Ge在Si(100)-2×1表面化学吸附的第一性原理研究

应用密度泛函理论,构造了具有非对称二聚体结构的Si(100)-2×1重构表面,在系统研究了其表面结构及特性的基础上,计算了Ge在不同吸附位置的表面吸附能,以及吸附前后的表面投影态密度.计算结果表明:Ge原子在基位(pedestal)吸附最稳定.另外,在吸附Ge原子之后,我们得到了一个具有完整对称性的特殊Si原子二聚体结构.此结构中相邻的两个二聚体链互相平行且分别与Si表面平行,每对二聚体Si原子呈对称分布.     

TFSI钝化晶体硅表面性质的第一性原理研究

晶体硅表面钝化是高效率晶体硅太阳能电池的核心技术,直接影响晶体硅器件的性能。本文采用第一性原理方法研究了一种超强酸-双三氟甲基磺酰亚胺(TFSI)钝化晶体硅(001)表面。研究发现,TFSI的四氧原子结构能够与Si(001)表面Si原子有效成键,吸附能达到-5.124 eV。电子局域函数研究表明,TFSI的O原子与晶体硅表面的Si的成键类型为金属键。由态密度和电荷差分密度分析可知,Si表面原子的电子向TFSI转移,从而有效降低了Si表面的电子复合中心,有利于提高晶体硅的少子寿命。Bader电荷显示,伴随着TFSI钝化晶体硅表面的Si原子,表面Si原子电荷电量减少,而TFSI中的O原子和S原子电荷电量相应增加,进一步证明了TFSI钝化Si表面后的电子转移。该工作为第一性原理方法预测有机强酸钝化晶体硅表面的钝化效果提供了数据支撑。     

Mn掺杂InP(111)-In极化面电子结构与磁性的第一性原理研究

本文利用密度泛函的广义梯度近似研究了Mn掺杂InP(111)-In极化面的电子结构与磁学性质.研究结果表明,随着Mn原子的掺杂位置靠近In极化面,Mn原子掺杂的形成能逐渐降低.并且所有Mn掺杂表面模型均表现出稀磁半导体特征.其原因主要在于费米能级附近的Mn-3d自旋态密度具有不对称性.通过对Mn不同掺杂位置的电子态密度、费米能级及Mn原子氧化态的对比分析发现,Mn原子的表面掺杂引起了In极化面的表面原子重构.通过对形成能与净磁矩的分析发现,所有掺杂在表面层的Mn原子的氧化态都是Mn2+.另外,随着Mn原子掺杂位置上移,费米能级向低能级方向移动,表面体系表现出明显的的P型半导体特征.     

CdTe(110)表面原子与电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理计算了CdTe(110)表面的原子和电子性质.结果表明,CdTe(110)理想表面在禁带中出现两个明显的表面态,弛豫后表层Cd原子和Te原子p态电子发生转移,Cd原子趋向于sp2平面杂化构型,Te原子趋向p3杂化的锥形构型.经过表面弛豫大大降低了表面能,增大了表面功函数,表面占据态和表面空态分别被推进价带顶之下和导带底之上,导致弛豫表面没有明显的表面态.     

非晶Cr/Ge多层膜的磁性和电性能研究

采用射频磁控溅射的方法制备了非晶[Crx/Ge10-x]20(x=0.1,0.3,0.5,0.8,1.1,1.6,2.7)多层膜.随着Cr层厚度增加,Ge层厚度减小,室温下薄膜磁性由抗磁性转变为铁磁性,同时样品出现了反常霍尔效应.扣除Ge层的抗磁性,制备态的[Cr2.7/Ge7.3]20的饱和磁化强度Ms可达33 emu/cm3.提出了磁性起源于Cr原子层间耦合的模型,薄膜的低温电阻导电机理属于自旋依赖电子变程跃迁机制.     

