Four-Terminal Electrochemistry: A Back-Gate Controls the Electrochemical Potential of a 2D Working Electrode

We demonstrate that ultrathin semiconductor working electrodes integrated into metal-insulator-semiconductor (MIS) stacks are an enabling platform for understanding non-Faradaic semiconductor electrochemistry. Here, 5 nm thick ZnO electrodes were deposited on 30 nm HfO ₂ dielectric on a Pd “gate” electrode. Application of a bias V _G between the Pd gate and the ZnO electrode causes electrons to accumulate in the ZnO layer as measured by recording the in-plane sheet conductance. By contacting the top surface of the ZnO layer with the electrolyte in a conventional three-electrode electrochemical cell, we show that the gate voltage V _G modulates the electrochemical potential V _ZnO of the ZnO film with respect to a reference electrode. Electrochemical potential changes ΔV _ZnO up to −1 V vs Ag/Ag ⁺ are achieved for V _G = +7 V. Furthermore, by measuring V _ZnO vs V _G, we extract the quantum capacitance C _Q of the ZnO film as a function of the Fermi-level position, which provides a direct measure of the ZnO electronic density of states (DOS). Finally, we demonstrate that the gated ZnO working electrodes can disentangle the two principal components of electrochemical potential, namely, the Fermi-level shift Δδ and the double-layer charging energy eΔϕ _EDL. This disentanglement hinges on a fundamental difference between back-gating and normal electrochemical control, namely, that electrochemical control requires double-layer charging, while back-gate control does not. Collectively, the results show that the backside gate electrode is an effective fourth terminal that enables measurements that are difficult to achieve in conventional three-terminal electrochemical setups.