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Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors
Dyes and Pigments 155 (2018) 233–240
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Multicolor-tunable emissions of YOF: Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors
T
Jovana Periša, Jelena Papan, Slobodan D. Dolić, Dragana J. Jovanović∗∗, Miroslav D. Dramićanin∗ Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, 11001, Serbia
A R T I C LE I N FO
A B S T R A C T
Keywords: YOF Downshifting emission Upconversion CIE coordinate Color Lanthanides
Color tuning of down-shifting and up-conversion emissions of YOF:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors is demonstrated. Nanophosphors were prepared by the modified sol-gel Pechini method and characterized by the X-ray diffraction, transmission electron microscopy, and photoluminescence spectroscopy. Samples consist of 20 nm particles crystallized in the rhombohedral crystal structure. Depending on the Ln3+/ Yb3+ concentration ratio and the type of excitation (UV/VIS or NIR) color of the particle's emission varied from the blue to red. Commission Internationale de L'Eclairage chromaticity coordinates of emission colors are given for the range of Ln3+/Yb3+ concentration ratios for both down-shifting and up-conversion luminescence. We showed that the emission color of these nanophosphors may be additional tuned by simultaneous excitation with UV-VIS and NIR radiation (in different proportion) which yields unique color labels for the anti-counterfeit and security applications.
1. Introduction Lanthanide doped nanophosphors have been a matter of interest over the past years for their utilization in many important technologies, such as lasers, electronics, therapeutics, displays, solar cells, catalysis, medicine [1–5]. In addition, there is a growing interest for their use as bio-markers or nano-sensors, instead of traditional ones [6]. The application of these materials is based on their unique optical characteristics which arise from partially filled f-orbitals of lanthanide ions [7,8]. Multicolor luminescence of trivalent lanthanide (Ln3+) doped nanomaterials can be realized using different ions whose f - f electronic transitions provide different-color emissions that span the complete visible–near-infrared spectral range. Also, it can be achieved by both types of excitation schemes, down-shifting (DS) and up-conversion (UC). In DS, the high-energy radiation (usually ultraviolet, UV) is used to excite phosphor's emission of lower energy photons. UC is an antiStokes emission process in which two or more low-energy photons (usually near-infrared, NIR) excite phosphor's emission of higher energy via multistep optical processess [2]. In both cases, phosphor luminescence is characterized by long-lived electronic excited states and narrow emission bands. Materials doped with activator Ln3+ ions (Ln3+ = Ho3+, Er3+, Tm3+, Tb3+) in different concentration ratios with Yb3+ (sensitizer
∗
ion) may provide multicolor-tunable emissions in both DS and UC excitation schemes [9]. Several Ln3+-activated nanophosphors with multi-color tunable emission have been reported so far, such as GdVO4:Ln3+/Yb3+ (Ln3+ = Ho, Er, Tm, Ho/Er/Tm), Ce3+-Mn2+ doped Y7O6F9, YOF:Ln3+(Ln = Tb, Eu, Tm, Dy, Ho, Sm), lanthanide metal-organic frameworks, Cu (Mn)-doped ZnInS, LuVO4:Tm3+/Dy3+/ Eu3+ [4,8,10–14]. However, to the best of our knowledge, there are no reports on the use of rare earth oxyfluorides for combined multi-color DS and UC emissions. These materials are promising hosts for preparation of phosphors due to their attractive chemical and physical properties which may be classified as in between characteristics of fluorides and oxides [1,15–17]. Alike fluorides, oxyfluorides have low phonon energy and high ionicity which leads to the efficient luminescence, while, on the other hand, these compounds have great chemical and thermal stabilities which resemble properties of oxides [2,3,18]. In addition, oxyfluoride nanomaterials are biocompatible and nearly nontoxic to live cells so they can be safely used for biomedical applications [5,19]. Zachariasen described oxyfluorides' (YOF and LaOF) crystal structures in 1950`s [20]. Generally, they can be found in three structural modifications: tetragonal, which is usually nonstoichiometric (REOnF3-2, 0.7 < n < 1), rhombohedral, and cubic (β-YOF). The rhombohedral structure (r-YOF) is the stable one, and it is strictly stoichiometric (n = 1). The cubic β-YOF structure occurs as the result of
Corresponding author. Corresponding author. E-mail addresses: [email protected] (D.J. Jovanović), [email protected] (M.D. Dramićanin).
∗∗
https://doi.org/10.1016/j.dyepig.2018.03.047 Received 12 February 2018; Received in revised form 14 March 2018; Accepted 23 March 2018 Available online 26 March 2018 0143-7208/ © 2018 Elsevier Ltd. All rights reserved.
Dyes and Pigments 155 (2018) 233–240
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2. Experimental
Table 1 Sample names and concentration ratio (Ln3+/Y3+) for preparation of different YOF: Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors. Samples name
Concentration ratio (mol %)
2.1. Reagents
mmol
All chemicals: yttrium(III) nitrate hexahydrate, Y(NO3)3 × 6H2O (99.9%, Alfa Aesar), ytterbium(III) nitrate pentahydrate, Yb(NO3)3 × 5H2O (99.9%, Alfa Aesar), holmium(III) nitrate pentahydrate, Ho (NO3)3 × 5H2O (99.9%, Alfa Aesar), erbium (III) nitrate hydrate, Er (NO3)3 × n H2O (n ˜ 5) (99.9%, Alfa Aesar), thulium(III) nitrate hydrate, Tm(NO3)3 × n H2O (n ˜ 5) (99.9%, Alfa Aesar), ammonium fluoride, NH4F (98%, Alfa Aesar), citric acid (99.6%, Acros Organics) and ethylene glycol (> 99%, Sigma Aldrich) were of the highest purity available and were used without any further purification.
