Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D17: Focus Session: Transport Through Molecules: Scanned Probe Methods |
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Sponsoring Units: DMP DCP Chair: Mark Hybertsen, Columbia University Room: LACC 404B |
Monday, March 21, 2005 2:30PM - 2:42PM |
D17.00001: Control of Relative Tunneling Rates in Single Molecule Bipolar Electron Transport Shiwei Wu, George Nazin, Xi Chen, Xiaohui Qiu, Wilson Ho The influence of relative electron tunneling rates on electron transport in a double-barrier single-molecule junction will be presented. The junction is defined by positioning a scanning tunneling microscope (STM) tip above a copper phthalocyanine (CuPc) molecule adsorbed on a thin oxide film grown on the NiAl(110) surface. Unlike the typical single-barrier (vacuum) junction for STM experiments, the finite voltage drops in both barriers, vacuum and oxide film, lead to conduction through the same vibronic states of the molecule at opposite bias polarities -- bipolar conduction. However, the differential conductance (dI/dV) spectra taken at opposite bias polarities show a distinct asymmetry. By tuning the tip-molecule separation, we control the ratio of electron tunneling rates through the two tunnel barriers. This results in dramatic changes in the relative intensities of individual conduction channels, associated with different vibronic states of the molecule. [Preview Abstract] |
Monday, March 21, 2005 2:42PM - 2:54PM |
D17.00002: Two-probe theory of scanning tunneling microscopy of single molecules John Buker, George Kirczenow Experiments in scanning tunneling microscopy of single molecules adsorbed on substrates have produced topographic maps showing how electron flow through a molecule depends on the position of the STM tip above the molecule. However, in some experimental situations, very different topographic maps are obtained when a molecule is adsorbed at different locations on the substrate \footnote{X.H. Qiu, G.V. Nazin, and W. Ho, Science {\bf 299}, 542 (2003).}. This suggests that the tip position is not the only important determining factor for electron flow through the molecule: It is possible that electron flow also depends strongly on the details of the coupling between the molecule and the substrate. However, theoretical work on STM imaging to date has focussed primarily on the role of the tip-molecule coupling. In this talk we re-examine scanning tunneling microscopy of molecules, treating the tip-molecule coupling and the molecule-substrate coupling on the same footing. Treating both the tip and substrate as probes coupled to the molecule, we find that the STM image of a molecule can be sensitive to the geometry of the molecule-substrate coupling. We obtain distinct topographic maps for various configurations of the stationary probe with respect to the molecule, and explain their differences in terms of the molecular orbitals that mediate electron flow in each case. Work supported by NSERC and the Canadian Institute for Advanced Research. [Preview Abstract] |
Monday, March 21, 2005 2:54PM - 3:06PM |
D17.00003: Controlling single-molecule negative differential resistance in a double-barrier tunnel junction Xiuwen Tu, Gareguin Mikaelian, Wilson Ho Negative differential resistance (NDR), the phenomenon of decreasing current with increasing voltage over certain voltage range, has found applications such as high frequency oscillators and high speed switches. NDR has been observed with a low temperature scanning tunneling microscope (STM) in the differential conductance spectra of single copper phthalocyanine (CuPc) molecules adsorbed on one and two atomic layers of NaBr film grown on a NiAl(110) surface. However, there is no NDR for single CuPc molecules adsorbed on three atomic layers of NaBr on NiAl(110). This is explained as the result of competing effects of increasing sample bias on tunnel barrier heights across vacuum junction and NaBr junction for resonant tunneling through the lowest unoccupied molecular orbital (LUMO) of CuPc. Whereas increasing positive sample bias increases tunnel barrier height across the vacuum junction, it decreases tunnel barrier height across the NaBr junction. The overall behavior, NDR versus the absence of NDR, depends on which of these two effects dominates. Numerical simulation supports this mechanism. Our results show that transition from NDR to the absence of NDR can occur as sharply as the addition of one atomic layer of NaBr. