Identifying the Alloy Structures of Germanene Grown on Al(111) Surface

TL;DR

This study uses advanced computational methods to identify the true structures of germanene grown on Al(111), revealing that the formation of germanene is energetically unfavorable and clarifying previous structural controversies.

Contribution

The paper combines DFT, RPA, and evolutionary algorithms to accurately identify alloy structures of germanene on Al(111), resolving existing structural ambiguities.

Findings

Germanene formation on Al(111) is energetically unfavorable.

Experimental phases are identified as Al3Ge3 and Al3Ge4 alloys.

Al3Ge4 structure explains STM dark clover pattern.

Abstract

While the growth of germanene has been claimed on many substrates, the exact crystal structures remain controversial. Here, we systematically explore the possible structures formed by Ge deposition onto Al(111) surface by combining density-functional theory (DFT) and global optimization algorithm. We show that, by high-level random-phase approximation (RPA) calculations, the formation of germanene on Al(111) is energetically unfavorable with positive formation energy. The two experimental phases are identified as honeycomb alloys Al3Ge3/Al(111)(r7xr7) and Al3Ge4/Al(111)(3x3), by combining ab initio evolutionary simulations, RPA calculations, and available experimental data from scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Al3Ge4/Al(111)(3x3) is an interesting structure with a vacancy in the substrate, which accounts for the dark clover pattern in the experimental STM image. Our results clarify the structural controversy of the Ge/Al(111) system and indicate the fabrication of germanene may remain challenging.