Aqueous-isopropanol-based green ink formulation of single-crystalline transition metal chalcogenides for fully-printed strain-insensitive flexible sensing electronics

The inkjet printing of two-dimensional transition metal chalcogenides (TMDs) holds great promise for the next-generation printed flexible electronics in the context of low-cost and large-scale manufacturing. Notwithstanding the noteworthy advancements witnessed in the formulation of inks and the development of devices, the process of printing high-performance flexible TMD-based electronics remains challenging. This is primarily attributable to the unsatisfactory crystallinity of TMDs and the utilisation of toxic solvents. In this study, we demonstrate the manufacturing process, performance, and applications of fully-printed flexible and multifunctional sensors. We propose the use of a zwitterion cocamidopropyl betaine (CAB) as a dispersant and surfactant to facilitate the liquid-phase exfoliation and uniform dispersion of single-crystalline TMD crystals in water and isopropanol (IPA) for green ink formulation. The absence of additives and binders in the dispersions is a key finding, as it enables the direct production of various TMD inks (i.e., MoS₂, MoTe₂, WS₂, WSe₂, and WTe₂) with high stability (more than one month) and concentrations (2 mg/mL). The “green” ink formulation allows for the uniform deposition of TMD single crystals on flexible substrates, paving the way for strain-insensitive wearable sensing electronics. The study demonstrates that a fully-printed MoSe₂/CAB humidity sensor exhibits superior sensitivity (ΔI/I₀ = 468.1) and rapid response/recovery times (27 s/0.42 s) under bending deformations. Furthermore, inkjet-printed WTe₂/CAB pads on ultrathin substrates (approximately 6 µm) demonstrate exceptional mechanical stability in acquiring high-quality biopotential signals, including electrocardiograph and electromyography. This proposed strategy facilitates the scalable fabrication of TMD-based flexible electronics, thereby significantly advancing their integration into industrial manufacturing processes. (Figure presented.)