锑烯吸附金属Li原子的密度泛函研究

锑烯(antimonene)是继石墨烯和磷烯之后出现的新型二维材料,在锂离子电池等领域受到关注.本文基于第一性原理的密度泛函理论,计算研究了锑烯对Li原子的吸附特性,包括Li原子的最稳定吸附构型、吸附密度以及吸附Li原子的扩散路径.结果表明:Li原子最稳定的吸附位置位于谷位,即底层Sb原子之上、顶层三个Sb原子中心位置,吸附能为1.69 eV,吸附距离为2.81?;能带计算发现,锑烯为带隙宽度1.08 eV的间接带隙半导体,吸附Li原子后费米能级上升进入导带,呈现出金属性;原子分波态密度分析发现, Sb原子的p电子态和Li原子的p和s电子态形成明显的共振交叠,表现出杂化成键的特征;随着吸附Li原子数量增加,锑烯晶格结构和电子结构发生较大变化.通过微动弹性带方法计算发现, Li原子在锑烯表面的扩散势垒为0.07 eV,较小的势垒高度有利于快速充放电过程.     

Mg~(2+)掺杂锰酸锂的第一性原理研究

利用基于密度泛函理论的第一性原理平面波超软赝势法对Mg~(2+)掺杂锰酸锂的晶格常数与能带结构、态密度、键布居进行计算和分析,计算结果表明:掺杂Mg~(2+)后将会促使Mn、 O原子的电荷重新分布且其相互作用加强,能带带隙减小,费米能级附近的带数增加,费米能由-1.29 eV增加到-1.02 eV, Mn、 O、 Mg在总态密度中贡献比较大,锂离子贡献比较小且峰型尖锐局域化严重,提高了Li~+的扩散效率, Mn—O键变短,共价性增强,形成的共价键较稳定,其相互作用形成的骨架较稳定不易坍塌.从而提高了材料的循环充放电性能和电池使用寿命.     

Na+替位掺杂对Li2MnSiO4的电子结构以及Li+迁移过程的影响

在锂二次电池中, 硅酸锰锂作为正极材料得到广泛研究, 但其固有的电子和离子电导率较低, 直接影响着电池的功率密度和充放电速率. 本文建立了不同浓度的Na⁺离子替位掺杂Li⁺离子形成的Li_1-xNa_xMnSiO₄(x=0, 0.125, 0.25, 0.5)结构, 采用第一性原理的方法, 研究了掺杂前后硅酸锰锂的电子结构以及Li⁺离子的跃迁势垒. 发现在Li⁺位替代掺杂Na⁺, 导带底的能级向低能方向发生移动, 降低了Li₂MnSiO₄ 材料的禁带宽度, 有利于提升材料的电子导电性能. 随着掺杂浓度的升高, 禁带宽度逐渐变窄. CI-NEB结果表明, 在Li₂MnSiO₄体系中具有两条有效的Li⁺离子迁移通道, 掺杂Na⁺以后扩大了Li⁺ 离子在[100]晶向上的迁移通道, Li⁺离子的跃迁势垒由0.64 eV降低为0.48, 0.52和0.55 eV. 掺杂浓度为 x=0.125时, 离子迁移效果最佳. 研究表明Na⁺掺杂有利于提高Li₂MnSiO₄材料的离子和电子电导率.     

KDP和尿素晶体电子结构的第一性原理研究

基于第一性原理的平面波超软赝势法对KDP(KH₂PO₄)和尿素(CH₄N₂O)晶体的能带结构、电子态密度、电荷差分密度以及布局分析进行了计算讨论.结果表明:尿素晶体中的C1-O1、C1-N1、N1-H2和N1-H1键都具有共价键特性,带隙值为4.636 eV,价带顶主要由H-1s与N、O的2p态贡献,导带底主要是H-1s与C、N、O的2p态贡献;KDP晶体的H1-O1键具有离子性而P1-O1则具有共价性,带隙宽度为5.713 eV,价带顶主要由O-2p以及P-3p贡献,导带底主要由H-1s、P-3s和3p以及K-4s和3p态贡献.     

