2D Electrets of Ultrathin MoO₂ with Apparent Piezoelectricity

Abstract

Since graphene, a variety of 2D materials have been fabricated in a quest for a tantalizing combination of properties and desired physiochemical behavior. 2D materials that are piezoelectric, i.e., that allow for a facile conversion of electrical energy into mechanical and vice versa, offer applications for sensors, actuators, energy harvesting, stretchable and flexible electronics, and energy storage, among others. Unfortunately, materials must satisfy stringent symmetry requirements to be classified as piezoelectric. Here, 2D ultrathin single‐crystal molybdenum oxide (MoO2) flakes that exhibit unexpected piezoelectric‐like response are fabricated, as MoO2 is centrosymmetric and should not exhibit intrinsic piezoelectricity. However, it is demonstrated that the apparent piezoelectricity in 2D MoO2 emerges from an electret‐like behavior induced by the trapping and stabilization of charges around defects in the material. Arguably, the material represents the first 2D electret material and suggests a route to artificially engineer piezoelectricity in 2D crystals. Specifically, it is found that the maximum out‐of‐plane piezoresponse is 0.56 pm V−1, which is as strong as that observed in conventional 2D piezoelectric materials. The charges are found to be highly stable at room temperature with a trapping energy barrier of ≈2 eV.

Ultrasonic activation of inert poly(tetrafluoroethylene) enables piezocatalytic generation of reactive oxygen species

“…These transient and extreme ultrasonic cavitation pressure waves can cause massive PTFE deformation resulting in permanent structural defects. As a consequence, the concurrent electric fields can generate charges that can be trapped in the PTFE structural defects creating an electret state 26 . In addition, although the initial PTFE as a whole is centrosymmetric, some of the local regions near these chemical or physical defects may cause PTFE to be in a localized noncentrosymmetric phase 35 .…”

Two-Dimensional Material-Based Biosensors for Virus Detection

“…Some 2D monolayer transition metal dichalcogenides such as MoS2, MoTe2, WTe2, and MoO2, are piezoelectric. [219][220][221] Specially, monolayers of post transition metal mono-chalchogenides, such as SnS, show very high piezoelectric coupling coefficients. 222 A precise control of the liquid flow is important in biosensing to direct the analytes onto the surface of 2DMs.…”

“…Piezoelectric materials allow conversion of mechanical energy into electrical energy and vice versa. Some 2D monolayer transition metal dichalcogenides such as MoS 2 , MoTe 2 , WTe 2 , and MoO 2 are piezoelectric. − Specially, monolayers of post-transition metal mono-chalcogenides, such as SnS, show very high piezoelectric coupling coefficients …”

Photocatalysis Within Intrinsic Spontaneous Polarization Electric Field

“…Ajayan et al confirmed that the out‐of‐plane piezoresponse value was 0.56 pm V − 1 for 2D ultrathin MoO 2 . [ 65 ] Extended piezoresponse and Kelvin probe force microscopy (KPFM) studies were conducted on the flakes to alleviate the chance of seeing hysteresis as a result of charge injection from the PFM tip. Zhang et al reported that a typical butterfly amplitude loop was obtained with a sudden phase change from 30 o to 210 o by a sweeping DC voltage from −28 to +28 V, suggesting the local switching of the ISPEF induced by ferroelectric polarization of KNbO 3 nanosheets (NS).…”