The past decade has seen rapid growth in the number of experimentally realized two-dimensional (2D) materials with diverse chemical and physical properties. However, information on their crystal structure, synthesis routes, and measured or predicted properties remains scattered across thousands of publications. Here, we consolidate this fragmented knowledge by establishing X2DB─an open infrastructure that integrates experimental and computational data on 2D materials. Using extensive literature mining and direct community uploads, we identify 370 unique 2D materials that have been realized in monolayer or few-layer form and link them to their digital counterparts in computational databases, enabling consistent ab initio characterization of their properties across monolayer, bilayer, and bulk forms. We describe the structure and content of the database, highlight its support for community uploads, illustrate how it can be used to generate scientific insight, and introduce a hierarchical classification of the known set of 2D materials. Our work supports the integration and cross-fertilization of experimental and theoretical knowledge and contributes to data-driven and predictive synthesis of 2D materials.

We report on a two-step ultrahigh vacuum chemical vapor deposition synthesis of a vertical Ir(111)/borophene/hexagonal boron nitride heterostructure, using borazine as a single-source precursor. The process takes advantage of the finite solubility of boron in Ir: low precursor pressure at high temperature first establishes a boron reservoir in the near-surface region of the substrate, whereas subsequent growth at higher precursor pressure promotes the formation of a closed hexagonal boron nitride monolayer. During cooldown, the reduced boron solubility drives segregation to the surface, resulting in the formation of a borophene monolayer beneath the hexagonal boron nitride overlayer. The heterostructure, with micron sized grains, homogeneously covers the entire Ir substrate. The study is performed by complementary spot profile analysis low-energy electron diffraction, low-energy electron microscopy, and scanning tunneling microscopy measurements. This intrinsic segregation-assisted growth concept provides a promising route toward scalable synthesis of high-quality, vertical heterostructures of two-dimensional materials.

Compared to bulk solids, defects in low-dimensional materials and, specifically, 2D systems are expected to have a stronger effect, detrimental or beneficial, on their properties. Owing to their geometry, defects in 2D materials can easily be formed due to the interaction with the environment or under impacts of energetic particles, such as ions and electrons. At the same time, many concepts of defect production under irradiation in bulk systems are not applicable for 2D materials or require substantial modifications. Various aspects of the physics and chemistry of defects in 2D materials have been addressed, and the results of these investigations are presented in hundreds of research papers and review articles. However, the challenges and open questions that still remain in the field have received relatively little attention. These topics were recently addressed at the symposium “Defect-mediated engineering of nanomaterials for energy and quantum applications” organized by the Beilstein-Institut. Following the discussions at the symposium, here, we present the challenges and open questions in our understanding of the behavior of defective 2D materials, interaction of energetic particles with low-dimensional targets, and defect-mediated engineering of the properties of 2D systems. We further discuss possible solutions to these problems or suggest “work-arounds”, which should accelerate the progress in the field.

Two-dimensional materials can stabilize crystal structures that are absent from their bulk counterparts, offering opportunities for materials design. Here, we report the synthesis of a previously unknown hexagonal Fe_2S_2 single layer with β-CuI structure, a buckled layer of two vertically stacked FeS honeycomb lattices, realized by thermally induced transformation of single layer mackinawite grown on graphene/Ir(111). In situ scanning tunneling microscopy and low-energy electron diffraction reveal a transition from a tetragonal to a hexagonal lattice accompanied by distinct morphological and electronic signatures. The hexagonal Fe_2S_2 forms reproducibly upon annealing and represents a new structural motif within the Fe-S material family. First-principles calculations identify the β-CuI structure as most consistent with experiment. The calculations suggest that on-site Coulomb interactions and magnetic order are relevant to understanding the stability of the new 2D Fe-S compound. The preferred nucleation of single-layer mackinawite, despite being energetically disfavored, is speculated to result from its low edge energy, analogous to the 3D case. Our results establish Fe_2S_2 as a platform for exploring structural polymorphism in two dimensions and demonstrate that reduced dimensionality can stabilize crystal structures not accessible in bulk materials.

Individual spin defects in solids are promising building blocks for quantum technologies, but their deterministic creation, individual addressability, and operation near surfaces remain major challenges. Two-dimensional materials provide an attractive alternative, as their single-layer thickness enables direct atomic-scale access to defect states. Here, we demonstrate single-spin control of solid-state defects in a two-dimensional semiconductor by a combination of scanning tunneling microscopy and electron spin resonance. We create and manipulate individual sulfur vacancies and carbon substitution defects in monolayer molybdenum disulfide and characterize their spin dynamics, including coherent control, at the single-defect level. Using atomic manipulation, we further engineer and probe spin-spin interactions between defect pairs. Our results demonstrate deterministic creation, addressability, coherent manipulation, and controlled coupling of individual spin defects within a single experimental platform. This establishes atomically engineered spin defects in two-dimensional semiconductors as a versatile class of controllable solid-state quantum systems and opens a route towards tailored quantum sensing experiments.

