Preparation And Mid-infrared Luminescence Properties Of Co²⁺: ZnSe Chalcogenide Nanocrystals

The 2⁵?m mid-infrared band covers many important molecular signature lines and is a relatively transparent window of the atmosphere in the air,so 2⁵?m mid-infrared laser has important applications in national defense,medical treatment,industries,communications and other aspects.Transition metal ions(TM²⁺)doped II-VI chalcogenide crystals have the advantages of wide tuning range,ultra-wide gain bandwidth,ultra-short pulse duration and large emission cross section,and is one of the excellent materials for middle infrared laser.However,at present,some difficulties have been encountered in the development of TM²⁺:II-VI laser materials.On the one hand,it is difficult to prepare TM²⁺:II-VI chalcogenide crystals with high doping concentration and uniform doping,and the process is complex.On the other hand,it is arduous to solve the problem of bulk crystals thermal management in order to further improve the output power.In recent years,TM²⁺:II-VI chalcogenide nanocrystals have been doped into sulfide glasses to form composite materials,which have been extensively studied.It has been found that the size and dispersion of nanocrystals will directly affect the optical properties of nanocrystalline glass composites.At present,the commonly used high temperature solid-phase ball-milling method needs to get the bulk crystals first,has a long experimental period and the grain size is in the order of microns.The laser sputtering method has some disadvantages such as low yield and serious particle agglomeration.The fluorescence intensity of nanocrystals prepared by organic phase chemical synthesis is weak due to the absorption of organic phase.To reduce the scattering loss of composite materials,it is urgent to find a new nanocrystals material with good dispersion and high mid-infrared emission intensity.In this paper,a series of Co²⁺:ZnSe and Co²⁺:ZnSe/ZnSe core/shell nanocrystals were successfully synthesized by a hydrothermal method.The 808 nm laser was used as pump source to obtain 2⁵?m mid-infrared fluorescence centered at 3.4 and 4.7?m in nanocrystals.The core/shell structure and post-heat treatment under reducing atmosphere were adopted to decrease the nanocrystal's surface quenching centers and improve the mid-IR photoluminescence.By analysis of the X-ray diffraction,X-ray photoelectron spectroscopy,transmission electron microscopy?TEM?,high-resolution TEM,energy-dispersive X-ray spectroscopy,Fourier transform IR,absorption spectra,and mid-IR emission spectra measurements of serial concentrations of Co²⁺-doped ZnSe nanocrystals,respectively to investigate the effects of doping concentration,core/shell structure and post-heat treatment temperature on the nanocrystal phase composition,grain size,the core/shell structure,crystal defect,Co²⁺ion distribution,hydroxyl content,the band gap value and mid-infrared emission of nanocrystals,and further explore the mechanism of the core/shell structure and post-heat treatment.Finally,the optimal hydrothermal preparation and post-heat treatment process were obtained to realize the controllable preparation technology of TM²⁺:II-VI nanocrystalline powders with nanometer scale,uniform size and excellent optical properties.The main research contents and results were summarized as follows:?1?Preparation and properties of Co²⁺:ZnSe nanocrystals:The nanocrystals with doped concentrations of 1¹0 mol%Co²⁺:ZnSe were prepared by hydrothermal synthesis.Co²⁺:ZnSe nanocrystals with different concentrations have cubic sphalerite structure,no other impurity phase,spherical morphology,grain size of about 16 nm,and good dispersion.Under 808 nm laser pumping,Co²⁺:ZnSe nanocrystals have 2⁵?m mid-infrared fluorescence emission centered at 3.4?m and 4.7?m.In the range from 1 mol%to 5 mol%,the mid-infrared fluorescence intensity of nanocrystals gradually increases with the increase of Co²⁺ion doping concentration.When the doping concentration reaches to10 mol%,the fluorescence intensities in the mid-infrared decreases due to concentration quenching.Co²⁺:ZnSe nanocrystals were post-heat treated at different temperature?200?,400?,800??under the reducing atmosphere.With the increase of heat treatment temperature,the hydroxyl content of nanocrystals gradually decreased,the mid-infrared fluorescence intensity increased significantly,the size of single grain first decreased and then increased,and the phenomenon of grain cluster aggregation intensified.1 mol%Co²⁺:ZnSe samples after heat treatment at 400?,the infrared fluorescence intensity increased 15 times;After heat treatment at 800?,grain size of nanocrystals increased significantly and grew into micron grade polycrystalline particles.?2?Preparation and properties of Co²⁺:ZnSe/ZnSe core/shell nanocrystals:Co²⁺:ZnSe/ZnSe core/shell nanocrystals were successfully prepared by hydrothermal method.The morphology of Co²⁺:ZnSe/ZnSe core/shell nanocrystals is still spherical,and the grain size is about 25 nm.FTIR results show that the core/shell structure can effectively reduce the content of nanocrystal surface groups.XPS and EDS results verify the successful coating of ZnSe shell.The core/shell structure can effectively improve the mid-infrared fluorescence intensity of Co²⁺:ZnSe nanocrystals.At low concentrations?1 mol%and 3 mol%?,the core/shell structure can significantly improve the fluorescence intensity of nanocrystals by promoting surface Co²⁺ions splitting.When the doping concentration increased to 5 mol%,the fluorescence intensity of nanocrystals was not significantly enhanced by core/shell structure.When the doping concentration is further increased to 10 mol%,the activation of surface Co²⁺ions by the core/shell structure will intensify the concentration quenching of mid-infrared fluorescence,resulting in a decrease in fluorescence intensity.For Co²⁺:ZnSe/ZnSe core/shell structure nanocrystals were post-heat treated with the same process.It was found that the core/shell structure nanocrystals after200?and 400?heat treatment still keep good dispersion and are not serious reunion phenomenon.Post-heat treatment can also effectively reduce the content of hydroxyl in core/shell nanocrystals.In addition,the core/shell structure and the heat treatment process can work together to further improve the mid-infrared fluorescence intensity.EDS results confirmed that the heat treatment process would cause the migration of Co element in nanocrystals with high doping concentration??3 mol%?,so the improvement of the mid-infrared fluorescence performance of nanocrystals by heat treatment would weaken with the increase of doping concentration.Experimental results confirm that 3 mol%Co²⁺:ZnSe/ZnSe after post-heat treatment at 400?has high dispersibility,low concentration of hydroxyl and higher mid-infrared fluorescence intensity.