Field Effect Modulation of Heterogeneous Charge Transfer Kinetics at Back-Gated Two-Dimensional MoS 2 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 (VBG ) 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 (Fc0/+ ) redox couple can be modulated by over 2 orders of magnitude from 4 × 10-6 to 1 × 10-3 cm/s, by varying VBG . 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.