Finally in combination with c-AFM and EFM, the high-k materials breakdown behaviors are also interrogated. The breakdown processes are classified into three stages: pre-breakdown (pre-BD), soft breakdown (SBD) and hard breakdown (HBD). And a HfOx nano-pattern is fabricated with the aid of AFM. The dot growth characteristics on the pulse amplitude, duration and humidity are scrutinized. / In this thesis, the single CdS nanobelt devices are fabricated successfully. The photosensitivity at 1V is up to 8 x 103 A/W and the electron mobility reaches to tens of cm/V·s. Based on these excellent optoelectronic properties, the CdS nanobelt becomes a good choice for interrogation on the charge transport characteristics on a nanometer scale. The transistor measurements show that the performance of CdS nanobelt device can be influenced by illuminations and ambient conditions, which result from the metal/CdS nanobelt contact and nanobelt surface redox reactions. / The intrinsic carrier transport characteristics in CdS nanobelt can be investigated by reconstructing of the local surface band diagram with the aid of SSPM. A ∼0.50 eV upward band bending can be obtained in the dark. The surface depletion length induced by the negative surface oxygen adsorbates is estimated to ∼66nm if a concentration of 1017 cm -3 shallow donors is assumed in the CdS nanobelt. This depletion length is close to the height of the ultra-thin CdS nanobelt. These adsorbates result in the surface depletion region expansion and the conduction channel reduction, which is responsible for the CdS conductance drop. Above the band-gap illumination or to the oxygen-deficient environment can effectively reduce the surface band bending and the depletion region, finally increase the conduction channel, which is one of the main reasons for the large photosensitivity and highly oxygen sensitivity for the single CdS nanobelt device. / To sustain Moore's law scaling trend beyond COMS, one-dimensional (1D) nanostructures, e.g. carbon nanotubes and semiconductor nanowires, are proposed to act as fundamental nanoscale blocks in the future electronic and optoelectronic devices. Therefore it is very crucial to understand the unique nature of electronic properties for 1D nanostructures in designing novel nanoelectronic devices and optimizing the device performance. In this thesis, the charge transport properties of nanostructure devices are studied. A method called photo-assisted scanning surface potential microscopy (SSPM) is developed, which yields a direct measurement of the electrostatic potential distributions across the 'biased' nanostructured device under different illumination conditions. Our efforts provide significant understanding of the nature of charge transport in nanoelectronics. / We can simply fabricate the MSM device using single CdS nanobelt. A positive Schottky barrier is found at the electrode/CdS nanobelt junction because of the unequal work function or the Fermi level pinning by the surface states. The barrier height is estimated to be 0.38 eV by fitting the temperature dependent I-V curves. A big potential drop at the junction can be visualized by SSPM. The calculated contact resistance for the electron injection is much larger than that for the CdS nanobelt, which illustrates that the transport properties of CdS nanobelt device are dominated by the charge injection process. The change in contact resistance and nanobelt resistance under the above bandgap illumination are measured by photo-assisted SSPM. The experimental results show that in the dark, the charge transport for the CdS nanobelt device is dominated by electron injection, while under high light intensity, the charge transport is governed by the intrinsic nature of CdS nanobelt. / With the aid of SPM, the charge injection and carrier transport characteristics of the individual CdS nanobelt device are systematically interrogated and comprehensively demonstrated, which are useful for designing and fabricating the nanostructured electronic and optoelectronic devices. / An, Jin. / Adviser: Jiambin Xu. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3674. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.

