Structure and dynamics of Al trimer on Al(111) surface

Trimer is the smallest cluster that can have a one-dimensional or a two-dim ensional structure on fee (111) surface. Using first-principles density-fun ctional-theory calculations, the structural and dynamical properties of Al trimer on Al(111) surface have been studied in detail. Al trimer on Al(111) surface has four close-packed (compact) triangular configurations, two lin ear configurations, and some other noncompact triangular configurations. Th e close-packed triangular trimers are more stable than the noncompact trian gular trimers as well as the linear trimers. For the dynamics of Al trimer on Al(111) surface, the diffusion processes are much more complicated than the adatom and dimer diffusions. There are three different kinds of diffusi on mechanisms: concerted translations and rotation of compact triangular tr imers (the energy barrier, E-d = 0.24 eV); back and forth transformation be tween compact triangular trimers and linear trimers (E-d = 0.21 eV); and tr anslation of linear trimers (E-d = 0.28 eV). Among these different mechanis ms with similar height of diffusion barriers, the concerted translations of the compact triangular trimers have the longest displacement of the center of mass in each step. Therefore, we expect that the long-range diffusion o f Al trimer on Al(111) surface is dominated by the concerted motion process of the compact triangular trimers.