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Enhanced down-conversion luminescence properties of CaSc2O4: Eu3+ crystals
Journal of Luminescence 214 (2019) 116526
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Enhanced down-conversion luminescence properties of CaSc2O4: Eu3+ crystals
T
Xue Wanga, Xiaoyu Zhanga, Lili Wangb,∗, Ling Huangc,∗∗ a
School of Chemistry and Life Sciences, Changchun University of Technology, Changchun, 130012, China School of Materials Science and Engineering, Changchun University of Technology, Changchun, 130012, China c Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing, 211816, China b
A B S T R A C T
CaSc2O4: Eu3+ crystals show intense red (612 nm/614 nm) down-conversion (DC) luminescence. In our paper, the red-shifting of Eu3+-O2- charge transfer (CT) band peaks position from 285 nm to 298 nm was investigated with increasing Ca2+ ion doping concentrations in CaSc2O4: Eu3+ matrix due to the low electronegativity of Ca2+ ion. Meanwhile the intensity ratios of 5D0 → 7F4/5D0 → 7F2 red emissions of Eu3+ ion were increased due to the distorted Cs point symmetry in CaSc2O4 matrix. The luminescence studies indicate that Sc2O3: Eu3+ and CaSc2O4: Eu3+ may serve as a potential red phosphor, where CaSc2O4: Eu3+ shows a better performance than Sc2O3: Eu3+ crystals.
1. Introduction The development of lanthanide-doped down-conversion (DC) luminescence materials have recently drawn great research attention due to their unique photophysical features, such as large anti-Stokes shifts, low toxicity, negligible autofluorescence background, high resistance to photobleaching, and long luminescence lifetime [1–6]. The intrinsic advantages make DC materials attractive for applications in anticounterfeiting, molecular sensing, bioimaging, and therapeutics [7–10]. In addition, Eu3+ ion has a simple energy level scheme and non-degenerative nature for 7F0 ground state and 5D0 excited state. They are well established as one of the best spectroscopic probes for getting an insight into the structure and nature of chemical bonds present in the host matrices [11,12]. Generally, it is vital to select appropriate compounds for lanthanide ion as luminescent activators [13]. Among them, lanthanide-doped fluorides, such as NaYF4, have attracted much research attention in the field of materials science because they have the low phonon frequency [14–16]. It is well known that the low phonon frequency can inhibit non-radiative multi-phonon relaxation and, in turn, yields a relatively high DC luminescence efficiency [17–19]. However, fluorides have poor chemical and thermal stability. In particular, they are easily converted to oxides at high temperature. In contrast to fluorides, oxide materials have attracted a great deal of attention due to their interesting physicochemical properties, such as high chemical and good thermal stabilities [20,21].
∗
Exploiting oxide materials can be an alternative route for obtaining DC crystals with desired emission characteristics. Over the past decade, many efforts have been devoted to controllable synthesis of lanthanidedoped oxide materials to explore their excellent optical properties. Very recently, Sc2O3 and CaSc2O4 have emerged as the new type of oxide host materials. For instance, Pan et al. reported a new Sc2O3: Eu2+/ Eu3+ nanoparticle-based ratiometric nano-thermometer. Our group reported efficient upconversion/downconversion luminescence of Sc2O3: Yb3+/Er3+, Yb3+/Tm3+, Eu3+ and Tb3+ microcrystals [21,22]. In particular, for CaSc2O4 crystal, it has been reported as a promising host for achieving efficient upconversion/downconversion luminescence, such as the red-emitting CaSc2O4: Eu3+, CaSc2O4: Yb3+/Er3+ and green emitting CaSc2O4: Ce3+ [23–25]. These results indicate that lanthanide-doped Sc2O3, CaSc2O4 and CaO materials are excellent DC luminescence host materials. Many researchers have been working on the issue of Eu3+-O2charge transfer (CT) in various oxide matrices for many years. Jiang et al. reported the blue shift of the charge-transfer band (CTB) in the excitation spectra of β-Ca2SiO4: Eu3+ [26]. Tian et al. reported the charge transfer band intensity ratio between Eu3+-O2- and Mo6+-O2increases with increasing Eu3+ ion doping concentrations in Y2(MoO4)3: Eu3+ red phosphors [27]. However, the red-shifts of band positions in Eu3+ ion doped Sc2O3 and CaSc2O4 matrix have not yet been demonstrated. In the current work, the strongest red 5D0 → 7F2 emission (612 nm/ 614 nm) in CaSc2O4: Eu3+ increased by 34 times than those of Sc2O3
Corresponding author. Corresponding author. E-mail addresses: [email protected] (L. Wang), [email protected] (L. Huang).
∗∗
https://doi.org/10.1016/j.jlumin.2019.116526 Received 30 January 2019; Received in revised form 26 April 2019; Accepted 29 May 2019 Available online 30 May 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
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crystals. Additionally, the red-shifting of Eu3+-O2- CT band peaks position from 285 nm to 298 nm was caused by Ca2+ ions substituting for a small amount Sc3+ ions in CaSc2O4 matrix. We also found the luminescence intensity ratio (5D0 → 7F4)/(5D0 → 7F2) gradually increased with increasing Ca2+ ion doping concentrations. 2. Experimental details All chemicals are of analytical grade and used without further purification. ScCl3·6H2O (99.99%), Ca(NO3)2·4H2O (99.99%) Yb (NO3)3·6H2O (99.99%), Eu(NO3)2·6H2O (99.99%) are supplied by Yutai Qingda Chemical Technology Co., Ltd. China. Ethanol is supplied by Beijing Fine Chemical Company. Sc2O3:Eu3+, x mol% Ca2+ (x = 0, 10, 20, 30, 40, 50, 60, 70, 80 and 90) samples were synthesized by a high-temperature solid-state method. Stoichiometric amounts of Sc(Cl)3·6H2O (99.99%), Ca (NO3)2·4H2O (99.99%), and Eu(NO3)3·6H2O (99.99%) was weighted and mixed thoroughly in an agate mortar using ethanol as a solvent. Then, the mixed solution was placed in an alumina crucible and reacted in a muffle furnace at 1100 °C for 4 h. Finally, the as-prepared samples were cooled to room temperature, regrounded for further characterization. The phase purity and crystal structure of the powder samples were examined by X-ray diffraction (XRD) analysis with a powder diffractometer (Model Rig-aku RU-200b), using Ni-filtered Cu-Ka radiation (λ = 1.5406 Å). The spectra were recorded with a Hitachi fluorescence spectrometer F-7000 equipped with a 150 W Xenon lamp. A digital oscilloscope (DPO4104B, bandwidth 1 GHz, sampling rate 5 GSs−1; Tektronix, Shanghai, China), a power-adjustable continuous wave laser diode, and a chopper were used to record decay curves. All tests were measured at a room temperature.
