Nowadays, for the synthesis of large-area graphene films, CVD is an effective method of producing graphene films and has attracted increased attention during the last decade. Research has demonstrated that the graphene film synthesized on transition metal foil by CVD process has a rough surface because of the high temperatures (up to 1000 °C) of deposition and heterogeneous crystallization during the cooling process. The existence of line-shaped surface defects (tears, cracks, folds, grain boundary, etc.) with hundred-microns-sized on the graphene films may be inevitable owing to the thermal treatment of the CVD process or other factors. Raman spectroscopy, atomic force microscopy (AFM) have been recognized as the most appropriate techniques for the qualitative analysis, thickness evaluation. Also, Raman Mapping has been used to evaluate the layers and uniformity of graphene films in a certain area of the Micron-scale. However, these characterization techniques are all microscopic characterization and cannot effectively reflect the line-shaped surface defects with the characterization range of hundreds of nanometers – dozens of microns on the graphene films. Therefore, finding a macroscopic way to measure the quality of graphene films is imperative. The beam size of the EM waves in THz band is in the millimeter to several hundreds of micrometer levels, if the THz waves are sensitive to the surface defect and the changes of the layer number of the graphene, whether the THz technology can be used as a new generation of quality testing technology for graphene films remains to be studied.
During my PhD stage, I am working on a new method for quality testing of graphene films based on the variation of the local electrical conductivity and anisotropy of electrical loss effect using THz vector network analyzing technology. Among our existing study results, we propose a method for detecting homogeneity of graphene films and a method for detecting line-shaped defect content on the surface of graphene film respectively. The calculation method of the quantitative index of these two detection methods was also studied. We also believe that this quality testing method can also be applied to the quality measurement of other 2D conductive materials, such as molybdenum disulfide, black phosphorus, etc.