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A new path to design near-infrared persistent luminescence materials using Yb3+-doped rare earth oxysulfides
Scripta Materialia 164 (2019) 57–61
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
A new path to design near-infrared persistent luminescence materials using Yb3+-doped rare earth oxysulfides Ian Pompermayer Machado a, Cássio Cardoso Santos Pedroso b, José Miranda de Carvalho c, Verônica de Carvalho Teixeira d, Lucas Carvalho Veloso Rodrigues a,⁎, Hermi Felinto Brito a a
Department of Fundamental Chemistry, Institute of Chemistry, University of São Paulo, 05508-000 São Paulo, SP, Brazil Department of Biochemistry, Institute of Chemistry, University of São Paulo, 05508-000 São Paulo, SP, Brazil Institute of Physics, University of São Paulo, 05508-090 São Paulo, SP, Brazil d Brazilian Synchrotron Light Laboratory (LNLS), Brazilian Center for Research in Energy and Materials (CNPEM), 13083-970 Campinas, SP, Brazil b c
a r t i c l e
i n f o
Article history: Received 12 December 2018 Accepted 21 January 2019 Available online xxxx Keywords: Optical materials Rare earth Photonic structure Optical microscopy Persistent luminescence
a b s t r a c t RE2O2S:Yb3+ and RE2O2S:Yb3+,Ti,Mg2+ near-infrared persistent luminescence materials (RE3+: La, Gd, Y) were designed based on the efficient red-emitting RE2O2S:Eu3+,Ti,Mg2+, due to the similar ground state level positions between Eu3+ and Yb3+. The Y2O2S:Yb3+,Ti,Mg2+ showed the longest near-infrared (~983 nm) persistent luminescence time (~1 h) when excited at the charge transfer transition, demonstrating that Ti,Mg2+ co-doping enhances the persistent luminescence only for the Y2O2S host. The unprecedented relationship between Eu3+ and Yb3+ persistent luminescence opens a new path in the systematic design of efficient Yb3+-activated persistent luminescence materials e.g. Y2O2S:Yb3+,Ti,Mg2+, which present potential for photonic applications such as bioimaging probes. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Persistent luminescence is the phenomenon where a material emits light for several minutes after ceasing the excitation sources [1]. Those materials are investigated in terms of self-sustained lighting devices which can be used both as room illumination and security lighting [2–4]. Moreover, near-infrared (NIR) persistent luminescence phenomenon is increasingly getting on focus for in vivo applications such as cell tagging and drug/tumor tracking since the emitted light falls into the therapeutic biological window (650–1350 nm) and can be detected for hours after ex-situ excitation [5–7]. However, efficient NIR emitting materials are still scarce in literature, and often are obtained through complex processes [7,8]. In this context, the microwave-assisted solidstate (MASS) method arises as a good alternative synthesis route. The advantages include short processing time, low energy consumption and the possibility of using inexpensive equipment (domestic microwave ovens), being a good alternative to yield high-purity photonic materials [9–11]. In the design of persistent luminescence materials, the knowledge of lanthanide energy level positions relative to the host lattice band structure is essential to understand persistent luminescence mechanisms [1,12–14]. According to the energy level diagrams obtained from host refereed binding energy model [15–18], the Eu3+ (4f6) and Yb3+ (4f13) ions presents similar electronic properties, e.g. their ground states (7F0 and 2F7/2, respectively) differ in energy by only 0.24 eV, and their ⁎ Corresponding author. E-mail address: [email protected] (L.C.V. Rodrigues).
https://doi.org/10.1016/j.scriptamat.2019.01.023 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
ligand-to-metal charge transfer (LMCT) energies are comparable. Aiming for alternative NIR persistent luminescence materials, we propose the substitution of Eu3+ by Yb3+ dopant to yield persistent luminescence from the 2F5/2 → 2F7/2 transition (~980 nm) of Yb3+ ion. The Y2O2S:Eu3+,Ti,Mg2+ is one of the most efficient red-emitting persistent luminescence materials due to the Eu3+ emission enhancement by the Ti,Mg2+ codoping [19–21]. This material is therefore a good candidate as a starting point to achieve efficient Yb3+ NIR persistent luminescence. Here, NIR persistent luminescence materials RE2O2S:Yb3+ and RE2O2S:Yb3+,Ti,Mg2+ (RE3+: La, Gd, Y) were obtained by a rapid microwave-assisted solid-state synthesis (MASS). NIR (~980 nm) persistent luminescence was observed for all materials, showing the highest efficiency for Y2O2S:Yb3+,Ti,Mg2+. The unprecedented persistent luminescence relationship established between Eu3+ and Yb3+ provides a new way for designing NIR persistent luminescence materials. Polycrystalline RE2O2S, RE2O2S:Yb3+, and RE2O2S:Yb3+,Ti,Mg2+ materials (RE3+: La, Gd, Y) were prepared by rapid microwave-assisted solid-state (MASS) method, similar to our previous reports [4,22]. Rare earth oxides (La2O3, Gd2O3, Y2O3 and Yb2O3, CSTARM 99.9%), titanium oxide (TiO2, Merck 99.5%) and magnesium carbonate (MgCO3, Merck 99.9%) were ground with sulfur (S, LabSynth 99.5%) and sodium carbonate (Na2CO3, Vetec 99.5%) used as flux. The RE2O3:S:Na2CO3 proportion for this reaction was 4:4:1 in the preparation of La2O2S- and Gd2O2S-based materials, with a 7.5 mol-% excess of sulfur. In the case
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of Y2O2S materials, the Y2O3:S:Na2CO3 molar ratio was 4:12:1. The RE2O2S:Yb3+ materials were doped with 0.5% Yb3+ ions, and the RE2O2S:Yb3+,Ti,Mg2+ with 0.5% Yb3+, 1.5% Ti and 4.5% Mg2+ in mol-% relative to RE3+ amount. For the MASS syntheses, 0.5 g of raw precursors was added to a 5 cm3 alumina crucible, which was previously surrounded with 11 g of activated charcoal placed inside a 50 cm3 alumina crucible. Both crucibles were partially covered by an alumina lid and then placed into the cavity of an aluminosilicate thermal insulation bricks. Finally, the samples were irradiated in a domestic microwave oven (Electrolux MEF41) using a program of 10 min at 900 W + 15 min at 800 W (totalizing 25 min). To obtain the final products, the samples were ground with a small amount of sulfur (7.5 mol-% relative to RE3+) and heated one more time at the same microwave conditions. Crystal structures and phase purities were analyzed by X-ray powder diffraction using a Rigaku Miniflex II instrument, with CuKα1 radiation (λ = 1.5406 Å) over the 10–80° 2θ range, employing a 0.05° step with integration time of 1 s. Photoluminescence excitation and emission spectra for RE2O2S: Yb3+ and RE2O2S:Yb3+,Ti,Mg2+ materials