Growth of Metal Chalcogenide Nanomaterials and Their Characterizations Yichao Zou Bachelor of Engineering A thesis submitted for the degree of Doctor of Philosophy at The University of Queensland in 2016 School of Mechanical and Mining Engineering Abstract Metal chalcogenides, such as IV-VI and V-VI compounds (SnTe, Bi2Te3, Bi2Se3), are ideal candidates for applications in thermoelectricity and topological (crystalline) insulators. This PhD thesis focuses on the controllable synthesis of metal chalcogenide nanostructures via chemical vapour deposition method (CVD), and on the understanding of the crystal structure, growth mechanism, and structure-property correlation in the as-grown nanomaterials. IV-VI and V-VI compounds have attracted extensive research interest because of their excellent thermoelectric properties and exotic physical properties. Nevertheless, there still exist unresolved issues that prevent the further applications of IV-VI and V-VI nanomaterials by rational design, including (1) it is still difficult to grow these nanomaterials with controllable morphology and (2) crystal structure; (3) limited investigations of their growth mechanisms; (4) limited study on structure-property relationships in the nanostructures. Therefore, in this thesis, the controllable growth technique, growth mechanism and structure-property relation in IV-VI and V-VI based nanostructures are explored. The objective is achieved in the following steps: Realizing the morphological control of the nanostructures. (i) By catalyst engineering in Au-catalysed CVD. For SnTe nanostructures, catalyst composition was found to be a key factor controlling the morphology. AuSn catalysts induce growth of triangular SnTe nanoplates, whereas Au5Sn catalysts result in <010> SnTe NWs. For Bi2Se3 nanostructures, catalyst-nanostructure interface was found to have an impact on their growth directions. A {0001} interface between the catalyst/nanostructure induces the growth of {0001} NWs, while when the interface is not defined, < 112̅0 > nanoribbons are grown. (ii) By doping in catalyst-free CVD. It has been found that Bi doping induces a switch of dominant surface facet in SnTe nanostructures, from {100} to {111}. This transition is driven by surface-energy minimization according to our energetic calculation results, which suggests that incorporation of Bi dopant reduces the surface energy of {111}Te facet in SnTe. Realizing the crystal-structure control of the nanostructures. Bi dopant was found to be an essential parameter that can control the crystal structure of SnTe based nanostructures. Bi dopants promote the formation of Sn planar vacancies in Sn1- I xBixTe nanoribbons. The density of the planar vacancies can be engineered by varying the Bi concentration. Through combination of sub-ångström-resolution imaging and calculation within density functional theory, these planar vacancies are found to be associated with Bi segregation, which significantly lowers the formation energies of the vacancies. The planar vacancies exhibit polymorphic structures with local variations in lattice relaxation level, determined by their proximity to nanoribbon surface. Understanding the growth mechanism. For the growth of Bi2Se3, Au catalyst was found to initiate the nucleation of nanostructures by absorbing V vapour species (Bi) via a vapour-solid-solid mechanism. For growth of SnTe, the catalyst was found to preferentially absorb IV resources (Sn) to form Au-Sn alloy particles to initiate the nanostructure growth, where both vapour-solid-solid and vapour- liquid-solid mechanisms may dominate. Understanding the structure-property correlation. (i) By experimental measurement of individual nanostructures. Bi2Te3 and Bi2Se3 have a layered rhombohedral crystal structure. Within each quintuple layer, atoms are joined by strong covalent bonding. While between neighboring quintuple layers, the interaction is only weak van der Waals. This anisotropic bonding is expected to result in a strong anisotropy in electronic properties. We demonstrate that such electronic anisotropy may be enhanced in nanoscale samples, by comparing the conductivities of <0001> and <112̅0> grown Bi2Se3 nanostructures using an in- situ TEM-STM facility. The conductivity anisotropic ratio at nanoscale is found to be ~50 times larger than their bulk counterparts. (ii) By electronic-structure calculation within density functional theory in combination of atomic-scale imaging. In planar-vacancy engineered Sn1-xBixTe nanoribbons, calculations show that the Bi segregated planar vacancies introduce newly localized distorted density of states into the system. In Bi2SnTe4 nanostructures, cation-atom disordering was observed by aberration-corrected STEM. Calculations show that such cation disordering may narrow the bandgap of this semiconductor system. II Declaration by author This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis. I have clearly stated the contribution of others to my thesis as a