Cooperative Electron–Phonon Coupling and Buckled Structure in Germanene on Au(111)

Abstract

Germanene, a single-atom-thick germanium nanosheet in a honeycomb lattice, was proposed to be a Dirac fermion material beyond graphene. We performed scanning tunneling microscopy and in situ Raman spectroscopy studies combined with first-principles calculations on the atomic structures and the electronic and phonon properties of germanene on Au(111). The low-buckled 1 × 1 germanene honeycomb lattice was determined to exist in an unexpected rectangular √7 × √7 superstructure. Through in situ Raman measurements, distinctive vibrational phonon modes were discovered in √7 × √7 germanene, revealing the special coupling between the Dirac fermion and lattice vibrations, namely, electron-phonon coupling (EPC). The significant enhancement of EPC is correlated with the tensile strain, which is evoked by the singular buckled structure of √7 × √7 germanene on the Au(111) substrate. Our results present clear evidence for the existence of epitaxial germanene and elucidate the exotic properties of germanene on Au(111).

Raman Studies on Silicene and Germanene

“…This structural mode is further confirmed by calculations of the vDos, as shown in Figure 7(b), where the peaks located at 179 and 228 cm −1 are correlated with the oTO and E 2g modes, respectively. 64 The E 2g peak position of the √7×√7 superstructure is far away from those of FS germanene and Ge(111), which is a result of the large tensile strain and electron doping effect of the underlying Au(111) substrate. In fact, the average Ge-Ge bond length in the √7×√7 superstructure is around 2·563 Å, giving a tensile strain of 4·91% compared to that of FS germanene (~2·443 Å).…”

“…In fact, the average Ge-Ge bond length in the √7×√7 superstructure is around 2·563 Å, giving a tensile strain of 4·91% compared to that of FS germanene (~2·443 Å). 24,28,64 The E 2g mode in FS germanene is softened by this biaxial tensile strain. Nevertheless, the oTO peak in the calculated result shows a weak dependence on the biaxial tensile strain, and its position is close to that in the experimental results.…”

“…28 Furthermore, another peak located at 165 cm −1 corresponds to the out-of-plane transverse optical (oTO) phonon branch at G, which is evoked by the larger buckling value compared to that of silicene. 28 Experimentally, germanene has been successfully fabricated on metallic and semiconducting substrates, including Au(111), [64][65][66] Pt(111), 67 Al(111), 68 Cu(111), 69 Sb(111) 70 and molybdenum disulfide, 71 as there is no bulk allotrope of FS germanene in nature. The superstructures are formed on the top of these substrates instead of 1×1 FS germanene, in which different buckling structures and phonon properties are expected.…”

“…Thus, the first layer is attributed to the formation of germanium-gold (Au) alloy, based on the absence of Ge-Ge covalent bonds that could contribute to the Raman results, as shown in Figure 7(a). 64 For the second layer, a rectangular superstructure emerges with two Raman peaks at 179 and 228 cm −1…”

Recent Progress on Two‐Dimensional Heterostructures for Catalytic, Optoelectronic, and Energy Applications

“…For 2D layered materials, the neighboring layers stack via weak van der Waals (vdW) interactions, [14,15] which allows to obtain their monolayer or few-layer nanosheets by exfoliating their bulk allotropes. Very recently, molecular beam epitaxial (MBE) deposition method has also been employed to produce elemental 2D materials, which is expected to be absent in nature, such as graphene's cousins, silicene, [34][35][36][37][38][39][40][41][42][43][44][45][46][47] germanene, [5,[48][49][50][51][52] phosphorene, [53,54] stanene, [55,56,57] and so on. In contrast, 2D non-layered materials cannot be simply achieved by exfoliation method due to the strong covalence bonds among crystal planes.…”