Ce3+-sensitized red Mn2+ luminescence in calcium aluminoborate phosphor material

Ce3+-sensitized red Mn2+ luminescence in calcium aluminoborate phosphor material

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Optical Materials xxx (2017) 1e10

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Optical Materials journal homepage: www.elsevier.com/locate/optmat

Ce3þ-sensitized red Mn2þ luminescence in calcium aluminoborate phosphor material M. Puchalska*, E. Zych Faculty of Chemistry, University of Wrocław, 14. F. Joliot-Curie Street, 50-383 Wrocław, Poland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 January 2017 Received in revised form 28 May 2017 Accepted 30 May 2017 Available online xxx

Ce3þ doped and Ce3þ,Mn2þ co-doped calcium aluminoborate (CAB) phosphors were synthesised by solid-state reaction method and their optical properties were studied. X-ray powder diffraction, SEM and TEM studies indicated the crystallization of the main trigonal CaAl2B2O7 phase and the presence of an additional non-crystalline phase. It was also observed that increasing dopant concentration promotes phase separation. Hence, both series of phosphors demonstrated the changes in luminescence properties via activator concentration variation. Upon UV excitation (lex ¼ 310 nm) Ce3þ doped and Ce3þ,Mn2þ codoped materials yielded intensive blue and pinkish luminescence, respectively. The spectra of CAB:Ce3þ samples showed a broad emission band due to 5d/4f transition of Ce3þ, which broadened and shifted to longer wavelengths with increasing dopant content. Mn2þ co-doping caused appearance of another broad-band emission with a maximum of 680 nm, resulting from the 4T1(4G) /6A1(6S) transition of Mn2þ. Detailed analysis of the emission and excitation spectra as well as decay time traces as a function of dopant concentration showed that efficient resonant energy transfer mainly occurs between Ce3þ and Mn2þ incorporated in the non-crystalline phase in CAB material. The estimated values of energy transfer efficiency of CAB:Ce3þ(3%),Mn2þ(4%) is close to 52%. © 2017 Elsevier B.V. All rights reserved.

Keywords: Calcium aluminoborate Ce3þ luminescence Mn2þ luminescence Energy transfer Phase separation

1. Introduction Mn(II) having a 3d5 electronic configuration is widely used as luminescent centers in many phosphors materials. It exhibits a broad-band emission (4T1/6A1) in wide spectral region from 470 to 750 nm depending on the crystal field of the host [1,2]. For this reason its luminescence may be useful in lighting devices, in particular for white light generation. However, direct excitation of Mn2þ ions is not very effective due to low absorption cross-section of the Mn2þ transitions from 6A1(6S) ground state to the excited quartet and doublet states, which are spin and parity forbidden [1,2]. Therefore it has to be sensitized by energy transfer from Ce3þ/ Eu2þ to Mn2þ [3e30]. Using Ce3þ/Eu2þ as a sensitizer is attractive due to their broad excitation bands in the UV and/or blue region corresponding to the spin and parity allowed 4f-5d interconfigurational electric-dipole transition and broad luminescence band, which is commonly resonant with Mn2þ excitation [1,2]. €rster-type energy transfer even for quite large Consequently, the Fo

* Corresponding author. E-mail address: [email protected] (M. Puchalska).

distances is possible [31]. Kim at all first reported that Me3MgSiO2O8:Eu2þ, Mn2þ (Me ¼ Ba, Sr) could be applied as a singlephase full color phosphors for fabrication of a warm white-light emitting diode [16,17]. The Ce3þ-sensitized Mn2þ luminescence has been studied in quite a few bulk materials including glasses [10,18], fluorides [19,20], silicates [4,5,8,9,21,25], phosphates [3,6,7,26], alkaline earth aluminates [11,13,27,28] as well as nanoparticles [29] and thin films [30]. These studies have shown that Ce3þ,Mn2þ co-doping can lead to luminescence tunable within a broad spectral range from blue, through green to red. Clearly, this can be useful in pcWLEDs. Another interesting and original approach was presented recently in Ref. [32]. There, Bi3þ luminescence could be tuned over the whole range of visible radiation using pure or mixed YNbO4-YVO4-ScVO4 hosts. The effect of the change of crystal field allowed to position the Bi3þ emission peak in the range of 450e650 nm. Till now, only scant studies have been performed on RE-doped calcium aluminoborate CaAl2B2O7. Blue luminescence was reported in CaAl2B2O7 powders activated with Eu2þ ions [33,34]. The long-lasting blue Eu2þ emission was investigated in Eu3þ,Nd3þ codoped calcium aluminoborate glass ceramic with CaAl2B2O7 being the crystalline, optically active phase [35]. The studies of singly

http://dx.doi.org/10.1016/j.optmat.2017.05.060 0925-3467/© 2017 Elsevier B.V. All rights reserved.

