Multidirection Piezoelectricity in Mono- and Multilayered Hexagonal α-In₂Se₃

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Piezoelectric materials are crucial for sensors, actuators, and energy conversion.
• Two-dimensional (2D) semiconductors like h-BN and MoS2 show piezoelectric properties, but often limited to in-plane polarization in ultrathin layers.
• Existing 2D materials have limitations in integrated nanoelectromechanical systems due to restricted polarization directions.

Purpose of the Study:

• To experimentally investigate the piezoelectric properties of alpha-indium selenide (α-In2Se3), particularly its out-of-plane and in-plane piezoelectricity.
• To explore the potential of α-In2Se3 in novel applications like energy harvesting and electronic skins.
• To understand the relationship between the noncentrosymmetric hexagonal stacking and the observed piezoelectric effects.

Main Methods:

• Experimental characterization of monolayer to bulk α-In2Se3.
• Measurement of piezoelectric coefficients (d33) across different thicknesses.
• Fabrication and testing of α-In2Se3-based flexible piezoelectric nanogenerators.

Main Results:

• Confirmed coexistence of out-of-plane and in-plane piezoelectricity in α-In2Se3.
• Observed a significant increase in the d33 piezoelectric coefficient from 0.34 pm/V (monolayer) to 5.6 pm/V (bulk) without an odd-even effect.
• Successfully demonstrated a flexible piezoelectric nanogenerator using α-In2Se3 for energy harvesting and electronic skin applications.

Conclusions:

• α-In2Se3 exhibits versatile piezoelectric behavior, overcoming limitations of other 2D materials.
• The noncentrosymmetry due to hexagonal stacking is key to the observed dual piezoelectricity.
• α-In2Se3 holds promise for developing advanced self-powered devices and adaptive, strain-tunable electronics and optoelectronics.