Orientation- and Pressure Dependence of the Vibrational Response of a Monolayer Crystal on a Vicinal Diamond Surface

We systematically investigate polarization-dependent Raman spectra of a monolayer crystal of WS2 on the (100) and (230) surfaces of a diamond. At ambient pressure, identical polarization dependence of the Raman spectra is observed on the different surfaces, independent of the orientation of the monolayer crystal relative to the diamond crystal. However, when monolayer WS2 is compressed to about 4 GPa, an abrupt drop of the intensity of the 2LA mode relative to the A’ mode occurs on the (100) surface and the (230) surface with the zigzag direction along the atomic step edges of the (230) surface. In contrast, no such drop is observed when the armchair direction is along or at 15° to the atomic steps of the (230) surface. We also observe a shift of the polarization angle of the intensity maxima of the 2LA and A’ modes on the (230) surface during compression. These results demonstrate that the atomic steps of a vicinal surface strongly modify the vibrational response under high pressure.