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.