VOLUME 81, NUMBER 13 PHYSICAL REVIEW LETTERS 28 SEPTEMBER 1998
Hexagonally Reconstructed Islands and Anisotropic Diffusion for AuAu(100)
Miki Nomura and Xiao-Qian Wang
Department of Physics and Center for Theoretical Studies of Physical Systems, Clark Atlanta University,
223 James P. Brawley Drive, Atlanta, Georgia 30314
(Received 27 January 1998)
The homoepitaxial island growth on hexagonally reconstructed Au(100) is studied using molecular
dynamics based on a well-tested many-atom interatomic potential. Our study reveals that the stable
islands of rectangular shape are hexagonally reconstructed in conformity with the patterns of the
reconstructed Au(100) surface and suggests the “magic” stable width for the reconstructed islands
in agreement with experimental observations. Furthermore, our results on the adatom diffusion indicate
that the experimentally observed strong anisotropic effect is attributed to the long-range exchange
diffusion. [S0031-9007(98)07233-0]
PACS numbers: 68.35.Fx, 68.10.Jy, 68.35.Bs, 68.55. – a
Surface adatoms and small cluster diffusion on metal
clusters have been the subject of recent experimental and
simulational attention due to their important role played
in controlling the formation of surface layers, thin film
growth, and catalysis. The nucleation and growth of
Au islands on the reconstructed Au(100) are both in-
trinsically interesting and technologically important. It
is expected that for the island growth, surface mor-
phology depends strongly on the reconstructed substrate.
Recently, Günther and co-workers [1] carried out a
scanning-tunneling microscope (STM) study of the nu-
cleation and growth of Au islands on the hexagonally
reconstructed Au(100) surface. It was shown that the
reconstructed substrate yields strong anisotropic effects,
specifically the rectangular shape of the island (island
shape anisotropy) and strongly anisotropic diffusion. The
latter was deduced from the rate equation analysis of
the experimental flux and temperature dependence of the
island density. Two typical sizes of the islands, namely,
(i) islands of widths of 30 Å and lengths ranging from
100 to 150 Å at high deposition flux rate and (ii) islands
of widths of less than 80 Å and lengths around 700 Å at
low deposition flux rate, were reported.
In spite of this intriguing picture, there has remained
a paucity of theoretical studies on the island growth and
anisotropic diffusions for AuAu(100). A clear under-
standing of the origin of anisotropic effects has been
hindered by the lack of quantitative theoretical calcula-
tions. Recently, Bonig, Liu, and Metiu [2] employed the
effective-medium theory to investigate the diffusion of
adatoms for a m-atom-wide 1 3 5 reconstructed island
on top of unreconstructed underneath. Their calculation
indicates that kinetics favors m 6, in that the adatom
diffusion along the long side of such a reconstructed island
is much faster than m fi 6. However, due to the limita-
tion of the employed interatomic potential (not capable of
predicting the reconstructed surface), their interpretation
of experiments needs to be reexamined with more realistic
models.
Inspired by the experimental results, we have carried
out a large-scale molecular-dynamics (MD) simulation
study of the island growth and anisotropic diffusion on
AuAu(100). Using a well-tested many-atom potential
of gold constructed by Ercolessi, Tosatti, and Parrinello
[3], our simulation study reveals that the optimal island
is “quantized” in concord to the “magic” size of 7, 13, . . .
(6n 1 1, where n is an integer) reconstructed rows. The
formation of a hexagonally reconstructed island in confor-
mity with the pattern of the reconstructed Au(100) surface
is energetically favored. Contrary to the proposed hop-
ping mechanism for adatom diffusion [2], our calculation
reveals that the long-range exchange effect plays a pre-
dominant role in the anisotropic diffusion and the growth
of hexagonally reconstructed islands.
The semiempirical many-body “glue model” [3] be-
longs to the same class as the embedded-atom method [4]
and “pair-functional” models [5]. The glue potential for
gold has been well tested and is known to provide good
results for diverse surface properties. Specifically, the
model is capable of explaining the structure and phases
of the reconstructed Au(100) surface. It is worthwhile to
mention that the simulation study of M 3 5 reconstruc-
tion configuration (i.e., M 1 1 first-layer atoms on top of
M second-layer atoms along [011], in addition to the 6-
onto-5 registry along 01
¯
1), reveals that the energies are
very close for the range 20 , M , 35, with a surface en-
ergy difference of 0.1 meVÅ
2
[3,6].
In carrying out our analysis, we have performed in-
tensive molecular-dynamics simulations, with in-plane
periodic boundary conditions, on 16-layer slab systems
consisting of 2 5 3 10
4
atoms. The experimentally
observed 28 3 5 structure [7,8] is used for the surface
throughout this study. The structure can be characterized
by edge dislocations along [011] and 01
¯
1, respectively,
with the dislocation lines corresponding to the corrugated
regions in connection with the excessive stress. It is
worth noting that the corrugated lines can be identified as
parallel stripes observed in STM experiments.
0031-9007 98 81(13) 2739(4)$15.00 © 1998 The American Physical Society 2739