Ho3+/Y3+ 0.005Ho 0.01Ho 0.015Ho 0.02Ho
0.005/0.895 0.01/0.89 0.015/0.885 0.02/0.88
0.00125/0.224 0.0025/0.222 0.00375/0.221 0.005/0.220
Er3+/Y3+ 0.01Er 0.015Er 0.02Er 0.025Er
0.01/0.89 0.015/0.885 0.02/0.88 0.025/0.875
0.0025/0.222 0.00375/0.221 0.005/0.220 0.625/0.219
2.2. Syntheses of YOF and YOF: Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors
Tm3+/Y3+ 0.02Tm 0.025Tm 0.03Tm
0.02/0.88 0.025/0.875 0.03/0.87
The modified sol-gel Pechini method, previously described by Grzyb and co-workers [28,29], was used to prepare YOF:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+,Tm3+) nanophosphors with different Ln3+/Yb3+ concentration ratios. Concentration of Yb3+ was kept constant in all the samples (10 mol% (0.1), 0.025 mmol), while concentrations of other Ln3+ ions (see Table 1) were chosen based on the experience with similar nanophosphors. The synthesis of undoped YOF nanophosphor. Weighted amount (0.25 mmol) of Y(NO3)3 was dissolved in 90 ml of water. Citric acid (as the chelating agent) and ethylene glycol (as complexing agent of Y3+ cations) were added into the solution. To prevent lanthanide fluorides from precipitating, a large excess of citric acid and ethylene glycol was used [1]. After the intense stirring of the solution at room temperature for 30 min, an aqueous solution of NH4F (0.31 mmol NH4F, 10 ml H2O) was slowly added drop-wise into solution. Since NH4F decomposes at 100 °C, the 25% excess to the stoichiometric amounts of nitrate precursor was used. The prepared mixture was additionally heated at 80 °C for 4 h (the time needed for the water evaporation and the gel formation). Final product was obtained through calcination of gel precursors at 700 °C in air for 4 h. After calcination, the sample was kept overnight in furnace to cool down to room temperature. The synthesis of YOF:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors. Lanthanide doped YOF were prepared in analogy to the procedure described for the undoped YOF system with only difference being the addition of the appropriate amount of the respective Ln-nitrates to the starting Y(NO3)3 solution (see Table 1).
0.005/0.220 0.00625/0.218 0.0075/0.217
the phase transition from the rhombohedral YOF at temperature of 560–570 °C [1,21]. Nonstoichiometric compounds usually have a tetragonal crystal form, but additional studies have shown that these materials are more complex than it was previously assumed; one example are compound with general formula REnOn-1Fn+2, 5 ≤ n ≤ 9 such as Y5O4F7, Y6O5F8 and Y7O6F9, which crystallize in an ortorhombohedral form [10,19,22–25]. To prepare oxyfluorides, several methods can be employed: thermolysis method, sol-gel method, fluorolytic sol-gel, hydrothermal method, co-precipitation method, solid state synthesis, combustion synthesis, stearic acid method, urea based homogenous precipitation [4,16,23,26,27]. Here, the goal was to prepare Ln3+/Yb3+-doped YOF (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors with multicolor-tunable DS and UC emissions under UV/VIS and NIR excitations. DS in Yb3+/ Ln3+ doped hosts is a considerably more complex process than DS in a single Ln3+ activated hosts. Yb3+,Ln3+ pairs offer a number of routes for the cross-relaxation between different Yb3+ and Ln3+ energy levels, depending on phonon energies of the host, temperature and excitation energy, which affect intensities of Ln3+ emissions (and sometimes providing Yb3+ emission via quantum-cutting) and consequently affect DS emission colors. In addition, we aimed to explore for the first time (to the best of our knowledge) the combined UV/VIS and NIR excitation of Yb3+/Ln3+ to facilitate the luminescence coding with unique emission colors suitable for anti-counterfeiting and security applications. To achieve this goal, nanophosphors were prepared by the optimized and modified sol-gel Pechini method, and their structural and luminescence properties are studied in details.
2.3. Characterization The X-ray diffraction (XRD) analysis of the obtained nanopowder samples was performed on Rigaku SmartLab system (Cu-Kα radiation, 30 mA current, 40 kV voltage) in the 2θ range from 10° to 90° (using continuous scan of 0.7 s−1). JEOL JEM-2100 LaB6 transmission electron
Fig. 1. a) and b) Typical TEM images for un-doped YOF nanoparticles at two different magnifications; c) size distribution histogram of YOF nanocrystals.
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Fig. 2. XRD patterns of (Y0.9-cLnc)OF:Yb0.1 (Ln = Ho, Er, Tm) nanophosphors doped with different concentration ratios (c) of: a) Ho/Y, b) Er/Y, c) Tm/Y; d) schematic representation of YOF rhombohedral crystal structure.
microscope (TEM) with accelerating voltage of 200 kV and equipped with the high-resolution (HR) style objective-lens pole piece has been used to study the microstructure of the obtained samples. TEM samples were prepared by dipping a carbon-coated copper grid into toluene or water dispersions and the grids with the nanocrystals were dried in air. Diffuse reflection spectra were recorded using Shimadzu UV-2600 (Shimadzu Corporation, Japan) spectrophotometer equipped with an integrated sphere (ISR-2600 Plus) in a 220–1300 nm range. UC emission spectra were measured upon excitation with 980-nm radiation (MDLH 980 3w diode laser with controller) and detected with an AvaSpec-2048 Fiber Optic Spectrometer system. DS emission measurements were performed on a spectrofluorometer system that comprises the optical parametric oscillator excitation source (EKSPLA NT 342), spectrograph FHR 1000 (Horiba Jobin-Yvon, 300 grooves mm−1 grating), and ICCD detector (Horiba Jobin-Yvon 3771).
