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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.

Funding Information: Financial support through NSF Grant No. 2140038 is acknowledged. In addition, we acknowledge funding from the German Research Foundation (DFG), Project KR 4866/6-1, and through the collaborative research center “Chemistry of Synthetic 2D Materials” SFB-1415-417590517. We further thank the Gauss Centre for Super-computing 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. Publisher Copyright: © 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 â¼0.2 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.

Funding Information: We want to thank all cooperation associates in the framework of this manuscript. Further, we especially thank the Institute Laue-Langevin, for the syntheses of the TMPT materials used in this work. We thank Gabriele Es-Samlaoui for successfully preparing the samples for TEM investigation. Further, we thank Hannu Pekka-Komsa for the fruitful discussions on Debye temperatures in TMPTs. This work is supported by a project funded by Carl Zeiss foundation. The computational support from the Technical University of Dresden computing cluster (TAURUS) and High-Performance Computing Center (HLRS) in Stuttgart is gratefully appreciated. We further thank the German Research Foundation (DFG) through projects KR 4866/8-1, the collaborative research center “Chemistry of Synthetic 2D Materials” CRC-1415-417590517, and the collaborative research center “Exploiting the Human Peptidome for Novel Antimicrobial and Anticancer Agents” CRC-1279-316249678 for financial support. Publisher Copyright: © 2022 The Authors. Published by American Chemical Society. Transition metal phosphorous trisulfides (TMPTs) are inorganic materials with inherent magnetic properties. Due to their layered structure, they can be exfoliated into ultra-thin sheets, which show properties different from their bulk counterparts. Herein, we present a detailed analysis of the interaction of the electron beam (30-80 kV) in a transmission electron microscope with freestanding few-layer TMPTs, with the aim of tailoring their properties. The irradiation-induced structure modifications were systematically investigated by various transmission electron microscopy methods on FePS3, MnPS3, and NiPS3, and the results are rationalized with the help of ab initio calculations, which predict that the knock-on threshold for removing sulfur is significantly lower than that for phosphorus. Therefore, a targeted removal of sulfur is feasible. Eventually, our experiments confirm the dose-dependent, predominant removal of sulfur by the impinging electrons, thus showing the possibility of tuning the sulfur concentration. Using ab initio calculations, we analyze the electronic structure of the TMPTs with single vacancies and oxygen impurities and predict distinct electronic properties depending on the type of defect. Therefore, our study shows the possibility of tuning the properties of ultra-thin freestanding TMPTs by controlling their stoichiometry.

Funding Information: We thank Dr. M. Ghorbani-Asl for fruitful discussion. A.V.K. acknowledges the German Research Foundation (DFG) for the support through project KR 4866/6-1 and the collaborative research center “Chemistry of Synthetic 2D Materials” SFB-1415-417590517. J.H.S. acknowledges financial support from the EU graphene flagship core 3 program. I.V.C. thanks the German Academic Exchange Service (DAAD) program “Mikhail Lomonosov” for support. The authors thank the HZDR Computing Center, HLRS, Stuttgart, Germany, and TU Dresden Cluster “Taurus” for generous grants of CPU time. Publisher Copyright: © 2022 American Chemical Society. Stacking various 2D materials in van der Waals heterostructures is a novel approach to design new systems, which can host alkali metal (AM) atoms to tune their electronic properties or store energy. Using state-of-the-art first-principles calculations, we systematically study the intercalation of the most widespread AMs (Li, Na, and K) into a graphene/MoS2 heterostructure. Contrary to the previous work on the intercalation of AMs into various heterostructures based on 2D materials, we consider not only single-, but also multi-layer configurations of AM atoms. We assess the intercalation energetics for various concentrations of AM atoms, calculate charge transfer from AM atoms to the host system, and show that although intercalation of AMs as a single layer is energetically preferable, multi-layer configurations can exist at high concentrations of AM atoms. We further demonstrate that the transition of the MoS2 layer from the H to T′ phase is possible upon Li intercalation, but not for Na or K. Our findings should help to better understand the behavior of heterostructures upon AM atom intercalation and may stimulate further experiments aimed at the tailoring of heterostructure properties and increasing the capacity of anode materials in AM ion batteries.

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.

Similar to their bulk 3D counterparts, the properties of two-dimensional (2D) materials can be tuned by controllable introduction of impurities and defects using beams of energetic ions, which requires complete microscopic understanding of their response to ion irradiation. The behavior of 2D materials under ion beam is also interesting in the context of their applications in radiation-hostile environments such as cosmic space. While irradiation effects in 3D systems are well understood, a growing body of experimental facts indicate that many concepts of energetic particle-solid interaction developed for 3D materials are not applicable to 2D systems due to their very geometry, or require substantial modifications. In this Chapter, we discuss at length the response of 2D materials to ion irradiation in different regimes with the main focus on the role of reduced dimensionality. We illustrate the differences between the impact of ions on 3D and 2D materials by examples taken from the recent theoretical and experimental studies on the response of 2D materials to ion irradiation and outline general trends in their behavior under irradiation. Finally, we discuss how ion beams can be used to engineer the structure and properties of 2D materials.

