Papers

AVS1996 Session NS+SS-FrM: Properties of Nanostructures III

Friday, October 18, 1996 8:20 AM in Room 202 A/B

Friday Morning

Start	Invited?	Item
8:20 AM		NS+SS-FrM-1 Phase Separation of a Model Alloy Mixture C\sub60\-C\sub70\ on a Cu(111) Surface Y. Suh, Y. Park (Seoul National University, Korea); T. Sakurai (Tohoku University, Japan); Y. Kuk (Seoul National University, Korea) Phase separation of C\sub60\, C\sub70\ binary mixture at various compositions was studied after quenching from 2D fluid temperature. Because of the shape difference, C\sub70\ molecules were imaged differently from C\sub60\ in STM images. Highly non-linear and irreversible phenomena, late stage growth, were observed, revealing time dependent order parameter. The 2D structure factor, calculated from time dependent compositional correlation function, was quite different from that measured by small angle neutron scattering due to finite domain size, symmetry and atomic resolution of STM. Although time dependence of the measured length scale could not be explained by either Lifshitz-Slyzov or Lifshitz-Allen-Cahn theory, the domain-size distribution could be dynamically scaled by a single function. The origin of the deviation and possible use of STM for late stage growth will be discussed.
8:40 AM		NS+SS-FrM-2 Molecule Corrals and Self Assembly on Surfaces: Probing Molecular Interactions D. Patrick (Western Washington University); V. Cee, T. Purcell, T. Beebe, Jr. (University of Utah) The highly-oriented pyrolytic graphite (HOPG) surface can be oxidized in a controlled etching reaction with a highly anisotropic etching rate that is essentially zero in the direction perpendicular to the surface plane, and variable in the in-plane direction. This process has been used to produce monolayer-deep, flat-bottomed pits surrounded by atomically flat terraces. The diameters, and to some extent the shapes, of these pits can be controlled on the nanometer scale. These pits, or "molecule corrals," have been used to study various aspects of molecular monolayer self assembly on the HOPG surface using the STM. The system studied follows a nucleation-growth mechanism, in which the rate of nucleation has been measured and found to be much slower than the rate of growth, depending not on the perimeter length of the corrals, but instead on their areas. The temperature dependence of the nucleation rate will be discussed. Since the nucleating corrals are doing so in the presence of an already nucleated and well ordered terrace, the terrace can exert some interesting and unexpected influences on the corrals. This presentation will describe some of the recent findings for organic liquid crystal systems, including the length scale of this influence, the symmetry-induced properties, and the transfer of chiral information to and from the surface.
9:00 AM		NS+SS-FrM-3 Images of Molecules with the Scanning Tunneling Microscope: A Theoretical Analysis of the Tunneling Mechanism P. Sautet (Ecole Normale Superieure de Lyon and Institut de Recherches sur la Catalyse, France) The scanning tunneling microscope is a very powerful tool for the study of individual molecules adsorbed on a conducting or semiconducting surface. In many cases, it can provide a distinct image shape for the molecule with "atomic-like" details. However the precise interpretation of these internal features in the image, and their relation with the molecule nature and structure on the surface, is not straitforward. It is not possible to directly relate the bumps on the image with atoms. More generally, the tunneling mechanism through a molecular adsorbate is not well understood. Theoretical calculations with the Electron Scattering Quantum Chemical approach will be presented on several examples. The dependence of the image on the nature of the molecule, its chemisorption geometry, and on the strength of the molecule-surface interaction will be underlined with the examples of benzene, CO, oxygene and ethylene on a Pt(111) surface. The tunnel current at the adsorbate can be decomposed in two contributions: the current through the surface atoms and the contribution of the molecular orbitals of the adsorbate. In most cases, the adsorbate induces a depression in the through-surface current and adds a positive through-adsorbate tunneling, the final current resulting from the superposition, with interferences effects, of these two contributions. The relative importance of the sigma and pi orbitals of the molecule will be discussed. Finaly the problem of the electron tunneling through long chain molecules, like alkylthiols will be adressed. We will show that the electron tunneling decays exponentially as a function of chain length, but at a slower rate than vaccum tunneling.