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(Tb3þ) and doubly (Ce3þ,Tb3þ) doped CaAl2B2O7 have shown enhanced green Tb3þ luminescence due to efficient energy transfer from Ce3þ to Tb3þ [36,37]. In this paper we present results of research on luminescence properties of Ce3þ doped and Ce3þ,Mn2þ co-doped CAB. The energy transfer from Ce3þ to Mn2þ is studied and discussed. To the best of our knowledge there has been no research reported regarding the Ce3þ-sensitized Mn2þ luminescence in CAB. 2. Experimental The aim of the experiment was to synthesize a single phase of the trigonal CaAl2B2O7. To obtain samples of composition: Ca1xCexAl2B2O7 (x ¼ 0.001e0.07) and Ca0.97-xCe0.3MnxAl2B2O7 (x ¼ 0.01e0.05) the conventional solid-state reaction technique was applied. Stoichiometric amounts of Al2O3 (99.8%, Aldrich), CaCO3 (99,99%, Alfa Aesar), CeO2 (99,99%, Stanford Materials) and MnCO3 (99,9% Sigma Aldrich) and an excess (3%) H3BO3 (99%, POCH 99.8% ROTH) were thoroughly mixed by grinding them in an agate mortar. The powders were then loaded into graphite crucibles, and subsequently fired in a vacuum at 1000  C for 8 h. Properties of the powders were examined by several methods. X-ray diffraction (XRD) analysis of the powders was completed using a Bruker D8 Advance Diffractometer, with Nickel-filtered Cu Ka1 radiation (l ¼ 1.540596 Å). The diffractograms were performed for 2q ranging from 10 to 80 and with 0.016 step. The morphology of the specimens was examined with an FEI Tecnai G2 20 X-TWIN transmission electron microscope (TEM) and a Hitachi S-3400N scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) analyzer. EDS measurements were performed using Noran System7 analyzer equipped with Thermo Scientific Ultra Dry detector. Electron paramagnetic resonance (EPR) spectra were recorded using a Bruker Elexsys 500 spectrometer operating at the X-band frequency (e9.7 GHz). A microwave power of 10 mW, modulation amplitude of 5 G, a sweep width of 1000 G, a time constant of 40 ms and conversion time of 122 were adopted. The central field was set to 3500 G. An analysis of the EPR spectra was carried out using the WinEPR software package, version 1.26b. Corrected excitation and emission spectra were carried out at room (298 K) and low temperatures (298 K - 13 K) using a FLS980 spectrofluorimeter (Edinburgh Instruments Ltd.) equipped with a 450 W Xenon lamp excitation source and a red-sensitive photomultiplier (Hamamatsu R-928P) operating within 200e870 nm. Low temperature measurements were accomplished by using an ARS cryostat that was optically coupled with the spectrometer. For the measurements of decay traces of Mn2þ and Ce3þ luminescences a microsecond Xenon flash lamp (mF2) and nanoseconds lamp (nF920) were used respectively. 3. Results and discussion 3.1. Structural characterization Fig. 1 shows the measured diffractograms for selected compositions of Ce3þ doped and Ce3þ,Mn2þ co-doped CAB. The XRD patterns of the obtained powders exhibit similar profiles and confirm formation of trigonal CaAl2B2O7 (PDF#00-057-0887) targeted in the synthesis procedure. CaAl2B2O7 crystalizes in R3c(h) space group with unit-cell parameters a ¼ 4.810(6) Å, c ¼ 46,633(5) Å and V ¼ 934(1) Å3 [38]. All Ca, Al and B atoms occupy single crystallographic sites. The Ca2þ ion is coordinated by six O atoms at the vertices of a distorted octahedron with S6 symmetry site. All CaO bonds have identical lengths (2.381 Å) but the O1-Ca-O1 bond angles deviate from 90 by a few degrees, which is consistent with a

Fig. 1. XRD patterns of the CAB:Ce3þ(3%) and CAB:Ce3þ(3%),Mn2þ(2%) samples.