Fig. 1. (a) XRD patterns of Sc2O3: 2 mol% Eu3+, x mol% Ca2+ (0, 10, 20, 30, 40, 50, 60, 70, 80 and 90) crystals with the standard XRD data of Sc2O3 (JCPDS NO.43–1028), CaSc2O4 (JCPDS NO.74–0499) and CaO (JCPDS NO.37–1497). (b), (c) and (d) Schematic of crystal structure of Sc2O3, CaSc2O4 and CaO. Some features diffraction peaks of Sc2O3, CaSc2O4 and CaO crystals are marked with •, ▲ and ◆, respectively.
3. Results and discussion 3.1. Phase and crystal structure The phase transition from Sc2O3 to CaSc2O4 to CaO crystals can be easily tuned by adjusting the content of Ca2+ ion in initial solutions. The XRD patterns of Sc2O3: 2 mol% Eu3+, x mol% Ca2+ (where x = 0, 10, 20, 30, 40, 50, 60, 70, 80 and 90) are illustrated in Fig. 1a. It can be seen that pure Sc2O3 crystals (which match quite well with JCPDS NO.43–1028) can be obtained without Ca2+ ion doping. Correspondingly, Fig. 1b shows the schematic of crystal structure of Sc2O3. It possesses a bixbyite-type structure of space group Ia3 (No. 206). The coordination number for Sc3+ ion is 6-fold, there are two coordinated Sc3+ ion sites. One of these is C2, while the other one is S6 symmetry. With the increase in Ca2+ ion content from 10 to 30 mol%, the samples partially change from Sc2O3 to CaSc2O4, which can be seen from the appearance of mixed phase of Sc2O3/CaSc2O4 in the XRD pattern. When the Ca2+ ion content was 40–60 mol%, all the diffraction peaks of samples were well indexed as pure CaSc2O4 (JCPDS No. 74–0449). The related crystal structure images are shown in Fig. 1c. The CaSc2O4 unit cell shows that there is only one position for Ca2+ ion and two for crystallographic Sc3+ ion, which has the calcium ferrite structure, space group Pnam (No. 62) [28,29]. All these positions have Cs point symmetry [30]. When the Ca2+ ion content reached ca. 80 mol%, the peaks of CaSc2O4 phase fell sharply, and overall, the CaO phase dominated. Finally, when the Ca2+ ion content increased to 90 mol%, the CaSc2O4 phase completely disappeared and CaO was obtained. The corresponding crystal structure image is shown in Fig. 1d. The structure of CaO crystal is in good agreement with the standard data (JCPDS NO.37–1497) with space group Fm3m (No. 225) [31].
mol% of Ca2+ (where x = 0, 10, 20, 30, 40, 50, 60, 70 and 80). It can be observed that the PLE spectra consisted of two main features. One broad band between 200 and 350 nm can be attributed to the CT band state transition from completely filled 2p orbital of O2− ion to partially filled 4f orbital of Eu3+ ion (Eu3+-O2−). Another series of sharp excitation bands between 350 and 550 nm were associated with the characteristic 4f-4f transition lines of Eu3+ ion in hosts, namely the 7F0 → 5D4, 7F0 → 5L7, 7F0 → 5L6, 7F0 → 5D2, and 7F0 → 5D1 at wavelengths of 363 nm, 383 nm, 395 nm, 465 nm and 531 nm, respectively. The PL spectra under CT band peaks and 395 nm laser excitation were composed of 5D0 → 7F < SUB > J < /SUB > (J = 1, 2, 3, and 4) emission lines of Eu3+ ion, which were dominated by the 5D0 → 7F2 electric dipole transitions at 612/614 nm. As shown in Fig. 3a and Fig. 3c, the enhancement trend of emissions spectra excited by 395 nm is similar to those excited by CT band peaks. The four transitions 5D0 → 7F < SUB > J < /SUB > (J = 1, 2, 3, and 4) emissions intensity gradually increased with the increase in Ca2+ ion doping concentrations from 0 to 50 mol%, and then, decreased with further increasing Ca2+ ion doping concentrations. Under 395 nm laser excitation, the integrated spectral (Fig. 3b) intensity of 5D0 → 7F2 red emission in CaSc2O4: Eu3+ crystal doped with 50 mol% Ca2+ ion is around 34 times than that in Ca2+absent Sc2O3: Eu3+ crystal. While under excitation of Eu3+-O2- CT band peak, the integrated spectral (Fig. 3c) intensity of 5D0 → 7F2 red emission in CaSc2O4: Eu3+ crystal is enhanced by 5.5 times than that in Ca2+-absent Sc2O3: Eu3+ crystal. The integral area ratios of 5D0 → 7F2/5D0 → 7F1 transitions, obtained from the emission spectra of the samples with doping different Ca2+ ion concentrations depicted in Table 1. The integral area ratios of
3.2. The luminescence properties of samples Fig. 2 shows the PLE and PL spectra of Sc2O3: 2 mol% Eu3+ with x 2
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Fig. 2. PLE and PL spectra of the Sc2O3: 2 mol% Eu3+, x mol% Ca2+ (0, 10, 20, 30, 40, 50, 60, 70, 80). (a) Sc2O3: 2 mol% Eu3+, (b) Sc2O3 + CaSc2O4: 2 mol% Eu3+, (c) Sc2O3 + CaSc2O4: 2 mol% Eu3+, (d) Sc2O3 + CaSc2O4: 2 mol% Eu3+, (e) CaSc2O4: 2 mol% Eu3+ (f) CaSc2O4: 2 mol% Eu3+, (g) CaSc2O4: 2 mol% Eu3+, (h) CaSc2O4 + CaO: 2 mol% Eu3+, (i) CaSc2O4 + CaO: 2 mol% Eu3+. 5 D0 → 7F2/5D0 → 7F1 transitions are often used to evaluate the variation degree of Eu3+ ion symmetry under different conditions, and a decrease in its value always corresponds to an increase of Eu3+ ion symmetry [32,33]. In our work, the high intensity ratio of at least 2.66 observed for all samples indicates the Eu3+ ion located in low-symmetry or more disordered sites in CaSc2O4 matrix. In oxidic host lattices, the coordination of Eu3+ ion is a determining factor for the position of CT band [34,35]. In octahedral VI coordination, the band position was more or less fixed, and in cubic VIII and XII co-ordinations, the band position varied as a function of the host lattice [35–37]. It can also be observed that PLE spectra (Fig. 2b and c; black line) exhibited two CT band peak positions: 250 nm and 285 nm, respectively. This is due to the reason that Ca2+ ion doping leads to the growth of partial Sc2O3 towards CaSc2O4. Eu3+ ion occupied the two kinds of luminescence centres: Sc3+ site (C2) in Sc2O3 matrix corresponding to 250 nm CT band