| The near infrared laser excited upconversion materials can be used as bio-fluorescent labels, which have the benefits of deep penetration in tissue, weak autofluorescence, no harmful to organisms, and so on. However, the low upconversion efficiency is still the most serious limitation for their applications. Therefore, the upconversion emission enhancement of nanocrystals is still a focus topic. In this paper, based on rare-earth-ion-doped fluoride materials, we have performed systematic investigations on upconversion emission and near infrared luminescence enhancement.NaREF4 (Gd, Y)as the host of rare-earth doped upconversion luminescent materials matrix, owing to its low phonon energy, physical and chemical stability, low photobleaching and toxicity has attracted many attentions of researchers, and they has been worked many researches about it. However, low efficiency is still the crucial factor, which limits the practical use of many fields, espescially in biological field. In this paper, in order to improve the efficiency, we propsed by preparing continuous solid solution, or introduce the third Ln3+ ions such as Li+, Ce3+. On this basis, our main results obtained as follows:1. Yb3+/Er3+ co-doped NaGdo.79-xYxF4 solid solutions were prepared through a facile EDTA-assisted hydrothermal method, the influence of Y3+ concentrations on UC fluorescence intensity was investigated. The results of XRD revealed that NaGd0.79-xYxF4 nanocrystals agreement with the hexagonal phase. EDX measurements revealed that the solid solubility of Yb3+ in NaYF4 is larger than that of in NaGdF4. Under 980 nm excitation, the blue (2H9/2â†'4I15/2), green (2H11/2,4S3/2â†'4I15/2) and red (4F9/2â†'4I15/2) up-conversion (UC) luminescence of Er3+ ions were observed. The intensity of these emissions strongly depends on ratio of Y3+/Gd3+. The emission intensity of green and red luminescence increases gradually with increasing Y3+ content and achieve a maximum value when Y3+ ions entirely replace the Gd3+ sites. This also can be explained by the different Yb3+ solid solubility in NaGdF4 and NaYF4. Our present work offers a different method to enhance the UC emission.2. β-NaYF4:Yb3+/Er3+/Li+ nanocrystals were prepared through a facile EDTA-assisted hydrothermal method. It is shown that the OH clusters reduced greatly by adding Li+ by fourier transform infrared spectrometry, while there is no change for phonon energy. OH clusters with a larger phonon energy can increase the multiphonon relaxation between two neighboring levels. This is one of the reasons for upconversion photoluminescence enhanced.3. In order to investigate the effect of Ce3+ concentration on the emission properties of Er3+/Yb3+ co-doped NaYF4 nanocrystals, Ce3+/Er3+/Yb3+ tri-doped NaYF4 nanocrystals were prepared through a facile EDTA-assisted hydrothermal method. Under 980 nm excitation, with the increasing of Ce3+ concentration, the emission intensity of Er3+ at 1550 nm (4I13/2â†'4I15/2) band increases firstly and then decreases. The increase of the fluorescence intensity of 1550 nm due to the energy transfer between Er3+ and Ce3+ ions:Er3+:4I11/2+Ce3+:2F5/2â†'Er3+:4I13/2+Ce3+:2F7/2. But when Ce3+ doping concentration is 2.0%, the cross relaxation:Er3+:4I13/2+Ce3+:2F5/2â†'Er3+: 4I15/2+Ce3+:2F7/2 happens, which depopulates the 4I13/2 level of Er3+ and results in the decrease of the emission intensity of Er3+ at 1550 nm band. The properties of this samples showed the potential application as an efficient 980 nm pumped infrared laser medium. |
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