Synthesis And Fluorescence Control Of Nd 3+ Ion Sensitized NaLa(MoO 4 ) 2 Upconversion Phosphor

The rare-earth?RE?doped up-conversion?UC?materials have been widely used in various fields such as three-dimensional?3D?display,information storage and bio-imaging.UC luminescent materials have been paid more attention in biology thanks to their merits in non-autofluorescence,low photobleaching,less photodamage and high light penetration depth in tissues.Among these UC materials,Yb³⁺ ions are usually used as sensitizers and 980 nm infrared laser diodes are used as pumping sources.However,strong absorption of the water in organisms at 980 nm not only causes thermal damage to the body,but also reduces the penetration depth in biological tissue.It is found that using 808 nm lasers as excitation light sources can effectively avoid the above problems.In addition,Nd³⁺ ions show large absorption cross section near 808 nm and are ability to take place Yb³⁺.Therefore,the study of Nd³⁺ sensitized upconversion luminescent materials is of great significance in biomechanics.A series of NaLa?MoO₄?₂:Nd³⁺,Yb³⁺,Ho³⁺ upconversion phosphors were synthesized by sol-gel method.The crystal structures and optimum doped concentration of samples were determined by the X-ray diffraction and up-conversion spectra.A group of sharp line emissions peaked at 543,658 and 1187 nm were observed under 808 nm excitation,respectively,which corresponds to the transitions of ⁵F₄/⁵S₂ ? ⁵I₈,⁵F₅ ? ⁵I₈ and ⁵Ie ? ⁵I₈ of Ho³⁺ ions.The energy transfers among Nd³⁺,Yb³⁺ and Ho³⁺ were confirmed by comparing the fluorescence spectra and the lifetime of Ho³⁺ solely doped,Nd³⁺-Ho³⁺ and Nd³⁺-Yb³⁺ co-doped and Nd³⁺-Yb³⁺-Ho³⁺tri-doped samples.It is further proved that the Yb³⁺ ions act as the bridge in the energy transfer process.The Nd³⁺,Yb³⁺ and TDm³⁺ tri-doped NaLa?MoO₄?₂ up-conversion phosphors were aslo synthesized by sol-gel method.Under the excitation of 808 nm CW excitation,the emission peaks at 478,653 and 692 nm from the 1G4 ? 3H6,1G4 ? 3F4 and 3F3?3H6 transitions of Tm³⁺were found.The UC spectra of Tm³⁺ single-doped,Nd³⁺-Tm³⁺ co-doped Nd³⁺-Yb³⁺-Tm³⁺tri-doped samples and the lifetime of Nd³⁺ and Yb³⁺ in the NaLa?MoO₄?₂:Nd³⁺,Yb³⁺,Tm³⁺system,which prove the occurance of Nd³⁺?Tm³⁺ and Nd³⁺?Yb³⁺?Tm³⁺ energy transfer processes.Yb³⁺ ion play an efficient bridge in the process of energy transfer Nd³⁺?Yb³⁺ ?Tm3.Moreover,on the tuning of the emission color in the NaLa?MoO₄?₂:Nd³⁺,Yb³⁺,Ho³⁺system were also realized through changing the concentration,changing the pulse width and doping Ce³⁺ ions three methods.For these methods,the modulation of emitting color is achieved by changing the relative intensity ratio of red to green emission,but their mechanisms are different.Among them,the emission color of the samples change from orange to yellowish and from yellow to green were realizing by the different population processes of red and green emitting states of Ho³⁺ by adjusting Yb³⁺ concentration and changing excitation pulse width.In addition,introducing Ce³⁺ ion into Nd³⁺/Yb³⁺/Ho³⁺ doped sample also enhances the R/G ratio through increasing probability of nonradiation?NR?processes?5F4/5S2 ? 5F5 and 5I6?5I7?by two cross relaxations between Ho³⁺ and Ce³⁺,which change the fluorescence color from orange to red.