Germanene is a two-dimensional (2D) sheet of Ge atoms arranged in a corrugated honeycomb lattice. The spin-orbit coupling opens a sizable bandgap between the Dirac cones, which could make germanene behave as a 2D topological insulator (a quantum spin Hall insulator) at experimentally accessible temperatures. In addition, its high carrier mobility as well as compatibility with the Si technology make germanene a promising candidate for electronics applications. However, lack of a layered form of Ge prevents germanene from being produced by exfoliation. So far, germanene has been mainly synthesized by Ge deposition on various substrates including Ag(111), and it can also be grown by Ge segregation on Ag(111) thin films from the Ge(111) substrate [1]. However, there seems to be controversy about the interpretation of the Ge-induced surface structures formed on Ag(111) [2,3]. High solvability of Ge in Ag causes Ag-Ge surface alloy, which complicates the structural determination.

Raman spectroscopy is a versatile tool to investigate 2D materials, and is expected to provide important clues to resolve the above controversy. However, germanene’s Raman spectra cannot be measured in air due to fast oxidation. In this paper, we first investigate formation processes of different surface structures on Ag(111) thin films during Ge segregation using low-energy electron microscopy (LEEM). Based on this knowledge, we fabricate various Ge-induced surface structures and identify their vibrational properties using ultrahigh vacuum Raman spectroscopy [4].

Figures 1(a)-1(c) show LEEM images during annealing the Ar-sputtered Ag(111)/Ge(111) sample at around 200 °C. In Fig. 1(d), the same region was imaged by the dark-field mode using the first-order diffraction spot of Ag(111). The bright and dark regions are Ag grains with different orientations. In Figs. 1(a)-1(c), the dark contrast around the grain boundaries corresponds to the “√3×√3” phase. Its spread from the grain boundaries indicates preferential diffusion of Ge atoms through them. The continuous annealing led the surface structure to develop into the √3×6√3 striped phase. Further annealing at higher temperatures caused the striped phase to change into the so-called disordered hexagonal or quasi-freestanding phases. When the sample was annealed at even higher temperatures like 600 °C, three-dimensional Ge islands were formed on the surface. Annealing at the highest temperature just before the Ge island formation caused the (7√7×7√7)R19.1° structure after cooling to room temperature.

Raman spectra of the striped phase are featureless in the range of 100-300 cm^-1, indicating that the striped phase is not germanene, but Ag-Ge surface alloy. Disordered hexagonal or quasi-freestanding phases showed Raman peaks assignable to germanene. The germanene Raman peaks intensified and shifted to higher wavenumbers as the annealing temperature increased, and the spectral shape continuously changed into that of the (7√7×7√7)R19.1° structure along with the emergence of new peaks. The peak shift could be due to the reduction of the tensile strain in germanene, and the zone folding of the phonon dispersion could cause the multiple peaks.

References

[1] J. Yuhara, H. Shimazu, K. Ito, A. Ohta, M. Araidai, M. Kurosawa, M. Nakatake, and G. Le Lay, ACS Nano 12, 11632 (2018).

[2] C.-H. Lin, A. Huang, W. W. Pai, W.-C. Chen, T.-Y. Chen, T.-R. Chang, R. Yukawa, C.-M. Cheng, C.-Y. Mou, I. Matsuda, T.-C. Chiang, H.-T. Jeng, and S.-J. Tang, Phys. Rev. Mater. 2, 024003 (2018).

[3] K. Zhang, R. Bernard, Y. Borensztein, H. Cruguel, and G. Prevot, Phys. Rev. B 102, 125418 (2020).

[4] S. Mizuno, A. Ohta, T. Suzuki, H. Kageshima, J. Yuhara, and H. Hibino, Appl. Phys. Express 14, 125501 (2021).