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|Title:||Chemical vapor deposition of 3D transition metal oxide and chalcogenide nanocrystals||Authors:||Hu, Dianyi||Keywords:||Engineering::Materials||Issue Date:||2021||Publisher:||Nanyang Technological University||Source:||Hu, D. (2021). Chemical vapor deposition of 3D transition metal oxide and chalcogenide nanocrystals. Doctoral thesis, Nanyang Technological University, Singapore. https://hdl.handle.net/10356/154249||Abstract:||This thesis aims to prepare 3d transition metal oxide and chalcogenide nanocrystals by the chemical vapor deposition (CVD) method and study their magnetic properties at a single nanocrystal level. Typically, the nanomaterials prepared in this thesis are two-dimensional (2D) structures, with the thickness down to several nanometers and the lateral size as large as tens of micrometers. The prepared materials include layered FeTe1-xSex, and nonlayered Fe3O4, MnS and MnSe. For the layered FeTe1-xSex, its 2D nanocrystals can be directly synthesized on the SiO2/Si substrate. A mixed precursor strategy was developed and enabled the tuning of composition and control of thickness. The size of the prepared nanosheets can be as large as 40 µm, and the thickness can be as low as 3.8 nm. The prepared FeTe1-xSex nanocrystals exhibited good quality with a superconducting transition temperature (Tc) close to the bulk counterparts. The study of using different precursors showed the relation between the defects and the quality of this unconventional superconductor. Besides, a stacking method utilizing the “stand-on” FeTe1-xSex samples can be easily used to prepare FeTe1-xSex-based van der Waals structures, giving hope to make use of these FeTe1-xSex nanosheets for fabricating complicated devices for topological quantum computing. For the nonlayered Fe3O4, MnS, and MnSe, the usage of mica, a van der Waals material, as the growth substrate was critical to growing the 2D crystals. The free of dangling bonding of the van der Waals substrate enables the easy migration of the absorbed species, thus facilitating the lateral growth and resulting in the formation of 2D crystals. Besides, in preparing 2D Fe3O4 nanocrystals, a water-assisted CVD method using CaSO4∙2H2O as the water precursor was developed, and it is capable of preparing ultrathin Fe3O4 nanocrystals. The prepared Fe3O4 nanocrystals possess very few antiphase boundaries (APBs), as confirmed by transmission electron microscope (TEM) results. This feature enables the study of the magnetic property of the single APB. In the preparation of the 2D MnS and MnSe nanocrystals, the mica also plays a significant role. The synthesis parameters were studied detailly, and their magnetic properties were also characterized. In addition, the morphology of the prepared MnSe nanocrystals can also change from triangular to dendritic shape by tuning the hydrogen concentration, which can also give new magnetic properties. The CVD synthesis strategies mentioned above are not limited to growing the materials mentioned above. They can also be used to synthesize other large-sized 3d transition metal oxide and chalcogenide nanocrystals with controllable thickness and defects, which can serve as a platform for further study the relation between their structures and properties on a single nanocrystal level.||URI:||https://hdl.handle.net/10356/154249||DOI:||10.32657/10356/154249||Rights:||This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).||Fulltext Permission:||embargo_20231216||Fulltext Availability:||With Fulltext|
|Appears in Collections:||MSE Theses|
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