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The effects of temperature and impurity phases on the luminescent properties of Ce3+-doped Ca3Sc2Si3O12 garnet
Journal of Luminescence 195 (2018) 24–30
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The effects of temperature and impurity phases on the luminescent properties of Ce3+-doped Ca3Sc2Si3O12 garnet
T
I.V. Berezovskayaa, Z.A. Khapkob, A.S. Voloshinovskiib, N.P. Efryushinaa, S.S. Smolaa, ⁎ V.P. Dotsenkoa, a b
A.V. Bogatsky Physico-Chemical Institute, National Academy of Sciences of Ukraine, 86 Lustdorfskaya doroga, 65080 Odessa, Ukraine Ivan Franko National University of Lviv, 8 Kirilo i Mefodii, 79005 Lviv, Ukraine
A R T I C L E I N F O
A B S T R A C T
Keywords: Oxides Chemical synthesis Luminescence Phosphors
The luminescent properties of Ce3+ ions in Ca3Sc2Si3O12 were studied in the temperature range of 77–800 K. It was found, for the first time, that at 77 K zero-phonon lines of 5d↔4f transitions and vibronic structure are distinctly observed in the Ce3+ emission and excitation spectra. The decay time of the Ce3+ emission in Ca3Sc2Si3O12 was found to be nearly constant (~ 71 ns) up to 700 K. This indicates that the thermal quenching of the Ce3+ emission in Ca3Sc2Si3O12 starts at high temperatures (≥ 750 K). The decrease of the Ce3+ integrated emission intensity (upon direct excitation of the lowest-energy Ce3+ 5d1 state) with increasing temperature in the range of 350–700 K, often attributed to the thermal quenching, probably arises from temperature changes of 4f→5d1 transition absorption strength. It was shown that typical impurity phases (β-Ca2SiO4, CaSiO3, CeO2) can cause competing "parasitic" absorption in the 250–380 nm range and they are expected to be responsible for variations in the luminescence excitation and diffuse-reflection spectra reported in the literature for Ce3+-doped Ca3Sc2Si3O12. A comparison with literature data on Ce3+-doped aluminum garnets (Y3Al5O12 and Lu3Al5O12) is also made.
1. Introduction In recent years, a number of alkaline earth silicates doped with Ce3+ or Eu2+ ions have been presented as potential phosphors for white light-emitting diodes (LEDs) [1]. Among them, material of composition Ca3Sc2Si3O12:Ce3+ is still one of the most promising phosphors because of its favorable luminescent properties and a weak thermal quenching of luminescence [2]. Ca3Sc2Si3O12 is known to have a garnet structure in which all the Ca atoms are coordinated by eight oxygen atoms forming a dodecahedron (D2 point symmetry) with four long Ca-O distances of 2.51 Å and four short Ca-O distances of 2.40 Å, whereas Sc atoms occupy octahedral positions [3]. Several groups of authors have studied the luminescent properties of Ce3+ ions in Ca3Sc2Si3O12 upon excitation in the range from 120 to 500 nm [2,4–6]. At room temperature Ce3+-doped Ca3Sc2Si3O12 was found to show a broadband emission with a maximum at 505 nm and a shoulder at about 550 nm. Since this emission is efficiently excited by photons in the 400–500 nm region, an efficient white LED with the high color rendering index (Ra = 92) was fabricated by using a combination of a blue (In,Ga)N chip emitting around 455 nm, Ca3Sc2Si3O12:Ce3+ and CaAlSiN3:Eu2+ as green and red phosphors, respectively [2]. Also, the luminescent
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properties of Ca3Sc2Si3O12 doped with some other lanthanides, such as Pr3+ [7], Eu3+ [8], Eu2+ [9], have been studied and reported. The Eu2+-doped Ca3Sc2Si3O12 exhibits a broadband emission with a maximum at about 840 nm, which is due to the 4f65d→4f7 transition of Eu2+ ions [9]. It was suggested that since the near infrared (NIR) emission band of Ca3Sc2Si3O12:Eu2+ is located in the highest spectral response region of c-Si photovoltaic solar cells, an optimized material may be used to convert the short-wavelength part of sunlight (300–550 nm) into the NIR emission. For this reason, the luminescence properties of double-doped with Eu2+ and Ce3+ ions Ca3Sc2Si3O12 have been also studied [6,10]. Upon the introduction of Ce3+ ions into the crystal lattice the Ce3+ 4f→5d band at 445 nm appears in the excitation spectra for the NIR emission of Eu2+ ions, indicating the presence of energy transfer between Ce3+ and Eu2+ ions and a possibility to increase an efficiency of converting the short-wavelength part of sunlight (300–550 nm) into the NIR emission. A deficit of red component in the emission spectrum of Ce3+-doped Ca3Sc2Si3O12 stimulated the attempts to improve the color characteristics of LEDs by incorporating N3− [11], substitution of Sc3+ and Ca2+ with other ions, such as Mg2+ [4], Mg2+ and Lu3+ [5,12]. The introduction of these ions into Ce3+-doped Ca3Sc2Si3O12 phosphors has
Corresponding author. E-mail address: [email protected] (V.P. Dotsenko).
https://doi.org/10.1016/j.jlumin.2017.11.002 Received 13 July 2017; Received in revised form 17 October 2017; Accepted 1 November 2017 Available online 02 November 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
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been reported to shift the Ce3+ emission band toward the longer wavelengths and to decrease a thermal stability of the luminescence [5,11,12]. However, due to disorder in the occupation of the cation and anion sites and, therefore, the presence of several different Ce3+ centers, the experimental data are quite complicated to interpret and an understanding of the reasons for the lower thermal stability of these phosphors requires additional studies. Despite the practical importance of Ce3+-doped Ca3Sc2Si3O12 phosphor, no detailed research has been performed on the influence of temperature on its luminescence properties. In this paper, we describe the results of the luminescence study on Ce3+-doped Ca3Sc2Si3O12 in the temperature range from 77 to 800 K. An analysis of fine structure in the emission and excitation spectra of Ce3+ ions at 77 K allowed to accurately determine the position of zero-phonon line for 5d→4f(2F5/2) transition, and the Stokes shift of the emission. Based on the decay time measurements, the intrinsic quenching temperature of the Ce3+ emission in Ca3Sc2Si3O12 was also evaluated for the first time. A comparison with literature data on Ce3+-doped aluminum garnets Y3Al5O12 (YAG) and Lu3Al5O12 (LuAG) is made. The results obtained are anticipated to be meaningful not only for interpretation of the influence of substitutional ions (N3−, Mg2+, Lu3+) on the luminescence properties of Ce3+doped Ca3Sc2Si3O12, but also for development of new garnet type phosphors for white LEDs.
Fig. 1. Comparison of XRD pattern of the as-prepared Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) (a) with data from JCPDS file no. 72-1969 for nominally pure Ca3Sc2Si3O12 (b). Peak from the impurity phase (Sc2O3) is denoted by the symbol (◊).
2. Experimental Three polycrystalline samples of general composition Ca3(1−xwith x = 0.00001, y = 0; and x = y = 0.005; 0.01 were prepared by solid state reaction method. To obtain samples with x = 0.005 and 0.01 an appropriate amounts of NaF were added to starting mixtures. One can expect that introduction of the charge compensator (Na+) should increase the solubility of Ce2O3 in the host. Starting mixtures of CaCO3, SiO2, Sc2O3, Ce(NO3)3·nH2O and NaF were thoroughly mixed and fired at a temperature of about 1300 °C for 4 h in air. After cooling to room temperature, the specimens were grounded by ball milling to insure homogeneity (Fritsch, PM 100), and fired twice at 1300 °C for 4 h in a reducing medium of CO. The final products were then washed with distilled water and dried at 105 °C. The samples were checked by powder X-ray diffraction (XRD) on a Shimadzu LabX XRD-6000 automated diffractometer using Cu Kα radiation (λ = 1.5418 Å). Morphological investigations were carried out by scanning electron microscopy (SEM) on a TESCAN VEGA 3 electron microscope. The emission and excitation spectra in UV–visible region were obtained at 77 K and room temperature using a Fluorolog Fl-3 (Horiba Jobin Yvon) spectrofluorometer equipped with a xenon lamp. The excitation spectra at wavelengths shorter than 330 nm were recorded at room temperature using synchrotron radiation and the equipment of the SUPERLUMI experimental station [13] at HASYLAB (Hamburg, Germany). The correction of these spectra for the wavelength dependent excitation intensity was performed with the use of sodium salicylate as a standard. The decay curves of the Ce3+ emission were recorded in the temperature range from 300 to 700 K with the step of 50 K using time correlated single photon counting method upon excitation with either a nanosecond LED at 370 nm or a flash lamp with discharge in air. y)Ce3xNa3ySc2Si3O12
Fig. 2. SEM images of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) sample annealed at 1300 °C for (a) − 4 h; (b) − 12 h.
impurity phases did not prevent selective excitation and luminescence detection of Ce3+ ions in Ca3Sc2Si3O12. As an example, the SEM images of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) are presented in Fig. 2. The SEM image of the sample annealed at 1300 °C for 4 h in air revealed the presence of particles with sizes of 10–30 µm, and with a poorly defined habitus, which can be an indication of a liquid phase formation during the synthesis process. These observations agree with the results of previous studies of Ca3Sc2Si3O12:Ce3+ obtained by different chemical methods [14–16]. As can be seen from Fig. 2(b), the grinding and further annealing at 1300 °C for 8 h resulted in the formation of the irregular shaped
3. Results and discussion 3.1. Phase and morphology identification XRD patterns of the final products were well matched with JCPDS File No. 72–1969 for pure Ca3Sc2Si3O12. Small amounts of impurity phases were also detected, however, only one of them (Sc2O3) was securely identified. The typical XRD pattern of the samples is shown in Fig. 1. Since the luminescence properties of Ce3+ ions in the garnets differ markedly from those of Ce3+ in other phases, the presence of the 25
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Fig. 3. Time-integrated excitation spectrum of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) for the Ce3+ emission at 510 nm at 293 K. The spectrum is a superposition of curves (a) and (b) obtained upon excitation with synchrotron radiation and optical photons, respectively. The inset shows the emission spectrum of the sample upon excitation with λexc = 445 nm.
