Effect of transition-metal chlorides on graphene properties

Abstract

The doping mechanisms of transition-metal chlorides such as NiCl2, CuCl2, CoCl2, ZnCl2, TiCl3, and InCl3 in graphene were investigated. The sheet resistance of graphene decreased from 780 to 400–700 Ω sq.−1 on doping; further, its transmittance at 550 nm also decreased from 97% to 90–96% owing to reduction of metal cations. Both the G and 2D peak in Raman spectra were shifted to higher wavenumbers after metal-chloride doping, which indicates a charge transfer from graphene to metal cations. The peak position of the CC bond in synchrotron radiation photoemission spectroscopy shifted to lower binding energy, suggesting that the Fermi level of doped graphene was lowered by spontaneous electron transfer between graphene and metal cations. Secondary cut-off spectra showed that the work function of doped graphene increased from 4.32 eV to 4.88, 4.85, 4.82, 4.81, 4.8, and 4.64 eV upon doping with 20 mM of NiCl2, CuCl2, CoCl2, ZnCl2, TiCl3, and InCl3, respectively. The tendency of an increasing work function is strongly related to the work function value of the bulk metallic state. It is considered that spontaneous electron transfer and strong carbon–chlorine bond formation induce a specific energy-level engineering in doped graphene sheets, thereby increasing its work function and conductivity.