Universität zu Köln

van Efferen, Camiel ORCID: 0000-0002-6237-8602 (2023). Growth, phases and correlation effects of single-layer MoS2 and VS2. PhD thesis, Universität zu Köln.

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Abstract

In this thesis the two-dimensional materials MoS2 and VS2 are investigated in their single-layer form. Different phases, dependent on the synthesis parameters, substrate or temperature are explored, with an emphasis on the correlated electronic and magnetic states that are present in these materials. Scanning tunnelling microscopy is used to obtain structural information with atomic-scale spatial resolution, while scanning tunnelling spectroscopy is employed to measure the local density of states. Experimental results on correlated phenomena are compared to density functional theory and numerical renormalization group calculations performed by cooperation partners. Single-layer VS2 was predicted to be a strongly-correlated material, with a possibly coexisting magnetic and charge density wave ground state. However, the metastability of VS2 makes it a difficult material to grow, and an experimental realisation of a freestanding single layer was yet to be achieved. We successfully synthesized single-layer VS2 on the weakly interacting graphene on Ir(111) substrate, using the molecular beam epitaxy method. With the scanning tunnelling microscope, we find a charge density wave with a unit cell of 9x√3R30°. It is an unusual charge density wave, as it involves a symmetry reduction from a hexagonal to a rectangular unit cell and is present even at room temperature. Using the experimental data as the basis for density-functional theory calculations, we find that the charge density wave transition leads to a drastic reconfiguration of the band structure. Contrary to the intuitive Peierls picture of a charge density wave, which is driven by the electrons at the Fermi level, we find that the formation of the charge density wave can be best explained with non-linear coupling between transverse and longitudinal phonon modes. The most striking feature of this phase is a large, full gap in the unoccupied density of states, which is measured with scanning tunnelling spectroscopy. Additionally, x-ray magnetic circular dichroism spectra show that there is no net magnetization in small islands of VS2, which could be explained by a coexistence of the charge density wave with an antiferromagnetic spin density wave. The metastability of VS2 can, however, also be utilized to create new compounds, since it can readily transform to V-rich phases via intercalation or desulphurization. Previously, the introduction of sulphur defects and the intercalation of vanadium atoms have been shown to lead to phase transitions or magnetism in similar compounds. We combine scanning tunnelling microscopy with X-ray photoelectron spectroscopy to demonstrate the synthesis of two new vanadium based materials on graphene on Ir(111). By annealing single-layer VS2 in ultra-high vacuum, we can create the one-dimensional patterned material V4S7, which has ordered rows of sulphur vacancies running along its surface. By increasing the amount of vanadium and sulphur on the sample sufficiently, we can alternatively grow ultimately thin V5S8, which has a 2x2 layer of vanadium atoms intercalated between sheets of VS2. In the thinnest V5S8-derived structures, a √3x√3 CDW is found at low temperatures, which is investigated with scanning tunnelling spectroscopy. While the semiconducting properties of single-layer MoS2 have been extensively studied, there are many open questions regarding its metallic state, which can only be reached by significant gating or doping. As typical back-gating cannot achieve this due to breakdown effects, MoS2 was previously made metallic using ionic liquids or doped via sulphur vacancies. However, these methods tend to lead to disorder and charge inhomogeneities. We instead exploit the unique properties of the graphene on Ir(111) substrate to adjust the MoS2 Fermi level without these drawbacks. Taking oxygen as a p-dopant and europium as a n-dopant, we intercalate these atoms between graphene and Ir(111). As a result of oxygen taking up electrons from, or europium donating electrons to graphene, the Fermi level of MoS2 is electrostatically shifted while remaining chemically pristine. Intercalating oxygen under graphene leads p-doping of MoS2, inducing a downwards Fermi level shift of 0.45 eV. Using europium, we can n-dope MoS2, shifting its Fermi level up by about 0.8 eV, inducing a metal-insulator transition. Due to the enhanced screening of the metallic state, a giant band gap renormalization of 0.67 eV takes place. We further explore the effect of the additional charge on the one-dimensional metallic states in MoS2 mirror twin boundaries. The depletion and charging via the substrate is shown to shift the periodicity of the states, while preserving their one-dimensional nature. Additional DFT calculations reproduce the metal-insulator transition and the shifts of the graphene Dirac point and MoS2 conduction band. The