small trigonal elongation of the octahedron. The Ca2þ ion is bonded to BO3 trigonal groups and distorted AlO4 tetrahedrals exhibiting C3 point-group symmetry [38]. It is expected that Ce3þ substitutes Ca2þ, because ionic radii of both ions are almost the same (for C.N. ¼ 6 r ¼ 1.00 Å and 1.01 Å for Ca2þ and Ce3þ, respectively [39]). However, Mn2þ (r ¼ 0.66 Å for C.N ¼ 4, r ¼ 0.67 Å for C.N. ¼ 6 [39]) could replace both Ca2þ and Al3þ ions. Furthermore, if Mn2þ replaces Al3þ and Ce3þ replaces Ca2þ, charge balance would be achieved in system. Thus, at least partial substitution of Al3þ by Mn2þ is likely. 3.2. SEM and TEM characterization The powder morphology and crystallite formation of the phosphors were further characterized by a SEM. Fig. 2 shows SEM images for CAB materials containing 0.2%, 3% and 7% of Ce3þ. Surprisingly, the obtained results show that the studied samples are not phase-pure. Two different phases (grey and white) can be easily observed. Moreover, these two phases are fused together. The grains formed are irregular in shape and their size varies from 5 to 50 mm. It is also clear that phase separation becomes more evident with increasing activator concentration. SEM image of 0.2%-doped sample reveals a few brighter areas, whilst in the case of 7%-doped CAB we can clearly distinguish a greater amount of white regions. These results together with XRD data suggest that the grey phase corresponds to trigonal CaAl2B2O7 targeted in the synthesis procedure. Taking into account that higher activator doping does not cause disappearance or emergence of new peaks in XRD patterns, it can be assumed that additional phase (white) is non-crystalline. The distribution and concentrations of B, O, Al, Ca and Ce in the samples were performed through EDS. These studies showed significantly different elemental compositions in both phases. Obtained results for CAB:Ce3þ(7%) together with respective SEM micrograph are presented in Fig. 3. It is clear from presented data that Ca/Al ratio as well as the concentration of dopant ions are severely larger in the second phase. Figs. 4 and 5 showing EDS elemental mapping of CAB:Ce3þ(7%) (Al, Ce) as well as CAB:Ce3þ(3%), Mn2þ(4%) (Al, Ce, Mn) reveal the differences very well in chemical compositions of both phases. It can easily be seen that the impurity phase contains a significantly smaller amount of aluminum and a much higher content of activator ions (Ce, Mn) compared to the main phase. To verify if the impurity phase (white) is indeed non-crystalline, electron diffraction was undertaken. Fig. 6a and b shows the high resolution TEM images of CAB:Ce3þ(3%),Mn2þ(4%) together with the results of elemental analysis and the selected area electron

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Fig. 2. SEM micrographs of CAB:Ce3þ(0.2%) (A), CAB:Ce3þ(3%) (B), CAB:Ce3þ(7%) (C, D).

Fig. 3. SEM micrograph of CAB:Ce3þ(7%) and the result of elemental analysis (Weight %).

diffraction (SAED) patterns. The diffraction spots are seen only in the SAED pattern presented in Fig. 6a. This indicates that the main (grey) phase with smaller Ca/Al ratio and very low dopant content is crystalline, whilst impurity phase (white) with larger Ca/Al ratio and much higher dopant content is non-crystalline.

3.3. Spectroscopic studies 3.3.1. Photoluminescence of Ce3þ doped CAB The photoluminescence properties as a function of Ce3þ content in CAB:Ce3þ(x%) (x ¼ 0.2e7%) powders were studied at 298 K. The

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Fig. 4. EDX elemental mapping of CAB:Ce3þ(7%) (Al and Ce).

Fig. 5. EDX elemental mapping of CAB:Ce3þ(3%),Mn2þ(4%) (Al, Mn and Ce).

recorded emission and excitation spectra are shown in Figs. 7 and 8a, respectively. In general, luminescence of all samples can be efficiently excited in the UV spectral range (240e370 nm) and the emission spectral position is slightly concentration-dependent appearing within about 350e550 nm with peak moving from about 365 nm (0.2%) to 415 nm (7%). The emission spectra consist of a broad band corresponding to the parity allowed 5d1/4f1 transition of the Ce3þ ions. Its intensity increases with the increasing Ce3þ concentration and shows a maximum at 2e3% (see inset in Fig. 7). For lower activator concentrations, 0.2e1%, we can

distinguish two components in emission band located at 372 nm and 402 nm, and resulting from the 5d1/2F5/2,2F7/2 transitions of the Ce3þ. The energy separation between them meets the typical energy of spin-orbit splitting (about 2200 cm1) of the two states of 4f1 configuration of Ce3þ (2F5/2 and 2F7/2). With increasing dopant concentration the emission band broadens and shifts to longer wavelengths, similarly to what was observed for Sr2B5O9X:Ce3þ (x ¼ Cl, Br) and Sr2AlO4F:Ce3þ [40,41]. At the same time the doublet becomes practically invisible above the concentration of 1%. Presumably, this is associated with phase separation, which becomes

