Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M26: Nanowires: Mechanical, Electronic, Optical, and Transport Properties |
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Sponsoring Units: DCMP Chair: Luis Balicas, Florida State University Room: 204A |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M26.00001: Synthesis and Photoresponse of Hydrothermally Grown ZnO Nanowires Ahmed Al-Asadi, Luke Henley, Sujoy Ghosh, Abdiel Quetz, Igor Dubenko, Nihar Pradhan, Luis Balicas, Saikat Talapatra, Naushad Ali We will present our results of hydrothermally grown ZnO nanowires (NWs) using ZnO nanoparticles as seeds. The seed layer was prepared simply by spraying commercially obtained ZnO nanoparticles with a diameter $\sim $20 nm mixed with Isopropanol (IPA) onto Si/SiO2. A detail structural characterization of the ZnO nanowires indicate that highly crystalline nanowires with an average diameter 45-55 nm and length 1 $\sim$ 1.3 $\mu$m with an optical band gap of $\sim$3.7 eV can be obtained using this method. We also show that a significant amount of photocurrent is generated in these nanowires when illuminated with UV radiation. The variation of photo response with light intensity as well as the nature of rise and decay of photocurrent will be presented and discussed in the light of available theoretical models. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M26.00002: Controlling the electrical impedance of nanomechanical oscillators by electromigration Fengpei Sun, Jie Zou, Ho Bun Chan Detection of nanomechanical motion is of fundamental and practical interests. For doubly clamped nanobeams, a common method is the magnetomotive reflection technique. However, this technique usually suffers from large signal background due to the mismatch of the electrical resistance ($R_{e})$ of the oscillators to the impedance (50ohm usually) of the cables for detection. The large signal background precludes the possibility of driving the device into self-sustaining oscillations using a phase-locked loop. We develop a reproducible method of minimizing the signal background in the magnetomotive reflection technique. A gold nanowire with a junction in the middle is fabricated on the top of a doubly-clamped SixNy nanobeam via e-beam lithography. By passing a large direct current through the nanowire, migration of the gold atoms around the junction is activated due to the heat dissipated. An analog feedback loop is designed to maintain a stable process of electromigration until the target $R_{e}$ is reached. Initially $R_{e}$ is smaller than 50ohm. The motional impedance of the nanowire shifts the total impedance closer to 50ohm so that the resonance of the nanobeam appears as a dip on a large background in the amplitude spectrum. As $R_{e}$ is increased to near 50ohm, the background reaches a minimum, and the resonance of the nanobeam turns into a peak. Self-sustaining oscillations of the nanobeam are successfully achieved via a phase-locked loop in this case. As $R_{e}$ is further increased, the background becomes higher again. The dependence of the background signal on $R_{e}$ agrees with calculations. [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M26.00003: In-situ electromechanical properties of suspended Poly-pyrrole (PPy) nanotube using metal coated AFM tip manipulations Sang Wook Lee, Hakseong Kim, Sung Ho Jhang Electromechanical properties of individual suspended poly-pyrrole (PPy) nanotube were investigated. PPy nanotubes were positioned and suspended using the ac electrophoresis and micro transfer method. The metal coated atomic force microscope tip was used as a pressure source of the suspended PPy tube and, at the same time, one of the contact electrodes for measuring in-situ current-voltage characteristics while the PPy tube is under stressed. The resistance of the PPy tube was decreased in electromechanical measurements with increasing pressure using a metal coated AFM tip. 1.36 GPa of maximum contact pressure on the tip-tube contact was estimated to reduce the contact resistance for making to 6.8 ohm cm in a lateral configuration. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M26.00004: GPU Accelerated Quantum Transport Modelling of Realistic Large Cross-Section Silicon Nanobeams Mohammed Harb, Hong Guo Understanding the quantum transport properties of silicon nano-beams or -wires is important for many practical applications. But for nanobeams with a cross section larger than 10 nm square or so, typical transport modelling techniques based on Green's functions and atomistic DFT Hamiltonians become very computationally demanding. The computational burden becomes even greater when electron scattering with phonons is included. In this work we report hardware acceleration of the computational algorithm using a cluster of CPUs and GPUs working together in a heterogenous computing scheme. The GPU accelerated transport technique is implemented for atomistic tight-binding Hamiltonian