Physical Adsorption and Charge Transfer of Molecular Br₂ on Graphene

Abstract

We present a detailed study of gaseous Br₂ adsorption and charge transfer on graphene, combining <i>in situ</i> 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 Br₂. 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 <i>versus</i> 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 (∼0.31 Torr^–1) and repulsive lateral interaction (∼20 meV) between adsorbed Br₂ molecules. The Br₂ molecule binding energy is ∼0.35 eV. We estimate that at monolayer coverage each Br₂ 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 Br₂ along the (somewhat rough) SiO₂/graphene interface. At high pressure, graphene supported on SiO₂ is doped by adsorbed Br₂ on both sides