论文题目：金属纳米线奇异结构和物理性质的理论研究
作者简介:
王保林，男，1957年03月出生，1998年09月师从于南京大学王广厚教授，于2002年03月获博士学位。
摘
要
金属纳米线因为在低维物理的基础研究方面的重要性和未来作为分子电子设备器件的应用前景而受到人们的广泛重视。最近，Kondo和Takayanagi提供了非常细的Au线类似纳米碳管的螺旋结构的实验证据。超过6 nm长、最小直径达到0.6 nm的悬浮金纳米线，已经在超高真空透射电子显微镜（ultrahigh vacuum transmission electron microscopy，UHVTEM）下，用电子束辐照在Au薄膜上成功制备出来。在充分粗和充分短时Au（110）纳米桥具有规则的fcc晶体结构,直径小于1.5nm时转变到规则但非晶、几个纳米长悬挂在顶针之间的纳米线。这些纳米线大都为离散的、特定幻数尺寸下的螺旋多壳结构，每个管由三角网格自相叠加而成。这种螺旋多壳的金属纳米线代表一种神奇的物质组织形态，类似于但又不同于团簇，人们还远没有充分了解这类物质结构。是什么决定了幻非晶纳米线的形貌和尺寸？除了Au以外的其它金属怎样？这种纳米线的物理性质，像力学、热学、电磁、光学、电导性质等等，以及在外部因素，包括温度、压力、及场等的作用下,线结构和性质的演化都具有潜在的兴趣。在本论文中，我们采用基于分子动力学的遗传算法，系统地研究了金、锆、铑、钛等金属纳米线的奇异结构。在结构优化的基础之上，我们比较系统地研究了这些金属纳米线各种奇异的物理性质。
采用基于分子动力学的遗传算法，模拟优化了自由状态的金纳米线的结构，发现比较细的纳米线结构是螺旋多壳的圆柱结构，而当直径增加到3nm以上时，金纳米线的结构呈现类晶体的fcc结构，这种非晶体－晶体转化的过程出乎预料地是从纳米线的核芯开始的。我们发现了中等尺寸的自由Au纳米线的幻闭壳数，是按照每个壳层增加6个原子绳的规律生长，这些纳米线的内外壳之间都是奇－奇或偶－偶匹配的耦合方式。基于优化的结构，采用动力学矩阵对角化的方法系统研究了Au纳米线的振动谱；采用有效核势的Dmol程序，用密度泛函理论（DFT）计算了金线的电子结构；用表面Green’s函数匹配方法的Landauer公式，还研究了金纳米线的电子输运性质。我们发现振动谱和电子性质随直径的增大而由分子型向类晶体行为转变。电导一般随线的直径的增加而增加，强烈地依赖于纳米线的结构。用常温分子动力学方法研究金纳米线的热力学行为，还发现了Au纳米线奇特的芯融化行为。
用基于分子动力学的遗传算法和紧束缚多体势，我们系统地研究了直径在0.574-2.787nm的自由锆纳米线的结构。发现Zr纳米线是按照不同序列的螺旋多壳圆柱体结构生长的幻闭壳数，Zr纳米线的壳间原子链股数从芯向外以5的倍数增加，按照奇－偶或偶－奇的螺旋匹配方式，形成幻的闭壳结构。这些多壳的圆柱结构由同轴的圆管在三角形、四边形、单链芯的中心五边形、中心六边形及平行双芯线内核芯上的曲面外延生长而成。角关联函数和振动谱研究进一步证明了这种奇特的纳米线结构生长序。
铑纳米线的结构，通过基于分子动力学的遗传算法结合紧束缚多体势的模拟发现也是螺旋多壳和五边形生长的结构。采用非限制Hartree-Fock近似下的Hubbard紧束缚哈密顿和递归算法，我们计算了Rh纳米线的电子态密度、平均磁矩和自旋极化电荷分布，结果表明超细的Rh纳米线和团簇一样是磁性的；发现了Rh纳米线中不等价原子间的电荷转移和spd电荷的重新分布；超细Rh纳米线的磁性质类似于Rh团簇、Rh的表面、薄片以及取向附生在其它金属表面的Rh薄膜；Rh纳米线的电子态密度的形貌在较细时与Rh团簇相似，而在尺寸较大时与Rh的表面接近；最大的纳米线的电子态密度也没有发现体材料电子态密度的三个特征峰。我们的计算结果表明， Rh纳米线的电子和磁性性质不仅由尺寸效应决定，还与结构及表面效应密切相关。特别是五边形和六边形生长的纳米线有不寻常的磁性。
我们选择不同的结构和生长序列的几个典型的超细钛纳米线，即四边形、五边形和六边形序列，以及六边形序列的两个较大体系，以分别考察结构和尺寸效应。我们计算了Ti纳米线的角关联函数和振动谱，电子态密度用平面波赝势的密度泛函电子结构的自洽场（SCF）来计算。发现Ti纳米线的振动性质和电子性质表现出渐进的尺寸演化和明显的结构关联，当纳米线的直径大于1nm时便表现为类似体材料的电子结构。常温分子动力学方法研究Ti纳米线的热力学行为，发现较大尺寸的Ti纳米线，观察到其融化过程伴随着系统从螺旋多壳的圆柱体到类似块体的结构相变，Ti纳米线的热力学性质表现出明显的尺寸演化和结构依赖性。
Abstract
Metallic nanowires have been intensively studied because of the fundamental interests in low-dimensional physics and technological applications as molecular electronic devices. Recently, Kondo and Tagayanagi provided evidence that thin gold wires form a helical structure, akin in some respects to carbon nanotubes. The suspended gold nanowires with 6 nm in length and diameters down to 0.6 nm were successfully prepared and characterized by electron-beam irradiation on a gold thin film in an ultrahigh vacuum transmission electron microscopy (UHVTEM). It was found that gold (110) nanobridges, which possess a regular crystalline fcc structure when sufficiently thick or short, can transform at diameters below ~ 1.5 nm into regular but noncrystalline wires several nanometers long, hanging between tips. These nanowires appear to form mostly discrete, helical multishell structures of specific “magic” sizes and shapes, and each tube consists of a triangular sheet folded cylindrically onto itself. The helical multishell metallic nanowires represent a novel organization of matter, similar but not identical to clusters, which is yet to be fully understood. What determines the magic helical multishell nanowire shapes and sizes? What about nanowires made of other metals besides gold? The physical properties of metallic nanowires, such as mechanical, thermodynamic, electronic, magnetic, optical, conductance, etc., and the evolution of the wire structure and properties under external agents, including temperature, stress, and fields, is potentially interesting. In this thesis, we have systematically studied the novel structures of gold, zirconium, rhodium, and titanium nanowires by using genetic algorithm (GA) based on molecular dynamics (MD). Based on the optimized structures of these metallic nanowires, their novel physical properties have been investigated.
