Layer-dependent dopant stability and magnetic exchange coupling of iron-doped MoS2 nanosheets

Transition Metal Dichalcogenides: Making Atomic-Level Magnetism Tunable with Light at Room Temperature

The capacity to manipulate magnetization in 2D dilute magnetic semiconductors (2D-DMSs) using light, specifically in magnetically doped transition metal dichalcogenide (TMD) monolayers (M-doped TX2 , where M = V, Fe, and Cr; T = W, Mo; X = S, Se, and Te), may lead to innovative applications in spintronics, spin-caloritronics, valleytronics, and quantum computation. This Perspective paper explores the mediation of magnetization by light under ambient conditions in 2D-TMD DMSs and heterostructures. By combining magneto-LC resonance (MLCR) experiments with density functional theory (DFT) calculations, we show that the magnetization can be enhanced using light in V-doped TMD monolayers (e.g., V-WS2 , V-WSe2 ). This phenomenon is attributed to excess holes in the conduction and valence bands, and carriers trapped in magnetic doping states, mediating the magnetization of the semiconducting layer. In 2D-TMD heterostructures (VSe2 /WS2 , VSe2 /MoS2 ), the significance of proximity, charge-transfer, and confinement effects in amplifying light-mediated magnetism is demonstrated. We attributed this to photon absorption at the TMD layer that generates electron-hole pairs mediating the magnetization of the heterostructure. These findings will encourage further research in the field of 2D magnetism and establish a novel design of 2D-TMDs and heterostructures with optically tunable magnetic functionalities, paving the way for next-generation magneto-optic nanodevices.

Reduction of Magnetic Interaction Due to Clustering in Doped Transition-Metal Dichalcogenides: A Case Study of Mn-, V-, and Fe-Doped WSe2

Using Hubbard-U-corrected density functional theory calculations, lattice Monte Carlo simulations, and spin Monte Carlo simulations, we investigate the impact of dopant clustering on the magnetic properties of WSe2 doped with period four transition metals. We use manganese (Mn) and iron (Fe) as candidate n-type dopants and vanadium (V) as the candidate p-type dopant, substituting the tungsten (W) atom in WSe2. Specifically, we determine the strength of the exchange interaction in Fe-, Mn-, and V-doped WSe2 in the presence of clustering. We show that the clusters of dopants are energetically more stable than discretely doped systems. Further, we show that in the presence of dopant clustering, the magnetic exchange interaction significantly reduces because the magnetic order in clustered WSe2 becomes more itinerant. Finally, we show that the clustering of the dopant atoms has a detrimental effect on the magnetic interaction, and to obtain an optimal Curie temperature, it is important to control the distribution of the dopant atoms.

High spin polarization, large perpendicular magnetic anisotropy and room-temperature ferromagnetism by biaxial strain and carrier doping in Janus MnSeTe and MnSTe

The emerging two-dimensional (2D) Janus systems with broken symmetry provide a new platform for designing ultrathin multifunctional spintronic materials. Recently, based on experimental monolayer MnSe2, ferromagnetism was predicted in Janus MnXY (X ≠ Y = S, Se, Te) monolayers; however, they exhibit low Curie temperatures and small magnetic anisotropic energies. To improve the Curie temperature and magnetic anisotropy, herein, we systemically explore the stability and electronic and magnetic properties of Janus MnSeTe and MnSTe monolayers under strain and carrier-doping using first-principles calculations and Monte Carlo simulations. It is found that both MnSeTe and MnSTe monolayers possess robustly high spin polarization with rational strain and carrier-doping. Both tensile strain and hole doping strengthen the ferromagnetic super-exchange interactions of the two nearest Mn atoms mediated by chalcogen atoms and exceedingly improve the perpendicular magnetic anisotropic energies (by up to 3.1 meV per f.u. for MnSeTe and 2.0 meV per f.u. for MnSTe). The Te-5p intraorbital hybridizations contributed to the main magnetic anisotropy. More remarkably, the tensile strain and hole doping collectively increase the Curie temperatures of MnSeTe and MnSTe to above and near room temperature (345 and 290 K, respectively). The present study reveals that Janus MnSeTe and MnSTe monolayers with robustly high spin polarization, room-temperature ferromagnetism and large perpendicular magnetic anisotropy are promising candidates for ultrathin multifunctional spintronic materials. This study will be of great interest for further experimental and theoretical explorations of 2D Janus manganese dichalcogenides.

