Developing approaches to effectively induce and control the magnetic states is critical to the use of magnetic nanostructures in quantum information devices but is still challenging. Here we have demonstrated, by employing the density functional theory calculations, the existence of infinite magnetic sheets with structural integrity and magnetic homogeneity. Examination of a series of transition metal dichalcogenides shows that the biaxial tensile strained NbS(2) and NbSe(2) structures can be magnetized with a ferromagnetic character due to the competitive effects of through-bond interaction and through-space interaction. The estimated Curie temperatures (387 and 542 K under the 10% strain for NbS(2) and NbSe(2) structures, respectively) suggest that the unique ferromagnetic character can be achieved above room temperature. The self-exchange of population between 4d orbitals of the Nb atom that leads to exchange splitting is the mechanism behind the transition of the spin moment. The induced magnetic moments can be significantly enhanced by the tensile strain, even giving rise to a half-metallic character with a strong spin polarization around the Fermi level. Given the recent progress in achieving the desired strain on two-dimensional nanostructures, such as graphene and a BN layer, in a controlled way, we believe that our calculated results are suitable for experimental verification and implementation, opening a new path to explore the spintronics in pristine two-dimensional nanostructures.

Voltage and capacity fading of layer structured lithium and manganese rich (LMR) transition metal oxide is directly related to the structural and composition evolution of the material during the cycling of the battery. However, understanding such evolution at atomic level remains elusive. On the basis of atomic level structural imaging, elemental mapping of the pristine and cycled samples, and density functional theory calculations, it is found that accompanying the hoping of Li ions is the simultaneous migration of Ni ions toward the surface from the bulk lattice, leading to the gradual depletion of Ni in the bulk lattice and thickening of a Ni enriched surface reconstruction layer (SRL). Furthermore, Ni and Mn also exhibit concentration partitions within the thin layer of SRL in the cycled samples where Ni is almost depleted at the very surface of the SRL, indicating the preferential dissolution of Ni ions in the electrolyte. Accompanying the elemental composition evolution, significant structural evolution is also observed and identified as a sequential phase transition of C2/m → I41 → Spinel. For the first time, it is found that the surface facet terminated with pure cation/anion is more stable than that with a mixture of cation and anion. These findings firmly established how the elemental species in the lattice of LMR cathode transfer from the bulk lattice to surface layer and further into the electrolyte, clarifying the long-standing confusion and debate on the structure and chemistry of the surface layer and their correlation with the voltage fading and capacity decaying of LMR cathode. Therefore, this work provides critical insights for design of cathode materials with both high capacity and voltage stability during cycling.

Two-dimensional boron materials have recently attracted extensive theoretical interest because of their exceptional structural complexity and remarkable physical and chemical properties. However, such 2D boron monolayers have still not been synthesized. In this report, the synthesis of atomically thin 2D γ-boron films on copper foils is achieved by chemical vapor deposition using a mixture of pure boron and boron oxide powders as the boron source and hydrogen gas as the carrier gas. Strikingly, the optical band gap of the boron film was measured to be around 2.25 eV, which is close to the value (2.07 eV) determined by first-principles calculations, suggesting that the γ-B28 monolayer is a fascinating direct band gap semiconductor. Furthermore, a strong photoluminescence emission band was observed at approximately 626 nm, which is again due to the direct band gap. This study could pave the way for applications of two-dimensional boron materials in electronic and photonic devices.

Capacity and voltage fading of Li2MnO3 is a major challenge for the application of this category of material, which is believed to be associated with the structural and chemical evolution of the materials. This paper reports the detailed structural and chemical evolutions of Li2MnO3 cathode captured by using aberration corrected scanning/transmission electron microscopy (S/TEM) after certain numbers of charge–discharge cycling of the batteries. It is found that structural degradation occurs from the very first cycle and is spatially initiated from the surface of the particle and propagates toward the inner bulk as the cyclic number increases, featuring the formation of the surface phase transformation layer and gradual thickening of this layer. The structure degradation is found to follow a sequential phase transformation: monoclinic C2/m → tetragonal I41 → cubic spinel, which is consistently supported by the decreasing lattice formation energy based on DFT calculations. For the first time, high spatial resolution quantitative chemical analysis reveals that 20% oxygen in the surface phase transformation layer is removed and such a newly developed surface layer is a Li-depleted layer with reduced Mn cations. This work demonstrates a direct correlation between structural degradation and the cell’s electrochemical degradation, which enhances our understanding of Li–Mn-rich (LMR) cathode materials.

