Structure of metal nanowires in nanoporous alumina membranes studied by EXAFS and X-ray diffraction

The structure of nanowires of different metals grown within nanoporous alumina membranes has been studied by EXAFS, WAXS and high energy X-ray diffraction. Nanowires of gold, silver, copper and iron adopt the lattice structure and bond distances of the bulk metals. Cobalt nanowires on the other hand were composed of a mixture of hcp phase, stable at room temperature, and fcc phase, which in bulk cobalt is normally stable only at high temperatures, in a ratio depending on the pore size. The nanowires are non-continuous but are made of nanocrystallites whose shape and size was found to depend strongly on the metal. All the metals except gold showed the presence of a preferred orientation which was slight in the case of Ag and Cu but much stronger in the case of iron and cobalt nanowires. Received 30 November 2000     

The influence of wire shape on surface plasmon mode distribution

The influence of the nanowire shape on the excitation of surface plasmon polaritons at metallic nanowire arrays is studied numerically. For a system of silver nanowires housed on a polymer substrate, nanowires with rectangular and elliptical cross sections are compared. It was found that in the case of rectangular nanowires the excitation efficiency is higher for surface plasmons at the polymer–metal interface than for surface plasmons at the air–metal interface. Conversely, in the case of elliptical nanowires the air–metal plasmon modes are stronger. Further, it is noted that the nanowire shape directly influences the position of the surface plasmon resonance.     

Formation of stable nanowires from ferromagnetic metals using 2-butyne-1,4-diol

We mechanically fabricated Fe, Co, and Ni nanowires showing conductance quantization under electrochemical potential control. The effects of 2-butyne-1,4-diol and saccharin on the stability of the nanowires were investigated by measuring the conductance and the stretched length of the nanowires. Nanowires of these ferromagnetic metals were stabilized by adding 2-butyne-1,4-diol. In Co, a monoatomic contact was stretched 0.4 nm in length, indicating the formation of a highly stable monoatomic contact of ferromagnetic metals. The improved stability of the nanowires could originate from specific adsorption of organic molecules on the contacts. Mechanical properties of these metals were considered to explain the origin of the metal-dependent stability of the nanowires.     

Specific features of the growth and stability of nanowires made of different metals

Ensembles of metal nanowires are prepared by the template method. The influence of electrodeposition conditions on the properties of the obtained samples is studied for some metals and alloys. It is shown that the observed change in the electrodeposition rate during electrolysis can be related to a change in the electrode potential as a result of an increase in the gradient potential in the near-electrode solution region and to diffusion limitations. A number of specific features of the deposition of iron, iron–cobalt alloy, and iron–nickel alloy are revealed (in particular, the dependence of the elemental composition of the obtained wires on the electrodeposition conditions). Both the topography of the nanowires and their stability are investigated by electron microscopy. It is found that the nanowire form does not always correspond to the form of growth pores. Nanowires exhibit rather high stability in alkali solutions and low stability in acid solutions.     

Stress analysis of multiple anisotropic elliptical inclusions in composites

A volume integral equation method is introduced for the solution of elastostatic problems in an unbounded isotropic elastic solid containing interacting multiple anisotropic elliptical inclusions subject to uniform remote tension or remote in-plane shear. This method is applied to two-dimensional problems involving long parallel elliptical cylindrical inclusions. A detailed analysis of the stress field at the interface between the matrix and the central elliptical inclusion is carried out for square and hexagonal packing of anisotropic inclusions. The effects of the number of anisotropic inclusions and various inclusion volume fractions on the stress field at the interface between the isotropic matrix and the central elliptical cylindrical inclusion are investigated in detail. The stress field at the interface between the isotropic matrix and the central elliptical inclusion is also compared with that between the isotropic matrix and the central circular inclusion.     

Predicted pressure-induced s-band ferromagnetism in alkali metals

First-principles density-functional-theory calculations show that compression of alkali metals stabilizes open structures with localized interstitial electrons which may exhibit a Stoner-type instability towards ferromagnetism. We find ferromagnetic phases of the lithium-IV-type, simple cubic, and simple hexagonal structures in the heavier alkali metals, which may be described as s-band ferromagnets. We predict that the most stable phases of potassium at low temperatures and pressures around 20 GPa are ferromagnets.     

Nanostructuring Germanium Nanowires by In Situ TEM Ion Irradiation

Once nanomaterials have been synthesized, inducing further structural modifications is challenging. However, being able to do so in a controlled manner is crucial. In this context, germanium nanowires are irradiated in situ within a transmission electron microscope (TEM) by a 300 keV xenon ion beam at temperatures ranging from room temperature (RT) to 500 °C. The ion irradiation is performed in situ and the evolution of nanowires during irradiation is monitored. At 300 °C and below, where the temperature is low enough to allow amorphization, the ion beam causes the formation of nanostructures within the nanowires. Formation of nanopores and swelling of nanowires is observed for a very low fluence of 2.2 × 10¹⁴ and up to 4.2 × 10¹⁵ ions cm⁻². At higher fluences, the thickness of the nanowires decreases, the nanowires lose their wire-like cylindrical shape and the nanostructuring caused by the ion beam becomes more complex. The nanostructures are observed to be stable upon crystallization when the nanowires are annealed at 530 °C. Furthermore, in situ imaging allows the growth of nanopores during irradiation to be followed at RT and at 300 °C providing valuable insights into the mechanism responsible for the nanostructuring.     