Heteroatom-Doped Molybdenum Disulfide Nanomaterials for Gas Sensors, Alkali Metal-Ion Batteries and Supercapacitors

Molybdenum disulfide (MoS2) is the second two-dimensional material after graphene that received a lot of attention from the research community. Strong S-Mo-S bonds make the sandwich-like layer mechanically and chemically stable, while the abundance of precursors and several developed synthesis methods allow obtaining various MoS2 architectures, including those in combinations with a carbon component. Doping of MoS2 with heteroatom substituents can occur by replacing Mo and S with other cations and anions. This creates active sites on the basal plane, which is important for the adsorption of reactive species. Adsorption is a key step in the gas detection and electrochemical energy storage processes discussed in this review. The literature data were analyzed in the light of the influence of a substitutional heteroatom on the interaction of MoS2 with gas molecules and electrolyte ions. Theory predicts that the binding energy of molecules to a MoS2 surface increases in the presence of heteroatoms, and experiments showed that such surfaces are more sensitive to certain gases. The best electrochemical performance of MoS2-based nanomaterials is usually achieved by including foreign metals. Heteroatoms improve the electrical conductivity of MoS2, which is a semiconductor in a thermodynamically stable hexagonal form, increase the distance between layers, and cause lattice deformation and electronic density redistribution. An analysis of literature data showed that co-doping with various elements is most attractive for improving the performance of MoS2 in sensor and electrochemical applications. This is the first comprehensive review on the influence of foreign elements inserted into MoS2 lattice on the performance of a nanomaterial in chemiresistive gas sensors, lithium-, sodium-, and potassium-ion batteries, and supercapacitors. The collected data can serve as a guide to determine which elements and combinations of elements can be used to obtain a MoS2-based nanomaterial with the properties required for a particular application.

Emerging Enhancement and Regulation Strategies for Ferromagnetic 2D Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides (2D TMDs) present promising applications in various fields such as electronics, optoelectronics, memory devices, batteries, superconductors, and hydrogen evolution reactions due to their regulable energy band structures and unique properties. For emerging spintronics applications, materials with excellent room-temperature ferromagnetism are required. Although most transition metal compounds do not possess room-temperature ferromagnetism on their own, they are widely modified by researchers using the emerging strategies to engineer or modulate their intrinsic properties. This paper reviews recent enhancement approaches to induce magnetism in 2D TMDs, mainly using doping, vacancy defects, composite of heterostructures, phase modulation, and adsorption, and also by electron irradiation induction, O plasma treatment, etc. On this basis, the produced effects of these methods for the introduction of magnetism into 2D TMDs are compressively summarized and constructively discussed. For perspective, research on magnetic doping techniques for 2D TMDs materials should be directed toward more reliable and efficient directions, such as exploring advanced design strategies to combine dilute magnetic semiconductors, antiferromagnetic semiconductors, and superconductors to develop new types of heterojunctions; and advancing experimentation strategies to fabricate the designed materials and enable their functionalities with simultaneously pursuing the upscalable growth methods for high-quality monolayers to multilayers.

Oxoiron(IV)-dominated Heterogeneous Fenton-like Mechanism of Fe-Doped MoS2

Oxoiron(IV) species are a critical intermediate in the Fe-based Fenton-like process at circumneutral pH, and its oxidative reactivity is closely related to the ligands. An optional inorganic host material, MoS₂ , is selected to construct a highly reactive sulfur ligand coordinated Fe species in this work. The Fe species doped in MoS₂ is presented as the Fe^II centre and triggers the transformation of the 2H phase to the octahedral 1T phase MoS₂ . The role of the interaction between doped Fe and the MoS₂ host lattice on the formation of oxoiron(IV) is studied. A significant Fenton-like reactivity and a remarkable accumulation of oxoiron(IV) species were observed for Fe-MoS₂ . The quenching experiment was implemented to disclose the predominant role of oxoiron(IV) species in the Fe-MoS₂ /H₂ O₂ Fenton-like system. Furthermore, oxoiron(IV) species could transform into the ⋅O₂ ^- and ¹ O₂ , which further expedites the Fenton-like reaction.

