Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D20: Focus Session: Transport Properties of Nanostructures II: Molecules & Surfaces |
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Sponsoring Units: DMP DCP Chair: Michelle Simmons, UNSW Room: LACC 407 |
Monday, March 21, 2005 2:30PM - 2:42PM |
D20.00001: Rotor-Rotor Interactions in a Three Dimensional Lattice of Dipolar Molecular Rotors R.D. Horansky, L.I. Clarke, E.B. Winston, J.C. Price, J. Nunez, M.A. Garcia-Garibay Crystals of dipolar molecular rotors may display novel phenomena with applications to memories and signal processing. We report on the characterization of crystals of synthesized dipolar molecular rotors. Crystal design provides for control over dipole moment, distance between dipoles, and barriers to rotation. We have previously reported characterization of the rotational potential for crystals with an asymmetric two-well potential in which the asymmetry was caused by steric interactions between the dipole and its surrounding crystal cage. In an effort to isolate the effects of rotor-rotor interactions, we now report on a system with a larger dipole moment and a sterically symmetric rotational potential. Dielectric spectroscopy results show that the two-well potential in this system is still asymmetric. Two models for the asymmetry are investigated through Monte Carlo simulations. In the first, we suppose that the asymmetry is caused by dipole-dipole interactions between rotors and evaluate the interaction strength needed to reproduce the observations. In the second, we consider the effects of a quenched random field. [Preview Abstract] |
Monday, March 21, 2005 2:42PM - 2:54PM |
D20.00002: The Electronic Structure of Wet DNA Richard Marsh, Hao Wang, James Lewis Currently there is vast interest in the field of nanotechnology. One particularly unique aspect is molecular electronics; the ability to take a single molecule and use it as a device (e.g. transistor). The structure and functionality of DNA make it a great potential for nanowire templates. Previous efforts have yielded the electronic properties of both the poly (A)-poly (T) and poly (G)-poly(C) DNA molecules. We continue this work by calculating the electronic structure of the DNA while taking into account surrounding solvation effects. We consider the electronic structure of DNA with varying solvation layers. This system contains over a thousand atoms and is the first ab initio calculation of wet DNA of this magnitude. Our results for both poly (A)-poly (T) and poly (G)-poly(C) DNA molecules will be presented. [Preview Abstract] |
Monday, March 21, 2005 2:54PM - 3:06PM |
D20.00003: Direct Measurements of Electrical Transport Through Single DNA Molecules of Complex Sequence Hezy Cohen, Danny Porath, Claude Nogues, Ron Naaman Seemingly contradicting results raised a debate over the ability of DNA to transport charge and the nature of the conduction mechanisms through it. We developed an experimental approach for measuring current through DNA molecules, chemically connected on opposite ends to a metal substrate and to a gold nanoparticle, using a conductive atomic force microscope.$^{1}$ Many samples could be made due to the experimental approach adopted here that enabled obtaining reproducible results in various samples, conditions and measurement methods. We present multileveled evidence for charge transport through 26 base-pairs long dsDNA of a complex sequence, characterized by S-shaped I-V curves that show currents higher than 220 nA at 2 V.$^{2}$ This significant observation implies that a coherent or band transport mechanism takes over for the high currents ($>$ 1 nA). 1. Claude Nogues, Sidney R. Cohen, Shirley S. Daube, and Ron Naaman, ``Electrical properties of short DNA oligomers characterized by conducting atomic force microscopy,'' PCCP, 2004, 18. 2. Hezy Cohen, Claude Nogues Ron Naaman and Danny Porath. ``Direct Measurement of Electrical Transport Through Single DNA Molecules,'' submitted. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:42PM |
D20.00004: Transport in Inorgainc Nanostructures and DNA: Molecular Welding, Switching and Gating Invited Speaker: . [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 4:18PM |
