Ferromagnetic and Half-Metallic FeC2 Monolayer Containing C2 Dimers

Magnetism of Otherwise Nonmagnetic Elements: From Clusters to Monolayers

Atomic clusters are known to exhibit properties different from their bulk phase. However, when assembled or supported on substrates, clusters often lose their uniqueness. For example, uranium and coinage metals (Cu, Ag, Au) are nonmagnetic in their bulk. Herein, we show that UX6 (X= Cu, Ag, Au) clusters, unlike their nonmagnetic bulk, are not only magnetic but also retain their magnetic character and structure when assembled into a two-dimensional (2D) material. The magnetic moment remains localized at the U site and is found to be 3μB in clusters and about 2μB in the 2D structure. In 2D UX4 (X = Cu, Ag, Au) monolayers, U atoms are found to be coupled antiferromagnetically through an indirect exchange coupling mediated by the coinage metal atoms. Furthermore, hydrogenation of these monolayers can induce a transition from the antiferromagnetic to the ferromagnetic phase. These results, based on density functional theory, have predictive capability and can motivate experiments.

Computational Discovery of High-Temperature Ferromagnetic Semiconductor Monolayer H-MnN2

In the past few years, two-dimensional (2D) high-temperature ferromagnetic semiconductor (FMS) materials with novelty and excellent properties have attracted much attention due to their potential in spintronics applications. In this work, using first-principles calculations, we predict that the H-MnN2 monolayer with the H-MoS2-type structure is a stable intrinsic FMS with an indirect band gap of 0.79 eV and a high Curie temperature (Tc) of 380 K. The monolayer also has a considerable in-plane magnetic anisotropy energy (IMAE) of 1005.70 μeV/atom, including a magnetic shape anisotropy energy induced by the dipole-dipole interaction (shape-MAE) of 168.37 μeV/atom and a magnetic crystalline anisotropy energy resulting from spin-orbit coupling (SOC-MAE) of 837.33 μeV/atom. Further, based on the second-order perturbation theory, its in-plane SOC-MAE of 837.33 μeV/atom is revealed to mainly derive from the couplings of Mn-dxz,dyz and Mn-dx2-y2,dxy orbitals through Lz in the same spin channel. In addition, the biaxial strain and carrier doping can effectively tune the monolayer's magnetic and electronic properties. Such as, under the hole and few electrons doping, the transition from semiconductor to half-metal can be realized, and its Tc can go up to 520 and 620 K under 5% tensile strain and 0.3 hole doping, respectively. Therefore, our research will provide a new, promising 2D FMS for spintronics devices.

2D antiferromagnetic semiconducting FeCN with interesting properties

Two-dimensional magnetic materials have demonstrated favorable properties (e.g., large spin polarization and net magnetization) for the development of next-generation spintronic devices. The discovery of such materials and insight into their magnetic coupling mechanism has become a research focus. Here, on the basis of first-principles structural search calculations, we have identified a fresh FeCN monolayer consisting of edge-sharing Fe triangle sublattices and FeC3N2 rings, which integrates antiferromagnetism, semiconductivity, and planarity. Interestingly, it possesses a large magnetic anisotropy energy (MAE) of 614 μeV per Fe atom, a narrow band gap (Eg) of 0.47 eV, a large magnetic moment of 3.15 μB, and a proper Néel temperature (TN) of 97 K. The direct exchange between the nearest-neighbor Fe atoms in the triangle sublattice is mainly responsible for the AFM ordering. Its high structural stability, stemming from the collective contribution of covalent C-C and C-N bonds, ionic Fe-N bonds, and metallic Fe-Fe bonds, provides a strong feasibility for experimental synthesis.

