You limited your search to:

  Partner: UNT Libraries
 Department: Department of Physics
Oligonucleotide guanosine conjugated to gallium nitride nano-structures for photonics.

Oligonucleotide guanosine conjugated to gallium nitride nano-structures for photonics.

Date: August 2008
Creator: Li, Jianyou
Description: In this work, I studied the hybrid system based on self-assembled guanosine crystal (SAGC) conjugated to wide-bandgap semiconductor gallium nitride (GaN). Guanosine is one of the four bases of DNA and has the lowest oxidation energy, which favors carrier transport. It also has large dipole moment. Guanosine molecules self-assemble to ribbon-like structure in confined space. GaN surface can have positive or negative polarity depending on whether the surface is Ga- or N-terminated. I studied SAGC in confined space between two electrodes. The current-voltage characteristics can be explained very well with the theory of metal-semiconductor-metal (MSM) structure. I-V curves also show strong rectification effect, which can be explained by the intrinsic polarization along the axis of ribbon-like structure of SAGC. GaN substrate property influences the properties of SAGC. So SAGC has semiconductor properties within the confined space up to 458nm. When the gap distance gets up to 484nm, the structure with guanosine shows resistance characteristics. The photocurrent measurements show that the bandgap of SAGC is about 3.3-3.4eV and affected by substrate properties. The MSM structure based on SAGC can be used as photodetector in UV region. Then I show that the periodic structure based on GaN and SAGC can have photonic ...
Contributing Partner: UNT Libraries
Emergence of Complexity from Synchronization and Cooperation

Emergence of Complexity from Synchronization and Cooperation

Date: May 2008
Creator: Geneston, Elvis L.
Description: The dynamical origin of complexity is an object of intense debate and, up to moment of writing this manuscript, no unified approach exists as to how it should be properly addressed. This research work adopts the perspective of complexity as characterized by the emergence of non-Poisson renewal processes. In particular I introduce two new complex system models, namely the two-state stochastic clocks and the integrate-and-fire stochastic neurons, and investigate its coupled dynamics in different network topologies. Based on the foundations of renewal theory, I show how complexity, as manifested by the occurrence of non-exponential distribution of events, emerges from the interaction of the units of the system. Conclusion is made on the work's applicability to explaining the dynamics of blinking nanocrystals, neuron interaction in the human brain, and synchronization processes in complex networks.
Contributing Partner: UNT Libraries
Microscopic Foundations of Thermodynamics and Generalized Statistical Ensembles

Microscopic Foundations of Thermodynamics and Generalized Statistical Ensembles

Date: May 2008
Creator: Campisi, Michele
Description: This dissertation aims at addressing two important theoretical questions which are still debated in the statistical mechanical community. The first question has to do with the outstanding problem of how to reconcile time-reversal asymmetric macroscopic laws with the time-reversal symmetric laws of microscopic dynamics. This problem is addressed by developing a novel mechanical approach inspired by the work of Helmholtz on monocyclic systems and the Heat Theorem, i.e., the Helmholtz Theorem. By following a line of investigation initiated by Boltzmann, a Generalized Helmholtz Theorem is stated and proved. This theorem provides us with a good microscopic analogue of thermodynamic entropy. This is the volume entropy, namely the logarithm of the volume of phase space enclosed by the constant energy hyper-surface. By using quantum mechanics only, it is shown that such entropy can only increase. This can be seen as a novel rigorous proof of the Second Law of Thermodynamics that sheds new light onto the arrow of time problem. The volume entropy behaves in a thermodynamic-like way independent of the number of degrees of freedom of the system, indicating that a whole thermodynamic-like world exists at the microscopic level. It is also shown that breaking of ergodicity leads to microcanonical ...
Contributing Partner: UNT Libraries
Multifunctional Organic-Inorganic Hybrid Nanophotonic Devices

