Direct Measurement of Dirac Point Energy at the Graphene/Oxide Interface

We report the direct measurement of the Dirac performing internal photoemission measurements on a point, the Fermi level, and the work function of graphene by graphene/SiO2/Si structure with a unique optical-cavity enhanced test structure. A complete electronic band alignment at the graphene/SiO2/Si interfaces is accurately established. The observation of enhanced photoemission from a one-atom thick graphene layer was possible by taking advantage of the constructive optical interference in the SiO2 cavity. The photoemission yield was found to follow the well-known linear density-of-states dispersion in the vicinity of the Dirac point. At the flat band condition, the Fermi level was extracted and found to reside 3.3 eV +/- 0.05 eV below the bottom of the SiO2 conduction band. When combined with the shift of the Fermi level from the Dirac point, we are able to ascertain the position of the Dirac point at 3.6 eV +/- 0.05 eV with respect to the bottom of the SiO2 conduction band edge, yielding a work function of 4.5 eV +/- 0.05 eV which is in an excellent agreement with theory. The accurate determination of the work function of graphene is of significant importance to the engineering of graphene-based devices, and the measurement technique we have advanced in this Letter will have significant impact on numerous applications for emerging graphene-like 2-dimensional material systems.