Conducting and semiconducting, pi-conjugated polymers are promising materials for micro- and nano-optoelectronic applications because of their widely tunable physical, electrical, and optical properties. These polymers have been used to fabricate a number of electronic devices including field-effect transistors, light-emitting diodes, and photovoltaic cells. However, widespread commercial application of these devices has yet to be realized, due in part to poor electronic transport characteristics and device degradation. Nanostructuring of conjugated polymers by various methods has demonstrated marked improvements in molecular ordering and electronic transport. In this research, nanoscale, tubular structures of semiconducting polymers fabricated by template wetting nanofabrication procedures are explored. In particular, confinement-induced effects on the electronic carrier transport property mobility, mu, were investigated for both highly ordered and amorphous polymers. Analysis of space-charge-limited currents provided the key means of monitoring transport characteristics and molecular order. The effects of chemical filtration, nanotube diameter, solvent selection, and temperature are examined in detail.

The dispersion characteristics of plate acoustic waves PAW) that propagate in the X direction of a Z-cut single crystal and periodically poled lithium niobate PPLN) wafers are investigated theoretically and experimentally. The numerical calculations are performed with the aid of Finite Element method FEM) and the method of partial waves. In the series of experiments, the vertical acoustical displacement and electric potential are measured off the plate surface. The dispersion relations between the frequency f and the wavenumber beta are calculated for the first eight acoustic modes by applying the discrete Fourier transform DFT) to the calculated and measured displacement. The phase and group velocity dispersion relations are calculated from the wavenumber dispersion betaf). The dispersion curves of PAW that propagate in a PPLN crystal are compared to the single crystal case. An increase in the cutoff frequency of some modes propagating in PPLN plate is found. The dispersion curves of PAW in PPLN wafer show stop bands at certain values of f. The stop band occurs where the wavelength of a propagating mode is equal to the period of ferroelectric domains. The group velocity of acoustic modes decreases to zero at stop bands. The piezoelectric coupling coefficient K2) is calculated for the first eight PAW modes in a lithium niobate LN) plate. It is shown to be higher than the K2 of surface waves. An ultrasonic delay line that operates on the modes with higher K2 is numerically simulated, fabricated, and tested. The dispersion curves for PAW in periodically corrugated lithium niobate wafer are calculated. A complete band gap in the dispersion curves of three zero order modes occurs at the frequencies where the stop bands of zero order modes coincide.

The thermoelectric power (TEP) and sheet resistance of epitaxial graphene were measured in the temperature range from 300 K to 500 K. Using thermal decomposition of 4H-SiC in an induction furnace, graphene multilayers were grown on the carbon terminated surfaces (C-face) while mono and bilayer graphene films were grown on the silicon terminated surfaces (Si-face). The electrostatic removal (ESR) technique was used to reduce the number of graphene layers from the C-face samples. The application of the electrostatic removal method allows us to achieve monolayer graphene on the C-face substrates. All investigated multilayer graphene samples showed a positive Seebeck coefficient in ambient conditions at 300 K and turned negative after vacuum annealing at a pressure of &sim；2 x 10-7 Torr and a temperature of 500 K. Monolayer graphene on C-faces showed a relatively small positive Seebeck coefficient under ambient conditions and an even greater negative Seebeck coefficient after vacuum annealing. The oxygen doping is found to be responsible for the observed positive thermopower. Electrons are transferred from graphene to a redox couple associated with mildly acidic moisture (water) and oxygen. The charge transfer mechanism is proposed to be electrochemically mediated. We also measured the response of thermoelectric power while exposing the annealed graphene to various gases. The negative behavior for the degassed graphene is speculated to be due to the pinning of the Fermi energy at a state associate with the dangling bonds of the SiC surface. Si-face terminated graphene is found to have no effect with ambient doping. However, it was easily doped by electrochemical top gating. The sign of the thermoelectric power could be reversed by tuning the top gate voltage.

