Their exceptional optical properties are a driving force for the persistent interest in atomically thin transition metal dichalcogenides such as MoS 2. The optical response is dominated by excitons. Apart from the bright excitons, which directly couple to light, it has been realized that dark excitons, where photon absorption or emission is inhibited by the spin state or momentum mismatch, are decisive for many optical properties. However, in particular the momentum dependence is difficult to assess experimentally and often remains elusive or is investigated by indirect means. Here we study the momentum dependent electronic structure experimentally and theoretically. We use angle-resolved photoemission as a one-particle probe of the occupied valence band structure and electron energy loss spectroscopy as a two-particle probe of electronic transitions across the gap to benchmark a single-particle model of the dielectric function ϵðq; ωÞ against momentum dependent experimental measurements. This ansatz captures key aspects of the data surprisingly well. In particular, the energy region where substantial nesting occurs, which is at the origin of the strong light-matter interaction of thin transition metal dichalcogenides and crucial for the prominent C-exciton, is described well and spans a more complex exciton landscape than previously anticipated. Its local maxima in ðq ≠ 0; ωÞ space can be considered as dark excitons and might be relevant for higher order optical processes. Our study may lead to a more complete understanding of the optical properties of atomically thin transition metal dichalcogenides.

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We have investigated the electronic properties of WSe2 surfaces covered by fluorinated cobalt phthalocyanine (F16CoPc) using photoemission spectroscopy. We show that a charge transfer occurs at this interface, which results in the creation of holes in the WSe2 surface while the Co center of the phthalocyanine is reduced to Co(I). We observe a potential change in WSe2 approaching the surface as a consequence of the induced positive charges near the surface. In addition, our data allow for a rough estimation of the induced charge density and suggest that the holes might be localized.

Many unusual and promising properties have been reported recently for the transition metal trichalcogenides of the type MPS3 (M = V, Mn, Fe, Ni ...), such as maintaining magnetic order to the atomically thin limit, ultra-sharp many-body excitons, metal-insulator transitions and, especially for Fe2P2S6, giant linear dichroism among others. Here we conduct a detailed investigation of the electronic structure of Fe2P2S6 using angle-resolved photoemission spectroscopy, q-dependent electron energy loss spectroscopy, optical spectroscopies and density functional theory. Fe2P2S6 is a Mott insulator with a gap of Egap ≈ 1.4 eV and zigzag antiferromagnetism below TN = 119 K. The low energy excitations are dominated by Fe 3d states. Large and sign-changing linear dichroism is observed. We provide a microscopic mechanism explaining key properties of the linear dichroism based on the correlated character of the electronic structure, thereby elucidating the nature of the spin-charge coupling in Fe2P2S6 and related materials.

Abstractα‐RuCl3 is a candidate material for the realization of a Kitaev spin liquid with envisioned applications for quantum computing. It is a van‐der‐Waals material with in‐plane honeycomb lattice equivalent to the CrX3 (X= Cl, Br, I) type 2D magnets. Here, possibilities of defect engineering and surface modification of thin crystals are explored by Ar+ dosing and vacuum annealing. Chlorine is easily removed from the surface, which reduces the Ru valence and eventually leaves a Ru rich surface layer behind. A peculiar thickness dependence of the work function emerges after Ar+ sputtering, which is ascribed to the remaining chlorine concentration. This work elucidates material properties of thin α‐RuCl3 and introduces concepts of property engineering to create homojunctions and control level alignment by standard in situ methods.

Atomically thin transition metal dichalcogenides (TMDCs) are promising candidates for implementation in next generation semiconducting devices, for which laterally homogeneous behavior is needed. Here, we study the electronic structure of atomically thin exfoliated WSe2, a prototypical TMDC with large spin–orbit coupling, by photoemission electron microscopy, electron energy-loss spectroscopy, and density functional theory. We resolve the inhomogeneities of the doping level by the varying energy positions of the valence band. There appear to be different types of inhomogeneities that respond differently to electron doping, introduced by potassium intercalation. In addition, we find that the doping process itself is more complex than previously anticipated and entails a distinct orbital and thickness dependence that needs to be considered for effective band engineering. In particular, the density of selenium vs tungsten states depends on the doping level, which leads to changes in the optical response beyond increased dielectric screening. Our work gives insight into the inhomogeneity of the electron structure of WSe2 and the effects of electron doping, provides microscopic understanding thereof, and improves the basis for property engineering of 2D materials.

The interface of manganese(II) phtalocyanine (MnPc) with an α-RuCl 3 single crystal is studied by means of photoelectron spectroscopy. The organic molecule was evaporated on top of an in situ cleaved α-RuCl 3 bulk crystal in steps. The work function and electron affinity of α-RuCl 3 is determined to be 6.3 and 5.4 eV, respectively, in line with common fluorinated acceptor molecules, but with the potential to absorb more electrons per area at interfaces with other materials. These favorable electronic properties lead to a charge transfer from MnPc to α-RuCl 3 which manifests itself mainly in peak shape changes of core level photoemission spectra and further filling of Ru 4d states as revealed by valence band photoemission. This work demonstrates the promising potential of α-RuCl 3 as a strong acceptor material for interfaces in general, and for organic molecules in particular.