Table 2 Structural parameters of (Y0.9-cLnc)OF:Yb0.1 (Ln = Ho, Er, Tm) nanophosphors calculated using Rietveld refinement of XRD data. Sample name
Average crystal size (nm)
a = b (Å)
c (Å)
V (Å3)
Microstrain (%)
YOF 0.005Ho 0.01Ho 0.015Ho 0.02Ho 0.01Er 0.015Er 0.02Er 0.025Er 0.02Tm 0.025Tm 0.03Tm 0.035Tm
22.2 (3) 15.6 (8) 16.9 (7) 15.7 (9) 12.7(14) 13.9 (2) 20.6(6) 12.4(16) 12.6(3) 13.9 (16) 14.5 (2) 19.8 (8) 15.7(8)
3.801 (11) 3.803 (15) 3.800 (7) 3.799 (3) 3.803 (8) 3.800 (3) 3.7960 (8) 3.801 (8) 3.800 (14) 3.799 (9) 3.799 (10) 3.793 (14) 3.796 (7)
18.879 (2) 18.840 (6) 18.827 (4) 18.830 (3) 18.841 (4) 18.848(12) 18.843 (4) 18.847 (5) 18.843 (7) 18.844 (5) 18.839 (5) 18.820 (3) 18.832 (4)
272.756 272.479 271.862 271.762 272.494 272.165 271.520 272.294 272.092 271.964 271.892 270.760 271.362
0.145 (7) 0.661 (11) 0.296 (16) 0.210 (3) 0.110 (2) 0.250(11) 0.126 (19) 0.278 (11) 0.270 (2) 0.198(11) 0.110 (3) 0.209 (19) 0.193 (4)
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3. Results and discussion 3.1. Microstructural and structural properties of YOF:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors All samples are nanocrystalline with similar morphology of particles. Fig. 1 shows the representative TEM image of undoped- YOF particles which exhibit a plate-like morphology and are about 20 nm in size (when measured from the edge to edge). The majority of nanoplates have hexagonal shapes, and the rest of particles are either irregular polygons or spheres. X-ray diffraction patterns of YOF:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors are shown in Fig. 2(a–c). All patterns clearly show materials of a rhombohedral structure (R-3m space group, reference card number: ICDD 00-025-1012). As it given on Fig. 2d, in rhombohedral structure, all ions occupy the six-fold 6c Wyckoff positions with the same C3v site symmetry; the Y3+ ions are coordinated by four oxide (O2−) and four fluoride (F−) anions in a bicapped trigonal antiprism arrangement [2]. The apparent absence of impurity phases indicates that doping with Ln3+ ions (in all
Fig. 3. Kubelka-Munk transformation of diffuse reflectance spectra of YOF:Yb3+,Ln3+ nanophosphors a) Ln = Ho, b) Ln = Er and c) Ln = Tm. Characteristic Ln3+ absorptions are indicated by arrows.
Fig. 4. DS emission spectra of (Y0.9-cLnc)OF:Yb0.1 (Ln = Ho, Er, Tm) nanophosphors doped with different concentration ratios (c) of: a) Ho/Y, b) Er/Y, c) Tm/Y (inset shows enlarged spectrum between 460 and 500 nm showing laser excitation contribution which was excluded from the calculations of emission color coordinates), d) The CIE 1931 chromaticity diagram of YOF:Ln3+/Yb3+ samples indicated emission color coordinates for each sample. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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bands from the 4I15/2 level to different excited levels: 4G11/2 (377 nm), 4S3/2, 2H11/2 (519 nm), 4F9/2 (649 nm) [32], while spectrum of Tm3+-doped shows absorption bands due electronic transitions from 3H6 level to 1G4 (467 nm), 3F2,3 (688 nm), 3H4 (794 nm) and 3H5 (1208 nm) levels [33].
Table 3 The summary of DS CIE emission color coordinates (x, y), emission branching's, and involved electronic transitions obtained with YOF:Yb3+ samples activated with different concentrations of Ho3+, Er3+, and Tm3+. Ln3+ ion/ Sample 3+
CIE coordinates
Emission branching (%)
3.3. Downshifting emissions of YOF:Ln3+/Yb3+ nanophosphors
Ho
x
y
Green (5F4, 5S2→5I8)
Red (5F5→5I8)
NIR (5F4,5S2→5I7)
0.005Ho 0.01Ho 0.015Ho 0.02Ho
0.285 0.276 0.286 0.281
0.702 0.711 0.701 0.707
88.1 90.1 87.5 91.3
4.0 3.1 4.4 2.5
7.9 6.8 8.1 6.2
Er3+
x
y
Green (2H11/2→4I15/2) (4S3/2→4I15/2)
Red (4F9/2→4I15/2)
0.01Er 0.015Er 0.02Er 0.025Er
0.299 0.318 0.343 0.336
0.685 0.666 0.641 0.644
86.4 79.2 70.5 73.9
13.6 20.8 29.5 26.1
Tm3+
x
y
Blue (1G4→3H6)
Red (1G4→3F4)
NIR (3H4→3H6)
0.02Tm 0.025Tm 0.03Tm 0.035Tm
0.124 0.141 0.187 0.230
0.198 0.205 0.241 0.289
80.8 80.2 73.9 65.7
6.1 6.9 9.7 13.2
13.1 12.9 16.4 21.1
DS emissions of (Y0.9-cLnc)OF:Yb0.1 (Ln = Ho3+, Er3+, and Tm3+) samples are shown in Fig. 4. In samples, the concentration of Ln3+ ions was varied depending on the type of doping ions as follows: Ho3+ (c = 0.005, 0.01, 0.015, 0.02), Er3+ (c = 0.01, 0.015, 0.02, 0.025), and Tm3+ (c = 0.02, 0.025, 0.03, 0.035). Under 446 nm excitation, Ho/Yb-doped YOFs show three emission bands (Fig. 4a): the strong green emission centered at 540 nm from electronic transitions from 5F4 and 5S2 levels to the 5I8 ground level of Ho3+; the weak red emission centered at 663 nm from 5F5 → 5I8 transition, and the NIR emission centered at 754 nm that can be assigned to 5F4 and 5S2 transitions to 5I7. Calculated CIE (Commission Internationale de l’Eclairage) chromaticity coordinates for Ho doped samples, given in Table 3, show that their DS emission is in the green spectral region (see Fig. 4d). Under 377 nm excitation, Er/Yb-doped YOF present two strong emission bands in the green spectral range (Fig. 4b) centered at 525 nm and 555 nm (2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions, respectively). In addition, the weak emission band in a red spectral region centered at 655 nm (4F9/2 → 4I5/2 transition) can be observed in spectrum. Emissions are green (see Fig. 3d) having CIE chromaticity coordinates given in Table 3. Tm/Yb -doped YOF nanophosphors excited by 465 nm radiation emit light in three bands (see Fig. 4c): the strong band in the blue and the weak band in the red spectral region at 483 nm (from 1G4 → 3H6 transition) and 657 nm (from 1G4 → 3F4 transition), respectively. The third weak emission band in the NIR spectral region is centered at 798 nm (3H4 → 3H6 transition) does not contribute to the emission color which is blue (see Fig. 3d) with CIE color coordinates given in Table 3.
concentrations) was successful. Due to the same valence and similar ionic radii between Y3+ (1.019 Å) ions in a host and Ln3+ (Ho3+1.015 Å, Er3+-1.004 Å, Tm3+-0.994 Å, Yb3+-0.985 Å) [30] dopant ions, doping is possible in the wide range of concentrations without significant distortion of the crystal lattice. In addition, the relatively intense reflection peaks suggest that as-synthesized nanoparticles are highly crystalline. Structural parameters of all samples were calculated by the Rietveld refinement of XRD data, and results of the analysis are summarized in Table 2. Calculated average crystallite sizes are in the range from 12 nm to 22 nm, which is consistent with values of particle sizes observed by TEM measurements. Similar values crystallites and particles sizes suggest that particles consist of one or two crystallites. Generally, microstrain values (0.110–0.661%) are higher than in microcrystalline particles, as expected, due to strong effect of the nanoparticle's surface which acts as a defect. The random variation of values does not facilitate drawing any conclusion on the effect of YOF doping with different lanthanide ions on material's crystal defects and ion ordering in the structure.
