Although the phenomenon that optical reflectivity of hard group IVB transition metal nitrides depends on stoichiometry has been reported, the microscopic origin of this behavior has not been well explored yet. Here we find that optical reflectivity of rocksalt hafnium nitride films (δ-HfNx) can be effectively tuned by stoichiometry x, and the underlying mechanism can be well elucidated by Drude–Lorentz fitting and first-principles calculations. It is shown that the observed tunability of optical reflectivity arises from a transition from N vacancies (VN) to Hf vacancies (VHf) in the films because this evolution from VN to VHf has important roles in changing electronic properties of the films in the following three aspects: (i) density of free electrons, wherein VN and VHf act as donor-like and acceptor-like defects, respectively; (ii) mean free path of free electrons, in which VN and VHf are the main electron scattering sites in sub- and overstoichiometric films, respectively; (iii) interband transition absorption of bound electrons, wherein three previously unreported absorption bands originating from VN and VHf are found to occur at ∼0.81, 2.27, and 3.75 eV. These point-defect-induced variations significantly affect the dielectric function of δ-HfNx films and thus drive the tailored evolution in reflectivity properties with x.

One key challenge facing room temperature Na-ion batteries lies in identifying earth-abundant, environmentally friendly and safe materials that can provide efficient Na+ storage sites in Na-ion batteries. Herein, we report such a material, polyoxometalate Na2H8[MnV13O38] (NMV), with entirely different composition and structure from those cathode compounds reported before. Ex-situ XPS and FTIR analyses reveal that NMV cathode behaves like an “electron/Na-ion sponge”, with 11 electrons/Na+ acceptability per mole, which has a decisive contribution to the high capacity. The extraordinary structural features, evidenced by X-ray crystallographic analysis, of Na2H8[MnV13O38] with a flexible 2D lamellar network and 1D open channels provide diverse Na ion migration pathways, yielding good rate capability. First-principle calculations demonstrate that a super-reduced state, [MnV13O38]20−, is formed with slightly expanded size (ca. 7.5%) upon Na+ insertion compared to the original [MnV13O38]9–. This “ion sponge” feature ensures the good cycling stability. Consequently, benefiting from the combinations of “electron/ion sponge” with diverse Na+ diffusion channels, when revealed as the cathode materials for Na-ion batteries, Na2H8[MnV13O38]/G exhibits a high specific capacity (ca. 190 mA h/g at 0.1 C), associates with a good rate capability (130 mA h/g at 1 C), and a good capacity retention (81% at 0.2 C). Our results promote better understanding of the storage mechanism in polyoxometalate host, enrich the existing rechargeable SIBs cathode chemistry, and enlighten an exciting direction for exploring promising cathode materials for Na-ion batteries.Keywords: first principle calculation; mechanism; polyoxometalates; sodium ion battery; sponge;

Two-dimensional crystals with weak layer interactions, such as twisted graphene, have been a focus of research recently. As a representative example, transitional metal dichalcogenides show a lot of fascinating properties due to stacking orders and spin–orbit coupling. We analyzed the dynamic energy barrier of possible phase transitions in MoX2 (X = S, Se and Te) with first-principles methods. In the structural transition from 2Hc to 2Ha, the energy barrier is found to be increased following an increase of pressure which is different from the phase transition in usual semiconductors. Among MoS2, MoSe2 and MoTe2, the energy barrier of MoS2 is the lowest and the stability of both 2Hc and 2Ha is reversed under pressure for MoS2. It is found that the absence of a phase transition in MoSe2 and MoTe2 is due to the competition between van der Waals interaction of layers and the coulomb interaction of Mo and X in nearest-neighbor layer of Mo in both phases.

Using first-principles DFT calculations, the pathway and the energy barrier of phase transition between 2H and 1T′ have been investigated for MoTe2 and WTe2 monolayers. The Phase transition is controlled by the simultaneous movement of metal atoms and Te atoms in their plane without the intermediate phase 1T. The energy barrier (less than 0.9 eV per formula cell) is not so high that the phase transition is dynamically possible. The relative stability of both 2H and 1T′ phases and the energy barrier for phase transition can be modulated by the biaxial and uniaxial strain. The dynamic energy barrier is decreased by applying the strain. The phase transition between 2H and 1T′ controlled by the strain can be used to modulate the electronic properties of MoTe2 and WTe2.

Using first-principle calculations, we studied the interaction between Li and graphene by considering two kinds of models, which are related to the configurations of Li adsorption and the concentration of Li on graphene. In a low concentration, the 2s state of Li is fully unoccupied due to charge transfer. With the increase of Li concentration, the 2s state is broadened and occupied partly by electrons. With a high concentration, such as Li:C = 1:6, Li cluster adsorption seems to become popular by the free formation energy of clusters with thermal effects.

