Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B16: Focus Session: Molecular-Scale Electronics I |
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Sponsoring Units: FIAP Chair: Udo Pernisz, Dow Corning Corporation, Midland MI Room: Baltimore Convention Center 312 |
Monday, March 13, 2006 11:15AM - 11:51AM |
B16.00001: Mechanically-Adjustable and Electrically-Gated Single-Molecule Transistors Invited Speaker: We describe the fabrication and characterization of single-molecule transistors whose properties can be tuned in two independent ways to achieve systematic measurements of electron transport. The spacing between the source and drain electrodes can be adjusted with better than 1 pm stability using the mechanical breakjunction technique -- the electrodes are freely suspended above a flexible substrate, and their spacing can be varied by bending the substrate. In the same devices, we are also able to apply a gate voltage to the molecule. This is done by employing lithographic techniques to suspend the breakjunction only 40 nm above the substrate surface and using the substrate as an electrostatic back gate. With the independent in-situ variations provided by these two experimental ``knobs'', we are able to achieve a more detailed characterization of electron transport through the molecule than is possible with either technique separately. To demonstrate the device capabilities, we have studied transport through single C$_{60}$ molecules at low temperature. We observe Coulomb blockaded transport and can resolve discrete energy levels of the molecule. We are able to mechanically tune the spacing between the electrodes (over a range of 5 \AA) to modulate the lead-molecule coupling, and we can electrostatically tune the energy levels on the molecule by up to 160 meV using the gate electrode. We will also present data of Kondo transport in single [Co(tpy-SH)$_2$]$^{2+}$ molecules. We are able to vary the strength of the Kondo resonance in these devices by changing the spacing between the source and drain electrodes. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B16.00002: Spontaneous oscillation of current in fullerene molecular junctions Chao-Cheng Kaun, Ryan Jorn, Tamar Seideman We study the correlated phenomena of resonant conduction and current-driven dynamics in Au--C$_{60}$--Au molecular junctions from first principles. A significant resonant component of the transmission gives rise to vibrations of the C$_{60}$ center of mass between the electrodes [C.-C. Kaun and T. Seideman, PRL 94, 226801 (2005)]. An interesting interplay between conductance channels of different symmetries and different degrees of spatial localization results in strong dependence of the transmission on the location of the fullerene within the junction. The distance-dependence, in turn, leads to oscillating current in the THz regime. Rotation of the C$_{60}$ about its axis likewise markedly modifies the transmission characteristics. [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B16.00003: Nano-electromechanical structures for single molecule transport studies and position sensing Shawn Tanner, Charles Rogers We have developed a process for making sub-micron cantilevers, clamped beams, and more complicated electro-mechanical structures that carry integrated electrical leads. Such objects are useful as test structures for measuring the electrical properties of molecular sized objects, as high frequency electromechanical components for radio frequency and microwave applications, and as sensor components for studying fluctuations in small systems. Our process uses two realigned electron beam lithography steps, a thin-film angled deposition system, and differential removal of sacrificial aluminum layers to produce freely suspended sub-micron electro-mechanical components. We have produced cantilevers and beams on a variety of substrates (silica, silicon, and polyimide) and have produced insulating, conductive, and multilayer electro-mechanical structures. The process allows the use of essentially any material that can be deposited from a thermal or electron-beam deposition source. We have constructed mechanically adjustable gold-vacuum-gold contacts and have demonstrated vacuum tunneling. The behavior of these contacts indicates a gold work function in excess of 4 eV, consistent with clean gold surfaces. [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B16.00004: Gold Nanogap Junctions Fabricated by Temperature-Controlled Electromigration G. Esen, M.S. Fuhrer Electromigration of gold nanowires of different cross-sectional areas are studied using a feedback-controlled electromigration algorithm. We have observed a linear correlation between the cross sectional area of the gold nanowire and the power dissipated in the junction during electromigration, indicating that the feedback mechanism primarily controls the junction temperature. We also show that the role of external feedback circuit is to prevent thermal runaway; minimization of series resistance allows control of electromigration with a simple voltage ramp. We also discuss the I-V characteristics of the junctions after they are formed. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B16.00005: Electron transport through OPE-based molecules in junctions formed by electromigration Xueqing Liu, Wei Chen, Zhongkui Tan, Konstantin K. Likharev, James Lukens, Andreas Mayr We have studied the self-assembly and electron transport properties of (i) simple oligo(phenylene ethynylene) (OPE) chains (2.2 nm) and (ii) OPE based molecules with a naphthalene diimide acceptor group (4.5 nm). Both are capped with terminal isocyanide groups. The molecules are self-assembled on gold wires with a cross-section $\sim $ 20$\times $100 nm$^{2}$. The junctions are formed by electromigration and the transport measurements are carried out at 4.2 K. For simple OPE chains, more than 40{\%} of the junctions have shown non-linear I-V curves with resistance R in the range from M$\Omega $ to G$\Omega $, due to trapping of single or multiple molecules. In $\sim $10{\%} of the junctions (R $\sim $ a few G$\Omega )$, we observe I-V curves with discrete current steps due to electron transport through one or a few molecules. The histogram of the step voltages shows grouping at certain levels. These levels are in a semi-quantitative agreement with our calculations based on the general theory of single-electron transport. For long OPE molecules