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DEPENDENCE OF QUANTUM YIELD OF UP-CONVERSION LUMINESCENCE ON THE COMPOSITION OF FLUORITE-TYPE SOLID SOLUTION NaYi-x-yYbxEryF4
D.S. Yasyrkina, S.V. Kuznetsov, A.V. Ryabova, D.V. Pominova, V. V. Voronov, R. P. Ermakov, P. P. Fedorov
A. M. Prokhorov General Physics Institute, Russian Academy of Sciences, 38 Vavilov Street, Moscow, 119991, Russia
PACS 78.20-e
A study of the properties of polycrystalline fluorite-type NaY1-x_y YbxEryF4 solid solutions demonstrated that NaY0.87Ybcu0Er0.03F4 and NaYo.885Ybo.1Ero.o15F4 samples produced up-conversion luminescence with 3.35% and 3.62% quantum yields, which were higher than quantum yields of the other NaY1-x-y YbxEryF4 samples. NaY1-x-y YbxEryF4 specimens were prepared by co-precipitation from aqueous solutions by drop-wise addition of rare earth nitrate solutions to aqueous sodium fluoride.
Keywords: fluorides, nanopowders, sodium, yttrium, up-conversion luminescence, quantum yield.
1. Introduction
At the present time, scientists are actively looking for materials that can efficiently transform infrared light to visible light by up-conversion for photodynamic cancer therapy (PDT) [1-3]. In order to satisfy the PDT strict requirements, these materials must have a high photochemical activity (i.e., they must effectively produce reactive oxygen species when excited); their particle size should be ca. 30-80 nm; their excitation should occur at the maximal transparency of human tissue; they should possess a specificity toward target tissues and low toxicity. Among the various substances that are used for photodynamic therapy (oxides, fluorides, CdSe-based quantum dots, etc.), Yb- and/or Er-doped NaYF4 matrices maintain a special place, because of their unusually high up-conversion quantum yields [4-6].
Properties of these matrices stem from the features intrinsic to NaYF4. According to the phase diagram of NaF-YF3 [7] (Fig. 1), NaYF4 exists in the form of two phases: a high-temperature non-stoichiometric cubic phase with fluorite structure (phase F) and a low-temperature hexagonal phase with Gagarinite-type structure (cubic phase F may be prepared in metastable state by low temperature synthesis [1]).
Researchers believe that the most promising up-converter matrix is the low-temperature hexagonal NaYF4 phase, due to its high quantum yield values. This phase can be synthesized by various methods [1], but all of them require special conditions. We have shown [9] that it was possible to prepare hexagonal NaYF4 at room temperature by co-precipitation from aqueous solutions in the presence of some structure-forming substances like polyethyleneimine (PEI), but the obtained products frequently were not thermally stable and were not suitable for fluorescence studies. Literature description of the aforementioned
Fig. 1. NaF-YF3 phase diagram [7, 8]: L — melt, F — cubic fluorite-type
Na0.5-x Y0.5+xF2+2x,
• , A — differential thermal analysis (DTA) data, o — single-phase samples,
® — two-phase samples (X-ray diffraction data)
doped NaYF4 matrices is vast, but limited in its scope [4]. It should be noted that other researchers [1] focused their attention primarily on two compositions only, NaYF4:Yb(20 mol. %):Er(2 mol. %) and NaYF4:Yb(17 mol. %):Er(3 mo. l%), without revealing the reasons for their choice. Also, various techniques were used to measure the quantum yields of their specimens as well as they prepared their samples by different methods [4]. As a result, the described optical materials possess different amounts of different impurities that seriously affect the obtained quantum yield data, and these circumstances make comparison of the existing literature data impossible.
Therefore, the aim of our study was a systematic research of the polycrystalline heat-resistant fluorite-type NaY1-x-yYbxEryF4 solid solutions, prepared by the same method, in order to find dopant concentrations which yielded the highest up-conversion quantum yields.
2. Experimental
We used 99.99 wt. % pure Y(NO3^6H2O, Yb(NO3^6H2O, Er(NO3)3-5H2O (Lanhit, Moscow, Russia), NaF (99 wt. % pure) and double distilled water were used as starting materials. All experiments were carried out in a polypropylene reactor with polypropylene stirring bar and polypropylene solution dispensers at room temperature unless otherwise specified. Dropwise addition of the starting solutions was done under vigorous stirring. Phase compositions of the solid specimens were evaluated by X-ray diffraction (DRON-4M diffractometer; CuKa radiation; graphite monochromator). Calculations of the lattice parameters were done using Powder 2.0 software. We used NVision 40 workstation to record scanning electron microscopy (SEM) images of the formed nanoparticles.
