The Enhanced Nonlinear Optical Properties And Mechanisms Of RGO Modified By Azobenzene Derivative

Graphene is a two-dimensional material that consists of carbon atoms with sp² Hybridized orbitals in a hexagonal honeycomb lattice.With excellent optical,electrical and mechanical properties,it has important application prospects in materials science,micro-nano processing,energy,biomedical and medical fields,and is considered to be a revolutionary material in the future.Due to its zero band gap linear molecular dispersion,high electron mobility and broadband absorption properties,it is a good nonlinear optical absorption material that can be used in the field of nonlinear optics.However,the optical properties of graphene still have certain deficiencies.Such as,the saturation absorption coefficient is small,the light response speed is slow and the performance is unstable.In order to further improve the nonlinear optical properties and response characteristics of graphene-based materials and devices,the construction of graphene composites and the enhancement of nonlinear optical effects have received extensive attention in recent years,which have become a hotspot in the field of nonlinear optical research.Herein,two different kinds of azobenzene derivatives?AOB-t8,BNB-t8?/reduced graphene oxide?RGO?composites?AOB-t8/RGO,BNB-t8/RGO?were prepared by functionalization of non-covalent bonds??-?stacking and hydrogen bonding?.The structure,morphology and basic properties of the researched objects were characterized by X-ray diffraction,Fourier transform infrared spectroscopy,Raman spectroscopy,Ultraviolet-visible absorption spectroscopy,Transmission electron microscopy and Steady-state fluorescence spectroscopy and so on.Moreover,we also got the nonlinearity of the two composites was by using femtosecond Z-scan technology.We found the nonlinear absorption of them is enhanced compared to the purely reduced graphene oxide RGO.Such as,the enhancement of RGO by AOB-t8 is approximately 2.7 times,and BNB-t8 is up to 38.6 times.In addition,the nonlinear enhancement mechanism of BNB-t8/RGO was also studied by femtosecond transient absorption spectroscopy.All the specific research contents and results are as follows:1.The reduced oxide graphene RGO and AOB-t8/RGO composite materials were prepared by solvent thermal method with GO and azobenzene derivative AOB-t8 with oxadiazole group as raw materials and sodium borohydride NaBH₄ was acted as reducing agent.The results show the composite of AOB-t8/RGO is mainly realized by the?-?stacking between AOB-t8 and RGO.Then the nonlinear optical properties of this composite were then tested by using femtosecond Z-scan technique.The results show that the third-order nonlinear absorption and refraction of AOB-t8/RGO composites have a certain degree of enhancement compared with RGO,and the enhancement of nonlinear absorption coefficient and nonlinear refractive index of AOB-t8/RGO are 2.7 times and 2.4 times of RGO respectively.The main mechanism of enhancement can be attributed to the?-?stacking between the oxadiazole and aromatic ring groups of AOB-t8 and RGO.Moreove the strong?-?interaction between AOB-t8 and RGO increases the?-electron ionization region of RGO and promotes the charge transfer between AOB-t8 and RGO.2.It was speculated that the difference in intermolecular force?hydrogen bonding?may have an important influence on the composite degree and optical nonlinearity of the composite based on the composite results of AOB-t8 and RGO.We prepared and optimized a composite material of RGO and BNB-t8 with Amide group by changing the mass ratio of substrate GO and BNB-t8.The results indicated a strong interaction and electron transfer between RGO and BNB-t8,and a new stronger complex absorption band appeared around 520 nm in the absorption spectroscopy.Compared with the RGO alone,the composite BNB-t8/RGO has more defects,lower crystallinity and larger interlayer spacing.BNB-t8 is bound to RGO by?-?stacking and hydrogen bonding,and BNB-t8 provides more hydrogen bonding sites?-C=O and-NH groups?.When the reaction mass of BNB-t8 is 80%,the binding of BNB-t8 to RGO is saturated.3.The nonlinear optical properties and mechanism of BNB-t8/RGO composites were investigated by Z-scan technique and femtosecond transient absorption technique.The results show BNB-t8 exhibites two-photon absorption at 532 nm.However,the RGO showes weaker saturated absorption characteristic.With the increasing of BNB-t8 content,the nonlinear absorption coefficient of BNB-t8/RGO increases gradually.When the content of BNB-t8 is 80%,the nonlinear saturated absorption coefficient of the composite is 38.3 times of RGO.The nonlinear optical absorption properties of the composite exhibite a certain wavelength dependence.The main mechanism of nonlinear absorption enhancement is due to the charge transfer from RGO to BNB-t8.The results also show us the charge separation process of the composite material under photoexcitation occurs in the 0.28 ps time domain,while the charge recombination process is completed at 2.0 ps.In summary,the combination of two kinds of azobenzene derivative molecules with RGO improved the nonlinear optical properties of RGO.Compared with pure RGO and AOB-t8/RGO composites,the nonlinear absorption characteristics of BNB-t8/RGO composites have greatly improved.The main reason can be attributed to the stronger hydrogen bonding between BNB-t8 and RGO,which is more conducive to the formation of a composite structure,then increases the delocalization of electrons.Compared with the weaker?-?stacking between AOB-t8 and RGO,the stronger hydrogen bonding between BNB-t8 and RGO greatly increases the charge transfer rate and response rate between BNB-t8 and RGO.The nonlinear optical properties and photo-response properties of BNB-t8/RGO composites are significantly improved.These research results have certain scientific significance for designing and constructing new graphene composites and improving their nonlinear optical properties.