9:40 AM		NS+SS-FrM-5 Predicting STM Images of Molecular Adsorbates D. Futaba, S. Chiang (University of California, Davis) We present a computational method, based on extended H\um u\ckel Molecular Orbital Theory, for predicting scanning tunneling microscope (STM) images of molecules adsorbed onto metals. H\um u\ckel theory uses several simplifying assumptions and approximations which focus its usefulness on determining structural information and make very large systems soluble. Through variational methods, molecular orbitals are constructed given the substrate species, binding site, and adsorption geometry. Images are calculated for both isolated molecules and for molecules adsorbed on a cluster of metal atoms, and the latter accurately incorporated surface effects. These images contain detailed structural information which has been compared with experiments, where available. These calculations have recently been successfully ported from an IBM mainframe computer to a Pentium-based PC. This method has recently been used to predict STM images for benzene, thiophene, furan, and pyrrole on Pd(111), and ethylene on Cu(110).
10:00 AM		NS+SS-FrM-6 Nanoscale Selective Adsorption of Disilane on the Si(111) Surface Partially Terminated by Ga Atoms K. Fujita, Y. Kusumi, M. Ichikawa (JRCAT - Angstrom Technology Partnership, Japan) The Ga termination of the Si(111) surface is attractive to form nanoscale structures such as quantum and doping wires by selective adsorption of gas molecules. This is because nanoscale Si(111)7x7 regions extend on Si(111)\sr\3x\sr\3-Ga surfaces by self-organization mechanism.In this study, we have investigated the nanoscale selective adsorption of disilane on Si(111)7x7 surfaces with nanoscale \sr\3x\sr\3-Ga regions. The shape of a typical Ga-terminated region is a triangle with sides of 19 nm. The selective adsorption is observed during disilane supply by scanning tunneling microscopy.At 400 \super o\C, disilane molecules dissociatively adsorb on Si(111)7x7 regions whereas almost no adsorption takes place on Ga-terminated regions after a dose of 6x10\super 14\ cm\super -2\. This indicates that the \sr\3x\sr\3-Ga reconstruction effectively passivates the Si(111)7x7 surface. The adsorbates on the 7x7 regions are silicon hydride. At this temperature, the adsorbates hardly migrate on the surface. At 430 \super o\C, 2-dimensional island growth takes place not only on 7x7 regions but also on Ga-terminated regions. Si atoms necessary for the growth on the Ga-terminated regions are supplied from the 7x7 regions by surface migration. The migrating species is likely to be Si atoms or Si monohydride, because tri- and dihydrides decrease due to the thermal desorption of hydrogen molecules above 400 \super o\C. In conclusion, nanoscale selective adsorption of disilane is achieved on Ga-terminated Si surfaces below 430 \super o\C.This work was partially supported by NEDO.
10:20 AM		NS+SS-FrM-7 The Controlled Growth of Nanonstructures T. Parker, L. Wilson, N. Condon (University of Liverpool, United Kingdom); F. Leibsle (University of Madison) The growth of nanoscale metal islands on a Cu(100) substrate has been controlled using a self-assembled template. This template takes the form of nanometer-scale islands of an atomic nitrogen-induced reconstruction of the surface. These islands passivate areas of the surface and form long-chains of islands separated by areas of the clean surface. Subsequent dosing with metals (Cu, Fe and Co) results in metal island growth only in certain areas between the islands of nitrogen. As the amount of nitrogen on the surface is varied so the template is changed allowing us to control the shape and size distributions of the metal islands. For example we can selectively create clusters of metal 25\Ao\ in diameter, in a cartesian like array spaced 52\Ao\ apart.
10:40 AM		NS+SS-FrM-8 Probing Potential-Tuned Resonant Tunneling through Single Molecules with Scanning Tunneling Microscopy N. Tao, Q. Jing (Florida International University) The electron transfer reaction of single redox molecules (Fe- protoporphyrin IX) at an electrode-solution interface has been studied with scanning tunneling microscopy (STM) by probing the tunneling current that flows between the electrode and the STM tip via the molecules. By adjusting the electrode potential towards the redox potential, the tunneling current increases drastically due to resonant enhancement as the redox states of the molecules align with the Fermi level of the electrode. Studying the enhanced tunneling current as a function of the electrode potential, the redox density of states and the reorganization energy of the single molecules can be extracted. This experiment, also, demonstrates a method for identifying structurally similar molecules with STM.
