Papers

AVS2001 Session NS+EL-WeA: Molecular Electronics and Patterning

Wednesday, October 31, 2001 2:00 PM in Room 133

Wednesday Afternoon

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2:00 PM		NS+EL-WeA-1 Molecular Electronics by the Numbers S.T. Pantelides (Vanderbilt University); M. Di Ventra (Virginia Tech); N.D. Lang (IBM) The paper gives an overview of recent work by the authors on first-principles, parameter-free calculations of electronic transport in molecules in the context of experimental measurements of current-voltage (I-V) characteristics of several molecules by Reed et al. The results show that the shape of I-V characteristics is determined by the electronic structure of the molecule in the presence of the external voltage whereas the absolute magnitude of the current is determined by the chemistry of individual atoms at the contacts. A three-terminal device has been modeled, showing gain. Finally, recent data that show large negative differential resistance and a peak that shifts substantially as a function of temperature have been accounted for.
2:40 PM		NS+EL-WeA-3 Controlled p-Doping of an Organic Molecular Semiconductor W. Gao, C. Chan, A. Kahn (Princeton University) Electrical doping is perceived as the key to enhance the performance and versatility of organic molecular devices.¹ Yet, few systematic investigations of the electronic structure of molecular films and interfaces doped with organic molecules have been published to date. We report here an investigation of controlled doping of zinc phthalocyanine (ZnPc) co-evaporated on Au with a strong acceptor, tetrafluoro-tetracyano-quinodimethane (F₄-TCNQ), using ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES). The 5.2 eV ionization energy of ZnPc is smaller than the electron affinity of F₄-TCNQ, suggesting host HOMO-to-guest LUMO charge transfer. Undoped ZnPc exhibits near mid-gap Fermi level (E_F) and flat bands away from the Au interface, indicative of quasi-intrinsic purity. In ZnPc doped with ~3% (molar ratio) F₄-TCNQ, E_F shifts toward the HOMO level by 0.72eV and reaches ~ 0.16 eV above the leading edge of ZnPc HOMO, as measured from the surface of a 100Å film. At the interface with Au, the ZnPc HOMO is 0.72 eV below E_F, leading to a depletion region with a 0.56eV band bending away from the interface, consistent with the p-type character of the film. The width of the depletion region in doped ZnPc is measured at approximately 32Å, consistent with a simple electrostatic model based on the doping concentration and a dielectric constant ε=3. The interface dipole barrier between Au and doped ZnPc is of the same sign and similar magnitude as for the undoped material. No evidence of chemical interaction can been seen, suggesting that pure charge transfer is the more likely mechanism for doping. The narrow depletion region in the doped layer is likely to lead to an increase in the tunneling of holes through the junction. I-V measurements will be performed to confirm this point. * Work supported by the NSF (DMR-0097133) ¹Zhou et al., Appl. Phys. Lett., 78, 410 (2001).
3:00 PM		NS+EL-WeA-4 Conductance Switching in Single Molecules Through Conformational Changes K.F. Kelly, Z.J. Donhauser, B.A. Mantooth, L.A. Bumm, J.D. Monnell, J.J. Stapleton (Penn State University); D.W. Price (Rice University); D.L. Allara (Penn State University); J.M. Tour (Rice University); P.S. Weiss (Penn State University) The viability of molecular electronics is being investigated with the aim of creating inexpensive, ultra-dense, high-capacity electronic devices. Conjugated phenylene-ethynylene oligomers have been extensively studied as candidate molecular devices. However, most experiments have required the assembly and study of these molecules in groups of thousands. We utilize self-assembly techniques in combination with scanning tunneling microscopy (STM) to study candidate molecular switches individually and in small bundles. Alkanethiol self-assembled monolayers (SAMs) on gold are used as a host two-dimensional matrix to isolate and to insulate electrically the molecular switches. The candidate molecules selectively adsorb into existing defect sites and at step edges. The molecules bind with a sulfur "alligator clip" to the underlying gold substrate, and the ordered SAM causes the molecules to adsorb nearly normal to the substrate. We then individually address and electronically probe each molecule using STM. The conjugated molecules exhibit reversible conductance switching, manifested as a change in the apparent height in STM images. The observed switching occurs randomly and reversibly, with persistence times for each state ranging from seconds (or less) to hours. Both individual molecules and bundles of molecules exhibit switching. We have demonstrated the ability to control the amount and rate of active switching by controlling the local environment of the guest molecules. Inserting the guest molecules into poorly ordered matrix films results in increased switching activity when compared to well-ordered films. Similarly, annealing the SAM after inserting the guest molecules results in decreased switching activity, when compared to unannealed SAMs. We ascribe the switching to conformational changes of the molecules that are either enhanced or reduced by the corresponding loosening or tightening of the surrounding matrix.
