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Based on density-functional calculations with Hubbard U correction for onsite Coulomb interactions, the structural, electronic, and magnetic properties of V, Mn, and Co-doped FeTe2 monolayers were investigated. Doping is more preferred in Fe-rich conditions than in Te-rich conditions, while Mn inclusion is the most thermodynamically stable in any environment, according to the formation energy. In all doped systems, the energy bandgap was widened, and the electron transport properties were improved. According to our predictions, the V, Mn, Co-doped FeTe2 monolayers are half metal in their ground states with enhanced magnetic moments. These V, Mn, and Co-doped FeTe2 monolayers with interesting electronic and magnetic properties can achieve novel spintronic functionalities.

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... Hence, to calculate electronics and magnetic properties, the GGA + U approach is implemented as well with the GGA to describe the 3d states of the TM. Based on the previous reports and findings the Hubbard U factor 4 eV is adopted for the Mn [44,[61][62][63][64][65][66][67]. In these calculations, we used the basis set triple zeta plus polarization (TZP) with a confinement radius up to 10 Bohr [68]. ...

Two-dimensional tin (II) monoxide (SnO) has shown great potential for future electronics, optoelectronics and thin film transistor devices. Using first-principles calculations, we investigated the structural, electronic, and magnetic properties of the Mn-doped SnO monolayer. The structural stability of the materials was examined which pointed to the feasibility of Mn substitutional doping in pure SnO monolayer. The doping-induced spin polarization revealed magnetic behavior which is due to the interaction between the dopants and the surrounding Sn and O atoms. The results show that the spin-split defect states are produced in the bandgap and a magnetic moment of 4.85 µB is observed. Along with the standard GGA approach, Hubbard U correction is also adopted to calculate the electronic and magnetic properties of the doped material, which unveiled the opening of the bandgap and an increase in the magnetic moment. The magnetic behavior of the dopant is discussed in the context of crystal field splitting in the square planner geometry of the host. The magnetic coupling between magnetic moments caused by two Mn atoms in the SnO monolayer is ferromagnetic, which is due to the p–d exchange interactions. It is found that Mn-doped monolayer SnO turns out to be a promising candidate for realizing a p-type diluted-magnetic-semiconducting metal oxide.

Recently, ferromagnetism observed in monolayer 2D materials draws attentions due to the promising applications in next-generation spintronics. Here, we predicted a novel symmetry-breaking phase in 2D FeTe2 which differs from conventional transition metal ditellurides shows superior stability and room-temperature ferromagnetism. Through DFT calculations, we found the exchange interactions in FeTe2 consist of short-range super-exchange and long-range oscillatory exchanges mediated by itinerant electrons. Up to 6 nearest neighbor (NN), the exchange constants are calculated to be 50.95meV, 33.41meV, 2.70meV, 11.02meV, 14.46meV and -4.12meV, respectively. Furthermore, the strong relativistic effects on Te2+ induce giant out-of-plane exchange anisotropy and open up a significantly large spin-wave gap of ΔSW=1.22meV. All these lead to the robust ferromagnetism with Tc surpassing 423K, which is predicted by renormalization group Monte Carlo method, sufficiently higher than room-temperature. The findings herewith shed a light for the promising future of FeTe2 in 2D magnetic researches and spintronic applications.

The emergence of low-dimensional nanomaterials has brought revolutionized development of magnetism, as the size effect can significantly influence the spin arrangement. Since the first demonstration of truly two-dimensional magnetic materials (2DMMs) in 2017, a wide variety of magnetic phases and associated properties have been exhibited in these 2DMMs, which offer a new opportunity to manipulate the spin-based devices efficiently in the future. Herein, we focus on the recent progress of 2DMMs and heterostructures in the aspects of their structural characteristics, physical properties, and spintronic applications. Firstly, the microscopy characterization of the spatial arrangement of spins in 2D lattices is reviewed. Afterwards, the optical probes in the light-matter-spin interactions at the 2D scale are discussed. Then, particularly, we systematically summarize the recent work on the electronic and spintronic devices of 2DMMs. In the section of electronic properties, we raise several exciting phenomena in 2DMMs, i.e., long-distance magnon transport, field-effect transistors, varying magnetoresistance behavior, and (quantum) anomalous Hall effect. In the section of spintronic applications, we highlight spintronic devices based on 2DMMs, e.g., spin valves, spin-orbit torque, spin field-effect transistors, spin tunneling field-effect transistors, and spin-filter magnetic tunnel junctions. At last, we also provide our perspectives on the current challenges and future expectations in this field, which may be a helpful guide for theorists and experimentalists who are exploring the optical, electronic, and spintronic properties of 2DMMs.

The existence of spontaneous magnetization in low dimensional magnetic systems has attracted intensive studies since the early 60s and research remains very active even now. Only recently, magnetic van der Waals (vdW) systems down to a few layers have been broadly discussed for their magnetic order ground states at finite temperature. The naturally inherited layered structure of the vdW magnetic systems possessing onsite magnetic anisotropy from band electrons can suppress the long‐range fluctuations. This provides an excellent vehicle to study the transition of magnetism to 2D limits both theoretically and experimentally. Here the current status of 2D vdW magnetic system and its potential applications are briefly summarized and discussed. Recently, magnetic van der Waals (vdW) systems down to the single layer limit have been studied intensively. Herein, the current status of 2D vdW magnetic systems is summarized from both a theoretical and an experimental point of view. The fundamental physics of 2D magnetism are discussed, along with the future outlook and potential applications of 2D vdW magnetic systems.

