Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q15: Focus Session: Energy and Electron Flow at Interfaces in Nanostructures |
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Sponsoring Units: DMP Chair: Gary Wiederrecht, Argonne National Laboratory Room: 008B |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q15.00001: Schottky barrier formation and reduction at Au/TiO$_2$ interfaces by dopants from quantum simulations Yang Jiao, Anders Hellman, Yurui Fang, Shiwu Gao, Mikael K\"all Excitation of localized surface plasmon resonances (LSPRs) in metallic nanoparticles, especially particles made of noble metals, results in efficient light absorption and strong field enhancement, thereby enabling a multitude of nanooptical applications of high current interest. Recently, the possibility of utilizing LSPRs to generate hot electrons has attracted considerable attention. One method to extract and make use of the hot electrons is by attaching the nanoparticles on a semiconductor surface such that excited electrons with proper energy and momentum can be transferred through the Schottky barrier at the interface. Using {\it ab initio} calculations for Au/TiO$_2$ interfaces, we investigate dopant induced Schottky barrier height reduction effects. We show that dopant induced polarization at the interface is the dominant reason behind the semiconductor band bending and Schottky barrier formation. Calculations for Nb-dopants at different depths ($d$) below the interface show that the Schottky barrier height reduction depends on the depth and varies from 0.1~eV at $d=4$~nm to up to 1.3~eV when the dopant is situated at the interfacial layer. The calculations also indicate that the Schottky barrier can be tuned by up to 1.5~eV by using different transition metal dopants. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q15.00002: Manipulating the charge state and conductance of a single molecule on a semiconductor surface by electrostatic gating Jesus Martinez-Blanco, Christophe Nacci, Steven C. Erwin, Kiyoshi Kanisawa, Elina Locane, Mark Thomas, Felix von Oppen, Piet Brouwer, Stefan Foelsch We studied the charge state and tunneling conductance of single phthalocyanine molecules adsorbed on InAs(111)A using scanning tunneling microscopy (STM) at 5 K. On the InAs(111)A surface, native $+$1 charged indium adatoms can be repositioned by the STM tip using atom manipulation. This allows us to electrostatically gate an individual adsorbed molecule by placing charged adatoms nearby or, alternatively, by repositioning the molecule within the electrostatic potential landscape created by an STM-engineered adatom corral. By stepwise increasing the gating potential, the molecular charge state can be tuned from neutral to -1, as well as to bistable intermediate states. We find that the molecule changes its orientational conformation when the charge state is switched. Scanning tunneling spectroscopy measurements reveal that the conductance gap of the single-molecule tunneling junction can be precisely controlled by the electrostatic gating. We discuss the observed gating-dependent single-molecule tunneling conductance in terms of charge transport through a gated quantum dot. [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q15.00003: Self-energy-corrected electronic energy level alignment in molecular junctions and at interfaces with hybrid functionals Michele Kotiuga, David Egger, Leeor Kronik, Jeffrey B. Neaton Accurate calculations of energy level alignment at complex interfaces are imperative for understanding a variety of transport and spectroscopy measurements, as well as for elucidating new interfacial electronic structure phenomena. However, standard approaches to such calculations, based on density functional theory (DFT), are well known to be deficient. In prior work on molecular junctions and physisorbed molecules on surfaces, an approximate GW approach, DFT+$\Sigma$, has been successful in describing the conductance and level alignment of amine and pyridine terminated molecules on gold surfaces and in junctions. Here, via the use of hybrid functionals, we preform quantitative studies of the level alignment of thiol- and carbon-terminated phenyls on gold, where the formation of a strong chemical bond and presence of gateway states limit the validity of the DFT+$\Sigma$ approximation as currently formulated. We contrast these systems to prior work on weakly-coupled molecules, including bipyridine or phenyl-diamines. Additionally, we compute transmission functions using both DFT-PBE and DFT-HSE starting points and predict conductance and thermopower with these methods, comparing to experiments where possible. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q15.00004: Interface Conductance Modal Analysis Kiarash Gordiz, Asegun Henry Reliably and quantitatively calculating the conductance of phonons across an interface between two materials has been one of the major unresolved questions in thermal transport physics for the last century. Theories have been presented in this regard, but their predictive power is limited. A new formalism to extract the modal contributions to thermal interface conductance with full inclusion of temperature dependent anharmonicity and all of the atom level topography is presented. The results indicate that when two materials are joined a new set of vibrational modes are required to correctly describe the transport across the interface. The new set of vibrational modes is inconsistent with the physical picture described by phonon gas model (PGM), because some of the most important modes are localized and non-propagating and therefore do not have a well-defined velocity nor do they impinge on the interface. Among these new modes, certain classifications emerge, as most modes extend at least partially into the other material. Localized interfacial modes are also present and exhibit a high conductance contribution on a per mode basis by strongly coupling to other types of vibrational modes. We apply our formalism to different interfaces and present thermal interface conductance accumulation functions, two-dimensional cross-correlation matrices, and a quantitative determination of the contributions arising from inelastic effects. The provided new perspective on interface thermal transport can open new gates towards deeper understanding of phonon-phonon and electron-phonon interactions around interfaces. [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q15.00005: \textit{In Situ} Soft X-ray Spectroscopy Characterization of Interfacial Phenomena in Energy Materials and Devices Jinghua Guo, Yi-Sheng Liu, Mukes Kapilashrami, Per-Anders Glans, Debajeet Bora, Artur Braun, Juan Jes\'us Velasco V\'elez, Miquel Salmeron Advanced energy technology arises from the understanding in basic science, thus rest in large on in-situ/operando characterization tools for observing the physical and chemical interfacial processes, which has been largely limited in a framework of thermodynamic and kinetic concepts or atomic and nanoscale. In many important energy systems such as energy conversion, energy storage and catalysis, advanced materials and functionality in devices are based on the complexity of material architecture, chemistry and interactions among constituents within. To understand and thus ultimately control the energy conversion and energy storage applications calls for in-situ/operando characterization tools. Soft X-ray spectroscopy offers a number of very unique features. We will present our development of the in-situ/operando soft X-ray spectroscopic tools of catalytic and electrochemical reactions in recent years, and reveal how to overcome the challenge that soft X-rays cannot easily peek into the high-pressure catalytic cells or liquid electrochemical cells. In this presentation a number of examples are given, including the nanocatalysts and the recent experiment performed for studying the hole generation in a specifically designed photoelectrochemical cell under operando conditions. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q15.00006: Determining level alignment and coupling strength in single-molecule junctions with chemically-enhanced Raman spectroscopy Pierre Darancet, Alexey Zayak Raman spectroscopy can be used at the nanoscale to probe binding geometries [1], molecule concentrations [2], carrier densities [3], and charging effects [4]. In this talk, we use finite-difference total-energy and self-energy corrected density functional theory calculations in conjunction with Landauer framework, to study the Raman spectra and transport properties of model nanoscale interfaces, single-molecule junctions -- individual molecules contacted with macroscopic metallic electrodes. In the cases of 4,4' bipyridine/Gold and polyphenylene venylene/Gold junctions, we will show how conductance and chemically-enhanced Raman measurement can be used in conjunction to determine the energy scales controlling electron transport i.e. frontier orbital energies and coupling strength. \\[4pt] [1] Zayak et al, Phys. Rev. Lett. 106, 083003 (2011)\\[0pt] [2] Zayak et al., J. Phys. Chem Lett. 3 (10), 1357-1362 (2012)\\[0pt] [3] Das et al. Nanotechnology , 3, 210– 215 (2008)\\[0pt] [4] Li et al., PNAS, 111 (4) 1282-1287 (2014) [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q15.00007: Nanoscale optical spectroscopy of CdTe photovoltaic devices Nikolai Zhitenev, Yohan Yoon, Jungseok Chae, Aaron Katzenmeyer, Heayoung Yoon, Andrea Centrone Thin film solar cells are based on polycrystalline materials such as CdTe and CIGS that are structurally and electronically non-uniform. To further advance the power conversion efficiency it is important to understand the properties of interfaces (p-n junction, contacts) and microsctructure (composition, grains) of these inhomogeneous devices. We apply two local optical techniques for spectroscopic characterization of CdTe devices. The samples are cross-sectional lamellas extracted from CdTe cell with sub-micron thickness prepared by focused ion beam. The first spectroscopic approach is based on the local light injection through a sub-wavelength aperture of optical fiber and the measurements of the transmitted / absorbed power. The optical wavelength was varied in the range from 400 nm to 900 nm. The contrast of the spatial maps of optical absorption is the