were carried out at room temperature using a Fluorolog 3 spectrofluorometer (Horiba), equipped with a 450 W xenon lamp and a Synapse E2V CCD30 detector (Horiba, resolution 1024 × 256 pixels). Persistent luminescence emission spectra and decay curves were recorded using the same system, and the optical data were acquired during 240 s after ceasing the irradiation source. In addition, persistent luminescence decay curves of Y2O2S:Yb3+ and Y2O2S:Yb3+,Ti,Mg2+ materials were recorded using a FLSP 920 photon counter (Edinburgh Instruments). It is equipped with a Hamamatsu H10330A-45 PMT apparatus, which was maintained at −20 °C by a CO2 thermo-electric cooler. The NIR light was detected by integrating all signal in the 900–2000 nm range, thus excluding any visible-light influences. NIR fluorescence micrographs for the La2O2S:Yb3+,Ti,Mg2+ material were obtained using a Nikon Eclipse microscope with 10× optical magnification. A CryLas YAG:Nd3+ 355 nm laser was used to excite the material. The image was recorded with a Nikon DS-Qi1 digital microscope camera (integration time 4.0 s), and a 715 nm cutoff filter was used to avoid any contributions of visible-light. RE2O2S materials were rapidly prepared with high crystal purity by the MASS method (Fig. S1, Supplementary material). The oxysulfides crystallized in trigonal system (space group P 3 m1), with RE3+ ions occupying seven-coordinate C3v site, each bonded to four oxygen and three sulfur atoms [23,24]. It was observed a phase segregation only for Y2 O 2S-based materials, which contain with a small portion of Y2O3. This impurity is due to the Y3+ smaller radius and consequently higher hard acid character [25], thereby the reaction with sulfur is enthalpically more favored for La3+ and Gd3+ ions. The higher molar ratio S:Y2O3 (3:1) was needed for generation of the Y2O2S with phase purity. The RE2O2S synthesis and particle formation mechanism by MASS method is detailed described in literature [4]. The excitation spectra of RE2O2S:Yb3+ and RE2O2S:Yb3+,Ti,Mg2+ materials (Fig. 1a and c) exhibit broad absorption bands assigned to the O2−(2p) → Yb3+(4f) and S2−(3p) → Yb3+(4f) ligand-to-metal charge transfer (LMCT) transitions (280–350 nm) as well as the host lattice absorption (260–280 nm). The two overlapped LMCT bands are clearly observed in Y2O2S-based materials. In addition, a broad absorption band at 370 nm is observed in the Y2O2S:Yb3+,Ti, Mg2+ (Fig. 1c), arising from Ti3+/IV ions [4,26]. The Ti species can thus also act as a sensitizer of Yb3+ ions in the Y2O2S host. This optical feature is less pronounced for Gd2O2S:Yb3+,Ti,Mg2+ and absent for La 2O 2 S:Yb 3+,Ti,Mg 2+ , indicating a dependence with the electronic structure of RE2O2S host. Lastly, Gd2O2S-based materials spectra also exhibit intraconfigurational Gd3+ 8S7/2 → 6P7/2 transitions, indicating a sensitization of Yb3+ by Gd3+ ions.
Fig. 1. Excitation (a) and emission (b) spectra of singly-doped RE2O2S:Yb3+ materials. Excitation (c) and emission (d) spectra of co-doped RE2O2S:Yb3+,Ti,Mg2+ materials – RE3+: La, Gd, Y. (e) NIR fluorescence optical micrograph of La2O2S:Yb3+,Ti,Mg2+ material.
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Yb3+-doped oxysulfides show narrow emission bands in the 900–1050 nm range, assigned to the Yb3+ 2F5/2 → 2F7/2 transitions (Fig. 1b and d). The crystal field splitting of Yb3+ Stark levels in RE2O2S hosts results in three main emission bands centered around 953, 983 and 1010 nm. Narrow emission bands can also be observed in 550–750 nm range, particularly in the Gd2O2S:Yb3+ and Gd2O2S: Yb3+,Ti,Mg2+ materials, attributed to Eu3+ impurities from the RE2O3 precursors. Eu3+ impurity in RE2O3 is due to the high ionic radius similarity among rare earths, La3+ (1.100 Å), Eu3+ (1.010 Å), Gd3+ (1.000 Å), and Y3+ (0.960 Å) for coordination number 7 [27], especially between trivalent gadolinium and europium ions [22]. It is worthy nothing that Eu3+ emission bands exhibit higher intensities than Yb3+ emissions in Gd-based materials despite its lower concentration, since RE2O2S:Eu3+ is reported as one of the most efficient red emitting materials [28]. In addition, the broad emission band centered at 610 nm of Y2O2S:Yb3+,Ti,Mg2+ material (Fig. 1d) is attributed to electronic transitions of Ti3+/IV ions [4,19,29]. In order to visualize NIR emissions in the Yb3+-doped oxysulfides, fluorescence optical micrographs of La2O2S:Yb3+,Ti,Mg2+ material were recorded at room temperature (Fig. 1e). A 715 nm cutoff filter was employed to avoid all possible visible-range contributions to the image. The material shows homogeneous NIR emissions, suggesting a good distribution of Yb3+ emitting ions through the particles. NIR persistent luminescence arises from Yb3+ 2F5/2 → 2F7/2 transitions when ceasing the irradiation source, as shown in the persistent luminescence emission spectra of RE2O2S:Yb3+,Ti,Mg2+ materials (Fig. 2). It is worth noting that Yb3+-activated NIR persistent luminescence materials are scarce [30–32]. Yb3+ emission profiles are similar when comparing the persistent luminescence and photoluminescence spectra (Fig. 1d). Persistent luminescence narrow emission bands in the 500–720 nm range observed in the RE2O2S:Yb3+,Ti,Mg2+ (RE3+: La and Gd), as well as in the Yb3+ singly-doped materials (Fig. S2, Supplementary material), are assigned to the 5D1 → 7F0–3 and 5D0 → 7F0–4 transitions of Eu3+ ion. In addition, it is observed in the same spectral range a persistent broad emission band originated from the Ti co-dopant. The titanium spectroscopic data are consistent with literature [4,19,33]. On the other hand, the emission spectra of the Y2O2S:Yb3+,Ti,Mg2+ material show only the broad emission band assigned to Ti species. As it can be seen, Ti persistent emission band intensity drastically increases moving through La → Gd → Y materials. According to the NIR persistent luminescence decay curves (Fig. 3a), RE2O2S:Yb3+ (RE3+: La, Gd, Y) exhibit short persistent luminescence decay times (~30 s). Moreover, the Ti and Mg2+ addition had no influence on the decay time and profile for La and Gd hosts. Serendipitously, the decay time was significantly improved by co-doping with Ti and Mg2+ in Y2O2S:Yb3+. To verify more precisely the decay time enhancement, long NIR persistent luminescence decay curves were acquired for the Y2O2S:Yb3+ and Y2O2S:Yb3+,Ti,Mg2+ (Fig. 3b). An enhancement of
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Fig. 3. (a) Persistent luminescence decay curves of RE2O2S:Yb3+ and RE2O2S:Yb3+,Ti,Mg2 materials – RE3+: La, Gd, Y. (b) Long-time persistent decay curves of Y2O2S:Yb3+ and Y2O2S:Yb3+,Ti,Mg2+ materials.
