Structure of Germanene/Al(111): A Two-Layer Surface Alloy

Abstract

Unlike silicene, for which the demonstration of its existence has been done through numerous independent studies, the possibility of growing epitaxial germanene remains highly controversial. It has been recently shown by scanning tunneling microscopy that the (3 × 3) surface reconstruction formed upon Ge deposition on Al(111) presents a honeycomb structure, and it was assigned to a pure germanene monolayer. Using quantitative measurements by surface X-ray diffraction compared to density functional theory calculations, we demonstrate that this Ge/Al­(111) (3 × 3) reconstruction corresponds, in fact, to a mixed Ge–Al honeycomb layer on top of an alloyed interfacial layer. The model of a germanene monolayer on top of the Al(111) surface can be completely excluded.

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

“…It is worth noting that the Al 3 Ge 3 /Al(√7×√7) was reported in very recent work by surface X-ray diffraction 56 , indicating the reliability of our method. Other structures proposed previously have much higher formation energies (see Fig 2a, 2b, and 2c), such as single-layer alloys Al 3 Ge 5 18 (here named R6) and Al 2 Ge 27 (here named R7-1), two-layer alloys Ge 8 /Al 2 Ge 27 (the top layer is Ge 8 and the sub-layer is Al 2 Ge; here named R7-2) and Al 4 Ge 4 /Al 7 Ge 2 26 (the top layer is Al 4 Ge 4 and the sub-layer is Al 7 Ge 2 ; here named R8).…”

“…Among the various substrates, Al(111) surface is one of the most widely studied. The formation of germanene on this surface is first reported by Delivaz et al in 2015 8 , and then attracts great interest [19][20][21][22][23][24][25][26][27][28][29][30] . Two ordered phases have been observed experimentally, corresponding to the √7×√7 reconstruction [19][20][21]25 and 3×3 reconstruction 8,[19][20][21][22][23][24] related to the Al(111) surface [hereafter named as (√7×√7) and (3×3), respectively].…”

“…Yet, Martinez et al 18 questioned the germanene model for the (3×3) structure and proposed an alloy structure Al 3 Ge 5 , since they detected the presence of Al atoms within the formed surface layer by surface analysis techniques. Recently, by surface X-ray diffraction, a two-layer surface alloy 26 and a honeycomb alloy 56 were suggested for the (3 × 3) structure and the ( √7 × √7 ) structure, respectively.…”

Combined surface x-ray diffraction and density functional theory study of the germanene/Al(111)- (7×7)R19.1∘ structure

“…In order to determine the exact structure and composition of Ge/Ag(111) and Ge/Al(111) layers previously assigned to germanene, we have recently combined surface x-ray diffraction (SXRD) measurements and DFT simulations [23,24]. For Ge/Ag(111), we have shown that the structure formed after deposition at 420 K and assigned previously to a striped honeycomb pattern [16] was in fact an Ag 2 Ge surface alloy, confirming STM observations of the growth dynamics [25].…”

“…It was initially proposed to correspond to a (2 × 2) reconstruction of a germanene honeycomb layer, i.e., with eight Ge atoms per unit cell [14]. Recent high resolution STM observations have also evidenced the presence, at the surface, of eight atoms per unit cell [24]. However, it was also suggested from DFT calculations that this structure could correspond to a surface alloy [26], in agreement with low energy electron diffraction and ion scattering measurements [20].…”