Effect of 3d Transition Metal Atom Intercalation Concentration on the Electronic and Magnetic Properties of Graphene/MoS2 Heterostructure: A First-Principles Study

The electronic and magnetic properties of graphene/MoS₂ heterostructures intercalated with 3d transition metal (TM) atoms at different concentrations have been systematically investigated by first principles calculations. The results showed that all the studied systems are thermodynamically stable with large binding energies of about 3.72 eV-6.86 eV. Interestingly, all the TM-intercalated graphene/MoS₂ heterostructures are ferromagnetic and their total magnetic moments increase with TM concentration. Furthermore, TM concentration-dependent spin polarization is obtained for the graphene layer and MoS₂ layer due to the charge transfer between TM atoms and the layers. A significant band gap is opened for graphene in these TM-intercalated graphene/MoS₂ heterostructures (around 0.094 eV-0.37 eV). With the TM concentration increasing, the band gap of graphene is reduced due to the enhanced spin polarization of graphene. Our study suggests a research direction for the manipulation of the properties of 2D materials through control of the intercalation concentration of TM atoms.

Strain-Modulated Magnetism in MoS2

Since the experiments found that two-dimensional (2D) materials such as single-layer MoS2 can withstand up to 20% strain, strain-modulated magnetism has gradually become an emerging research field. However, applying strain alone is difficult to modulate the magnetism of single-layer pristine MoS2, but applying strain combined with other tuning techniques such as introducing defects makes it easier to produce and alter the magnetism in MoS2. Here, we summarize the recent progress of strain-dependent magnetism in MoS2. First, we review the progress in theoretical study. Then, we compare the experimental methods of applying strain and their effects on magnetism. Specifically, we emphasize the roles played by web buckles, which induce biaxial tensile strain conveniently. Despite some progress, the study of strain-dependent MoS2 magnetism is still in its infancy, and a few potential directions for future research are discussed at the end. Overall, a broad and in-depth understanding of strain-tunable magnetism is very necessary, which will further drive the development of spintronics, straintronics, and flexible electronics.

Ferromagnetism of two-dimensional transition metal chalcogenides: both theoretical and experimental investigations

In recent years, with the fast development of integrated circuit electronic devices and technologies, it has become urgent to improve the density of data storage and lower the energy losses of devices. Under these circumstances, two-dimensional (2D) materials, which have a smaller size and lower energy loss compared with bulk materials, are becoming ideal candidates for future spintronic devices. Among them, 2D transition metal chalcogenides (TMCs), which have excellent electronic and optical properties, have attracted great attention from researchers. However, most of them are intrinsically non-magnetic, which severely hinders their further applications in spintronics. Therefore, introducing intrinsic room-temperature ferromagnetism into 2D TMC materials has become an important issue in spintronics. In this work, we review the introduction of intrinsic ferromagnetism into typical 2D TMCs using various strategies, such as defect engineering, doping with transition metal elements, and phase transfer. Additionally, we found that their ferromagnetism could be adjusted via changing the experimental conditions, such as the nucleation temperature, ion irradiation dose, doping amount, and phase ratio. Finally, we provide some insight into prospective solutions for introducing ferromagnetism into 2D TMCs, hoping to shed some light on future spintronics development.

Intrinsic Polarization-Induced Enhanced Ferromagnetism and Self-Doped p-n Junctions in CrBr3/GaN van der Waals Heterostructures

Two-dimensional (2D) ferromagnetic (FM) semiconductors with a high Curie temperature and tunable electronic properties are a long-term pursuing target for the development of high-performance spin-dependent optoelectronic devices. Herein, on the basis of density functional theory calculations, we report a new strategy to tune the Curie temperature and electronic structures of a ferromagnetic CrBr₃ monolayer through the formation of CrBr₃/GaN van der Waals heterostructures. Our calculated results demonstrate that the Curie temperature and band alignment of CrBr₃/GaN heterostructures strongly depend on the thickness and polarization direction of the GaN layer. The combination of the CrBr₃ monolayer with N-terminated GaN nanosheets leads to enhanced FM coupling via superexchange interactions between the Cr-t_2g and Cr-e_g orbitals, consequently resulting in a Curie temperature of CrBr₃ of up to 67 K. Moreover, self-doped p-n junctions can be naturally formed in the heterostructures without additional modulation of external fields. The enhanced FM coupling and self-doping effect in the heterostructures are associated with the intrinsic polarization of the GaN layer that drives interfacial electron transfers from GaN to CrBr₃. Therefore, this work not only offers an efficient scheme to boost the Curie temperature of the CrBr₃ monolayer but also opens up a new route to realize nonvolatile van der Waals p-n junctions.

