abstracts.pdf

Understanding molecular conduction

Supriyo Datta School of Electrical and Computer Engineering

Purdue University, West Lafayette, IN 47907

It is common to differentiate between two ways of building a nanodevice: a top-down approach where we start from something big and chisel out what we want and a bottom-up approach where we start from something small like atoms or molecules and assemble what we want. When it comes to describing electrical resistance, the standard approach could be called a “top-down” one where we start from big complicated resistors and work our way down to molecules primarily because our understanding has evolved in this top-down fashion. But I believe it is instructive to take a bottom-up view of the subject starting from the conductance of something really small, like a molecule, and then discussing the issues that arise as we move to bigger conductors. That is what I will try to do in this tutorial lecture [1].

Remarkably enough, no serious quantum mechanics is needed to understand electrical conduction through something really small, except for unusual things like the Kondo effect that are seen only for a special range of parameters. I will (1) start with energy level diagrams, (2) show that the broadening that accompanies coupling limits the conductance to a maximum of (q^2/h) per level, (3) describe how a change in the shape of the self-consistent potential profile can turn a symmetric current-voltage characteristic into a rectifying one, (4) show that many interesting effects in molecular electronics can be understood in terms of a simple model, and (5) introduce the non-equilibrium Green’s function (NEGF) formalism as a sophisticated version of this simple model with ordinary numbers replaced by appropriate matrices. Finally I will describe the distinction between the self-consistent field regime and the Coulomb blockade regime and the issues involved in modeling each of these regimes.

1. S. Datta, “Electrical Resistance: An Atomistic View,” Nanotechnology, 15, S433 (2004).

Figure: What is the

resistance of a molecule?

Quantum chemistry

George C. Schatz and Mark A. Ratner Department of Chemistry

Northwestern University, Evanston IL 60208-3113 This tutorial will provide an overview of electronic structure calculations from a chemist’s perspective. This will include a review of the basic electronic structure theories: Hartree-Fock and beyond, density functional theories and semiempirical theories; the atomic orbital basis sets used to represent the wavefunction; and properties that can be obtained from solutions to the Schrodinger equation. Much of the discussion will center on quantum chemistry methods that use Gaussian orbital basis functions to determine molecular orbitals, equilibrium geometries and the thermochemical properties of small molecules. We will also consider extensions relevant to molecular electronics.

Probing molecular conduction with scanning probe microscopy

Mark C. Hersam Department of Materials Science and Engineering Northwestern University, Evanston IL 60208-3108

This tutorial will provide an overview of scanning probe microscopy (SPM) and its application towards problems in molecular conduction. In an effort to communicate the power and limitations of these instruments, the tutorial will describe design considerations and reveal the detailed construction of a cryogenic variable temperature ultra-high vacuum scanning tunneling microscope. With the microscope complete, the tutorial will then discuss its use for a variety of techniques that have been used to study the properties and performance of molecular-scale electronic devices. Specific examples include Kelvin probe microscopy, conductive atomic force microscope potentiometry, scanning tunneling spectroscopy, and inelastic electron tunneling spectroscopy.

Controlled molecular adsorption on Si: Laying a foundation for molecular devices

Bob Wolkow Department of Physics, University of Alberta

and National Institute for Nanotechnology, Edmonton Alberta

Our understanding of and control over molecular adsorption on silicon has

advanced very significantly in the last several years. It is now possible to provide a microscopic picture of structure and bonding in covalently attached molecule-silicon surface systems. This detailed understanding of adsorbate-surface structures was entirely lacking when the first wave of enthusiasm for molecular devices crested roughly 20 years ago. While many ideas for molecule-scale devices have been put forward in the past, the tools - both synthetic and analytical - to pursue those ideas did not exist. Now, the control necessary to begin exploring ways to incorporate organic function into existing technologies or, eventually, to make new molecule-scale devices is within reach [1]. Experimental and modeling methods have emerged that effectively extend the resolution of STM to see the details of adsorbed molecule structure and bonding. In the next several years it is now realistic to expect structures and concepts dreamed about for decades to begin to be realized. This talk will focus on a self-directed growth process for creating molecular nanostructures on silicon [2] and extensions of that process [3-7].