Effect of stretching-contraction deformations on the magnetic ordering state of mixed Pd-Fe nanowires

The performed ab initio calculations indicate the possibility of forming stable, uniformly mixed Pd-Fe nanowires. The effect of stretching-contraction deformations on the electronic structure of Pd-Fe nanowires has been revealed. Using the calculation results, a transition from ferromagnetic to antiferromagnetic state in a uniformly mixed Pd-Fe nanowire subjected to tension has been predicted.     

A new theory of compressible ions — structures of the alkali halides

Ions in ionic crystals are considered to exist in compressible space-filling polyhedral cells analogous to the Wigner-Seitz cell in metals. Repulsion arises from the compression energy of the ions written as a surface integral over the ionic cells. Two adjustable parameters are introduced per ion with the provision that the same parameters can be used in any crystal of any structure in which the ion occurs. The 18 parameters for the 5 alkali and 4 halogen ions have been determined from PV data on the 20 alkali halides. The important successes of the theory are: (i) All the twenty alkali halides are correctly predicted to occur in their observed structures (ii) The thermal transition in CsCl is explained (iii) The pressure transitions in the alkali halides are predicted well (iv) The calculated values of the variation of transition pressures with temperature agree well with experiment. These results are much better than those obtained by earlier theories.     

涂覆石墨烯的椭圆形电介质纳米线光波导的模式特性分析

设计了一种涂覆石墨烯的椭圆形电介质纳米线光波导.采用分离变量法,在椭圆柱坐标系中,借助Mathieu函数,得到了色散方程.通过数值求解色散方程,可以得到模式的有效折射率和场分布,从而得到模式的传播长度.研究了工作波长、结构参数以及石墨烯的费米能对模式特性的影响,并给出了前五个模式的品质因数.计算表明,当波长从4.3μm增加到8.8μm,这5个模式的有效折射率的实部减小,基模和一阶模的传播长度增大,二阶模的传播长度先增大后减小.当改变纳米线结构参数半长轴和半短轴时,对基模和一阶模的模式特性影响较小,对二阶模的模式特性影响较大.当石墨烯的费米能从0.45 eV增加到0.72 eV时,有效折射率的实部减小,传播长度可以达到2μm左右.分离变量法得到的结果与有限元方法得到的结果完全一致.本文工作可以为基于涂覆石墨烯的电介质纳米线的光波导的设计、制作和应用提供理论基础.     

一种新的NMR法测定溶液中碱土金属混合物的方法原理(英文)

本文研究了碱土金属离子-锶、钡及钙可以形成在NMR标尺上是稳定配合物的条件。找到了最适合得到这样的配合物的配体-C222穴醚。研究并归属了碱土金属离子以及碱金属-钠、钾及锂配合物在氯仿中的~(13)C NMR谱。检测出了可用于分析自由配体及其与碱土金属配合物的~(13)C NMR信号,还研究了不同比例混合物的碱土金属及碱金属在氯仿中的~(13)C NMR谱,得到了当自由配体过量时在钡、钙及锶每种金属浓度为1×10~(-3)M的谱中可观测到相当于自由配体及每种离子配合的配体的分立信号。这些信号的积分值的精度可以定量地表征金属离子的浓度,所得结果为首次观测到的。它可以作为用~(13)C NMR波谱建立新的无机分析方法。文中并阐明了作溶液中碱土金属混合物NMR测定方法的具体条件,并讨论了将这一方法扩展到包括一系列碱金属在内的更复杂的混合物的可能性。试验在VARIAN XL-200波谱仪上进行。     

像散椭球高斯光束的理论分析与实验模拟

从理论上讨论了具有简单像散特必的椭 图 束的处理方法,在实验中利用两个正交的柱透镜模拟球面双轴透镜对半导体激光器的输出光束进行变换,得较为理想的结果,即把椭圆高斯光束自理为近似圆高斯光束。     

Acoustic elliptical cylindrical cloaks

By making a comparison between the acoustic equations and the 2-dimensional (2D) Maxwell equations, we obtain the material parameter equations (MPE) for acoustic elliptical cylindrical cloaks. Both the theoretical results and the numerical results indicate that an elliptical cylindrical cloak can realize perfect acoustic invisibility when the spatial distributions of mass density and bulk modulus are exactly configured according to the proposed equations. The present work is the meaningful exploration of designing acoustic cloaks that are neither sphere nor circular cylinder in shape, and opens up possibilities for making complex and multiplex acoustic cloaks with simple models such as spheres, circular or elliptic cylinders.     