Electronic and optical properties of pure and Ce3+-doped CaS single crystals: a first-principles prediction

This paper investigates the electronic and optical properties for pure and Ce 3+-doped CaS crystals by using the first-principles total energy calculations.The results show that CaS:Ce has a direct band gap of 2.16 eV,and the top of the valence band is determined by S 3p states and the bottom of the conduction band is determined by Ce 4f states,respectively.Our results validate that the yellow emission from CaS:Ce is produced by doped cerium and the green emission quenches at 12.5% cerium concentration.The Ce-S bond shows more covalent character than the Ca-S bond.     

H,F修饰单层GeTe的电子结构与光催化性质

采用基于密度泛函理论的第一性原理平面波赝势方法,计算了单层GeTe、表面氢化及氟化单层GeTe的晶体结构、稳定性、电子结构和光学性质.计算结果表明,经过修饰后, GeTe的晶格常数、键角、键长增大,且均具有较好的稳定性.电子结构分析表明,单层GeTe为间接带隙半导体,全氢化修饰、全氟化修饰以及氢氟共修饰(F, Ge同侧;H, Te同侧)则转变为直接带隙半导体,且修饰后的能隙均不同程度减小.载流子有效质量表明,全氢化、全氟化以及氢氟共修饰GeTe (F, Ge同侧;H, Te同侧)的有效质量减小,其载流子迁移率增强.带边势分析结果显示,单层GeTe能够光裂解水制氢和析氧,而修饰后的GeTe的价带带边势明显下移,其氧化性明显增强,能够光裂解水析O2, H2O2, O3以及OH·等产物.光学性质表明,修饰后的GeTe对可见光区和红、紫外区的光谱吸收效果明显增强,表明其在光催化领域有着广阔的应用前景.     

LSDA+U方法研究PuO2电子结构和成键特征

采用LSDA(局域自旋密度近似) U(有效库仑相关能)方法计算研究了PuO2的电子结构和成键特征。计算的平衡体积和半导体带隙分别为0.03875 nm3和0.18 eV,与实验结果符合得很好。能态密度和电子密度的分析表明PuO2并不是纯粹的离子晶体,Pu5f和O2p轨道杂化形成共价键。计算结果有助于理解PuO2晶体中Pu5f电子的关联效应。     

First-principle study of nanostructures of functionalized graphene

We present first-principle calculations of 2D nanostructures of graphene functionalized with hydrogen and fluorine, respectively, in chair conformation. The partial density of states, band structure, binding energy and transverse displacement of C atoms due to functionalization (buckling) have been calculated within the framework of density functional theory as implemented in the SIESTA code. The variation in band gap and binding energy per add atom have been plotted against the number of add atoms, as the number of add atoms are incremented one by one. In all, 37 nanostructures with 18C atoms, 3 × 3 × 1 (i.e., the unit cell is repeated three times along x-axis and three times along y-axis) supercell, have been studied. The variation in C–C, C–H and C–F bond lengths and transverse displacement of C atoms (due to increase in add atoms) have been tabulated. A large amount of buckling is observed in the carbon lattice, 0.0053–0.7487 Å, due to hydrogenation and 0.0002–0.5379 Å, due to fluorination. As the number of add atoms (hydrogen or fluorine) is increased, a variation in the band gap is observed around the Fermi energy, resulting in change in behaviour of nanostructure from conductor to semiconductor/insulator. The binding energy per add atom increases with the increase in the number of add atoms. The nanostructures with 18C+18H and 18C+18F have maximum band gap of 4.98 eV and 3.64 eV, respectively, and binding energy per add atom –3.7562 eV and –3.3507 eV, respectively. Thus, these nanostructures are stable and are wide band-gap semiconductors, whereas the nanostructures with 18C+2H, 18C+4H, 18C+4F, 18C+8F, 18C+10F and 18C+10H atoms are small band-gap semiconductors with the band gap lying between 0.14 eV and 1.72 eV. Fluorine being more electronegative than hydrogen, the impact of electronegativity on band gap, binding energy and bond length is visible. It is also clear that it is possible to tune the electronic properties of functionalized graphene, which makes it a suitable material in microelectronics.     

