The Effect of Thermal Annealing Processes on Graphene

K. Kumar, Y.S. Kim, X. Li, Y. Tian, A. Pallikaras and E.H. Yang
Stevens Institute of Technology, US

Keywords: graphene, annealing, CVD

Summary:

We demonstrate the impact of PMMA removal from CVD graphene by annealing in H2/Ar and ambient atmospheres as well as under vacuum on the Raman spectra and AFM topography of graphene. We find a broad signal from 1200 to 1600 cm-1 superimposed upon the D and G bands after H2/Ar annealing in samples transferred both with and without PMMA, which we attribute it to a combination of amorphous carbon and physisorption of H2 with the graphene plane, forming sp3 bonds during annealing. Upon ambient annealing, this signal is removable. AFM scans of gas annealed graphene confirm this effect; amorphous carbon content on the graphene layer after the H2/Ar anneal increased step height. Vacuum annealing increased hole-doping levels, demonstrated by blueshifts of G and G’ bands, caused by the interplay between hole-doping reduction as PMMA was evaporated and hole-doping increase from substrate injection as interfacial distance decreased. Our results demonstrate that annealing under gas or vacuum increases doping levels in graphene as residual PMMA is burned off, interfacial water is evaporated, and graphene conformally contacts the substrate. This is corroborated by AFM measurements, indicating a need for further research on graphene transfer or growth methods where substrate-induced doping is minimized.