Optimizing Electrical Properties of Graphene Film by Chemical Doping Methods

Abstract

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 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. For studying doping effect of graphene, we used APPE (aminophenyl propargyl ether) as a novel n-type dopant for graphene. From the result of Raman spectroscopy and electrical measurement, we suppose that the electron donating property of amine group (-NH2) is the origin of such intense n-doping effect. Also we studied a simple method to tune the electrical properties of CVD graphene through n-doping by vaporized molecules at 70 oC, where the dopants in vapor phase are mildly adsorbed on graphene surface. 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. Moreover, we studied strong modulaton method for enhancement of the electrical conductivity of graphene by dual-side molecular n-doping with diethylenetriamines (DETAs) on top and amine-functionalized self-assembled monolayers (SAMs) at bottom. We constructed a self-assembled monolayers (SAMs) for modification of SiO2 substrate resulting in enhancement of mobility and conductivity of graphene. Graphene p-n junction was constructed by e-beam lithographic technique with NH2-SAMs and confirmed n-type and p-type simultaneously. Our approach would be a promising for modifying graphene surface at a desired region of electronic devices due to easy to make p-n junction. SAMs with different functional group show temperature dependence in electrical properties. For atual application of graphene, we studied the mass-productive graphene films synthesized by hydrogen-free rapid thermal chemical vapor deposition (RT-CVD), roll-to-roll etching, and transfer methods, which enabled faster and larger production of homogeneous graphene films by installing the RT-CVD films to resistive multi-touch devices. Lastly, we studied a giant orbital diamagnetism in graphene films grown by chemical vapor deposition (CVD), where the diamagnetic susceptibility in perpendicular direction is measured to be ~100 times greater than the strongest diamagnetic materials such as bismuth.