Structural identification of silicene on the Ag(111) surface by atomic force microscopy

Abstract

Silicene is a two-dimensional atomic layer material with buckled honeycomb arrangements of Si atoms. The diversity of those arrangements, which expands its potential applications, makes it difficult to determine its structure in any particular case. In this paper, we show that atomic force microscopy (AFM) has the capability of structural determination of unknown phases of silicene. We carried out an AFM observation of $(\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13})R13.{9}^{\ensuremath{\circ}}$ silicene of unknown structures on Ag(111). Remarkably, it was shown that all constituent Si atoms forming a honeycomb lattice can be resolved by AFM whereas scanning tunneling microscopy (STM) can image only the topmost Si atoms. High-resolution AFM imaging allowed us to identify two types of buckled structure of $(\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13})R13.{9}^{\ensuremath{\circ}}$ silicene on Ag(111), which had not been previously discriminated. The structure models obtained by theoretical simulation reproduced AFM images as well as previous STM images. In addition, the mechanism of high-resolution AFM imaging was elucidated by force spectroscopy combined with first-principles calculations. Namely, attractive interaction with the tip pulls up buckled down Si atoms, causing local flips of the buckled structures.