摘  要

石墨烯作为低维材料表现出了卓越的性能，科研人员一直希望能充分利用其稳定、低密度、超高强度的特性。当我们用这类碳结构来构筑宏观三维材料的骨架或增强组分时，维持这类基本单位结构高强度的共价键无法得到保留：结构之间，或结构与基体之间的结合通常被较弱的相互作用取代，如范德华尔斯作用，这样一来，我们实际得到的宏观三维材料的强度比其碳构筑单元的强度低2~3数量级。因此，由低维向三维尺度扩展时如何完美的继承低维碳材料的性能对碳材料的研究就是一项巨大的挑战。同时石墨烯制备过程中缺陷是不可避免的，不管是晶界缺陷还是Stone-Wales缺陷均是以5-7环缺陷的形式存在，而缺陷的存在对石墨烯的面内相互作用和面外表界面作用均会产生显著的影响。

该论文通过密度泛函理论给出了稳定三维碳蜂窝结构的微观构型，对其力学性能分析发现此结构完美的继承了石墨烯的优良性能，结合连续介质力学推导对其非线性力学响应和强各向异性作出了理论解释，此材料属于超轻超高比强度材料。在对其电学性能的计算中，我们得到扶椅型碳蜂窝结构当壁长参数N=3p+1时表现出了金属性，壁长参数N=3p 和N=3p+2时表现出了半导体性，而锯齿型和杂交型碳蜂窝结构均表现出了金属性。

关于石墨烯晶界与曲率效应对于锂吸附的影响研究中，通过计算我们发现晶界与曲率的存在对于锂在石墨烯表面的吸附起到了极大的促进作用，结合差分电荷密度从微观角度分析得到锂更倾向于吸附在石墨烯的晶界缺陷位置。曲率效应和晶界缺陷的耦合作用下，会使碳材料的储锂容量远远高于标准石墨烯的水平，这为高性能锂电池的发展奠定了理论基础。

通过气相沉积生长石墨烯后，由于石墨烯与基底的热膨胀系数不同，在冷却时过程中会使得制备得到的石墨烯产生了褶皱，通过经典分子动力学方法我们研究了在不同铜基底上此应力释放过程中石墨烯与基底的界面作用，得出Cu（111）面上由于较大的粘附能而有利于生长无褶皱石墨烯。在对含缺陷石墨烯在基底上的生长情况研究中，我们得到缺陷方向与缺陷对分布取向对于褶皱产生与消除有较大的影响，并予以理论解释，所得到的结果与实验结果完全一致，为制备完美石墨烯给出了准确的的实验建议与理论指导。

外延生长AlN通常是在蓝宝石表面，当在蓝宝石表面铺上一层石墨烯后不仅没有破坏AlN生长的外延性，而且还显著提高了AlN的生长速率。我们通过模拟计算得到AlN可以穿透石墨烯与蓝宝石基底成键，从而保证了其外延性。导入石墨烯缓冲层后AlN内应力得到了极大的释放，因此更容易快速生长。对于硼烯的力学行为研究，我们发现其具有高延展性和负泊松比的特性，具有广阔的功能材料应用前景。

Abstract

Low-dimensional carbon allotropes, such as graphene, have been broadly explored due to their outstanding and special properties. While the existent of defects in large-area graphene films is inevitable and grain boundaries (GBs) and Stone-Wales defect are commonly presented in form of pentagon and heptagon rings. These defects may significantly influence the interaction between graphene and the other interfaces. Although graphene has been broadly explored due to their outstanding mechanical properties, there exist significant challenges in retaining such properties of basic building blocks when scaling them up to three-dimensional materials and structures for many technological applications. The realized mechanical properties of 3-D carbon materials, by staggering graphene sheets or vertically grown carbon nanotube arrays, are significantly lower than those of individual graphene sheets or individual CNTs. The huge gap in mechanical properties between the low-dimensional carbon allotropes and their 3-D derivatives originates from the dissimilar bonding characteristics between carbon atoms within graphene or CNTs and the architectured 3-D engineering materials: The intra-structure bonding is covalent in nature, while van der Waals bonding dominates between different layers/tubes or with other materials. Such heterogeneous bonding leads to property inheritance that is a mission impossible.

We demonstrate the feasibility of constructing stable 3-D architectured C-honeycomb with covalent bonding. The specific strength of C-honeycomb could be the best in structural carbon materials. Its strong anisotropic Poisson’s effect may be utilized to design multi-functional structures with applications ranging from biomedical engineering to energy and environment systems. With the growing interest for 3-D nano-architectured functional materials, the well patterned two-level hexagonal structures in C-honeycomb pave a new strategy in achieving desirable properties that are comparable with carbon allotropes.

We also reporte a first-principles study on how grain boundary defects in graphene may influence the adsorption of lithium. The adsorption energy for Li atoms trapping in 5-, 7-, and 8-rings is much lower than the counter-part of Li atoms and pristine graphene. Such defective graphene could adsorb more Li atoms, and may reach the speculated ratio of 1:1 for C-Li adsorption. In a contrast study of lithium on fullerenes of different size, we find that the adsorption energy decreases with increasing size of fullerenes, but does not approach the energy when Li atoms adsorb on flat graphene. The energy in carbon nanotubes, however, converges to the adsorption energy between Li atoms and flat graphene if the radius of carbon nanotubes is sufficiently large. It hence indicates that while curvature plays a role in the enhanced adsorption in fullerenes, the twelve 5 rings in a fullerene ball is the primary factor accounting for the enhanced lithium adsorption.

Besides, we also show that the wrinkle formation of graphene grown on Cu substrates is strongly dependent on the crystallographic orientations. The wrinkle-free feature of graphene is attributed to the strong interaction of graphene with Cu(111) that enable strain energy retaining in graphene lattice instead of wrinkles formation. Moreover, the defects direction and orieation distribution of defects in graphene can influence the formation and elimination of wrinkle on different crystallographic orientations of Cu. All of our research provide a theoretical basis for experimental growth of wrinkle-free graphene.

The graphene interlayer also can weaken the interaction between AlN and sapphire, thus the compressive strain in AlN and tensile strain in sapphire is largely relaxed. The effective relaxation of strain further leads to fast epitaxial growth of AlN on sapphire. We also investigate the mechanics of borophene under uniaxial and biaxial tension. We find that the failure behavior, Young’s modulus and Poisson’s ratio of both the monolayer and bilayer borophene are highly anisotropic: Poisson’s ratio is negative when tension is along the atomic ridge direction.