Structural study of two-dimensional semiconductors using four-dimensional scanning transmission electron microscopy
Graphene has shown superior mechanical properties, electronic and thermal conductivity, and room temperature quantum hall effect that enables its wide application in nanoelectronics. Due to its gapless feature, other two-dimensional (2D) materials like transition metal dichalcogenides (TMDs) have dr...
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| Format: | Thesis |
| Language: | English |
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2021
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| _version_ | 1797112931302768640 |
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| author | Wen, Y |
| author2 | Warner, J |
| author_facet | Warner, J Wen, Y |
| author_sort | Wen, Y |
| collection | OXFORD |
| description | Graphene has shown superior mechanical properties, electronic and thermal conductivity, and room temperature quantum hall effect that enables its wide application in nanoelectronics. Due to its gapless feature, other two-dimensional (2D) materials like transition metal dichalcogenides (TMDs) have drawn attention and have been extensively studied for their application in optoelectronics and photonics. However, the atomic defect structures brought in during synthesis will affect these properties and thus a thorough understanding is needed. 4D-STEM is a characterization method that captures the convergent beam electron diffraction (CBED) pattern to allow post-acquisition reconstruction including orientation and strain mapping, position averaged CBED, and ptychography. This doctoral project focused on using 4D-STEM to characterize graphene, MoS2, and WS2 with atomic defective structures by producing a series of maps of the local electronic environment with atomic resolution.
Monolayer MoS2 and WS2 were first investigated to form the basis of this project. The projected electric field map was first extracted since it is linearly proportional to the centre of mass shift of the CBED pattern caused by the atomic electric field, which can then be used to calculate the charge density map and electrostatic potential. Simple defective structures were subsequently investigated, leading to a detailed study of the enhanced electric field on TMD zigzag edges and MoS nanowire. Next, more complex structures, bilayer and trilayer twisted graphene were investigated. The electric field map showed that the bilayer with a large twist angle of 28.4° has decoupled based on comparison between the independent atom model (IAM) simulation and experimental results. A study on a backfolded trilayer further showed that, even though the top and bottom layer have a small twist angle of 3°, they show no strong interlayer coupling in the electric field due to a layer with large twist angle in between, though the observation may be obscured by the insufficient sensitivity of the current technique.
Post-acquisition reconstruction modes of 4D-STEM were also investigated and compared. Results showed that the annular bright field (ABF), ptychographic phase, and divergence of differential phase contrast (DPC) can identify low and high atomic number atoms simultaneously with high resolution, surpassing the annular dark field (ADF)-STEM mode under the same imaging conditions. Series of reconstructed low angle ADF (LAADF)-STEM images from confined angular range showed potential in locating and identifying surface adsorbed atoms due to their different intensity profile as a function of reconstruction angular range, and combined with simulation, it will be possible to achieve atomic resolution elemental mapping after data acquisition. |
| first_indexed | 2024-03-07T07:41:09Z |
| format | Thesis |
| id | oxford-uuid:908de875-85b4-41e7-90cf-16f68ab28b8b |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-04-09T03:56:10Z |
| publishDate | 2021 |
| record_format | dspace |
| spelling | oxford-uuid:908de875-85b4-41e7-90cf-16f68ab28b8b2024-03-18T08:03:39ZStructural study of two-dimensional semiconductors using four-dimensional scanning transmission electron microscopyThesishttp://purl.org/coar/resource_type/c_db06uuid:908de875-85b4-41e7-90cf-16f68ab28b8bmaterialsEnglishHyrax Deposit2021Wen, YWarner, JGraphene has shown superior mechanical properties, electronic and thermal conductivity, and room temperature quantum hall effect that enables its wide application in nanoelectronics. Due to its gapless feature, other two-dimensional (2D) materials like transition metal dichalcogenides (TMDs) have drawn attention and have been extensively studied for their application in optoelectronics and photonics. However, the atomic defect structures brought in during synthesis will affect these properties and thus a thorough understanding is needed. 4D-STEM is a characterization method that captures the convergent beam electron diffraction (CBED) pattern to allow post-acquisition reconstruction including orientation and strain mapping, position averaged CBED, and ptychography. This doctoral project focused on using 4D-STEM to characterize graphene, MoS2, and WS2 with atomic defective structures by producing a series of maps of the local electronic environment with atomic resolution. Monolayer MoS2 and WS2 were first investigated to form the basis of this project. The projected electric field map was first extracted since it is linearly proportional to the centre of mass shift of the CBED pattern caused by the atomic electric field, which can then be used to calculate the charge density map and electrostatic potential. Simple defective structures were subsequently investigated, leading to a detailed study of the enhanced electric field on TMD zigzag edges and MoS nanowire. Next, more complex structures, bilayer and trilayer twisted graphene were investigated. The electric field map showed that the bilayer with a large twist angle of 28.4° has decoupled based on comparison between the independent atom model (IAM) simulation and experimental results. A study on a backfolded trilayer further showed that, even though the top and bottom layer have a small twist angle of 3°, they show no strong interlayer coupling in the electric field due to a layer with large twist angle in between, though the observation may be obscured by the insufficient sensitivity of the current technique. Post-acquisition reconstruction modes of 4D-STEM were also investigated and compared. Results showed that the annular bright field (ABF), ptychographic phase, and divergence of differential phase contrast (DPC) can identify low and high atomic number atoms simultaneously with high resolution, surpassing the annular dark field (ADF)-STEM mode under the same imaging conditions. Series of reconstructed low angle ADF (LAADF)-STEM images from confined angular range showed potential in locating and identifying surface adsorbed atoms due to their different intensity profile as a function of reconstruction angular range, and combined with simulation, it will be possible to achieve atomic resolution elemental mapping after data acquisition. |
| spellingShingle | materials Wen, Y Structural study of two-dimensional semiconductors using four-dimensional scanning transmission electron microscopy |
| title | Structural study of two-dimensional semiconductors using four-dimensional scanning transmission electron microscopy |
| title_full | Structural study of two-dimensional semiconductors using four-dimensional scanning transmission electron microscopy |
| title_fullStr | Structural study of two-dimensional semiconductors using four-dimensional scanning transmission electron microscopy |
| title_full_unstemmed | Structural study of two-dimensional semiconductors using four-dimensional scanning transmission electron microscopy |
| title_short | Structural study of two-dimensional semiconductors using four-dimensional scanning transmission electron microscopy |
| title_sort | structural study of two dimensional semiconductors using four dimensional scanning transmission electron microscopy |
| topic | materials |
| work_keys_str_mv | AT weny structuralstudyoftwodimensionalsemiconductorsusingfourdimensionalscanningtransmissionelectronmicroscopy |