Understanding the origin of valence band splitting is important because it governs the unique spin and valley physics in few-layer MoS2. We explore the effects of spin–orbit coupling and interlayer coupling on few-layer MoS2 using first-principles methods. We find spin–orbit coupling has a major contribution to the valence band splitting at K in multilayer MoS2. In double-layer MoS2, the interlayer coupling leads to the widening of the gap between the already spin–orbit split states. This is also the case for the bands of the K-point in bulk MoS2. In triple-layer MoS2, the strength of interlayer coupling of the spin-up channel becomes different from that of spin-down at K. This combined with spin–orbit coupling results in the band splitting in two main valence bands at K. With the increase of pressure, this phenomenon becomes more obvious with a decrease of main energy gap in the splitting valence bands at the K valley.

In two-dimensional transition-metal dichalcogenides, both spin–orbit coupling and interlayer coupling play critical roles in the electronic band structure and are desirable for the potential applications in spin electronics. Here, we demonstrate the pressure characteristics of the exciton absorption peaks (so-called excitons A, B and C) in monolayer, bilayer, and trilayer molybdenum disulfide (MoS2) by studying the reflectance spectra under hydrostatic pressure and performing the electronic band structure calculations based on density functional theory to account for the experimental observations. We find that the valence band maximum splitting at the K point in monolayer MoS2, induced by spin–orbit coupling, remains almost unchanged with increasing pressure applied up to 3.98 GPa, indicating that the spin–orbit coupling is insensitive to the pressure. For bilayer and trilayer MoS2, however, the splitting shows an increase with increasing pressure due to the pressure-induced strengthening of the interlayer coupling. The experimental results are in good agreement with the theoretical calculations. Moreover, the exciton C is identified to be the interband transition related to the van Hove singularity located at a special point which is approximately 1/4 of the total length of Γ–K away from the Γ point in the Brillouin zone.Keywords: hydrostatic pressure; interlayer coupling; molybdenum disulfide; spin−orbit coupling; valence band maximum splitting;

•We identify a new ternary compound ReCN with theoretical calculation.•The ternary compound ReCN is with two stable structures with P63mc and P3m1.•ReCN is a semiconductor from the calculation of electronic structures.•ReCN is found to possess the outstanding mechanical properties.•ReCN may be synthesized relatively easily.We identify a new ternary compound, ReCN and characterize its properties including structural stability and indicators of hardness using first principles calculations. We find that there are two stable structures with space groups P63mc (HI) and P3m1 (HII), in which there are no C–C and N–N bonds. Both structures, H1 and III are elastically and dynamically stable. The electronic structures show that ReCN is a semiconductor, although the parent compounds, ReC2 and ReN2 are both metallic. ReCN is found to possess the outstanding mechanical properties with the large bulk modulus, shear modulus and excellent ideal strengths. In addition, ReCN may perhaps be synthesized relatively easily because it becomes thermodynamic stable with respect to decomposition at very low pressures.

With first-principles density functional theory calculations, we demonstrate that quantum capacitance of graphene-based electrodes can be improved by the N-doping, vacancy defects, and adsorbed transition-metal atoms. The enhancement of the quantum capacitance can be contributed to the formation of localized states near Dirac point and/or shift of Fermi level induced by the defects and doping. In addition, the quantum capacitance is found to increase monotonically following the increase of defect concentrations. It is also found that the localized states near Fermi level results in the spin-polarization effect.

We analyze the changes in the electronic structures of single-layer and multilayer MoS2 under pressure using first-principles methods including van der Waals interactions. For single-layer MoS2, the bond angle is found to control the electronic structure around the band gap under the pressure. For multilayer and bulk MoS2, the changes in electronic structure under pressure are mainly controlled by the coupling of layers. Under pressure, the band gap of single-layer MoS2 changes from direct to indirect, while multilayer MoS2 becomes a band metal. Analysis of the real-space distribution of band-decomposed charge density shows that this behavior can be understood in terms of the different Mo d-electron orbitals making up the states near band gap including those at the top of valence band of Γ and K points and the bottom of conduction band along Λ and at the K point.

We analyzed the adsorption of Li on graphene in the context of anodes for lithium-ion batteries (LIBs) using first-principles methods including van der Waals interactions. We found that although Li can reside on the surface of defect-free graphene under favorable conditions, the binding is much weaker than to graphite and the concentration on a graphene surface is not higher than in graphite. At low concentration, Li ions spread out on graphene because of Coulomb repulsion. With increased Li content, we found that small Li clusters can be formed on graphene. Although this result suggests that graphene nanosheets can conceivably have a higher ultimate Li capacity than graphite, it should be noted that such nanoclusters can potentially nucleate Li dendrites, leading to failure. The implications for nanostructured carbon anodes in batteries are discussed.Keywords: adsorption of Li; first-principles calculations; formation of Li cluster; graphene; rechargeable lithium-ion batteries;

With first-principle DFT calculations, the catalytic activity of heteroatom-doped carbon nanostructures in oxygen reduction reaction is investigated by exploring the active site of B-doped, N-doped and (B, N)-codoped and analyzing the kinetic pathways of oxygen reduction with the participation of protons. It is found that the heteroatom-doped graphene can become the effective catalysis materials for ORR with four-electron pathway. Especially, the formation of epoxide groups may be important for the four-electron processes on B-doped and (B, N)-codoped graphene. By the analysis of charge redistribution, the formation of active catalytic sites is attributed to the localized positive charge and electronic dipole induced by the dopant.

