Clean graphene surface through high temperature annealing

The cleanliness of the surface of graphene is important for its proper functioning in devices and sensors. Impurities including residual poly(methyl methacrylate) (PMMA) and hydrocarbon contaminants can alter its electronic and chemical properties. In this study, we used two surface-sensitive techniques, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), to monitor the chemical composition of the surface of graphene after washing it with acetone and annealing at high temperatures. The concentration of residual PMMA and hydrocarbon contaminants decreased as the annealing temperature increased. The atomic ratio of sp³ carbons to sp² carbons of a clean graphene surface determined using XPS can be used to estimate the amounts of sp³ defects in graphene. ToF-SIMS spectra indicate that residual PMMA was removed from the surface of graphene at 400 °C, while hydrocarbon contaminants required a higher temperature of 500 °C to remove. In ToF-SIMS spectra obtained at 500 °C, the characteristic ions for graphene, which are related to cleavage of ring structure, include C_x⁺ (x = 1, 2, 3), C_xH⁺ and C_xH₂⁺ as well as C_x^- and C_xH^-. For the first time, we developed a process to produce a very clean graphene surface which was verified by ToF-SIMS and XPS analyses.