Investigating the Redox Properties of Two-Dimensional MoS2 Using Photoluminescence Spectroelectrochemistry and Scanning Electrochemical Cell Microscopy.

Control over photophysical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) is the key to advance their applications in next-generation optoelectronics. Although chemical doping and surface modification with plasmonic metals have been reported to tune the photophysical and catalytic properties of 2D TMDs, there have been few reports of tuning optical properties using dynamic electrochemical control of electrode potential. Herein, we report (1) the photoluminescence (PL) enhancement and red-shift in the PL spectrum of 2D MoS2, synthesized by chemical vapor deposition and subsequent transfer onto an indium tin oxide electrode, upon electrochemical anodization and (2) spatial heterogeneities in its photoelectrochemical (PEC) activities. Spectroelectrochemistry shows that positive electrochemical bias causes an initial ten-fold increase in the PL intensity followed by a quick decrease in the enhancement. The PL enhancement and spectrum red-shift are associated with the decrease in nonradiative decay rates of excitons formed upon electrochemical anodization of 2D MoS2. Additionally, scanning electrochemical cell microscopy (SECCM) study shows that the 2D MoS2 crystal is spatially sensitive to PEC oxidation at positive potentials. SECCM also shows a photocurrent increase caused by spatially heterogeneous edge-type defect sites of the crystal.