and Ca2+ site (Cs) in CaSc2O4 matrix corresponding to 285 nm CT band. For Sc2O3 matrix, the coordination number for Sc3+ ion is 6-fold. Consequently, it can be observed that the Eu3+-O2- CT band position was almost constant at 250 nm (Fig. 2a, b and 2c; black line). Additionally, when the Ca2+ ion doping concentrations increased from 10 mol% to 20 mol%, the increasing occupancy probability of Eu3+ ion at Ca2+ ion position resulted in the intensity enhancement of CT band (285 nm) in PLE spectra Fig. 2b than that in Fig. 2c. As shown in Fig. 2a–c (red line), all samples were excited by 250 nm wavelength UV light, the main emission peak was present at 612 nm (5D0 → 7F2) due to the reason that Eu3+ ion occupied the Sc3+ (C2) site with lack of inversion symmetry in Sc2O3 matrix. This result is similar to those reported on the Eu3+ doped Y2O3 [38,39]. Besides, in the PL
spectra (both Fig. 2b and c; blue line) under 285 nm laser excitation, the D0 → 7F2 transition of Eu3+ ion exhibited relatively broader emission lines than those under 250 nm laser excitation (both Fig. 2b and c; red line). It further confirmed that Eu3+ ion occupied the Ca2+ ion site (Cs) in CaSc2O4 matrix, and Sc3+ ion site (C2) in Sc2O3 matrix, respectively [40]. The Eu3+ ion formed Eu3+-O2--Sc3+ in the complex oxide (CaSc2O4). The energy required to transfer an electron from O2− ion to the Eu3+ ion depended strongly on the potential field generated between the O2− ion and its nearest cation Sc3+ [39]. If the potential decreases, the energy required to transfer an electron from O2− ion to Eu3+ ion decreases, and the CT band moves to lower energy [41]. This means that the mixing of Eu3+ ion and O2− ion orbitals increases [42]. Therefore, the migration zone will move to low-energy long-wavelength region. When the Ca2+ ion doping concentrations increased from 30 mol% to 70 mol%, a small amount of Sc3+ ion was replaced by Ca2+ ion. Ca2+ ion exhibited larger radius (1.12 Å) than the Sc3+ ion (0.745 Å), its electronegativity was lower than that of the Sc3+ ion, increasing the mixing of Eu3+ ion and O2− ion orbitals. Due to this reason, the peak positions of Eu3+-O2- CT band shifted from 285 nm to 294 nm–300 nm in Fig. 2d–h. All The positions of the CT band in different components are listed in Table 2. The results also showed that the intensity ratio (5D0 → 7F4)/(5D0 → 7 F2) gradually increased (for the Ca2+ ion doping concentrations ≤ 80 mol%), as shown in Fig. 4. When the Eu3+ ion (4f6 configuration) occupied a position (Cs) with non-inversion symmetry, the 5D0 → 7F2 and 5D0 → 7F4 transition intensities were governed by the forced electric dipole (FED) and dynamic coupling (DC) mechanisms [43]. The gradual increase in intensity ratio for 5D0 → 7F4/5D0 → 7F2 transitions 5
3
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Fig. 3. (a) PL spectra of Sc2O3: Eu3+ crystals doped with diverse Ca2+ ion under 395 nm laser excitation. (b) The ratios of enhancement at 5D0 → 7F2 transition emission intensity as a function of Ca2+ ion doping concentrations under 395 nm laser excitation. (c) PL spectra of Sc2O3: Eu3+ crystals doped with diverse Ca2+ ion under the excitation of Eu3+-O2- CT band peaks. (d) The ratios of enhancement at 5D0 → 7F2 transitions emission intensity as a function of Ca2+ ion doping concentrations under the excitation of Eu3+-O2- CT band peaks. In (b) and (c), the integrated emission intensity of the 5D0 → 7F2 transition in Ca2+-absent Sc2O3 is normalized to unity. Table 1 The integral emission intensity ratios of 5D0 → 7F2/5D0 → 7F1 transitions in samples with doping different Ca2+ ion concentrations. Samples (mol%) integral emission intensity ratios
395 nm excitation CT band peaks excitation
30
40
50
60
70
80
90
3.06
2.79
2.84
3.01
2.66
2.84
2.75
3.06
2.82
2.84
2.91
2.66
2.77
2.85
Table 2 The positions of the CT band in different components. Ca2+ ion concentrations (mol %)
Compound
Position of CT (λmax; nm)
0 10 20 30 40 50 60 70 80
Sc2O3: 2% Eu3+ Sc2O3+CaSc2O4: 2%Eu3+ Sc2O3+CaSc2O4: 2%Eu3+ Sc2O3+CaSc2O4: 2%Eu3+ CaSc2O4: 2% Eu3+ CaSc2O4: 2% Eu3+ CaSc2O4: 2% Eu3+ CaSc2O4+CaO: 2% Eu3+ CaSc2O4+CaO: 2% Eu3+
250 250, 285 250, 285 285 294 298 298 300 300
Fig. 4. Under the excitation of Eu3+-O2- CT band peaks, the corresponding integral emission intensity ratios of 5D0 → 7F4/5D0 → 7F2 transitions as a function of Ca2+ ion doping concentration.
where the quantities Bλtp (t = 1, 3, 5 and 7) have been described in detail elsewhere [44,46], and may be expressed using Eq. (2).
can be rationalized as follows. The Ωλ intensity (Judd–Ofelt intensity [44]) parameters depend on both the chemical environment and the lanthanide ion [45], and theoretically, are given by Eq. (1).
Ωλ = (2λ + 1)
Bλtp
(2)
where FED Bλtp = Ξ (t , λ ) γpt
2
∑ 2t + 1 t,p
FED DC Bλtp = Bλtp + Bλtp
(1)
and 4
(3)
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Fig. 5. (a) Decay fitting curves of Eu3+ ion characteristic emissions in Sc2O3: 2 mol% Eu3+, x mol% Ca2+ (0, 10, 20, 30, 40, 50, 60, 70, 80, 90). (b) Lifetime changing trends of 5D0 → 7F2 transition emission versus Ca2+ ion concentrations.
DC Bλtp = −⎡ ⎣
nm/614 nm) of Eu3+ ion in CaSc2O4: Eu3+ crystals were enhanced by factors of 34. With the Ca2+ ion doping concentrations increasing from 30 mol% to 60 mol%, the peak positions of Eu3+-O2- CT band shifted from 285 nm to 298 nm. The extremely strong red luminescence studies indicate that CaSc2O4 crystals can work as excellent host materials that require pure red luminescence emission. The results may open new avenues of research on the development of novel and technologically promising red-emitting phosphor for solid-state lighting and optoelectronic devices.