strongest at excitation energies close to the band gap of CdTe and it can be associated with the composition variation throughout the device. The second technique uses the photo-thermal effect as a local measurement of absorption and can be used for broader range of wavelengths. Pulsed laser with variable wavelength is used for the excitation, and the local thermal expansion is detected by an atomic force microscope. We compare the resolution and the sensitivity of these two approaches in the range of photon energies close to the band gap where both techniques can be used. [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q15.00008: Variable temperature shot noise measurements in mechanically controlled gold break junctions Ruoyu Chen, Leland Richardson, Douglas Natelson Shot noise originates from the discreteness of charge carriers when a finite bias is applied. The noise spectral density reflects the effective charge and the Fano factor. The former may be modified by electron-electron interactions, while the latter can be affected by both interactions and the microscopic nature of transport. The temperature dependence of shot noise is interesting due to the fact that both interactions and the microscopic dynamics and geometries can vary with temperature, as well as length scales associated with scattering. Previous excess noise measurements demonstrate the existence of shot noise in ballistic atomic-scale contacts at room temperature, indicating that the quantum coherence length is at least larger than atomic scale even at 300 K. A detailed temperature dependent study from cryogenic conditions to room temperature is still absent. Here we will present progress on using lithographically patterned gold bowtie junctions on mechanically-bended substrates to study the broadband rf excess noise over different conductance values, electric biases and temperatures. [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q15.00009: Dissipation and heating in C$_{60}$ molecular junctions Pavlo Zolotavin, Charlotte Evans, Douglas Natelson We present a novel experimental approach to study energy dissipation during electron transport through the molecular scale junction containing a C$_{60}$ molecule. One of the pathways for a tunneling electron to dissipate energy in the junction is to excite vibrations of the molecule. Previously, such vibrational heating had been observed by measuring the intensity of anti-Stokes modes in the surface enhanced Raman spectra (SERS). A complimentary electron-focused approach is to use inelastic electron tunneling spectroscopy (IETS) to study the electron-vibronic interactions by tracking the effects of vibrations upon the electronic current. A combination of these two techniques should allow for a quantitative study of the energy dissipation in molecular junctions. The preliminary results of simultaneous IETS and SERS measurements in C$_{60}$ molecular junctions will be presented. We discuss the vibrational heating of C$_{60}$ molecule and future expansion of this work to junctions containing semiconductor nanocrystals. (ARO award W911 NF-13-1-0476) [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q15.00010: Low temperature high bias enhanced noise in atomic-scale Au junctions Loah Stevens, Pavlo Zolotavin, Ruoyu Chen, Douglas Natelson We report measurements on STM-style Au break junctions, investigating the bias dependence of current noise at room temperature, 77K, and 4K. Previous experiments at room temperature observed that low bias noise (\textless 150mV) agrees well with predictions for shot noise at fixed electronic temperature, but at high biases, noise was found to have a nonlinear dependence on the scaled bias. Possible sources of this deviation are nonequilibrium electron-phonon effects or local heating of the electronic distribution. In order to expand upon the understanding of the enhanced noise at high bias, we have measured current noise for a range of biases as a function of environmental temperature. This allows for distinction between electron-electron and electron-vibrational contributions to the shot noise. We will discuss differences in the bias dependence of the noise between cryogenic and room temperature conditions. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q15.00011: Edge-Localized Spin-Polarized State in Nanofacet Formed on SiC(0001) Surfaces Keisuke Sawada, Jun-Ichi Iwata, Atsushi Oshiyama The nanometer-scale facet (nanofacet) are self-organized on the SiC(0001) surfaces being slightly misoriented toward the (11\={2}0) direction[1, 2]. It is known that the nanostructure induces the novel electronic property such as localized states at zigzag graphene edges[3]. In this study, we perform density-functional calculations on the nanofacet formed on the SiC(0001) surface. We find peculiar electron states without dispersion along the step edges (SEs) near the Fermi level. To explore possibilities of the observation of these peculiar states, we examine the situation that the nanofacet is covered by H atoms and calculate the H absorption energy at the several positions on the nanofacet. We then find that absorbed H atoms energetically prefer terraces to