whole, including statistical assistance, survey design, data analysis, significant technical procedures, professional editorial advice, and any other original research work used or reported in my thesis. The content of my thesis is the result of work I have carried out since the commencement of my research higher degree candidature and does not include a substantial part of work that has been submitted to qualify for the award of any other degree or diploma in any university or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been submitted to qualify for another award. I acknowledge that an electronic copy of my thesis must be lodged with the University Library and, subject to the policy and procedures of The University of Queensland, the thesis be made available for research and study in accordance with the Copyright Act 1968 unless a period of embargo has been approved by the Dean of the Graduate School. I acknowledge that copyright of all material contained in my thesis resides with the copyright holder(s) of that material. Where appropriate I have obtained copyright permission from the copyright holder to reproduce material in this thesis. III Publications during candidature Published papers included in this Thesis: 1. Zou, Y.; Chen, Z.-G.; Lin, J.; Zhou, X.; Lu, W.; Drennan, J.; Zou, J., Morphological Control of SnTe Nanostructures by Tuning Catalyst Composition. Nano Res. 2015, 8, 3011-3019. 2. Zou, Y.; Chen, Z.-G.; Huang, Y.; Yang, L.; Drennan, J.; Zou, J., Anisotropic Electrical Properties from Vapor–Solid–Solid Grown Bi2Se3 Nanoribbons and Nanowires. J. Phys. Chem. C 2014, 118, 20620-20626. 3. Zou, Y.; Chen, Z.-G.; Kong, F.; Lin, J.; Drennan, J.; Cho, K. J.; Wang, Z.; Zou, J., Planar Vacancies in Bi1-xSnxTe Nanoribbons. ACS Nano 2016, 10, 5507-5515. Published co-authored papers: 1. Chen, Z.-G.; Zhang, C.; Zou, Y.; Zhang, E.; Yang, L.; Hong, M.;. Xiu, F.-X; Zou, J., Scalable Growth of High Mobility Dirac Semimetal Cd3As2 Microbelts. Nano Lett. 2015, 15, 5830−5834 2. Yang, L.; Chen, Z.-G.; Han, G.; Hong, M.; Zou, Y.; Zou, J., High-Performance Thermoelectric Cu2Se Nanoplates Through Nanostructure Engineering. Nano Energy 2015, 16, 367-374. 3. Yuan, X.; Tang, L.; Liu, S.; Wang, P.; Chen, Z.; Zhang, C.; Liu, Y.; Wang, W.; Zou, Y.; Liu, C.; Guo, N.; Zou, J.; Zhou, P.; Hu, W.-D.; Xiu, F.-X., Arrayed van der Waals Vertical Heterostructures Based on 2D GaSe Grown by Molecular Beam Epitaxy. Nano Lett. 2015, 15, 3571-3577. 4. Yuan, X.; Tang, L.; Wang, P.; Chen, Z.; Zou, Y.; Su, X.; Zhang, C.; Liu, Y.; Wang, W.; Liu, C.; Chen, F.-S.; Zou, J.; Zhou, P.; Hu, W.-D.; Xiu, F.-X., Wafer-Scale Arrayed p-n Junctions Based on Few-Layer Epitaxial GaTe. Nano Res. 2015, DOI: 10.1007/s12274-015-0833-8. 5. Han, G.; Chen, Z.-G.; Ye, D.; Wang, B.; Yang, L.; Zou, Y.; Wang, L.; Drennan, J.; Zou, J., In3Se4 and S-Doped In3Se4 Nano/Micro-Structures as New Anode Materials for Li-Ion Batteries. J. Mater. Chem. A 2015, 3, 7560-7567. 6. Han, G.; Chen, Z.-G.; Zou, Y.; Drennan, J.; Zou, J., Long Wavelength Emissions 4+ of Se -Doped In2O3 Hierarchical Nanostructures. J. Mater. Chem. C 2014, 2, 6529-6535. IV Conference abstract 1. Zou, Y. C.; Chen, Z. G.; and Drennan, J.; Zou, J.; Morphology Control of SnTe Nanostructures by Catalyst Engineering. ACMM 2016. (Canberra, Oral presentation) 2. Zou, J.; Zou, Y. C.; Chen, Z. G.; and Drennan, J.; Catalyst Impact on Morphologies of SnTe Nanostructures. ICONN 2016. (Canberra, Poster presentation) 3. Zou, Y. C.; Chen, Z. G.; and Drennan, J.; Zou, J.; Gold Catalyzed and Catalyst- Free Growth of One-Dimensional Bi2Se3 Nanostructures. COMMAD 2014. (Perth, Oral presentation) 4. Zou, Y. C.; Chen, Z. G.; and Drennan, J.; Zou, J.; Vapor-Solid-Solid Grown Bi2Se3 Nanoribbons and Nanowires with Anisotropic Electronic Properties. ICAMP8 2014. (Gold Coast, Oral presentation) V Publications included in this thesis Zou, Y.; Chen, Z.-G.; Kong, F.; Lin, J.; Drennan, J.; Cho, K. J.; Wang, Z.; Zou, J., Planar Vacancies in Bi1-xSnxTe Nanoribbons. ACS Nano 2016, 10, 5507-5515. Contributions Statement of contribution Yichao Zou (Candidate) Designed experiments (60%) Carried out materials synthesis (100%) Carried out characterization (60%) Carried out data analysis (60%) Wrote the paper (60%) Zhigang Chen Designed experiments (15%) Carried out characterization (10%) Carried out data analysis (5%) Wrote the paper (15%) Supervision (20%) Fantai Kong Carried out modelling and calculations (50%) Carried out data analysis (25%) Wrote the paper (5%) Jing Lin Wrote the paper (5%) John Drennan Supervision (20%) Kyeongjae Cho Carried out modelling and calculations (10%) Zhongchang Wang Designed experiments (10%) Carried out characterization (30%) Carried out modelling and calculations (40%) Jin Zou Designed experiments (15%) Carried out data analysis (10%) Wrote the paper (15%) Supervision (60%) VI Zou, Y.; Chen, Z.-G.; Lin, J.; Zhou, X.; Lu, W.; Drennan, J.; Zou, J., Morphological Control of SnTe Nanostructures by Tuning Catalyst Composition.