Effects of stacking and twisting on the optical and electronic properties of transition metal dichalcogenide bilayers
File(s)
Author(s)
Scharnke, Miriam
Type
Thesis
Abstract
Moiré lattices formed by bilayers of atomically thin semiconductors with relative twist angle or
lattice mismatch host a plethora of novel electronic and optical phenomena. The Bethe–Salpeter
equation has been used very successfully to describe optical properties of semiconductors, however,
the unit cells of these moiré systems can be very large, including hundreds of atoms. The
resulting Bethe–Salpeter basis size is not currently computationally accessible, which means
that standard ab initio codes can not yet be used. Using tight-binding instead of DFT as the
basis for the Bethe–Salpeter Hamiltonian significantly reduces basis size.
In this work, I use a sophisticated tight-binding model in order to calculate electronic structure
and optical properties of MoS2 monolayers and homobilayers with different high-symmetry
stackings and stackings with relative twist angles of 3.89◦-21.79◦.
I first generalize a tight-binding model with eleven bands per spin and per monolayer unit
cell first presented by Fang et al. to include up to sixth-nearest neighbour intralayer hopping.
I then use this tight-binding model to calculate band structures of MoS2 monolayers,
and twisted and untwisted bilayers. Finally, I compute exciton binding energies and absorption
spectra of these systems in vacuum by using the tight-binding eigenenergies and wavefunctions
to set up and solve an effective Bethe–Salpeter Hamiltonian, using the Rytova–Keldysh
interaction to describe static dielectric screening in atomically thin semiconductors.
lattice mismatch host a plethora of novel electronic and optical phenomena. The Bethe–Salpeter
equation has been used very successfully to describe optical properties of semiconductors, however,
the unit cells of these moiré systems can be very large, including hundreds of atoms. The
resulting Bethe–Salpeter basis size is not currently computationally accessible, which means
that standard ab initio codes can not yet be used. Using tight-binding instead of DFT as the
basis for the Bethe–Salpeter Hamiltonian significantly reduces basis size.
In this work, I use a sophisticated tight-binding model in order to calculate electronic structure
and optical properties of MoS2 monolayers and homobilayers with different high-symmetry
stackings and stackings with relative twist angles of 3.89◦-21.79◦.
I first generalize a tight-binding model with eleven bands per spin and per monolayer unit
cell first presented by Fang et al. to include up to sixth-nearest neighbour intralayer hopping.
I then use this tight-binding model to calculate band structures of MoS2 monolayers,
and twisted and untwisted bilayers. Finally, I compute exciton binding energies and absorption
spectra of these systems in vacuum by using the tight-binding eigenenergies and wavefunctions
to set up and solve an effective Bethe–Salpeter Hamiltonian, using the Rytova–Keldysh
interaction to describe static dielectric screening in atomically thin semiconductors.
Version
Open Access
Date Issued
2023-04
Date Awarded
2024-03
Copyright Statement
Creative Commons Attribution Licence
License URL
Advisor
Lischner, Johannes
Horsfield, Andrew
Sponsor
Royal Society (Great Britain)
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)