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Funding Information: This work has been partially supported by the MIUR (PRIN 2017: Multi-e, 20179337R7) and the Cariparo Foundation (project Synergy, Progetti di Eccellenza 2018). M.B. acknowledges support from the U.S. National Science Foundation under award 2140038. The National Science Centre, Poland, is acknowledged for funding (grant no. 2021/43/D/ST3/02873). The University of Padova is acknowledged for support through the grant P-Disc 2022 (MUSYCA). The “Excellence Initiative – Research University” program is acknowledged for support. Publisher Copyright: © 2023 The Authors. Published by American Chemical Society. 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.

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.

Funding Information: The Jena group received financial support of the Deutsche Forschungsgemeinschaft (DFG) through a research infrastructure grant INST 275/257‐1 FUGG (313713174), CRC 1375 NOA (Project B2, 398816777) and SPP2244 (Project TU149/13‐1, 443361515). This project has also received funding from the joint European Union's Horizon 2020 and DFG research and innovation programme FLAG‐ERA under grant TU149/9‐1 (397373225). Ulm and Jena acknowledge financial support of the joint DFG project within grant 464283495. The authors from Toulouse received funding from the Institute for Quantum Technologies Occitanie, ANR IXTASE and the Institut Universitaire de France. Growth of hexagonal boron nitride crystals was supported by the Elemental Strategy Initiative conducted by the MEXT, Japan, Grant Number JPMXP0112101001, JSPS KAKENHI Grant Number JP20H00354, and the CREST (JPMJCR15F3), JST. A.V.K. further thanks DFG for the support through Project KR 4866/8‐1 and the Collaborative Research Center “Chemistry of Synthetic 2D Materials” SFB‐1415‐417590517. The authors also thank the HZDR Computing Center, HLRS, Stuttgart, Germany, and TU Dresden Cluster “Taurus” for generous grants of CPU time. The authors thank Stephanie Höppener and Ulrich S. Schubert for enabling the Raman spectroscopy and microscopy studies at the Jena Center for Soft Matter (JCSM). Publisher Copyright: © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH. One-pot chemical vapor deposition (CVD) growth of large-area Janus SeMoS monolayers is reported, with the asymmetric top (Se) and bottom (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms. The formation of these 2D semiconductor monolayers takes place upon the thermodynamic-equilibrium-driven exchange of the bottom Se atoms of the initially grown MoSe2 single crystals on gold foils with S atoms. The growth process is characterized by complementary experimental techniques including Raman and X-ray photoelectron spectroscopy, transmission electron microscopy, and the growth mechanisms are rationalized by first principle calculations. The remarkably high optical quality of the synthesized Janus monolayers is demonstrated by optical and magneto-optical measurements which reveal the strong exciton–phonon coupling and enable an exciton g-factor of −3.3.

Funding Information: J.L. and T.J. contributed equally to this work. The USF‐group acknowledges financial support from the NSF through award no. 2140038. A.V.K. thanks the German Research Foundation (DFG), projects KR 4866/2‐1, and KR 4866/7‐1 for the support. The authors further acknowledge the Gauss Centre for Supercomputing e.V. (www.gauss‐centre.eu) for providing computing time on the GCS Supercomputer HAWK at Höchstleistungsrechenzentrum Stuttgart (www.hlrs.de) and TU Dresden (Taurus cluster) for generous grants of CPU time. Publisher Copyright: © 2022 Wiley-VCH GmbH Edges and point defects in layered dichalcogenides are important for tuning their electronic and magnetic properties. By combining scanning tunneling microscopy (STM) with density functional theory (DFT), the electronic structure of edges and point defects in 2D-PtSe2 are investigated where the 1.8 eV bandgap of monolayer PtSe2 facilitates the detailed characterization of defect-induced gap states by STM. The stoichiometric zigzag edge terminations are found to be energetically favored. STM and DFT show that these edges exhibit metallic 1D states with spin polarized bands. Various native point defects in PtSe2 are also characterized by STM. A comparison of the experiment with simulated images enables identification of Se-vacancies, Pt-vacancies, and Se-antisites as the dominant defects in PtSe2. In contrast to Se- or Pt-vacancies, the Se-antisites are almost devoid of gap states. Pt-vacancies exhibit defect induced states that are spin polarized, emphasizing their importance for inducing magnetism in PtSe2. The atomic-scale insights into defect-induced electronic states in monolayer PtSe2 provide the fundamental underpinning for defect engineering of PtSe2-monolayers and the newly identified spin-polarized edge states offer prospects for engineering magnetic properties in PtSe2 nanoribbons.