3.4. Upconversion emission of YOF:Ln3+/Yb3+ nanophosphors UC emission spectra of YOF:Ln3+/Yb3 (Ln3+ = Ho3+, Er3+, and Tm3+) samples excited by 980 nm radiation are shown in Fig. 5. As in case of DS emission, Ho/Yb-doped YOF exhibit three emission bands (Fig. 5a). Differences between UC and DS spectra (Figs. 4 and 5, respectively) occur due to differences in spectral resolutions of instruments utilized for UC and DS emission measurements (which does not affect calculated emission colors), and, more importantly, due to significantly different emission branching which produces the difference in emission and emission color coordinates, see Table 4. One should note that DS in Yb3+/Ln3+ doped hosts is a considerably more complex process than DS in a single Ln3+ activated hosts, so that several routes for the cross-relaxation between different Yb3+ and Ln3+ energy levels is possible significantly affecting intensities of Ln3+ emissions from different excited levels [34]. Also, due to the nature of excitation of upconversion process, the red emission from 5F5 → 5I8 transition gains on relative intensity compared to green emission (5F4, 5S2 → 5I8 transitions). This effect may vary depending on the excitation power and the Yb3+ level [35,36]. The difference between DS and UC emission is even more pronounced in Er/Yb doped YOF (Fig. 5b and d, Table 4). The red component of UC emission accounts more than 80% of total emission,
3.2. Absorption of Yb3+,Ln3+ activated YOF nanoparticles Absorption spectra (Kubelka-Munk transformation of diffuse reflection spectra) of Ln3+/Yb3+-doped YOF nanoparticles in UV-VISNIR spectral range (220–1300 nm) are shown in Fig. 3. For all nanoparticles, the dominant absorption is in NIR and comes from the Yb3+ ions (2F5/2 level). In addition, a number of weaker-intensity absorption bands from f-f electron transitions of Ho3+, Er3+ and Tm3+ ions can be observed. The absorption spectrum of Ho3+-doped sample is characterized by absorptions the 5I8 level to different excited levels: 3K8,5G6 (446 nm), 5S2,5F4 (541 nm), 5F5 (643 nm) and 5I6 (1140 nm) [31]. Spectrum of Er3+-doped sample shows absorption
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Fig. 5. UC emission spectra (λex = 980 nm) of (Y0.9-cLnc)OF:Yb0.1 (Ln = Ho, Er, Tm) nanophosphors doped with different concentration ratios (c) of: a) Ho/Y, b) Er/ Y, c) Tm/Y, d) The CIE 1931 chromaticity diagram of YOF:Ln3+/Yb3+ samples indicated emission color coordinates for each sample. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
4. Conclusions
which is considerable higher than 20% in DS emission. NIR component of UC emission (3H4 → 3H6 transition) of Tm/Yb -doped YOF nanophosphors is very strong (see Fig. 5c, Table 4), leaving weak blue and deep-red emissions to contribute to the feeble UC blue light (see Fig. 5d). The analyzed YOF powders exhibit both DS and UC emissions upon UV/VIS and 980 nm excitation, respectively. In both cases their emission color can be tuned by changing the Yb3+/Ln3+ concentration. In addition, these multicolor-tunable nanophosphors may provide unique colors by simultaneous excitation with UV-VIS and NIR radiation (in different proportions), Fig. 6, that may be quite beneficial for the anticounterfeit and security applications [37]. They may also find use in lamps, display devices, and medical diagnostic devices [38,39].
To conclude, modified sol-gel Pechini technique provides 20 nm YOF: Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanoparticles over the wide range of doping concentrations. These nanoparticles show both DS and UC emissions of different colors upon UV/VIS and NIR excitation; the color being tunable by changing Yb3+/Ln3+ concentration ratio and/or by combination of excitation radiation. Multicolor-tunable emissions make these luminescent nanoparticles good candidates for the anti-counterfeit and security applications, optoelectronic devices, and display systems.
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Appendix A. Supplementary data
Table 4 The summary of UC CIE emission color coordinates (x, y), emission branching's, and involved electronic transitions obtained with YOF:Yb3+ samples activated with different concentrations of Ho3+, Er3+, and Tm3+. Ln3+ ion/ Sample
CIE coordinates
Ho3+
x
y
Green (5F4, 5S2→5I8)
Red (5F5→5I8)
NIR (5F4,5S2→5I7)
0.005Ho 0.01Ho 0.015Ho 0.02Ho
0.333 0.325 0.316 0.330
0.606 0.623 0.631 0.584
59.2 61.9 67.6 59.7
32.7 30.7 23.2 27.5
8.1 7.4 9.2 12.8
Er3+
x
y
Green (2H11/2→4I15/2) (4S3/2→4I15/2)
Red (4F9/2→4I15/2)
0.01Er 0.015Er 0.02Er 0.025Er
0.442 0.486 0.432 0.447
0.455 0.445 0.426 0.418
18.3 12.6 17.8 19.6
81.7 87.4 82.2 80.4
Tm3+
x
y
Blue (1G4→3H6)
Red (1G4→3F4)
NIR (3H4→3H6)
0.02Tm 0.025Tm 0.03Tm 0.035Tm
0.367 0.369 0.370 0.369
0.412 0.414 0.414 0.414
8.0 16.0 15.6 17.4
20.5 20.4 22.8 30.3
71.5 63.6 61.6 52.3
Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.dyepig.2018.03.047.
Emission branching (%)
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Fig. 6. The CIE 1931 chromaticity diagram of 0.01Er sample emission colors upon simultaneous excitation by UV/VIS and NIR radiation; the arrow indicates the emission color change with the variation of intensities of UV/VIS and NIR radiation (IDS/IUC). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Acknowledgment The authors acknowledge the financial support of the Ministry of Education, Science and Technological Development of the Republic of Serbia (Projects no 45020 and 172056). The authors thank to Scott P. Ahrenkiel for TEM measurements.
