【作者】 贯佳；

【导师】 黄旭日；

【作者基本信息】 吉林大学 ， 物理化学， 2013， 博士

【摘要】 随着石墨烯在实验上得以成功分离，基于碳的纳米材料家族，包括富勒烯、碳纳米管、石墨烯纳米带等，成为了化学、物理学、生物学、材料学等学科的实验与理论研究的重要交叉点，这其中，石墨烯与其一维同素异形体石墨烯纳米带无疑是当下最耀眼的成员。在日益进步的计算模拟方法和飞速提升的计算机性能的双重推动下，人们现在已经可以对更具实际意义的低维纳米材料的结构和物理化学性质在原子水平上进行更精确的设计和研究。本论文借助系统的密度泛函理论（DFT）计算，研究了不同的表面修饰手段下石墨烯纳米带及其无机类似物碳化硅纳米带的几何结构、电学性质、磁学性质和稳定性，并通过考察某些典型缺陷结构的存在对这些性质的影响，进一步提升相关纳米材料在未来多功能自旋纳米器件中的实际应用价值。首先，本文提出了一种能够简便而有效地调控锯齿型石墨烯纳米带的电学和磁学性质的新型非共价表面修饰方法。利用单个或多个C≡C桥连两条聚二炔长链而成离域性优良的π共轭骨架，改性的梯形聚二炔衍生物能借助π π相互作用稳定吸附到纳米带上，并通过长链上不同拉/推电子功能基团之间电荷转移诱导出的偶极场打破原本锯齿型石墨烯纳米带能带结构的能量简并边缘态，将其变成半金属或金属。逐步改变纳米带的宽度或聚二炔衍生物的桥链中C≡C的数目会影响诱导偶极场的作用，进而在体系中实现自旋无带隙半导体性半金属性金属性以及反铁磁态到铁磁态的丰富电学和磁学性质转变。甚至当锯齿型石墨烯纳米带的边缘发生57重构而变形时，仍然能在聚二炔衍生物的作用下观察到体系展现反铁磁态自旋无带隙半导体铁磁态半金属反铁磁态金属非磁态金属的多重性质变化。进一步，通过对锯齿型石墨烯纳米带的57重构边缘进行氢化，能够消除边缘重构对体系性质的影响，使其大致恢复到基于代表其中剩余未氢化部分的相应的窄的边缘光滑氢饱和锯齿型石墨烯纳米带的体系的性质，并针对一系列宽度的纳米带得到类似的自旋无带隙半导体行为半金属行为的电学性质转变。受启发于石墨烯长期积累的广泛研究和无机纳米材料发展的日新月异，本论文接下来主要考察了石墨烯纳米带的一类无机类似物碳化硅纳米带分别在典型的共价表面修饰手段氢化和典型的拓扑缺陷Stone-Wales（SW）缺陷的作用下其电学和磁学性质可能发生的变化，分别着重阐述了氢化的不同模式和比例以及SW缺陷的不同方向和位置对碳化硅纳米带的电学和磁学行为的影响。首先，计算结果表明，全氢化的碳化硅纳米带以其椅式构型较船式和马镫式构型更为稳定。无论边缘是扶手椅型还是锯齿型，全氢化的碳化硅纳米带均展现非磁性的半导体行为，带隙随宽度增加呈现降低趋势。随着氢化分别从纳米带的硅边、碳边和两边开始以及氢化比例的一步步增加，我们能够在锯齿型碳化硅纳米带中准确地实现一系列丰富的电学和磁学行为：半金属、自旋无带隙半导体、金属、宽带半导体及有磁性、无磁性，甚至有一些部分氢化的碳化硅纳米带结构能展现与其剩余未氢化的纯碳化硅纳米带部分几乎完全相同的性质，这能为实验上制造“窄”的碳化硅纳米带提供一些有益的思路。通过计算氢化的碳化硅纳米带的形成能和氢原子在碳化硅纳米带表面受到的束缚能可知，所有这些结构都具有很高的热力学稳定性，论证了它们能通过氢化纯的碳化硅纳米带的方式在实验上得以实现的可能性。众所周知，各种缺陷的产生是纳米材料在生长、分离和处理过程中不可避免的，而SW缺陷就是其中一种典型的拓扑缺陷，因为完美结构中的某个键发生90°旋转而产生。我们通过计算发现，当SW缺陷在碳化硅纳米带中（按照硅碳键相对于碳化硅纳米带周期性方向的取向不同，分别以SW-1和SW-2表示平行/垂直于和倾斜于其周期性方向的两种SW缺陷）形成时，无论缺陷方向如何，都会因为引入杂化态能带而显著降低扶手椅型碳化硅纳米带的带隙，但不影响其原有的非磁性半导体行为；与之类似，带有SW缺陷的锯齿型碳化硅纳米带依旧表现出能量简并的铁磁态和反铁磁态以及各自分别对应的金属性和半金属性，而在某些结构中，半金属行为能在铁磁态和反铁磁态同时观察到。这会有助于保护半金属性避免在外界微扰影响下减弱甚至消失的危险。计算结果同时表明，SW缺陷在碳化硅纳米带中的形成能具有方向和位置依赖性。无论边缘手性如何，SW缺陷位于纳米带中心时其形成能随宽度增加而增大；相比之下，位于锯齿型碳化硅纳米带边缘的SW缺陷其形成能总体较中心时降低，而SW-2总比SW-1在能量上更有利。通过考察SW缺陷形成的动态过程，我们发现，相比于已经在实验上观察到SW缺陷的石墨，存在于碳化硅纳米带中的SW缺陷具有明显更低的计算能垒，以及相当大的逆向能垒以稳定形成后的缺陷，这进一步论证了它们在碳化硅纳米带中存在的可能性。更多还原

【Abstract】 The family of carbon-based nanomaterials, including fullerene, carbon nanotubes,and graphene nanoribbons, has become the crucial intersection linkinginterpdisciplinary researches of chemistry, physics, biology, and material science fromboth the experimental and theoretical viewpoints ever since the successful fabricationof graphene in experiment. Amongst, graphene, along with its one-dimensionalallotrope graphene nanoribbons, is undoubtedly the brightest star instantly. Driven bythe more advanced computational methods and more upgraded computer capacity, thegeometric structures and physicochemical properties of more practicallow-dimensional nanostructures can be designed and studied at atomic level moreprecisely right now. With employing systematic density functional theory (DFT)computations, here, we have investigated the geometry, electronic and magneticfeatures, as well as the stabilities