1 (λ + 1)(2λ + 3) 2 ⎤ (2λ + 1) ⎦
× 〈4f r λ 4f 〉 (1 − σλ ) 〈f ‖C (λ) ‖f 〉Γ pt δt , λ + 1
(4)
is called the odd-rank ligand field parameter, and (where where t = 1, 3, 5 and 7) contains the dependence on the coordination geometry and on the nature of the chemical environment around the lanthanide ion. The quantities 4f |r f |4f , (1 − σ ) and f ∥c λ∥f refer to radial integral, shielding factor and one-electron reduced matrix element, respectively. All the quantities in Eq. (3) and Eq. (4) are described in detail in a previous study [43]. Due to excessive doping of Ca2+ ion with lower electronegativity, the environment of Eu3+ ion is more polarized chemical environment with local symmetry corresponding to a distorted coordination polyhedron. The γ pt and Γ tp will be likely to be the lower rank ones (t = 1, 3), then affecting primarily the Ω4 and Ω6 intensity parameters [43,46]. In this case, it is conceivable that the value of Ω4 parameter may be higher than that of Ω2. According to the standard 4f–4f intensity theory [47], the 5D0 → 7F2 transition is governed by the effective operator Ω4U(2), while the 5D0 → 7F4 transition is governed by Ω4U(2), where U(λ) is the unit tensor operator. This would lead to a higher intensity ratio of 5D0 → 7F4/5D0 → 7F2. And, the intensity ratio 5D0 → 7F4/5D0 → 7 F2 is expected to be smaller than unity, which is usual for low symmetries. Fig. 5a shows the decay fitting curves of Eu3+ ion characteristic emission in Sc2O3: 2 mol% Eu3+, x mol% Ca2+ (0, 10, 20, 30, 40, 50, 60, 70, 80, 90) excited by 250 nm laser, which can be well fitted using a single-exponential equation as given by Eq. (5).
γ pt
I = I0 exp(−t / τ )
Γ tp
Acknowledgements This work was supported by the National Natural Science Foundation of China (grants 61405016 and 21371095), Scientific and Technological Developing Project of Jilin Province (20170101038JC) and Jilin Province Education (JJKH20170539KJ, JJKH20181017KJ). References [1] K. Kömpe, H. Borchert, J. Storz, A. Lobo, S. Adam, T. Möller, M. Haase, Angew. Chem. Int. Ed. 42 (44) (2003) 5513–5516. [2] H. Dong, L.D. Sun, Y.F. Wang, J. Ke, R. Si, J.W. Xiao, G.M. Lyu, S. Shi, C.H. Yan, J. Am. Chem. Soc. 137 (20) (2015) 6569–6576. [3] Z. Chen, W. Zheng, P. Huang, D. Tu, S. Zhou, M. Huang, X. Chen, Nanoscale 7 (10) (2015) 4274–4290. [4] J. Shin, Y. Kim, J. Lee, S. Kim, H.S. Jang, Part. Part. Syst. Char. 34 (1) (2017) 1600183. [5] H. Dong, S.R. Du, X.Y. Zheng, G.M. Lyu, L.D. Sun, L.D. Li, P.Z. Zhang, C. Zhang, C.H. Yan, Chem. Rev. 115 (19) (2015) 10725–10815. [6] S. Lahtinen, Q. Wang, T. Soukka, Anal. Chem. 88 (1) (2016) 653–658. [7] M. Li, W. Yao, J. Liu, Q. Tian, L. Liu, J. Ding, Q. Xue, Q. Lu, W. Wu, J. Mater. Chem. C. 7 (26) (2017) 6512–6520. [8] J. Rocha, L.D. Carlos, F.A.A. Paz, D. Ananias, Chem. Soc. Rev. 40 (2) (2011) 926–940. [9] X. Zheng, X. Zhu, Y. Lu, J. Zhao, W. Feng, G. Jia, F. Wang, F. Li, D. Jin, Anal. Chem. 88 (7) (2016) 3449–3454. [10] Y. Liu, D. Tu, H. Zhu, X. Chen, Chem. Soc. Rev. 42 (16) (2013) 6924–6958. [11] A.J. Kenyon, Prog. Quant. Electron. 26 (4–5) (2002) 225–284. [12] A. Kumar, D.K. Rai, S.B. Rai, Spectrochim. Acta A. 58 (10) (2002) 2115–2125. [13] K. Li, X. Liu, Y. Zhang, X. Li, H. Lian, J. Lin, Inorg. Chem. 54 (1) (2015) 323–333. [14] X. Liu, X. Li, X. Qin, X. Xie, L. Huang, X. Liu, Adv. Mater. 29 (37) (2017) 1702315. [15] C. Li, Z. Quan, J. Yang, P. Yang, J. Lin, Inorg. Chem. 46 (16) (2007) 6329–6337. [16] F.T. Rabouw, P.T. Prins, P. Villanueva-Delgado, M. Castelijns, R.G. Geitenbeek, A. Meijerink, ACS Nano 12 (5) (2018) 4812–4823. [17] F. Wang, Y. Han, C.S. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M. Hong, X. Liu, Nature 463 (7284) (2010) 1061–1065. [18] F. Wang, X. Liu, J. Am. Chem. Soc. 130 (17) (2008) 5642–5643. [19] D.J. Gargas, E.M. Chan, A.D. Ostrowski, S. Aloni, M.V.P. Altoe, E.S. Barnard, B. Sanii, J.J. Urban, D.J. Milliron, B.E. Cohen, P.J. Schuck, Nat. Nanotechnol. 9 (4) (2014) 300. [20] G. Xiang, Y. Ma, W. Liu, J. Wang, Z. Gu, Y. Jin, S. Jiang, X. Luo, L. Li, X. Zhou, Y. Luo, J. Zhang, Inorg. Chem. 56 (22) (2017) 13955–13961. [21] D. Li, W. Qin, S. Liu, W. Pei, Z. Wang, P. Zhang, L. Wang, L. Huang, J. Alloy. Comp. 653 (2015) 304–309.
