硅团簇的结构及生长模式——紧束缚分子动力学：Si11—Si32

采用紧束缚分子动力学模拟硅团簇的结构,通过比较它们的结合能来确定基态结构,最后描绘出不同尺寸所对应的径向分布函数、角分布函数.模拟表明硅团簇在n=27处发生结构转变,从结构图上看,是由扁长结构向近球形结构转变.从径向分布函数图像、键角分布函数图像上也可以得到团簇结构在n=27处发生了变化,结构变得越来越紧密.关键词：硅团簇紧束缚分子动力学模拟退火基态结构     

金刚石表面无定形碳氢薄膜生长的分子动力学模拟

利用分子动力学模拟方法研究了CH₂基团轰击金刚石(111)面所形成的无定形碳氢薄膜(a-C:H)的生长过程. 结构分析表明, 得到的无定形碳氢薄膜中碳原子的局域结构(如C–C第一近邻数)与其中氢原子的含量密切相关. CH₂ 基团入射能量的增加会导致得到的薄膜的氢含量降低, 从而改变薄膜中类sp³成键碳原子的比例.     

Molecular dynamics simulation of nanoscale mechanical behaviour of ZnO under nanoscratching and nanoindentation

Molecular dynamics is used to simulate the mechanical behaviour of zinc oxide under nanoscratching and nanoindentation. The effects of indenter speed and substrate temperature on the structure-phase formation, slip vector, radial distribution function, and residual stresses are investigated. Simulation results show that the dislocation loops nucleate and propagate, forming a body-centred tetragonal lattice structure along the slip direction due to high local stress. Furthermore, the dislocation loops nucleate and propagate due to the resolved shear stress along the 45° slip direction under nanoscratching. The average mean biaxial stress and the normal stress of the O layers are –9.35 and –4.36 GPa, respectively, and those of the Zn layers are –0.80 and –0.30 GPa, respectively. This may be attributed to the energetic O atoms, with which unstable atoms have high activation.     

非晶Ti3Al合金的变形晶化机理的原子模拟

利用分子动力学方法研究了非晶Ti₃Al合金拉伸过程中的晶化行为,模拟结果表明局部塑性变形导致非晶合金晶化.从微观结构演化的角度分析了拉伸过程中的晶化机理,局部剪切导致拉伸过程中晶粒发生成核与合并,最终生成的晶粒具有面心立方结构.晶核的生长过程伴随着应力强化现象,非晶相中的纳米晶粒能提高非晶合金材料的强度.关键词：非晶合金变形晶化分子动力学     

脉冲激光晶化超薄非晶硅膜的分子动力学研究

利用准分子脉冲激光晶化非晶硅薄膜是制备高密度尺寸可控的硅基纳米结构的有效方法之一.本文将脉冲激光对非晶硅超薄膜的影响处理为热传导问题,采用了基于Tersoff势函数的分子动力学方法模拟了在非晶氮化硅衬底上2.7 nm超薄非晶硅膜的脉冲激光晶化过程.研究了不同激光能量对非晶硅薄膜晶化形成纳米硅的影响,发现在合适的激光能量窗口下,可以获得高密度尺寸可控的纳米硅薄膜,进而模拟了在此能量作用下非晶硅膜中成核与生长的机理与微观过程,并对晶化所获得的纳米硅薄膜的微结构进行了分析.关键词：非晶硅分子动力学脉冲激光晶化     

Ionic liquid confined in silica nanopores: molecular dynamics in the isobaric–isothermal ensemble

