Structural and dynamical behavior of Al trimer on Al(111) surface

Trimer is the smallest cluster that can have a one-dimensional or two-dimensional structure on surfaces, and it can diffuse and transform between these structures. Using first-principles density-functional theory (DFT) calculations, the structural and dynamical behaviors of Al trimer on Al(111) surface have been studied in detail. Al trimer on Al(111) surface has three different kinds of structure conformations (groups with similar configurations): close-packed (compact) triangular trimers, non-compact triangular trimers, and linear trimers. The close-packed triangular trimers are more stable than the non-compact triangular trimers and the linear trimers, while most of the non-compact triangular trimers are as stable as the linear trimers. For the dynamics of Al trimer on Al(111) surface, there are three different kinds of diffusion mechanisms: (1) concerted translations and rotation of compact triangular trimers (the highest energy barrier by DFT calculation, Ed=0.24 eV); (2) back-and-forth transformation between compact triangular trimers and linear trimers (Ed=0.21 eV); and (3) translation of linear trimers (Ed=0.28 eV). Among these different mechanisms with similar height of diffusion barriers, the concerted translations of the compact triangular trimers have the longest displacement of the center of mass in the least steps. Therefore, we expect the long-range diffusion of Al trimer on Al(111) surface is dominated by the concerted motion process of the compact triangular trimers. The concerted translations and concerted rotations of Al trimer on Al(111) surface have also been observed in the molecular dynamics simulations using the embedded atom method.