Aqueous-Isopropanol-Based Green Ink Formulation of Single-Crystalline Transition Metal Chalcogenides for Fully-Printed Strain-Insensitive Flexible Sensing Electronics
Inkjet printing of two-dimensional transition metal chalcogenides (TMDs) is promising for low-cost, large-scale flexible electronics, yet challenges persist due to poor crystallinity and toxic solvents. Here, we report a green ink formulation using zwitterionic cocamidopropyl betaine (CAB) as a dispersant and surfactant for liquid-phase exfoliation of single-crystalline TMDs in water and isopropanol (IPA). The dispersions contain no additives or binders, enabling direct production of stable (over one month) and concentrated (2 mg/mL) inks for MoS2, MoTe2, WS2, WSe2, and WTe2. Fully-printed MoSe2/CAB humidity sensors exhibit superior sensitivity (ΔI/I0 = 468.1) and rapid response/recovery times (27 s/0.42 s) under bending. Inkjet-printed WTe2/CAB pads on 6-μm-thick substrates demonstrate exceptional mechanical stability, with resistance variations of 1.4% under single bending and 2% after 1,000 cycles, and acquire high-quality electrocardiogram (ECG) and electromyography (EMG) signals. This strategy enables scalable fabrication of TMD-based flexible electronics, advancing industrial integration.