(Mg(BH_4)_2)_n团簇结构与性质的密度泛函理论研究

采用了密度泛函理论中的杂化密度泛函B3LYP方法,在6-31G*基组水平上对[Mg(BH_4)_2]_n(n=1-3)团簇的结构进行了优化,几何结构优化时自旋多重度取了1、3、5等多种情况进行计算.最后对最稳定结构的振动特性、成键特性、电荷特性和稳定性等进行了研究.结论表明:团簇最稳定结构易形成链状结构,其中Mg-B键长为0.210～0.253 nm,-BH_4基中靠近Mg原子处的B-H键长约为0.125 nm、远离Mg原子处的B-H键长为0.119 nm.对比团簇和晶体的光谱结构表明,-BH_4基在团簇和晶体中结构基本一致. Mg原子的自然电荷在1.687～1.757e之间,B原子的自然电荷在-0.702～-0.788e之间,H原子基本不参与电荷转移,其自然电荷在-0.094～0.070e之间.团簇中Mg原子和-BH_4基之间相互作用呈现较强的离子性,Mg(BH_4)_2团簇具有相对较高的动力学稳定性.     

First-principles Study of Mechanical and Electronic Properties of Co-Sn Intermetallics for Lithium Ion Battery Anode

The equilibrium structures, formation energy, mechanical properties and electronic properties of Co-Sn intermetallics have been systemically studied by first-principles study. The results show that the CoSn phase is more thermodynamically stable than any other stoichiometry of Co-Sn intermetallics. With the increasing of Co content in Co-Sn intermetallics, the mechanical properties change into brittle behavior from ductility character. Adding proper amount of Co to Co-Sn intermetallics can improve the cycle performance for lithium ion battery anode. However, high Co content will lead to a poor cycle performance for Co-Sn intermetallics.     

第一性原理研究氧空位对Ag在MgO(001)面吸附的影响

采用基于密度泛函理论的第一性原理研究方法,应用Vienna Ab-initio Simulation Package (VASP),计算了氧空位对Ag原子在MgO(001)面吸附的影响．通过成键过程中电荷密度的变化以及电荷转移的讨论,从原子尺度上分析了MgO(001)面空位点Fs和Fs+对其吸附、聚集与成核属性的影响以及吸附的能量属性.结果表明,相对清洁的MgO表面而言,Ag原子吸附在O空位时,能够更牢固地与MgO表面结合,并吸引更多的Ag原子聚集在一起,形成一个个独立的Ag原子岛.     

Instability Origin and Improvement Scheme of Facial Alq3 for Blue OLED Application

Degradation phenomenon and poor stability of tris(8-hydroxyquinoline) aluminum(III)(Alq₃)-based organic light-emitting diodes(OLEDs) have attracted much attention. In this paper, we discussed the origin of instability of the facial Alq₃-based blue luminescent OLEDs with the help of first-principles calculation. The results show that environmental humidity seriously affects the luminescence stability of Alq₃-based OLEDs. H₂O molecules in environment can be firmly bound to the oxygen atoms of the facial Alq₃, which then act as starting points for further degradation of Alq₃. Moreover, the interactions between facial Alq₃ and different cathode metal layers were investigated to explain the experiment phenomenon. A design guideline for diminishing the strong attraction from oxygen atoms can be proposed to protect Alq₃ and improve the stability of materials applied in OLEDs.     

Ion-exchange between Na2Ti3O7 and H2Ti3O7 nanosheets at different pH levels: An experimental and first-principles study

The models of Na_2−xH_xTi₃O₇ (x=0, 1, and 2) nanosheets were proposed to investigate the formation energies of ion-exchange using first-principles calculations. The calculated results demonstrated that sodium titanate nanosheet is energetically favorable for ion-exchange in a wide pH range, from acidic solution to even highly concentrated alkaline aqueous solution due to the negative formation energies. Therefore, the composition of sodium titanate nanosheet in alkaline solution should be Na_2−xH_xTi₃O₇ (0<x≤2) rather than Na₂Ti₃O₇. The formation energies of ion-exchange decrease with the pH decreasing. As a result, the thermodynamic driving force of ion-exchange is enhanced at low pH level. To further verify the calculated results, the ion-exchange properties of a series of titanate nanosheets in aqueous solutions at different pH levels were investigated. The experimental results are in good agreement with the theoretical deduction.     