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. This journal is

The adsorption and dissociation of water molecules on two-dimensional transition metal dichalcogenides (TMDs) is expected to be dominated by point defects, such as vacancies, and edges. At the same time, the role of grain boundaries, and particularly, mirror twinboundaries (MTBs), whose concentration in TMDs can be quite high, is not fully understood. Using density functional theory calculations, we investigate the interaction of water, hydroxyl groups, as well as oxygen and hydrogen molecules with MoSe2 monolayers when MTBs of various types are present. We show that the adsorption of all species on MTBs is energetically favorable as compared to that on the basal plane of pristine MoSe2, but the interaction with Se vacancies is stronger. We further assess the energetics of various surface chemical reactions involving oxygen and hydrogen atoms. Our results indicate that water dissociation on the basal plane should be dominated by vacancies even when MTBs are present, but they facilitate water clustering through hydroxyl groups at MTBs, which can anchor water molecules and give rise to the decoration of MTBs with water clusters. Also, the presence of MTBs affects oxygen reduction reaction (ORR) on the MoSe2 monolayer. Unlike Se vacancies which inhibit ORR due to a high overpotential, it is found that the ORR process on MTBs is more efficient, indicating their important role in the catalytic activity of MoSe2 monolayer and likely other TMDs.

openaire: EC/H2020/720964/EU//npSCOPE 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.

The variation of the electronic structure normal to 1D defects in quasi-freestanding MoS$_2$, grown by molecular beam epitaxy, is investigated through high resolution scanning tunneling spectroscopy at $5$K. Strong upwards bending of valence and conduction bands towards the line defects is found for the 4|4E mirror twin boundary and island edges, but not for the 4|4P mirror twin boundary. Quantized energy levels in the valence band are observed wherever upwards band bending takes place. Focusing on the common 4|4E mirror twin boundary, density functional theory calculations give an estimate of its charging, which agrees well with electrostatic modeling. We show that the line charge can also be assessed from the filling of the boundary-localized electronic band, whereby we provide a measurement of the theoretically predicted quantized polarization charge at MoS$_2$ mirror twin boundaries. These calculations elucidate the origin of band bending and charging at these 1D defects in MoS$_2$. The 4|4E mirror twin boundary not only impairs charge transport of electrons and holes due to band bending, but holes are additionally subject to a potential barrier, which is inferred from the independence of the quantized energy landscape on either side of the boundary.

npj 2D materials and applications 6(1), 42 (2022). doi:10.1038/s41699-022-00318-4, Published by Nature Publishing Group, London

