表面支撑的钽掺杂硅团簇几何和电子结构性质研究

【作者】 郑琳琳；

【导师】 任兆玉；

【作者基本信息】 西北大学 ， 光学， 2010， 硕士

【摘要】 团簇是介于微观和宏观之间的一物质结构新层次,由于量子尺寸效应,团簇的稳定性、磁学和光学等常会表现出很多新颖的性质。硅作为微电子领域一种重要的半导体材料,其团簇形式已被众多的理论和实验所研究,纯的硅团簇不像炭的团簇那样稳定。通过掺杂适当的过渡金属原子,不但有效改善了纯的硅团簇的稳定性,而且大大改善了硅团簇的电子特性。之前对过渡金属硅团簇的理论研究多集中于对自由团簇的研究,本文考虑到实验中的具体环境(如：衬底),建立了一系列graphene表面支撑的TaSin(n=1-3,12)团簇的计算模型,并采用密度泛函方法对该体系的几何和电子结构进行计算研究。另外,论文最后报道了d电子数目互补的过渡金属Irn和Tan(n=4-10,12,13)自由团簇的对比研究。第三章主要运用MedeA-VASP软件中的PAW密度泛函理论方法研究了graphene表面支撑的TaSin(n=1-3,12)团簇的几何稳定性、电子转移特性、差分电荷密度。结果表明,TaSin (n=1,2,3)团簇最稳定的吸附位置都是Ta原子在芯(H)位上,而TaSi12只能吸附在顶(T)位上。前者的Ta—-Si键长以及它们距graphene表面的距离,发生了较大的变化,而TaSi12的几何结构几乎没有变化。这是因为Si原子数目较小的时候,如n=1或2时,团簇中的Ta原子与graphene表面的C原子相互作用比较强,因而团簇与表面的相互作用大；随着Si原子数目增大,Ta原子慢慢被Si原子团簇包围,减弱了与graphene表血的相互作用,当n=12时,TaSi12中Ta原子已经被Si原子完全包裹在中心,因此团簇几乎和表面之间没有相互作用。从吸附能的数值上看,平均吸附能随着Si原子数目增加而减小。TaSi和TaSi2在graphene表而是化学吸附,TaSi3和TaSi12则是物理吸附。在吸附过程中,由于过渡金属钽与石墨烯之间存在电荷转移,这就导致了石墨烯电子结构将被掺杂,随着团簇尺寸的不同,这种掺杂方式也不同,当团簇尺寸较小时,这种掺杂表现出n型掺杂,而团簇较大尺寸时,则为p型掺杂。第四章主要运用ADF软件中考虑相对论的密度泛函方法对同处于第六周期、但d电子数目互补的过渡金属Irn和Tan(n=4-10,12,13)团簇的的几何结构稳定性,电子结构和磁性等性质。Irn团簇的基态结构主要以非紧凑的棱柱、简立方等中空结构为增长方式,而Ta。团簇的基态结构则按紧凑密堆积结构为增长方式。Irn和Ta。的平均结合能都表现出尺寸效应,随尺寸增大,团簇的结合能也不断增大,但增加方式有所不同,Irn团簇的结合能曲线表现出微弱的奇偶振荡,而Tan团簇的增长曲线比较平滑,总体看来,Tan团簇的平均结合能远大于Irn团簇的。Irn(n=4-10)的能量二阶差分,最低未占据轨道和最高占据轨道之间的能隙(HOMO-LUMO gap)及原子平均磁矩都随团簇尺寸表现出比较明显的奇偶振荡,而Tan(n=4-10)的这些参量曲线振荡表现较弱, Irn的偶数团簇相对稳定,而Tan的奇数团簇更稳定。另外,除Ta4团簇LUMO-HOMO能隙较大外,其余的Ir。和Tan团簇的能隙都很小,说明它们都有较强的活性；Irn(n=4-7)的原子平均磁矩明显高于相应尺寸的Tan团簇,n>8的Irn团簇的平均磁矩和相应尺寸的Tan团簇都相对较低。更多还原

【Abstract】 Nanocluster is a unique mesoscope structure between micro and macro-scaled particles. With the quantum size effect of the clusters, some novel phenomena related with the geometrical, electrical, magnetic and optical properties appear. Silicon clusters have been investigated extensively in experimental and theoretical reports as a significant semi-conducter material. Since the pure silicon clusters are geometrically unstable, dopping appropriate transition metals (TM) not only can effectively stabilize the clusters, also improve the electronic properties of them. The previous researches mainly focused on free TM adulterating silicon clusters (TM@Sin). Considering the experimently specific condition (such as:the substrate), this paper have constructed a series of computational models of TM@Sin clusters supported on graphene and calculated them by density functional theory (DFT). Additionally, the properties of Irn and Tan(n=4-10.12,13) clusters are contrastively studied for their complementary d electrons.The third chapter mainly studied the geometrical and electrical properties of TaSin(n=1-3,12) supported on graphene by PAW density functional theory(DFT) in MedeA-VASP software package, including the structural stability, charge transfer property as well as difference charge density. It is showed that Ta atom prefers locating over the hollow site of graphene for TaSin(n=1-3) clusters with obvious shifts of the Ta-Si bonds and binding distances between the clusters and subetrate. However, TaSi12 only can be physically adsorbed above the top sits of grapheme with almost original structrue. It is rationalized when considering the interaction between Ta and C is gradually weakened with the increasing of Si atoms which surrounded the Ta. The interaction is comparatively strong between TaSin (n=1-3) and grapheme. While, Ta atom nearly interacts with C atom until Si atoms increases to 12 since Ta atom is capsulated in the center of 12 Si atoms. From the point of adsorption energy (Ea), the average Ea decreased with the nuber of silicon atoms increasing, leading to chemisorption between TaSin(n=1,2) and graphene and physisorption between TaSin(n=3,12) and grapheme.During the adsorption, charges transfer appears between transitional metal and grapheme, leading to a p or n doping in the electronic structure of grapheme. Graphene will be n doped for TaSin (n=1,2) adsorption, on the contrary, p doped for TaSin (n=3,12).The geometrical and electrical structures of Irn and Tan(n=4-10,12 and 13) clusters, both period 6 elements with complementary d electrons, are contrastively investigated by relativistic density functional method in the fourth chapter. The ground state structures of Irn favor non-compact structures growth pattern like prism or simple cubic, while Tan clusters are more supportive of compact structures. The average binding energies of Irn and Tan (n=4-10) clusters show size effect. The curve of Irn displays a little oscillates, while that of Tan ascends smoothly. The second-order difference of energy, the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and the atomic average magnetic moments for the lowest-energy Irn (n=4-10) geometries all show obvious odd-even alternative behaviors which is weak for Tan (n=4-10) clusters. And Irn clusters with even number are more stable than those with odd number, but it is converse for Tan. In addition, the gaps between HOMO and LUMO are generally smaller than 0.3 eV except Ta4, indicating a strong chemical activity. The atomic average magnetic moments for lrn(n=4-7) are far higher than those of Tan, while they are very close and comparatively low when n>8.更多还原