Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D24: Superlattices and Nanostructures: Electronic Properties I |
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Sponsoring Units: DCMP Chair: Alexander Govorov, Ohio University Room: Morial Convention Center 216 |
Monday, March 10, 2008 2:30PM - 2:42PM |
D24.00001: Recent Progress on Modeling H Passivated CdS Nanocrystals using ab initio techniques Chad Junkermeier, Jinling Zhou, James P. Lewis Spherical CdS nanocrystals with shells of H atoms are studied via an \textit{ab initio} tight-binding analysis. Starting from the bulk zinc blende structure of CdS, these nanocrystals undergo relaxation as the geometries optimize to configurations that minimize internal forces. H atoms are then attached, to passivate the surface, and then whole structure is relaxed. We will present our latest results. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D24.00002: Measurements of Charge Transport in Arrays of Lead Selenide Nanocrystals Kenneth MacLean, Tamar Mentzel, Scott Geyer, Venda Porter, Moungi Bawendi, Marc Kastner We report electrical transport measurements of self-assembled arrays of PbSe nanocrystals (NC). NCs $\sim $6.2 nm in diameter are colloidally synthesized and drop cast onto an inverted field effect structure. The NCs self assemble into hexagonal close-packed arrays with $\sim $2 nm inter-particle spacing. The current is immeasurable in as deposited arrays. After annealing at 400K for $\sim $30 minutes, the arrays become less ordered and the inter-particle spacing decreases to $\sim $1 nm as evinced from TEM images and glancing incidence small angle x-ray scattering experiments. As a result of these changes, the conductance increases by more than 6 orders of magnitude. We measure the current in these devices as a function of source-drain voltage, gate voltage and temperature. We find that the temperature dependence of the conduction is strong at zero-bias and grows weaker with application of a source-drain bias. This implies that the conductance is thermally activated and the field serves to reduce the activation energy. We also find that the gate modulates the activation energy to conduction. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D24.00003: Structural and Electronic Properties of IV-VI Semiconductor Nanodots Roman Leitsmann, Friedhelm Bechstedt The characterization of nanostructure properties versus dimension and surface passivation is of increasing importance for the nanotechnology. Especially the stoichiometry, geometry, and the electronic states of IV-VI semiconductor nanodots are of special interest [1,2]. We use ab initio methods to calculate structural and electronic properties of colloidal IV-VI semiconductor nanodots as a function of the dot diameter. A method to passivate the non-directional dangling bonds at the nanodot surfaces is derived and used to study the confinement effect on the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) states. In addition we take the influence of relativistic (spin-orbit coupling -- SOC ) and excitonic effects into account. While the SOC leads to a considerable decrease of the HOMO-LUMO gap, excitonic effects play a minor role. [1] JACS 128, 10337 (2006) [2] JACS 129, 11354 (2007) [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D24.00004: Density functional theory studies of core-shell semiconductor nanoparticle quantum dots Brent Walker, Shaun Hendy, Richard Tilley In going from the macroscale to the nanoscale, quantum-mechanical effects become increasingly important and may mean that nanostructures of a material exhibit very different properties from the corresponding bulk. This is especially noticeable in the case of the optical properties of semiconductor nanoparticles (or quantum dots), which display a number of remarkable features (including very distinct peaks, and tunability across a broad range of wavelengths), due to quantum confinement. Our work involves modeling Si-Ge core-shell nanoparticles using large-scale computer simulations based on the density functional and time-dependent density functional theories. These simulations in particular provide us with predictions of the geometric structures and optical absorption spectra of nanoparticles in an accurate and computationally efficient way, and allow us to study the systematic trends in these properties as the composition and size of the nanoparticle change. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D24.00005: Conduction Mechanism in Arrays of Lead Selenide Nanocrystals Tamar Mentzel, Kenneth MacLean, Scott Geyer, Venda Porter, Moungi Bawendi, Marc Kastner We perform transport measurements of a PbSe nanocrystal solid which serves as the channel of a field-effect transistor. We find that a simple model of hopping between intrinsic localized states describes the conduction mechanism. From the field effect, we see that the majority carriers are holes, which are thermally released from acceptor states. At low source-drain voltages, the activation energy for the conductivity is given by the energy required to generate holes plus the activation over barriers resulting from site disorder. At high source-drain voltages the activation energy is given by the former only. The thermal activation energy of the zero-bias conductance indicates that the Fermi energy is close to the highest-occupied valence level, the 1S$_{h}$ state, and this is confirmed by field-effect measurements, which give a density of states of approximately 8 per nanocrystal as expected from the degeneracy of the 1S$_{h}$ state. Using the Thomas-Fermi screening length in the NC solid, we find that the gate serves to modulate the charge density in the monolayer closest to the gate, while successive monolayers are screened from the field. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D24.00006: Quasi 1-D electronic structure of silver nanowires A. Sekharan, W. Zhao, F. Womack, F. Wang, O. Kizilkaya, R. Kurtz, P. Sprunger Epitaxial Ag nanowires have been found to self-assemble on Cu(110) at coverages exceeding 1.2 ML. The low energy electronic structure of these nanowires has been characterized by ARPES. Previous STM, LEED, and LEEM data reveal that the Ag nanowires grown on Cu(110) are approximately 2 nm ($\sim $12 nm) in height (width). The nanowires orient with the long axis parallel to the [\={ }110] substrate direction The ARPES results reveal that the valence bands within the Ag nanowire are strongly anisotropic with clear band dispersion in the along-wire direction, but no dispersion in the across-wire direction. ARPES identified two low-energy electronic bands, with strong dispersion close to the Fermi energy. The first band, which crosses the Fermi energy, suggests the metallic nature of nanowires. However, there is an avoided crossing of the second band, perhaps due to many-body effects. While discussing the quasi 1-D electronic structure, we will emphasize its connection to many-body effects and the one dimensional nature of nanowires. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D24.00007: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 3:54PM - 4:06PM |
D24.00008: Measurements of the conduction band energy surrounding individual InGaAs quantum dots by Cross-Sectional Ballistic Electron Emission Microscopy (XBEEM) C. Marginean, J.P. Pelz, S.Y. Lehman, J.G. Cederberg Cross-sectional ballistic electron emission microscopy (XBEEM) at room temperature was used to measure the conduction band (CB) energy in the ``wetting layer'' around and \textit{behind} cleaved InGaAs quantum dots (QDs). Samples with a $\sim $2 nm thick In$_{0.4}$Ga$_{0.6}$As layer embedded in n-doped (5 x 10$^{16}$ cm$^{-3})$ GaAs were grown by organometallic vapor phase epitaxy, then cleaved \textit{ex situ} and 7nm-thick Au Schottky barrier (SB) contacts deposited on the cleaved edge using a shadow mask [1]. With reverse bias $V_{rev}$ = 0V, Schottky barrier heights (SBHs) over different QDs were measured to range from $\sim $0.78 eV - 0.82 eV, compared to $\sim $ 0.84 eV over the wetting layer next to the QDs and $\sim $0.913 eV SBH over the adjacent GaAs. With $V_{rev}$ = 1V, the SBH over the QDs were reduced by $\sim $30 -- 50 meV compared to a much smaller ($<$ 5 meV) measured decrease over the GaAs due to image force lowering, indicating that SBH over the QDs was due to the CB of the wetting layer at an estimated depth of 6 -- 9 nm \textit{behind} the QDs. The XBEEM transmission over the QDs was also strongly enhanced by the applied reverse bias, for reasons that are not yet clear. Work supported by NSF Grant No. DMR-0505165. [1] C. Tivarus \textit{et al}., PRL \textbf{94}, 206803 (2005). [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D24.00009: Shifting the Reference: Co on Cu(775) M.B. Yilmaz, S. Wang, K.R. Knox, N. Zaki, J.I. Dadap, R.M. Osgood, T. Valla, P.D. Johnson Bimetallic metal stepped surfaces enable easy and controlled variation of surface electronic structure. Use of vicinal substrates varies the electronic structure compared to that of flat surfaces directly as a result of the ordered step array and indirectly by affecting the growth mode of the adlayer. We have used ARUPS measurement to probe the coverage-dependent electronic structure of the Co/Cu(775). The regular step structure on our substrate gives rise to Umklapp features and shift in the surface state binding energy ($\Delta $E=110 meV). It also displays a shift in the reference frame, which is in agreement with earlier vicinal-angle guidelines. However, in the presence of even 0.03 ML Co, a dramatic shift in the reference plane occurs, which is in accord with a change in the surface atomic structure. At 0.06 ML, new electronic features are observed, namely a quantum well state that results from the hybridization of Co with the Cu(775) and a nondispersive d-band due to island growth of Co. This latter band smoothly evolves into a Co d-band similar to that previously observed on Co/Cu(111). [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D24.00010: Electronic nanoholography Christopher R. Moon, Laila S. Mattos, Brian K. Foster, Gabriel Zeltzer, Hari C. Manoharan We have developed a quantum holographic method to advance information density beyond the areal limits set by the discreteness of matter. We present experiments on information encoding using nanoscale writing with degenerate two-dimensional electrons. We show ``pages'' (letters encoded at specific energies) materialized by precisely engineering electron scattering environments with the tip of a scanning tunneling microscope. We then demonstrate that multiple pages can be encoded into the same region of space, using energy as a third holographic dimension. This form of holography produces non-volatile subnanometer features, tens of times smaller than the most precise optical or scanned-probe lithography, and information densities exceeding 5 bits per square nanometer. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D24.00011: Exploiting Resonances in Laser Photoemission J.I. Dadap, M.B. Yilmaz, K. Knox, N. Zaki, R.M. Osgood, P.D. Johnson Laser photoemission is attracting new interest due to its ability to increase photoemission probe depth and to gain insight into surface-electron dynamics. We present new data on the use of energy levels that are resonant with the excitation-photon energy. Our surface system is the regular array of nanometer-scale steps on Cu(775), which yield information on surface electron confinement. We probe this nanostructured system with 2-photon photoemission. Our tunable, fs optical parametric amplifier (OPA) source allows resonance mapping with photon energies $\sim $ 4.2-4.6 eV to obtain a comprehensive map of the unoccupied state manifold. We observe, in addition to the surface state and image states, bandfolding from Umklapp due to the periodic steps, and, for the first time, the existence of a weak unoccupied state. The OPA allows observation of even this relatively weak unoccupied state as well as the rapid interrogation of electronic structure. The origin of this intermediate state is discussed. In addition, we present the intensity dependence of the measured linewidth and the position of the resonances. [Preview Abstract] |
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