Molecular dynamics simulations in the isobaric–isothermal ensemble are used to investigate the structure and dynamics of an ionic liquid confined at ambient temperature and pressure in hydroxylated amorphous silica nanopores. The use of the isobaric–isothermal ensemble allows estimating the effect of confinement and surface chemistry on the density of the confined ionic liquid. The structure of the confined ionic liquid is investigated using density profiles and structural order parameters while its dynamics is assessed by determining the mobility and ionic conductivity of the confined phase. Despite the important screening of the electrostatic interactions (owing to the small Debye length in ionic liquids), the local structure of the confined ionic liquid is found to be mostly driven by electrostatic interactions. We show that both the structure and dynamics of the confined ionic liquid can be described as the sum of a surface contribution arising from the ions in contact with the surface and a bulk-like contribution arising from the ions located in the pore centre; as a result, most properties of the confined ionic liquid are a simple function of the surface-to-volume ratio of the host porous material. In contrast, the ionic conductivity of the confined ionic liquid, which is a collective dynamical property, is found to be similar to the bulk. This study sheds light on the complex behaviour of hybrid materials made up of ionic liquid confined in inorganic porous materials.     

基于缠结微结构的非晶态玻璃态高分子材料屈服行为的分子动力学研究

非晶态玻璃态高分子材料作为结构材料在工程领域应用广泛,其机械力学性能特别是屈服变形行为受到热处理、加载应变率和环境温度的影响.采用分子动力学模拟方法研究非晶态玻璃态高分子材料不同工况下的单轴拉伸变形,基于分子链缠结微结构的概念,阐明了非晶态玻璃态高分子材料屈服和应变软化过程的内在变形机制.结果表明,拓扑缠结具有较为稳定的空间结构,难以发生解缠,决定了非晶态高分子材料屈服后的软化平台.由相邻分子链的局部链段相互作用形成的次级缠结在一定外界条件下可发生破坏或重新生成,次级缠结微结构及其演化是非晶态高分子材料发生屈服及软化的内在物理原因.     

摩擦导致的聚合物表层微观结构改变

应用大规模分子动力学方法, 模拟了锥形探头在非晶态聚合物薄膜表面的滑动摩擦过程, 研究了摩擦导致的聚合物薄膜表层微观结构改变, 以及探头与基体间黏着作用、滑动速度和分子链长度对基体表层微观结构改变的影响. 当探头与基体之间为黏着作用时, 摩擦导致基体表面滑痕区域的键取向沿滑动方向重新取向, 导致表层分子链回转半径沿滑动方向伸长, 并且这些表层微观结构的改变程度随滑动速度的减小而增大. 在摩擦导致结构改变的过程中, 链端单体和链中单体的贡献作用不同, 形成了不同的分子链拉伸变形机制. 当样本缠结度较大或探头滑动速度较小时, 相比于链中单体, 探头对链端单体的拖曳作用使更多分子链发生拉伸变形. 研究还发现, 在探头与聚合物薄膜系统中, 使薄膜表层微观结构发生改变是摩擦能量耗散的重要途径.     

开口悬挂端对单壁碳纳米管电子输运特性的影响

采用紧束缚模型研究了悬挂端对单壁碳纳米管电子输运特性的影响.结果表明：有限长悬挂端开口碳纳米管的电导在费米能级附近作周期性振荡.椅型(armchair)碳纳米管的振荡同时具有快、慢两个准周期,而锯齿型(zigzag)碳纳米管的振荡仅有一个周期;碳纳米管电导在费米能级附近的振荡周期随着悬挂端的增长而减小.研究还发现：有限长悬挂端开口碳纳米管的平均电导随探针与碳纳米管间耦合强度的增加而增大,其大小约为无限长悬挂端开口碳纳米管平均电导的两倍.关键词：输运特性碳纳米管紧束缚模型     

单壁碳纳米管单原子空位缺陷的紧束缚理论研究

文章在紧束缚势模型基础上系统地研究了非手性单壁碳纳米管上单原子空位缺陷结构和电子结构性质.计算表明,单原子空位缺陷会自发地形成5-1DB型缺陷, 且该缺陷的局域结构和形成能强烈地依赖于碳纳米管的尺寸、旋度和电学性质.同时作者发现这类缺陷在费米能级以上约0.2eV处产生局域的电子态.     