+
at least one order of magnitude is observed by Ti,Mg2+ co-doping, leading to an efficient NIR persistent luminescence for the Y2O2S:Yb3+,Ti, Mg2+ material, which lasts for almost 1 h. From the experimental data above it is noticed that the persistent luminescence and the photoluminescence properties arising from Yb3+
Fig. 2. (a) Persistent luminescence emission spectra of RE2O2S:Yb3+,Ti,Mg2+ materials – RE3+: La (a), Gd (b), Y (c).
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is dependent on Ti3+ d-d transitions, the different crystal fields in the series could be a reason for Y host higher efficiency. The electron trapping process is directly related to the concentration of positive defects, which are generated by Mg2+ co-doping via charge compensation. This is supported by C-C. Kang et al. and X. Wang et at. which showed that Ti and Mg2+ dopants randomly occupy the Y3+ sites in Y2O2S [19], and that the persistent luminescence of Y2O2S:Eu3+,Ti,Mg2+ is much longer than for Y2O2S:Eu3+,Ti [37].
Fig. 4. Energy level diagram for the Yb3+-activated persistent luminescence mechanism in rare earth oxysulfides. CT energies were obtained from the excitation spectra and band gap values are consistent with Ref [4]. Eu3+ and Yb3+ ground states were placed according to the references [16–18].
ions and Ti species are dependent on the RE2O2S host electronic structure. According to literature [34,35], the persistent luminescence mechanism for RE2O2S:Eu3+ materials is a hole-trapping process, where holes act as the charge carries created after excitation at band gap or LMCT O2−(S2−) → Eu3+ energies. Due to the similar energy positions of ground states and CT states between Eu3+ and Yb3+, persistent luminescence of Yb3+ in rare earth oxysulfides works in a similar fashion to those for RE2O2S:Eu3+, as illustrated in the persistent luminescence mechanism (Fig. 4). Firstly, the excitation at Yb3+ CT energy, or at band-gap energy, promotes an electron (e−) from the top of valence band (VB) to CT state Yb2+ 1S0, or to conduction band (CB). This excitation process creates a hole (h+) in VB, which can be captured by nearby hole traps (e.g. cation vacancies or interstitial O2– and S2− in the lattice). Then, the return of h+ to VB and subsequent e−–h+ recombination transfers energy to the Yb3+ 2F5/2 level, producing 2F5/2 → 2F7/2 NIR persistent luminescence emission. This mechanism (Fig. 4) shows how NIR persistent luminescence works for the singly-doped RE2O2S:Yb3+ materials. However, Ti,Mg2+ co-doping strongly enhances NIR persistent luminescence in Y2O2S host, even if the effect on La- and Gd-oxysulfides is very small. Two distinct processes, considering either Ti3+ or TiIV valences, can explain the longest persistent luminescence of Y2O2S:Yb3+,Ti,Mg2+ material:
Both mechanisms might simultaneously occur since mixed Ti3+/IV valence states are quite probable due to the reducing CO (g) atmosphere provided by the MASS synthesis and the better charge/radius match between Ti3+ and RE3+ ions [4]. Therefore, additional EPR and thermoluminescence studies will be carried out to elucidate the interplay of Ti3+/TiIV to understand the dominant persistent luminescence mechanism in rare earth oxysulfide materials. In summary, RE2O2S:Yb3+ and RE2O2S:Yb3+,Ti,Mg2+ NIR persistent luminescence materials (RE3+: La, Gd, Y) were rapidly prepared (two 25-min steps) by microwave-assisted solid-state synthesis (MASS). Based on RE2+/3+ energy level diagrams of Eu3+-doped oxysulfide materials, the Yb3+ persistent luminescence in RE2O2S systems was validated and a new NIR persistent luminescence material was designed. Yb3+ persistent luminescence (λem: 983 nm) was observed for all materials, showing the highest efficiency for Y2O2S:Yb3+,Ti,Mg2+ (NIR emission detected for almost 1 h). The persistent luminescence enhancement by Ti,Mg2+ co-doping was shown to be an exclusive effect on the Y2O2S host. These results suggest that the energy storage and transfer processes involving Ti and Yb species are intrinsically related to the host electronic structure. The relationship between Eu3+ and Yb3+ persistent luminescence mechanisms provides a systematic approach for the design of new Yb3+-activated NIR persistent luminescence materials. Owing to its simple preparation and efficient NIR persistent luminescence, the Y2O2S:Yb3+,Ti,Mg2+ is a promising material for photonic applications such as night vision devices and bioimaging probes. The authors thank the financial support from the Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq (IPM #141446/2016-1, LCVR #427312/2016-7 and HFB #427733/2016-2) and the Fundação de Amparo à Pesquisa do Estado de São Paulo - FAPESP (JMC #2017/05195-5 and CCSP #2017/09774-0), Brazil. The authors sincerely thank Prof. Dr. Maurício Baptista (IQ-USP), Dr. Helena Couto Junqueira (NAP-Phototech) and Dr. Divinomar Severino (TechScientific/Edinburgh Instruments) for the NIR fluorescence micrograph studies. Finally, the authors deeply thank Prof. Dr. Paolo Di Mascio and Dr. Fernanda Manso Prado from the Laboratório Lesões em Biomoléculas (IQ-USP) for the long-time NIR persistent luminescence decay experiments. Appendix A. Supplementary data
