Nanoscale Investigations into the Structural and Electromagnetic Properties of Novel Two-Dimensional Materials

Abstract

The field of research into two-dimensional (2D) materials is rapidly expanding into areas of wide functional significance, such as optoelectronics and spintronics, where they promise to serve as a basis for future electronic devices. The functional properties of a 2D material are intrinsically linked to its structural properties and the local atomic structure. Scanning transmission electron microscopy (STEM) provides a key method for the imaging of 2D materials, enabling the direct analysis of structural features down to atomic resolution, and can additionally be validated through computational methods and simulation. This thesis details the application of STEM to novel 2D materials using both standard high resolution imaging techniques, supported by structural modelling and image simulation, and state-of-the-art differential phase contrast (DPC) STEM to directly image the structural and internal electromagnetic field states within a specimen. The first work investigates the presence and behaviour of defects in atomically thin post transition metal chalchogenides, gallium selenide (GaSe) and indium selenide (InSe), protected with graphene encapsulation. Point defects, identified via image simulation, were observed to form and heal under electron irradiation, and the extended defect structure observed to differ between the two materials. The second work details the first atomic resolution imaging and defect analysis of few layer thick chromium tribromide (CrBr3), a ferromagnetic 2D material, identifying both faceted edge formation and local monoclinic stacking, the latter confirmed through structural modelling and image simulation. The third work provides insights into the magnetic domain structure in cross-sectional lamella of the 2D ferromagnet, triiron germanide ditelluride (Fe3GeTe2), using DPC-STEM. Using cryogenic conditions, the stripe domains were observed to possess Bloch type domain walls, becoming mixed Bloch-Néel type in regions below 50 nm lamella thickness, with local loss of ferromagnetism observed over time associated with surface oxidation and crystal damage.

Date of Award	31 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Sarah Haigh (Supervisor) & David Lewis (Supervisor)