Self-assembly of two-dimensional nanoclusters observed with STM: From surface molecules to surface superstructure

A type of stable and identical two dimensional (2D) Au nanoclusters are discovered to exist on the Si(111)-3×3-Ag surface at submonolayer Au coverage. Self-assembly of these Au nanoclusters leads to a 2D superstructure Si(111)-21×21-(Ag+Au). This discovery helps us to obtain definite evidence that all the previously proposed atomic structure models for the 21×21 structure can be excluded. A model in which three Au adatoms locate on Ag triangles in the 21×21 unit cell is built based on the low-temperature scanning tunneling microscopy observations. Additionally, the stability and the uniformity of the Au nanocluster are discussed in terms of a substrate-mediated interaction among the Au adatoms, and the self-assembling process is understood by analogizing the Au nanocluster with a kind of surface molecule.