Effects of enhanced electronic correlation on magnetic properties of light non-metallic element (B,C, N,and O)-doped CuCl: A first-principles study

Based on density functional calculations within both standard generalized gradient approximation and plus on-site Coulomb interactions approaches, we have investigated the electronic structure and magnetic properties of the first-row element-doped CuCl semiconductors. The electronic correlations in both 2p and 3d orbitals are enhanced by adding the on-site Coulomb repulsion (Hubbard U and Hund exchange J). After a comparative study, we find that, for both standard and beyond approaches, B-doped CuCl is a half-metallic magnet with majority-spin impurity bands touching the Fermi level, C-doped CuCl is a magnetic semiconductor, and N-doped CuCl is a half-metallic magnet with minority-spin impurity bands crossing the Fermi level. Nevertheless, for O-doped CuCl, it transforms from a nonmagnetic semiconductor to a half-metallic magnet with metallic up-spins by considering the correlation effects. The calculation shows that the enhanced electronic correlation not only corrects the error of band-gap, but also influences the magnetic ground state and the distribution of local magnetic moments. The location of impurity bands with different dopants was understood based on the elements' electronegativity and interaction between dopant and host atoms. Strong hybridization between the dopant's 2p states and the filled 3d orbitals of adjacent Cu yields the main contribution to magnetization.     

Electronic properties of anatase TiO2 doped by lanthanides: A DFT+U study

The effects of mono-doping of 4f lanthanides with and without oxygen vacancy defect on the electronic structures of anatase TiO₂ have been studied by first-principles calculations with DFT+U (DFT with Hubbard U correction) to treat the strong correlation of Ti 3d electrons and lanthanides 4f electrons. Our results revealed that dopant Ce is easy to incorporate into the TiO₂ host by substituting Ti due to its lower substitutional energy (∼−2.0 eV), but the band gap of the system almost keeps intact after doping. The Ce 4f states are located at the bottom of conduction band, which mainly originates from Ti 3d states. The magnetic moment of doped Ce disappears due to electron transfer from Ce to the nearest O atoms. For Pr and Gd doping, their substitutional energies are similar and close to zero, indicating that both of them may also incorporate into the TiO₂ host. For Pr doping, some 4f spin-down states are located next to the bottom of the conduction band and narrow the band gap of the doping system. However, for Gd doping, the 4f states are located in deep valence band and there is no intermediate band in the band gap. The magnetic moment of dopant Gd is close to the value of isolated Gd atom (∼7 μB), indicating no overlapping between Gd 4f with other orbitals. For Eu, it is hard to incorporate into the TiO₂ host due to its very higher substitutional energy. The results also indicated that oxygen vacancy defect may enhance the adsorption of the visible light in Ln-doped TiO₂ system.     

First-principles study of two-dimensional van der Waals heterostructure based on ZnO and Mg(OH)2: A potential photocatalyst for water splitting

In this study, the structural, electronic and optical properties of the two-dimensional heterostructure based on ZnO and Mg(OH)₂ are investigated by first-principle calculations. The ZnO/Mg(OH)₂ heterostructure, formed by van der Waals (vdW) interaction, possesses a type-II band structure, which can separate the photogenerated electron–holes constantly. The heterostructure has decent band edge positions for the redox reaction to decompose the water at pH 0 and 7. As for the interfacial properties of the heterostructure, the trend of band bending of the ZnO and Mg(OH)₂ layers in the heterostructure is addressed, which will result a built-in electric field. Besides, the charge-density difference and potential drop across the interface of the ZnO/Mg(OH)₂ vdW heterostructure are also calculated. Finally, the heterostructure is demonstrated that it not only has excellent ability to capture the light near the visible spectrum region, but also can improve the optical performance for the monolayered ZnO and Mg(OH)₂.     

The structure of self-assembled N2O multilayers

Based on the structure of the monolayer, the structure of multilayered nitrous oxide (N₂O) was determined by applying the first-principle technique. Several potential structures of the N₂O multilayer were evaluated. The results showed that the multilayer consisted of many layers and that the distance between neighboring layers was equal to 4.1 Å.     

First-principle study of the electronic and optical properties of BInGaAs quaternary alloy lattice-matched to GaAs

The first-principle calculations based on density functional theory have been used to study the electronic and optical properties of zinc-blende BInGaAs quaternary alloy lattice-matched to GaAs. The calculated results show the band gap of BInGaAs alloy are direct and the band gap will reduce with the increment of boron and indium composition. The electronic structures of BInGaAs alloy are analyzed via the calculation of density of states. The variation of optical properties including dielectric function, absorption coefficient, reflectivity, refractive index and energy loss function of the alloy were also investigated in detail.