Preparation And Mid-infrared Luminescence Properties Of TM²⁺:ZnSe Nanocrystals/Chalcogenide Glass Composite

The 2?5?m mid-infrared waveband contains the characteristic absorption lines of many molecules,which has a very important application in the fields of molecular spectroscopy,national defense and security,medical and other fields.Transition metal ions doped?-?sulphide crystals have wide laser tunable range and large emission cross section,which are suitable to be used as gain media for broadband tunable mid-infrared laser sources.Zn Se has been recognized as an excellent MIR laser matrix material with a wide range of light transmission and good chemical stability,and the crystal structure of Zn Se is not easy to change by doping ions,TM²⁺:Zn Se crystal has become a potential medium for mid-infrared laser gain and have been widely studied by scientists.Chalcogenide glass has very good glass forming ability,thermal stability,and also has very good light transmittance in the mid-infrared waveband.In this paper,As₄₀S₅₇Se₃ glass which matches the refractive index of TM²⁺:Zn Se crystal is selected to prepare TM²⁺:Zn Se/As₄₀S₅₇Se₃ composites.Doping TM²⁺:Zn Se nanocrystals into glass matrix to form composite laser matrix materials,and the composite materials can be further drawn into fiber.The structure of fiber is helpful to solve the problem of thermal management of transition metal ions in crystal matrix.Therefore,the preparation of TM²⁺:Zn Se/As₄₀S₅₇Se₃ composites with excellent properties is of great significance for the development of mid-infrared fiber laser,the preparation of TM²⁺:Zn Se/As₄₀S₅₇Se₃ composites with excellent properties must first obtain TM²⁺:Zn Se nanocrystals with excellent properties.The preparation methods of TM²⁺:Zn Se nanocrystals are mainly divided into physical method and chemical method.TM²⁺:Zn Se nanocrystals prepared by physical method must first obtain bulk crystals,but the preparation period of bulk crystals is long and the cost is high.The yield of nanocrystals prepared by futher preparation of bulk crystal is low,the particle size distribution of nanocrystals is uneven,and the agglomeration of high energy particles can not be avoided.The preparation of micro-nano crystals based on chemical methods has the advantages of simple preparation process,short experimental period,easy adjustment of matrix component and transition metal ion doping concentration.However,in the previous report,TM²⁺:Zn Se nanocrystals prepared by chemical method introduced organic solvents as crystal stabilizers during nucleation,and the residual organic solvents on the surface of the nanocrystal formed fluorescence quenching centers such as C-H;On the other hand,when water or ethanol is used as a solvent in the hydrothermal process,fluorescence quenching centers such as hydroxyl groups and surface defects will inevitably be introduced into the surface of the nanocrystal,resulting in a relatively weak luminescence intensity of TM²⁺:Zn Se nanocrystals.In order to improve the mid-infrared luminescence of TM²⁺:Zn Se crystal powders prepared by chemical methods,the use of organic solvents as stabilizers should be avoided in the selection of raw materials,and the crystal crystallinity should be further improved and the concentration of surface defects should be reduced.In this paper,Co²⁺:Zn Se nanocrystals and Fe²⁺:Co²⁺:Zn Se nanocrystals were prepared by simple hydrothermal method without introducing organic phase,and Co²⁺:Zn Se/As₄₀S₅₇Se₃,Fe²⁺:Co²⁺:Zn Se/As₄₀S₅₇Se₃composite were prepared by melting pressing method.The morphology and structure of Co²⁺:Zn Se nanocrystals,Fe²⁺:Co²⁺:Zn Se nanocrystals,Co²⁺:Zn Se/As₄₀S₅₇Se₃ and Fe²⁺:Co²⁺:Zn Se/As₄₀S₅₇Se₃composite were characterized by XRD,TEM and SEM.The optical properties of nanocrystals and composite were characterized by UV-vis absorption spectra,infrared transmission spectra and fluorescence spectra.The main contents and achievements of this paper are as follows:The 3 mol%Co²⁺:Zn Se nanocrystals prepared by hydrothermal method have cubic sphalerite structure,the grain size of these nanocrystals are about 15.5 nm and the particles are uniformly distributed,and the doping ions are uniforming distributed in the nanocrystals.Under 1550 nm laser pump,the fluorescence peaks of 3 mol%Co²⁺:Zn Se nanocrystals lies about 3.4?m and 4.7?m,corresponding to the ⁴T₂?F??⁴A₂?F?and ⁴T₁?F??⁴T₂?F?energy transitions of Co²⁺ions,respectively.Co²⁺:Zn Se/As₄₀S₅₇Se₃ composite materials were prepared by vacuum melting pressing method.XRD results show that the melting pressing process does not destory the structure of nanocrystals and glass matrix,but the melting pressing process will lead to the growth of nanocrystals from 15.5 nm to 32.8 nm,Co²⁺:Zn Se nanocrystals are uniformly diepersed in the composite,and there are fewer pores and defects in the composite.Under 1550 nm laser pump,the fluorescence peak position and shape of composite is consistent with Co²⁺:Zn Se nanocrystals.A series of Fe²⁺:Co²⁺:Zn Se nanocrystals with cubic sphalerite structure were prepared by hydrothermal method.With the doping concentration of Fe²⁺ions increasing from 0.3 mol%to 3 mol%,the shape of nanocrystals changes from spherical to irregular sphere,the particle size of nanocrystals decreasing from 15.3 nm to 14.3nm,and the doping elements are partially aggregated,the diffraction peak position of XRD shifts slightly to the low angle,and the lattice spacing increases,which proves that the Fe²⁺ions successfully replaced Zn²⁺ions and doped into Zn Se lattice.Through the energy transfer between Co²⁺and Fe²⁺ions,the fluorescence emission peaks at 3.3?m and 4.4?m were observed in Fe²⁺:Co²⁺:Zn Se nanocrystals pumped by 1550 nm laser at room temperature.Compared with Co²⁺:Zn Se nanocrystals,an obvious fluorescence peak broadening and blue shift at 3.3?m and 4.6?m can be observed due to the change of crystal field environment.The fluorescence band of 4.4?m in the Fe²⁺:Co²⁺:Zn Se nanocrystals is thought to be an overlapping emission involving the peak centered at 4648 nm corresponding to the Co²⁺:⁴T₁?F??⁴T₂?F?energy transition and the peak centered at 4273 nm corresponding to Fe²⁺:⁵T₂?D??⁵E?D?energy transition.In the 0.3-3 mol%Fe²⁺3 mol%Co²⁺:Zn Se nanocrystals co-doped samples,the 0.6 mol%Fe²⁺3 mol%Co²⁺:Zn Se nanocrystal exhibits the strongest fluorescence intensity at 4.4?m in the serial co-doping samples.Fe²⁺:Co²⁺:Zn Se/As₄₀S₅₇Se₃composite were prepared by vacuum melting pressing method,XRD results show that the melting pressing process does not destory the structure of nanocrystals and glass matrix.Under 1550 nm laser pump,the fluorescence peak shape of the composite is consistent with Fe²⁺:Co²⁺:Zn Se nanocrystals.