Transmission Electron Microscopy Imaging Alkali Ion (De)intercalation in van der Waals Crystals

Abstract

Layered van der Waals crystals have been extensively studied because of their attractive physical and chemical properties. Alkali ion intercalation as a unique avenue for tuning the fundamental properties of these layered materials is a rapidly expanding field of research to develop the novel materials applied in the next generation nanoelectronics, catalyst, and energy storage device. Understanding the atomic structures of the intercalated materials and the (de)intercalation process are prerequisites to produce the desirable materials with expected properties. In-situ scanning/transmission electron microscopy (S/TEM) allows researchers to dynamically observe structural and chemical variations of the specimen under external stimuli at very high resolution, providing an ideal platform to study intercalation process and intercalated materials. This thesis details the application of in-situ S/TEM combined with energy dispersive X-ray spectroscopy (EDS) spectral imaging and four-dimensional (4D) STEM to directly image the structural evolution and chemical variation within the intercalated van der Waals crystal under heating and biasing. The first work discusses structural and chemical variations of the Kdoped MoS2 flake heated at different temperatures using in-situ TEM. Studying the dynamics of the K deintercalation in a van der Waals crystal is applied in order to understand the intercalation process and the accompanying phase and compositional changes. Selected area electron diffraction (SAED) confirms the change of local K ordering supported by structural modelling and diffraction simulation, and STEM-EDS suggests K deintercalation in the flake follows first-order kinetics. The second work introduces the 4DSTEM imaging technique and the principle of the 4DSTEM dataset processing methods through which the superstructures appearing in the K-intercalated MoS2 were first characterized at the nanoscale, supporting the hypothesis purposed in the first work. The third work provides insight into Eu intercalation into bilayer graphene. The intercalation process was imaged by STEM in real time and we observed the morphologic changes of the suspended bilayer graphene membrane induced by intercalation. Both intercalation of Eu and ionic liquid cations have been achieved at low applied bias (-4V) and high applied bias (-8V) respectively, as evidenced using SAED and STEM-EDS.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Irina Grigorieva (Supervisor) & Sarah Haigh (Supervisor)