As organic semiconductors gain more importance for application, research into their properties has become necessary. This work investigated the exciton and charge transport properties of organic semiconducting crystals. Based on a hopping approach, protocols have been developed for the calculation of Charge mobilities and singlet exciton diffusion coefficients. The protocols do not require any input from experimental data except for the x-ray crystal structure, since all needed quantities can be taken from high-level quantum chemical calculations. Hence, they allow to predict the transport properties of yet unknown compounds for given packings, which is important for a rational design of new materials. Different thermally activated hopping models based on time-dependent perturbation theory were studied for the charge and exciton transport; i. e. the spectral overlap approach, the Marcus theory, and the Levich-Jortner theory. Their derivations were presented coherently in order to emphasize the different levels of approximations and their respective prerequisites. A short reference was made to the empirical Miller-Abrahams hopping rate. Rate equation approaches to calculate the stationary charge carrier mobilities and exciton diffusion coefficients have been developed, which are based on the master equation. The rate equation approach is faster and more efficient than the frequently used Monte Carlo method and, therefore, provides the possibility to study the anisotropy of the transport parameters and their three-dimensional representation in the crystal. The Marcus theory, originally derived for outer sphere electron transfer in solvents, had already been well established for charge transport in organic solids. It was shown that this theory fits even better for excitons than for charges compared with the experiment. The Levich-Jortner theory strongly overestimates the charge carrier mobilities and the results deviate even stronger from the experiment than those obtained with the Marcus theory. The latter contains larger approximations by treating all vibrational modes classically. The spectral overlap approach in combination with the developed rate equations leads to even quantitatively very good results for exciton diffusion lengths compared to experiment. This approach and the appendant rate equations have also been adapted to charge transport. The Einstein relation, which relates the diffusion coefficient with the mobility, is important for the rate equations, which have been developed here for transport in organic crystals. It has been argued that this relation does not hold in disordered organic materials. This was analyzed within the Framework of the Gaussian disorder model and the Miller-Abrahams hopping rate. / Organische Halbleiter gewinnen immer größere Bedeutung für Anwendungen in der Elektronik. In dieser Arbeit wurden deren Eigenschaften bezüglich des Exzitonen- und Ladungstransports untersucht. Diese beiden Prozesse sind wesentlich für viele Bauteile der organischen Elektronik, wie zum Beispiel Solarzellen. Ausgehend von einem Sprungmodell wurden Verfahren zur Berechnung von Ladungsträgerbeweglichkeiten und Diffusionskoeffizienten von Singulettanregungen entwickelt, wofür bis auf die Röntgenstruktur des Kristalls keine weiteren experimentellen Daten benötigt werden, da alle notwendigen Größen durch quantenchemische Rechnungen auf hohem Niveau bestimmt werden können. Dies ermöglicht die Vorhersage der Transporteigenschaften von noch unbekannten Materialien mit bekannter Struktur, was eine Voraussetzung für das Maßschneidern neuer Materialien darstellt. Verschiedene, auf der zeitabhängigen Störungstheorie basierende thermisch aktivierte Sprungmodelle - der spektrale Überlappungsansatz, die Marcus- und die Levich-Jortner-Theorie - wurden für die Anwendung auf den Ladungs- und Energietransport hin untersucht. Ausgehend von Fermis Goldener Regel wurden die Sprunggleichungen konsistent hergeleitet, um die verschiedenen Abstufungen der jeweils vorgenommenen Näherungen und deren Voraussetzungen deutlich zu machen. Zusätzlich dazu wurde ein kurzer Exkurs zur empirischen Miller-Abrahams-Sprungrate und deren Anwendung in amorphen Systemen gemacht. Unter Verwendung der Mastergleichung wurden Ratengleichungsansätze zur Berechnung der stationären Ladungsträgerbeweglichkeiten und Exzitonendiffusionskoeffizienten entwickelt. Die Berechnung der Transportgrößen über Ratengleichungen ist wesentlich schneller und effizienter als die häufig angewendete Monte-Carlo-Simulation. Dies ermöglicht die Analyse der Anisotropie des Transports im Kristall und ihre dreidimensionale Darstellung. Die Marcustheorie, die ursprünglich für Elektronentransfer in Lösungen entwickelt wurde, hat sich auch für Ladungstransport in organischen Festkörpern bewährt. Hier wurde diese Theorie auch auf Exzitonentransport übertragen und gezeigt, daß sie im Vergleich zum Experiment für Exzitonen sogar bessere Ergebnisse liefert als für Ladungsträger. Die Levich-Jortner-Theorie überschätzt die Ladungsträgerbeweglichkeiten im Falle der Acene sehr stark. Ihre Ergebnisse weichen sogar stärker vom Experiment ab als die der Marcustheorie. Letztere enthält deutlich stärkere Näherungen, weil alle Molekülschwingungen klassisch behandelt werden. Der spektrale Überlappungsansatz führt zusammen mit den hier entwickelten Ratengleichungen sogar zu quantitativ guten Ergebnissen für die Exzitonendiffusion. Dieser Ansatz und die Ratengleichungen wurden auch für die Berechnung der Ladungsträgerbeweglichkeiten angepaßt. Für die in dieser Arbeit entwickelten Ratengleichungen ist die Einsteinrelation, welche die Diffusion mit der Drift in Beziehung setzt, von zentraler Bedeutung. Es ist umstritten, ob diese Beziehung auch in amorphen, ungeordneten Materialien gültig ist. Dieser Frage wurde im Rahmen des Gaußschen Unordnungsmodells und der Miller-Abrahams-Sprungrate nachgegangen.

