Karol Szczodrowski*a,
Alicja Chruścińskab,
Justyna Barzowskaa,
Krzysztof Przegiętkab,
Krzysztof Andersc,
Ryszard Piramidowiczc and
Marek Grinberga
aInstitute of Experimental Physics, University of Gdańsk, Wita Stwosza 57, 80-952 Gdańsk, Poland. E-mail: fizks@ug.edu.pl
bInstitute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5/7, 87-100 Torun, Poland
cInstitute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland
First published on 23rd July 2015
The Sr2Si0.95Ti0.05O4:Eu2+ phosphor was synthesized using titanium modified silica SBA-15 or titania as titanium precursors via a solid state synthesis method. The phase composition of samples was investigated by powder XRD technique. The SEM pictures enabled study of the influence of Ti4+ precursors on the materials' morphology. The results of versatile optical characterization pointed out that titanium incorporation causes the extending of the persistent luminescence time and does not change the basic spectral properties of the parent phosphor. The persistent luminescence phenomenon results from the presence of point defects in the lattice (ion vacancies) that play a role in electron or hole traps. It was found that both the concentration and distribution of traps in Sr2Si0.95Ti0.05O4:Eu2+ depends on the form of the titanium precursor used in the synthesis. The trap parameters were characterized by thermoluminescence measurements and luminescence kinetics analysis.
In the literature there can be found also the reports on long lasting luminescent materials doped with transition metal ions,14,15 but the luminescence efficiency in such materials is relatively low comparing to those doped with Eu2+. Taking the all above mentioned facts into account it may be concluded that there is still a lot of room for research and development of new optical materials enabling persistent luminescence and investigation of effects supporting the effective transfer of charge carriers from the traps to the active ion. In this contribution the synthesis strategy, luminescence (excitation and emission spectra) as well as luminescence kinetics of a new material – Sr2Si0.95Ti0.05O4:Eu2+ are presented.
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Fig. 1 Schemes of the synthesis of titanium modified silica SBA-15 (a) and all luminescent materials (b). |
Moreover the strontium silicate samples doped only with Eu2+ ions at different concentration (0.5, 1, 2, 3% of mol) were synthesized to exhibit the influence of Ti4+ on the luminescent properties of Sr2SiO4:Eu2+ as well as to study the influence of europium concentration on the presence of afterglow luminescence phenomenon. The materials were obtained by solid state synthesis method using the same synthesis conditions and reagents except TiO2, as in the case of sample B. Since the strontium orthosilicate doped with 1% of Eu2+ had no detectable persistent luminescence we have chosen sample doped with 2% of Eu2+ as a reference material. In that case the concentration of activator is as close as possible to the concentration of Eu2+ in investigated materials co-doped with Ti4+ and is enough to observe the persistent luminescence (see inset of Fig. 6). The Sr2SiO4:Eu2+ already has been shown to present the long lasting luminescence.17 Quality and purity of samples were examined with X-ray diffraction method (XRD) using BRUKER D2PHASER equipment employing Cu Kα radiation and operated at 30 kV and 10 mA. The XRD patterns were collected using scanning step of 0.02° and counting time of 0.4 s per step. Morphology of the sample A, sample B and reference material was examined with Scanning Electron Microscope TM – 1000 (Hitachi).
To obtain time resolved luminescence spectra and luminescence kinetics the sample was excited using PL 2143 A/SS laser followed by PG 401/SH parametric optical generator generating 30 ps pulses with frequency 10 Hz. The emission spectra were collected using a 2501S (Bruker Optics) spectrograph followed by a C4334-01 Hamamatsu Streak Camera. Time resolved luminescence spectra and decay profiles were obtained by integration of the streak camera images over the time and wavelength intervals, respectively. This apparatus allowed to measure time resolved spectra and luminescence decays for the time scale shorter than 10 ms.
For the measurements of persistent luminescence kinetics at room temperature the modified PTI QuantaMasterTM spectrofluorimetric setup equipped with PMT followed by analog/digital converter was used. In this system the sample was excited directly by laser diodes or LEDs (depending on the spectral range and required optical power) controlled by programmable signal generator. To avoid detector's saturation, the fast electro-mechanical shutter synchronized with excitation source triggering was applied at the entrance slit of the emission path monochromator. The recorded data were cumulated and averaged to maximize signal-to-noise ratio.
