Papers

AVS2002 Session NS-ThM: Single Molecule Devices

Thursday, November 7, 2002 9:40 AM in Room C-207

Thursday Morning

Start	Invited?	Item
9:40 AM		NS-ThM-5 Toward the DNA Electronics T. Kawai, H. Tabata (Osaka University, Japan) DNA is one of the most promising molecules as the scaffold for molecular nanotechnology toward nanoelectronics. DNA has the special double helix structure with p-electron cores of well-stacking bases for one-dimensional charge transport. The investigations of DNA on the nanostructure, electrical conductivity and electronic states have significant implications for the application of DNA in electronic devices and in DNA-based electrochemical biosensors. It is worthily noted that divergent and controversial conclusions were reported in DNA-mediated charge transport. The direct measurements of the intrinsic electrical characteristics of polynucleotides using a conducting probe atomic force microscope have been performed using self-assembled two dimensional DNA networks. It has been revealed that DNA without carrier doping is a wide-gap semiconductor. Upon carrier doping, poly(dG)?poly(dC) show the p-type behaviors, presumably due to the shallow ionization potentials of DNA bases. The conductivity of these molecules has been successfully controlled by chemical doping, electric field doping and photo-doping. It is found that the poly(dG)?poly(dC) has the best conductivity and can act as a conducting nanowire. The conductive mechanism is discussed by the charge hopping model based on the SPM observation of DNA nanostructure. For the advanced construction of DNA based molecular memories and circuits, gold and cobalt particles have been assembled within the two-dimensional DNA networks. Gold particles are arranged artificially with DNA molecular template as an average separation distance of 260nm. The pattern of the complex is controlled by changing the concentration of the DNA solution, suggesting that this method is effective in achieving the positional control of nano-scale molecular memories and circuits. T.Kawai et al; Appl.Phys.Lett.,77,3848(2000), Appl.Phys.Lett., 77,3105(2000), Surf.Sci.Lett,432,L611(1999), J.Vac.Sci.Technol.B17,1313(1999), Jpn.J.Appl.Phys. 39, 581(2000), 38,L606(1999), 38,L1211(1999)
10:20 AM		NS-ThM-7 Lander Molecules Acting as Nanomolds on Cu(110) F. Rosei, Y. Naitoh, P. Thostrup, M. Schunack (University of Aarhus, Denmark); P. Jiang, A. Gourdon (CEMES-CNRS, France); E. Laegsgaard, I. Stensgaard (University of Aarhus, Denmark); C. Joachim (CEMES-CNRS, France); F. Besenbacher (University of Aarhus, Denmark) The adsorption of a large organic C₉₀H₉₈ molecule, known as the Lander molecule, is studied by Scanning Tunneling Microscopy (STM) on a Cu(110)surface.¹ Manipulation experiments on isolated Landers anchored at step edges at low temperatures, reveal a restructuring of the Cu steps. Surprisingly, when the molecule is removed from the step, a tooth-like structure appears (two atomic rows in width), corresponding to the distance between the spacer legs within the molecule. Scanning Tunneling Spectroscopy measurements are in progress to investigate the electronic states of the Lander on Cu(110). This is the first prototype of more complex molecular machines able to selffabricate nanostructures with the prospect of developing planar and atomically precise interconnections of molecular nanodevices. Furthermore, by nanopatterning the substrate via O₂ chemisorption and using this template for Lander adsorption, we show that it is possible to self-assemble long 1D molecular wires. This type of assembly opens new possibilities for ordering organic molecules on surfaces. ¹ F. Rosei et al., Science 296, 328 (2002).
10:40 AM		NS-ThM-8 Electronic Properties of Individual Defects in Molecular Circuit Elements S.V. Kalinin, M. Freitag, A.T. Johnson, D.A. Bonnell (University of Pennsylvania) The local property variations of nanowires, nanotubes, and functional molecules dictate the behavior of nanoelectronic devices. Scanning Gate Microscopy (SGM) provides information regarding individual defects during dc transport. To determine local properties of defects during ac transport, we have developed Scanning Impedance Microscopy (SIM). In combination these measurements quantify the electronic structures of individual defects in nanocircuits. A circuit is configured with a molecule or nanotube on an oxidized Si wafer with metal contacts at each end and a back electrode. In the case of a semiconducting single walled carbon nanotube the defects become depleted at a gate voltage that is related to the local electronic structure of the defect. The depletion voltage for each individual defect can be accessed in several ways. In SGM the current through the circuit is measured and the scanning probe tip provides a local gate voltage. In SIM an ac signal is applied across the circuit while the tip measures the local potential amplitude. The gate voltage in this case can be applied both from the back electrode (back gate) and the tip (tip gate). In both SGM and SIM the defects are manifest as sharp discontinuities in the image when they are depleted. The gate voltage dependence of the image contrast is a direct measure of the difference in Fermi energies at these defects. A comparison of results from nanotube circuits and molecular circuits will be presented and implications to local electronic structure and transport mechanisms will be discussed.
11:00 AM		NS-ThM-9 Single Molecular Switches P.S. Weiss (The Pennsylvania State University) We use intermolecular interactions to direct molecules into desired positions to create nanostructures, to connect functional molecules to the outside world, and to serve as test structures for measurements on single or bundled molecules. We use and develop scanning probe microscopes to determine both local structures and the electronic and other local properties. We have applied these to isolate molecules with electronic function to determine the mechanisms of function, and the relationships between molecular structure, environment, connection, coupling, and function. We have been able to demonstrate that single molecules can function as multistate switches, and have determined important aspects of the mechanism, function, and persistence of switching. We will discuss the origins of switching and the relevant aspects of the molecular structure and environment required.
11:40 AM		NS-ThM-11 High-bias Conductance in Single-atom Contacts of Au Alloys J. Mizobata, A. Fujii, S. Kurokawa, A. Sakai (Kyoto University, Japan) Single-atom contacts of Au have a strong tendency to exhibit a conductance quite close to 1G0, one quantum unit of conductance, as long as a bias voltage is less than 1 V. They show a well-defined peak at 1G0 when their conductance data are plotted in a histogram. With increasing the bias, however, the 1G0 peak decreases in height and disappears at 1.9 V at room temperature. This result suggests that a single-atom 1G0 contact of Au becomes unstable under high biases, perhaps due to an extremely high current density in the contact, which may cause electromigration or current-induced bond weakening. In an effort to improve the stability of single-atom contacts of Au, we have recently carried out experiments on Au alloy contacts, containing Ag and Pt as solute atoms, and compared their high-bias conductance with that of pure Au. All measurements were performed at room temperature with varying the bias from 0.2 V to 2.0 V. In the case of an Au20wt%Ag alloy, we found that the 1G0 peak is systematically higher than that of pure Au for 0.2-1.2 V. The Ag alloying is thus effective for improving the stability of Au single-atom contacts against high contact current. However, the 1G0 peak height difference between AuAg and pure Au disappears for higher biases, and the positive effect of Ag alloying is somehow limited for biases lower than 1.4 V. On the other hand, an alloying with 20wt%Pt yields no enhancement in the 1G0 peak height, and the peak disappears at 1.0 V. Therefore the effect of alloying on the high-bias 1G0 conductance of Au depends on an alloying element.