  The first metallic glass of Au-Si alloy has been discovered for over half a
century, but its atomic structure is still puzzling. Herein, Au 8 Si
dodecahedrons with local five-fold symmetry are revealed as building blocks in
Au-Si metallic glass, and the interconnection modes of Au 8 Si dodecahedrons
determine the medium-range order. With dimensionality reduction, the surface
ordering is attributed to the motif transformation of Au 8 Si dodecahedrons
into planar Au 5 Si pyramids with five-fold symmetry, and thus the
self-assembly of Au 5 Si pyramids leads to the formation of the ordered Au 2 Si
monolayer with the lowest energy. Furthermore, the structural similarity
analysis is performed to unveil the physical origin of structural
characteristics in different dimensions. The amorphism of Au-Si is due to the
smooth energy landscape around the global minimum, while the ordered surface
structure occurs due to the steep energy landscape.
