Mechanism for phase transitions and vacancy island formation in alkylthiol/Au(111)self-assembled monolayers based on adatom and vacancy-induced reconstructions
E. Torres, A. T. Blumenau, P. U. Biedermann.
Physical Review B, 79, 075440, 1-6, (2009)
The structural and energetic effects due to gold adatoms and/or gold surface vacancies in ethylthiol/Au(111) self-assembled monolayers (SAMs) in the high-density regime have been studied. The stability of these SAM structures was evaluated based on adsorption and surface energies that allow a direct comparison of structures with different compositions. We have found another energetically more favorable c(4x2) structure that includes two adatoms. These adatoms may initially be taken from the gold surface creating vacancies. However our results indicate that the SAM may further stabilize by stepwise filling the surface vacancies. A plausible mechanism for the formation of gold vacancy islands, as seen in experiments during the growth of high-density domains, arises from this stabilization process. The best structure including two adatoms agrees with many structural data derived from the experiments. In particular, the simulated scanning tunneling microscope (STM) image exhibits a zigzag modulation that is characteristic of the δ phase frequently found in STM experiments.