1. Controlled Molecular Adsorption on Si: Laying a Foundation for Molecular Devices, R.A. Wolkow, Annual Review of Physical Chemistry, 50, 413-41, 1999.

2. Self-Directed Growth of Molecular Nano-Structures on Silicon, G.P Lopinski, D.D.M. Wayner and R.A. Wolkow, Nature 406, 48 (2000).

3. Electronic structure and STM images of self-assembled styrene lines on a Si(100) surface, W.A. Hofer, A.J. Fisher, G.P. Lopinski and R.A. Wolkow, Chem Phys Lett, 365, 129–134 (2002)

4. Patterning of vinyl ferrocene on H-Si(100) via self directed growth of molecular lines and STM induced decomposition, Peter Kruse and R.A.Wolkow, Nano Lett.; 2, 807-810 (2002).

5. Organic modification of hydrogen terminated silicon surfaces, Danial D. M. Wayner and Robert A. Wolkow, J. Chem. Soc., Perkin Trans. 2, 23-34 (2002).

6. Reversible Passivation of Silicon Dangling Bonds with the Stable Radical TEMPO, Pitters, J. L.; Piva, P. G.; Tong, X.; Wolkow, R. A. Nano Lett.;3, 1431-1435 (2003).

7. “Gentle lithography'' with benzene on Si(100), Peter Kruse and Robert A. Wolkow, Appl. Phys. Lett. 81, 4422-24 (2002).

Molecular electronics: Theoretical hits and misses

A. W. Ghosh, G-C. Liang, T. Rakshit, K. Bevan, S. Pati, M. Paulsson, F. Zahid, D. Kienle, E. Polizzi, B. Muralidharan and S. Datta

Department of Electrical and Computer Engineering Purdue University

Over the last few years, there have been a lot of experimental and theoretical activity in the emerging field of molecular electronics. As in any new area of research, progress has been stymied at times by experimental glitches including artifacts, characterization problems and lack of reproducibility by different groups. In the theoretical world as well, there has been similar confusion, with lack of proper benchmarking and consistency between theory and experiments, and indeed among different theories. In this talk, I will concentrate on our general theoretical understanding of available experiments, concentrating on what is understood and what is not.

The non-equilibrium Green's function approach (NEGF) gives us a formally rigorous platform for studying quantum kinetics, including effects of scattering and many-body correlations. Our past endeavors in this direction have included making NEGF part of the silicon device theorists' toolkit, as well as investigating various molecular I-Vs [1]. Our approach consists of self-consistently combining the electronic structures of the conducting species and the contacts with the NEGF transport formalism, allowing us to explore various interesting physics as well as device prospects. Encouraging quantitative agreements have been achieved with experiments showing ohmic conduction on quantum point contacts [2], resonant conduction in phenyl [2] and xylyl dithiol [3], and rectifying behavior arising from charging in other organic molecules [4]. Furthermore, we have a clearer understanding of factors responsible for various aspects of conduction, such as (a) the role of the potential profile [5], (b) the importance of the conformational flexibility [6], (c) a proper understanding of level broadening by the contacts [7], and (d) the position of the Fermi energy relative to the molecular levels [8]. Our recent endeavors in this direction include incorporating electron-electron interactions to develop a proper Coulomb Blockade theory of molecular conduction within a model Hamiltonian for benzene, and a treatment of electron-phonon interactions to study various observed inelastic tunneling (IETS) spectra.

