Upconverting SrF₂:Er³⁺ Nanoparticles for Optical Temperature Sensors

Abstract

The upconversion (UC) phenomenon is one of the most frequently investigated in lanthanide ions (Ln 3+ )-doped phosphors. Usually, to obtain UC luminescence, two types of dopants are used: one is a sensitizer, and the other one is an activator, e.g., Yb 3+ /Er 3+ . Exceptional UC luminescence can also be presented by systems doped with one type of Ln 3+ ions, which are characterized by self-sensitization properties, for instance, Er 3+ ions. Herein, the structural and spectroscopic properties of SrF 2 nanoparticles (NPs) doped with Er 3+ ions in concentration from 2.1 to 29.0% were investigated. The SrF 2 :Er 3+ samples, synthesized using the hydrothermal method, were characterized by the crystalline structure and sizes of their formed NPs below 18 nm. The UC emission under 975 and 1532 nm excitations was observed, wherein the most intense luminescence was registered for SrF 2 :11.7% Er 3+ NPs under 975 nm excitation and for SrF 2 :4.7%Er 3+ NPs under 1532 nm excitation. The UC mechanisms were proposed on the basis of the measured dependencies of integral luminescence intensities on the laser power densities and luminescence lifetimes. The thermometric properties of ratiometric luminescent thermometers based on luminescence intensity ratios (LIRs) from 2 H 11/2 and 4 S 3/2 thermally coupled levels in SrF 2 :Er 3+ were evaluated in the 273−373 K temperature range. The interesting temperature dependencies of the UC emission from 4 I 9/2 and 4 I 11/2 states were registered; these have not been previously reported.

External-Field-Independent Thermometric Sensitivity and Green Emission of Upconversion Phosphor Sr2InF7: Yb3+, Er3+

“…Their populations abide by the Boltzmann thermal equilibrium. LIR values of the two green emissions satisfy the following formula LIR = I normalH I normalS = B 0.25em exp ( − Δ E kT ) where I S and I H are the integrated emission intensities at 545 and 520 nm, respectively, k is Boltzmann’s constant, and T is the absolute temperature. The LIR values of phosphor at various temperatures under different external fields were obtained using formula (), as revealed in Figure a,c.…”

“…In addition, absolute and relative sensitivities ( S a , S r ) are indispensable significant parameters for thermometers. The specific calculation formula is as follows S normalr = d L I R L I R × d T = normalΔ E k T 2 S normala = d L I R d T = L I R normalΔ E k T 2 …”

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

“…UC phosphors (UCPs) have found applications in four main aspects of modern days, that is, energy, security, health, and environment. UCPs are effective in improving the conversion efficiency in solar cells, bioimaging, and disinfecting bacteria present in water. − Near-infrared (NIR) excitation of UCPs is particularly useful for bioimaging owing to deep penetration depth, low or no autofluorescence, and reduced light scattering compared to ultraviolet (UV)/blue-green light excitation . Generation of violet-blue photons from NIR light is also quite important for biomedical applications, photoinduced drug delivery, photodynamic therapy, and optogenetics where large anti-Stokes shift is needed .…”

“…Proper selection of host matrices has a strong influence on UC and DC efficiencies since the distance between lanthanide ions and their relative spatial location, local coordination number, and environment is determined by host lattices . An ideal host matrix should be transparent in the spectral range of interest, possess a high optical damage threshold with low maximum phonon energy, and maintain chemical stability. , Lanthanum fluoride (LaF 3 ) is considered to be an excellent UC/DC host as it meets the above-mentioned criteria to endow both efficient NIR and MIR DC emissions and visible and UV UC emissions . As reported, LaF 3 is optically transparent from the UV (250 nm, n = 1.63) to far infrared (up to 7 μm) range. , Based on its index of refraction value, that is, 1.4872 at 2.8 μm, LaF 3 is considered as an excellent host for MIR emission compared to that of visible UC.…”

“…11 An ideal host matrix should be transparent in the spectral range of interest, possess a high optical damage threshold with low maximum phonon energy, and maintain chemical stability. 2,12 Lanthanum fluoride (LaF 3 ) is considered to be an excellent UC/DC host as it meets the above-mentioned criteria to endow both efficient NIR and MIR DC emissions and visible and UV UC emissions. 13 As reported, LaF 3 is optically transparent from the UV (250 nm, n = 1.63) to far infrared (up to 7 μm) range.…”