Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session B44: Focus Session: Nanoscale Transport - Molecules I |
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Sponsoring Units: DMP Chair: J.B. Neaton, Lawrence Berkeley National Laboratory Room: Colorado Convention Center 507 |
Monday, March 5, 2007 11:15AM - 11:51AM |
B44.00001: Conductance of Single Molecule Junctions: Dependence on Structure and Conformation Invited Speaker: We recently demonstrated that the conductance of single molecule junctions formed by breaking Au point contacts in an environment of molecules with amine linkages can be measured reliably and reproducibly$^{1}$. We have now studied junctions formed by approximately 30 different amine terminated molecules, allowing systematic study of the correlation between molecular properties and single molecule junction conductance. This talk will focus on the relation between molecular conductance and molecule conformation for the simple case of a biphenyl, two benzene rings linked together by a single C-C bond. Our results from a series of seven biphenyl derivatives show that the molecular junction conductance depends on the twist angle. Specifically, we find that the planar molecule has the highest conductance, and the conductance for the series decreases with increasing twist angle, consistent with a cosine squared relation predicted theoretically$^{2}$. 1. L. Venkataraman, J.E. Klare, I.W. Tam, C. Nuckolls, M.S Hybertsen and M. Steigerwald, \textit{Nano Letters}, vol. 5, pp. 458-462, 2006. 2. L. Venkataraman, J.E. Klare, C. Nuckolls, M.S Hybertsen and M. Steigerwald, \textit{Nature}, vol. 442, pp. 904-907, 2006. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B44.00002: DFT-based transport calculations for single molecules: Can Coulomb blockade effects be reproduced by local functionals? Max Koentopp, Kieron Burke In principle, time-dependent current density functional theory (TDCDFT) allows for exact calculations of the electronic transport properties of single molecules. In practice, one is forced to make approximations for the exchange-correlation functional employed, and the computationally less costly ground-state DFT in a local approximation (GGA) is used. This introduces errors that can lead to an overestimation of the calculated current by one to two orders of magnitude. The use of local approximations to the exchange-correlation functional also leads to the inability to reproduce Coulomb blockade effects. We will discuss the origin and scope of these errors. Then, model calculations for molecules where Coulomb blockade effects have been observed experimentally will be presented, and the mechanism for the failure to reproduce Coulomb blockade effects will be explained. \newline \newline [1] M.Koentopp, K.Burke, F.Evers, PRB Rapid Comm. {\bf 73}, 121403(R) (2006) [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B44.00003: First-principles Studies of Single-molecule Conductance in Amine Linked Junctions Su Ying Quek, Mark S. Hybertsen, Latha Venkataraman, Michael Steigerwald, Colin Nuckolls, Steven G. Louie, J.B. Neaton Recently, it was discovered that the conductance of single molecule junctions with amine linkages to Au electrodes can be reliably and reproducibly measured. We compute and examine the conductance of prototypical single molecule junctions formed with amine-Au links using a first-principles scattering state method based on density functional theory. In particular, we elucidate the nature of the scattering states that give rise to the computed conductance, and relate the transmission spectra of each junction to intrinsic molecular and amine link properties. We explore the sensitivity of our results to specific contact geometries. The results are discussed relative to the measured distribution of conductance for each molecule. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B44.00004: Electronic transport through alkane chains: the case of end group functionalization G. Kim, W. Lu, S. Wang, M. Buongiorno Nardelli, J. Bernholc Using first-principles calculations, we have investigated the mechanism of metal/molecule coupling and its influence on the electronic transport properties in the prototypical case of long hydrocarbon (alkane) chains sandwiched between gold contacts. In our study, 1-Pentanethiol [CH$_3$-(CH$_2$)$_4$- SH] , 1-Pentylamine [CH$_3$-(CH$_2$)$_4$-NH$_2$], octanediamine [C$_8$H$_{16}$(NH$_2$)$_2$] and octanedithiol [C$_8$H$_{16}$(SH) $_2$] are anchored to ideally terminated Au (111) surfaces in order to investigate the effects of the functionalization of the end groups on the conduction properties. The results indeed show that the end group functionalization plays a crucial role in controlling the electronic transport through the molecule: the effective contact resistance of the amine/Au system is much smaller than that of the thiol/Au one, giving rise to a large difference in the I-V characteristics. Our results are in good agreement with recent experimental measurements of the tunneling current through these functional groups [1]. \\ \\ $[1]$ C. Chu and G. Parsons, to be published (2006) [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B44.00005: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 12:39PM - 12:51PM |
