Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor | Department of Applied Physics | en_US |
| dc.contributor.advisor | Hao, Jianhua (AP) | en_US |
| dc.creator | Io, Weng Fu | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/13663 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | en_US |
| dc.rights | All rights reserved | en_US |
| dc.title | Piezoelectricity and ferroelectricity in two-dimensional layered metal thiophosphates and their applications | en_US |
| dcterms.abstract | Emerging two-dimensional (2D) van der Waals (vdW) layered materials possessing unprecedented properties, including atomically thin thickness, outstanding mechanical durability, and dangling bond-free surfaces, have garnered significant attention from fundamental research to various potential applications. In particular, they hold great promise to address the intrinsic size effect and constraints of lattice mismatch in ultrathin traditional piezoelectric and ferroelectric materials, opening up new opportunities for exploring ferroelectric and piezoelectric characteristics at low dimensions. However, most 2D layered piezoelectric materials discovered to date only exhibit piezoelectricity along in-plane direction, largely inhibiting their ability to be incorporated into vertical nanoelectronic device systems. Some out-of-plane piezoelectric materials have also been theoretically and experimentally reported but illustrate relatively weak piezoelectric response and low piezoelectric coefficients (d33) compared to traditional piezoelectric materials. On the other hand, various 2D vdW materials are theoretically predicted with intrinsic ferroelectricity including In2Se3, monolayer group IV chalcogenides, and metal thiophosphates. Nevertheless, the experimental observations of robust room-temperature ferroelectricity in 2D layered materials, especially along the vertical direction, are still rather limited. Therefore, there is an urgent need to search for strong vertical piezoelectricity in 2D materials with systematic investigations. Moreover, the experimental achievements in realizing intrinsic room-temperature ferroelectricity in 2D materials and studying the underlying mechanism are highly desired in developing multifunctional nanoelectronic devices. | en_US |
| dcterms.abstract | As one of the most studied layered metal thiophosphates materials, CuInP2S6 (CIPS), with numerous intriguing characteristics including strong vertical piezoelectricity and ferroelectricity, shows immense promise for integration into vertical nanoelectronic device systems. In contrast to the extensive research to investigate its ferroelectric mechanisms and applications, the study on the piezoelectric effect of 2D CIPS and its relevant piezotropic device application is still in the early phases. In this thesis, a quantitative analysis of the high vertical piezoelectric responses in 2D CIPS nanosheets was performed and the potential device application as 2D piezoelectric nanogenerators (PENGs) fabricated on silicon substrates was demonstrated. Experimental result indicates a high d33 piezoelectric coefficient of 17.4 pm V-1 in few-layer CIPS that outperforms most 2D piezoelectric materials previously explored. Moreover, a prototype 2D CIPS-based PENG device illustrates a maximum piezoelectric power density of 70.4 nW cm-2. The outstanding piezoelectric responses of 2D CIPS nanoflakes warrant their prospective utilizations in the next generation strain-modulable nanoscale electronic and piezotronic devices integrated with the conventional silicon-based chips. | en_US |
| dcterms.abstract | Recently, the isostructural layered metal thiophosphate CuCrP2S6 (CCPS) has been evidenced theoretically and experimentally the possibility of realizing 2D multiferroicity, which is of great significance from both fundamental physics and device application perspectives. Nevertheless, incorporating magnetic Cr cations in the ferroelectric framework results in antiferromagnetic and antiferroelectric orderings, whereas macroscopic spontaneous polarization is always lacking. Here, the direct observation of robust longitudinal ferroelectricity in 2D vdW layered CCPS at room temperature is reported with an in-depth investigation. Modification of the ferroelectric domain phase in 2D CCPS nanosheets is achieved experimentally by scanning probe technique. External electric field-induced polarization switching loops are acquired in CCPS down to 2.6 nm (around four layers). More importantly, the atomically resolution scanning transmission electron microscopy study unveiled the origin of the emergent room-temperature out-of-plane ferroelectricity in 2D CCPS layers. This comprehensive experimental study shall promote the development of multifunctional nanoelectronic devices and shed light on understanding the fundamentals of ferroelectric order of this 2D novel vdW material. As a proof-of-concept demonstration for the possible applications of ultrathin ferroelectric CCPS, various prototype devices including ferroelectric tunneling junction, ferroelectric diode, and ferroelectric field-effect transistor were further constructed and characterized. Their applications as non-volatile memory devices operating at room temperature were presented and the feasibility of constructing heterostructures for multifunctional device applications was demonstrated. These findings signify the promising applications of 2D CCPS nanosheets in efficient nanoelectronic devices. | en_US |
| dcterms.extent | xxiv, 140 pages : color illustrations | en_US |
| dcterms.isPartOf | PolyU Electronic Theses | en_US |
| dcterms.issued | 2025 | en_US |
| dcterms.educationalLevel | Ph.D. | en_US |
| dcterms.educationalLevel | All Doctorate | en_US |
| dcterms.LCSH | Two-dimensional materials | en_US |
| dcterms.LCSH | Piezoelectricity | en_US |
| dcterms.LCSH | Ferroelectricity | en_US |
| dcterms.LCSH | Hong Kong Polytechnic University -- Dissertations | en_US |
| dcterms.accessRights | open access | en_US |
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