Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T11: Focus Session: Transport Properties of Nanostructures V: Wires and Films |
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Sponsoring Units: DMP Chair: Mark Hybertsen, Brookhaven National Laboratory Room: 305 |
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T11.00001: Platform for Measurement of Phonon Scattering from the Surface of Silicon Nanostructures. J. P. Sullivan, T. A. Friedmann, E. S. Piekos, S. L. Shinde, J. R. Wendt We've created a micro-platform for measuring thermal phonon surface scattering in single-crystal Si nanostructures, specifically specular-to-diffuse surface scattering in the long phonon mean-free-path regime. The platform consists of three suspended co-linear monocrystalline Si islands with the center island resistively heated and connected to its neighbors by Si nanoligaments (one ligament straight, the other bent). The ligaments have a blade-like geometry with length, width, and depth of 1000 nm, 100 nm, and 2500 nm, respectively. Heat conducts from the center island across the ligaments in proportion to the ligament thermal conductance, which is lower for the bent ligament due to increased surface scattering. Monte Carlo simulations indicate that the heat flux differs between straight and bent nanoligaments by 10{\%} for diffuse (rough surface) phonon reflection and by almost 40{\%} for specular (smooth surface) reflection. Acknowledgment: DOE BES Div. of Mat. Sci. {\&} Eng. and LDRD (Sandia is operated by Sandia Corp. for the US DOE's NNSA, contract DE-AC04-94AL85000). [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T11.00002: Thermal Effects in Precision Nano-Electronics Construction Stephen Johnson, Douglas Strachan The development of high precision nano-electronics requires a detailed understanding of the non-equilibrium thermal effects during their construction and use. To better understand the dynamics of these nano-scale thermal effects, we investigate nanowires and nano-scale junctions of various dimensions with applied electric currents. During the application of current, significant joule heating occurs which induces the structures to evolve through thermo- and electromigration processes. A distinct change in the symmetry of these processes indicates that different mechanisms occur at the various stages of evolution. The results are compared to detailed thermal modeling of our structures and have implications on the development of a wide range of novel nanoscale devices. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program, the University of Kentucky Center for Advanced Materials, and the University of Kentucky Center for Nanoscale Science and Engineering. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T11.00003: First-principles parameter-free calculations of electron mobilities in silicon: phonon and Coulomb scattering Oscar D. Restrepo, Kalman Varga, Sokrates T. Pantelides Mobility is a key factor in charge transport since it describes how the motion of an electron is affected by an applied electric field. As such, it is an important element in the design of new devices. Mobilities are generally modeled using methods that suppress atomic-scale detail (effective mass theory or bulk energy bands for electron velocities, empirical deformation potentials, macroscopic roughness, etc). Parameter fitting to experimental data is needed. As new technologies require modeling of transport at the nanoscale and new materials are introduced, predictive parameter-free mobility modeling is needed. The main scattering mechanisms that limit mobilities are due to phonon, ionized impurities, and interface roughness. A first-principles calculation of mobilities limited by atomic scale roughness with atomic-scale detail was reported recently [1]. We report the development of parameter-free quantum-mechanical methods to calculate scattering rates and electron mobilities limited by phonon and ionized-impurity scattering in a self-consistent way. Results for n-doped silicon are in good agreement with experimental data. This work was supported by NSF Grant ECS-0524655. [1] M. H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T11.00004: Reconstructing Fourier's law from disorder in quantum wires Massimiliano Di Ventra, Yonatan Dubi The validity of Fourier's law in nano-scale wires poses a fundamental theoretical challenge, with both scientific and technological implications. In this work, a novel theory of open quantum systems is used to study the local temperature and heat currents in metallic nanowires connected to leads at different temperatures. We show that for ballistic wires the local temperature is almost uniform along the wire and Fourier's law is invalid. By gradually increasing disorder, a uniform temperature gradient ensues inside the wire and the thermal current linearly relates to this local temperature gradient, in agreement with Fourier's law. Finally, we show that while disorder is responsible for the onset of Fourier's law, the non-equilibrium energy distribution function is determined solely by the heat baths. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T11.00005: A Single-Molecule Phonon Field-Effect Transistor Marcos Menezes, Brenda Moreira, Jordan Del Nero, Rodrigo Capaz Controlling phonons in the same way we control electrons in materials has been an old but elusive dream for physicists. In particular, it would be extremely desirable to control the thermal (phonon) flux between two reservoirs using a gate electric field, i.e., to construct a field-effect transistor for phonons. However, in most materials, electric fields do not couple strongly to lattice vibrations. Moreover, at the molecular and nano scale, in which the ballistic regime is dominant, thermal conductance of acoustic modes is universal, independent of the sound velocity. Therefore, modulating the sound velocity does not change the thermal conductance, thus making even more difficult the conception of such device. In this work, we propose a realizable architecture for a phonon field-effect transistor based on a single polar polymeric molecule placed between two reservoirs. An applied transverse electric field transforms the acoustic torsion mode into optical. For feasible temperatures and electric field magnitudes, this coupling can virtually suppress the contribution from this mode to the thermal conductance, therefore modulating the conductance by as much as 25{\%}. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T11.00006: Hypersonic Phononic Crystal Based on 2D Single Crystalline Nanoporous Alumina Akihiro Sato, Yan Pennec, Takashi Yanagishita, Bahram Djafari-Rouhani, Fytas George, Wolfgang Knoll, Hideki Masuda Periodic nanocomposite media consisted of alumina matrix and infiltrated cylindrical nanopores exhibit rich elastic wave propagation behaviour including the localization of phonons, anisotropic propagation and the formation of phononic band gaps at GHz frequencies. We have examined the translational symmetry dependence of dispersion relations on 2D single crystalline phononic crystals based on nanoporous alumina using Brillouin light scattering. The propagation of elastic waves is significantly different between native and filled with fluids alumina matrix. For the latter, the dispersion relations become independent of the propagation direction, as opposed to the native alumina scaffold. Theoretical band diagrams and the displacement fields describe well the experimental results. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T11.00007: Charge Carrier Confinement in a Nano-patterned Silicon Film Zheng Liu, Wenhui Duan, Feng Liu, Jian Wu Impurity scattering is becoming a critical problem in sub- micrometer MOSFET. One way to reduce the impurity scattering is by separating carriers from dopants, as used in the modulation- doping technique. From first-principles calculation, we find that by etching channels along (001) direction on the surface of a thin (110) silicon film, the hole states can be strongly confined in the film underneath the patterned layer. Therefore, by seletive doping in the top patterned layer, a modulation- doping-like effect can be achieved which is expected to greatly enhance the hole mobility. This effect arises from matching between carrier wavefunction orientation and quantum confinement direction determined by film and pattern geometry. It will be functional as long as the patterned feature size is within a few nanometers. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T11.00008: Magnetotransport of Bi nanowires: Evidence for surface carriers in bismuth. Tito Huber, Alla Nikolaeva, Leonid Konopko, Michael J. Graf Angle resolved photoemission spectroscopy studies (Hirahara et al, Phys. Rev. Lett. 97, 146803 (2006)) provide evidence of quantum-confined bulk-like states and surface states in ultrathin Bi films. Can these states be observed in electronic transport? We studied magnetotransport of trigonal Bi nanowires (30 nm $<$ diameter $<$ 200 nm) for fields up to 14 T. Bulklike states (M.R. Black et al, Phys. Rev. B68, 235417 (2003)) are identified because of its anisotropic Fermi surface and low effective mass. A two-dimensional behavior was expected of high-effective mass surface carriers; we found instead a three-dimensional behavior, with a rich spectrum of Landau levels in a nearly spherical Fermi surface. This behavior is related to the long penetration length of surface states in non-basal surfaces. On the basis of similarity of spectra, we show that recent observations of sharp peaks in the bulk Bi Nernst thermopower near the 9 T quantum limit, attributed to charge fractionalization (K. Behnia, L. Balicas and Y. Kopelevich, Science 317, 1729 (2007)), can be more plausibly interpreted in terms of quasiparticles that are based on surface states. Bismuth true quantum limit is 70 T. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T11.00009: Nanoscale Charge Transport in Realistic Organic Thin-Films: Beyond Variable-Range Hopping and Percolation Networks Geoffrey Hutchison, Marcus Hanwell, Xialing Chen, Aaron Crandall We are building up experimental and computational model systems for charge transport in nanoscale organic electronic devices. In particular, our combined approach is aimed at addressing questions as to the effect of impurities, traps, and other defects on electronic conductivity. Experimentally, we have designed thin films and monolayers to which we can controllably add known quantities of defects with known electronic properties. In tandem, we focus on a new Monte Carlo style simulation of charge transport in these imperfect devices with an aim to move beyond simple variable-range hopping models. Our goal is to establish all parameters for our simulations from first-principles calculations and detailed experimental results. I will describe initial results and comparisons with other organic electronic materials and existing charge transport models. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T11.00010: Computational Simulation of Charge Transport in Metal Terpyridine Monolayer FETs Marcus Hanwell, Geoffrey Hutchison Understanding the roles of charge traps and defects in electronic transport in organic materials is becoming increasingly important. Computational studies have been undertaken, using an agent-based Monte Carlo method, of the active region of a monolayer FET. Charge transport is assumed to be due to thermally activated, variable-range hopping between neighboring sites. This model system allows us to probe the role of charge traps/defects both computationally and experimentally. We do this by using multiple metal terpyridine complexes, each having known electronic structure. Using Marcus Theory and quantum calculations, the hopping rate between neighboring complexes can be predicted. Results from computational simulations of this system will be discussed, with special attention being paid to the results that can be experimentally verified, such as voltage-current curves. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T11.00011: Random telegraph signal and low frequency noise in molecular tunnel junctions Dominique Vuillaume, Nicolas Clement, David Guerin, Stephane Pleutin, David Cahen Monolayers of organic molecules present one of the main systems studied in molecular electronics. We report the observation and study of a low frequency noise and Random Telegraph Signal (RTS) in self-assembled alkyl chain junctions on silicon. The 2 levels of current can be clearly distinguished. With a sufficiently long recording time ($>$ 500 events), statistics can be performed on the current level and on the upper and lower times. The RTS amplitude is usually few {\%} of the average current and the process follows poissonian statistics. This RTS signal is also modulated by another RTS with a much longer time constant. This allowed us evaluation of the change of noise in the frequency domain from 1/f noise to Lorentzian like spectrum. In inorganic tunnel junctions, such signal can only be observed in sub-micrometric junctions whereas we observe it in almost millimetric junctions. This precludes mechanisms involving electron trapping / detrapping in single isolated trap. We propose several hypotheses leading to long-range fluctuations including molecular dynamics and relaxation processes. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T11.00012: Maximum energy transfer in nanoscale thermal radiation Soumyadipta Basu, Zhuomin Zhang Radiation heat transfer between closely spaced objects has received much attention lately because of the emerging applications of near-field thermophotovoltaics, thermal radiation scanning tunneling microscopy, and nanothermal manufacturing. The energy transfer in nanoscale radiation can exceed that of blackbody radiation by several orders of magnitude due to photon tunneling and the excitation of surface polaritons. An outstanding question remains as whether there exists an upper limit of near-field radiation for arbitrarily selected material properties. We examine the maximum radiative energy flux between two parallel plates separated by a vacuum gap from 0.1 and 100 nm distance. An upper bound is imposed to the parallel wavevector component in the analysis based on fluctuational electrodynamics. By assuming a frequency-independent dielectric function, we find that the maximum heat flux depends on the chosen complex permittivity and the distance. The determination of the achievable heat flux at nanometer distances will benefit future research and applications of near-field radiation for energy harvesting. [Preview Abstract] |
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