B44.00006: Transport in Molecular Junctions with Different Metallic Contacts John Lawson, Charles Bauschlicher Ab initio calculations of phenyl dithiol connected to Au, Ag, Pd, and Pt electrodes are performed using non-equilibrium Green's functions and density functional theory. For each metal, the properties of the molecular junction are considered both in equilibrium and under bias. In particular, we consider in detail charge transfer, changes in the electrostatic potential, and their subsequent effects on the IV curves through the junctions. Gold is typically used in molecular junctions because it forms strong chemical bonds with sulfur. We find however that Pt and Pd make better electrical contacts than Au. The zero-bias conductance is found to be greatest for Pt, followed by Pd, Au, and then Ag. (Physical Review B, 74, (2006), p 125401) [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B44.00007: Effects of -NO$_{2}$ substitution on charge addition and reorganization energies in phenylene ethynylene oligomers Steven Robey, N. E. Gruhn, J. Cizek, J. M. Tour Reports of non-linear transport in molecular-scale junctions have stimulated suggestions for computing and switching applications based on molecular electronics. One of the most widely referenced results is reported negative differential resistance (NDR) behavior in -NO$_{2}$ substituted oligo- phenylene ethynylenes (OPE). Theoretical work has invoked the importance of charge addition effects on conformation and electronic structure and polaronic effects to provide potential explanations for this behavior. We have investigated charge addition effects for pristine versus -NO$_{2}$ substituted OPE self-assembled monolayers using photoelectron spectroscopy in combination with ``doping'' with K. Results are consistent with differences arising from filling of levels associated with the - NO$_{2}$ group. We have investigated polaronic effects using photoelectron spectroscopy and optical absorption to guide calculations of reorganization energies for pristine versus -NO$_ {2}$ substituted OPE's. We find theoretical evidence for increased reorganization energy with -NO$_{2}$ substitution for anionic species by about 33 percent, with experimental values of the reorganization energy ranging from about 0.2 eV to 0.4 eV and theoretical values about 0.2 eV. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B44.00008: Theoretical Analysis of the Trends in Single Molecule Junction Conductance Formed Using Amine-Gold Links Mark Hybertsen, Michael Steigerwald, Latha Venkataraman, Colin Nuckolls The conductance of single molecule junctions using amine-gold links has now been measured for several families of molecules. Systematic trends are revealed with length, conjugation, conformation and substituents. The amine link group binds to undercoordinated gold atoms in the junction through a donor- acceptor type bond. The frontier orbitals that result are consistent with flexible and reproducible electronic coupling between the electrodes and the molecules. A simple proxy for the Au link site in each electrode allows direct calculation of the tunnel coupling through the molecule. The calculated trends in tunnel coupling are in excellent agreement with experiment within each family studied. Surprisingly, the trends between different families are also reproduced, albeit with a modest offset between families. The comparison between families is proposed to also involve small changes in the alignment of the gateway states controlling tunnel coupling with the electrode Fermi Energy. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B44.00009: Electronic transport through single molecules: effects of strain and contacts Helio Chacham, Ronaldo Batista, Mario Mazzoni, Ignacio Garzon, Marcela Beltran, Pablo Ordejon, Emilio Artacho We will present theoretical investigations on single-molecule electron transport. We will focus on the following systems: a) Connected Au nanoparticles: we performed a first principles study [1] of the electronic properties of lattices of Au nanoparticles functionalized by the conjugated molecules BDMT and BDCT. Distinct behaviors of the electron hopping matrix elements between particles as a function of compression are predicted for functionalized lattices. b) Current rectification with symmetric molecules: In an interesting experiment, Reichet et al. [2] measured the current through symmetric organic molecules and obtained asymmetric IV curves when the Au contacts are pulled apart. We show, by means of first-principles calculations, that this effect can originate from the formation of small Au chains between the molecule and the Au surfaces in an asymmetric way. [1] R. J. C. Batista et al, Phys. Rev. Lett. 96, 116802 (2006). [2] J. Reichet et al., Phys. Rev. Lett. 88, 176804 (2002). [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B44.00010: Controlled Fabrication and High-Resolution Imaging of Molecular-Scale Three-Terminal Devices Douglas R. Strachan, Danvers E. Johnston, Beth S. Guiton, T.