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Vol. 8, No. 7 | 1 Jul 2018 | OPTICAL MATERIALS EXPRESS 1863

Infrared-to-visible upconversion luminescence in SrF 2:Er powders upon excitation of the 4 I 13/2 level A. A. LYAPIN,1,* S. V. GUSHCHIN,1 S. V. KUZNETSOV,2 P. A. RYABOCHKINA,1 A. S. ERMAKOV,1 V. YU. PROYDAKOVA,2 V. V. VORONOV,2 P. P. FEDOROV,2 S. A. ARTEMOV,1 A. D. YAPRYNTSEV,3 AND V. K. IVANOV3 1

National Research Ogarev Mordovia State University, 68 Bolshevistskaya Str., Saransk 430005, Republic of Mordovia, Russia 2 Prokhorov General Physics Institute, RAS, 38 Vavilova str., Moscow 119991, Russia 3 Kurnakov Institute of General and Inorganic Chemistry, RAS, 31 Leninsky pr., Moscow 119991, Russia *[email protected]

Abstract: Fluorite-type SrF2:Er powders were synthesized by using a co-precipitation from the aqueous solution technique. X-ray powder diffraction, scanning electron microscopy, absorption and luminescence spectroscopy were used to characterize the samples. For the first time upconversion luminescence of SrF2:Er powders in the visible and near-infrared spectral region upon excitation of 4I13/2 level Er3+ ions was investigated. The decrease in the slopes of the visible upconversion luminescence with increasing excitation power density and concentration of Er3+ ions were experimentally observed and discussed. The most intensive visible luminescence was obtained for SrF2:Er (8.8%) with a maximum quantum yield of 0.19%. Chromaticity coordinates and color temperature of yellow-green upconversion luminescence of SrF2:Er were calculated. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (160.2540) Fluorescent and luminescent materials; (190.7220) Upconversion.

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#327445 Journal © 2018

https://doi.org/10.1364/OME.8.001863 Received 2 Apr 2018; revised 6 Jun 2018; accepted 7 Jun 2018; published 13 Jun 2018

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1. Introduction Lanthanide-doped photon upconversion (UC) powders are rapidly developing photonics materials since they have numerous applications in bioimaging, biometrics for laser scanning microscopy, temperature sensors, infrared quantum counters, multimodal diagnostics and therapy [1–3]. In addition, UC powders are used in security printing due to the sharp luminescence bands from ultraviolet to near-infrared spectral range [4].

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At present, a large number of papers are devoted to the study of upconversion luminescence (UCL) of rare-earth (RE) ions doped powders upon excitation by laser radiation at about 980 nm to the 2F5/2 level of Yb3+ ions [3–13]. To a lesser extent UC properties of REdoped materials were studied upon excitation by laser radiation at around 1.5 µm (4I15/2→4I13/2 of Er3+) and 2 µm (5I8→5I7 of Ho3+) [14–28]. However, the extension of knowledge about the properties of UC materials upon excitation by short-wavelength infrared (SWIR) radiation will allow to find new applications of these materials in science and technology. Fluorite-type materials MF2 (M = Ca, Sr, Ba) doped with RE ions show highly efficient UC luminescence because they have low phonon energy (~366 cm−1 of SrF2 [29]) and the tendency to form multiple cluster configurations of dopant ions [20, 30, 31]. Multiphonon relaxation (MPR) is important UC luminescence quenching mechanism for RE ions. MPR rate decreases with the number of phonons required to bridge the energy gap between the excited level and next lower level of RE ions. Thus, RE ions doped in low phonon fluoride hosts have efficient UC luminescence. Clustering effect shorten the distance between RE ions and thus enhances the probability of energy transfer process between them, which is beneficial for achieving efficient UC luminescence. From available literature data, it is known that the UC luminescence of RE-doped (Er3+, Ho3+, Yb3+, Tm3+) fluoride MF2 materials in the visible and near-IR wavelength regions upon different excitation has been widely studied [3, 11–26]. However, we have not found publications of UC luminescence of SrF2:Er powders upon excitation by laser radiation at ~1.5 µm to the 4I13/2 level of Er3+ ions. Thus the aims of the present work are to study the UC luminescence of SrF2:Er powders upon excitation by laser at 1.5 µm and quantify the material performance. 2. Experimental The SrF2:Er (CEr = 1.58, 3.16, 5.25, 7.35, 11.52, 13.60, 15.68 mol.%) powders were synthesized by using a co-precipitation from aqueous nitrate solution technique [31]. The initial reagents for the synthesis of fluoride powders were strontium nitrate (99.99% for metallic impurities), erbium nitrate fivehydrate (99.99% for metallic impurities) produced by LANHIT (Moscow, Russia), ammonium fluoride and bidistilled water. Erbium and strontium nitrate aqueous solution of 0.08 M concentration was added dropwise to 7% excess of 0.16 M aqueous ammonium fluoride under intense stirring. After precipitation of SrF2:Er solid solution the matrix solution was decanted. Obtained powders were dried in air at 45 °C (5 hours) and annealed in platinum crucibles in air at 600 °C (1 hour). The crystalline phases of the synthesized samples were examined by X-ray powder diffraction (XRD) patterns recorded on a diffractometer Bruker D8 Advanced (Cu Kα radiation). Scanning electron microscopy (SEM) images were determined using a Carl Zeiss NVision 40 microscope. The diffuse reflection spectra from the UV to the NIR spectral range of the fluoride powder samples were recorded using a spectrophotometer Lambda 950 Perkin–Elmer. The UC luminescence was investigated by use of continuous-wave fiber laser (λex = 1531.8 nm). The focused excitation beam diameter on the samples was 200 μm. The incident excitation power was 150 mW. The luminescence of the Er3+ ions was recorded using a Horiba FHR1000 spectrometer. Absolute upconversion luminescence quantum yield (ΦUC) in the visible spectral range in SrF2:Er powders was estimated using an OL IS-670-LED integrating sphere, OL-770 UV/VIS (Gooch & Housego) spectroradiometer and monochromator-spectrograph M833 Solar LS. The incident excitation power was measured using an UP19K-110F-H9-D0 (Standa) power meter. All measurements were performed at room temperature. 3. Results and discussion Figure 1(a) presents XRD pattern of the SrF2:Er (6.0%) powder after annealing at 600 °C. The XRD patterns of the synthesized powders correspond to JCPDS database 06-0262 SrF2 (a = 5.800 Å), and other phases are not observed. The unit cell parameters of SrF2:Er powders