Optical spectroscopy studies included the registration of up-conversion luminescence spectra at 400-900 nm and diffusely reflected excitation laser radiation, followed by calculation of the absolute quantum yields. We used our own assembly that included fiber optic LESA-01-BIOSPEC spectrometer (BIOSPEC, Russia) with a modified integrating sphere (Avantes, Netherlands) and Y-shaped fiber catheter (974 nm laser excitation wavelength; 1 W laser power output; UnoMomento processing software) [10]. The spectrometer was calibrated with LEDs of different wavelengths and known outputs as well as with LabMax®-TO power meter (Coherent, USA). Powder samples were placed between two cover slips, which were then fastened together and placed inside the integrating sphere. The scattered radiation was captured by the optical fibers connected to the spectrometer.
The quantum yield (QY) of the produced up-conversion luminescence was calculated [9] according to equation (1):
„Sample DSample
QY _ Pemitted _ _P emitted__(i)
^ DSample DReference DSample ' V '
P 974_absorbed P974scattered P974scattered
where:
— Penned is power of radiation emitted by the sample and
— P^mbsar^ is the output power of 974 nm laser, absorbed by the sample. The latter is equal to the difference between:
— PR4SaMered - the power of the scattered radiation of the reference specimen and
— P97T£ttered - the power of the scattered radiation of the studied sample. Reference samples were chosen among specimens containing no activator ions (e.g.,
intrinsic NaYF4) in order to enhance the accuracy of our measurements.
3. Sample preparation
Synthesis of fluoride nanopowders was done by co-precipitation from aqueous solution described in detail in [11, 12]. We used aqueous rare earth nitrates and sodium fluoride solutions of the same concentrations (0.35 M). Yttrium and dopant rare earth solutions were premixed before aqueous NaF was added to them under vigorous stirring. All experiments were carried out with 10-fold excess of NaF (calculated for NaRF4 stoichiometry). Precipitates were separated from their mother solutions by decanting. Collected precipitates were washed twice with double distilled water and dried at 35°C under air. Calcination of the obtained samples was performed in an oven at 600°C for 1 hour at a heating rate of 10°C/min. In some cases, we used an Eppendorf 5804 centrifuge to separate precipitates from the mother liquors in order to reduce time of synthetic experiments.
4. Results and Discussion
Samples of NaY1-x-yYbxEryF4 were synthesized with an Yb content (x) which varied from 2 to 90 mol. %, and an Er content (y) that varied from 0.5 to 20 mol. %. According to X-ray analysis, all samples contained only one fluorite-type phase (Table 1). A typical X-ray diffraction pattern of one of the synthesized samples (nominal composition NaY0.6Yb0.3Er0.1 F4) is shown in Fig. 2.
Fig. 2. Typical X-ray diffraction pattern of fluorite-type NaY0 6Yb0.3Er01F4 sample
These single phase specimens contained 50-90 nm particles that formed 200-350 nm agglomerates (Fig. 3). The size of the latter is higher than the required particle size for photodynamic therapy (30-80 nm), but breaking up the agglomerates and dispersion of the individual particles were left outside of the scope of the present paper.
We evaluated NaY1-x-yYbxEryF4 sample compositions with the use of correlation (2) between lattice parameter of Na0.5-xR0.5+xF2+2x fluorite-type solid solutions and content of the rare earth ion [13]:
a = 5.398 + (6.7238 x rcp - 7.259) x (x + 0.13), (2)
Fig. 3. Typical SEM image of the single phase sample with total
NaY06Yb0.3Er01F4 composition
where a is a cubic unit cell lattice parameter (A) and r is the effective RE3+ ionic radius.
Because our samples were doped with ytterbium and erbium, we had to adjust the effective ionic radius of the rare-earth cation according to the following formula:
rcp = Xy * ry + Xyb * ryb + Xet * rEr, (3)
where ry = 1.159 A, ryb = 1.125 A, and rEr = 1.144 A are ionic radii of the corresponding elements [13], and Xy, Xyb and XEr are their molar parts.
Estimated compositions of synthesized samples Na0.5-xR0.5+xF2+2x are given in Table 1.
