ISIS
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          institute for surface and interface science
southern california symposium on surface science   
__________________________________________________________________________________________________________


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Southern California Symposium on
  Surface Science
April 25, 2005
Natural Science I Building, Room 1114
University of California, Irvine (campus map)

Symposium Program Details   |   Online Registration (deadline:  March 31, 2005)
program abstracts   |   local accommodations   |   parking   |   directions  

Program Abstracts
(updated 4-7-2005)

......................................................................................................................................................................
9:00 am - 9:40 am

D.W. Goodman
 Texas A & M University

Title:
Catalysis by Metal Alloys:  From Single Crystals to Nanoparticles

Abstract:
Model mixed-metal catalysts consisting of Pd alloyed with Au, Cu, and Ag as bulk films on refractory metal single crystals and as nanoparticles supported on oxides have been characterized using an array of surface techniques including X-ray photoemission spectroscopy (XPS), low energy ion scattering spectroscopy (LEIS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), infrared reflection absorption spectroscopy (IRAS), metastable impact electron spectroscopy (MIES), scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and reaction kinetics.  The surface sensitivity of LEIS and IRAS has been exploited for elucidating atomic composition of the outermost surface layer.  Of special interest is the composition of the surface compared to the overall composition, particularly in transitioning from planar surfaces to nanoparticles, in the presence and absence of adsorbates.   The mechanistic details of the vinyl acetate synthesis reaction, used to probe the structure-function relationship of these alloy surfaces, will also be discussed.

......................................................................................................................................................................
9:40 am - 10:20 am

Francisco Zaera
University of California Riverside
Title:
Characterization of Redox-Active Monolayers for use in Molecular Memory Devices
Amir Yasseri,1 Dennis Syomin,1 John S. Lindsey,2 David F. Bocian,1 Francisco Zaera1
1 Department of Chemistry, University of California, Riverside, CA 92521
2 Department of Chemistry, North Carolina State University, Raleigh, NC 27695
Abstract:
Construction of hybrid molecular-based devices may provide the most tractable approach in bridging the gap between modern day semiconductor materials and novel molecular based devices. Toward this goal, we have examined a number of functionalized monolayers of ferrocene- and porphyrin-terminated alcohols, thiols, and RSeH compounds both self-assembled on gold surfaces and covalently linked to monocrystalline semiconductor substrates. Several synthetic methods have been explored to optimize the electronic properties and chemical stability of these monolayers, including activation of hydride-terminated surfaces by HF or NH4F followed by reaction with iodine and organic film deposition, and direct thermal activation of the organic reactant on the hydrogen-capped semiconductor substrate. The resulting monolayers have then been characterized by a number of techniques, including X-ray photoelectron and Fourier transform infrared spectroscopies, and the rates of electron transfer for oxidation in the presence of applied potentials and of charge dissipation after the applied potential is disconnected have both been probed by using modified AC voltammetric and amperometric techniques and redox-kinetic measurements.

Electron-transfer rates were found to be in most cases similar on both gold and semiconductor surfaces, but to differ from the charge-dissipation rates by six orders of magnitude, and to strongly depend on the surface concentration of the electroactive species. Two alternative hypotheses are advanced to explain the diminution in rates with increased surface coverage, one based on space-charge effects at the monolayer-solution interface, and a second relying on changes in distance of the redox centers from the surface as modulated by the orientation of the linking chains. These have been probed by systematic experiments varying the anchoring
chalcogen atom used, the length and nature of the tethering group, the number of substitutions in the porphyrin ring, and the nature of the substrate. It was found that surface binding can be readily achieved with all methylene- and benzyl-based tethers tried regardless of the nature of the anchor atom (C, O, S, Se). However, the longer tethers afford monolayers that are somewhat more upright on the surface, leading to a higher packing density and more homogeneous redox thermodynamics. Also, it was found that the kinetics of electron transfer does not depend on the type of anchor atom to the surface, but that the monolayers bearing the methylene class of linker exhibit systematically faster rates. On the other hand, both electron-transfer and chargedissipation rates do decrease monotonically as the length of the linker increases.

In an attempt to control the adsorption geometry and with that the electronic properties of the monolayers, additional studies were performed with a family of multithiol-functionalized zinc porphyrins. Unfortunately, those were found to bind to the surface via a single anchoring group regardless of the number of appendages available per molecular unit. On the other hand, some differences were found in terms of adsorption geometry connected with the steric effects of the unbound tethering groups.

......................................................................................................................................................................