Tomography has made revolutionary impacts in a number of fields ranging from medical imaging, magnetic resonance imaging to electron microscopy. Conventional tomography reconstructs a 3D object from a set of equally angled 2D projections. Since the set of projections are in polar coordinates and the object in Cartesian coordinates, interpolation has to be used in the reconstruction process, which introduces artifacts in the reconstructed 3D object. In application to biology and medicine, there are two more difficulties: i) a limited number of projections due to radiation damage to biological specimens and the patients； and ii) the missing wedge problem i.e. specimens cannot be tilted beyond ＋/- 70&deg； in cryo-electron microscopy). Here we apply equally-sloped tomography EST) to significantly alleviate these difficulties and to demonstrate that EST can dramatically reduce the required radiation dose for achieving a desired resolution. We applied EST to reconstructing frozen-hydrated keyhole limpet hemocyanin molecules, a frozen-hydrated bacterial cell and a single human immunodeficiency virus HIV). In comparison with traditional weighted back-projection WBP), the algebraic reconstruction technique ART) and the simultaneous algebraic reconstruction technique SART), EST reconstructions exhibited higher contrast, less peripheral noise, more easily detectable molecular boundaries and reduced missing wedge effects. More importantly, EST reconstructions including only two-thirds the original images appeared to have the same resolution as full WBP reconstructions, suggesting that EST can either reduce the dose required to reach a given resolution or allow higher resolutions to be achieved with a given dose. The development of the tomographic implementation of x-ray phase contrast imaging holds great promise for biological and medical imaging； however, the radiation dose imparted to biological specimens and patients presents a major obstacle in such an implementation. By using an experimental data set from the European Synchrotron Radiation Facility, we show that EST can reduce the radiation dose by 60%, while obtaining the comparable images reconstructed by the conventional WBP at a full dose. This work may hence open the possibility of significantly radiation dose reduction in biological and medical imaging.

In this research, we studied the optical properties of a variety of pi-conjugated polymers and pi-conjugated polymers/fullerene blends, using various continuous wave optical spectroscopies. We found an illumination-induced metastable polaron-supporting phase in films of a soluble derivative of poly-p-phenylene vinylene MEH-PPV). Pristine, MEH-PPV polymer films in the dark do not show long-lived photogenerated polarons. Prolonged UV illumination, however, is found to induce a reversible, metastable phase characterized by its ability to support abundant long-lived photogenerated polarons. We also discovered a photobleaching band in our photomodulation measurement around 0.9eV that scales with and thus is related to the observed polaron band. In the dark, the illumination-induced metastable phase reverts back to the phase of the original MEH-PPV within about 30 min at room temperature. We also applied our experimental techniques in polymer/fullerene blends for studying the photophysics of bulk heterostructures with below-gap excitation. In contrast to the traditional view, we found that below-gap excitation, which is incapable of generating intrachain excitons, nevertheless efficiently generates polarons on the polymer chains and fullerene molecules. Using frequency dependence photomodulation, we distinguished between the two mechanisms of photoinduced charge transfer using above-gap and below-gap excitations, and found a distinguishable long polaron lifetime when photogenerated with below-gap excitation. The polaron action spectrum extends deep inside the gap as a result of a charge-transfer complex state formed between the polymer chain and fullerene molecule. Using the electroabsorption technique, we were able to detect the optical transition of the charge transfer complex state that lies below the gap of the polymer and the fullerene. With appropriate design engineering the long-lived polarons might be harvested in solar cell devices. Another system studied was platinum-containing conjugated polymers in which the intrachain Pt atom was incorporated into the polymer either in each Pt-1) or in every three Pt-3) monomer units. The heavy metal Pt atom that is incorporated in the polymer chain dramatically increases the spin-orbit coupling, and this influences both the intersystem crossing rate, and the phosphorescence emission intensity. We discuss an interesting effect for the photoexcited triplets, which dramatically affects the phosphorescence spectrum and intensity at high temperature.

Novel metal nano-clusters are always being an interest of scientists and researchers because of their unique optical and chemical properties. This thesis studies the formation mechanism of gold nanoisland film by studying transport properties. We used layer-by-layer self-assembled multilayer gold samples and annealed them at the temperature ranging from room temperature to 625&deg；C. Transport properties, particularly the resistance and capacitance, were measured in situ during annealing and compared with the surface morphology and UV-vis studies. Five films of the 8-layer gold and one film of the 5-layer silver and 5-layer gold nanoparticle sequentially self-assembled samples were measured. Temperature dependent resistance curves were plotted and analyzed. From the resistance curves, we were able to identify the actual temperature for polymer evaporation and nanoisland formation. These data were re-verified by comparing them with the temperature dependent studies of surface morphology and UV-vis spectroscopy. The effect of measuring condition, like heating rate and pre-annealing time factor, was also analyzed. Particularly, the slow heating and long pre-annealing time effected nanoisland growth mechanism.

