Graphene, one of the most popular allotropes of carbon, has sparked broad interest in the field of material science since it was first isolated in 2004 by Professors Geim and Novoselov (University of Manchester). Currently, the synthesis of large-scale graphene on copper surfaces by chemical vapor deposition (CVD) is being explored by the scientific community. Despite considerable efforts, CVD graphene in different growth conditions exhibits various morphologies such as the presence of hillocks, defects, grain boundaries and multilayer island formation, effects which researchers are attempting to mitigate. But to be able to exhaustively study the composition of these samples, hyperspectral Raman imaging was required, and was carried out on CVD monolayer graphene with bilayer islands. Raman spectroscopy is a non-destructive analysis method that provides microscopic structural information by comparing a sample’s spectrum with reference spectra. Here, we present selected results from Prof. Martel’s group at Université de Montréal obtained during the investigation of the formation of graphene multilayer islands during Chemical Vapor Deposition growth with methane as feedstock. Known Raman signatures of the different configurations of graphene were used in this study to map the number of layers of the samples.
Raman imaging was performed with the hyperspectral Raman imaging platform RIMA™ based on Bragg tunable filter technology. In these measurements, a CW laser at λ = 532 nm illuminated 130 × 130 μm2 and 260 × 260 μm2 sample surface areas through 100X and 50X microscope objectives respectively. In this configuration, the sample was excited with 120 μW/μm2 and 30 μW/μm2 and the resolution was diffraction limited.
FIG. 1 (a) presents a 130 × 130 μm2 Raman map of graphene’s G band (∼1590 cm-1) in three different families: monolayer graphene (blue), bilayer graphene in resonance (red) and bilayer graphene out of resonance (green). Their typical associated Raman spectra are presented in FIG. 1 (b-c). The intensity variations of the G band reveal information on the stacking of the layers. The most significant changes in intensity observed in FIG. 1 (b) can be explained by resonance resulting from the twisted angle (13.5° at λexc = 532 nm ) of the bilayer graphene. FIG. 1 (d-f) presents similar results as in FIG. 1 (a-c), but data were acquired from a larger area: 260 × 260 μm2. The intrinsic specificity of Raman scattering combined with global imaging capabilities allows users to assess large maps (hundreds of microns) of defects, number of layers and stacking order, etc.
Results kindly provided by: Vincent Aymong, Minh Nguyen, and Richard Martel, at Université de Montréal, Canada.
 Havener R. W., Zhuang H. L., Brown, L., Hennig, R. G., Park, J., Angle-Resolved Raman Imaging of Inter layer Rotations and Interactions in Twisted Bilayer Graphene. DOI: 10.1021/nl301137k. Nano Letters. 2012, 12.