原子尺度金属纳米线的结构和性质

当物质尺度减少到几层原子时,形成超细的纳米结、纳米线、或者纳米团簇,原有凝聚态物质的结构和物理性质将不再保持,而呈现出许多令人惊奇的奇异特性。本文重点讨论直径大约3 nm以下,具有足够长度的、原子结构往往不同于体材料的准一维金属纳米结构,我们称之为原子尺度金属纳米线或超细金属纳米线(也称为金属原子线)。近年来实验上已经制备和表征出在超高真空中悬挂在两个顶针尖端的Au、Pt、Cu等金属纳米线和纳米管,金属线直径达到1 nm以下而长度为6 nm以上。通过高分辩电子显微镜观察,它们是同轴圆管(或壳)组成的、类似纳米碳管的单壳或多壳结构,管由绕着线轴的螺旋原子绳构成。理论工作围绕这种新奇结构形态的形成机制、奇异的物理性质和可能的应用前景而同时展开。这是一个崭新的纳米世界,无论是对基础的低维物理还是未来分子电子设备的应用,都将产生深远的影响,有许多奇妙的现象正等待人们去发现。本文将对最近几年原子尺度金属纳米线研究工作的主要进展和发展趋势作一个概述,并重点介绍本组有关的具有螺旋结构的纳米线的各类新奇结构和物理性质。     

AuPd纳米粒子作为催化剂制备硼纳米线及其场发射性质

利用化学气相沉积法,采用不同组分的金属合金纳米粒子AuPd作为催化剂,在Si(111)基底上成功制备大面积、高密度的硼纳米线薄膜.纳米线的平均长度约为10μm,直径在50—130 nm之间.结构分析表明,纳米线为单晶结构,硼纳米线的直径随着元素Pd在合金催化剂中比例的增加而减少.场发射特性测试结果表明,通过调整催化剂组分可以实现对硼纳米线的尺寸和密度的调控.     

表面纳米结构的自组装生长

无论是对低维基本物理以及其中新奇量子现象的探索与认识,还是微电子工业水平的持续发展,都迫切地需要掌握一种能够精确、可靠地操控表面纳米结构的方法.自组织生长,即粒子聚集时由于介观尺度力场或受限运动作用而导致的自发有序现象,在原子尺度上可以实现对纳米结构的精确控制,而在介观尺度上又可以调节这些微观结构单元的组织构型.文章结合作者近年来在表面纳米结构生长与物理性质研究方面所做过的工作,从自组织生长的原理出发,介绍了对金属纳米线、有序分子薄膜以及合金量子点阵列生长进行人工操控的方法.     

超细Pt纳米线结构和熔化行为的分子动力学模拟研究

基于EAM原子嵌入势, 对临界尺寸下的自由Pt纳米线的奇异结构和熔化行为进行分子动力学模拟. 模拟结果显示, 超细Pt纳米线的熔点随径向尺寸和结构的不同而发生明显改变; 引入林德曼因子, 令其临界值为0.03, 以此得到对应熔点值大小与通过势能-温度变化曲线找出的一致, 又比较了纳米线各层粒子平均林德曼指数的大小, 对各层纳米结构的热稳定性进行定量标度; 综合分析发现螺旋结构纳米线的熔化从内核开始, 而多边形结构的纳米线的熔化从外壳层开始.     

热效应对纳米焊接界面结构的影响（英文）

采用分子动力学方法深入研究热效应对纳米焊接界面结构的影响。具体分析了碳管直径和时间的变化对界面结构的影响规律,并详细给出了1530K下的焊接过程。结果表明,除碳管(5,5)外,碳管(6,6),(7,7),(9,9),(11,11)中纳米线形成的时间分别为13.8,14.6,17.5,19.6ps。纳米线是由单根或多根Ni原子链组成,碳管(6,6),(7,7),(9,9),(11,11)中Ni原子链数分别为1,3,7和16。界面结构包括内部焊接和外部焊接。内部焊接的临界直径由碳管(6,6)决定,其值为0.814nm。在同一时刻,大直径的碳管可获得更大的外部接触长度。外部接触长度的增长速率随碳管直径的增加而增大,接触长度的最大增长速率可达0.013nm/ps。最后确定了界面结构形成临界温度,发现对于相同直径碳管,外部焊接的临界温度高于内部焊接的临界温度,临界温度与碳管直径无关。     

多壳层铜纳米线结构和电子特性的第一性原理研究

基于密度泛函理论框架下的第一性原理计算,系统地研究了多壳层Cu纳米线的稳定结构和电子特性.得到不同线径多壳层Cu纳米线的平衡态晶格常数相差不大,都表现出金属特性,且其单原子平均结合能和量子电导随着纳米线直径的增加而增加.纳米线中内壳层Cu原子表现出体相结构Cu原子相似的电子特性,而表面壳层由于配位数的减少,其3d态能量范围变窄且整体向费米能级发生移动.电荷密度分析表明,相对于体相Cu晶体中原子间的相互作用,纳米线表面壳层Cu原子与其最近邻原子间的相互作用明显增强.     