Fig. 4. Emission spectrum of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.005) recorded upon excitation at 445 nm at 77 K. The inset shows the onset of excitation spectrum for the emission at 510 nm (T = 77 K). Arrows indicate the positions of the ZPLs.
particles with evidently better crystallinity. The size of particles was found to vary from 2 to 25 µm. 3.2. Luminescence properties at 77 K Fig. 3 shows the emission and excitation spectra of the sample Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) at 293 K. The emission spectrum consists of a band with a maximum at about 506 nm and a shoulder at 548 nm, which are due to transitions from the lowest 5d excited state to the 4f ground state levels 2F5/2 and 2F7/2 of Ce3+ ions on the calcium dodecahedral sites [2]. The time-integrated excitation spectrum for this emission (curves a, b) contains bands at 237, 308 and 445 nm. It is evident that these bands are due to direct excitation of the Ce3+ ions via transitions from the 4f ground state to the three components (5d1,2,3) of the Ce3+ 5d configuration. Also, there are intense band with a maximum at about 174 nm (7.13 eV), a shoulder at 203 nm and a relatively weak band with a maximum at about 357 nm. The band at 174 nm was observed previously in the excitation spectra for emission of self-trapped excitons in non-doped Ca3Sc2Si3O12 [7]. This indicates that this band is caused by host lattice absorption with a subsequent energy transfer to the Ce3+ ions. From its maximum, the onset of interband transitions in Ca3Sc2Si3O12 was roughly estimated to be ≥ 7.10 eV. The short-time excitation spectrum, measured in time-window of 2–12 ns after excitation pulse, has the same structure, however the relative intensity of the shoulder at 203 nm was found to be lower as compared to that in the time-integrated excitation spectrum, so this feature can be attributed to absorption of exciting radiation by lattice defects. This interpretation coincides with the results reported earlier [6,7]. It was shown that upon excitation at 206 nm, nominally pure Ca3Sc2Si3O12 exhibits a broadband emission in the range of 300–550 nm with a maximum at 385 nm. The decay time of this emission was found to be 5.5 μs [6]. The possible origin of the low intensity band at 357 nm will be discussed below. The emission and excitation spectra of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.005) at 77 K recorded upon excitation at 445 nm are presented in Fig. 4. It is seen that at 77 K the emission band of Ce3+ ions is narrowed and a fine structure is observed in the spectrum. The two emission bands are clearly resolved and zero-phonon lines (ZPLs) are observed at 473.2 nm (21,133 cm−1) and 526.7 nm (18,986 cm−1). The energy gap between the ZPLs (2147 cm−1) coincides with the theoretical spin-orbit splitting of Ce3+ ground state. The Stokes shift (δ) of the Ce3+ emission was roughly determined from the spectral position of the maxima of the excitation and emission bands. In this way the Stokes shift of the emission was found to be 2312 cm−1. The close value of δ (2346 cm−1) was also obtained taking into account that the Stokes shift can be evaluated as twice the energy gap between the ZPL (473.2 nm) and the maximum of the emission band (501 nm). Note that
Fig. 5. Fine structure in the emission and excitation spectra of Ca3(1−x)Ce3xSc2Si3O12 (x = 0.00001) at 77 K. The emission spectrum (a) was recorded upon excitation with λexc = 445 nm and the excitation spectrum (b) was obtained for the emission at 510 nm. Arrows indicate the positions of the vibronic replicas.
the fine structure is also observed in the luminescence excitation spectrum. The onsets of both the excitation and emission spectra for Ca3(1−x)Ce3xSc2Si3O12 (x = 0.00001) at 77 K are shown in Fig. 5. It is seen that the mirror symmetry at about 473.2 nm is observed, supporting that this feature is the ZPL of the Ce3+ 5d↔4f (2F5/2) transitions. Vibronic replicas are present at about 245 and 501 cm−1 from the ZPL, suggesting a coupling with ~ 245 cm−1 local vibrational mode. Although low temperature (3–15 K) luminescence properties of Ce3+doped Ca3Sc2Si3O12 were reported earlier [6,17], no fine structure in the emission and excitation spectra has been observed. It is well known that inhomogeneous broadening and reabsorption of emission can effectively prevent the appearance of the ZPL and vibronic structure [18,19]. Note that the luminescence excitation spectra of Ce3+-doped aluminum garnets (YAG, LuAG) exhibit sharp ZPLs of 5d1→4f(2F5/2) transitions of Ce3+ ions, whereas for mixed Y3(1−x)Lu3xAl5O12 (x = 0.10–0.90) crystals no ZPLs were revealed [20]. This observation was attributed to the lattice disorder caused by statistical distribution of Y and Lu atoms in garnet lattice. One can suppose that the induced crystal field fluctuations result in strong inhomogeneous broadening and the absence of fine structure [20]. Comparison of the excitation spectra for Ce3+-doped Ca3Sc2Si3O12 at 293 and 77 K indicated that the intensity of the band at about 308 nm relative to that of band at ~ 445 nm decreases with decreasing temperature to 77 K. Similar observations were made previously for Ce3+-doped YAG [21], LuAG [19] and Gd3Al5O12 [22], and this effect was ascribed to differences in temperature dependences of the absorption strength for 4f→5d transitions to the two lowest-energy sublevels of Ce3+ 5d configuration. According to Robinson [21], these
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Fig. 6. Decay curves of the Ce3+ emission (λem = 510 nm) for Ca3(1−xy)Ce3xNa3ySc2Si3O12 (x = y = 0.01) recorded in the temperature range from 300 to 700 K upon excitation at 450 nm.
Fig. 8. Temperature dependences of the integrated emission intensity and luminescence decay time of Ce3+ ions in Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) recorded upon excitation with λexc = 440 nm.
differences are related with a temperature dependent population of the lowest crystal field components of the 2F5/2 ground state. 3+
3.3. Temperature dependences of the Ce luminescence decay time
see that upon excitation of the lowest-energy Ce3+ 5d1 state (λexc = 440 nm), the emission intensity decreases steadily with increasing temperature and at 700 K, the intensity amounts to ~ 40% of its value at room temperature. This behavior agrees with the data of previous reports on Ce3+-doped Ca3Sc2Si3O12 [5,12,17]. In Table 1, the luminescent characteristics of Ce3+ ions in Ca3Sc2Si3O12 at 77 K are presented and compared with those of Ce3+ in rare earth aluminum garnets (YAG, LuAG). The maxima for the 4f→5d1 excitation and emission bands (Eexc, Eem), the Stokes shifts (δ) for YAG, LuAG at ~ 4.5 K are taken from Refs. [19,23,24]. It is clear that the luminescence properties of Ce3+ ions in Ca3Sc2Si3O12 are very similar to those of Ce3+ ions in LuAG. The found value of the Stokes shift for Ce3+ ions in Ca3Sc2Si3O12 is close to that for LuAG and somewhat smaller than that of Ce3+ ions in YAG. One can see that the thermal quenching of the Ce3+ emission in Ca3Sc2Si3O12 starts at essentially higher temperatures than that of Ce3+ emission in YAG. This observation is in line with the results on the Pr3+-doped garnets. Ivanovskikh et al. [7] have revealed that the decay time of the Pr3+ emission in Ca3Sc2Si3O12 remains practically unchanged up to 500 K, significant temperature quenching was observed above 600 K, whereas the onset of thermal quenching of Pr3+ emission was found to be at 150 and ~ 450 K for YAG and LuAG, respectively [23]. The mechanism for temperature quenching of Ce3+ emission in the most practically important series of compounds with the garnet structure is still under discussion in the literature [23–27]. In general, two mechanisms can cause the temperature quenching of the 5d→4f luminescence of Ce3+ in inorganic compounds: (i) quenching by thermally activated cross-over from the 5d1 excited stated to the 4f ground state and (ii) thermally activated photoionization from the 5d1 state to the conduction band. New experimental data supporting the view that temperature quenching of the Ce3+ emission in mixed Y3(1−x)Gd3xAl5O12 (YGdAG) garnets is caused by thermally induced ionization of the Ce3+ 5d1 excited state have been recently presented [25,27]. For example, by an analysis of thermoluminescence (TL) excitation spectra at 300–700 K for YAG:Ce3+ Ueda et al. have found out that there is a good correspondence between the temperature quenching of the Ce3+ emission in YAG and the appearance and rise of a TL signal in the 500–700 K range [25]. The obtained persistent luminescence excitation spectra of Ce3+-doped YGdAG crystals at 300–400 K imply that this mechanism also takes place for Y3(1−x)Gd3xAl5O12 (x = 0.50, 0.75) garnets [27]. In contrast, the recent measurements of temperature dependence of delayed radiative recombination indirectly indicate that the main mechanism responsible for the thermal quenching of the Ce3+ emission in mixed YGdAG single crystals is the thermally activated cross-over from the 5d1 excited stated
emission intensity and
The decay curves of the Ce3+ emission were recorded upon excitation at 450 nm in the temperature range from 300 to 700 K with the step of 50 K. All decay curves appeared to be close to single exponential with a time constant of ~ 71 ns. As an example, the decay curves measured at 300, 500 and 700 K are presented in Fig. 6. Depending upon the synthesis conditions, nominal Ce concentration etc. the reported decay times of the Ce3+ emission in Ca3Sc2Si3O12 at room temperature vary from 56 ns [11,16] to 77 ns [15]. This variation of the decay time may be partly explained by differences in the effective refractive index for polycrystalline samples obtained by various authors. From Fig. 6 it follows that the thermal quenching of the Ce3+ emission in Ca3Sc2Si3O12 can start at high temperatures (≥ 750 K). The room temperature decay curves of the Ce3+ emission upon excitation into three different 4f→5d1,2,3 bands are compared in Fig. 7. It is seen that the decay curves are similar and, in first approximation, can be characterized by a time constant of 71 ± 2 ns. For λexc = 238 nm, the decay curve has a higher constant background level than the other curves. Since the microsecond time scale emission associated with native defects of Ca3Sc2Si3O12 (λmax = 385 nm) is not excited at 238 nm [6], this may be connected with photoionization of Ce3+ ions and retrapping processes with the participation of shallow traps. Fig. 8 shows the temperature dependences of the integrated emission intensity and luminescence decay time of Ce3+ ions in Ca3(1−xy)Ce3xNa3ySc2Si3O12 (x = y = 0.01) in the range of 300–800 K. One can
Fig. 7. Room temperature decay curves of the Ce3+ emission (λem = 510 nm) for Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) recorded upon excitation at 238, 310, 410 nm.