Multifunctional Organic-Inorganic Hybrid Nanophotonic Devices

Date: May 2008
Creator: Garner, Brett William
Description: The emergence of optical applications, such as lasers, fiber optics, and semiconductor based sources and detectors, has created a drive for smaller and more specialized devices. Nanophotonics is an emerging field of study that encompasses the disciplines of physics, engineering, chemistry, biology, applied sciences and biomedical technology. In particular, nanophotonics explores optical processes on a nanoscale. This dissertation presents nanophotonic applications that incorporate various forms of the organic polymer N-isopropylacrylamide (NIPA) with inorganic semiconductors. This includes the material characterization of NIPA, with such techniques as ellipsometry and dynamic light scattering. Two devices were constructed incorporating the NIPA hydrogel with semiconductors. The first device comprises a PNIPAM-CdTe hybrid material. The PNIPAM is a means for the control of distances between CdTe quantum dots encapsulated within the hydrogel. Controlling the distance between the quantum dots allows for the control of resonant energy transfer between neighboring quantum dots. Whereby, providing a means for controlling the temperature dependent red-shifts in photoluminescent peaks and FWHM. Further, enhancement of photoluminescent due to increased scattering in the medium is shown as a function of temperature. The second device incorporates NIPA into a 2D photonic crystal patterned on GaAs. The refractive index change of the NIPA hydrogel as ...
Contributing Partner: UNT Libraries
Perturbation of renewal processes

Perturbation of renewal processes

Date: May 2008
Creator: Akin, Osman Caglar
Description: Renewal theory began development in the early 1940s, as the need for it in the industrial engineering sub-discipline operations research had risen. In time, the theory found applications in many stochastic processes. In this thesis I investigated the effect of seasonal effects on Poisson and non-Poisson renewal processes in the form of perturbations. It was determined that the statistical analysis methods developed at UNT Center for Nonlinear Science can be used to detect the effects of seasonality on the data obtained from Poisson/non-Poisson renewal systems. It is proved that a perturbed Poisson process can serve as a paradigmatic model for a case where seasonality is correlated to the noise and that diffusion entropy method can be utilized in revealing this relation. A renewal model making a connection with the stochastic resonance phenomena is used to analyze a previous neurological experiment, and it was shown that under the effect of a nonlinear perturbation, a non-Poisson system statistics may make a transition and end up in the of Poisson basin of statistics. I determine that nonlinear perturbation of the power index for a complex system will lead to a change in the complexity characteristics of the system, i.e., the system will reach ...
Contributing Partner: UNT Libraries
Anderson Localization in Two-Channel Wires with Correlated Disorder: DNA as an Application

Anderson Localization in Two-Channel Wires with Correlated Disorder: DNA as an Application

Date: December 2007
Creator: Bagci, V. M. Kemal
Description: This research studied the Anderson localization of electrons in two-channel wires with correlated disorder and in DNA molecules. It involved an analytical calculation part where the formula for the inverse localization length for electron states in a two-channel wire is derived. It also involved a computational part where the localization length is calculated for some DNA molecules. Electron localization in two-channel wires with correlated disorder was studied using a single-electron tight-binding model. Calculations were within second-order Born-approximation to second-order in disorder parameters. An analytical expression for localization length as a functional of correlations in potentials was found. Anderson localization in DNA molecules were studied in single-channel wire and two-channel models for electron transport in DNA. In both of the models, some DNA sequences exhibited delocalized electron states in their energy spectrum. Studies with two-channel wire model for DNA yielded important link between electron localization properties and genetic information.
Contributing Partner: UNT Libraries
Computational studies of selected ruthenium catalysis reactions.

Computational studies of selected ruthenium catalysis reactions.

Date: December 2007
Creator: Barakat, Khaldoon A.
Description: Computational techniques were employed to investigate pathways that would improve the properties and characteristics of transition metal (i.e., ruthenium) catalysts, and to explore their mechanisms. The studied catalytic pathways are particularly relevant to catalytic hydroarylation of olefins. These processes involved the +2 to +3 oxidation of ruthenium and its effect on ruthenium-carbon bond strengths, carbon-hydrogen bond activation by 1,2-addition/reductive elimination pathways appropriate to catalytic hydrogen/deuterium exchange, and the possible intermediacy of highly coordinatively unsaturated (e.g., 14-electron) ruthenium complexes in catalysis. The calculations indicate a significant decrease in the Ru-CH3 homolytic bond dissociation enthalpy for the oxidation of TpRu(CO)(NCMe)(Me) to its RuIII cation through both reactant destabilization and product stabilization. This oxidation can thus lead to the olefin polymerization observed by Gunnoe and coworkers, since weak RuIII-C bonds would afford quick access to alkyl radical species. Calculations support the experimental proposal of a mechanism for catalytic hydrogen/deuterium exchange by a RuII-OH catalyst. Furthermore, calculational investigations reveal a probable pathway for the activation of C-H bonds that involves phosphine loss, 1,2-addition to the Ru-OH bond and then reversal of these steps with deuterium to incorporate it into the substrate. The presented results offer the indication for the net addition of aromatic C-H ...
Contributing Partner: UNT Libraries
Neutron Transmutation and Hydrogenation Study of Hg₁₋xCdxTe

