【作者】 宋薇；

【导师】 吕文彩；

【作者基本信息】 吉林大学 ， 物理化学， 2011， 博士

【摘要】 团簇作为由原子、分子向宏观材料的过渡层次,它们的结构和特性随尺度大小的变化在一定程度上可反映由微观向宏观体系的特殊转变过程。近年来,过渡金属团簇以其在化学催化,激光物理,晶体生长,有机金属化学,纳米尺度的电镀材料等方面的广泛应用,引起了实验和理论研究者的关注,并对其产生了浓厚的兴趣。在30多种单一成分的过渡金属团簇中,镍团簇以其广泛的催化效应和重要的磁性得到了最为广泛和深入的研究。本文中我们采用了遗传算法（Genetic Algorithm,GA）与密度泛涵理论（Density Functional Theory,DFT）相结合的方法对镍团簇进行了系统的理论研究,找到了尺度介于20-35的镍团簇低能异构体,并确定了其最稳定几何结构,进而总结了它们的生长模式。以此为基础,我们对镍团簇的相对稳定性,电子性质及磁性等进行了系统地分析和研究,并得到以下的结论:（1）首先我们基于紧束缚势的遗传算法,对镍团簇Ni_n （20≤n≤35）的结构进行了全局搜索,当得到低能量的稳定结构后,采用第一性原理的密度泛函理论在VASP软件程序包中,对候选的低能量结构进行进一步的优化,确定其最稳定的几何结构。通过研究我们发现,中等尺度的镍团簇Ni_n （20≤n≤35）的结构是支持类双二十面结构的。比如说Ni₂₀的最稳定结构可以看做是在Ni₁₉（双二十面体1-5-1-5-1-5-1结构）的腰部添加一个原子构成的,Ni₂₁的最稳定结构是在Ni₁₉的相连的腰部添加两个原子构成的。Ni₂₃是由三个Ni₁₃（二十面体1-5-1-5-1结构）相互贯穿而构成的,Ni₂₆则可是看成是由两个Ni₁₉相互贯穿而构成的,而Ni₂₉则是由一个Ni₁₉和一个Ni₂₂（双二十四面体1-6-1-6-1-6-1结构）相互贯穿而构成的。（2）我们系统分析了镍团簇Ni_n （n≤30）的相对稳定性,包括结合能,二次差分能。对于中性的镍团簇而言,在n≤6时平均原子结合能随着团簇尺寸的增加迅速升高,在n = 6-10范围内,平均原子结合能呈现中等速度的增加,当原子数为11或者12时,平均原子结合能呈现缓慢增加的趋势。当n≥13时平均原子结合能变化很小。对于镍团簇来说,Ni₂, Ni₆, Ni₁₀, Ni₁₁, Ni₁₃, Ni₂₀, Ni₂₃, Ni₂₆,和Ni₂₉比相邻的团簇有更好的稳定性。对于离子团簇来说,阳离子镍团簇n = 3, 4, 11, 13, 23, 26和28处;阴离子镍团簇n = 6, 9, 11, 16, 18, 23, 26和29处表现出更好的稳定性。（3）我们详细计算了镍团簇Ni_n （n≤30）的磁性,对于镍团簇Ni₁-Ni₃₀来说,理论计算和实验上Ni_{3,6,13,16,19,23}有相对较小的磁矩值。对于离子团簇而言Ni_{4, 13, 15, 19, 21, 24}⁺和Ni_{4, 13, 19, 23}^-相对于其邻近团簇有相对较小的磁矩值。Ni)（13,19）的中性和离子结构均具有最小的磁矩值,我们可以认为二十面体结构Ni₁₃和双二十面体结构Ni₁₉具有特殊的性质即相对于相邻的团簇而言具有较低的磁矩值。（4）我们对Ni_n （n≤30）团簇的绝热电离势（adiabatic ionization potentials）和绝热电子亲和势（adiabatic electron affinities）进行了系统的分析,并与实验值做了对比。我们计算得到的整个AIP曲线与实验值在变化趋势上是一致的,只是我们计算得到的AIP值略低于实验值,团簇的原子数在3-10之间时AIP值出现振荡行为,在Ni₆和Ni₈处出现了一个极大的AIP值。而对于镍团簇Ni_n（n = 6 - 30）而言AIP值随着团簇原子个数的增加而缓慢的降低。我们计算所得的AEA值在趋势上与实验值基本一致,只是个别尺度上有略微的偏差,Ni₂具有最小的AEA值,且Ni_10,16,19较相邻的团簇来说,其AEA值略小。更多还原

