|Author:||Io, Weng Fu|
|Title:||Robust room-temperature ferroelectric ultrathin chalcogenide layers and van der Waals heterostructures|
|Advisors:||Hao, Jianhua (AP)|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Physics|
|Pages:||xvi, 98 pages : color illustrations|
|Abstract:||Emerging two-dimensional (2D) ferroelectric materials with unique structures and extraordinary electrical properties have drawn enormous research attention for the applications in nanoelectronic and optoelectronic devices. In traditional ferroelectrics, their potential nanoscale electronic applications are seriously hindered by the size effect. With the merit of van der Waals (vdW) stacking layer structure, 2D materials without the restriction of the fundamental size effect are promising to overcome the increasing effect of depolarization field in ultrathin ferroelectric materials, therefore opening up a new route for exploring ferroelectric phenomena at low dimensionality. Although various potential 2D layered ferroelectric materials have been predicted with theoretical calculations, plenty of them still requires to be implemented with experimental works. To date, experimental achievements of ultrathin layered ferroelectrics are still very limited, and further investigations are urgently needed. In this thesis, the robust high-temperature 2D out-of-plane ferroelectricity of α-In₂Se₃ nanoflakes with the existence of switchable electric polarization above 200 °C is demonstrated. First, high-quality single-crystallized α-In2Se3 nanosheets were fabricated via chemical vapor deposition (CVD), and their phase and crystal quality were examined using optical microscopy and Raman spectroscopy. The non-centrosymmetric structure of the as-grown α-In₂Se₃ samples was evident under second harmonic generation (SHG) microscopy. The crystal structure and chemical composition of α-In₂Se₃ were studied by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), and the morphology was determined with atomic force microscopy (AFM). The ferroelectric properties were investigated on α-In2Se3/Pt heterostructure at different temperatures using piezoresponse force microscopy (PFM) equipped with a polyheater. Distinctive polarization switching and ferroelectric domains are observable in ultrathin α-In2Se3 films down to 10 nm in a wide range of temperatures. The coercive field causing polarization reversal in 2D α-In2Se3 demonstrates a thickness dependency at room temperature and increases significantly when reducing the film thickness. When the temperature of samples varies, there shows no obvious changes in the value of coercive voltage for α-In2Se3 nanosheets with identical thickness. The overall results effectively verify the high stability of ferroelectric phenomena in CVD-grown α-In2Se3 nanoflakes at high temperature reaching above 200 °C and provide critical insights for 2D materials to apply in next-generation high-temperature nanoelectronic devices.|
On the other hand, we demonstrate the out-of-plane ultrathin ferroelectricity in two vdW layered transition metal thiophosphates: CuInP2S6 (CIPS) and CuCrP2S6 (CCPS), both possessing critical temperatures above room temperature and are not comprehensively investigated with experiments. The crystal structure and phase of CIPS and CCPS crystal were studied by Raman spectroscopy, X-ray diffraction (XRD) and TEM, and their broken inverse symmetry was verified using second harmonic imaging microscopy (SHIM). The ferroelectric phenomena at room temperature were characterized by PFM. In experiment, we discover stable out-of-plane ferroelectricity in 2D CIPS and CCPS samples at different thicknesses, in which switchable spontaneous polarization field and butterfly-shaped amplitude signals are observed in CIPS and CCPS nanoflakes down to 9 nm and 6 nm, respectively. Furthermore, the origin of 2D ferroelectricity in CIPS and CCPS is discussed. In summary, the outstanding thermal stability of ferroelectricity in CVD-grown α-In2Se3 nanosheets is verified in this thesis with desirable ferroelectric properties. Also, robust room-temperature ferroelectric phenomena are observed in ultrathin CIPS and CCPS thin films with a thickness of less than 10 nm. It is believed our work presented in this thesis broadens the 2D ferroelectric family that accomplished with experiments and facilitates the potential 2D layered ferroelectric materials in nanoelectronic applications.
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