金属Ni纳米线凝固行为的分子动力学模拟

采用EAM镶嵌原子作用势，通过经典的分子动力学模拟方法研究了不同冷却速度下的金属Ni纳米线的凝固行为，并给出了纳米线在凝固区域的结构演变过程.利用键对分析技术研究了在不同冷却速度下体系中的原子团簇在降温过程中的变化情况.研究表明，纳米线的凝固起始于表面原子，并且随着冷却速度的降低，Ni纳米线的微观结构从非晶态过渡到多壳螺旋结构，最终达到稳定的面心立方结构.多壳螺旋结构同时具有确定的结晶温度和长程无序、短程有序的非晶结构的特征. 关键词： 纳米线 凝固行为 分子动力学 键对分析     

热效应对纳米焊接界面结构的影响

采用分子动力学方法深入研究热效应对纳米焊接界面结构的影响。具体分析了碳管直径和时间的变化对界面结构的影响规律,并详细给出了1530 K下的焊接过程。结果表明,除碳管(5,5)外,碳管(6,6),(7,7),(9,9),(11,11)中纳米线形成的时间分别为13.8, 14.6, 17.5, 19.6 ps。纳米线是由单根或多根Ni原子链组成,碳管(6,6),(7,7),(9,9),(11,11)中Ni原子链数分别为1,3,7和16。界面结构包括内部焊接和外部焊接。内部焊接的临界直径由碳管(6,6)决定,其值为0.814 nm。在同一时刻,大直径的碳管可获得更大的外部接触长度。外部接触长度的增长速率随碳管直径的增加而增大,接触长度的最大增长速率可达0.013 nm/ps。最后确定了界面结构形成临界温度,发现对于相同直径碳管,外部焊接的临界温度高于内部焊接的临界温度,临界温度与碳管直径无关。     

AlN纳米线宏观阵列的制备

借助二次模板法成功的合成了AlN纳米线宏观阵列,并进行了表征.主要研究CVD法制备有一定取向,直径均匀的AlN纳米线宏观阵列的过程.通过气相沉积法和利用PS球自组装模板制备了金属纳米颗粒模板;再以模板上的金属纳米颗粒作为催化剂,利用化学气相沉积在模板上合成AlN纳米线宏观阵列.借助SEM,TEM观察所得样品,AlN纳米线阵列面积约为0.3 mm×0.2 mm,直径和长度分布均匀,平均直径约为41 nm,平均长度为1.8 μm左右,分散密度和覆盖率大的六角结构AlN纳米线宏观阵列.得到了可控制备AlN纳米线 关键词： AlN纳米线阵列 模板法 CVD法 SEM     

Cu/C核/壳纳米结构的气相合成、形成机理及其光学性能研究

采用乙酰丙酮铜为原料, 通过化学气相沉积大批量制备出Cu/C核/壳纳米颗粒和纳米线. 研究结果表明, 通过控制沉积温度可对Cu/C核/壳纳米材料的形貌和结构进行很好的控制. 比如, 沉积温度为400 ℃时可获得直径约200 nm的Cu/C核/壳纳米线, 沉积温度为450 ℃ 时可获得直径约200 nm的Cu/C核/壳纳米颗粒和纳米棒的混合产物, 沉积温度为600 ℃时可获得直径约22 nm的Cu/C核/壳纳米颗粒. 获得的Cu/C核/壳纳米结构是由一个新颖的凝聚机理形成的, 而这种机理不同于著名的溶解-析出机理. 紫外-可见光谱和荧光光谱分析结果表明: Cu/C核/壳纳米线和纳米颗粒均在225 nm处出现Cu的吸收峰, 同时在620 和616 nm处分别出现了纳米线和纳米颗粒的表面等离子共振吸收峰. Cu/C核/壳纳米线在312 和348 nm处、 Cu/C核/壳纳米颗粒在304 和345 nm处出现荧光发射谱峰. 关键词： Cu/C核/壳结构 纳米线 纳米颗粒 光学性能     

Structures of Si and Ge nanowires in the subnanometer range

We report an ab initio investigation of several structures of pristine Si and Ge nanowires with diameters between 0.5 and 2.0 nm. We consider nanowires based on the diamond structure, high-density bulk structures, and fullerenelike structures. Our calculations indicate a transition from sp3 geometries to structures with higher coordination, for diameters below 1.4 nm. We find that diamond-structure nanowires are unstable for diameters smaller than 1 nm, undergoing considerable structural transformations towards amorphouslike wires. For diameters between 0.8 and 1 nm, filled-fullerene wires are the most stable. For even smaller diameters (approximately 0.5 nm), we find that a simple hexagonal structure is particularly stable for both Si and Ge.     