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Fig. 6. a, b TEM image of CAB: Ce3þ(3%), Mn2þ(4%), the result of elemental analysis and selected area electron diffraction (SAED) of CAB: Ce3þ(3%), Mn2þ(4%).

more and more evident with increasing dopant concentration. The luminescence of 0.2% - doped CAB results mainly from Ce3þ ions incorporated into trigonal CaAl2B2O7. As shown by SEM studies, with the increase in dopant content increasingly more Ce3þ ions are introduced into the second (non-crystalline) phase. Consequently, an increasing variety of Ce3þ sites with different symmetry of their local environment appear and the observed luminescence band reflects these effects. Comparison of the excitation spectra of the samples also confirms this conclusion (see Fig. 8a). For the lowest activator concentration (0.2%) the components located in the range of longer wavelengths at 330 and 350 nm are strongly dominant. As the Ce3þ content increases the components at shorter wavelengths get more intensive, showing the change of relative population of different Ce3þ sites in CAB as well as energy transfer between the Ce3þ ions of slightly different symmetry of their local environment. Consequently, when the Ce content increases the luminescence is generated by Ce3þ ions emitting at slightly different energies and

their luminescence is superimposed within one emission band [1]. To further trace this effect the excitation spectra of the sample containing the lowest Ce3þ concentration (0.2%) monitoring the emissions at 370 nm, 400 nm, 445 and 475 nm were recorded (Fig. 8b). These spectra show noticeable differences in the relative intensities of the excitation components. Monitoring the 5d1/4f1 Ce3þ emission at shorter wavelengths, 370 and 400 nm, bands A (350 nm) and B (325 nm) shows the highest intensities. Yet, in the excitation spectra of luminescence monitored at longer wavelengths of 445 nm and 475 nm, the band A almost disappears, while the band C (310 nm) and D (270 nm) become much more intensive and the former becomes as intense as the band B. These results encouraged us to measure and compare the emission spectra at room and low (12 K) temperatures under different excitation wavelengths for the 0.2% sample. The results are given in Fig. 9. As expected, the obtained spectra differ noticeably, proving again the co-existence of Ce3þ ions with different environment symmetries in the studied CAB material,

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dopant concentration. At moderate Ce contents, when concentration quenching does not yet occur and the energy migration is already present the measured decay time reflects the elongated time between excitation and emission due to the energy migration. At higher concentrations this effect coexists with Ce emission concentration quenching and the decay time gets shorter [42].

Fig. 7. Normalized emission spectra of CAB:Ce3þ(x%) under 310 nm excitation at 298 K. The inset presents the dependence of integrated intensity of Ce3þ luminescence band versus concentration of active ion.

even when the dopant content is relatively low. We have also studied the lifetime behavior of the Ce3þ as a function of activator concentration. Upon excitation at 310 nm, the 5d1/4f1 luminescence decay curves at 410 nm were recorded for all Ce3þ doped CAB powders. Some of these are presented in Fig. 10. It was found that only for the lowest dopant concentration (0.2%) experimental data follow a single exponential dependence with lifetime value close to 30.5 ns. Then, with increasing Ce3þ content the decay profiles become more and more non-exponential, though the disparity is not strong. Observed effect may mainly result from changing with concentration distribution of activator ions between the main and the impurity phases. Thus, biexponential (1e3% Ce3þ) or three-exponential (3e7% Ce3þ) functions had to be used in the fitting procedure and obtained luminescence lifetimes are presented in Table 1. As can be seen, the average luminescence decay t slightly increases with increasing Ce3þ content up to 34.5 ns (4%) and at yet higher Ce3þ concentrations it gets reduced to 30.0 ns (7%). This is in line with the already concluded increasing energy transfer between Ce ions of different symmetries with increasing

