Raman studies on silicene and germanene

Two-dimensional (2D) group IV elemental materials are expected to have similar electronic features to their lightest analogue, graphene. The favorable hybridization state of heavier group IV elements is the sp 3 state, leading to buckled structures instead of a planar one. These buckled structures could modulate the vibrational properties of these 2D materials by introducing a strain effect. In this review, the authors focus on the recent research on the Raman spectra of silicene and Germanene from both the theoretical and experimental sides. The abundant superstructures of silicene and Germanene are formed due to the different interaction strengths between silicene/germanene and underlying substrate, providing a way to modulate their phonon vibrational properties. Several factors affecting the vibrational modes are discussed, including the strain, doping, coverage and defect effects. Furthermore, the relationship between electron-phonon coupling strength and these factors is established based on the variation of Raman peak position and linewidth. Finally, the authors provide an overview of the general outlook and challenges for this field.