An antiferromagnetic semiconducting FeCN2 monolayer with a large magnetic anisotropy and strong magnetic coupling

Two-dimensional antiferromagnetic (AFM) materials with an intrinsic semiconductivity, a high critical temperature, and a sizable magnetic anisotropy energy (MAE) have attracted extensive attention because they show promise for high-performance spintronic nanodevices. Here, we have identified a new FeCN2 monolayer with a unique zigzag Fe chain through first-principles swarm structural search calculations. It is an AFM semiconductor with a direct band gap of 2.04 eV, a Néel temperature (TN) of 176 K, and a large in-plane MAE of 0.50 meV per Fe atom. More interestingly, the intrinsic antiferromagnetism, contributed by the strong magnetic coupling of neighbouring Fe ions, can be maintained under the external biaxial strains. A large cohesive energy and high dynamical stability favor synthesis and application. Therefore, these fascinating properties of the FeCN2 monolayer make it a promising nanoscale spintronic material.

Electronic structures and photovoltaic applications of vdW heterostructures based on Janus group-IV monochalcogenides: insights from first-principles calculations

The van der Waals integration can help 2D materials modulate their properties and provide more opportunities for 2D materials in the next-generation high-performance optoelectronic devices. Using first-principles calculations, we explored the atomic and electronic structures of 2D pristine and Janus group-IV monochalcogenides and found the internal vertical electric field at Janus group-IV monochalcogenides. Then, we constructed vdW heterostructures with pristine and Janus group-IV monochalcogenides monolayers as building blocks and explored their atomic structures and band alignments. Our results demonstrate that these vdW heterostructures can be synthesized experimentally, and the surface termination of the Janus monolayer at the interface can significantly help the heterostructure realize the transition from type I to type II due to the intrinsic electric field. Moreover, we found eight vdW heterostructures with a mismatch of less than 5% exhibiting type II band alignment with charge densities of VBM and CBM mainly localized at different domains of heterostructures, and excellent power conversion efficiency (∼19%) in photovoltaics are also predicted for these heterostructures with type II band alignment. Our results not only give an idea to use the Janus monolayer as building blocks to construct vdW heterostructures and modulate their band alignment but also provide a guide to the experimental researcher to design more efficient photovoltaic devices with Janus group-IV monochalcogenides.

First-principles Density Functional Theory Elucidation of the Hydrogen Evolution Reaction on TM-promoted TiC2 (TM=Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au)

Single-atom-catalyst-based systems have been attractive by virtue of their desirable catalytic performance. Herein, the possibility of the 15 transition-metal (TM)-promoted (TM=Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Os, Ir, Pt, Au, and Hg) and their hydrogen evolution reaction (HER) performance were investigated on two-dimensional titanium carbides (TiC₂ ). It is found that the adsorption strength of TMs on TiC₂ is stronger than that of TMs on γ-graphyne and weaker than that of TMs on Ti₃ C₂ . Among the fifteen investigated catalysts, Ru-TiC₂ , Ag-TiC₂ , Ir-TiC₂ , Au-TiC₂ , and Fe-TiC₂ exhibits overpotential of -0.18, -0.15, -0.18, -0.17, and -0.04 V, respectively. In addition, the Volmer-Tafel step was preferred to the Volmer-Heyrovsky step on Fe-TiC₂ . This work suggests that Fe-TiC₂ is possibly a superior HER electrocatalyst.

An intrinsic room-temperature half-metallic ferromagnet in a metal-free PN2 monolayer

In spintronics, the embodiment of abundance availability, long spin relaxation time, complete spin-polarization and high Curie temperature (T_C) in intrinsic metal-free half-metallic ferromagnets (MFHMFs) are highly desirable and challenging. In this work, employing density functional theory, we first propose a dynamically, thermally, and mechanically stable two-dimensional (2D) intrinsic MFHMF, i.e. a MoS₂-like PN₂ monolayer, which possesses not only completely spin-polarized half-metallicity, but also an above-room-temperature T_C (385 K). The half-metallic gap is calculated to be 1.70 eV, which can effectively prevent the spin-flip transition caused by thermal agitation. The mechanism of magnetism in the PN₂ monolayer is mainly derived from the p electron direct exchange interaction that separates from usual d-state magnetic materials. Moreover, the robustness of the ferromagnetism and half-metallicity is observed against an external strain and carrier (electron or hole) doping. Surprisingly, electron doping can effectively increase the Curie temperature of the PN₂ monolayer. The proposed research work provides an insight that PN₂ can be a promising candidate for realistic room-temperature metal-free spintronic applications.