Title:
On the dynamics of defects on metal surfaces

Abstract:
By analyzing STM movies of steps and islands on metal surfaces in vacuum as well as in liquid environment we determine basic energy parameters such as diffusion barriers, kink formation energies and step line tensions as a function of temperature in case of vacuum measurements and as a function of the electrode potential in case electrochemical environment. For the analysis we make use of theoretical concepts known from statistical physics and equilibrium and non-equilibrium thermodynamics. Studies in vacuum were performed on Cu(100), Cu(111), Ag(111) and Pt(111). Here we studied the decay of islands, island coalescence, the island equilibrium shape and shape fluctuations. Studies in liquid environment were performed for Au(100) in 50 mM H2SO4 and in 100 mM HClO4 + 100 µM HCl. In both cases, the mean shape of the Au islands, the step line tension and the kink energy show a substantial dependence on the electrode potential. We attribute this potential dependence partly to electrical contributions from the double layer at the solid/liquid interface to the charge dipole moments at steps under the influence of specifically adsorbed sulfate and chloride ions.

......................................................................................................................................................................
11:20 a.m. - 12:00 p.m.

Bruce Koel
University of Southern California

Title:
Site-Directed Chemistry at Pt-Sn Alloy Surfaces

Abstract:
Achieving higher activity and selectivity of heterogeneous catalysts, electrocatalysts, and sensors requires advances in controlling structure and chemistry relevant to interfacial reactions at the nanoscale. One can now exploit an unprecedented ability to investigate such phenomena on alloy surfaces to obtain new information about how and why composition, structure, and defects alter chemical reactions that occur at specific sites. We have been probing this “site-directed” chemistry at alloy surfaces in a wide range of chemisorption and catalytic reactivity studies. The talk today will focus on how recent results for several Sn/Pt(111) and Sn/Pt(100) ordered surface alloys have helped to define the overall chemical reactivity of Pt-Sn bimetallic surfaces, clarified the role of a second metal in altering the chemistry of Pt alloys, and led to general principles for understanding the reactivity and selectivity of alloy catalysts. Specifically, I will discuss exploiting Pt-Sn alloys for selective hydrogenation of 1,3-butadiene based on studies of well-defined model catalysts, i.e., Pt(111) and the (2¥2)-Sn/Pt(111) and (÷3¥÷3)R30∞-Sn/Pt(111) surface alloys, that probe the influence of alloyed Sn on the reaction barrier to butadiene hydrogenation and the effect of surface Sn concentration on hydrogenation activity and selectivity. Fundamental concepts emerging from such studies enhance our understanding and ability to tailor local properties of alloy surfaces, which should facilitate the design of new catalysts and sensors.

Surface Chemistry Group webpage: http://www-rcf.usc.edu/~koel/
Lab for Molecular Robotics webpage: http://www-lmr.usc.edu/~lmr/

......................................................................................................................................................................


2:00 - 2:40

William A. Goddard, III
Charles and Mary Ferkel Professor of
Chemistry, Materials Science, and Applied Physics
Director, Materials and Process Simulation Center (MSC)
California Institute of Technology (139-74)

Title:
de Novo Multi-Paradigm Simulations of Surface Processes

Abstract:
Advances in theoretical and computational chemistry are making it practical to consider fully first principles (de novo) predictions of important systems and processes in the Chemical, Biological, and Materials Sciences.  Quantitative models based on theory and computation are starting to become the basis for design and operations in industry.
We will highlight some recent advances in methodology and will illustrate them with recent applications to problems involving Surface Processes. Topics will be selected from:

 -Mechanisms of heterogeneous reactions relevant for fuel cell anodes and cathodes
 -De novo Force Fields from QM (ReaxFF) to describe reaction processes at surfaces
 -Simulations of proton transfer processes from anode to polymer electrolyte to cathode
 -Nanoelectronic switches, self-assembled monolayers and switching performance


......................................................................................................................................................................


2:40 - 3:20
Horia Metiu, S. Chretien
University of California, Santa Barbara

Title:
Catalysis by metal ions in oxides

Abstract:
We use density functional theory to explore the possibility that oxides doped with a metal  provide us with a new class of catalysts.  The present, preliminary work ,explores titania and ceria doped with gold.   We find that the substitution of a cation in the oxide with Au weakens dramatically the bond of oxygen to the oxide and facilitates oxidation reactions. For the catalyst to work, one needs a good balance between two conflicting tendencies: that of giving away the oxygen and that of taking oxygen from the gas-phase.


......................................................................................................................................................................


4:20 - 5:00 p.m.

