Nonlinear Absorption Properties Of Bi₂S₃ And Bi₂S₃/MoS₂ Heterostructure Films

Compared with bulk materials,two-dimensional materials such as graphene,black phosphorus,metal chalcogenidesA₂^V B₃^VI（A=Sb,Bi;B=S,Se,Te）,transition metal dichalcogenidesAB₂（A=Mo,W;B=S,Se,Te）,etc.have special properties such as high on/off ratio,high surface area and so on.Owing to these excellent performances,two-dimensional materials have been widely researched and applied in some fields such as photoelectrochemistry,photocatalysis,photovoltaic cells,electrochemical hydrogen storage,and nonlinear optical devices.As a typical metal chalcogenides,due to larger Bohr’s excitonic radius,bismuth sulfide（Bi₂S₃）has a strong quantum confinement effect,which is introduced into sodium borosilicate glass to improve the nonlinear absorption performance.Bi₂S₃ has great application potential in nonlinear photonic devices.However,there are few studies on Bi₂S₃ in the field of nonlinear optics.The lack of nonlinear parameters greatly limits the application of Bi₂S₃ in nonlinear photonics devices.The study of the nonlinear absorption property of Bi₂S₃ paves the way for future development in the nonlinear field and also provides a comparison for researching the nonlinear optical process in van der Waals heterostructures based on Bi₂S₃.Based on summarizing the research progress of the nonlinear absorption properties for two-dimensional materials,firstly,this thesis studies the influence of film thickness and pump intensity on the nonlinear absorption properties of Bi₂S₃ films.Then,the influence of nanosheet size on the nonlinear absorption properties of Bi₂S₃ films and Bi₂S₃ solutions was studied under the same thickness and pump intensity.Finally,the nonlinear absorption property of Bi₂S₃ film was further improved by constructing type-Ⅲheterostructure bismuth sulfide/molybdenum sulfide（Bi₂S₃/MoS₂）.The mainly contents are shown as follows:（1）The influence of thickness and pump intensity on the nonlinear absorption property of Bi₂S₃ thin films:Using liquid-phase exfoliation method,Bi₂S₃ films with different thickness were prepared by controlling the filtration volume.The thicknesses of Bi₂S₃ films were accurately confirmed by atomic force microscope.The data of Z-scan for Bi₂S₃ films indicate that the saturable absorption response increases with the thickening of films,when the thickness for Bi₂S₃ films is below 40 nm.The saturable absorption response decreases with the increasing of films,when the thickness for Bi₂S₃ films is larger than 40 nm.This thickness-dependence phenomenon is related to the strength of the coupling effect between layers and the interaction between light and matter.The saturable absorption response of the Bi₂S₃ film increases with the increase of the pump intensity,which may be related to the number of electrons participating in the nonlinear absorption process.These results provide references for the thickness and pump intensity in the design of nonlinear photonic devices based on Bi₂S₃ films.The results also provide an example for the mechanism explanation of the nonlinear absorption properties of other materials.（2）The influence of nanosheet size on the nonlinear absorption property of Bi₂S₃films and Bi₂S₃ solutions:The Bi₂S₃ solutions with 100 nm-600 nm of the nanosheets size were prepared by controlling the centrifugal speed,and the Bi₂S₃ films with the same thickness as well as different nanosheets sizes were prepared by using vacuum filtration technique.With the decreasing of nanosheets size,the Z-scan results indicate a variation trend from saturable absorption response to reverse saturable absorption response in both Bi₂S₃ solution and film.This phenomenon is probably related to the two photon absorption caused by the edge defects.These results give a guide for the design of ultrafast photonic devices based on Bi₂S₃ in the selection of nanosheet size,and pave the way for the applications in laser protection,optical limiting,and other fields for Bi₂S₃.（3）The nonlinear absorption property of Bi₂S₃/MoS₂ heterostructure films:The Bi₂S₃/MoS₂ heterostructure films were prepared by mixing the supernatants of pure Bi₂S₃and MoS₂ with the ratio 1:1 followed by a vacuum filtration.The results of X-ray photoelectron spectroscopy and UV–Vis absorption spectra indicate that Bi₂S₃/MoS₂ is a type-Ⅲheterostructure.The experiment results of Z-scan show that the nonlinear absorption coefficient of the Bi₂S₃/MoS₂ film is twice larger than that of the Bi₂S₃ film.This enhancement of the nonlinear absorption property comes from the interlayer electron transition in the Bi₂S₃/MoS₂ heterostructure,and the direction of electron transition lies on the direction of the special built-in electric field for type-Ⅲheterostructure.The results in this work provide a reliable way to further improve the nonlinear absorption property of materials,and the mechanism provides a reference for the explanation on the enhancement of optical performance for type-Ⅲheterostructure.This work has been published in Applied Surface Science.