MnGa合金体的电子结构与磁性质的研究

基于密度泛函理论方法系统研究了四方结构MnGa合金体的结构、形成、电子结构和磁性质。结果表明,四方MnGa合金晶胞的生成焓为-4.85 eV,高于一些不含d电子的体系。其呈现导体的能带结构,其中d电子主要形成深能级价带,定域性最强。四方MnGa合金存在着明显的自旋极化,靠近费米能级两侧的s电子和靠近费米能级下方的p电子具有较弱的自旋极化。形成浅能级价带和导带的d电子产生高强度的自旋极化,对磁性质贡献较大。Mn的s电子和Mn的p电子自旋极化作用较弱,Mn的d电子形成浅能级价带和导带,自旋极化作用最强。形成深能级价带的Ga的d电子自旋极化作用较弱,不同位置的Ga原子的自旋极化不同。四方MnGa合金体具有净有效磁矩,呈弱的亚铁磁性。     

稀土金属Y掺杂锐钛矿TiO2(101)表面的改性

为了掌握Y原子掺杂在锐钛矿TiO₂(101)表面的稳定吸附位置和电子结构变化,提高其表面光催化活性,本文利用基于密度泛函理论的第一性原理计算研究了Y原子掺杂在完美的、带有亚表层氧空位和带有表层氧空位的锐钛矿TiO₂(101)表面的结构稳定性和电子性能。结构优化和电荷密度结果表明,Y原子可以稳定吸附在三种不同的表面上。在完美表面吸附时,Y原子最稳定的吸附位置是两个三配位O原子之间的空位;与完美表面类似,在带有亚表层氧空位表面吸附时,Y原子最稳定吸附位置是与氧空位邻近的两个三配位O原子之间的空位;而在带有表层氧空位表面吸附时,Y原子则停留于氧空位邻近的四配位Ti原子位置上最稳定。电荷密度计算结果也表明Y原子与这三种表面结合非常稳固。电子态密度计算结果表明,在带有表层氧空位的锐钛矿TiO₂(101)表面引入Y原子会在费米面附近的带隙中引入缺陷态,带隙从1.67 eV降至1.44 eV,这有可能引起电子的分级跃迁,提高表面光催化能力。本文的研究为利用单原子Y掺杂提高TiO₂(101)表面光催化能力提供了理论...     

气体分子吸附调控CdG电子结构和磁性的第一性原理研究

采用基于密度泛函理论的第一性原理方法,深入研究单个气体分子CO、NO、NO2、SO2、O2和H2S吸附对Cd掺杂石墨烯(CdG)电子结构和磁性的影响及调控机理.结果 表明:六种气体分子以较大的吸附能与CdG基底中的Cd原子键合并形成Cd-X键(X代表C,O,N,S),属于化学吸附;由气体吸附注入的空穴使得复合体系的电子分布发生重构,致使复合体系电子结构和磁性明显改变.CO吸附后,体系仍保持CdG基底原有的半导体性,但带隙宽度有所变化;NO、NO2、SO2和O2吸附的CdG基底转变为金属性,H2S@CdG为半金属性.CO的吸附使得原本自旋极化的CdG基底磁性消失;NO2、H2S、NO和O2吸附基底时,产生局域自旋极化.对于NO2@CdG和H2S@CdG,自旋极化主要分布于基底之上且极化方向相同,NO2和H2S发生弱极化且自旋方向相反;而NO@CdG和O2@CdG的自旋分布特征与之相反;SO2@CdG呈现完全自旋极化特征,即SO2和基底皆有显著的自旋分布但自旋方向不同.据此,由各气体分子引起体系电子结构和磁性的不同变化特征,可有效检测和甄别气体分子.     

B掺杂BaTiO3的电子结构和磁性的第一性原理研究

利用基于第一性原理的自旋极化密度泛函理论计算了B掺杂BaTiO3的稳定性、电子结构和磁学性质.结果表明B掺杂BaTiO3体系稳定,并表现为铁磁性.B替位掺杂BaTiO3体系的磁性机制可归结为部分B2p电子的自旋极化和B2p/O2p与Ti3d电子的p-d耦合作用,B间隙掺杂BaTiO3体系中磁性源于未配对Ti3d电子的自旋极化.     

Fe对InP材料电子结构及光学性质的调控机理研究

采用基于密度泛函理论的线性缀加平面波(FLAPW)方法,研究了3d族过渡金属元素Fe对Ⅲ-Ⅴ族半导体InP的电子结构和光学性质的调控机理,并对其能带结构和电荷密度分布进行了分析.结果表明,InP为直接带隙半导体,其价带主要由P-3s和3p态构成,而导带则由In-5s电子态构成.当Fe元素替代In原子后,由于Fe和P原子的轨道杂化作用,InP带隙中出现杂质态,Fe-3d态产生自旋极化效应.随着Fe的掺杂浓度增大,Fe-P原子之间轨道杂化作用明显增加,费米能级逐渐进入价带,这导致了材料的电子跃迁几率提高,光学吸收边明显增强,跃迁峰发生红移.     