Asymmetries in optical fibres

Two methods for treating the effects of small asymmetries in the cross sections of roughly cylindrical optical fibres are described. These predict the presence of stable power distributions, ribbons for example, within the fibre core. Experimental results on actual fibres showing these effects are given. To be effectively asymmetric a fibre of elliptical cross section need have major and minor diameters differing by only 0.08 μm.     

一种新的NMR法测定溶液中碱土金属混合物的方法原理

本文研究了碱土金属离子-锶、钡及钙可以形成在NMR标尺上是稳定配合物的条件。找到了最适合得到这样的配合物的配体-C222穴醚。研究并归属了碱土金属离子以及碱金属-钠、钾及锂配合物在氯仿中的¹³C NMR谱。检测出了可用于分析自由配体及其与碱土金属配合物的¹³C NMR信号,还研究了不同比例混合物的碱土金属及碱金属在氯仿中的¹³C NMR谱,得到了当自由配体过量时在钡、钙及锶每种金属浓度为1×10^-3M的谱中可观测到相当于自由配体及每种离子配合的配体的分立信号。这些信号的积分值的精度可以定量地表征金属离子的浓度,所得结果为首次观测到的。它可以作为用¹³C NMR波谱建立新的无机分析方法。文中并阐明了作溶液中碱土金属混合物NMR测定方法的具体条件,并讨论了将这一方法扩展到包括一系列碱金属在内的更复杂的混合物的可能性。试验在VARIAN XL-200波谱仪上进行。     

Fermi energy and optical conductivity of metal quantum wires

The size dependence of the Fermi energy of conduction electrons has been analytically evaluated and its oscillations have been calculated in the framework of the cylindrical well model. The conductivity tensor components for a metal wire with the radius ρ₀ have been calculated with the use of the expansion in powers of ρ₀/λ, where λ is the wavelength of incident light. The influence of the dimension of the systems has been established by comparing the results of the calculations of the optical conductivity of thin wires and films for Au, Al, and Pb metals. According to the results of the calculations, the real and imaginary parts of the conductivity depend substantially on the size and frequency. The difference between the results obtained for Pb and Au metals has been explained by the different relaxation times of electrons in nanowires that are homogeneous in the thickness.     

基于轴棱锥产生近似无衍射Mathieu光束的新方法

提出了一种基于轴棱锥产生零阶近似无衍射Mathieu光束的新方法,利用轴棱锥聚焦具有椭圆高斯振幅调制的平面波,得到近似零阶无衍射Mathieu光束.根据椭圆高斯平面波经轴棱锥衍射的衍射积分公式,对光强分布进行了数值模拟,依据几何光学模型计算了近似无衍射Mathieu光束的最大无衍射距离,并设计了实验对理论模拟的结果进行了验证.实验采用柱透镜和准直扩束系统变换圆高斯光束产生具有椭圆高斯振幅调制的平面波,用轴棱锥聚焦该平面波后得到近似无衍射Mathieu光束,实验结果与理论模拟和计算相符.     

Scattering from an elliptical cylindrical plasma for electromagnetic waves with wavelength much greater than the dimensions of the plasma cross-section

The distribution of the electric potential inside and outside a cold plasma column with elliptical cross-section when electromagnetic waves scatter from it are obtained. It is assumed that the wavelength of incident wave is much greater than the dimensions of the cross section of the plasma column. The electrical potential inside and outside a magnetized elliptical plasma column for an anisotropic scattering process are obtained. The graphs of the ratio of the electric field inside the plasma and the incident electric field versus the incident angle, the geometrical dimension of the elliptical plasma column and cyclotron frequency are presented separately. Finally, scattering of the same waves from an elliptical cylindrical inhomogeneous isotropic cold plasma is also investigated.     

Novel structures and properties of gold nanowires

The structures of free-standing gold nanowires are studied by using molecular-dynamics-based genetic algorithm simulations. Helical and multiwalled cylindrical structures are found for the thinner nanowires, while bulk-like fcc structures eventually form in the thicker nanowires up to 3 nm in diameter. This noncrystalline-crystalline transition starts from the core region of nanowires. The vibrational, electronic, and transport properties of nanowires are investigated based on the optimal structures. Bulklike behaviors are found for the vibrational and electronic properties of the nanowires with fcc crystalline structure. The conductance of nanowires generally increases with wire diameter and depends on the wire structure.     

Pressure effect on the melting point of alkali metals

The mean-square displacement of alkali metals is studied theoretically using our local Heine-Abarenkov-type model potential in the perturbational scheme. The temperature-dependent mean-square displacement of alkali metals decreases as function of the compressed volume. Lindemann's criterion for melting x_m, which is defined as the ratio of two times the root-mean-square displacement to the nearest-neighbour distance, is found to be nearly constant for five alkali metals. The volume effect on the melting temperature of alkali metals is studied by keeping x_m constant. The obtained melting curve increases as function of the compressed volume and are qualitatively in good agreement with the observed tendency for alkali metals.