Browsing by Author "Di Ventra, Massimiliano"

Now showing 1 - 7 of 7
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    DNA Electronics
    Zwolak, Michael Philip (Virginia Tech, 2003-05-07)
    DNA is a potential component in molecular electronics. To explore this end, there has been an incredible amount of research on how well DNA conducts and by what mechanism. There has also been a tremendous amount of research to find new uses for it in nanoscale electronics. DNA's self-assembly and recognition properties have found a unique place in this area. We predict, using a tight-binding model, that spin-dependent transport can be observed in short DNA molecules sandwiched between ferromagnetic contacts. In particular, we show that a DNA spin-valve can be realized with magnetoresistance values of as much as 26% for Ni and 16% for Fe contacts. Spin-dependent transport can broaden the possible applications of DNA as a component in molecular electronics and shed new light into the transport properties of this important biological molecule.
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    From Cluster to Grid: A Case Study in Scaling-Up a Molecular Electronics Simulation Code
    Ribbens, Calvin J.; Bora, Prachi; Di Ventra, Massimiliano; Hauck, J.; Prabhakar, Sandeep; Taylor, C. (Department of Computer Science, Virginia Polytechnic Institute & State University, 2002-11-01)
    This paper describes an ongoing project whose goal is to significantly improve the performance and applicability of a molecular electronics simulation code. The specific goals are to (1) increase computational performance on the simulation problems currently being solved by our physics collaborators; (2) allow much larger problems to be solved in reasonable time; and (3) expand the set of resources available to the code, from a single homogeneous cluster to a campus-wide computational grid, while maintaining acceptable performance across this larger set of resources. We describe the sequential performance of the code, the performance of two parallel versions, and the benefits of problem-specific load balancing strategies. The grid context motivates the need for runtime algorithm selection; we present a component-based software framework that makes this possible.
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    Optical studies of GaAs:C grown at low temperature and of localized vibrations in normal GaAs:C
    Vijarnwannaluk, Sathon (Virginia Tech, 2002-04-25)
    Optical studies of heavily-doped GaAs:C grown at low temperature by molecular beam epitaxy were performed using room-temperature photoluminescence, infrared transmission, and Raman scattering measurements. The photoluminescence experiments show that in LT-GaAs:C films grown at temperatures below 400 °C, nonradiative recombination processes dominate and photoluminescence is quenched. When the growth temperature exceeds 400 °C, band-to-band photoluminescence emission appears. We conclude that the films change in character from LT-GaAs:C to normal GaAs:C once the growth temperature reaches 400 °C. Annealing, however, shows a different behavior. Once grown as LT-GaAs:C, this material retains its nonconducting nonluminescing LT characteristics even when annealed at 600 °C. The Raman-scattering measurements showed that the growth temperature and the doping concentration influence the position, broadening, and asymmetry of the longitudinal-optical phonon Raman line. We attribute these effects to changes in the concentration of interstitial carbon in the films. Also, the shift of the Raman line was used to estimate the concentration of arsenic-antisite defects in undoped LT-GaAs. The infrared transmission measurements on the carbon-doped material showed that only a fraction of the carbon atoms occupy arsenic sites, that this fraction increases as the growth temperature increases, and that it reaches about 100% once the growth temperature reaches 400 °C. The details of all these measurements are discussed. Infrared transmission and photoluminescence measurements were also carried out on heavily-doped GaAs:C films grown by molecular beam epitaxy at the standard 600 C temperature. The infrared results reveal, for dopings under 5 x 10⁹ cm⁻³, a linear relation between doping concentration and the integrated optical absorption of the carbon localized-vibrational-mode band. At higher dopings, the LVM integrated absorption saturates. Formation of CAs-CAs clusters is proposed as the mechanism of the saturation. The photoluminescence spectra were successfully analyzed with a simple model assuming thermalization of photoelectrons to the bottom of the conduction band and indirect-transition recombination with holes populating the degenerately doped valence band. The analysis yields the bandgap reduction and the Fermi-level-depth increase at high doping.