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.

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.

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: 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.

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.

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.

Phase engineering has extensively been used to achieve metallization of two-dimensional (2D) semiconducting materials, as it should boost their catalytic properties or improve electrical contacts. In contrast, here we demonstrate compositional phase change by incorporation of excess metals into the crystal structure. We demonstrate post-synthesis restructuring of the semiconducting MoTe2 or MoSe2 host material by unexpected easy incorporation of excess Mo into their crystal planes, which causes local metallization. The amount of excess Mo can reach values as high as 10% in MoTe2 thus creating a significantly altered material compared to its parent structure. The incorporation mechanism is explained by density functional theory in terms of the energy difference of Mo atoms incorporated in the line phases as compared to Mo ad-clusters. Angle resolved photoemission spectroscopy reveals that the incorporated excess Mo induces band gap states up to the Fermi level causing its pinning at these electronic states...

The morphology and electronic properties of single and few-layer graphene films nanostructured by the impact of heavy high-energy ions have been studied. It is found that ion irradiation leads to the formation of nano-sized pores, or antidots, with sizes ranging from 20 to 60 nm, in the upper one or two layers. The sizes of the pores proved to be roughly independent of the energy of the ions, whereas the areal density of the pores increased with the ion dose. With increasing ion energy (>70 MeV), a profound reduction in the concentration of structural defects (by a factor of 2–5), relatively high mobility values of charge carriers (700–1200 cm2 V−1 s−1) and a transport band gap of about 50 meV were observed in the nanostructured films. The experimental data were rationalized through atomistic simulations of ion impact onto few-layer graphene structures with a thickness matching the experimental samples. We showed that even a single Xe atom with energy in the experimental range produces a considerable amount of damage in the graphene lattice, whereas high dose ion irradiation allows one to propose a high probability of consecutive impacts of several ions onto an area already amorphized by the previous ions, which increases the average radius of the pore to match the experimental results. We also found that the formation of “welded” sheets due to interlayer covalent bonds at the edges and, hence, defect-free antidot arrays is likely at high ion energies (above 70 MeV).

Two-dimensional transition metal dichalcogenides (TMDCs) exhibit excellent optoelectronic properties. However, the large band gaps in many semiconducting TMDCs make optical absorption in the near-infrared (NIR) wavelength regime impossible, which prevents applications of these materials in optical communications. In this work, we demonstrate that Ar+ ion irradiation is a powerful post-synthesis technique to tailor the optical properties of the semiconducting tungsten disulfide (WS2) by creating S-vacancies and thus controlling material stoichiometry. First-principles calculations reveal that the S-vacancies give rise to deep states in the band gap, which determine the NIR optical absorption of the WS2 monolayer. As the density of the S-vacancies increases, the enhanced NIR linear and saturable absorption of WS2 is observed, which is explained by the results of first-principles calculations. We further demonstrate that by using the irradiated WS2 as a saturable absorber in a waveguide system, the passively Q-switched laser operations can be optimized, thus opening new avenues for tailoring the optical response of TMDCs by defect-engineering through ion irradiation.

Our scanning tunneling microscopy and X-ray photoelectron spectroscopy experiments along with first-principles calculations uncover the rich phenomenology and enable a coherent understanding of carbon vapor interaction with graphene on Ir(111). At high temperatures, carbon vapor not only permeates to the metal surface but also densifies the graphene cover. Thereby, in addition to underlayer graphene growth, upon cool down also severe wrinkling of the densified graphene cover is observed. In contrast, at low temperatures the adsorbed carbon largely remains on top and self-organizes into a regular array of fullerene-like, thermally highly stable clusters that are covalently bonded to the underlying graphene sheet. Thus, a new type of predominantly sp2-hybridized nanostructured and ultrathin carbon material emerges, which may be useful to encage or stably bind metal in finely dispersed form.

Grain boundaries in 2D materials can have marked influence on the material properties. The effects can be not only detrimental, but also beneficial in transition metal dichalcogenides (TMDs), so that controlling the density and type of the boundaries in these systems should be important for engineering their properties. However, this is often possibly only during the growth stage. Molybdenum and tungsten dichalcogenides feature a particular set of 60° mirror twin boundaries, which are reported to occur upon merging of the growing flakes, to appear during growth to accommodate for the nonstoichiometry of the sample, or to be produced a posteriori by electron irradiation or thermal annealing. Furthermore, different preparation conditions lead to different atomic structure of the boundary, which consequently exhibit different electronic properties. This has obviously garnered interest for the ability to control grain boundary types and densities. In this progress report, the recent experimental and theoretical work related to the characterization of mirror twin boundaries is reviewed. A consistent set of formation energies for the mirror twin boundaries is provided, which then allows a coherent picture on the formation mechanisms under different conditions to be drawn. Finally, the electronic structure of these boundaries is analyzed and their potential applications are discussed.

