【作者】 李伟峰；

【导师】 赵明文；

【作者基本信息】 山东大学 ， 凝聚态物理， 2008， 硕士

【摘要】 本论文采用密度泛函理论计算,研究了低维碳纳米材料(石墨和碳纳米管)的电子结构调控、磁性机理以及制造磁性材料的可能性。内容包括碳离子注入石墨后产生点缺陷,单层石墨片(graphene)表面化学修饰,碳纳米管表面化学修饰,碳纳米管管内金属银原子掺杂等方面。重点研究了各种功能化修饰对体系电子结构的影响,通过自旋极化的计算分析了各种情况下体系产生局域磁矩的可能性,提出了制造基于石墨和碳纳米管的磁性纳米材料和电子输运器件的可能性和方法。本论文主要包括以下几个方面的内容:一、系统分析了碳离子注入石墨的电子性质和诱发磁性产生的原因。系统分析了离子注入对石墨材料的影响,注入过程中可能产生的各种缺陷的结构和稳定性。通过我们的计算,发现各种缺陷中只有空位缺陷的局域磁矩能够有效的铁磁耦合,对宏观磁性起作用;笼形缺陷在特定密度下能够产生宏观磁矩。而其余各种缺陷如间隙Spiro缺陷、间隙原子-空位复合对、Stone-wale缺陷等虽然能够大量存在,但是都不具有局域磁矩。二、讨论了通过单层石墨(graphene)表面单价修饰和双价修饰的方法制造磁性材料的可能性。我们发现单纯依靠单价修饰来产生磁性需要极高的修饰浓度并且修饰位置必须统一在相同石墨次晶格的碳原子上,技术上实现几乎不可能。而双价修饰的磁性对修饰浓度要求不高,并且修饰位也没有限制。但是,局域磁矩依赖于双价修饰基上未成对的p电子,这些p电子往往具有很强的化学不稳定性,在空气中很容易被饱和,而失去磁性。我们指出,如果实验上能够很好的保护修饰基不被饱和,双价修饰的石墨将有可能在纳米材料和自旋输运材料上有潜在的应用价值。三、研究了在单壁碳纳米管内制备纳米级超细导线的可能性。因为从实验上发现,拉成纳米量级粗细的金属导线,在空气中很容易被氧化,这就提出了如何对这些超细的金属导线进行保护的问题。而单壁碳纳米管具有优良的力学特性和独特的中空结构,自然成为制造纳米导线的首选。我们系统的研究了在不同粗细的金属性和非金属性单壁碳纳米管内部掺杂不同浓度银原子的稳定结构和电学性质,发现只要掺杂银原子的浓度达到一定比例,整个的复合体系具有很好的电导性和热稳定性,从而使制备超细金属导线成为可能。我们还从物理和化学两方面分析了金属原子进入碳纳米管内部的可能性并提出了实验上实现的方法。四、研究了单壁碳纳米管外壁氢原子修饰的电子结构和磁性。发现外壁氢原子修饰后,导致碳纳米管外壁相关碳原子sp~3杂化,使得金属性碳纳米管出现带隙,并在费米能级附近产生非常定域的杂质能带。在一定程度上,由于带隙的出现碳纳米管丧失导电性。如果氢原子修饰的浓度足够大,并且氢原子的修饰位置统一在相同次晶格的碳原子上时,由于定域电子态之间的直接耦合将会发生自旋极化而具有磁性,这为制备最小的纳米磁性材料提供了一种可行的手段。五、研究了单壁碳纳米管((5,5)和(8,0))外壁存在含氮本征缺陷时的电子结构和磁性,缺陷结构包括双价氮原子外壁吸附、氮原子替位缺陷、氨基吸附以及外壁含氮原子的空位缺陷。碳纳米管外壁双价氮原子的存在往往会产生局域磁矩,和石墨片表面的双键修饰诱发磁性一样具有对缺陷浓度要求不高、没有修饰位限制的优越性;并且修饰后的碳纳米管仍然呈现金属性,从而使得单壁碳纳米管在制造纳米导线和自旋输运器件有潜在的应用价值。在我们的计算中,含氮空位缺陷只能在(8,0)管(半导体性)中产生局域磁矩,而(5,5)管含氮空位缺陷的磁矩为0。但是,当悬挂氮原子上未配对的p电子被氢饱和之后,(8,0)管中的磁性随之消失。更多还原