acase@tulane.edu

As a hydrophilic biopolymer, a DNA molecule is surrounded by water molecules in aqueous solution. The charge hopping mechanism indicates the competition between radical cation quenching by water molecules and migration along DNA partially determines the distance and efficiency of charge transport in DNA. Lipid can effectively bind DNA to induce hydrophobic environment around the DNA helix and reduce the water contact with bases in the DNA duplex. Therefore, the effect of water molecules on charge transport can be studied by comparison between nature DNA and DNA-lipid complexes. We synthesized several cationic lipids with various lengths of dialkyl chain (2, 8, 18) and spermine (Sp4+) binding core in this research, which posses strong DNA binding affinity due to their multi-charged spermine head-groups. Among those, C8GlySp4+ and C2GlySp4+ can form stable complex with DNA oligomer in aqueous solution, characterized by time dependent UV and CD spectrometry. C2GlySp4+ showed the similar inhibition on oxidative damage in GG steps as spermine while C8GlySp4+ demonstrated much more significant prohibitive effect at the same concentration. Since all the lipids bear the same binding core, they should afford the similar binding affinity towards DNA duplexes. we attributed the observation to the longer length of dialkyl group in C8GlySp4+, which can more effectively shield the DNA duplex from the water molecules than either spermine or C2GlySp4+. A kinetic model based on phonon-assist polaron hopping mechanism was proposed to rationalize the experimental results. The finding may give insight on the protection of DNA oxidative damage by reducing the access of the water molecule to DNA duplex and may have potential impact on the application of DNA as conducting biopolymer and protection of DNA in biological system.

Abstract The first hyperpolarizability(£]) of five charge-transfer molecules are determined using the hyper-Rayleigh scattering (HRS) technique at two excitation wavelengths : 1064nm and 1907nm. The 1064nm excitation wavelength is derived from a Nd: YAG pulsed laser, and the 1907nm excitation wavelength is obtained by shifting the 1064nm laser light by stimulated Raman scattering of pressurized H2 gas. Four of the five samples contains thiophene and thiazole are synthetized by Prof. Shu Ching-Fong at the National Chiao Tung University (NCTU) and the other sample is synthetized by Prof. Hong, Jin-Long at the National Sun Yat-sen University (NSYSU). The measured £] values are used to calculate the intrinsic molecular hyperpolarizabilities using the two-level model previously developed by Oudar and Chemla. The results are related to molecular structure.