Valley degree of freedom in the first Brillouin zone of Bloch electrons offers an innovative approach to information storage and quantum computation. Broken inversion symmetry together with the presence of time-reversal symmetry endows Bloch electrons non-zero Berry curvature and orbital magnetic moment, which contribute to the valley Hall effect and optical selection rules in valleytronics. Furthermore, the emerging transition metal dichalcogenides (TMDs) materials naturally become the ideal candidates for valleytronics research attributable to their novel structural, photonic and electronic properties, especially the direct bandgap and broken inversion symmetry in the monolayer. However, the mechanism of inter-valley relaxation remains ambiguous and the complicated manipulation of valley predominantly incumbers the realization of valleytronic devices. In this review, we systematically demonstrate the fundamental properties and tuning strategies (optical, electrical, magnetic and mechanical tuning) of valley degree of freedom, summarize the recent progress of TMD-based valleytronic devices. We also highlight the conclusion of present challenges as well as the perspective on the further investigations in valleytronics.

Using first-principles calculation, a systematic study on the effect of transition metal (TM: V, Cr, Mn, Fe, Co, Ni) dopant atoms in the MoS2 monolayer as potential spintronic, catalytic and optoelectronic materials were carried out. The electronic and magnetic properties of this monolayer changed due to the presence of the TM ion dopants. The calculated substitutional energies indicate that it is energetically favourable to introduce TM ions into the MoS2 lattice under the S-rich condition compared to the Mo-rich condition. The calculated binding energies also show that TM ions exhibit a dispersive distribution rather than the suggested multi-site configurations considered in the MoS2 lattice. This is because most of the considered dopant multi-site configurations of a particular dopant ion are not energetically favourable compared to the single site configuration. Generally, there is a reduction in the electronic band gap of doped MoS2 compounds as well extra absorption peaks in the absorption spectra. The calculated redox potentials of H2O splitting show that Cr doped MoS2 monolayer can be potential photo-reductants. This theoretical investigation provides further insight into the application of MoS2 as ultra-thin spintronic material in the case of V, Fe and Mn doped monolayers.

Triggered by the growing needs of developing semiconductor devices at ever‐decreasing scales, strain engineering of 2D materials has recently seen a surge of interest. The goal of this principle is to exploit mechanical strain to tune the electronic and photonic performance of 2D materials and to ultimately achieve high‐performance 2D‐material‐based devices. Although strain engineering has been well studied for traditional semiconductor materials and is now routinely used in their manufacturing, recent experiments on strain engineering of 2D materials have shown new opportunities for fundamental physics and exciting applications, along with new challenges, due to the atomic nature of 2D materials. Here, recent advances in the application of mechanical strain into 2D materials are reviewed. These developments are categorized by the deformation modes of the 2D material–substrate system: in‐plane mode and out‐of‐plane mode. Recent state‐of‐the‐art characterization of the interface mechanics for these 2D material–substrate systems is also summarized. These advances highlight how the strain or strain‐coupled applications of 2D materials rely on the interfacial properties, essentially shear and adhesion, and finally offer direct guidelines for deterministic design of mechanical strains into 2D materials for ultrathin semiconductor applications. The strain engineering of 2D materials is particularly exciting, because an individual sheet can survive remarkably large mechanical strain and its atomic thinness allows mechanical deformations like a piece of paper. These exceptional circumstances create opportunities for the study of new fundamental physics and applications of 2D materials emerging at the large strain level.

Reduced dimensionality and interlayer coupling in van der Waals materials gives rise to fundamentally different electronic1, optical2and many-body quantum3-5properties in monolayers compared with the bulk. This layer-dependence permits the discovery of novel material properties in the monolayer regime. Ferromagnetic order in two-dimensional materials is a coveted property that would allow fundamental studies of spin behaviour in low dimensions and enable new spintronics applications6-8. Recent studies have shown that for the bulk-ferromagnetic layered materials CrI3(ref.9) and Cr2Ge2Te6(ref.10), ferromagnetic order is maintained down to the ultrathin limit at low temperatures. Contrary to these observations, we report the emergence of strong ferromagnetic ordering for monolayer VSe2, a material that is paramagnetic in the bulk11,12. Importantly, the ferromagnetic ordering with a large magnetic moment persists to above room temperature, making VSe2an attractive material for van der Waals spintronics applications.

Recently, two dimensional transition metal dichalcogenides MX2 (M=Mo, W, etc; X=S, Se, Te) have ignited immense interests because of their unique structural and physical properties for the potential applications in the nano-optoelectronics, valley-spintronics etc. In terms of the structural compatibility and van der Waals interaction, 2D MX2 layers can be fabricated into various lateral and vertical hetero-structures. The atomically-thin hetero-structures comprising different layered MX2 provide a new platform for exploring fundamental physics and device technologies with unprecedented phenomenon and extraordinary functionalities. In this review, we report the recent progress about the fabrication, properties and applications of 2D hetero-structures based on transition metal dichalcogenides.