-H. Park, M.J. Therien, Peter K. Davies, Dawn A. Bonnell, A.T. Charlie Johnson One of the biggest challenges to developing molecular-scale three terminal devices is to precisely fabricate and monitor the formation of the nanometer-scale electrodes (nanogaps). Recently, electromigrated nanogaps have been developed which provide sufficient gate-coupling to produce such devices. We have developed a technique for forming electromigrated nanogaps in a transmission electron microscope (TEM) in order to monitor their formation with high-resolution imaging in real time. The technique relies on computer-controlled electromigration using feedback to produce the nanogaps at room temperature. This TEM imaging allows us to monitor the dynamics of the device evolution, where the gaps remain ordered and clear of residue during the process. Using this technique, we find clear visual evidence for the importance of joule heating in the formation of electromigrated nanogaps. These electrodes can also be directly used in the construction of three terminal nanometer-scale devices. Our results have implications on the development of a wide range of novel molecular-scale devices. This work was supported by the National Science Foundation (NIRT Grant No. 0304531 and MRSEC award DMR05-20020). [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B44.00011: Movies of Electromigration During the Formation of Break Junctions Thiti Taychatanapat, K. I. Bolotin, F. Kuemmeth, D. C. Ralph Breaking metal wires by electromigration is a useful technique for making contacts for single-molecule devices. However, some research groups have found that a high percentage of gaps formed during electromigration (10-30{\%}) can contain metal nanoparticles which produce artifacts in the device's electrical characteristics that might be mistaken for molecular signals. Other groups, using slightly different electromigration protocols, observe these artifacts at much lower rates. Here we investigate this issue by examining the electromigration process in real-time using a scanning electron microscope. We provide direct confirmation for arguments that the amount of series resistance in the electromigration circuit is a critical parameter in controlling whether nanoparticles are formed within the device. By observing devices to which metal nanoparticles have been attached using linker molecules, we are also able to estimate the effective temperature experienced by molecular adsorbates during electromigration. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B44.00012: Formation of a Metallic Contact: Jump to Contact Revisited. C. Untiedt, M.J. Caturla, M.R. Calvo, J.J. Palacios, R.C. Segers, J.M. van Ruitenbeek The process of adhesion between two metallic surfaces has been described so far as involving an instability leading to a jump from tunneling into contact[1,2]. In the last decade some experiments have shown that this is not always the case, and sometimes the transition from tunnelling to metallic contact goes smoothly[3-5]. We have observed that the configuration and material composition of the electrodes before contacts largely determines the presence or absence of a jump. Moreover, when jumps are found preferential values of conductance have been identified. Through combination of experiments, molecular dynamics, and first-principles transport calculations these conductance values are identified with atomic contacts of either monomers, dimmers or double-bond contacts. These results provide basic understanding of fundamental interactions between surfaces at the nanoscale. [1]N. Agrait, et al. Phys. Rep {\bf 377},81 (2003) [2]U. Landman et al. Science {\bf 248}, 454 (1990) [3]G. Cross et al. Phys. Rev. Lett. {\bf 80}, 4685 (1998) [4]A. Halbritter et al. Phys. Rev. B. {\bf 68}, 035417 (2003) [5]L. Limot et al. Phys. Rev. Lett. {\bf 94}, 126102 (2005). [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B44.00013: High Quality Nanogap Electrodes for Electronic Transport Measurements of Single Molecules Danvers E. Johnston, Douglas R. Strachan, Beth S. Guiton, Peter K. Davies, Tae Hong Park, Michael J. Therien, A. T. Charlie Johnson Electromigrated metal electrodes and resulting devices have shown great promise in moving towards the realization of single molecule-based electronic devices holding the potential for a wide range of electronic applications. At present, a major concern is that the electronic behavior of such devices may be greatly influenced by residual nanoscale metal particles. We have developed a computer controlled electromigration (CCE) process for creating nanogaps at room temperature which allows us to characterize a bare nanogap before putting a molecule into the nanogap.$^{1}$ This is very different from other approaches used in the field where nanogaps are formed at low temperature with molecules already attached to the nanowire by employing a simple ramp up in voltage. Among the bare nanogaps we produced using CCE, tunneling behavior is observed with no indication of transport signatures associated with metal particle formation. Details of molecular measurements utilizing these clean gaps will be discussed. This work was supported by the National Science Foundation (NIRT Grant No. 0304531 and MRSEC award DMR05-20020). $^{1}$D. R. Strachan, D. E. Smith, D. E. Johnston et al., Appl. Phys. Lett. \textbf{86} 043109 (2005). [Preview Abstract] |
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