(5)
where I0 is a constant, t is the time, and τ corresponds to the decay lifetime for the exponential component. As shown in Fig. 5b, when the samples are pure CaSc2O4 matrix, the decay lifetime of 5D0 → 7F2 transition emission is shortest. Increased disorder in the Cs site of CaSc2O4 matrix may increase the Eu3+ ion radiative transition probabilities for ions excited in the wings of the inhomogeneously broadened transition, resulting in shorter lifetimes [48,49]. However, when Ca2+ ion contents are over 60 mol%, the phase transition of samples happens from CaSc2O4 to CaO matrix, and the lifetimes of 5D0 → 7F2 transition are prolonged. 4. Conclusions In our work, the phase transition and DC luminescence properties from Sc2O3 to CaSc2O4 to CaO have been investigated after Ca2+ ion doping. In comparison with Sc2O3: Eu3+ crystals, the red emission (612 5
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Enhanced down-conversion luminescence properties of CaSc2O4: Eu3+ crystals
T
Xue Wanga, Xiaoyu Zhanga, Lili Wangb,∗, Ling Huangc,∗∗ a
School of Chemistry and Life Sciences, Changchun University of Technology, Changchun, 130012, China School of Materials Science and Engineering, Changchun University of Technology, Changchun, 130012, China c Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing, 211816, China b
A B S T R A C T
CaSc2O4: Eu3+ crystals show intense red (612 nm/614 nm) down-conversion (DC) luminescence. In our paper, the red-shifting of Eu3+-O2- charge transfer (CT) band peaks position from 285 nm to 298 nm was investigated with increasing Ca2+ ion doping concentrations in CaSc2O4: Eu3+ matrix due to the low electronegativity of Ca2+ ion. Meanwhile the intensity ratios of 5D0 → 7F4/5D0 → 7F2 red emissions of Eu3+ ion were increased due to the distorted Cs point symmetry in CaSc2O4 matrix. The luminescence studies indicate that Sc2O3: Eu3+ and CaSc2O4: Eu3+ may serve as a potential red phosphor, where CaSc2O4: Eu3+ shows a better performance than Sc2O3: Eu3+ crystals.
1. Introduction The development of lanthanide-doped down-conversion (DC) luminescence materials have recently drawn great research attention due to their unique photophysical features, such as large anti-Stokes shifts, low toxicity, negligible autofluorescence background, high resistance to photobleaching, and long luminescence lifetime [1–6]. The intrinsic advantages make DC materials attractive for applications in anticounterfeiting, molecular sensing, bioimaging, and therapeutics [7–10]. In addition, Eu3+ ion has a simple energy level scheme and non-degenerative nature for 7F0 ground state and 5D0 excited state. They are well established as one of the best spectroscopic probes for getting an insight into the structure and nature of chemical bonds present in the host matrices [11,12]. Generally, it is vital to select appropriate compounds for lanthanide ion as luminescent activators [13]. Among them, lanthanide-doped fluorides, such as NaYF4, have attracted much research attention in the field of materials science because they have the low phonon frequency [14–16]. It is well known that the low phonon frequency can inhibit non-radiative multi-phonon relaxation and, in turn, yields a relatively high DC luminescence efficiency [17–19]. However, fluorides have poor chemical and thermal stability. In particular, they are easily converted to oxides at high temperature. In contrast to fluorides, oxide materials have attracted a great deal of attention due to their interesting physicochemical properties, such as high chemical and good thermal stabilities [20,21].
∗
Exploiting oxide materials can be an alternative route for obtaining DC crystals with desired emission characteristics. Over the past decade, many efforts have been devoted to controllable synthesis of lanthanidedoped oxide materials to explore their excellent optical properties. Very recently, Sc2O3 and CaSc2O4 have emerged as the new type of oxide host materials. For instance, Pan et al. reported a new Sc2O3: Eu2+/ Eu3+ nanoparticle-based ratiometric nano-thermometer. Our group reported efficient upconversion/downconversion luminescence of Sc2O3: Yb3+/Er3+, Yb3+/Tm3+, Eu3+ and Tb3+ microcrystals [21,22]. In particular, for CaSc2O4 crystal, it has been reported as a promising host for achieving efficient upconversion/downconversion luminescence, such as the red-emitting CaSc2O4: Eu3+, CaSc2O4: Yb3+/Er3+ and green emitting CaSc2O4: Ce3+ [23–25]. These results indicate that lanthanide-doped Sc2O3, CaSc2O4 and CaO materials are excellent DC luminescence host materials. Many researchers have been working on the issue of Eu3+-O2charge transfer (CT) in various oxide matrices for many years. Jiang et al. reported the blue shift of the charge-transfer band (CTB) in the excitation spectra of β-Ca2SiO4: Eu3+ [26]. Tian et al. reported the charge transfer band intensity ratio between Eu3+-O2- and Mo6+-O2increases with increasing Eu3+ ion doping concentrations in Y2(MoO4)3: Eu3+ red phosphors [27]. However, the red-shifts of band positions in Eu3+ ion doped Sc2O3 and CaSc2O4 matrix have not yet been demonstrated. In the current work, the strongest red 5D0 → 7F2 emission (612 nm/ 614 nm) in CaSc2O4: Eu3+ increased by 34 times than those of Sc2O3
Corresponding author. Corresponding author. E-mail addresses: [email protected] (L. Wang), [email protected] (L. Huang).
∗∗
https://doi.org/10.1016/j.jlumin.2019.116526 Received 30 January 2019; Received in revised form 26 April 2019; Accepted 29 May 2019 Available online 30 May 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
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crystals. Additionally, the red-shifting of Eu3+-O2- CT band peaks position from 285 nm to 298 nm was caused by Ca2+ ions substituting for a small amount Sc3+ ions in CaSc2O4 matrix. We also found the luminescence intensity ratio (5D0 → 7F4)/(5D0 → 7F2) gradually increased with increasing Ca2+ ion doping concentrations. 2. Experimental details All chemicals are of analytical grade and used without further purification. ScCl3·6H2O (99.99%), Ca(NO3)2·4H2O (99.99%) Yb (NO3)3·6H2O (99.99%), Eu(NO3)2·6H2O (99.99%) are supplied by Yutai Qingda Chemical Technology Co., Ltd. China. Ethanol is supplied by Beijing Fine Chemical Company. Sc2O3:Eu3+, x mol% Ca2+ (x = 0, 10, 20, 30, 40, 50, 60, 70, 80 and 90) samples were synthesized by a high-temperature solid-state method. Stoichiometric amounts of Sc(Cl)3·6H2O (99.99%), Ca (NO3)2·4H2O (99.99%), and Eu(NO3)3·6H2O (99.99%) was weighted and mixed thoroughly in an agate mortar using ethanol as a solvent. Then, the mixed solution was placed in an alumina crucible and reacted in a muffle furnace at 1100 °C for 4 h. Finally, the as-prepared samples were cooled to room temperature, regrounded for further characterization. The phase purity and crystal structure of the powder samples were examined by X-ray diffraction (XRD) analysis with a powder diffractometer (Model Rig-aku RU-200b), using Ni-filtered Cu-Ka radiation (λ = 1.5406 Å). The spectra were recorded with a Hitachi fluorescence spectrometer F-7000 equipped with a 150 W Xenon lamp. A digital oscilloscope (DPO4104B, bandwidth 1 GHz, sampling rate 5 GSs−1; Tektronix, Shanghai, China), a power-adjustable continuous wave laser diode, and a chopper were used to record decay curves. All tests were measured at a room temperature.