With first-principles DFT calculations, the interaction between Li and carbon in graphene-based nanostructures is investigated as Li is adsorbed on graphene. It is found that the Li/C ratio of less than 1/6 for the single-layer graphene is favorable energetically, which can explain what has been observed in Raman spectrum reported recently. In addition, it is also found that the pristine graphene cannot enhance the diffusion energetics of Li ion. However, the presence of vacancy defects can increase the ratio of Li/C largely. With double-vacancy and higher-order defects, Li ion can diffuse freely in the direction perpendicular to the graphene sheets and hence boost the diffusion energetics to some extent.Keywords: adsorption of Li; defects and nanostructures; diffusion of Li; first-principles calculations; graphene; rechargeable Li batteries;

By the incorporation of C into (BN)12 fullerene, our theoretical investigation shows that the hydrogenation reaction on carbon doped B11N12C cluster is both thermodynamically favored and kinetically feasible under ambient conditions. Without using the metal catalyst, the C atom can work as an activation center to dissociate H2 molecule and provide the free H atom for further hydrogenation on the B11N12C fullerene, which saves the materials cost in practical applications for hydrogen storage. Moreover, the material curvature also plays an important role in reducing the activation barrier for the hydrogen dissociation on the BN fullerenes.Highlights► The kinetics of the hydrogenation reaction on C-doped BN fullerene was studied. ► The C atom works as an activation center for the hydrogen dissociation. ► The hydrogenation on C-doped BN fullerene is a metal free and self-catalyzed process. ► C doping can effectively modify the hydrogen storage property of the BN fullerene. ► There is a curvature effect on the hydrogenation kinetics of BN fullerenes.

Using first-principles DFT calculations, the pathway and the energy barrier of phase transition between 2H and 1T′ have been investigated for MoTe2 and WTe2 monolayers. The Phase transition is controlled by the simultaneous movement of metal atoms and Te atoms in their plane without the intermediate phase 1T. The energy barrier (less than 0.9 eV per formula cell) is not so high that the phase transition is dynamically possible. The relative stability of both 2H and 1T′ phases and the energy barrier for phase transition can be modulated by the biaxial and uniaxial strain. The dynamic energy barrier is decreased by applying the strain. The phase transition between 2H and 1T′ controlled by the strain can be used to modulate the electronic properties of MoTe2 and WTe2.

Two-dimensional crystals with weak layer interactions, such as twisted graphene, have been a focus of research recently. As a representative example, transitional metal dichalcogenides show a lot of fascinating properties due to stacking orders and spin–orbit coupling. We analyzed the dynamic energy barrier of possible phase transitions in MoX2 (X = S, Se and Te) with first-principles methods. In the structural transition from 2Hc to 2Ha, the energy barrier is found to be increased following an increase of pressure which is different from the phase transition in usual semiconductors. Among MoS2, MoSe2 and MoTe2, the energy barrier of MoS2 is the lowest and the stability of both 2Hc and 2Ha is reversed under pressure for MoS2. It is found that the absence of a phase transition in MoSe2 and MoTe2 is due to the competition between van der Waals interaction of layers and the coulomb interaction of Mo and X in nearest-neighbor layer of Mo in both phases.

Although the phenomenon that optical reflectivity of hard group IVB transition metal nitrides depends on stoichiometry has been reported, the microscopic origin of this behavior has not been well explored yet. Here we find that optical reflectivity of rocksalt hafnium nitride films (δ-HfNx) can be effectively tuned by stoichiometry x, and the underlying mechanism can be well elucidated by Drude–Lorentz fitting and first-principles calculations. It is shown that the observed tunability of optical reflectivity arises from a transition from N vacancies (VN) to Hf vacancies (VHf) in the films because this evolution from VN to VHf has important roles in changing electronic properties of the films in the following three aspects: (i) density of free electrons, wherein VN and VHf act as donor-like and acceptor-like defects, respectively; (ii) mean free path of free electrons, in which VN and VHf are the main electron scattering sites in sub- and overstoichiometric films, respectively; (iii) interband transition absorption of bound electrons, wherein three previously unreported absorption bands originating from VN and VHf are found to occur at ∼0.81, 2.27, and 3.75 eV. These point-defect-induced variations significantly affect the dielectric function of δ-HfNx films and thus drive the tailored evolution in reflectivity properties with x.