FeSi2: a two-dimensional ferromagnet containing planar hexacoordinate Fe atoms

As an unconventional bonding pattern different from conventional chemistry, the concept of planar hypercoordinate atoms was first proposed in the molecular system, and it has been recently extended to 2D periodic systems. Using first-principles calculations, herein we predict a stable FeSi₂ monolayer with planar hexacoordinate Fe atoms. Due to its abundant multicenter bonds, the FeSi₂ monolayer shows excellent thermal and kinetic stability, anisotropic mechanical properties and room-temperature ferromagnetism (T _C ∼360 K). Furthermore, we have demonstrated the feasibility of directly growing an FeSi₂ monolayer on a Si (110) substrate while maintaining the novel electronic and magnetic properties of the freestanding monolayer. The FeSi₂ monolayer synthesized in this way would be compatible with the mature silicon semiconductor technology and could be utilized for spintronic devices.

MC2 (M = Y, Zr, Nb, and Mo) monolayers containing C2 dimers: prediction of anode materials for high-performance sodium ion batteries

Seeking novel high performance anode materials for sodium ion batteries (SIBs) is an attractive theme in developing energy storage devices. In this work, by means of density functional theory computations, we predicted a family of MC₂ (M = Y, Zr, Nb, and Mo) monolayers containing C₂ dimers to be promising anode materials for SIBs. The stability, electronic structure, and adsorption/diffusion/storage behavior of sodium atoms in MC₂ (M = Y, Zr, Nb, and Mo) monolayers were explored. Our computations revealed that Na adsorbed MC₂ (M = Y, Zr, Nb, and Mo) monolayers show metallic characteristics that give rise to excellent electrical conductivity and Na mobility with low activation energies for diffusion (0.21, 0.04, 0.20, and 0.22 eV, respectively) in these materials, indicative of a high charge/discharge rate. In addition, the theoretical capacities of Na-adsorbed on YC₂, ZrC₂, NbC₂, and MoC₂ monolayers are 478, 697, 687, and 675 mA h g^-1, respectively, higher than that of commercial graphite (284 mA h g^-1), and the open-circuit voltages are moderate (0.11-0.25 V). Our results suggest that MC₂ (M = Y, Zr, Nb, and Mo) monolayers have great potential to serve as anode materials for SIBs.

Room-temperature ferromagnetism in two-dimensional transition metal borides: a first-principles investigation

It is currently technologically important to predict new two-dimensional (2D) ferromagnetic materials for next-generation information storage media. However, discovered 2D ferromagnetic materials are still rare. Here, we explored the fact that 2D transition metal borides are potential room-temperature 2D ferromagnetic materials. By performing first-principles calculations, we found that the CrB monolayer is a ferromagnetic (FM) metal, while the FeB monolayer is a typically antiferromagnetic (AFM) semiconductor. Interestingly, both CrB and FeB monolayers are FM metals with a moderate magnetic anisotropy energy by saturating with functional groups. Monte Carlo simulations show that the Curie temperature (Tc) of the CrB monolayer is about 520 K, which is further increased to 580 K and 570 K through -F and -OH chemical modification, while Tc is about 250 K, 275 K and 300 K for the FeBF, FeBO and FeBOH monolayer, respectively. Thus, the 2D transition metal borides have great potential applications in information storage devices.

Highly Stable Two-Dimensional Iron Monocarbide with Planar Hypercoordinate Moiety and Superior Li-Ion Storage Performance