Fig. 1. (a) XRD patterns of Sc2O3: 2 mol% Eu3+, x mol% Ca2+ (0, 10, 20, 30, 40, 50, 60, 70, 80 and 90) crystals with the standard XRD data of Sc2O3 (JCPDS NO.43–1028), CaSc2O4 (JCPDS NO.74–0499) and CaO (JCPDS NO.37–1497). (b), (c) and (d) Schematic of crystal structure of Sc2O3, CaSc2O4 and CaO. Some features diffraction peaks of Sc2O3, CaSc2O4 and CaO crystals are marked with •, ▲ and ◆, respectively.
3. Results and discussion 3.1. Phase and crystal structure The phase transition from Sc2O3 to CaSc2O4 to CaO crystals can be easily tuned by adjusting the content of Ca2+ ion in initial solutions. The XRD patterns of Sc2O3: 2 mol% Eu3+, x mol% Ca2+ (where x = 0, 10, 20, 30, 40, 50, 60, 70, 80 and 90) are illustrated in Fig. 1a. It can be seen that pure Sc2O3 crystals (which match quite well with JCPDS NO.43–1028) can be obtained without Ca2+ ion doping. Correspondingly, Fig. 1b shows the schematic of crystal structure of Sc2O3. It possesses a bixbyite-type structure of space group Ia3 (No. 206). The coordination number for Sc3+ ion is 6-fold, there are two coordinated Sc3+ ion sites. One of these is C2, while the other one is S6 symmetry. With the increase in Ca2+ ion content from 10 to 30 mol%, the samples partially change from Sc2O3 to CaSc2O4, which can be seen from the appearance of mixed phase of Sc2O3/CaSc2O4 in the XRD pattern. When the Ca2+ ion content was 40–60 mol%, all the diffraction peaks of samples were well indexed as pure CaSc2O4 (JCPDS No. 74–0449). The related crystal structure images are shown in Fig. 1c. The CaSc2O4 unit cell shows that there is only one position for Ca2+ ion and two for crystallographic Sc3+ ion, which has the calcium ferrite structure, space group Pnam (No. 62) [28,29]. All these positions have Cs point symmetry [30]. When the Ca2+ ion content reached ca. 80 mol%, the peaks of CaSc2O4 phase fell sharply, and overall, the CaO phase dominated. Finally, when the Ca2+ ion content increased to 90 mol%, the CaSc2O4 phase completely disappeared and CaO was obtained. The corresponding crystal structure image is shown in Fig. 1d. The structure of CaO crystal is in good agreement with the standard data (JCPDS NO.37–1497) with space group Fm3m (No. 225) [31].
mol% of Ca2+ (where x = 0, 10, 20, 30, 40, 50, 60, 70 and 80). It can be observed that the PLE spectra consisted of two main features. One broad band between 200 and 350 nm can be attributed to the CT band state transition from completely filled 2p orbital of O2− ion to partially filled 4f orbital of Eu3+ ion (Eu3+-O2−). Another series of sharp excitation bands between 350 and 550 nm were associated with the characteristic 4f-4f transition lines of Eu3+ ion in hosts, namely the 7F0 → 5D4, 7F0 → 5L7, 7F0 → 5L6, 7F0 → 5D2, and 7F0 → 5D1 at wavelengths of 363 nm, 383 nm, 395 nm, 465 nm and 531 nm, respectively. The PL spectra under CT band peaks and 395 nm laser excitation were composed of 5D0 → 7F < SUB > J < /SUB > (J = 1, 2, 3, and 4) emission lines of Eu3+ ion, which were dominated by the 5D0 → 7F2 electric dipole transitions at 612/614 nm. As shown in Fig. 3a and Fig. 3c, the enhancement trend of emissions spectra excited by 395 nm is similar to those excited by CT band peaks. The four transitions 5D0 → 7F < SUB > J < /SUB > (J = 1, 2, 3, and 4) emissions intensity gradually increased with the increase in Ca2+ ion doping concentrations from 0 to 50 mol%, and then, decreased with further increasing Ca2+ ion doping concentrations. Under 395 nm laser excitation, the integrated spectral (Fig. 3b) intensity of 5D0 → 7F2 red emission in CaSc2O4: Eu3+ crystal doped with 50 mol% Ca2+ ion is around 34 times than that in Ca2+absent Sc2O3: Eu3+ crystal. While under excitation of Eu3+-O2- CT band peak, the integrated spectral (Fig. 3c) intensity of 5D0 → 7F2 red emission in CaSc2O4: Eu3+ crystal is enhanced by 5.5 times than that in Ca2+-absent Sc2O3: Eu3+ crystal. The integral area ratios of 5D0 → 7F2/5D0 → 7F1 transitions, obtained from the emission spectra of the samples with doping different Ca2+ ion concentrations depicted in Table 1. The integral area ratios of
3.2. The luminescence properties of samples Fig. 2 shows the PLE and PL spectra of Sc2O3: 2 mol% Eu3+ with x 2
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Fig. 2. PLE and PL spectra of the Sc2O3: 2 mol% Eu3+, x mol% Ca2+ (0, 10, 20, 30, 40, 50, 60, 70, 80). (a) Sc2O3: 2 mol% Eu3+, (b) Sc2O3 + CaSc2O4: 2 mol% Eu3+, (c) Sc2O3 + CaSc2O4: 2 mol% Eu3+, (d) Sc2O3 + CaSc2O4: 2 mol% Eu3+, (e) CaSc2O4: 2 mol% Eu3+ (f) CaSc2O4: 2 mol% Eu3+, (g) CaSc2O4: 2 mol% Eu3+, (h) CaSc2O4 + CaO: 2 mol% Eu3+, (i) CaSc2O4 + CaO: 2 mol% Eu3+. 5 D0 → 7F2/5D0 → 7F1 transitions are often used to evaluate the variation degree of Eu3+ ion symmetry under different conditions, and a decrease in its value always corresponds to an increase of Eu3+ ion symmetry [32,33]. In our work, the high intensity ratio of at least 2.66 observed for all samples indicates the Eu3+ ion located in low-symmetry or more disordered sites in CaSc2O4 matrix. In oxidic host lattices, the coordination of Eu3+ ion is a determining factor for the position of CT band [34,35]. In octahedral VI coordination, the band position was more or less fixed, and in cubic VIII and XII co-ordinations, the band position varied as a function of the host lattice [35–37]. It can also be observed that PLE spectra (Fig. 2b and c; black line) exhibited two CT band peak positions: 250 nm and 285 nm, respectively. This is due to the reason that Ca2+ ion doping leads to the growth of partial Sc2O3 towards CaSc2O4. Eu3+ ion occupied the two kinds of luminescence centres: Sc3+ site (C2) in Sc2O3 matrix corresponding to 250 nm CT band