The Application of Scanning Precession Electron Diffraction to 2D Materials Microstructure

Abstract

Owing to their unique single or few-atom thick sheet-like structure, two dimensional (2D) materials provide a plethora of opportunities for electronic device applications. However, during the preparation and transfer steps of device fabrication, 2D materials often undergo mechanical stress that can induce structural defects either within the crystal or at the interfaces between the different crystals, via strain or surface effects. Ripples, folds, bends, and cracks can form within the crystal and surface contamination or bubbles can form between layers, affecting the electronic behaviour. Transmission electron microscopy (TEM) provides detailed information on materials at imaging resolutions from tens of microns down to sub-angstrom through the use of a high energy electron probe. Within a periodic structure, such as a crystal, the probe elec-trons get elastically scattered when the Bragg condition is satisfied for a given atomic spacing-a process known as diffraction, and the diffracted electrons will then converge to form a diffraction pattern. Through TEM, both crystallographic information and an image of the morphology of a specific area can be obtained. Uneven topography may cause unexpected dynamical diffraction which will be present in the diffraction pattern and can complicate image interpretation. Precession electron diffraction (PED) allows the effect of dynamical diffraction to be reduced to obtain pseudo-kinematical datasets. This technique is popular for structure solutions in some ceramic and metal materials, but its application has not yet been revealed for the investigation of structural and morphological information for 2D materials. In this project, scanning precession electron diffraction (SPED) is used to obtain the crystal structure information for 2D InSe and the results are compared to zone-axis diffraction data for the same region of interest. Multislice simulation is then used to interpret the results as well as to assess the wider feasibility of the application of scanning precession electron diffraction to obtain structural and morphological in-formation in 2D materials. It is demonstrated through simulation and experimental data analysis that SPED reduces local artefacts arising from a range of factors including tilt, disorder, displacements etc. As a result, the intensities of diffraction spots in the same ring tend to be more similar with a smaller offset value of incident beam (equivalent to the sample tilt) or the larger precession angle, resulting from the more kinematical nature of the SPED datasets. Based on the diffraction pattern the sample is identified as bilayer Gamma-InSe with some trilayer Gamma-InSe areas. A comparison of experimental data and simulation reveals a clearer structure interpretation is obtained through SPED than with zone axis diffraction. The results suggest that the SPED technique can be used to enhance structure interpretation even in 2D materials, where the dynamical diffraction is relatively low compared to thicker specimens.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Roman Gorbachev (Supervisor) & Sarah Haigh (Supervisor)