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Table 1 Comparison of the luminescence properties of Ce3+ ions in Ca3Sc2Si3O12 and some compounds with the garnet structure. Compound
Еexc (5d1) (сm−1)
Еem (5d1) (сm−1)
ZPL (сm−1)
Stokes shift (сm−1)
Quenching onset (K)
Ref.
Y3Al5O12 Lu3Al5O12 Ca3Sc2Si3O12
21,716 22,217 22,272
18,904 19,877 19,960
20,424 21,137 21,133
2800 2380 2340
~ 600 ~ 700 ≥ 750
[23,24] [19,23] This work
to the 4f ground state [26]. Both the above-mentioned mechanisms were also used to explain the decrease in the integrated emission intensity of Ce3+ ions in Ca3Sc2Si3O12 with increasing temperature from 350 to 600 K [5,12,17]. However, as shown in Figs. 6 and 8, the observed luminescence intensity decrease is not accompanied by a decrease of the luminescence decay time, and therefore this effect is not due to cross-over or photoionization from the 5d1 state. The possible explanation is related to a temperature dependence of the absorption strength of 4f→5d transition for the lowest-energy sublevel of Ce3+ 5d configuration. A very similar situation was observed earlier for Ce3+-doped YAG single crystals [24]. With increasing temperature from 300 to 600 K, the absorption intensity at the Ce3+ absorption maximum (λmax = 460 nm) was found to decrease steadily to ~60% of its value at room temperature [24], whereas the decay time and quantum yield of the Ce3+ emission in YAG start to decrease at ca. 600 K [23–25]. To our knowledge, Ce3+-doped Ca3Sc2Si3O12 and LuAG exhibit the highest thermal stability of the emission among the known Ce3+-doped phosphors of garnet family. In recent years, the luminescence properties of numerous Ce3+-doped silicates with garnet structure, such as CaY2Al4SiO12 [28], CaLu2Mg2Si3O12 [5,12], Lu3(Al,Mg)2(Al,Si)3O12 [29], Y3MgAl2(BSi)O12 (B= Al, Sc, Ga) [30], have been studied and reported. It was found that a considerable broadening of Ce3+ emission bands and large Stokes shifts (3000–4300 cm−1) are characteristic for these garnets. This was attributed to the multisite nature of Ce3+ emission caused by the disorder on the dodecahedral and octahedral sites. The onset quenching temperatures for these phosphors were found to be lower by 200–400 K compared with those for Ce3+-doped Ca3Sc2Si3O12, LuAG [12,28,29]. In addition to the cross-over or photoionization from the 5d1 state [12,29], a thermally activated trapping of the excitation energy by complex defects involving antisite atoms and vacancies was also supposed to explain the lower thermal stability of these phosphors [28]. It can be excluded that a competition between different processes may determine temperature dependences of Ce3+ luminescence in such multicomponent phosphors.
Fig. 9. Comparison of the emission spectra of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.005) upon excitation at (a) 355 nm and (b) at 322 nm. The insets show the excitation spectra for the emission at (a) 450 nm and (b) 380 nm (T = 293 K).
that the excitation spectrum for the emission at 450 nm differs markedly from that of the Ce3+ emission in Ca3Sc2Si3O12. It consists of bands with maxima at 292 and 357 nm. Upon 322 nm excitation, the emission spectrum comprises a band with a maximum at 374 nm and a shoulder at 351 nm. The excitation spectrum of this emission contains bands at 252, 307 and 324 nm. In general, the shape and position of the excitation bands at 320–360 nm indicate that these features can belong to the Ce3+ ions. The decay curve of the emission at 410 nm recorded under excitation at 370 nm is presented in Fig. 10. The decay curve is seen to deviate significantly from a purely exponential one and it could not be well fitted to a sum of two exponential functions. However, the
3.4. Role of impurity phases As mentioned above, the excitation spectra for the Ce3+ emission in Ca3Sc2Si3O12 contain a weak band with a maximum at 357 nm (see Fig. 3). More or less intense bands in the 320–360 nm range are present in the luminescence excitation spectra of Ce3+ in Ca3Sc2Si3O12 reported by numerous authors [4–6,11,17]. Based on the first-principles calculations, Ding et al. [31] have supposed that an excitation band in the 320–360 nm range can be due to 4f→5d2 transition of Ce3+ ions, with Sc3+ occupying Si4+-positions in their nearest environment. A little later, such the features were attributed to Ce3+ ions located in the vicinity of point defects of Ca3Sc2Si3O12 [6]. In order to clarify the origin of the band at 357 nm, we compared the emission spectra at different excitation wavelengths from 320 to 370 nm. The typical spectra are shown in Fig. 9. The spectrum of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.005) recorded upon excitation at 355 nm contains a broad band extending from 360 nm to 520 nm and peaking at about 406 nm. There is also a band with a maximum at 506 nm and a shoulder at 548 nm, which are evidently due to transitions from the lowest 5d excited state to the 4f ground state levels 2F5/2 and 2F7/2 of Ce3+ ions in Ca3Sc2Si3O12. From Figs. 3 and 9 one can see
Fig. 10. Decay curve of the Ce3+ emission in Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.005) at 410 nm recorded upon excitation at 370 nm.
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275–320 nm. It is important to note the obtained DR spectra of Cedoped Ca3Sc2Si3O12 in the 270–380 nm range differ markedly from those for nominally pure Ca3Sc2Si3O12 [11,16]. Therefore, it cannot be excluded that even at low nominal concentrations some amounts of Ce exist as CeO2. Also, several studies have revealed that for Eu2+ and Ce3+ co-doped Ca3Sc2Si3O12 the relative intensity of the high-energy Eu2+ 4f7→4f65d excitation band at 360 nm decreases with the increase of Ce concentration [6,10]. This observation was tentatively ascribed to photoionization of Eu2+ ions, electron trapping on Ce3+-related defects and subsequent non-radiative relaxation [10]. Later, based on comparison of the luminescence properties of Eu2+ and Ce3+ co-doped Ca3Sc2Si3O12 and reference samples with general composition Ca3Sc2Si3O12:Eu2+/CeO2, we came to the conclusion that this effect, at least in part, was caused by an increase of CeO2 content, which resulted in enhancement of competing absorption in the UV range. Finally, the presence of impurity phases (β-Ca2SiO4, CaSiO3, CeO2) can be the reason for competing absorption in the 250–380 nm range and, therefore, for variations in the luminescence excitation and diffuse-reflection spectra reported in the literature for Ce3+-doped Ca3Sc2Si3O12.
