Tattoo electronics has attracted intensive interest in recent years due to its comfortable wearing and imperceivable sensing, and has been broadly applied in wearable healthcare and human–machine interface. However, the tattoo electrodes are mostly composed of metal films and conductive polymers. Two-dimensional (2D) materials, which are superior in conductivity and stability, are barely studied for electronic tattoos. Herein, we reported a novel electronic tattoo based on large-area Mo₂C film grown by chemical vapor deposition (CVD), and applied it to accurately and imperceivably acquire on-body electrophysiological signals and interface with robotics. High-quality Mo₂C film was obtained via optimizing the distribution of gas flow during CVD growth. According to the finite element simulation (FES), bottom surface of Cu foil covers more stable gas flow than the top surface, thus leading to more uniform Mo₂C film. The resulting Mo₂C film was transferred onto tattoo paper, showing a total thickness of ~ 3 μm, sheet resistance of 60–150 Ω/sq, and skin-electrode impedance of ~ 5 × 10⁵ Ω. Such thin Mo₂C electronic tattoo (MCET in short) can form conformal contact with skin and accurately record electrophysiological signals, including electromyography (EMG), electrocardiogram (ECG), and electrooculogram (EOG). These body signals collected by MCET can not only reflect the health status but also be transformed to control the robotics for human–machine interface.

As a direct-bandgap semiconductor, single-layer MoS₂ has gained great attention in optoelectronics, especially wearable photodetectors. However, MoS₂ exhibits poor photoresponsivity on a stretchable substrate due to intrinsic low carrier density and a large number of scattering centers on polymer substrates. Few air-stable yet strong dopants on MoS₂ has been reported. In addition, the roughness, hydrophobicity and susceptibility to organic solvents of polymer surface are critical roadblocks in the development of stretchable high-performance MoS₂ photodetectors. Here, we realize a stretchable and stable photodetector with high photoresponsivity by combining n-type dopant ((4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl) phenyl) dimethylamine, N-DMBI) with MoS₂ and assembly transfer technique. It is found electron tends to transfer from N-DMBI to MoS₂ and the effect is maintained after the integrable photodetector transferred directly by elastic substrate styrene-ethylene-butylene-styrene (SEBS), even after being exposed to the air for 20 days, which benifits greatly from the encapsulation of SEBS. The increased carrier density greatly promotes carrier injection efficiency and photogenerated electron–hole separation efficiency at the metal–semiconductor interface, thus offering a significantly improved photoresponsivity in MoS₂ photodetectors. Moreover, such photodetector shows great durability to stretch, which can remain functional after stretched 100 cycles within its stretch limit. Our strategy opens a new avenue to fabricate high-photoresponsivity stretchable electronics or optoelectronics of two-dimensional (2D) materials.

The variation of interlayer coupling can greatly affect the bandstructure of few layered transition metal dichalcogenides (TMDs), for instance, transition of indirect-to-direct bandgap and vice versa, which is correlated with the charge carrier and optical density. However, methods that can modulate the coupling strength in a controllable way are still lacking. Here, we report a fluidic dynamic strategy to tune the interlayer coupling of folded bi-layer MoS₂. By controlling the flow direction and particle size of the fluid, mono-layer MoS₂ can be folded into bi-layer with a controlled folding direction for designated twist angles as well as tunable interlayer coupling. Compared with normally folded bi-layer MoS₂, the photoluminescence (PL) peak of the direct-bandgap transition for folded bi-layer MoS₂ by fluid flow is weakened accompanied with the re-appearance of indirect-bandgap transition peak. Besides, the fluid flow creates a clear trajectory on the folded MoS₂, exhibiting various degrees of interlayer coupling along it. Field-effect transistors (FETs) were further fabricated on tunably coupled folded-bi-layers, proving that the bandstructure and electrical property is strongly correlated with the degree of interlayer coupling. This fluidic dynamic strategy can be extended to other TMDs on any substrate, and together with its excellent capability in controlled interlayer coupling, it will provide a new way for the development of TMDs optoelectronics.