Because of the combined advantages of both porous materials and two-dimensional (2D) graphene sheets, superior mechanical properties of three-dimensional (3D) graphene foams have received much attention from material scientists and energy engineers. Here, a 2D mesoscopic graphene model (Modell. Simul. Mater. Sci. Eng. 2011, 19, 054003), was expanded into a 3D bonded graphene foam system by utilizing physical cross-links and van der Waals forces acting among different mesoscopic graphene flakes by considering the debonding behavior, to evaluate the uniaxial tension behavior and fracture mode based on in situ SEM tensile testing (Carbon 2015, 85, 299). We reasonably reproduced a multipeak stress–strain relationship including its obvious yielding plateau and a ductile fracture mode near 45° plane from the tensile direction including the corresponding fracture morphology. Then, a power scaling law of tensile elastic modulus with mass density and an anisotropic strain-dependent Poisson’s ratio were both deduced. The mesoscopic physical mechanism of tensile deformation was clearly revealed through the local stress state and evolution of mesostructure. The fracture feature of bonded graphene foam and its thermodynamic state were directly navigated to the tearing pattern of mesoscopic graphene flakes. This study provides an effective way to understand the mesoscopic physical nature of 3D graphene foams, and hence it may contribute to the multiscale computations of micro/meso/macromechanical performances and optimal design of advanced graphene-foam-based materials.Keywords: coarse-grained molecular dynamics; ductile fracture; graphene foam; mesoscopic model; multipeak stress−strain curve; multiscale physics−mechanics; power scaling law;

Correction for ‘First-principles prediction on bismuthylene monolayer as a promising quantum spin Hall insulator’ by Run-Wu Zhang, et al., Nanoscale, 2017, 9, 8207–8212.

Exploring two-dimensional (2D) materials with magnetic ordering is a focus of current research. It remains a challenge to achieve tunable magnetism in a material of one-atom-thickness without introducing extrinsic magnetic atoms or defects. Here, based on first-principles calculations, we propose that tunable ferromagnetism can be realized in the recently synthesized holey 2D C2N (h2D-C2N) monolayer via purely electron doping that can be readily achieved by gating. We show that owing to the prominent van Hove singularity in the band structure, the material exhibits spontaneous ferromagnetism at a relatively low doping density. Remarkably, over a wide doping range of 4 × 1013 cm−2 to 8 × 1013 cm−2, the system becomes half-metallic, with carriers fully spin-polarized. The estimated Curie temperature can be up to 320 K. Besides gating, we find that magnetism can also be effectively tuned by lattice strain. Our result identifies h2D-C2N as the first material with single-atom-thickness that can host gate-tunable room-temperature half-metallic magnetism, suggesting it as a promising platform to explore nanoscale magnetism and flexible spintronic devices.

Two-dimensional (2D) large band-gap topological insulators (TIs) with highly stable structures are imperative for achieving dissipationless transport devices. However, to date, only very few materials have been experimentally observed to host the quantum spin Hall (QSH) effect at low temperature, thus obstructing their potential application in practice. Using first-principles calculations, herein, we predicted a new 2D TI in the porous allotrope of a bismuth monolayer, i.e. bismuthylene, its geometrical stability was confirmed via phonon spectrum and molecular dynamics simulations. Analysis of the electronic structures reveal that bismuthylene is a native QSH state with a band gap as large as 0.28 eV at the Γ point, which is larger than that (0.2 eV) of the buckled Bi (111) and suitable for room temperature applications. Note that it has a much lower energy than buckled Bi (111) and flattened Bi films; thus, bismuthylene is feasible for experimental realization. Interestingly, the topological properties can be retained under strains within the range of −6%–3% and electrical fields up to 0.8 eV Å−1. A heterostructure was constructed by sandwiching bismuthylene between BN sheets, and the non-trivial topology of bismuthylene was retained with a sizable band gap. These findings provide a platform to design a large-gap QSH insulator based on the 2D bismuthylene films, which show potential applications in spintronic devices.

