Universität zu Köln

Ehlen, Niels ORCID: 0000-0002-8581-8359 (2021). Electronic Structure Engineering via Functionalization of Two-Dimensional Quantum Materials. PhD thesis, Universität zu Köln.

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Abstract

The goal of this thesis is to investigate and establish methods of electronic structure engineering in (quasi-)two-dimensional quantum materials. Since the inception of the field of two-dimensional matter, many production methods have been established for a broad range of materials. While these crystals by themselves show many interesting properties from a fundamental physics perspective, one of their main advantages is their "all-surface" nature. This property allows for manipulation of the inherent electronic behaviour of a given material from the outside, often leading to fundamental changes in the electronic structure. As part of the maturation of the field, it is important to establish methods for band structure engineering and investigate their effects on the known materials. This thesis thus focuses on ways to manipulate the electronic properties in three distinct material families. In black phosphorus, the electronic structure of the pristine bulk crystal is established using angle-resolved photoemission spectroscopy (ARPES) and investigated via a tight-binding (TB) model fitted to the experimental dispersion. From this bulk fit, the layer-dependent band gap is determined with a zone-folding approach. Comparing the zone-folded band structure with a direct calculation of the few-layer bands shows good agreement between both methods. The agreement confirms that interlayer hybridization and surface effects barely affect the dispersion of few-layer samples and allows to infer many properties of few-layer phosphorene from the bulk crystal. The tight-binding model is used to predict the doping dependent Fermi surface of bulk and few-layer black phosphorus. Modifying the band structure of bulk black phosphorus by caesium doping is shown leading to a band inversion at the Gamma-point. Angle-dependent X-ray photoemission spectroscopy confirms the caesium is adsorbed on top of black phosphorus rather than intercalated. A bilayer version of the TB model developed for the pristine crystal is used to explain the experimental observation of the band inversion. A strong reduction of the interlayer interaction is inferred. This reduction is explained with density functional theory calculations. The best fit to the experimental observation is reproduced by assuming a stacking fault of the topmost black phosphorus layer, thereby reducing the interlayer interaction. These calculations confirm the surface nature of the downshifted conduction band and thus establish the experimental observation of a surface resonance state. The optoelectronic properties of a MoS2/Graphene/Iridium(111) heterostructure are investigated using ARPES, photoluminescence and Raman spectroscopy. A sharp photoluminescence peak is observed. The lack of quenching is explained with a low interaction between MoS2 and graphene. The growth of the MoS2/graphene heterostructure consequently allows for photoluminescent properties where they would usually be quenched on a metallic substrate due to non-radiative recombination channels. Combining photoluminescence spectroscopy of the pristine sample with the electronic band gap of lithium doped MoS2 determined from ARPES in conjunction with theoretical calculations for the doping-dependent band gap renormalization, the exciton binding energy in the heterostructure is predicted. Investigating the high-doping regime of bilayer graphene by deposition of large amounts of caesium leads to the observation of a strained alkali metal quantum well structure grown on the bilayer graphene substrate. The resulting band structure is investigated using ARPES. A 2x2 superstructure is observed. Combining the experimental results with theoretical calculations elucidates the microscopic structure of the resulting sample. The two most likely structures are determined from theoretically evaluated total energy considerations and good agreement with the experimental band structure. The broadening of the bands arising from the Cs quantum well structure is found to be close to the resolution limit of the instrument suggesting only very small many-body renormalization in the Cs derived states.

Item Type:	Thesis (PhD thesis)
Translated title:	Title Language Manipulation der elektronischen Struktur via Funktionalisierung von zweidimensionaler Quantenmaterie German
Creators:	Creators Email ORCID ORCID Put Code Ehlen, Niels nehlen@outlook.de orcid.org/0000-0002-8581-8359 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-522494
Date:	2021
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 two-dimensional matter; graphene; molybdenum disulfide; MoS2; black phosphorus; electronic structure engineering; chemical doping; quantum matter; Fermi liquid; Angle-resolved photoemission spectroscopy; ARPES; molecular beam epitaxy; MBE English
Date of oral exam:	16 June 2021
Referee:	Name Academic Title Grüneis, Alexander Prof. Dr. Grüninger, Markus Prof. Dr. Damascelli, Andrea Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/52249