AFM and STM Characterization of Electrochemically Synthesized Few-Layer Graphene Nano-Sheets

Abstract

The aim of this work is electrochemical exfoliation of pyrolytic graphite for mass production of few-layer graphene nano sheets. It is synthesized by intercalation of graphite sheets in the electrolyte of two different
types of concentrations, one molar and two molar concentrations of nitric acid by application of positive bias. The voltage is gradually increased with an increment of 0.5V upto 8V and an interval of 3 minutes. The X-ray diffraction peaks corresponding to graphene sheet ((002) plane) were observed at 2θ positions of 26.35°. The morphology of as-synthesized FLGNSs is characterized by field emission scanning electron microscopy. The transparent layers of FLGNSs are observed in transmission electron microscopy. The number of layers in transparent graphene sheets is confirmed by the HRTEM. Through FTIR studies, the presence of functional groups of O-H and C-O has been identified. AFM topography revealed that the thickness of the single layer is in the range of 1 nm, and for few-layer graphene nano sheets are in the range of 5-6 nm only. However, FLGNSs could be readily distinguished through phase imaging of tapping-mode AFM, because of differences in hydrophobicity arising from their different oxygen contents. STM studies of graphene nanosheets revealed atomic scaled periodicity at very low tunneling currents (∼1 pA). Phase imaging showed distinct contrast difference between FLGNSs to the graphite substrate (HOPG), a result that was attributed to their extremely low conductivity. The atomically flatness of the graphene nano sheets and electronic properties were measured by scanning tunneling
microscopy. Scanning probe spectroscopy revealed the electronic properties like the density of states (DOS) and Dirac point (DP) of graphene sheets. The synthesized material can be used as a base material for the future applications such as desalination of sea water,
supercapacitors, sensors, solar cells, and coatings.