Adsorption of Ammonia on Graphene*

We report on experimental studies of NH$_{3}$ adsorption/desorption kinetics on graphene surfaces. The study employs bottom-gated graphene field effect transistors (FETs) supported on Si/SiO$_{2}$ substrates. Detection of NH$_{3}$ occurs through the shift of the source-drain resistance maximum (``Dirac peak'') with gate voltage. The observed shift of the Dirac peak toward negative gate voltages in response to NH$_{3}$ exposure is attributed to the charge transfer from adsorbed NH$_{3}$, with the amount of charge estimated to be $\sim $ 0.06 electrons per molecule. The desorption kinetics of our FET devices is well described by the sum of two exponential terms corresponding to a fast and a much slower process, whose time constants differ by a factor of $\sim $ 9. The two-time constant desorption kinetics is consistent with Fickian-type diffusion of NH$_{3}$ from the interstitial pockets formed at the interface between the graphene and the supporting SiO$_{2}$ gate dielectric.