for graphene nanoribbons and the inorganicanalogue silicon carbide nanoribbons under different surface-modification strategies,where the effects of the formation of some typical defects on these performances havebeen explored, aiming at further facilitating the practical application potentials ofrelevant nanomaterials in the future multi-functional and spintronic nanodevices.Initially, we identified a new noncovalent surface-modification strategy toconveniently and effectively modulate the electronic and magnetic properties ofzigzag graphene nanoribbons (zGNRs). Taking advantage of the excellent delocalizedπ–conjugated backbone with single/multiple C≡C bonds bridging twopolydiacetylene (PDA) chains, the modified ladder-structure PDA derivatives canphysisorb stably on zGNRs via π π interactions, and more interestingly, break theenergetically degenerated edge states in the original band structures of zGNRs by thedipole field induced through the charge transfer between different acceptor/donor groups that decorate the PDA chains, rendering them half-metallic or metallic. Byaltering the width of zGNRs or the number of C≡C bond in the linking bridge ofPDA, the effect of the induced dipole field can be altered, leading to the abundantelectronic transition of spin gapless semiconductor (SGS) half-metal metal alongwith the magnetic conversion of antiferromagnetic ferromagnetic. Even with the57-reconstruction at the zGNR edges, multiple transformation of antiferromagneticSGS ferromagnetic half-metal antiferromagnetic metal nonmagnetic metal can alsobe observed with the PDA functionalization. Further, hydrogenation at the57-reconstructed edges of zGNRs can eliminate the effect of the edge-reconstructionand generally recover their properties to those based on the corresponding perfectnarrow H-terminated zGNRs representing the remaining pristine zGNRs inside,realizing similar SGS half-metal transition for zGNRs with a series of widths.Inspired by the accumulated extensive researches in graphene and rapidlyadvanced cognition in the inorganic nanomaterials, subsequently, we mainlyaddressed how the electronic and magnetic properties of silicon carbide nanoribbons(SiCNRs), one inorganic counterpart of graphene nanoribbons, may be affected underthe functionalization of hydrogenation one typical covalent surface-modificationstrategy and the formation of Stone-Wales (SW) defects one typical topologicaldefect, respectively, focusing on elaborating the effects of different hydrogenationpatterns/ratios and different defect orientations/sites on the relevant performances.Initially, the computed results reveal that the fully hydrogenated SiCNRs favor thechair configuration over boat and stirrup, where they are all nonmagneticsemiconductors with a descending trend of their band gaps as a function of the ribbonwidth, regardless of the edge chirality. With hydrogenation