【Abstract】 In this thesis, we performed first-principles calculations based on density functional theory (DFT) by utilizing the SIESTA and VASP codes, focusing on functional modification and manufacturing the electronic properties of low-dimensional carbon nano-sized materials (graphite and carbon nanotubes). The systems under study are: point defects produced by carbon ion implantation of graphite, graphite monolayer films (graphene) surface chemical modification, surface chemical modification of carbon nanotubes, silver-containing carbon nanotubes. We have analyzed the various features of the modification of electronic structures of these systems. By performing spin polarized calculations, we discussed the magnetic mechanisms in these systems. Finally, we made conclusions of the possibility and ways of manufacturing magnetic nanomaterials, electron transport devices based on graphite and carbon nanotubes.This thesis covers the following aspects:(1) We have studied the magnetic mechanisms of graphite induced by ion implantation. We analyzed the possible point defects produced in the implantation process and related structure stability. Point defects like interstitials, vacancies, interstitial-vacancy (I-V) pair and Stone-Wales defect have been systematically studied. It is amazing that only the vacancies contribute to the magnetization of the graphite. SRIM03 simulations indicate that heavy ions like C, N can cause more vacancies than light ions like H, and thus give a more significant magnetic signal. We hope this phenomenon is helpful for the researchers to prepare magnetic materials.(2) The possibility of tuning the magnetic properties of graphene layer by surface monovalent- and divalent- modifications was then studied. It was found that the monovalent addations induced magnetism depends on two aspects: (i) high modification concentration; (ii) topological distributions of the addations. As contrast, divalent addations can induce magnetism even at low concentration independent of addation distribution. The advantages of divalent additions make it a potentional candidate for the future magnetic materials and spintronic devices.(3) The atomic and electronic structures of silver-filled (n, 0) single-walled carbon nanotubes (Ag@SWCNTs) for n - 6, 7, 8, and 10 were explored by performing first-principle calculations to search for an efficient way to tune the electronic properties of SWNTs. We found that silver atoms encapsulated in SWCNTs self-aggregate to form ultrathin nanowires, of which the atomic arrangement depends on the diameter of the SWCNTs as well as the silver content. The electronic structures of the Ag@SWCNTs can be changed from semiconducting to metallic by controlling the silver content. The hybridization of electronic states and the charge transfer between the encapsulated silver nanowires (AgNWs) and the SWCNTs were also addressed.(4) The magnetic properties of hydrogenated single walled carbon nanotubes (SWNTs) were studied at different hydrogen concentrations. The hydrogen adsorption on SWNTs generates quasi- localized states near the Fermi level and opens substantial gaps. The magnetic properties of the compounds depend on the hydrogen concentration. At high hydrogen concentration, the localized states split into spin-up and spin-down branches locating above and below the Fermi level, making the systems spontaneous magnetic. However, at low hydrogen concentration, the spin-up and spin-down branches of the localized states are energetically degenerated and the systems are nonmagnetic. This result is understandable from the point of view of the direct interaction between the local states at adjacent adsorption sites.(5) The electronic structures of nitrogen-doped single-walled carbon nanotubes ((5,5) and (8,0) tubes) were calculated. Various forms of nitrogen-doped defects, including divalent nitrogen adatoms, nitrogen substitutions, amido adsorptions and vacancy defects on tube walls were systematically considered. Divalent nitrogen addations can induce magnetism even at low concentration on these two kinds of CNT, independent of addition distribution. The spin polarized (5,5) CNTs with nitrogen adatoms have good electron transport abilities, thus would have potential applications in spin transport devices. Vacancies, from our calculations, can trigger magnetism only in (8, 0) carbon nanotube (semiconductor). If the unpaired p electrons localized on the dangling C/N atoms are saturated by hydrogen atoms, the magnetic moment will vanish.更多还原