Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J32: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures: Nanowires |
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Sponsoring Units: DMP Chair: Leigh Smith, University of Cincinnati Room: C144 |
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J32.00001: Finite Size Effects in Electrical Transport in Nanowires and Nanowire-like Devices. Invited Speaker: Semiconductor nanowires posses many unique qualities including high crystallinity, simple growth techniques compatible with variety of low cost substrates, increased ability to accommodate strain, and nanoscale dimensions not easily accessible by `top-down' lithographic means. These and other interesting characteristics have made nanowires an attractive topic for fundamental research, as well as for potential applications in nanoelectronics, photonics, sensors, and more recently energy conversion and storage. Most of these applications rely on charge transport, which can be profoundly affected by the high aspect ratio, high surface to volume ratio and nanoscale diameter typical of semiconductor nanowires. Properly identifying the factors that influence electrical transport characteristics is important for device design but also because extraction of material parameters such as the mobility relies on analysis with specific models. In my talk I will discuss several specific examples where nanoscale dimensions and geometry profoundly affect transport and device characteristics, including GaN and GaN/III-N core/shell nanowires; Si radial pn-junction photovoltaics; and all-nanowire Li-ion batteries.. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J32.00002: Photocurrent Spectroscopy of single ZB, WZ InP Nanowire devices K. Pemasiri, S. Perera, A. Wade, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, S. Paiman, Q. Gao, H.H. Tan, C. Jagadish Photocurrent spectroscopy was performed on single InP nanowire devices having either zinc-blende (ZB) or wurtzite (WZ) crystal structures at 300~K and 10~K. Photolithography was used to fabricate Ohmic Ti/Al metal contact pads separated by 5 $\mu $m. Using a monochromatic white light set up or a tunable (1.30 to 1.55 eV) CW laser, the photocurrent is measured as a function of bias voltage and excitation energy. At room temperature, the lowest energy band of In WZ (1.408~eV) is found to be 70 meV above the ZB band gap (1.338~eV), consistent with previous photoluminescence measurements. At low temperatures (10 K), the ZB device shows strong evidence for a broadened excitonic resonance peak at 1.432 eV and the WZ device shows three excitonic peaks at 1.504 eV, 1.56 eV, and 1.65 eV corresponding to the A,B and C valence band energies, respectively, which coincide with recent photoluminescence excitation measurements. Support for this work was provided by the NSF ({\#}0701703, {\#}0806700 and {\#}0806572) and the Australian Research Council. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J32.00003: Photoresponse of solution-synthesized PbSe nanowire devices Todd Brintlinger, Edward E. Foos, Joseph G. Tischler, Janice E. Boercker, Thomas J. Zega, Rhonda M. Stroud, Steve C. Erwin PbSe nanowires are of interest for photovoltaic applications due to their variable band gap in the near infrared and potential for efficient multiexciton generation. Contributing to this effort, we present our ongoing studies of the photoresponse of PbSe nanowire devices. These materials are synthesized in solution by reaction of Pb and Se precursors in the presence of stabilizing organic ligands, and possess 10-20 nm diameters with lengths $>$1 $\mu $m. The nanowires are then dispersed on transparent silicon nitride membranes, allowing for both transmission electron microscopy and optical spectroscopy of individual devices. Devices show up to two-fold increases in current under illumination, with current density in the $\sim $1x10$^{9}$ A/m$^{2}$ range. Electrical transport and characterization of nanowires will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J32.00004: Energy and Spatially Resolved Measurements of Plasmonically Enhanced Photocurrent in a Single Si Nanowire with Au Nanoparticles Jerome Hyun, Lincoln Lauhon Hybrid assemblies of nanowires and metallic particles have attracted great interest because of their potential as light harvesting systems. Optoelectronic measurements of the most basic light absorbing unit in such systems, consisting of a single nanowire and plasmonic particles, would provide further guidance for performance optimization schemes. Here, we present spatially and energy resolved photocurrent measurements across the visible spectrum on a Si nanowire device with Au nanoparticles using a confocal scanning microscope and a tunable wavelength laser source. A 50 nm diameter nanowire was used due to its monotonic optical response in the wavelength range of interest, and 50 nm size Au nanoparticles were selected in order to neglect the effects of Mie scattering. The photocurrent is shown to depend on the azimuthal location of the nanoparticles on the nanowire. Nanoparticles resting on the substrate adjacent to the nanowire can significantly modify the absorption with a strong polarization-dependent plasmonic response while nanoparticles resting directly in the line of sight between the nanowire and light source show minimal contribution to the photocurrent. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J32.00005: Charge injection and transport in nanowires Invited Speaker: Semiconductor nanowires show promise in electronic, optoelectronic, and sensing devices. To realize this promise, a fundamental understanding of charge injection and electronic transport in these novel nanomaterials is necessary. In this presentation, I will discuss recent work that couples experiment and theory to address this topic. For example, in GaN and InAs nanowires, we achieve efficient charge injection and find that space-charge-limited currents are unusually strong. In contrast, charge transport across individual Au-nanoparticle/Ge-nanowire interfaces is injection-limited, and surprisingly, the conductance increases with decreasing nanowire diameter due to a dominance of electron-hole recombination. Furthermore, we find that transport in GaAs nanowires is governed by charge traps, which can be activated to reveal the nature of the charge injection at the contacts. More generally, our results indicate that a broad range of electronic transport regimes can be observed in semiconducting nanowires depending on the particular material system and growth process. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J32.00006: Photocurrent spectroscopy of CdS nanosheets P. Kumar, A. Wade, H.E. Jackson, L.M. Smith, J. Yarrison Rice, Y.-J. Choi, J.-G. Park We study the photocurrent from photoexcited charge carriers in CdS nanosheet (NS) structures. Metal-semiconductor-metal nanodevices are made with both Schottky and Ohmic contacts using photolithography followed by Ti/Al (20nm/200nm) metal evaporation and lift-off. Ohmic contacts are formed by Ar ion bombardment before the metal deposition to create donor sulfur vacancies which increases the electron concentration. Photocurrent spectra using a white light source filtered by a monochrometer show excitonic resonances at low temperatures corresponding to each of the A, B, and C hole bands. The photocurrent increases linearly with power for above gap excitation, and nonlinearly (quadratic) with laser power for below gap excitation, consistent with two-photon absorption with a nonlinear coefficient of $\beta $ = 2 cm/GW. A wavelength dependence of the photocurrent with sub-band gap excitation to find the resonances and hence band structure is in progress. We acknowledge the financial support of the National Science Foundation through grants DMR-0806700, 0806572 and ECCS-0701703, and the KIST institutional research program 2E21060R. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J32.00007: Electronic Devices with Dichalcogenide Nanolayers Andras Kis, Branimir Radisavljevic, Mohamed Benameur, Jacopo Brivio We have exfoliated single, two-dimensional layers 6.5 Angstrom thick from a number of dichalcogenide materials such as MoS2, using the micromechanical cleavage technique commonly used for the production of graphene. Optical microscopy together with AFM was used to characterize the nanolayers and establish optimal conditions for rapid identification of monolayers using optical methods. Our nanolayers are mechanically and chemically stable under ambient conditions. We have electrically contacted nanolayers using electron-beam lithography and fabricated field-effect transistors. Electrical transport measurements show that our devices have high on/off ratios and high mobilities. Our results indicate that two-dimensional dichalcogenide nanolayers could be interesting building blocks for nanoelectronic applications. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J32.00008: Electric Field Dependent Photocurrent Decay Length in Single Lead Sulfide Nanowire Field Effect Transistors Dong Yu, Rion Graham, Chris Miller, Eunsoon Oh We determined the minority carrier diffusion length to be $\sim $1 $\mu $m in single PbS nanowire field effect transistors by scanning photocurrent microscopy. PbS nanowires grown by the vapor-liquid-solid method were p-type with hole mobilities up to 49 cm$^{2}$/Vs. We measured a photo-response time faster than 14 $\mu $s with near-unity charge separation efficiency at the contacts. For the first time, we also observed a field dependent photocurrent decay length, indicating a drift dominant carrier transport at high bias. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J32.00009: Exciton Diffusion Measurements in III/V Nanowires using Spatially and Time Resolved Photoluminescence M.A. Fickenscher, L.M. Smith, H.E. Jackson, J.M. Yarrison-Rice, J.H. Kang, Q. Gao, H.H. Tan, C. Jagadish We present an optical investigation of transport in GaAs/AlGaAs core shell nanowires utilizing low temperature spatial and time resolved photoluminescence (PL). We use a solid immersion lens (SIL) to achieve a laser spot size and image resolution of 600 nm. With the laser spot fixed on the nanowire, the image of the wire is scanned across the entrance slit of the spectrometer taking time-decays at each point. Thus, we measure the spatial profiles of the exciton distribution in the wire as a function of time. We then extract the diffusion constant from the width squared of each spatial distribution as a function of time. The measured exciton diffusion constants are of the order of 100 cm$^{2}$s$^{-1}$, equivalent to a mobility of 100,000 cm$^{2}$ V$^{-1}$s$^{-1}$ by using the Einstein relation. These values are comparable to the best hole mobilities seen in modulation doped two dimensional GaAs/AlGaAs heterostructures. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J32.00010: Electron transport in coupled InGaAs quantum wires Vasyl Kunets, Sergey Prosandeev, Sabina Koukourinkova, Vitaliy Dorogan, Yuriy Mazur, Marcio Teodoro, Morgan Ware, Mourad Benamara, Peter Lytvyn, Gregory Salamo Remotely doped InGaAs/GaAs heterostructures were grown by molecular beam epitaxy on the (311)A plane of GaAs. Applying strain driven epitaxy on the (311)A GaAs surface, two-dimensional quantum wells (QW) and quasi-one-dimensional quantum wires (QWr) were formed by varying InGaAs coverage between 6 and 11 monolayers. Polarization dependent photoluminescence and electrical conductivity experiments revealed a remarkable anisotropy in the QWr samples, which was insignificant in the QWs, the dimensionality of which was confirmed by atomic force and cross-sectional transmission electron microscopies. The resulting complex behavior of the electric current anisotropy as function of InGaAs coverage, doping and temperature is explained through a multi-band conductivity model, which is supported by magneto-transport measurements at low and high magnetic fields along with the Hall effect theory in anisotropic media with multi-band conduction. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J32.00011: Probing charge density wave transition at the nanoscale in NbSe$_{2 }$using NEMS resonators Shamashis Sengupta, Hari Solanki, Vibhor Singh, Sajal Dhara, Mandar Deshmukh We study nanoelectromechanical (NEMS) resonators fabricated from suspended flakes of NbSe$_{2}$ (thickness $\sim $ 30--50 nm) to probe charge density wave (CDW) physics at nanoscale. Variation in elastic and electronic properties accompanying the CDW phase transition (around 30 K) are investigated simultaneously using the devices as self-sensing heterodyne mixers. Elastic modulus is observed to change by 10{\%}, an amount significantly larger than what had been reported earlier in the case of bulk crystals. We also study the modulation of conductance by electric field effect, and examine its relation to the order parameter and the CDW energy gap at the Fermi surface. [Preview Abstract] |
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