Synthesis of emerging 2D layered magnetic materials

van der Waals atomically thin magnetic materials have been recently discovered. They have attracted enormous attention as they present unique magnetic properties, holding potential to tailor spin-based device properties and enable next generation data storage and communication devices. To fully understand the magnetism in two-dimensions, the synthesis of 2D materials over large areas with precise thickness control has to be accomplished. Here, we review the recent advancements in the synthesis of these materials spanning from metal halides, transition metal dichalcogenides, metal phosphosulphides, to ternary metal tellurides. We initially discuss the emerging device concepts based on magnetic van der Waals materials including what has been achieved with graphene. We then review the state of the art of the synthesis of these materials and we discuss the potential routes to achieve the synthesis of wafer-scale atomically thin magnetic materials. We discuss the synthetic achievements in relation to the structural characteristics of the materials and we scrutinise the physical properties of the precursors in relation to the synthesis conditions. We highlight the challenges related to the synthesis of 2D magnets and we provide a perspective for possible advancement of available synthesis methods to respond to the need for scalable production and high materials quality.

Two-dimensional Janus semiconductor BiTeCl and BiTeBr monolayers: a first-principles study on their tunable electronic properties via an electric field and mechanical strain

Motivated by the recent successful synthesis of highly crystalline ultrathin BiTeCl and BiTeBr layered sheets [Debarati Hajra et al., ACS Nano, 2020, 14, 15626], herein for the first time, we carry out a comprehensive study on the structural and electronic properties of BiTeCl and BiTeBr Janus monolayers using density functional theory (DFT) calculations. Different structural and electronic parameters including the lattice constant, bond lengths, layer thickness in the z-direction, different interatomic angles, work function, charge density difference, cohesive energy and Rashba coefficients are determined to acquire a deep understanding of these monolayers. The calculations show good stability of the studied single layers. BiTeCl and BiTeBr monolayers are semiconductors with electronic bandgaps of 0.83 and 0.80 eV, respectively. The results also show that the semiconductor-metal transformation can be induced by increasing the number of layers. In addition, the engineering of the electronic structure is also studied by applying an electric field, and mechanical uniaxial and biaxial strain. The results show a significant change of the bandgaps and that an indirect-direct band-gap transition can be induced. This study highlights the positive prospect for the application of BiTeCl and BiTeBr layered sheets in novel electronic and energy conversion systems.

Universal In Situ Substitutional Doping of Transition Metal Dichalcogenides by Liquid-Phase Precursor-Assisted Synthesis

Doping lies at the heart of modern semiconductor technologies. Therefore, for two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), the significance of controlled doping is no exception. Recent studies have indicated that, by substitutionally doping 2D TMDs with a judicious selection of dopants, their electrical, optical, magnetic, and catalytic properties can be effectively tuned, endowing them with great potential for various practical applications. Herein, and inspired by the sol-gel process, we report a liquid-phase precursor-assisted approach for in situ substitutional doping of monolayered TMDs and their in-plane heterostructures with tunable doping concentration. This highly reproducible route is based on the high-temperature chalcogenation of spin-coated aqueous solutions containing host and dopant precursors. The precursors are mixed homogeneously at the atomic level in the liquid phase prior to the synthesis process, thus allowing for an improved doping uniformity and controllability. We further demonstrate the incorporation of various transition metal atoms, such as iron (Fe), rhenium (Re), and vanadium (V), into the lattice of TMD monolayers to form Fe-doped WS₂, Re-doped MoS₂, and more complex material systems such as V-doped in-plane W_xMo_1-xS₂-Mo_xW_1-xS₂ heterostructures, among others. We envisage that our developed approach is universal and could be extended to incorporate a variety of other elements into 2D TMDs and create in-plane heterointerfaces in a single step, which may enable applications such as electronics and spintronics at the 2D limit.

