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

Abstract

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.

Electrochemically probing exciton transport in monolayers of two-dimensional semiconductors

“…Recently, our laboratory has been exploring how pipet-based electrochemical techniques, such as scanning electrochemical cell microscopy (SECCM), [26][27][28][29][30][31][32][33][34][35][36][37] can be employed as powerful tools for visualizing carrier transport in 2DSCs. In this approach, carrier generation-tip collection (CG-TC) SECCM, an electrolyte-lled pipet is utilized to detect photogenerated carriers as they drive reactions at a nm-scale electrochemical interface.…”

Directly visualizing carrier transport and recombination at individual defects within 2D semiconductors

“…SECCM has been successfully employed to study the catalytic and photoelectrochemical properties of a variety of materials, including 2DSCs. [56][57][58][59] In the studies presented here, we demonstrate a new "Carrier Generation-Tip Collection" (CG-TC) mode of SECCM designed to quantify carrier diffusion lengths within semiconducting materials and locally probe recombination at individual, well-defined defects. This method is applied to visualize carrier transport within mechanicallyexfoliated n-WSe 2 nanosheets, directly revealing the distance photogenerated holes travel within this material and the dramatic impact individual nanoscale defects can have on transport.…”

Locally Engineering and Interrogating the Photoelectrochemical Behavior of Defects in Transition Metal Dichalcogenides

“…58−65 Recent work from our group has applied SECCM to image photoelectrochemical processes within TMDs, providing direct insights into the role that different native defects play in carrier recombination, catalysis, and stability. 66,67 Here, we demonstrate that SECCM can also be used as a multifunctional tool for engineering defects within TMD materials and locally probing their photoelectrochemical behavior. Inspired by atomic force microscopy (AFM) patterning experiments, 68 SECCM probes are utilized to locally anodize exfoliated p-type WSe 2 nanosheets, generating arrays of controllable defects in a localized, deliberate manner within individual basal planes.…”