RESUMO
We report a comprehensive study of the structural, morphological, and optical properties, and UC-based ratiometric temperature sensing behavior of (α) cubic and (ß) hexagonal phases of NaYF4:Yb3+/Er3+ nanoparticles. The α-NaYF4:Yb3+/Er3+ and ß-NaYF4:Yb3+/Er3+ nanoparticles were synthesized using co-precipitation and hydrothermal methods, respectively. Powder X-ray diffraction studies confirmed the phase purity of the samples. The morphological studies show uniform particle sizes of both phases; the average particle size of α-NaYF4:Yb3+/Er3+ and ß-NaYF4:Yb3+/Er3+ was 9.2 nm and 29 nm, respectively. The Raman spectra reveal five sharp peaks at 253 cm-1, 307 cm-1, 359 cm-1, 485 cm-1, and 628 cm-1 for ß-NaYF4:Yb3+/Er3+, whereas α-NaYF4:Yb3+/Er3+ shows two broad peaks centred at 272 cm-1 and 721 cm-1. The optical property measurements show that α- and ß-NaYF4:Yb3+/Er3+ phases have distinct upconversion emission and temperature sensing behavior. The upconversion emission measurements show that ß-NaYF4:Yb3+/Er3+ has higher overall emission intensities and green/red emission intensity ratio. The temperature-dependent upconversion emission measurements show that α-NaYF4:Yb3+/Er3+ has higher energy separation between 2H11/2 and 4S3/2 energy states. The temperature sensing performed utilizing these thermally coupled energy levels shows a maximum sensitivity of 0.0069 K-1 at 543 K and 0.016 K-1 at 422 K for ß-NaYF4:Yb3+/Er3+ and α-NaYF4:Yb3+/Er3+, respectively.
RESUMO
We demonstrate an enhancement in the upconversion (UC) emission and temperature sensing property of a CaMoO4:Er/Yb phosphor via distortion of the local symmetry environments and reduction in no-radiative channels. Bi3+ ion co-doping creates a local distortion while the average tetragonal structure of CaMoO4 remains intact. This creates asymmetry around the Er3+ ions which improves the UC emission. Furthermore, our calculations on XRD data show a reduction in the dislocation density and the micro-strain in the crystal with the introduction of Bi3+, which also favours the enhancement of UC emission as it reduces the non-radiative channels. Furthermore, the effect of this enhancement on the temperature sensing property of Er3+ ion has also been revealed. Our results show that the UC emission is enhanced about 25 times for Bi3+ co-doped samples which improves the temperature sensitivity significantly. The samples, both with and without Bi3+ co-doping, exhibited relative sensitivities of 0.0068 K-1 at 300 K and 0.0057 K-1 at 298 K which is a significant improvement and indicates the potential of the material for temperature sensing applications. This proof-of-concept provides a deeper understanding of the effect of Bi3+ doping on UC emission and opens new avenues for the development of high-performance temperature sensing materials.
RESUMO
Optical temperature sensing is widely realized by using upconversion (UC) emission in lanthanide-doped phosphors. There are various parameters that are responsible for UC intensity of the phosphor like particle shape and size, type of symmetry that exist at the site position, distribution of lanthanide ions in the phosphor, and so on. However, a comparative study of the bulk and nanostructure on the temperature sensing ability of such phosphor is rare. In the present work, we have taken Ca0.79Er0.01Yb0.2MoO4phosphors as a model system and synthesized its bulk (via solid-state reaction method, named SCEY) and nanostructures (via solution combustion route, named CCEY). We further studied their phase, crystal structure, phonon frequency, optical excitation, and emission (upconversion & downshifting) properties. Finally, the optical temperature sensing behavior of SCEY and CCEY, in the range 305 K-573 K, have been compared. The maximum relative sensitivity of the phosphor SCEY and CCEY are 0.0061 K-1at 305 K and 0.0094 K-1at 299 K, respectively, while, the maximum absolute sensitivities are 0.0150 K-1at 348 K, and 0.0170 K-1at 398 K, respectively. We thus conclude that the temperature sensing ability of nanoparticle-based Ca0.79Er0.01Yb0.2MoO4phosphor is better compared to its bulk phosphor.