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Multicolor-tunable emissions of YOF: Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors
T
Jovana Periša, Jelena Papan, Slobodan D. Dolić, Dragana J. Jovanović∗∗, Miroslav D. Dramićanin∗ Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, 11001, Serbia
A R T I C LE I N FO
A B S T R A C T
Keywords: YOF Downshifting emission Upconversion CIE coordinate Color Lanthanides
Color tuning of down-shifting and up-conversion emissions of YOF:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors is demonstrated. Nanophosphors were prepared by the modified sol-gel Pechini method and characterized by the X-ray diffraction, transmission electron microscopy, and photoluminescence spectroscopy. Samples consist of 20 nm particles crystallized in the rhombohedral crystal structure. Depending on the Ln3+/ Yb3+ concentration ratio and the type of excitation (UV/VIS or NIR) color of the particle's emission varied from the blue to red. Commission Internationale de L'Eclairage chromaticity coordinates of emission colors are given for the range of Ln3+/Yb3+ concentration ratios for both down-shifting and up-conversion luminescence. We showed that the emission color of these nanophosphors may be additional tuned by simultaneous excitation with UV-VIS and NIR radiation (in different proportion) which yields unique color labels for the anti-counterfeit and security applications.
1. Introduction Lanthanide doped nanophosphors have been a matter of interest over the past years for their utilization in many important technologies, such as lasers, electronics, therapeutics, displays, solar cells, catalysis, medicine [1–5]. In addition, there is a growing interest for their use as bio-markers or nano-sensors, instead of traditional ones [6]. The application of these materials is based on their unique optical characteristics which arise from partially filled f-orbitals of lanthanide ions [7,8]. Multicolor luminescence of trivalent lanthanide (Ln3+) doped nanomaterials can be realized using different ions whose f - f electronic transitions provide different-color emissions that span the complete visible–near-infrared spectral range. Also, it can be achieved by both types of excitation schemes, down-shifting (DS) and up-conversion (UC). In DS, the high-energy radiation (usually ultraviolet, UV) is used to excite phosphor's emission of lower energy photons. UC is an antiStokes emission process in which two or more low-energy photons (usually near-infrared, NIR) excite phosphor's emission of higher energy via multistep optical processess [2]. In both cases, phosphor luminescence is characterized by long-lived electronic excited states and narrow emission bands. Materials doped with activator Ln3+ ions (Ln3+ = Ho3+, Er3+, Tm3+, Tb3+) in different concentration ratios with Yb3+ (sensitizer
∗
ion) may provide multicolor-tunable emissions in both DS and UC excitation schemes [9]. Several Ln3+-activated nanophosphors with multi-color tunable emission have been reported so far, such as GdVO4:Ln3+/Yb3+ (Ln3+ = Ho, Er, Tm, Ho/Er/Tm), Ce3+-Mn2+ doped Y7O6F9, YOF:Ln3+(Ln = Tb, Eu, Tm, Dy, Ho, Sm), lanthanide metal-organic frameworks, Cu (Mn)-doped ZnInS, LuVO4:Tm3+/Dy3+/ Eu3+ [4,8,10–14]. However, to the best of our knowledge, there are no reports on the use of rare earth oxyfluorides for combined multi-color DS and UC emissions. These materials are promising hosts for preparation of phosphors due to their attractive chemical and physical properties which may be classified as in between characteristics of fluorides and oxides [1,15–17]. Alike fluorides, oxyfluorides have low phonon energy and high ionicity which leads to the efficient luminescence, while, on the other hand, these compounds have great chemical and thermal stabilities which resemble properties of oxides [2,3,18]. In addition, oxyfluoride nanomaterials are biocompatible and nearly nontoxic to live cells so they can be safely used for biomedical applications [5,19]. Zachariasen described oxyfluorides' (YOF and LaOF) crystal structures in 1950`s [20]. Generally, they can be found in three structural modifications: tetragonal, which is usually nonstoichiometric (REOnF3-2, 0.7 < n < 1), rhombohedral, and cubic (β-YOF). The rhombohedral structure (r-YOF) is the stable one, and it is strictly stoichiometric (n = 1). The cubic β-YOF structure occurs as the result of
Corresponding author. Corresponding author. E-mail addresses: [email protected] (D.J. Jovanović), [email protected] (M.D. Dramićanin).
∗∗
https://doi.org/10.1016/j.dyepig.2018.03.047 Received 12 February 2018; Received in revised form 14 March 2018; Accepted 23 March 2018 Available online 26 March 2018 0143-7208/ © 2018 Elsevier Ltd. All rights reserved.
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2. Experimental
Table 1 Sample names and concentration ratio (Ln3+/Y3+) for preparation of different YOF: Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors. Samples name
Concentration ratio (mol %)
2.1. Reagents
mmol
All chemicals: yttrium(III) nitrate hexahydrate, Y(NO3)3 × 6H2O (99.9%, Alfa Aesar), ytterbium(III) nitrate pentahydrate, Yb(NO3)3 × 5H2O (99.9%, Alfa Aesar), holmium(III) nitrate pentahydrate, Ho (NO3)3 × 5H2O (99.9%, Alfa Aesar), erbium (III) nitrate hydrate, Er (NO3)3 × n H2O (n ˜ 5) (99.9%, Alfa Aesar), thulium(III) nitrate hydrate, Tm(NO3)3 × n H2O (n ˜ 5) (99.9%, Alfa Aesar), ammonium fluoride, NH4F (98%, Alfa Aesar), citric acid (99.6%, Acros Organics) and ethylene glycol (> 99%, Sigma Aldrich) were of the highest purity available and were used without any further purification.
