Theoretical Studies On The Second-Order Nonlinear Optical Properties Of Spiral Graphene Nanoribbons With Azulene Defects And BN Doping

Since the discovery of nonlinear optical phenomena,nonlinear optical materials have been widely used in more and more fields,such as optical communication,biological imaging,tunable lasers and so on.In view of the application requirements of nonlinear optical materials,it is still urgent and necessary to find nonlinear optical materials with strong nonlinear optical response.Because of seeking nonlinear optical materials with strong nonlinear optical response through experiment needs long period of time and high cost,and theoretical simulation not only does not have these problems,but also predicts some aspects that cannot be done in the experiment.The first hyperpolarizability of micromolecules is often used as a measure of the second-order nonlinear optical response of materials,and the first hyperpolarizability is not zero if it has the noncentric symmetry.Spiral graphene(HGN)not only has unique helicity,chirality and large ? conjugated system,but also has some excellent properties of graphene.On the other hand,spiral molecules can be used as molecular springs in some special fields.The electrons of the spiral structure can be transferred from the three dimensional direction to facilitate the transfer of electrons than the one dimensional direction of the planar molecule,which makes the nonlinear optical response of the structure significantly enhanced.This also provides a new prospect for the application of spiral molecules in the field of nonlinear optics.Azulene is a noncentral symmetrical structure,which is inherently polar and BN nanomaterials have a large band gap and often have the properties of insulators.Therefore,doping azulene defects or BN into spiral graphene nanoribbons(HGN)not only changes the symmetry but also makes the molecules polarized.Structure and properties are inseparable.Therefore,the first hyperpolarizability of the nanoribbons can be enhanced by introducing azulene defects and BN into the nanoribbons.A series of different structures are designed by changing the doping pattern and the doping amount,in order to obtain a structure with good dynamic stability and a large first hyperpolarizability.The influence factors of doping on the first hyperpolarizability are found by the frontier molecular orbital,electronic spectrum.The main content of this thesis is divided into two parts.The first part(Section 3)mainly studies the introduction of azulene defects into spiral graphene nanoribbons,and changes the doping pattern and amount of azulene defects to find the structure with large first hyperpolarizability,and through the analysis of the relationship between the static first hyperpolarizability and the length of the nanoribbons,the frontier molecular orbital,and the electron transition to find out how the azulene defects affects the first hyperpolarizability of the structure.The doping pattern plays a decisive role in the first hyperpolarizability,and the azulene defects in the structure mainly contribute to the first hyperpolarizability.The AHGN-a structure with the best first hyperpolarizability was obtained.The second part(Section 4)introduced BN or BN and azulene defects into the spiral graphene nanoribbons,and a series of different structures are designed.According to the energy gap and the first hyperpolarizability,the best structures BNHGN-a and BNAHGN-a with good dynamic stability and large first hyperpolarizability are selected.The influence factors of the static first hyperpolarizability of the structure are studied.It is found that the doping mode plays a decisive role in the first hyperpolarizability of the structure.The frontier molecular orbital analysis shows that BN doping or BN and azulene defect co-doping have different effects in the molecule.At the same time,the electronic spectrum of the best structure is also analyzed.In short,AHGN-a,BNHGN-a and BNAHGN-a of good dynamic stability and a large first hyperpolarizability are obtained by investigating azulene defect or BN doping.It is clear that the doping pattern is the main factor affecting the first hyperpolarizability.It is found that azulene defect,BN or BN and azulene defect doping have different effects on the improvement of the nonlinear optical response of the structure.The second order nonlinear optical process of the optimal structure under different external fields was explored,and the two dimensional second order spectrum was used for characterization.