Investigation On The Electrical Properties Of Graphene Oxide And Its Reduced Product Via Scanning Probe Microscope

In the paper, the Surface Potential(SP), Work Function(WF) and local electric conductivity of Graphene Oxide(GO) and its reduction products were investigated by using Scanning Probe Microscope(SPM).At first, we measured the SP of GO prepared by the modified Hummers method via Kelvin Force Microscope(KFM). It was found that the GO film exhibited the uniform SP distribution and the SP of GO is 20 mV higher than that of substrate. It is interesting that the SP at the edges and wrinkles of GO film were much lower than that at the in-plane flat area, which is proposed to be contributed to the bonds’ unsaturated and disorder at these zones. These results provide insight into understanding electronic properties of GO-based composites and GO-based devices.Secondly, the time-dependent WF evolution of graphene oxide under ultraviolet(UV) irradiation were investigated via KFM after spin coating GO on Highly Oriented Pyrolytic Graphite(HOPG). Fourier Transform Infrared Spectrum(FT-IR) and Raman Spectrum(Raman) measurements were carried out to assess the effects of UV irradiation on the structure of GO. The experimental results described that under UV irradiation, the thickness of GO reduced from the 1.06 nm to 0.79 nm and the WF decreased from 4.617 eV to 4.55 eV, in which the change of WF mainly occurred within the first 20 minutes. The change of GO were contributed to the reduction reaction of oxygen functional groups under UV irradiation. Because of the exist of more oxygen functional groups, and more oxygen functional groups would be reduced, resulting in the sharp decrease of the work function of GO; and after that the initial drastic reduction process, little functional groups were resided in GO, resulting in the little change of GO’s work function.Finally, we measured the local conductivity of GO attached to HOPG substrate. Using Conductive Atomic Force Microscope(CAFM), the current distribution image of GO and the I/V curve of different feature zones were given. The experimental results demonstrated that the graphene oxide nanosheet composed by large rich-oxygen insulating regions and graphene-like semiconductor regions. Single point I/V curve measurement were using to investigate the effect of local conductivity of GO under local bias voltage. By increasing the range of bias voltages, we found several critical voltages, such as 0.8 V, 1.5 V, 2.2 V, 3.2 V and so on. When the largest voltage is lower than 0.8V, there is no current; when the largest voltage lying in 1.5-2.1 V, the conductivity of GO can be improved slightly, but the GO structure didn’t change significantly; when the largest voltage lying in 2.2-3.1 V, the conductive of GO were improved greatly, while GO is still a semiconductor; when the largest voltage increases to 3.2 V, the conductivity of GO local change, behaving metal feature. The strong electric field caused by voltage, is consider as the key reason of the change of local conductivity, which destroys the oxygen functional group and leads to the reduction of graphene, finally improving the local conductivity.Our results would be proposed to give an insight on the structure, electrical properties and reduction mechanism of graphene oxide.