Stable planar hypercoordinate motifs have been recently demonstrated in two-dimensional (2D) confinement systems, while perfectly planar hypercoordinate motifs in 2D carbon-transition metal systems are rarely reported. Here, by using comprehensive ab initio computations, we discover two new iron monocarbide (FeC) binary sheets stabilized at 2D confined space, labeled as tetragonal-FeC (t-FeC) and orthorhombic-FeC (o-FeC), which are energetically more favorable compared with the previously reported square and honeycomb lattices. The proposed t-FeC is the global minimum configuration in the 2D space, and each carbon atom is four-coordinated with four ambient iron atoms, considered as the quasi-planar tetragonal lattice. Strikingly, the o-FeC monolayer is an orthorhombic phase with a perfectly planar pentacoordinate carbon moiety and a planar seven-coordinate iron moiety. These monolayers are the first example of a simultaneously pentacoordinate carbon and planar seven-coordinate Fe-containing material. State-of-the-art theoretical calculations confirm that all these monolayers have significantly dynamic, mechanical, and thermal stabilities. Among these two monolayers, the t-FeC monolayer shows a higher theoretical capacity (395 mAh g^-1) and can stably adsorb Li up to t-FeCLi₄ (1579 mAh g^-1). The low migration energy barrier is predicted as small as 0.26 eV for Li, which results in the fast diffusion of Li atoms on this monolayer, making it a promising candidate for lithium-ion battery material.

First-principles investigation of a new 2D magnetic crystal: Ferromagnetic ordering and intrinsic half-metallicity

The development of two-dimensional (2D) magnetic materials with half-metallic characteristics is of great interest because of their promising applications in spintronic devices with high circuit integration density and low energy consumption. Here, by using density functional theory calculations, ab initio molecular dynamics, and Monte Carlo simulation, we study the stability, electronic structure, and magnetic properties of a OsI₃ monolayer, of which crystalline bulk is predicted to be a van der Waals layered ferromagnetic (FM) semiconductor. Our results reveal that the OsI₃ monolayer can be easily exfoliated from the bulk phase with small cleavage energy and is energetically and thermodynamically stable at room temperature. Intrinsic half-metallicity with a wide bandgap and FM ordering with an estimated T_C = 35 K are found for the OsI₃ monolayer. Specifically, the FM ordering can be maintained under external biaxial strain from -2% to 5%. The in-plane magnetocrystalline anisotropy energy of the 2D OsI₃ monolayer reaches up to 3.89 meV/OsI₃, which is an order larger than that of most magnetic 2D materials such as the representative monolayer CrI₃. The excellent magnetic features of the OsI₃ monolayer therefore render it a promising 2D candidate for spintronic applications.

Two-dimensional stable Mn based half metal and antiferromagnets promising for spintronics

In this paper, we predict that the tetragonal MnSi and MnC0.5Si0.5 monolayers are mechanically stable metallic ferromagnetic materials. The thermal stability of the MnC0.5Si0.5 monolayer is verified by our ab initio molecular dynamics (AIMD) result at 300 K. Both MnSi and MnC0.5Si0.5 monolayers exhibit room temperature half-metallic properties, which is very promising for spintronic applications. Both monolayers exhibit large perpendicular magnetic anisotropy, which is desirable for maintaining magnetic order and for high density storage spintronics. A bilayer of the MnSi nanosheet has obviously enhanced thermal stability and exhibits antiferromagnetic metal properties. The Néel temperature could be effectively manipulated and improved by surface functionalization. In addition, monolayer and bilayer MnSi nanosheets exhibit nodal lines in the reciprocal space, and the nodal lines are robust against spin orbit coupling.

Recent advances of novel ultrathin two-dimensional silicon carbides from a theoretical perspective

Compared to graphene with semimetallic features, two-dimensional (2D) silicon carbide (Si-C) materials constitute another highly promising family for opto-electronic applications owing to their intrinsic electronic gaps. Recent theoretical studies of 2D Si-C materials thoroughly investigated their structure and properties. Herein, we overview these high-throughput approaches aiming to theoretically design 2D Si-C crystals. Graphene-like siligraphene and non-siligraphene are described in terms of morphology, physicochemical properties and potential applications based on the insights provided by simulations. In addition, the current progress of experimental exploration of 2D Si-C materials and underlying challenges are assessed as well.

