边界对石墨烯量子点非线性光学性质的影响

石墨烯作为一种新型非线性光学材料,在光子学领域具有重要的应用前景,引起研究人员的极大兴趣.本文运用量子化学计算方法研究了边界引入碳碳双键(C=C)和掺杂环硼氮烷(B₃N₃)环对石墨烯量子点非线性光学性质和紫外-可见吸收光谱的影响.研究发现,扶手椅边界上引入C=C双键后,六角形石墨烯量子点分子结构对称性降低,电荷分布对称性发生破缺,导致分子二阶非线性光学活性增强.石墨烯量子点在从扶手椅型边界向锯齿型边界过渡的过程中,随着边界C=C双键数目的增加,六角形石墨烯量子点和B3N3掺杂六角形石墨烯量子点的极化率和第二超极化率分别呈线性增加.此外,边界对石墨烯量子点的吸收光谱也有重要影响.无论是石墨烯量子点还是B₃N₃掺杂石墨烯量子点,扶手椅型边界上引入C=C双键导致最高占据分子轨道能级升高,最低未占分子轨道能级的降低,前线分子轨道能级差减小,因而最大吸收波长发生了红移.中心掺杂B₃N₃环后会增大石墨烯量子点的分子前线轨道能级差,导致B3N3掺杂后的石墨烯量子点紫外-可见吸收光谱发生蓝移.本文研究为边界修饰调控石墨烯量子点非线性光学响应提供了一定的理论指导.

Effect of edge on nonlinear optical property of graphene quantum dots

Graphene is a two-dimensional material with single-layer honeycomb lattice structure formed by sp2 hybrid connection of carbon atoms.Graphene has excellent optical,electrical,thermal and mechanical properties,and it is considered to be an ideal material for future flexible optoelectronic devices.In recent years,the nonlinear optical properties and regulation of graphene nanostructures have attracted experimental and theoretical interest.Graphene has good delocalization ofπ-electrons and its unique plane structure,showing good nonlinear optical properties.Graphene quantum dots can be regarded as small graphene nanoflakes.Their unique electronic structure is closely related to the non-bond orbitals on the boundary/edge.Therefore,it is very important to study the boundary/edge effect on the electronic and optical properties of nanographene.In this paper,effects of the number of edge C=C double bonds and Borazine(B₃N₃)doping on the nonlinear optical properties and UV-Vis absorption spectrum of graphene quantum dots are studied by the quantum chemical calculation methods,respectively.It is found that the symmetry of hexagonal graphene quantum dots decreases and the symmetry of charge distribution is broken when C=C double bond is introduced into the armchair edge,which leads the second-order nonlinear optical activity to be enhanced.During the transition from armchair to zigzag edge,the polarizability and the second hyperpolarizability of hexagonal graphene quantum dots and B₃N₃-doped graphene quantum dots increase linearly with the number of introduced C=C double bonds incrrasing.In addition,the edge also has an important influence on the absorption spectrum of graphene quantum dots.For graphene quantum dots and B₃N₃-doped graphene quantum dots,the introduction of C=C double bond at the armchair edge increases the highest occupied molecular orbital energy level and also reduces the lowest unoccupied molecular orbital energy level,which reduces the energy gap between the frontier molecular orbitals,and thus resulting in the red-shift of the maximum absorption wavelength.The doping of B₃N₃ ring will increase the energy gap between molecular frontier orbitals of graphene quantum dots,leading the UV-Vis absorption spectrum of graphene quantum dots to be blue-shifted.This study provides theoretical guidance for controlling the nonlinear optical response of graphene quantum dots by edge modification.