Charged Impurity Scattering in Graphene

We have measured the impact of charged impurity scattering on the transport properties of graphene sheets [1]. We vary the density of adsorbed potassium atoms in our experiment up to $5 \times 10^{12} K/cm^2$ on the surface of graphene based-devices which are otherwise devoid of any surface adsorbates [2] in ultra high vacuum environment. Adsorbed potassium decreases the charge carrier mobility, renders the gate-dependent conductivity linear, shifts the minimum conductivity point in gate voltage, broadens the width of minimum conductivity region, and lowers the minimum conductivity. Our results are in qualitative agreement with a recent Boltzmann transport calculation [3]. New features, such as asymmetric response of electron-hole mobility and the observation of a ``residual'' conductivity (the extrapolation of the linear gate-voltage dependent conductivity to the minimum conductivity point) near 2 $e^2/h$, indicate transport properties beyond the simple Boltzmann picture. [1] J.H.Chen et al., http://xxx.lanl.gov/abs/0708.2408. [2] M.Ishigami et al., Nano Letters, 7, 1643 (2007). [3] S. Adam et al., PNAS 104, 18392 (2007).