Influence of the synthesis method on preferential clustering of Yb3+ in CaF2:Yb3+/Er3+ upconverting nanoparticles

Enhancement of Vibrant Up-Conversion in (Ca,Sr)F2:Er,Yb,Al Phosphors via Energy Transfer, Localized Interstitial Distortion, and Au Nanostructures

“…The phase identification and emission spectra of up-conversion CaF 2 :Er,Yb phosphors were determined from the powder XRD patterns and under 980 nm diode laser radiation with a power of 77 mW, respectively, as shown in Figure S1. The CaF 2 host lattice, featuring the Ca 2+ site coordinated with eight fluorine atoms, adopts a well-known cubic fluorite structure (Fd 3̅ m). − When smaller Er 3+ ( r = 1.004 Å, CN = 8) and Yb 3+ ions ( r = 0.985 Å, CN = 8) occupy the Ca 2+ site ( r = 1.12 Å, CN = 8), the XRD patterns of the phosphors, particularly at 2θ = 28–29°, shift to higher angles corresponding to Bragg reflections with increasing dopant content. Effective emission spectra at the concentrations of 1 mol % Er 3+ and 10 mol % Yb 3+ ions in the CaF 2 host lattice were obtained under excitation with a 980 nm wavelength diode laser.…”

“…Additionally, Yb 3+ ions, acting as sensitizers, can be co-doped to enhance up-conversion luminescence through energy transfer to the activators. − The integration of localized surface plasmon resonance (LSPR) from metal nanostructures, which absorb and scatter light with the up-conversion phosphors, further improves luminescence intensity. − In a previous report, the Er 3+ -Yb 3+ -doped NaYF 4 up-conversion phosphor, considered one of the most efficient luminescent materials, exhibited over 1000-fold luminescence enhancement when incorporated with metal–insulator–metal nanostructures . Up-conversion phosphors with low excitation power are attractive for applications in bioimaging, displays, sensors, therapeutics, photovoltaics, etc. − CaF 2 , an alkaline earth fluoride, is acknowledged as an efficient host due to its excellent stability and lower phonon energy, preventing nonradiative decay. − Furthermore, Er 3+ -Yb 3+ -doped CaF 2 phosphors, with a cubic fluorite structure in the Fd 3̅ m space group, are considered a compatible up-conversion luminescence material compared to cubic NaYF 4 :Er 3+ ,Yb 3+ phosphors in previous reports. − The presence of interstitial ion in optical materials is crucial as it significantly decreases the local symmetry of rare-earth ion, leading to increasing homogeneity in the host lattice and effectively enhancing up-conversion intensity. − This occurs because interstitial ions reduce the phonon energy, thereby reducing the nonradiative relaxation rate. LSPR effect, caused by collective oscillation of free electrons, especially in Au metal nanoparticles, further enriches the emission intensity of up-conversion phosphors. − The shape and size of the Au nanoparticles play a crucial role in the LSPR effect as their irradiations are specific to certain wavelengths. − In this study, the photoluminescent emission of up-conversion CaF 2 :Er phosphors was dramatically enhanced to approximately over 1100-fold under excitation with a 980 nm laser diode, showing the synergistic effects of energy transfer, interstitial, distortion, and LSPR.…”