Strain-tunable magnetic anisotropy in two-dimensional Dirac half-metals: nickel trihalides

The recent discovery of intrinsic two-dimensional (2D) ferromagnetism has sparked intense interest due to the potential applications in spintronics. Magnetic anisotropy energy defines the stability of magnetization in a specific direction with respect to the crystal lattice and is an important parameter for nanoscale applications. In this work, using first-principles calculations we predict that 2D NiX3 (X = Cl, Br, and I) can be a family of intrinsic Dirac half-metals characterized by a band structure with an insulator gap in one spin channel and a Dirac cone in the other. The combination of 100% spin polarization and massless Dirac fermions renders the monolayer NiX3 a superior candidate material for efficient spin injection and high spin mobility. The NiX3 is dynamically and thermodynamically stable up to high temperature and the magnetic moment of about 1 μ B per Ni3+ ion is observed with high Curie temperature and large magnetic anisotropy energy. Moreover, detailed calculations of their energetics, atomic structures, and electronic structures under the influence of a biaxial strain ε have been carried out. The magnetic anisotropy energy also exhibits a strain dependence in monolayer NiX3. The hybridization between Ni d xy and d x 2-y 2 orbitals gives the largest magnetic anisotropy contribution, whether for the off-plane magnetized NiCl3 (NiBr3) or the in-plane magnetized NiI3. The outstanding attributes of monolayer NiX3 will substantially broaden the applicability of 2D magnetism for a wide range of applications.

High-Temperature Ferromagnetism in an Fe3P Monolayer with a Large Magnetic Anisotropy

For the development of high-performance spintronic nanodevices, one of the most urgent and challenging tasks is the preparation of two-dimensional materials with room-temperature ferromagnetism and a large magnetic anisotropic energy (MAE). Through first-principles swarm-intelligence structural search calculations, we identify an ideal ferromagnetic Fe₃P monolayer, in which Fe atoms show a perfect Kagome lattice, leading to strong in-plane Fe-Fe coupling. The predicted Curie temperature of Fe₃P reaches ∼420 K, and its MAE is comparable to those of ferromagnetic materials, such as Fe and Fe₂Si. Moreover, the Fe₃P monolayer remains as an above room-temperature ferromagnet under biaxial strains as large as 10%. Its lattice can be retained at temperatures of ≤1000 K, exhibiting a high thermodynamic stability. All of these desirable properties make the Fe₃P monolayer a promising candidate for applications in spintronic nanodevices.

Large Tunneling Magnetoresistance in VSe2/MoS2 Magnetic Tunnel Junction

Two-dimensional (2D) van der Waals (vdW) materials provide the possibility of realizing heterostructures with coveted properties. Here, we report a theoretical investigation of the vdW magnetic tunnel junction (MTJ) based on VSe₂/MoS₂ heterojunction, where the VSe₂ monolayer acts as a ferromagnet with room-temperature ferromagnetism. We propose the concept of spin-orbit torque (SOT) vdW MTJ with reliable reading and efficient writing operations. The nonequilibrium study reveals a large tunneling magnetoresistance of 846% at 300 K, identifying significantly its parallel and antiparallel states. Thanks to the strong spin Hall conductivity of MoS₂, SOT is promising for the magnetization switching of VSe₂ free layer. Quantum-well states come into being and resonances appear in MTJ, suggesting that the voltage control can adjust transport properties effectively. The SOT vdW MTJ based on VSe₂/MoS₂ provides desirable performance and experimental feasibility, offering new opportunities for 2D spintronics.

Perfect planar tetra-coordinated MC6 monolayer: superior anode material for Li-ion battery

A hitherto unknown atomic-thin planar-structured transition metal carbide sheet denoted as MC₆ (M = Cu, Ag, Au) is reported via a structure-swarm intelligence algorithm.

VC2 and V1/2Mn1/2C2 nanosheets with robust mechanical and thermal properties as promising materials for Li-ion batteries

In this paper, vanadium carbides VC₂ and bi-transition-metal carbides V_1/2Mn_1/2C₂ are predicted to be stable metallic nanosheets showing promising mechanical properties.

