Identifying the alloy structures of germanene grown on Al(111)

Abstract

While the growth of germanene has been extensively claimed on many substrates, their exact crystal structures remain controversial. Herein, we systematically explore the surface structures of Ge deposition onto the Al(111) surface by theoretical calculations combined with available scanning tunneling microscopy (STM) and low-energy electron diffraction data. We show that the formation of germanene on Al(111) is energetically unfavorable by $ab\phantom{\rule{0.16em}{0ex}}initio$ evolutionary simulations and high-level random-phase approximation calculations, and the two experimental phases are identified as honeycomb alloys ${\mathrm{Al}}_{3}{\mathrm{Ge}}_{3}/\mathrm{Al}(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})$ and ${\mathrm{Al}}_{3}{\mathrm{Ge}}_{4}/\mathrm{Al}(3\ifmmode\times\else\texttimes\fi{}3)$. It is interesting to find a vacancy at the interface of ${\mathrm{Al}}_{3}{\mathrm{Ge}}_{4}/\mathrm{Al}(3\ifmmode\times\else\texttimes\fi{}3)$, which is responsible for the dark clover pattern in STM experiments. Our results clarify the structural controversy of the Ge/Al(111) system and indicate that the fabrication of germanene on Al(111) remains challenging.