Atomic study of defects in two-dimensional transition metal chalcogenides using electron microscopy

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are layered semiconductors with unique electronic and optical properties which have shown immense potential in ultrathin (opto-)electronic devices. Structural defects that are ubiquitous in 2D materials have demonstrated to exert significant impacts on the materials' properties. The in-depth comprehension of structural defects at the atomic level is vital for the rational exploration of their exceptional properties, which is the objective of my DPhil project. By conducting advanced scanning transmission electron microscopy (STEM), this project resolves the structures and dynamics of defects with the atomic-scale clarity in 2D TMDs of different phases, and the use of in-situ heating holder in the STEM enables the direct visualization structural dynamics at elevated temperatures. The first part focuses on the grain boundary (GB)-involved defective structures in the typical 2H-phase MoS₂/WS₂ monolayers/bilayers at the thermal condition. For the monolayers, the high-temperature mechanism is elucidated for the dynamics of 60° GB-coupled large inversion domains. In the bilayers, the existence of dual localized GBs is demonstrated with high-temperature stability. This co-localization causes one layer to adopt novel dislocation cores, not found in monolayers, due to van der Waals strain of maintaining 2H and 3R interlayer stacking either side. Then, the defects and GB structures in TMDs of another significant phase, 1T phase, are also investigated systematically, taking the emerging1T-PtSe₂ as the study object. The atomic structures and dynamics of point vacancies, 1D defects, GBs and dislocations exhibit distinct "1T-feature" that differs from those found in 2H-phase MoS₂/WS₂. Apart from imaging the intrinsic defects, this project also explores the controlled in-situ structural modification methods by manipulating focused electron beam and the heating conditions in STEM with atomic-resolution monitoring of the process. The precise controllability is demonstrated in intentionally patterning 2D nanowell arrays at bilayer WS₂ in terms of shapes, depth and locations, using the combination of convergent beam and the high temperature. Besides, the phase transformation is triggered by the controlled in-situ heating process for promoting Se loss from few-layered PdSe₂, obtaining the novel phase 2D Pd₂Se₃ monolayers. The structural defects and their beam-driven behaviours in Pd2Se3 are visualized, which are sharply distinguished from those in the conventional TMDs.