Determining the number of layers in few-layer graphene by combining Raman spectroscopy and optical contrast
Résumé
Raman spectroscopy is commonly used to determine the number of layers of few-layer graphene (FLG)
samples. In this work, we focus on the criteria based on the G-band integrated intensity and on the laser
optical contrast. Limitations due to stacking order are discussed and lead to the conclusion that it is
necessary to combine Raman and optical contrast to avoid misinterpretation. Both methods enable to
distinguish unambiguously between single layer graphene and multilayer graphene. However, neither
each method separately nor the combination of the two enable a determination of the number of layers for
all possible stacking orientations. Importantly, since the two methods always significantly disagree when
they fail, the comparison of the values deduced by each method allows to discriminate if the determined
number of layers can be specified or not. Other important parameters (substrate, laser wavelength,
objective numerical aperture) are discussed to define a reliable method to determine the number of
graphene layers in FLG and its domain of validity. The proposed method which combines Raman and
optical contrast measurements, carried out with a 532 nm laser and using a 100x objective with a
numerical aperture of 0.9, allows the determination of the number of layers for (up to 5) FLG on the
following substrates: (i) glass (soda lime glass or similar with refractive index between 1.50 and 1.55) and
(ii) oxidized silicon (SiO2 on silicon, with a SiO2 thickness of 90 ± 5 nm). The method is however limited to
high quality graphene and FLG with small defect density and low residue.