Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V26: Focus Session: Charge Transport in Nanostructures II |
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Sponsoring Units: DCP Chair: Hongkun Park, Harvard University Room: Colorado Convention Center 205 |
Thursday, March 8, 2007 11:15AM - 11:51AM |
V26.00001: Electronic transport in semiconductor nanowires: physics studies and possible device applications Invited Speaker: Semiconductor nanowires are attractive for physics as well as for applications due to the highly ideal character of their electronic and structural properties. We grow our III-V nanowires by what can be described as guided self-assembly, by which we can accurately control location as well as dimensions of epitaxially nucleated nanowires. The level of control of growth allows controlled formation of axial as well as radial heterostructures. I will describe studies of charge transport via single, double and multiple quantum dots positioned inside InAs/InP nanowires. Such studies have allowed detailed studies of the addition of electrons one-by-one, from the very first electron into an empty quantum dot to the addition of up to 50 electrons. By replacing the one-dimensional emitter by a small quantum dot in a double-dot configuration, the discrete character of the injecting state allows ever more detailed spectroscopic studies of the charge additions to the second dot. Comparisons will be made with transport through quantum dots defined by tunnel barriers induced via gating techniques. Finally, a recently developed technique for the formation vertical wrap-gate field-effect transistors around InAs nanowires will be described, suggesting interesting opportunities for the realization of high-speed and low-power transistors and circuits. The geometrical design of such nanowire wrap-gate field-effect transistors, offers exciting ways of formation of ultra-short transistor gate-lengths as well as the use of heterostructures to further enhance the performance of such devices. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V26.00002: Charge Transport in Semiconductor Nanocrystal Solids Dmitri Talapin, Elena Shevchenko, Jong Soo Lee, Jeffrey Urban, David Mitzi, Christopher Murray Self-assembly of chemically-synthesized nanocrystals can yield complex long-range ordered structures which can be used as model systems for studying transport phenomena in low-dimensional materials [1]. Treatment of close-packed PbSe nanocrystal arrays with hydrazine enhanced exchange coupling between the nanocrystals and improved conductance by more than ten orders of magnitude compared to native nanocrystal films [2]. The conductivity of PbSe nanocrystal solids can be switched between n- and p-type transports by controlling the saturation of electronic states at nanocrystal surfaces. Nanocrystal arrays form the n- and p-channels of field-effect transistors with electron and hole mobilities of 2.5 cm$^{2}$V$^{-1}$s$^{-1}$ and 0.3 cm$^{2}$V$^{-1}$s$^{-1}$, respectively, and current modulation I$_{on}$/I$_{off} \quad \sim $10$^{3}$-10$^{4}$. The field-effect mobility in PbSe nanocrystal arrays is higher than the mobility of organic transistors while the easy switch between n- and p-transport allows realization of complimentary circuits and p-n junctions for nanocrystal-based solar cells and thermoelectric devices. [1] E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O'Brien, C. B. Murray. \textit{Nature} \textbf{439}, 55 (2006). [2] D. V. Talapin, C. B. Murray. \textit{Science} \textbf{310}, 86 (2005). [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V26.00003: Tuning the Height of the Tunnel Barrier in Colloidal Semiconductor Nanoparticle Films Venda Porter, Scott Geyer, Jonathan Halpert, Moungi Bawendi, Tamar Mentzel, Marc Kastner Much of the work in the field of charge transport through arrays of semiconductor nanoparticles has focused on improving the conductivity by tuning the organic ligand spacer between particles. We present a study in which we enhance the conductivity in nanoparticles films by instead tuning the energetic height of the tunnel barrier by removing the organic ligand spacer and tuning the inorganic shell around each particle. Experimentally, we modify the height of the tunnel barrier by depositing an array of core/shell nanoparticles and burning away all of the organic ligands. The height of the tunnel barrier is now the energy difference between the conduction band of the core and the conduction band of the shell, rather than the much larger energy difference between the conduction band of the nanoparticle and the LUMO of the organic ligand. In addition, this method may reduce the impact of surface states on conductivity as the shell may provide better passivation than organic ligands that may leave surface trap sites unbound. These unpassivated sites can trap charge carriers, lowering the mobility in nanoparticles films. The reduction of charge trapping is also critical to raising the efficiency of nanoparticle solar devices. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V26.00004: Transport measurements of semiconductor nanocrystals and nanorods using nanoscale devices. Claudia Querner, Michael D. Fischbein, Marija Drndic Efficient charge transport through nanocrystal arrays is important for many applications in electronics or optoelectronics. Various parameters can influence the transport in nanocrystals. Beside the material itself, other parameters such as shape (spherical, rod-shaped or branched structures), surface capping (insulating or electroactive surface-ligands), as well as the nanocrystal assembly may affect the observed transport phenomena. We carry out transport measurements of semiconductor nanoparticles using devices made on silicon nitride membranes. This approach enables both transport measurements and device imaging using high-resolution transmission electron microscopy, allowing a direct correlation of the measured transport phenomena with the local structure of the nanomaterial on the device surface. We will discuss the effects that we observe by varying parameters such as shape, size and surface capping of the nanoparticles, in particular CdSe, as well as temperature and photo-excitation. This work was supported by ONR Young Investigator Award (N000140410489), NSF Career Grant (DMR-0449553), NSF NSEC Grant (DMR-0425780), and