First-principles calculations are performed to study the magnetic anisotropy of monolayer VS2. The magnetic anisotropy energy (MAE) for H-VS2 is −0.213 meV, and the magnetic preferential direction is in the monolayer plane, while the corresponding value for T-VS2 is only 0.004 meV, which can be ignored for two-dimensional materials. According to the second-order perturbation of the spin–orbit coupling (SOC) interactions, the physical origin of magnetic anisotropy for H-VS2 is derived from the occupied and unoccupied dxy/dx 2-y 2 states with different spin channels in light of the electronic structure. In the reciprocal space, the negative contributions mainly stem from the corners of hexagonal Brillouin zone. Interestingly, there are non-equivalent K and K′ for the MAEs, which are observed for the first time for MAE in the reciprocal space. We predict that the lack of inversion symmetry results in the different signs of MAEs in the K and K′. Our studies open up broad prospects to trace the physical origin of magnetic anisotropy in the reciprocal space.

Theoretically it has been long known that breaking spin-degeneracy and realizing so-called 'spinless fermions' is a promising path to a topological superconductor. However, topological superconductors are rare to date. We propose a new strategy for realizing 'spinless fermions' by splitting the spin degeneracy in momentum-space ($k$-space). Specifically, we identify monolayer hole-doped (p-type) transition metal dichalcogenide (TMD)s as candidates that can materialize topological superconductors out of such $k$-space-split 'spinless fermions'. In fact, superconductivity in electron-doped (n-type) TMDs is by now well established. On the other hand, the possibility of superconductivity in the p-type TMDs remains unexplored. Employing a renormalization group analysis, we propose that the unusual spin-valley locking in the p-type TMDs will selectively favor two topological superconducting states, an inter-pocket pairing state with Chern number $|C|=2$ and an intra-pocket pairing with finite pair momentum, in the presence of repulsive interactions expected from the $d-$orbital character of the carriers. A confirmation of our predictions will open up new possibilities for manipulating topological superconductors on the device friendly platform of monolayer TMDs.

Manipulating electronic and magnetic properties of two-dimensional (2D) transitional-metal dichalcogenides (TMDs) MX2 by doping has raised a lot of attention recently. By performing the first-principles calculations, we have investigated the structural, electronic, and magnetic properties of transitional metal (TM)-doped MoS2 at low and high impurity concentrations. Our calculation result indicates that the five elements of V-, Mn-, Fe-, Co-, and Cu-doped monolayer MoS2 at low impurity concentration all give rise to the good diluted magnetic semiconductors. By studying various configurations with different TM-TM separations, we found that the impurity atoms prefer to stay together in the nearest neighboring (NN) configuration, in which the doped TM atoms are FM coupling except for Fe doping at 12 % concentration. For V, Mn, and Fe doping, the total magnetic moment is smaller than the local magnetic moment of the dopants because the induced spins on the nearby host atoms are antiparallel to that of the doped atoms. In contrast, Co and Cu doping both give the higher total magnetic moment. Especially, Cu doping induces strong ferromagnetism relative to the local spins. However, the atomic structures of Co- and Cu-doped MoS2 deviate from the original prismatic configuration, and the magnetic moments of the doped systems decrease at 12 % impurity concentration although both elements give higher magnetic moments at 8 % impurity concentration. Our calculations indicate that V and Mn are promising candidates for engineering and manipulating the magnetism of the 2D TMDs.
Electronic supplementary material
The online version of this article (doi:10.1186/s11671-016-1376-y) contains supplementary material, which is available to authorized users.

Via the hydrothermal method, we synthesized MoS2 nanosheets with varying Co dopant concentrations of 0%, 3%, 7%, using cobaltous acetate as a promoter, and marked as A, B, and C, respectively. We found that the thickness and flatness of the nanosheets increased with the increase of the co dopant concentrations. Meanwhile, the BET Surface Area of samples (A, B, and C) decreased with the increase of the Co dopant concentrations. Optical absorption spectroscopy showed that, compared to sample A, the A1 and B1 excitons of samples B and C were 10 and 23 meV redshifted, respectively. Then, we performed magnetization measurement to investigate the effect of co-doping; the unique result implied that the values of the magnetic moment decreased with the increase of the Co dopant concentrations. We performed DFT computations to address the above magnetic result. The computational result indicated that the value of the magnetic moment decreased with the increase of the Co dopant concentrations, which is in agreement with results of the experiments described above.

Transition metal dichalcogenides (TMDCs) have emerged as a new two
dimensional materials field since the monolayer and few-layer limits show
different properties when compared to each other and to their respective bulk
materials. For example, in some cases when the bulk material is exfoliated down
to a monolayer, an indirect-to-direct band gap in the visible range is
observed. The number of layers $N$ ($N$ even or odd) drives changes in space
group symmetry that are reflected in the optical properties. The understanding
of the space group symmetry as a function of the number of layers is therefore
important for the correct interpretation of the experimental data. Here we
present a thorough group theory study of the symmetry aspects relevant to
optical and spectroscopic analysis, for the most common polytypes of TMDCs,
i.e. $2Ha$, $2Hc$ and $1T$, as a function of the number of layers. Real space
symmetries, the group of the wave vectors, the relevance of inversion symmetry,
irreducible representations of the vibrational modes, optical selection rules
and Raman tensors are discussed.

