Synthesis And Spectral Modulation Of Erbium-Doped Upconversion Phosphors

Erbium ion is a typical rare-earth(RE)up-conversion(UP)activator and widely used in optical temperature sensor and bio-optical imaging,etc.However,the requirements for the properties of RE-doped UC phosphors are different in the practical applications.Therefore,effectively regulating the emission spectrum of Er³⁺-doped UC phosphors can enrich their functions and broaden their applications.However,since the fluorescent emissions of Er³⁺ion originate from the inner layer 4f electrons suffering from the shielding effect of the outer5s and 5p electrons,the emission wavelength of Er³⁺ion is basically unchanged,but the intensities of each emission peaks will change with the changes of the phosphor's chemical composition,lattice structure parameters and external environment.Herein,the optical characteristics of Er³⁺are regulated by adjusting the intensity of each emission peak in the up-conversion emission spectrum,and the adopted methods of regulation include changing the concentration of dopant,co-doped ion and concentration,changing the synthesis conditions and environmental temperature,etc.The mechanism is to regulate the interaction processes between the dopant ions,including energy transfer,cross relaxation and non-radiation relaxation,which affect the electron population on each excited state level of Er³⁺and the emission intensity of corresponding peaks.As a result,the spectra of Er³⁺doped up-conversion phosphors are successfully modulated.First of all,the UC emission spectrum of Er³⁺was regulated by changing the chemical composition of the phosphor,which was achieved in Gd₂Mg Ti O₆:Yb³⁺/Er³⁺.Firstly,the optical characteristics of Gd₂Mg Ti O₆:Yb³⁺/Er³⁺were successfully regulated by adjusting the doping concentration of Yb³⁺/Er³⁺.The mechanism is to regulate the electron population of each Er³⁺excited state energy level,which cause a change in the relative intensities of the emission peaks corresponding to each energy level.Based on the train of thought about regulating the spectrum of Er³⁺,the co-doping of Mn⁴⁺was implemented to reduce the number of electrons on the ²H_11/2/⁴S_3/2 energy levels of Er³⁺,owning to the energy transfer from Er³⁺to Mn⁴⁺.As a result,the intensities of green emission peaks decreased with increasing Mn⁴⁺concentration,and the spectra of Gd₂Mg Ti O₆:Yb³⁺/Er³⁺are successfully modulated.The chemical composition modulation technology is relatively mature,but the optical characteristics are irreversible and controllable.In addition,the UC emission spectrum of Er³⁺was regulated by adjusting the environmental conditions of the fluoride NaBiF₄:Yb³⁺/Er³⁺microsphere phosphors.Firstly,the modulation of UC emission spectrum by changing reaction temperature was studied.In addition,the optical characteristics of NaBiF₄:Yb³⁺/Er³⁺and its hydrolysate BiF₃:Yb³⁺/Er³⁺were found to be different.By adjusting the water content of the surrounding environment of NaBiF₄:Yb³⁺/Er³⁺microspheres,the content ratio of NaBiF₄:Yb³⁺/Er³⁺and BiF₃:Yb³⁺/Er³⁺in the hydrolysate was changed,and the UC emission spectrum of Er³⁺was successfully regulated.The modulation method has good controllability,but the optical characteristics are also irreversible.In order to further optimize the modulation method,the fluoride Rb₂KInF₆:Yb³⁺/Er³⁺nanospheres were used as the research object,and the UC emission spectrum of Er³⁺was regulated by adjusting the environmental conditions of phosphor.The modulation method has the advantages of fast response speed and reversible optical characteristics.The mechanism of the above modulation method is temperature-induced phase transition of Rb₂KInF₆ which affects the interaction between the dopant ions.As a result,the UC emission spectrum of Rb₂KInF₆:Yb³⁺/Er³⁺phosphor was regulated by adjusting the ambient temperature.Furthermore,the effect of Rb⁺partial substitution by Cs⁺on the crystal structure of Rb₂KInF₆ was studied.By changing the proportion of partial substitution,the UC emission spectrum of Rb₂KInF₆:Yb³⁺/Er³⁺phosphor was regulated successfully,which proved that the essence of spectral modulation of Rb₂KInF₆:Yb³⁺/Er³⁺phosphor spectrum by changing the temperature is the temperature-induced reversible phase transition of the Rb₂KInF₆.