Phase transition of germanene prepared by atomic segregation epitaxy at Ag(111) thin films on Ge(111)

Yuhara, Junji*; Maeda, Shota*; Katsube, Daiki*; Suzuki, Seiya   ; Terasawa, Tomoo   ; Takakura, Shoichi*; Nakatake, Masashi*; Le Lay, G.*

This study aims to investigate the temperature-dependent structure of germanene prepared by atomic segregation epitaxy on an Ag(111) thin film surface. A germanene (7 7) superstructure, obtained upon heating at 500 C followed by cooling at room temperature, transforms into a germanene ( ) superstructure after further annealing at 250 C and subsequently evolves into the Ag-Ge surface alloy upon annealing at 350 C, as evidenced from the thorough analysis by scanning tunneling microscopy, low-energy electron diffraction, and core-level (CL) photoemission spectroscopy. Angle-resolved CL spectra reveal that germanium preferentially occupies the topmost atomic layer, forming the (7 7) and ( ) superstructures. In contrast, germanium is distributed within at least a few layers for the striped incommensurate surface alloy phase. From angle-resolved photoemission spectroscopy, the electronic structures of the (7 7) and ( ) superstructures appear to be almost the same. Based on these considerations, the surface phase diagram of Ge/Ag(111)/Ge(111) has been constructed. These findings provide insights into the relationship between germanene thermal stability and Ge concentration at the surface region of Ag(111).