Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D31: Focus Session: Computational Nanoscience II: Nanowires and Transport |
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Sponsoring Units: DMP DCOMP Chair: Serdar Ogut, University of Illinois at Chicago Room: Morial Convention Center 223 |
Monday, March 10, 2008 2:30PM - 3:06PM |
D31.00001: First Principles Studies of Tapered Silicon Nanowires: Fundamental Insights and Practical Applications Invited Speaker: Nanowires (NWs) are often observed experimentally to be tapered rather than straight-edged, with diameters (d) shrinking by as much as 1 nm per 10 nm of vertical growth. Previous theoretical studies have examined the electronic properties of straight-edged nanowires (SNWs), although the effects of tapering on quantum confinement may be of both fundamental and practical importance. We have employed ab initio calculations to study the structural and electronic properties of tapered Si NWs. As one may expect, tapered nanowires (TNWs) possess axially-dependent electronic properties; their local energy gaps vary along the wire axis, with the largest gap occurring at the narrowest point of the wire. In contrast to SNWs, where confinement tends to shift valence bands more than conduction bands away from the bulk gap, the unoccupied states in TNWs are much more sensitive to d than the occupied states. In addition, tapering causes the band-edge states to be spatially separated along the wire axis, a consequence of the interplay between a strong variation in quantum confinement strength with diameter and the tapering-induced charge transfer. This property may be exploited in electronic and optical applications, for example, in photovoltaic devices where the separation of the valence and conduction band states could be used to transport excited charges during the thermalization process. In order to gain insight into TNW photovoltaic properties, we have also carried out calculations of the dipole matrix elements near the band edges as well as the role of metal contacts on TNW electronic properties. Finally, a combination of ab initio total energy calculations and classical molecular dynamics (MD) simulations are employed to suggest a new technique for bringing nanoscale objects together to form ordered, ultra high-aspect ratio nanowires. This work was supported in part by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D31.00002: Structure and electronic properties of silicon nanowires grown along the [110] direction: role of surface reconstruction T. Akiyama, K. Nakamura, T. Ito Silicon single-crystal nanowires (SiNWs) are attracting great interest for future nanoscale devises in recent years. So far, the [110] grown SiNWs with diameters below 4 nm have been successfully fabricated by various methods\footnote{Ma {\it et al.}, Science {\bf 299}, 1874(2003); Wu {\it et al.}, Nano Lett. {\bf 4}, 433(2004).}. Although the size dependence in electronic and optical properties for the [110] grown SiNWs terminated by H-atoms have been intensively studied\footnote{Zhao {\it et al.}, Phys. Rev. Lett. {\bf 92}, 236805(2004).}, effects of surface reconstructions on the electronic structure have been rarely examined. Here, we investigate the atomic and electronic structures of SiNWs along the [110] direction with \{001\} and \{111\} facets using first-principles pseudopotential method. The calculations for SiNWs whose diameters are $\sim$4 nm demonstrate that the reconstructions on these facets are strongly dependent on H-chemical potential $\mu_{\rm H}$: The SiNW consisting of monohydride \{001\} and H-terminated \{111\} facets is stabilized for high $\mu_{\rm H}$($\geq$-0.75 eV) while the pristine SiNW stabilized for low $\mu_{\rm H}$($\leq$-0.82 eV). The reconstructions with partially hydrogenated facets appear for -0.82$<$$\mu_{\rm H}$$<$-0.75 eV. Peculiar features in the electronic structure are also found in partially hydrogenated SiNWs. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D31.00003: Modeling of Polycrystalline and Wurtzite Si Nanowires with Symmetry-Adapted Objective Molecular Dynamics Traian Dumitrica, Dong-Bo Zhang, Ming Hua The stability and properties of the most promising ground state candidate Si nanowires with less than $10$~nm in diameter is comparatively studied with molecular dynamics coupled with non-orthogonal tight-binding and classical potential models. The computationally-expensive tight-binding treatment becomes tractable due to the substantial simplification in the number of atoms introduced by the presented symmetry-adapted objective molecular dynamics scheme. It indicates that the achiral polycrystalline of five-fold symmetry and the wurtzite wires of three-fold symmetry are the most favorable quasi one-dimensional Si arrangements. Quantitative differences with the classical model description are noted over the whole diameter range. Using a Wulff energy decomposition approach it is revealed that these differences are caused by the inability of the classical potential to accurately describe the interaction of Si atoms on surfaces and strained morphologies. