Investigation of Thermal Annealing Effect on Bilayer Graphene by Isotope‐Labeling‐Assisted Raman Spectroscopy

Thermal annealing is a necessary step in the standard fabrication process of graphene for removal of polymethyl methacrylate (PMMA). The influence of thermal annealing on monolayer graphene (MLG) has been intensively studied, but that on bilayer graphene (BLG) remains unclear. A key challenge for such investigation is that the Raman spectroscopic method that is usually used on MLG encounters the overlap of featured peaks from the two graphene layers. Herein, the independent featured Raman peaks of the two layers can be detected using isotope‐labeled BLG samples, so that the shift in each spectrum affected by the annealing process can be monitored. Results show that there are compression and doping in MLG and twisted‐stacked BLG (t‐BLG) after annealing. Compared with that in MLG, the annealing‐induced doping level in the adlayer of t‐BLG is lower, and in the top layer of t‐BLG, only a low level of doping and compression remains after thermal annealing. Moreover, the annealing‐induced doping level in AB‐stacked BLG (AB‐BLG) is almost the same with that in t‐BLG, and the originally induced compression during a transfer process is unloaded. These results can provide more understanding for the post‐treatment of graphene on substrates and the characterization of each layer.