From a scientific and technical point of view, one of the interesting directions to pursue is molecules on silicon substrates. This is a system of inherent applied interest with memory elements and sensors in mind. Furthermore, it promises new physics due to the silicon bandstructure, making the contact an active player in the conduction process rather than a passive electron injector. In particular, the silicon band-edge acts as a filter which leads to peaks in the I-V (negative differential resistance, NDR) thereby accentuating features intrinsic to the molecule. Based on a simple band-diagram, we predicted a doping-dependent one-sided NDR [9] which was shortly thereafter demonstrated through a powerful set of experiments from Mark Hersam's group at Northwestern University [10]. Doing quantitative justice to this problem, however, has been elusive for some time. One of the challenges was to formally integrate a semi-empirical description of silicon surface physics (including band-bending, reconstruction and surface state effects) with a first-principles description of the molecule, ie, bringing

the physicists' and chemists' worlds together within one formalism. I will outline some of our endeavors in this manner, which have allowed us to obtain an atomistic theory of molecular I-Vs on silicon with encouraging quantitative agreements with experiments (such as with STM data on the bare silicon surface and with various measurements on buckyballs on silicon). However, one of the outstanding puzzles remains the quantitative explanation of the experiments from Northwestern University, where the NDRs seem to appear prematurely compared to our theoretical estimates. [1] P. S. Damle, A. W. Ghosh and S. Datta, in "Molecular Nanoelectronics," Ed. Mark Reed and Takhee Lee. [2] P. S. Damle, A. W. Ghosh and S. Datta, Phys. Rev. B 64, R201403 (2001). [3] A. W. Ghosh, P. Damle, S. Datta and A. Nitzan, MRS Bulletin 29, 391 (2004). [4] F. Zahid, A. W. Ghosh, M. Paulsson, E. Polizzi and S. Datta, cond-mat/0403401. [5] G-C. Liang, A. W. Ghosh, M. Paulsson and S. Datta, Phys. Rev. B 69, 115302 (2004). [6] A. W. Ghosh, T. Rakshit and S. Datta, Nano Letters 4, 565 (2004). [7] P. S. Damle, A. W. Ghosh and S. Datta, Chem. Phys. 281, 171 (2002). [8] A. W. Ghosh and S. Datta, Journal of Computational Electronics 1, 515 (2002). [9] T. Rakshit, G-C. Liang, A. W. Ghosh and S. Datta, cond-mat/0305695. [10] N. P. Guisinger, M. E. Greene, R. Basu, A. S. Baluch, and M. C. Hersam, Nano Letters 4, 55 (2004).

Ballistic emission electron microscopy studies of metal-molecule-semiconductor diodes

J. W. P. Hsu,1 W.-J. Li,2 K. L. Kavanagh,2 C. M. Matzke,1 A. A. Talin3

1 Sandia National Laboratories, Albuquerque, NM 87123 2 Department of Physics, Simon Fraser University, BC, V5A 1S6, Canada

3 Sandia National Laboratories, Livermore, CA 94551

Using ballistic electron emission microscopy (BEEM), an application of scanning tunneling microscopy (STM), we measured the transport through alkanedithiol molecules that are chemically bonded to GaAs and Au. In these devices, the molecules are sandwiched between the two electrodes and are not easily accessible. BEEM is one of the very few experimental techniques that is capable of measuring the local transport through such buried interfaces. Carriers that are transmitted ballistically across the thin Au layer, through the molecular layer, and into the GaAs are detected. The BEEM current is measured as a function of the tip bias and thus, the local interfacial barrier height is determined without applying a bias across the metal-semiconductor interface. Since BEEM is a local measurement, it should be able to distinguish artifacts such as metallic filaments from purely molecular transport. Our preliminary results on Au/alkanedithiol/GaAs diodes with electron-beam evaporated Au contacts onto the molecular layer show a large increase in the I-V barrier height (1.1 eV) in the presence of the molecular layer, with a repeatable BEEM threshold voltage 1.4 eV occurring at scattered locations. This is compared with 0.8 eV barrier heights for the Au/GaAs control diodes.

