Theoretical Study On The Structures And Nonlinear Optical Properties Of Carbon-Boron Nitride Planar Nanofragments

In recent years,two-dimensional materials have attracted much attention due to their unique and fascinating optoelectronic properties.Boron nitrogen nanosheet derivatives with ionic bond characteristics show great potential applications in optoelectronic devices.Recent studies have shown that the doped h-BN/graphene hybrid structure has good nonlinear optical properties.By changing the ratio of h-BN/graphene fragments,the energy gap and the second-order nonlinear optical properties of the material can be effectively adjusted.In this thesis,the relationship between the structure and the first hyperpolarizability of a series of planar carbon-boron-nitrogen nanomolecules was systematically studied by using quantum chemistry calculation method.The specific work is as follows:1.On the basis of the boron nitrogen nanofragments of the same size,a series of new carbon-boron nitrogen heterostructures(BNC-n)were designed by doping the ever-increasing vertical carbon-segments on boron nitrogen nanofragments.The results show that compared with the original boron nitrogen fragments,the BNC-n series have higher polarizability and the first hyperpolarizability.It is worth noting that the values of the polarizability and the first hyperpolarizability increase with the increase of the longitudinal carbon fragments in the heterostructure.Further analysis shows that BNC-N has a small transition energy and a large transition dipole moment,which confirms that boron nitrogen nanofragments replaced by carbon fragments can obtain better nonlinear optical properties.2.Based on the highly symmetrical graphene carbon fragments,we designed a series of vertical boron nitrogen fragments“clipping”graphene structures,and used density functional theory to investigate the effect of the ratio of the two carbon fragments(C_L/C_R)separated by boron nitrogen fragments on the first hyperpolarizability.It is gratifying that the BN/C molecules all have the larger first hyperpolarizability.At the same time,as the ratio of C_L/C_Rapproaches,the first hyperpolarizability of the system increases gradually.Wherein,when the ratio of BN/C is 1:1,the first hyperpolarizability value of the structure is the largest,and the calculation results of time-dependent density functional theory(TD-DFT)shows that the structure with the highest first hyperpolarizability corresponds to the largest transition dipole moment.In this paper,the structure and nonlinear optical properties of heteroatom substituted graphene are theoretically explored,which provides a new design idea for the experimental study of developing high-performance nonlinear optical materials.