Electronic structure of single and few layered graphene studied by angle resolved photoemission spectro-microscopy.

This thesis reports the study of electronic band structure of single and few layered graphene grown by thermal decomposition of SiC at the surface and by C-sublimation on Ru single crystals. Growth conditions were optimized in order to obtain big few micrometer sized graphene domains. For the first system twisted multilayer graphene domains were found and chosen for study. On ruthenium only single layer graphene domains and also the domains with incorporated bilayer patches were obtained and their electronic properties were investigated after oxidation-reduction reactions at graphene/Ru interface. The electronic band structure was analyzed using high resolution angle resolved photoelectron spectroscopy. In order to obtain spectra from individual domains novel spectromicroscopy end station was used for focusing synchrotron radiation beam to sub-micrometer spot on the sample surface. Experimental results on twisted graphene confirmed interlayer coupling and resulting van Hove singularities, graphene Dirac fermions velocity renormalization and other exotic phenomena predicted by theoretical calculations and partially observed by scanning tunneling spectroscopy technique. Particular attention has been paid to poorly studied interlayer coupling in trilayer systems where middle layer has two different couplings being sandwiched between differently twisted layers. These multilayer graphene domains were also investigated in detail upon alkali metal intercalation and unexpected splitting of upper part of Dirac cone, related to graphene sublattice symmetry breaking in the middle graphene layer was found. In graphene on Ru it was first confirmed that oxidation of Ru under graphene decouples its strongly hybridized \u3c0 orbitals making graphene p-doped. Our observations indicate that bilayer patches incorporated into single layer background remain n-doped and decorated by intercalated oxygen, thereby forming lateral p-n junctions in the same graphene layer. It was found that hydrogen atmosphere helps to reduce RuOx without the formation of carbon vacancy defects. However, structural wrinkle patterns appeared due to loss of original graphene/Ru epitaxial order remain, and in big graphene domains they can trap H2+RuOx reaction products, making graphene fully decoupled and undoped

Spectroscopic characterization of charge carrier anisotropic motion in twisted few-layer graphene

Graphene, a layer of carbon atoms in a honeycomb lattice, captures enormous interest as probably the most promising component of future electronics thanks to its mechanical robustness, flexibility, and unique charge carrier quasiparticles propagating like massless high energy Dirac fermions. If several graphene layers form a stack, the interaction between them is, on the one hand, weak, allowing realization of various registries between the layers and, on the other hand, strong enough for a wide range tuning of the electronic properties. Here we grow few layer graphene with various number of layers and twist configurations and address the electronic properties of individual atomic layers in single microscopic domains using angle-resolved photoelectron spectromicroscopy. The dependence of the interlayer coupling on the twist angle is analyzed and, in the domains with tri-layers and more, if different rotations are present, the electrons in weaker coupled adjacent layers are shown to have different properties manifested by coexisting van Hove singularities, moir\ue9 superlattices with corresponding superlattice Dirac points, and charge carrier group velocity renormalizations. Moreover, pronounced anisotropy in the charge carrier motion, opening a possibility to transform strongly coupled graphene bilayers into quasi one-dimensional conductors, is observed

Visualizing electrostatic gating effects in two-dimensional heterostructures

The ability to directly observe electronic band structure in modern nanoscale field-effect devices could transform understanding of their physics and function. One could, for example, visualize local changes in the electrical and chemical potentials as a gate voltage is applied. One could also study intriguing physical phenomena such as electrically induced topological transitions and many-body spectral reconstructions. Here we show that submicron angle-resolved photoemission (micro-ARPES) applied to two-dimensional (2D) van der Waals heterostructures affords this ability. In graphene devices, we observe a shift of the chemical potential by 0.6 eV across the Dirac point as a gate voltage is applied. In several 2D semiconductors we see the conduction band edge appear as electrons accumulate, establishing its energy and momentum, and observe significant band-gap renormalization at low densities. We also show that micro-ARPES and optical spectroscopy can be applied to a single device, allowing rigorous study of the relationship between gate-controlled electronic and excitonic properties.Comment: Original manuscript with 9 pages with 4 figures in main text, 5 pages with 4 figures in supplement. Substantially edited manuscript accepted at Natur

Ghost anti-crossings caused by interlayer umklapp hybridization of bands in 2D heterostructures

In two-dimensional heterostructures, crystalline atomic layers with differing lattice parameters can stack directly one on another. The resultant close proximity of atomic lattices with differing periodicity can lead to new phenomena. For umklapp processes, this opens the possibility for interlayer umklapp scattering, where interactions are mediated by the transfer of momenta to or from the lattice in the neighbouring layer. Using angle-resolved photoemission spectroscopy to study a graphene on InSe heterostructure, we present evidence that interlayer umklapp processes can cause hybridization between bands from neighbouring layers in regions of the Brillouin zone where bands from only one layer are expected, despite no evidence for Moiré-induced replica bands. This phenomenon manifests itself as ‘ghost’ anti-crossings in the InSe electronic dispersion. Applied to a range of suitable two-dimensional material pairs, this phenomenon of interlayer umklapp hybridization can be used to create strong mixing of their electronic states, giving a new tool for twist-controlled band structure engineering

Determination of band offsets, hybridization, and exciton binding in 2D semiconductor heterostructures

Combining monolayers of different two-dimensional semiconductors into heterostructures creates new phenomena and device possibilities. Understanding and exploiting these phenomena hinge on knowing the electronic structure and the properties of interlayer excitations. We determine the key unknown parameters in MoSe2/WSe2 heterobilayers by using rational device design and submicrometer angle-resolved photoemission spectroscopy (μ-ARPES) in combination with photoluminescence. We find that the bands in the K-point valleys are weakly hybridized, with a valence band offset of 300 meV, implying type II band alignment. We deduce that the binding energy of interlayer excitons is more than 200 meV, an order of magnitude higher than that in analogous GaAs structures. Hybridization strongly modifies the bands at Γ, but the valence band edge remains at the K points. We also find that the spectrum of a rotationally aligned heterobilayer reflects a mixture of commensurate and incommensurate domains. These results directly answer many outstanding questions about the electronic nature of MoSe2/WSe2 heterobilayers and demonstrate a practical approach for high spectral resolution in ARPES of device-scale structures

Revealing the conduction band and pseudovector potential in 2D moir\'e semiconductors

Stacking monolayer semiconductors results in moir\'e patterns that host many correlated and topological electronic phenomena, but measurements of the basic electronic structure underpinning these phenomena are scarce. Here, we investigate the properties of the conduction band in moir\'e heterobilayers using submicron angle-resolved photoemission spectroscopy with electrostatic gating, focusing on the example of WS2/WSe2. We find that at all twist angles the conduction band edge is the K-point valley of the WS2, with a band gap of 1.58 +- 0.03 eV. By resolving the conduction band dispersion, we observe an unexpectedly small effective mass of 0.15 +- 0.02 m_e. In addition, we observe replicas of the conduction band displaced by reciprocal lattice vectors of the moir\'e superlattice. We present arguments and evidence that the replicas are due to modification of the conduction band states by the moir\'e potential rather than to final-state diffraction. Interestingly, the replicas display an intensity pattern with reduced, 3-fold symmetry, which we show implicates the pseudo vector potential associated with in-plane strain in moir\'e band formation.Comment: Main text: 12 pages, 4 figures. Appended Supporting Information: 10 pages, 11 figure