Fig. 1. Schematic illustrations of the (a) epitaxial structure, (b) flexible sensor after detaching the original wafer, and (c) potential application on the hand. (d) A side-view AC-STEM image of (In,Ga)N/GaN MQWs, and (e) a magnified image of the MQWs. AC-STEM images of (f) GaN and (g) (In,Ga)N crystals.

Fig. 2. (a) Optical switching characteristics of monolithic devices illuminated at 365 nm across various incident power densities. (b) Relationship among optical current density, responsivity, and incident optical power density. (c) Optical response profile of the sensor when illuminated under zero-bias conditions.

Fig. 3. (a) A schematic representation of the bending model for the flexible sensor. (b), (c) Optical images of the flexible sensor under different bending angles. (d) The flexible sensor exhibits photoswitching behavior when illuminated with 365 nm light, demonstrating variations in response to different incident light power intensities. (e) Optical switching response of the sensor under different bending degrees. (f) Optical switching response of the sensor after multiple bending times.

Fig. 4. Schematic illustrations of a flexible sensor (a) in the flat state and (b) in the bending condition. (c) Schematic illustration of a flexible sensor in the bending condition contacting with liquid. (d)–(f) Energy band diagrams of sensors corresponding to those conditions in (a)–(c), respectively.

Fig. 5. Photoswitching behavior of the sensor for detecting (a) various concentrations of sodium chloride (NaCl) in artificial sweat and (b) different sweat volumes with the same NaCl concentration (∼80 mmol). A schematic image is employed to showcase the feasible applications of the wearable sensor.