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    Optical studies of highly-doped GaAs:C
    Songprakob, Wantana (Virginia Tech, 2001-08-31)
    Infrared reflectivity and transmittance measurements (200-5000 cm⁻¹) were carried out on heavily-doped GaAs:C films grown by molecular beam epitaxy. With increasing carbon concentration, a broad reflectivity minimum develops in the 1000-3000 cm⁻¹ region and the one-phonon band near 270 cm⁻¹ rides on a progressively increasing high-reflectivity background. An effective-plasmon/one-phonon dielectric function with only two free parameters (plasma frequency ωp and damping constant γp) gives a good description of the main features of the reflectivity spectra. The dependence of effective plasma frequency on hole concentration p is linear. At each doping, the effective-plasmon damping constant γp is large and corresponds to an optical hole mobility that is about half the Hall mobility at that p. Secondary-ion mass spectroscopy and localized-vibrational-mode measurements indicate that the Hall-effect-derived hole concentration is close to the carbon concentration and that the Hall factor is close to unity, so that the Hall mobility provides a good estimate of the actual dc mobility. Also, analysis shows that, for our highly-doped samples, the observed dichotomy between the dc and infrared mobilities is not a statistical-averaging artifact of the approximations involved in the model. The explanation of the small infrared mobility resides in the influence of intervalence band absorption on the effective-plasmon fit, which operationally defines that mobility via the effective-plasmon damping. The optical properties obtained with the use of the effective-plasmon model for GaAs:C yield a phenomenological, approximate, overall picture of the infrared spectra. But the neglect of intervalenceband transitions, for this p-type semiconductor, is shown (in this dissertation) to be a serious drawback of this simple model. In order to obtain the optical properties of GaAs:C in a model-independent way, and to attempt to resolve the apparent dc/infrared mobility dichotomy, we made use of a recently-developed spectroscopic-analysis procedure. Using direct numerical-solution techniques for the reflectance (R) and transmittance (T) equations of a multilayer structure, we analyzed our infrared R and T results for highly-doped films having hole concentrations from 2 × 10¹⁹ up to 1.4 × 10²⁰ cm⁻³. The optical properties were determined for photon energies from 0.07 to 0.6 eV, in which region plasmon (intraband) and intervalenceband contributions are in competition. Our results for the optical absorption coefficient resolve two separate peaks located (at high doping) at about 0.1 and 0.2 eV. (The effective-plasmon model necessarily missed the two-peak character of the actual absorption spectrum.) By carrying out theoretical calculations of the intervalenceband (IVB) absorption processes for our dopings, we identify the peak near 0.2 eV with light-hole to heavy-hole IVB transitions, and we attribute the lower-energy peak to the hole plasmon. Our experimental absorption spectra are very well described by a model combining the intervalenceband contribution to the dielectric function with a plasmon contribution. The hole-plasmon parameters ωplasmon and γplasmon that we obtain for highly-doped p-GaAs yield an infrared mobility which (unlike the too-small IVB-entangled infrared mobility implied by the use of the usual effective-plasmon model) is in substantial agreement with the dc mobility. Therefore, in actuality, there is no dc/infrared mobility discrepancy. The discrepancy implied by the use of the usual, standard-operating-procedure, effective-plasmon model is a consequence of the inadequacy of that model for p-type semiconductors exhibiting intervalenceband infrared absorption. Raman-scattering measurements were carried out on the GaAs:C films. Only the phononlike coupled plasmon-phonon mode is observed. The non-occurrence of the plasmonlike mode is due to the large damping of the hole plasmon and the competition with strong Raman scattering by intervalenceband transitions among the heavy-hole, light-hole, and split-off bands. Analysis of the phononlike coupled mode, within the framework of the wavevector-dependent Lindhard-Mermin dielectric function, supports the hole properties that we determined by Hall and infrared studies. Photoluminescence measurements showed that the split-off band also participates in the photoluminescence of GaAs:C, giving rise to an above-bandgap emission band corresponding to transitions from the conduction band to the split-off valence band.