D20.00005: Probing charge transport through individual molecules on silicon surfaces Invited Speaker: The ultra-high vacuum (UHV) scanning tunneling microscope (STM) allows individual molecules to be imaged, addressed, and manipulated on semiconducting surfaces with atomic resolution. In particular, this paper considers three different molecules on the Si(100) surface: styrene, cyclopentene, and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). In all cases, room temperature STM current-voltage characteristics on individual molecules mounted on degenerately n-type Si(100) show multiple negative differential resistance (NDR) events at negative sample bias. On the other hand, at positive sample bias, the current-voltage characteristics do not show NDR, although a discontinuity in the differential conductance is observed. When the Si(100) substrate is changed to degenerate p-type doping, multiple NDR events are observed at positive sample bias while the discontinuity in the differential conductance occurs at negative sample bias. These empirical observations can be qualitatively explained by considering the energy band diagram for a semiconductor-molecule-metal junction [1]. More sophisticated nonequilibrium Green's function theoretical treatments also confirm the experimental data [2]. This paper will also describe recent efforts to characterize the temperature dependence of charge transport through molecule-semiconductor junctions using cryogenic UHV STM at temperatures between 10 K and 300 K [3]. In addition, using multi-step feedback controlled lithography [4], heteromolecular nanostructures consisting of both styrene and TEMPO molecules have been fabricated on hydrogen passivated Si(100). Atomic-scale characterization of these structures will be discussed in the context of silicon-based molecular electronics. [1] N. P. Guisinger, M. E. Greene, R. Basu, A. S. Baluch, and M. C. Hersam, \textit{Nano Letters}, \textbf{4}, 55 (2004). [2] T. Rakshit, G.-C. Liang, A. W. Ghosh, and S. Datta, \textit{Nano Letters}, \textbf{4}, 1803 (2004). [3] E. T. Foley, N. L. Yoder, N. P. Guisinger, and M. C. Hersam, \textit{Rev. Sci. Instrum.}, \textbf{75}, 5280 (2004). [4] R. Basu, N. P. Guisinger, M. E. Greene, and M. C. Hersam, \textit{Appl. Phys. Lett.}, \textbf{85}, 2619 (2004). [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D20.00006: Electron transport in molecular devices Simone Piccinin, Ralph Gebauer, Roberto Car We apply a novel quantum kinetic approach [1] to study transport in molecular devices. Our scheme is based on a master equation for the reduced electronic density operator and includes dissipation by inelastic scattering with the phonons. A proper choice of the gauge allows us to use periodic boundary conditions. Here we adopt a plane-wave pseudopotential scheme within time-dependent Density Functional theory. We apply the scheme to the well studied case of a benzene-dithiol molecule sandwiched between two gold electrodes. Our results are in general agreement with previous calculations based on open boundary scattering schemes. Our calculated I-V characteristics is also close to recent experimental results. We discuss how transport through the molecular device is affected by dissipative processes in the metallic electrodes. Work partially supported by NSF through Grant DMR-0213706 to the MRSEC PCCM and by DOE through Grant DOE-DE-FG02-01ER45928. \newline \newline [1] R. Gebauer, R. Car, Phys. Rev. B \bf{70}, 125324 (2004) [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D20.00007: Smeagol: the ultimate computational tool for spin transport at the nanoscale Stefano Sanvito The ability of manipulating electron spin in organic molecular materials offers a new and extremely tantalizing roadmap for both spin and molecular electronics. The modeling of molecular spin-devices however requires a level of sophistication never reached before since both accurate electrostatics and the description of the magnetic state are needed. In this talk I will present our newly developed {\it ab initio} quantum transport code {\it Smeagol} (Spin and Molecular Electronics in an Atomically-Generated Orbital Landscape. www.smeagol.tcd.ie), which has been specifically constructed for dealing with these issues. {\it Smeagol} combines density functional theory in the numerical optimization contained in SIESTA, with non-equilibrium Green's function transport method. It has been completely designed for dealing with magnetic systems including both non-collinear spin, spin-orbit interaction and strong correlated functionals (LDA+U and LDA+SIC). A demonstration of its capabilities will be presented. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D20.00008: Direct-Write Molecular Layer Epitaxy by Thermal Dip-Pen Nanolithography M. Yang, P. E. Sheehan, W. P. King, L. J. Whitman Achieving nanometer-scale control of structure in organic thin films is crucial to understand charge transport and thereby develop reliable devices such as organic FETs and LEDs. Although a variety of methods can be used to reliably deposit thin polymer films, fabrication of polymer nanostructures remains a significant challenge. We have developed a new technique, thermal dip-pen nanolithography (tDPN),$^{1}$ that can be used to directly write such nanostructures. In tDPN a custom AFM cantilever with an integral tip heater is pre-coated with a solid ``ink,'' which can then be precisely deposited onto a substrate by heating the tip above the ink’s melting temperature. Using this technique, poly(3-dodecylthiophene) nanostructures have been deposited on silicon oxide surfaces with layer-by-layer thickness control. By adjusting the tip heating power and the writing speed, we can vary the polymer thickness from a single monolayer (about 2.8 nm) to tens of monolayers with lateral dimensions $<200$ nm. \\ $^{1}$P. A. Sheehan, {\em et al.}, Appl. Phys. Lett. {\bf 85}, 1589 (2004). [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D20.00009: Molecular Transport in Dip-Pen Nanolithography of Alkanethiols on Gold Peter Schwartz, Erik Peterson, Ivan Hromada, Matthew Leyden, Jamie Romnes, Brandon Weeks The direct patterning of Octadecanethiol (ODT) and mercaptohexadecanoic acid (MHA) from an AFM tip by Dip-Pen Nanolithography (DPN) is investigated as a function of humidity, temperature, total elapsed time, and protocol for coating the AFM tip, and the process is directly observed by scanning electron microscopy (SEM). Rather than being independent of AFM tip speed, the molecular transport rates of ODT and MHA decrease for slower AFM tip translation rates, consistent with Fickian Diffusion. Both molecules can be patterned under a dry atmosphere in apparent absence of a water meniscus and exhibit Arrhenius temperature dependence, consistent with a ``dry patterning'' process driven by thermal motion. Unlike ODT, the molecular transport rate of MHA both decreases over time in a near exponential fashion with an approximately 1-hour decay time, and substantially increases at very high relative humidity indicating that MHA transports my means of both dry patterning as well as solvation in a water meniscus. SEM results indicate that surface characteristics strongly affect the presence of a meniscus. [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D20.00010: Manipulation of metallic and semiconducting nanowires with FETEM Shengyong Xu, Mingliang Tian, Jinguo Wang, Moses H. W. Chan We demonstrate that a field-emission transmission electron microscope (FETEM) can be applied to in situ manipulation of metallic, semiconducting nanowires and nanoparticles. With a highly convergent electron beam, we are able to create various patterns, including holes and gaps of 0.2-0.3 nm in width and complex features (e.g. letters) smaller than 10 nm by 15 nm in size, on a single nanowire. We can also weld individual nanoparticles and nanowires together to form metal-metal (e.g., Au-Au, Au-Sn) and metal-semiconductor (e.g., Au-Si) junctions or contacts. This in situ manipulation technique has an applicable length scale ranging from a few angstroms to over 100 nm therefore it may bridge the gap between the limitations of STM/AFM and electron beam lithographic techniques. The method may also open up an alternative approach for in situ patterning, modification and connection of nano-materials in nanoscale devices and circuits. [Preview Abstract] |
Monday, March 21, 2005 5:18PM - 5:30PM |
D20.00011: Hall effect induced by electron-surface scattering on thin gold films deposited on preheated mica substrates under high vacuum Raul Munoz, Juan Pablo Garcia, Ricardo Henriquez, Ana Maria Moncada, German Kremer, Luis Moraga, Simon Cancino, Miguel Chesta, Pedro Miranda, Jose Roberto Morales We report the first measurement of the Hall effect where the signal can be unequivocally attributed to electron-surface scattering. The measurement of both the Hall effect and the resistivity of the samples was carried out on a family of 4 gold films (thickness of 69 nm, 93 nm, 150 nm and 185 nm) evaporated onto preheated mica substrates under high vacuum. The experiment was performed at low temperatures T (4K $\le $ T $\le $ 50K) under high magnetic field strengths \textbf{B} (1.5 T $\le $ B $\le $ 9 T), with \textbf{B} oriented perpendicular to the films. The Hall tangent E$_{y}$/E$_{x}$ turns out to depend on film thickness, as predicted by Sondheimer [E. H. Sondheimer, Pys. Rev. \textbf{80} (1950) 401; Adv. Phys. \textbf{1} (1952) 1] and by Calecki [D. Calecki, Phys. Rev. \textbf{B42} (1990) 6906]. However, both Sondheimer's and Calecki's model fail to describe the temperature as well as the thickness dependence of both the resistivity of the films and of the Hall tangent. Work funded by FONDECYT 1040723. [Preview Abstract] |
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