11:00 AM		NS+SS-FrM-9 STM Investigations of Two-Dimensional Molecular Ordering of Perylene Tetracarboxylic Acid Diimides with Alkyl Tails (PTCDI-C\sub n\) Y. Kaneda, D. Sampson (Colorado State University); N. Brunkan (Northeast Missouri State University); B. Parkinson (Colorado State University) We report on scanning tunneling microscopy (STM) investigations of perylene tetracarboxylic acid diimides with alkyl tails with varying chain lengths (PTCDI-C\sub n\: n is alkyl chain length) ordered on two-dimensional substrate. PTCDI with no alkyl tails have been reported to order by the intermolecular interactions to form structures nearly independent of the substrate. On the other hand, alkyl tails are known to play an important role in forming two-dimensional structures by registry with the substrate lattice. We have introduced alkyl tails to PTCDI to change the domination of intermolecular or molecular-substrate interactions to yield different ordered structures on the surface. STM investigations of these perylene dye molecules are important because of their electronic and optical properties. It also provides general information about mechanisms of molecular ordering at the interface. Four different PTCDI-C\sub n\ (C\sub n\ = C\sub 4\, C\sub 8\, C\sub 12\ and C\sub 18\) molecules were synthesized for our studies. STM measurements were performed in air at room temperature after they were dissolved in 1-phenyloctane and adsorbed onto highly oriented pyrolytic graphite(HOPG) or MoS\sub 2\. STM images of molecules with high spatial resolution were obtained for all samples. These images allow us to measure the two dimensional lattice constants and to derive a model of the molecular alignment with the lattice. Both square lattices and row structures have been observed even with the same compounds. PTCDI-C\sub 4\ shows a square lattice structure, while a row structure is observed for C\sub 18\. The effect of alkyl chain lengths on the two dimensional ordering will be discussed.
11:20 AM		NS+SS-FrM-10 Hydrodynamic Effects on the Reaction of Calcite in Water - An AFM Study Y. Liang, D. Baer (Pacific Northwest National Laboratory) The dissolution reaction of calcite (CaCO\sub3\) with water has been investigated in situ, under different flow rates, using atomic force microscopy (AFM). Surface dynamics such as movement of steps are obtained from real-time images and are used to analyze the rate of reaction of calcite with water. The study shows that on increasing the flow rate, the surface step velocity increases rapidly in the low flow rate regime, then it approaches a constant when the flow rate exceeds 0.15 mL/min. This result indicates that the reaction is controlled by diffusion when the flow rate is low, but by surface reaction when the flow rate exceeds 0.15 mL/min. In the low-flow-rate regime, the variation of the step velocity with flow rate was found to approximately obey the Levich theory, suggesting that the hydrodynamic boundary-layer theory is valid even at nano-meter scale. In addition to monatomic steps and shallow pits (several layers deep), AFM images also show deeper pits (>20 layers deep) which are initiated by the reaction at dislocation lines terminated at the surface. In the surface-reaction regime, for the shallow pits, the associated steps retreat at constant velocities which are independent of the flow rate. However, for the deeper pits, the velocities of the steps near the bottom of the pits depend upon flow rate and the distance of the steps from the surface, although those deeper pits are very open with the angles of the sides only several degrees. Such depth and flow rate dependence suggests that the effects of diffusion on reaction take place in the more occluded parts of the pits. By modeling the variation of the pits geometry with flow rates, the concentration profile within the pits will be presented and the consequent back-reaction effect will be discussed.
11:40 AM		NS+SS-FrM-11 Comparative Studies of Domain Structures and Switching Behaviour of PZT Ferroelectric Thin Films by Scanning Force Microscopy A. Gruverman (National Institute for Advanced Interdisciplinary Research, Japan); O. Auciello (Microelectronics Center of North Carolina); H. Tokumoto (National Institute for Advanced Interdisciplinary Research, Japan) Recently, scanning force microscopy (SFM) was found to be an effective tool for nanoscale visualization and control of domain structures in ferroelectric materials. The SFM piezoresponse technique based on the detection of piezoelectric vibration of the ferroelectric sample allows one to visualize ferroelectric domains even in samples with rough surfaces. In the present study we exploit the SFM piezoresponse imaging method to perform comparative studies of domain structures and switching behaviour of PZT ferroelectric thin films with various crystalline structures and integrated into capacitors with different electrodes, which yield different fatigue characteristics. These measurements have revealed significant difference in the domain structures and polarization reversal dynamics of the PZT films at the nanometer scale. Obtained data were used to analyse the correlation between domain structure dynamics and fatigue characteristics of thin films. SFM was used to demonstrate its capability for writing and imaging complex domain patterns on PZT film surfaces. The resolution power of the SFM piezoresponse technique is discussed.