ABSTRACT

1. PATTERN FORMATION OF FUNCTIONALIZED FULLERENES ON GOLD SURFACES: ATOMISTIC AND MODEL CALCULATIONS Greg Bubnis, Sean Cleary and Howard Mayne Department of Chemistry, University of New Hampshire, Durham, NH 03824 r1 r2 r=0.25 r3 r4 ABSTRACT ATOMISTIC CALCULATIONS MODEL CALCULATIONS We have developed simple potential energy functions to mimic adsorbate-adsorbate and adsorbate-substrate interactions for functionalized fullerenes adsorbed on a flat gold surface. Exploratory calculations, including Monte Carlo simulations and conformational energy minimizations, have been carried out which reveal several possible types of adsorbate pattern formation. Preliminary results suggest that local pattern formation, driven by carboxylic acid hydrogen bonding, on a randomly populated surface can occur at approximately 500K but higher temperatures are necessary to drive global organization. Adsorbate – adsorbate (united atom) interactions. The Girifalco [3] fullerene-fullerene potential is used. Site lattice spacing =1 t Au(111) Surface D re INTRODUCTION Adsorbate-adsorbate interactions are modeled by a tip-to-tip Morse potential (shown above). Basin Hopping Monte Carlo calculations are used to locate the configurations that have the lowest potential energy. For this parameter set, “herringbone” patterns emerge. Two structures for a lattice containing a single defect site (N=24) are also shown. Parameters D=1 t=5 re=0.7 Recent work [1] has shown that pattern formation on solid surfaces can be achieved using nonbonding forces between adsorbate molecules. The Miller group has synthesized functionalized fullerenes of the form shown above. These molecules can chemisorb to gold surfaces via the (yellow) S-Au interaction. These molecules interact with each other on the Au substrate through intermolecular forces – in particular through hydrogen bonds. (The hydrogen bonding centers, carboxylic acids, are shown in red.) We have constructed simple potential energy functions to mimic adsorbate-adsorbate and adsorbate-substrate interactions. Preliminary calculations have been carried out to explore the possibility of surface pattern formation with these potential functions. Illustrative results are shown in the center panels. In order to explore the underlying principles governing pattern formation, we have also devised a simple model. Rigid adsorbate molecules are sited on a hexagonal lattice. Each molecule can rotate freely on its site. Molecules interact through a pairwise-additive Morse potential between the tips. The most stable energy configurations for 3x3, 4x4 and 5x5 lattices are shown for one parameter set. Also shown is a model calculation showing the likely effect of defect sites. N=9 2D Metropolis Monte Carlo calculations (50 million steps) carried out at 500K with surface corrugation. N=16 Sulfur interaction with periodic Au surface [2]. The minimum energy at each (x,y) is found by allowing z to relax. N=24 N=25 CONCLUSIONS We have begun to develop potential energy functions to simulate the behavior of functionalized fullerenes on a gold substrate. Pattern formation has been observed in both atomistic and model calculations. We are beginning to be able to predict physical properties of adsorbates which will lead to desired pattern formation. REFERENCES 2 coordinate hydrogen bonding (red dots) and C60-C60 coordination are observed. More detailed calculations exploring surface corrugation are underway for larger clusters. [1] B. Xu, C. Tao, W. G. Cullen, J. E. Reutt-Robey, and E. D. Williams, Nano Lett. 5 (2005) 2207 [2] R. Bhatia and B.J. Garrison, Langmuir, 13 (1997) 4038 [3] L. A. Girifalco, J. Phys. Chem. 96 (1992) 858 Corrugated atom surface potential energy contours (kcal/mol) This work was supported under the Nanoscale Science and Engineering Centers Program of the National Science Foundation (Award # NSF-0425826)