Funding Information: We acknowledge funding from the German Research Foundation (DFG), project KR 48661/1, and through the collaborative research center “Chemistry of Synthetic 2D Materials” SFB-1415-417590517. We further thank the Gauss Centre for Supercomputing e.V. ( www.gauss-centre.eu ) for providing computing time on the GCS Supercomputer HAWK at Höchstleistungsrechenzentrum Stuttgart ( www.hlrs.de ) and also TU Dresden (Taurus cluster) for generous grants of CPU time. The USF group acknowledges financial support from NSF through awards 1801199 and 2140038. Publisher Copyright: © 2021 American Chemical Society. Owing to the relatively strong interlayer interaction, the platinum dichalcogenides exhibit tunability of their electronic properties by controlling the number of layers. Both PtSe2 and PtTe2 display a semimetal to semiconductor transition as they are reduced to bi-or single layers. The value of the fundamental band gap, however, has been inferred only from density functional theory (DFT) calculations, which are notoriously challenging, as different methods give different results, and currently, there is no experimental data. Here, we determine the band gap as a function of the number of layers by local scanning tunneling spectroscopy on molecular beam epitaxy (MBE)-grown PtSe2 and PtTe2 islands. We find band gaps of 1.8 and 0.6 eV for mono-and bilayer PtSe2, respectively, and 0.5 eV for monolayer PtTe2. Trilayer PtSe2 and bilayer PtTe2 are semimetallic. The experimental data are compared to DFT calculations carried out at different levels of theory. The calculated band gaps may differ significantly from the experimental values, emphasizing the importance of the experimental work. We further show that the variations in the calculated fundamental band gap in bilayer PtSe2 are related to the computed separation of the layers, which depends on the choice of the van der Waals functional. This sensitivity of the band gap to interlayer separation also suggests that the gap can be tuned by uniaxial stress, and our simulations indicate that only modest pressures are required for a significant reduction of the gap, making Pt dichalcogenides suitable materials for pressure sensing.

Funding Information: The authors thank the HZDR Computing Center, HLRS, Stuttgart, Germany, and TU Dresden Cluster “Taurus” for generous grants of CPU time. The authors acknowledge the Leibniz Foundation (project ENGRAVE Nr. K335/2020) for support. A.V.K. also thanks the German Research Foundation (DFG) for funding through project KR 4866/6‐1 and the collaborative research center “Chemistry of Synthetic 2D Materials” SFB‐1415‐417590517. The authors thank Dr. M. Ghorbani‐Asl for fruitful discussions. The authors also thank the European Cooperation in Science and Technology (COST) action CA19140 “FIT4NANO” ( https://www.fit4nano.eu ) for support. Publisher Copyright: © 2022 The Authors. physica status solidi (RRL) Rapid Research Letters published by Wiley-VCH GmbH. Using first-principles and analytical potential atomistic simulations, production of defects in epitaxial graphene (EG) on SiC upon ion irradiation for ion types and energies accessible in helium-ion microscope is studied. Graphene-SiC systems consisting of the buffer (zero) graphene layer and SiC substrate, as well as one (monolayer) and two (bilayer) additional graphene layers, are focused on. The probabilities for single, double, and more complex vacancies to appear upon impacts of energetic ions in each graphene layer as functions of He- and Ne-ion energies are calculated and the data are compared with those obtained for free-standing graphene. The results indicate that the role of the substrate is minimal for He-ion irradiation with energies above 5 keV, which can be associated with a low sputtering yield from this system upon ion irradiation, as compared with the common Si/SiO2 substrate. In contrast, SiC substrate has a significant effect on defect production upon Ne-ion irradiation. The results can serve as a guide to the experiments on ion irradiation of EG to choose the optimum ion beam parameters for defect-mediated engineering of such systems, for example, for creating nucleation centers to grow other 2D materials, such as h-BN, on top of the irradiated EG.

Trions, quasi-particles consisting of two electrons combined with one hole or of two holes with one electron, have recently been observed in transition metal dichalcogenides (TMDCs) and drawn increasing attention due to potential applications of these materials in light-emitting diodes, valleytronic devices as well as for being a testbed for understanding many-body phenomena. Therefore, it is important to enhance the trion emission and its stability. In this study, we construct a MoSe2/FePS3 van der Waals heterostructure (vdWH) with type-I band alignment, which allows for carriers injection from FePS3 to MoSe2. At low temperatures, the neutral exciton (X0) emission in this vdWH is almost completely suppressed. The ITrion/Ix0 intensity ratio increases from 0.44 in a single MoSe2 monolayer to 20 in this heterostructure with the trion charging state changing from negative in the monolayer to positive in the heterostructure. The optical pumping with circularly polarized light shows a 14% polarization for the trion emission in MoSe2/FePS3. Moreover, forming such type-I vdWH also gives rise to a 20-fold enhancement of the room temperature photoluminescence from monolayer MoSe2. Our results demonstrate a novel approach to convert excitons to trions in monolayer 2D TMDCs via interlayer doping effect using type-I band alignment in vdWH., 21 pages, including the suppl. materials, accepted at Adv. Fun. Mater