3:20 PM		NS+EL-WeA-5 Epitaxial Growth of Self-Assembled Dots and Wires S. Williams (Hewlett-Packard Laboratories) Various structures with nanometer-scale dimensions can be grown on surfaces by taking advantage of lattice mismatch, crystal symmetry and surfactant species. This provides experimentalists with an array of control parameters to tune the size and shapes of the structures that form. The general principals for attaining this control will be illustrated for various types of Ge nano-islands on Si(001) and also for nano-wires of various silicides on Si(001).
4:00 PM		NS+EL-WeA-7 Micro and Nanoscale Patterning of SAMs and Their Functionalisation S. Sun, K. Chong, G.J. Leggett (University of Manchester Institute of Science and Technology, UK) Lithography methods are at the heart of modern-day microfabrication, nanotechnology and molecular electronics. These methods always rely on patterning of a resistive film followed by a chemical etching of the substrates. Self-assembled-monolayers (SAMs) of alkanethiols on metals (Au, Ag or Cu) have been found to be good resists to protect underneath metals from wet etching. In addition, due to the chemical reactivity of some groups on SAMs, various kind of materials can be immobilized onto it. Therefore patterning and functionalisation of SAMs have attracted great interest since the last decade. Various methods have been employed for this purpose. Here we report experiments that generate micro- and nanometer size patterns of SAMs. Micrometer size features have been obtained through masked photo-oxidation either by a high-pressure mercury arc lamp or a lamp that only emits 254 nm light. Proteins have been immobilised onto these features successfully and can exhibit both a lateral force and topography contrast. Nanometer scale patterns have been achieved by scanning alkanethiol coated atomic force microscope (AFM) tips across Ag/Au film surface, a method called dipped-pen-nanolithography (DPN). It has been found the transportation rate of alkanethiols from AFM tip to metal surface not only depend strongly on the humidity of the environment, but also on the quantity of alkanethiol adsorbed on the tip and properties of substrate of interest. Different feature sizes from several micrometers to less than 50 nm have been obtained by controlling the scan speed and the environment humidity. Similar with the SAMs on Au, the SAMs on Ag formed by this method can also be used as resist layer to protect underneath Ag film from chemical wet etching.
4:20 PM		NS+EL-WeA-8 Diffusion of Alkanethiols in the Presence of Water P.E. Sheehan, M.L. Stevens, L.J. Whitman (Naval Research Laboratory) The patterning of alkanethiols has become a cornerstone in the burgeoning field of nanotechnology. Several patterning techniques have been developed, the more popular of which include stamping using polymer masters, known as microcontact printing (mCP) and, more recently, the direct writing of the thiols using an AFM tip, known as Dip Pen Nanolithography (DPN). Importantly, in both techniques, diffusion of the thiol away from the contact area fundamentally limits the spatial resolution obtained. Obtaining the highest resolution possible from these techniques will require a full understanding of the rate and nature of thiol diffusion. To address this need, the radii of octadecanethiol spots deposited via DPN were studied as a function of tip-surface contact time and relative humidity. The increase in spot size with time was well described by two-dimensional radial diffusion from a constant source of finite radius. Fits using this formula revealed a diffusion constant of approximately 2500 nm2/sec with little dependence on humidity. Analysis of published images¹ showing the spread of hexadecanethiol on gold after microcontact printing leads to comparable diffusion constants. Significantly, these values are four orders of magnitude smaller than that expected for diffusion through bulk water. Mechanisms that would explain such a low diffusion coefficient will be discussed. ¹ Y. Xia and G.M. Whitesides, J. Am. Chem. Soc. 117 (1995) 3274.
4:40 PM		NS+EL-WeA-9 Chemical Nanolithography A. Gölzhäuser, W. Geyer, A. Küller, V. Stadler, W. Eck, M. Grunze (Universität Heidelberg, Germany); T. Weimann, P. Hinze (Physikalisch Technische Bundesanstalt, Germany); K. Edinger (University of Maryland) The efficient fabrication of chemically defined surface nanostructures is an important objective in fields such as molecular electronics, biochips or biosensors. Chemical nanolithography utilizes electron beams to selectively modify self-assembled monolayers, for example to convert NO₂ end groups in monolayers of nitrobiphenylthiol to NH₂ groups.^1,2 In this presentation, we show that chemical nanolithography can fabricate chemical features with lateral dimensions down to ~20 nm on a variety of different surfaces (noble metals, semiconductors or oxides). E-beam lithography as well as low energy electron proximity printing are used to fabricate chemical surface structures. These are then used as high resolution templates for the laterally controlled electrochemical deposition and for the immobilization of molecules (or macromolecular objects) on predefined surface regions. ¹ ¹ W. Eck, V. Stadler, W. Geyer, M. Zharnikov, A. Gölzhäuser, M. Grunze, Adv. Mater. 12, 805 (2000). ² A. Gölzhäuser, W. Eck, W. Geyer, V. Stadler, T. Weimann, P. Hinze, M. Grunze, Adv. Mater. in press.