1/e decay time constant was found to be 12.5 ns, which is close to typical values for 5d→4f transitions of Ce3+ in inorganic compounds. The distinct nonexponentiality of the decay can be explained by the existence of several kinds of Ce3+-related centers responsible for the emission at 410 nm. This is in line with the lack of the characteristic doublet structure in the Ce3+ emission spectrum. The prominent feature of the Ce3+-related emission at 330–450 nm is that at room temperature it is effectively excited in the range of Ce3+ 4f→5d transitions, but not excited in the fundamental absorption region of Ca3Sc2Si3O12 (≥ 7.0 eV). The same conclusion can be made from the luminescence excitation spectra of Ce3+-doped Ca3Sc2Si3O12 at 15 K reported by Zhou et al. [6]. Since there is some analogy between the observations made above and the results reported in the literature for some nanoand microscale Ce3+-containing luminescent species dispersed in different inorganic matrices (see, for example, [26,32]), we came to the conclusion that, at least in the case of the samples studied in this work, the observed excitation bands at 320–360 nm are not due to irregular Ce3+ centers in Ca3Sc2Si3O12, but come from Ce3+ ions in impurity phases. Note that the Ce3+ emission in Ca3Sc2Si3O12 might partly arise due to radiative energy transfer from impurity phase inclusions to Ce3+ in the garnet phase. As mentioned in Section 3.1, the XRD patterns of the samples revealed, in addition to Ca3Sc2Si3O12, the presence of small amounts of Sc2O3. This implies that some amounts of calcium silicates, such as CaSiO3, β-Ca2SiO4, Ca3SiO5, can be also present in the samples under study, and their appearance as impurity phases in Ca3Sc2Si3O12 has been really detected by several authors [2,16]. Comparison with literature data on luminescence properties of Ce3+ in different calcium silicates indicates that the most probable candidates are β-Ca2SiO4 and CaSiO3. It is known that the emission spectra of Ce3+ ions in β-Ca2SiO4 are strongly dependent on the excitation wavelength due to the presence of two non-equivalent sites for Ca atoms in the lattice. Upon excitation at 355 nm, Ce3+ ions in β-Ca2SiO4 exhibit a broadband emission with a maximum at about 405 nm. Its excitation spectrum contains bands at 292 and 356 nm [33]. The emission spectrum of Ce3+ ions in CaSiO3 consists of a band with a maximum at about 350 nm and a shoulder at 380 nm. The broad band at ~ 320 nm was found to be dominant in the excitation spectra for this emission [34]. Thus, the emission and luminescence excitation spectra of Ce3+ ions in β-Ca2SiO4 and CaSiO3 are very similar to those presented in Fig. 9. In the context of influence of possible impurity phases, the role of CeO2 should be also mentioned. By means of Ce LIII-edge X-ray absorption near-edge structure (XANES) method Shimomura et al. [2] have analyzed the chemical composition of Ce-doped Ca3Sc2Si3O12 prepared in a reducing medium (4%H2/96%N2). The obtained XANES spectra did not reveal any amounts of Ce4+ in the samples under study. In contrast, the X-ray photoelectron spectroscopy data for Ce-doped Ca3Sc2Si3O12 samples prepared in air by the freeze-drying precursor method [16] indicated that a large fraction (up to 60%) of the cerium ions is stabilized in the oxidation state +4. Probably, most of them exist as CeO2. With a few exceptions, in the above-mentioned studies on Cedoped Ca3Sc2Si3O12 phosphors, no presence of CeO2 was revealed by XRD analysis. Perhaps, concentrations of this impurity phase in the materials studied were too low to be detected by this method. Taking into account that CeO2 exhibits a strong absorption below 400 nm with a diffuse maximum at 290–320 nm, which is due to a charge transfer from the fully occupied 2p O orbitals to the empty 4f orbitals of Ce4+ [35], diffuse-reflection (DR) spectroscopy seems to be more sensitive to its presence as a separate phase than XRD method. Several groups of authors have reported DR spectra of Ce-doped Ca3Sc2Si3O12 prepared by different methods [11,16,17]. Below 380 nm, the spectra were found to be very complex and difficult to interpret, due to overlap of the Ce3+ absorption band at 308 nm and various bands associated with point defects and, probably, impurity phases. In spite of the fact that there are distinct differences between the spectra presented, one can conclude that the reflectance of Ce-doped Ca3Sc2Si3O12 phosphors decreases sharply below 380 nm and approaches to a local minimum at
4. Conclusions Ce3+-doped Ca3Sc2Si3O12 is still one of the most promising phosphors for white LEDs. In the present work, the luminescent properties of Ce3+ ions in Ca3Sc2Si3O12 were studied in the temperature range of 77–800 K. It is shown that at 77 K zero-phonon lines of 5d↔4f transitions and vibronic structure are distinctly observed in the Ce3+ emission and excitation spectra. The decay time of the Ce3+ emission in Ca3Sc2Si3O12 was found to be nearly constant (~ 71 ns) up to 700 K. This indicates that the thermal quenching of the Ce3+ emission in Ca3Sc2Si3O12 starts at higher temperature (≥ 750 K) than that of the Ce3+ emission in YAG. The decrease of the Ce3+ integrated emission intensity (upon direct excitation of the lowest-energy Ce3+ 5d1 state) with increasing temperature in the range 350–700 K, often attributed to the thermal quenching, probably arises from a temperature dependence of the absorption strength of 4f→5d transition for the lowest-energy sublevel of Ce3+ 5d configuration. It is also shown that typical impurity phases (β-Ca2SiO4, CaSiO3, CeO2) can cause competing extra absorption in the 250–380 nm range and they are expected to be responsible for differences in the luminescence excitation and diffuse-reflection spectra reported in the literature for Ce3+-doped Ca3Sc2Si3O12. One can hope that an improvement in powder preparation techniques can promote enhancement in the luminescence characteristics of Ca3Sc2Si3O12:Ce3+ phosphors for white LEDs. References [1] Z. Xia, Q. Liu, Prog. Mater. Sci. 84 (2016) 59–117. [2] Y. Shimomura, T. Honma, M. Shigeiwa, T. Akai, K. Okamoto, N. Kijima, J. Electrochem. Soc. 154 (2007) J35–J38. [3] B.V. Mill, E.I. Belokoneva, M.A. Simonov, N.V. Belov, J. Struct. Chem. 18 (1977) 321. [4] Y. Shimomura, T. Kurushima, M. Shigeiwa, N. Kijima, J. Electrochem. Soc. 155 (2008) J45–J49. [5] M.S. Kishore, N.P. Kumar, R.G. Chandran, A.A. Setlur, Electrochem. Solid-State Lett. 13 (2010) J77–J80. [6] L. Zhou, W. Zhou, F. Pan, R. Shi, L. Huang, H. Liang, P. Tanner, X. Du, Y. Huang, Y. Tao, L. Zheng, Chem. Mater. 28 (2016) 2834–2843. [7] K.V. Ivanovskikh, A. Meijerink, F. Piccinelli, A. Speghini, E.I. Zinin, C. Ronda, M. Bettinelli, J. Lumin. 130 (2010) 893–901. [8] M. Bettinelli, A. Speghini, F. Piccinelli, A.N.C. Neto, O.L. Malta, J. Lumin. 131 (2011) 1026–1028. [9] I.V. Berezovskaya, V.P. Dotsenko, A.S. Voloshinovskii, S.S. Smola, Chem. Phys. Lett. 585 (2013) 11–14. [10] V.P. Dotsenko, I.V. Berezovskaya, I.V. Zatovsky, S.S. Smola, N.P. Efryushina, J. Nano, Electron. Phys. 7 (2015) 02041. [11] Y. Liu, X. Zhang, Z. Hao, X. Wang, J. Zhang, J. Mater. Chem. 21 (2011) 6354–6358. [12] Y. Liu, X. Zhang, Z. Hao, Y. Luo, X. Wang, J. Zhang, J. Lumin. 132 (2012) 1257–1260. [13] G. Zimmerer, Radiat. Meas. 42 (2007) 859–864. [14] Y. Suzuki, M. Kakihana, Y. Shimomura, N. Kijima, J. Mater. Sci. 43 (2008)
29
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[25] J. Ueda, P. Dorenbos, A.J.J. Bos, A. Meijerink, S. Tanabe, J. Phys. Chem. C 119 (2015) 25003–25008. [26] K. Bartosiewicz, V. Babin, K. Kamada, A. Yoshikawa, J.A. Mares, A. Beitlerova, M. Nikl, Opt. Mater. 63 (2017) 134–142. [27] J. Ueda, M. Yagi, S. Tanabe, ECS J. Solid State Sci. Technol. 5 (2016) R219–R222. [28] A. Katelnikovas, S. Sakirzanovas, D. Dutczak, J. Plewa, D. Enseling, H. Winkler, A. Kareiva, T. Jüstel, J. Lumin. 136 (2013) 17–25. [29] H. Ji, L. Wang, M.S. Molokeev, N. Hirosaki, R. Xie, Z. Huang, Z. Xia, O.M. ten Kate, L. Liu, V.V. Atuchin, J. Mater. Chem. C 4 (2016) 6855–6863. [30] N.M. Khaidukov, V.N. Makhov, Q. Zhang, R. Shi, H. Liang, Dyes Pigments 142 (2017) 524–529. [31] W. Ding, J. Wen, J. Cheng, L. Ning, Y.-C. Huang, C.-K. Duan, M. Yin, Chin. J. Chem. Phys. 28 (2015) 150–154. [32] V.V. Vistovskyy, P.V. Savchyn, G.B. Stryganyuk, A.S. Voloshinovskii, M.S. Pidzyrailo, J. Phys.: Condens. Matter 20 (2008) 325218. [33] Y. Liu, Q. Fang, L. Ning, Y. Huang, S. Huang, H. Liang, Opt. Mater. 44 (2015) 67–72. [34] S. Ye, X.-M. Wang, X.-P. Jing, J. Electrochem. Soc. 155 (2008) J143–J147. [35] S. Mochizuki, F. Fujishiro, Phys. Status Solidi B 246 (2009) 2320–2328.
