  We report valence band electronic structure evolution of graphene oxide (GO)
upon its thermal reduction. Degree of oxygen functionalization was controlled
by annealing temperatures, and an electronic structure evolution was monitored
using real-time ultraviolet photoelectron spectroscopy. We observed a drastic
increase in density of states around the Fermi level upon thermal annealing at
~600 oC. The result indicates that while there is an apparent band gap for GO
prior to a thermal reduction, the gap closes after an annealing around that
temperature. This trend of band gap closure was correlated with electrical,
chemical, and structural properties to determine a set of GO material
properties that is optimal for optoelectronics. The results revealed that
annealing at a temperature of ~500 oC leads to the desired properties,
demonstrated by a uniform and an order of magnitude enhanced photocurrent map
of an individual GO sheet compared to as-synthesized counterpart.
