【作者】 罗强；

【导师】 王新强；

【作者基本信息】 重庆大学 ， 凝聚态物理， 2005， 硕士

【摘要】 半导体团簇由于其独特的物理性质和重要的应用前景,在过去十年里引起了人们高度的重视。近年来,通过胶体化学已成功合成出高质量的II～VI 簇半导体量子点（CdSe 及HgTe 等）,并应用于可调谐发光器件上,进一步激发起人们对量子点及其“裸”的同类—团簇的研究兴趣。已有的理论研究主要采用的是半经验的方法,用第一性原理研究由于计算量的限制相对较少,因此,进一步开展这方面的研究对我们充分认识量子点及团簇的性质是十分必要的。本文采用基于密度泛函理论的第一性原理方法,在GGA 下,计算了团簇（CdSe）_n 和（HgTe）_n n=1 到7 的基态几何结构、态密度、结合能及能隙等。为了对比分析,同时也采用LDA 进行计算。我们得到以下的主要结论: （1）对于团簇（CdSe）_n（n=1-7）,我们的计算结果和M.C.Troparevsky等人采用LDA 研究的结果吻合的非常好。这说明我们的计算方案是可行的。（2）团簇（CdSe）_n（n=1-6）基态几何结构出现高对称性排列,而对于n=7 则没有那么规则了;团簇（HgTe）_n 和团簇（CdSe）_n 具有类似的结构对称性,但其键长和键角还是呈现出一定的差异。这不仅表明了这两种团簇基态几何结构跟其对称性相关,而且也表明了导致材料性质（CdSe 体材料为闪锌矿,HgTe 为纤锌矿）差异的原因。（3）团簇（CdSe）₄ 在基态附近存在三种不同对称性的几何结构（4A、4B 和4C 结构）,4C 结构最稳定,4A 和4B 结构的结合能比4C 结构的分别小7.10%和3.54%;团簇（CdSe）₅ 在基态附近存在两种不同对称性的几何结构（5A、5B 结构）,5A 较稳定,5B 结构的结合能比5A 的小1.27%;相应的（HgTe）₄ 在基态附近也存在三种几何结构,也是4C 结构最稳定,4A和4B结构的结合能比4C结构的分别小3.95%和15.26%;但团簇（HgTe）₅ 的基态附近的两种结构中,则是5B 结构较稳定,5A 结构的结合能比5B 结构的小6.19%;（CdSe）₇ 和（HgTe）₇ 在基态附近也存在两种不同对称性的几何结构（7A、7B 结构）,均是7B 较稳定。这不仅表明团簇结合能跟其结构对称性相关,而且也表明了导致材料性质差异的原因。（4）团簇（CdSe）_n（n=1-7）能隙,在n=4 时出现了极大值,在n=5时出现了极小值。团簇（HgTe）_n（n=1-7）的能隙随尺寸的变化规律跟团簇更多还原

【Abstract】 Semiconductor cluster have attracted much attention over the last decade due to their unique physical properties and potential use in a wide range of applications. Recently, high-quality II～VI semiconductor （CdSe and HgTe） quantum dots（QDs） synthesized by colloidal chemistry have emerged as attractive candidates for use in light emitting devices with tunable emission. In many previous theoretical studies, the semiempirical methods have been applied to investigate properties QDs. However, there have been much fewer first principles studies due to the computational demands. Therefore, it is significant to further study the structures and electronic properties of QDs and clusters by using first principles. In this paper, using the first principles method which base on density function theory（DFT）, we calculated the structures and electronic properties of ground states of （CdSe）_n and （HgTe）_n（n=1⁷） clusters with the generalized gradient approximation（GGA）. To compare the results, we also carried out the calculations within the local density approximation （LDA）. We come to the following conclusions: （1） Our results of （CdSe）_n（n=1-7） are in good agreement with those of M.C.Troparevsky et al.. This shows that our calculation scheme is reasonable. （2） The results show that high symmetry arrangements occur in geometrical structures of the ground state for the （CdSe）_n （n=1-6） clusters. However, for n=7, the symmetry is much lower. （HgTe）_n and （CdSe）_n exhibit similar structural symmetries. However, there are small discrepancies in both the length of bonds and bond angles between them, which lead to the difference of the material （i.e., CdSe crystal structure is zincblende and HgTe is wurtzite）. （3） There are three different structures near the ground state of （CdSe）₄ （4A, 4B and 4C）. Structure 4C is more stable, and the binding energy of which is 7.10% higher than that of 4A and 3.54% higher than that of 4B. There are two different structures around the ground state of （CdSe）₅（5A and 5B）. Structure 5A is more stabe, and the binding energy of which is 1.27% higher than that of 5B. Accordingly, there are three different structures near the ground state of （HgTe）4 （4A, 4B and 4C）, and structure 4C is also more stable, while the binding energy of which is 3.95% higher than that of 4A and 15.26% higher than that of 4B. For （HgTe）5 cluster, structure 5B is more stable, and the binding energy of which is 6.19% higher than that of 5A. The relation between （CdSe）7 and （HgTe）7 is similar with （CdSe）5 and （HgTe）5. There are two different structures around the ground state both （CdSe）7 and （HgTe）7 （7A and 7B）, and structure 7B is more stable . Therefor, we come to conclusion that the binding energy of clusters depends not only on the symmetry, but also lead to the difference of the material. （4） （CdSe）n（n=1-7） clusters with n=4 have substantially higher energy gaps and those with n=5 have lower ones. The variation in the energy gaps as a function of cluster size for （HgTe）n clusters exhibits different trends with that for （CdSe）n clusters besides that peak occurs with n=4 and without lower ones in the size which we study on, which indicates that different quantum-size effects occur between the two different material clusters. As for correlation between binding energy and energy gaps, results within GGA exhibit more clearly than those within LDA. We have investigated （CdSe）n and （HgTe）n clusters with n ranging just from 1 to 7 due to the computational demands. In order to gain full information of II-VI semiconductor （semimetal）, it is necessary to investigate larger size and the other semiconductor （semimetal） clusters such as （CdTe）n and （HgSe）n, which is what we need to do for further investigation.更多还原