Coexistence of strongly buckled germanene phases on Al(111)

We report a study of structural and electronic properties of a germanium layer on Al(111) using scanning tunneling microscopy (STM), low energy electron diffraction and core-level photoelectron spectroscopy. Experimental results show that a germanium layer can be formed at a relatively high substrate temperature showing either (3$\times$3) or ($\sqrt{7}$$\times$$\sqrt{7}$)R$\pm$19.1{\deg} reconstructions. First-principles calculations based on density functional theory suggest an atomic model consisting of a strongly buckled (2$\times$2) germanene layer, which is stable in two different orientations on Al(111). Simulated STM of both orientations fit nicely with experimental STM images and the Ge 3d core-level data decomposed into four components is consistent with the suggested model.