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decreases con ntinuously (Tab ble 1) with an increasing conncentration of Er3+ ions. Thee unit cell parameters deecrease due to the action of additional a interrstitial fluorinee ions and smaaller ionic radii of the RE R dopant comp pared with stro ontium ion [322]. The same eeffect was obseerved and discussed for SrF2-YF3 nano opowders [33]. Experimentall a values correelate very welll with the Sr1-xErxF2+x soliid solution, preepared by data obtained by Sobolev ett al. [34] on thee samples of S nthesis. solid state syn

Fig. 1. 1 XRD pattern (a)) and SEM imagee (b) of the SrF2:E Er (6.0%) powder after annealing att 600 °C C. Table 1. The unit cell parameters p of thee SrF2:Er powderrs Sample Nominal N compositiion*

Unit cell parameter, aexp Å

1 SrF S 2: Er (1.58%) 5.7984(2 2) 2 SrF S 2: Er (3.16%) 5.7953(5) 5.7840(5) 3 SrF S 2: Er (5.25%) 5.7726(7 4 7) SrF S 2: Er (7.35%) 5 SrF S 2: Er (11.52%) 5.7627(2 2) 6 SrF S 2: Er (13.60%) 5.7525(5) 5.745(1) 7 SrF S 2: Er (15.68%) * - Er3+ concen ntration is presented in mol. % **- calculation n according to data of Sobolev at al [34].

Composition baseed on EDX* SrF2: Er (1.6%) SrF2: Er (3.4%) SrF2: Er (6.0%) SrF2: Er (8.8%) SrF2: Er (14.2%) SrF2: Er (18.3%) SrF2: Er (21.3%)

