Field Effect Modulation of Heterogeneous Charge Transfer Kinetics at Back-Gated Two-Dimensional MoS₂ Electrodes

The ability to improve and to modulate the heterogeneous charge transfer kinetics of two-dimensional (2D) semiconductors, such as MoS₂, is a major challenge for electrochemical and photoelectrochemical applications of these materials. Here we report a continuous and reversible physical method for modulating the heterogeneous charge transfer kinetics at a monolayer MoS₂ working electrode supported on a SiO₂/p-Si substrate. The heavily doped p-Si substrate serves as a back gate electrode; application of a gate voltage (V_BG) to p-Si tunes the electron occupation in the MoS₂ conduction band and shifts the conduction band edge position relative to redox species dissolved in electrolyte in contact with the front side of the MoS₂. The gate modulation of both charge density and energy band alignment impacts charge transfer kinetics as measured by cyclic voltammetry (CV). Specifically, cyclic voltammograms combined with numerical simulations suggest that the standard heterogeneous charge transfer rate constant (k⁰) for MoS₂ in contact with the ferrocene/ferrocenium (Fc^0/+) redox couple can be modulated by over 2 orders of magnitude from 4 × 10^-6 to 1 × 10^-3 cm/s, by varying V_BG. In general, the field effect offers the potential to tune the electrochemical properties of 2D semiconductors, opening up new possibilities for fundamental studies of the relationship between charge transfer kinetics and independently controlled electronic band alignment and band occupation.