2213–2216. [15] Y.-F. Wu, Y.-H. Chan, Y.-T. Nien, I.-G. Chen, J. Am. Ceram. Soc. 96 (2013) 234–240. [16] R. Fernandez-Gonzalez, J.J. Velazquez, V.D. Rodriguez, F. Rivera-Lopez, A. Lukowiak, A. Chiasera, M. Ferrari, R.R. Goncalves, J. Marrero-Jerez, F. Lahoz, P. Nūňez, RSC Adv. 6 (2016) 15054–15061. [17] Y. Chen, J. Li, S. Zeng, H. Fan, J. Feng, L. Tan, Opt. Mater. 37 (2014) 464–469. [18] I. van Pieterson, M.F. Reid, R.T. Wegh, S. Soverna, A. Meijerink, Phys. Rev. B 65 (2002) 045113. [19] J.M. Ogieglo, A. Zych, K.V. Ivanovskikh, T. Jüstel, C.R. Ronda, A. Meijerink, J. Phys. Chem. A 116 (2012) 8464–8474. [20] S. Feofilov, A. Kulinkin, K. Ovanesyan, A. Petrosyan, C. Dujardin, Phys. Chem. Chem. Phys. 16 (2014) 22583–22587. [21] D.J. Robbins, J. Electrochem. Soc. 126 (1979) 1550–1555. [22] V.P. Dotsenko, I.V. Berezovskaya, A.S. Voloshinovskii, B.I. Zadneprovski, N.P. Efryushina, Mater. Res. Bull. 64 (2015) 151–155. [23] K.V. Ivanovskikh, J.M. Ogieglo, A. Zych, C. Ronda, A. Meijerink, ECS J. Solid State Sci. Technol. 2 (2013) R3148–R3152. [24] S. Arjoca, E.G. Villora, D. Inomata, K. Aoki, Y. Sugahara, K. Shimamura, Mater. Res. Express 2 (2015) 0555503.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
The effects of temperature and impurity phases on the luminescent properties of Ce3+-doped Ca3Sc2Si3O12 garnet
T
I.V. Berezovskayaa, Z.A. Khapkob, A.S. Voloshinovskiib, N.P. Efryushinaa, S.S. Smolaa, ⁎ V.P. Dotsenkoa, a b
A.V. Bogatsky Physico-Chemical Institute, National Academy of Sciences of Ukraine, 86 Lustdorfskaya doroga, 65080 Odessa, Ukraine Ivan Franko National University of Lviv, 8 Kirilo i Mefodii, 79005 Lviv, Ukraine
A R T I C L E I N F O
A B S T R A C T
Keywords: Oxides Chemical synthesis Luminescence Phosphors
The luminescent properties of Ce3+ ions in Ca3Sc2Si3O12 were studied in the temperature range of 77–800 K. It was found, for the first time, that at 77 K zero-phonon lines of 5d↔4f transitions and vibronic structure are distinctly observed in the Ce3+ emission and excitation spectra. The decay time of the Ce3+ emission in Ca3Sc2Si3O12 was found to be nearly constant (~ 71 ns) up to 700 K. This indicates that the thermal quenching of the Ce3+ emission in Ca3Sc2Si3O12 starts at high temperatures (≥ 750 K). The decrease of the Ce3+ integrated emission intensity (upon direct excitation of the lowest-energy Ce3+ 5d1 state) with increasing temperature in the range of 350–700 K, often attributed to the thermal quenching, probably arises from temperature changes of 4f→5d1 transition absorption strength. It was shown that typical impurity phases (β-Ca2SiO4, CaSiO3, CeO2) can cause competing "parasitic" absorption in the 250–380 nm range and they are expected to be responsible for variations in the luminescence excitation and diffuse-reflection spectra reported in the literature for Ce3+-doped Ca3Sc2Si3O12. A comparison with literature data on Ce3+-doped aluminum garnets (Y3Al5O12 and Lu3Al5O12) is also made.
1. Introduction In recent years, a number of alkaline earth silicates doped with Ce3+ or Eu2+ ions have been presented as potential phosphors for white light-emitting diodes (LEDs) [1]. Among them, material of composition Ca3Sc2Si3O12:Ce3+ is still one of the most promising phosphors because of its favorable luminescent properties and a weak thermal quenching of luminescence [2]. Ca3Sc2Si3O12 is known to have a garnet structure in which all the Ca atoms are coordinated by eight oxygen atoms forming a dodecahedron (D2 point symmetry) with four long Ca-O distances of 2.51 Å and four short Ca-O distances of 2.40 Å, whereas Sc atoms occupy octahedral positions [3]. Several groups of authors have studied the luminescent properties of Ce3+ ions in Ca3Sc2Si3O12 upon excitation in the range from 120 to 500 nm [2,4–6]. At room temperature Ce3+-doped Ca3Sc2Si3O12 was found to show a broadband emission with a maximum at 505 nm and a shoulder at about 550 nm. Since this emission is efficiently excited by photons in the 400–500 nm region, an efficient white LED with the high color rendering index (Ra = 92) was fabricated by using a combination of a blue (In,Ga)N chip emitting around 455 nm, Ca3Sc2Si3O12:Ce3+ and CaAlSiN3:Eu2+ as green and red phosphors, respectively [2]. Also, the luminescent
⁎
properties of Ca3Sc2Si3O12 doped with some other lanthanides, such as Pr3+ [7], Eu3+ [8], Eu2+ [9], have been studied and reported. The Eu2+-doped Ca3Sc2Si3O12 exhibits a broadband emission with a maximum at about 840 nm, which is due to the 4f65d→4f7 transition of Eu2+ ions [9]. It was suggested that since the near infrared (NIR) emission band of Ca3Sc2Si3O12:Eu2+ is located in the highest spectral response region of c-Si photovoltaic solar cells, an optimized material may be used to convert the short-wavelength part of sunlight (300–550 nm) into the NIR emission. For this reason, the luminescence properties of double-doped with Eu2+ and Ce3+ ions Ca3Sc2Si3O12 have been also studied [6,10]. Upon the introduction of Ce3+ ions into the crystal lattice the Ce3+ 4f→5d band at 445 nm appears in the excitation spectra for the NIR emission of Eu2+ ions, indicating the presence of energy transfer between Ce3+ and Eu2+ ions and a possibility to increase an efficiency of converting the short-wavelength part of sunlight (300–550 nm) into the NIR emission. A deficit of red component in the emission spectrum of Ce3+-doped Ca3Sc2Si3O12 stimulated the attempts to improve the color characteristics of LEDs by incorporating N3− [11], substitution of Sc3+ and Ca2+ with other ions, such as Mg2+ [4], Mg2+ and Lu3+ [5,12]. The introduction of these ions into Ce3+-doped Ca3Sc2Si3O12 phosphors has
Corresponding author. E-mail address: [email protected] (V.P. Dotsenko).
https://doi.org/10.1016/j.jlumin.2017.11.002 Received 13 July 2017; Received in revised form 17 October 2017; Accepted 1 November 2017 Available online 02 November 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
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been reported to shift the Ce3+ emission band toward the longer wavelengths and to decrease a thermal stability of the luminescence [5,11,12]. However, due to disorder in the occupation of the cation and anion sites and, therefore, the presence of several different Ce3+ centers, the experimental data are quite complicated to interpret and an understanding of the reasons for the lower thermal stability of these phosphors requires additional studies. Despite the practical importance of Ce3+-doped Ca3Sc2Si3O12 phosphor, no detailed research has been performed on the influence of temperature on its luminescence properties. In this paper, we describe the results of the luminescence study on Ce3+-doped Ca3Sc2Si3O12 in the temperature range from 77 to 800 K. An analysis of fine structure in the emission and excitation spectra of Ce3+ ions at 77 K allowed to accurately determine the position of zero-phonon line for 5d→4f(2F5/2) transition, and the Stokes shift of the emission. Based on the decay time measurements, the intrinsic quenching temperature of the Ce3+ emission in Ca3Sc2Si3O12 was also evaluated for the first time. A comparison with literature data on Ce3+-doped aluminum garnets Y3Al5O12 (YAG) and Lu3Al5O12 (LuAG) is made. The results obtained are anticipated to be meaningful not only for interpretation of the influence of substitutional ions (N3−, Mg2+, Lu3+) on the luminescence properties of Ce3+doped Ca3Sc2Si3O12, but also for development of new garnet type phosphors for white LEDs.
Fig. 1. Comparison of XRD pattern of the as-prepared Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) (a) with data from JCPDS file no. 72-1969 for nominally pure Ca3Sc2Si3O12 (b). Peak from the impurity phase (Sc2O3) is denoted by the symbol (◊).
2. Experimental Three polycrystalline samples of general composition Ca3(1−xwith x = 0.00001, y = 0; and x = y = 0.005; 0.01 were prepared by solid state reaction method. To obtain samples with x = 0.005 and 0.01 an appropriate amounts of NaF were added to starting mixtures. One can expect that introduction of the charge compensator (Na+) should increase the solubility of Ce2O3 in the host. Starting mixtures of CaCO3, SiO2, Sc2O3, Ce(NO3)3·nH2O and NaF were thoroughly mixed and fired at a temperature of about 1300 °C for 4 h in air. After cooling to room temperature, the specimens were grounded by ball milling to insure homogeneity (Fritsch, PM 100), and fired twice at 1300 °C for 4 h in a reducing medium of CO. The final products were then washed with distilled water and dried at 105 °C. The samples were checked by powder X-ray diffraction (XRD) on a Shimadzu LabX XRD-6000 automated diffractometer using Cu Kα radiation (λ = 1.5418 Å). Morphological investigations were carried out by scanning electron microscopy (SEM) on a TESCAN VEGA 3 electron microscope. The emission and excitation spectra in UV–visible region were obtained at 77 K and room temperature using a Fluorolog Fl-3 (Horiba Jobin Yvon) spectrofluorometer equipped with a xenon lamp. The excitation spectra at wavelengths shorter than 330 nm were recorded at room temperature using synchrotron radiation and the equipment of the SUPERLUMI experimental station [13] at HASYLAB (Hamburg, Germany). The correction of these spectra for the wavelength dependent excitation intensity was performed with the use of sodium salicylate as a standard. The decay curves of the Ce3+ emission were recorded in the temperature range from 300 to 700 K with the step of 50 K using time correlated single photon counting method upon excitation with either a nanosecond LED at 370 nm or a flash lamp with discharge in air. y)Ce3xNa3ySc2Si3O12
Fig. 2. SEM images of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) sample annealed at 1300 °C for (a) − 4 h; (b) − 12 h.
impurity phases did not prevent selective excitation and luminescence detection of Ce3+ ions in Ca3Sc2Si3O12. As an example, the SEM images of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) are presented in Fig. 2. The SEM image of the sample annealed at 1300 °C for 4 h in air revealed the presence of particles with sizes of 10–30 µm, and with a poorly defined habitus, which can be an indication of a liquid phase formation during the synthesis process. These observations agree with the results of previous studies of Ca3Sc2Si3O12:Ce3+ obtained by different chemical methods [14–16]. As can be seen from Fig. 2(b), the grinding and further annealing at 1300 °C for 8 h resulted in the formation of the irregular shaped
3. Results and discussion 3.1. Phase and morphology identification XRD patterns of the final products were well matched with JCPDS File No. 72–1969 for pure Ca3Sc2Si3O12. Small amounts of impurity phases were also detected, however, only one of them (Sc2O3) was securely identified. The typical XRD pattern of the samples is shown in Fig. 1. Since the luminescence properties of Ce3+ ions in the garnets differ markedly from those of Ce3+ in other phases, the presence of the 25
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Fig. 3. Time-integrated excitation spectrum of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) for the Ce3+ emission at 510 nm at 293 K. The spectrum is a superposition of curves (a) and (b) obtained upon excitation with synchrotron radiation and optical photons, respectively. The inset shows the emission spectrum of the sample upon excitation with λexc = 445 nm.
