Hall, Joshua ORCID: 0000-0002-7009-5602 (2019). Controlling Growth and Electronic Properties of Transition Metal Disulphide Layers via Molecular Beam Epitaxy. PhD thesis, Universität zu Köln.

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Transition metal dichalcogenides are an emergent class of layered materials with a broad range of properties that make them interesting for fundamental studies and applications ranging from nanoelectronics, to optoelectronics and spintronics. The weak van der Waals interaction present between successive sheets allows isolation of single layers of the material and grants access to their thickness-dependent properties. The incompatibility of sulphur powder with ultra-high vacuum conditions creates a lack of in situ growth methods, which produce single to few-layer samples of sulphur based transition metal dichalcogenides that grow epitaxially and maintain their intrinsic, freestanding properties. Investigations have been conducted either on ex situ grown samples – those are inevitably exposed to ambient conditions – or on layers that are supported on metallic substrates – which alter the pristine properties of the materials through doping and hybridisation. Here, we introduce a two-step molecular beam epitaxy based synthesis, where we supply elemental sulphur by thermal decomposition of FeS2, a solid-state compound which is compatible with use in ultra-high vacuum. As a substrate, we use high-quality graphene on Ir(111), which only interacts via weak van der Waals forces. Using the example of semiconducting 2H-MoS2, we demonstrate that the transition metal disulphide layers remain in their pristine state. A large band gap of (2.53 ± 0.08) eV and the ability to move flakes with the scanning tunnelling microscope tip both document the weak interaction of 2H-MoS2 with its substrate. This is corroborated by angle-resolved photoemission spectroscopy displaying the absence of hybridisation with the substrate. Further, Raman spectroscopy indicates independent thermal expansion, and finally, photoluminescence is observed despite the metallic substrate. Since growth studies on transition metal disulphides are rare, we use the potential of molecular beam epitaxy to systematically study the growth process. Using scanning tunnelling microscopy and low-energy electron diffraction we investigate the influence of the single synthesis parameters and provide insight into the growth and annealing mechanisms. We further study the effect of repeated growth cycles, which we find to facilitate keeping epitaxial alignment at high coverages. Finally, the detrimental effect of inadvertent sulphur intercalation underneath graphene on the growth quality of 2H-MoS2 is considered. The method also works for the synthesis of other transition metal sulphides, and in the following part of the thesis, we investigate layers of metallic V1+xS2. Since the system vanadium-sulphur has many stable phase equilibria, the structural scanning tunnelling microscopy investigations reveal three distinct single-layer phases and multilayer phases. The single-layer phases include stoichiometric 1T-VS2 which neither in scanning tunnelling microscopy shows signs of a charge density wave nor of magnetism in circular x-ray dichroism. By thermal annealing, we induce irreversible transformations into two phases with increasing sulphur depletion. The first phase develops upon annealing to 600 K and shows an ordered superstructure, which we interpret as an array of sulphur vacancies. The second sets in when the sample is annealed to temperatures above 800 K and shows a reconstructed surface with a 3x1 superstructure. Atomic models describing the arrangement of the top sulphur layer are given for both phases. We introduce ways to tune the growth to multilayer morphologies of VS2, on which two superstructures are found. The superstructures spatially exclude themselves and by comparing the appearance of the occupied and unoccupied states in scanning tunnelling microscopy we resolve their origin. The 2x2 superstructure is ascribed to self-intercalation of V atoms in the van der Waals gap, which cause local stoichiometry V1+xS2. The [sqrt(3) x sqrt(3)]R30� superstructure shows clear indications of a charge density wave present at room temperature. Finally, we investigate the charge density wave phase of 2H-TaS2. For the pristine monolayer, scanning tunnelling microscopy shows a periodicity close to 3x3 and scanning tunnelling spectroscopy determines the partial charge density wave energy gap to be (32 ± 9) meV. Quasiparticle interference patterns and angle-resolved photoemission spectroscopy in tandem determine parts of the occupied and unoccupied band structure and make a precise tight-binding fit to the experimental data possible. The absence of hybridisation is documented and a carrier concentration of (1.10 ± 0.02) electrons per unit cell is determined. Exposure to Li vapour causes a [sqrt(7) x sqrt(7)]R19.1� adatom superstructure, presumably accompanied by intercalation. Removing the adatoms with the STM tip reveals a 2x2 superstructure, which together with a reduced energy gap of (18 ± 9) meV implies the presence of a 2x2 charge density wave in n doped 2H-TaS2. Bilayer 2H-TaS2 also shows a 2x2 charge density wave which, however, is of poor order compared to the monolayer case. A theoretical analysis based on density functional theory, density functional perturbation theory and many-body perturbation theory provides insight into phonon renormalisation as a function of doping and hybridisation with the substrate. This enables us to present a phase diagram of the charge density wave as a function of these parameters and to infer how they affect lattice dynamics and stability. Our theoretical considerations are consistent with the experimental work presented and shed light on previous experimental and theoretical investigations of related systems.

Item Type: Thesis (PhD thesis)
CreatorsEmailORCIDORCID Put Code
Hall, Joshuajoshua.hall@posteo.deorcid.org/0000-0002-7009-5602UNSPECIFIED
URN: urn:nbn:de:hbz:38-105231
Date: 2019
Language: English
Faculty: Faculty of Mathematics and Natural Sciences
Divisions: Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics II
Subjects: Physics
Uncontrolled Keywords:
Molecular Beam EpitaxyEnglish
Transition Metal DisulphidesEnglish
Electronic PropertiesEnglish
Date of oral exam: 18 September 2019
NameAcademic Title
Michely, ThomasProf. Dr.
Bürgler, DanielPD Dr.
Refereed: Yes
URI: http://kups.ub.uni-koeln.de/id/eprint/10523


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