First-principles study on the structural and electronic properties of LiB and its hydrides (Li2BnHn, n=5, 8, 12, LiBH4)

Structural, electronic and vibrating properties of LiB and its hydrides (Li₂B_nH_n, n=5, 8, 12, LiBH₄) were calculated by the first-principles using density functional theory in its generalized gradient approximation. The calculated results are in good agreement with experimental studies. The deviation between theory and experimental results are also discussed. With the increasing of H atoms in range of 5-12, the band gap energy increases and the width of the conduction band decreases. Comparing with LiB, the band gap of LiBH₄ is broadened, which indicates the enhancement of Li-B and Li-H bond strength. Valence electrons mainly transfer from Li atoms to B and H atoms. As a result, Li atoms are thought to be partially ionized as Li⁺ cations. There is little contribution of Li orbital to the occupied states, resulting in Li-H and Li-B bond exhibiting an ionic nature, and B-H bond showing a covalent nature.     

Co,Zn共掺铌酸锂电子结构和吸收光谱的第一性原理研究

利用基于密度泛函的第一性原理的计算方法,研究了Co单掺及Co和Zn共掺LiNbO_3晶体的电子结构和吸收光谱.研究显示,各掺杂体系铌酸锂晶体的带隙均较纯铌酸锂晶体变窄. Co:LiNbO_3晶体禁带宽度为3.32 eV; Co:Zn:LiNbO_3晶体, Zn的浓度低于阈值或达到阈值时,禁带宽度分别为2.87或2.75 eV. Co:LiNbO_3晶体在可见-近红外光波段2.40, 1.58, 1.10 eV处形成吸收峰,这些峰归结于Co 3d分裂轨道的跃迁;加入抗光折变离子Zn~(2+),在1.58, 1.10 eV处的吸收峰增强,可以认为Zn~(2+)与Co~(2+)之间存在电荷转移,使e_g轨道电子减少,但并不影响t_(2g)轨道电子.结果表明,晶体中的Co离子在不同共掺离子下可充当深能级中心(2.40 eV),或可充当浅能级中心(1.58 eV),两种情况下,掺入近阈值的Zn离子均有助于实现优化存储.     

Antisite defects and Mg doping in LiFePO<Subscript>4</Subscript>: a first-principles investigation

Atomic and electronic structures of LiFePO₄ with the antisite defect and Mg doping at Li and Fe sites have been investigated using first-principles density-functional theory with the on-site Coulomb interaction taken into account. It is demonstrated that the most favorable antisite defect type is the exchange defect, in which Li and Fe ions exchange positions. The resultant longer Fe–O bond and narrower band gap drop a hint that the electronic and ionic transport properties may be improved. For the case of Mg doping, Mg is preferentially doped at the Fe site instead of the Li site to form a new LiFe_1−y Mg _y PO₄ solid solution, leading to a higher ionic conductivity. Moreover, the dependence of the electrochemical properties on the concentration of Mg dopant has also been discussed.     

A GGA+U study of lithium diffusion in vanadium doped LiFePO4

Recent experiments showed beneficial influence of vanadium doping on the electrochemical performance of lithium iron phosphate (LiFePO₄) cathode materials. First-principles calculations have been performed to investigate the stability, electronic structure and lithium diffusivity of vanadium-doped LiFePO₄ and to elucidate the origin of such improvement. It is found that vanadium prefers occupying Fe sites and leads to additional density of states within the intrinsic bandgap. By the nudged elastic band method, we show that the barrier of Li ions diffusion along the one dimensional channel in both LiFePO₄ and FePO₄ phases can be effectively reduced by vanadium doping. Structural analysis shows the lower diffusion barrier ties closely to a volumetric expansion of the diffusion channel.     

