High resolution imaging of few-layer graphene

In this work, we successfully demonstrate how imaging ellipsometry can be applied to obtain high-resolution thickness maps of few-layer graphene (FLG) samples, with the results being thoroughly validated in a comparative study using several complementary techniques: Optical reflection microscopy (ORM), atomic force microscopy (AFM), and scanning confocal Raman microscopy. The thickness map, revealing distinct terraces separated by steps corresponding to mono- and bilayers of graphene, is extracted from a pixel-to-pixel fitting of ellipsometric spectra using optical constants (n = 2.7 and k = 1.2) derived by fitting slab model calculations to averaged Ψ and Δ spectra collected in large homogenous sample areas. An analysis of reflection spectra and contrast images acquired by ORM confirm the results by quantifying the number of graphene layers and retrieving the FLG optical constants using a simple Fresnel-law-based slab model. The morphology results are further corroborated with AFM and Raman images, the latter unambiguously verifying that the thinnest part of the FLG consists of a graphene bilayer and providing additional information of electronic origin that might help identifying subtle FLG features, such as the presence of impurities, variations in stacking order, or rolling and folding at the FLG edges.