The detection of Néel vector switching in a single-layer A-type antiferromagnet marks an important step toward functional two-dimensional spintronics. Here, Cr 2 S 3 -2D, grown on graphene on Ir(110), is established as a first single-layer A-type antiferromagnet. Spin-polarized scanning tunneling microscopy reveals hysteresis loops with a large switching field and a pronounced dependence on island size. X-ray magnetic circular dichroism at the Cr L 2 , 3 edges exhibits a tiny signal with a linear magnetic field dependence, consistent with an antiferromagnetic ground state with an estimated net moment below 10 - 2 μ B per Cr and a Néel temperature of about 160 K. Quantitative analysis of the island-size dependence of the switching field, together with first-principles calculations, indicates a slight imbalance between the magnetic moments of the two Cr planes of Cr 2 S 3 -2D when supported on a substrate. This imbalance results in a net magnetization for the A-type antiferromagnet, which enables the 180 ∘ rotation of the Néel vector. Moreover, Cr 2 S 3 -2D retains its magnetic properties after several days of exposure to air.

Single-layer MnBr 2 ${\rm MnBr}_2$ is grown on graphene (Gr) supported by Ir(110) and investigated using low-energy electron diffraction, scanning tunneling microscopy, and spectroscopy. The structure and epitaxial relationship with the substrate are systematically characterized. The growth morphology strongly depends on the growth temperature, evolving from fractal to dendritic and eventually to compact dendritic-skeletal islands, reflecting changes in the underlying surface diffusion processes. The pronounced variation in the apparent height with tunneling conditions for the magnetic insulator is explained based on the measured electronic density of states. MnBr 2 ${\rm MnBr}_2$ on Gr/Ir(110) constitutes a three-lattice system, giving rise to a super-moiré pattern - a moiré of moirés. The super-moiré of MnBr 2 ${\rm MnBr}_2$ /Gr/Ir(110) is unique, as it involves a virtual moiré of MnBr 2 ${\rm MnBr}_2$ with the Ir(110) surface lattice - two lattices not in contact with each other. Using a careful Fourier analysis, the known properties of Gr/Ir(110), and the results of ab initio calculations, the origin of the virtual moiré is uncovered and related to the inhomogeneous binding of Gr to Ir(110). Comparative experiments with MnBr 2 ${\rm MnBr}_2$ on Gr/Ir(111) show similar growth and structure, but highlight the unique properties of the MnBr 2 ${\rm MnBr}_2$ /Gr/Ir(110) super-moiré.

2D chromium‐sulfides are synthesized by molecular beam epitaxy using graphene as a substrate. Structure characterization by employing scanning tunneling microscopy and low energy electron diffraction indicates that there are two 2D phases, Cr 2 S 3 ‐2D and 4 ‐2D, which have not been reported before. 4 ‐2D is related to bulk Cr 5 S 6 , but thinner than a bulk unit cell. For Cr 2 S 3 ‐2D, an even thinner material, no bulk counterpart exists. Both 2D materials are found to be structurally stable under ambient conditions and exhibit interesting electronic properties. Extensive first‐principles calculations provide further insight into the electronic structure of these systems and indicate that they should be magnetic. Although single layers of CrS 2 are predicted to be stable by density functional theory calculations and reported in previous experimental studies, CrS 2 is unable to synthesize under the range of experimental conditions.

Cluster superlattice membranes constitute a novel 2D material, comprising a cluster superlattice sandwiched between a graphene support and an amorphous carbon embedding matrix. They offer a platform for investigating phenomena at the few atom to small cluster level with lateral averaging techniques. The amorphous carbon matrix provides mechanical and thermal stability to the cluster array, but alternative and nonconductive embedding materials are being sought. Such alternative embedding materials might, for instance, be advantageous for use in catalytic reactions and the exploration of charge transport in a cluster array. Here, the embedding of iridium and platinum cluster superlattices in elemental boron is characterized by scanning tunneling microscopy and X-ray photoelectron spectroscopy. The embedding in B preserves the superlattice order and provides mechanical stability comparable to that of amorphous C while maintaining thermal stability with the cluster superlattice order preserved up to 650 K.

