Multidirection Piezoelectricity in Mono- and Multilayered Hexagonal α-In₂Se₃

Abstract

Piezoelectric materials have been widely used for sensors, actuators, electronics, and energy conversion. Two-dimensional (2D) ultrathin semiconductors, such as monolayer h-BN and MoS 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-InSe. Here, we experimentally report the coexistence of out-of-plane and in-plane piezoelectricity in monolayer to bulk α-InSe, attributed to their noncentrosymmetry originating from the hexagonal stacking. Specifically, the corresponding d piezoelectric coefficient of α-InSe 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-InSe-based flexible piezoelectric nanogenerator as an energy-harvesting cell and electronic skin. The out-of-plane and in-plane piezoelectricity in α-InSe 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.

2D Electrets of Ultrathin MoO2 with Apparent Piezoelectricity

“…[ 26 ] Xue et al obtained an out‐of‐plane piezoelectric coefficient of 0.34 pm V −1 for monolayer α‐In 2 Se 3 , which itself contains odd (five) number of atomic layers. [ 27 ] Another interesting category of 2D materials are Janus monolayers, which can show stronger piezoelectricity than their standard TMDC counterparts. [ 28 ] These developments bring to fore artificial approaches to create piezoelectricity in materials that are not intrinsically so.…”

Piezoelectricity in Multilayer Black Phosphorus for Piezotronics and Nanogenerators

“…However, monolayer BP devices are almost impossible to fabricate because of their rapid oxidation in air. Xue et al reported the existence of piezoelectricity in multilayered α‐In 2 Se 3 and were the first to explain the piezoelectric mechanism in multilayer 2D materials . Similarly, the structure of multilayer BP can be analyzed in terms of non‐neutralized polarity of bilayers and the piezoelectricity of multilayer BP devices can then be elaborated on detail.…”

Confined van der Waals Epitaxial Growth of Two-Dimensional Large Single-Crystal In2Se3 for Flexible Broadband Photodetectors

“…Third, unlike black phosphorus which is also a direct bandgap 2D material, thin In 2 Se 3 flakes are very stable in air, which is very important for practical applications. Recently, there has been pioneering work on the use of 2D In 2 Se 3 in piezoelectronics [24,25], optoelectronics [26], and photovoltaics [27]. We note that these reported 2D In 2 Se 3 materials have small domain sizes (from a few to tens of micrometers, Figure 1(a) and Table S1) [28][29][30].…”