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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
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
(N24) are also shown.
Parameters D1 t5 re0.7
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.
N9
2D Metropolis Monte Carlo calculations (50
million steps) carried out at 500K with surface
corrugation.
N16
Sulfur interaction with periodic Au surface 2.
The minimum energy at each (x,y) is found by
allowing z to relax.
N24
N25
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)