Ferromagnetic thin films of Heusler compounds are highly relevant for spintronic applications owing to their predicted half-metallicity, that is, 100% spin polarization at the Fermi energy. However, experimental evidence for this property is scarce. Here we investigate epitaxial thin films of the compound Co2MnSi in situ by ultraviolet-photoemission spectroscopy, taking advantage of a novel multi-channel spin filter. By this surface sensitive method, an exceptionally large spin polarization of () % at room temperature is observed directly. As a more bulk sensitive method, additional ex situ spin-integrated high energy X-ray photoemission spectroscopy experiments are performed. All experimental results are compared with advanced band structure and photoemission calculations which include surface effects. Excellent agreement is obtained with calculations, which show a highly spin polarized bulk-like surface resonance ingrained in a half metallic bulk band structure.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit novel
electrical and optical properties and are emerging as a new platform for
exploring 2D semiconductor physics. Reduced screening in 2D results in
dramatically enhanced electron-electron interactions, which have been predicted
to generate giant bandgap renormalization and excitonic effects. Currently,
however, there is little direct experimental confirmation of such many-body
effects in these materials. Here we present an experimental observation of
extraordinarily large exciton binding energy in a 2D semiconducting TMD. We
accomplished this by determining the single-particle electronic bandgap of
single-layer MoSe2 via scanning tunneling spectroscopy (STS), as well as the
two-particle exciton transition energy via photoluminescence spectroscopy (PL).
These quantities yield an exciton binding energy of 0.55 eV for monolayer
MoSe2, a value that is orders of magnitude larger than what is seen in
conventional 3D semiconductors. This finding is corroborated by our ab initio
GW and Bethe Salpeter equation calculations, which include electron correlation
effects. The renormalized bandgap and large exciton binding observed here will
have a profound impact on electronic and optoelectronic device technologies
based on single-layer semiconducting TMDs.

Quantum systems in confined geometries are host to novel physical phenomena. Examples include quantum Hall systems in semiconductors and Dirac electrons in graphene. Interest in such systems has also been intensified by the recent discovery of a large enhancement in photoluminescence quantum efficiency and a potential route to valleytronics in atomically thin layers of transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se, Te), which are closely related to the indirect-to-direct bandgap transition in monolayers. Here, we report the first direct observation of the transition from indirect to direct bandgap in monolayer samples by using angle-resolved photoemission spectroscopy on high-quality thin films of MoSe2 with variable thickness, grown by molecular beam epitaxy. The band structure measured experimentally indicates a stronger tendency of monolayer MoSe2 towards a direct bandgap, as well as a larger gap size, than theoretically predicted. Moreover, our finding of a significant spin-splitting of ∼180 meV at the valence band maximum of a monolayer MoSe2 film could expand its possible application to spintronic devices.

A broad classification scheme is proposed for half-metallic ferromagnets which embraces the possibilities of itinerant and localized electrons, as well as semimetallic and semiconducting electronic structure. Examples of each type are given. The problems of defining and measuring spin polarization are discussed and some characteristics of half-metals are reviewed with reference to chromium dioxide. © 2002 American Institute of Physics.

The quasiparticle (QP) band structures of both strainless and strained
monolayer MoS$_{2}$ are investigated using more accurate many body perturbation
\emph{GW} theory and maximally localized Wannier functions (MLWFs) approach. By
solving the Bethe-Salpeter equation (BSE) including excitonic effects on top of
the partially self-consistent \emph{GW$_{0}$} (sc\emph{GW$_{0}$}) calculation,
the predicted optical gap magnitude is in a good agreement with available
experimental data. With increasing strain, the exciton binding energy is nearly
unchanged, while optical gap is reduced significantly. The sc\emph{GW$_{0}$}
and BSE calculations are also performed on monolayer WS$_{2}$, similar
characteristics are predicted and WS$_{2}$ possesses the lightest effective
mass at the same strain among monolayers Mo(S,Se) and W(S,Se). Our results also
show that the electron effective mass decreases as the tensile strain
increases, resulting in an enhanced carrier mobility. The present calculation
results suggest a viable route to tune the electronic properties of monolayer
transition-metal dichalcogenides (TMDs) using strain engineering for potential
applications in high performance electronic devices.

We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.

We demonstrate how by taking better account of electron correlations in the 3d shell of metal ions in nickel oxide it is possible to improve the description of both electron energy loss spectra and parameters characterizing the structural stability of the material compared with local spin density functional theory.

The energy of a large number of oxidation reactions of 3d transition metal oxides is computed using the generalized gradient approach (GGA) and GGA+U methods. Two substantial contributions to the error in GGA oxidation energies are identified. The first contribution originates from the overbinding of GGA in the O-2 molecule and only occurs when the oxidant is O-2. The second error occurs in all oxidation reactions and is related to the correlation error in 3d orbitals in GGA. Strong self-interaction in GGA systematically penalizes a reduced state (with more d electrons) over an oxidized state, resulting in an overestimation of oxidation energies. The constant error in the oxidation energy from the O-2 binding error can be corrected by fitting the formation enthalpy of simple nontransition metal oxides. Removal of the O-2 binding error makes it possible to address the correlation effects in 3d transition metal oxides with the GGA+U method. Calculated oxidation energies agree well with experimental data

The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blöchl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.