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D31.00004: \textit{Ab initio} calculations for the electronic properties of zinc-doped indium phosphide nanowires Manuel Alemany, Xiangyang Huang, Murilo L. Tiago, L.J. Gallego, James R. Chelikowsky $p$-type indium phosphide nanowires are known to function as working devices when assembled with $n$-type nanowires, and thus are seen as very promising building blocks for highly integrated electronic devices within the semiconductor industry. In this work, we have characterized the impurity state responsible for current flow in zinc-doped indium phosphide nanowires through first-principles calculations based on a real-space implementation of density-functional theory and pseudopotentials. The binding energy of the acceptor state is predicted to range from the value of the acceptor state in the bulk to up to values of approximately 0.2 eV in the thinner nanowires as a result of the two-dimensional quantum confinement. Our results show that, in thin nanowires, quantum confinement can move the defect level deep into the energy gap. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D31.00005: Molecular-Dynamics Simulations of Nanowire Growth Tomorr Haxhimali, Dorel Buta, Mark Asta, Jeffrey Hoyt This talk will present results of molecular dynamics simulations investigating the mechanisms of nanowire growth from a liquid. We investigate the model system of elemental Si, modeled with the classical Stillinger-Weber potential. The work aims to investigate the effect of nanowire size on the intrinsic growth mechanisms and the relations between solid-liquid interface velocity, growth direction and driving force. Results will be presented for nanowires with diameters ranging from 5-10 nm, and will be compared with simulations for bulk Si modeled with the same potential. The consequences of these findings for the mechanisms of nanowire growth from liquid catalysts by the vapor-liquid-solid mechanism will be discussed. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D31.00006: First-principles study of the electronic and magnetic properties of Fe-Co nanowires Dangxin Wu, Ping Liu, Qiming Zhang, Ruqian Wu Fe-Co nanowires provide a potential way to produce high-performance nanocomposite permanent magnets due to their high Curie temperature, large magnetization and appreciable anisotropy. In this talk we present our recent results of first-principles investigation of this matter. The calculations use both PAW method and FLAPW method, based on density functional theory. The structures of Fe-Co nanowires were optimized by PAW method and then the electronic structure and magnetic properties such as saturation magnetization and anisotropy energies are studied by FLAPW method. The effects of size and composition of the nanowires on the magnetic properties are also studied and compared with those of bulk Fe-Co materials. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D31.00007: Magnetic Co impurity in Gold Nanowires Edison da Silva, Renato Pontes, Antonio J.R. da Silva, Adalberto Fazzio Nanoscale electric contacts using suspended gold nanowires (NWs) have recently been made and were imaged by electron microscopy. Using tools derived from Density Functional Theory (DFT) we study the role of magnetic impurities in these NWs with the possibility of spintronic applications. Here we study structural and transport properties of a gold nanowire with one Co impurity as function of tension applied to the NW. Co added new features to the physics of this system. We present studies of structure and also electronic transport using the same DFT formalism [1] that show the effect of the spin anisotropy introduced by Co. In particular, we present results of two geometries, one where the Co atom is connected to two Au atoms of the lead and another where it is in the middle of the suspended neck, in a linear configuration. In the former case we observe an interference between the s and d channels, leading to a Fano-like structure in the transmittance, whereas in the latter configuration due to the local symmetry there is a decoupling between these two channels and the transmittance has a simple peak around the Co d-states, leading to a large spin polarized transport. [1] F. D. Novaes, A.J.R. da Silva, and A. Fazzio, Braz. Jou. Phys. \textbf{36 }(3A): 799-807 (2006) [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D31.00008: Transport in Carbon Nanotube Junctions K.H. Khoo, James R. Chelikowsky There is growing interest in the use of carbon nanotube thin films as transparent electrical conductors and thin-film transistors owing to their high optical transmittance, low sheet resistivity, and ease of fabrication. [1,2] A major contribution to the sheet resistivity originates at nanotube junctions, as electrical contact is typically poor between adjacent nanotubes. It is thus important to characterize carbon nanotube junctions in order to understand the conduction properties of nanotube thin films. To this end, we have performed \textit{ab initio} density functional theory calculations to investigate the structural, electronic and transport properties of carbon nanotube junctions as a function of nanotube chirality and contact geometry \newline [1] Z. Wu \textit{et al.