Probing the Band Splitting near the Γ Point in the van der Waals Magnetic Semiconductor CrSBr.

This study investigates the electronic band structure of chromium sulfur bromide (CrSBr) through comprehensive photoluminescence (PL) characterization. We clearly identify low-temperature optical transitions between two closely adjacent conduction-band states and two different valence-band states. The analysis on the PL data robustly unveils energy splittings, band gaps, and excitonic transitions across different thicknesses of CrSBr, from monolayer to bulk. Temperature-dependent PL measurements elucidate the stability of the band splitting below the Néel temperature, suggesting that magnons coupled with excitons are responsible for the symmetry breaking and brightening of the transitions from the secondary conduction band minimum (CBM2) to the global valence band maximum (VBM1). Collectively, these results not only reveal splitting in both the conduction and valence bands but also highlight a significant advance in our understanding of the interplay between the optical, electronic, and magnetic properties of antiferromagnetic two-dimensional van der Waals crystals.

Molybdenum Chloride Nanostructures with Giant Lattice Distortions Intercalated into Bilayer Graphene.

The nanospace of the van der Waals (vdW) gap between structural units of two-dimensional (2D) materials serves as a platform for growing unusual 2D systems through intercalation and studying their properties. Various kinds of metal chlorides have previously been intercalated for tuning the properties of host layered materials, but the atomic structure of the intercalants remains still unidentified. In this study, we investigate the atomic structural transformation of molybdenum(V) chloride (MoCl5) after intercalation into bilayer graphene (BLG). Using scanning transmission electron microscopy, we found that the intercalated material represents MoCl3 networks, MoCl2 chains, and Mo5Cl10 rings. Giant lattice distortions and frequent structural transitions occur in the 2D MoClx that have never been observed in metal chloride systems. The trend of symmetric to nonsymmetric structural transformations can cause additional charge transfer from BLG to the intercalated MoClx, as suggested by our density functional theory calculations. Our study deepens the understanding of the behavior of matter in the confined space of the vdW gap in BLG and provides hints at a more efficient tuning of material properties by intercalation for potential applications, including transparent conductive films, optoelectronics, and energy storage.

Ferromagnetic Interlayer Coupling in CrSBr Crystals Irradiated by Ions.

Layered magnetic materials are becoming a major platform for future spin-based applications. Particularly, the air-stable van der Waals compound CrSBr is attracting considerable interest due to its prominent magneto-transport and magneto-optical properties. In this work, we observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr crystals exposed to high-energy, non-magnetic ions. Already at moderate fluences, ion irradiation induces a remanent magnetization with hysteresis adapting to the easy-axis anisotropy of the pristine magnetic order up to a critical temperature of 110 K. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggests that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favors ferromagnetic order between the layers.

A van der Waals Heterostructure with an Electronically Textured Moiré Pattern: PtSe2/PtTe2.

The interlayer interaction in Pt-dichalcogenides strongly affects their electronic structures. The modulations of the interlayer atom-coordination in vertical heterostructures based on these materials are expected to laterally modify these interlayer interactions and thus provide an opportunity to texture the electronic structure. To determine the effects of local variation of the interlayer atom coordination on the electronic structure of PtSe2, van der Waals heterostructures of PtSe2 and PtTe2 have been synthesized by molecular beam epitaxy. The heterostructure forms a coincidence lattice with 13 unit cells of PtSe2 matching 12 unit cells of PtTe2, forming a moiré superstructure. The interaction with PtTe2 reduces the band gap of PtSe2 monolayers from 1.8 eV to 0.5 eV. While the band gap is uniform across the moiré unit cell, scanning tunneling spectroscopy and dI/dV mapping identify gap states that are localized within certain regions of the moiré unit cell. Deep states associated with chalcogen pz-orbitals at binding energies of ∼ -2 eV also exhibit lateral variation within the moiré unit cell, indicative of varying interlayer chalcogen interactions. Density functional theory calculations indicate that local variations in atom coordination in the moiré unit cell cause variations in the charge transfer from PtTe2 to PtSe2, thus affecting the value of the interface dipole. Experimentally this is confirmed by measuring the local work function by field emission resonance spectroscopy, which reveals a large work function modulation of ∼0.5 eV within the moiré structure. These results show that the local coordination variation of the chalcogen atoms in the PtSe2/PtTe2 van der Waals heterostructure induces a nanoscale electronic structure texture in PtSe2.