Metal/molecule/semiconductor device structures

David B. Janes School of Electrical and Computer Engineering

And Birck Nanotechnology Center Purdue University, W. Lafayette, IN 47906 USA

This talk will describe the development and electrical characterization of

molecular electronic components involving metal/molecule/semiconductor (MMS) device structures. The devices are lithographically defined and fabricated using an indirect evaporation technique for the metal (top) contact and p+ GaAs for the bottom contact. In these structures, the electronic conduction between the metal and semiconductor can be modulated by choice of molecular species. Several alkyl thiol and aromatic thiol molecules have been employed in order to determine the effects of molecular length, conjugation and intrinsic dipole moment. The MMS devices exhibit larger current densities than metal-GaAs control samples. The current-voltage characteristics and conductance of the devices have been studied versus temperature. Generally, the MMS devices exhibit lower observed activation energies than that of a metal/semiconductor Schottky barrier. These results reflect previous studies in which nanoscale metal/molecule/semiconductor structures exhibited low resistance contacts, implying that effective coupling and control of the surface electrical properties can be achieved using a molecular layer1. An electrostatic model for the MMS structure has been developed, in order to determine the band diagram of the structure. Two important observations arise from this model. First, due to the relatively low dielectric constant of the molecular layers, the majority of both the built-in and applied potentials is dropped across the molecular layer. Second, there is generally a depletion region in the semiconductor layer, with a width which is significantly smaller than that of the metal-semiconductor control sample. Therefore, the overall conduction appears to be determined by a compound barrier consisting of both the molecular states and a modest depletion region in the semiconductor. These results indicate that the interface Fermi level is not pinned, consistent with our prior studies on surface Fermi level unpinning in GaAs structures2. The model 1) T. Lee, et al., APL 76, 212 (2000). 2) S. Lodha, et al., Appl. Phys. Lett. 80, 4452 (2002).

Current-induced effects in molecular junctions

Max Di Ventra Department of Physics

University of California at San Diego

I will discuss current-induced effects in molecular junctions such as electromigration, local heating, inelastic current and current fluctuations. The main focus will be on their description at the atomic level. Particular attention will be given to open questions regarding these effects, and their impact on the stability of molecular structures under current flow and as possible diagnostic tools.

Reading chemical information of a single molecule electronically

N. J. Tao Department of Electrical Engineering

Arizona State University, Tempe, Arizona 85287

The ability to measure and control electron transport through a single molecule electrically wired to two electrodes is a basic requirement towards the goal of building an electronic device using single molecules. It also allows one to read the chemical and biological information of the molecule electronically, which opens the door to chemical sensor applications based on electrical measurement of individually wired molecules. To reliably measure the conductance, one must provide a reproducible electronic coupling between the molecule and the probing electrodes. One must also find a signature to identify that the measured conductance is due to not only the sample molecules but also a single sample molecule. Finally, for biologically relevant molecules, it is highly desired to carry out the measurement in aqueous solutions in order to preserve the native conformation of the molecule. We have developed a method to attach a single molecule to two electrodes via covalent bonds, which allows us to reliably measure single molecule conductance of many systems, including peptides and DNA in aqueous solutions. By simultaneously measuring the conductance and the force required to break a molecule from contacting the electrodes, we can identify how many molecules are involved in the measurements and if the molecules are indeed covalently bonded to two electrodes. We have studied the dependence of the conductance on the sequence, length as well as specific bindings of the molecules with other species.

Inelastic effects in molecular conduction

Abe Nitzan Department of Chemistry

Tel Aviv University

This work will give an overview of work done at Tel Aviv University and at Northwestern University on effects of electron-phonon coupling on molecular conduction, including dephasing, dissipation, heating and heat conduction, then describe some recent observations, interpretations and predictions on inelastic electron tunneling spectroscopy. (with M. Galperin and M. Ratner, NWU, and D. Segal Weizmann Institute)

Non-linear conductance in molecular electronics

R. Shashidhar, David Long, and Jason Lazorcik Geo-Centers Inc, Washington D.C. 20003

B. R. Ratna, J. Kushmerick, and A. Blum

Naval Research Laboratory, Washington D.C. 20375

Non-linear conductance is a very important factor in the development of molecular electronic devices. We have carried out a variety of studies aimed at understanding the fundamental aspects associated with conductance switching and other non-linear molecular electronic behavior in two-terminal devices. The following results covering both these aspects will be presented: Results on conductance switching and non-linear electronic behavior:

1. Using molecules with asymmetric attachment groups on the two ends and step-wise self-assembly; it is shown that molecular rectification is derived solely from the internal molecular asymmetry and not from the nature of metal-molecule contacts.