Blue phosphorene (BP) and gray arsenene (GA), consisting of phosphorus and arsenic atoms in two-dimensional (2D) low-buckled honeycomb lattices, respectively, have received great interest because of their excellent electronic and optoelectronic performances. Here, using first-principles density functional theory, we investigate magneto-optical (MO) Kerr and Faraday effects in BP and GA under hole doping. Ferromagnetic ground states are found in hole-doped monolayer and bilayer BP and GA due to the Stoner electronic instability, which originates from the van Hove singularity of the density of states at the valence band edge. The Kerr and Faraday effects strongly depend on the doping concentration and therefore are electrically controllable by adjusting the number of holes via the gate voltage. The influences of the thin film thickness, spin-polarized direction, and the substrate on the MO effects are further studied. We find that the MO effects are weakened remarkably as the thin film thickness increases and can be negligible more than three single-layers; the MO effects are much more prominent when spin polarization is along the out-of-plane direction and will decrease more than one order of magnitude on turning the spin in the crystal plane; the insulating substrates with small refractive indices are favorable to generate large MO effects and appropriate compressive strains applied on BP and GA due to lattice mismatch with substrates are further beneficial. The MO effects in GA are generally larger than those in BP because the strength of spin–orbit coupling in the arsenic atom is larger than that in the phosphorus atom. Monolayer GA possesses the largest Kerr and Faraday rotation angles, which are comparable to or even larger than those of well-known MO materials such as 3d-transition-metal multilayers and compounds. Our results indicate that BP and GA are a promising material platform for MO device applications.

Silicene, a silicon analogue of graphene, has attracted increasing attention during the past few years. As early as in 1994, the possibility of stage corrugation in the Si analogs of graphite had already been theoretically explored. But there were very few studies on silicene until 2009, when silicene with a low buckled structure was confirmed to be dynamically stable by ab initio calculations. In spite of the low buckled geometry, silicene shares most of the outstanding electronic properties of planar graphene (e.g., the “Dirac cone”, high Fermi velocity and carrier mobility). Compared with graphene, silicene has several prominent advantages: (1) a much stronger spin–orbit coupling, which may lead to a realization of quantum spin Hall effect in the experimentally accessible temperature, (2) a better tunability of the band gap, which is necessary for an effective field effect transistor (FET) operating at room temperature, (3) an easier valley polarization and more suitability for valleytronics study. From 2012, monolayer silicene sheets of different superstructures were successfully synthesized on various substrates, including Ag(1 1 1), Ir(1 1 1), ZrB2(0 0 0 1), ZrC(1 1 1) and MoS2 surfaces. Multilayer silicene sheets have also been grown on Ag(1 1 1) surface. The experimental successes have stimulated many efforts to explore the intrinsic properties as well as potential device applications of silicene, including quantum spin Hall effect, quantum anomalous Hall effect, quantum valley Hall effect, superconductivity, band engineering, magnetism, thermoelectric effect, gas sensor, tunneling FET, spin filter, and spin FET, etc. Recently, a silicene FET has been fabricated, which shows the expected ambipolar Dirac charge transport and paves the way towards silicene-based nanoelectronics. This comprehensive review covers all the important theoretical and experimental advances on silicene to date, from the basic theory of intrinsic properties, experimental synthesis and characterization, modulation of physical properties by modifications, and finally to device explorations.

In recent years, the Berry phase as a fascinating concept has made a great success for interpreting many physical phenomena. In this paper, we review our past works on the Berry phase effect in solid materials with spin–orbit coupling. Firstly, we developed an exact method for directly evaluating the Berry curvature and anomalous Hall conductivity, then the intrinsic mechanism of anomalous Hall effect was quantitatively confirmed in bcc Fe and CuCr2Se4−xBrx. An effective method was proposed to decompose the anomalous Hall effect into the intrinsic and extrinsic contributions. We also developed computational methods for the spin Hall conductivity and anomalous Nernst conductivity. Secondly, we developed a powerful method for computing the Z2 topological invariant without consideration of special symmetry. We predicted many topological insulators in three-dimensions, including half-Heusler, chalcopyrite, strained InSb, core-holed Ge and InSb, Bi2Te3/BiTeI heterostructure, and in two-dimensions, including silicene, germanene, stanene, X-hydride/halide (X = N–Bi) monolayers and Bi4Br4. We also predicted the quantum anomalous Hall effect in magnetic atoms adsorbed graphene and half-passivated BiH, the quantum valley Hall effect and topological superconducting in silicene and n-dopped BiH. Finally, the summary of our past works and the further outlooks are discussed.

Searching for suitable anodes with good performance is a key challenge for rechargeable Na-ion batteries (NIBs). Using the first-principles method, we predict that 2D nitrogen electride materials can be served as anode materials for NIBs. Particularly, we show that Ca2N meets almost all the requirements of a good NIB anode. Each formula unit of a monolayer Ca2N sheet can absorb up to four Na atoms, corresponding to a theoretical specific capacity of 1138 mAh·g–1. The metallic character for both pristine Ca2N and its Na intercalated state NaxCa2N ensures good electronic conduction. Na diffusion along the 2D monolayer plane can be very fast even at room temperature, with a Na migration energy barrier as small as 0.084 eV. These properties are key to the excellent rate performance of an anode material. The average open-circuit voltage is calculated to be 0.18 V vs Na/Na+ for the chemical stoichiometry of Na2Ca2N and 0.09 V for Na4Ca2N. The relatively low average open-circuit voltage is beneficial to the overall voltage of the cell. In addition, the 2D monolayers have very small lattice change upon Na intercalation, which ensures a good cycling stability. All these results demonstrate that the Ca2N monolayer could be an excellent anode material for NIBs.Keywords: 2D electride; anode material; diffusion barrier; Na-ion batteries; open-circuit voltage