The electronic structure and the band gap of nano-sized Si particles: competition between quantum confinement and surface reconstruction

The electronic structure and especially the band gap of Si_n clusters (n=3–45 atoms) is studied by photoelectron spectroscopy. Contrary to expectations of quantum confinement, almost all clusters studied here have a band gap smaller than that of crystalline Si or even display a continuous (metallic) density of states. We attribute this to covalent bond formation analogous to the reconstructions observed on single-crystal surfaces. Additionally, for Si₃₀ and Si₃₃ a gap size of 0.6 eV (0.4 eV) is observed, supporting the prediction of stable, spherically symmetric structures of these particular clusters. Received: 18 November 1999 / Accepted: 24 November 1999 / Published online: 5 April 2000     

剪切形变下掺杂及缺陷对MoS2电子结构的影响

本文利用密度泛函理论,研究剪切形变下掺杂改性及不同类型缺陷对MoS2电子结构的影响。发现：剪切形变下,MoS2+P体系为相对最稳定的结构,掺杂改性相较于缺陷对模型稳定性影响更小;模型MoS2+P+Se中P-Mo键易形成共价键,而其中的Se-Mo键和MoS2+P-Mo-S模型中的P-Mo键,易形成离子键;掺杂使MoS2模型能隙变大,而缺陷使能隙减小,且S和Mo原子共缺陷的模型带隙为0;缺陷相较于掺杂改性模型,更能使Mo原子周围增加电荷聚集度,带隙值更低,更能影响或调控模型的电子结构。     

CuInSe2电子结构与光学性质的第一性原理计算

从头计算了CuInSe₂(CIS)体相的性质，参数设定和性质计算都基于密度泛函理论，交换相关能采用GGA，泛函形式为PBE，原子间相互作用的描述采用超软赝势.计算发现CIS中存在共价键，是一种非典型的离子型晶体，在整个晶体内存在共用电子对，Cu原子和Se原子的作用大于Se原子和In原子.CIS是一种典型的直接带隙半导体，计算得到了光学性质的各项参数，包括折射指数和反射率，吸收系数以及介电函数与光子能量的关系，发现CIS的主要光吸收峰有6个，分别为：3.1，7.6，10.0，16.1，19.0，21.0eV，理论上最强吸收峰在紫外光区.     

Band engineering of honeycomb monolayer CuSe via atomic modification

Two-dimensional monolayer copper selenide(Cu Se) has been epitaxially grown and predicted to host the Dirac nodal line fermion(DNLF). However, the metallic state of monolayer Cu Se inhibits the potential application of nanoelectronic devices in which a band gap is needed to realize on/off properties. Here, we engineer the band structure of monolayer Cu Se which is an analogue of a p-doped system via external atomic modification in an effort to realize the semiconducting state.We find that the H and Li modified monolayer Cu Se shifts the energy band and opens an energy gap around the Fermi level.Interestingly, both the atomic and electronic structures of monolayer Cu HSe and Cu Li Se are very different. The H atoms bind on top of Se atoms of monolayer Cu Se with Se–H polar covalent bonds, annihilating the DNLF band of monolayer Cu Se dominated by Se orbitals. In contrast, Li atoms prefer to adsorb at the hexagonal center of Cu Se, preserving the DNLF band of monolayer Cu Se dominated by Se orbitals, but opening band gaps due to a slight buckling of the Cu Se layer. The realization of metal-to-semiconductor transition from monolayer Cu Se to Cu X Se(X = H, Li) as revealed by first-principles calculations makes it possible to use Cu Se in future electronic devices.     

