Substrate Dependent Anisotropic Diffusion Of Indium Atoms On Ptcda Thin Films Studied by Peem

Anisotropic surface diffusion of In atoms was observed by photoelectron emission microscopy (PEEM) when a square-shaped microstructure of In metal was deposited onto thin films of perylene-3,4,9,10-tetracarboxyric acid-dianhydride (PTCDA) grown on cleaved MoS2 and GeS surfaces. For In microstructure on PTCDA/MoS2 system, the PEEM images showed a triangle pattern, although the deposited shape of the In microstructure was square, indicating that In atoms diffuse to three directions on PTCDA/MoS2. Such triangle pattern was not observed on PTCDA/GeS but an oval structure was observed, revealing that In atoms diffuse to two directions on PTCDA/GeS with different diffusion constant. The two directions correspond well with surface crystal axes of GeS. Since the unit cells of PTCDA adsorbed on these two crystal surfaces are nearly the same, it is suggested that different anisotropic diffusion of In in these systems is related to strong In-PTCDA interaction which may be affected by the surface structure of underlying substrate.