Thermoluminescence (TL) measurements were carried out using Risø TL/OSL System TL-DA-12 equipped with the EMI 9235QA photomultiplier. 90Sr/90Y beta sources (beta dose rate calibrated for quartz—about 40 mGy s−1) were used for TL excitation. TL signal was detected in different spectral windows using Schott BG 39 (2 mm) and Schott BG 3 (3 mm) filters. Two 2 mg portions of each of investigated samples were carefully scattered onto stainless steel discs, covered previously by the thin layer of silicon oil spray. Experiments were performed in argon atmosphere, in the temperature range from 273 K to 723 K. The TL curves were obtained with the heating rate 0.5 K s−1 unless other values are given.
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Fig. 2 SEM micrographs for the samples: upper pictures – reference material; middle pictures – sample A; lower pictures – sample B. |
Sr2SiO4 can exist in two crystallographic phases: high temperature—orthorhombic α′ and low temperature—monoclinic β. The temperature of α′ ↔ β phase transition is equal to ∼358 K, and is relatively low comparing to other silicates. The α′ phase as a meta-stable form can exist below the phase transition temperature if small amount of Ba2+ or Eu2+ is added to the Sr2SiO4 lattice. In both phases Sr2+ ions occupy two kinds of non equivalent sites: ten-coordinated SI and nine-coordinated SII sites.18,19 The X-ray diffraction (XRD) patterns collected for synthesized materials (sample A, sample B and reference material) prove that α′ form of strontium silicate is a predominant phase in all cases – the recorded XRD patterns are in good agreement with PDF 00-039-1256 card (Fig. 3). However, in the case of samples co-doped with titanium ions the α-Sr2TiO4 phase (PDF 01-072-2040) can be distinguished and the contribution of this phase is comparable in both materials. This shows that small concentration of Eu2+ and Ti4+ ions introduced into the silicate lattice has no effect on Sr2SiO4 basic crystal structure and the excess of titanium, not incorporated in the silicate, takes part in creation of strontium titanate regardless of Ti4+ precursor used. Moreover the small amount of Sr(OH)2 (PDF 00-027-0847) exists in all samples as an impurity phase. The detailed phase composition of products is presented in Table 1. Sr2SiO4:Eu2+ obtained as a reference material as well as Sr2SiO4:Eu2+ co-doped with titanium (sample A and sample B), exhibit strong luminescence which consists of two broad bands with maxima at about 490 nm and 570 nm [Fig 4]. These bands are attributed to the parity allowed 4f65d1 → 4f7 transitions in Eu2+ ions occupying in Sr2SiO4 crystal lattice two different Sr2+ sites: ten-coordinated SI and nine-coordinated SII, respectively.17,20–25 Since the emission excitation bands are partially overlapped (Fig. 4), both types of Eu2+ luminescence centers can be excited simultaneously and the relative intensity of the signals from Eu2+ ions occupying SI site and SII site can be easily controlled by changing the excitation wavelength.17,26 When emission is excited with 442 nm, a single emission band with maximum at 570 nm is observed. Under excitation at wavelengths shorter than 420 nm, the second emission band appears in the blue-green spectral region, with maximum at 490 nm. However, this later band, originating from Eu2+ (SI) centers, is always (i.e. independently of the excitation wavelength) accompanied by the yellow-orange emission band from Eu2+ (SII).27 For 325 nm excitation the yellow-orange emission band is seen only as a shoulder on the longer wavelengths side of the dominating emission from Eu2+ (SI) (Fig. 4). Both mentioned emission bands have comparable intensities under 355 nm excitation, therefore this wavelength was chosen to investigate decays in the time resolved emission spectra (TRES) experiment. The emission decay profiles of the reference material, sample A and sample B, obtained at room temperature are presented in Fig. 5. In the case of the band with maximum at 490 nm the decay profiles were obtained by integration of the emission signal over 475–505 nm wavelength range. Such a range was chosen to minimize the contribution of the second, yellow-orange band. To get decay profiles of the yellow-orange band, the emission signal was integrated over 545–595 nm wavelength range. As it can be seen in Fig. 5 the luminescence intensity decreases approximately exponentially. A slight deviation from the exponential dependence, observed for the blue-green emission decay is related to the inter site SI → SII energy transfer and has been discussed in our previous paper.17 Decay constants estimated by fitting an exponential function to the decay profiles of the blue-green emission band are equal to: 0.55 μs for the reference material, 0.49 μs for the sample A, 0.56 μs for the sample B, and for the yellow-orange emission band: 1.06 μs, 1.16 μs and 1.19 μs for the reference material, sample A and sample B, respectively.
