Electronic Structure Engineering via Functionalization of Two-Dimensional Quantum Materials

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

Synthesis and spectroscopic characterization of alkali-metal intercalated ZrSe2

We report on the synthesis and spectroscopic characterization of alkali metal intercalated ZrSe2 single crystals. ZrSe2 is produced by chemical vapour transport and then Li intercalated. Intercalation is performed from the liquid phase (via butyllithium) and from the vapour phase. Raman spectroscopy of intercalated ZrSe2 reveals phonon energy shifts of the Raman active A1g and Eg phonon modes, the disappearance of two-phonon modes and new low wavenumber Raman modes. Angle-resolved photoemission spectroscopy is used to perform a mapping of the Fermi surface revealing an electron concentration of 4.7 × 1014 cm−2. We also perform vapour phase intercalation of K and Cs into ZrSe2 and observe similar changes in the Raman modes as for the Li case

Resonance Raman spectrum of doped epitaxial graphene at the Lifshitz transition

We employ ultra-high vacuum (UHV) Raman spectroscopy in tandem with angle-resolved photoemission (ARPES) to investigate the doping-dependent Raman spectrum of epitaxial graphene on Ir(111). The evolution of Raman spectra from pristine to heavily Cs doped graphene up to a carrier concentration of 4.4*10^14cm^-2 is investigated. At this doping graphene is at the onset of the Lifshitz transition and renormalization effects reduce the electronic bandwidth. The optical transition at the saddle point in the Brillouin zone then becomes experimentally accessible by ultraviolet (UV) light excitation which achieves resonance Raman conditions in close vicinity to the van Hove singularity in the joint density of states. The position of the Raman G band of fully doped graphene/Ir(111) shifts down by ~60cm^-1. The G band asymmetry of Cs doped epitaxial graphene assumes an unusual strong Fano asymmetry opposite to that of the G band of doped graphene on insulators. Our calculations can fully explain these observations by substrate dependent quantum interference effects in the scattering pathways for vibrational and electronic Raman scattering

Environmental Control of Charge Density Wave Order in Monolayer 2H-TaS2

Contains fulltext : 208621.pdf (publisher's version ) (Open Access

Charge density wave phase of VSe2 revisited

Scanning tunneling microscopy and spectroscopy are used to image the charge density wave at the surface of cleaved VSe2 and to probe its local density of states at 5 K. The main features in the spectrum are linked to the contributions of the p-like and d-like bands of VSe2 found in angle-resolved photoemission spectroscopy and tight-binding calculations. Different from previous tunneling spectroscopy work, we find a narrow partial gap at the Fermi level that we associate with the charge density wave phase. The energy scale of the gap found in the experiment is in good agreement with the charge density wave transition temperature of VSe2, under the assumption of weak electron-phonon coupling, consistent with the Peierls model of Fermi surface nesting. The role of defects is investigated, which reveals that the partial gap in the density of states and hence the charge density wave itself is extremely stable, though the order, phase, and amplitude of the charge density waves on the surface are strongly perturbed by defects

Comprehensive tunneling spectroscopy of quasifreestanding MoS2 on graphene on Ir(111)

We apply scanning tunneling spectroscopy to determine the band gaps of mono-, bi-, and trilayer MoS2 grown on a graphene single crystal on Ir(111). Besides the typical scanning tunneling spectroscopy at constant height, we employ two additional spectroscopic methods giving extra sensitivity and qualitative insight into the k vector of the tunneling electrons. Employing this comprehensive set of spectroscopic methods in tandem, we deduce a band gap of 2.53 +/- 0.08 eV for the monolayer. This is close to the predicted values for freestanding MoS2 and larger than is measured for MoS2 on other substrates. Through precise analysis of the comprehensive tunneling spectroscopy we also identify critical point energies in the mono- and bilayer MoS2 band structures. These compare well with their calculated freestanding equivalents, evidencing the graphene/Ir(111) substrate as an excellent environment upon which to study the many celebrated electronic phenomena of monolayer MoS2 and similar materials. Additionally, this investigation serves to expand the fledgling field of the comprehensive tunneling spectroscopy technique itself

Photothermal Bottom-up Graphene Nanoribbon Growth Kinetics

We present laser-induced photothermal synthesis of atomically precise graphene nanoribbons (GNRs). The kinetics of photothermal bottom-up GNR growth are unravelled by in situ Raman spectroscopy carried out in ultrahigh vacuum. We photothermally drive the reaction steps by short periods of laser irradiation and subsequently analyze the Raman spectra of the reactants in the irradiated area. Growth kinetics of chevron GNRs (CGNRs) and seven atoms wide armchair GNRs (7-AGNRs) is investigated. The reaction rate constants for polymerization, cyclodehydrogenation, and interribbon fusion are experimentally determined. We find that the limiting rate constants for CGNR growth are several hundred times smaller than for 7-AGNR growth and that interribbon fusion is an important elementary reaction occurring during 7AGNR growth. Our work highlights that photothermal synthesis and in situ Raman spectroscopy are a powerful tandem for the investigation of on-surface reactions

Origin of the Flat Band in Heavily Cs-Doped Graphene

A flat energy dispersion of electrons at the Fermi level of a material leads to instabilities in the electronic system and can drive phase transitions. Here we show that the flat band in graphene can be achieved by sandwiching a graphene monolayer by two cesium (Cs) layers. We investigate the flat band by a combination of angle-resolved photoemission spectroscopy experiment and the calculations. Our work highlights that charge transfer, zone folding of graphene bands, and the covalent bonding between C and Cs atoms are the origin of the flat energy band formation. Analysis of the Stoner criterion for the flat band suggests the presence of a ferromagnetic instability. The presented approach is an alternative route for obtaining flat band materials to twisting bilayer graphene which yields thermodynamically stable flat band materials in large areas

Reversible crystalline-to-amorphous phase transformation in monolayer MoS2 under grazing ion irradiation

| openaire: EC/H2020/648589/EU//SUPER-2DBy combining scanning tunneling microscopy, low-energy electron diffraction, photoluminescence and Raman spectroscopy experiments with molecular dynamics simulations, a comprehensive picture of the structural and electronic response of a monolayer of MoS 2 to 500 eV Xe + irradiation is obtained. The MoS 2 layer is epitaxially grown on graphene/Ir(1 1 1) and analyzed before and after irradiation in situ under ultra-high vacuum conditions. Through optimized irradiation conditions using low-energy ions with grazing trajectories, amorphization of the monolayer is induced already at low ion fluences of 1.5 × 10 14 ions cm -2 and without inducing damage underneath the MoS 2 layer. The crystalline-to-amorphous transformation is accompanied by changes in the electronic properties from semiconductor-to-metal and an extinction of photoluminescence. Upon thermal annealing, the re-crystallization occurs with restoration of the semiconducting properties, but residual defects prevent the recovery of photoluminescence.Peer reviewe