Theoretical Study On Photoelectric Properties Of Graphene Nanostructures

Because of the important application values of graphene-based nonlinear optical materials in the field of the photonics and the optoelectronics,the nonlinear optical properties and its regulation of graphene have aroused intense interest for the experimental and theoretical researchers in recent years.The quantum chemistry methods are used in this paper to calculate the nonlinear optical properties of graphene quantum dots and to discuss the factors that affect the nonlinear optical properties: including size,substituents,frequency dispersion effect,external electric field and doping groups,which provides a theoretical basis for the design and synthesis of high performance graphene quantum dot photoelectric materials.The main results are as follows:1.We studied the static and dynamic linear polarizabilities and the second hyperpolarizabilities of hexagonal graphene quantum dots,and discussed the relationship of the linear polarizabilities and the second hyperpolarizabilities with the size,substituents and incident light frequency.It was found that with the increase of the size of graphene quantum dot,the frontier molecular orbital energy gap is decreased,which leads to the increase of the polarizabilities and the static second hyperpolarizabilities.In addition,introducing the electron donors/acceptors can significantly increase the(hyper)polarizabilities of graphene quantum dots.Finally,this thesis also discussed the linear polarizabilities of graphene quantum dots and the frequency dispersion effect of second hyperpolarizabilities,meanwhile,and we found the third-order nonlinear optical properties of graphene quantum dots were strongly influenced by incident light frequency,so the nonlinear optical response can be controlled by changing the frequency of incident light.2.We also studied the electronic properties of the first hyperpolarizabilities and its structure,the frontier molecular orbital and the density of states and so on of hexagonal graphene quantum dots under the external electric field.It was also been found that the magnitude and direction of the external electric field has an important influence on the structure of the graphene quantum dots.Imposing electric field is an effective way to adjust and control the the first hyperpolarizabilities of hexagonal graphene quantum dots.With the enhancement of the electric field intensity,the charge density of the molecules will separate as the direction of electric field,which leads to the enhancement of the electron transition in the laser field,the decrease of the frontier molecular orbital energy gap and the significant increase of the static first hyperpolarizabilities.Therefore,the electron density redistribution induced by strong electric field can significantly reduce the orbital energy gap of the quantum dots,and so evidently enhance the second-order nonlinear optical effect.The control of electric field has provided a good strategy for optimizing and designing of the second order nonlinear optical properties of molecular materials with the centrosymmetrics structure.3.The second hyperpolarizabilities,frontier molecular orbital energy gaps and ultraviolet-visible absorption spectra of graphene quantum dots and three borazine-doped graphene quantum dots were studied in this paper.It was found that the model molecules have both the large third-order nonlinear optical response and the good transparency of the visible light waveband.The doped location of boron-nitride has a great influence on the frontier molecular orbital energy gap,energy density of states distribution and the second hyperpolarizabilities.The research showed that the doping of borazine can effectively regulate the nonlinear optical response and ultraviolet-visible absorption spectra on the graphene quantum dots,which provides useful guidance for the study on the photoelectric properties devices of other graphene-based materials.