Determination of Carrier Type Doped from Metal Contacts to Graphene by   Channel-Length-Dependent Shift of Charge Neutrality Points
  A method for determining the type of charge carrier, electron or hole, which is transferred from metal contacts to graphene, is described. The Dirac point is found to shift toward more negative (positive) gate voltages for electron (hole) doping by shortening of the interelectrode spacing. The shift of the Dirac point is accompanied by an enhancement of the electron-hole conductivity asymmetry. Experimentally determined carrier types may be explained in terms of the metal work functions modified by interactions with graphene. 
Ryo Nouchi, Tatsuya Saito, and Katsumi Tanigaki
cond-mat.mtrl-sci