Graphene, so far, is the most promising material for the new generation of THz devices, from lasers to detectors. In this work, we employ a THz scattering-type scanning near-field optical microscope (s-SNOM) to investigate the conductivity of different types of graphene, exfoliated, and CVD. As substrates, we used a cyclic olefin copolymer (TOPAS) film widely used for THz applications and serves as a flexible substrate, hexagonal boron nitride (hBN), and silicon. Figure 1 shows the amplitude and phase of exfoliated graphene's first three tip harmonics on the TOPAS substrate. For accurate conductivity determination, the analytical model for quantitative prediction of material contrasts in scattering-type near-field optical microscopy was implemented
Further characterization methods were used to study our samples: Raman spectroscopy and atomic force microscopy (AFM). As expected, graphene's conductivity on hBN is the highest and on TOPAS, it is the lowest. This can be attributed to the surface roughness of the TOPAS substrate and the strains. However, TOPAS, as an easily accessible material, can still be a suitable substrate for future THz devices.