Graphene Films

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REELS of Graphene Films

The presence of peak due to sp2 hybridization at 4-6 eV lower than the elastic scattering peak is a very fast way to confirm the presence of graphene. The position and intensity of this peak are directly proportional to the thickness of graphene films, which opens the potential for developing a quantitative calibration curve for graphene thickness based on energy loss spectra. 

Left: REELS spectra from a bare substrate (SiO2) and monolayer graphene samples supported on quartz and on SiO2. Right: REELS spectra as a function of the number of layers of graphene – from monolayer to unsupported graphene sheet.

XPS on Graphene Films

The X-ray beam induced secondary electron image (SXI) is sensitive to inhomogeneity in graphene. The D-parameter, extracted from the C KLL Auger transition, is very useful for differentiating between sp3 and sp2 character of carbon.

SXI image of multilayered Graphene on SiO2.  Contrast due to inhomogeneity in graphene coverage is observed. Middle: D-parameter extracted from C KLL Auger transition. D parameter value of 16-19 confirms sp2 character of carbon due to graphene.

Thicknesses of graphene layers, as well as adventitious carbon on top of it, were calculated for all samples using PHI’s StrataPHI algorithm for thin film structures. Normalized peak intensities from single spectral acquisitions at 20° TOA and narrow angular acceptance mode were used to calculate thicknesses. The thickness of mono, 2L and multi-layered graphene are within 10-20% of known values.

Estimated structure with thicknesses for 2L/SiO2 sample.  Graphene thicknesses calculated for multiple samples. The red line shows the known thickness.