纳米金丝的生长序列、奇特结构和性质

文章扼要报道了用遗传算法和分子动力学以及密度泛函理论对纳米金丝的结构和电子性质随直径变化进行系统研究的结果,发现对于较细的纳米金丝、主要呈螺旋和多层壁的圆柱结构,而当纳米金丝的直径大于3mm时,则呈fcc块体类结构,并且晶化过程是从轴心开始的,振动谱和电子性质和也随纳米金丝半径增大而由分子型向块体行为转变。     

Transport properties of boron nanotubes investigated by ab initio calculation

We investigate atomic and electronic structures of boron nanotubes (BNTs) by using the density functional theory (DFT). The transport properties of BNTs with different diameters and chiralities are studied by the Keldysh nonequilibrium Green function (NEGF) method. It is found that the cohesive energies and conductances of BNTs decrease as their diameters decrease. It is more diffcult to form (N,0) tubes than (M,M) tubes when the diameters of the two kinds of tubes are comparable. However, the (N,0) tubes have a higher conductance than the (M,M) tubes. When the BNTs are connected to gold electrodes, the coupling between the BNTs and the electrodes will affect the transport properties of tubes significantly.     

Nanowire formation by coalescence of small gold clusters inside carbon nanotubes

The coalescence of Au₁₃, Au₅₅ and Au₁₄₇ icosahedral clusters encapsulated inside single walled carbon nanotubes (CNTs) of different diameters are investigated using molecular dynamics simulation with semi-empirical potentials. Three steps needed for the formation of encapsulated nanowires are followed in detail, namely, the penetration of clusters in CNTs, the coalescence between two clusters inside CNTs and their accumulation to form wires. It is suggested that no significant energy barrier is encountered during the penetration of free clusters into CNTs provided the CNT radius is large enough, that is, about 0.3 nm larger than the cluster radius. The relative orientation of clusters imposed by the CNT favors their spontaneous coalescence. After coalescence of two clusters, the Au atoms are rearranged to form new structures of cylindrical symmetry that may be seven fold, six fold, five fold, helical or fcc depending on the CNT diameter. The thermal stability of these structures is discussed and the structural properties of nanowires formed by accumulation of many clusters in CNTs are analyzed in detail. A geometrical method is presented which allows the prediction of the structure of multi-shell helical wires, when knowing only the CNT radius. These modeling results suggest the possibility of synthesizing metallic nanowires with controlled diameter and structure by embedding clusters into nanotubes with suitable diameters.     

Tailoring optical properties of TiO2 nanowires coated with Ag nanoparticles by plasmon coupling of Ag nanoparticles

TiO₂ nanowires were grown on titanium foil by an alkali hydrothermal growth method. The as-synthesized nanowires are structurally uniform with diameters of 50-100 nm and lengths of up to a few micrometers. The as-prepared TiO₂ nanowires were coated with Ag nanoparticles by reducing AgNO₃ in solution. The experimental results indicate that the Ag nanoparticles can aggregate together on the surfaces of TiO₂ nanowires by interconnection between nanoparticles. The degree of aggregation of Ag nanostructures can be controlled by changing the concentrations of Ag nanoparticles. The as-prepared nanostructures exhibit a wide optical absorption from 387 to 580 nm that can be easily tuned by controlling the degree of aggregation of Ag nanostructures. The results reveal that optical properties of the Ag-coated TiO₂ nanowires can be enhanced by plasmon coupling of Ag nanoparticles. The as-prepared nanostructures may find potential applications in the field of solar cells.     

Elastic and plastic deformations of nickel nanowires under uniaxial compression

Using atomistic molecular dynamics simulation with a Sutton–Chen many body potential, we studied the structural evolution and deformation mechanisms of nickel nanowires under homogeneous uniaxial compressions. Nickel nanowires with helical multi-shell structure and fcc-like crystalline structures have been considered. Elastic and plastic behaviors of nickel nanowires under compression were observed and their elastic limits were determined. Our simulations show that the nickel nanowires with helical multi-shell structure have greater yield strength than that of macroscopic solid. Above elastic limit, the plastic deformation of the nanowires shows behavior that is associated with superplasticity. The final atomic structures for the two kinds of nanowires are resemblant crystalline-like.     

体超导铟对一维锌超导纳米线超导电性的抑制效应

文章细致研究了超导体铟／一维锌超导纳米线阵列／超导体铟夹心结构的超导电性．实验发现，当锌纳米线的长度在2—6μm、直径等于40nm时，宏观尺寸的超导体铟电极对中间的锌纳米线的超导电性具有反常的抑制作用,即当铟处在超导态时，中间的锌纳米线则停留在正常态．如果施加一个磁场，使超导体铟电极变为正常态，锌纳米线则恢复其超导电性，这种奇异的现象与超导电极材料的类型及锌纳米线的直径和长度有关。     

Synthesis and photoluminescence properties of Bi2S3 nanowires via surfactant micelle-template inducing reaction

Single-crystalline Bi₂S₃ nanowires, with diameters in the range of 80-200 nm and lengths up to tens of micrometers, have been successfully synthesized through surfactant micelle-template inducing reaction at ambient-pressure and low-temperature. The synthetic route is simple, effective and can provide great opportunities for both fundamental and technological applications. The optical properties of the Bi₂S₃ nanowires with different diameters were firstly examined by means of photoluminescence spectroscopy at room temperature. The representative photoluminescence spectrum exhibits a great blue-shift from the band gap of 1.30 eV of bulk Bi₂S₃ to high energy of 1.44 eV, which indicated that these nanostructures showed quantum confinement effects.