Sample | Formula | Ti4+ precursor | Dopant concentration | Composition |
---|---|---|---|---|
Reference material | Sr1.98Eu0.02SiO4 | — | Eu2+ – 2% mol | 95% α′-Sr2SiO4, 5% Sr(OH)2 |
Sample A | Sr1.99Eu0.01Si0.95Ti0.05O4 | SiTi-20 | Eu2+ – 1% mol, Ti4+ – 5% mol | 93% α′-Sr2SiO4, 2% α-Sr2TiO4, 5% Sr(OH)2 |
Sample B | Sr1.99Eu0.01Si0.95Ti0.05O4 | TiO2 | Eu2+ – 1% mol, Ti4+ – 5% mol | 97% α′-Sr2SiO4, 2% α-Sr2TiO4, 1% Sr(OH)2 |
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Fig. 5 Decay profiles of the two emission bands, measured at room temperature. Luminescence was excited at 355 nm and monitored at 480 nm (a) and 570 nm (b). |
The high similarity in the shape of the emission and excitation spectra, as well as similarity in fluorescence decay profiles indicate that in samples A and B the active luminescence centers are the same as the luminescence centers present in the reference material. In all investigated samples the luminescence is a result of d–f transitions in Eu2+ ions occupying SI and SII sites. These results suggest also that the introduction of Ti4+ ions into the strontium silicate lattice as well as the presence of additional, impurity phases such as Sr(OH)2 and Sr2TiO4, at least at given in Table 1 concentration, do not influence the basic spectral properties of Eu2+ in the investigated materials what is advisable due to maintaining the good spectral properties of Sr2SiO4:Eu2+.
Besides of described above two strong emission bands, which decays in microseconds range, Sr2SiO4:Eu2+ exhibits also luminescence that lasts at room temperature for several seconds after stopping the excitation.17 The luminescence decay profiles collected for materials undoped with Ti4+ (inset of Fig. 6) show that the presence of the long lasting luminescence depends on the concentration of europium ions. The persistent luminescence is practically observed only in Sr2SiO4:Eu2+ nominally doped with 2 and 3% mol of activator. As described in our previous paper persistent luminescence is represented by the single band, with maximum at 570 nm, independently of the excitation wavelength. It leads to the conclusion, that traps responsible for long lasting luminescence of Sr2SiO4:Eu2+ observed at room temperature, deactivate directly and only through Eu2+ ions occupying nine coordinated SII sites. The traps type was attributed to the oxygen vacancies formed during synthesis of material to compensate the additional negative charge created after reduction of Eu3+ to Eu2+.17
The samples of the Sr2SiO4:Eu2+ co-doped with Ti4+ exhibit the persistent luminescence in the same spectra region as the Sr2SiO4:Eu2+, however it is significantly enhanced. The decay profiles obtained after switching off the excitation source (λexc =445 nm) for both Sr2Si0.95Ti0.05O4:Eu2+ materials are shown in Fig. 6. It can be seen from these decay profiles that the persistent luminescence is characterized by much longer decay time comparing to the reference material. All phosphors show firstly a rapid intensity decrease and then a longer lasting yellow luminescence. The introduction of Ti4+ into the Eu2+ doped silicate matrix significantly extends the time of persistent luminescence from few seconds up to several minutes. It should be noted that the long lasting luminescence is observed in the samples doped with 1% mol of Eu2+ while in the parent material with the same concentration of Eu2+ it is practically not observable (Fig. 6). Taking into account the similarity of the spectroscopic properties between all samples it can be assumed that the long lasting luminescence observed in Ti4+ doped materials is a result of the existence of traps that are localized or active only at the SII emission center as in Sr2SiO4:Eu2+.
Furthermore, it was found that the decay time of persistent luminescence observed in Sr2Si0.95Ti0.05O4:Eu2+ phosphor depends strongly on the titanium precursor used during synthesis route. In the case of sample obtained using modified mesoporous silica SiTi-20 the decay time of long lasting luminescence was increased up to several tens of minutes comparing to the sample modified using TiO2 (Fig. 6). The reason seems to be that the distribution of Ti4+ ions among silicon in modified SBA-15 silica is much more homogeneous and is controlled by appropriate rate of hydrolysis and condensation reactions of alcoxides whereas the distribution of the ions in sample obtained using SiO2 and TiO2 was controlled only by mechanical milling.
