Bulletin of the American Physical Society
2006 Four Corners Section of the APS Fall Meeting
Friday–Saturday, October 6–7, 2006; Logan, Utah
Session H2: Condensed Matter Electronic Properties |
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Chair: T.C. Shen, Utah State University Room: Eccles Conference Center Room 205/207 |
Saturday, October 7, 2006 8:30AM - 8:42AM |
H2.00001: Femtosecond transient studies of charge transfer in~polymers doped with acceptor molecules; applications~for organic solar cells Josh Holt, Chuanxiang Sheng, Tomer Drori, Z. Valy Vardeny Current developments in organic solar cells ($\sim $5{\%} efficiency nowadays)~require understanding and control of charge carrier transfer and electronic state dynamics of donor-acceptor pairs. One current drawback to organic solar cell efficiency is negligible absorption in the near infrared region of the solar spectrum. We provide evidence that poly(2-methoxy-5(2'-ethyl)hexoxy-phenylenevinylene) (MEH-PPV) doped with 2,7-dinitrofluoronone (DNF) forms a charge transfer complex state that can extend absorption into the near infrared. We found that photoluminescence and the~photoinduced absorption (PA)~band of excitons~are simultaneously quenched. Ultrafast spectroscopic measurements with spectral range from 0.2 to 1.2 eV provide insights into polaron and exciton band dynamics~for these complexes. We also suggest a mechanism for bimolecular charge transfer in this system. [Preview Abstract] |
Saturday, October 7, 2006 8:42AM - 8:54AM |
H2.00002: Optical and Transport studies of isolated and aggregated molecular wires Alexandre Ndobe, Vladimir Burtman, Golda Hukic, Valy Vardeny We have studied the optical and transport properties of self assembled monolayer of a mixture of conducting molecules methyl-bezenedithiol (Me-BDT-wire) and non conducting molecules Pentathiol (PT-spacer). The I-V characteristic dependence of the fabricated diodes on the ratio, r of wire/spacers reveals that at low ratio 10-8 $<$ r $<$ 10-3, the transport studies of this mixture can provide us with single molecule resistance of Me-BDT; With the knowledge of the number of conducting molecules which we estimated by multiple self assembly and titration, we found that the single molecule resistance of Me-BDT is 600 M$\Omega $. At high ratio r $>$ 10-4, we found that the conducting molecule tend to aggregate and form a broad resonant state at mid gap that is detectable through the differential conductance measurements as well as by optical absorption and photoluminescence emission. [Preview Abstract] |
Saturday, October 7, 2006 8:54AM - 9:06AM |
H2.00003: Surface Green function calculations in the infinite number of principal layer approach: A non-recursive scheme Aleksey Kletsov, Yuri Dahnovsky, Vinz Ortiz A novel computational method for a surface Green function matrix is determined for the calculation of electrical current in molecular wires. The proposed non-recursive scheme approach allows one to find the imaginary part of the surface Green matrix by the method that includes the infinite number of principal layers. It is shown that the solution of the second order matrix equation gives the spectral density matrix which for noninteracting electrons determines the density of states. Single and double principle layer models are studied both analytically and numerically. [Preview Abstract] |
Saturday, October 7, 2006 9:06AM - 9:18AM |
H2.00004: Coherent Excitation of the Optic Phonon in Si Measured with Femtosecond Spectroscopy D.M. Riffe, A.J. Sabbah Using 28 fs, 800 nm laser pulses from a Ti:sapphire oscillator we have coherently excited, and subsequently probed with time dependent reflectivity, the Si zone-center optic phonon. The reflectivity modulations are well described by an underdamped oscillator $\left( {{\Delta R} \mathord{\left/ {\vphantom {{\Delta R} R}} \right. \kern-\nulldelimiterspace} R} \right)\exp \left( {{-t} \mathord{\left/ {\vphantom {{-t} \tau }} \right. \kern-\nulldelimiterspace} \tau } \right)\cos \left( {{2\pi t} \mathord{\left/ {\vphantom {{2\pi t} T}} \right. \kern-\nulldelimiterspace} T+\phi } \right)$ with amplitude $\Delta R$/$R \quad \approx $ 5 $\times $ 10$^{-6}$, phase $\phi $ = 86 $\pm $ 14 degrees, period $T$ = 64.07 $\pm $ 0.07 fs, and decay time $\tau $ = 2.75 $\pm $ 0.15 ps. The phase indicates that transiently stimulated Raman scattering (TSRS) is responsible for the coherent-phonon generation: our result are in good agreement with a recent theory of TSRS for opaque materials [T. E. Stevens \textit{et al}., Phys. Rev. B \textbf{65}, 144304 (2002)] when we extend the theory to include the finite lifetime of the excited charge density that drives the oscillation. Additionally, the period and decay time of the coherent oscillations are consistent with carrier-density dependent Raman-scattering measurements. [Preview Abstract] |
Saturday, October 7, 2006 9:18AM - 9:30AM |