NSF-IGERT (DGE 022166). [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V26.00005: Electron Transport in Arrays of Lead Selenide Nanocrystals Tamar Mentzel, Venda Porter, Scott Geyer, Sophie Charpentier, Moungi Bawendi, Marc Kastner We report on measurements of electron transport in self-assembled arrays of PbSe nanocrystals (NCs). NCs $\sim $8 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 $1.5 nm interdot spacing after annealing. The field-effect device enables us to measure the dependence of current on gate voltage (V$_{g})$ as well as source-drain voltage (V$_{ds})$. At high temperature we find that the conductance is exponentially dependent on both V$_{ds}$ and temperature. At low temperature the conductance is still exponentially dependent on V$_{ds}$, but is independent of temperature indicating a tunneling mechanism. While the conductance is independent of V$_{g}$ at high temperatures, it decreases with V$_{g}$ at low temperature suggesting that holes are the dominant carriers. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V26.00006: Charge Transport in Magnetite Nanoparticle Arrays Seongjin Jang, Hao Zeng Charge transport properties of magnetite (Fe$_{3}$O$_{4})$ nanoparticle arrays were studied as a function of annealing conditions. These arrays were prepared by self-assembling chemically synthesized nanoparticles with micro-gaps between lateral electrodes. Annealing removed surfactant molecules and varied the interparticle spacing systematically. Arrays annealed under 200 $^{o}$C are insulating. Arrays annealed between 200 $^{o}$C to 500 $^{o}$C show thermally assisted tunneling behavior, with the tunneling barrier decreasing with increasing annealing temperatures. Above 500 $^{o}$C, a transition from tunneling to hopping mechanism is observed. Magnetoresistance decreases with increasing annealing temperature. For the hopping samples, Verway transition is observed from both the resistivity and magnetoresistance measurements. Work supported by NSF DMR 0547036 [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V26.00007: Single Step Growth and Low Resistance Interconnecting of Metallic Nanowires Bret Flanders, Birol Ozturk We present an innovative approach to nanowire growth and interconnecting with external circuitry. Depositing salt-solution over a pair of on-chip electrodes and applying an alternating voltage induces the growth of metallic nanowires between the electrode tips. The voltage-signal provides sensitive control over the metal deposition process. For example, precise specification of the nanowire-diameter is attained through the frequency $\omega $ of the alternating voltage that induces the wire-growth process. For indium wires, increasing $\omega $ from 0.5 to 3.5 MHz increases the growth velocity of the wires from 11 to 78 $\mu $m/s and reduces their diameter from 770 to 114 nm. Gold wires exhibit diameter-tunability that extends below 100 nm. By the feedback-controlled application of the alternating voltage, it becomes possible to produce electrode-nanowire-electrode assemblies with contact-resistances of less than 25 $\Omega $, which would not be possible were the voltage terminated manually. This combination of capabilities enables study of the intrinsic transport properties of metallic nanowires. An area of particular interest is the contribution of electron-surface scattering to the total resistivity, an effect that is expected to increase with decreasing diameter. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V26.00008: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V26.00009: Electronic-structure and quantum conductance of pristine and defective graphene layers and ribbons Giovanni Cantele, Young-Su Lee, Domenico Ninno, Nicola Marzari Graphene has recently emerged as a fascinating alternative to carbon nanotubes as a subject both of fundamental research and of promising technological applications. In this work, we study the electronic structure and the transport properties of graphene layers and of graphene ribbons in the presence of several defects - from vacancies to topological defects to substitutional impurities. Very large systems with random distributions of defects are treated fully from first-principles and with chemical accuracy thanks to a formulation that combines density-functional theory and maximally-localized Wannier functions \footnote{Young-Su Lee et al, Phys. Rev. Lett. 95, 076804 (2005)}. Our results are also compared with previous tight-binding calculations, when available. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 2:03PM |
V26.00010: Theory of transport through molecular magnets Invited Speaker: Quantum transport through single molecular magnets (SMM) is starting to become a new exciting field in molecular spin electronics. Recent experiments [1,2] have shown that magnetic excitations can be identified in transport measurements and that NDC effects and complete current suppression can be explained by charge dependent anisotropies. Recent theoretical investigations [3,4,5] are presented which demonstrate fingerprints of quantum tunneling of magnetization (QTM). For weak tunneling, the violation of spin-selection rules leads to the occurence of fake resonances with temperature-dependent position [3]. For strongtunneling, it is show that a pseudo spin-1/2 Kondo effect is induced by QTM. If the Kondo temperature T$_K$ is smaller than the distance to excited magnetic states, selection rules depending on spin and symmetry of the SMM are derived for the Kondo effect to occur [4]. If T$_K$ exceeds the anisotropy barrier, it is shown that a reentrant Kondo effect can be induced by application of a longitudinal magnetic field for SMM with half-integer or integer spin [5]. This effect can be used for transport spectroscopy of the various anisotropies characterizing a SMM. \newline \newline [1] H.B. Heersche et al., Phys. Rev. Lett. 96, 206801 (2006). \newline [2] Moon-Ho Jo et al., Nano Lett. 6, 2014 (2006). \newline [3] C. Romeike, M.R. Wegewijs, H. Schoeller, Phys. Rev. Lett. 96, 196805 (2006). \newline [4] C. Romeike, M.R. Wegewijs, W. Hofstetter, H. Schoeller, Phys. Rev. Lett. 96, 196601 (2006). \newline [5] C. Romeike, M.R. Wegewijs, W. Hofstetter, H. Schoeller, to be published in Phys. Rev. Lett., cond-mat/0605514. [Preview Abstract] |
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