Preparation And Linear-nonlinear Optical Properties Of ZnO Composite MoS₂ Nano-film Materials

With the rapid development of science and the continuous improvement of demand,the application characteristics of semiconductor materials in different fields have gradually attracted the attention of researchers.Nano-sized semiconductor materials have shown excellent properties in the fields of optoelectronics,biology,chemistry,etc.,which has aroused the scientific interest of researchers.Graphene,with its excellent electrical properties,has been used as an electron transfer/accommodating layer between various materials.However,due to its band gap structure(Dirac cone),its optical behavior is suppressed,so it is necessary to find materials whose band gap can be easily modulated.Since 2005,researchers have discovered that the unidirectional movement of electrons in molecular layers of two-dimensional materials is restricted,which leads to significant quantum confinement effects.In view of this characteristic,the preparation and design strategies of 2D materials may lead to a tunable optoelectronic property and exhibit its multifunctional application characteristics.Although the band gaps of transition metal dichalcogenides in 2D materials are clearly affected by quantum confinement effects,the weak absorption capacity due to the low absorption cross section and the high carrier recombination rate lead to lower quantum yields.This undoubtedly limits its application flexibility in the field of optoelectronic devices.To solve this problem,researchers adopt the modification strategy of metal or semiconductor combined with transition metal dichalcogenide,which has been widely reported in several years.In this paper,taking the transition metal sulfide MoS₂ prepared by radio frequency magnetron sputtering as an example,based on scanning electron microscope,transmission electron microscope,X-ray diffractometer,atomic force microscope and other equipment,the modulation effect of the sputtering environment on its growth state was investigated,and a possible stress mechanism was used to elucidate the effect of different sputtering conditions on the film state.Furthermore,based on confocal Raman spectrometer,UV-Vis spectrophotometer,fluorescence spectrophotometer,femtosecond transient absorption spectroscopy and Z-scan technology with ultra-short pulse width,the excellent linear and nonlinear optical behaviors brought by MoS₂@ZnO thin films were investigated.To gain an in-depth understanding of this excellent optical behavior,the electron transfer effect between material interfaces is discussed in detail.ZnO,MoS₂ and MoS₂@ZnO films with excellent linear-nonlinear absorption properties were obtained by magnetron sputtering(MS)method.First,the growth orientation and nucleation process of MoS₂ and MoS₂@ZnO thin films were studied.By observing the scanning electron microscope(SEM)images of MoS₂ under different sputtering power and time,the MoS₂ film exhibits obvious morphology change process(cluster-grain-worm-like).The reasons for this phenomenon can be attributed to two points:First,with the increase of sputtering power,the kinetic energy of molybdenum atoms and sulfur atoms reaching the surface of the substrate increases,and the particles are more likely to move to the lowest point of their thermodynamic lattice,resulting in an increase in crystallinity and the generation of worm-like MoS₂.Second,the longer sputtering time provides sufficient collision nucleation time for the molybdenum and sulfur atoms reaching the surface of the substrate,which in turn leads to a great change in the growth orientation and morphology of MoS₂.Third,the linear absorption characteristics of the samples were characterized by UV-Vis spectrophotometer and the interfacial energy band structure of the composite films was theoretically calculated.The results show that the interband absorption peak intensity of MoS₂ is significantly promoted with the increase of sputtering power.Compared with pure ZnO and MoS₂ films,the linear optical absorptivity of MoS₂@ZnO films is several times stronger than that of pure materials.Combined with the photoluminescence phenomenon and the interfacial band structure of the sample under the excitation laser field with the excitation wavelength of 325 nm,the electron transfer mechanism between the interfaces of the composite film was analyzed.Finally,a femtosecond Z-scan system was used to characterize the third-order nonlinear optical behavior of the samples before and after recombination.At the excitation wavelength of 800 nm,the nonlinear optical absorption behavior of pure MoS₂ films exhibits the reverse saturable absorption induced by two-photon absorption.When the sputtering power increases,MoS₂ exhibits enhanced two-photon absorption and modulation depth;When the laser energy was increased from 50n J to 500 n J,the nonlinear phenomenon of MoS₂@ZnO film changed from saturable absorption(SA)to reverse saturable absorption(RSA).Therefore,MoS₂@ZnO films with lower transmittance of strong light and higher transmittance of weak light can be used as the main alternative material for optoelectronic protection devices in the field of optoelectronic limiting devices.In addition,according to nonlinear optics theoretical calculations,the two-photon absorption coefficient of MoS₂@ZnO films is 2–5 times higher than that of ZnO films,which means that MoS₂ films significantly promote the nonlinear absorption properties of ZnO.The above phenomenon can be attributed to two points:First,the reverse saturable absorption behavior guided by the two-photon absorption feature of pure ZnO films is significantly promoted by the MoS₂-based film;Second,the excited-state absorption cross-section of the ZnO film is significantly extended by the MoS₂-based film,which leads to an optimized reverse saturable absorption performance.This work realizes the excellent reverse saturable absorption properties of MoS₂@ZnO homogeneous films with controllable morphology and provides an important reference for other researchers to follow-up research in nonlinear limiting functional materials.