▎ 摘　　要

Tunable electrical transport properties of graphene field-effect transistors (GFETs) are achieved by spin-coating a poly(ethylene imine) (PEI) layer on graphene surface. The initially p-doped graphene recovers the ambipolar characteristics with the PEI overlayer. When increasing the PEI concentration in a methanol solvent, a systematic evolution of the transport properties of GFETs is observed. The carrier mobility of graphene is greatly improved by several tens of times and the hole/electron conductivity saturation is shown. The voltage of the neutrality point V-Dirac gets closer to 0 V and the plateau width around the neutrality point Delta V-Dirac becomes much smaller. It is proposed that the long-range Coulomb scattering in graphene is suppressed due to the screening effect of PEI and the performances of GFETs are consequently improved. The hysteretic behaviors of the transfer characteristic curves of GFETs are also influenced by the PEI coating. The gradual reversion of the hysteresis direction is observed when increasing the concentration of PEI, which is probably due to the two competing mechanisms between the charge trapping at the graphene/SiO2 interface and the capacitive coupling of graphene and the PEI overlayer. (C) 2014 Elsevier Ltd. All rights reserved.