【作者】 赵丽珍；

【导师】 吕文彩；

【作者基本信息】 吉林大学 ， 物理化学， 2009， 博士

【摘要】 纳米半导体材料一直是材料科学领域研究的热点之一,在电子、冶金、化工、等领域有广泛的潜在用途。纳米材料研究的主要目标是合成,设计,和分析具有特殊功能的纳米级的微结构材料。团簇随尺度大小的变化在一定程度上可反应微观向宏观体系转变这个特殊的过程。团簇的结构和其独特的物理化学性质,吸引着许多科学工作者浓厚的兴趣。本文中,我们使用密度泛函理论(Density Functional Theory, DFT)结合紧束缚(Tight-Binding,TB)势与遗传算法(Genetic Algorithm, GA)搜索的方法对半导体硅锗团簇进行了系统的理论研究,确定了它们的最稳定几何结构,总结了它们的生长模式。在此基础上我们还分析了半导体硅锗团簇的相对稳定性,电子特性,解离通道等,并对具有特殊稳定性的团簇做了深入研究。另一方面,我们对金属原子与氧气的反应势能曲线进行了细致的理论研究,计算分析了碱金属和过渡金属原子与氧气的最佳反应路径,以及体系加入一个负电荷后,相应反应能垒和反应路径所受的影响。主要结论如下:对于25≤n≤33尺度范围的中性Sin与Gen团簇我们进行了全局最优结构计算。基于GA/TB搜索与DFT计算组合的方法,获得了Sin与Gen团簇的低能量异构体,比较了这一尺度范围硅锗团簇的生长模式.结果表明硅团簇拥有多种结构特征,低能量异构体包含棒状、类球形、和Y形结构。B3LYP计算结果显示Sin (n = 25 ~ 33)团簇的生长模式在n = 31处经历一个从棒状到笼形结构的转换,而对于PBE计算结果这个转化是发生在n = 26的位置,该结果与实验观测的结构转换范围( 24≤n≤34)相吻合;而在n = 25 ~ 33的尺寸范围,相应的锗团簇却一直保持着棒状的结构模式。Sin与Gen团簇在25≤n≤33的尺度范围,其生长模式具有显著差异。使用全电子DFT方法对Ge2-Ge33进行了系统研究。详细分析了锗团簇的各种性质:包括结合能,二次差分能,HOMO-LOMO能隙,特别是锗团簇的解离能和解离通道。n≤11尺寸范围的小型锗团簇具有较大的解离能,11 < n≤33尺寸范围的中小型锗团簇有较小的解离能,可以推断11 < n≤33尺度范围的锗团簇很容易分解为稳定的小团簇,这样的连续分解可将产物解离到n≤11的小团簇。在解离产物中,Ge6, Ge7, Ge10显示较大的丰度,表明这类小团簇非常稳定,计算结果和分析与实验光电离质谱测量结果一致。用第一性原理计算研究了Si70团簇的结构和稳定性。虽然来自DFT-PBE计算结果显示内嵌富勒烯Si16@Si54是Si70的最低能量结构;但DFT-BLYP计算结果表明一个具有硅金刚石体结构片段的结构是最稳定的几何。因为BLYP泛函计算的Si70团簇最稳定结构的电离势更接近实验值,我们认为硅团簇由笼形结构向体结构模式的转变点很有可能在n = 70附近。同时,我们也预测了Si70团簇的多种性质,包括电子态密度,IR振动谱,离子反向迁移率等,为实验提供了重要的理论信息。用MP2/6-311+G(d)计算方法,分别对过渡金属原子Ti﹑碱金属原子Na与O2的反应势能曲线进行了细致研究。比较了Ti﹑Na垂直O-O键和沿着O-O键逼近O2的不同反应路径的势能曲线的差异,预测了最佳反应方式;同时还比较了Ti/Na+O2和(Ti/Na+O2)–反应势能面的不同,结果显示体系带负电后反能垒降低了280.3 KJ/mol.更多还原