Coexistence of the Kondo effect and spin glass physics in Fe-doped NbS2

We report the coexistence of the Kondo effect and spin glass behavior in Fe-doped NbS₂ single crystals. The Fe _x NbS₂ shows the resistance minimum and negative magnetoresistance due to the Kondo effect, and exhibits no superconducting behavior at low temperatures. The resistance curve follows a numerical renormalization-group theory using the Kondo temperature [Formula: see text] K for x  =  0.01 as evidence of Kondo effect. Scanning tunneling microscope/spectroscopy (STM/STS) revealed the presence of Fe atoms near sulfur atoms and asymmetric spectra. The magnetic susceptibility exhibits a feature of spin glass. The static critical exponents determined by the universal scaling of the nonlinear part of the susceptibility suggest a three-dimensional Heisenberg spin glass. The doped-Fe atoms in the intra- and inter-layers revealed by the x-ray result can realize the coexistence of the Kondo effect and spin glass.

Tuning the spin polarization in monolayer MoS2 through (Y,Yb) co-doping

Yb-Doped monolayer MoS₂ is ferromagnetic at room temperature, and this ferromagnetic state can be stabilized by Y co-doping.

The influence of dopants on aW-phase antimonene: theoretical investigations

We systemically investigate the effect of dopants on the geometrics, electronic and magnetic properties of asymmetric washboard structure of antimonene (aW-Sb) by using density functional theory (DFT) calculations. The large binding energies and short bond lengths indicate the doped systems still maintain high stability. Pristine aW-Sb is a nonmagnetic semiconductor with a narrow band gap, while the doped aW-Sb exhibit metallic by doping. Furthermore, the Ti, V, Cr, Mn and Fe doping induced magnetic states, and the result of spin density indicates that the magnetic moments are mainly localized at dopant and the adjacent Sb atoms.

We systemically investigate the effect of dopants on the geometrics, electronic and magnetic properties of asymmetric washboard structure of antimonene (aW-Sb) by using density functional theory (DFT) calculations.

Challenges and recent advancements of functionalization of two-dimensional nanostructured molybdenum trioxide and dichalcogenides

Atomically thin two-dimensional (2D) semiconductors are the thinnest functional semiconducting materials available today. Among them, both molybdenum trioxide and chalcogenides (MT&Ds) represent key components within the family of different 2D semiconductors for various electronic, optoelectronic and electrochemical applications due to their unique electronic, optical, mechanical and electrochemical properties. However, despite great progress in research dedicated to the development and fabrication of 2D MT&Ds observed within the last decade, there are significant challenges that affected their charge transport behavior and fabrication on a large scale as well as there is high dependence of the carrier mobility on the thickness. In this article, we review the recent progress in the carrier mobility engineering of 2D MT&Ds and elaborate devised strategies dedicated to the optimization of MT&D properties. Specifically, the latest physical and chemical methods towards the surface functionalization and optimization of the major factors influencing the extrinsic transport at the electrode-2D semiconductor interface are discussed.

Two-dimensional MoS2/Fe-phthalocyanine hybrid nanostructures as excellent electrocatalysts for hydrogen evolution and oxygen reduction reactions

Two-dimensional (2D) MoS2 nanostructures have been extensively investigated in recent years because of their fascinating electrocatalytic properties. Herein, we report 2D hybrid nanostructures consisting of 1T' phase MoS2 and Fe-phthalocyanine (FePc) molecules that exhibit excellent catalytic activity toward both the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). X-ray absorption spectra revealed an increased Fe-N distance (2.04 Å) in the hybrid complex relative to the isolated FePc. Spin-polarized density functional theory calculations predicted that the Fe center moves toward the MoS2 layer and induces a non-planar structure with an increased Fe-N distance of 2.05 Å, which supports the experimental results. The experiments and calculations consistently show a significant charge transfer from FePc to stabilize the hybrid complex. The excellent HER catalytic performance of FePc-MoS2 is characterized by a low Tafel slope of 32 mV dec-1 at a current density of 10 mA cm-2 and an overpotential of 0.123 V. The ORR catalytic activity is superior to that of the commercial Pt/C catalyst in pH 13 electrolyte, with a more positive half-wave potential (0.89 vs. 0.84 V), a smaller Tafel slope (35 vs. 87 mV·dec-1), and a much better durability (9.3% vs. 40% degradation after 20 h). Such remarkable catalytic activity is ascribed to the HER-active 1T' phase MoS2 and the ORR-active nonplanar Fe-N4 site of FePc.

