Atomic and electronic structures of Ag/Si(111)-c(12x2) surface: A first-principles study

Structural models for the Ag/Si(111)-c(12x2) phase were identified from a systematic search using tight-binding molecular dynamics and first-principles total-energy calculations. Our calculations showed that the low-energy c(12x2) and 6x1 structures can be derived from the well-known honeycomb chain-channel 3x1 model. Two c(12x2) models are found to have lower surface energies than both the 3x1 and 6x1 models, which is consistent with the experimental observation. A distinct feature of the c(12x2) structure is the dimer formation induced by lateral displacements of Ag atoms within the same row in the 6x1 phase. Different packing patterns of the Ag dimers with the same surface area can lead to either 6x2 or c(12x2) phase with very similar energies. Simulated scanning tunneling microscope images from our c(12x2) structure are in excellent agreement with the experimental measurement. Moreover, the lowest-energy c(12x2) structure identified from our calculations also reproduces the key features in the angle-resolved photoemission measurement.