Theoretical Studies On Second-order Nonlinear Optical Properties Of Azulene Defected Spiral Graphene Nanoribbons

In recent years,nonlinear optical materials have been widely used in laser,communication and other fields.Searching or designing new materials with good nonlinear optical properties has been the ultimate goal of experimental and theoretical research.As a special material,spiral graphene has the structural characteristics of single-layer graphene and multi-layer graphene.There is a special interlayer electron coupling.This unique geometric structure and electronic features make it widely used in the fields of optics and chemistry.In this thesis,azulene units are introduced into spiral graphene nanoribbons.Because the azulene unit itself has polarity,and the introduction of azulene units destroy the central symmetry of spiral graphene nanoribbons,it significantly enhances the first hyperpolarizabilities of those nanoribbons and makes the nanoribbons potential nonlinear optical materials.The details are as follows:Based on density functional theory and semiempirical method combined with sum-over-states(SOS)model,the electronic spectral properties and nonlinear optical properties of azulene defect spiral graphene nanoribbons under different stresses and different spiral lengths were calculated.The results show that under different stresses,the electronic spectral properties,dipole and nonlinear optical properties of the structure change slightly.With the increase in spiral length,major absorption peaks show an overall redshift,and the dipole of azulene defect spiral graphene nanoribbon increases roughly linearly with nanoribbon length.The static first hyperpolarizability of the structure also increases with nanoribbon length except for eight units is substantially equal to nine units.It can be clearly seen from the two-dimensional second-order nonlinear optical specta that azulene defect spiral graphene nanoribbons exhibit a very strong second-order nonlinear optical response under external fields,especially in the visible region of about 600.0 nm.The detailed study of nine units spiral graphene nanoribbon structure shows that the introduction of azulene defects causes the charge distribution of the spiral graphene nanoribbons to be significantly polarized.The highest occupied frontier molecular orbital localize on the pentagon end and the lowest unoccupied frontier molecular orbital mainly localize on the heptagon end,which makes the spiral graphene nanoribbons a polar material,and the second-order nonlinear optical response is significantly improved.The overlap of interlayer ?-electrons in the spiral graphene nanoribbon structure opens up a new charge transport channel,but the change in strain causes the overlap of the interlayer ?-electrons of this structure.Studies have shown that,for azulene-defected spiral graphene nanoribbons with nine units,a strong interlayer ?-electrons overlap can be clearly seen when compressed to 3.52 (?),when the structure is stretched to 4.27 (?),this ?-electrons overlap between the layers has largely disappeared.Therefore,the adjustment of the transport properties of electrons can be achieved by adjusting the layer spacing of the spiral graphene nanoribbons.In summary,the introduction of azulene structure can effectively improve the second-order nonlinear optical response of spiral graphene,thus providing some guidance for the design of new nonlinear optical materials.