Quantum spin Hall (QSH) insulators have gapless topological edge states inside the bulk band gap, which can serve as dissipationless spin current channels. The major challenge currently is to find suitable materials for this topological state. Here, we predict a new large-gap QSH insulator with bulk direct band gap of ∼0.18 eV, in single-layer Bi4Br4, which could be exfoliated from its three-dimensional bulk material due to the weakly bonded layered structure. The band gap of single-layer Bi4Br4 is tunable via strain engineering, and the QSH phase is robust against external strain. Moreover, because this material consists of special one-dimensional molecular chain as its basic building block, the single layer Bi4Br4 could be torn to ribbons with clean and atomically sharp edges. These nanoribbons, which have single-Dirac-cone edge states crossing the bulk band gap, are ideal wires for dissipationless transport. Our work thus provides a new promising material for experimental studies and practical applications of the QSH effect.

A large bulk band gap is critical for the application of quantum spin Hall (QSH) insulators or two-dimensional (2D) topological insulators (TIs) in spintronic devices operating at room temperature (RT). On the basis of first-principles calculations, we predicted a group of 2D TI BiX/SbX (X=H, F, Cl and Br) monolayers with extraordinarily large bulk gaps from 0.32 eV to a record value of 1.08 eV. These giant-gaps are entirely due to the result of the strong spin-orbit interaction related to the px and py orbitals of the Bi/Sb atoms around the two valleys K and K′ of the honeycomb lattice, which is significantly different from that consisting of the pz orbital as in graphene/silicene. The topological characteristic of BiX/SbX monolayers is confirmed by the calculated nontrivial Z2 index and an explicit construction of the low-energy effective Hamiltonian in these systems. We demonstrate that the honeycomb structures of BiX monolayers remain stable even at 600 K. Owing to these features, the giant-gap TIs BiX/SbX monolayers are an ideal platform to realize many exotic phenomena and fabricate new quantum devices operating at RT. Furthermore, biased BiX/SbX monolayers become a quantum valley Hall insulator, exhibiting valley-selective circular dichroism.

First-principles calculations are performed to study the electronic properties and Li storage capability of V2C and its corresponding fluoride and hydroxide. We find that the V2C monolayer is metallic with antiferromagnetic configuration, while its derived V2CF2 and V2C(OH)2 in their the most stable configurations are small-gap antiferromagnetic semiconductors. Li adsorption could enhance the electric conductivity of V2C fluoride and hydroxide. The bare V2C monolayer shows fast Li diffusion with low diffusion barrier height and very high Li storage capacity (with theoretical value ∼940 mAh/g), while the passivated F or OH atoms on its surface tend to impede Li diffusion and largely reduce the Li storage capacity. Moreover, the average intercalation potentials for V2C-based materials are calculated to be relatively low. Our results suggest that V2C monolayer could be a promising anode material for Li-ion batteries.

Benefiting from the advantages of environmental friendliness, easy purification, and high thermal stability, the recently synthesized two-dimensional (2D) material MoN2 shows great potential for clean and renewable energy applications. Here, through first-principles calculations, we show that monolayered MoN2 is promising as a high capacity electrode material for metal ion batteries. Firstly, identified by phonon dispersion and exfoliation energy calculations, MoN2 monolayer is proved to be a structurally stable material that can be exfoliated from its bulk counterpart in experiments. Secondly, all the studied metal atoms (Li, Na and K) can be adsorbed on MoN2 monolayer; both the pristine and doped MoN2 are metallic. Thirdly, the metal atoms possess moderate/low migration barriers on MoN2, which ensures excellent cycling performance as battery electrodes. In addition, the calculated average voltages suggest that MoN2 monolayer is a suitable cathode for Li-ion batteries and a suitable anode for Na-ion and K-ion batteries. Most importantly, as a cathode for Li-ion batteries, MoN2 possesses a comparable average voltage but a capacity 1 to 2 times larger (432 mA h g−1) than that of standard commercial cathode materials; as an anode for Na-ion batteries, the theoretical capacity (864 mA h g−1) of MoN2 is 2 to 5 times larger than that of typical 2D anode materials, such as MoS2 and most MXenes. Finally, we also provide an estimation of the capacities of other transition-metal dinitride materials. Our work suggests that the transition-metal dinitride MoN2 is an appealing 2D electrode material with high storage capacity.