Fabrication of Graphene FET.

The top side of graphene on a copper film was spin-coated with PMMA to protect the top side of graphene, whereas the bottom side of graphene was etched away. PMMA acted as the supporting layer of the graphene after etching the copper. The back side of graphene was removed by oxygen plasma etching. The sample was cut into 4 × 6 mm pieces with scissors. Copper was etched by floating on 0.1 M ammonium persulfate for about 5 h and rinsed in deionized (DI) water overnight. Graphene supported by PMMA was then transferred on a SiO₂-coated silicon wafer. PMMA layer was removed by acetone at 60 °C for 1 h. The sample was annealed at 300 °C for 2 h under a hydrogen/argon atmosphere (52). To fabricate transistor, conducting silver paste was used as source and drain electrodes at the two ends of the graphene. Silicone rubber was used to insulate source and drain electrodes from liquid and construct solution reservoir.