Chemical environment dominated Fermi level pinning of a graphene gas sensor

The time evolution of the Fermi level of the graphene channel during a gas sensing process is systematically investigated. A Fermi level converging behavior at negative back-gate voltage and a Fermi level pinning behavior at positive back-gate voltage are observed during NH3 gas sensing process for an originally p-type doped graphene channel. The experimental results confirm that the original p-type doping level has a significant effect on the energy level where the Fermi level converges (CFL). An empirical model is proposed for the Fermi level converging and pinning behavior: the up-shift of Fermi level suppresses the electron injection from NH3 to graphene while it enhances the electron extraction from graphene to the original p-type doping agent such as H2O. At positive back-gate voltage, the significantly suppressed electron injection from the freshly absorbed NH3 is complemented by the electron extraction from graphene to the original p-type doping agent. That is why a graphene channel shows no response to NH3 when the Fermi level is pushed above CFL even though it is still significantly below the donor level of NH3. The results in this report reveal a general interplay behavior between the freshly introduced testing gas and the original dopant of the graphene.