Synthesis of Eu3+- and Er3+/Yb3+-doped GdVO4 nanoparticles in reverse micelles and their multifunctional luminescence properties are presented. Using cyclohexane, Triton X-100, and n-pentanol as the oil, surfactant, and co-surfactant, respectively, crystalline nanoparticles with ~4 nm diameter are prepared at low temperatures. The particle size assessed using transmission electron microscopy is similar to the crystallite size obtained from X-ray diffraction measurements, suggesting that each particle comprises a single crystallite. Eu3+-doped GdVO4 nanoparticles emit red light through downconversion upon UV excitation. Er3+/Yb3+-doped GdVO4 nanoparticles exhibit several functions; apart from the downconversion of UV radiation into visible green light, they act as upconvertors, transforming near-infrared excitation (980 nm) into visible green light. The ratio of green emissions from 2H11/2 → 2I15/2 and 4S3/2 → 4I15/2 transitions is temperature dependent and can be used for nanoscale temperature sensing with near-infrared excitation. The relative sensor sensitivity is 1.11%K−1, which is among the highest sensitivities recorded for upconversion-luminescence-based thermometers.
This paper provides the detailed study of (nano)particle's size effect on structural and luminescent properties of LaPO 4 :Eu 3+ synthesized by four different methods: high temperature solid-state, co-precipitation, reverse micelle and colloidal. These methods delivered monoclinic monazite-phase submicron particles (> 100 nm), 4 × 20 nm nanorods and 5 nm spheres (depending on the annealing temperature), 2 × 15 nm nanorods, and ultrasmall spheres (2 nm), respectively. The analysis of emission intensity dependence on Eu 3+ concentration showed that quenching concentration increases with a decrease of the particle size. The critical distance for energy transfer between Eu 3+ ions is found to be 18.2 Å, and the dipole-dipole interaction is the dominant mechanism responsible for the concentration quenching of emission. With the increase in Eu 3+ concentration, the unit-cell parameter slightly increases to accommodate larger Eu 3+ ions at sites of smaller La 3+ ions. Photoluminescent emission spectra presented four characteristic bands in the red spectral region: at 592 nm (5 D 0 → 7 F 1), at 612 nm (5 D 0 → 7 F 2), at 652 nm (5 D 0 → 7 F 3) and at 684 nm (5 D 0 → 7 F 4), while in small colloidal nanoparticles additional emission bands from host defects appear at shorter wavelengths. Intensities of f-f electronic transitions change with particles size due to small changes in symmetry around europium sites, while emission bandwidths increase with the reduction of particle size due to increased structural disorder. Judd-Ofelt analysis showed that internal quantum yield of Eu 3+ emission is strongly influenced by particle's morphology.
The figures of merit of luminescence intensity ratio (LIR) thermometry for Er3+ in 40 different crystals and glasses have been calculated and compared. For calculations, the relevant data has been collected from the literature while the missing data were derived from available absorption and emission spectra. The calculated parameters include Judd–Ofelt parameters, refractive indexes, Slater integrals, spin–orbit coupling parameters, reduced matrix elements (RMEs), energy differences between emitting levels used for LIR, absolute, and relative sensitivities. We found a slight variation of RMEs between hosts because of variations in values of Slater integrals and spin–orbit coupling parameters, and we calculated their average values over 40 hosts. The calculations showed that crystals perform better than glasses in Er3+-based thermometry, and we identified hosts that have large values of both absolute and relative sensitivity.
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