Optical detection of graphene nanoribbons synthesized on stepped SiC surfaces

Graphene nanoribbons (GNRs) are nanostructures considered to be promising building blocks for the realization of graphene-based devices. The optical properties of GNRs are hard to determine due to their nanoscopic dimensions. Reflectance Anisotropy Spectroscopy/Reflectance Difference Spectroscopy (RAS/RDS) is a powerful optical tool to characterize highly anisotropic structures. RAS/RDS has shown to be very useful to measure the optical response of materials including semiconductor heterostructures. The technique is non-destructive and can be used in air or in vacuum conditions. Considering the highly anisotropic geometry of the GNRs, the RAS/RDS becomes a quite convenient technique to characterize the optical properties of GNRs and in general to study the dependence on the thickness of the optical properties of graphene. The GNRs used in the present work were synthesized on 6H-SiC stepped substrates and annealed in air to obtain quasi-free-standing bilayer graphene (widths: 240 nm, 210, and 120 nm). For this system, the isolation of the optical signal coming from the GNRs in the RAS spectra is not an easy task due to the fact that both GNRs and the 6H-SiC stepped substrate are highly anisotropic. To study and characterize the GNRs, we present and discuss an experimental approach to isolate the RAS signal coming from the GNRs. We also have performed nano-RAS measurements by using a near-field scanning optical microscopy technique (nanometric resolution) that supports our method. We show that RAS and nano-RAS are powerful complementary optical probes that can be used to characterize GNRs and also properties such as the visual transparency of one-, two-, or few-layer thick graphene. Published by AIP Publishing.