and Ca2+ site (Cs) in CaSc2O4 matrix corresponding to 285 nm CT band. For Sc2O3 matrix, the coordination number for Sc3+ ion is 6-fold. Consequently, it can be observed that the Eu3+-O2- CT band position was almost constant at 250 nm (Fig. 2a, b and 2c; black line). Additionally, when the Ca2+ ion doping concentrations increased from 10 mol% to 20 mol%, the increasing occupancy probability of Eu3+ ion at Ca2+ ion position resulted in the intensity enhancement of CT band (285 nm) in PLE spectra Fig. 2b than that in Fig. 2c. As shown in Fig. 2a–c (red line), all samples were excited by 250 nm wavelength UV light, the main emission peak was present at 612 nm (5D0 → 7F2) due to the reason that Eu3+ ion occupied the Sc3+ (C2) site with lack of inversion symmetry in Sc2O3 matrix. This result is similar to those reported on the Eu3+ doped Y2O3 [38,39]. Besides, in the PL
spectra (both Fig. 2b and c; blue line) under 285 nm laser excitation, the D0 → 7F2 transition of Eu3+ ion exhibited relatively broader emission lines than those under 250 nm laser excitation (both Fig. 2b and c; red line). It further confirmed that Eu3+ ion occupied the Ca2+ ion site (Cs) in CaSc2O4 matrix, and Sc3+ ion site (C2) in Sc2O3 matrix, respectively [40]. The Eu3+ ion formed Eu3+-O2--Sc3+ in the complex oxide (CaSc2O4). The energy required to transfer an electron from O2− ion to the Eu3+ ion depended strongly on the potential field generated between the O2− ion and its nearest cation Sc3+ [39]. If the potential decreases, the energy required to transfer an electron from O2− ion to Eu3+ ion decreases, and the CT band moves to lower energy [41]. This means that the mixing of Eu3+ ion and O2− ion orbitals increases [42]. Therefore, the migration zone will move to low-energy long-wavelength region. When the Ca2+ ion doping concentrations increased from 30 mol% to 70 mol%, a small amount of Sc3+ ion was replaced by Ca2+ ion. Ca2+ ion exhibited larger radius (1.12 Å) than the Sc3+ ion (0.745 Å), its electronegativity was lower than that of the Sc3+ ion, increasing the mixing of Eu3+ ion and O2− ion orbitals. Due to this reason, the peak positions of Eu3+-O2- CT band shifted from 285 nm to 294 nm–300 nm in Fig. 2d–h. All The positions of the CT band in different components are listed in Table 2. The results also showed that the intensity ratio (5D0 → 7F4)/(5D0 → 7 F2) gradually increased (for the Ca2+ ion doping concentrations ≤ 80 mol%), as shown in Fig. 4. When the Eu3+ ion (4f6 configuration) occupied a position (Cs) with non-inversion symmetry, the 5D0 → 7F2 and 5D0 → 7F4 transition intensities were governed by the forced electric dipole (FED) and dynamic coupling (DC) mechanisms [43]. The gradual increase in intensity ratio for 5D0 → 7F4/5D0 → 7F2 transitions 5
3
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Fig. 3. (a) PL spectra of Sc2O3: Eu3+ crystals doped with diverse Ca2+ ion under 395 nm laser excitation. (b) The ratios of enhancement at 5D0 → 7F2 transition emission intensity as a function of Ca2+ ion doping concentrations under 395 nm laser excitation. (c) PL spectra of Sc2O3: Eu3+ crystals doped with diverse Ca2+ ion under the excitation of Eu3+-O2- CT band peaks. (d) The ratios of enhancement at 5D0 → 7F2 transitions emission intensity as a function of Ca2+ ion doping concentrations under the excitation of Eu3+-O2- CT band peaks. In (b) and (c), the integrated emission intensity of the 5D0 → 7F2 transition in Ca2+-absent Sc2O3 is normalized to unity. Table 1 The integral emission intensity ratios of 5D0 → 7F2/5D0 → 7F1 transitions in samples with doping different Ca2+ ion concentrations. Samples (mol%) integral emission intensity ratios
395 nm excitation CT band peaks excitation
30
40
50
60
70
80
90
3.06
2.79
2.84
3.01
2.66
2.84
2.75
3.06
2.82
2.84
2.91
2.66
2.77
2.85
Table 2 The positions of the CT band in different components. Ca2+ ion concentrations (mol %)
Compound
Position of CT (λmax; nm)
0 10 20 30 40 50 60 70 80
Sc2O3: 2% Eu3+ Sc2O3+CaSc2O4: 2%Eu3+ Sc2O3+CaSc2O4: 2%Eu3+ Sc2O3+CaSc2O4: 2%Eu3+ CaSc2O4: 2% Eu3+ CaSc2O4: 2% Eu3+ CaSc2O4: 2% Eu3+ CaSc2O4+CaO: 2% Eu3+ CaSc2O4+CaO: 2% Eu3+
250 250, 285 250, 285 285 294 298 298 300 300
Fig. 4. Under the excitation of Eu3+-O2- CT band peaks, the corresponding integral emission intensity ratios of 5D0 → 7F4/5D0 → 7F2 transitions as a function of Ca2+ ion doping concentration.
where the quantities Bλtp (t = 1, 3, 5 and 7) have been described in detail elsewhere [44,46], and may be expressed using Eq. (2).
can be rationalized as follows. The Ωλ intensity (Judd–Ofelt intensity [44]) parameters depend on both the chemical environment and the lanthanide ion [45], and theoretically, are given by Eq. (1).
Ωλ = (2λ + 1)
Bλtp
(2)
where FED Bλtp = Ξ (t , λ ) γpt
2
∑ 2t + 1 t,p
FED DC Bλtp = Bλtp + Bλtp
(1)
and 4
(3)
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Fig. 5. (a) Decay fitting curves of Eu3+ ion characteristic emissions in Sc2O3: 2 mol% Eu3+, x mol% Ca2+ (0, 10, 20, 30, 40, 50, 60, 70, 80, 90). (b) Lifetime changing trends of 5D0 → 7F2 transition emission versus Ca2+ ion concentrations.
DC Bλtp = −⎡ ⎣
nm/614 nm) of Eu3+ ion in CaSc2O4: Eu3+ crystals were enhanced by factors of 34. With the Ca2+ ion doping concentrations increasing from 30 mol% to 60 mol%, the peak positions of Eu3+-O2- CT band shifted from 285 nm to 298 nm. The extremely strong red luminescence studies indicate that CaSc2O4 crystals can work as excellent host materials that require pure red luminescence emission. The results may open new avenues of research on the development of novel and technologically promising red-emitting phosphor for solid-state lighting and optoelectronic devices.
