Effect of the gate voltage scan rate on charge transport in graphene that is gated with an ionic liquid*

Charge transport in CVD graphene was investigated at room temperature using a field effect transistor platform with an ionic liquid (IL) as the gate material. Using an IL lowers the gate voltage needed for device operation due to its high specific capacitance. Impurity charges adsorbed on the graphene surface during growth and post processing act as dopants and influence charge transport. Applying a voltage to the IL (V_G) changed this impurity charge concentration and shifted the charge neutrality point (CP) in graphene. In this work we varied the gate voltage scan rates from 100 mV/s to 1 mV/s and examined its effect on the channel current (I). Lowering the scan rate led to the following observations in the I-V_G plots: (i) a non-linear shift in gate voltage at the CP voltage from a positive value toward V_G = 0 (i.e. n-doping), (ii) an increase in the electron and hole mobilities and (iii) a narrowing of the spread in the current near the CP. We believe that a slower scan rate helped create a more uniform electric double layer at the graphene/IL interface which could neutralize impurity charges. The result was a homogeneous redistribution of  un-neutralized impurity charges and a reduction in charge scattering of carrier charges. By using an IL that selectively absorbs gas species, our device can also be used as a gas sensor. The low applied voltages in combination with transistor operation make this device multifunctional and suitable in battery powered electronics.