Piezoelectricity is the electric charge which accumulates in certain materials in response to mechanical stimuli, while piezoelectric nanogenerators (PENGs) converting mechanical energy into electricity can be widely used for energy harvesting and self-powered systems. The group IV-VI monochalcogenides may exhibit strong piezoelectricity because of their puckered C_2v symmetry and electronic structure, making them promising for flexible PENG. Herein, we investigated the synthesis and piezoelectric properties of multilayer SnSe nanosheets grown by chemical vapor deposition (CVD). The SnSe nanosheets exhibited high single-crystallinity, large area, and good stability. The strong layer-dependent in-plane piezoelectric coefficient of SnSe nanosheets showed a saturated trend to be ~ 110 pm/V, which overcomes the weak piezoelectric response or odd-even effects in other layered nanosheets. A high energy conversion efficiency of 9.3% and a maximum power density of 538 mW/cm² at 1.03% strain have been demonstrated in a SnSe-based PENG. Based on the enhanced piezoelectricity of SnSe and attractive output performance of the nanogenerator, a self-powered sensor for human motion monitoring is further developed. These results demonstrate the strong piezoelectricity in high quality CVD-grown SnSe nanosheets, allowing for application in flexible smart piezoelectric sensors and advanced microelectromechanical devices.

Two-dimensional (2D) ferroelectric materials with unique structure and extraordinary optoelectrical properties have attracted intensive research in the field of nanoelectronic and optoelectronic devices, such as optical sensors, transistors, photovoltaics and non-volatile memory devices. However, the transition temperature of the reported ferroelectrics in 2D limit is generally low or slightly above room temperature, hampering their applications in high-temperature electronic devices. Here, we report the robust high-temperature ferroelectricity in 2D α-In₂Se₃, grown by chemical vapor deposition (CVD), exhibiting an out-of-plane spontaneous polarization reaching above 200 °C. The polarization switching and ferroelectric domains are observed in In₂Se₃ nanoflakes in a wide temperature range. The coercive field of the CVD grown ferroelectric layers illustrates a room-temperature thickness dependency and increases drastically when the film thickness decreases; whereas there is no large variance in the coercive field at different temperature from the samples with identical thickness. The results show the stable ferroelectricity of In₂Se₃ nanoflakes maintained at high temperature and open up the opportunities of 2D materials for novel applications in high-temperature nanoelectronic devices.