Tracking atomic processes throughout the formation of heteroepitaxial interfaces

Understanding the atomic processes governing the formation a heteroepitaxial interface is central to predict and control the basic physical and chemical properties of a variety of hetero‐structures. With this perspective, we address in this work the dynamic behavior of Ge atoms deposited on Si‐surfaces by molecular dynamics simulations using enhanced bond order potentials. We demonstrate that the deposition of Ge atoms on Si surface induces the competition between several processes including adsorption, desorption, and bulk and surface diffusion involving atomic exchange, substitution, and clustering. By tracking these process, the simulations provide unprecedented insights onto the assembly of the first atomic layer of Ge on Si, the nucleation, growth, and relaxation of islands and quantum dots as well as of defect generation in the bulk.     

Mechanism for CuPt-type ordering in mixed III–V epitaxial layers

A model is proposed to rationalize the occurence of CuPt-type ordering in mixed III–V epitaxial layers grown on (001) substrates. It is invoked that 2× surface reconstruction occuring on group V terminated (001) surfaces produces dilated and compressed regions in sub-surface layers. The presence of these regions biases the occupation of certain sites during the growth of a layer consisting of atoms differing in their tetrahedral radii. By assuming that the reconstruction always occurs at the growing surface, it is possible to explain the evolution of CuPt-type ordering in epitaxial layers. Also, based on the model, experimental observations pertaining to the influence of growth conditions, surface misorientation and steps on ordering can be rationalized.     

完全Heusler合金Cr2ZrSb/Sc2FeSn(100)异质结的结构、电磁特性及电子性质

本文采用基于密度泛函理论的第一性原理计算,系统研究了完全Heusler合金Cr₂ZrSb/Sc₂FeSn(100)异质结中六种界面CrCr-ScFe-T、ZrSb-ScSn-T、CrCr-ScSn-B、ZrSb-ScFe-B、CrCr-ScFe-V和 ZrSb-ScSn-V的电磁特性及电子性质。结果表明,界面原子间的相互作用造成了界面间原子层的不均匀,导致界面层的力学失配率加大。与块体中的高自旋极化率相比,异质结的自旋极化率遭到不同程度的破坏。但是,ZrSb-ScFe-B界面保留了较高的自旋极化率值,通过Julliere模型预测该异质结在低温下隧道磁电阻值约为429.29%,在自旋电子学器件中具有潜在的应用前景。     

Influence of the B and P content on the thermal stability and crystallization of cast iron based bulk amorphous alloys

The possibility of synthesizing bulk amorphous alloys with additions of B and P to commercial cast iron (Ci) of the chemical composition Fe_81.5Si_3.8C₁₄Tm_0.7 (at.%) was investigated. The effect of the B and P concentration on thermal stability, bulk glass forming ability (BGFA) and microstructure was studied by calorimetric (DSC and DTA) measurements as well as by X-ray diffraction. With the addition of 8.42 and 12.17 B to the commercial Ci bulk amorphous alloys with very wide supercooled liquid regions (77 and 81 K, respectively) could be produced. The partial replacement of Ci by B and P atoms increases the thermal stability and the BGFA. The kinetics of crystallization was investigated both by linear heating and by isothermal DSC measurements. The changes in the microstructure of the crystalline phases formed during linear heating were studied during further isothermal annealing at 833 K.     

Structure and magnetic properties of bulk amorphous Fe60Co10Ni10Zr7B13 alloy formed by mechanical synthesis and hot pressing

Formation and magnetic properties of the bulk amorphous Fe₆₀Co₁₀Ni₁₀Zr₇B₁₃ alloy are described. The amorphous powder was prepared by high-energy ball milling of the amorphous ribbon and then was pressed under high pressure and temperature to obtain a bulk alloy in the form of a disc 5 mm in diameter and 2.5 mm thick. The compacted discs are fully amorphous as confirmed by X-ray diffraction and Mössbauer spectroscopy measurements. Differential scanning calorimetry curve recorded for the amorphous ribbon shows two exothermal peaks related to two stages of crystallization. Similar thermal behavior is observed for the powder samples. Hysteresis loops obtained for the amorphous ribbon, as-milled and compacted powders reveal their soft magnetic properties.     

