Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U26: Focus Session: Charge Transport in Nanostructures I |
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Sponsoring Units: DCP Chair: Troy van Voorhis, Massachusetts Institute of Technology Room: Colorado Convention Center 205 |
Thursday, March 8, 2007 8:00AM - 8:36AM |
U26.00001: Transport in nanoscale systems: hydrodynamics, turbulence, and local electron heating Invited Speaker: Transport in nanoscale systems is usually described as an open-boundary scattering problem. This picture, however, says nothing about the dynamical onset of steady states, their microscopic nature, or their dependence on initial conditions [1]. In order to address these issues, I will first describe the dynamical many-particle state via an effective quantum hydrodynamic theory [2]. This approach allows us to predict a series of novel phenomena like turbulence of the electron liquid [2], local electron heating in nanostructures [3], and the effect of electron viscosity on resistance [4]. I will provide both analytical results and numerical examples of first-principles electron dynamics in nanostructures using the above approach. I will also discuss possible experimental tests of our predictions. Work supported in part by NSF and DOE. \newline \newline [1] N. Bushong, N. Sai and M. Di Ventra, ``Approach to steady-state transport in nanoscale systems'' Nano Letters, 5 2569 (2005); M. Di Ventra and T.N. Todorov, ``Transport in nanoscale systems: the microcanonical versus grand-canonical picture,'' J. Phys. Cond. Matt. 16, 8025 (2004). \newline [2] R. D'Agosta and M. Di Ventra, ``Hydrodynamic approach to transport and turbulence in nanoscale conductors,'' cond-mat/05123326; J. Phys. Cond. Matt., in press. \newline [3] R. D'Agosta, N. Sai and M. Di Ventra, ``Local electron heating in nanoscale conductors,'' cond-mat/0605312; Nano Letters, in press. \newline [4] N. Sai, M. Zwolak, G. Vignale and M. Di Ventra, ``Dynamical corrections to the DFT-LDA electron conductance in nanoscale systems,'' Phys. Rev. Lett. 94, 186810 (2005). [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U26.00002: Molecular conductance simulations with a `hybrid' DFT-NEGF approach Alexander Prociuk, Barry Dunietz A time propagated DFT-NEGF methodology for describing molecular conductance through extended metal-molecule-metal systems is developed. This innovative method calculates transient currents in the presence of time dependent perturbations applied to the molecular junction. Steady state currents can be calculated in the presence of finite temporal perturbations. The electronic density, represented by the lesser GF, is recast into a form that expresses the temporal propagation of the energy spectrum. The effects of the potential biased metal electrodes are expressed with energy dependent `self-energy' terms. This results in a manageable and compact expression for the electron density. This density can be propagated by a specialized scheme that elucidates the transport properties of the system. Propagation, in the absence of an applied temporal perturbation, reduces to an alternative and novel NEGF transport methodology. In addition, applied perturbations can be propagated fully or approximated to any order in time dependent perturbation theory. Calculations are performed for various DFT functionals with a LANL2DZ ECP basis set. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U26.00003: Local electron heating in nanoscopic conductors Roberto D'Agosta, Na Sai, Massimiliano Di Ventra The electron current density in nanoscale junctions is typically several orders of magnitude larger than the corresponding one in bulk electrodes. Consequently, the electron-electron scattering rate increases substantially in the junction. This leads to local electron heating of the underlying Fermi sea [1] in analogy to the local ionic heating that is due to the increased electron-phonon scattering rates [2]. By using a novel hydrodynamic formulation of transport [3], we predict the bias dependence of local electron heating in quasi-ballistic nanoscale conductors [1], its effect on ionic heating [1], and the consequent observable changes in the inelastic conductance [4]. \noindent [1] R. D'Agosta, N. Sai and M. Di Ventra, accepted in Nano Letters (2006). \noindent [2] Y.-C. Chen, M. Zwolak, and M. Di~Ventra, Nano Lett. {\bf 3}, 1961 (2003); Nano Lett. {\bf 4}, 1709 (2004); Nano Lett. {\bf 5}, 621 (2005). M. J.~Montgomery, T. N. Todorov, and A. P. Sutton, J. Phys. Cond. Matt. {\bf 14}, 5377 (2002). \noindent [3] R. D'Agosta and M. Di Ventra, J. Phys. Cond. Matt. in press. \noindent [4] R. D'Agosta and M. Di Ventra, in preparation. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U26.00004: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U26.00005: Electroluminescence from individual CdS/CdSe nanowires contacted by poly(3-hexylthiophene) Kristin Maher, Lian Ouyang, Dong Yu, Yong-Joo Doh, Chun Yu, Hongkun Park Nanocrystal-polymer composites have been shown to exhibit interesting optoelectronic properties. However, only bulk properties of these materials have been measured, and the single nanocrystal-polymer interface is poorly understood. We report the fabrication and characterization of light-emitting transistors incorporating individual CdS/CdSe heterostructure nanowires and a thin film of poly(3-hexylthiophene) (P3HT). The nanowire is contacted at one end by a metal electrode and at the other by a thin film of P3HT, a $p$-type conducting polymer. The devices show rectifying current-voltage behavior and light emission can be observed at forward bias. The peak wavelength and the full width at half-maximum of the electroluminescence were 1.68 eV and 0.08 eV, respectively. The mechanism for light emission will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U26.00006: Photoconductivity and multiple exciton generation in arrays of coupled semiconductor nanoparticles Matthew Beard, Joseph