Structure and phase engineering of 2D transition metal chalcogenides

• Main Author: Tang, Bijun
• Other Authors: Liu Zheng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials::Microelectronics and semiconductor materials
• Online Access: https://hdl.handle.net/10356/150580
• Institution: Nanyang Technological University

As an indispensable member of the two-dimensional (2D) family as well as a perfect complement to graphene, 2D transition metal chalcogenides (TMCs) have long been the center of research attention, attributed to their large spectrum of fascinating properties along with the various crystal structures. In parallel with the discovery of new candidate materials and exploration of their unique characteristics, engineering the 2D TMCs into designated structures and phases are also of great importance for meeting the requirement for different applications. This thesis focuses on the structure and phase engineering of 2D TMCs to tune their physicochemical properties. Four strategies are adopted to tune the properties of 2D TMCs by fabricating them into desired structures, architectures, or defined phases: construction of heterostructure, alloying, phase-selective growth, and dimension tuning. Moreover, this thesis also describes and validates the feasibility and potential of introducing ML to guide the synthesis and engineering of 2D TMCs. In the first project, through the construction of MoS2-WS2 lateral heterostructures, semiconductor p-n junctions are successfully obtained, which are essential building blocks for modern electronic and optoelectronic devices. Moreover, the morphology of heterostructures can be engineered by fine-tuning the synthesis conditions. WS2 quantum well is also identified in the synthesized heterostructures, providing opportunities for studying novel optical properties and quantum confinement effects. In the second project, monolayer WTe2xS2(1-x) alloys with tunable chemical compositions and phases are fabricated using a carefully designed one-step CVD method. By controlling the synthesis condition, both semiconducting 1H and metallic 1T´ phase 2D WTe2xS2(1-x) alloys are obtained. Bandgap engineering of WTe2xS2(1-x) alloys in the 1H phase is achieved as well. Moreover, the generalizability of the proposed approach in preparing phase tunable TMCs alloys, is demonstrated for the growth of 2D WTe2xSe2(1-x) alloys. In the third project, the strategy of phase-selective growth is applied to the study of 2D Cr5Te8. Phase-tunable growth of 2D ferromagnetic Cr5Te8 is achieved via a facile CVD route. By fine-tuning the synthesis condition, both trigonal and monoclinic phase Cr5Te8 down to a few nanometers are synthesized for the first time and their ferromagnetic properties are respectively examined. Compared with the trigonal phase, monoclinic Cr5Te8 possesses a higher Curie temperature and coercivity field. Phase-dependent characteristics that existed in many 2D TMCs make phase-selective growth more useful. In the last project, the feasibility and capability of ML techniques to guide the synthesis and engineering of 2D TMCs are demonstrated. ML-guided synthesis and dimension tuning of few-layer 1T´ WTe2 are realized. An ML model with a high AUROC of 0.93 is established, to optimize the CVD synthesis conditions of few-layer 1T´ WTe2. Feature importance extracted from the model further reveals that source ratio plays a dominating role in governing the morphology of the synthesized WTe2 flakes. WTe2 nanoribbons are eventually obtained. This work suggests that ML is a powerful and efficient approach to guide the synthesis and dimension tuning of 2D materials, opening up new opportunities for boosting the diversified nanostructures derived from the 2D TMCs family.