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 LiClO 4, 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 10 12 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, 10 13 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 10 6 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.