Probing the exfoliation mechanism of 2D materials by X-ray scattering techniques

Abstract

The exceptional properties of two-dimensional (2D) materials have garnered significant attention from both industry and academia. However, while much research has focused on their synthesis and performance, the underlying exfoliation mechanisms remain insufficiently understood. While common nanomaterial characterization techniques such as Raman microscopy and electron microscopy can provide structural and bonding information of 2D materials, their application for in-situ measurements is limited. In light of these limitations, we sought to explore the use of synchrotron X-ray scattering for in-situ characterization of exfoliation processes, with a particular focus on the potential of both X-ray diffraction (XRD) and X-ray pair distribution function (XPDF). We used XRD to investigate the thermal exfoliation process of hexagonal boron nitride (h-BN), observing a linear thermal expansion in the c-axis direction consistent with previous findings, but also a contraction in the a-axis direction at the beginning of the heating process. We identified a "trigger point" in the cooling process based on a retardation in the integrated intensity recovery rate, which we interpret as the starting point for flakes peeling off. To further analyse the structural transformation, we developed a characterization parameter based on Bragg peak intensities, which we used to construct a characterization map based on particle size and temperature. To probe the details of atomic bond lengths during liquid-phase exfoliation, we investigated the potential of XPDF. By using high-energy X-ray beams, we were able to measure total scattering data, from which we derived XPDF results in real space. Our experiments demonstrate the feasibility of using XPDF to investigate the rippled carbon hexagon structure of graphite, graphene in solvent, and dried graphene flakes. Overall, our study highlights the potential of synchrotron X-ray scattering for in-situ characterization of exfoliation processes in 2D materials, offering insights into the underlying mechanisms and aiding the development of optimized synthesis methods.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Brian Derby (Supervisor), Wajira Mirihanage (Supervisor) & Cinzia Casiraghi (Supervisor)