2D Electrets of Ultrathin MoO2 with Apparent Piezoelectricity

• Published in: Advanced materials (Weinheim) 2020-06, Vol.32 (24), p.e2000006-n/a
• Main Authors: Apte, Amey, Mozaffari, Kosar, Samghabadi, Farnaz Safi, Hachtel, Jordan A., Chang, Long, Susarla, Sandhya, Idrobo, Juan Carlos, Moore, David C., Glavin, Nicholas R., Litvinov, Dmitri, Sharma, Pradeep, Puthirath, Anand B., Ajayan, Pulickel M.
• Format: Article
• Language: English
• Subjects: 2D electrets; 2D materials; Actuators; Crystal defects; defects; Electrets; Energy harvesting; Energy storage; Flakes (defects); Flexible components; Graphene; MATERIALS SCIENCE; Molybdenum oxides; Physiochemistry; Piezoelectricity; Room temperature; trapped charges; Trapping; Two dimensional materials
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.202000006

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. Chemical vapor deposition (CVD) synthesis of ultrathin single‐crystal MoO2 flakes with monoclinic structure results in the formation of voids in the structure, which act as sources for trapping and stabilizing charges. The emergent electret‐like behavior in 2D of an otherwise non‐piezoelectric bulk material is capable of artificially engineering piezoelectricity in 2D materials.