Ho3+/Y3+ 0.005Ho 0.01Ho 0.015Ho 0.02Ho
0.005/0.895 0.01/0.89 0.015/0.885 0.02/0.88
0.00125/0.224 0.0025/0.222 0.00375/0.221 0.005/0.220
Er3+/Y3+ 0.01Er 0.015Er 0.02Er 0.025Er
0.01/0.89 0.015/0.885 0.02/0.88 0.025/0.875
0.0025/0.222 0.00375/0.221 0.005/0.220 0.625/0.219
2.2. Syntheses of YOF and YOF: Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors
Tm3+/Y3+ 0.02Tm 0.025Tm 0.03Tm
0.02/0.88 0.025/0.875 0.03/0.87
The modified sol-gel Pechini method, previously described by Grzyb and co-workers [28,29], was used to prepare YOF:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+,Tm3+) nanophosphors with different Ln3+/Yb3+ concentration ratios. Concentration of Yb3+ was kept constant in all the samples (10 mol% (0.1), 0.025 mmol), while concentrations of other Ln3+ ions (see Table 1) were chosen based on the experience with similar nanophosphors. The synthesis of undoped YOF nanophosphor. Weighted amount (0.25 mmol) of Y(NO3)3 was dissolved in 90 ml of water. Citric acid (as the chelating agent) and ethylene glycol (as complexing agent of Y3+ cations) were added into the solution. To prevent lanthanide fluorides from precipitating, a large excess of citric acid and ethylene glycol was used [1]. After the intense stirring of the solution at room temperature for 30 min, an aqueous solution of NH4F (0.31 mmol NH4F, 10 ml H2O) was slowly added drop-wise into solution. Since NH4F decomposes at 100 °C, the 25% excess to the stoichiometric amounts of nitrate precursor was used. The prepared mixture was additionally heated at 80 °C for 4 h (the time needed for the water evaporation and the gel formation). Final product was obtained through calcination of gel precursors at 700 °C in air for 4 h. After calcination, the sample was kept overnight in furnace to cool down to room temperature. The synthesis of YOF:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors. Lanthanide doped YOF were prepared in analogy to the procedure described for the undoped YOF system with only difference being the addition of the appropriate amount of the respective Ln-nitrates to the starting Y(NO3)3 solution (see Table 1).
0.005/0.220 0.00625/0.218 0.0075/0.217
the phase transition from the rhombohedral YOF at temperature of 560–570 °C [1,21]. Nonstoichiometric compounds usually have a tetragonal crystal form, but additional studies have shown that these materials are more complex than it was previously assumed; one example are compound with general formula REnOn-1Fn+2, 5 ≤ n ≤ 9 such as Y5O4F7, Y6O5F8 and Y7O6F9, which crystallize in an ortorhombohedral form [10,19,22–25]. To prepare oxyfluorides, several methods can be employed: thermolysis method, sol-gel method, fluorolytic sol-gel, hydrothermal method, co-precipitation method, solid state synthesis, combustion synthesis, stearic acid method, urea based homogenous precipitation [4,16,23,26,27]. Here, the goal was to prepare Ln3+/Yb3+-doped YOF (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors with multicolor-tunable DS and UC emissions under UV/VIS and NIR excitations. DS in Yb3+/ Ln3+ doped hosts is a considerably more complex process than DS in a single Ln3+ activated hosts. Yb3+,Ln3+ pairs offer a number of routes for the cross-relaxation between different Yb3+ and Ln3+ energy levels, depending on phonon energies of the host, temperature and excitation energy, which affect intensities of Ln3+ emissions (and sometimes providing Yb3+ emission via quantum-cutting) and consequently affect DS emission colors. In addition, we aimed to explore for the first time (to the best of our knowledge) the combined UV/VIS and NIR excitation of Yb3+/Ln3+ to facilitate the luminescence coding with unique emission colors suitable for anti-counterfeiting and security applications. To achieve this goal, nanophosphors were prepared by the optimized and modified sol-gel Pechini method, and their structural and luminescence properties are studied in details.
2.3. Characterization The X-ray diffraction (XRD) analysis of the obtained nanopowder samples was performed on Rigaku SmartLab system (Cu-Kα radiation, 30 mA current, 40 kV voltage) in the 2θ range from 10° to 90° (using continuous scan of 0.7 s−1). JEOL JEM-2100 LaB6 transmission electron
Fig. 1. a) and b) Typical TEM images for un-doped YOF nanoparticles at two different magnifications; c) size distribution histogram of YOF nanocrystals.
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Fig. 2. XRD patterns of (Y0.9-cLnc)OF:Yb0.1 (Ln = Ho, Er, Tm) nanophosphors doped with different concentration ratios (c) of: a) Ho/Y, b) Er/Y, c) Tm/Y; d) schematic representation of YOF rhombohedral crystal structure.
microscope (TEM) with accelerating voltage of 200 kV and equipped with the high-resolution (HR) style objective-lens pole piece has been used to study the microstructure of the obtained samples. TEM samples were prepared by dipping a carbon-coated copper grid into toluene or water dispersions and the grids with the nanocrystals were dried in air. Diffuse reflection spectra were recorded using Shimadzu UV-2600 (Shimadzu Corporation, Japan) spectrophotometer equipped with an integrated sphere (ISR-2600 Plus) in a 220–1300 nm range. UC emission spectra were measured upon excitation with 980-nm radiation (MDLH 980 3w diode laser with controller) and detected with an AvaSpec-2048 Fiber Optic Spectrometer system. DS emission measurements were performed on a spectrofluorometer system that comprises the optical parametric oscillator excitation source (EKSPLA NT 342), spectrograph FHR 1000 (Horiba Jobin-Yvon, 300 grooves mm−1 grating), and ICCD detector (Horiba Jobin-Yvon 3771).