The quantum yield of up-conversion luminescence in the visible range of the spectrum (QY) was calculated by summarizing the integrated luminescence decay intensities corresponding to the following Er3+ transitions: 2P3/2 111/2 at 480 nm; 2H11/2 ^ 4I15/2 at 525 nm; 4S3/2 ^4I15/2 at 545 nm; and 4F9/2 ^4I15/2 at 665 nm (1 W/cm2 the pump power density). The quantum yields of the other Er3+ transitions in the red region of the spectrum (4F9/2 ^4I15/2 at 665 nm) (QYr) as well as in the green region of the spectrum (2H11/2 ^4I15/2 at 525 nm and 4S3/2 ^4I15/2 at 545 nm) (QYg) and fr/g factor value, which characterizes the ratio of the intensities of the decay up-conversion luminescence in the red and green areas, were also accounted for.
The results of our studies for the up-conversion luminescence quantum yields as a function of ytterbium and erbium dopant concentrations are presented in Table 2 and Figs. 4(a,b).
The intensity of up-conversion luminescence was proportional to the amount of absorbed IR photons [14], but it exhibited a nonlinear dependency from the pump power density: quantum yields were lower at 800 mW/cm2 pump density than at 1000 mW/cm2 (Table 2).
Table 1. The lattice parameters of fluorite-type NaY1-x_yYbxEryF4 samples, their estimated compositions and quantum yields
Samples Sample nominal composition Cubic lattice parameter a, A Calculated composition, Na0.5-x R0.5+xF2+2x QY, % QYr % QYg % fr/g
F391 NaY0.80Yb0.17Er0.03F4 5.483(8) Na0.457R0.543F2.086 0.32 0.29 0.03 2.36
F393 NaYb0.90Er0.10F4 5.448(1) Na0.466R0.534F2.068 0.02 0.02 < 0.01 4.11
F394 NaY0.60Yb0.30Er0.10F4 5.474(1) Na0.463R0.537F2.074 0.07 0.06 0.01 1.27
F398 NaY0.20Yb0.60Er0.20F4 5.454(1) Na0.476R0.524F2.048 0.01 0.01 < 0.01 2.28
F406 NaY0.70Yb0.10Er0.2F4 5.4861(6) Na0.477R0.523F2.046 0.05 0.05 < 0.01 2.25
F407 NaY0.80Yb0.10Er0.10F4 5.438(1) Na0.55R0.45F1.9 0.08 0.07 0.01 2.08
F442 NaY0.63Yb0.17Er0.20F4 5.474(2) Na0.459R0.541F2.082 0.22 0.19 0.03 1.49
F443 NaY0.73Yb0.17Er0.10F4 5.4840(4) Na0.451R0.549F2.098 0.04 0.04 < 0.01 2.10
F461 NaY0.50Yb0.30Er0.20F4 5.474(1) Na0.459R0.518F2.082 0.05 0.05 < 0.01 2.80
F462 NaY0.67Yb0.30Er0.03F4 5.464(3) Na0.487R0.513F2.026 0.22 0.21 0.01 3.89
F465 NaY0.685Yb0.30 Er0.015F4 5.463(1) Na0.49R0.51F2.02 0.41 0.30 0.11 0.69
F471 NaY0.37Yb0.60Er0.03F4 5.4642(5) Na0.462R0.538F2.076 0.11 0.11 < 0.01 5.86
F472 NaY0.385Yb0.60Er0.015F4 5.4632(8) Na0.465R0.535F2.07 0.04 0.04 < 0.01 2.94
F473 NaY0.07Yb0.90Er0.03F4 5.446(1) Na0.482R0.518F2.036 0.07 0.07 < 0.01 5.06
F474 NaY0.085Yb0.90Er0.015F4 5.446(1) Na0.489R0.511F2.022 0.07 0.07 < 0.01 6.18
F647 NaY0.95Yb0.02Er0.03F4 5.493(7) Na0.541R0.459 F 1.918 0.31 0.16 0.15 0.28
F648 NaY0.935Yb0.05Er0.015F4 5.485(2) Na0.357R0.643F2.286 1.41 0.70 0.71 0.25
F685 NaY0.77Yb0.2Er0.03F4 5.476(2) Na0.47R0.53F2.06 1.53 1.11 0.42 0.67
F693 NaY0.87Yb0.10Er0.03F4 5.487(6) Na0.454R0.546F2.092 3.35 2.73 0.62 1.12
F702 NaY0.885Yb0.10Er0.015F4 5.481(2) Na0.466R0.534F2.068 3.62 2.48 1.14 0.55
F704 NaY0.815Yb0.17Er0.015F4 5.499(2) Na0.431R0.569F2.138 2.03 1.62 0.41 1.01
F740 NaY0.895Yb0.1Er0.005F4 5.4887(8) Na0,452 R0,548F2,096 0.99 0.61 0.38 0.41
F741 NaY0.825Yb0.17Er0.005F4 5.4860(7) Na0,452R0,548F2,096 1.18 0.77 0.41 0.47
F743 NaY0.695Yb0.3Er0.005F4 5.4800(7) Na0,454R0,546F2,092 1.79 1.30 0.49 0.67
F744 NaY0.395Yb0.6Er0.005F4 5.464(1) Na0,463R0,537F2,074 0.56 0.51 0.05 2.55
F745 NaY0.095Yb0.9Er0.005F4 5.451(1) Na0,438R0,562F2,124 0.25 0.24 0.01 4.24
Table 2. Sample quantum yield at 800 mW/cm2 and 1000 mW/cm2 pump power
