Tunning carrier concentration and Fermi-level in substrate-supported graphene monolayers: Effect of laser power

Tunning the carrier concentration n and the position of Fermi-level EF in graphene monolayers has a great impact on the design and fabrication of next generation graphene-based devices. Raman spectral properties of graphene/SiO2/Si and graphene/Al2O3 are investigated over a wide range of laser power (PL= 1 to 25 mW). The increase in PL results in significant variations in position and half-width of the G-band, allowing the determination of n and EF using nonadiabatic fitting. The initially formed p-type graphene is converted to n-type with increasing PL due to charge transfer to/from graphene with the substrate interfacial defect states and/or redox interactions with environmental gas. Whereas n and EF of the graphene/SiO2/Si reach saturation at PL=12mW, those of graphene/Al2O3 exhibit a monotonic increase before saturating at PL>21mW. A noticeable enhancement in graphene quality concurrent with a decrease in defect density is observed at PL>17 mW and >20 mW for graphene/Al2O3 and graphene/SiO2/Si, respectively, which are attributed to thermal laser effects at high PL. This work provides a contactless easy approach to monitor EF and tune the doping state of graphene against unintentional environment or substrate doping and gives an insight to the significant role played by the supported substrate.