Unit cell parameteer, acalc Å** 5.7957 5.7908 5.7838 5.7762 5.7617 5.7506 5.743

EM image for synthesized SrF S 2:Er powderrs after annealing at 600 °C is shown Typical SE in Fig. 1(b). Average diam meter of particlles with spheriical shape is aabout 140 nm.. Energyopy (EDX) was w used to evvaluate the chhemical compoosition of dispersive X--ray spectrosco SrF2:Er powd ders (Table 1). The content of o erbium fluooride in the sollid solution is higher in comparison with w the aqueeous solution. It is a norm mal situation ffor samples w which are synthesized by co-precipitattion from waterr solution. Figure 2(aa) presents the room-temperaature diffuse reeflection specttrum of Er3+ ioons in the range 300–18 800 nm of the SrF2:Er (8.8% %) powder. Abbsorption bandds correspondinng to the transitions of Er3+ ions from m ground level 4I15/2 to the exccitation levels 4G11/2, 2H9/2, 4F3/2, 4F5/2, 4 F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2 and 4I11/2 are seen s clearly onn the diffuse reeflection specttrum. The dashed arrow w indicates the excitation of the t 4I13/2 level in SrF2:Er pow wders by radiaation of a fiber laser at 1531.8 1 nm (Fig g. 2(a)). Upon excitation of the 4I13/2 level the UC luminesccence from neaar-infrared to vvisible of 3+ S 2:Er (8.8%) powder at 300 K corresponnding to 4G11/2 → 4I15/2, 2H9/22 → 4I15/2, Er ions in SrF 4 4 F5/2 → I15/2, 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, 4F9/2 → 4 I15/2, 4I9/2 → 4I15/2 and 4I11/22 → 4I15/2 transitions weere recorded (F Fig. 2(b, c)). The T peaks wavvelengths of the SrF2:Er are 3378, 406, 449, 521, 539 9, 666, 867, an nd 982 nm. Thee most intensivve luminescencce of Er3+ ionss is in the red region (6 630–700 nm, 4F9/2→4I15/2 traansition). The same UC lum minescence speectra was observed for all SrF2:Er pow wders. The energy level diaggram of SrF2:E Er powders is shown in

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Fig. 2(d). The absorption transition 4I15/2→4I13/2 and UC luminescence transitions of Er3+ ions are indicated by arrows.

Fig. 2. The diffuse reflection spectra (a) of Er3+ in the spectral range 300–1800 nm for SrF2:Er (8.8%) powder at 300 K. UC luminescence spectra of the SrF2:Er (8.8%) powder in visible (b) and near-infrared spectral range (c). Partial energy-level diagram (d) of Er3+ in SrF2 [23].

We have studied the excitation power density (P)-dependent upconversion luminescence at the 2H9/2→4I15/2 (406 nm), 4S3/2→4I15/2 (539 nm) and 4F9/2→4I15/2 (666 nm) transitions of Er3+ ions in SrF2:Er powders at 300 K (Fig. 3). It is well known [35] that the upconversion emission intensity IUC depends on the excitation power density P as IUC ∝ Pn, where n is the number of absorbed photons needed to populate a certain luminescence energy level. Our experiments show that the slopes of the blue, green and red UCL of SrF2:Er powders strongly depend on the concentration of Er3+ ions and excitation power density. Increasing concentration of Er3+ions leads to a decrease in the slopes of the blue, green and red UCL. For example, slopes of the blue UCL of SrF2:Er (1.6%), SrF2:Er (3.4%) and SrF2:Er (8.8%) are 4.2, 2.1 and 1.8 in the excitation range 200-340 W/cm2, respectively (Fig. 3(a)). With further increasing excitation power density (450-850 W/cm2), the slopes of the blue UCL of SrF2:Er (1.6%), SrF2:Er (3.4%) and SrF2:Er (8.8%) also decrease to 3, 1.8 and 1.5, respectively (Fig. 3(b)). The slopes of green and red UCL in SrF2:Er powders also decrease with increasing excitation power density and concentration of Er3+ ions. Significant decrease in the slopes of the visible UCL with increasing excitation power density was observed for Cs3Lu2Cl9:Er crystals upon excitation of 4I13/2 level [35]. Pollnau et al [35] show that this phenomenon is explained by the competition between linear decay and UC processes for the depletion of the intermediate excited levels. From literature it is know that energy transfer upconversion (ETU) processes are dominated mechanisms of UCL in SrF2:Er single crystals upon excitation of 4I13/2 level [23– 25]. Figure 2(d) presents ETU1(4I13/2 + 4I13/2→4I15/2 + 4I9/2), ETU2(4I13/2 + 4I9/2→4I15/2 + 2H11/2), ETU3(4I13/2 + 4S3/2→ 4I15/2 + 2H9/2) and ETU4(4I13/2 + 2H9/2→4I15/2 + 2G7/2) processes on energy level diagram of Er3+ ions. Er3+ ions in strontium fluoride have a pronounced tendency to associate in clusters. At low Er3+ concentrations (0.01 mol.%), oppositely charged point defects R3+ and Fint combine to form dipole pairs [30, 31]. The concentration of clusters increases with increasing the Er3+ concentration and phenomenon of percolation begins approximately from 6% [30, 31]. As a result of this phenomenon, clusters come to inevitable spatial contact with each other. Thus, superclusters are formed, which reach micron size. Er3+ ions are concentrated in these superclusters. Therefore, the processes of ion–ion interaction

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between Er3+ ions in neighboring clusters are very effective. In this case ETU processes significantly change the slopes of UCL in Er-doped strontium fluoride powders with increasing concentration of Er3+ ions and excitation power density.