IV
i) The LMCT excitation of Ti is more efficient to trap the holes than Yb3+ LMCT. Thus, more energy is stored and since Ti can sensitize Yb3+ ions, this would increase the NIR persistent luminescence decay time. A similar sensitizing process was described by Sontakke et al. for the Ce3+-Yb3+ pair in borate glasses [36]. Yb3+ sensitization by Ti ions increases through La → Gd → Y oxysulfides (Fig. 1c). Thus, the increase in crystal field splitting leads to a decrease of O2−(2p) → TiIV(3d) and S2−(3p) → TiIV(3d) CT energies values [34] which can find an optimal level position to transfer the energy to Yb3+ in Y2O2S host. Besides, TiIV increases the concentration of hole traps through charge compensation, contributing to a higher energy storage. ii) Ti3+ d-d excitation leads to electron trapping by oxide and sulfide vacancies via CB. After the kT bleaching of traps, the electron returns to Ti3+ and then the Ti3+-Yb3+ energy transfer occurs, increasing the NIR persistent luminescence efficiency. Since the energy transfer
Supplementary data to this article can be found online at https://doi. org/10.1016/j.scriptamat.2019.01.023. References [1] H.F. Brito, J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, L.C.V. Rodrigues, Opt. Mater. Express 2 (2012) 371–381. [2] M. Saito, N. Adachi, H. Kondo, Opt. Express 15 (2007) 1621–1626. [3] J. Xu, J. Ueda, K. Kuroishi, S. Tanabe, Scr. Mater. 102 (2015) 47–50. [4] J. Miranda de Carvalho, C.C.S. Pedroso, I.P. Machado, J. Hölsä, L.C.V. Rodrigues, P. Głuchowski, M. Lastusaari, H.F. Brito, J. Mater. Chem. C 6 (2018) 8897–8905. [5] T. Maldiney, A. Bessiere, J. Seguin, E. Teston, S.K. Sharma, B. Viana, A.J.J. Bos, P. Dorenbos, M. Bessodes, D. Gourier, D. Scherman, C. Richard, Nat. Mater. 13 (2014) 418–426. [6] S.K. Singh, RSC Adv. 4 (2014) 58674–58698. [7] Z. Pan, Y.-Y. Lu, F. Liu, Nat. Mater. 11 (2011) 58–63. [8] K. Van den Eeckhout, D. Poelman, P.F. Smet, Materials (Basel) 6 (2013) 2789–2818. [9] K.J. Rao, B. Vaidhyanathan, M. Ganguli, P.A. Ramakrishnan, Chem. Mater. 11 (1999) 882–895.
I.P. Machado et al. / Scripta Materialia 164 (2019) 57–61 [10] H.J. Kitchen, S.R. Vallance, J.L. Kennedy, N. Tapia-Ruiz, L. Carassiti, A. Harrison, A.G. Whittaker, T.D. Drysdale, S.W. Kingman, D.H. Gregory, Chem. Rev. 114 (2014) 1170–1206. [11] C.C.S. Pedroso, J.M. Carvalho, L.C.V. Rodrigues, J. Holsa, H.F. Brito, ACS Appl. Mater. Interfaces 8 (2016) 19593–19604. [12] T. Aitasalo, J. Hölsä, H. Jungner, M. Lastusaari, J. Niittykoski, J. Phys. Chem. B 110 (2006) 4589–4598. [13] L.C.V. Rodrigues, H.F. Brito, J. Hölsä, R. Stefani, M.C.F.C. Felinto, M. Lastusaari, T. Laamanen, L.A.O. Nunes, J. Phys. Chem. C 116 (2012) 11232–11240. [14] J. Ueda, R. Maki, S. Tanabe, Inorg. Chem. 56 (2017) 10353–10360. [15] P. Dorenbos, J. Phys. Condens. Matter 15 (2003) 8417–8434. [16] P. Dorenbos, J. Lumin. 111 (2005) 89–104. [17] P. Dorenbos, Phys. Rev. B: Condens. Matter Mater. Phys. 85 (2012) 1–10. [18] P. Dorenbos, J. Lumin. 135 (2013) 93–104. [19] C.-C. Kang, R.-S. Liu, J.-C. Chang, B.-J. Lee, Chem. Mater. 15 (2003) 3966–3968. [20] J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, J. Niittykoski, E. Zych, Opt. Mater. (Amst.) 129 (2009) 1661–1663. [21] S. Deng, Z. Xue, Y. Liu, B. Lei, Y. Xiao, M. Zheng, J. Alloys Compd. 542 (2012) 207–212. [22] I.P. Machado, V.C. Teixeira, C.C.S. Pedroso, H.F. Brito, L.C.V. Rodrigues, J. Alloys Compd. 777 (2019) 638–645. [23] O.J. Sovers, T. Yoshioka, J. Chem. Phys. 51 (1969) 5330–5336. [24] H.A. Eick, J. Am. Chem. Soc. 80 (1958) 43–44.
61
[25] R.G. Pearson, J. Am. Chem. Soc. 85 (1963) 3533–3539. [26] L.H.C. Andrade, S.M. Lima, A. Novatski, A.M. Neto, A.C. Bento, M.L. Baesso, F.C.G. Gandra, Y. Guyot, G. Boulon, Phys. Rev. B: Condens. Matter Mater. Phys. 78 (2008), 224202. (1-11). [27] R.D. Shannon, Acta Crystallogr. Sect. A 32 (1976) 751–767. [28] C. Guo, L. Luan, C. Chen, D. Huang, Q. Su, Mater. Lett. 62 (2008) 600–602. [29] I. Norrbo, J.M. Carvalho, P. Laukkanen, J. Mäkelä, F. Mamedov, M. Peurla, H. Helminen, S. Pihlasalo, H. Härmä, J. Sinkkonen, M. Lastusaari, Adv. Funct. Mater. 27 (2017) 1–9. [30] C. Li, J. Wang, H. Liang, Q. Su, J. Appl. Phys. 101 (2007) 1–5. [31] V. Caratto, F. Locardi, G.A. Costa, R. Masini, M. Fasoli, L. Panzeri, M. Martini, E. Bottinelli, E. Gianotti, I. Miletto, ACS Appl. Mater. Interfaces 6 (2014) 17346–17351. [32] Y.-J. Liang, F. Liu, Y.-F. Chen, X.-J. Wang, K.-N. Sun, Z. Pan, Light Sci. Appl. 5 (2016), e16124. . [33] B. Lei, Y. Liu, J. Zhang, J. Meng, S. Man, S. Tan, J. Alloys Compd. 495 (2010) 247–253. [34] H. Luo, A.J.J. Bos, P. Dorenbos, J. Phys. Chem. C 121 (2017) 8760–8769. [35] F. Clabau, X. Rocquefelte, T. Le Mercier, P. Deniard, S. Jobic, M.H. Whangbo, Chem. Mater. 18 (2006) 3212–3220. [36] A.D. Sontakke, J. Ueda, Y. Katayama, Y. Zhuang, P. Dorenbos, S. Tanabe, J. Appl. Phys. 117 (2015), 013105. . [37] X. Wang, Z. Zhang, Z. Tang, Y. Lin, Mater. Chem. Phys. 80 (2003) 1–5.
