【作者】 巩文斌；

【导师】 朱志远；

【作者基本信息】 中国科学院研究生院（上海应用物理研究所） ， 核技术及应用， 2014， 博士

【摘要】 十世纪九十年代以来,低维碳纳米结构材料的研究一直是物理、化学、材料学等多学科交叉的热点之一。碳原子以sp2杂化形式构成六角形蜂窝状结构,并有序排列组成包括一维碳纳米管和二维石墨烯在内的低维碳纳米结构材料。由于其独特的结构特征,这些低维度碳纳米结构材料具有优异的性质：例如高弹性模量,高机械强度,高热导率,高载流子迁移率以及大比表面积等。低维度碳纳米材料目前被认为是后硅时代的重要替代材料,其与金属构成的复合纳米结构不但拥有原先单体各自的物理化学特性,同时还具有特殊的增效协同作用,从而进一步拓宽了碳纳米材料及其复合体系的应用范围,有望在电学、光电、力学、传感、能源、催化等诸多领域展现其迷人的色彩。本论文基于第一性原理以及分子动力学方法,对碳/金属复合纳米结构的空间构型、电子掺杂特性、自旋轨道耦合机理以及热输运过程进行了研究,在此基础上对调节其结构性质及利用复合结构收集核衰变能应用方面也进行了探索性研究。基于密度泛函理论,本论文进行了石墨烯/金属复合纳米结构的基本性质以及张力作用对界面吸附和电性掺杂调控的研究。石墨烯/金属接触面的物理性吸附不会破坏石墨烯狄拉克点附近独特的线性关系,但却面临巨大的应用挑战,即石墨烯和金属之间非常弱的界面结合(～30meV)使其在常温条件下会出现解离问题。计算发现施加适当的界面张力,可以明显减小石墨烯和金属的面问距并极大地增强界面结合(最大增强315%),从而简单有效地克服界面解离问题；同时张力作用可以在102量级上调制石墨烯中的载流子浓度。研究表明该界面变化的物理机制为增强的pd相互作用和张力所致的界面偶极子相互作用。采用全势线性缀加平面波方法,本论文研究了石墨烯/金属复合纳米结构的自旋轨道耦合效应及其增强机理,探讨了原子序数和pd相互作用对自旋轨道耦合劈裂的影响。研究中,我们采用了低原子序数的金属铜与石墨烯构成复合纳米结构,重点研究了金属的存在对石墨烯狄拉克点附近的自旋轨道耦合效应的影响。计算结果表明,复合结构中金属铜使石墨烯π态产生了-2meV的自旋轨道耦合劈裂。自旋轨道耦合劈裂在不改变金属类型而只增加界面3%张力的条件下,最大值出乎意料地增加到-83meV。对石墨烯π态成分的分析表明此张力调控下的巨大自旋轨道耦合劈裂是碳原子pz态与铜dz2态混合所致。结果首次证明了在低原子序数的3d金属上,石墨烯可以在狄拉克点实现巨大的旋轨劈裂,对金属衬底上石墨烯旋轨耦合效应增强机理的认识有进一步推进。基于含时密度泛函理论,本论文对石墨烯/金属复合纳米结构用于收集α核衰变能进行了模拟研究,重点关注电子在辐射受激条件下的转移和重分布过程,该研究的目的是了解碳/金属复合纳米结构能量转化的基本物理机制,为利用碳/金属复合纳米结构实现衰变能向电能的直接转化提供理论基础。研究结果表明,衰变α粒子入射石墨烯/金属复合结构后,大量衰变粒子能量被复合结构吸收,其中绝大部分的能量沉积由金属贡献。结果表明,复合结构吸收衰变粒子能量后金属层的受激电荷能够获得足够高的能量克服界面势垒而积聚于石墨烯层；石墨烯层上的受激电子能量由于电声相互作用受到抑制,使得其能量传递主要在层内以电子传递为主运用分子动力学方法,通过精确控制碳纳米管中晶格的振动模式,本论文还研究了碳纳米管中的热脉冲的传输过程和机理。结果表明不同振动模式激发的热脉冲在碳纳米中的传播以不同种波包形式进行,传播的波包构型强烈依赖于初始振动模式。研究还表明传播速度最大的纵声学波具有孤子特定,其传播过程对空位缺陷具有较强的稳定性。更多还原

