Monolayer Solid-State Electrolyte for Electric Double Layer Gating of Graphene Field-Effect Transistors

The electrostatic gating of graphene field-effect transistors is demonstrated using a monolayer electrolyte. The electrolyte, cobalt crown ether phthalocyanine (CoCrPc) and LiClO₄, is deposited as a monolayer on the graphene channel, essentially creating an additional two-dimensional layer on top of graphene. The crown ethers on the CoCrPc solvate lithium ions and the ion location is modulated by a backgate without requiring liquid solvent. Ions dope the channel by inducing image charges; the doping level (i.e., induced charge density) can be modulated by the backgate bias with the extent of the surface potential change being controlled by the magnitude and polarity of the backgate bias. With a crown ether to Li⁺ ratio of 5:1, programming tests for which the backgate is held at -V_BG shift the Dirac point by ∼15 V, corresponding to a sheet carrier density on the order of 10¹² cm^-2. This charge carrier density agrees with the packing density of monolayer CoCrPc on graphene that would be expected with one Li⁺ for every five crown ethers (at the maximum possible Li⁺ concentration, 10¹³ cm^-2 is predicted). The crown ethers provide two stable states for the Li⁺: one near the graphene channel (low-resistance state) and one ∼5 Å away from the channel (high-resistance state). Initial state retention measurements indicate that the two states can be maintained for at least 30 min (maximum time monitored), which is 10⁶ times longer than polymer-based electrolytes at room temperature, with at least a 250 Ω μm difference between the channel resistance in the high- and low-resistance states.