Up- and Down-Convertible LaF₃:Yb,Er Nanocrystals with a Broad Emission Window from 350 nm to 2.8 μm: Implications for Lighting Applications

Abstract

In this study, emissions between 350 nm and 2.8 μm are produced from molten-salt-synthesized LaF3:Yb3+,Er3+ (LFYE) nanocrystals (NCs) under 980 nm excitation. In fact, such wide spectral emissions from a single phosphor would be highly favorable in lighting applications ranging from light-emitting diodes, optical fibers, and telecommunications. Upconversion luminescence (UCL) spectra show intense red emission compared to the green band below 6 mW, wherein a strong green emission is achieved compared to a red emission when the laser power is increased beyond 85 mW. To further enhance their UCL and downconversion luminescence (DCL), the LFYE NCs are annealed at 400 °C for 30, 60, 90, and 120 min. Their UCL and DCL intensities are improved with up to 90 min of annealing but drastically reduced after annealing for 120 min. The latter is caused by both phase transition and shape changes, that is, from the cubic LFYE NCs to tetragonal LaOF:Yb3+,Er3+ (LOFYE) NCs with a square pyramidal shape. Density functional theory calculations show that the LOFYE NCs are more thermodynamically favorable to form due to their lower defect formation energy than the LFYE NCs. Moreover, LaF3 is dynamically more unstable in comparison to LaOF, as indicated by a greater number of imaginary modes for LaF3 in the phonon dispersion plot. Interestingly, the highest phonon energy for LaF3 is found to be 439 cm–1 while that of LaOF is 527 cm–1, which is expected to play a role in the different emission behaviors between LaF3 and LaOF. Our work shows exemplary potential of the LFYE NCs with near-ultraviolet to mid-infrared emissions as UCL and DCL for laser power-induced color tunability for a wide range of applications.

Highly thermostable fluoride nanocrystal-in-glass composites (NGCs) for mid-infrared emission

“…The primary MIR transition of Er 3+ : 4 I 11/2 → 4 I 13/2 at ∼2.7 μm covers the atmosphere communication window and strong absorption band of water. 29,30 Meanwhile, this simple MIR transition process can effectively avoid the introduction of additional factors affecting the MIR emission of the as-prepared NGC materials, which is advantageous for the demonstration of our strategies. Accordingly, Er 3+ was chosen as the target activator.…”

Tunable upconversion in ZnAl2−xGaxO4:Er,Yb phosphors by modulating the Al/Ga ratio and application in optical thermometry

“…Mostly NIR-II emission is auto fluorescence-free, having high signal to noise ratio and deeper penetration depth in biological tissues. 7…”

“…16 The combination of the Er and Yb system has been very well known for many years in the UC process and it is the most efficient UC system which is known. 5–8,14,17…”

“…Among this the choice of host lattice plays a significant role for UC. Many reported UC materials are composed of fluoride-based host systems such as YF 3 , 19 NaYF 4 , 5,8 NaBiF 4 , 20 NaGdF 4 , 21 NaLuF 4 , 15 LaF 3 , 7 etc. Even though they have a low phonon energy and high fluorescence quantum yield, they are not suitable for real situations in terms of their poor chemical stability, complex synthesis procedure, and environmental pollution.…”

Utilizing Energy Transfer in Mn2+/Ho3+/Yb3+ Tri-doped ZnAl2O4 Nanophosphors for Tunable Luminescence and Highly Sensitive Visual Cryogenic Thermometry

“…Upconversion (UC) materials have garnered global attention of researchers due to their applications in the field of bioimaging, image-guided cancer therapy, luminescence thermometry, solid-state lighting, and anti-counterfeiting. − In the past decades, lanthanide (Ln 3+ )-doped phosphors emerged as potential UC materials on account of their high photostability, large anti-Stokes shift, longer lifetimes, sharp tunable emissions, and ladder-like 4f energy levels. The use of near-infrared (NIR) excitations lying in the “optical transparency window” of biological tissues (usually 700–1100 nm) is promising for bioapplications and has several well-known merits over downconversion (DC) materials such as deep tissue penetration, less autofluorescence, safe to DNA and tissues, and lower phototoxicity. ,, …”