立方(Ba0.5Sr0.5)TiO3高压诱导带隙变化的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理计算研究了 (Ba_0.5Sr_0.5)TiO₃ (BST) 晶体在高压下的电子结构及能带变化行为. 研究结果发现,随着压强的增加,BST能带间隙先增加,在压强为55 GPa时达到最大值,然后减小,这些有趣的结果将有助于开发与设计新的BST铁电器件. 进一步地,通过电子态密度和密度分布图的研究分析可知:在低压区域(055 GPa),则是出现的离域现象占主导(电子的离域作用超过键态的作用),从而使带隙减小. 关键词： 钛酸锶钡 第一性原理 高压 能带间隙     

TiS2-graphene heterostructures enabling polysulfide anchoring and fast electrocatalyst for lithium-sulfur batteries: A first-principles calculation

Lithium-sulfur batteries have attracted attention because of their high energy density. However, the "shuttle effect" caused by the dissolving of polysulfide in the electrolyte has greatly hindered the widespread commercial use of lithium-sulfur batteries. In this paper, a novel two-dimensional TiS₂/graphene heterostructure is theoretically designed as the anchoring material for lithium-sulfur batteries to suppress the shuttle effect. This heterostructure formed by the stacking of graphene and TiS₂ monolayer is the van der Waals type, which retains the intrinsic metallic electronic structure of graphene and TiS₂ monolayer. Graphene improves the electronic conductivity of the sulfur cathode, and the transferred electrons from graphene enhance the polarity of the TiS₂ monolayer. Simulations of the polysulfide adsorption show that the TiS₂/graphene heterostructure can maintain good metallic properties and the appropriate adsorption energies of 0.98-3.72 eV, which can effectively anchor polysulfides. Charge transfer analysis suggests that further enhancement of polarity is beneficial to reduce the high proportion of van der Waals (vdW) force in the adsorption energy, thereby further enhancing the anchoring ability. Low Li₂S decomposition barrier and Li-ion migration barrier imply that the heterostructure has the ability to catalyze fast electrochemical kinetic processes. Therefore, TiS₂/graphene heterostructure could be an important candidate for ideal anchoring materials of lithium-sulfur batteries.     

空位缺陷对β-AgVO_3电子结构和光吸收性能的影响

基于密度泛函的第一性原理研究了Ag空位、O空位和Ag-O双空位对β-AgVO_3的电子结构及光学性质的影响.采用广义梯度近似平面波超软赝势GGA+U方法,对不同缺陷体系的形成能、能带结构、电子态密度、差分电荷密度和吸收光谱进行了计算和分析.通过比较不同Ag空位和O空位的形成能,确定了β-AgVO_3中主要形成Ag3空位和O1空位,并且Ag空位较O空位更容易形成.Ag3空位和O1空位的存在都使得β-AgVO_3带隙有一定程度的减小;Ag3空位使β-AgVO_3呈现p-型半导体性质,而O1空位和Ag3-O1双空位使β-AgVO_3呈现n-型半导体性质.Ag3和O1空位对晶体在可见光范围内的光吸收影响较小.     

Spin-filter effect and spin-polarized optoelectronic properties in annulene-based molecular spintronic devices

Using Fe, Co or Ni chains as electrodes, we designed several annulene-based molecular spintronic devices and investigated the quantum transport properties based on density functional theory and non-equilibrium Green's function method.Our results show that these devices have outstanding spin-filter capabilities and exhibit giant magnetoresistance effect,and that with Ni chains as electrodes, the device has the best transport properties. Furthermore, we investigated the spinpolarized optoelectronic properties of the device with Ni electrodes and found that the spin-polarized photocurrents can be directly generated by irradiating the device with infrared, visible or ultraviolet light. More importantly, if the magnetization directions of the two electrodes are antiparallel, the photocurrents with different spins are spatially separated, appearing at different electrodes. This phenomenon provides a new way to simultaneously generate two spin currents.     

N-F共掺杂锐钛矿二氧化钛(101)面纳米管的第一性原理研究

共掺杂是提高二氧化钛纳米管可见光催化性能的一种有效方式. 采用基于密度泛函理论的第一性原理方法, 研究了N单掺杂、F单掺杂及N-F共掺杂二氧化钛纳米管的原子结构、电子性质和光学性质. 计算结果表明, 相比N单掺杂和F单掺杂, N-F共掺杂二氧化钛纳米管的形成能更低, 掺杂后的体系热力学稳定性更好. 此外, 相比未掺杂时的带隙, N-F共掺杂后体系的带隙变化最多, 减少了0.557 eV, 而这主要源于价带顶附近的杂质能级的贡献. 此外, 通过分析掺杂后的光催化活性发现, N-F共掺杂时纳米管的还原性和氧化性都有所降低, 但并没有丧失活性, 并且光吸收谱表明, 共掺杂体系的红移现象最为明显. 因此, N-F共掺杂可有效提高二氧化钛纳米管可见光的光催化性能.     

含扭转晶界位错Al金属拉伸强度第一性原理预测

本文基于密度泛函理论第一原理方法,从影响力学性能本质的电子结构计算上,对含Σ 5{001}扭转晶界位错Al金属拉伸强度进行了预测,发现其理论拉伸强度达到8.73 GPa,临界应变为 24%.拉伸强度低于文献报道(Phys. Rev. B 75, 174101 (2007))的倾斜晶界位错Al金属的理论拉伸强度9.5 GPa,但其临界应变却远大于倾斜晶界的16%.本研究结果表明,通过工艺参数控制,改变缺陷形态,可极大地改变其力学性能.进一步地,从电子结构层次上, 分析了含晶界位错Al金属拉伸断裂行为的实质,通过分析电荷密度分布、键长变化等,发现其断裂处发生在晶界处;理论计算结果将对Al金属结构设计及力学性能改善具有重要的指导作用.