Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor | Department of Mechanical Engineering | en_US |
| dc.contributor.advisor | Zheng, Guangping (ME) | en_US |
| dc.creator | Jiang, Shujuan | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/13392 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | en_US |
| dc.rights | All rights reserved | en_US |
| dc.title | Theoretical predictions on the electrical and magnetic properties of novel two-dimensional materials | en_US |
| dcterms.abstract | In 2004, the discovery of graphene has opened creative opportunities in two-dimensional (2D) materials science and other research fields, which developed rapidly in recent years. Compared to their bulk counterparts, 2D materials have ultrathin structure and perfect boundaries, endowing them with extraordinary mechanical, electrical, magnetic, and optical properties. In addition to the most prevalent 2D materials including graphene, hexagonal boron nitride, transition metal dichalcogenides, and black phosphorene, many other outstanding 2D materials have been studied in the past five years. However, there are some challenges, such as limited types of experimentally synthesized 2D materials, poor material stability, and low Curie temperatures of the 2D magnetic materials. First-principles calculations based on density functional theory can effectively discover and predict 2D materials, and study their electrical and magnetic properties, which are much related to their electron density and atomic structure. In this thesis work, a series of novel 2D materials with excellent electronic, mechanical, and magnetic performances are predicted. The underlying mechanisms are also explained. | en_US |
| dcterms.abstract | 1) A novel GaOCl monolayer with excellent thermal and dynamical stability at room temperature is predicted and studied systematically, and its wide direct bandgap (4.46 eV) can be further modulated under applied strains. The 2D semiconductor exhibits high mechanical flexibility, and anisotropy in Poisson’s ratio and carrier mobilities, endowing it with a broad spectrum of electronic and optoelectronic applications. More importantly, the GaOCl monolayer has spontaneous magnetization induced by hole doping and shows outstanding multidirectional piezoelectricity, which are comparable with those of either magnetic or piezoelectric 2D materials. Our calculations indicate that the GaOCl monolayer with wide bandgaps and tunable piezoelectricity and ferromagnetism could be promising for applications in multifunctional integrated nanodevices with high performance. | en_US |
| dcterms.abstract | 2) Novel 2D Janus GaOClX (X = F, Br, and I) monolayers with superior dielectric energy storage properties are predicted. They are indirect-bandgap semiconductors with bandgaps of 2.18 ~ 4.36 eV, and possess anisotropic carrier mobility, strong mechanical flexibility, as well as excellent out-of-plane piezoelectricity. More importantly, it is found that the GaOCl monolayer and Janus GaOClX monolayers could exhibit ultrahigh energy storage density (as high as 893.32 J/cm3) compatible to those of electrochemical supercapacitors and batteries, unparallel by other dielectric materials reported to date. This work opens a new window in searching for novel dielectric materials, which could be used in dielectric capacitors with superior energy storage density and power density, excellent efficiency and thermal stability. | en_US |
| dcterms.abstract | 3) The effects of carrier doping on magnetic and electronic properties of 2D Janus TiXO (X = S, Se) systems are investigated. It is found the electronic instability of systems is related with the flat conduction-band edge and high density of states near Fermi energy. At a concentration of electron doping n ranging from ~1013 to ~1014 cm-2, the system maintains in a fully spin-polarized state and the Curie temperature (TC) is estimated to be above room temperature; while the nonmagnetic semiconductor TiXO monolayer could become a magnetic half-metal when n is varied. The doping-induced magnetisms and TC could be modulated substantially by applying strains and alternating the number of monolayers in Janus TiXO multilayers. The results indicate that the Janus TiXO possesses tunable magnetic properties, providing promising 2D materials for electronics and spintronics applications. Moreover, a series of novel Janus XYP2 (X/Y = Si, Ge, Sn and Pb; X≠Y) monolayers with good stability, electronic, mechanical, piezoelectric properties as well as ferromagnetic properties are predicted. They are all indirect-bandgap semiconductors with bandgaps varying from 0.56 to 2.18 eV. The monolayers exhibit in-plane and out-of-plane piezoelectricity with large piezoelectric coefficients, suggesting that they are promising piezoelectric materials for nano-electromechanical systems. In the SiGeP2, GeSnP2, and SnPbP2 monolayers, hole doping is found to be an effective approach in inducing or tuning different ferromagnetic states resulting from the electronic instability as reflected by the van Hove singularity in the 2D systems. The results suggest that Janus XYP2 monolayers could be promising 2D materials for applications in nano-electronic, spintronic and nano-electromechanical devices. | en_US |
| dcterms.abstract | 4) With transition- metal (TM) and rare-earth (RE) elements, (MoWFeCoX)S2 (X=Gd, Sm, and Nd) high-entropy (HE) disulfide monolayers are designed and studied. Through local distortion, the 2D HE TM-RE disulfides with disordered structures can be stable at elevated temperatures. It is feasible to synthesize these disulfides at moderate temperatures similar to those of previously reported layered HE compounds. More importantly, they could be promising 2D materials with high absorption coefficients (105 cm-1) in a broad range of frequency domain and anisotropic mechanical properties that are used in multifunctional integrated devices. | en_US |
| dcterms.extent | xx, 175 pages : color illustrations | en_US |
| dcterms.isPartOf | PolyU Electronic Theses | en_US |
| dcterms.issued | 2024 | en_US |
| dcterms.educationalLevel | Ph.D. | en_US |
| dcterms.educationalLevel | All Doctorate | en_US |
| dcterms.LCSH | Two-dimensional materials | en_US |
| dcterms.LCSH | Two-dimensional materials -- Mechanical properties | en_US |
| dcterms.LCSH | Two-dimensional materials -- Electric properties | en_US |
| dcterms.LCSH | Hong Kong Polytechnic University -- Dissertations | en_US |
| dcterms.accessRights | open access | en_US |
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