Functionalization of chemical vapor deposition-grown graphene

Abstract

Graphene has been a wonder material since its discovery in 2004. Due to its interesting properties such as excellent thermal and electrical conductivities, flexibility and high strength, many researchers around the globe have turned their attention to this carbon allotrope. To synthesize graphene with high quality, chemical vapor deposition (CVD) is an industrially feasible method. Generally, methane is the most widely used carbon precursor. However, high purity grade is expensive in Thailand. Instead, acetylene should be a possible replacement. Moreover, operating CVD at atmospheric pressure would also dramatically reduce the cost. Still, growth conditions need to be optimized for new carbon sources using experimental design. First, seven factors in CVD process were screened by Plackett–Burman design and six Raman parameters of graphene were dependent variables. The results suggested that growth temperature and Ar-H2 total flow rates were significant at confidence intervals of 70-80%. The two significant factors together with acetylene flow rate were then subject to constructing response surface model with Box–Behnken design. Four relationships were developed by multiple linear regression (MLR) with R2 = 0.987 – 0.942 and the best quality graphene could be obtained at growth temperature of 1050 °C with Ar/H2/C2H2 flow rates of 900/100/0.4 sccm. Until now, graphene has been mostly used as a transparent conductor. Its application as a semiconductor has been limited due to its zero bandgap. Covalent modification was suggested as a possible way to open the bandgap in graphene. Although functionalization of graphene with diazonium salts using surfactant in aqueous solution was trivial for single-layer graphene, degree of functionalization was relatively low in case of multilayer graphene. Herein, substrate and photoinduced effects were investigated. While functionalized graphene on polar SiO2/Si substrate was contaminated with unreacted diazonium salts, functionalization of graphene on Cu growth substrate afforded the cleaner product. We also found that UVA light from a blacklight lamp or sunlight played important roles in functionalization presumably by generation of hot electrons and phenyl cations from graphene and diazonium salts, respectively. Nonetheless, the reaction could occur slowly without light. In conclusion, diazonium grafting could be enhanced under irradiation of light.