Raman spectroscopy of substrate-induced compression and substrate doping in thermally cycled graphene

By thermally cycling single layer graphene in air, we observe irreversible upshifts of the Raman G and 2D bands of 24 and 23 cm(-1), respectively. These upshifts are attributed to an in-plane compression of the graphene induced by the mismatch of thermal expansion coefficients between the graphene and the underlying Si/SiO2 substrate, as well as doping effects from the trapped surface charge in the underlying substrate. Since the G and the 2D band frequencies have different responses to doping, we can separate the effects of compression and doping associated with thermal cycling. By performing thermal cycling in an argon gas environment and by comparing suspended and on-substrate regions of the graphene, we can separate the effects of gas doping and those of doping from the underlying substrate. Variations in the ratio of the 2D-to-G band Raman intensities provide an independent measure of the doping in graphene that occurs during thermal cycling. During subsequent thermal cycles, both the G and the 2D bands downshift linearly with increasing temperature and then upshift reversibly to their original frequencies after cooling. This indicates that no further compression or doping is induced after the first thermal cycle. The observation of ripple formation in suspended graphene after thermal cycling confirms the induction of in-plane compression. The amplitude and wavelength of these ripples remain unchanged after subsequent thermal cycling, corroborating that no further compression is induced after the first thermal cycle.