Two-dimensional (2D) few-layer VSe2, V1−xFexSe2 nanosheets have been synthesized by a high-temperature organic solution-phase method. The thickness of VSe2 nanosheets can be tuned from 12 to 5 layers by decreasing the precursor concentrations. The few-layer VSe2 nanosheets show the room-temperature ferromagnetism. The coercivity and magnetization reach 0.024 T and 0.036 mA·m2·g−1 at room temperature. The charge-density wave behavior is also confirmed in VSe2 by the hysteresis loops and zero-field-cooling curve. V1−xFexSe2 nanosheets can be obtained by doping Fe(acac)3 in the reaction process. The room-temperature coercivity and magnetization of V0.8Fe0.2Se2 nanosheets are 5 times higher than those of the pure VSe2 nanosheets without destroying the structures. The enhancement of magnetization is due to the coupling interaction of 3d orbits between V and Fe atoms. Higher Fe concentration is beneficial to improve the coercivity, which is attributed to the formation of the second phase Fe3Se4. This simple chemical preparation method can be extended to prepare the other 2D materials.

We performed the first-principles calculation on common thermoelectric semiconductors Bi 2 Te 3 , Bi 2 Se 3 , SiGe, and PbTe in bulk three-dimension (3D) and two-dimension (2D). We found that miniaturisation of materials does not generally increase the thermoelectric figure of merit ( ZT ) according to the Hicks and Dresselhaus (HD) theory. For example, ZT values of 2D PbTe (0.32) and 2D SiGe (0.04) are smaller than their 3D counterparts (0.49 and 0.09, respectively). Meanwhile, the ZT values of 2D Bi 2 Te 3 (0.57) and 2D Bi 2 Se 3 (0.43) are larger than the bulks (0.54 and 0.18, respectively), which agrees with HD theory. The HD theory breakdown occurs because the band gap and band flatness of the materials change upon dimensional reduction. We found that flat bands give a larger electrical conductivity ( σ ) and electronic thermal conductivity ( κ el ) in 3D materials, and smaller values in 2D materials. In all cases, maximum ZT values increase proportionally with the band gap and saturate for the band gap above 10 k B T . The 2D Bi 2 Te 3 and Bi 2 Se 3 obtain a higher ZT due to the flat corrugated bands and narrow peaks in their DOS. Meanwhile, the 2D PbTe violates HD theory due to the flatter bands it exhibits, while 2D SiGe possesses a small gap Dirac-cone band.

Understanding and predicting the thermodynamic properties of point defects in semiconductors and insulators would greatly aid in the design of novel materials and allow tuning the properties of existing ones. As a matter of fact, first-principles calculations based on density functional theory (DFT) and the supercell approach have become a standard tool for the study of point defects in solids. However, in the dilute limit, of most interest for the design of novel semiconductor materials, the “raw“ DFT calculations require an extensive post-processing. Spinney is an open-source Python package developed with the aim of processing first-principles calculations to obtain several quantities of interest, such as the chemical potential limits that assure the thermodynamic stability of the defect-laden system, defect charge transition levels, defect formation energies, including electrostatic corrections for finite-size effects, and defect and carrier concentrations. In this paper we demonstrate the capabilities of the Spinney code using c-BN, GaN:Mg, TiO2 and ZnO as examples.
Program summary
Program Title: Spinney
CPC Library link to program files: https://doi.org/10.17632/2xp4ddwmgx.1
Developer’s repository link: https://gitlab.com/Marrigoni/spinney
Code Ocean capsule: https://codeocean.com/capsule/4970623
Licensing provisions: MIT
Programming language: Python 3
External libraries: NumPy [1], SciPy [2], Pandas [3], Matplotlib [4], ASE [5]
Nature of problem: Post-processing of first-principles calculations in order to obtain important properties of defect laden systems in the dilute-limit: chemical potential values ensuring thermodynamic stability, thermodynamic charge transition levels, defect formation energies and corrections thereof using state-of-the-art corrections schemes for electrostatic finite-size effects, equilibrium defect and carriers concentrations.
Solution method: Flexible low-level interface for allowing the post-processing of the raw fist-principles data provided by any computer code. High-level interface for parsing and post-processing the first-principles data produced by the popular computer codes VASP and WIEN2k.
Additional comments including restrictions and unusual features: An extensive documentation is available at: https://spinney.readthedocs.io

Two-dimensional (2D) van der Waals transition metal dichalcogenides (TMDs) are a new class of electronic materials offering tremendous opportunities for advanced technologies and fundamental studies. Similar to conventional semiconductors, substitutional doping is key to tailoring their electronic properties and enabling their device applications. Here, we review recent progress in doping methods and understanding of doping effects in group 6 TMDs (MX2, M = Mo, W; X = S, Se, Te), which are the most widely studied model 2D semiconductor system. Experimental and theoretical studies have shown that a number of different elements can substitute either M or X atoms in these materials and act as n- or p-type dopants. This review will survey the impact of substitutional doping on the electrical and optical properties of these materials, discuss open questions, and provide an outlook for further studies.