Andrew C. Kummel
University of California, San Diego

Title:
Chemisorption Dynamics on Chemical Sensor Materials

Abstract:
While several groups have reported the use of metallophthalocyanines (MPcs) as thin film resistive sensors for analyte gases such as NOx, CO, O3 and NH3, the chemisorption energies and mechanisms for these gases reacting with various MPcs have not been studied.  We have studied the gas-surface reaction dynamics of NO on FePc (iron phthalocyanine).  The sticking probability of a monoenergetic 0.11 eV molecular beam of NO was measured on an ordered 6x6 monolayer of FePc on Au(111) and on clean Au(111) as a function of temperature.  At low temperature (100K), a direct chemisorption channel with an initial sticking probability of more than 25% exists.  This direct channel is observed even at 300K.  Although NO binds to the Fe metal center, these Fe atoms only represent 3% of the surface.  This suggests that the NO molecules are steered to the reactive metal centers by other portions of the FePc molecule.   Density functional theory (DFT) calculations performed in this study show that chemisorption energies vary strongly with metal center and analyte.  The calculated potential curves are consistent with two chemisorption channels  (a) a direct chemisorption channel with spatial steering whereby a barrierless potential extends out from the metal center towards the inner nitrogen atoms and (b) a multi-step absorption mechanism in which NO initially binds to the inner or outer ring nitrogens and subsequently migrates to the deep chemisorption well on the metal center.


......................................................................................................................................................................


4:20 - 5:00 p.m.

Ludwig Bartels
University of California, Riverside

Title:
Two Macroscopic Concepts Found to Govern the Dynamics of Individual Molecules on Cu(111)_

Abstract:
Since D. Eigler showed that Xe-atoms can be rearranged on a surface to form letters, there has been the dream of assembling functional machinery consisting of individual atoms and molecules on surfaces. For this dream to come true, efficient methods for control of the dynamics and reactivity of molecules on surfaces are required. I will present two instances, in which concepts from macroscopic chemistry and biology can be transferred directly to experiments addressing individual molecules: the Hammett Equation (which describes the acidity of substituted benzoic acids and which is a widely-used tool in drug discovery, toxicology and related areas of chemistry) and the walking motion of a human being (which distinguishes itself from random motion of a species across a surface by the way the substrate linkers are moved in a guided and alternate fashion).

......................................................................................................................................................................


7:00-7:40 pm

H. Ibach
Institute of Thin Films & Interfaces (ISG)


Title:  Spin dependent electron energy loss spectroscopy and its application to spin waves in ultra-thin films

Abstract:
Spin waves in ultra thin layers provide information on the exchange coupling at surfaces, on the number of neighbors contributing to the coupling, on the  magnetic magnetic moments at surfaces, on the magnon / e-h pair coupling, and on the  transport of collective spin properties.

To study the dispersion of spin waves inelastic scattering of low energy electrons can be employed. Until recently the available spectrometers where not optimal for the purpose. In my talk, describe a novel spectrometer design. Several examples for spin wave dispersion will be discussed. The importance of the interaction with Stoner excitations will be pointed out. Future applications of the spectroscopy may include magnetic insulators and nanopatterned systems.

......................................................................................................................................................................


7:40 - 8:20 p.m.

 John C. Hemminger
University of California, Irvine

Title:  Surface Composition of Aqueous Solutions:  New Experiments and Comparison with Simulations

Abstract:
It has been well established that the surface composition of condensed phase systems is often not identical to the bulk composition.  However, while experimental measurements of surface segregation for solid systems have been carried out for many years, experiments that ask similar questions about liquid phase systems have been quite limited.  Recently molecular dynamics simulations of aqueous alkali halide solutions have suggested that the halide ion concentration in the top molecular layer of the solution is enhanced over that of the bulk solution.  It has also been pointed out that such an effect could have important implications for reactivity at the liquid/vapor interface of atmospheric aerosol droplets and particles.  We have carried out x-ray photoelectron spectroscopy experiments under conditions that allow the sample to be in contact with ambient vapor pressures during analysis.  The experiments were carried out using a new spectroscopy facility at the Advanced Light Source synchrotron facility.  Experiments on saturated solutions of KBr and KI show that the solution composition at the liquid/vapor interface is indeed enhanced in the anion.  The enhancement is more pronounced for the larger, more polarizable I– anion, as predicted by MD simulations.  By carrying out the experiments at different x-ray wavelengths (resulting in different photoelectron kinetic energies) the probe depth of the XPS experiments was varied.  This allows us to obtain a “depth profile” of the anion from the liquid/vapor interface into the bulk of the solution.  Results from recent experiments that utilized a Br– doped NaCl sample show that the Br– ion is preferentially at the liquid/vapor interface of such a solution.

























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