Redox-Active 2D Molecular-Transition Metal Dichalcogenides Bilayer Crystals

Understanding the interfacial behaviors is key to electrochemistry. Two dimensional (2D) materials, such as graphene and transition metal dichalcogenides (TMDs), are atomically flat surfaces where electrical and mass transport under electrochemical conditions could be well defined, <i>in situ</i> monitored, and precisely engineered. What’s more, 2D materials can be used as a synthetic template affording atomic precision for the growth of new 2D systems that are otherwise impossible.<br/><br/>In this work, we present the electrochemical studies on a unique hybrid 2D system, where large-scale, crystalline redox-active 2D molecules were grown on monolayer TMDs as a <i>2D molecular-TMD bilayer crystal</i>. This system features ultrathin geometry, atomic precision, and chemical cleanness, and can be used as an ideal model system for electrochemical investigation at 2D interface. As a hybrid 2D system, the intimate interaction between the 2D molecular crystal and the underlying 2D TMD represents a unique interfacial problem, which remains less understood.<br/><br/>Here, toward this goal, we developed an <i>in situ</i> electrochemical setup, combining optical microscopy/spectroscopy and electrical transport measurement, to study the electrochemical behaviors of atomically thin 2D hybrid crystals with spatial, temporal and spectral resolution. Specifically, we introduced redox control of 2D molecular crystals made of ordered monolayer perylene diimide (PDI) and PDI-derivatives on monolayer MoS₂ with electrolyte gating. The uniformity, crystallinity and monolayer thickness of the 2D molecular crystals were confirmed by polarization microscopy, scanning-tunneling microscopy and cross-section transmission electron microscopy. Because of the wafer-scale crystallinity and distinct polarized optical responses, the electrochemical properties (such as redox states) of the 2D molecular crystals were accessible via far-field microscopic observations and electrical transport measurements. As the doping level was well controlled electrochemically, we optically and electrically monitored the band filling behaviors of 2D molecular crystals. Coupled with conventional electrochemical testing methods, such as electrochemical impedance spectroscopy, our integrative setup and high-quality growth of 2D molecular crystals are highly versatile, which unlocks new potentials in the electrochemical applications at 2D interface, such as electrochemical transistors, organic batteries, etc.