|Title:||2D ferroelectricity and piezoelectricity for electronic devices|
|Advisors:||Hao, Jianhua (AP)|
|Subject:||Layer structure (Solids)|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Physics|
|Pages:||xxiii, 148 pages : color illustrations|
|Abstract:||Two-dimensional (2D) layered materials have attracted enormous interests in fundamental research and industrial applications due to their excellent physical properties, such as electronic, optoelectric, thermal, optical, mechanical and magnetic properties. Recently, ferroelectricity and piezoelectricity in 2D layered material family were addressed, which leads to great potential applications for atomic-scale smart electronic devices. The emerging 2D ferroelectricity and piezoelectricity can yield fascinating behaviors with various functional devices. In this thesis, firstly, the MoTe₂ samples were prepared using mechanical exfoliation and laser process. By combining first-principle calculations and experimental studies, the robust room-temperature out-of-plane ferroelectricity is realized in the monolayer MoTe₂ with unexploited distorted 1T (d1T) phase. The ferroelectricity origin in d1T-MoTe₂ results from the spontaneous symmetry breaking due to the relative atomic displacements of Mo atoms and Te atoms. A large ON/OFF resistance ratio is achieved in ferroelectric devices composed of MoTe₂ based van der Waals (vdW) heterostructure. Secondly, piezoelectricity in 2D material family has largely been predicted by theoretical calculations, but still few experimental observations from them. The out-of-plane piezoelectric response can be achieved in 2D layered material In₂Se3. In addition, a high-precision actuator device based on the piezoelectricity in 2D atomic scale is demonstrated. Moreover, the vertical ferroelectric polarization characteristic of In2Se3 is investigated by piezoresponse force microscopy, which can retain the ferroelectric behaviour down to 6 nm. Thirdly, the vertical piezoelectric response of several 2D layered materials is theoretically absent via electric field or strain applied perpendicular to their surface due to the inversion symmetry. Recently, vertical piezoelectric properties of In₃Se₃ were reported, which exhibits low piezoelectric response. A remarkably enhanced out-of-plane piezoelectric performance in 2D layered vdW heterostructure is reported. In particular, the vdW heterostructure based piezoelectricity is not restricted by layer-dependence and edge-effect. The enhanced piezoelectric properties result from the band offset. In addition, the vdW heterostructure can be extended to other 2D materials like WS₂. The heterostructure is fabricated onto silicon substrate which is compatible with state-of-the-art microfabrication technology. The 2D ferroelectricity and piezoelectricity have been studied. The ferroelectric, piezoelectric properties and related device applications of 2D materials have been investigated. These studies provide a platform for designing new functionalities and achieving unexpected applications in atomic-scale smart electronic devices and electromechanical systems.|
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