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
A new path to design near-infrared persistent luminescence materials using Yb3+-doped rare earth oxysulfides Ian Pompermayer Machado a, Cássio Cardoso Santos Pedroso b, José Miranda de Carvalho c, Verônica de Carvalho Teixeira d, Lucas Carvalho Veloso Rodrigues a,⁎, Hermi Felinto Brito a a
Department of Fundamental Chemistry, Institute of Chemistry, University of São Paulo, 05508-000 São Paulo, SP, Brazil Department of Biochemistry, Institute of Chemistry, University of São Paulo, 05508-000 São Paulo, SP, Brazil Institute of Physics, University of São Paulo, 05508-090 São Paulo, SP, Brazil d Brazilian Synchrotron Light Laboratory (LNLS), Brazilian Center for Research in Energy and Materials (CNPEM), 13083-970 Campinas, SP, Brazil b c
a r t i c l e
i n f o
Article history: Received 12 December 2018 Accepted 21 January 2019 Available online xxxx Keywords: Optical materials Rare earth Photonic structure Optical microscopy Persistent luminescence
a b s t r a c t RE2O2S:Yb3+ and RE2O2S:Yb3+,Ti,Mg2+ near-infrared persistent luminescence materials (RE3+: La, Gd, Y) were designed based on the efficient red-emitting RE2O2S:Eu3+,Ti,Mg2+, due to the similar ground state level positions between Eu3+ and Yb3+. The Y2O2S:Yb3+,Ti,Mg2+ showed the longest near-infrared (~983 nm) persistent luminescence time (~1 h) when excited at the charge transfer transition, demonstrating that Ti,Mg2+ co-doping enhances the persistent luminescence only for the Y2O2S host. The unprecedented relationship between Eu3+ and Yb3+ persistent luminescence opens a new path in the systematic design of efficient Yb3+-activated persistent luminescence materials e.g. Y2O2S:Yb3+,Ti,Mg2+, which present potential for photonic applications such as bioimaging probes. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Persistent luminescence is the phenomenon where a material emits light for several minutes after ceasing the excitation sources [1]. Those materials are investigated in terms of self-sustained lighting devices which can be used both as room illumination and security lighting [2–4]. Moreover, near-infrared (NIR) persistent luminescence phenomenon is increasingly getting on focus for in vivo applications such as cell tagging and drug/tumor tracking since the emitted light falls into the therapeutic biological window (650–1350 nm) and can be detected for hours after ex-situ excitation [5–7]. However, efficient NIR emitting materials are still scarce in literature, and often are obtained through complex processes [7,8]. In this context, the microwave-assisted solidstate (MASS) method arises as a good alternative synthesis route. The advantages include short processing time, low energy consumption and the possibility of using inexpensive equipment (domestic microwave ovens), being a good alternative to yield high-purity photonic materials [9–11]. In the design of persistent luminescence materials, the knowledge of lanthanide energy level positions relative to the host lattice band structure is essential to understand persistent luminescence mechanisms [1,12–14]. According to the energy level diagrams obtained from host refereed binding energy model [15–18], the Eu3+ (4f6) and Yb3+ (4f13) ions presents similar electronic properties, e.g. their ground states (7F0 and 2F7/2, respectively) differ in energy by only 0.24 eV, and their ⁎ Corresponding author. E-mail address: [email protected] (L.C.V. Rodrigues).
https://doi.org/10.1016/j.scriptamat.2019.01.023 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
ligand-to-metal charge transfer (LMCT) energies are comparable. Aiming for alternative NIR persistent luminescence materials, we propose the substitution of Eu3+ by Yb3+ dopant to yield persistent luminescence from the 2F5/2 → 2F7/2 transition (~980 nm) of Yb3+ ion. The Y2O2S:Eu3+,Ti,Mg2+ is one of the most efficient red-emitting persistent luminescence materials due to the Eu3+ emission enhancement by the Ti,Mg2+ codoping [19–21]. This material is therefore a good candidate as a starting point to achieve efficient Yb3+ NIR persistent luminescence. Here, NIR persistent luminescence materials RE2O2S:Yb3+ and RE2O2S:Yb3+,Ti,Mg2+ (RE3+: La, Gd, Y) were obtained by a rapid microwave-assisted solid-state synthesis (MASS). NIR (~980 nm) persistent luminescence was observed for all materials, showing the highest efficiency for Y2O2S:Yb3+,Ti,Mg2+. The unprecedented persistent luminescence relationship established between Eu3+ and Yb3+ provides a new way for designing NIR persistent luminescence materials. Polycrystalline RE2O2S, RE2O2S:Yb3+, and RE2O2S:Yb3+,Ti,Mg2+ materials (RE3+: La, Gd, Y) were prepared by rapid microwave-assisted solid-state (MASS) method, similar to our previous reports [4,22]. Rare earth oxides (La2O3, Gd2O3, Y2O3 and Yb2O3, CSTARM 99.9%), titanium oxide (TiO2, Merck 99.5%) and magnesium carbonate (MgCO3, Merck 99.9%) were ground with sulfur (S, LabSynth 99.5%) and sodium carbonate (Na2CO3, Vetec 99.5%) used as flux. The RE2O3:S:Na2CO3 proportion for this reaction was 4:4:1 in the preparation of La2O2S- and Gd2O2S-based materials, with a 7.5 mol-% excess of sulfur. In the case
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of Y2O2S materials, the Y2O3:S:Na2CO3 molar ratio was 4:12:1. The RE2O2S:Yb3+ materials were doped with 0.5% Yb3+ ions, and the RE2O2S:Yb3+,Ti,Mg2+ with 0.5% Yb3+, 1.5% Ti and 4.5% Mg2+ in mol-% relative to RE3+ amount. For the MASS syntheses, 0.5 g of raw precursors was added to a 5 cm3 alumina crucible, which was previously surrounded with 11 g of activated charcoal placed inside a 50 cm3 alumina crucible. Both crucibles were partially covered by an alumina lid and then placed into the cavity of an aluminosilicate thermal insulation bricks. Finally, the samples were irradiated in a domestic microwave oven (Electrolux MEF41) using a program of 10 min at 900 W + 15 min at 800 W (totalizing 25 min). To obtain the final products, the samples were ground with a small amount of sulfur (7.5 mol-% relative to RE3+) and heated one more time at the same microwave conditions. Crystal structures and phase purities were analyzed by X-ray powder diffraction using a Rigaku Miniflex II instrument, with CuKα1 radiation (λ = 1.5406 Å) over the 10–80° 2θ range, employing a 0.05° step with integration time of 1 s. Photoluminescence excitation and emission spectra for RE2O2S: Yb3+ and RE2O2S:Yb3+,Ti,Mg2+ materials were carried out at room temperature using a Fluorolog 3 spectrofluorometer (Horiba), equipped