Vibrational properties of a monolayer silicene sheet studied by tip-enhanced Raman spectroscopy

Combining ultrahigh sensitivity, spatial resolution and capability to resolve chemical information, tip-enhanced Raman spectroscopy (TERS) is a powerful tool to study molecules or nanoscale objects. Here we show that TERS can also be a powerful tool in studying two-dimensional (2D) materials. We have achieved a 10⁹ Raman signal enhancement and a 0.5 nm spatial resolution using monolayer silicene on Ag(111) as a prototypical 2D material system. Due to the selective enhancement on Raman modes with vertical vibrational components in TERS, our experiment provides a direct evidence of the origination of Raman modes in silicene. Furthermore, the ultrahigh sensitivity of TERS allow us to identify different vibrational properties of silicene phases, which differ only in the bucking direction of the Si-Si bonds. Local vibrational features from defects and domain boundaries in silicene can also be identified.