This thesis summarizes our work in the past few years in the field of transport studies of carbon nanotubes and graphene. The first half of the thesis focuses on carbon nanotube CNT) Josephson junctions JJ) formed by coupling CNTs to superconducting electrodes. They exhibited Fabry Perot resonance patterns, enhanced differential conductance peaks, multiple Andreev reflection peaks, gate-tunable supercurrent transistor behaviors, hysteretic current-voltage line shape and “superconductor-insulator” transition. The junction behavior can be understood based on the dissipation dynamics and phase diffusion on the model of resistively and capacitively shunted junctions RCSJ). In addition, we investigated Fano resonance on a particular device. The transport spectroscopy exhibited “inverse” Coulomb blockade structures superimposed on Fabry-Perot resonance patterns, indicating quantum interference between a channel that is well-coupled to the electrodes and another channel that is poorly-coupled channel. Our transport data was reproduced reasonably by the simulation. The second half of the thesis discusses our results on graphene. Firstly, by developing a technique to fabricate suspended top gates, we were able to fabricate exceedingly clean, high quality graphene pnp junctions. In the high magnetic fields, we observed quantum hall plateaus at fractional values, which arise from edge state propagation and equilibration in regions with different filling factors, in agreement with the theoretical predictions. In zero magnetic fields, we observed Fabry-Perot conductance oscillations in the bipolar regime, demonstrating the high quality of our devices. Secondly, we explored specular Andreev reflection and have observed conductance peaks at the superconducting energy gap in normal metal — graphene — superconductor NS) junctions. However, the intended goal of the project, observation of specular Andreev reflection, was not achieved. As significant progress has been made towards fabrication of high quality suspended devices, we expect that specular Andreev reflection could be observed in the near future.

In order to find/design porous materials that could be used in practical applications involving adsorption, it is important to investigate the basic properties i.e. isosteric heat, specific surface area, binding energy, pore size, pore volume, etc.) of each material. With this aim in mind we have looked at three different types of materials: single-walled carbon nanotubes prepared by the HiPco and laser methods), single-walled nanohorns dahlia-like and bud-like) and metal-organic frameworks Cu-BTC and RPM-1). For these substrates we have measured volumetric adsorption isotherms using several gases such as neon, argon, tetrafluoromethane CF4), xenon, and methane not all gases for all substrates). Experimental adsorption isotherms were measured using methane, argon, xenon, and neon gases on unpurified single-walled carbon nanotubes prepared by the HiPco method. The main idea behind these experiments was to investigate, using different size gas molecules, the sites available for adsorption on this type of porous material. We found that surface area occupied by these adsorbates on the sample is the same, regardless of their size. This means that all the gases have access to the same group of adsorption sites. Since the biggest adsorbate in this experiment was Xe, and since it is unlikely that it could penetrate the interstitial channels in the nanotube bundles, we conclude that none of the gases, including the smallest one – Ne, are able to adsorb in the interstitial channels in bundles of single-walled carbon nanotubes. For the case of argon on laser produced single-walled carbon nanotubes we measured 21 adsorption isotherms using argon gas temperatures between 40 and 153 K that were used to determine the isosteric heat of adsorption for this system. Our experimental results were compared to the ones from computer simulations performed by J. K. Johnson from the University of Pittsburgh) for the same gas on heterogeneous and homogenous bundles. It was observed that the isosteric heat data matches better with data computed for heterogeneous nanotube bundles. This indicates that at the lowest pressure and coverages argon might be adsorbing in the defect-induced interstitial channels. We studied Cu3Benzene–1,3,5–tricarboxylate) 2H2O)3 abbreviated as Cu-BTC) metal-organic framework with argon to determine the sites available for adsorption on this material. Volumetric adsorption isotherms were measured at temperatures between 66 and 143 K. We found two substeps in the isotherm data, indicating that there are two types of pores present in the material: tetrahedrally-shaped side pockets and the main channels. Our experimental results were compared with data from simulations conducted using the Grand Canonical Monte Carlo method. We determined that the theoretical results match reasonably well with ours if the coverage is scaled down by a factor of 1.6. We explored the potential of two different metal-organic framework materials Cu-BTC and RPM–1) for gas separation application. We used argon and tetrafluoromethane CF4) gases to check if this can be achieved through kinetic and steric mechanisms. We found that Cu-BTC has excellent potential in gas separation using a steric mechanism, since argon easily adsorbs into the small pores present in the sample, while CF4 is excluded from them. Adsorption properties of RPM–1 showed that it could be employed in gas separation using a kinetic mechanism — argon gas adsorbs and reaches equilibrium in the pores of the sample more than the order of magnitude faster than CF4. Closed-ended dahlia-like nanohorns were studied with neon and tetrafluoromethane gases. In the first layer of neon and tetrafluoromethane adsorbed on dahlia-like nanohorns we found two substeps. These results were compared with results of computer simulations performed by Prof. M. Calbi. We determined, after comparison with the simulation isotherms, that the lower pressure substeps correspond to adsorption of Ne and CF4 in the narrowest parts of interstitial channels of the aggregates. Surface area calculated from neon isotherms was found to be higher than the one obtained using CF4, meaning that the smaller Ne molecule has the access to the parts of the interstitial channels that are not accessible for the bigger CF4 molecule. Features that appeared in neon adsorption isotherms on bud-like nanohorn aggregates were quite different from the ones on dahlia-like aggregates. We measured neon adsorption isotherms on this type of sample at temperatures between 22 and 49 K. In the monolayer regime we observed one single substep whose origin we can not definitely identify, because the structure of the bud-like nanohorns is not well-known. The binding energy value that was calculated from the isotherm data was lower than the value for neon adsorbed in the grooves of nanotube bundles but higher than for neon on graphite.