Fig. 3. P-dependent upconversion luminescence at the 2H9/2→4I15/2 (а, b), 4S3/2→4I15/2 (c, d) and F9/2→4I15/2 (e, f) transitions of Er3+ ions for two power density ranges. The diagram is in a double logarithmic scale. 4

Figure 4 presents the spectral power of the UC luminescence of Er3+ ions in the visible range upon laser excitation at 1531.8 nm for SrF2:Er (CEr = 1.6, 3.4, 6.0, 8.8, 14.2, 18.3, 21.3 mol.%) powders. From Fig. 4 we can conclude that the spectral power of the UC luminescence in range 360-700 nm increases with the increase of the concentration of the Er3+ ions until to 8.8%. Upon further increasing in the concentration of Er3+ ions, the spectral power of UC luminescence begins to decrease. The ratio of red to green luminescence of Er3+ ions is the similar for all Er-doped strontium fluoride samples. The same dependence of UCL on concentration of Er3+ ions was observed for CaF2:Er crystals [23].

Fig. 4. (A) The spectral power of the UC luminescence of SrF2:Er powders. (B) The CIE chromaticity diagram of SrF2:Er powders. Excitation power density is 0.51 kW/cm2.

SrF2:Er powders can be used as infrared quantum counters, visualizers of infrared laser radiation and phosphors for light-emitting diodes. For these applications photoluminescence

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quantum yield, chromaticity coordinates and color temperature of UCL of SrF2:Er powders were estimated (Table 2). The photoluminescence quantum yield ΦUC is defined as the ratio of the number of photons emitted to the number of photons absorbed by UC materials [36]. We developed system for measuring of ΦUC based on absolute method [36]. The absolute quantum yield of SrF2:Er increases with doping concentration of Er3+ ions until to 8.8%. Maximum quantum yield of 0.19% was achieved for SrF2:Er (8.8 mol.%) with incident laser power density of 0.51 kW/cm2. As mentioned above, ETU processes responsible for the UCL in SrF2:Er. Efficiency of ETU increases with doping concentration of Er3+ ions in SrF2:Er. Dependences of quantum yield and intensity of UCL on the concentration of Er3+ ions until to 8.8% can be explained by increasing the efficiency of ETU processes. Upon further increasing in the concentration of Er3+ ions concentration quenching of UCL is observed. Table 2. The quantum yields, chromaticity coordinates and color temperature of SrF2:Er powders CEr (mol.%)

ΦUC (%)

X

Y

T (K)

1.6 3.4

0.10 0.11

0.456 0.440

0.475 0.500

3221 3589

6.0

0.13

0.432

0.519

3792

8.8

0.19

0.407

0.553

4305

14.2

0.15

0.400

0.542

4389

18.3

0.08

0.411

0.532

4178

21.3

0.05

0.414

0.519

4082

The chromaticity of the SrF2:Er powders was calculated by use of the Commission International de l’Eclairage (CIE) chromaticity coordinates (x, y) and the results are presented in the Table 2. The color temperatures of yellow-green UCL of SrF2:Er powders with 1.6, 3.4, 6.0, 8.8, 14.2, 18.3 and 21.3 mol.% concentration of Er3+ ions were 3221, 3589, 3792, 4305, 3993, 4389, 4178 and 4082 K, respectively. 4. Conclusions In summary, Er-doped strontium fluoride powders with different concentration of Er3+ ions from 1.6 mol.% to 21.3 mol.% have been successfully prepared by using a co-precipitation from aqueous solution technique. For the first time upconversion luminescence of SrF2:Er powders upon excitation of the 4I13/2 level of Er3+ ions were investigated. Upconversion luminescence was observed from 4G11/2, 2H9/2, 4F5/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2 and 4I11/2 levels to the ground 4I15/2 level. ETU processes significantly change the slopes of blue, green and red upconversion luminescence in Er-doped strontium fluoride powders with increasing concentration of Er3+ ions and excitation power density. The most intensive visible luminescence was obtained for SrF2:Er (8.8%) powder. Maximum quantum yield of 0.19% was achieved for the SrF2:Er (8.8%) powder with incident laser power of 0.51 kW/cm2. Also chromaticity coordinates and color temperature of yellow-green UC of SrF2:Er were calculated. The present results indicate that SrF2:Er powder is a promising upconversion material. Funding Russian Science Foundation (17-72-10163). Acknowledgments This work is supported by the Russian Science Foundation under grant 17-72-10163.