Characterizing the properties of graphene in a standardized manner is a prerequisite to industrial adoption of the material. Although Raman spectroscopy has proven to be a tool ideally matched to the requirements of graphene characterization, widescale industrial use of Raman metrics will hinge on reducing measurement uncertainty due to different tools and analysis procedures. An international team coming from 22 different academic, industrial and government institutions has made large steps towards this goal by comparing Raman spectroscopy data on the same samples, measured with different tools and in different labs. The product used for the measurements was Graphenea CVD graphene.
The study, recently published in the journal 2D Materials, showed that certain metrics that were taken for granted to be indicative of graphene quality can have widely varying values, depending on the Raman tool and analysis software used. With data gathered from 17 participants, there were inconsistent reports on Raman peak intensity ratios, peak widths, and the coverage of graphene. Due to a lack of relative intensity calibration, the relative difference reported in the ratio of the intensities of the 2D and G peaks in the Raman spectra, which is often taken as a precise measure of graphene thickness, was up to 200%. The number of layers in a graphene sample, i.e. material thickness, is also measured through the width of the 2D peak. The standard deviation of that measure was shown to be 15 times larger if common Lorentzian fitting was used on the peak, compared to using pseudo-Voigt functions.
The international team, led by the National Physical Laboratory (NPL) in the UK, crafted a path to standardizing Raman spectroscopy of graphene, by adopting a relative intensity calibration and consistent peak fitting and data analysis methodologies. If labs worldwide were to adopt the proposed methods, variations in measurements would be significantly reduced, which would support the industrial adoption of this exquisite material.
The findings directly underpin the development of the ISO/IEC standard 'DTS 21356-2—Nanotechnologies—Structural Characterisation of CVD-grown Graphene'. Standardisation is one of the Work Packages of the Graphene Flagship, a billion-euro program of the European Commission to bring graphene from research labs to commercialization.