Table 2 Structural parameters of (Y0.9-cLnc)OF:Yb0.1 (Ln = Ho, Er, Tm) nanophosphors calculated using Rietveld refinement of XRD data. Sample name
Average crystal size (nm)
a = b (Å)
c (Å)
V (Å3)
Microstrain (%)
YOF 0.005Ho 0.01Ho 0.015Ho 0.02Ho 0.01Er 0.015Er 0.02Er 0.025Er 0.02Tm 0.025Tm 0.03Tm 0.035Tm
22.2 (3) 15.6 (8) 16.9 (7) 15.7 (9) 12.7(14) 13.9 (2) 20.6(6) 12.4(16) 12.6(3) 13.9 (16) 14.5 (2) 19.8 (8) 15.7(8)
3.801 (11) 3.803 (15) 3.800 (7) 3.799 (3) 3.803 (8) 3.800 (3) 3.7960 (8) 3.801 (8) 3.800 (14) 3.799 (9) 3.799 (10) 3.793 (14) 3.796 (7)
18.879 (2) 18.840 (6) 18.827 (4) 18.830 (3) 18.841 (4) 18.848(12) 18.843 (4) 18.847 (5) 18.843 (7) 18.844 (5) 18.839 (5) 18.820 (3) 18.832 (4)
272.756 272.479 271.862 271.762 272.494 272.165 271.520 272.294 272.092 271.964 271.892 270.760 271.362
0.145 (7) 0.661 (11) 0.296 (16) 0.210 (3) 0.110 (2) 0.250(11) 0.126 (19) 0.278 (11) 0.270 (2) 0.198(11) 0.110 (3) 0.209 (19) 0.193 (4)
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3. Results and discussion 3.1. Microstructural and structural properties of YOF:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors All samples are nanocrystalline with similar morphology of particles. Fig. 1 shows the representative TEM image of undoped- YOF particles which exhibit a plate-like morphology and are about 20 nm in size (when measured from the edge to edge). The majority of nanoplates have hexagonal shapes, and the rest of particles are either irregular polygons or spheres. X-ray diffraction patterns of YOF:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanophosphors are shown in Fig. 2(a–c). All patterns clearly show materials of a rhombohedral structure (R-3m space group, reference card number: ICDD 00-025-1012). As it given on Fig. 2d, in rhombohedral structure, all ions occupy the six-fold 6c Wyckoff positions with the same C3v site symmetry; the Y3+ ions are coordinated by four oxide (O2−) and four fluoride (F−) anions in a bicapped trigonal antiprism arrangement [2]. The apparent absence of impurity phases indicates that doping with Ln3+ ions (in all
Fig. 3. Kubelka-Munk transformation of diffuse reflectance spectra of YOF:Yb3+,Ln3+ nanophosphors a) Ln = Ho, b) Ln = Er and c) Ln = Tm. Characteristic Ln3+ absorptions are indicated by arrows.
Fig. 4. DS emission spectra of (Y0.9-cLnc)OF:Yb0.1 (Ln = Ho, Er, Tm) nanophosphors doped with different concentration ratios (c) of: a) Ho/Y, b) Er/Y, c) Tm/Y (inset shows enlarged spectrum between 460 and 500 nm showing laser excitation contribution which was excluded from the calculations of emission color coordinates), d) The CIE 1931 chromaticity diagram of YOF:Ln3+/Yb3+ samples indicated emission color coordinates for each sample. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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bands from the 4I15/2 level to different excited levels: 4G11/2 (377 nm), 4S3/2, 2H11/2 (519 nm), 4F9/2 (649 nm) [32], while spectrum of Tm3+-doped shows absorption bands due electronic transitions from 3H6 level to 1G4 (467 nm), 3F2,3 (688 nm), 3H4 (794 nm) and 3H5 (1208 nm) levels [33].
Table 3 The summary of DS CIE emission color coordinates (x, y), emission branching's, and involved electronic transitions obtained with YOF:Yb3+ samples activated with different concentrations of Ho3+, Er3+, and Tm3+. Ln3+ ion/ Sample 3+
CIE coordinates
Emission branching (%)
3.3. Downshifting emissions of YOF:Ln3+/Yb3+ nanophosphors
Ho
x
y
Green (5F4, 5S2→5I8)
Red (5F5→5I8)
NIR (5F4,5S2→5I7)
0.005Ho 0.01Ho 0.015Ho 0.02Ho
0.285 0.276 0.286 0.281
0.702 0.711 0.701 0.707
88.1 90.1 87.5 91.3
4.0 3.1 4.4 2.5
7.9 6.8 8.1 6.2
Er3+
x
y
Green (2H11/2→4I15/2) (4S3/2→4I15/2)
Red (4F9/2→4I15/2)
0.01Er 0.015Er 0.02Er 0.025Er
0.299 0.318 0.343 0.336
0.685 0.666 0.641 0.644
86.4 79.2 70.5 73.9
13.6 20.8 29.5 26.1
Tm3+
x
y
Blue (1G4→3H6)
Red (1G4→3F4)
NIR (3H4→3H6)
0.02Tm 0.025Tm 0.03Tm 0.035Tm
0.124 0.141 0.187 0.230
0.198 0.205 0.241 0.289
80.8 80.2 73.9 65.7
6.1 6.9 9.7 13.2
13.1 12.9 16.4 21.1
DS emissions of (Y0.9-cLnc)OF:Yb0.1 (Ln = Ho3+, Er3+, and Tm3+) samples are shown in Fig. 4. In samples, the concentration of Ln3+ ions was varied depending on the type of doping ions as follows: Ho3+ (c = 0.005, 0.01, 0.015, 0.02), Er3+ (c = 0.01, 0.015, 0.02, 0.025), and Tm3+ (c = 0.02, 0.025, 0.03, 0.035). Under 446 nm excitation, Ho/Yb-doped YOFs show three emission bands (Fig. 4a): the strong green emission centered at 540 nm from electronic transitions from 5F4 and 5S2 levels to the 5I8 ground level of Ho3+; the weak red emission centered at 663 nm from 5F5 → 5I8 transition, and the NIR emission centered at 754 nm that can be assigned to 5F4 and 5S2 transitions to 5I7. Calculated CIE (Commission Internationale de l’Eclairage) chromaticity coordinates for Ho doped samples, given in Table 3, show that their DS emission is in the green spectral region (see Fig. 4d). Under 377 nm excitation, Er/Yb-doped YOF present two strong emission bands in the green spectral range (Fig. 4b) centered at 525 nm and 555 nm (2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions, respectively). In addition, the weak emission band in a red spectral region centered at 655 nm (4F9/2 → 4I5/2 transition) can be observed in spectrum. Emissions are green (see Fig. 3d) having CIE chromaticity coordinates given in Table 3. Tm/Yb -doped YOF nanophosphors excited by 465 nm radiation emit light in three bands (see Fig. 4c): the strong band in the blue and the weak band in the red spectral region at 483 nm (from 1G4 → 3H6 transition) and 657 nm (from 1G4 → 3F4 transition), respectively. The third weak emission band in the NIR spectral region is centered at 798 nm (3H4 → 3H6 transition) does not contribute to the emission color which is blue (see Fig. 3d) with CIE color coordinates given in Table 3.
concentrations) was successful. Due to the same valence and similar ionic radii between Y3+ (1.019 Å) ions in a host and Ln3+ (Ho3+1.015 Å, Er3+-1.004 Å, Tm3+-0.994 Å, Yb3+-0.985 Å) [30] dopant ions, doping is possible in the wide range of concentrations without significant distortion of the crystal lattice. In addition, the relatively intense reflection peaks suggest that as-synthesized nanoparticles are highly crystalline. Structural parameters of all samples were calculated by the Rietveld refinement of XRD data, and results of the analysis are summarized in Table 2. Calculated average crystallite sizes are in the range from 12 nm to 22 nm, which is consistent with values of particle sizes observed by TEM measurements. Similar values crystallites and particles sizes suggest that particles consist of one or two crystallites. Generally, microstrain values (0.110–0.661%) are higher than in microcrystalline particles, as expected, due to strong effect of the nanoparticle's surface which acts as a defect. The random variation of values does not facilitate drawing any conclusion on the effect of YOF doping with different lanthanide ions on material's crystal defects and ion ordering in the structure.
