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Atomic structures of symmetric/distorted "X-sub" configurations, along with the "TM@X″ configuration are illustrated. Formation energies and magnetic moment for different atomic structures of TM atoms deposited on Mo X 2 layers are presented. Spin-polarization densities and electronic structures are shown, as obtained from first-principles calculations (PDF)■ AUTHOR INFORMATION

potentially to exploit them in spintronics applications. Such potential applications of magnetic vdW materials have recently sparked considerable interest in investigating magnetism of bulk ferromagnetic materials thinned to a single layer [1][2][3][4] or the emergence of ferromagnetism of paramagnetic bulk materials at the monolayer limit. [5,6] In addition, defect-and dopant-induced ferromagnetism has been predicted theoretically. [7][8][9][10][11][12] Recent experimental results suggest that 1T-2H phase boundaries in MoSe 2 , [13][14][15] edges in WS 2 or MoS 2 , [16][17][18][19] adsorbate induced defect states, [20] or substitutional doping of transition metals in 2D materials [21][22][23] can result in defect-or dopant-induced ferromagnetic ordering in these systems. Recently, long-range magnetic order has also been observed in both MoTe 2 and MoSe 2 , which has been suggested to be induced by intrinsic ferromagnetic defects, such as Mo antisites, that is, Mo atoms at chalcogen lattice sites. [24] This indicates that in these materials even dilute defects can cause long-range ferromagnetic ordering, making them promising diluted magnetic semiconductor (DMS) materials. Here, we investigate a new doping mechanism for 2H-MoTe 2 that enables altering the monolayer or surface layer of a MoTe 2 crystal with transition metal impurities. We have recently demonstrated that 2H-MoTe 2 and MoSe 2 can be modified by incorporation of transition metals into the host's interstitial site. [25] We have shown experimentally and by density functional theory (DFT) calculations that excess Mo atoms on MoTe 2 are energetically favored at interstitial sites as compared to adsorbed atoms at the surface. At elevated temperatures, these excess transition metal atoms are mobile and can undergo site-exchange with lattice Mo atoms. For high enough mobility (temperature), the interstitials rearrange into 1D Mo-rich crystal modifications, known as mirror twin grain boundaries. [26][27][28] These grain boundaries have shown no ferromagnetic properties. We demonstrate here, that this doping mechanism can also be expanded to other transition metals, in particular with the goal of inducing magnetism into MoTe 2 . Titanium or vanadium has been used as ferromagnetic dopants in diluted semiconductor systems [29,30] and thus, in this study we explore if V can be introduced into MoTe 2 interstitial Figure 4. Magnetization measurements of MoTe 2 with different V concentrations. a) M-H hysteresis loops taken at 10 K for MoTe 2 with 0.2, 0.3, and 0.8% of V coverage. The variation of the linear diamagnetic background is a consequence of different substrate thicknesses. b) Variation in magnetization saturation with the V concentration. c) Temperature dependences of H C and M S for the 0.8% V-doped sample. The error bars reflect the experimental uncertainty related to background noise. www.advancedsciencenews.com

Transition metal chalcogenides (TMCs) are a large family of 2D materials that are currently attracting intense interest. TMCs with 3d transition metals provide opportunities for introducing magnetism and strong correlations into the material with manganese standing out as a particularly attractive option due to its large magnetic moment. Here we report on the successful synthesis of monolayer manganese selenide on a NbSe 2 substrate. Using scanning tunneling microscopy and spectroscopy experiments and global structure prediction calculations at the density functional theory level, we identify the atomic structure and magnetic and electronic properties of the layered Mn 2 Se 2 phase. The structure is similar to the layered bulk phase of CuI or a buckled bilayer of h -BN. Interestingly, our results suggest that the monolayer is antiferromagnetic, but with an unusual out-of-plane ordering that results in two ferromagnetic planes.

Herein, we employed first-principles density functional theory calculations to understand the structural, electronic, and magnetic properties of pristine and lithiated zinc blende (ZB) SiC(111) surface slabs. Our calculations on below four layer thick slabs reveal the spontaneous formation of a graphitic SiC layer which mimics the two-dimensional boron nitride structure. Though this monolayer shows a direct band gap, the energy bands in bi- and trilayer slabs are nondegenerated owing to weak van der Waal’s interaction between the layers, and they show indirect band gap for those cases. In a pristine slab, the surface states presented in both sides originate magnetism, and they are coupled antiferromagnetically. Its strength decreases with increasing layer thickness. This magnetism is quenched during lithiation and exfoliation of layers. The latter is observed, even for thicker ZB slabs during lithiation. The average lithium intercalation potential is calculated to be 0.20 V, which is quite comparable with the anodic potential of high capacity SiC nanoparticles as reported in experiment. Thus, the mechanism of lithiation in SiC nanoparticles is proposed to be intercalation, rather than alloying.

processes such as ion implantation can be used, but this approach tends to create undesirable defects, whose removal then requires additional annealing steps. Recently, lots of research attention has been focused on 2D materials, [1,2] as they not only exhibit great variety in electronic characteristics ranging from insulators to metals, but also possess unique properties related to their reduced dimensionality. While 2D materials can be doped with the same methods as bulk systems, there are approaches that are unique to them. Due to the surface-only geometry, the doping in 2D materials can also be attained by: 1) physical/chemical adsorption; 2) ionicliquid-gating; and 3) direct atomic substitution. [3,4] The surface adsorption and ionic-liquid-gating are basically equivalent to the implementation of charge transfer between the environment and the 2D materials, which are both very effective due the high surface to volume ratio of the 2D materials. However, the difficulties in integration of the system limit the practical applications of these approaches. The direct atomic substitution in 2D materials can be done via, e.g., sulfurization/selenization. [5] Alternatively, vacancies can be produced by irradiation [6,7] or thermal evaporation during annealing, [8] followed by the deposition of doping species. Direct substitution can also be achieved via ion implantation, but it is technically difficult, as it requires very low ion energies (below 100 eV) or needs an additional coating of buffer layer and The ORCID identification number(s) for the author(s) of this article can be found under

The replacement of platinum with nonprecious metal electrocatalysts for hydrogen evolution reaction (HER) remains an important challenge. We report facile synthesis of precious-metal-free HER electrocatalysts that are made up of metal–organic framework-derived cobalt/carbon nanostructures and semicrystalline ultrathin MoS2 or WS2 nanosheets. The as-synthesized catalysts MoS2/Co@NC and WS2/Co@NC delivered an electrochemical HER current density of 25 mA cm–2 at overpotentials of 0.23 and 0.28 V, respectively. Both the catalysts were found to be highly stable in 0.5 M H2SO4 with small Tafel slope values. The high-performance HER activity can be related to (i) covalent cobalt doping into MoS2 and WS2 layers confirmed by X-ray photoelectron spectroscopy, (ii) the presence of cobalt nanoparticles in close vicinity of MoS2 and WS2 layers, (iii) the presence of bridging disulfide S2 2– into MoS2 and WS2 layers, and (iv) synergistic cooperation among multicomponents present in the catalyst structure. Density functional theory calculations suggested that Co doping at Mo sites in MoS2 has a favorable Gibb’s free energy (ΔG) value for HER. Interestingly, the interface between the Co nanocluster and MoS2 is found to be a favorable HER active site with localization of electrons. To the best of our knowledge, the simultaneous effect of single metal substitution and metal clusters in/on MoS2 and WS2 layers has not been studied for HER. Moreover, we have also demonstrated a durable acid–base water electrolyzer using MoS2/Co@NC and WS2/Co@NC as cathodes, generating 10 mA cm–2 current density at a cell voltage of ∼0.89 V.

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