Electron-Beam- and Thermal-Annealing-Induced Structural Transformations in Few-Layer MnPS3.

Quasi-two-dimensional (2D) manganese phosphorus trisulfide, MnPS3, which exhibits antiferromagnetic ordering, is a particularly interesting material in the context of magnetism in a system with reduced dimensionality and its potential technological applications. Here, we present an experimental and theoretical study on modifying the properties of freestanding MnPS3 by local structural transformations via electron irradiation in a transmission electron microscope and by thermal annealing under vacuum. In both cases we find that MnS1-xPx phases (0 ≤ x < 1) form in a crystal structure different from that of the host material, namely that of the α- or γ-MnS type. These phase transformations can both be locally controlled by the size of the electron beam as well as by the total applied electron dose and simultaneously imaged at the atomic scale. For the MnS structures generated in this process, our ab initio calculations indicate that their electronic and magnetic properties strongly depend on both in-plane crystallite orientation and thickness. Moreover, the electronic properties of the MnS phases can be further tuned by alloying with phosphorus. Therefore, our results show that electron beam irradiation and thermal annealing can be utilized to grow phases with distinct properties starting from freestanding quasi-2D MnPS3.

Catalytic Activity of Defect-Engineered Transition Me tal Dichalcogenides Mapped with Atomic-Scale Precision by Electrochemical Scanning Tunneling Microscopy.

Unraveling structure-activity relationships is a key objective of catalysis. Unfortunately, the intrinsic complexity and structural heterogeneity of materials stand in the way of this goal, mainly because the activity measurements are area-averaged and therefore contain information coming from different surface sites. This limitation can be surpassed by the analysis of the noise in the current of electrochemical scanning tunneling microscopy (EC-STM). Herein, we apply this strategy to investigate the catalytic activity toward the hydrogen evolution reaction of monolayer films of MoSe2. Thanks to atomically resolved potentiodynamic experiments, we can evaluate individually the catalytic activity of the MoSe2 basal plane, selenium vacancies, and different point defects produced by the intersections of metallic twin boundaries. The activity trend deduced by EC-STM is independently confirmed by density functional theory calculations, which also indicate that, on the metallic twin boundary crossings, the hydrogen adsorption energy is almost thermoneutral. The micro- and macroscopic measurements are combined to extract the turnover frequency of different sites, obtaining for the most active ones a value of 30 s-1 at -136 mV vs RHE.

Formation of In-Plane Semiconductor-Metal Contacts in 2D Platinum Telluride by Converting PtTe2 to Pt2Te2.

Monolayer PtTe2 is a narrow gap semiconductor while Pt2Te2 is a metal. Here we show that the former can be transformed into the latter by reaction with vapor-deposited Pt atoms. The transformation occurs by nucleating the Pt2Te2 phase within PtTe2 islands, so that a metal-semiconductor junction is formed. A flat band structure is found with the Fermi level of the metal aligning with that of the intrinsically p-doped PtTe2. This is achieved by an interface dipole that accommodates the ∼0.2 eV shift in the work functions of the two materials. First-principles calculations indicate that the origin of the interface dipole is the atomic scale charge redistributions at the heterojunction. The demonstrated compositional phase transformation of a 2D semiconductor into a 2D metal is a promising approach for making in-plane metal contacts that are required for efficient charge injection and is of particular interest for semiconductors with large spin-orbit coupling, like PtTe2.

Controlling Stoichiometry in Ultrathin van der Waals Films: PtTe2, Pt2Te3, Pt3Te4, and Pt2Te2.

