不同孪晶界密度银纳米线拉伸形变行为的分子动力学模拟

采用分子动力学方法模拟了不同孪晶界密度银纳米线的拉伸形变行为,分析了孪晶界密度对多晶银纳米线屈服强度、弹性模量和塑性变形机理的影响.在弹性形变区域,孪晶界的存在对杨氏模量变化的作用不明显.在塑性形变阶段,首先从表面边缘开始产生位错成核,然后延伸并受阻于孪晶界.在进一步拉伸载荷作用下,孪晶界将作为位错源产生新的位错.模拟结果表明,银纳米线的强度与孪晶界和晶粒的尺寸有关.孪晶界密度较小(即晶粒的长径比大于1)时,此纳米线的屈服应力比单晶纳米线还要小,只有当孪晶界密度较大时(即晶粒的长径比小于1),孪晶界使得纳米线得到强化.综合分析了孪晶界和晶粒尺寸对银纳米线的影响,为构建高强度金属纳米线打下基础.最后讨论了温度和拉伸速度对孪晶纳米线屈服应力所产生的影响,随着温度的升高,孪晶纳米线与单晶纳米线的屈服应力差先升高后趋于稳定;当拉伸速度逐渐增大,孪晶纳米线与单晶纳米线的屈服应力差先稳定后增大.     

不同孪晶界密度银纳米线拉伸形变行为的分子动力学模拟

采用分子动力学方法模拟了不同孪晶界密度银纳米线的拉伸形变行为, 分析了孪晶界密度对多晶银纳米线屈服强度、弹性模量和塑性变形机理的影响. 在弹性形变区域, 孪晶界的存在对杨氏模量变化的作用不明显. 在塑性形变阶段, 首先从表面边缘开始产生位错成核, 然后延伸并受阻于孪晶界. 在进一步拉伸载荷作用下, 孪晶界将作为位错源产生新的位错. 模拟结果表明, 银纳米线的强度与孪晶界和晶粒的尺寸有关. 孪晶界密度较小(即晶粒的长径比大于1)时, 此纳米线的屈服应力比单晶纳米线还要小, 只有当孪晶界密度较大时(即晶粒的长径比小于1), 孪晶界使得纳米线得到强化. 综合分析了孪晶界和晶粒尺寸对银纳米线的影响, 为构建高强度金属纳米线打下基础. 最后讨论了温度和拉伸速度对孪晶纳米线屈服应力所产生的影响, 随着温度的升高, 孪晶纳米线与单晶纳米线的屈服应力差先升高后趋于稳定; 当拉伸速度逐渐增大, 孪晶纳米线与单晶纳米线的屈服应力差先稳定后增大.     

电子显微镜技术表征聚乙二醇包覆的银纳米线的显微结构特征

以缓慢的水热合成法制备聚乙二醇(PEG)包覆的银纳米线,并用电子显微镜对其微观形貌和结构进行了表征。PEG是一种对银表面呈弱亲和性的水溶性聚合物,它在银纳米线合成中既作为Ag+的还原剂,又作为银线的包覆剂。扫描电镜和透射电镜表征显示,这种银纳米线的直径为80～300 nm,长度大于100μm;并且其中一部分银纳米线首尾相接形成了闭合纳米线圈,是首例以化学法制取的金属闭合纳米线圈。闭合环状纳米线的形成进一步支持了金属纳米线的融合生长机制。透射电镜和选区电子衍射以及银纳米线横截面切片的透射电镜表征显示,这种PEG包覆的银纳米线具有轴对称的五重挛晶结构。银纳米线经过长时间的水热孵化而保持其五重挛晶结构,这一点与某些文献报道的情况有所不同,说明银纳米线结构及其演化具有一定的复杂性。     

Size-dependent orientation growth of large-area ordered Ni nanowire arrays

Large-area ordered Ni nanowire arrays with different diameters have been fabricated by the direct current electrodeposition into the holes of porous anodic alumina membrane. The crystal structure and micrograph of nanowire arrays are characterized by X-ray diffraction, field-emission scanning electron microscopy and high-resolution transmission electron microscopy. The results indicate that the growth orientation of Ni nanowires turns from [110] to [111] direction with increasing diameters of nanowires. The mechanism of the growth was discussed in terms of interface energy minimum principle. The size-dependent orientation of Ni nanowire arrays has the important significance for the design and control of nanostructures.     