Developing approaches to effectively induce and control the magnetic states is critical to the use of magnetic nanostructures in quantum information devices but is still challenging. Here MoS2-based nanostructures including atomic defects, nanoholes, nanodots and antidots are characterized with spin-polarized density functional theory. The S-vacancy defect is more likely to form than the Mo-vacancy defect due to the form of Mo-Mo metallic bonds. Among different shaped nanoholes and nanodots, triangle ones associated with ferromagnetic characteristic are most energetically favorable, and exhibit unexpected large spin moments that scale linearly with edged length. In particular, S-terminated triangle nanodots show strong spin anisotropy around the Fermi level with a substantial collective characteristic of spin states at edges, enabling it to a desired spin-filtering structure. However, in the antidot, the net spin, coupled order and stability of spin states can be engineered by controlling type and distance of internal nanoholes. Based on the analysis of the spin coupled mechanism, a specific antidot structure, the only S-terminated antidot, was determined to exhibit a large net spin with long-range ferromagnetic coupling above room temperature. Given the recent achievement of graphene- and BN-based nanohole, nanodot and antidot structures, we believe that our calculated results are suitable for experimental verification and implementation opening a new path to explore MoS2-based magnetic nanostructures.

Tw o-dimensional boron materials have recently attracted extensive theoretical interest because of their exceptional structural complexity and remarkable physical and chemical properties.H owever,s uch 2D boron monolayers have still not been synthesized. In this report, the synthesis of atomically thin 2D g-boron films on copper foils is achieved by chemical vapor deposition using amixture of pure boron and boron oxide powders as the boron source and hydrogen gas as the carrier gas.S trikingly,t he optical band gap of the boron film was measured to be around 2.25 eV,w hich is close to the value (2.07 eV) determined by first-principles calculations, suggesting that the g-B 28 monolayer is afascinating direct band gap semiconductor.Furthermore,astrong photoluminescence emission band was observed at approximately 626 nm, which is again due to the direct band gap.This study could pave the way for applications of two-dimensional boron materials in electronic and photonic devices.Two-dimensional (2D) materials are attractive components of atomic-layer field-effect transistors (FETs), sensors,a nd photovoltaic and photoelectric devices. [1][2][3][4][5] Although graphene has been shown to be au seful material for highperformance electronics owing to its very high carrier mobility,i ts uffers from the lack of as ignificant band gap, which limits its application in digital electronics. [3][4][5] Hence, three-atom-thick 2D semiconducting transition-metal dichalcogenides (TMDs;e .g., MoS 2 ,W S 2 ,M oSe 2 ,W Se 2 )a nd oneatom-thick non-metal/metal layers with ah oneycomb structure (e.g., silicene,g ermanene,p hosphorene,a rsenene,a ntimonene,a nd stannene) have attracted wide interest. [3][4][5][6][7]

Due to its unique electronic properties and wide spectrum of promising applications, graphene has attracted much attention from scientists in various fields. Control and engineering of graphene's semiconducting properties is considered to be key to its applications in electronic devices. Here, we report a novel method to prepare in situ nitrogen-doped graphene by microwave plasma assisted chemical vapor deposition (CVD) using PDMS (polydimethylsiloxane) as a solid carbon source. Based on this approach, the concentration of nitrogen-doping can be easily controlled via the flow rate of nitrogen during the CVD process. X-ray photoelectron spectroscopy results indicated that the nitrogen atoms doped into the graphene lattice were mainly in the forms of pyridinic and pyrrolic structures. Moreover, first-principles calculations show that the incorporated nitrogen atoms can lead to p-type doping of graphene. This in situ approach provides a promising strategy to prepare graphene with controlled electronic properties.

Defects play an important role on the unique properties of the sp2-bonded materials, such as graphene. The creation and evolution of monovacancy, divacancy, Stone-Wales (SW), and grain boundaries (GBs) under irradiation in graphene are investigated using density functional theory and time-dependent density functional theory molecular dynamics simulations. It is of great interest that the patterns of these defects can be controlled through electron irradiation. The SW defects can be created by electron irradiation with energy above the displacement threshold energy (T d, ∼19 eV) and can be healed with an energy (14–18 eV) lower than T d. The transformation between four types of divacanciesV2(5–8–5), V2(555–777), V2(5555–6–7777), and V2(55–77)can be realized through bond rotation induced by electron irradiation. The migrations of divancancies, SW defects, and GBs can also be controlled by electron irradiation. Thus, electron irradiation can serve as an important tool to modify morphology in a controllable manner and to tailor the physical properties of graphene.