Collision-induced diffusion and vacancy migration in alkanethiol monolayers on Au(1 1 1)

Scanning tunneling microscopy is used to characterize the collision-induced migration of molecules within well-ordered octanethiol and nonanethiol self-assembled monolayers. A seeded molecular beam is used to create xenon atoms with a kinetic energy of 1.3 eV, and collisions with these atoms cause measurable changes in alkanethiol monolayer surface structure. Migration rates are calculated and compared for molecules in close-packed domains, at domain-boundary defects, and along the perimeter of vacancy-island defects. The number of nearest-neighbor molecules (within the 5 Å lattice distance) is strongly predictive of molecular stability with respect to xenon bombardment, and the overall dependence of stability on nearest neighbors is well fit by a simple exponential curve. The incident direction of the molecular beam is not observed to influence the direction of molecular motion; however, in some cases, migration correlates strongly to surface lattice directions. Finally, there is no evidence that substrate restructuring or gold-atom diffusion accompanies alkanethiol migration under these non-equilibrium conditions.