Two-dimensional (2D) sheets of transition metal phosphorus trichalcogenides (TMPTs) offer unique magnetic and optical properties that can complement those found in other 2D materials. Insights into the structure and properties of these materials can be obtained by a juxtaposition of the experimental and calculated Raman spectra, but there is very little theoretical knowledge of the vibrational properties of TMPTs. Using first-principles calculations, we study mechanical and vibrational properties of a large set of monolayer TMPTs. From the phonon dispersion curves, we assess the dynamical stabilities and general trends on the atomic character of the vibrational modes. We determine Raman active modes from group theory, calculate Raman intensities, and analyze them with the help of the corresponding atomic displacements. We evaluate how the mode frequencies shift in response to a biaxial strain. We also determine elastic properties, which show that these systems are softer than many other layered materials....

Atomic chains are perfect systems for getting fundamental insights into the electron dynamics and coupling between the electronic and ionic degrees of freedom in one-dimensional metals. Depending on the band filling, they can exhibit Peierls instabilities (or charge density waves), where equally spaced chain of atoms with partially filled band is inherently unstable, exhibiting spontaneous distortion of the lattice that further leads to metal-insulator transition in the system. Here, using high-resolution scanning transmission electron microscopy, we directly image the atomic structures of a chain of iodine atoms confined inside carbon nanotubes. In addition to long equidistant chains, the ones consisting of iodine dimers and trimers were also observed, as well as transitions between them. First-principles calculations reproduce the experimentally observed bond lengths and lattice constants, showing that the ionic movement is largely unconstrained in the longitudinal direction, while naturally confined by thenanotube in the lateral directions. Moreover, the trimerized chain bears the hallmarks of a charge density wave. The transition is driven by changes in the charge transfer between the chain and the nanotube and is enabled by the charge compensation and additional screening provided by the nanotube.

Raman spectroscopy is a widely used, powerful, and nondestructive tool for studying the vibrational properties of bulk and low-dimensional materials. Raman spectra can be simulated using first-principles methods but due to the high computational cost calculations are usually limited only to fairly small unit cells, which makes it difficult to carry out simulations for alloys and defects. Here, we develop an efficient method for simulating Raman spectra of alloys, benchmark it against full density-functional theory calculations, and apply it to several alloys of two-dimensional (2D) transition metal dichalcogenides. In this method, the Raman tensor for the supercell mode is constructed by summing up the Raman tensors of the pristine system weighted by the projections of the supercell vibrational modes to those of the pristine system. This approach is not limited to 2D materials and should be applicable to any crystalline solid with defects and impurities. To efficiently evaluate vibrational modes of very large supercells, we adopt mass approximation, although it is limited to chemically and structurally similar atomic substitutions. To benchmark our method, we first apply it to the ${\mathrm{Mo}}_{x}{\mathrm{W}}_{(1\ensuremath{-}x)}{\mathrm{S}}_{2}$ monolayer in the $\mathrm{H}$ phase where several experimental reports are available for comparison. Second, we consider ${\mathrm{Mo}}_{x}{\mathrm{W}}_{(1\ensuremath{-}x)}{\mathrm{Te}}_{2}$ in the ${\mathrm{T}}^{\ensuremath{'}}$ phase, which has been proposed to be a 2D topological insulator but where experimental results for the monolayer alloy are still missing. We show that the projection scheme also provides a powerful tool for analyzing the origin of the alloy Raman-active modes in terms of the parent system eigenmodes. Finally, we examine the trends in characteristic Raman signatures for dilute concentrations of impurities in ${\mathrm{MoS}}_{2}$.

Thermal properties of molybdenum disulfide (MoS$_2$) have recently attracted attention related to fundamentals of heat propagation in strongly anisotropic materials, and in the context of potential applications to optoelectronics and thermoelectrics. Multiple empirical potentials have been developed for classical molecular dynamics (MD) simulations of this material, but it has been unclear which provides the most realistic results. Here, we calculate lattice thermal conductivity of single- and multi-layer pristine MoS$_2$ by employing three different thermal transport MD methods: equilibrium, nonequilibrium, and homogeneous nonequilibrium ones. These methods allow us to verify the consistency of our results and also facilitate comparisons with previous works, where different schemes have been adopted. Our results using variants of the Stillinger-Weber potential are at odds with some previous ones and we analyze the possible origins of the discrepancies in detail. We show that, among the potentials considered here, the reactive empirical bond order (REBO) potential gives the most reasonable predictions of thermal transport properties as compared to experimental data. With the REBO potential, we further find that isotope scattering has only a small effect on thermal conduction in MoS$_2$ and the in-plane thermal conductivity decreases with increasing layer number and saturates beyond about three layers. We identify the REBO potential as a transferable empirical potential for MD simulations of MoS$_2$ which can be used to study thermal transport properties in more complicated situations such as in systems containing defects or engineered nanoscale features. This work establishes a firm foundation for understanding heat transport properties of MoS$_2$ using MD simulations., 14 pages, 6 figures