The impact of ultraviolet laser excitation during Raman spectroscopy of hexagonal boron nitride thin films

Abstract

We utilized excitation in the ultraviolet (UV) spectral range for the study of hexagonal boron nitride (h-BN) thin films on different substrates by Raman spectroscopy. Whereas UV excitation offers fundamental advantages for the investigation of h-BN and heterostructures with graphene, the actual Raman spectra recorded under ambient conditions reveal a temporal decay of the signal intensity. The disappearance of the Raman signal is found to be induced by thermally activated chemical reactions with ambient molecules at the h-BN surface. The chemical reactions could be strongly suppressed under vacuum conditions which, however, favor the formation of a carbonaceous surface contamination layer. For the improvement of the signal-to-noise ratio under ambient conditions, we propose a line-scan method for the acquisition of UV Raman spectra in atomically thin h-BN, a material which is expected to play a key role in future technologies based on 2D van der Waals heterostructures.