with a 450 W xenon lamp and a Synapse E2V CCD30 detector (Horiba, resolution 1024 × 256 pixels). Persistent luminescence emission spectra and decay curves were recorded using the same system, and the optical data were acquired during 240 s after ceasing the irradiation source. In addition, persistent luminescence decay curves of Y2O2S:Yb3+ and Y2O2S:Yb3+,Ti,Mg2+ materials were recorded using a FLSP 920 photon counter (Edinburgh Instruments). It is equipped with a Hamamatsu H10330A-45 PMT apparatus, which was maintained at −20 °C by a CO2 thermo-electric cooler. The NIR light was detected by integrating all signal in the 900–2000 nm range, thus excluding any visible-light influences. NIR fluorescence micrographs for the La2O2S:Yb3+,Ti,Mg2+ material were obtained using a Nikon Eclipse microscope with 10× optical magnification. A CryLas YAG:Nd3+ 355 nm laser was used to excite the material. The image was recorded with a Nikon DS-Qi1 digital microscope camera (integration time 4.0 s), and a 715 nm cutoff filter was used to avoid any contributions of visible-light. RE2O2S materials were rapidly prepared with high crystal purity by the MASS method (Fig. S1, Supplementary material). The oxysulfides crystallized in trigonal system (space group P 3 m1), with RE3+ ions occupying seven-coordinate C3v site, each bonded to four oxygen and three sulfur atoms [23,24]. It was observed a phase segregation only for Y2 O 2S-based materials, which contain with a small portion of Y2O3. This impurity is due to the Y3+ smaller radius and consequently higher hard acid character [25], thereby the reaction with sulfur is enthalpically more favored for La3+ and Gd3+ ions. The higher molar ratio S:Y2O3 (3:1) was needed for generation of the Y2O2S with phase purity. The RE2O2S synthesis and particle formation mechanism by MASS method is detailed described in literature [4]. The excitation spectra of RE2O2S:Yb3+ and RE2O2S:Yb3+,Ti,Mg2+ materials (Fig. 1a and c) exhibit broad absorption bands assigned to the O2−(2p) → Yb3+(4f) and S2−(3p) → Yb3+(4f) ligand-to-metal charge transfer (LMCT) transitions (280–350 nm) as well as the host lattice absorption (260–280 nm). The two overlapped LMCT bands are clearly observed in Y2O2S-based materials. In addition, a broad absorption band at 370 nm is observed in the Y2O2S:Yb3+,Ti, Mg2+ (Fig. 1c), arising from Ti3+/IV ions [4,26]. The Ti species can thus also act as a sensitizer of Yb3+ ions in the Y2O2S host. This optical feature is less pronounced for Gd2O2S:Yb3+,Ti,Mg2+ and absent for La 2O 2 S:Yb 3+,Ti,Mg 2+ , indicating a dependence with the electronic structure of RE2O2S host. Lastly, Gd2O2S-based materials spectra also exhibit intraconfigurational Gd3+ 8S7/2 → 6P7/2 transitions, indicating a sensitization of Yb3+ by Gd3+ ions.
Fig. 1. Excitation (a) and emission (b) spectra of singly-doped RE2O2S:Yb3+ materials. Excitation (c) and emission (d) spectra of co-doped RE2O2S:Yb3+,Ti,Mg2+ materials – RE3+: La, Gd, Y. (e) NIR fluorescence optical micrograph of La2O2S:Yb3+,Ti,Mg2+ material.
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Yb3+-doped oxysulfides show narrow emission bands in the 900–1050 nm range, assigned to the Yb3+ 2F5/2 → 2F7/2 transitions (Fig. 1b and d). The crystal field splitting of Yb3+ Stark levels in RE2O2S hosts results in three main emission bands centered around 953, 983 and 1010 nm. Narrow emission bands can also be observed in 550–750 nm range, particularly in the Gd2O2S:Yb3+ and Gd2O2S: Yb3+,Ti,Mg2+ materials, attributed to Eu3+ impurities from the RE2O3 precursors. Eu3+ impurity in RE2O3 is due to the high ionic radius similarity among rare earths, La3+ (1.100 Å), Eu3+ (1.010 Å), Gd3+ (1.000 Å), and Y3+ (0.960 Å) for coordination number 7 [27], especially between trivalent gadolinium and europium ions [22]. It is worthy nothing that Eu3+ emission bands exhibit higher intensities than Yb3+ emissions in Gd-based materials despite its lower concentration, since RE2O2S:Eu3+ is reported as one of the most efficient red emitting materials [28]. In addition, the broad emission band centered at 610 nm of Y2O2S:Yb3+,Ti,Mg2+ material (Fig. 1d) is attributed to electronic transitions of Ti3+/IV ions [4,19,29]. In order to visualize NIR emissions in the Yb3+-doped oxysulfides, fluorescence optical micrographs of La2O2S:Yb3+,Ti,Mg2+ material were recorded at room temperature (Fig. 1e). A 715 nm cutoff filter was employed to avoid all possible visible-range contributions to the image. The material shows homogeneous NIR emissions, suggesting a good distribution of Yb3+ emitting ions through the particles. NIR persistent luminescence arises from Yb3+ 2F5/2 → 2F7/2 transitions when ceasing the irradiation source, as shown in the persistent luminescence emission spectra of RE2O2S:Yb3+,Ti,Mg2+ materials (Fig. 2). It is worth noting that Yb3+-activated NIR persistent luminescence materials are scarce [30–32]. Yb3+ emission profiles are similar when comparing the persistent luminescence and photoluminescence spectra (Fig. 1d). Persistent luminescence narrow emission bands in the 500–720 nm range observed in the RE2O2S:Yb3+,Ti,Mg2+ (RE3+: La and Gd), as well as in the Yb3+ singly-doped materials (Fig. S2, Supplementary material), are assigned to the 5D1 → 7F0–3 and 5D0 → 7F0–4 transitions of Eu3+ ion. In addition, it is observed in the same spectral range a persistent broad emission band originated from the Ti co-dopant. The titanium spectroscopic data are consistent with literature [4,19,33]. On the other hand, the emission spectra of the Y2O2S:Yb3+,Ti,Mg2+ material show only the broad emission band assigned to Ti species. As it can be seen, Ti persistent emission band intensity drastically increases moving through La → Gd → Y materials. According to the NIR persistent luminescence decay curves (Fig. 3a), RE2O2S:Yb3+ (RE3+: La, Gd, Y) exhibit short persistent luminescence decay times (~30 s). Moreover, the Ti and Mg2+ addition had no influence on the decay time and profile for La and Gd hosts. Serendipitously, the decay time was significantly improved by co-doping with Ti and Mg2+ in Y2O2S:Yb3+. To verify more precisely the decay time enhancement, long NIR persistent luminescence decay curves were acquired for the Y2O2S:Yb3+ and Y2O2S:Yb3+,Ti,Mg2+ (Fig. 3b). An enhancement of
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Fig. 3. (a) Persistent luminescence decay curves of RE2O2S:Yb3+ and RE2O2S:Yb3+,Ti,Mg2 materials – RE3+: La, Gd, Y. (b) Long-time persistent decay curves of Y2O2S:Yb3+ and Y2O2S:Yb3+,Ti,Mg2+ materials.