3.3.2. Photoluminescence of Ce3þ,Mn2þ doped CAB7 The spectroscopic properties of the series of CAB:Ce(3%), Mn(x %) (x ¼ 0e5) were studied at 298 K. The presence of the divalent manganese ion was confirmed by an EPR measurement. The room temperature EPR spectrum of CAB:Ce(3%), Mn(3%) presented in Fig. 11 shows a six-line hyperfine signal centered at g ¼ 2.006 characteristic for Mn2þ. By fixing the Ce3þ concentration at 3%, the effect of increasing Mn2þ content on luminescence properties was observed. It was found (see Fig. 12 a) that upon excitation into the Ce3þ absorption band (310 nm), with increasing Mn content the blue-violet luminescence of Ce3þ got complemented with a deep red (partially infrared) broad band emission characteristic for Mn2þ (4T1(4G)/6A1(6S)). Above 4% of Mn its luminescence got strongly quenched. Thus, the CAB:Ce, Mn materials, upon excitation into the efficient f/d Ce absorption, produced two luminescent bands, each of which was in significant fraction located outside the visible part of spectrum. Consequently, this composition does not seem to be useful for white light generation. However, the observed deep red emission of Mn2þ could be used to enrich the red component in white LEDs. It is well known that the energy of the 4T1(4G) state depends strongly on the Mn2þ bonding environment which consequently results in different emission energies [1,2]. A weaker crystal field, as is typical for tetrahedral coordination, leads to green luminescence, while a stronger crystal field of octahedral coordination makes orange or red luminescence more probable [1,2]. Already the luminescence spectra make it clear that the excited Ce3þ transfers a fraction of its energy to Mn2þ. Let us yet note that the shapes and positions of emission features change with Mn2þ content. Namely, with increasing Mn concentration the Ce3þrelated luminescence (~400 nm) gets continuously blue-shifted from ~410 nm (0% Mn) to 385 nm (4% Mn) and its shape indicates that the long-wavelength fraction got reduced. At the same time, the Mn2þ luminescence band gets red-shifted though this

Fig. 8. Normalized excitation spectra of CAB:Ce3þ(x%) recorded monitoring Ce3þ emission at 410 nm at 298 K (a), normalized excitation spectra of CAB:Ce3þ(0.2%) recorded monitoring Ce3þ emission at different wavelengths at 298 K (b).

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Fig. 9. Normalized emission spectra of CAB:Ce3þ(0.2%) under different wavelength excitation at 298 K and 12 K.

Fig. 10. Ce3þ luminescence decay curves of CAB:Ce3þ samples excited at 310 nm and monitored at 410 nm. (T ¼ 298 K).

Table 1 Ce3þ luminescence decay times and their contributions of CAB:Ce3þ(x%) and CAB:Ce3þ(3%), Mn2þ(x%) samples excited at 310 nm and monitored at 410 nm. Sample

t1

A1

(ns) CAB:Ce3þ(x%) 0.2% 30.48 0.5% 9.240 0.034 1% 11.46 0.035 2% 8.97 0.042 3% 13.71 0.033 4% 8.53 0.040 5% 5.37 0.035 7% 4.50 0.04 3þ 2þ CAB:Ce (3%),Mn (x%) 1% 3.62 0.06 2% 3.29 0.059 3% 2.86 0.069 4% 2.55 0.073

t2

A2

(ns)

t3 (ns)

< t> (ns)

A3

34.92 37.73 37.37 41.20 29.16 25.07 21.43

0.042 0.036 0.035 0.038 0.032 0.037 0.034

50.25 51.44 47.51

0.013 0.012 0.013

30.48 30.38 31.74 31.02 32.77 34.47 32.33 29.98

17.22 15.66 14.11 12.38

0.031 0.03 0.028 0.027

40.05 38.14 36.73 33.82

0.012 0.008 0.007 0.006

23.72 20.26 18.34 15.86

hT (%)

27.70 38.16 44.10 51.60

Fig. 11. EPR spectrum of CAB:Ce3þ(3%), Mn2þ(3%) at 298 K.

effect is the most profound when Mn content changes from 1% to 2%. Thus, these data give a consistent picture with a preferential Ce/Mn energy transfer occurring for Ce3þ ions emitting and absorbing at longer wavelengths. The Ce3þ dopants which give luminescence at shorter wavelengths appear less prone to share their energy with the Mn co-dopant. This results suggest that energy transfer process is efficient mainly between Ce3þ ions (donor) and Mn2þ ions (acceptor) incorporated into non-crystalline phase. This conclusion is further supported by the excitation spectra of Mn2þ (lem ¼ 680 nm), which are more intensive at shorter wavelengths range when compared to excitation spectra of Ce3þ emission (lem ¼ 410 nm) (Fig. 13). This was already presented as characteristic for Ce3þ ions emitting at the longer wavelengths, see Figs. 8b and 9. The excitation spectrum of Mn2þ luminescence contains also a low intensity component located around 420 nm which can be ascribed to the 6A1(6S) / 4A1(G), 4E(G), 4T2(G), 4T1(4G) transitions of Mn2þ. They are parity and spin forbidden so their efficiencies are very low and thus not suitable for direct excitation in LEDs. The coincidence