We study the electronic structure and magnetism of monolayer 3d transition-metal ditellurides MTe2 (M = Ti, V, Cr, Mn, Fe, Co and Ni) in trigonal prismatic H- and / or octahedral T-phase by means of the first-principles calculations. The results show that H-VTe2, T-VTe2, H-FeTe2 and T-MnTe2 monolayers exhibit intrinsic ferromagnetism, and the others have no ferromagnetism. The exchange splitting of V, Mn and Fe 3d orbitals is responsible for ferromagnetism. The exchange constant and the Curie temperature are estimated by using 2D Ising model and mean field theory. Among the four ferromagnetic monolayers, the H-VTe2 monolayer has the largest exchange constant and the corresponding Curie temperature is near room temperature. Calculations also show that the T-VTe2, H-VTe2 and H-FeTe2 monolayers have in-plane easy magnetization direction, while the easy direction of the T-MnTe2 monolayer is perpendicular to the layer. Moreover, we analyze the relative exchange strength of the four ferromagnetic monolayers based on the competition between the through-bond ferromagnetic interaction and the through-space antiferromagnetic interaction according to the Goodenough−Kanamori rules. The magnetocrystalline anisotropy is explained qualitatively based on the second-order perturbation theory from the spin-orbit coupling between 3d orbitals of M atoms.

Our analysis based on the results of hybrid and semilocal density-functional calculations with and without Hubbard U correction for on-site Coulomb interactions reveals the true magnetic ground states of three transition-metal dichalcogenide monolayers, viz., FeTe2,VS2, and NiTe2, which comprise inhomogeneous magnetic moment configurations. In contrast to earlier studies considering only the magnetic moments of transition-metal atoms, the chalcogen atoms by themselves have significant, antiparallel magnetic moments owing to the spin polarization through p−d hybridization. The latter is found to be true for both H and T phases of FeTe2,VS2, and NiTe2 monolayers. Our predictions show that the FeTe2 monolayer in its lowest-energy structure is a half metal, which prevails under both compressive and tensile strains. Half metallicity occurs also in the FeTe2 bilayer but disappears in thicker multilayers. The VS2 monolayer is a magnetic semiconductor; it has two different band gaps of different character and widths for different spin polarization. The NiTe2 monolayer, which used to be known as a nonmagnetic metal, is indeed a magnetic metal with a small magnetic moment. These monolayers with intriguing electronic and magnetic properties can attain new functionalities for spintronic applications.

The ultimate in thin-film magnetism
The alignment of the magnetic properties of atoms gives rise to a wealth of simple and exotic properties that can be exploited. As the dimension of the material is reduced, such that the atoms are in a single monolayer, it was widely believed that thermal fluctuations overwhelm and prevent magnetic ordering. Gong and Zhang review the developments that have followed the recent discovery of magnetism in two-dimensional materials. Recognizing that magnetic anisotropy can be used to induce stable magnetism in atomic monolayers, they provide an overview of the materials available and the physical understanding of the effects and then discuss how these effects could be exploited for widespread practical applications.
Science , this issue p. eaav4450

Monolayer van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for scientific and technological advances, but the intrinsic ferromagnetism has only been observed at low temperatures. Here, we report the observation of room temperature ferromagnetism in manganese selenide (MnSe$_x$) films grown by molecular beam epitaxy (MBE). Magnetic and structural characterization provides strong evidence that in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe$_2$) monolayer, while for thicker films it could originate from a combination of vdW MnSe$_2$ and/or interfacial magnetism of $\alpha$-MnSe(111). Magnetization measurements of monolayer MnSe$_x$ films on GaSe and SnSe$_2$ epilayers show ferromagnetic ordering with large saturation magnetization of ~ 4 Bohr magnetons per Mn, which is consistent with density functional theory calculations predicting ferromagnetism in monolayer 1T-MnSe$_2$. Growing MnSe$_x$ films on GaSe up to high thickness (~ 40 nm) produces $\alpha$-MnSe(111), and an enhanced magnetic moment (~ 2x) compared to the monolayer MnSe$_x$ samples. Detailed structural characterization by scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), and reflection high energy electron diffraction (RHEED) reveal an abrupt and clean interface between GaSe(0001) and $\alpha$-MnSe(111). In particular, the structure measured by STEM is consistent with the presence of a MnSe$_2$ monolayer at the interface. These results hold promise for potential applications in energy efficient information storage and processing.

Two dimensional (2D) single crystal layered transition materials have got extensive considerations owing to their interesting magnetic properties originated from their lattices and strong spin-orbit coupling, which make them of vital importance for spintronic application. Herein, we present synthesis of a highly crystalline tungsten diselenide layered single crystals grown by chemical vapor transport technique and doped with nickel (Ni) to tailor its magnetic properties. The pristine WSe2 single crystal and Ni doped one were characterized and analyzed for magnetic properties from both experimental and computational aspects. It is found that the magnetic behavior of 2D layered WSe2crystal changes from diamagnetic to ferromagnetic after Ni doping at all tested temperatures. Moreover, first principle density functional theory (DFT) calculations further confirmed the origin of room temperature ferromagnetism of Ni doped WSe2, where d-orbitals of doped Ni atom promotes the spin moment and thus largely contributes the magnetism change in the 2D layered material.