2. By a combination of experimental techniques on the same molecular system, it is

demonstrated that voltage controlled molecular switching is distinct from stochastic switching. Scanning tunneling spectroscopy and crossed-wire tunnel junction measurements on the same molecule demonstrate that the switching is indeed an inherently molecular event.

3. Inelastic tunneling spectroscopy studies on insulating alkyl and π-conjugated

molecular wires are presented. The results provide experimental insight into the coupling between tunnel charge carriers and molecular vibrations in molecular electronic systems.

4. Systematic studies on conductance switching have been carried out (by using a

new test bed) on molecules with different chemical structures. These studies allow us to correlate the extent of non-linear switching behavior with the specifics of the molecular structure.

Perspectives on molecules and nanostructures for molecular conductance

James Yardley Department of Chemical Engineering

Columbia University

Molecules offer attractive alternatives to silicon devices for use in for information processing. The Columbia University Center for Electron Transport in Molecular Nanostructures was created by the National Science Foundation of the United States as one of six Nanoscale Science and Engineering Centers. The purpose of the Center is to understand fundamental aspects of the conduction of electrons through molecules and molecular nanostructures. The Nanocenter is bringing some new perspectives in molecular constructs and some new perspectives in the fabrication of nanoscale structures. I will describe the overall program and I will provide an overview of some of these perspectives.

Charge transport in molecular wires

Hong Guo Center for the Physics of Materials and Department of Physics

McGill University, Montreal, PQ, Canada H3A 2T8

Carrying out density functional theory (DFT) analysis within the Keldysh nonequilibrium Green's function (NEGF) formalism, we have calculated nonlinear and nonequilibrium charge transport properties of various molecular scale conductors without involving phenomenological parameters. In this talk, I will report our recent investigations using the NEGF-DFT formalism, on resistance of several molecular wires, and compare our results with the corresponding experimental data. Time permitting, I will also report our preliminary results on calculating switching speed of molecular tunnel junctions when a pulsed bias is applied to an electrode.

Molecular materials and devices

Cherie R. Kagan IBM T. J. Watson Research Center

Yorktown Heights, NY 10598

Efforts to fabricate devices based on active molecular components have been driven by both the fundamental interest in using chemistry to build function at the molecular level and the looming technological expectation of the end of Moore’s law. In this talk, we describe the directed assembly of organic and metal-metal bonded supramolecular systems that are interesting materials for potential electronic and memory device applications. Molecules are chosen with head groups that bind to metal or oxide surfaces and tail groups that bind to metal electrodes or that template the growth of the particular molecular system. Optical spectroscopy, scanning probe microscopy, and electrochemistry are used to characterize the chemistry and physics of molecular assemblies. We incorporate these molecular systems in nanometer scale device “test” structures aimed at understanding the electronic characteristics of two- and three-terminal molecular devices.

Gated transport in single crystal organic semiconductors probed with elastomeric stamps

John Rogers

Department of Materials Science and Engineering University of Illinois at Urbana-Champaign

Physical lamination of organic crystals against elastomeric elements that support

gate electrodes, gate dielectrics and source/drain electrodes produces high performance transistors in a manner that avoids exposing the fragile crystals to any form of conventional processing. Van der Waals forces drive this completely reversible soft contact lamination process. We describe a range of results, including temperature dependent measurements of field effect mobilities that reach 32 cm2/V-s in p type rubrene and 1.6 cm2/Vs in n type TCNQ. We also present data that reveal large anisotropies in the transport in the a-b plane of rubrene crystals.

Spectroscopic probes of molecular junctions

Xiaoyang Zhu Department of Chemistry

University of Minnesota, Minneapolis, MN 55455

Charge transport at or across molecule-electrode interfaces is central to the operation of a wide variety of molecule-based devices. The critical charge transporting interfaces in most systems are buried interfaces which are not readily accessible to conventional structural or spectroscopic probes. For any given device, two critical questions are: (1) What is the structural and chemical nature of the molecules at the buried interface? (2) How is the electrical conductance across an interface related to physical properties such as electronic energy level alignment and charge redistribution? My lab is addressing both questions in spectroscopic experiments. The first experiment relies on attenuated total internal reflection spectroscopy to establish the structure and conformation of molecules at buried interfaces. The second experiment probes electronic structure and electron transfer rates at molecule-metal interfaces using femtosecond time-resolved two-photon photoemission spectroscopy. These experiments are starting to provide molecular insights into molecular electronics.