Single-layer MnBr_2 is grown on graphene (Gr) supported by Ir(110) and investigated using low-energy electron diffraction, scanning tunneling microscopy, and spectroscopy. The structure and epitaxial relationship with the substrate are systematically characterized. The structure and epitaxial relationship with the substrate are systematically characterized. The growth morphology strongly depends on the growth temperature, evolving from fractal to dendritic and eventually to compact dendritic skeletal islands, reflecting changes in the underlying surface diffusion processes. The pronounced variation in the apparent height with tunneling conditions for the magnetic insulator is explained based on the measured electronic density of states. MnBr_2 on Gr/Ir(110) constitutes a three-lattice system, giving rise to a super-moiré pattern – a moiré of moirés. The super-moiré of MnBr_2/Gr/Ir(110) is unique, as it involves a virtual moiré of MnBr_2 with the Ir(110) surface lattice – two lattices not in contact with each other. Using a careful Fourier analysis, the known properties of Gr/Ir(110), and the results of ab initio calculations, the origin of the virtual moiré is uncovered and related to the inhomogeneous binding of Gr to Ir(110). Comparative experiments with MnBr_2 on Gr/Ir(111) show similar growth and structure, but highlight the unique properties of the MnBr_2/Gr/Ir(110) super-moiré.

Single-layer transition metal dihalides grown on conducting substrates were shown to host stable polarons. Here, we investigate polarons in insulating single-layer MnBr_2 grown by molecular beam epitaxy on three different substrates, namely graphene on Ir(110), graphene on Ir(111), and Au(111). The number densities and species of polarons observed vary strongly as a function of the substrate. For MnBr_2 grown on Ir(110) the largest number of polaron species is observed, namely four, of which three show clear similarities with the species observed for CoCl_2 on graphite. Polarons in single-layer MnBr_2 are observed up to 300K. They can be created, converted, and moved by the STM tip when a tunneling current flows at a proper bias voltage. For graphene on Ir(110) as a substrate, mobile polarons in MnBr_2 are guided through the periodic potential imposed from the super-moiré resulting from the interaction of MnBr_2 with graphene and Ir(110). Our findings indicate that modeling of polarons in such single-layer insulators in contact with a conducting substrate requires to take the substrate explicitly into account.

Skyrmions are particle-like spin textures that arise from spin spiral states in an external magnetic field. These spirals can originate from frustrated Heisenberg exchange or from the interplay of exchange and the Dzyaloshinskii-Moriya interaction, leading to atomic- and mesoscale textures. However, converting exchange-stabilized spin spirals into single skyrmions typically requires fields exceeding laboratory limits. Here, we demonstrate a strategy that uses hydrogen adsorption to expand the range of magnetic films hosting skyrmions. In two pseudomorphic Fe layers on Ir(110), spin-polarized scanning tunneling microscopy and ab initio calculations reveal that an exchange-stabilized Néel-type Yoshimori spin spiral with a 1.3 nm period transforms upon hydrogen adsorption into a Dzyaloshinskii-Moriya spiral with a 7-fold longer period of 8.5 nm. This transition enables elliptical skyrmions at moderate fields. Hydrogenation thus provides a nonvolatile mechanism to toggle between distinct magnetic states, offering a versatile platform for controlling spin textures.

Polarons are quasiparticles that arise from the interaction of electrons or holes with lattice vibrations. Though polarons are well-studied across multiple disciplines, experimental observations of polarons in two-dimensional crystals are sparse. We use scanning tunneling microscopy and spectroscopy to measure inelastic excitations of polaronic bound states emerging from coupling of non-polar zone-boundary phonons to Bloch electrons in n-doped metallic single-layer MoS_2. The latter is kept chemically pristine via contactless chemical doping. Tunneling into the vibrationally coupled polaronic states leads to a series of evenly spaced peaks in the differential conductance on either side of the Fermi level. Combining density functional (perturbation) theory with a recently developed ab initio electron-lattice downfolding technique, we show that the energy spacing stems from the longitudinal-acoustic phonon mode that flattens at the Brillouin zone edge and is responsible for the formation of stable multipolarons in metallic MoS_2.

