Two-dimensional transition metal dichalcogenides: additive-mediated synthesis, doping and related applications
Open Access
- Author:
- Zhang, Tianyi
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 07, 2021
- Committee Members:
- Nasim Alem, Major Field Member
Mauricio Terrones, Chair & Dissertation Advisor
Saptarshi Das, Outside Unit & Field Member
Joan Redwing, Major Field Member
John Mauro, Program Head/Chair - Keywords:
- 2D materials
transition metal dichalcogenides
chemical vapor deposition
doping
van der Waals heterostructures - Abstract:
- Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are an emerging family of 2D materials beyond graphene. 2D semiconducting TMDs possess a series of unique structural and functional properties, such as the presence of atomically flat surfaces without dangling bonds, layer-dependent electronic band structure, and pronounced excitonic effects, thus making them very intriguing both fundamentally and technologically. Apart from these excellent properties, another important feature of 2D TMDs is that these materials are extremely “tunable”. For example, the physicochemical properties of TMDs can be effectively modulated by lattice defects (e.g., vacancies, dopants, grain boundaries) and external perturbations (e.g., strain, substrate effect, van der Waals heterostacks), providing rich opportunities for materials engineers to tailor TMD properties by means of doping, alloying, coupling TMDs with predesigned substrates, etc. Therefore, the research presented in my thesis mainly focuses on the synthesis of 2D semiconducting TMDs, the investigation of their intrinsic defects, and the development of effective substitutional doping and material transfer techniques to engineer their properties for functional applications. Chapter 1 provides an introduction to structures, properties, synthesis techniques, and defect engineering of 2D TMDs. In Chapter 2, two different additive-mediated chemical vapor deposition (CVD) approaches, involving sodium bromide and sodium cholate powders as growth promoters, are demonstrated. Pristine TMDs, alloyed MoxW1-xS2, and in-plane MoxW1-xS2-WxMo1-xS2 heterostructures are synthesized using our methods with improved grain size, yield, and reproducibility when compared to the conventional solid precursor CVD approach. Chapter 3 studies intrinsic defects and their distributions within CVD-synthesized TMD monolayers utilizing a combination of various microscopic and spectroscopic characterization techniques. The results indicate that 3d- and 4d-transition metal impurities (e.g., Cr, Fe, V, Mo) are often nonuniformly distributed within single-crystalline WS2 monolayers, leading to the photoluminescence inhomogeneity that is common in WS2. In addition, scanning tunneling microscopy/spectroscopy studies of CVD-grown WS2 have also unambiguously identified carbon-hydrogen (CH) complex as a common type of intrinsic defects. Chapter 4 reports an effective, convenient, and generalized method for in situ substitutional doping of 2D TMDs. This method is based on spin-coating and high-temperature chalcogenization of a mixture of water-soluble host precursor, dopant precursor, and growth promoter. Using this liquid phase precursor-assisted CVD method, we demonstrate the successful growth of Fe-doped WS2, Re-doped MoS2, and more complex structures such as V-doped in-plane MoxW1-xS2-WxMo1-xS2 heterostructures. In Chapter 5, we develop a clean and deterministic transfer method of 2D TMDs. We report a cellulose acetate-assisted method that transfers TMDs onto various substrates with improved micro- and nano-scale cleanliness. A deterministic transfer system is built up for placing a selected monolayer TMD to target locations on the substrate. The development of 2D TMD transfer techniques facilitates the investigation of their functional applications. As an example, the fabrication and ionic transport properties of monolayer MoS2 nanopore arrays are demonstrated in Chapter 5, and the correlation between ionic conductance and nanopore diameter distributions is carefully analyzed by combined experimental studies and molecular dynamic simulations. Finally, we provide a summary of main findings in this thesis and an outlook of future directions that can be pursued.