Fig. 4. Emission spectrum of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.005) recorded upon excitation at 445 nm at 77 K. The inset shows the onset of excitation spectrum for the emission at 510 nm (T = 77 K). Arrows indicate the positions of the ZPLs.
particles with evidently better crystallinity. The size of particles was found to vary from 2 to 25 µm. 3.2. Luminescence properties at 77 K Fig. 3 shows the emission and excitation spectra of the sample Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) at 293 K. The emission spectrum consists of a band with a maximum at about 506 nm and a shoulder at 548 nm, which are due to transitions from the lowest 5d excited state to the 4f ground state levels 2F5/2 and 2F7/2 of Ce3+ ions on the calcium dodecahedral sites [2]. The time-integrated excitation spectrum for this emission (curves a, b) contains bands at 237, 308 and 445 nm. It is evident that these bands are due to direct excitation of the Ce3+ ions via transitions from the 4f ground state to the three components (5d1,2,3) of the Ce3+ 5d configuration. Also, there are intense band with a maximum at about 174 nm (7.13 eV), a shoulder at 203 nm and a relatively weak band with a maximum at about 357 nm. The band at 174 nm was observed previously in the excitation spectra for emission of self-trapped excitons in non-doped Ca3Sc2Si3O12 [7]. This indicates that this band is caused by host lattice absorption with a subsequent energy transfer to the Ce3+ ions. From its maximum, the onset of interband transitions in Ca3Sc2Si3O12 was roughly estimated to be ≥ 7.10 eV. The short-time excitation spectrum, measured in time-window of 2–12 ns after excitation pulse, has the same structure, however the relative intensity of the shoulder at 203 nm was found to be lower as compared to that in the time-integrated excitation spectrum, so this feature can be attributed to absorption of exciting radiation by lattice defects. This interpretation coincides with the results reported earlier [6,7]. It was shown that upon excitation at 206 nm, nominally pure Ca3Sc2Si3O12 exhibits a broadband emission in the range of 300–550 nm with a maximum at 385 nm. The decay time of this emission was found to be 5.5 μs [6]. The possible origin of the low intensity band at 357 nm will be discussed below. The emission and excitation spectra of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.005) at 77 K recorded upon excitation at 445 nm are presented in Fig. 4. It is seen that at 77 K the emission band of Ce3+ ions is narrowed and a fine structure is observed in the spectrum. The two emission bands are clearly resolved and zero-phonon lines (ZPLs) are observed at 473.2 nm (21,133 cm−1) and 526.7 nm (18,986 cm−1). The energy gap between the ZPLs (2147 cm−1) coincides with the theoretical spin-orbit splitting of Ce3+ ground state. The Stokes shift (δ) of the Ce3+ emission was roughly determined from the spectral position of the maxima of the excitation and emission bands. In this way the Stokes shift of the emission was found to be 2312 cm−1. The close value of δ (2346 cm−1) was also obtained taking into account that the Stokes shift can be evaluated as twice the energy gap between the ZPL (473.2 nm) and the maximum of the emission band (501 nm). Note that
Fig. 5. Fine structure in the emission and excitation spectra of Ca3(1−x)Ce3xSc2Si3O12 (x = 0.00001) at 77 K. The emission spectrum (a) was recorded upon excitation with λexc = 445 nm and the excitation spectrum (b) was obtained for the emission at 510 nm. Arrows indicate the positions of the vibronic replicas.
the fine structure is also observed in the luminescence excitation spectrum. The onsets of both the excitation and emission spectra for Ca3(1−x)Ce3xSc2Si3O12 (x = 0.00001) at 77 K are shown in Fig. 5. It is seen that the mirror symmetry at about 473.2 nm is observed, supporting that this feature is the ZPL of the Ce3+ 5d↔4f (2F5/2) transitions. Vibronic replicas are present at about 245 and 501 cm−1 from the ZPL, suggesting a coupling with ~ 245 cm−1 local vibrational mode. Although low temperature (3–15 K) luminescence properties of Ce3+doped Ca3Sc2Si3O12 were reported earlier [6,17], no fine structure in the emission and excitation spectra has been observed. It is well known that inhomogeneous broadening and reabsorption of emission can effectively prevent the appearance of the ZPL and vibronic structure [18,19]. Note that the luminescence excitation spectra of Ce3+-doped aluminum garnets (YAG, LuAG) exhibit sharp ZPLs of 5d1→4f(2F5/2) transitions of Ce3+ ions, whereas for mixed Y3(1−x)Lu3xAl5O12 (x = 0.10–0.90) crystals no ZPLs were revealed [20]. This observation was attributed to the lattice disorder caused by statistical distribution of Y and Lu atoms in garnet lattice. One can suppose that the induced crystal field fluctuations result in strong inhomogeneous broadening and the absence of fine structure [20]. Comparison of the excitation spectra for Ce3+-doped Ca3Sc2Si3O12 at 293 and 77 K indicated that the intensity of the band at about 308 nm relative to that of band at ~ 445 nm decreases with decreasing temperature to 77 K. Similar observations were made previously for Ce3+-doped YAG [21], LuAG [19] and Gd3Al5O12 [22], and this effect was ascribed to differences in temperature dependences of the absorption strength for 4f→5d transitions to the two lowest-energy sublevels of Ce3+ 5d configuration. According to Robinson [21], these
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Fig. 6. Decay curves of the Ce3+ emission (λem = 510 nm) for Ca3(1−xy)Ce3xNa3ySc2Si3O12 (x = y = 0.01) recorded in the temperature range from 300 to 700 K upon excitation at 450 nm.
Fig. 8. Temperature dependences of the integrated emission intensity and luminescence decay time of Ce3+ ions in Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) recorded upon excitation with λexc = 440 nm.
differences are related with a temperature dependent population of the lowest crystal field components of the 2F5/2 ground state. 3+
3.3. Temperature dependences of the Ce luminescence decay time
see that upon excitation of the lowest-energy Ce3+ 5d1 state (λexc = 440 nm), the emission intensity decreases steadily with increasing temperature and at 700 K, the intensity amounts to ~ 40% of its value at room temperature. This behavior agrees with the data of previous reports on Ce3+-doped Ca3Sc2Si3O12 [5,12,17]. In Table 1, the luminescent characteristics of Ce3+ ions in Ca3Sc2Si3O12 at 77 K are presented and compared with those of Ce3+ in rare earth aluminum garnets (YAG, LuAG). The maxima for the 4f→5d1 excitation and emission bands (Eexc, Eem), the Stokes shifts (δ) for YAG, LuAG at ~ 4.5 K are taken from Refs. [19,23,24]. It is clear that the luminescence properties of Ce3+ ions in Ca3Sc2Si3O12 are very similar to those of Ce3+ ions in LuAG. The found value of the Stokes shift for Ce3+ ions in Ca3Sc2Si3O12 is close to that for LuAG and somewhat smaller than that of Ce3+ ions in YAG. One can see that the thermal quenching of the Ce3+ emission in Ca3Sc2Si3O12 starts at essentially higher temperatures than that of Ce3+ emission in YAG. This observation is in line with the results on the Pr3+-doped garnets. Ivanovskikh et al. [7] have revealed that the decay time of the Pr3+ emission in Ca3Sc2Si3O12 remains practically unchanged up to 500 K, significant temperature quenching was observed above 600 K, whereas the onset of thermal quenching of Pr3+ emission was found to be at 150 and ~ 450 K for YAG and LuAG, respectively [23]. The mechanism for temperature quenching of Ce3+ emission in the most practically important series of compounds with the garnet structure is still under discussion in the literature [23–27]. In general, two mechanisms can cause the temperature quenching of the 5d→4f luminescence of Ce3+ in inorganic compounds: (i) quenching by thermally activated cross-over from the 5d1 excited stated to the 4f ground state and (ii) thermally activated photoionization from the 5d1 state to the conduction band. New experimental data supporting the view that temperature quenching of the Ce3+ emission in mixed Y3(1−x)Gd3xAl5O12 (YGdAG) garnets is caused by thermally induced ionization of the Ce3+ 5d1 excited state have been recently presented [25,27]. For example, by an analysis of thermoluminescence (TL) excitation spectra at 300–700 K for YAG:Ce3+ Ueda et al. have found out that there is a good correspondence between the temperature quenching of the Ce3+ emission in YAG and the appearance and rise of a TL signal in the 500–700 K range [25]. The obtained persistent luminescence excitation spectra of Ce3+-doped YGdAG crystals at 300–400 K imply that this mechanism also takes place for Y3(1−x)Gd3xAl5O12 (x = 0.50, 0.75) garnets [27]. In contrast, the recent measurements of temperature dependence of delayed radiative recombination indirectly indicate that the main mechanism responsible for the thermal quenching of the Ce3+ emission in mixed YGdAG single crystals is the thermally activated cross-over from the 5d1 excited stated
emission intensity and
The decay curves of the Ce3+ emission were recorded upon excitation at 450 nm in the temperature range from 300 to 700 K with the step of 50 K. All decay curves appeared to be close to single exponential with a time constant of ~ 71 ns. As an example, the decay curves measured at 300, 500 and 700 K are presented in Fig. 6. Depending upon the synthesis conditions, nominal Ce concentration etc. the reported decay times of the Ce3+ emission in Ca3Sc2Si3O12 at room temperature vary from 56 ns [11,16] to 77 ns [15]. This variation of the decay time may be partly explained by differences in the effective refractive index for polycrystalline samples obtained by various authors. From Fig. 6 it follows that the thermal quenching of the Ce3+ emission in Ca3Sc2Si3O12 can start at high temperatures (≥ 750 K). The room temperature decay curves of the Ce3+ emission upon excitation into three different 4f→5d1,2,3 bands are compared in Fig. 7. It is seen that the decay curves are similar and, in first approximation, can be characterized by a time constant of 71 ± 2 ns. For λexc = 238 nm, the decay curve has a higher constant background level than the other curves. Since the microsecond time scale emission associated with native defects of Ca3Sc2Si3O12 (λmax = 385 nm) is not excited at 238 nm [6], this may be connected with photoionization of Ce3+ ions and retrapping processes with the participation of shallow traps. Fig. 8 shows the temperature dependences of the integrated emission intensity and luminescence decay time of Ce3+ ions in Ca3(1−xy)Ce3xNa3ySc2Si3O12 (x = y = 0.01) in the range of 300–800 K. One can
Fig. 7. Room temperature decay curves of the Ce3+ emission (λem = 510 nm) for Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.01) recorded upon excitation at 238, 310, 410 nm.
