Manipulating Upconversion Luminescence Of Lanthanide Fluoride Nanoparticles

Photon upconversion in lanthanide-doped upconversion nanoparticles?Ln-UCNPs?is promising for applications such as display,solar cell,and bio-imaging.Despite these attractions,the applications of UCNPs are still restricted due to their low UC intensity.On the one hand,weak upconversion luminescene is typically caused by the parity-forbidden characteristics of f–f transitions for lanthanide ions;on the other hand,concentration quenching occurs in heavily doped Ln-UCNPs containing sensitizer and activator because nonradiative energy losses including surface quenching and cross-relaxation between neighboring dopant ions,will dominate in this case.Thus,it still remains an open challenge to increase the luminescence intensity of UC materials.In this paper,transition metal ion doping and manipulation of energy transfer have been adopted to overcome the above obstacles and to enhance the upconversion luminescence for Ln-UCNPs.The main results are as follows:We report the enhancement of green upconversion luminescence in tetrahedral LiYF₄:Yb/Er nanoparticles by Mn²⁺ion doping,which is different from the enhanced single-band red emission dominated by Mn²⁺ions in cubic NaLnF₄:Yb/Er?Ln=Y,Gd,Lu?nanoparticles.The energy levels of the first excited state ⁴T₁ of Mn²⁺in tetrahedral LiYF₄:Gd and cubic NaGd?Y?F₄ are compared by detection of emissions from Mn²⁺via the energy transition from Gd³⁺to Mn²⁺with excitation at 275 nm.The coordination environments of Mn²⁺in these two host lattices have been investigated by X-ray absorption fine structure measurements.The results demonstrate that the formation of tetrahedral MnF₄ in tetragonal LiYF₄ arising from the re-placement of Ln³⁺ions with Mn²⁺ions leads to a higher energy level of the Mn²⁺⁴T₁ state than that in octahedral MnF₆ in cubic NaYF₄.The high-lying excited state of tetrahedral MnF₄ is close to the green emitting ⁴S_3/2 state of Er³⁺and thus enhances green upconversion emission in tetragonal LiYF₄:Yb/Er,while the low-lying excited state of octahedral MnF₆ dominates red emission in cubic NaYF₄:Yb/Er.These findings provide direct evidence for the key roles of the host lattices and more possibilities in modulating the upconversion behaviour of lanthanide-based nanoparticles by transition-metal ion doping to achieve the desired goals of specific applications.Photon upconversion in lanthanide-doped upconversion nanoparticles offers a wide variety of applications such as deep-tissue biophotonics.However,the upconversion luminescence and efficiency,especially involving multiple photons,is still limited by the concentration quenching effect.Here,we demonstrate a multilayered core-shell-shell structure for lanthanide doped NaYF₄,where Er³⁺activators and Yb³⁺sensitizers are spatially separated,which can enhance the multiphoton emission from Er³⁺by 100-fold compared with the multiphoton emission from canonical core-shell nanocrystals.This difference is due to the excitation energy transfer at the interface between activator core and sensitizer shell being unexpectedly efficient,as revealed by the structural and temperature dependence of the multiphoton upconversion luminescence.Therefore,the concentration quenching is suppressed via alleviation of cross-relaxation between the activator and the sensitizer,resulting in a high quantum yield of up to 6.34%for this layered structure.These findings will enable versatile design of multiphoton upconverting nanoparticles overcoming the conventional limitation.