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Fig. 12. Normalized emission spectra of CAB:Ce3þ(3%),Mn2þ(0e4%) under 310 nm excitation at 298 K (a), Ce3þ luminescence decay curves of CAB:Ce3þ(3%),Mn2þ(0e4%) samples excited at 310 nm and monitored at 410 nm at 298 K (b), Mn2þ luminescence decay curves of CAB:Ce3þ(3%),Mn2þ(0e4%) samples excited at 310 nm and monitored at 680 nm at 298 K (c).

hT ¼ 1 

Fig. 13. Excitation spectra of CAB:Ce3þ(3%),Mn2þ(2%) recorded monitoring Ce3þ emission (410 nm) and Mn2þ emission (680 nm) at 298 K.

of this band and the Ce3þ 5d1/4f1 emission is most effective for the long-wavelength part of the overall Ce luminescence what makes the just described preferential energy transfer understandable. To better recognize the energy transfer from Ce3þ to Mn2þ the decay traces of the Ce3þ luminescence for materials with different Mn2þ concentrations were measured and are presented in Fig. 12 b. The lifetimes of Ce3þ luminescence were determined with bi- or three-exponential decay profile and obtained values are presented in Table 1. Clearly, Mn drains the energy from the excited Ce3þ ions and produces luminescence at the expense of the latter. This effect is more efficient for higher Mn concentrations as then the average Ce3þ-Mn2þ distance gets reduced and more energy-accepting ions of Mn2þ participate in the process. Efficiency of the energy transfer (hT) was determined using equation (1):

ts tso

(1)

where tS and tS0 are the lifetimes of Ce3þ luminescence in the presence and absence of Mn2þ [1]. Table 1 presents the calculated values of hT. As shown, for CAB:Ce3þ(3%), Mn2þ(4%) hT ¼ 51.6%. The lifetime of the Mn2þ ions photoluminescence as a function of dopant concentration was also investigated by measuring the decay traces of the 680 nm emission band (see Fig. 12 c). The obtained curves are strongly not monoexponential (boat-shaped). As shown in Fig. 12 c and Table 2 the red Mn2þ emission decays faster with increasing dopant content and the time constant changes from 7.9 ms for 1% sample to 5.6 ms when Mn content is 4%. These observations are indicative of strongly diverse Mn2þ site distribution, which is typical for non-crystalline materials. Furthermore, for CAB:Ce3þ(3%), Mn2þ(2%) sample the luminescence and lifetime of Mn2þ measurements were performed at temperature range of 13e300 K. Fig. 14 presents the obtained normalized emission spectra and thermal dependence of an integrated intensity of both Ce3þ and Mn2þ luminescences (inset a in Fig. 14). It appears that the red (Mn2þ) luminescence is much more temperature dependent than the blue (Ce3þ) luminescence in the composition. Consequently, the relative intensity of the Mn2þ red luminescence decreases with increasing temperature related to the

Table 2 Mn2þ luminescence decay times of CAB:Ce3þ(3%), Mn2þ(x%) samples excited at 310 nm and monitored at 680 nm. CAB:Ce3þ(3%), Mn2þ(x%)

< t> (ms) 298 K

1% 2% 3% 4%

7.89 6.91 6.22 5.64

150 K

80 K

50 K

13.5 K

9.64

10.42

10.64

10.90

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Fig. 14. The thermal dependence of emission spectra of CAB:Ce3þ(3%),Mn2þ(2%) under excitation at 310 nm. The inset (a) presents the thermal dependence of integrated intensity of Ce3þ and Mn2þ luminescence and inset (b) presents the thermal dependence of Mn2þ luminescence decay curves excited at 310 nm and monitored at 680 nm.