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D17.00004: Ferrocenyl-alkanethiolate on the Au(111) surface: Electronic and transport properties in density functional theory Shuchun Wang, Wenchang Lu, Jerry Bernholc Self-assembled monolayers of organic molecules on the gold surface have shown a number of interesting features, such as non-linear conductance and negative differential resistance (NDR). We focus here on ferrocenyl-alkanethiolate on the Au(111) surface, for which NDR has been observed in experiments. In order to understand conduction through the ferrocenyl-alkanethiolate, we investigate its atomic and electronic structure on the surface, as well as its electron transport properties within density functional theory. The real-space multigrid method and ultrasoft pseudopotentials are used to examine various surface configurations with different molecular coverages. We find that the attachment of ferrocenyl significantly reduces the packing density of alkanethiolates on the gold surface. The saturation coverages for different molecular species will be discussed. The conductances and I-V curves, calculated using non-equilibrium Green's functions expanded in variationally optimized localized orbitals, are analyzed in terms of resonant tunneling and modification of molecular levels under bias. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D17.00005: Scanning Tunneling Spectroscopy of Molecular Electronics Switches J.W. Gadzuk Theoretical aspects of STS pertinent to molecular electronics systems are considered. The role of multiple-state scattering resonances localized at variable distances along a one-dimensional wire (part of a parallel array lined upright to form a finite-thickness monolayer adsorbed on a metal electrode) is examined. This provides a plausible model for a mobile ligand or attachment responsible for switching and/or negative differential conductance in a MolE device. It equates to the formal problem of a discrete state (with internal structure) coupled to a 1-D discretized continuum (of molecular orbital/band states). When the continuum level spacing, an inverse function of the wire length, becomes comparable with other characteristic energies of the system such as the coupling strength, vibrational energies, or resonance width, then the position-dependent spectroscopic signature of the switch will be qualitatively different from the usual Fano resonance profile, in a fashion similar to STM line shapes of Kondo impurities in quantum corrals. This dependence provides a useful systematic quantitative tool helpful in the characterization of the electronic conduction properties of MolE constructs. [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D17.00006: Scanning Tunneling Microscopy studies of single conjugated diblock co-oligomers Maria Iavarone, Goran Karapetrov, Wai K. Kwok, Gustavo Morales, Ping Jiang, Yu Luping The electrical properties of single conjugated diblock co- oligomers were studied with a low temperature Scanning Tunneling Microscope (STM) at 4.2 K and 77 K. The molecules consist of an electron rich biphenyl segment and an electron poor bipyrimidine segment with two different protected terminal thiol groups that can be sequentially connected to gold electrodes. They were first self-assembled on a gold substrate Au(111) embedded in an insulating matrix of dodecanethiol and then a gold nanoparticle was connected as the second gold electrode on the top thiol. Thus the diode molecules were connected at both ends to gold electrodes and ready for the electron-transport study. In STM topography we observed a hexagonal closed packed lattice of the matrix molecules and we could locate single gold nanoparticles. Scanning tunneling spectroscopy on the gold nanoparticles and away from them were performed. Symmetric I-V characteristic were observed on the SAM matrix by switching the bias between --2.0 V to +2.0 V at 4.2 K and 77 K. Strongly asymmetric I-V were recorded on the top of gold nanoparticles suggesting that the diblock acts as a molecular rectifier. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 4:18PM |
D17.00007: Selectively Doping Individual C60 Molecules and Clusters Atom by Atom Invited Speaker: Future molecular technologies are expected to rely on the ability to locally control the electronic properties of single molecules and molecular complexes. It is critical therefore to understand the influence on molecular properties of different molecular spatial and electronic configurations in the presence of metal electrodes. Here we present a novel local probe technique for constructing and investigating electronically modified molecular complexes. Our technique is based on the molecular manipulation capabilities of a scanning tunneling microscope for controllably changing the charge state of C$_{60}$ molecules [1]. By moving a single C$_{60}$ molecule over successive K atoms adsorbed onto the Ag(001) surface, charge-donating K atoms are attached to the fullerenes' outer surface. This allows the synthesis of isolated K$_{x}$C$_{60}$ complexes in a controllable and reversible manner for $x$ ranging from 0 to 6. The doped C$_{60 }$molecules can then be combined using molecular manipulation