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Band alignments within 2-dimensional van der Waals heterostructures
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Teutsch, Natalie C. (2020) Band alignments within 2-dimensional van der Waals heterostructures. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3494482~S15
Abstract
Since graphene’s discovery in 2004, a new world of 2-dimensional (2D) materials has been discovered. These new materials range from metals, semi-metals, semiconductors and insulators, and reveal new physics and possibilities in the world of 2D electronics. Transition metal dichalcogenides (TMDs) are a family of materials which stand out as potential candidates for 2D device design. A subgroup within this family, MX2 layers (M for the transition metal element, X for the chalcogen element) are made up of covalently bonded MX2 sheets held together vertically by weaker van der Waals bonds. They are easy to isolate as monolayers (MLs) and have a number of interesting electronic features, including being direct bandgap semiconductors in their ML form. These MLs can be stacked into van der Waals heterostructures (HSs) to form a range of functioning 2D devices.
Here, in situ angle-resolved photoemission spectroscopy (ARPES) with electrostatic doping has been used to directly measure the electronic structure of MX2 MLs and HSs and study both carrier concentration and field dependent effects. The band structure of exfoliated ML MX2 materials have been characterised, along with the layer dependent electronic structure of WS2 and WSe2. Using this technique, the band alignments within a number of 2D HSs have been measured, including gr/MX2 and MX2/MX2 HSs.
In situ gated ARPES measurements of electrostatically gated MX2 MLs and HSs are reported. By controlling the carrier concentration within these flakes, the conduction band of these materials has been directly resolved, enabling measurement of the bandgap for WS2, WSe2, MoS2 and MoSe2. These values have been long disputed in literature, in part due to bandgap renormalisation; an effect observed here in WSe2. By controlling the carrier concentration, direct measurements of the layer dependent transition from indirect to direct bandgap were made. textitIn situ gating probed the field dependent effects of MX2 MLs by observing the gate dependent band shifts across HSs, showing that the flakes within these HSs act as capacitors in series to each other, a fact which could help future device design.
In situ ARPES was used here to study the origin and behaviour of photocurrent in 2D HSs. By measuring the current and the photon-assisted tunnelling through a functioning device, a model was formed to describe the generation and behaviour of carriers within the sample. Combining these measurements with other surface science techniques, such as Kelvin force gradient microscopy (KFGM), gave insight into the electronic behaviour within HSs and can be used to identify conductive and insulating regions, as well as study gate dependent effects and effective geometry for charge dissipation within HSs.
Combining these results demonstrates that in situ gated ARPES is a powerful technique for studying 2D materials and HSs as it effectively probes their band structure, band alignments and interlayer effects. This helps to develop our understanding of the fundamental physics behind these 2D materials and could help improve future device design.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QC Physics Q Science > QD Chemistry T Technology > TK Electrical engineering. Electronics Nuclear engineering |
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Library of Congress Subject Headings (LCSH): | Emission spectroscopy, Heterostructures, Van der Waals forces, Chalcogenides, Two-dimensional materials, Electronics -- Materials | ||||
Official Date: | January 2020 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Department of Physics | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Wilson, Neil R. | ||||
Format of File: | |||||
Extent: | xiii, 118 leaves : colour illustrations | ||||
Language: | eng |
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