Critical current density (Jc) has been identified as one of the most critical parameters for the practical application of high temperature superconductors such as YBa2Cu3O7-delta (YBCO). Unfortunately, the Jc of optimized un-doped YBCO films barely satisfies the criteria for these applications. High Jc can be achieved by introducing strong artificial pinning centers in YBCO which can inhibit flux motion and prevent dissipation. However, insertion of strong pins has been observed to strain and poison the YBCO lattice resulting in unnecessary degradation of Tc and low field Jc. In this work, two types of strong pinning centers with negligible effect on the T c and low field Jc were incorporated in YBCO films via strain engineering on the nanoscale. The nanotube pores were generated by depositing YBCO films on vicinal SrTiO3 (STO) substrates. A close correlation between Jc and the magnetic pinning potential Up of the nanotube pores has been demonstrated below the accommodation field, suggesting that nanotube pores are strong pins on the magnetic vortices. Splayed BaZrO 3 nanorords (BZO-NRs) were generated in YBCO film by depositing 2 vol.% BZO-doped YBCO on vicinal STO substrates. The interplay between the lattice strain caused by the large lattice mismatch between YBCO and BZO and the anisotropic strain due to vicinal growth resulted in the dispersed orientation of BZO-NRs. The splayed BZO-NRs led to an enhanced Jc in the entire range of the magnetic field orientation up to 5 T as compared to the non-splayed case of YBCO/BZO-NRs films.

Strongly correlated materials exhibit some of the most complex and technologically useful phenomena in condensed matter. We use the world line and determinant quantum Monte Carlo techniques to explore the phases of the Hubbard Hamiltonian, a model describing strong interactions, in a number of different contexts. We first consider the spin, charge, and bond order correlations of the one-dimensional extended fermion Hubbard model in the presence of a coupling to the lattice. A static alternating lattice distortion leads to enhanced charge density wave correlations at the expense of antiferromagnetic order. When the lattice degrees of freedom are dynamic, we show that a similar effect occurs even though the charge asymmetry must arise spontaneously. Although the evolution of the total energy with lattice coupling is smooth, the individual components exhibit sharp crossovers at the phase boundaries. Finally, we observe a tendency for bond order in the region between the charge and spin density wave phases. Second, we examine mixtures of bosons and fermions in one-dimensional optical lattices. We evaluate the density profiles and bosonic visibility Vb, resolving the discrepancy between theory and experiment by identifying parameter regimes where Vb is reduced and increased. We present a simple qualitative picture of the different response to the fermion admixture in terms of the superfluid and Mott-insulating domains before and after the fermions are included. Finally, we show that Vb exhibits kinks which are tied to the domain evolution present in the pure bosonic case, and also additional structure arising from the formation of boson-fermion molecules, a prediction for future experiments. Third, we report large scale calculations of the effective bandwidth, momentum distribution, and magnetic correlations of the square lattice fermion Hubbard Hamiltonian. The sharp Fermi surface of the non-interacting limit is significantly broadened by the electronic correlations, but retains signatures of the approach to the edges of the first Brillouin zone as the density increases. Finite size scaling of simulations on large lattices allows us to extract the interaction dependence of the antiferromagnetic order parameter, exhibiting its evolution from weak-coupling to the strong-coupling Heisenberg limit. Our lattices provide improved resolution of the momentum distribution, allowing a more quantitative comparison with time-of-flight optical lattice experiments.