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Table 1 Comparison of the luminescence properties of Ce3+ ions in Ca3Sc2Si3O12 and some compounds with the garnet structure. Compound
Еexc (5d1) (сm−1)
Еem (5d1) (сm−1)
ZPL (сm−1)
Stokes shift (сm−1)
Quenching onset (K)
Ref.
Y3Al5O12 Lu3Al5O12 Ca3Sc2Si3O12
21,716 22,217 22,272
18,904 19,877 19,960
20,424 21,137 21,133
2800 2380 2340
~ 600 ~ 700 ≥ 750
[23,24] [19,23] This work
to the 4f ground state [26]. Both the above-mentioned mechanisms were also used to explain the decrease in the integrated emission intensity of Ce3+ ions in Ca3Sc2Si3O12 with increasing temperature from 350 to 600 K [5,12,17]. However, as shown in Figs. 6 and 8, the observed luminescence intensity decrease is not accompanied by a decrease of the luminescence decay time, and therefore this effect is not due to cross-over or photoionization from the 5d1 state. The possible explanation is related to a temperature dependence of the absorption strength of 4f→5d transition for the lowest-energy sublevel of Ce3+ 5d configuration. A very similar situation was observed earlier for Ce3+-doped YAG single crystals [24]. With increasing temperature from 300 to 600 K, the absorption intensity at the Ce3+ absorption maximum (λmax = 460 nm) was found to decrease steadily to ~60% of its value at room temperature [24], whereas the decay time and quantum yield of the Ce3+ emission in YAG start to decrease at ca. 600 K [23–25]. To our knowledge, Ce3+-doped Ca3Sc2Si3O12 and LuAG exhibit the highest thermal stability of the emission among the known Ce3+-doped phosphors of garnet family. In recent years, the luminescence properties of numerous Ce3+-doped silicates with garnet structure, such as CaY2Al4SiO12 [28], CaLu2Mg2Si3O12 [5,12], Lu3(Al,Mg)2(Al,Si)3O12 [29], Y3MgAl2(BSi)O12 (B= Al, Sc, Ga) [30], have been studied and reported. It was found that a considerable broadening of Ce3+ emission bands and large Stokes shifts (3000–4300 cm−1) are characteristic for these garnets. This was attributed to the multisite nature of Ce3+ emission caused by the disorder on the dodecahedral and octahedral sites. The onset quenching temperatures for these phosphors were found to be lower by 200–400 K compared with those for Ce3+-doped Ca3Sc2Si3O12, LuAG [12,28,29]. In addition to the cross-over or photoionization from the 5d1 state [12,29], a thermally activated trapping of the excitation energy by complex defects involving antisite atoms and vacancies was also supposed to explain the lower thermal stability of these phosphors [28]. It can be excluded that a competition between different processes may determine temperature dependences of Ce3+ luminescence in such multicomponent phosphors.
Fig. 9. Comparison of the emission spectra of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.005) upon excitation at (a) 355 nm and (b) at 322 nm. The insets show the excitation spectra for the emission at (a) 450 nm and (b) 380 nm (T = 293 K).
that the excitation spectrum for the emission at 450 nm differs markedly from that of the Ce3+ emission in Ca3Sc2Si3O12. It consists of bands with maxima at 292 and 357 nm. Upon 322 nm excitation, the emission spectrum comprises a band with a maximum at 374 nm and a shoulder at 351 nm. The excitation spectrum of this emission contains bands at 252, 307 and 324 nm. In general, the shape and position of the excitation bands at 320–360 nm indicate that these features can belong to the Ce3+ ions. The decay curve of the emission at 410 nm recorded under excitation at 370 nm is presented in Fig. 10. The decay curve is seen to deviate significantly from a purely exponential one and it could not be well fitted to a sum of two exponential functions. However, the
3.4. Role of impurity phases As mentioned above, the excitation spectra for the Ce3+ emission in Ca3Sc2Si3O12 contain a weak band with a maximum at 357 nm (see Fig. 3). More or less intense bands in the 320–360 nm range are present in the luminescence excitation spectra of Ce3+ in Ca3Sc2Si3O12 reported by numerous authors [4–6,11,17]. Based on the first-principles calculations, Ding et al. [31] have supposed that an excitation band in the 320–360 nm range can be due to 4f→5d2 transition of Ce3+ ions, with Sc3+ occupying Si4+-positions in their nearest environment. A little later, such the features were attributed to Ce3+ ions located in the vicinity of point defects of Ca3Sc2Si3O12 [6]. In order to clarify the origin of the band at 357 nm, we compared the emission spectra at different excitation wavelengths from 320 to 370 nm. The typical spectra are shown in Fig. 9. The spectrum of Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.005) recorded upon excitation at 355 nm contains a broad band extending from 360 nm to 520 nm and peaking at about 406 nm. There is also a band with a maximum at 506 nm and a shoulder at 548 nm, which are evidently due to transitions from the lowest 5d excited state to the 4f ground state levels 2F5/2 and 2F7/2 of Ce3+ ions in Ca3Sc2Si3O12. From Figs. 3 and 9 one can see
Fig. 10. Decay curve of the Ce3+ emission in Ca3(1−x-y)Ce3xNa3ySc2Si3O12 (x = y = 0.005) at 410 nm recorded upon excitation at 370 nm.
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275–320 nm. It is important to note the obtained DR spectra of Cedoped Ca3Sc2Si3O12 in the 270–380 nm range differ markedly from those for nominally pure Ca3Sc2Si3O12 [11,16]. Therefore, it cannot be excluded that even at low nominal concentrations some amounts of Ce exist as CeO2. Also, several studies have revealed that for Eu2+ and Ce3+ co-doped Ca3Sc2Si3O12 the relative intensity of the high-energy Eu2+ 4f7→4f65d excitation band at 360 nm decreases with the increase of Ce concentration [6,10]. This observation was tentatively ascribed to photoionization of Eu2+ ions, electron trapping on Ce3+-related defects and subsequent non-radiative relaxation [10]. Later, based on comparison of the luminescence properties of Eu2+ and Ce3+ co-doped Ca3Sc2Si3O12 and reference samples with general composition Ca3Sc2Si3O12:Eu2+/CeO2, we came to the conclusion that this effect, at least in part, was caused by an increase of CeO2 content, which resulted in enhancement of competing absorption in the UV range. Finally, the presence of impurity phases (β-Ca2SiO4, CaSiO3, CeO2) can be the reason for competing absorption in the 250–380 nm range and, therefore, for variations in the luminescence excitation and diffuse-reflection spectra reported in the literature for Ce3+-doped Ca3Sc2Si3O12.
