Enhancing Upconversion Luminescence of LiYF₄:Yb,Er Nanocrystals by Cd²⁺ Doping and Core–Shell Structure

Abstract

In this work, we report a phenomenon that upconversion emission intensity of Er3+ was enhanced by cadmium. Cd2+ was taken with 0, 3, 10, 20, 30, and 40 mol % as a new dopant to codope LiY0.78Yb0.20Er0.02F4, meanwhile the doping effects on the crystal structure, morphology, and upconversion fluorescence emission are investigated by XRD, Rietveld refinement, TEM, and upconversion spectroscopy analysis methods in detail. The green upconversion emission intensity was enhanced, and the maximum emission intensities were enhanced to twice when 10 mol % Cd2+ was codoped into tetragonal LiYF4, while its decay lifetime was reduced. Additionally, we further designed the synthesis of the homogeneous LiYF4:20%Yb,2%Er,10%Cd@LiYF4 of which the emission intensity is 11 times higher than that of the Cd2+-free nanocrystals, meanwhile an intriguing morphology of core–shell nanocrystals was obtained. This investigation may be useful for the design and improvement of upconversion materials for fulfilling the prerequisite of the practical application.

Extremely straightforward room temperature co-precipitation method to synthesize cubic KYF4:Yb/Er up-conversion nanoparticles in deionized water-ethanol solution

“…With further research, many advantages of UC nanomaterials have been presented, such as low autofluorescence background, the absence of blinking and photobleaching, superior photostability, tunable and sharp-band emission, long lifetimes, large anti-Stokes shifts and so on. [5][6][7][8][9][10] As a result, it was studied by many researchers. Among the multitude of reported host materials, especially the fluoride nanoparticles get much attention mainly for their low lattice phonon energy which can reduce thermal lensing effects under strong pumping conditions to reduce non-radiative losses and therefore enhance luminescence efficiency.…”

“…11 Combining with its high chemical and thermal stability and narrow emission lines makes it particularly suitable for IR conversion. Rare earth doped fluoride up-conversion nanoparticles have a potential application in a wide range of fields: 8,12 solid-state lasing, optical data storage, display, photovoltaics and biological imaging.…”

LiYF4:Yb/LiYF4 and LiYF4:Yb,Er/LiYF4 core/shell nanocrystals with luminescence decay times similar to YLF laser crystals and the upconversion quantum yield of the Yb,Er doped nanocrystals

“…Despite these disadvantages, the above mentioned reactions are frequently used to synthesize both, the LiYF4 particles as well as the LiYF4 shell. To nevertheless minimize the nucleation of new LiYF4 particles, the solution of the shell precursor is often added very slowly or in small portions to the solution of the core particles [78,81,88].…”

“…For example, X. Xue et al determined  UC values of about 0.3% and 0.4% for nanoparticles without shell and sizes of 30 and 100 nm, respectively [72].  UC measurements of relatively large diamondshaped LiYF4:20%Yb,2%Er/LiYF4 core/shell nanocrystals (length about 200 nm, width ≥ 70 nm) in cyclohexane revealed a value of 1.05% [78]. Significantly higher  UC values of 2.28% and 5.7% (P = 6.2 W•cm −2 ) have been reported, however, for Cd 2+ co-doped LiYF4:20%Yb,2%Er,10%Cd/LiYF4 core/shell nanocrystals and for single crystals of LiYF4 doped with only 2% Figure 6 Upconversion quantum yield of powders of LiYF4:Yb,Er core and core/shell particles as function of excitation power density P. The quantum yield of the 18 nm LiYF4:Yb(18%),Er(2%) core particles is one to two orders of magnitude lower than the quantum yield of the corresponding LiYF4:Yb(18%),Er(2%)/LiYF4 core/shell particles.…”

Constructing a “Native” Oxyfluoride Layer on Fluoride Particles for Enhanced Upconversion Luminescence

“…The surface shell can be grown epitaxially or nonepitaxially, depending on the degree of core/shell lattice match and the preparing method . Epitaxial inert shells generally show high photochemical stability and can separate spatially lanthanide ions from surface quenching sites . Several homogeneous core/shell nanocomposites have been synthesized, such as LaF 3 :Yb/Ho@LaF 3 , NaYF 4 :Yb/Tm(Er)@NaYF 4 , YOF:Yb/Er@YOF, and NaYF 4 :Yb/Er@CaF 2 .…”