Vapor-phase molecular doping of graphene for high-performance transparent electrodes

Doping is an essential process to engineer the conductivity and work-function of graphene for higher performance optoelectronic devices, which includes substitutional atomic doping by reactive gases, electrical/electrochemical doping by gate bias, and chemical doping by acids or reducing/oxidizing agents. Among these, the chemical doping has been widely used due to its simple process and high doping strength. However, it also has an instability problem in that the molecular dopants tend to gradually evaporate from the surface of graphene, leading to substantial decrease in doping effect with time. In particular, the instability problem is more serious for n-doped graphene because of undesirable reaction between dopants and oxygen or water in air. Here we report a simple method to tune the electrical properties of CVD graphene through n-doping by vaporized molecules at 70 °C, where the dopants in vapor phase are mildly adsorbed on graphene surface without direct contact with solution. To investigate the dependence on functional groups and molecular weights, we selected a series of ethylene amines as a model system, including ethylene diamine (EDA), diethylene triamine (DETA), and triethylene tetramine (TETA) with increasing number of amine groups showing different vapor pressures. We confirmed that the vapor-phase doping provides not only very high carrier concentration but also good long-term stability in air, which is particularly important for practical applications.