Elastic constant anomaly in Fe80−xMoxB20 metallic glasses

The extensional sound velocities V_E of Fe_80−xMo_xB₂₀(x = 6, 7, 8) metallic glasses have been measured between 100 and 600 K in a saturating magnetic field. Young's modulus (E = _E², = density) shows anomalous temperature dependence at and below the magnetic transition temperature of each glass studied. These anomalies are interpreted as arising from the change of long-range spin coupling energy at the Curie temperature in a molecular field approximation. Comparison is made with similar effects in other crystalline and non-crystalline magnetic materials.     

Neutron measurements of magnetic correlations in metallic glasses

Neutron diffraction experiments have been made on FeB and PdSi transition metal alloy glasses prepared by melt-spinning. The magnetic neutron scattering has been analysed to provide information about the magnetic order in the specimens. An Fe₈₃B₁₇ metallic glass has magnetic correlations at room temperature with a range commensurate with the variations in atomic structure. These correlations appear to have a structure based on fcc gamma-iron, and the magnetic disorder is small - given that the specimen was in a field of 1 kG. Magnetic scattering was observed from Pd₇₀Si₂₀Co₁₀ and Pd₆₇Si₂₀Fe₁₃ at room temperature, which indicated a large magnetic moment value for the transition metal atoms. Cooling the PdSiCo sample to 80 K induced an antiparallel moment correlation, while further cooling to 7 K produced little obvious change in the scattering. These latter results are discussed in the light of the considerable disagreement which exists between different investigations of these alloys.     

室温铁磁性Sr3YCo4O10.5+δ多晶有序化的烧结工艺及电磁性能

采用固相反应法制备了不同烧结温度(950~1 180 ℃)、烧结时间、烧结次数共7种工艺的Sr₃YCo₄O_10.5+δ多晶块材,通过热分析、XRD、SEM确定了有序化相变和最佳烧结工艺(1 180 ℃/24 h+1 180 ℃/24 h),并研究了多晶的电磁性能。结果表明,964 ℃完全晶化的四方相Sr₃YCo₄O_10.5在1 042 ℃吸氧(δ)完成有序化,生成Sr₃YCo₄O_10.5+δ,而1 100 ℃和1 180 ℃烧结的样品均出现(103)、(215)超结构峰,验证了其结构的有序性。块材均呈半导体电输运行为,二次烧结晶格完整性提高,晶粒长大,300 K时电阻率仅为0.06 Ω·cm,居里温度(T_c)~335 K,零场冷曲线(ZFC)上的Hopkinson峰源于低温时被冻结的磁矩随温度升高转向磁场方向,磁化强度在298 K达到最大,随后受热扰动的影响减小。室温铁磁性源于有序结构导致的中自旋或高自旋态Co³⁺的e_g轨道有序。     

A criterion of the existence of the anomalous Kondo effect in amorphous magnetic materials

A theory of the anomalous Kondo effect in amorphous magnetic materials below the Curie temperature is presented. It was found that whether or not a particular spin is a Kondo scatterer, when the long-range magnetic order prevails, depends on the ratio of its local temperature of magnetic ordering and the bulk Curie temperature. By changing the external magnetic field or temperature we can obtain information about higher correlations of spins.     

应变对Mo2C(001)表面电子结构及光学性质的影响

采用第一性原理方法研究应变对Mo₂C(001)表面电子结构及光学性质的影响。研究表明,在应变作用下Mo₂C(001)表面均为间接带隙半导体,带隙随着压应变和拉应变的增加而减小。当应变为-20%时,Mo₂C(001)表面由间接带隙半导体转变为金属性质。当应变为-20%、-15%、-10%、-5%、0%、5%、10%、15%、20%时,其带隙分别为0 eV、0.162 eV、0.376 eV、0.574 eV、0.696 eV、0.708 eV、0.604 eV、0.437 eV、0.309 eV。带隙变化的原因主要是Mo 4p、4d、5s态电子和C 3p态电子对应变敏感,在应变作用下受激发,活性增强导致价带顶在布里渊区G、A、L、M点之间变化,导带底在K、H点之间变化;当应变由-15%逐渐变化到20%时,吸收谱的第一峰逐渐减弱,并且第一峰对应的光电子能量减小,吸收带边向低能方向移动,表明光吸收随着压应变增大而增加,吸收带边随着拉应变增加向低能方向移动。其他光学性质表现出类似的变化规律,光学性质计算结果表明应变能够有效调节光吸收特性,增强光学利用率,研究结果为Mo₂C(001)作为新型光电子材料的应用提供理论支撑。