Funding Information: We acknowledge funding from the German Research Foundation (DFG), project KR 48661/1, and through the collaborative research center “Chemistry of Synthetic 2D Materials” SFB-1415-417590517. We acknowledge the German Research Foundation (DFG) and the Ministry of Science, Research and the Arts (MWK) of the federal state of Baden-Württemberg, Germany, in the frame of the SALVE (sub-Angström low-voltage electron microscopy) project (KA1295/21-1). We further thank HLRS, Stuttgart, Germany, and TU Dresden (Taurus cluster) for the generous grant of CPU time. Publisher Copyright: © Atom migrations in single-layer 1H-MoTe2 are studied with Cc/Cs-corrected high-resolution transmission electron microscopy at an electron energy of 40 keV using the electron beam simultaneously for material modification and imaging. After creating tellurium vacancies and vacancy lines, we observe their migration pathways across the lattice. Furthermore, we analyze phase transformations from the 1H- to the 1T′-phase associated with the strain induced due to the formation of Te vacancy lines. Combining the experimental data with the results of first-principles calculations, we explain the energetics and driving forces of point- and line-defect migrations and the phase transformations due to an interplay of electron-beam-induced energy input, atom ejection, and strain spread. Our results enhance the understanding of defect dynamics in 2D transition metal dichalcogenides, which should facilitate tailoring their local optical and electronic properties.

The adsorption and dissociation of water molecules on two-dimensional transition metal dichalcogenides (TMDs) is expected to be dominated by point defects, such as vacancies, and edges. At the same time, the role of grain boundaries, and particularly, mirror twinboundaries (MTBs), whose concentration in TMDs can be quite high, is not fully understood. Using density functional theory calculations, we investigate the interaction of water, hydroxyl groups, as well as oxygen and hydrogen molecules with MoSe

Porous single layer molybdenum disulfide (MoS$_2$) is a promising material for applications such as DNA sequencing and water desalination. In this work, we introduce irradiation with highly charged ions (HCIs) as a new technique to fabricate well-defined pores in MoS$_2$. Surprisingly, we find a linear increase of the pore creation efficiency over a broad range of potential energies. Comparison to atomistic simulations reveals the critical role of energy deposition from the ion to the material through electronic excitation in the defect creation process, and suggests an enrichment in molybdenum in the vicinity of the pore edges at least for ions with low potential energies. Analysis of the irradiated samples with atomic resolution scanning transmission electron microscopy reveals a clear dependence of the pore size on the potential energy of the projectiles, establishing irradiation with highly charged ions as an effective method to create pores with narrow size distributions and radii between ca. 0.3 and 3 nm., 22 pages, 4 figures

Funding Information: The authors thank Benjamin I. Weintrub, Nele Stetzuhn, and Abhijeet Kumar for their great comments on the paper. The authors acknowledge the German Research Foundation (DFG) for financial support through the Collaborative Research Center TRR 227 Ultrafast Spin Dynamics (project B08), SfB 951 (projects A6, A12, B15, Z2). AVK further thanks DFG (projects KR 4866/6‐1 and SFB‐1415‐417590517) for support. The authors thank HRLS Stuttgart, Germany, and TU Dresden (Taurus cluster) for generous grants of computing time. Publisher Copyright: © 2022 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH. A new localized excitonic state is demonstrated in patterned monolayer 2D semiconductors. The signature of an exciton associated with that state is observed in the photoluminescence spectrum after electron beam exposure of several 2D semiconductors. The localized state, which is distinguished by non-linear power dependence, survives up to room temperature and is patternable down to 20 nm resolution. The response of the new exciton to the changes of electron beam energy, nanomechanical cleaning, and encapsulation via multiple microscopic, spectroscopic, and computational techniques is probed. All these approaches suggest that the state does not originate from irradiation-induced structural defects or spatially non-uniform strain, as commonly assumed. Instead, it is shown to be of extrinsic origin, likely a charge transfer exciton associated with the organic substance deposited onto the 2D semiconductor. By demonstrating that structural defects are not required for the formation of localized excitons, this workopens new possibilities for further understanding of localized excitons as well as their use in applications that are sensitive to the presence of defects, e.g. chemical sensing and quantum technologies.