【Abstract】 Since the1990s, the low-dimensional carbon nanostructures have alreadydrawn quite a lot of attentions in physics, chemistry, materials science andother disciplines. Carbon atoms are densely packed in a regular sp2-bondedhexagonal pattern and form the low-dimensional carbon nanostructures includ-ing the1-dimensional carbon nanotube and2-dimensional graphene. Due to theunique structure, the low-dimensional carbon nanostructures possess outstand-ing properties, such as high tensile strength and elastic modulus, high thermalconductivity, high electronic mobility and high specific surface area. Up to thepresent, the low-dimensional carbon nanostructures have been considered as oneof the most important materials in the post-silicon era. The combination ofgraphene and metals produce nanocomposites, which possess not only both func-tions of graphene and metals, but also some extraordinary synergetic efects. Thenanocomposites spread the applications of graphene in quite a lot of fields, in-cluding electronics, optics, mechanics, catalysis and energy. In this dissertation,we focus our attentions on the theoretical studies on carbon-based nanocom-posites. Based on the density functional theory and molecular dynamics, weinvestigated the structure configuration, electronic doping, spin-orbit coupling ofthe nanocomposites, as well as the modulations of the properties. In addition,the application of graphene-metal nanocomposites in harvesting nuclear energywas also studied.Based on the density functional theory, the properties of graphene/metalinterface and the influences of strain on the interface were studied. It is foundthat the physical adsorption of graphene on metal does not sacrifice graphene’sunique linear dispersion around the Dirac point. However, a huge challenge inthe potential applications is the weak binding (～30meV) at the contacts. Arealizable tensile strain is found to be very efective in enhancing the interfacebinding as well as shifting the Fermi level. Particularly, an enhancement ofthe binding energy up to315%can be achieved because of the dipole-dipole interaction.Based on the full potential linearized augmented plane wave method, thespin-orbit splitting around the Dirac point and its enhancement were studied.We mainly focus our attentions on the roles of the substrate’s atomic numberand the p d interaction between C and metals. It is found that the presenceof light metal substrate Cu generates a small spin-orbit splitting about2meVaround the Dirac point. When a tensile strain of about3%is applied to theinterface, the spin-orbit splitting is greatly enhanced to a maximum value of83meV. The analysis of the compositions of the π-derived band suggests that the Cpzand Cu dz2interaction is responsible for the induced giant spin-orbit splitting.The results confirm, for the first time, that a sufcient spin-orbit splitting canbe achieved in graphene on substrates with small atomic numbers.Using time-dependent density functional theory, the process and mechanismof the conversion from radiation energy into electric power at graphene/metalinterface were studied. It is found that the energetic α particle loses its kineticenergy when it passes through the graphene/metal interface. Most of the αparticle’s energy is absorbed by the metal atoms, resulting in excited electrons.The excited electrons have high enough energy to overcome the potential barrierbetween graphene and metal, and then accumulate at the graphene layer. Dueto the bottleneck that limits electronic energy redistribution into the lattice, theexcited electrons maintain their energies during transport in graphene.The propagation of heat pulses along single-wall carbon nanotube (SWCNT)is also studied by using molecular dynamics. Controlled heat pulses were usedin this work to excite heat wave packets which propagate along SWCNT. It isfound that the propagation of excited wave packets significantly depend on theinitial components of the heat pulses. The excited leading wave packets or heatsignals keep their shapes and amplitudes during propagation and have robustnessagainst vacancy defects, which indicates the stability of the heat wave packetsduring the propagation.更多还原