starting from the Si edge,C edge, and both edges of zSiCNRs, respectively, a series of substantial electronic andmagnetic behaviors can be precisely achieved as increasing the hydrogenation ratio:half-metal, SGS, metal, wide-band-gap semiconductor, magnetic, and nonmagneticfeatures. There even exist some structures among the partially hydrogenated zSiCNRsexhibiting almost the same electronic and magnetic behaviors as that of the remainingpristine zSiCNRs without hydrogenation, which may provide some useful insights into producing “narrow” SiCNRs in experiment. As inferred from the computedformation energies and binding energies per hydrogen atom to the SiCNRs, all ofthese hydrogenated SiCNRs present high thermodynamic stability, stronglysuggesting the feasibility to realize these structures by hydrogenating the pristineSiCNRs experimentally.It’s well-known that the formation of various defects is inevitable during thegrowth, fabrication, and processing of nanomaterials, amongst, SW defects is onetypical class of topological defects, created through rotating one bond of the pristinestructure by90°. It’s inferred from the computed results that when SW defects occurin SiCNRs (According to the different orientations of Si C bond relative to theperiodical direction of the nanoribbon, SW-1and SW-2are labeled to differentiate theSW defects with parallel/vertical and slanted orientations, respectively), the band gapsof aSiCNRs can be significantly reduced due to the inpurity band states introduced bythe defects, independent of the defect orientation, yet the original nonmagnetic andsemiconducting features are not affected; similarly, the SW-defective zSiCNRs stillexhibit energetically degenerated ferromagnetic and antiferromagnetic states with thecorresponding metallic and half-metallic behaviors, respectively, even half-metallicitymay be observed in both states, simultaneously. This can be advantageous for theprotection of the intriguing half-metallicity, inhibiting the drawback of beingvulnerable or even removed under small extra disturbances. Meanwhile, it’s foundthat the formation energies of SW defects in SiCNRs have defect orientation-andsite-dependency. Regardless of the edge chirality, the formation energies of SWdefects in the center of SiCNRs increase as widening the ribbon; comparatively, SWdefects at the edges of zSiCNRs generally present lower formation energies than thosein the center, yet the formation of SW-2is always energetically more favored thanSW-1. Moreover, we infer from the kinetic process of the formation of SW defects inSiCNRs that its barrier is remarkably lower than that computed for SW defects ingraphite, which has already been observed experimentally. Along with the largereverse energy barriers to stabilize the formed SW defects, these results can furthervalidate the possibility of the existence of SW defects in SiCNRs.更多还原