The platinum-tellurium phase diagram exhibits various (meta)stable van der Waals (vdW) materials that can be constructed by stacking PtTe2 and Pt2Te2 layers. Monophase PtTe2, being the thermodynamically most stable compound, can readily be grown as thin films. Obtaining the other phases (Pt2Te3, Pt3Te4, Pt2Te2), especially in their ultimate thin form, is significantly more challenging. We show that PtTe2 thin films can be transformed by vacuum annealing-induced Te-loss into Pt3Te4- and Pt2Te2-bilayers. These transformations are characterized by scanning tunneling microscopy and X-ray and angle resolved photoemission spectroscopy. Once Pt3Te4 is formed, it is thermally stable up to 350°C. To transform Pt3Te4 into Pt2Te2, a higher annealing temperature of 400°C is required. The experiments combined with density functional theory calculations provide insights into these transformation mechanisms and show that a combination of the thermodynamic preference of Pt3Te4 over a phase segregation into PtTe2 and Pt2Te2 and an increase in the Te-vacancy formation energy for Pt3Te4 compared to the starting PtTe2 material is critical to stabilize the Pt3Te4 bilayer. To desorb more tellurium from Pt3Te4 and transform the material into Pt2Te2, a higher Te-vacancy formation energy has to be overcome by raising the temperature. Interestingly, bilayer Pt2Te2 can be retellurized by exposure to Te-vapor. This causes the selective transformation of the topmost Pt2Te2 layer into two layers of PtTe2, and consequently the synthesis of e Pt2Te3. Thus, all known Pt-telluride vdW compounds can be obtained in their ultrathin form by carefully controlling the stoichiometry of the material.

Self-Driven Broadband Photodetectors Based on MoSe2/FePS3 van der Waals n-p Type-II Heterostructures.

Two-dimensional (2D) van der Waals materials with broadband optical absorption are promising candidates for next-generation UV-vis-NIR photodetectors. FePS3, one of the emerging antiferromagnetic van der Waals materials with a wide bandgap and p-type conductivity, has been reported as an excellent candidate for UV optoelectronics. However, a high sensitivity photodetector with a self-driven mode based on FePS3 has not yet been realized. Here, we report a high-performance and self-powered photodetector based on a multilayer MoSe2/FePS3 type-II n-p heterojunction with a working range from 350 to 900 nm. The presented photodetector operates at zero bias and at room temperature under ambient conditions. It exhibits a maximum responsivity (Rmax) of 52 mA W-1 and an external quantum efficiency (EQEmax) of 12% at 522 nm, which are better than the characteristics of its individual constituents and many other photodetectors made of 2D heterostructures. The high performance of MoSe2/FePS3 is attributed to the built-in electric field in the MoSe2/FePS3 n-p junction. Our approach provides a promising platform for broadband self-driven photodetector applications.

Data-Driven Quest for Two-Dimensional Non-van der Waals Materials.

Two-dimensional (2D) materials are frequently associated with the sheets forming bulk layered compounds bonded by van der Waals (vdW) forces. The anisotropy and weak interaction between the sheets have also been the main criteria in the computational search for new 2D systems, predicting ∼2000 exfoliable compounds. However, some representatives of a new type of non-vdW 2D systems, without layered 3D analogues, were recently manufactured. For this novel materials class, data-driven design principles are still missing. Here, we outline a set of 8 binary and 20 ternary candidates by filtering the AFLOW-ICSD database according to structural prototypes. The oxidation state of the surface cations regulates the exfoliation energy with low oxidation numbers leading to weak bonding─a useful descriptor to obtain novel 2D materials also providing clear guidelines for experiments. A vast range of appealing electronic, optical, and magnetic properties make the candidates attractive for various applications and particularly spintronics.

Boosting the Electrocatalytic Conversion of Nitrogen to Ammonia on Metal-Phthalocyanine-Based Two-Dimensional Conjugated Covalent Organic Frameworks.

The electrochemical N2 reduction reaction (NRR) under ambient conditions is attractive in replacing the current Haber-Bosch process toward sustainable ammonia production. Metal-heteroatom-doped carbon-rich materials have emerged as the most promising NRR electrocatalysts. However, simultaneously boosting their NRR activity and selectivity remains a grand challenge, while the principle for precisely tailoring the active sites has been elusive. Herein, we report the first case of crystalline two-dimensional conjugated covalent organic frameworks (2D c-COFs) incorporated with M-N4-C centers as novel, defined, and effective catalysts, achieving simultaneously enhanced activity and selectivity of electrocatalytic NRR to ammonia. Such 2D c-COFs are synthesized based on metal-phthalocyanine (M = Fe, Co, Ni, Mn, Zn, and Cu) and pyrene units bonded by pyrazine linkages. Significantly, the 2D c-COFs with Fe-N4-C center exhibit higher ammonia yield rate (33.6 µg h-1 mgcat-1) and Faradaic efficiency (FE, 31.9%) at -0.1 V vs reversible hydrogen electrode than those with other M-N4-C centers, making them among the best NRR electrocatalysts (yield rate >30 µg h-1 mgcat-1 and FE > 30%). In situ X-ray absorption spectroscopy, Raman spectroelectrochemistry, and theoretical calculations unveil that Fe-N4-C centers act as catalytic sites. They show a unique electronic structure with localized electronic states at Fermi level, allowing for stronger interaction with N2 and thus faster N2 activation and NRR kinetics than other M-N4-C centers. Our work opens the possibility of developing metal-nitrogen-doped carbon-rich 2D c-COFs as superior NRR electrocatalyst and provides an atomic understanding of the NRR process on M-Nx-C based electrocatalysts for designing high-performance NRR catalysts.