The structures of free-standing gold nanowires with diameters from 0.4 to 3.0 nm are systematically studied by using molecular-dynamics-based genetic algorithm simulations with glue potential. Helical and multiwalled cylindrical structures are found for the thinner nanowires. As the wire diameter increase up to about 3nm, the emerge of fcc crystal-like structures in the Au nanowires are observed. This noncrystalline-crystalline transition starts from the core region of the nanowires. The magic closed-shell numbers of helical multi-shell cylindrical growth sequences are found for the intermediate size gold nanowires. The numbers of atomic strands show the even-even or odd-odd coupling between the inner- and outer-shell with the differences of six strands. Based on the optimized structures of nanowires, the vibrational densities of states for these nanowires are obtained by diagonalizing the dynamical matrix for the equilibrium structures with glue potential. We performed the density functional theory (DFT) electronic structure calculations by using the Dmol program with effective core potential. We also investigated transport properties by using Landauer formalism with surface Green’s function matching method. 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. Studies of the thermal stability of some helical cylindrical titanium nanowires by using molecular dynamical simulation (MD) starting from the optimized structures have found an interesting interior core melting phenomena in the gold nanowire.
By using genetic algorithm simulations based on molecular dynamics with a tight-binding many body potential, the structures of zirconium nanowires with 0.574-2.787 nm in diameter are systematically studied. The magic closed-shell numbers of helical multi-shell cylindrical growth sequences are found for zirconium nanowires. The numbers of atomic strands show the even-odd or odd-even coupling between the inner- and outer-shell with the differences of five strands. These multi-shell structures composed of coaxial tubes in the three- and four-strands helical, centered pentagonal and hexagonal, and double-chain-core parallel curved surface epitaxy. Studies of the angular correlation functions and vibrational properties of zirconium nanowire have farther proved such novel structural growth order.
The structural properties of ultrathin rhodium nanowires are studied by using empirical molecular dynamics simulations with genetic algorithm. Helical multi-shell cylindrical and pentagonal packing structures are found. The electronic and magnetic properties of those rhodium nanowires are calculated by using a spd tight-binding Hamiltonian in unrestricted Hartree-Fock approximation. The average magnetic moment, spin-polarized charge distribution, and electronic density of state（DOS） are obtained. All these nanowires studied are magnetic. Significant charge transfer between different sites and spd charge redistribution are found. The electronic and magnetic behavior of ultrathin nanowire is similar to that of Rh clusters, slabs, and surfaces. The DOS’s of the thinner nanowire show Rh cluster features, in particular, the For the thicker wires, the DOS’s looks like the feature of the Rh bare surface and slabs surface, whereas deviations from the bulk DOS are three dominant peaks, even in thicker nanowires. Our results indicate that the electronic and magnetic properties of the rhodium nanowires are related not only to size but also to structure and surface effect. In particular, the centered pentagonal and hexagonal structures can be unusually magnetic.
Some typical structures of titanium nanowires, i.e. four-strands helical, centered pentagonal, hexagonal and the thicker hexagonal structures are chosen to study the structural and size effects of ultrathin nanowires. The angular correlation functions and vibrational properties are discussed. We have further calculated the electronic structures of the titanium nanowire by using density functional electronic structure self-consistent-field (SCF) calculations with the plane-wave pseudopotential method. The vibrational and electronic properties of titanium nanowire depend on the structure and size. Bulk-like continuous electronic band are formed in the Ti wires thicker than 1 nm. We have studied the thermal stability of the titanium nanowires by using molecular dynamical simulation (MD) starting from the optimized structures. The structural transformation from the helical multi-shell cylindrical to the bulk-like for the thicker nanowire is observed which takes place in the melting process. The thermal properties of titanium nanowires are significantly dependent on the structure and size of the nanowire.
