石墨烯及其掺杂体系电子结构性质的理论研究

【作者】 杜恭贺；

【导师】 任兆玉；

【作者基本信息】 西北大学 ， 凝聚态物理， 2010， 博士

【摘要】 石墨烯是一种新型二维平面碳纳米材料,其特殊的单原子层结构决定了它具有丰富而新奇的性质。过去几年中,石墨烯已经成为了备受瞩目的国际前沿和热点,在众多领域有着潜在的巨大应用价值。本文利用基于第一性原理的密度泛函理论,对石墨烯及其掺杂体系电子结构性质进行理论模拟和计算,为其在纳米电子器件,储能,新型传感器等领域的应用提供理论指导。对一定尺寸的石墨烯研究发现,随着尺寸的增大,能隙逐渐减小,材料由半导体性质逐渐向金属性质过渡,电荷密度主要集中在边缘部分,特别是集中于zigzag边缘。设计了一系列石墨烯量子点并对其进行了理论模拟计算。研究发现,当石墨烯量子点包含六个苯环且呈正三角形结构排列时,其同时具有磁性和光致发光现象。用含时密度泛函理论对其进行激发态的计算,得到了该量子点的电子吸收光谱,找到了和实验观测到的波长相一致的吸收光谱,提出石墨烯量子点的光致发光源于zigzag边缘效应。对其磁性分析发现,由于三重态比单重态更稳定,就可以提供具有未成对电子的顺磁中心而使其具有磁性,因而要想使石墨烯具有磁性,就需要有一定的孤对电子。研究H2在石墨烯表面的吸附发现,H2在本征的单层石墨烯表面是很弱的物理吸附作用,H2和石墨烯表面的距离较远,吸附能也很小。当有Al或Li+掺杂后,吸附能大大增加,体系的几何构型和电子结构等都有了显著的改变,H2与掺杂的石墨烯之间具有较强的物理吸附,从而大大增强了石墨烯基探测器对H2的灵敏度。对石墨烯储锂机理研究发现,锂离子吸附在石墨烯碳环心位时最稳定,其吸附能相比碳纳米管大大增加,通过对Li+-石墨烯体系的电子态密度分析发现,有一部分电子从石墨烯转移到锂离子上,增强了体系的导电性质。锂离子要想穿越单层石墨烯势垒,需要1.726eV的能量,而穿越有空穴的石墨烯势垒则需要更大的能量。研究发现,石墨烯作为锂离子电池的新型负极材料,有着很大的优越性和应用性。并五苯分子也可以看作是zigzag边缘的石墨烯量子点,因为其高的电荷迁移率而成为一种倍受重视的有机半导体材料,为了增加其在有机溶剂中的溶解性同时又保持高的电荷迁移率,我们在并五苯分子中引入极性取代基F,Cl,Br。采用密度泛函理论研究了它们的分子构型,偶极矩,前线分子轨道,电离势和亲和能以及重组能。研究发现,这些卤代并五苯和并五苯一样具有很低的重组能(＜0.2eV),因而具有很高的电荷迁移率。更为重要的是,当取代基位于2位置或者2,9位置时卤代并五苯是极性分子,因而易溶于有机溶剂而便于器件的加工和性能的稳定(并五苯不溶于有机溶剂),这表明它们是一种优于并五苯的新型有机半导体材料。更多还原

【Abstract】 Graphene, a single layer of carbons, is found to exist as a free-standing form and exhibits many unusual and intriguing physical, chemical and mechanical properties. In the past few years, graphene which has caused researchers’ extensive attention is becoming the leading edge and hotspot in various relative fields. In this paper, electronic structures of intrinsic and doped graphene have been simulated and computed based on the first principles of density functional theory(DFT), which could provide theoretical guidance for their applications in nano-electronic devices, energy storage and novel gas sensors.It indicated that finite size graphene had a band gap which decreased on increasing the dimension of the sheet to near metallic situation through compairing with three dimension graphene sheets by DFT. In our investigation we also observed high electron density along edges especially at the zigzag edges. Furthermore, we deviced a serials of graphene dots. By using DFT we found that it had both luminescent and ferromagnetic properties when graphene quantum dot was consists of six equilateral triangle structures of the benzene rings. We also obtained the electronic absorption spectra of the quantum dot by time-dependent density functional theory(TD-DFT) and our results were well agreement with experimental observations. We proposed that strong Luminescence of graphene quantum dot from the zigzag edge effect. In addition, the energy of the triplet state was lower than the singlet one which could be explained as the spin containing units with unpaired electrons for synthesis and design of the molecular magnetic materials.The adsorptions of H2 molecules on the intrinsic and doped graphenes were also investigated. It is found that H2 molecules are only weakly adsorbed onto the intrinsic graphene with small binding energy value and large distance between the H2 molecules and graphene. The electronic structure and electrical conductivity of the intrinsic graphene have a limited change caused by the adsorption of H2 molecules. However, the H2 molecule has strong interaction with the Al or Li+ doped graphene, forming strong physisoption that introduces a large amount of shallow acceptor states into the system. In this case, the remarkable variation of the electrical conductivity is induced by the H2 adsorption, possessing an excellent characteristic of high sensitivity for H2 gas detection.The structures and electronic states of sodium ion (Li+) trapped on graphene have been investigated by means of density functional theory (DFT) calculation to elucidate the nature of interaction between Li+ and graphenes. The VASP calculation showed that the Li+ ion is stabilized in hexagonal site and the height of Li+ ion at the energy minimum is 1.8A., Li+ ion is preferentially bound to a hexagonal site of the graphene, In addition, the DOS indicated that there were some charges transferred from graphene surface to Li ion, and we also found that there was a high potential barrier about 1.726eV for Li ion cross the carbon ring, and the potential barrie increased sharply as the increase of grapheme layers. All results showed that graphene as a new type of lithium-ion battery anode material, had great advantages and applicability.Pentacene(C22H12) can also be known as a graphene dot.We introduce polar substituents to pentacene such as F, Cl, Br, to enhance their dissolubility in common organic solvents while keeping the high charge-carrier mobilities of pentacene. Geometric structures, dipole moments, frontier molecule orbits, ionization potentials and electron affinities, as well as reorganization energies of those molecules, and of pentacene as a comparision, have been successively calculated by the density functional theory. The results indicate that halopentacenes have rather small reorganization energies (<0.2eV), and when the substituents in the 2 position or 2,9 positions they are polarity molecules, so we conjecture they can easily dissolve in common organic solvents, and are promising candidates for organic semiconductors.更多还原