blue Ce3þ emission. For the lowest temperature (13.5 K) the ratio I(Mn2þ)/I(Ce3þ) is close to 3 but at room temperature it is 1.2. Thus, it can be concluded that thermal quenching of CAB:Ce3þ,Mn2þ affects the most the luminescence of Mn2þ ions. The thermal quenching of Mn2þ emission was also confirmed by temperature dependence of Mn2þ luminescence decays (see inset b in Fig. 14). As seen in Table 2 the red luminescence monitored at 680 nm decays faster with increasing temperature and the time constant changes from 10.9 ms (13.5 K) to 6.9 ms (298 K). One of the possible quenching processes is non-radiative relaxation from the excited state to the ground state due to crossing of the parabolas in the configurational coordinate diagram. When the temperature is raised the higher vibrational levels of the excited states are populated, and finally the crossover point between both parabolas can be reached allowing the electrons to get coupled with the high-vibrational states of the ground electronic state and consequently to decay nonradiatively [1,42].

4. Conclusions We have investigated the spectroscopic properties of Ce3þ doped and Ce3þ,Mn2þ co-doped calcium aluminoborate materials, focusing on the sensitized Mn2þ luminescence. SEM and TEM studies showed that obtained materials are not phase-pure and that increasing concentration of activator ions facilitates the formation of non-crystalline impurity phase. It was found that the luminescence spectra of the studied materials are concentration dependent, what mainly results from changing distribution of dopant ions between both phases. Under UV excitation singly Ce3þ activated powders exhibited intensive blue luminescence due to interconfigurational 5d1/4f1 transition of Ce3þ. Part of the emission was positioned in UV, however. The studies also showed that Ce3þ luminescence is resonant with Mn2þ excitation. Consequently, generation of Mn2þ emission band centered around 680 nm in doubly (Ce3þ, Mn2þ) doped CAB samples was possible by excitation

of Ce3þ which shared its excessive energy with Mn2þ. It was also found that sensitized Mn2þ luminescence is mainly associated with impurity phase. The Ce3þ/Mn2þ energy transfer efficiency increased with increasing Mn2þ concentration and reached the value close to 52% for CAB:Ce3þ(3%),Mn2þ(4%) sample. The studies of the emission spectra in the function of temperature showed thermal quenching of Mn2þ luminescence. Acknowledgment This work was supported by POIG.01.01.02-02-006/09 project co-funded by European Regional Development Fund within the Innovative Economy Program. Priority I, Activity 1.1. Sub-activity 1.1.2, which is gratefully acknowledged. References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994. [2] W.M. Yen, S. Shionoya, H. Yamamoto, Phosphor Handbook, second ed., CRC Press, Boca Raton, 2007. [3] Xinguo Zhang, Jungu Xu, Menglian Gong, J. Lumin 183 (2017) 348. [4] Junhe Zhou, Ting Wang, Xue Yu, Dacheng Zhou, Jianbei Qiu, Mater. Res. Bull. 73 (2016) 1. [5] Wenzhen Lv, Yongchao Jia, Qi Zhao, Mengmeng Jiao, Baiqi Shao, Lue Wei, Hongpeng You, RSC Adv. 4 (15) (2014) 7588. [6] Z.W. Zhang, H. Zhong, S.S. Yang, X.J. Chu, J. Alloy Compd. 708 (2017) 671. [7] Anxiang Guan, Guofang Wang, Siyu Xia, Xinguo Zhang, Xiaoxi Hu, Liya Zhou, Yingbin Meng, Chunying Yao, Haiman Pan, J. Electr. Mater 46 (3) (2017) 1451. [8] Lili Wang, Byung Kee Moon, Sung Heum Park, Jung Hwan Kim, Jinsheng Shi, Kwang Ho Kim, Jung Hyun Jeong, RSC Adv. 6 (83) (2016) 79317. [9] Fei Wang, Wanrong Wang, Ye Jin, Ceram. Inter 42 (15) (2016) 16626. [10] S. Gomez-Salces, J.A. Barreda-Argueso, R. Valiente, F. Rodriguez, J. Mater. Chem. C Mater. Opt. Electron. Devices 4 (38) (2016) 9021. [11] Mengqiao Li, Jilin Zhang, Jin Han, Zhongxian Qiu, Wenli Zhou, Liping Yu, Zhiqiang Li, Shixun Lian, Inorg. Chem. 56 (1) (2017) 241. [12] Sangmoon Park, Sena Koh, Hyuntae Kim, Displays 48 (2017) 29. [13] N. Singh, Vijay Singh, G. Sivaramaiah, J.L. Rao, Pramod K. Singh, M.S. Pathak, S.J. Dhoble, M. Mohapatra, J. Lumin 178 (2016) 479. [14] Zaifa Yang, Denghui Xu, Jiangnan Du, Xuedong Gao, Jiayue Sun, RSC Adv. 6 (90) (2016) 87493.

