Time-of-flight secondary ion mass spectrometry as a tool for evaluating the plasma-induced hydrogenation of graphene

Bandgap modification in graphene has received immense attention in the quest to develop postsilicon electronic materials. Hydrogenation of graphene has largely been the focus of bandgap modification; however, current research has relied solely upon the appearance of structural defects in Raman spectroscopy to evaluate the extent of hydrogenation. Here, the authors present time-of-flight secondary ion mass spectrometry (ToF-SIMS) as a complementary tool to Raman in determining the chemical changes occurring concurrently with structural defects. Multivariate analysis, through the use of principal component analysis (PCA) is coupled to ToF-SIMS analysis for the rapid determination of differences in surface chemistry between samples. Untreated graphene was found to exhibit a minimal defect intensity in the Raman with the most prominent ion intensities in the ToF-SIMS originating from unsaturated, graphene-like carbon fragments. However, exposure to H₂ plasma results in the evolution of a structural defect in the Raman which coincides with a mass shift toward ions representing hydrogen-rich, saturated hydrocarbons in addition to aliphatic oxides. The use of PCA on the resultant ion images and spectra allowed for rapid differentiation between samples based upon chemical species, rather than the intensity of a lattice defect alone, with high intra- and intersample reproducibility.