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 {\deg}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 A_1g 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., Comment: 32 pages, 10 figures

Funding Information: This work was supported in China by the National Natural Science Foundation of China (No. 11604047), the Natural Science Foundation of Jiangsu Province (No. BK20160694), the Jiangsu Planned Projects for Postdoctoral Research Funds (No. 2019K010A), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Postgraduate Research and Practice Innovation Program of Jiangsu Province (KYCX20_1065), the Fundamental Research Funds for the Central Universities, and the open research fund of Key Laboratory of MEMS of Ministry of Education, Southeast University, and in USA by NASA (Grant Number 80NSSC19M0236) and NSF Center for the Advancement of Wearable Technologies (Grant 1849243). A.V.K. acknowledges funding from the German Research Foundation (DFG), Project KR 48661/2. The authors are thankful for the computational resources from the High Performance Computing Center of Nanjing Tech University, National Supercomputer Center in Tianjin. Publisher Copyright: © Copyright: Copyright 2021 Elsevier B.V., All rights reserved. Among first experimentally discovered two-dimensional (2D) ferromagnetic materials, chromium triiodide (CrI3) monolayers have attracted particular attention due to their potential applications in electronics and spintronics. However, the Curie temperature Tc of the CrI3 monolayer is below room temperature, which greatly limits practical development of the devices. Herein, using density functional theory calculation, we explore how the electronic and magnetic properties of CrI3 monolayers change upon adsorption of 3d transition-metal (TM) atoms (from Sc to Zn). Our results indicate that the electronic properties of the TM-CrI3 system can be tuned from semiconductor to metal/half-metal/spin gapless semiconductor depending on the choice of the adsorbed TM atoms. Moreover, the adsorption can improve the ferromagnetic stability of CrI3 monolayers by increasing both magnetic moments and Tc. Notably, Tc of CrI3 with Sc and V adatoms can be increased by nearly a factor of 3. We suggest postsynthesis doping of 2D CrI3by deposition of TM atoms as a new route toward potential applications of TM-CrI3 systems in nanoelectronic and spintronic devices.

Funding Information: Y.‐C.L. and K.S. acknowledge JSPS‐KAKENHI (JP16H06333 and 18K14119), the JST‐CREST program (JPMJCR20B1, JMJCR20B5, and JPMJCR1993), the JSPS A3 Foresight Program, and the Kazato Research Encouragement Prize. H.A. acknowledges JSPS‐KAKENHI (18H03864 and 19K22113) and the JST‐CREST program (JPMJCR20B1). A.V.K. thanks the German Research Foundation (DFG; project KR 4866/2‐1) for support. The authors thank HRLS Stuttgart, Germany, TU Dresden (Taurus cluster), and CSC Finland for generously providing computing time. The authors acknowledge Dr. Pablo Solís‐Fernández and Prof. Rika Matsumoto for providing high‐quality BLG and facilitating valuable discussions. Publisher Copyright: © 2021 Wiley-VCH GmbH 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.

Doping of materials beyond the dopant solubility limit remains a challenge, especially when spatially nonuniform doping is required. In 2D materials with a high surface-to-volume ratio, such as transition metal dichalcogenides, various post-synthesis approaches to doping have been demonstrated, but full control over spatial distribution of dopants remains a challenge. A post-growth doping of single layers of WSe2 is performed by adding transition metal (TM) atoms in a two-step process, which includes annealing followed by deposition of dopants together with Se or S. The Ti, V, Cr, and Fe impurities at W sites are identified by using transmission electron microscopy and electron energy loss spectroscopy. Remarkably, an extremely high density (6.4–15%) of various types of impurity atoms is achieved. The dopants are revealed to be largely confined within nanostripes embedded in the otherwise pristine WSe2. Density functional theory calculations show that the dislocations assist the incorporation of the dopant during their climb and give rise to stripes of TM dopant atoms. This work demonstrates a possible spatially controllable doping strategy to achieve the desired local electronic, magnetic, and optical properties in 2D materials.

