Raman Spectroscopy of Two-Dimensional Borophene Sheets.

The successful fabrication of two-dimensional boron sheet, which features triangular lattice with periodic hole arrays, has stimulated great interest in its unique structure as well as properties such as possible superconductivity. Here, we report a study on the vibrational spectra and electron-phonon coupling (EPC) in monolayer boron sheets by in-situ Raman and tip-enhanced Raman spectroscopy (TERS) in low temperature and ultrahigh vacuum. The gap-mode TERS gives a 3x10^9, selective enhancement on vertical vibrational Raman modes. A spatial resolution of 1 nm is achieved in this system. Combined with first principle calculations, the vibrational properties as well as EPC in borophene are determined. The results are helpful for further study on the mechanical, electronic and possible superconducting properties of two-dimensional boron.