We elucidate the origin of the phonon-mediated superconductivity in 2$H$-NbS$_2$ using the ab initio anisotropic Migdal-Eliashberg theory including Coulomb interactions. We demonstrate that superconductivity is associated with Fermi surface hot spots exhibiting an unusually strong electron-phonon interaction. The electron-lattice coupling is dominated by low-energy anharmonic phonons, which place the system on the verge of a charge density wave instability. We also provide definitive evidence for two-gap superconductivity in 2$H$-NbS$_2$, and show that the low- and high-energy peaks observed in tunneling spectra correspond to the $\Gamma$- and $K$-centered Fermi surface pockets, respectively. The present findings call for further efforts to determine whether our proposed mechanism underpins superconductivity in the whole family of metallic transition metal dichalcogenides.

Materials with large magnetocrystalline anisotropy and strong electric field effects are highly needed to develop new types of memory devices based on electric field control of spin orientations. Instead of using modified transition metal films, we propose that certain monolayer transition metal dichalcogenides are the ideal candidate materials for this purpose. Using density functional calculations, we show that they exhibit not only a large magnetocrystalline anisotropy (MCA), but also colossal voltage modulation under external field. Notably, in some materials like CrSe_2 and FeSe_2, where spins show a strong preference for in-plane orientation, they can be switched to out-of-plane direction. This effect is attributed to the large band character alteration that the transition metal d-states undergo around the Fermi energy due to the electric field. We further demonstrate that strain can also greatly change MCA, and can help to improve the modulation efficiency while combined with an electric field.

Two-dimensional (2D) materials with intrinsic and robust ferromagnetism and half-metallicity are of great interest to explore the exciting physics and applications of nanoscale spintronic devices, but no such materials have been experimentally realized. In this study, we predict several M2NTx nitride MXene structures that display these characteristics based on a comprehensive study using a crystal field theory model and first-principles simulations. We demonstrate intrinsic ferromagnetism in Mn2NTx with surface terminations (T = O, OH, and F), as well as in Ti2NO2, and Cr2NO2. High magnetic moments (up to 9 B per unit cell), high Curie temperatures (1877 K to 566 K), robust ferromagnetism and intrinsic half-metallic transport behavior of these MXenes suggest that they are promising candidates for spintronic applications, which should stimulate interest in their synthesis.

Planar composite structures formed from the stripes of transition metal dichalcogenides joined commensurately along their zigzag or armchair edges can attain different states in a two-dimensional (2D), single-layer, such as a half metal, 2D or one-dimensional (1D) nonmagnetic metal and semiconductor. Widening of stripes induces metal-insulator transition through the confinements of electronic states to adjacent stripes, that results in the metal-semiconductor junction with a well-defined band lineup. Linear bending of the band edges of the semiconductor to form a Schottky barrier at the boundary between the metal and semiconductor is revealed. Unexpectedly, strictly 1D metallic states develop in a 2D system along the boundaries between stripes, which pins the Fermi level. Through the δ doping of a narrow metallic stripe one attains a nanowire in the 2D semiconducting sheet or narrow band semiconductor. A diverse combination of constituent stripes in either periodically repeating or finite-size heterostructures can acquire critical fundamental features and offer device capacities, such as Schottky junctions, nanocapacitors, resonant tunneling double barriers, and spin valves. These predictions are obtained from first-principles calculations performed in the framework of density functional theory.

Recent research has revealed a gamut of interesting properties present in layered two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as photoluminescence, comparatively high electron mobility, flexibility, mechanical strength and relatively low toxicity. The large surface to area ratio inherent in these materials also allows easy functionalization and maximal interaction with the external environment. Due to its unique physical and chemical properties, much work has been done in tailoring TMDCs through chemical functionalization for use in a diverse range of biomedical applications as biosensors, drug delivery carriers or even as therapeutic agents. In this review, current progress on the different types of TMDC functionalization for various biological applications will be presented and its future outlook will be discussed.

Based on density functional theory, we investigated electronic and magnetic properties of X-doped (Group 4) WS2 monolayer for 6.25% and 12.5% X concentration. Numerical results show that one X-doped WS2 monolayer is non-magnetic, while two X-doped systems of the next nearest neighbor configuration are ferromagnetic (FM). The hybridization between the X dopant and its neighboring W and S atoms results in the splitting of the energy levels near the Fermi energy. These results suggest the p(d)-d(d) hybridization mechanism for the magnetism of the X-doped WS2 monolayer structures. The asymmetric charge density distribution induces to magnetism for two next nearest neighbor X-doped WS2 systems. The studies find that the two next nearest X-doped WS2 monolayers to be candidates for magnetic metallic material. Moreover, the formation energy calculations also indicate that it is energy favorably and relatively easier to incorporate X atom into the WS2 monolayer under S-rich experimental conditions. Our results show that substitutional doping from IVB group is an effective way to modulate electronic and magnetic properties of tungsten disulphide monolayer.

