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 | Yang, Fumei | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/13842 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | en_US |
| dc.rights | All rights reserved | en_US |
| dc.title | Synthesis of layered nanosheets and their application in flexible nanogenerators | en_US |
| dcterms.abstract | Flexible nanogenerators capable of efficiently converting mechanical energy into electricity have diverse applications in communication, healthcare, environmental monitoring and beyond. Since the first discovery of piezoelectricity in quartz, conventional piezoelectric crystals such as lead zirconate titanate (PZT) and barium titanate (BTO) have attracted great interest due to their exceptional electromechanical conversion efficiency. Unfortunately, these ceramics suffer from vulnerability in flexibility and biocompatibility for advanced consumer electronics such as wearable and bio-implantable devices. Recently, layered materials have emerged as the promising candidates for flexible electronics due to their ultrathin nature, desirable electrical and mechanical properties, and attractive biocompatibility. To date, various layered materials, including hexagonal boron nitride (h-BN), transition metal chalcogenides (TMDs), and group III-IV metal chalcogenides, have been applied in developing flexible nanogenerators, but the intrinsic piezoelectricity generally only exists in the monolayer or few-odd layers due to the polarization cancellation between adjacent layers, leading to low output signals and poor mechanical stability. While extensive designs like Janus structures were proposed to enhance the performance by inducing higher symmetry breaking, the complexity and uncontrollability of these designs limit their broad practical applications. In contrast, novel multi-layer piezoelectric materials show great potential in achieving considerable output performance and long-term durability for practical applications. Furthermore, the scalability of these layered materials is highly desired for facilitating the widespread practical applications. | en_US |
| dcterms.abstract | This thesis delves into the synthesis of novel multi-layer nanosheets, including strong-piezoresponse layered SnSe and high-entropy MXene, and their application in advanced flexible nanogenerators. Firstly, SnSe nanosheets were synthesized using bottom-up vapor transport deposition (VTD). By fine-tailoring the specific growth parameters, such as the growth temperature, growth duration, carrier gas and so on, the resulting SnSe nanosheets on mica substrates exhibit large area, homogeneous surface and high crystalline quality, confirmed by systematically morphological and structural characterization. In addition, the enhanced in-plane piezoelectric properties of SnSe nanosheets with varying thickness is evidenced through piezoresponse force microscope (PFM), which attributes to their unique puckered C2v symmetry. Specifically, effective piezoelectric coefficient in 10 nm SnSe nanosheet (d11 = ~45.82 pm/V) surpassed those in most investigated TMDs with odd-even effects, showing great potential for practical flexible-nanogenerator applications. The electrical properties of SnSe nanosheets were also investigated by fabricating field-effect-gating devices, displaying the clear p-type transfer behavior with a considerable hole mobility of 19.35 cm² V⁻¹ s⁻¹. On these bases, the SnSe nanosheets were further developed into flexible PENG device, exhibiting distinct piezotronic effect, high energy conversion efficiency and sensibility in energy harvesting and human-motion monitoring, suggesting their capability for smart wearable electronics and healthcare applications. In addition to VTD technique, top-down electrochemical etching approach is also employed for high-yield synthesis of layered nanosheets, and the target material here is the high-entropy MXene (TiVCrMoC₃Tₓ). Different from the commonly used hydrofluoric acid (HF)-etching method, electrochemical etching can produce MXene with low bio-toxicity, which is very suitable for the future development of biocompatible devices. Flexible nanogenerators were further fabricated based on a composite structure of high-entropy MXene nanosheets with PVA polymer relying on a direct self-assembly path. The output performance of MXene/PVA self-powered flexible nanogenerators reaches up to 500 mV in voltage and 790 pA in current at 3.47 N, with stable working over 1,500 cycles, indicating the desirable mechanical durability and potential for practical applications in wearable and bio-implantable electronics. | en_US |
| dcterms.abstract | In summary, this thesis explores the synthesis of high-quality SnSe and MXene layered nanosheets with high yield and further develops their applications in innovative flexible nanogenerators and sensors. These findings drive the exploration of novel multi-layer materials for the advancement of next-generation electronic devices with attractive high-performance, satisfying energy-efficiency, and desirable flexibility and functionality across diverse applications and industries. | en_US |
| dcterms.extent | xxi, 150 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.accessRights | open access | en_US |
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