within the non-equilibrium Green's function formalism. As examples, we calculate charge transport properties of realistic large cross-section Si nanobeams with several defect configurations, and report how local density of states can be significantly perturbed by the presence of the atomic impurity. [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M26.00005: Thermal properties of semiconductor nanowires using electromechanics John Mathew, Mandar Deshmukh We present low temperature measurements of thermal expansion and diffusivity of InAs nanowires using electromechanical response of the nanowire. To realize this, we fabricate InAs nanowire devices in doubly clamped, suspended field effect transistor geometry, and use direct radio frequency detection of the nanowire electromechanics. The resonant frequency of the nanowire is studied at different temperatures as a function of applied source-drain bias voltage. Due to joule heating and the non-monotonic thermal expansion of InAs we observe positive and negative dispersion of the nanowire resonant frequency with applied bias. We also study dynamical response to heating to understand the thermal diffusivity in these sub-micron structures. We show that the resonant frequency of NEMS devices acts as a good indicator of thermal properties of semiconducting materials providing information on thermal conductivity, expansion and diffusivity. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M26.00006: Optimizing electronic characteristics of SnO2 nanobelts for FET devices Timothy Keiper, Jorge Barreda, Jim P. Zheng, Peng Xiong Oxide semiconductors are attractive channel materials for nanoscale field effect transistors (FETs), especially for applications in chemical and biological sensing. Here we focus on optimizing the current-voltage relationship and gating response of SnO$_2$ nanobelt (NB) FETs, a widely used sensor material. The NBs are grown by a physical vapor-liquid-solid process, with dimensions are desirable for FET application, however the electrical characteristics of the as-grown materials are often not optimum for high-performance FETs. We have developed a multistep thermal annealing procedure in low vacuum ranging from 150 to 250 $^{\circ}$C and oxygen environment at atmospheric pressure and 600 $^{\circ}$C to increase the conductivity by more than 10$^3$. The multistep annealing process is necessary to consistently obtain FETs with low resistance, Ohmic contacts which differ by $<$5\%. Utilizing a typical backgate geometry the device is transitioned from the on state to the off state over a gate voltage range of less than 30 V through a thick 250 nm SiO$_2$ dielectric layer. The On/Off ratio is as large as 10$^4$. We surmise the oxygen annealing effectively activates the NBs while the vacuum annealing both helps clean the material and tune the carrier density at the surface, affecting metallization. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M26.00007: Optoelectronic Properties of Hybrid Titania Nanotubes/Hematite Nanoparticles Structures Lili Wang, Eugen Panaitescu, Latika Menon TiO$_{2}$/Fe$_{2}$O$_{3}$ nanostructures are becoming promising alternatives for improving cost effectiveness (in {\$}/W) of emerging photovoltaic devices such as dye sensitized or metal-insulator-semiconductor solar cells, combining the low cost, earth abundance and stability of the materials with the enhanced performance offered by the nanoscale architecture. We investigated novel, high quality titania/hematite composites, namely hematite nanoparticle decorated titania nanotube arrays, which were obtained by a simple, inexpensive and easily scalable two-step process, electrochemical anodization of titanium followed by forced hydrolysis. The titania nanotubular scaffold provides a large active surface area, while the iron oxide nanoparticles significantly broaden the light absorption range into the visible region. The morphological and structural characteristics of the samples were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The light absorption efficiency was measured by diffuse reflectance spectroscopy (DRS), and the optoelectronic behavior of the hybrid structures was analyzed by IV measurements under simulated solar illumination. The influence of the synthesis process and the structure design on the photovoltaic performance is currently investigated for optimal device prototyping. [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M26.00008: Structural and electronic properties of CdS/ZnS core/shell nanowires: A first-principles study Hyo Seok Kim, Yong-Hoon Kim Carrying out density functional theory (DFT) calculation, we studied the relative effects of quantum confinement and strain on the electronic structures of II-IV semiconductor compounds with a large lattice-mismatch, CdS and ZnS, in the core/shell nanowire geometry. We considered different core radii and shell thickness of the CdS/ZnS core/shell nanowire, different surface facets, and various defects in the core/shell interface and surface regions. To properly describe the band level alignment at the core/shell boundary, we adopted the self-interaction correction (SIC)-DFT scheme. Implications of our findings in the context of device applications will be also discussed.