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    Performance of a Parallel Transport Code for Molecular Electronics Simulations
    Metz, Brent; Wienckowski, Justin; Ribbens, Calvin J.; Di Ventra, Massimiliano (Department of Computer Science, Virginia Polytechnic Institute & State University, 2002-02-01)
    We describe the sequential and parallel performance of a nonlinear transport simulation code. This code is used by researchers at Virginia Tech to investigate phenomena underlying the emerging eld of molecular electronics. The computational requirements of the code are summarized, and an initial distributed-memory parallel implementation of the code is evaluated. We conclude with several suggestions for improving the parallel performance and scalability of the code.
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    Polymer/Fullerene Photovoltaic Devices - Nanoscale Control of the Interface by Thermally-controlled Interdiffusion
    Drees, Martin (Virginia Tech, 2003-05-12)
    In this thesis, the interface between the electron donor polymer and the electron acceptor fullerene in organic photovoltaic devices is studied. Starting from a bilayer system of donor and acceptor materials, the proximity of polymer and fullerene throughout the bulk of the devices is improved by inducing an interdiffusion of the two materials by heating the devices in the vicinity of the glass transition temperature of the polymer. In this manner, a concentration gradient of polymer and fullerene throughout the bulk is created. The proximity of a fullerene within 10 nm of any photoexcitation in the polymer ensures that the efficient charge separation occurs. Measurements of the absorption, photoluminescence, and photocurrent spectra as well as I-V characteristics are used to study the interdiffusion and its influence on the efficiency of the photovoltaic devices. In addition, the film morphology is studied on a microscopic level with transmission electron microscopy and with Auger spectroscopy combined with ion beam milling to create a depth profile of the polymer concentration in the film. Initial studies to induce an interdiffusion were done on poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) as the electron donor polymer and the buckminsterfullerene C60 as the electron acceptor. Interdiffused devices show an order of magnitude photoluminescence quenching with concomitant increase in the photocurrents by an order of magnitude. Variation of the polymer layer thickness shows that the photocurrents increase with decreasing thickness down to 70 nm due to charge transport limitation. The choice of layer thickness in organic photovoltaic devices is critical for optimization of the efficiency. The interdiffusion process is also monitored in situ and a permanent increase in photocurrents is observed during the heat treatment. Transmission electron microscopy (TEM) studies on cross sections of the film reveal that C60 interdiffuses into the MEH-PPV bulk in the form of >10 nm clusters. This clustering of C60 is a result of its tendency to crystallize and the low miscibility of C60 in MEH-PPV, leading to strong phase separation. To improve the interdiffusion process, the donor polymer is replaced by poly(3-octylthiophene-2,5-diyl) (P3OT), which has a better miscibility with C60. Again, the photocurrents of the interdiffused devices are improved significantly. A monochromatic power conversion efficiency of 1.5 % is obtained for illumination of 3.8 mW/cm2 at 470 nm. The polymer concentration in unheated and interdiffused films is studied with Auger spectroscopy in combination with ion beam milling. The concentration profile shows a distinct interface between P3OT and C60 in unheated films and a slow rise of the P3OT concentration throughout a large cross-section of the interdiffused film. TEM studies on P3OT/C60 films show that C60 still has some tendency to form clusters. The results of this thesis demonstrate that thermally-controlled interdiffusion is a viable approach for fabrication of efficient photovoltaic devices through nanoscale control of composition and morphology. These results are also used to draw conclusions about the influence of film morphology on the photovoltaic device efficiency and to identify important issues related to materials choice for the interdiffusion process. Prospective variations in materials choice are suggested to achieve better film morphologies.
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    Role of heating and current-induced forces in the stability of atomic wires
    Yang, Z.; Chshiev, Mairbek; Zwolak, Michael; Chen, Y. C.; Di Ventra, Massimiliano (American Physical Society, 2005-01)
    We investigate the role of local heating and forces on ions in the stability of current-carrying aluminum wires. For a given bias, we find that heating increases with wire length due to a redshift of the frequency spectrum. Nevertheless, the local temperature of the wire is relatively low for a wide range of biases provided good thermal contact exists between the wire and the bulk electrodes. On the contrary, current-induced forces increase substantially as a function of bias and reach bond-breaking values at about 1 V. These results suggest that local heating promotes low-bias instabilities if dissipation into the bulk electrodes is not efficient, while current-induced forces are mainly responsible for the wire breakup at large biases. We compare these results to experimental observations.