Topographic studies using scanning probes have found that graphene surfaces are often covered by micron-scale domains of periodic stripes with a 4 nm pitch. These stripes have been variously interpreted as structural ripples or as self-assembled adsorbates. We show that the stripe domains are optically anisotropic by imaging them using a polarization-contrast technique. Optical spectra between 1.1 and 2.8 eV reveal that the anisotropy in the in-plane dielectric function is predominantly real, reaching 0.6 for an assumed layer thickness of 0.3 nm. The spectra are incompatible with a rippled graphene sheet but would be quantitatively explained by the self-assembly of chainlike organic molecules into nanoscale stripes.
Keywords: Graphene; adsorbates; friction; polarized light; self-assembly.