Growth of Germanene on Al(111) Hindered by Surface Alloy Formation

Abstract

Obtaining pure group IV 2D films on well-behaved substrates is at present a major goal in materials science and of great interest for the associated industries. This goal still represents a challenge in surface science because often these materials tend to form alloys. As a consequence, some of the proposed 2D films resulted in topics of controversy regarding the top-layer elemental composition and interpretation of the honeycomb patterns measured by STM. Very recently, germanene on Al(111) was proposed to be a system having a larger gap than silicene and a quantum-spin Hall effect. This system was studied by several techniques including scanning tunnel microscopy, low-energy electron diffraction, photoemission, and density functional theory. None of the techniques used until now have the capability to detect unambiguously the presence of substrate atoms within the ultrathin film (i.e., separated from the corresponding substrate), thus leaving open the question of the composition or purity of the layer. Here we follow previous guidelines to grow a Ge film on Al(111) with the expected 3 × 3 arrangement that was assumed to be characteristic of germanene, and then we study in situ the properties of the films with ion scattering and recoiling spectrometry, a technique particularly suited for determining the elemental composition of the last surface layer. Our results unambiguously show the formation of a mixture of well-ordered Ge and Al atoms for all of the temperatures and conditions tested, in clear disagreement with the pure single germanene layer proposed in previous works. These conclusions led us to investigate by DFT calculations other possible structures compatible with our present results and the previously reported ones. The most favorable alloyed structures obtained by DFT were then compared with new I-V low-energy electron diffraction curves, and from this comparison, a top surface model composed of five Ge atoms and three Al atoms is proposed to replace the germanene model.