Field-Effect Modulation of Electrochemical Reactions on Ultrathin Semiconductor Electrodes using High-k Dielectric Layer

Abstract

This thesis focuses on the modulation of heterogeneous charge transfer on ultrathin semiconducting electrodes for enhanced electrochemical reaction kinetics. Instead of changing the chemical composition of a semiconductor, a physical approach is adopted to modulate the solution electrochemistry on ultrathin semiconductor working electrodes by altering the electronic occupation of the electrode. A high-k local gate field-effect transistor (FET) structure is used in this study to realize large modulation ability under low applied gate voltage (V_G). Specifically, the structure in this thesis is fabricated with Pd as the local gate, HfO2 as the dielectric, and an ultrathin ZnO semiconductor (~ 5 nm) as the working electrode. By back-gating this metal/insulator/semiconductor (MIS) FET, charge carriers electrostatically induced by the back-gate lead to band edge shift on the ultrathin semiconductor within the space charge region, thereby manipulating charge transfer kinetics at the electrode/electrolyte interface. The use of HfO2 with a high dielectric constant boosted the injected charge density in the semiconductor, and realized low operation V_G (< 10 V). Cyclic voltammograms (CV) are used to probe reactivity modulation of several outer-sphere redox couples in ionic liquids, demonstrating a continuous and reversible control of electrochemical reactions. Furthermore, steady-state voltammetry was carried out on outer-sphere redox species at back-gated ultrathin ZnO working electrodes to determine electron transfer rate constants (k_ET) as a function of V_G. The Pd/HfO2/ZnO stack's enhanced gating power allowed observation of a non-monotonic dependence of k_ET on V_G, revealing the inherent density of redox acceptor states in solution. This work highlights the independent tuning of electrochemical kinetics at ultrathin working electrodes byV_G, irrespective of the conventional working electrode potential. Additionally, the shift of working electrode potential by back-gating is studied by probing the working electrode potential (V_ZnO) with respect to fixed reference. By this approach, band edge shift on the ultrathin ZnO electrode (∆δ) by back-gating can be determined. This also allows the estimation of quantum capacitance (C_Q), representing the density of states distribution, and separation of electric double layer charging potential (∆ϕ_EDL) at the ZnO/electrolyte interface from the band edge shift (∆δ). In conclusion, the field-effect modulation approach in this thesis essentially constitutes a unique four-electrode electrochemical system, which offers a valuable platform to investigate electronic properties and charge transport on semiconductor electrodes.