Atomic and electronic structures ofAg/Si(111)-c(12×2)surface: A first-principles study

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Structural models for the $\text{Ag}/\text{Si}(111)\text{-c}(12\ifmmode\times\else\texttimes\fi{}2)$ 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 $\text{c}(12\ifmmode\times\else\texttimes\fi{}2)$ and $6\ifmmode\times\else\texttimes\fi{}1$ structures can be derived from the well-known honeycomb chain-channel $3\ifmmode\times\else\texttimes\fi{}1$ model. Two $\text{c}(12\ifmmode\times\else\texttimes\fi{}2)$ models are found to have lower surface energies than both the $3\ifmmode\times\else\texttimes\fi{}1$ and $6\ifmmode\times\else\texttimes\fi{}1$ models, which is consistent with the experimental observation. A distinct feature of the $\text{c}(12\ifmmode\times\else\texttimes\fi{}2)$ structure is the dimer formation induced by lateral displacements of Ag atoms within the same row in the $6\ifmmode\times\else\texttimes\fi{}1$ phase. Different packing patterns of the Ag dimers with the same surface area can lead to either $6\ifmmode\times\else\texttimes\fi{}2$ or $\text{c}(12\ifmmode\times\else\texttimes\fi{}2)$ phase with very similar energies. Simulated scanning tunneling microscope images from our $\text{c}(12\ifmmode\times\else\texttimes\fi{}2)$ structure are in excellent agreement with the experimental measurement. Moreover, the lowest-energy $\text{c}(12\ifmmode\times\else\texttimes\fi{}2)$ structure identified from our calculations also reproduces the key features in the angle-resolved photoemission measurement.