Self-Powered 2D Material-Based pH Sensor and Photodetector Driven by Monolayer MoSe2 Piezoelectric Nanogenerator.

The large piezoelectricity of monolayer MoSe2, which is predicted to be stronger than that of all of the other group VIB transition-metal dichalcogenides (including MoS2), has only been theoretically investigated. Here, we report experimental evidence of in-plane piezoelectricity in MoSe2. Monolayer single-crystalline MoSe2 flake derived from chemical vapor deposition demonstrates a peak output voltage of 60 mV at 0.6% strain, which is ∼50% larger than that of MoS2. Piezoelectric signal along the armchair orientation of MoSe2 is ∼6 times larger than that along the zigzag orientation, indicative of strong anisotropic piezoelectricity. Piezoelectric nanogenerator based on a single MoSe2 flake illustrates remarkable electromechanical conversion ability, and thus is able to noninvasively monitor vital health signs, such as respiratory rate and heart rate. Despite the extremely small size, MoSe2 nanogenerator is able to drive pH sensor based on MoS2 and photodetector based on MoS2/WSe2 heterojunction due to the outstanding piezoelectricity of MoSe2 and the ultralow power consumption of two-dimensional (2D) material sensors. The self-powered, solely 2D-material-based sensor units demonstrate superb sensing performance. Therefore, the discovery of piezoelectricity in monolayer MoSe2 provides a route for achieving self-powered atomic-scale electromechanical systems that could stimulate further fundamental research and potential applications.