1/e decay time constant was found to be 12.5 ns, which is close to typical values for 5d→4f transitions of Ce3+ in inorganic compounds. The distinct nonexponentiality of the decay can be explained by the existence of several kinds of Ce3+-related centers responsible for the emission at 410 nm. This is in line with the lack of the characteristic doublet structure in the Ce3+ emission spectrum. The prominent feature of the Ce3+-related emission at 330–450 nm is that at room temperature it is effectively excited in the range of Ce3+ 4f→5d transitions, but not excited in the fundamental absorption region of Ca3Sc2Si3O12 (≥ 7.0 eV). The same conclusion can be made from the luminescence excitation spectra of Ce3+-doped Ca3Sc2Si3O12 at 15 K reported by Zhou et al. [6]. Since there is some analogy between the observations made above and the results reported in the literature for some nanoand microscale Ce3+-containing luminescent species dispersed in different inorganic matrices (see, for example, [26,32]), we came to the conclusion that, at least in the case of the samples studied in this work, the observed excitation bands at 320–360 nm are not due to irregular Ce3+ centers in Ca3Sc2Si3O12, but come from Ce3+ ions in impurity phases. Note that the Ce3+ emission in Ca3Sc2Si3O12 might partly arise due to radiative energy transfer from impurity phase inclusions to Ce3+ in the garnet phase. As mentioned in Section 3.1, the XRD patterns of the samples revealed, in addition to Ca3Sc2Si3O12, the presence of small amounts of Sc2O3. This implies that some amounts of calcium silicates, such as CaSiO3, β-Ca2SiO4, Ca3SiO5, can be also present in the samples under study, and their appearance as impurity phases in Ca3Sc2Si3O12 has been really detected by several authors [2,16]. Comparison with literature data on luminescence properties of Ce3+ in different calcium silicates indicates that the most probable candidates are β-Ca2SiO4 and CaSiO3. It is known that the emission spectra of Ce3+ ions in β-Ca2SiO4 are strongly dependent on the excitation wavelength due to the presence of two non-equivalent sites for Ca atoms in the lattice. Upon excitation at 355 nm, Ce3+ ions in β-Ca2SiO4 exhibit a broadband emission with a maximum at about 405 nm. Its excitation spectrum contains bands at 292 and 356 nm [33]. The emission spectrum of Ce3+ ions in CaSiO3 consists of a band with a maximum at about 350 nm and a shoulder at 380 nm. The broad band at ~ 320 nm was found to be dominant in the excitation spectra for this emission [34]. Thus, the emission and luminescence excitation spectra of Ce3+ ions in β-Ca2SiO4 and CaSiO3 are very similar to those presented in Fig. 9. In the context of influence of possible impurity phases, the role of CeO2 should be also mentioned. By means of Ce LIII-edge X-ray absorption near-edge structure (XANES) method Shimomura et al. [2] have analyzed the chemical composition of Ce-doped Ca3Sc2Si3O12 prepared in a reducing medium (4%H2/96%N2). The obtained XANES spectra did not reveal any amounts of Ce4+ in the samples under study. In contrast, the X-ray photoelectron spectroscopy data for Ce-doped Ca3Sc2Si3O12 samples prepared in air by the freeze-drying precursor method [16] indicated that a large fraction (up to 60%) of the cerium ions is stabilized in the oxidation state +4. Probably, most of them exist as CeO2. With a few exceptions, in the above-mentioned studies on Cedoped Ca3Sc2Si3O12 phosphors, no presence of CeO2 was revealed by XRD analysis. Perhaps, concentrations of this impurity phase in the materials studied were too low to be detected by this method. Taking into account that CeO2 exhibits a strong absorption below 400 nm with a diffuse maximum at 290–320 nm, which is due to a charge transfer from the fully occupied 2p O orbitals to the empty 4f orbitals of Ce4+ [35], diffuse-reflection (DR) spectroscopy seems to be more sensitive to its presence as a separate phase than XRD method. Several groups of authors have reported DR spectra of Ce-doped Ca3Sc2Si3O12 prepared by different methods [11,16,17]. Below 380 nm, the spectra were found to be very complex and difficult to interpret, due to overlap of the Ce3+ absorption band at 308 nm and various bands associated with point defects and, probably, impurity phases. In spite of the fact that there are distinct differences between the spectra presented, one can conclude that the reflectance of Ce-doped Ca3Sc2Si3O12 phosphors decreases sharply below 380 nm and approaches to a local minimum at
4. Conclusions Ce3+-doped Ca3Sc2Si3O12 is still one of the most promising phosphors for white LEDs. In the present work, the luminescent properties of Ce3+ ions in Ca3Sc2Si3O12 were studied in the temperature range of 77–800 K. It is shown that at 77 K zero-phonon lines of 5d↔4f transitions and vibronic structure are distinctly observed in the Ce3+ emission and excitation spectra. The decay time of the Ce3+ emission in Ca3Sc2Si3O12 was found to be nearly constant (~ 71 ns) up to 700 K. This indicates that the thermal quenching of the Ce3+ emission in Ca3Sc2Si3O12 starts at higher temperature (≥ 750 K) than that of the Ce3+ emission in YAG. The decrease of the Ce3+ integrated emission intensity (upon direct excitation of the lowest-energy Ce3+ 5d1 state) with increasing temperature in the range 350–700 K, often attributed to the thermal quenching, probably arises from a temperature dependence of the absorption strength of 4f→5d transition for the lowest-energy sublevel of Ce3+ 5d configuration. It is also shown that typical impurity phases (β-Ca2SiO4, CaSiO3, CeO2) can cause competing extra absorption in the 250–380 nm range and they are expected to be responsible for differences in the luminescence excitation and diffuse-reflection spectra reported in the literature for Ce3+-doped Ca3Sc2Si3O12. One can hope that an improvement in powder preparation techniques can promote enhancement in the luminescence characteristics of Ca3Sc2Si3O12:Ce3+ phosphors for white LEDs. References [1] Z. Xia, Q. Liu, Prog. Mater. Sci. 84 (2016) 59–117. [2] Y. Shimomura, T. Honma, M. Shigeiwa, T. Akai, K. Okamoto, N. Kijima, J. Electrochem. Soc. 154 (2007) J35–J38. [3] B.V. Mill, E.I. Belokoneva, M.A. Simonov, N.V. Belov, J. Struct. Chem. 18 (1977) 321. [4] Y. Shimomura, T. Kurushima, M. Shigeiwa, N. Kijima, J. Electrochem. Soc. 155 (2008) J45–J49. [5] M.S. Kishore, N.P. Kumar, R.G. Chandran, A.A. Setlur, Electrochem. Solid-State Lett. 13 (2010) J77–J80. [6] L. Zhou, W. Zhou, F. Pan, R. Shi, L. Huang, H. Liang, P. Tanner, X. Du, Y. Huang, Y. Tao, L. Zheng, Chem. Mater. 28 (2016) 2834–2843. [7] K.V. Ivanovskikh, A. Meijerink, F. Piccinelli, A. Speghini, E.I. Zinin, C. Ronda, M. Bettinelli, J. Lumin. 130 (2010) 893–901. [8] M. Bettinelli, A. Speghini, F. Piccinelli, A.N.C. Neto, O.L. Malta, J. Lumin. 131 (2011) 1026–1028. [9] I.V. Berezovskaya, V.P. Dotsenko, A.S. Voloshinovskii, S.S. Smola, Chem. Phys. Lett. 585 (2013) 11–14. [10] V.P. Dotsenko, I.V. Berezovskaya, I.V. Zatovsky, S.S. Smola, N.P. Efryushina, J. Nano, Electron. Phys. 7 (2015) 02041. [11] Y. Liu, X. Zhang, Z. Hao, X. Wang, J. Zhang, J. Mater. Chem. 21 (2011) 6354–6358. [12] Y. Liu, X. Zhang, Z. Hao, Y. Luo, X. Wang, J. Zhang, J. Lumin. 132 (2012) 1257–1260. [13] G. Zimmerer, Radiat. Meas. 42 (2007) 859–864. [14] Y. Suzuki, M. Kakihana, Y. Shimomura, N. Kijima, J. Mater. Sci. 43 (2008)
29
Journal of Luminescence 195 (2018) 24–30
I.V. Berezovskaya et al.
[25] J. Ueda, P. Dorenbos, A.J.J. Bos, A. Meijerink, S. Tanabe, J. Phys. Chem. C 119 (2015) 25003–25008. [26] K. Bartosiewicz, V. Babin, K. Kamada, A. Yoshikawa, J.A. Mares, A. Beitlerova, M. Nikl, Opt. Mater. 63 (2017) 134–142. [27] J. Ueda, M. Yagi, S. Tanabe, ECS J. Solid State Sci. Technol. 5 (2016) R219–R222. [28] A. Katelnikovas, S. Sakirzanovas, D. Dutczak, J. Plewa, D. Enseling, H. Winkler, A. Kareiva, T. Jüstel, J. Lumin. 136 (2013) 17–25. [29] H. Ji, L. Wang, M.S. Molokeev, N. Hirosaki, R. Xie, Z. Huang, Z. Xia, O.M. ten Kate, L. Liu, V.V. Atuchin, J. Mater. Chem. C 4 (2016) 6855–6863. [30] N.M. Khaidukov, V.N. Makhov, Q. Zhang, R. Shi, H. Liang, Dyes Pigments 142 (2017) 524–529. [31] W. Ding, J. Wen, J. Cheng, L. Ning, Y.-C. Huang, C.-K. Duan, M. Yin, Chin. J. Chem. Phys. 28 (2015) 150–154. [32] V.V. Vistovskyy, P.V. Savchyn, G.B. Stryganyuk, A.S. Voloshinovskii, M.S. Pidzyrailo, J. Phys.: Condens. Matter 20 (2008) 325218. [33] Y. Liu, Q. Fang, L. Ning, Y. Huang, S. Huang, H. Liang, Opt. Mater. 44 (2015) 67–72. [34] S. Ye, X.-M. Wang, X.-P. Jing, J. Electrochem. Soc. 155 (2008) J143–J147. [35] S. Mochizuki, F. Fujishiro, Phys. Status Solidi B 246 (2009) 2320–2328.
2213–2216. [15] Y.-F. Wu, Y.-H. Chan, Y.-T. Nien, I.-G. Chen, J. Am. Ceram. Soc. 96 (2013) 234–240. [16] R. Fernandez-Gonzalez, J.J. Velazquez, V.D. Rodriguez, F. Rivera-Lopez, A. Lukowiak, A. Chiasera, M. Ferrari, R.R. Goncalves, J. Marrero-Jerez, F. Lahoz, P. Nūňez, RSC Adv. 6 (2016) 15054–15061. [17] Y. Chen, J. Li, S. Zeng, H. Fan, J. Feng, L. Tan, Opt. Mater. 37 (2014) 464–469. [18] I. van Pieterson, M.F. Reid, R.T. Wegh, S. Soverna, A. Meijerink, Phys. Rev. B 65 (2002) 045113. [19] J.M. Ogieglo, A. Zych, K.V. Ivanovskikh, T. Jüstel, C.R. Ronda, A. Meijerink, J. Phys. Chem. A 116 (2012) 8464–8474. [20] S. Feofilov, A. Kulinkin, K. Ovanesyan, A. Petrosyan, C. Dujardin, Phys. Chem. Chem. Phys. 16 (2014) 22583–22587. [21] D.J. Robbins, J. Electrochem. Soc. 126 (1979) 1550–1555. [22] V.P. Dotsenko, I.V. Berezovskaya, A.S. Voloshinovskii, B.I. Zadneprovski, N.P. Efryushina, Mater. Res. Bull. 64 (2015) 151–155. [23] K.V. Ivanovskikh, J.M. Ogieglo, A. Zych, C. Ronda, A. Meijerink, ECS J. Solid State Sci. Technol. 2 (2013) R3148–R3152. [24] S. Arjoca, E.G. Villora, D. Inomata, K. Aoki, Y. Sugahara, K. Shimamura, Mater. Res. Express 2 (2015) 0555503.
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