplay between the electronic (V) and the vibronic (L) coupling. One can obtain both parameters from a Hush analysis of the intervalence band that arises upon optical intramolecular electron transfer if the band is intense and well-separated from other bands. This is quite often the case for mixed-valence triarylamines. As such, both Hush analysis and simulation of the intervalence band are widely used to classify these compounds as charge localized (class-II) or delocalized (class-III). Yet one must estimate the diabatic electron transfer distance (R) to calculate V in the Hush formalism. For mixed-valence triarylamines, R is commonly taken as the N-N distance; we show this to be a poor approximation in many cases. The activation barrier to thermal intramolecular electron transfer in a class-II mixed-valence compound is also related to the parameters V and L. Thus, if one can capture the rate of thermal electron transfer at multiple temperatures, then two experimental methods exist by which to extract the microscopic parameters. One technique that is widely used for organic mixed-valence compounds is variable-temperature electron spin resonance (ESR) spectroscopy. But this method is only rarely used to determine thermal electron transfer rates in mixed-valence triarylamines, as the electron transfer in most of the class-II compounds with distinct intervalence bands is too fast to observe on the ESR timescale. We show, for the first time, that one can use ESR spectroscopy to measure thermal electron transfer rates in such compounds. Simulation of ESR spectra based on density functional theory calculation and comparison with optical data also uncover the nature (i.e., adiabatic or nonadiabatic) of the electron transfer process.

Structure-property relationships for two-photon absorption (2PA) in branched organic chromophores is a topic of current interest, as is the design of chromophores with advantageous properties for two-photon laser scanning microscopy (2PLSM). The main goals of this dissertation were to study and explain the one-photon absorption (1PA) and 2PA properties of cruciform chromophores based on 1,4-distyryl-2,5-bis(phenylethynyl)benzene with varying electron donor (D) and acceptor (A) groups, and to characterize the 2PLSM-relevant response of some of these chromophores and a set of dipolar chromophores to binding with zinc ions. The compounds were studied by 1PA, fluorescence and 2PA spectroscopy. A ππ* exciton model was developed to explain the spectral properties of the 1,4-distyryl-2,5-bis(phenylethynyl)benzene cruciform with no D or A groups or with four identical D groups at the termini of the linear arms of the chromophore. This model indicated that there is some coupling and mixing of the lowest excited states e of the linear arms, leading to splitting of the 1PA spectrum of the cruciform. There was little coupling or mixing of the higher excited states e′ accessed in 2PA, leading to a two-band 2PA spectrum for the chromophore, in contrast to cruciform compounds in the literature with identical conjugated arms, which have one visible 2PA band. For cruciforms with D groups on the styryl arm and A character on the terminal phenyls of the phenylethynyl arms (D/A cruciforms), the ππ* exciton model was complemented with a charge-transfer (CT) exciton model describing interactions of charge-transfer pathways between the D and A groups. This model explained the broadness of the 1PA band of D/A cruciforms as well as the two 2PA bands observed for these chromophores. The fluorescence and 2PA spectral responses to binding of Zn²⁺ ions to the D or A groups of some cruciform compounds were also assessed, to provide insight into the design of new analyte-sensing cruciforms for 2PLSM that take advantage of enhancement or reduction of D/A character upon analyte binding. It was found that canceling charge donation from the D groups in differing D/A cruciforms resulted in fluorescence and 2PA spectra nearly indistinguishable from each other, suggesting that turn-off of D groups is not an optimal modality of 2PLSM analyte sensing in cruciforms. Binding Zn²⁺ to A groups was shown to result in an increase in the D/A character of the cruciform, with fluorescence peak energies that changed depending on the location of the A group. It is suggested that the use of non-binding donors and analyte-binding A groups in differing patterns on the arms could be a valuable design motif to achieve 2PLSM sensor compounds based on this cruciform structure. The 2PA spectra of a set of dipolar Zn²⁺ sensing dyes designed for ratiometric imaging in 2PLSM were also studied. These dyes had moderate 2PA strength, with redshifts of fluorescence 2PA spectra on Zn²⁺ binding. The isosbestic point of 2PA of most chromophores was within the range of 2PLSM excitation sources commonly used, rendering these dyes good candidates for use in ratiometric sensing in 2PLSM.

Thesis (M. Phil.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 80-85).

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph.D.)--Ohio University, November, 2009. / Release of full electronic text on OhioLINK has been delayed until December 1, 2014. Title from PDF t.p. Includes bibliographical references.