Musheng Wu

Strain-induced semimetal-metal transition in silicene

The effect of the tensile strain on the electronic structure of the silicene is studied by using first-principles density functional theory. It is found that a semimetal-metal transition occurs when an in-plane strain larger than 7.5% is applied in silicene. The downward movement of the lowest conduction band at Γ-point, which originates from the weakened interaction between neighboring Si atoms, leads to the transition. The proposed mechanical control of the electronic properties will widen the application of the silicene in Si-based nanotechnology.

Is silicene stable in O2???First-principles study of O2 dissociation and O2-dissociation?induced oxygen atoms adsorption on free-standing silicene

The stability of free-standing silicene in O2 is an open question. In this letter, the O2 dissociation and O2-dissociation–induced O atoms adsorption on free-standing silicene are studied by using first-principles calculations. Our results show that the O2 molecule dissociates on the free-standing silicene surface easily from both the thermodynamic and kinetic points of view, which is different from the case of graphene. The dissociation reaction is an exothermic process, and the dissociated O atoms form strong bonds with Si atoms, which lowers the energy of the system substantially. On the other hand, the dissociation reaction occurs spontaneously on the free-standing silicene without overcoming any energy barrier. Furthermore, the migration and desorption of O atoms are relatively difficult under room temperature due to the strong Si-O bonds in the O-adsorbed silicene, which is in favor of forming silicon oxides. Our results provide convictive evidence to show that free-standing silicene is unstable in O2.

Comparison of the stability of free-standing silicene and hydrogenated silicene in oxygen: a first principles investigation.

The stability of free-standing silicene in oxygen is worthwhile discussing. In this letter, the oxygen adsorption and dissociation on free-standing silicene is studied using first principles. It is found that free-standing silicene is not stable in oxygen because O2 molecules can be easily adsorbed and dissociated into O atoms on a silicene surface without overcoming any energy barrier. Moreover, dissociated oxygen atoms are difficult to migrate on and desorb from silicene surfaces, leading to the formation of Si-O compounds. To enhance the stability of free-standing silicene in oxygen, fully hydrogenated silicene is used as a stabiliser. Interestingly, compared with no energy barrier on pristine silicene, there are two minor energy barriers of O2 molecule adsorption and dissociation on fully hydrogenated silicene, indicating that hydrogenated silicene has higher stability than free-standing silicene in oxygen. However, once the O2 molecule dissociates into two O atoms on hydrogenated silicene, desorption of O atoms will be very difficult due to its high energy barrier. This work will be helpful to understand the detail of O2 molecule dissociation and dissociation-induced O atoms adsorption on free-standing and hydrogenated silicene in oxygen and will be useful to the application of silicene.

The role of Cu in degrading adsorption of CO on the PtnCu clusters.

The platinum copper alloy nanocrystals (NCs) have generated much interest because of their wide applications in fuel cells due primarily to their good catalytic performance and to decreasing sensitivity toward CO poisoning. The exact atomic-level morphology of platinum copper alloy NCs is still not clear in the literature, and research to understanding the poisoning mechanism is still insufficient to date. In this article, we report on density functional calculations of small PtnCu clusters and their adsorption of a CO molecule that provide evidence for degrading adsorption of the CO molecule compared to pure platinum clusters. The lowest-energy geometries of PtnCu and PtnCuCO clusters have been identified. The CO molecule prefers to be adsorbed on the nearest platinum atom by the C-end-on mode, forming linear or quasi-linear O-C-Pt structures. The adsorption energies indicate that the introduction of a copper atom decreases the adsorption ability of the CO molecule. The local density of states of the representative clusters is used to characterize the adsorption properties of the CO molecule on the PtnCu clusters. Results from our theoretical calculations can be helpful for understanding the poisoning mechanism of the CO molecule on the platinum copper alloy NCs.

Comparisons between adsorption and diffusion of alkali, alkaline earth metal atoms on silicene and those on silicane: Insight from first-principles calculations*

The adsorption and diffusion behaviors of alkali and alkaline-earth metal atoms on silicane and silicene are both investigated by using a first-principles method within the frame of density functional theory. Silicane is staler against the metal adatoms than silicene. Hydrogenation makes the adsorption energies of various metal atoms considered in our calculations on silicane significantly lower than those on silicene. Similar diffusion energy barriers of alkali metal atoms on silicane and silicene could be observed. However, the diffusion energy barriers of alkali-earth metal atoms on silicane are essentially lower than those on silicene due to the small structural distortion and weak interaction between metal atoms and silicane substrate. Combining the adsorption energy with the diffusion energy barriers, it is found that the clustering would occur when depositing metal atoms on perfect hydrogenated silicene with relative high coverage. In order to avoid forming a metal cluster, we need to remove the hydrogen atoms from the silicane substrate to achieve the defective silicane. Our results are helpful for understanding the interaction between metal atoms and silicene-based two-dimensional materials.

