Abstract
Er3+ and Er3+/Yb3+ melilite-based SrLaAl3O7 (SLA) phosphors were synthesized by a facile Pechine method. The differences in emission intensities of 4I13/2 → 4I15/2 transition in NIR region when excited with Ar+ and 980 nm lasers were explained in terms of energy transfer mechanisms. Temperature and power dependence of upconversion bands in the visible region centered at 528, 548 and 660 nm pertaining to 2H11/2, 4S3/2 and 4F9/2 → 4I15/2 transitions were investigated. Fluorescence intensity ratio (FIR) technique was used to explore temperature sensing behaviour of the thermally coupled levels 2H11/2/4S3/2 of Er3+ ions in the phosphors within the temperature range 14–300 K and the results were extrapolated up to 600 K. Anomalous intensity trend observed in Er3+ doped SLA phosphor was discussed using energy level structure. Cytotoxicity of phosphors has been evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay in Bluegill sunfish cells (BF-2). The non-cytotoxic nature and high sensitivity of the present phosphors pay a way for their use in vitro studies and provide potential interest as a thermo graphic phosphor at the contact of biological products.
Highlights
Recent interest on rare earth (RE3+) photoluminescence (PL) is because of the efficient down conversion (DC) and about the wide range application of their upconversion (UC) in the fields of color displays, LEDs, bio-imaging, temperature sensors etc.[1,2,3,4,5]
We describe the experiments done to evaluate the application of SLA doped with Er3+ ion or with the Er3+-Yb3+ ion pair, synthesized by Penchini process, to monitor the temperature by Fluorescence intensity ratio (FIR) method
The crystallite size of the phosphors were calculated using the Debye-Scherrer equation[33], D = kλ / (B cos θ), where D is the average grain size, k (0.9) is the shape factor of the average crystallite, λ represents the Kα1 radiation wavelength of copper, B is the full width at half maximum (FWHM) of the diffraction peak, and θ represents the angle of diffraction, i.e., half of the diffraction angle 2θ
Summary
Followed by a non-radiative (NR) relaxation process as shown in Fig. 5 (b), the Er3+ ions de-excite to 2H11/2 and due to a small energy gap between 2H11/2 and 4S3/2 states, the Er3+ ions can relax fast to the 4S3/2 state resulting in the observed two green emission bands G1 and G2. The increase of R band with Yb3+ concentration is more pronounced This may be due to the cross-relaxation (CR) process 4F7/2 + 4I11/2 → 4F9/2 + 4F9/2 between nearby Er3+ ions due to availability of high number of excited Er3+ ions with effective energy transfer from Yb3+ in their own vicinities as shown in Fig. 5 (b)[40,41].
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