Preparation And Optical Properties Of Rare Earth-doped K-Lu-F Up-conversion Material System Based On Energy Transfer

Benefiting from the unique 4f electronic layer structures,Rear-earth ions(Ln³⁺)are endowed with unique and fascinating optical properties.Especially the high-quality lanthanide doped upconverting nanoparticles?UCNPs?that can convert near-infrared light to shorter wavelength radiation.Therefore,the UCNPs offer excellent prospects in luminescent labeling,displays,bioimaging,sensors,and anticounterfeiting applications.At present,more researches are forced on the enhancement of the luminescence efficiency of the UCNPs and modulated the multicolor output.It is worth mentioning that intrinsic energy transfers in Ln³⁺-doped UCNPs play a dominant role in determining the UC emission efficiency.Consequently,it is essential to rationally modify the energy transfer routes in order to obtain satisfactory UC emission efficiencies and desired color outputs.In this paper,K₂Lu F₅ and KLu₂F₇ UC luminescent materials were prepared by hydrothermal method and co-precipitation method.And the optical properties of UC luminescent materials were studied in detail by modifying the energy transfer routes.The main contents of this paper are as follows:Firstly,a series of K₂LuF₅:Yb³⁺,Er³⁺micron crystals doped with different Ce³⁺ions have been synthesized via a facile and enviro nmentally-friendly hydrothermal route.It is obvious that the the emission intensity located at 1.54?m of 15mol%Ce³⁺doped sample is enhanced.in comparison with that of Ce³⁺-free sample.But,the UC luminescence intensities of Ce³⁺doped samples are higher than Ce³⁺-free sample.However,the UC emission intensity decreases with the increase of Ce³⁺concentration?5-15%?.In addition,the cross-relaxation process between Ce³⁺ions and Er³⁺ions are promoted by increasing the excitation power density.Thus,the electrons can be excited to higher energy level(²H_9/2)of Er³⁺ions,which leads to the multicolor output in Ce³⁺doped K₂LuF₅:Yb³⁺,Er³⁺micro-crystals.This result also proves that the cross relaxation between Ce³⁺ions and Er³⁺ions has an important influence on the optical properties of the UCNPs.The application of upconversion materials in biology is greatly limited due to the large size of microcrystal synthesized in the above work.Therefore,on the basis of the above work,we have successfully synthesized the well-dispersed KLu₂F₇:Nd³⁺,Yb³⁺,Er³⁺nanoparticles with smaller size via a co-precipitation method,by optimizing the experimental scheme.By monitoring the intensities of UC luminescence under 980 and 808 nm excitation,the optical temperature sensing properties of the as-synthesized nanoparticles are achieved over a wide temperature range,of which the maximum sensitivity is approximately 0.0044 K^-1at 533K and 0.0041K^-1 at 413K,respectively.The results of cycling process of heating and cooling over the temperature range from 293 to 573K indicate the excellent thermal stability of KLu₂F₇:Nd³⁺/Yb³⁺/Er³⁺nanoparticles.Furthermore,a laser induced thermal effect has been investigated.The thermal effect induced by808 nm laser excitation is negligible compared with that of induced by 980 nm excitation with increasing pump power,which ensures the accuracy of the sample as a thermometry under 808 nm excitation.Therefore,Yb³⁺,Er³⁺,Nd³⁺tri-doped KLu₂F₇ nanoparticles can be prominent candidates as temperature sensor and optical heater for multifunctional application both in thermometers and biosensors.In addition,in order to further enhance the emission intensity of KLu₂F₇upconversion nanoparticles and control the emission color,a series of 2 mol%Ho³⁺or Er³⁺and 18-98 mol%Yb³⁺doped KLu₂F₇ nanoparticles were prepared via a co-precipitation method.KLu₂F₇ has a special crystal structure.The lanthanide doped ions are distributed in arrays of tetrad clusters while doping high concentration of Yb³⁺ions.More importantly,the distinct configuration enables preserve the excitation energy within the Yb³⁺sub-lattice and efficiently minimizes the migration of excitation energy to lattice or surface defects.Thus,the overall integrated UC emission intensity increases with the increasing of Yb³⁺ions doping concentration.And the UC luminescence to be enhanced by⁷ times while the Lu³⁺ions in the KLu₂F₇ host matrix were replaced in full by the Yb³⁺ion.Meanwhile,different activators are sensitized by Yb³⁺ions can realize the reversible variety from green to red emission due to the impact of back energy transfer process of activators Ho³⁺or Er³⁺to Yb³⁺ions.This study will offer a general strategy to precise control of color emission and could provide potential applications in the fields of display systems and optoelectronic devices.