Modeling conduction through photoswitching molecules

Otto F. Sankey1, Jun Li1, Gil Speyer2 1Department of Physics and Astronomy 2Department of Electrical Engineering

Arizona State University, Tempe, AZ 85287-1504

We study theoretically a model photochromic molecule (dithienylethene derivatives) which can switch from an “off” state to an “on” enhanced conductivity state by the application of light. The light-induced intra-molecular conformation conversion drives a swapping of the HOMO-LUMO molecular orbitals between the two distinct conjugated structures. The shuffling of single and double bonds after conversion produces a significant change in the single molecule conductance when the molecule is sandwiched between metal electrodes. We model the switching event using quantum molecular dynamics and the conductance changes using Green's function electronic transport theory. We find large on-off conductance ratios (between 10 and over 100) depending on the side-group outside the switching core.

Valence electronic structure of phenylene-ethynylene oligomers

Roger van Zee Process Measurements Division NIST, Gaithersburg, Maryland

The electronic structure of phenylene ethynylene oligomers chemisorbed on gold

have been studied using photoelectron spectroscopy and optical absorption spectroscopy. In one series of experiments, the number of phenyl rings was varied from a single ring to three rings in unsubstituted phenylene ethynylene-thiol oligomers. This provides a qualitative picture of the extent of perturbation of the electronic structure caused by thiol coupling and the changes to the molecular levels important for electron transport with length. In another study, the effect of substitution on the middle ring was investigated. Shifts that scale with the electronegativity of substitutent were observed. Finally, the effect of the molecular linker was investigated by substituting isocyanide for thiol. This change led to a shift of the occupied state of the molecule without significantly changing its overall electronic structure. Together, these data begin to give a picture of the effects of linker-group, substitution, and metal-molecule coupling chemistry on electron transport through these model systems.

TBA

Jie Han

NASA Ames

Electrical transport properties of nanoscale molecular junctions

Theresa Mayer, Lintao Cai, Marco Cabassi, Thomas Mallouk, Yoram Selzer, and David Allara

Department of Electrical Engineering Pennsylvania State University

This talk will present temperature-dependent current-voltage characteristics and

inelastic electron tunneling (IET) spectra measured using sub 40-nm diameter in-wire metal-molecule-metal junctions containing oligo-(phenylene ethynylene) (OPE) molecular wires and their –NO2 derivatives (NOPE). The in-wire junctions are synthesized by replicating the pores of polycarbonate mesoporous membranes using sequential electrodeposition and molecule self assembly steps. Following synthesis, the 5 µm long nanowires are released from the membrane and electrofluidically aligned between pairs of lithographically defined large area electrodes. Room temperature electrical measurements on junctions containing these and other molecular wires such as oligo-(phenylene vinylene) (OPV) show differences in conductance that are expected based on their differing molecular structure. In particular, the current of OPV is approximately one order of magnitude larger than that of the OPE and NOPE junctions. IET spectra were obtained by cooling these junctions to 10 K and collecting the second harmonic of the current as a function of bias using a digital lock-in amplifier. Several predominant peaks were observed at positive and negative biases for the OPE and NOPE junctions, which further confirm that our in-wire junctions are free of process induced artifacts and can be confidently used to study transport mechanisms in metal-molecule junctions.

The authors would like to acknowledge the Tour group (Rice University) for providing the molecules used in this study.