Kondo effect, where the spin of a magnetic impurity is screened by the itinerant electrons in a metal, is one of the best understood phenomena in many-body physics. Its microscopic description allows one to predict the properties of the system at any energy scale, using only a few parameters. Nevertheless, these parameters, which pertain to the magnetic impurity, are inaccessible in the standard scanning tunnelling microscopy realisation of the Kondo effect, which makes use of magnetic adatoms on metal surfaces. We present a unique Kondo system in the mirror twin boundaries of single-layer MoS2. Using voltage pulses applied with the tip of the scanning tunnelling microscope, the doubly occupied, degenerate one-dimensional metallic states in the mirror twin boundaries can be shifted to the Fermi level, where they split due to strong Coulomb interactions. This leads to the formation of a singly occupied state with spin-1/2. Coupling of the magnetic moment of this state to electrons in the substrate results in a Kondo resonance at the Fermi level. Our ability to resolve the spectral function of the full Kondo system, consisting of the magnetic mirror twin boundary states together with the resonance, allows us to compare the experimental data directly with numerical renormalization group calculations. Another benefit of this system is that the variety of boundary lengths and energy states leads to a wide range of accessible Kondo coupling strengths, with Kondo temperatures spanning six orders of magnitude. We also resolve the relation between the magnetic states and the resonance in real space for the first time. When single-layer MoS2 is made metallic, theoretical and experimental works find evidence for strong electron-phonon coupling, which can lead to various correlation effects, like superconductivity or the formation of polarons, a quasiparticle consisting of an excess charge carrier (electron or hole) localized within a potential well formed by the ions of the lattice. We provide a scanning tunnelling microscopy and density functional theory study of electron-phonon coupling effects in metallic single-layer MoS2. Conductance spectra of two metallic MoS2 samples, with europium or caesium intercalated below the graphene substrate, show series of evenly-spaced peaks around the Fermi level. Density functional theory calculations for n-doped single-layer MoS2 demonstrate strong coupling between the occupied K-band of MoS2 and the unoccupied Q-band via a M-phonon, with an energy corresponding to the conductance peak spacing. The coupling is shown to be strong enough to lead to the formation of polarons. We fit the conductance spectra to get an estimate of the electron-phonon coupling strength. We furthermore show that the conductance peaks, related to the local creation of polarons with the STM tip, can be shifted by charges at the edges of the MoS2 islands. We use this dependence on the local environment to confine polarons to a quantum well, which allows us to image them directly with the scanning tunnelling microscope. In the Scientific Appendix, two additional studies on single-layer MoS2 are presented. With transport studies exhibiting signs of localization effects in MoS2, which could lead to a Coulomb gap at the Fermi level, we conducted low-temperature (0.35 K) scanning tunnelling spectroscopy measurements of metallic MoS2 on the graphene/europium/Ir(111) sample. The spectra reveal a V-shaped gap at the Fermi level. The gap is only present on MoS2 and is absent on graphene. It does not respond to external magnetic fields, ruling out that it has a superconducting origin. Instead, it is found that the gap size depends on the area of the MoS2 islands, which is in line with it being a Coulomb gap driven primarily by electron-electron correlations. Finally, we have investigated single-layer MoS2 on hBN/Ir(111). Exchanging graphene for the large band gap insulator hBN leads to a substantial band gap increase in MoS2, at least in regions where sulphur intercalation below hBN flattens out the strong hBN/Ir(111) moiré. In these regions, we can resolve the maximum of the valence band at the K-point with constant-current scanning tunnelling spectroscopy, further pointing towards an enhanced decoupling from the substrate compared to graphene/Ir(111). Simultaneously, in regions with less or no sulphur intercalation, the moiré of hBN/Ir(111) modulates the band structure of MoS2, giving rise to possible localized states in the quantum wells formed by the moiré potential.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code van Efferen, Camiel UNSPECIFIED orcid.org/0000-0002-6237-8602 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-717895
Date:	4 December 2023
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:	Keywords Language 2D materials English MoS2 English VS2 English Scanning tunneling microscopy English Graphene English
Date of oral exam:	3 November 2023
Referee:	Name Academic Title Michely, Thomas Prof. Dr. van Loosdrecht, Paul Prof. Dr. Khajetoorians, Alexander Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/71789