Electronics

Chemistry

Materials science

Stacking

Optoelectronics

Composite material

Physics

Energy (signal processing)

Piezoelectricity

Nanotechnology

Organic chemistry

Energy harvesting

Quantum mechanics

Piezoelectric coefficient

Nanogenerator

Monolayer

Semiconductor

Condensed matter physics

Physical chemistry

ACS Nano

Piezoelectric materials have been widely used for sensors, actuators, electronics, and energy conversion. Two-dimensional (2D) ultrathin semiconductors, such as monolayer h-BN and MoS2 with their atom-level geometry, are currently emerging as new and attractive members of the piezoelectric family. However, their piezoelectric polarization is commonly limited to the in-plane direction of odd-number ultrathin layers, largely restricting their application in integrated nanoelectromechanical systems. Recently, theoretical calculations have predicted the existence of out-of-plane and in-plane piezoelectricity in monolayer α-In2Se3. Here, we experimentally report the coexistence of out-of-plane and in-plane piezoelectricity in monolayer to bulk α-In2Se3, attributed to their noncentrosymmetry originating from the hexagonal stacking. Specifically, the corresponding d33 piezoelectric coefficient of α-In2Se3 increases from 0.34 pm/V (monolayer) to 5.6 pm/V (bulk) without any odd-even effect. In addition, we also demonstrate a type of α-In2Se3-based flexible piezoelectric nanogenerator as an energy-harvesting cell and electronic skin. The out-of-plane and in-plane piezoelectricity in α-In2Se3 flakes offers an opportunity to enable both directional and nondirectional piezoelectric devices to be applicable for self-powered systems and adaptive and strain-tunable electronics/optoelectronics.

Multidirection Piezoelectricity in Mono- and Multilayered Hexagonal α-In<sub>2</sub>Se<sub>3</sub>

Multidirection Piezoelectricity in Mono- and Multilayered Hexagonal α-In2Se3

Multidirection Piezoelectricity in Mono- and Multilayered Hexagonal α-In₂Se₃