As one of paradigmatic phenomena in condensed matter physics, the quantum anomalous Hall effect (QAHE) in stoichiometric Chern insulators has drawn great interest for years. By using model Hamiltonian analysis and first-principle calculations, we establish a topological phase diagram and map on it with different two-dimensional configurations, which is taken from the recently-grown magnetic topological insulators MnBi4Te7 and MnBi6Te10 with superlattice-like stacking patterns. These configurations manifest various topological phases, including quantum spin Hall effect with and without time-reversal symmetry, as well as QAHE. We then provide design principles to trigger QAHE by tuning experimentally accessible knobs, such as slab thickness and magnetization. Our work reveals that superlattice-like magnetic topological insulators with tunable exchange interaction serve as an ideal platform to realize the long-sought QAHE in pristine compounds, paving a new avenue within the area of topological materials.

Ideal Li-ion battery materials should have a low diffusion barrier, which can realize fast charging/discharging. Here, density function theory (DFT) computations were performed to investigate the possibility of phosphorene and its nanoribbons as anode materials for Li-ion battery. Both phosphorene and its nanoribbons are direct band gap semiconductors, and show semiconductor-to-metal transition upon lithiation with significant charge transfer. Importantly, the computed energy barrier of Li diffuses on phosphorene monolayer along zigzag direction is only 0.09 eV, which indicates the possibility of ultrafast charging/discharging. Furthermore, the fast Li ion mobility is robust and well kept in phosphorene nanoribbons. The ultrahigh Li-ion mobility well distinguishes phosphorene from other typical two-dimensional (2D) materials, such as graphene and MoS 2 , and makes phosphorene a promising anode material for Li ion batteries. ■ INTRODUCTIONBlack phosphorus (BP) is the most stable phosphorus allotrope at room temperature that was first synthesized from white phosphorus under high pressure and high temperature in 1914. 1 It is a layered material coupled together by weak van der Waals (vdW) forces, structurally well resembles graphite. However, instead of a layered planar structure in graphite, BP is a puckered structure due to the sp 3 hybridization character of tetraphosphorus. Recently, two groups reported that bulk BP can be mechanically exfoliated to atomic layers which they called phosphorene. 2,3 Phosphorene turns out to be a promising material with wide potential applications. For example, phosphorene based field effect transistors (FETs) show a high charge-carrier mobility up to 1000 cm 2 V −1 s −1 , much better than other typical two-dimensional (2D) semiconductors, such as MoS 2 (around 200 cm 2 V −1 s −1 ). 3 In contract to graphene, phosphorene is a direct band gap semiconductor whose band gap strongly depending on the number of layers. 2,4 The band gap of BP can be monotonously tunable from ∼0.3 eV in bulk to ∼1.6 eV for a monolayer. 2,4 Additionally, the band gap can also be effectively modified by strain, 5−7 stack orders, 8 electric field 8 and so on. However, the most particularly characters of phosphorene are its high anisotropic electric conductance and optical responses, 2,4,5,9,10 which distinguish it from graphene and MoS 2 .On the other hand, 2D materials attract a lot of attention in the field of lithium-ion batteries (LIBs) because of their large surface-to-volume ratio and unique electronic properties. For today's LIBs, it is highly desired with high capacity and fast charging/discharging, which naturally requires strong binding and low diffusion barriers. Graphene and MoS 2 have been considered to be novel electrode materials for advanced LIBs. 11−15 Fan et al. have reported that Li atoms can be adsorbed on the hollow position with a large binding energy of 1.3 eV. 16 The large binding energy of Li atoms ensures its high capacity. However, they also demonstrated that the energy barrier of L...

MnBi 2 Te 4 is an antiferromagnetic topological insulator that has stimulated intense interest due to its exotic quantum phenomena and promising device applications. The surface structure is a determinant factor to understand the magnetic and topological behavior of MnBi 2 Te 4 , yet its precise atomic structure remains elusive. Here we discovered a surface collapse and reconstruction of few-layer MnBi 2 Te 4 exfoliated under delicate protection. Instead of the ideal septuple-layer structure in the bulk, the collapsed surface is shown to reconstruct as a Mn-doped Bi 2 Te 3 quintuple layer and a Mn x Bi y Te double layer with a clear van der Waals gap in between. Combined with first-principles calculations, such surface collapse is attributed to the abundant intrinsic Mn−Bi antisite defects and the tellurium vacancy in the exfoliated surface, which is further supported by in situ annealing and electron irradiation experiments. Our results shed light on the understanding of the intricate surface-bulk correspondence of MnBi 2 Te 4 and provide an insightful perspective on the surface-related quantum measurements in MnBi 2 Te 4 few-layer devices.

Scalable and sustainable solar hydrogen production via photocatalytic water splitting requires extremely active and stable light‐harvesting semiconductors to fulfill the stringent requirements of suitable energy band position and rapid interfacial charge transfer process. Motivated by this point, increasing attention has been given to the development of photocatalysts comprising intimately interfaced photoabsorbers and cocatalysts. Herein, a simple one‐step approach is reported to fabricate a high‐efficiency photocatalytic system, in which single‐site dispersed iron atoms are rationally integrated on the intrinsic structure of the porous crimped graphitic carbon nitride (g‐C3N4) polymer. A detailed analysis of the formation process shows that a stable complex is generated by spontaneously coordinating dicyandiamidine nitrate with iron ions in isopropanol, thus leading to a relatively complicated polycondensation reaction upon thermal treatment. The correlation of experimental and computational results confirms that optimized electronic structures of Fe@g‐C3N4 with an appropriate d‐band position and negatively shifting Fermi level can be achieved, which effectively gains the reducibility of electrons and creates more active sites for the photocatalytic reactions. As a result, the Fe@g‐C3N4 exhibits a highlighted intramolecular synergistic effect, performing greatly enhanced solar‐photon‐driven activities, including excellent photocatalytic hydrogen evolution rate (3390 µmol h−1 g−1, λ > 420 nm) and a reliable apparent quantum efficiency value of 6.89% at 420 nm.

Since the recent experimental discovery of the CrI 3 and CrGeTe 3 monolayers, van der Waals (vdW) layered transition metal compounds have been recognized as promising candidates to realize 2D ferromagnetic (FM) semiconductors. However, until now, only limited compounds have been proposed to be ferromagnetic semiconductors. Here, on the basis of first-principles calculations, we report that the monolayer, Janus monolayer, and bilayer of NiX 2 (X = Cl, Br, I) are intrinsic 2D FM semiconductors. Our results show that exfoliation energy of the NiX 2 monolayer is smaller than that of graphene, and all studied NiX 2 layers show semiconducting band gaps. The predicted Curie temperature values for NiX 2 (X = Cl, Br, I) monolayers ranged from 120 to 170 K with Monte Carlo simulations. For the Janus monolayer, we found that the spin interaction shows a very strong magnetoelectric coupling under an external electric field. Furthermore, for the bilayer of NiX 2 , our results show that the interlayer coupling is quite weak, indicating the possibility of tuning the magnetic coupling through external manipulations.