石墨烯和六方氮化硼纳米片的几何结构、电子结构、磁性及边缘特性研究

Study on the geometrical structures,electronic structures,magnetic properties and edge effects of graphene and hexagonal boron nitride nanosheets

采用基于密度泛函理论(DFT)的第一性原理计算方法,计算了多种尺寸的石墨烯和六方氮化硼纳米片的性质,系统研究了其中的量子尺寸效应.研究的最大尺寸纳米片的直径约为5.5 nm,包含816个原子.对纳米片及其边缘的几何结构、电子结构、磁性性质以及电子分布进行了深入探讨,发现石墨烯和六方氮化硼纳米片最外层原子有由锯齿形向圆形变化的趋势,使得纳米片最外层更加平滑.随着纳米片尺寸的增加,能级由分立逐渐变得连续,纳米片由孤立分子态逐渐变得接近无限的晶体;禁带宽度总体有下降的趋势,符合量子尺寸效应.纳米片存在明显的磁性,磁矩主要集中在最外层原子上,且在相对平滑的地方容易出现磁性,相对弯曲的地方不易出现磁性.当增加体系的电子数时,增加的电子主要分布在最外层,使得纳米片整体磁性呈递减的趋势;当减少体系的电子数时,减少的电子的分布逐渐由最外层向内收缩,体系的总磁矩略有增加.研究结果对石墨烯和六方氮化硼纳米片的应用有参考价值.

This research was performed by first-principles calculations based on the density functional theory （DFT）.The properties of graphene and h-BN nanosheets with different sizes were calculated.The quantum size effect and edge effect are studied systematically.In the calculation, n×n （n = 6, 7, 8, ？？？, 24） supercell constructed from the primitive cell and taking out one of the largest regular hexagon or approximate hexagonal nanosheets.In order to increase the stability of the structure, each atom in the structure must form a six-atom ring with other atoms.The diameter of the largest nanosheet is about 5.5nm, containing 816 atoms.The geometrical structures, electronic structures, magnetic properties and electron distributions of the edge are discussed.The results show that outermost atoms of the nanosheets are more likely to change positions, to make the edge of the nanosheets smoother.For the electronic structures, with the increasing of the size of the nanosheets, the energy levels change from discrete to continuous.For the magnetic properties, the total magnetic moments increase with the size of the nanosheets.The magnetic moments of the nanosheets are mainly located on the outermost atoms.In the investigation of charge doping, when additional electrons are added to the nanosheets, the total magnetic moments will decrease.Further calculations show that the added electrons are mainly distributed on the outermost atoms.The added electrons are filled in the breaking bonds of the atoms on the edge of the nanosheets, thereby causing the magnetic moments disappear.The results are also given when electrons are subtracted from the nanosheets. In conclusion, we have systematically investigated graphene and h-BN nanosheets.The quantum size effects of different size nanosheets and edge properties have been studied, which may have important reference value for the application of graphene and h-BN nanosheets.