Irradiation engineering on hexagonal boron nitride reveals insights into the microscopic nature of quantum emitters., Luminescent centers in the two-dimensional material hexagonal boron nitride have the potential to enable quantum applications at room temperature. To be used for applications, it is crucial to generate these centers in a controlled manner and to identify their microscopic nature. Here, we present a method inspired by irradiation engineering with oxygen atoms. We systematically explore the influence of the kinetic energy and the irradiation fluence on the generation of luminescent centers. We find modifications of their density for both parameters, while a fivefold enhancement is observed with increasing fluence. Molecular dynamics simulations clarify the generation mechanism of these centers and their microscopic nature. We infer that VNCB and VB− are the most likely centers formed. Ab initio calculations of their optical properties show excellent agreement with our experiments. Our methodology generates quantum emitters in a controlled manner and provides insights into their microscopic nature.

Publisher Copyright: © 2021 IOP Publishing Ltd. Recently, as a new representative of Heisenberg's two-dimensional (2D) ferromagnetic materials, 2D Cr2Ge2Te6 (CGT), has attracted much attention due to its intrinsic ferromagnetism. Unfortunately, the Curie temperature (T C ) of CGT monolayer is only 22 K, which greatly hampers the development of the applications based on the CGT materials. Herein, by means of density functional theory computations, we explored the electronic and magnetic properties of CGT monolayer under the applied strain. It is demonstrated that the band gap of CGT monolayer can be remarkably modulated by applying the tensile strain, which first increases and then decreases with the increase of tensile strain. In addition, the strain can increase the Curie temperature and magnetic moment, and thus largely enhance the ferromagnetism of CGT monolayer. Notably, the obvious enhancement of T C by 191% can be achieved at 10% strain. These results demonstrate that strain engineering can not only tune the electronic properties, but also provide a promising avenue to improve the ferromagnetism of CGT monolayer. The remarkable electronic and magnetic response to biaxial strain can also facilitate the development of CGT-based spin devices.

Quasi-two-dimensional (2D) sodium chloride (NaCl) crystals of various lateral sizes between graphene sheets were manufactured via supersaturation from a saline solution. Aberration-corrected transmission electron microscopy was used for systematic in situ investigations of the crystals and their decomposition under an 80 kV electron beam. Counterintuitively, bigger clusters were found to disintegrate faster under electron irradiation, but in general no correlation between crystal sizes and electron doses at which the crystals decompose was found. As for the destruction process, an abrupt decomposition of the crystals was observed, which can be described by a logistic decay function. Density-functional theory molecular dynamics simulations provide insights into the destruction mechanism, and indicate that even without account for ionization and electron excitations, free-standing NaCl crystals must quickly disintegrate due to the ballistic displacement of atoms from their surface and edges during imaging. However, graphene sheets mitigate damage development by stopping the displaced atoms and enable the immediate recombination of defects at the surface of the crystal. At the same time, once a hole in graphene appears, the displaced atoms escape, giving rise to the quick destruction of the crystal. Our results provide quantitative data on the stability of encapsulated quasi 2D NaCl crystals under electron irradiation and allow the conclusion that only high-quality graphene is suitable for protecting ionic crystals from beam damage in electron microscopy studies.