【Abstract】 Nano- semiconductor materials are an important member of the nanomaterial family. When the size of materials is reduced to a nano-scale, it will show a lot of marvellous physical and chemical properties. The main goals of nanomaterial research are to synthesize, design, and analyze the micro-structural materials with special functions. 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, the studies of clusters have got rapid development.In this thesis, we have investigated semiconductor silicon and germanium clusters using the genetic algorithm (GA) with TB (tight-binding) method and density functional theory (DFT) calculations. We try to determine the most stable geometries of Si and Ge clusters, and explore their growth patterns. The energies, HOMO-LUMO gaps, occupations on HOMO shells, densities of states, ionization potentials, IR vibrational spectra, fragmentation behaviors, and mobilities of cluster ions have been studied. On other hand, the reaction mechanisms of Ti, Na with O2 were investigated at the MP2/6-311+G(d) level. We try to understand the optimal reaction pathways of alkali and transitional metal atoms with O2, and the effect of a negative charge on the reaction barriers and pathways.The main results are listed as follows:(1) We performed an unbiased search for low-energy structures of medium-sized neutral Sin and Gen clusters (n = 25 ~ 33) using a genetic algorithm (GA) coupled with tight-binding (TB) interatomic potentials. The structural candidates obtained from our GA search were further optimized by first-principles calculations using density functional theory (DFT). Our approach reproduces well the lowest-energy structures of Sin and Gen clusters of n = 25 ~ 29 as compared to previous studies, showing the accuracy and reliability of our approach. In the present study, we pay more attention to determine low-lying isomers of Sin and Gen (n = 29 ~ 33) and their relative stabilities, and study the growth patterns of these clusters. The B3LYP calculations suggest that the growth pattern of Sin (n = 25 ~ 33) clusters undergoes a transition from prolate to cage at n = 31, while this transition appears at n = 26 from the PBE calculated results. In the size range of 25 ~ 33, the corresponding Gen clusters hold the prolate growth pattern. The changes of small cluster structures from the free standing states to the building blocks of large clusters, were also studied.(2) The lowest-energy structures of Ge2-Ge33 have been optimized using DFT method. The properties of the germanium clusters including binding energies, second differences in energy, HOMO-LOMO gaps, and especially fragmentation energies and fragmentation behaviors have been studied. Our calculation results show that Ge6, Ge7, and Ge10 species display large abundances in the fragmentation products. According to the large fragmentation energies of the small germanium clusters with n≤11 and the small fragmentation energies of the medium-sized clusters with 11 < n≤33, we can conclude that the germanium clusters with 11 < n≤33 can be easily dissociated into small stable germanium clusters. Such dissociations would occur successively until the sizes of fragmentation products are reduced to n≤11. The present calculated results and analyses are consistent with the experimental observations of photo-ionization mass spectra. (3) We have performed DFT calculations to study the structure and stability of the Si70 cluster. The results from the density functional theory (DFT) calculation with BLYP exchange-correlation functional suggest that a bulk-fragment Si70 isomer is the most stable structure, in contrast to the cage structure of Si60, showing that the transition from cage structure to bulk-like motif for silicon clusters would require only about 70 atoms. In addition, an endohedral fullerene of Si16@Si54 was found to be competitive to bulk-like Si70 if the PBE functional is used. The calculated density of states, IR vibration spectra, ionization potential (IP) and inverse motilities were also calculated and discussed.(4) The reaction mechanisms of Ti, Na with O2 were investigated at the MP2/6-311+G(d) level. Ti and Na atom were assumed to approach O2 in the manner of horizontally or vertically with respect to the O-O bond, respectively. The binding energy and the charge curves for each reaction pathway have been analyzed in detail, to validate the optimal reaction mode. The vertical pathway is shown to be more favorable than the horizontal one. Both neutral Ti/Na + O2 and anionic (Ti/Na + O2)– systems were considered. The reaction of metal atom (Ti and Na) with O2 is easier for negatively charged system (Ti/Na + O2)– than for the neutral Ti/Na+O2. Meanwhile, the calculated results show that Ti atom prefers to react with the singlet O2 in both neutral and negatively charged systems. Na is easy to react with the singlet O2 in the neutral system, but with triplet O2 in negatively charged system. The potential energy surfaces of Ti with O2 reaction were also calculated at CCSD(T)/6-311++G(3df)//MP2/6-311+G(d) for Ti + O2 and (Ti + O2)– systems.更多还原