Transmission electron microscopy study of pseudoperiodically twinned Zn2SnO4 nanowires

The ternary oxide functional nanomaterial Zn2SnO4 has been synthesized by the thermal evaporation method. The products in general contain numerous kinds of nanowires. In the present work, a remarkable type of Zn2SnO4 nanowires with a pseudoperiodical twinning structure has been investigated by transmission electron microscopy (TEM). These nanowires with a diameter of about 100 nm grow along the 111 direction. High-resolution TEM examinations suggest that a large fraction of the (111) twin boundaries are extended to a thickness of a few nanometers. The twining plane for the perfect case is localized on the Zn atom layer.     

High-resolution transmission electron microscopy study of nanostructured hydroxyapatite

Crystalline properties of synthetic nanostructured hydroxyapatite (n-HA) were studied using high-resolution transmission electron microscopy. The focal-series-restoration technique, obtaining exit-plane wavefunction and spherical aberration-corrected images, was successfully applied for the first time in this electron-beam-susceptible material. Multislice simulations and energy dispersive X-ray spectroscopy were also employed to determine unequivocally that n-HA particles of different size preserve stoichiometric HA-like crystal structure. n-HA particles with sizes of twice the HA lattice parameter were found. These results can be used to optimize n-HA sinterization parameters to improve bioactivity.     

基于分子动力学方法的孪晶银纳米线拉伸形变模拟

采用分子动力学方法研究了具有不同孪晶界密度的< 111>晶向孪晶Ag纳米线在拉伸载荷下的形变行为,讨论了孪晶界对纳米线力学强度的影响,并分别阐明具有不同孪晶界密度Ag纳米线的塑性形变机理.结果表明,与单晶Ag纳米线的强度进行对比可知,基于不同的形变模式,孪晶界的引入对纳米线可以起到弱化作用或者强化作用.以孪晶片层厚度的倒数(1/TBS)作为临界值,当1/TBS小于0.2 nm^-1时,孪晶界作为位错源,表现为对纳米线的弱化作用;当1/TBS大于0.2 nm^-1时,孪晶界阻碍位错运动表现为强化作用.强化作用机理分为两种:当1/TBS介于0.2到0.5 nm^-1时,形变机理以孪晶界和位错相互作用为主,断裂开口均在纳米线内部产生,随着位错增殖形成孔洞,进而向四周蔓延;当1/TBS大于0.5 nm^-1时,孪晶界发生迁移以容纳位错活动,位错不断增殖穿过孪晶界形成剪切带,进而导致纳米线的颈缩.由孪晶界密度不同引起的强化作用和弱化作用均随温度升高而减弱.     

孔洞和孪晶界对银纳米线形变行为联合影响的分子动力学模拟

Based on molecular dynamics simulations, the plastic deformation of silver nanowires under uniaxial tension has been studied systematically. In this paper, the mechanical properties of [111]-oriented twin nanowires with different hole sizes have been studied. The existence of holes has no effect on the elastic deformation stage. The hole on the twin boundary has two main roles in the plastic deformation stage. During the initial stages of plastic deformation, the main function of the hole is to produce new dislocations as dislocation sources at small hole sizes.Upon increasing the hole size, the main effect changes to stop dislocation slip. During the late stages of plastic deformation, the two functions of the hole complement each other, upon increasing the hole size, the function of the hole as dislocation sources becomes obvious, leading to weakening of the plasticity of the nanowires.     

应变率诱发镍纳米丝的非晶化

采用镶嵌原子法(EAM)作用势对镍纳米丝的拉伸过程进行了分子动力学模拟. 体系施加应变率范围在5×10^7 - 1×10^10 s^-1, 观察体系的应力-应变曲线、平均原子能量、原子轨迹, 偶相关函数随应变率的变化. 发现当应变率低于1×10^8 s^-1时, 体系发生塑性变形, 最终颈缩断裂, 偶相关函数表明体系一直保持晶体结构; 而当应变率高于1×10^9 s^-1时, 体系发生了由晶体向非晶的持续转变, 偶相关函数出现了典型的非晶峰, 高应变率诱发了Ni纳米丝的非晶化.     

聚吡咯/硫化镉核壳纳米线的制备及整流特性

以无机多孔氧化铝膜为模板,利用气相沉积和原位电化学沉积方法成功地制备了有机-无机杂化聚吡咯/硫化镉核壳纳米线;采用扫描电子显微镜和透射电子显微镜分析了聚吡咯/硫化镉核壳纳米线的表面形貌和微结构.结果表明,内部的聚吡咯纳米线紧紧依附在外部的硫化镉纳米管中,并且硫化镉纳米管被聚吡咯全部填充.与此同时,在聚吡咯/硫化镉核壳纳米线中,外部硫化镉壳与内部聚吡咯核之间存在电荷转移;聚吡咯和硫化镉之间形成有机-无机杂化的P-N界面,从而导致单根聚吡咯/硫化镉核壳纳米线显示出不同于外部壳和内部核的整流特性.     