Polymorphic Phases of Metal Chlorides in the Confined 2D Space of Bilayer Graphene.

Unprecedented 2D metal chloride structures are grown between sheets of bilayer graphene through intercalation of metal and chlorine atoms. Numerous spatially confined 2D phases of AlCl3 and CuCl2 distinct from their typical bulk forms are found, and the transformations between these new phases under the electron beam are directly observed by in situ scanning transmission electron microscopy (STEM). The density functional theory calculations confirm the metastability of the atomic structures derived from the STEM experiments and provide insights into the electronic properties of the phases, which range from insulators to semimetals. Additionally, the co-intercalation of different metal chlorides is found to create completely new hybrid systems; in-plane quasi-1D AlCl3 /CuCl2 heterostructures are obtained. The existence of polymorphic phases hints at the unique possibilities for fabricating new types of 2D materials with diverse electronic properties confined between graphene sheets.

Atomistic Simulations of Defect Production in Monolayer and Bulk Hexagonal Boron Nitride under Low- and High-Fluence Ion Irradiation.

Controlled production of defects in hexagonal boron nitride (h-BN) through ion irradiation has recently been demonstrated to be an effective tool for adding new functionalities to this material, such as single-photon generation, and for developing optical quantum applications. Using analytical potential molecular dynamics, we study the mechanisms of vacancy creation in single- and multi-layer h-BN under low- and high-fluence ion irradiation. Our results quantify the densities of defects produced by noble gas ions in a wide range of ion energies and elucidate the types and distribution of defects in the target. The simulation data can directly be used to guide the experiment aimed at the creation of defects of particular types in h-BN targets for single-photon emission, spin-selective optical transitions and other applications by using beams of energetic ions.

Chlorine doping of MoSe2 flakes by ion implantation.

The efficient integration of transition metal dichalcogenides (TMDs) into the current electronic device technology requires mastering the techniques of effective tuning of their optoelectronic properties. Specifically, controllable doping is essential. For conventional bulk semiconductors, ion implantation is the most developed method offering stable and tunable doping. In this work, we demonstrate n-type doping in MoSe2 flakes realized by low-energy ion implantation of Cl+ ions followed by millisecond-range flash lamp annealing (FLA). We further show that FLA for 3 ms with a peak temperature of about 1000 °C is enough to recrystallize implanted MoSe2. The Cl distribution in few-layer-thick MoSe2 is measured by secondary ion mass spectrometry. An increase in the electron concentration with increasing Cl fluence is determined from the softening and red shift of the Raman-active A1g phonon mode due to the Fano effect. The electrical measurements confirm the n-type doping of Cl-implanted MoSe2. A comparison of the results of our density functional theory calculations and experimental temperature-dependent micro-Raman spectroscopy data indicates that Cl atoms are incorporated into the atomic network of MoSe2 as substitutional donor impurities.

Correction: Mirror twin boundaries in MoSe2 monolayers as one dimensional nanotemplates for selective water adsorption.

Correction for 'Mirror twin boundaries in MoSe2 monolayers as one dimensional nanotemplates for selective water adsorption' by Jingfeng Li et al., Nanoscale, 2021, 13, 1038-1047, DOI: 10.1039/D0NR08345C.

Mirror twin boundaries in MoSe2 monolayers as one dimensional nanotemplates for selective water adsorption.