\\[4pt] This work was supported by the Basic Science Research Grant (No. 2012R1A1A2044793), Global Frontier Program (No. 2013-073298), and Nano-Material Technology Development Program (2012M3A7B4049888) of the National Research Foundation funded by the Ministry of Education, Science and Technology of Korea. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M26.00009: Surface States and Transport in Bismuth Nanowires with Strong Magnetic Field Shiang Fang, Bertrand Halperin, Efthimios Kaxiras Bismuth nanowires have attracted attention due to the enhancement of thermoelectricity in the one-dimensional geometry. Because of the large surface-bulk ratio and gapped bulk-like states, the transport in Bismuth nanowires is dominated by surface states, which may be probed by measurement in magnetic field $\sim$14T. However, due to the spiral motion of electrons between various nanowire facets, the electron would bounce between multiple wire surface with different crystal orientations. Shubnikov-de Haas oscillations in magneto-resistance in transport encode the convoluted information for different surface states. To study these effects and the mechanism for transport in strong magnetic field, we employ a density functional theory calculation for the band structure. Maximally-Localized Wannier Functions are used to construct an empirical tight-binding model, which provides numerically accurate results at reduced computational cost. In this way, the surface states in a semi-infinite geometry can be obtained from the efficient iteration of Green’s functions. The formation of Landau levels can also be studied by Peierls substitution. [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M26.00010: Transport through double quantum dots in Ge/Si core/shell nanowires Azarin Zarassi, Zhaoen Su, Dharmraj K. Patil, Sergey M. Frolov, Moira Hocevar, Minh Nguyen, Jinkyoung Yoo, Shadi A. Dayeh In the studies of spin qubits long dephasing times are crucial. Qubits made of materials with low abundance of nuclear spin, group IV semiconductors, have shown long spin coherence times. On top of this, the predicted strong spin-orbit interaction in the valence band of Ge/Si makes it a good platform to electrically manipulate spin states. We have formed stable and tunable double quantum dot by confining holes with the help of bottom gate electrodes in Ge/Si core/shell nanowires. Hole transport through the dots exhibits excited hole states from Zeeman splitting of which g-factors can be extracted. We are searching for spin blockade in the transport between double dots, which can be used to readout spin qubits and study spin-orbit interaction. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M26.00011: Intrinsic and extrinsic effects on electron-phonon coupling strength in individual ZnTe nanowires: The effects of laser annealing and metallic Te Jason Marmon, Tao Sheng, Haitao Zhang, Yong Zhang Electron-phonon coupling is typically studied as an intrinsic property for a given bulk material, and modifying the coupling has been explored in a nanostructure. We point out that the coupling strength can be easily perturbed both significantly and unintentionally. Nanowires, with their large surface-to-volume ratio, are more susceptible to extrinsic perturbations that affect coupling strength, although literature assumes that coupling is intrinsic. This work uses Raman spectroscopy under a near resonant condition to probe the coupling strength of individual ZnTe nanowires. Using the intensity ratio of the first and second order Raman peaks, R $=$ I$_{2LO}$/I$_{1LO}$, as a measure of the electron-phonon coupling strength (proportional to the Huang-Rhys factor), we find that the ratio can change greatly when varying either the sample or measurement condition, for instance, the presence of defects in the as-grown sample and their removal though laser illumination. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M26.00012: Single Schottky junction FETs based on Si:P nanowires with axially graded doping Jorge Barreda, Timothy Keiper, Mei Zhang, Peng Xiong Si nanowires (NWs) with a systematic axial increase in phosphorus doping have been synthesized via a vapor-liquid-solid method. Silane and phosphine precursor gases are utilized for the growth and doping, respectively. The phosphorous doping profile is controlled by the flow ratio of the precursor gases. After the as-grown product is ultrasonically agitated into a solution, the Si NWs are dispersed on a SiO$_{2}$ substrate with a highly doped Si back gate. Individual NWs are identified for the fabrication of field-effect transistors (FETs) with multiple Cr/Ag contacts along the NW. Two-probe and