Electric Double-Layer Gating of Two-Dimensional Field-Effect Transistors Using a Single-Ion Conductor

Electric double-layer (EDL) gating using a custom-synthesized polyester single-ion conductor (PE400-Li) is demonstrated on two-dimensional (2D) crystals for the first time. The electronic properties of graphene and MoTe2 field-effect transistors (FETs) gated with the single-ion conductor are directly compared to a poly(ethylene oxide) dual-ion conductor (PEO:CsClO4). The anions in the single-ion conductor are covalently bound to the backbone of the polymer, leaving only the cations free to form an EDL at the negative electrode and a corresponding cationic depletion layer at the positive electrode. Because the cations are mobile in both the single- and dual-ion conductors, a similar enhancement of the n-branch is observed in both graphene and MoTe2. Specifically, the single-ion conductor decreases the subthreshold swing in the n-branch of the bare MoTe2 FET from 5000 to 250 mV/dec and increases the current density and on/off ratio by two orders of magnitude. However, the single-ion conductor suppressed the p-branch in both the graphene and the MoTe2 FETs, and finite element modeling of ion transport shows that this result is unique to single-ion conductor gating in combination with an asymmetric gate/channel geometry. Both the experiments and modeling suggest that single-ion conductor-gated FETs can achieve sheet densities up to 1014 cm-2, which corresponds to a charge density that would theoretically be sufficient to induce several percent strain in monolayer 2D crystals and potentially induce a semiconductor-to-metal phase transition in MoTe2.