This dissertation investigates the photophysics of bis(diarylamino)biphenyl (TPD) and silver nanoparticles (AgNP). A main goal of this work was to develop an understanding of the relaxation pathways involved in the deactivation of photoexcited TPD chromophores in close proximity to silver nanoparticles. The TPD chromophores were attached to the silver nanoparticle core via an alkylthiol group. The TPD-AgNP systems were synthesized and characterized using Transmission Electron Microscopy (TEM), UV-visible absorption, infrared spectroscopy, and Nuclear Magnetic Resonance (NMR) spectroscopy, Inductively Coupled Plasma - Emission Spectroscopy (ICP-ES) and Thermogravimetric Analysis (TGA). Time-resolved photophysical processes in these systems were studied using femtosecond transient absorption spectroscopy. Initial studies of the interaction of the TPD and AgNP addressed the linker length dependence of the dye excited state decay kinetics, wherein alkyl linker chains of 3, 4, 8 and 12 carbon atoms were used. These results showed that an ultrafast deactivation of the excited state of the TPD chromophore, which is three orders of magnitude faster than that of the free chromophore in solution, occurred in all of the systems. However, an unexpected new transient species was observed for the systems with three and four carbon linker chains. Further studies showed this species to be spectroscopically very similar to the TPD radical cation, suggesting a charge separation pathway in the excited state relaxation. Possible pathways for formation of the cation-like state were examined through comparisons to the photophysics of alkyl substituted TPD in solution and in solid films, investigation of the pulse energy and TPD surface coverage dependence of the yield of the cation-like TPD species, transient absorption anisotropy decay dynamics, and kinetic modeling studies. Taken together, these investigations provide support for exciton-exciton annihilation being responsible for the formation of cation-like species. The packing of the TPD chromophores is concluded to be of critical importance in the generation of the cation like species but it is also possible that proximity to the silver nanoparticle plays a role in facilitating charge separation as well.

The electronic material graphene has attracted much attention for its unique physical properties such as, linear band structure, high electron mobility, and room temperature ballistic conduction. The possibilities for device applications utilizing graphene show great variety, from transistors for computing to chemical sensors. Yet, there are still several basic physical properties such as thermal conductivity that need to be determined accurately. This work examines the thermal properties of graphene grown by the chemical vapor deposition technique. The thermoelectric power of graphene is studied in ambient and vacuum environments and is shown to be highly sensitive to surface charge doping. Exploiting this effect, we study the change in thermoelectric power due to introduction of gaseous species. The temperature dependent thermal conductivity of graphene is measured using a comparison method. We show that the major contribution to the thermal conductivity is the scattering of in-plane phonons. Graphene also shows promise as an optoelectronic material. We probe the Landau level structure of graphene in high magnetic fields using a differential photoconductivity technique. Using this method we observed the lifting of spin and valley degeneracies of the lowest Landau level in graphene.

Understanding organic semiconductor interfaces is critical to developing organic photovoltaics (OPV). OPV interfaces are disordered due to weak intermolecular interactions, resulting in diverse charge transfer micro-environments. I present experimental data isolating high-order intermolecular interactions controlling interfacial energy level alignment and describe new instrumental capabilities providing access to the local electronic and kinetic landscape at organic semiconductor interfaces. Interface formation between vanadyl naphthalocyanine (VONc) and highly ordered pyrolytic graphite (HOPG) is investigated. Ultraviolet photoemission spectroscopy (UPS) shows that the VONc binding energy (BE) decouples from the work function, shifting in an opposite direction and contradicting the standard interface dipole model. This effect is quantitatively described using an electrostatic depolarization model and confirmed by simulations which show an inhomogeneous potential at the interface. New data and literature values suggest orthogonality between polarizability and molecular dipole in polar porphyrazines. Their potential for interface engineering is discussed. The electron-rich Au(111)/VONc interface is investigated. The organic layer induces a large interface dipole in Au(111) which can be fit to a depolarization model. Ionization potential and depolarization data suggest that the second VONc layer on Au(111) adopts a tilted geometry. Electrostatic differences between Au(111)/VONc and HOPG/VONc are discussed, demonstrating that interface dipole contributions are not interchangeable. The surface states of the Au(111)/VONc interface are characterized by angle resolved 2-photon photoemission to determine the magnitude of the perturbation. The measured free-electron-like effective mass and BE destabilization of the Shockley state is attributed to step edges caused by lifting the Au(111) (22 x √3) reconstruction. The Shockley state is accessible primarily through resonance with the n = 1 image state. Another resonance between the image state and a molecular state of VONc is tentatively identified. Design and construction of a confocal fluorescence microscope capable of single molecule detection in ultrahigh vacuum is described. Initial images and fluorescence trajectories demonstrate the ability to measure charge transfer kinetics between an individual organic semiconductor molecule and well-characterized insulating surfaces. Progress towards completion of a scanning photoionization microscope is presented. The microscope demonstrates diffraction-limited imaging capabilities using fs-laser-generated photoelectron current as contrast. Recommendations are given towards achieving spectral resolution and for future experimental systems.

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