Atomic structure and dynamics of defects in transition metal dichalcogenide bilayers
<p>Transition metal dichalcogenides (TMDs), such as MoS<sub>2</sub> and WS<sub>2</sub>, are direct band gap semiconductors in their monolayer form and indirect band gap semiconductors in bulk. They offer band gaps in the red visible spectrum and semiconducting propertie...
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| Format: | Thesis |
| Language: | English |
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2019
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| _version_ | 1797076833487814656 |
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| author | Zhou, S |
| author2 | Warner, JH |
| author_facet | Warner, JH Zhou, S |
| author_sort | Zhou, S |
| collection | OXFORD |
| description | <p>Transition metal dichalcogenides (TMDs), such as MoS<sub>2</sub> and WS<sub>2</sub>, are direct band gap semiconductors in their monolayer form and indirect band gap semiconductors in bulk. They offer band gaps in the red visible spectrum and semiconducting properties that expand the application of the two-dimensional (2D) materials beyond what graphene alone can achieve. These layered materials are stacked through weak interlayer van der Waals interactions. Their bilayer structures offer an extra layer degree of freedom, which introduces performance enhancements compared to their monolayer counterpart. Interaction between MoS<sub>2</sub> or WS<sub>2</sub> layers as well as the specific stacking arrangement can significantly modify their electrical, optical and vibrational properties. The presence of defective structures has been proved to exert significant influences on TMD monolayers. Understanding the atomic structures and the dynamics of these defects in layered materials and their differences from those in monolayers is important for revealing their impact on TMD bilayers, leading approaches for their future device application.</p>
<p>Aberration-corrected transmission electron microscopy (AC-TEM) is one of the leading approaches to study the atomic structure of defects on 2D materials, laying the foundation for the first experiment, which involves the production and aggregation as well as observation of vacancies and line defects in differently stacked MoS<sub>2</sub> bilayers at atomic-level resolution. DFT calculated models prove the main discrepancy between defective monolayer and bilayer systems, which is assigned to the interlayer van der Waals force in the bilayer system. In the next experiment, we move forward to probe the atomic structure of a more complex defective structure, the anti-phase grain boundary in MoS<sub>2</sub> bilayers using annular dark field scanning transmission electron microscopy (ADF-STEM). Our results show the anti-phase GB in thebottom layer leads to the coexistence of both 2H and 3R stacking in the same bilayer domain, which is attributed to the lattice compression in the dislocation core due to the interlayer van der Waals force in the bilayer system. In the next experiment, the use of in-situ heating holder in the ADF-STEM makes the direct visualization of atomic structures and dynamics at elevated temperatures possible. Surface ripples, which are also denoted as stacking boundaries due to their morphologies in bilayer systems are analyzed at both micrometer and nanometer levels. These surface ripples introduce two perfect stacking orders with nanometer-wide transition regions, the morphology of which is decided by the interlayer van der Waals forces. The reversibility of these structures under temperature perturbation and beam irradiation is also studied.</p> |
| first_indexed | 2024-03-07T00:09:23Z |
| format | Thesis |
| id | oxford-uuid:78af15ba-f88a-42a0-b066-f9d4dd478c62 |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-03-07T00:09:23Z |
| publishDate | 2019 |
| record_format | dspace |
| spelling | oxford-uuid:78af15ba-f88a-42a0-b066-f9d4dd478c622022-03-26T20:32:22ZAtomic structure and dynamics of defects in transition metal dichalcogenide bilayersThesishttp://purl.org/coar/resource_type/c_db06uuid:78af15ba-f88a-42a0-b066-f9d4dd478c62STEMTEM2D materialsTMDsEnglishHyrax Deposit2019Zhou, SWarner, JH<p>Transition metal dichalcogenides (TMDs), such as MoS<sub>2</sub> and WS<sub>2</sub>, are direct band gap semiconductors in their monolayer form and indirect band gap semiconductors in bulk. They offer band gaps in the red visible spectrum and semiconducting properties that expand the application of the two-dimensional (2D) materials beyond what graphene alone can achieve. These layered materials are stacked through weak interlayer van der Waals interactions. Their bilayer structures offer an extra layer degree of freedom, which introduces performance enhancements compared to their monolayer counterpart. Interaction between MoS<sub>2</sub> or WS<sub>2</sub> layers as well as the specific stacking arrangement can significantly modify their electrical, optical and vibrational properties. The presence of defective structures has been proved to exert significant influences on TMD monolayers. Understanding the atomic structures and the dynamics of these defects in layered materials and their differences from those in monolayers is important for revealing their impact on TMD bilayers, leading approaches for their future device application.</p> <p>Aberration-corrected transmission electron microscopy (AC-TEM) is one of the leading approaches to study the atomic structure of defects on 2D materials, laying the foundation for the first experiment, which involves the production and aggregation as well as observation of vacancies and line defects in differently stacked MoS<sub>2</sub> bilayers at atomic-level resolution. DFT calculated models prove the main discrepancy between defective monolayer and bilayer systems, which is assigned to the interlayer van der Waals force in the bilayer system. In the next experiment, we move forward to probe the atomic structure of a more complex defective structure, the anti-phase grain boundary in MoS<sub>2</sub> bilayers using annular dark field scanning transmission electron microscopy (ADF-STEM). Our results show the anti-phase GB in thebottom layer leads to the coexistence of both 2H and 3R stacking in the same bilayer domain, which is attributed to the lattice compression in the dislocation core due to the interlayer van der Waals force in the bilayer system. In the next experiment, the use of in-situ heating holder in the ADF-STEM makes the direct visualization of atomic structures and dynamics at elevated temperatures possible. Surface ripples, which are also denoted as stacking boundaries due to their morphologies in bilayer systems are analyzed at both micrometer and nanometer levels. These surface ripples introduce two perfect stacking orders with nanometer-wide transition regions, the morphology of which is decided by the interlayer van der Waals forces. The reversibility of these structures under temperature perturbation and beam irradiation is also studied.</p> |
| spellingShingle | STEM TEM 2D materials TMDs Zhou, S Atomic structure and dynamics of defects in transition metal dichalcogenide bilayers |
| title | Atomic structure and dynamics of defects in transition metal dichalcogenide bilayers |
| title_full | Atomic structure and dynamics of defects in transition metal dichalcogenide bilayers |
| title_fullStr | Atomic structure and dynamics of defects in transition metal dichalcogenide bilayers |
| title_full_unstemmed | Atomic structure and dynamics of defects in transition metal dichalcogenide bilayers |
| title_short | Atomic structure and dynamics of defects in transition metal dichalcogenide bilayers |
| title_sort | atomic structure and dynamics of defects in transition metal dichalcogenide bilayers |
| topic | STEM TEM 2D materials TMDs |
| work_keys_str_mv | AT zhous atomicstructureanddynamicsofdefectsintransitionmetaldichalcogenidebilayers |