to form larger molecular complexes where the local doping level is precisely controlled. I will discuss the topographic and spectroscopic properties of such selectively doped K$_{x}$C$_{60}$ complexes and implications for possible devices. [1] R. Yamachika, M. Grobis, A. Wachowiak and M. F. Crommie, Science \textbf{304}, 281 (2004). [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D17.00008: Single Molecule Conductance Measurement of Photochromic Molecules and Carotenoids Jin He, Fan Chen, Jun Li, Otto Sankey, Yuichi Terazono, Paul Liddell, Joakim Andreasson, Stephen Straight, Deven Gust, Thomas Moore, Ana Moore, Stuart Lindsay We report data for the single molecule conductance of (a) photochromic molecules in the `open' and `closed' forms and (b) carotenoid polyenes. The photochromic molecules we studied switch between an open state (that absorbs in the UV to become closed) and a closed state (that absorbs in the visible to become open) through light-induced isomerization. The molecular resistance is 526$\pm $90 M$\Omega $ in the open form and 4$\pm $1 M$\Omega $ in the closed form when attached to gold break junction electrodes via thiol linkages. Carotenoid polyenes play an essential role as `molecular wires' in photosynthesis. We measured the electrical conductance of a series of carotenoids with 5, 7, 9 and 11 double-single bond pairs. The electronic decay constant, $\beta $, is determined to be 0.224$\pm $0.036{\AA}$^{-1}$ in close agreement with the value obtained from first principles simulations (0.217$\pm $0.01 {\AA}$^{-1})$. The absolute values of the molecular conductance are within a factor three of those calculated from first-principles. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D17.00009: STM Spectroscopy of Individual Doping Centers in a Monolayer Organic Crystal Gueorgui Nazin, Xiaohui Qiu, Wilson Ho A scanning tunneling microscope (STM) is used to study individual Ag doping centers in a monolayer of C$_{60}$ molecules supported on a thin Al$_{2}$O$_{3}$ film grown on the NiAl(110) surface. The Al$_{2}$O$_{3 }$film acts as a spacer reducing the interaction of the C$_{60}$ layer with the metal substrate. Vibronic states of the doping centers are observed with differential conductance spectroscopy. Charging of individual doping centers becomes possible upon applying sufficiently high bias voltage to the junction. Differential conductance spectroscopy shows that charging affects the conduction through C$_{60}$ molecules located around the doping centers. This effect is used to observe the electrostatic interaction of a pair of Ag doping centers. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D17.00010: Vibrational Spectroscopy of Biomolecules via Scanning-Thermal Conductance Sarah Dunning, Michael Geller Since the advent of the Scanning Tunneling Microscope (STM), considerable progress has been made in the use of scanning tunneling microscopy to image and spectroscopically identify single biomolecules. However, for molecules and substrates of poor electrical conductivity, tunneling microscopy is not effective. Here we propose a scanning thermal-conductance microscope, which measures the transport of thermal energy carried by phonons, from a temperature controlled atomic-force microscope tip to a substrate. We apply this idea to a model DNA strand and show that such a thermal conductance measurement can be used to identify the base-pair sequence. [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D17.00011: STM Characterization of Si Surface Functionalized with Organic Layers Hongbin Yu, Lauren Webb, Peigen Cao, Santiago Solares, Ryan Ries, William Goddard III, Nathan Lewis, James Heath High quality, high density Si nanowires can be used for making high density memory and logic circuits, and for biomolecule sensing when nanowires are selectively functionalized. An important step towards successful functionalization of Si wires is to control and understand the covalently bound monolayer of hydrocarbon molecules onto a silicon surface. Low temperature scanning tunneling microscopy (STM) data have been obtained on a series of alkyl group-terminated crystalline Si(111) surfaces that were prepared through a two-step chlorination/alkylation technique. The data from CH$_{3}$-terminated surface revealed a well-ordered structure commensurate with the atop sites of an unreconstructed 1x1 overlayer on the silicon (111) surface. Images collected at 4.7 K revealed bright spots, separated by 0.18 $\pm $ 0.01 nm, which are assigned to adjacent H atoms on the same methyl group. The orientation of the methyl group with respect to the Si lattice can subsequently be determined as well as the interactions among different surface species. Ethyl- terminated Si surface will also be discussed. [Preview Abstract] |
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