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Monolayer VSe 2 , featuring both charge density wave and magnetism phenomena, represents a unique van der Waals magnet in the family of metallic two-dimensional transition-metal dichalcogenides (2D-TMDs). Herein, by means of in-situ microscopic and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X-ray and angle-resolved photoemission, and X-ray absorption, direct spectroscopic signatures are established, that identify the metallic 1T-phase and vanadium 3d 1 electronic configuration in monolayer VSe 2 grown on graphite by molecular-beam epitaxy. Element-specific X-ray magnetic circular dichroism, complemented with magnetic susceptibility measurements, further reveals monolayer VSe 2 as a frustrated magnet, with its spins exhibiting subtle correlations, albeit in the absence of a long-range magnetic order down to 2 K and up to a 7 T magnetic field. This observation is attributed to the relative stability of the ferromagnetic and antiferromagnetic ground states, arising from its atomic-scale structural features, such as rotational disorders and edges. The results of this study extend the current understanding of metallic 2D-TMDs in the search for exotic low-dimensional quantum phenomena, and stimulate further theoretical and experimental studies on van der Waals monolayer magnets. Endowed by the many possible combinations of their constituting elements, two-dimensional transition-metal dichalcogenides (2D-TMDs) can exhibit a multitude of exotic properties. [1, 2] For the much-studied group-VI 2D semiconductors, these include coupled spin and valley Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff))

This article reviews experimental studies on 'bridging electronic structure and charge transport property of organic semiconductors' performed using ultraviolet photoelectron spectroscopy (UPS) and related methods mainly in Chiba University, Japan, in particular on the investigation of the origin and the role of electronic states existing in the highest occupied molecular orbital bandlowest unoccupied molecular orbital band (HOMO-LUMO) gap. We summarize experimental observations including direct measurements of 'invisible' gap states with ultrahigh sensitivity UPS, which demonstrate that there exist intrinsic gap states in organic semiconductors. We firstly describe the nature of organic molecular solids to understand features of organic semiconductors because such intrinsic gap states are a result of the interplay of these features, which give the principal difference between the organic semiconductor and inorganic counterpart. We then discuss (i) the origin and role of the band gap states in relation to intermolecular interaction/ band dispersion and electron-phonon coupling, (ii) the Fermi level pinning issue in organic semiconductors, and (iii) the method of computing the Fermi level position within the HOMO-LUMO gap for experimental groups. The gap states of organic semiconductors appear easily when a weak perturbation is applied to the organic system, namely by contact with other material, by injecting a charge, by elevating temperature, and by exposure to 1 atm gas. What we finally found is that tailing states of HOMO and LUMO always exist, and their energy distributions must not be symmetric; they thus produce a larger Fermi level shift from the mid gap position than previously thought. Furthermore, as shown by computational work, Fermi level pinning, which is a wellknown phenomena in semiconductor devices field, occurs in weakly interacting organic/conductor systems without any gap states if the system temperature is not zero (T > 0). We also describe the experimental knowhow of the ultrahigh-sensitivity UPS of organic systems, which are very fragile upon ultraviolet-light irradiation, and some updates on our previously published results.

We studied gas-exposure effects on pentacene (Pn) films on SiO2 and Au(111) substrates by ultrahigh sensitivity photoelectron spectroscopy, which can detect the density of states of ∼10(16) states eV-1 cm-3 comparable to electrical measurements. The results show the striking effects for Pn/SiO2: exposure to inert gas (N2 and Ar) produces a sharp rise in gap states from ∼10(16) to ∼10(18) states eV-1 cm-3 and pushes the Fermi level closer to the valence band (0.15-0.17 eV), as does exposure to O2 (0.20 eV), while no such gas-exposure effect is observed for Pn/Au(111). The results demonstrate that these gap states originate from small imperfections in the Pn packing structure, which are induced by gas penetration into the film through the crystal grain boundaries.

Doping plays a crucial role in semiconductor physics, with n-doping being controlled by the ionization energy of the impurity relative to the conduction band edge. In organic semiconductors, efficient doping is dominated by various effects that are currently not well understood. Here, we simulate and experimentally measure, with direct and inverse photoemission spectroscopy, the density of states and the Fermi level position of the prototypical materials C and zinc phthalocyanine n-doped with highly efficient benzimidazoline radicals (2-Cyc-DMBI). We study the role of doping-induced gap states, and, in particular, of the difference Δ between the electron affinity of the undoped material and the ionization potential of its doped counterpart. We show that this parameter is critical for the generation of free carriers and influences the conductivity of the doped films. Tuning of Δ may provide alternative strategies to optimize the electronic properties of organic semiconductors.

The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288 eV photon energy, due to absorption to pi* virtual orbitals, and broader structures at higher energy, involving sigma* virtual orbitals. The sharp absorption structures to the pi* empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of pi* symmetry, from the six chemically shifted C 1s core orbitals.

There have been several recent conflicting reports on the ferromagnetism of clean monolayer VSe2. Herein, the controllable formation of 1D defect line patterns in vanadium diselenide (VSe2) monolayers initiated by thermal annealing is presented. Using scanning tunneling microscopy and q‐plus atomic force microscopy techniques, the 1D line features are determined to be 8‐member‐ring arrays, formed via a Se deficient reconstruction process. The reconstructed VSe2 monolayer with Se‐deficient line defects displays room‐temperature ferromagnetism under X‐ray magnetic circular dichroism and magnetic force microscopy, consistent with the density functional theory calculations. This study possibly resolves the controversy on whether ferromagnetism is intrinsic in monolayer VSe2, and highlights the importance of controlling and understanding the atomic structures of surface defects in 2D crystals, which could play key roles in the material properties and hence potential device applications.

Single-layer molybdenum disulfide (MoS) has attracted significant attention due to its electronic and physical properties, with much effort invested toward obtaining large-area high-quality monolayer MoS films. In this work, we demonstrate a reactive-barrier-based approach to achieve growth of highly homogeneous single-layer MoS on sapphire by the use of a nickel oxide foam barrier during chemical vapor deposition. Due to the reactivity of the NiO barrier with MoO, the concentration of precursors reaching the substrate and thus nucleation density is effectively reduced, allowing grain sizes of up to 170 μm and continuous monolayers on the centimeter length scale being obtained. The quality of the monolayer is further revealed by angle-resolved photoemission spectroscopy measurement by observation of a very well resolved electronic band structure and spin-orbit splitting of the bands at room temperature with only two major domain orientations, indicating the successful growth of a highly crystalline and well-oriented MoS monolayer.