+
at least one order of magnitude is observed by Ti,Mg2+ co-doping, leading to an efficient NIR persistent luminescence for the Y2O2S:Yb3+,Ti, Mg2+ material, which lasts for almost 1 h. From the experimental data above it is noticed that the persistent luminescence and the photoluminescence properties arising from Yb3+
Fig. 2. (a) Persistent luminescence emission spectra of RE2O2S:Yb3+,Ti,Mg2+ materials – RE3+: La (a), Gd (b), Y (c).
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is dependent on Ti3+ d-d transitions, the different crystal fields in the series could be a reason for Y host higher efficiency. The electron trapping process is directly related to the concentration of positive defects, which are generated by Mg2+ co-doping via charge compensation. This is supported by C-C. Kang et al. and X. Wang et at. which showed that Ti and Mg2+ dopants randomly occupy the Y3+ sites in Y2O2S [19], and that the persistent luminescence of Y2O2S:Eu3+,Ti,Mg2+ is much longer than for Y2O2S:Eu3+,Ti [37].
Fig. 4. Energy level diagram for the Yb3+-activated persistent luminescence mechanism in rare earth oxysulfides. CT energies were obtained from the excitation spectra and band gap values are consistent with Ref [4]. Eu3+ and Yb3+ ground states were placed according to the references [16–18].
ions and Ti species are dependent on the RE2O2S host electronic structure. According to literature [34,35], the persistent luminescence mechanism for RE2O2S:Eu3+ materials is a hole-trapping process, where holes act as the charge carries created after excitation at band gap or LMCT O2−(S2−) → Eu3+ energies. Due to the similar energy positions of ground states and CT states between Eu3+ and Yb3+, persistent luminescence of Yb3+ in rare earth oxysulfides works in a similar fashion to those for RE2O2S:Eu3+, as illustrated in the persistent luminescence mechanism (Fig. 4). Firstly, the excitation at Yb3+ CT energy, or at band-gap energy, promotes an electron (e−) from the top of valence band (VB) to CT state Yb2+ 1S0, or to conduction band (CB). This excitation process creates a hole (h+) in VB, which can be captured by nearby hole traps (e.g. cation vacancies or interstitial O2– and S2− in the lattice). Then, the return of h+ to VB and subsequent e−–h+ recombination transfers energy to the Yb3+ 2F5/2 level, producing 2F5/2 → 2F7/2 NIR persistent luminescence emission. This mechanism (Fig. 4) shows how NIR persistent luminescence works for the singly-doped RE2O2S:Yb3+ materials. However, Ti,Mg2+ co-doping strongly enhances NIR persistent luminescence in Y2O2S host, even if the effect on La- and Gd-oxysulfides is very small. Two distinct processes, considering either Ti3+ or TiIV valences, can explain the longest persistent luminescence of Y2O2S:Yb3+,Ti,Mg2+ material:
Both mechanisms might simultaneously occur since mixed Ti3+/IV valence states are quite probable due to the reducing CO (g) atmosphere provided by the MASS synthesis and the better charge/radius match between Ti3+ and RE3+ ions [4]. Therefore, additional EPR and thermoluminescence studies will be carried out to elucidate the interplay of Ti3+/TiIV to understand the dominant persistent luminescence mechanism in rare earth oxysulfide materials. In summary, RE2O2S:Yb3+ and RE2O2S:Yb3+,Ti,Mg2+ NIR persistent luminescence materials (RE3+: La, Gd, Y) were rapidly prepared (two 25-min steps) by microwave-assisted solid-state synthesis (MASS). Based on RE2+/3+ energy level diagrams of Eu3+-doped oxysulfide materials, the Yb3+ persistent luminescence in RE2O2S systems was validated and a new NIR persistent luminescence material was designed. Yb3+ persistent luminescence (λem: 983 nm) was observed for all materials, showing the highest efficiency for Y2O2S:Yb3+,Ti,Mg2+ (NIR emission detected for almost 1 h). The persistent luminescence enhancement by Ti,Mg2+ co-doping was shown to be an exclusive effect on the Y2O2S host. These results suggest that the energy storage and transfer processes involving Ti and Yb species are intrinsically related to the host electronic structure. The relationship between Eu3+ and Yb3+ persistent luminescence mechanisms provides a systematic approach for the design of new Yb3+-activated NIR persistent luminescence materials. Owing to its simple preparation and efficient NIR persistent luminescence, the Y2O2S:Yb3+,Ti,Mg2+ is a promising material for photonic applications such as night vision devices and bioimaging probes. The authors thank the financial support from the Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq (IPM #141446/2016-1, LCVR #427312/2016-7 and HFB #427733/2016-2) and the Fundação de Amparo à Pesquisa do Estado de São Paulo - FAPESP (JMC #2017/05195-5 and CCSP #2017/09774-0), Brazil. The authors sincerely thank Prof. Dr. Maurício Baptista (IQ-USP), Dr. Helena Couto Junqueira (NAP-Phototech) and Dr. Divinomar Severino (TechScientific/Edinburgh Instruments) for the NIR fluorescence micrograph studies. Finally, the authors deeply thank Prof. Dr. Paolo Di Mascio and Dr. Fernanda Manso Prado from the Laboratório Lesões em Biomoléculas (IQ-USP) for the long-time NIR persistent luminescence decay experiments. Appendix A. Supplementary data
