| Author: | Chen, Changsheng |
| Title: | Quantitative electron diffraction and spectroscopy and their application on polymers and 2D materials |
| Advisors: | Zhu, Ye (AP) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Electron spectroscopy Electrons -- Diffraction Crystallization Polymers Two-dimensional materials Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xvi, 135 pages : color illustrations |
| Language: | English |
| Abstract: | The determination of structures and phases in functional materials is crucial for understanding and optimizing their properties. However, traditional direct-imaging electron microscopy techniques often fall short in identifying structural orderings and phases of dose-sensitive materials at the nanoscale, such as some semicrystalline polymers and emerging 2D materials. Their intrinsic structure can be rapidly damaged under electron beam irradiation, like a few e/Å2 for polymers and hundreds of e/Å2 for some 2D materials, and can only be identified on certain zone axes. Within the developing low-dose methods, four-dimensional scanning transmission electron microscope (4D-STEM) has demonstrated the potential to unveil the hierarchical structures and spatial distribution of degree of crystallinity in polymers. Additionally, thanks to single-electron direct detection and low-sampling conditions, electron energy loss spectroscopy (EELS), which contains rich and strong signals within the energy range of valence electron, becomes another dose-efficient avenue for phase identification at the nanoscale and on various zone axes. Furthermore, quantitative analysis of electron diffraction and EEL spectra is implemented to fully reveal nuanced differences in these data and elucidate underlying reasons. In this work, we first investigated the electron beam effects on spherulite thin films made of polyethylene (PE), polycaprolactone (PCL), and poly(3-hexylthiophene) (P3HT) at room temperature and cryogenic condition for identification of polymeric structures and phases with minimized artifacts. The beam effects on structure and chemical bonding evolution are demonstrated by quantitative electron diffraction and spectroscopy. Thus, both amorphization and mass loss from electron beam irradiation have been clearly identified, most of which can be suppressed effectively by cryoprotection. Moreover, the different degradation paths are revealed on PE/PCL and on P3HT which shows an individual two-stage damage process at cryogenic temperature. Furthermore, the methodology employed in this study also establishes a robust framework for quantitatively analyzing the crystalline and amorphous components paving a path for the spatial distribution of degree of crystallinity in polymers. With knowledge of polymer damage under electron beams, we optimized the acquisition conditions of 4D-STEM technique on PE and PCL spherulite thin films. Our quantitative algorithm for 4D electron diffraction allowed us to examine the microstructures of these films, including their circular symmetry and degree of crystallinity distribution. In PE, we observed a growth direction along [1-10], and a twisting motion approximately around the crystallographic b axis in the lamellae within the spherulites. Besides, its orientation distribution exhibited continuous circular symmetry. Similarly, PCL also displays a growth direction along the [1-10] in the radial direction. However, PCL does not exhibit a twist in the lamellae. Instead, an inhomogeneous crystallinity pattern is observed in the tangent direction, forming a radial arrangement of crystalline and amorphous regions. Furthermore, an increase of crystallinity from spherulite cores to peripheries is revealed in both PE and PCL. Hence, our developed technology using the 4D-STEM system showcases great potential in uncovering local information, but additional research is necessary to fully exploit its capabilities. For another dose-sensitive material, namely the 2D In2Se3, its second phase can form down to a single quintuple layer (~ 1.0 nm) thus requiring high-resolution characterization. However, the crystal structures of the various phases show only subtle differences on specific zone axes, and defects tend to occur more frequently under high electron dose irradiation of conventional STEM imaging. As a result, these factors pose significant obstacles to overcome in terms of quantitative analysis and TEM techniques. In this case, the 4D-STEM technique becomes ineffective owing to the large overlap of diffraction disks from a small probe. Fortunately, EELS can identify the phase of 𝛼/𝛽′ In2Se3 on both 2D plane and cross-section with a slightly lower dose than STEM imaging. The phase identification using quantitative EELS has a resolution of approximately 1.1 nm. Besides, the in-situ results of EELS and XRD not only confirm the robustness of our observations but also reveal two factors that contribute to the difference in plasmon energy of EELS between the 𝛼/𝛽′phases: unit cell volumes in a single quintuple layer and equivalent valence electrons. Finally, our findings are further supported by first-principles calculations and may have broader implications for other chalcogenides and transition metal materials. |
| Rights: | All rights reserved |
| Access: | open access |
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