Silicon carbide with optically and magnetically active point defects offers unique opportunities for quantum technology applications. Since interaction with these defects commonly happens through optical excitation and de-excitation, a complete understanding of their light-matter interaction in general and optical signatures, in particular, is crucial. Here, we employ quantum mechanical density functional theory calculations to investigate the photoluminescence lineshapes of selected, experimentally observed color centers (including single vacancies, double vacancies, and vacancy impurity pairs) in 4H-SiC. The analysis of zero-phonon lines as well as Huang-Rhys and Debye-Waller factors are accompanied by a detailed study of the underlying lattice vibrations. We show that the defect lineshapes are governed by strong coupling to bulk phonons at lower energies and localized vibrational modes at higher energies. Generally, good agreement to the available experimental data is obtained, and thus we expect our theoretical work to be beneficial for the identification of defect signatures in the photoluminescence spectra and thereby advance the research in quantum photonics and quantum information processing., 8 pages

openaire: EC/H2020/648589/EU//SUPER-2D By combining scanning tunneling microscopy, low-energy electron diffraction, photoluminescence and Raman spectroscopy experiments with molecular dynamics simulations, a comprehensive picture of the structural and electronic response of a monolayer of MoS 2 to 500 eV Xe + irradiation is obtained. The MoS 2 layer is epitaxially grown on graphene/Ir(1 1 1) and analyzed before and after irradiation in situ under ultra-high vacuum conditions. Through optimized irradiation conditions using low-energy ions with grazing trajectories, amorphization of the monolayer is induced already at low ion fluences of 1.5 × 10 14 ions cm -2 and without inducing damage underneath the MoS 2 layer. The crystalline-to-amorphous transformation is accompanied by changes in the electronic properties from semiconductor-to-metal and an extinction of photoluminescence. Upon thermal annealing, the re-crystallization occurs with restoration of the semiconducting properties, but residual defects prevent the recovery of photoluminescence.

Correction to: Nature Communications https://doi.org/10.1038/s41467-020-15141-y, published online 16 March 2020. The original version of this Article did not acknowledge Renhao Dong as a corresponding author. This has now been corrected in both the PDF and HTML versions of the Article.

Silicon carbide is a very promising platform for quantum applications because of extraordinary spin and optical properties of point defects in this technologically-friendly material. These properties are strongly influenced by crystal vibrations, but the exact relationship between them and the behavior of spin qubits is not fully investigated. We uncover the local vibrational modes of the Si vacancy spin qubits in as-grown 4H-SiC. We apply the resonant microwave field to isolate the contribution from one particular type of defects, the so-called V2 center, and observe the zero-phonon line together with seven equally-separated phonon replicas. Furthermore, we present first-principles calculations of the photoluminescence lineshape, which are in excellent agreement with our experimental data. To boost up the calculation accuracy and decrease the computation time, we extract the force constants using machine learning algorithms. This allows us to identify dominant modes in the lattice vibrations coupled to an excited electron during optical emission in the Si vacancy. The resonance phonon energy of 36 meV and the Debye-Waller factor of about 6% are obtained. We establish experimentally that the activation energy of the optically-induced spin polarization is given by the local vibrational energy. Our findings give insight into the coupling of electronic states to vibrational modes in SiC spin qubits, which is essential to predict their spin, optical, mechanical and thermal properties. The approach described can be applied to a large variety of spin defects with spectrally overlapped contributions in SiC as well as in other 3D and 2D materials., 9 pages, 6 figures, corrected references

Highly effective electrocatalysts promoting CO2 reduction reaction (CO2RR) is extremely desirable to produce value-added chemicals/fuels while addressing current environmental challenges. Herein, we develop a layer-stacked, bimetallic two-dimensional conjugated metal-organic framework (2D c-MOF) with copper-phthalocyanine as ligand (CuN4) and zinc-bis(dihydroxy) complex (ZnO4) as linkage (PcCu-O8-Zn). The PcCu-O8-Zn exhibits high CO selectivity of 88%, turnover frequency of 0.39 s−1 and long-term durability (>10 h), surpassing thus by far reported MOF-based electrocatalysts. The molar H2/CO ratio (1:7 to 4:1) can be tuned by varying metal centers and applied potential, making 2D c-MOFs highly relevant for syngas industry applications. The contrast experiments combined with operando spectroelectrochemistry and theoretical calculation unveil a synergistic catalytic mechanism; ZnO4 complexes act as CO2RR catalytic sites while CuN4 centers promote the protonation of adsorbed CO2 during CO2RR. This work offers a strategy on developing bimetallic MOF electrocatalysts for synergistically catalyzing CO2RR toward syngas synthesis., Effective electrocatalyst is crucial in promoting CO2 reduction to address current energy/environmental issue. Here, the authors develop bimetallic layered two-dimensional conjugated metal-organic framework to synergistically and efficiently electro-catalyze CO2 to CO toward syngas synthesis.