H2.00005: Electron transport in laterally confined phosphorus $\delta $-layers in silicon S.J. Robinson, J.R. Tucker, T.-C. Shen Carrier transport in 1D semiconductor structures has not been much studied experimentally because of the difficulty of confining dopant atoms in a quasi-1D configuration in a crystal. In the past few years we have developed a UHV-STM-based fabrication scheme to create 2D nanoscale patterns buried in crystalline silicon. By selectively desorbing H from a Si surface and dosing the dangling bonds with PH$_{3}$, we can create laterally confined conductive P $\delta $-layers with widths on the order of 10 nm. These nanowires are connected to arrays of As-implanted contacts for transport characterization. Electrical measurements at cryogenic temperatures show ohmic behavior and magnetoconductance in accordance with weak localization theory. In addition, by lowering the temperature continuously, we find a clear 2D to 1D transition as the phase coherence length approaches the wire width. In 1D, the nanowire resistance becomes independent of temperature, indicating a saturation of the phase coherence and thermal lengths due to inelastic boundary scattering in the wire. Large-scale integration of $\delta $-layer devices and potential 3D architectures will become possible by employing the UHV-photolithography currently under development. [Preview Abstract] |
Saturday, October 7, 2006 9:30AM - 9:42AM |
H2.00006: Calculation of efficiency losses in Cu(In,Ga)Se2/CdS solar cells with ultra-thin absorbers Ana Kanevce, James Sites One of the main obstacles for commercialization of solar cells based on Cu(In,Ga)Se2 absorbers is the price and unavailability of indium. An obvious way to reduce the amount of indium required is to reduce the thickness of the Cu(In,Ga)Se2 absorber. This work uses numerical simulations to investigate the physical aspects of the changes that should occur with thinner absorbers. As the thickness becomes smaller than the diffusion length, the back-contact recombination losses increase. Increased Ga content towards the back contact can improve the carrier collection and lower the recombination loss. The influence of Ga/In distribution throughout the absorber and the necessary Ga/In ratio are calculated. The effect of nonuniformities on the solar cell behavior has been investigated, with emphasis on comparison between thinner (below 500 nm) and thicker (1 micron and above) devices. Devices thinner than 500 nm are less forgiving to thickness nonuniformities than the thicker ones. Band-gap nonuniformities cause collection problems and lower the fill-factor. [Preview Abstract] |
Saturday, October 7, 2006 9:42AM - 9:54AM |
H2.00007: E-Field Conditioning and Charging Memory in Low Density Polyethylene Jerilyn Brunson, J.R. Dennison Accurate measurement of electronic properties in extremely high resistivity materials must take into account a number of ways in which the measurements influence the materials properties being probed. These can include the strength of the applied electric field, the number of successive exposures to an applied field, the duration of exposure, and recovery time allowed during exposure cycles. An extensive series of constant voltage measurements of the resistivity of low density polyethylene samples were taken to determine consistency of measured resistivity results, the effects of varying electric field amplitude, and the extent of charging memory. Higher electric fields were found to lower the resistivity, as predicted by hopping conductivity models of polymers. Measurements at a particular voltage showed that the dark current resistivity approach successively lower values with repeated exposure. [Preview Abstract] |
Saturday, October 7, 2006 9:54AM - 10:06AM |
H2.00008: Electric Field Induced Hopping Conductivity: An Investigation of Electric Field-Dependent Resistivity in Polymers S.R. Hart, J. Brunson, J.R. Dennison The resistivity of highly insulating materials exhibits a dependence on electric field strength. Mott and Davis as well as Poole and Frankle describe theoretically the resistivity of disordered semiconductors, when subject to a changing electric field, in terms of hopping conductivity models. While these models have often been applied to polymers, there is little direct experimental evidence to confirm the validity of the theories for polymers. We present such results for a newly-developed block co-polymer Hytrel, a highly insulating material similar to Teflon. The constant voltage resistivity test method has been used to study Hytrel for a range of electric fields up to electrostatic breakdown. We consider whether the Hytrel results are consistent with existing models of electric-field induced hopping conductivity. [Preview Abstract] |
Saturday, October 7, 2006 10:06AM - 10:18AM |
H2.00009: Photodarkening of Te-modified TiO2 Nanocrystals Steven Phillips The photocatalytic properties of TiO2 nanocrystals can be enhanced by doping, which can result in increased absorption in the visible range. After annealing Te-modified TiO2 nanocrystals, the powder becomes photo-sensitive, changing from an off-white color to a dark-red color under UV and visible illumination. This color change is stable, but can be reversed by annealing the powder again. We analyze the nanocrystals with x-ray diffraction, x-ray photoelectron spectroscopy, electron spin resonance, and transmission electron microscopy. Currently, the main question we are investigating is whether the Te is in the interior of the nanocrystals or on the surface. [Preview Abstract] |
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