Solvent-Mediated Chemical Hole Doping of Graphene by Iodine

We use micro-Raman spectroscopy to investigate how solvation affects chemical hole doping of single-layer graphene by iodine. Measuring the graphene hole density as a function of iodine concentration in cyclohexane, benzene, and water generates solvent-dependent isotherms. Fitting these data to Langmuir isotherms provides equilibrium constants of adsorption and maximum hole densities. Raman spectra of bilayer graphene in water at intermediate iodine concentrations reveal a split in the graphene G peak, indicating asymmetric doping. This result shows that discrepancies between the Langmuir fits and the data are explained by different adsorption thermodynamics on the top and bottom graphene surfaces. The equilibrium constant is largest in water and equal for benzene and cyclohexane. In contrast, the maximum hole density decreases from water to cyclohexane to benzene. Equilibrium constants in all solvents and the maximum hole density in water are explained by solvent polarity. More hole doping occurs in cyclohexane than in benzene, and density functional theory calculations suggest a mechanism where this result is caused by solvent modulation of the energies and shapes of donor and acceptor orbitals, rather than by competitive charge transfer with the solvent.