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From Surface Magic Clusters to a Lattice of Surface Magic Clusters Y.
L. Wang,1,2 M. Y. Lai,1,2 H. H.
Chang,1,2 J. H. Wei,1,2 and C. M. Wei3 1
Institute
of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan 2
Department of Physics, National Taiwan University, Taipei, Taiwan, P. O. Box 23-166, Tapei 106, Taiwan 3 Institute
of Physics,
Academia Sinica, Nankang,
Taipei, Taiwan The
discovery of magic number clusters formation on a solid surface [1]
has triggered many studies attempting to understand the unusual
structure and properties of such ‘surface magic clusters’ (SMC)
[2-4]. It has also lead to the proposal that the enhanced stability
and abundance of SMC be exploited for the formation of mondispersed
nanostructures on surfaces [5]. Recently, we have found that, under
a well controlled deposition condition, gallium induces almost
exclusive formation of a type of SMC on the 7´7-reconstructed
silicon(111) surface without destroying the surface lattice
structure [5]. The SMC has a triangular closed-shell structure and
resides on the center of a 7´7
half unit-cell, as revealed by the scanning tunneling microscopy (STM)
shown Fig. 1a. When the gallium coverage is increased, the cluster
density increases proportionally, leading essentially to the
complete filling of the 7´7
half unitcells and the creation of an unprecedented two-dimensional
lattice of SMC (Fig.2). The extraordinary stability of the SMC can
be qualitatively explained by a structure model (Fig. 1b) that
employs three silicon atoms to link six gallium atoms into a cluster
configuration with completely satisfied bonding. Ab
inito calculation of the density of states of the cluster model
also shows similar results to the STM observations and therefore
supports the validity the model.
FIG. 1. (a) Empty-state STM image (Vtip= -2.0 V) of four surface
magic clusters. Inset is filled-state image (Vtip= 2.2 V)
of a pair of surface magic clusters surrounded by six corner Si
adatoms. Scale bar is 2 nm. (b) Model for surface magic clusters on
faulted and unfaluted half unit cells of the Si (111) 7x7 surface.
FIG. 2. Empty-state STM image (Vtip= -1.9 V) of a two dimensional
lattice of Ga-induced surface magic clusters grown on the Si(111)-7´7
surface at 350 °C. A 7´7
lattice unit containing two clusters is marked on the image. References [1]
M. Y. Lai and Y. L. Wang, Phys. Rev. Lett. 81,164 (1998). [2]
M. Y. Lai and Y. L. Wang, Phys. Rev. B 60, 1764 (1999). [3]
Ing-Shouh Hwang, Mon-Shu Ho, and Tien T. Tsong, Phys. Rev. Lett. 83,
120 (1999). [4]
Mon-Shu Ho, Ing-Shouh
Hwang, and
Tien T. Tsong, Phys. Rev. Lett. 84, 6792 (2000). [5]
Y. L Wang and M. Y. Lai, J.
Phys.: Condens. Matter 13,
R589–R618 (2001). [6]
M. Y. Lai and Y. L. Wang, Phys. Rev. B 64, R241404 (2001). |