Tunable magnetic coupling in Mn-doped monolayer MoS2 under lattice strain

First-principles calculations are conducted to study the electronic and magnetic states of Mn-doped monolayer MoS₂ under lattice strain. Mn-doped MoS₂ exhibits half-metallic and ferromagnetic (FM) characteristics in which the majority spin channel exhibits metallic features but there is a bandgap in the minority spin channel. The FM state and the total magnetic moment of 1 µ _B are always maintained for the larger supercells of monolayer MoS₂ with only one doped Mn, no matter under tensile or compressive strain. Furthermore, the FM state will be enhanced by the tensile strain if two Mo atoms are substituted by Mn atoms in the monolayer MoS₂. The magnetic moment increases up to 0.50 µ _B per unit cell at a tensile strain of 7%. However, the Mn-doped MoS₂ changes to metallic and antiferromagnetic under compressive strain. The spin polarization of Mn 3d orbitals disappears gradually with increasing compressive strain, and the superexchange interaction between Mn atoms increases gradually. The results suggest that the electronic and magnetic properties of Mn-doped monolayer MoS₂ can be effectively modulated by strain engineering providing insight into application to electronic and spintronic devices.

Long range of indirect exchange interaction on the edges of MoS2 flakes

We study the Ruderman-Kittel-Kasuya-Yosida interaction between two magnetic impurities connected to the edges of zigzag-terminated MoS₂ flakes. When the impurities lie on the edges of the flake, the effective exchange interaction exhibits sizable noncollinear Dzyaloshinskii-Moriya character that competes with a strong Ising coupling. We analyze the characteristic decay exponent for doping levels inside the band gap of the infinite layer, corresponding to edge states of the flake at the Fermi level. The characteristic exponents show sub-two-dimensional behavior for these band fillings, with decays much slower than quadratic. The Ising interaction has effectively one-dimensional long range, while the noncollinear component that grows for short impurity separation becomes comparable in magnitude. The resulting tunable exchange interaction on these systems opens the way for the study of interesting phases of impurity arrays with long-range stable helical order.

Structural stability and magnetic-exchange coupling in Mn-doped monolayer/bilayer MoS2

Ferromagnetic (FM) two-dimensional (2D) transition metal dichalcogenides (TMDs) have potential applications in modern electronics and spintronics and doping of TMDs with transition metals can enhance the magnetic characteristics.

Intriguing electronic and optical properties of two-dimensional Janus transition metal dichalcogenides

Two-dimensional Janus transition metal dichalcogenides with an asymmetric structure present intriguing electronic, transport, and optical properties, which make them ideally suitable for electronic and optoelectronic applications.

Magnetically Ordered Transition-Metal-Intercalated WSe2

Introducing magnetic behavior in nonmagnetic transition metal dichalcogenides is essential to broaden their applications in spintronic and nanomagnetic devices. In this article, we investigate the electronic and magnetic properties of transition-metal-intercalated tungsten diselenide (WSe₂) using density functional theory. We find that intercalation compounds with composition of T_1/4WSe₂ (T is an iron-series transition-metal atom) exhibit substantial magnetic moments and pronounced ferromagnetic order for late transition metals. The densities of states of the T atoms and the magnetic moments on the W sites indicate that the moments of the intercalated atoms become more localized with increasing atomic number. A large perpendicular magnetocrystalline anisotropy of about 9 meV per supercell has been found for Fe_1/4WSe₂. Furthermore, using mean field theory, we estimated high Curie temperatures of 660, 475, and 379 K for Cr, Mn, and Fe, respectively. The predicted magnetic properties suggest that WSe₂ may have applications in spin electronics and nanomagnetic devices.