The increase of the decay time is attributed to the changes in electron/hole traps distribution. The changes should give the additional contribution to thermoluminescence. Therefore the thermoluminescence measurements were carried out to investigate the traps occurring in the reference material, sample A and sample B. To distinguish the emission from Eu2+ occupying SI and SII, the TL signal was detected in two different spectral windows using Schott BG39 and Schott BG3 filters (Fig. 8). The comparison of the TL intensities measured with different filters indicates that the Eu2+ (SII) emission dominates also in the TL spectrum.
Trap parameters such as a trap depth E (or thermal activation energy) and a frequency factor s determine the lifetime of carriers in a trap (τ):
![]() | (1) |
![]() | (2) |
The above relation is valid for the first order kinetics model. In the case of the investigated materials the first-order kinetics of the TL process can be supposed because of the stable position of the TL peaks in the TL curves measured for different initial trap populations (after different excitation doses). Three heating rates (0.5, 1 and 2 K s−1) were applied in order to estimate the E values for peaks at about 350 K and 450 K (see Fig. 7). For each peak a line was plotted in the coordinates 1/T and ln(Tmax2/β). Then the line slope, which is equal E/k, was estimated. The E values were used in the next step of TL analysis – the glow curve decomposition into first-order TL peaks:
![]() | (3) |
Peak | E [eV] | n0 | s [s−1] | τ [s] |
---|---|---|---|---|
1 | 0.79 | 9.4 × 105 | 4.2 × 1010 | 9.9 × 102 |
2 | 0.70 | 1.2 × 105 | 7.9 × 1010 | 1.2 × 101 |
3 | 0.92 | 3.5 × 105 | 1.1 × 1012 | 5.0 × 103 |
4 | 1.30 | 4.0 × 105 | 2.4 × 1013 | 9.1 × 108 |
5 | 0.92 | 1.4 × 105 | 4.3 × 109 | 1.4 × 106 |
6 | 1.07 | 7.0 × 104 | 2.4 × 1013 | 1.0 × 105 |
7 | 1.10 | 6.5 × 104 | 7.3 × 1012 | 1.2 × 106 |
Peak | E [eV] | n0 | s [s−1] | τ [s] |
---|---|---|---|---|
1 | 0.79 | 5.8 × 105 | 6.9 × 1010 | 6.0 × 102 |
2 | 0.70 | 2.8 × 105 | 8.2 × 1010 | 1.1 × 101 |
3 | 0.83 | 3.6 × 105 | 7.1 × 1010 | 2.8 × 103 |
4 | 1.29 | 3.0 × 104 | 2.3 × 1013 | 6.3 × 108 |
5 | 0.92 | 2.2 × 104 | 4.4 × 109 | 1.3 × 106 |
6 | 1.07 | 2.0 × 105 | 4.1 × 1013 | 6.0 × 104 |
7 | 0.96 | 4.7 × 104 | 8.0 × 1010 | 3.6 × 105 |
8 | 0.83 | 2.1 × 105 | 1.0 × 1012 | 2.0 × 102 |
9 | 1.08 | 7.4 × 104 | 1.4 × 1013 | 2.5 × 105 |
Peak | E [eV] | n0 | s [s−1] | τ [s] |
---|---|---|---|---|
1 | 0.79 | 3.1 × 105 | 5.2 × 1010 | 8.0 × 102 |
2 | 0.70 | 9.8 × 104 | 6.8 × 1010 | 1.4 × 101 |
3 | 0.83 | 3.9 × 105 | 6.2 × 1010 | 2.8 × 103 |
4 | 1.30 | 6.1 × 104 | 2.7 × 1013 | 8.2 × 108 |
5 | 0.92 | 4.4 × 104 | 5.2 × 109 | 1.1 × 106 |
6 | 1.07 | 1.2 × 105 | 4.2 × 1013 | 5.9 × 104 |
7 | 1.10 | 5.3 × 104 | 7.7 × 1012 | 1.1 × 106 |
8 | 0.83 | 2.1 × 104 | 6.9 × 1011 | 3.0 × 102 |
9 | 1.08 | 5.6 × 104 | 2.4 × 1013 | 1.5 × 105 |
The most conspicuous feature of the TL curves measured for the investigated samples and presented in Fig. 8 and Fig. 9 is the TL signal below 300 K especially high for sample A. The time between the end of excitation and starting the TL measurement is 60 s. This is the time needed for moving the sample in the reader from the position under the beta source to the position of luminescence detection. The TL below 300 K may fade significantly during this period, so what is observed only a small part of this signal. The traps responsible for this luminescence are identical to traps that are the source of carriers for the fastest components of the long-lasting luminescence presented in Fig. 6. Their parameters E and s cannot be estimated by the fitting procedure. One can suppose that their lifetime at RT should be a bit smaller than the lifetime of trap 2 that was identified by the fitting procedure, so of the order of a few seconds. From Fig. 8 it is clear that these traps are most highly populated in sample A but are also present in sample B and (in the slightest degree) in the reference material. A high concentration of the defects responsible for the shallow traps in sample A and B can be the first reason of the strong TL signal below 300 K and efficient long-lasting luminescence. There is, however, also another factor that favors the higher occupation of the shallow traps after excitation and enhances the long-lasting luminescence. In the TL curves of both samples, A and B, peak 1 is significantly weaker than in reference material. Moreover, the peaks present in the TL curve of reference material above 400 K are only slightly marked in TL curves of samples B and they sink in the background in the case of sample A. The deeper traps present in reference material are strong competitors for the shallower traps during the process of trap filling. They may also diminish the long-lasting luminescence by retrapping the carriers freed from shallow traps. The lack of the deeper traps in samples A and B enhances the signal originating from shallow traps.