SEs. This leads to a situation in which H atoms at SE C atoms are desorbed. In this case, we find an electronic state distributed along the SE and is spin-polarized. Implication of magnetic transport in such nanofacet will be discussed. [1] H. Nakagawa \textit {et al}., PRL \textbf {91}, 226107 (2003). [2] M. Fujii and S. Tanaka, PRL \textbf {99}, 016102 (2007). [3] M. Fujita \textit {et al}., JPSJ \textbf {65}, 1920 (1996). [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q15.00012: Thermal conductivity and interfacial thermal conductance of HfN/ScN superlattices Bo Sun, Jeremy Schroeder, YeeKan Koh Metal/semiconductor superlattices are known for their potential application as thermionic devices. Understanding thermal properties of such superlattices is essential for the design of new material structures and devices. Here, we measured the cross-plane thermal conductivity of HfN/ScN metal/semiconductor superlattices using time-domain thermoreflectance (TDTR). HfN/ScN superlattices with different period thickness (2nm to 24 nm) were grown on MgO substrate using reactive magnetron sputtering. We found that the minimum thermal conductivity is 4.3 W/m K when the period thickness is 6 nm. By changing the ratio of layer thickness of HfN and ScN (1:4 to 4:1), we studied the contributions electrons and phonons to the thermal conductivity of superlattices. Use a simple thermal resistance calculation, we extract the interfacial thermal conductance between HfN and ScN. The interfacial thermal conductance is 1.8 GW/m$^{\mathrm{2}}$ K, which is 3 times higher than that of AlN/GaN. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q15.00013: Resistivity of thiol-modified Au thin films Patricio H\"{a}berle, Jonathan Correa-Puerta, Valeria Del Campo, Ricardo Henr\'{i}quez Electrical transport in conductors with sizes in the nanoscale range is indeed a surface dependent feature. We report on the modification of electrical transport in thin gold films by the functionalization of alkanethiols, which form a self-assembled monolayer. Theoretical models such as the Fuchs-Sondheimer-Lucas and Namba [1], have been used to describe the electrical conduction, including size effects. Within these models, the resistivity can be attributed to an electron-surface scattering mechanism. Measurements performed in different films display an increased resistivity in the functionalized films. This increment depends mainly of the gold surface topography and not necessarily on the film thickness.\\[4pt] [1] J. Correa-Puerta, V. Del Campo, R. Henr\'{i}quez, P. H\"{a}berle, Thin Solid Films, 570, Part A, 150 ( 2014). [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q15.00014: Direct measurement of the intrinsic linewidth of a resonant state Zachary Kobos, Mark Reed We have applied inelastic electron tunneling spectroscopy (IETS) techniques to a resonantly-coupled system to determine quantitative differences in resonant versus non-resonant IETS. We use as a model system a set of GaAs-AlGaAs resonant tunneling diodes (RTDs)(footnote: with different barrier widths to tune resonant state linewidths and transmission coefficients. Modulation-broadening studies confirm theoretical predictions;\footnote{Klein, et al., \textbf{Phys. Rev. B} 7(6), 1973} however, the thermal dependence is markedly different than expected from classical IETS theory.\footnote{J. Lambe \& R.C. Jaklevic, \textbf{Phys. Rev.} 165(3), 1968} An analysis of resonance shut-off reveals that the thermal dependence reflects the thermal broadening of the injector and resonant state density of states. Using this analysis, we show that one can extract both the transmission coefficient and the intrinsic linewidth of the resonant state. This is compared for RTDs of different tunneling barrier widths, and we observe the expected increase in resonance width for thinner barriers. [Preview Abstract] |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q15.00015: The Role of Joule Heating and Defect Chemistry in Memristor Modeling Brian Tierney, Harold Hjalmarson, Michael McLain, Denis Mamaluy Resistive switching in electroformed metal/metal-oxide/metal memristive devices involves the growth and dissolution of conductive filaments within the metal-oxide. These filaments are typically formed/dissolved by applying a voltage pulse of the appropriate polarity across the metal contacts. The induced electric field across the oxide causes Joule heating. This heating is a significant contributor to the migration of lattice defects such as charged oxygen vacancies, which modulate the time-evolution of the conductive filaments, and hence the device resistance. In this talk, continuum calculations are presented that model the temporal evolution of conductive filaments in tantalum oxide devices. The effects of Joule heating, chemical species migration and pulsed ionizing radiation from an external source are included in the model. Interface tunneling current is determined via a WKB model, in conjunction with a lattice defect generation scheme. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contractDE-AC04-94AL85000. [Preview Abstract] |
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