four-probe measurements are taken systematically under vacuum conditions at room temperature and the contribution from each contact and each NW section between adjacent contacts is determined. The graded doping level, produced by a systematic reduction in dopant density along the length of the NWs [1], is manifested in the regular increases in the channel and contact resistances. Our Si NWs facilitate the fabrication of asymmetric FETs with one ohmic and one Schottky contact. A significant increase in gate modulation is obtained due to the single Schottky-barrier contact. Characterization details and the applicability for sensing purposes will be discussed. \\[4pt] [1] D. E. Perea et al., Nature Nanotechnology 4, 315 (2009). [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M26.00013: Fabry Perot phenomena in nanowire cavities Michelle Tomczyk, Guanglei Cheng, Shicheng Lu, Joshua Veazey, Mengchen Huang, Patrick Irvin, Hyungwoo Lee, Sangwoo Ryu, Chang-Beom Eom, Jeremy Levy A solid-state geometry analogous to an optical Fabry-Perot cavity gives evidence for coherent transport on the order of microns through nanowires at the LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface. Conductive AFM lithography is used to create both the nanowires and the two confining barriers which define the cavity. These two barriers act as the primary scattering centers so that as the chemical potential is tuned through the conducting state of the device, partial reflections from the barriers interfere in the cavity, resulting in quasi-periodic oscillations of the conductance at low temperatures. Full and extended single-mode periodicity is not observed in all devices; however, the conductance oscillations are only observed in cavity structures, suggesting that the effects of the two manufactured barriers dominate over any random scattering sites from disorder. The conductance oscillations from interference of coherently scattered electrons give evidence for ballistic transport on much longer length scales than implied by mobility measurements in two-dimensional LAO/STO. [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M26.00014: Quantized conductance through reconfigurable 1D channels Shicheng Lu, Anil Annadi, Guanglei Cheng, Michelle Tomczyk, Mengchen Huang, Hyungwoo Lee, Sangwoo Ryu, Chang-Beom Eom, Patrick Irvin, Jeremy Levy In recent years, a high mobility two-dimensional electron gas LaAlO$_3$/SrTiO$_3$ (LAO/STO) system has become a model system to investigate various exotic ground states of condensed matter physics. This system can co-host superconductivity, magnetism, and strong spin-orbit coupling at 2D interfaces which led to predictions of exotic phenomena such as unconventional superconductivity, helical/chiral modes, and Majorana phases in these interfaces. In order to explore these exotic phases high quality 1D devices are desirable. We demonstrate the realization of a gate tunable quantum point contact (QPC) structure embedded in a LAO/STO nanowire created using conductive AFM lithography. We observe integer quantized conductance in the units of $e^2/h$ at high magnetic fields ($B=9$ Tesla, $T=50$ mK),a signature of the existence of 1D quantum channels. Significantly, we observe quantized conduction for nanowires as long as 1 $\mu$m, implying that transport is ballistic along the magnetic-field induced chiral edge states in these devices. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M26.00015: Magnetoconductance signatures of subband structure in semiconductor nanowires Gregory Holloway, Chris Haapamaki, Ray LaPierre, Jonathan Baugh Understanding the subband structure due to radial confinement in semiconductor nanowires can benefit technologies ranging from optical sensors to quantum information processing. An axial magnetic field couples to the orbital angular momentum, giving rise to non-trivial features in electronic transport as a function of magnetic field. Previous reports focused on conduction electrons confined to a thin shell near the nanowire surface, which lead to flux-periodic energies and conductance oscillations. Here, we calculate the eigenstates for more general radial potentials with moderate to low surface band bending such that electrons are distributed more uniformly across the nanowire cross-section. It is found that the energy spectrum becomes aperiodic in both gate voltage and magnetic field as the radial potential becomes flatter. The behavior of an energy level is dictated by its angular momentum, and this allows, in principle, each state to be identified based on its dependence on magnetic field and the chemical potential. We experimentally investigate a short-channel InAs nanowire FET in search of conductance features that reveal this subband structure. A quantitative measure for assigning conductance features to specific transverse states is introduced and applied to this device. [Preview Abstract] |
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