Intrinsic Ferromagnetism in Mn-Substituted MoS2 Nanosheets Achieved by Supercritical Hydrothermal Reaction

Doping atomically thick nanosheets is a great challenge due to the self-purification effect that drives the precipitation of dopants. Here, a breakthrough is made to dope Mn atoms substitutionally into MoS₂ nanosheets in a sulfur-rich supercritical hydrothermal reaction environment, where the formation energy of Mn substituting for Mo sites in MoS₂ is significantly reduced to overcome the self-purification effect. The substitutional Mn doping is convincingly evidenced by high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine spectroscopy characterizations. The Mn-doped MoS₂ nanosheets show robust intrinsic ferromagnetic response with a saturation magnetic moment of 0.05 µ_B Mn^-1 at room temperature. The intrinsic ferromagnetism is further confirmed by the reversibility of the magnetic behavior during the cycle of incorporating/removing Li codopants, showing the critical role of Mn 3d electronic states in mediating the magnetic interactions in MoS₂ nanosheets.

Doping two-dimensional materials: ultra-sensitive sensors, band gap tuning and ferromagnetic monolayers

The successful isolation of graphene from graphite in 2004 opened up new avenues to study two-dimensional (2D) systems from layered materials. Since then, research on 2D materials, including graphene, hexagonal-BN (h-BN), transition metal dichalcogenides (TMDs) and black phosphorous, has been extensive, thus leading to various possible applications in the fields of optoelectronics, biomedicine, spintronics, electrochemistry, energy storage and catalysis. However, certain barriers still need to be overcome when dealing with real applications, such as graphene's lack of a bandgap, restricting its use in semiconductor electronics. In this context, a possible solution is to tailor the electronic and optical properties of 2D materials by introducing defects or elemental doping. Although defects play a major role in modifying materials properties, the fact that we call them "defects" might have a negative impact. There has been a long debate on whether structurally perfect materials are equally relevant for modifying the properties and for applications. In this focus article, we clarify that although extra large amounts of defects could be detrimental to the materials properties, well-designed defects might lead to unprecedented properties and interesting applications that pristine materials do not have. Given the relatively short history of research on doped 2D layered materials, our objective is to answer and clarify the following fundamental questions: why does nanomaterial doping offer improved physico-chemical properties? What new applications arise from doping? And what are the current challenges along this line?

Orienting spins in dually doped monolayer MoS2: from one-sided to double-sided doping

Single- and double-sided doped monolayer MoS₂show electron spin alignments with their origins beyond explanations of the existing models.

Magnetic MoS2 pizzas and sandwiches with Mnn (n = 1-4) cluster toppings and fillings: A first-principles investigation

The inorganic layered crystal (ILC) MoS₂ in low dimensions is considered as one of the most promising and efficient semiconductors. To enable the magnetism and keep intrinsic crystal structures, we carried out a first-principles study of the magnetic and semiconductive monolayer MoS₂ adsorbed with the Mn_n (n = 1–4) clusters, and bilayer MoS₂ intercalated with the same clusters. Geometric optimizations of the Mn_n@MoS₂ systems show the complexes prefer to have Mn_n@MoS₂(M) pizza and Mn_n@MoS₂(B) sandwich forms in the mono- and bi-layered cases, respectively. Introductions of the clusters will enhance complex stabilities, while bonds and charge transfers are found between external Mn clusters and the S atoms in the hosts. The pizzas have medium magnetic moments of 3, 6, 9, 4 μ_B and sandwiches of 3, 2, 3, 2 μ_B following the manganese numbers. The pizzas and sandwiches are semiconductors, but with narrower bandgaps compared to their corresponding pristine hosts. Direct bandgaps were found in the Mn_n@MoS₂(M) (n = 1,4) pizzas, and excitingly in the Mn₁@MoS₂(B) sandwich. Combining functional clusters to the layered hosts, the present work shows a novel material manipulation strategy to boost semiconductive ILCs applications in magnetics.

First-principles investigation of the Schottky contact for the two-dimensional MoS2 and graphene heterostructure

The electronic properties of an MoS₂ and graphene heterostructure are investigated by density functional calculations.

Tuning the Schottky contacts in the phosphorene and graphene heterostructure by applying strain

The structures and electronic properties of the phosphorene and graphene heterostructure are investigated by density functional calculations using the hybrid Heyd–Scuseria–Ernzerhof (HSE) functional.

Effect of strain on the electronic and magnetic properties of an Fe-doped WSe2 monolayer

We investigate the electronic and magnetic properties of an Fe-doped single-layer WSe₂ sheet with strain from −10% to 10% using first-principles methods based on density functional theory.