The interpretation of helium ion microscopy (HIM) images of crystalline metal clusters requires microscopic understanding of the effects of He ion irradiation on the system, including energy deposition and associated heating, as well as channeling patterns. While channeling in bulk metals has been studied at length, there is no quantitative data for small clusters. We carry out molecular dynamics simulations to investigate the behavior of gold nanoparticles with diameters of 5–15 nm under 30 keV He ion irradiation. We show that impacts of the ions can give rise to substantial heating of the clusters through deposition of energy into electronic degrees of freedom, but it does not affect channeling, as clusters cool down between consecutive impact of the ions under typical imaging conditions. At the same time, high temperatures and small cluster sizes should give rise to fast annealing of defects so that the system remains crystalline. Our results show that ion-channeling occurs not only in the principal low-index, but also in the intermediate directions. The strengths of different channels are specified, and their correlations with sputtering-yield and damage production is discussed, along with size-dependence of these properties. The effects of planar defects, such as stacking faults on channeling were also investigated. Finally, we discuss the implications of our results for the analysis of HIM images of metal clusters.

Recent discovery of intrinsic ferromagnetism in Fe3GeTe2 (FGT) monolayer [ Deng, Y.; et al. Nature 2018, 563, 94-99; Fei, Z.; et al. Nat. Mater. 2018, 17, 778-782 ] not only extended the family of two-dimensional (2D) magnetic materials but also stimulated further interest in the possibility to tune their magnetic properties without changing the chemical composition or introducing defects. By means of density functional theory computations, we explore strain effects on the magnetic properties of the FGT monolayer. We demonstrate that the ferromagnetism can be largely enhanced by the tensile strain in the FGT monolayer due to the competitive effects of direct exchange and superexchange interaction. The average magnetic moments of Fe atoms increase monotonically with an increase in biaxial strain from -5 to 5% in FGT monolayer. The intriguing variation of magnetic moments with strain in the FGT monolayer is related to the charge transfer induced by the changes in the bond lengths. Given the successful fabrication of the FGT monolayer, the strain-tunable ferromagnetism in the FGT monolayer can stimulate the experimental effort in this field. This work also suggests an effective route to control the magnetic properties of the FGT monolayer. The pronounced magnetic response toward the biaxial strain can be used to design the magnetomechanical coupling spintronics devices based on FGT.

Defects and their spatial distribution are crucial factors in controlling the electronic and optical properties of semiconductors. By using scanning transmission electron microscopy and electron energy loss spectroscopy, the type of impurities/defects in WS2 subdomains with different optical properties is successfully assigned. A higher population of Cr impurities is found in the W-terminated edge domain, while the S-terminated domain contains more Fe impurities, in accordance with the luminescence characteristics of chemical-vapor-grown WS2 of a hexagonal shape. In agreement with the first-principles calculations, the domains with Cr substitutional dopants exhibit strong trion emission. Fe atoms tend to gather into trimer configuration and introduce deep acceptor levels which compensate the n-type doping and suppress trion emission. It is also discovered that the domain with higher luminescence but smaller defect concentration tends to get oxidized more rapidly and degrade the 2D structure with many triangular holes. Excitons tend to accumulate at the edges of the oxidized triangular holes and results in enhanced PL emission. The findings indicate that choosing stable elements as dopant and controlling the number of specific edge structures within a crystal domain of 2D transitional metal dichalcogenides can be a new route to improve the optical properties of these materials.