Water adsorption on transition metal dichalcogenides and other 2D materials is generally governed by weak van der Waals interactions. This results in a hydrophobic character of the basal planes, and defects may play a significant role in water adsorption and water cluster nucleation. However, there is a lack of detailed experimental investigations on water adsorption on defective 2D materials. Here, by combining low-temperature scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, we study in that context the well-defined mirror twin boundary (MTB) networks separating mirror-grains in 2D MoSe2. These MTBs are dangling bond-free extended crystal modifications with metallic electronic states embedded in the 2D semiconducting matrix of MoSe2. Our DFT calculations indicate that molecular water also interacts similarly weak with these MTBs as with the defect-free basal plane of MoSe2. However, in low temperature STM experiments, nanoscopic water structures are observed that selectively decorate the MTB network. This localized adsorption of water is facilitated by functionalization of the MTBs by hydroxyls formed by dissociated water. Hydroxyls may form by dissociating of water at undercoordinated defects or adsorbing of radicals from the gas phase in the UHV chamber. Our DFT analysis indicates that the metallic MTBs adsorb these radicals much stronger than on the basal plane due to charge transfer from the metallic states into the molecular orbitals of the OH groups. Once the MTBs are functionalized with hydroxyls, molecular water can attach to them, forming water channels along the MTBs. This study demonstrates the role metallic defect states play in the adsorption of water even in the absence of unsaturated bonds that have been so far considered to be crucial for adsorption of hydroxyls or water.

Polyoxoplatinates as covalently dynamic electron sponges and molecular electronics materials.

In organic systems, dynamic covalent chemistry provides an adaptive approach (i.e., "covalent dynamics") where thermodynamic equilibria are used to tailor structural and electronic changes in molecular assemblies. The covalent dynamics finds utility in the design of novel self-healing materials, sensors, and actuators. Herein, using density functional theory (DFT) we explore the structural, electronic and transport properties of the Pt-based polyoxometalate (POM) [PtIII 12O8(SO4)12]4- and its derivatives. The latter POM has six redox responsive {O-Pt-Pt-O} moieties and prospects for storage of up to twelve electrons, thus exemplifying how dynamic covalent chemistry may manifest itself in fully inorganic systems. Simulations of the Au/POM/Au junction show that the electron conduction strongly depends on the redox of the POM but more weakly on its rotations with respect to the Au surface. Moreover, the POM shows promising spin-polarized current behaviour, which can be modulated using bias and gate voltages.

Effect of interfacial defects on the electronic properties of MoS2 based lateral T-H heterophase junctions.

The coexistence of semiconducting (2H) and metallic (1T) phases of MoS2 monolayers has further pushed their strong potential for applications in the next generation of electronic devices based on two-dimensional lateral heterojunctions. Structural defects have considerable effects on the properties of these 2D devices. In particular, the interfaces of two phases are often imperfect and may contain numerous vacancies created by phase engineering techniques, e.g. under an electron beam. Here, the transport behaviors of the heterojunctions with the existence of point defects are explored by means of first-principles calculations and non-equilibrium Green's function approach. While vacancies in semiconducting MoS2 act as scattering centers, their presence at the interface improves the flow of the charge carriers. In the case of VMo, the current has been increased by two orders of magnitude in comparison to the perfect device. The enhancement of transmission was explained by changes in the electronic densities at the T-H interface, which open new transport channels for electron conduction.

Freestanding and Supported MoS2 Monolayers under Cluster Irradiation: Insights from Molecular Dynamics Simulations.

Two-dimensional (2D) materials with nanometer-size holes are promising systems for DNA sequencing, water purification, and molecule selection/separation. However, controllable creation of holes with uniform sizes and shapes is still a challenge, especially when the 2D material consists of several atomic layers as, e.g., MoS2, the archetypical transition metal dichalcogenide. We use analytical potential molecular dynamics simulations to study the response of 2D MoS2 to cluster irradiation. We model both freestanding and supported sheets and assess the amount of damage created in MoS2 by the impacts of noble gas clusters in a wide range of cluster energies and incident angles. We show that cluster irradiation can be used to produce uniform holes in 2D MoS2 with the diameter being dependent on cluster size and energy. Energetic clusters can also be used to displace sulfur atoms preferentially from either top or bottom layers of S atoms in MoS2 and also clean the surface of MoS2 sheets from adsorbents. Our results for MoS2, which should be relevant to other 2D transition metal dichalcogenides, suggest new routes toward cluster beam engineering of devices based on 2D inorganic materials.

Publisher Correction: Synergistic electroreduction of carbon dioxide to carbon monoxide on bimetallic layered conjugated metal-organic frameworks.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.