Luther, Kelly Knutsen, Qing Song, Randy Ellingson, Arthur Nozik Three dimensional arrays of semiconductor nanocrystals (NCs) in p-i-n structures are a novel approach to solar energy conversion that offers the potential to control the microscopic charge generation, separation, and transport so as to maximize solar energy conversion efficiencies. A necessary characteristic of the NC arrays is that they exhibit very high conductivity for electrons and holes; this requires strong inter-NC electronic coupling and the subsequent formation of extended electronic states. Many factors, such as inter-NC spacing, site energy dispersion, NC size and shape, cross linking, and Coulomb charging determine the inter-NC coupling. In addition, efficient carrier transport in NC solids requires minimization of carrier loss processes such as surface trapping. All of these factors are highly interdependent. Time-resolved THz spectroscopy (TRTS) is a powerful experimental tool that measures both photoconductivity, in a non-contact fashion, and carrier dynamics simultaneously, with sub-picosecond temporal resolution. We report TRTS for a series of chemically treated PbSe NCs where the inter-NC separation has been varied in a systematic manner. We also report multiple exciton generation (MEG) QYs within the coupled arrays. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U26.00007: Photoconductivity of Nanotapes Self-assembled from meso-Tri(4-sulfonatophenyl)monophenylporphine A. L. Yeats, B. Massare, W. F. Smith, A. D. Schwab, J. C. de Paula, D. E. Johnston, A. T. Johnson We have shown$^{1}$ that meso-Tetra(4-sulfonatophenyl)porphine (TPPS$_{4})$ forms well-defined nanorods in acidic solution. Experiments on related molecules provide insight into the mechanisms for self-assembly and photoconduction. Meso-Tri(4-sulfonatophenyl)monophenylporphine (TPPS$_{3})$ has one fewer sulfonate group than TPPS$_{4}$, but is otherwise identical. Previous work$^{2}$ has shown that, when deposited onto substrates by rotary evaporation, it forms folded nano-ribbons. We have found that, when deposited via immersion and spin-drying, it forms tape-like aggregates of two distinct heights. The larger width of these nanotapes (compared to TPPS$_{4}$ nanorods) is expected from the smaller in-solution charge of the monomer. The TPPS$_{3}$ aggregates exhibit photoconductive properties very similar to those of TPPS$_{4}$. The aggregates ordinarily form in solution, but can also be surface-catalyzed. $^{1}$ A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004). $^{2}$ J. Crusats \textit{et al.,} Chem Commun. 1588 (2003). [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U26.00008: Inelastic electron tunneling -induced light emission from a metal - quantum dot - metal double barrier tunnel junction Antti Makinen, Alan Wan, James Long A double barrier tunnel junction formed by a scanning tunneling microscope (STM) tip together with the substrate-tethered quantum dots provides a flexible architecture to explore the characteristics of envisaged nanoparticle-based optoelectronic devices. STM-induced light emission (STM-LE) measurements of ligand-capped CdSe/ZnS quantum dots, tethered to a gold substrate, reveal a light generation mechanism, which is driven by inelastic electron tunneling (IET) at a threshold voltage. The threshold voltage for STM-LE is found to depend on the optical gap of the quantum dots, stipulating a simple energy conservation rule for light emission through IET. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U26.00009: Break-down of the density-of-states description of scanning tunneling spectroscopy in supported metal clusters Martin Garcia, Mario De Menech, Ulf Saalmann Low-temperature scanning tunneling spectroscopy allows to probe the electronic properties of clusters at surfaces with unprecedented accuracy. By means of quantum transport theory, using realistic tunneling tips, we obtain differential conductance curves which considerably deviate from the cluster's density of states. Our study explains the remarkably small number of peaks in the conductance spectra observed in recent experiments. We demonstrate that the unambiguous characterization of the states on the supported clusters can be achieved with energy-resolved images, which we are able to construct with a complete simulation of the experimental imaging procedure. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:48AM |
U26.00010: Measurement of the Conductance of Single Conjugated Molecules Invited Speaker: Electrical conduction through molecules depends critically on the delocalization of the molecular orbitals, and their weight on the metallic contacts. Thiolated conjugated organic molecules are therefore often considered as good candidates for molecular conductors. In such molecules the orbitals are delocalized throughout the molecular backbone, with substantial weight on the sulfur-metal bonds. However, their relatively small size, typically 1 nm, calls for innovative approaches to realize a functioning single molecule device. In this paper we report a new approach for contacting a single molecule and use it to study the effect of localizing groups within a conjugated molecule on the electrical conduction. Our method is based on synthesizing a dimer structure, consisting of two gold colloids connected by a di-thiolated short organic molecule, and electrostatically trapping it between two metal electrodes. We study the electrical conduction through three short organic molecules: A fully conjugated molecule, 4,4'-biphenyldithiol (BPD), 4,4'-biphenyletherdithiol (BPED) in which the conjugation is broken at the center by an oxygen atom, and 1,4-benzenedimethanethiol (BDMT), where the conjugation is broken near the contacts by a methylene group. We find that the oxygen in the BPED and the methylene groups in the BDMT suppress the electrical conduction relative to the BPD. [Preview Abstract] |
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