Assembling a Photoactive 2D Puzzle: From Bulk Powder to Large- Area Films of Semiconducting Transition-Metal Dichalcogenide Nanosheets

CONSPECTUS: Two-dimensional (2D) semiconducting materials are poised to revolutionize ultrathin, high-performance optoelectronic devices. In particular, transition-metal dichalcogenides (TMDs) are well-suited for applications requiring robust and stable materials such as electrocatalytic, photocatalytic, and photo-electrochemical devices. One of the most compelling assets of these materials is the ability to produce and process 2D TMDs in the nanosheet form using solution-based (SB) exfoliation methods. Compared to other methods, SB techniques are typically inexpensive, efficient, and more suitable for scale-up and industrial implementation. In acknowledgment of the importance of this area, much work has been done to develop various SB methods starting from the exfoliation of bulk crystalline TMD materials to the chemical modification of final devices consisting of thin films of semiconducting 2D TMD nanosheets. However, not all SB methods are equally compatible or interchangeable, and they result in very diverse material and device properties. Therefore, the aim of this Account is to provide an overview of the developed SB techniques that can serve as a guide for assembling high-performance thin films of 2D TMDs. We start by introducing the most popular methods for producing 2D TMDs using liquid-phase exfoliation (LPE), discussing their working mechanisms as well as their advantages and disadvantages. Notably we highlight a recently developed LPE technique using electro-intercalation that draws on the advantages of previously presented methods. Next, we discuss processing the as-produced 2D TMD nanosheets via SB separating techniques designed for size and morphology selection while also presenting the ongoing challenges in this area. We then examine SB methods for processing the selected 2D nanomaterial dispersions into semiconducting thin films. Various methods are compared and contrasted, and special attention is paid to a recently developed method that carefully deposits 2D TMD nanoflakes with preferential alignment and has been shown scalable to the meter-squared size range. Finally, we explore strategies for increasing the optoelectronic performance of the TMD films via device engineering and defect management. We scrutinize these post treatments based on the final device application, which are explicitly discussed. In all of the discussed processes we present the most promising SB techniques giving critical analysis and insight from experience. While we provide our own "best practices", we stress the use of adaptability and critical thinking when designing specifically tailored procedures. By providing examples of different uses and measured improvements in one comprehensive guide, we hope to simplify process-development and aid researchers in making their own unique photoactive 2D "puzzles".