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dc.contributor.authorChen, Qiranen_US
dc.identifier.citationChen, Q. (2021). Optical properties of semi-conductive MXene Sc2COx, CsPbBr3 nanoplatelets and quantum dots. Doctoral thesis, Nanyang Technological University, Singapore.
dc.description.abstractSemiconductors in the shape of nanostructures, including quantum dots (0-dimension) and nanoplatelets (2-dimension) demonstrate unique electronic and optoelectronic properties because of the quantum mechanism related to excitons confined in low dimensions. Such confinement can provide intriguing properties like high capability of photo-activated carriers, band gap tunability, splendid photoluminescence (PL) yield, narrow emission bandwidth, high absorption efficiency of light, fast fluorescence lifetime, etc. These properties attract great attention due to their high potential in applications like solar cells, light-emitting diodes (LEDs), lasers, photodetectors, catalysts, etc. Novel semiconductors including MoS2, MXene and perovskite have received increasing research interest in recent years. Unlike traditional semiconductor materials like Si, Ge, II-VI and III-V materials, the novel ones usually have low-dimensional form natively, which have emerged as very promising candidates for advanced electronic and optoelectronic devices. Among the various novel semiconductor materials, this thesis focuses on two types of them, which are MXene Sc2CO2 and cesium lead halide perovskite CsPbBr3. The two materials have some features in common. Both of them are direct band gap materials, which makes them available for optical applications. And they can be synthesized directly into two-dimensional structure, which o ers the opportunity to learn the influence of one-dimensional confinement. To extend the study into another dimension, the quantum dots of CsPbBr3 are also studied. The first part of the thesis focuses on the study of Sc2COx which is a type of semi-conductive MXene. The material is synthesized for the first time by using magnetron sputtering. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are performed to determine the composition of the sample. The band gap of Sc2COx sample is determined by ultraviolet-visible (UV-VIS) range absorption and photoluminescence (PL) spectrum. The Tauc plot indicates that Sc2COx is a direct band gap material. The high sensitivity of scandium to oxygen makes the sample contain both scandium carbide and scandium oxide. Thermal annealing introduces more oxidation into the sample. The structure of MXene will be broken when the annealing temperature is higher than 600C. The second part of the thesis focuses on the study of perovskite CsPbBr3 in forms of nanoplatelets and quantum dots. For CsPbBr3 nanoplatelets, the effect of thickness of cesium lead halide perovskite CsPbBr3 nanoplatelets (NPLs) on their electronic structure and optical properties are investigated using an 8-band k · p model which is based on effective-mass envelope function theory with exciton binding energy consideration. We first reported the CsPbBr3 nanoplatelets' band structure and optical gain with exciton effect. As the thickness of NPLs decreases, their band gap increases and the band mixing is more obvious which influences the transition matrix element (TME). The optical gain of CsPbBr3 nanoplatelets is calculated by taking into account TME, Fermi factor, injected carrier density and thickness. A blue shift of peak position in optical gain can be observed as the thickness of NPLs decreases. For any given NPL with a certain thickness, there is a slight blue shift in the peak position of optical gain as the carrier density increases because of band filling effect. The maximum optical gain of thinner NPLs needs higher carrier density to reach saturation. To obtain high differential gain, the carrier density in thick NPLs has to maintain in a small range. Experimental work is carried out and the results agree with our theoretical results very well. For quantum dot, the size effect on the electronic structure and optical properties of cubic CsPbBr3 perovskite quantum dots are investigated by using an 8-band k · p model which is also based on effective-mass envelope function theory with exciton binding energy consideration. Quantum dots with smaller sizes have larger band gaps due to quantum confinement. The transition matrix element (TME) is also influenced by the size. The optical gain of such material is calculated by considering TME, Fermi factor, injected carrier density, quantum dot size, and dephasing rate. A higher density of the injected carrier is needed for smaller quantum dots to get a positive optical gain. The peak of optical gain has a blue shift as the size of a quantum dot decreases. When the carrier density increases for a quantum dot with a certain size, a blue shift in emission peak position can be observed due to the band filling effect. Smaller quantum dots have higher differential optical gain at carrier densities within a certain range. In a word, two direct band gap semiconductor materials, Sc2COx and CsPbBr3 are studied via theoretical and experimental approaches. Their band-structure and optical properties are presented, followed by the analysis of their potential in applications.en_US
dc.publisherNanyang Technological Universityen_US
dc.rightsThis work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).en_US
dc.subjectEngineering::Electrical and electronic engineeringen_US
dc.titleOptical properties of semi-conductive MXene Sc2COx, CsPbBr3 nanoplatelets and quantum dotsen_US
dc.typeThesis-Doctor of Philosophyen_US
dc.contributor.supervisorFan Weijunen_US
dc.contributor.schoolSchool of Electrical and Electronic Engineeringen_US
dc.description.degreeDoctor of Philosophyen_US
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