Ab initio investigation of Jahn–Teller-distortion-tuned Li-ion migration in λ-MnO2

The migration of Li ions in electrode materials is the main limiting factor to determine the rate capability of Li-ion batteries. In this work, the influences of Jahn–Teller (JT) distortion on Li migration in full delithiated LiMn2O4 (λ-MnO2) were systematically studied by a first-principles computational approach. Our results unravel the direction of JT distortion strongly affecting the activation barrier of Li-ion migration in λ-MnO2. In particular, Li-ion migration has the lowest activation barrier when the two elongated Mn–O bonds of Mn3+ ion are quasi-collinear with the linked Li–O bonds in the transition state, in contrast with the highest barrier when the elongated Mn–O bonds are approximately perpendicular to the linked Li–O bonds. In addition, lattice-strain-induced variation of the Li-migration barrier in λ-MnO2 exhibits either upward or downward trends, depending on the detailed coupling with JT distortion. Further analysis showed that the difference in activation barriers can be explained by the different Li–O distances in terms of the Coulomb interaction energies, which is induced by the different position and direction of JT distortion. Finally, the Li-ion migration in the whole λ-MnO2 system is also discussed by considering the influences of JT polarons.

First-principles study of nitrogen and carbon monoxide adsorptions on silicene

Atomic adsorptions of N, C and O on silicene and molecular adsorptions of N2 and CO on silicene have been investigated using the density functional theory (DFT) calculations. For the atomic adsorptions, we find that the N atom has the most stable adsorption with a higher adsorption energy of 8.207 eV. For the molecular adsorptions, we find that the N2 molecule undergoes physisorption while the CO molecule undergoes chemisorption, the corresponding adsorption energies for N2 and CO are 0.085 and 0.255 eV, respectively. Therefore, silicene exhibits more reactivity towards the CO adsorption than the N2 adsorption. The differences of charge density and the integrated charge calculations suggest that the charge transfer for CO adsorption ( ∼0.015e) is larger than that for N2 adsorption ( ∼0.005e). This again supports that CO molecule is more active than N2 molecule when they are adsorbed onto silicene.

STRUCTURAL AND ELECTRONIC EVOLUTION FROM SiC SHEET TO SILICENE

The evolution of the structural and electronic properties from SiC sheet to silicene is studied by using first-principles density functional theory. It is found that the planar configurations of the Si–C monolayer systems are basically kept except for the increase of the buckling of the planar structure when the substitution ratio of Si increases. Band gaps of the Si–C monolayer system decrease gradually when the substitution ratio of Si atoms ranges from 0% to 100%. The energy and type of the band gaps are closely related with the substitution ratio of Si atoms and the Si–C order. Further analysis of density of states reveals the orbital contribution of Si and C atoms near the Fermi level. The discussion of the electronic evolution from SiC sheet to silicene would widen the application of the Si–C monolayer systems in the optoelectronic field in the future nanotechnology.

Bulk properties and transport mechanisms of a solid state antiperovskite Li-ion conductor Li3OCl: insights from first principles calculations

The excellent Li+ conductivity (0.85 × 10−3 S cm−1 at room temperature) and wide electrochemical stability window (>6 V) of the antiperovskite Li3OCl material make it a promising candidate electrolyte for rechargeable all-solid-state Li batteries. In this study, we systematically evaluate the electronic, mechanical, and thermodynamic properties of Li3OCl by first-principles density functional theory calculations. The defect chemistry and Li+ migration mechanisms are also discussed in the context since Li+ diffusion is strongly influenced by defects in the material. Our results show that Li3OCl is an indirect wide-band gap insulator in the equilibrium state with a band gap of ∼6.26 eV. Phonon spectral data confirm that Li3OCl is dynamically stable at its ground state. It is also revealed that Li3OCl is mechanically brittle. The bulk modulus of Li3OCl is greater than that of Li10GeP2S12, while it is comparable to that of Li0.5La0.5TiO3 and Li7La3Zr2O12. From quasi-harmonic approximation, the linear thermal expansion coefficient and thermal conductivity of the material are found to be 3.12 × 10−6 K−1 and 22.49 W m−1 K−1 at room temperature, respectively. With four types of point defect pairs in Li3OCl being considered, it is revealed that the LiCl defect pair has the lowest formation energy compared to Li2O, O substituted Cl, and Li/Li-vacancy Frenkel defect pairs. The LiCl and Frenkel defect pairs are the most important point defects responsible for the fast Li diffusion. Overall, our study provides fundamental and comprehensive insights into the bulk properties and transport mechanisms of Li3OCl for its practical application as a solid-state electrolyte in all-solid-state Li batteries.

Structural, electronic, sodium diffusion and elastic properties of Na–P alloy anode for Na-ion batteries: Insight from first-principles calculations

Sodium-ion batteries (NIBs) as an alternative to lithium-ion batteries (LIBs) have recently received great attentions because of the relatively high abundance of sodium. Searching for suitable anode materials has always been a hot topic in the field of NIB study. Recent reports show that phosphorus-based materials are potential as the anode materials for NIBs. Using first-principles calculations, herein, we study the atomic and electronic structures, diffusion dynamics and intrinsic elastic properties of various Na–P alloy compounds (NaP5, Na3P11, NaP and Na3P) as the intermediate phases during Na extraction/insertion in phosphorus-based anode materials. It is found that all the crystalline phases of Na–P alloy phases considered in our study are semiconductors with band gaps larger than that of black phosphorus (BP). The calculations of Na diffusion dynamics indicate a relatively fast Na diffusion in these materials, which is important for good rate performance. In addition, the diffusion channels of sodium ions are one-dimensional in NaP5 phase and three-dimensional in other three phases (Na3P11, NaP and Na3P). Elastic constant calculations indicate that all four phases are mechanically stable. Among them, however, NaP5, Na3P11 and NaP alloy phases are ductile, while the fully sodiated phase Na3P is brittle. In order to improve the electrochemical performance of Na–P alloy anodes for NIBs, thus, promoting ductility of Na–P phase with high sodium concentration may be an effective way.