1 (λ + 1)(2λ + 3) 2 ⎤ (2λ + 1) ⎦
× 〈4f r λ 4f 〉 (1 − σλ ) 〈f ‖C (λ) ‖f 〉Γ pt δt , λ + 1
(4)
is called the odd-rank ligand field parameter, and (where where t = 1, 3, 5 and 7) contains the dependence on the coordination geometry and on the nature of the chemical environment around the lanthanide ion. The quantities 4f |r f |4f , (1 − σ ) and f ∥c λ∥f refer to radial integral, shielding factor and one-electron reduced matrix element, respectively. All the quantities in Eq. (3) and Eq. (4) are described in detail in a previous study [43]. Due to excessive doping of Ca2+ ion with lower electronegativity, the environment of Eu3+ ion is more polarized chemical environment with local symmetry corresponding to a distorted coordination polyhedron. The γ pt and Γ tp will be likely to be the lower rank ones (t = 1, 3), then affecting primarily the Ω4 and Ω6 intensity parameters [43,46]. In this case, it is conceivable that the value of Ω4 parameter may be higher than that of Ω2. According to the standard 4f–4f intensity theory [47], the 5D0 → 7F2 transition is governed by the effective operator Ω4U(2), while the 5D0 → 7F4 transition is governed by Ω4U(2), where U(λ) is the unit tensor operator. This would lead to a higher intensity ratio of 5D0 → 7F4/5D0 → 7F2. And, the intensity ratio 5D0 → 7F4/5D0 → 7 F2 is expected to be smaller than unity, which is usual for low symmetries. Fig. 5a shows the decay fitting curves of Eu3+ ion characteristic emission in Sc2O3: 2 mol% Eu3+, x mol% Ca2+ (0, 10, 20, 30, 40, 50, 60, 70, 80, 90) excited by 250 nm laser, which can be well fitted using a single-exponential equation as given by Eq. (5).
γ pt
I = I0 exp(−t / τ )
Γ tp
Acknowledgements This work was supported by the National Natural Science Foundation of China (grants 61405016 and 21371095), Scientific and Technological Developing Project of Jilin Province (20170101038JC) and Jilin Province Education (JJKH20170539KJ, JJKH20181017KJ). References [1] K. Kömpe, H. Borchert, J. Storz, A. Lobo, S. Adam, T. Möller, M. Haase, Angew. Chem. Int. Ed. 42 (44) (2003) 5513–5516. [2] H. Dong, L.D. Sun, Y.F. Wang, J. Ke, R. Si, J.W. Xiao, G.M. Lyu, S. Shi, C.H. Yan, J. Am. Chem. Soc. 137 (20) (2015) 6569–6576. [3] Z. Chen, W. Zheng, P. Huang, D. Tu, S. Zhou, M. Huang, X. Chen, Nanoscale 7 (10) (2015) 4274–4290. [4] J. Shin, Y. Kim, J. Lee, S. Kim, H.S. Jang, Part. Part. Syst. Char. 34 (1) (2017) 1600183. [5] H. Dong, S.R. Du, X.Y. Zheng, G.M. Lyu, L.D. Sun, L.D. Li, P.Z. Zhang, C. Zhang, C.H. Yan, Chem. Rev. 115 (19) (2015) 10725–10815. [6] S. Lahtinen, Q. Wang, T. Soukka, Anal. Chem. 88 (1) (2016) 653–658. [7] M. Li, W. Yao, J. Liu, Q. Tian, L. Liu, J. Ding, Q. Xue, Q. Lu, W. Wu, J. Mater. Chem. C. 7 (26) (2017) 6512–6520. [8] J. Rocha, L.D. Carlos, F.A.A. Paz, D. Ananias, Chem. Soc. Rev. 40 (2) (2011) 926–940. [9] X. Zheng, X. Zhu, Y. Lu, J. Zhao, W. Feng, G. Jia, F. Wang, F. Li, D. Jin, Anal. Chem. 88 (7) (2016) 3449–3454. [10] Y. Liu, D. Tu, H. Zhu, X. Chen, Chem. Soc. Rev. 42 (16) (2013) 6924–6958. [11] A.J. Kenyon, Prog. Quant. Electron. 26 (4–5) (2002) 225–284. [12] A. Kumar, D.K. Rai, S.B. Rai, Spectrochim. Acta A. 58 (10) (2002) 2115–2125. [13] K. Li, X. Liu, Y. Zhang, X. Li, H. Lian, J. Lin, Inorg. Chem. 54 (1) (2015) 323–333. [14] X. Liu, X. Li, X. Qin, X. Xie, L. Huang, X. Liu, Adv. Mater. 29 (37) (2017) 1702315. [15] C. Li, Z. Quan, J. Yang, P. Yang, J. Lin, Inorg. Chem. 46 (16) (2007) 6329–6337. [16] F.T. Rabouw, P.T. Prins, P. Villanueva-Delgado, M. Castelijns, R.G. Geitenbeek, A. Meijerink, ACS Nano 12 (5) (2018) 4812–4823. [17] F. Wang, Y. Han, C.S. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M. Hong, X. Liu, Nature 463 (7284) (2010) 1061–1065. [18] F. Wang, X. Liu, J. Am. Chem. Soc. 130 (17) (2008) 5642–5643. [19] D.J. Gargas, E.M. Chan, A.D. Ostrowski, S. Aloni, M.V.P. Altoe, E.S. Barnard, B. Sanii, J.J. Urban, D.J. Milliron, B.E. Cohen, P.J. Schuck, Nat. Nanotechnol. 9 (4) (2014) 300. [20] G. Xiang, Y. Ma, W. Liu, J. Wang, Z. Gu, Y. Jin, S. Jiang, X. Luo, L. Li, X. Zhou, Y. Luo, J. Zhang, Inorg. Chem. 56 (22) (2017) 13955–13961. [21] D. Li, W. Qin, S. Liu, W. Pei, Z. Wang, P. Zhang, L. Wang, L. Huang, J. Alloy. Comp. 653 (2015) 304–309.
(5)
where I0 is a constant, t is the time, and τ corresponds to the decay lifetime for the exponential component. As shown in Fig. 5b, when the samples are pure CaSc2O4 matrix, the decay lifetime of 5D0 → 7F2 transition emission is shortest. Increased disorder in the Cs site of CaSc2O4 matrix may increase the Eu3+ ion radiative transition probabilities for ions excited in the wings of the inhomogeneously broadened transition, resulting in shorter lifetimes [48,49]. However, when Ca2+ ion contents are over 60 mol%, the phase transition of samples happens from CaSc2O4 to CaO matrix, and the lifetimes of 5D0 → 7F2 transition are prolonged. 4. Conclusions In our work, the phase transition and DC luminescence properties from Sc2O3 to CaSc2O4 to CaO have been investigated after Ca2+ ion doping. In comparison with Sc2O3: Eu3+ crystals, the red emission (612 5
Journal of Luminescence 214 (2019) 116526
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