3.4. Upconversion emission of YOF:Ln3+/Yb3+ nanophosphors UC emission spectra of YOF:Ln3+/Yb3 (Ln3+ = Ho3+, Er3+, and Tm3+) samples excited by 980 nm radiation are shown in Fig. 5. As in case of DS emission, Ho/Yb-doped YOF exhibit three emission bands (Fig. 5a). Differences between UC and DS spectra (Figs. 4 and 5, respectively) occur due to differences in spectral resolutions of instruments utilized for UC and DS emission measurements (which does not affect calculated emission colors), and, more importantly, due to significantly different emission branching which produces the difference in emission and emission color coordinates, see Table 4. One should note that DS in Yb3+/Ln3+ doped hosts is a considerably more complex process than DS in a single Ln3+ activated hosts, so that several routes for the cross-relaxation between different Yb3+ and Ln3+ energy levels is possible significantly affecting intensities of Ln3+ emissions from different excited levels [34]. Also, due to the nature of excitation of upconversion process, the red emission from 5F5 → 5I8 transition gains on relative intensity compared to green emission (5F4, 5S2 → 5I8 transitions). This effect may vary depending on the excitation power and the Yb3+ level [35,36]. The difference between DS and UC emission is even more pronounced in Er/Yb doped YOF (Fig. 5b and d, Table 4). The red component of UC emission accounts more than 80% of total emission,
3.2. Absorption of Yb3+,Ln3+ activated YOF nanoparticles Absorption spectra (Kubelka-Munk transformation of diffuse reflection spectra) of Ln3+/Yb3+-doped YOF nanoparticles in UV-VISNIR spectral range (220–1300 nm) are shown in Fig. 3. For all nanoparticles, the dominant absorption is in NIR and comes from the Yb3+ ions (2F5/2 level). In addition, a number of weaker-intensity absorption bands from f-f electron transitions of Ho3+, Er3+ and Tm3+ ions can be observed. The absorption spectrum of Ho3+-doped sample is characterized by absorptions the 5I8 level to different excited levels: 3K8,5G6 (446 nm), 5S2,5F4 (541 nm), 5F5 (643 nm) and 5I6 (1140 nm) [31]. Spectrum of Er3+-doped sample shows absorption
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Fig. 5. UC emission spectra (λex = 980 nm) of (Y0.9-cLnc)OF:Yb0.1 (Ln = Ho, Er, Tm) nanophosphors doped with different concentration ratios (c) of: a) Ho/Y, b) Er/ Y, c) Tm/Y, d) The CIE 1931 chromaticity diagram of YOF:Ln3+/Yb3+ samples indicated emission color coordinates for each sample. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
4. Conclusions
which is considerable higher than 20% in DS emission. NIR component of UC emission (3H4 → 3H6 transition) of Tm/Yb -doped YOF nanophosphors is very strong (see Fig. 5c, Table 4), leaving weak blue and deep-red emissions to contribute to the feeble UC blue light (see Fig. 5d). The analyzed YOF powders exhibit both DS and UC emissions upon UV/VIS and 980 nm excitation, respectively. In both cases their emission color can be tuned by changing the Yb3+/Ln3+ concentration. In addition, these multicolor-tunable nanophosphors may provide unique colors by simultaneous excitation with UV-VIS and NIR radiation (in different proportions), Fig. 6, that may be quite beneficial for the anticounterfeit and security applications [37]. They may also find use in lamps, display devices, and medical diagnostic devices [38,39].
To conclude, modified sol-gel Pechini technique provides 20 nm YOF: Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+) nanoparticles over the wide range of doping concentrations. These nanoparticles show both DS and UC emissions of different colors upon UV/VIS and NIR excitation; the color being tunable by changing Yb3+/Ln3+ concentration ratio and/or by combination of excitation radiation. Multicolor-tunable emissions make these luminescent nanoparticles good candidates for the anti-counterfeit and security applications, optoelectronic devices, and display systems.
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Appendix A. Supplementary data
Table 4 The summary of UC CIE emission color coordinates (x, y), emission branching's, and involved electronic transitions obtained with YOF:Yb3+ samples activated with different concentrations of Ho3+, Er3+, and Tm3+. Ln3+ ion/ Sample
CIE coordinates
Ho3+
x
y
Green (5F4, 5S2→5I8)
Red (5F5→5I8)
NIR (5F4,5S2→5I7)
0.005Ho 0.01Ho 0.015Ho 0.02Ho
0.333 0.325 0.316 0.330
0.606 0.623 0.631 0.584
59.2 61.9 67.6 59.7
32.7 30.7 23.2 27.5
8.1 7.4 9.2 12.8
Er3+
x
y
Green (2H11/2→4I15/2) (4S3/2→4I15/2)
Red (4F9/2→4I15/2)
0.01Er 0.015Er 0.02Er 0.025Er
0.442 0.486 0.432 0.447
0.455 0.445 0.426 0.418
18.3 12.6 17.8 19.6
81.7 87.4 82.2 80.4
Tm3+
x
y
Blue (1G4→3H6)
Red (1G4→3F4)
NIR (3H4→3H6)
0.02Tm 0.025Tm 0.03Tm 0.035Tm
0.367 0.369 0.370 0.369
0.412 0.414 0.414 0.414
8.0 16.0 15.6 17.4
20.5 20.4 22.8 30.3
71.5 63.6 61.6 52.3
Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.dyepig.2018.03.047.
Emission branching (%)
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Fig. 6. The CIE 1931 chromaticity diagram of 0.01Er sample emission colors upon simultaneous excitation by UV/VIS and NIR radiation; the arrow indicates the emission color change with the variation of intensities of UV/VIS and NIR radiation (IDS/IUC). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Acknowledgment The authors acknowledge the financial support of the Ministry of Education, Science and Technological Development of the Republic of Serbia (Projects no 45020 and 172056). The authors thank to Scott P. Ahrenkiel for TEM measurements.
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