with acceptor groups, the yield is low ($<$15{\%}), and the data show Coulomb blockade with threshold voltages from 30 to 200meV. Currently we are working on different support nano-structures that will promise higher yield. This work is supported by AFOSR and NSF. [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B16.00006: Electronic Quantum Interference in Molecular Devices Feng Miao, Douglas Ohlberg, R. Stanley Williams, C.N. Lau Understanding the mechanisms for electrical transport and conductance switching in molecular devices is necessary for developing molecular electronics. Here we use pressure-modulated conductance microscopy to characterize Pt/stearic acid monolayer/metal molecular heterostructures. We use either titanium or chromium as the top electrodes. By using atomic force microscope (AFM) to apply a localized force to the junction while monitoring device conductance, we observe nanoscale conductance peaks in response to applied pressure, indicating nanoscale conductance channels through the devices. Additionally, for devices with conductance of $\sim $1 to 2 conductance quantum, we also observe conductance dips and oscillations under localized pressure. The results are consistent with quantum interference of electrons between partially transmitting electrodes. [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B16.00007: Molecular conductance measurements through printed Au nano-dots. Weirong Jiang, Nikolai Zhitenev, Zhenan Bao, David Abusch-Magder, Don Tennant, Eric Garfunkel Gold pads with $\sim $100 nm diameter are imprinted on self-assembled monolayers of alkane dithiols of different lengths using nano-transfer technique. The fabrication technique ensures formation of chemical bonds at both ends of molecules while minimizes defect creations compared to other metallization methods. The pads are contacted by conductive atomic force microscope (CAFM) to study electron transport through the SAM as a function of contact force. We found that atomic scale topography at the metal-molecules interface is essential to describe the conductance-stress relationship. In as-fabricated devices, only small percentage of molecules (below 1{\%}) is wired to both contacts. A finite force (1-10 nN) deforms devices resulting in two competing effects: (a) contacting larger number of molecules leading; (b) deforming interfacial bonds and/or tilting the molecules. The estimated conductance of molecules is significantly smaller than in previous CAFM experiments and calculations. [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B16.00008: Single-Molecule STM Studies on Atomically-Flat Nanoparticles D.H. Dahayanaka, D.W. Kelle, D.J. Wasielewski, E.S. Day, D.R. White, L.A. Bumm, C.M. Waite, J.L. Moore, R.L. Halterman The scanning tunneling microscope (STM) has been broadly applied to measure electronic characteristics of individual molecules supported in an inert monolayer matrix, which is typically grown on gold thin films on mica or bulk single crystal substrates. Although these substrates are excellent for electronic measurements, they have serious disadvantages for optical measurements because they are not optically transparent and the metal surface can quench the molecular excited state. We demonstrate that single molecule electronic measurements can also be performed using atomically-flat gold nanoparticles (FGNPs) supported on indium tin oxide coated glass as a replacement for the typical gold substrate. These substrates are optically transparent and each of the FGNP ``nanosubstrates'' is an optically resonant photonic antenna, thus they have the added advantage that optical measurements can be performed. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B16.00009: Theory of Molecular Conformational Switching Shashi Karna We have investigated stability and current switching as a function of conformational change in tolane molecule by ab initio Hartree-Fock and density functional theory approaches. The planar conformation of the molecule in which the two benzene rings are coplanar is calculated to be more stable than a twisted conformation. The current ($I)$ with respect to the applied external potential ($V)$ increases from minimum to maximum as the two $\pi $-rings become planar, suggesting the ``ON'' and ``OFF'' mechanism of the molecular switch at planar and perpendicular conformations, respectively. [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B16.00010: Comparison of Transport and Switching Characteristics of Ti/Molecule/Pt and Cr/Molecule/Pt Devices Michael Taber, Feng Miao, Douglas Ohlberg, R. Stanley Williams, C.N. Lau Molecular devices that consist of a monolayer of stearic acid molecules sandwiched between Pt and Ti electrodes can be switched ``on'' and ``off'' reversibly. To gain insight into the switching and mechanism, we fabricated and measured similar molecular junctions by replacing Ti with chromium as the top electrode. These devices can be switched ``off'' controllably, but not ``on.'' The conductances of both types of devices display similar dependence on temperature. Latest data will be discussed in terms of various theoretical models. [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 1:51PM |
B16.00011: Controlling Negative Differential Resistance in Molecular Electronic Devices by Means of Designer Transition Metal Interfaces Hugh Dalgleish, George Kirczenow Observations of negative differential resistance (NDR) have been reported for a number of molecular junctions with potential for device applications and have helped fuel the promise of viable molecular nano-electronic technologies. Here we present predictions of non-linear transport phenomena in molecular junctions where single organic molecules bridge transition metal nanocontacts. We predict the transmission to be mediated by interface states that appear within the HOMO-LUMO gap due to hybridization between thiol-terminated ends of the molecules and the d-orbitals of the transition metal. Our calculations reveal resonant enhancement and reduction in the interface state transmission under the application of moderate bias that result in NDR in molecular junctions with Pd nanocontacts. We show that this NDR can be tailored by suitably choosing the nanocontact materials: If a Rh electrode is substituted for one Pd contact we predict the NDR of the molecular junction to be strongly enhanced. [Preview Abstract] |
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