}, Science \textbf{305}, 1273 (2004) \newline [2] E. S. Snow, J. P. Novak, P. M. Campbell, and D. Park, Appl. Phys. Lett. \textbf{82}, 2145 (2003). [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D31.00009: Schottky Barrier Heights in CNT-Metal Junctions from First-principles Nicholas Singh-Miller, Nicola Marzari Fundamental understanding of the electronic properties at the junction between a carbon nanotube (CNT) and a substrate is important for the practical application of CNT-based devices. Here, we use density functional theory (DFT) to probe the properties of the CNT-metal interface, paying particular attention to the Schottky barrier heights (SBH). We focus on the junction between a semiconducting (8,0)CNT and aluminum or palladium, chosen as paradigmatic examples of a simple metal and a transition metal, respectively. We obtain SBHs from the potential lineup, examining the effects of geometry at the interface and the functionalization of the CNT on the SBH. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D31.00010: Transport properties of transition-metal-encapsulated Si cages Lingzhu Kong, James R. Chelikowsky We performed density functional pseudopotential calculations of the spin dependent transport through transition-metal-atom-encapsulated Si cages Si$_{12}X$($X$=Mn, Fe and Co). The effect of the metal atom on conductance is studied. Mn and Fe doped systems show highly spin polarized transmission whereas the magnetization in Co doped system is quenched. It is found that electrons are transferred from Si atoms into the minority $d$ orbitals of the metal atoms. The conductance decreases as these electrons become localized around the encapsulated atoms. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D31.00011: Transport Properties through Nanomaterials by First-principles Calculations Hiroshi Mizuseki, Rodion Belosludov, Sang Uck Lee, Yoshiyuki Kawazoe Nanoscale molecular devices are potential candidates for this next step, and they would make it possible to realize the most advantageous devices. Our group has covered a wide range of nanoscale materials[1] such as self-assembled nanowires on Si(001) [2, 3], quantum length dependence of conductance in oligomers [4] and single-molecule rotation switch [5] and so on. In this presentation, we will present our recent study on the transport properties of these nanoscale materials using the nonequilibrium Green's function formalism for quantum transport and the density functional theory (DFT) of electronic structures using local orbital basis sets. References 1. http://www-lab.imr.edu/$\sim $mizuseki/nanowire.html 2. J.-T. Wang, C. Chen, E. G. Wang, D.-S. Wang, H. Mizuseki, and Y. Kawazoe, Phys. Rev. Lett., 97 (2006) 046103. 3. R. V. Belosludov, A. A. Farajian, H. Mizuseki, K. Miki, and Y. Kawazoe, Phys. Rev. B, 75 (2007) 113411. 4. Y. X. Zhou, F. Jiang, H. Chen, R. Note, H. Mizuseki, and Y. Kawazoe, Phys. Rev. B, 75 (2007) 245407. 5. Y. Y. Liang, F. Jiang, Y. X. Zhou, H. Chen, R. Note, H. Mizuseki, and Y. Kawazoe, J. Chem. Phys. 127 (2007) 084107. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D31.00012: Ab initio non-equilibrium Green's function study on the growth of metallic bridge in mixed conductor atomic switch Tomofumi Tada, Zhongchang Wang, Tingkun Gu, Satoshi Watanabe A novel atomic switch [1] composed of a mixed conductor, Ag$_{2}$S or Cu$_{2}$S, has attracted much attention. To investigate the electronic properties of the atomic switch, we have examined interface structure and electron transport of Ag/Ag$_{2}$S/Ag and Cu/Cu$_{2}$S/Cu using ab initio non-equilibrium Green's function method. In Ag/Ag$_{2}$S/Ag, we found a spontaneous growth of a metallic bridge composed of a Ag atomic chain when a unidirectional stress is applied to Ag$_{2}$S [2]. On the contrary, a metallic bride does not appear in Ag$_{2}$S and Cu$_{2}$S without stress. We also examined the influence of Ag/Cu addition on the structural and transport properties of Ag$_{2}$S and Cu$_{2}$S atomic switches, and found that the Ag/Cu addition leads to the metallization in the both systems. However, clear growth of the atomic bridge is confirmed only in Ag/Ag$_{2+\delta }$S/Ag. The metallic nature in Cu/Cu$_{2+\delta }$S/Cu is related to the growth of electron charge network at the Fermi level. 1) K. Terabe, et al., Nature 433, 47 (2005). 2) Z. Wang, T. Kadohira, T. Tada, S. Watanabe, Nano Letters 7, 2688 (2007). [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D31.00013: Ab initio transport properties of platinum chains calculated by taking into account spin orbit effects Victor Garcia-Suarez, David Zsolt Manrique, Colin Lambert, Jaime Ferrer The transport properties of infinite and finite platinum chains are calculated by using a combination of Density Functional Theory and Non-Equilibrium Green's Functions Formalism, as implemented in the Smeagol Code. We show that spin orbit effects, which are included fully self-consistently in our calculations, are of paramount importance to determine accurately the electronic and transport characteristics of these systems. For infinite chains we find that under special circumstances which depend on the type of chain (linear or zigzag), length and spin orientation relative to the chain, the conductance can be totally suppressed, giving rise to large magnetoresistive ratios. In the case of finite chains between bulk electrodes the spin-orbit effect plays also a crucial role and gives results which agree better with experiments. [Preview Abstract] |
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