Structure determination of atomically controlled crystal architectures grown within single wall carbon nanotubes.

Indirect high resolution electron microscopy using one of several possible data-set geometries offers advantages over conventional high-resolution imaging in enabling the recovery of the complex wavefunction at the specimen exit plane and simultaneously eliminating the aberrations present in the objective lens. This article discusses results obtained using this method from structures formed by inorganic materials confined within the bores of carbon nanotubes. Such materials are shown to be atomically regulated due to their confinement, leading to integral layer architectures that we have termed "Feynman crystals." These one-dimensional (1D) crystals also show a wide range of structural deviations from the bulk, including unexpected lattice distortions, and in some cases entirely new forms have been observed.     

Multiwfn: a multifunctional wavefunction analyzer

Multiwfn is a multifunctional program for wavefunction analysis. Its main functions are: (1) Calculating and visualizing real space function, such as electrostatic potential and electron localization function at point, in a line, in a plane or in a spatial scope. (2) Population analysis. (3) Bond order analysis. (4) Orbital composition analysis. (5) Plot density-of-states and spectrum. (6) Topology analysis for electron density. Some other useful utilities involved in quantum chemistry studies are also provided. The built-in graph module enables the results of wavefunction analysis to be plotted directly or exported to high-quality graphic file. The program interface is very user-friendly and suitable for both research and teaching purpose. The code of Multiwfn is substantially optimized and parallelized. Its efficiency is demonstrated to be significantly higher than related programs with the same functions. Five practical examples involving a wide variety of systems and analysis methods are given to illustrate the usefulness of Multiwfn. The program is free of charge and open-source. Its precompiled file and source codes are available from http://multiwfn.codeplex.com.     

β-FeOOH纳米线的制备及表征

通过无机铁(III)盐的水解在常温条件下制备了β-FeOOH 纳米线, 并用透射电子显微镜(TEM)、高分辨透射电镜(HRTEM)、X射线衍射(XRD)及选区电子衍射(SAED)对其形貌及结构进行了表征. 电镜结果表明, 所得到的纳米线直径约 60 nm, 长度为4～5 μm, 且沿[001]方向生长. XRD结果表明纳米线为四方相β-FeOOH, 在常温下结晶性良好. 研究表明FeCl₃浓度对纳米线生长有很大影响, 只有当FeCl₃浓度合适时, 才能制备出高质量的纳米线.     

Bicrystalline hematite nanowires

Bicrystalline nanowires of hematite (alpha-Fe(2)O(3)) have been successfully synthesized by the oxidation of pure iron. The product was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM in combination with focal series reconstruction, energy-dispersive X-ray spectroscopy, and electron energy-loss spectroscopy. The bicrystalline nanowires have diameters of 20-80 nm and lengths up to 20 microm. All of the investigated materials are found to be alpha-Fe(2)O(3) with a rhombohedral crystal structure. Investigations indicate that most of the bicrystalline nanowires are nanotwins with ellipsoidal heads. The orientation relationship between the nanotwins can be described as (110)(M)//(110)(T), [110](M)//[0](T). An energy-filtered TEM investigation indicates that the ellipsoidal head is iron-rich. The growth mechanism of such unique nanostructures is considered to be a solid-phase growth via surface and internal diffusions of molecules from base to tip.     

Polysynthetic Twinning in RbIn3S5

The real structure of the new ternary compound RbIn₃S₅ is characterized by polysynthetic twinning of nanosized domains separated by coherent twin boundaries. Based on electron microscopy, a model of the structure at the twin interface is derived which allows a convincing simulation of experimental images. The details of twinning are described in a unified model. As a rule, a disordered arrangement of twin lamellas with a random distribution of their widths is observed with ordering of twin boundaries only in nanosized regions of the crystallites. For a minimal distance of the twin boundaries (back-to-back twinning) the close relationship between twinning and shearing is discussed. The distinction of these two cases is only possible by close inspection of the atomic structure at the boundaries. Twinning is not the only crystal defect in the structure of RbIn₃S₅. Variations in the connection of the characteristic structural units occur which lead to the formation of different real structures.     

