The influencing factors of chelator, reaction times, and Yb 3+ /Tm 3+ -doped concentration on the formation of NaYF 4 :Yb,Tm crystals with various morphologies have been investigated and determined. It is found that NaYF 4 :Yb,Tm with various morphologies and different phases can be tuned. Compared with the addition of ethylene diamine tetra acetic acid, hexagonal nanoprism-like β-NaYF 4 :Yb,Tm with a mean length of ~ 1300 nm and diameter of ~ 100 nm is prepared in the presence of oleic acid, which preferentially orients along with the [0001] direction more quickly than others. Systematical research on the emission of Yb 3+ /Tm 3+ -doped NaYF 4 crystals with microrod, hexagonal microprism, and nanoplate has implied that these obtained crystals with unique optical properties are closely related to their morphologies, phases, and sizes. In addition, a possible growth mechanism of co-existence of α- and β-NaYF 4 phases is proposed. It can be expected to illustrate the formation mechanism for the upconversion crystals or other rare-earth upconversion materials and to import these highly luminescent microcrystals or nanocrystals to environmental remediation by full solar energy spectra.
In the field of disease diagnosis and treatment, achieving highly sensitive temperature sensing via non-contact methods based on fluorescence continues to present a significant challenge. This study investigated the influence of the cross-relaxation (CR) process of Tm 3+ ions at varying concentrations and the negative thermal expansion (NTE) property of the matrix on the luminescent properties of Sc 2 (MoO 4 ) 3 :Yb 3+ /Tm 3+ materials under diverse temperature conditions, and further evaluated their potential application value in the field of temperature sensing. At room temperature, the analysis of the concentration dependence of the emission spectra reveals that the intensities of the emission peaks originating from higher energy levels ( 1 G 4 , 3 H 4 , and 3 H 5 ) and the lowest excited state ( 3 F 4 ) exhibit significant variation with the doping concentration. As the doping concentration increases, the temperature enhancement factor for the fluorescence intensity of high-energy level transition ( 1 G 4 → 3 H 6 ) progressively decreases from 21.6 to 1.2. In contrast, the temperature enhancement factor for the lowest excited state transition ( 3 F 4 → 3 H 6 ) markedly increases from 471 to 2964. At a concentration of 8%, the temperature extension factor of the fluorescence lifetime at 1627 nm ( 3 F 4 → 3 H 6 ) can reach 7.2, whereas at a concentration of 0.5%, this factor is 2.7. These suggests that the CR process between Tm 3+ ions exerts markedly distinct influences on the luminescence intensities and lifetime in Sc 2 (MoO 4 ) 3 :Yb 3+ /Tm 3+ materials. In addition, this study employs the fluorescence intensity ratio of I 1629 nm / I 1197 nm for temperature sensing, thereby achieving high sensitivity in temperature measurement.
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