Intrinsic electron conduction mechanisms in molecules

Wenyong Wang, Takhee Lee, and Mark A. Reed Departments of Electrical Engineering and Applied Physics

Yale University

Electron devices containing molecules as the active region have been an active area of research over the last few years. This talk presents measurements in a variety of molecular systems to elucidate the transport mechanisms, specifically in self-assembled monolayers (SAMs) using nanometer scale devices. Detailed kinetic studies are necessary to distinguish between different conduction mechanisms; for example, in alkanes temperature-independent electron transport is observed, proving tunneling as the dominant conduction mechanism when defects are eliminated from the device structure. This is distinct from Langmuir-Blodgett films, where only defect or filamentary conduction has been observed. From the bias-dependence of b, a barrier height FB of 1.39 ± 0.01 eV and a zero field decay coefficient b0 of 0.79 ± 0.01 Å-1 are determined for alkanethiols. Inelastic electron tunneling spectroscopy of the molecules in the junction exhibits well-defined modes of the molecules in the junction, and yield a measurement of the intrinsic linewidths of these modes. Deviation from this classic behavior for more complex molecule structures, and a comparison of the differences and pitfalls of various fabrication and characterization approaches, will be discussed.

What can we learn about electronic interfaces from single molecule measurements?

Stuart Lindsay Department of Physics

Arizona State University

Single molecule measurements, with molecules attached to electrodes in a reasonably well-defined way, leave no wiggle room for comparison with theory because the number of molecules, their geometry and (hopefully) the attachments are defined. In a number of cases investigated to date, the behavior of small molecules in which ballistic transport dominates appears to be well described by theory. More challenging are cases where intrinsic molecular properties might play a role, for example redox-active molecules and photo-excitable molecules. I will outline some initial results with electroactive and photochromic molecules.

Alkane monolayers: Temperature dependent tunneling and template-stripped surfaces

Duncan Stewart

Quantum Structures Research Initiative Department Hewlett-Packard Laboratories

I will discuss two related topics. First, I will show temperature dependent

transport through alkane monolayers sandwiched between upper and lower platinum electrodes. The transport shows a large low-bias conductance anomaly and an anomalous exponential temperature dependence. The exponential dependence can be partially fit with two models: a standard thermionic field emission model with anomalous parameters, or a less-conventional but general vibrating tunnel barrier model that can be associated with thermal molecular vibrations. Second, I will describe AFM and STM physical characterization of atomically-flat template-stripped platinum surfaces, and IR spectroscopy of alkane monolayers self-assembled thereon.

Theoretical principles of single-molecule electronics: A chemical and mesoscopic view

Yongqiang (Alex) Xue

Department of Chemistry Northwestern University

The recent surge of activity in molecular electronics represents the convergence

of the trend of top-down device miniaturization and the progress in bottom-up single-molecule manipulation and supramolecular self-assembly techniques. Detailed understanding of the electronic and transport properties of single-molecules contacted by electrodes is crucial for the development of this emerging technology.

In this talk, we summarize our understanding of the physical principles involved in the operation of single-molecule devices. Our analysis is based on the extrapolation of the concepts and techniques of mesoscopic physics into the molecular regime. We emphasize insights obtained using a real-space microscopic theory which combines rigorous transport theory with atomic-scale description of electronic processes within the molecular junction. Key concepts are introduced and illustrated with detailed microscopic studies regarding the nature of electron transport at the ultimate limit of device scaling and the prospect of device design through bottom-up atom engineering. We conclude the talk discussing some open issues and pointing out new directions in molecular electronics.

Apparent conductance of molecular nanojunctions: Roles of surface topography and metal contacts

Nikolai Zhitenev Bell Laboratories

Lucent Technologies

Systematic conductivity measurements of nanoscale junctions containing self-assembled monolayer of conjugated molecules are carried out for a variety of metal electrodes. The monolayers are assembled on 25-100 nm electrodes. Another 10-100 nm electrode is defined on top of the monolayers by metal evaporation through apertures. Unexpectedly, the characteristic energy scales of the dominant conductance channels are small in comparison with the molecular level spacing. In all cases, the dominant room temperature conductance is hopping with characteristic energy of the order of 10-100 meV determined by the nature of metal contacts. Relative contribution of tunneling conductance strongly depends on the surface topography of the metal electrodes. For smallest junctions assembled on a single grain of metal electrode, the direct tunneling through the molecules is negligibly small. In the case of multi-grain junctions, tunnel conductance is observed with low-energy Coulomb-blockade features.