Analysis of fluctuations in the Raman spectra of suspended and supported graphene films

Abstract

The repeatability of the Raman scattering (RS) spectra was analyzed in monolayer and bilayer CVD-grown graphene films. Position and width of the two main RS lines (G-line and 2D-line) together with their intensity ratio I2D/IG were measured in samples where graphene film was supported by a SiO2/Si substrate and where the film was suspended over pits in the substrate. The results are presented as histograms averaged by a Gaussian with the width W, considered as a measure of non-repeatability. Relatively large fluctuations in the I2D/IG value and linewidth were observed upon repeated measurements at the same point both in supported and suspended samples. The RS measurements in crystalline graphite and mechanically exfoliated graphene deposited on hexagonal boron nitride showed higher repeatability which means that increased W in CVD-grown films reflects the sensitivity of RS processes in these films to changes in charge and deformation. In bilayer CVD samples, the values of W are higher due to additional fluctuations in the twist angle between two misoriented layers. The average position of the RS lines is shifted in opposite directions and makes it possible to distinguish between bilayer and monolayer films, despite the similarity in the RS spectra.