IV
i) The LMCT excitation of Ti is more efficient to trap the holes than Yb3+ LMCT. Thus, more energy is stored and since Ti can sensitize Yb3+ ions, this would increase the NIR persistent luminescence decay time. A similar sensitizing process was described by Sontakke et al. for the Ce3+-Yb3+ pair in borate glasses [36]. Yb3+ sensitization by Ti ions increases through La → Gd → Y oxysulfides (Fig. 1c). Thus, the increase in crystal field splitting leads to a decrease of O2−(2p) → TiIV(3d) and S2−(3p) → TiIV(3d) CT energies values [34] which can find an optimal level position to transfer the energy to Yb3+ in Y2O2S host. Besides, TiIV increases the concentration of hole traps through charge compensation, contributing to a higher energy storage. ii) Ti3+ d-d excitation leads to electron trapping by oxide and sulfide vacancies via CB. After the kT bleaching of traps, the electron returns to Ti3+ and then the Ti3+-Yb3+ energy transfer occurs, increasing the NIR persistent luminescence efficiency. Since the energy transfer
Supplementary data to this article can be found online at https://doi. org/10.1016/j.scriptamat.2019.01.023. References [1] H.F. Brito, J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, L.C.V. Rodrigues, Opt. Mater. Express 2 (2012) 371–381. [2] M. Saito, N. Adachi, H. Kondo, Opt. Express 15 (2007) 1621–1626. [3] J. Xu, J. Ueda, K. Kuroishi, S. Tanabe, Scr. Mater. 102 (2015) 47–50. [4] J. Miranda de Carvalho, C.C.S. Pedroso, I.P. Machado, J. Hölsä, L.C.V. Rodrigues, P. Głuchowski, M. Lastusaari, H.F. Brito, J. Mater. Chem. C 6 (2018) 8897–8905. [5] T. Maldiney, A. Bessiere, J. Seguin, E. Teston, S.K. Sharma, B. Viana, A.J.J. Bos, P. Dorenbos, M. Bessodes, D. Gourier, D. Scherman, C. Richard, Nat. Mater. 13 (2014) 418–426. [6] S.K. Singh, RSC Adv. 4 (2014) 58674–58698. [7] Z. Pan, Y.-Y. Lu, F. Liu, Nat. Mater. 11 (2011) 58–63. [8] K. Van den Eeckhout, D. Poelman, P.F. Smet, Materials (Basel) 6 (2013) 2789–2818. [9] K.J. Rao, B. Vaidhyanathan, M. Ganguli, P.A. Ramakrishnan, Chem. Mater. 11 (1999) 882–895.
I.P. Machado et al. / Scripta Materialia 164 (2019) 57–61 [10] H.J. Kitchen, S.R. Vallance, J.L. Kennedy, N. Tapia-Ruiz, L. Carassiti, A. Harrison, A.G. Whittaker, T.D. Drysdale, S.W. Kingman, D.H. Gregory, Chem. Rev. 114 (2014) 1170–1206. [11] C.C.S. Pedroso, J.M. Carvalho, L.C.V. Rodrigues, J. Holsa, H.F. Brito, ACS Appl. Mater. Interfaces 8 (2016) 19593–19604. [12] T. Aitasalo, J. Hölsä, H. Jungner, M. Lastusaari, J. Niittykoski, J. Phys. Chem. B 110 (2006) 4589–4598. [13] L.C.V. Rodrigues, H.F. Brito, J. Hölsä, R. Stefani, M.C.F.C. Felinto, M. Lastusaari, T. Laamanen, L.A.O. Nunes, J. Phys. Chem. C 116 (2012) 11232–11240. [14] J. Ueda, R. Maki, S. Tanabe, Inorg. Chem. 56 (2017) 10353–10360. [15] P. Dorenbos, J. Phys. Condens. Matter 15 (2003) 8417–8434. [16] P. Dorenbos, J. Lumin. 111 (2005) 89–104. [17] P. Dorenbos, Phys. Rev. B: Condens. Matter Mater. Phys. 85 (2012) 1–10. [18] P. Dorenbos, J. Lumin. 135 (2013) 93–104. [19] C.-C. Kang, R.-S. Liu, J.-C. Chang, B.-J. Lee, Chem. Mater. 15 (2003) 3966–3968. [20] J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, J. Niittykoski, E. Zych, Opt. Mater. (Amst.) 129 (2009) 1661–1663. [21] S. Deng, Z. Xue, Y. Liu, B. Lei, Y. Xiao, M. Zheng, J. Alloys Compd. 542 (2012) 207–212. [22] I.P. Machado, V.C. Teixeira, C.C.S. Pedroso, H.F. Brito, L.C.V. Rodrigues, J. Alloys Compd. 777 (2019) 638–645. [23] O.J. Sovers, T. Yoshioka, J. Chem. Phys. 51 (1969) 5330–5336. [24] H.A. Eick, J. Am. Chem. Soc. 80 (1958) 43–44.
61
[25] R.G. Pearson, J. Am. Chem. Soc. 85 (1963) 3533–3539. [26] L.H.C. Andrade, S.M. Lima, A. Novatski, A.M. Neto, A.C. Bento, M.L. Baesso, F.C.G. Gandra, Y. Guyot, G. Boulon, Phys. Rev. B: Condens. Matter Mater. Phys. 78 (2008), 224202. (1-11). [27] R.D. Shannon, Acta Crystallogr. Sect. A 32 (1976) 751–767. [28] C. Guo, L. Luan, C. Chen, D. Huang, Q. Su, Mater. Lett. 62 (2008) 600–602. [29] I. Norrbo, J.M. Carvalho, P. Laukkanen, J. Mäkelä, F. Mamedov, M. Peurla, H. Helminen, S. Pihlasalo, H. Härmä, J. Sinkkonen, M. Lastusaari, Adv. Funct. Mater. 27 (2017) 1–9. [30] C. Li, J. Wang, H. Liang, Q. Su, J. Appl. Phys. 101 (2007) 1–5. [31] V. Caratto, F. Locardi, G.A. Costa, R. Masini, M. Fasoli, L. Panzeri, M. Martini, E. Bottinelli, E. Gianotti, I. Miletto, ACS Appl. Mater. Interfaces 6 (2014) 17346–17351. [32] Y.-J. Liang, F. Liu, Y.-F. Chen, X.-J. Wang, K.-N. Sun, Z. Pan, Light Sci. Appl. 5 (2016), e16124. . [33] B. Lei, Y. Liu, J. Zhang, J. Meng, S. Man, S. Tan, J. Alloys Compd. 495 (2010) 247–253. [34] H. Luo, A.J.J. Bos, P. Dorenbos, J. Phys. Chem. C 121 (2017) 8760–8769. [35] F. Clabau, X. Rocquefelte, T. Le Mercier, P. Deniard, S. Jobic, M.H. Whangbo, Chem. Mater. 18 (2006) 3212–3220. [36] A.D. Sontakke, J. Ueda, Y. Katayama, Y. Zhuang, P. Dorenbos, S. Tanabe, J. Appl. Phys. 117 (2015), 013105. . [37] X. Wang, Z. Zhang, Z. Tang, Y. Lin, Mater. Chem. Phys. 80 (2003) 1–5.