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[15] Yue Tian, Ningning Feng, Maria Wierzbicka-Wieczorek, Ping Huang, Lei Wang, Qiufeng Shi, Cai'e Cui, Dyes Pigm. 131 (2016) 91. [16] J.S. Kim, P.E. Jeon, J.C. Choi, H.I. Park, S.I. Mho, G.C. Kim, Appl. Phys. Lett. 84 (2004) 2931. [17] J.S. Kim, P.E. Jeon, Y.H. Park, J.C. Choi, H.I. Park, G.C. Kim, T.W. Kim, Appl. Phys. Lett. 85 (2004) 3696. [18] R. Martínez-Martínez, A. Speghini, M. Bettinelli, C. Falcony, U. Caldino, J. Lumin 120 (2009) 1276. [19] M.Y. Xie, L. Yu, H. He, X.F. Yu, J. Solid State Chem. 182 (2009) 597. [20] Yun Ding, Liang-Bo Liang, Min Li, Ding-Fei He, Liang Xu, Pan Wang, XueFeng Yu, Nanoscale Res. Lett. 6 (2011) 119. [21] W.B. Dai, RSC Adv. 4 (2014) 11206. [22] M. Müller, T. Jüstel, J. Lumin 155 (2014) 398. [23] Sanjith Unitrhrattil, Kyoung Hwa Lee, Won Bin Im, J. Am. Ceram. Soc. 97 (3) (2014) 874. [24] M.A. Mickens, Z. Assefa, J. Lumin 143 (2014) 498. [25] Xia Zhang, Yongfu Liu, Jian Li, Zhendong Hao, Yongshi Luo, Qingzhe Liu, Jiahua Zhang, J. Lumin 146 (2014) 321.  ~ o, E. Aílvarez, [26] U. Caldin A. Speghini, M. Bettinelli, J. Lumin 132 (2012) 2077. [27] Xuhui Xu, Yuhua Wang, Xue Yu, Yangin Li, Yu Gong, J. Am. Ceram. Soc. 94 (1) (2011) 160.

[28] N. Suriyamurthya, B.S. Panigrahi, J. Lumin 127 (2007) 483. [29] V.C. Teixeira, P.J.R. Montes, M.E.G. Valerio, Opt. Mater 36 (2014) 1580.  lito, L. Huerta, J. Rickards, U. Caldin ~ o, [30] R. Martínez-Martínez, M. García-Hipo C. Falcony, Thin Solid Films 515 (2006) 607. €rster, Ann. Phys. 2 (1948) 55. [31] T. Fo [32] Fengwen Kang, Haishan Zhang, Lothar Wondraczek, Xiaobao Yang, Yi Zhang, Dang Yuan Lei, Mingying Peng, Chem. Mater. 28 (2016) 7807. [33] Yingli Zheng, Donghua Chen, Wei Li, Phys. B Condens. Matter 406 (2011) 996. [34] S.J. Camardello, P.J. Toscano, M.G. Brik, A.M. Srivastava, Opt. Mater. 37 (2014) 404. [35] Chengyu Li, Qiang Su, J. Alloy Compd. 9 (2006) 875. [36] Hongpeng You, Guangyan Hong, Mater. Res. Bull. 32 (1997) 785. [37] Hucheng Yang, Chengyu Li, Hong He, Guobin Zhang, Zeming Qi, Qiang Su, J. Lumin 124 (2007) 235. [38] Ki-Seong Chang, Douglas A. Keszler, Mater. Res. Bull. 33 (1998) 299. [39] R.D. Shannon, Acta Cryst. A32 (1976) 751. [40] V.P. Dotsenko, I.V. Berezovskaya, N.P. Efryushina, A.S. Voloshinovskii, P. Dorenbos, C.W.E. van Eijk, J. Lumin 93 (2001) 137. [41] A.A. Setlur, D.G. Porob, U. Happek, M.G. Brik, J. Lumin 133 (2013) 66. [42] J. Garcia Sole, L.E. Baus a, D. Jaque, An Introduction to the Optical Spectroscopy of Inorganic Solids, John Wiley and Sons Ltd, 2005.

Please cite this article in press as: M. Puchalska, E. Zych, Ce3þ-sensitized red Mn2þ luminescence in calcium aluminoborate phosphor material, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.05.060