Samples Sample nominal composition Capacity, mW/cm2 QY, % fr/g QYr, % QYfl, %
F693 NaY0.87Yb0.10Er0.03F4 800 1.10 1.38 0.93 0.17
1000 3.35 1.12 2.73 0.62
F685 NaY0.77Yb0.2Er0.03F4 800 1.22 0.75 0.91 0.31
1000 1.53 0.67 1.11 0.42
F702 NaY0.885Yb0.10Er0.015F4 800 2.21 0.55 1.51 0.70
1000 3.64 0.55 2.48 1.14
F704 NaY0.815Yb0.17Er0.015F4 800 1.74 0.97 1.38 0.36
1000 2.03 1.01 1.62 0.41
o.ca 0« 0» m oi o.ii ou ois o.w 0.2
Er. mol. fraction
(a)
Er, mol. fraction
(b)
Fig. 4. The color-coded absolute quantum yields of the up-conversion luminescence NaY1-x-yYbxEryF4 samples vs. doping Yb and Er concentrations: 0.1-0.6 mol. fraction Yb and 0-0.2 mol. fraction Er (a); 0.1-0.4 mol. fraction Yb and 0.005-0.07 mol. fraction Er (b)
Fig. 4 data clearly indicated the composition area with the highest quantum yields correspond to the samples containing 0.5-3 mol. % Er and 10-20 mol. % Yb as well as about 0.15 mol. % Er and about 10 mol. % Yb. NaY0.g7Yb0.10Er0.03F4 (3.35 %) and NaY0 885Yb0.1Er0015F4 (3.62 %) specimens demonstrated the highest quantum yields achieved during our experiments. Their composition was significantly different from the compositions of the samples described in the literature [1]. It is very hard to compare our results with previous data [1, 4], for, as we have already mentioned above, different researchers
utilized different synthetic methods (including different conditions of incongruent crystallization of the samples) and, therefore, impurity content and sample compositions were also different. However, it is worth noting that some authors [15, 16] reported a quantum yield decrease with decreased particle size for sub-micron crystals, whereas the quantum yields of their micron-sized agglomerates were the same as those obtained for bulk samples. All of their samples [15, 16] did not provide up-conversion quantum yields above 3.5% while still being unsuitable for photodynamic therapy because of their large size. In the present study, we achieved quantum yields above 3% using much smaller 50-90 nm particles. Further study of Yb/Er-doped NaYF4 samples as prospective optical materials for use in photodynamic therapy are ongoing, and results will be reported soon.
5. Conclusions
NaY1-x-yYbxEryF4 specimens were prepared by co-precipitation from aqueous solutions by dropwise addition of rare earth nitrate solutions to aqueous sodium fluoride. Study of the properties of polycrystalline fluorite-type NaY1-x-yYbxEryF4 solid solutions demonstrated that NaY087Yb0.10Er0 03F4 and NaY0 885Yb0.1 Er0 015F4 samples produced up-conversion luminescence with 3.35% and 3.62% quantum yields, which were higher than the quantum yields of the other NaY1-x-yYbxEryF4 samples. The 50-90 nm particle size of the synthesized NaY1-x-yYbxEryF4 specimens makes them useful for photodynamic therapy once a practical method for the dispersion of their 200-350 nm agglomerates is found.
Acknowledgements
This work was supported by the grants of the Federal Program "Scientific and scientific-pedagogical personnel of innovative Russia" (State contract N 14.740.12.1343), RFBR (12-02-00851/a, N 12-02-12080-ofi-m, 13-02-12162 ofi-m) and a grant of the leading scientific schools (NSH-341.2012.2). Authors are very grateful to A. Baranchikov for his assistance in scanning electron microscopy studies and A. I. Popov and E. Chernova for their help in the manuscript preparation.
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