Ultrathin magnetic films on heavy metal substrates with strong spin-orbit coupling provide versatile platforms for exploring novel spin textures. So far, structurally open fcc(110) substrates remain largely unexplored. Here, we stabilize a metastable, unreconstructed Ir(110)-(1 x 1) surface on which Fe forms a stable pseudomorphic double layer. Combining spin-polarized scanning tunneling microscopy and ab initio calculations, we reveal a right-handed N & eacute;el-type spin spiral along the [110] crystallographic direction with a period of 1.27 nm as the magnetic ground state. Our analysis reveals this spiral is of the Yoshimori type, i.e., driven by frustrated Heisenberg interactions, with the Dzyaloshinskii-Moriya interaction determining its cycloidal nature and handedness.

The growth of monolayer hexagonal boron nitride (h-BN) on Ir(110) through low-pressure chemical vapor deposition is investigated using low energy electron diffraction and scanning tunneling microscopy. We find that the growth of aligned h-BN on Ir(110) requires a growth temperature of 1500 K, whereas lower growth temperatures result in coexistence of aligned h-BN with twisted h-BN. The presence of the h-BN overlayer suppresses the formation of the nanofaceted ridge pattern known from clean Ir(110). Instead, we observe the formation of a (1 x n) reconstruction, with n such that the missing rows are in registry with the h-BN/Ir(110) moire pattern. Our moire analysis showcases a precise methodology for determining both the moire periodicity and the h-BN lattice parameter on an fcc(110) surface. Aligned h-BN on Ir(110) is found to be slightly compressed compared to bulk h-BN, with a monolayer lattice parameter of a(h-BN) = (0.2489 +/- 0.0006) nm. The lattice mismatch with the substrate along [1 (1) over bar0] gives rise to a moire periodicity of a(m) = 2.99 +/- 0.08 nm.

Two new ultimately thin vanadium rich 2D materials based on VS2 are created via molecular beam epitaxy and investigated using scanning tunneling microscopy, X-ray photoemission spectroscopy and density-functional theory calculations. The controlled synthesis of stoichiometric single-layer VS2 or either of the two vanadium-rich materials is achieved by varying the sample coverage and the sulphur pressure during annealing. Through annealing of small stoichiometric single-layer VS2 islands without S pressure, S-vacancies spontaneously order in 1D arrays, giving rise to patterned adsorption. Via the comparison of density-functional theory calculations with scanning tunneling microscopy data, the atomic structure of the S-depleted phase, with a stoichiometry of V4S7, is determined. By depositing larger amounts of vanadium and sulphur, which are subsequently annealed in a S-rich atmosphere, self-intercalated ultimately thin V5S8-derived layers are obtained, which host 2x2 V-layers between sheets of VS2. We provide atomic models for the thinnest V5S8-derived structures. Finally, we use scanning tunneling spectroscopy to investigate the charge density wave observed in the 2D V5S8-derived islands.

A metallic line defect in a layer of molybdenum disulfide can serve as an atomically narrow gate electrode demonstrating how to further miniaturize two-dimensional field effect transistors.

Starting from a single layer of NbS_2 grown on graphene by molecular beam epitaxy, the single unit cell thick 2D materials Nb_5/3S_3-2D and Nb_2S_3-2D are created using two different pathways. Either annealing under sulfur-deficient conditions at progressively higher temperatures or deposition of increasing amounts of Nb at elevated temperature result in phase-pure Nb_5/3S_3-2D followed by Nb_2S_3-2D. The materials are characterized by scanning tunneling microscopy, scanning tunneling spectroscopy and X-ray photoemission spectroscopy. The experimental assessment combined with systematic density functional theory calculations reveals their structure. The 2D materials are covalently bound without any van der Waals gap. Their stacking sequence and structure are at variance with expectations based on corresponding bulk materials highlighting the importance of surface and interface effects in structure formation.

This repository contains the data and source code associated with the paper: *Unconventional charge-density-wave gap in monolayer NbS₂*. Experimental setup STM and STS were carried out at a base operating temperature of T₀ = 0.4 K after in-situ transfer from the preparation chamber. STS was performed with the lock-in technique. STM images were taken in constant current mode. Computational setup All DFT and DFPT calculations were performed using Quantum ESPRESSO 7.1. Further information in the README file.

Das Anderson‐Modell (Anderson Impurity Model) wird in der Festkörperphysik zur Beschreibung von korrelierten, also stark wechselwirkenden Elektronen genutzt. Es besteht aus lokalen Energieniveaus, die aufgrund der Wechselwirkung mit delokalisierten Leitungselektronen eine nach Yun Kondo benannte Kondo‐Resonanz hervorrufen. Nun ist es erstmals gelungen, diese Energieniveaus zusammen mit der Kondo‐Resonanz präzise zu vermessen, wodurch eine quantitative Übereinstimmung zwischen Theorie und Experiment demonstriert wurde.