Neutron Transmutation and Hydrogenation Study of Hg₁₋xCdxTe

Date: December 2007
Creator: Zhao, Wei
Description: Anomalous Hall behavior of HgCdTe refers to a "double cross-over" feature of the Hall coefficient in p-type material, or a peak in the Hall mobility or Hall coefficient in n-type material. A magnetoconductivity tensor approach was utilized to identify presence of two electrons contributing to the conduction as well as transport properties of each electron in the material. The two electron model for the mobility shows that the anomalous Hall behavior results from the competition of two electrons, one in the energy gap graded region near the CdZnTe/HgCdTe interface with large band gap and the other in the bulk of the LPE film with narrow band gap. Hg0.78Cd0.22Te samples grown by LPE on CdZnTe(111B)-oriented substrates were exposed to various doses of thermal neutrons (~1.7 x 1016 - 1.25 x 1017 /cm2) and subsequently annealed at ~220oC for ~24h in Hg saturated vapor to recover damage and reduce the presence of Hg vacancies. Extensive Magnetotransport measurements were performed on these samples. SIMS profile for impurities produced by neutron irradiation was also obtained. The purpose for this study is to investigate the influence of neutron irradiation on this material as a basis for further study on HgCdTe74Se. The result shows that total ...
Contributing Partner: UNT Libraries
A Novel Process for GeSi Thin Film Synthesis

A Novel Process for GeSi Thin Film Synthesis

Date: December 2007
Creator: Hossain, Khalid
Description: A unique process of fabricating a strained layer GexSi1-x on insulator is demonstrated. Such strained heterostructures are useful in the fabrication of high-mobility transistors. This technique incorporates well-established silicon processing technology e.g., ion implantation and thermal oxidation. A dilute GeSi layer is initially formed by implanting Ge+ into a silicon-on-insulator (SOI) substrate. Thermal oxidation segregates the Ge at the growing oxide interface to form a distinct GexSi1-x thin-film with a composition that can be tailored by controlling the oxidation parameters (e.g. temperature and oxidation ambient). In addition, the film thickness can be controlled by implantation fluence, which is important since the film forms pseudomorphically below 2×1016 Ge/cm2. Continued oxidation consumes the underlying Si leaving the strained GeSi film encapsulated by the two oxide layers, i.e. the top thermal oxide and the buried oxide. Removal of the thermal oxide by a dilute HF etch completes the process. Strain relaxation can be achieved by either of two methods. One involves vacancy injection by ion implantation to introduce sufficient open-volume within the film to compensate for the compressive strain. The other depends upon the formation of GeO2. If Ge is oxidized in the absence of Si, it evaporates as GeO(g) resulting in spontaneous ...
Contributing Partner: UNT Libraries
Fractional Brownian motion and dynamic approach to complexity.

Fractional Brownian motion and dynamic approach to complexity.

Date: August 2007
Creator: Cakir, Rasit
Description: The dynamic approach to fractional Brownian motion (FBM) establishes a link between non-Poisson renewal process with abrupt jumps resetting to zero the system's memory and correlated dynamic processes, whose individual trajectories keep a non-vanishing memory of their past time evolution. It is well known that the recrossing times of the origin by an ordinary 1D diffusion trajectory generates a distribution of time distances between two consecutive origin recrossing times with an inverse power law with index m=1.5. However, with theoretical and numerical arguments, it is proved that this is the special case of a more general condition, insofar as the recrossing times produced by the dynamic FBM generates process with m=2-H. Later, the model of ballistic deposition is studied, which is as a simple way to establish cooperation among the columns of a growing surface, to show that cooperation generates memory properties and, at same time, non-Poisson renewal events. Finally, the connection between trajectory and density memory is discussed, showing that the trajectory memory does not necessarily yields density memory, and density memory might be compatible with the existence of abrupt jumps resetting to zero the system's memory.
Contributing Partner: UNT Libraries