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Fig. 9 TL curves decomposition into first order TL peaks, (a) – reference material, (b) – sample A, (c) – sample B. |
Outcomes of the fitting procedure reveal an additional trap, beside the shallower traps mentioned above, that may be responsible for the clear difference between the shapes of long-lasting luminescence of sample A and B. The first maximum of TL curve of sample A is shifted into lower temperatures with respect to the first maximum in reference material and sample B (Fig. 8). This is due to the presence of peak 8 that does not occur in the TL curve of reference material and is very weak in the TL curve of sample B (Fig. 9b). This peak, having the lifetime of about 200–300 s, fades significantly faster than peaks 1 and 3 (lifetimes about 1000 s and 3000 s respectively), so the luminescence produced by the carriers freed from the trap related to this peak is a medium component of long-lasting luminescence in RT. Peaks 1 and 3 contribute to the slowest component of long-lasting luminescence of both samples.
In view of the analysis of TL spectra have not gave the answer about the life time of the most shallow traps, we have also analyzed the persistent luminescence decay assuming that each trap is an individual emission source and contributes to the persistent luminescence intensity by its individual light output where τ is the lifetime of the trap, and A(τ) is the number of the of the traps having lifetime τ. Thus the intensity of the total emission is given by the following sum:
![]() | (4) |
When considering no homogeneously broadened system, the quantity A(τ) can be viewed as the distribution of luminescence lifetimes in the sample. Thus, instead of summation, one can consider the integration
![]() | (5) |
![]() | (6) |
One can recover the distribution function A(τ) from the experimental luminescence decay by minimizing the χ2 function defined as:
![]() | (7) |
Ref. sample | Sample A | Sample B |
---|---|---|
3 × 10−3 | 0.02 | 0.02 |
0.1 | 0.3 | 0.15 |
— | 1.2 | 1.2 |
6.0 | 20 | 10 |
— | 200 | 300 |
One found the correlation between the results of analysis of luminescence decay and thermoluminescence experiments. Especially in both experiments one obtained the traps characterized by 10 s lifetime (in reference sample the luminescence kinetics yield 6 s). Additionally the appearance of traps characterized by the lifetime 200 s and 300 s for sample A and B (not observed for reference sample) obtained from thermoluminescence experiment were confirmed by luminescence kinetics analysis. Probably this traps are related to Ti3+.
The extending of the time of persistent luminescence is a result of the traps distribution changing observed in thermoluminescence spectra and luminescence kinetics analysis. The Ti4+ incorporated into the silicate matrix can play the role of electron acceptor and the possibility of Ti4+ to Ti3+ reduction after excitation of Eu2+ should be considered. Thus, we propose tentatively the model in which Eu2+ after excitation give the electron to Ti4+. This process can be described by following relation:
Eu2+ + Ti4+ → (Eu2+)* + Ti4+ → Eu3+ + Ti3+ |
Energy of the [Eu3+ + Ti3+] is smaller than [(Eu2+)* +Ti4+] by value of 0.83 eV and the [Eu3+ + Ti3+] electron trap is responsible for enhancement of persistent luminescence in Ti doped Sr2SiO4:Eu2+. Taking into account that the typical persistent luminescent materials doped with Eu2+ need another expensive lanthanide ions to extend the time of luminescence,12 obtaining a similar effect by Ti4+ codoping can be considered as an advantage.
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