Work-function decrease of graphene sheet using alkali metal carbonates

A chemical approach was applied to decrease the work function of few-layer graphene. Li₂CO₃, K₂CO₃, Rb ₂CO₃, and Cs₂CO₃ were used as n-doping materials. The sheet resistance of graphene doped with carbonate salt slightly increased from 1100 to 1700-2500 Ω/sq, and the transmittance of doped graphene with 0.1 M alkali metal at 550 nm decreased from 96.7 to 96.1-94% due to the formation of metal particles on the surface of graphene. A higher sheet resistance and lower transmittance were obtained at a higher concentration of alkali metal carbonate. The G peak in the Raman spectra was shifted to a lower wavenumber after alkali metal carbonate doping and the intensity ratio of the carbon double bond to the carbon single bond decreased with doping in the X-ray photoemission spectroscopy spectra, suggesting the charge transfer from metal ions to graphene sheets. Ultraviolet photoemission spectroscopy data showed that the work function of the graphene sheets decreased from 4.25 eV to 3.8, 3.7, 3.5, and 3.4 eV for graphene doped with Li₂CO₃, K₂CO₃, Rb₂CO₃, and Cs ₂CO₃, respectively. This suggested that spontaneous chemical combination occurred between the carbon atoms and alkali metal, thereby decreasing the work function.