▎ 摘　　要

The effect of atmospheric adsorbates, composed of dry oxygen (O-2) and ambient air, on electrical transport in high-quality and moderate-quality graphene samples was investigated. Atmospheric doping is seen to affect different samples at different rates but in all samples hole-doping is observed within several minutes of O-2 or ambient exposure. After an initial rapid increase, the doping rate falls off dramatically. It is shown that carrier mobility (mu) can be either improved or degraded depending on the nature of the impurities affecting transport. The former case represents the first experimental demonstration of an improvement in carrier mobility with increased doping in graphene. A model is derived to understand the interplay between charge screening and impurity scattering. The study points to the possibility that oxygen and water molecules mitigate carrier scattering from negatively-charged impurities but cause increased carrier scattering from positivelycharged impurities. (C) 2011 Elsevier Ltd. All rights reserved.