Graphene-like two-dimensional materials have garnered tremendous interest as emerging device materials for nanoelectronics due to their remarkable properties. However, their applications in spintronics have been limited by the lack of intrinsic magnetism. Here, using hybrid density functional theory, we predict ferromagnetic behavior in a graphene-like two-dimensional Cr2C crystal that belongs to the MXenes family. The ferromagnetism, arising from the itinerant Cr d electrons, introduces intrinsic half-metallicity in Cr2C MXene, with the half-metallic gap as large as 2.85 eV. We also demonstrate a ferromagnetic-antiferromagnetic transition accompanied by a metal to insulator transition in Cr2C, caused by surface functionalization with F, OH, H or Cl groups. Moreover, the energy gap of the antiferromagnetic insulating state is controllable by changing the type of functional groups. We further point out that the localization of Cr d electrons induced by the surface functionalization is responsible for the ferromagnetic-antiferromagnetic and metal to insulator transitions. Our results highlight a new promising material with tunable magnetic and electronic properties towards nanoscale spintronics and electronics applications.

A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.

Finite-size corrections for charged defect supercell calculations typically
consist of image-charge and potential alignment corrections. A wide variety of
schemes for both corrections have been proposed for decades. Regarding the
image-charge correction, Freysoldt, Neugebauer, and Van de Walle (FNV) recently
proposed a novel method that enables us to accurately estimate the correction
energy a posteriori through alignment of the defect-induced potential to the
model charge potential [Freysoldt et al., Phys. Rev. Lett. 102, 016402 (2009)].
This method, however, still has two issues in practice. Firstly, it uses
planar-averaged potential for determining the potential offset, which cannot be
readily applied to relaxed system. Secondly, the long-range Coulomb interaction
is assumed to be screened by a macroscopic dielectric constant. This is valid
only for cubic systems and can bring forth huge errors for defects in
anisotropic materials. In this study, we use the atomic site electrostatic
potential as a potential marker instead of the planar-averaged potential, and
extend the FNV scheme by adopting the point charge model in an anisotropic
medium for estimating long-range interactions. We also revisit the conventional
potential alignment correction and show that it is fully included in the
image-charge correction and therefore unnecessary. In addition, we show that
the potential alignment corresponds to a part of first-order and full of
third-order image-charge correction; thus the third-order image-charge
contribution is absent after the potential alignment. Finally, a systematic
assessment of the accuracy of the extended FNV correction scheme is performed
for a wide range of material classes. The defect formation energies calculated
using around 100-atom supercells are successfully corrected even after atomic
relaxation within a few tenths of eV compared to those in the dilute limit.

We demonstrate the continuous tuning of the electronic structure of atomically thin MoS<sub>2</sub> on flexible substrates by applying a uniaxial tensile strain. A redshift at a rate of ~70 meV per percent applied strain for direct gap transitions, and at a rate 1.6 times larger for indirect gap transitions, have been determined by absorption and photoluminescence spectroscopy. Our result, in excellent agreement with first principles calculations, demonstrates the potential of two-dimensional crystals for applications in flexible electronics and optoelectronics. <sub>

Semiconducting transition metal dichalcogenides (TMDs) are emerging as the potential alternatives to graphene. As in the case of graphene, the monolayer of TMDs can easily be exfoliated using mechanical or chemical methods, and their properties can also be tuned. At the same time, semiconducting TMDs (MX(2); M = Mo, W and X = S, Se, Te) possess an advantage over graphene in that they exhibit a band gap whose magnitude is appropriate for applications in the opto-electronic devices. Using ab initio simulations, we demonstrate that this band gap can be widely tuned by applying mechanical strains. While the electronic properties of graphene remain almost unaffected by tensile strains, we find TMDs to be sensitive to both tensile and shear strains. Moreover, compared to that of graphene, a much smaller amount of strain is required to vary the band gap of TMDs. Our results suggest that mechanical strains reduce the band gap of semiconducting TMDs causing an direct-to-indirect band gap and a semiconductor-to-metal transition. These transitions, however, significantly depend on the type of applied strain and the type of chalcogenide atoms. The diffuse nature of heavier chalcogenides require relatively more tensile and less shear strain (when the monolayer is expanded in y-direction and compressed in x-direction) to attain a direct-to-indirect band gap transition. In addition, our results demonstrate that the homogeneous biaxial tensile strain of around 10% leads to semiconductor-to-metal transition in all semiconducting TMDs, while through pure shear strain this transition can only be achieved by expanding and compressing the monolayer of MTe(2) in the y- and x-directions, respectively. Our results highlight the importance of tensile and pure shear strains in tuning the electronic properties of TMDs by illustrating a substantial impact of the strain on going from MS(2) to MSe(2) to MTe(2).

We calculate absolute formation energies of native defects in GaAs. The formation energy and hence the equilibrium concentration of the defects depends strongly on the atomic chemical potentials of As and Ga as well as the electron chemical potential. For example, the Ga vacancy concentration changes by more than ten orders of magnitude as the chemical potentials of As and Ga vary over the thermodynamically allowed range. This result indicates that the rate of self-diffusion depends strongly on the surface-annealing conditions.

Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}

We describe monocrystalline graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions,
metallic, and of remarkably high quality. The films are found to be a two-dimensional semimetal with a tiny overlap between
valence and conductance bands, and they exhibit a strong ambipolar electric field effect such that electrons and holes in
concentrations up to 1013 per square centimeter and with room-temperature mobilities of ∼10,000 square centimeters per volt-second can be induced by
applying gate voltage.

Electronic structure and magnetism of MTe2 (M=Ti, V, Cr, Mn, Fe, Co and Ni) monolayers

- Chen