【Abstract】 Clusters, as a transition state from molecular to macroscopic materials, have attracted much attention and interest in both theoretical and experimental studies. The structural and property changes of clusters with cluster size can reflect the micro to macro transformation at some extent. In recent years, transition metal clusters are known to play an important role in homogeneous and heterogeneous catalysis, laser physics, crystal and thin film growth, organometallic chemistry, and nanoscale electronic materials and devices. Among the 30 kinds of pure transition metal clusters, nickel clusters are the primary target of many research groups because of their extensive catalytic and important magnetic properties.In this work, we use the genetic algorithm （GA） coupled with a TB potential of Ni to search for low-energy candidates of Ni clusters. The low-energy candidate structures from the GA/TB search were further optimized by using the density functional theory calculations with the PBE exchange-correlation energy functional. We search for stable structures of medium-sized Ni_n （n = 20-35） clusters, and the binding energies, second differences in energies, magnetic properties, the adiabatic electronic affinities, adiabatic ionization potentials have been caculated in order to analyze the stabilities and electronic structures of the clusters. The main research results are listed as follows:（1）: The lowest-energy structures of the neutral nickel clusters Ni_n （n = 20-35） were studied by a combination method of the genetic algorithm searching with a tight-binding potential and the density functional theory calculations. The DFT-PBE calculations were performed using the VASP code. The medium-sized nickel clusters in this size range from 20 to 35 atoms are found to favor the double-icosahedron-like structures. For example, Ni₂₀ can be viewed as adding one atom to the side face in the waist of Ni₁₉; Ni₂₁ by adding two atoms on the adjacent waist sites of Ni₁₉; Ni₂₃ by adding a Ni₄ segment on Ni₁₉, etc. Ni₂₃ and Ni₂₆ can be seen as three interpenetrating 13-atom icosahedrons and two interpenetrating 19-atom double-icosahedrons, respectively. The structure of Ni₂₉ is considered to be composed of two interpenetrating double-icosahedron and double-icositetrahedron （with three parallel hexagonal rings）.（2） We have carried out a systematic study on relative stabilities of nickel clusters Ni_n （n = 2-30）, including binding energies and second differences in energies. The binding energy curve can be roughly divided into four regions: the binding energy increase rapidly for n < 6; moderately for 6 < n < 10; smoothly for Ni₁₁ and Ni₁₂; and slowly for n > 13. The slight humps on the curve indicate higher stability for some specific clusters. The curve of the second differences in energies shows an oscillating behavior for Ni_n at n = 2, 6, 10, 11, 13, 20, 23, 26, and 29, suggesting that these clusters are more stable compared to their neighbors. For ionic clusters, there are several peaks on the binding energy curve at n =3, 4, 11, 13, 23, 26 and 28 for the cationic clusters, and at n = 6, 9, 11, 16, 18, 23, 26 and 29 for the anionic clusters, showing these cluster ions are relatively more stable. （3） The calculated magnetic moments of Ni_n （n = 1-30） reveal that Ni3, 6,13,16,19,23 have relatively smaller magnetic moments both from the experimental and theoretical results. The magnetic moments of the cationic and anionic nickel clusters have been shown that Ni_{4, 13, 15, 19, 21, 24}⁺ and Ni_{4, 13, 19, 23}^- have relatively smaller magnetic moments compared with their neighbors. All the Ni_13,19 neutral and ionic clusters have smaller magnetic moments. We can consider that the icosahedron and double icosahedron structures of Ni13,19 have special characters which can lower their magnetic moment of the whole system.（4） The adiabatic ionization potentials （AIPs） and the adiabatic electron affinities （AEAs） of the Ni_n （ n-30） clusters have been calculated based on the lowest-energy structures of the neutral and ionic clusters of the Ni_n （n-30）. The behaviors of the calculated ionization potentials for the Ni_n （n-30） clusters agree well with the experimental results, though the calculated AIPs are smaller than the experiment data. It can be seen from the plot that there is an oscillating behavior at the size range of 3-10. In particular, there correspond to the sharp rises at Ni6 and Ni8 on the AIP curve. For the Ni_n （n = 6-30）, the AIP decreases slowly as the cluster size increases. The tendency of the calculated electron affinity is generally consistent but has small deviations compared with the experimental result. Ni2 has the lowest AEA value. Moreover, Ni_10,16,19 shows slightly smaller AEAs than their neighbors.更多还原