Graham, Abigail Jade (2022) Interlayer effects revealed in two-dimensional heterostructures by angle-resolved photoemission spectroscopy. PhD thesis, University of Warwick.

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Abstract

Since the isolation of the first monolayer material, graphene, in 2004, the field of two-dimensional (2D) materials has evolved significantly. Semiconductors, metals, insulators and superconductors have now all been fabricated in the monolayer form. The weak van der Waals forces between the layers allows any 2D material to be stacked upon any other 2D material, creating a wide range of systems that can be studied for new emergent quantum phenomena that depend upon the interlayer coupling and the twist angle between the layers. Twist-dependent interlayer interactions, such as moire potentials, have been shown to give rise to novel physical properties such as moire excitons and electronic bands with a at dispersion and a high density of states. These twist-dependent properties can be exploited to create new 2D twistronic devices, adding to the variety of 2D electronic, spintronic, valleytronic and optoelectronic devices that have already been created.

In this thesis, angle-resolved photoemission spectroscopy with micrometre spatial resolution (μARPES) has been used to visualise interlayer interactions in 2D van der Waals heterostructures. We observe twist-dependent interlayer Umklapp scattering in heterostructures of graphene combined with monolayer and multi-layer post transition metal chalcogenides, such as InSe and GaSe.
Additionally, we combine μARPES measurements with in-situ electrostatic gating to study the conduction band in heterobilayers of transition metal dichalcogenides, a popular family of semiconducting 2D materials with chemical composition MX2, where M is a transition metal atom and X is a chalcogen atom. We determine band gaps and band alignments for the heterobilayers; MoSe2/WSe2, WS2/MoSe2 and WS2/WSe2. At high carrier concentrations, intralayer band gap renormalisation is observed within the layer which charge is injected, leaving the heterobilayer interlayer band gap unaltered.
μARPES with in-situ electrostatic gating was used to study moire effects within the conduction band of WS2/WSe2 heterobilayers. Furthermore, we observe moire induced replica bands in graphene on WS2/WSe2, due to the moire potential in the heterobilayer. In this work, it is important to distinguish between replica bands observed in μARPES spectra due to initial state effects and those due to final state photoelectron diffraction.

Overall, we show μARPES combined with in-situ electrostatic gating to be a powerful technique in characterising 2D materials and their interlayer interactions.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	Heterostructures, Emission spectroscopy, Two-dimensional materials, Monomolecular films, Moiré method, Van der Waals forces
Official Date:	March 2022
Dates:	Date Event March 2022 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Wilson, Neil R.
Format of File:	pdf
Extent:	xiv, 138 leaves : colour illustrations
Language:	eng

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