水对无定形SiO2拉伸特性影响的反应分子动力学模拟

潮湿对SiO₂的强度有重要影响. 采用反应场分子动力学模拟方法,研究液态水对无定形SiO₂ （a-SiO₂）准静态拉伸特性的影响. 准静态拉伸模拟的结果表明,在干燥条件下,a-SiO₂的拉伸强度为9.4 GPa,而在含液态水时则下降为4.7 GPa,表明液态水使得a-SiO₂拉伸强度发生显著下降. 根据应力-应变曲线分析可知,干燥条件下a-SiO₂结构的刚度随着拉伸应变的增加保持稳定,而含液态水的a-SiO₂刚度随着拉伸应变的增加而逐步降低,并且应变为16%–20%时的应力-应变曲线类似于金属的屈服现象. 通过对拉伸过程的原子图像分析可知,含液态水a-SiO₂的拉伸过程并没有发生塑性变形,而是因为应力增大加速了水解反应,使得应力-应变曲线表现出上述塑性现象.关键词：2')\" href=\"#\">无定形SiO₂机械强度水分子动力学模拟     

Study of mechanical properties of amorphous copper with molecular dynamics simulation

The formation and mechanical properties of amorphous copper are studied using molecular dynamics simulation. The simulations of tension and shearing show that more pronounced plasticity is found under shearing, compared to tension. Apparent strain hardening and strain rate effect are observed. Interestingly, the variations of number density of atoms during deformation indicate free volume creation, especially under higher strain rate. In particular, it is found that shear induced dilatation does appear in the amorphous metal.     

分子动力学模拟H原子与Si的表面相互作用

通过分子动力学模拟了入射能量对H原子与晶Si表面相互作用的影响. 通过模拟数据与实验数据的比较, 得到H原子吸附率随入射量的增加 呈先增加后趋于平衡的趋势. 沉积的H原子在Si表面形成一层氢化非晶硅薄膜, 刻蚀产物(H₂, SiH₂, SiH₃和SiH₄)对H原子吸附率趋于平衡有重要影响, 并且也决定了样品的表面粗糙度. 当入射能量为1 eV时, 样品表面粗糙度最小. 随着入射能量的增加, 氢化非晶硅薄膜中各成分(SiH, SiH₂, SiH₃)的量以及分布均有所变化.关键词：分子动力学吸附率表面粗糙度氢化非晶硅薄膜     

Molecular dynamics simulation of atomic hydrogen diffusion in strained amorphous silica

Understanding hydrogen diffusion in amorphous SiO_2(a-SiO_2),especially under strain,is of prominent importance for improving the reliability of semiconducting devices,such as metal-oxide-semiconductor field effect transistors.In this work,the diffusion of hydrogen atom in a-SiO_2 under strain is simulated by using molecular dynamics(MD) with the ReaxFF force field.A defect-free a-SiO_2 atomic model,of which the local structure parameters accord well with the experimental results,is established.Strain is applied by using the uniaxial tensile method,and the values of maximum strain,ultimate strength,and Young's modulus of the a-SiO_2 model under different tensile rates are calculated.The diffusion of hydrogen atom is simulated by MD with the ReaxFF,and its pathway is identified to be a series of hops among local energy minima.Moreover,the calculated diffusivity and activation energy show their dependence on strain.The diffusivity is substantially enhanced by the tensile strain at a low temperature(below 500 K),but reduced at a high temperature(above500 K).The activation energy decreases as strain increases.Our research shows that the tensile strain can have an influence on hydrogen transportation in a-SiO_2,which may be utilized to improve the reliability of semiconducting devices.     

Assessment of delocalized and localized molecular orbitals through electron momentum spectroscopy

Recently, there was a hot controversy about the concept of localized orbitals, which was triggered by Grushow＇s work titled ＂Is it time to retire the hybrid atomic orbital？＂ [J. Chem. Educ. 88, 860 （2011）]. To clarify the issue, we assess the delocalized and localized molecular orbitals from an experimental view using electron momentum spectroscopy. The delocalized and localized molecular orbitals based on various theoretical models for CH4, NH3, and H20 are compared with the experimental momentum distributions. Our results show that the delocalized molecular orbitals rather than the localized ones can give a direct interpretation of the experimental （e, 2e） results.