Physical Adsorption and Charge Transfer of Molecular Br2 on Graphene

We present a detailed study of gaseous Br-2 adsorption and charge transfer on graphene, combining in situ Raman spectroscopy and density functional theory (DFT). When graphene is encapsulated by hexagonal boron nitride (h-BN) layers on both sides, in a h-BN/graphene/h-BN sandwich structure, it is protected from doping by strongly oxidizing Br-2. Graphene supported on only one side by h-BN shows strong hole doping by adsorbed Br2. Using Raman spectroscopy, we determine the graphene charge density as a function of pressure. DFT calculations reveal the variation in charge transfer per adsorbed molecule as a function of coverage. The molecular adsorption isotherm (coverage versus pressure) is obtained by combining Raman spectra with DFT calculations. The Fowler-Guggenheim isotherm fits better than the Langmuir isotherm. The fitting yields the adsorption equilibrium constant (similar to 0.31 Torr(-1)) and repulsive lateral interaction (similar to 20 meV) between adsorbed Br-2 molecules. The Br-2 molecule binding energy is similar to 0.35 eV. We estimate that at monolayer coverage each Br-2 molecule accepts 0.09 e(-) from single-layer graphene. If graphene is supported on SiO2 instead of h-BN, a threshold pressure is observed for diffusion of Br-2 along the (somewhat rough) SiO2/graphene interface. At high pressure, graphene supported on SiO2 is doped by adsorbed Br-2 on both sides.