Browsing by Author "Zhang, Junwei"

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Multidirection Piezoelectricity in Mono- and Multilayered Hexagonal alpha-In2Se3
(2018-05-01)
Xue, Fei
;
Zhang, Junwei
;
Hu, Weijin
;
Hsu, Wei-Ting
;
Han, Ali
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 alpha-In2Se3. Here, we experimentally report the coexistence of out-of-plane and in-plane piezoelectricity in monolayer to bulk alpha-In2Se3, attributed to their noncentrosymmetry originating from the hexagonal stacking. Specifically, the corresponding d(33) piezoelectric coefficient of alpha-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 alpha-In2Se3-based flexible piezoelectric nanogenerator as an energy-harvesting cell and electronic skin. The out-of-plane and in-plane piezoelectricity in alpha-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.
Room-Temperature Ferroelectricity in Hexagonally Layered alpha-In2Se3 Nanoflakes down to the Monolayer Limit
(2018-12-12)
Xue, Fei
;
Hu, Weijin
;
Lee, Ko-Chun
;
Lu, Li-Syuan
;
Zhang, Junwei
2D ferroelectric material has emerged as an attractive building block for high density data storage nanodevices. Although monolayer van der Waals ferroelectrics have been theoretically predicted, a key experimental breakthrough for such calculations is still not realized. Here, hexagonally stacking alpha-In2Se3 nanoflake, a rarely studied van der Waals polymorph, is reported to exhibit out-of-plane (OOP) and in-plane (IP) ferroelectricity at room temperature. Ferroelectric multidomain states in a hexagonal alpha-In2Se3 nanoflake with uniform thickness can survive to 6 nm. Most strikingly, the electric-field-induced polarization switching and hysteresis loop are, respectively, observed down to the bilayer and monolayer (approximate to 1.2 nm) thicknesses, which designates it as the thinnest layered ferroelectric and verifies the corresponding theoretical calculation. In addition, two types of ferroelectric nanodevices employing the OOP and IP polarizations in 2H alpha-In2Se3 are developed, which are applicable for nonvolatile memories and heterostructure-based nanoelectronics/optoelectronics.