Synthesis and characterization of indium-doped ZnO nanowires with periodical single-twin structures

In-doped ZnO (IZO) nanowires have been synthesized by a thermal evaporation method. The morphology and microstructure of the IZO nanowires have been extensively investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). The products in general contain several kinds of nanowires. In this work, a remarkable type of IZO zigzag nanowire with a periodical twinning structure has been investigated by transmission electron microscopy (TEM). HRTEM observation reveals that this type of IZO nanowire has an uncommonly observed zinc blend crystal structure. These nanowires, with a diameter about 100 nm, grow along the [111] direction with a well-defined twinning relationship and a well-coherent lattice across the boundary. In addition, an IZO nanodendrite structure was also observed in our work. A growth model based on the vapor-liquid-solid mechanism is proposed for interpreting the growth of zigzag nanowires in our work. Due to the heavy doping of In, the emission peak in photoluminescence spectra has red-shifted as well as broadened seriously.     

Synthesis and Characterization of SnO2 One-dimensional Nanostructures

Single-crystalline SnO2 nanowires have been successfully prepared in large scale on Au-coated silicon substrate by heating the mixture of self-made high-purity SnO2 powders and graphite powders at 900℃. Besides the line type nanowires some more features were observed. The products were characterized by means of X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and Raman spectrum techniques. The results indicate that the tin dioxide nanowires have a rutile structure with diameters ranging from 30 to 120 nm and lengths up to several tens of micrometers. The possible mechanism of the growth and reaction for the nanowires was also discussed.     

Layering at an ionic liquid-vapor interface: a molecular dynamics simulation study of [bmim][PF6

The structure of the planar liquid-vapor interface of a room-temperature ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), is studied using atomistic molecular dynamics simulations. Layering of the ions at the interface is observed as oscillations in the corresponding number density profiles. These oscillations, however, are diminished in amplitude in the electron density profile, due to a near cancellation in the contributions from the anions and the cations. An enhancement by 12% in the electron density at the interface over its value in the bulk liquid is observed, in excellent agreement with X-ray reflectivity experiments. The anions are found to predominantly contribute to this increase in the interfacial electron density. The cations present at the interface are oriented anisotropically. Their butyl chains are observed to be preferentially oriented along the interface normal and to project outside the liquid surface, thus imparting a hydrophobic character. In the densest region of the interface, the imidazolium ring plane is found to lie parallel to the surface normal, in agreement with direct recoil spectroscopy experiments.     

High-quality ultralong Bi2S3 nanowires: structure, growth, and properties

A simple one-step hydrothermal method for large-scale synthesis of ultralong single-crystalline Bi2S3 nanowires was reported, and the nanowires were comprehensively characterized. The diameters of the nanowires are about 60 nm, and their lengths range from tens of microns to several millimeters. The structure of the nanowires was determined to be of the orthorhombic phase, the growth direction was along [001], and the growth mechanism was investigated based on extensive high-resolution transmission electron microscopy observations. Optical absorption experiments revealed that the Bi2S3 nanowires are narrow-band semiconductors with a band gap E(g) approximately 1.33 eV. Electrical transport measurements on individual nanowires gave a resistivity of about 1.2 ohms cm and an emission current of 3.5 microA at a bias field of 35 V/microm. This current corresponds to a current density of about 10(5) A/cm2, which makes the Bi2S3 nanowire a potential candidate for applications in field-emission electronic devices.     

Analyzing Grid-Based Direct Quantum Molecular Dynamics Using Non-Linear Dimensionality Reduction

Grid-based schemes for simulating quantum dynamics, such as the multi-configuration time-dependent Hartree (MCTDH) method, provide highly accurate predictions of the coupled nuclear and electronic dynamics in molecular systems. Such approaches provide a multi-dimensional, time-dependent view of the system wavefunction represented on a coordinate grid; in the case of non-adiabatic simulations, additional information about the state populations adds a further layer of complexity. As such, wavepacket motion on potential energy surfaces which couple many nuclear and electronic degrees-of-freedom can be extremely challenging to analyse in order to extract physical insight beyond the usual expectation-value picture. Here, we show that non-linear dimensionality reduction (NLDR) methods, notably diffusion maps, can be adapted to extract information from grid-based wavefunction dynamics simulations, providing insight into key nuclear motions which explain the observed dynamics. This approach is demonstrated for 2-D and 9-D models of proton transfer in salicylaldimine, as well as 8-D and full 12-D simulations of cis-trans isomerization in ethene; these simulations demonstrate how NLDR can provide alternative views of wavefunction dynamics, and also highlight future developments.