Characteristics of Li diffusion on silicene and zigzag nanoribbon

We perform a density functional study on the adsorption and diffusion of Li atoms on silicene sheet and zigzag nanoribbons. Our results show that the diffusion energy barrier of Li adatoms on silicene sheet is 0.25 eV, which is much lower than on graphene and Si bulk. The diffusion barriers along the axis of zigzag silicene nanoribbon range from 0.1 to0.25 eV due to an edge effect, while the diffusion energy barrier is about 0.5 eV for a Li adatom to enter into a silicene nanoribbon. Our calculations indicate that using silicene nanoribbons as anodes is favorable for a Li-ion battery.     

表面氢化的双层氮化硼的结构和电子性质

采用第一性原理方法研究了表面氢化的双层氮化硼的结构和电子性质.考虑了表面氢化的双层BN可能存在的六种主要构型,计算结果表明:AB-BN和AA-BN两种构型最为稳定.进一步分析了氢化后的双层BN最稳定构型的能带和电子性质.AB-BN和AA-BN两种构型的原子薄片均为直接带隙半导体,GGA计算的带隙值分别为1.47 eV和1.32 eV.因为GGA通常严重低估带隙值,采用hybrid泛函计算得到带隙值分别为2.52eV和2.34 eV.在最稳定的AB-BN和AA-BN两种构型中,B-N键呈现共价键,而B-H和N-H则具有明显的离子键的特点.在双轴应变下氢化双层BN原子薄片可以被连续地调节带隙,当晶格常数被压缩约8%时,原子薄片由半导体性转变为金属性.     

Mn掺杂LiFePO4的第一性原理研究

采用基于密度泛函理论的第一性原理方法, 计算了不同Mn掺杂浓度LiFe_1-xMn_xPO₄ (x=0,0.25,0.50,0.75) 的电子结构. 同时采用流变相辅助高温固相碳热还原法制备了LiFe_1-xMn_xPO₄ (x= 0,0.25,0.50,0.75) 材料. 理论计算表明: LiFePO₄具有E_g = 0.725 eV的带隙宽度, 为半导体材料. 通过Fe位掺杂25%的Mn离子可最大程度地 减小材料带隙宽度、降低Fe---O键及Li---O键键能, 进而提高材料的电子电导率及锂离子扩散速率. 实验结果亦表明, 当Mn掺杂量x=0.25时, 材料具有最优的电化学性能, 其具有约为158 mAh· g^-1的放电比容量以及551 Wh· kg^-1的能量密度. 理论计算与实验结果非常符合.     

强激光照射对2H-SiC晶体电子特性的影响

使用基于密度泛函微扰理论的第一原理赝势法, 计算了纤锌矿结构2H-SiC晶体在强激光照射下的电子特性, 分析了其能带结构和电子态分布. 计算结果表明: 2H-SiC平衡晶格参数a 和c随电子温度T_e的升高逐渐增大; 电子温度在0–2.25 eV范围内时, 2H-SiC仍然是间接带隙的半导体晶体, 当T_e超过2.25 eV达到2.5 eV以上时, 2H-SiC变为直接带隙的半导体晶体; 随着电子温度升高, 导带底和价带顶向高能量或低能量方向发生了移动, 当电子温度T_e大于3.5 eV以后, 价带顶穿越费米能级; 电子温度T_e在0–2.0 eV变化时, 带隙随电子温度升高而增大; T_e在2.0–3.5 eV范围变化时, 带隙随电子温度升高而快速地减少, 表明2H-SiC晶体的金属性随电子温度T_e的继续升高而增强. 在T_e =0, 5.0 eV 处, 计算了2H-SiC晶体总的电子态密度和分波态密度. 电子结构表明T_e =0 eV 时, 2H-SiC 是一个带隙为2.3 eV的半导体; 在T_e =5.0 eV时, 带隙已经消失而呈现出金属特性, 表明当电子温度升高时晶体的共价键变弱、金属键增强, 晶体经历了一个熔化过程, 过渡到金属状态.