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

Structural and Dynamical Behavior of Al Trimer on Al(111) Surface. Chang, C.M., Wei, C.M., Chen S.P.: Surface Science, 2000, 465[1-2], 65-75