Screening and Design of Novel 2D Ferromagnetic Materials with High Curie Temperature above Room Temperature

Two-dimensional (2D) intrinsic ferromagnets with high Curie temperature ( T_C) are desirable for spintronic applications. Using systematic first-principles calculations, we investigate the electronic and magnetic properties of 22 monolayer 2D materials with layered bulk phases. From these candidates, we screen out five ferromagnetic monolayer materials belonging to three types of structures: type i (ScCl, YCl, LaCl), type ii (LaBr₂), and type iii (CrSBr). Type i is a kind of metallic ferromagnetic material, whereas LaBr₂ and CrSBr of type ii and iii are small-bandgap ferromagnetic semiconductors with T_C near room temperature. Moreover, the ferromagnetic CrSBr monolayer possesses a large magnetic moment of ∼3 μ_B per Cr atom, originating from its distorted octahedron coordination. The robust ferromagnetism of the CrSBr monolayer is ascribed to the halogen-mediated (Cr-Br-Cr) and chalcogen-mediated (Cr-S-Cr) superexchange interactions; then, an isoelectronic substitution strategy is proposed to tailor the magnetic coupling strength. Hence, monolayer structures of CrSI, CrSCl, and CrSeBr with notably enhanced Curie temperature up to 500 K as well as favorable formation energy are designed. The moderate interlayer binding energy and high T_C make these monolayer ferromagnetic materials feasible for experimental synthesis and attractive as 2D spintronic devices.

MBene (MnB): a new type of 2D metallic ferromagnet with high Curie temperature

We extend the 2D MXene family into the boride world, namely, MBenes. High-throughput calculations screen twelve MBenes with excellent stability. Among them, 2D MnB MBene exhibits robust metallic ferromagnetism (∼3.2 μ_B per Mn atom) and high Curie temperature (345 K). After functionalization with the -F and -OH groups, the ferromagnetic ground state of 2D MnB is well preserved. The Curie temperature is increased to 405 and 600 K, respectively, providing a novel and feasible strategy to tailor the T_C of 2D magnetic materials.

Two-dimensional stable transition metal carbides (MnC and NbC) with prediction and novel functionalizations

MnC and NbC monolayers are predicted to be stable and promising for Li-ion battery, by functionalization, they exhibit half-metallic property and quantum spin Hall effect, respectively.

Two-Dimensional Co2S2 monolayer with robust ferromagnetism

Design and synthesis of two-dimensional (2D) materials with robust intrinsic ferromagnetism is highly desirable due to their potential applications in spintronics devices. In this work, we identify a new 2D cobalt sulfide (Co₂S₂) material by using first-principles calculations and particle swarm optimization (PSO) global structure search. We show that the 2D Co₂S₂ is most stable in the litharge type tetragonal structure with space group of P4/nmm. The elastic constants, phonon spectrum, and molecular dynamics simulation confirm its mechanical, dynamical and thermal stability, respectively. It is also found that Co₂S₂ monolayer is a ferromagnetic metal with a Curie temperature up to 404 K. In addition, we propose a feasible procedure to synthesize the Co₂S₂ monolayer by chemically exfoliating from bulk TlCo₂S₂ phase.

Phase and Facet Control of Molybdenum Carbide Nanosheet Observed by In Situ TEM

Transition metal carbides are of great potential for electrochemical applications. The phase and facet of molybdenum carbides greatly affect the electrochemical performance. Carburization of MoO₃ inside a transmission electron microscope to monitor the growth process of molybdenum carbides is performed. Carbon sources with different activities are used and the controllable growth of molybdenum carbides is investigated. The results show that the relatively inert amorphous carbon film produces Mo₂ C, where the interstitial sites formed by hexagonal closed packing molybdenum atoms are partially occupied by carbon atoms. In contrast, the carbon decomposed from the sucrose has a high portion of sp³ hybridized and crosslinked carbon atoms with high reactivity, leading to the formation of MoC with full occupation of interstitial sites by carbon atoms. In addition, the MoC growth experiences a (111) to (100) facets change with the increase of temperature. The (111) facet formed at low temperature has Mo-terminated or C-terminated surface with higher surface energy and higher reactivity, while the (100) facet with 1:1 C/Mo ratio on the surface exhibits enhanced stability. The phase and facet control by carbon source and temperature allow us to tune the crystal structures and surface atoms as well as their electrochemical properties.

NiX2 (X = Cl and Br) sheets as promising spin materials: a first-principles study

In order to achieve paper-like spin devices, we explored two promising two-dimensional (2D) spin materials, namely NiCl₂ and NiBr₂.