The Study of Atomic Dynamics of Monolayer and Twisted 2D Materials at High-Times/Spatial Resolution

Abstract

Phase transitions, defects, and single atoms doping in 2D materials are widely studied in many research areas, such as interface chemistry, chemical synthesis, and functional applications. The 2D materials can give rise to local catalysis because they can provide large active sites. In this work, we have carried out an electron beam stimulation of atom dynamics analysis in scanning transmission electron (STEM, focus beam) mode and transmission electron microscopy (TEM, broad beam) mode. A JEOL ARM300F2, equipped with a pulse electron beam, external laser stimuli, and double aberration correctors has been used for these analyses in my research.

In Chapter 1, the physical properties and layered structure of MoS2 and WS2 are briefly introduced. The electron beam mediates the phase transition of monolayer/ twisted 2D MoS2 materials quantitatively investigated by the STEM imaging mode under irradiation of the electron beam. The TEM sample preparation for atom dynamics observation is given in Chapter 2. When identifying the phase structures of the monolayers MoS2, it is found that some metastable polymorphs often coexist in a monolayer. Interestingly, in-situ observations can record the dynamic process of phase transition and unveil the electron beam (e-beam) interactions with atoms, the corresponding results are presented in Chapter 3.

It is well-known that the secret of the single-atom Au dynamics on 2D materials is encoded in the movement of the single atoms. Studying the single-atom dynamic migration can determine the favored anchoring point of the adatom and evaluate its stability as single-atom catalysts (SACs). The trajectory of the single-doped Au atoms on MoS2 can be analyzed from snapshots recorded with the fast-recording system. Basically, the Au atoms loaded on the edge (T_s) and above the center of the hexagonal ring (V_H), show Brownian motion due to the electron beam (e-beam) irradiation. Statistically analyzed, the one loaded on top of S (T_s) is very stable and occupies 40% of the Au atoms investigated (392 in total), which means the movement trace of the single atoms largely depends on atomic bonding and the local structural environment between Au and MoS₂. The strongest interactions between the Au and S atoms make TS sites energetically favorable. The atom dynamic result studied by HRTEM is presented in Chapter 4.

The information along the z direction basically is hindered due to the projected image recorded in both broad beam (HRTEM) and focus beam (STEM) imaging modes. In Chapter 4, exit wave reconstruction based on focal series images coupled with the simulation annealing method was adopted to retrieve the information of 3D dynamics at the atomic scale. Here we used the pulse electron gun and a synchronized camera to acquire focal series images. It was noted that the pulse electron was reported to be a softer toucher to the sample, not only the dose and dose rate can be controlled by the pulse length, but also the pulse gap offers relaxation of excited phonon. In this thesis, the pulse length is set to be around 100 ns to control low-dose imaging to optimize radiation damage and signal-to-noise, and the pulse gap is set to be about 1.8 μs allowing excited phonon has enough time to relax. The radiation damage was mitigated by a low-dose acquisition (2 e-/pixel/sec) with a camera recording speed of 0.2 sec/frame in real space.

It has been reported that the 3D atomic structure can be reconstructed when a nanoparticle is oriented along the crystallographic zone axis. In this case, rather isolated atomic columns are resolved, and the moiré pattern formed by twisted MoS2 layers is visible. In my thesis, a genetic evolution method utilizing iterative simulation annealing and energy minimization (SA-EM) is to extract all spatiotemporal information encoded in the TEM data up to the limits of the counting statistics. The detail of the method is discussed in Chapter 5 and the results are also presented in Chapter 5. We can identify the z-height of the twisted layers of MoS2. After extracting z-heights, the 3D atomic structure for the twisted MoS2 was constructed. This effective method wound unravel intrinsic dynamic properties and capture more details about the sample within high-time/spatial resolution, which also shows the capability to solve the atomic structures of the other beam-sensitive soft materials.

Date of Award	30 Oct 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Fu-Rong CHEN (Supervisor) & Chunyi ZHI (Co-supervisor)