Electrochemically Probing Exciton Transport in Monolayers of Two-Dimensional Semiconductors

Description:
Two-dimensional semiconductors (2DSCs) such as the transition metal dichalcogenides (TMDs) are attractive for a variety of optoelectronic and catalytic applications due to their unique optical and electronic properties which arise in the 2D limit. TMDs are of particular interest as absorbing layers in ultrathin photovoltaic devices, and while these materials are known to exhibit excellent photovoltaic properties at the bulk level, it is still unclear how different types of defects influence optoelectronic processes at thicknesses approaching the monolayer limit. Here, it is demonstrated that Carrier Generation-Tip Collection Scanning Electrochemical Cell Microscopy (CG-TC SECCM), which utilizes spatially-offset optical and pipet-based electrochemical probes to generate carriers and detect their concentration, can be employed to visualize excitonic transport behavior at the single defect level within individual monolayers. n-WSe2 samples were prepared at bulk (>10 nm), bilayer, and monolayer thicknesses via mechanical exfoliation or chemical vapor deposition (CVD) and CG-TC SECCM studies were carried out to map carrier transport within these samples using the I3-/I- couple as a probe reaction. Data from these experiments directly reveal how carrier diffusion lengths and recombination processes vary as layer thicknesses approach the monolayer limit. SECCM studies are compared to photoluminescence lifetime imaging studies of the same structures, allowing meaningful contrasts to be drawn between electrochemical imaging and more conventional optical methods. These studies provide novel insights into mechanisms that govern carrier transport in monolayer TMD materials and demonstrate electrochemical imaging to be a powerful technique for visualizing carrier transport behavior in 2DSCs.

Speaker: Chloe Tolbert - University of Wyoming