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Synthesis and optical properties of Y2Mo4O15 doped by Pr3+
Journal of Luminescence 190 (2017) 525–530
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Synthesis and optical properties of Y2Mo4O15 doped by Pr3+ a
b
Indre Mackeviciute , Ausra Linkeviciute , Arturas Katelnikovas a b
a,⁎
MARK
Faculty of Chemistry and Geosciences, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania State Research Institute, Centre for Physical Sciences and Technology, Sauletekio Avenue 3, LT-10257 Vilnius, Lithuania
A R T I C L E I N F O
A B S T R A C T
Keywords: Solid state synthesis Praseodymium Photoluminescence Optical band gap Thermal quenching
A set of Y2Mo4O15 compounds doped with 0.1–30% Pr3+ was synthesized by conventional high temperature solid state synthesis. The structural, morphological and optical characteristics of the compounds were investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) analysis, thermal quenching (TQ), photoluminescence (PL) and fluorescence lifetime techniques. It turned out that Pr3+ emission spectra are very sensitive to the concentration and temperature in the Y2Mo4O15 host matrix. Samples showed mainly red and deep red luminescence when excited with blue radiation. A severe concentration quenching was observed in samples with higher Pr3+ content due to cross relaxation. Y2Mo4O15:Pr3+ luminescent materials possess rather unique emission spectra, which can be tuned by adopting the appropriate Pr3+ concentration and/or temperature. Thus these materials could be used as luminescent security markers where such properties are desirable.
1. Introduction Pr3+ is one of the most unique rare earth ions that can luminesce in a very broad spectrum ranging from deep UV through the visible to the infrared spectral range [1]. The type of luminescence spectrum of Pr3+ doped inorganic compounds is governed by the covalent character of the respective bonds between activator and the neighbouring anions, and the crystal field splitting. For instance, if the chosen host matrix has low crystal field strength and wide enough optical band gap, then all crystal field states of [Xe]4f15d1 configuration are located above the 1S0 state and efficiently populate it leading to efficient line emission in the UV region. Besides, in these cases the photon cascade emission is also sometimes observed, leading to external quantum yields larger than 100% [2–5]. If the stronger crystal field strength shifts the lowest crystal-field component of the [Xe]4f15d1 configuration below 1S0 state, then broad emission bands to the terminal 3HJ and 3F2 levels are observed in the UV region. The direct excitation of 3PJ, 1D2 and other levels of the [Xe]4f2 configuration yields characteristic sharp emission lines from cyan spectral region to infrared [6,7]. Moreover, in some compounds an efficient energy transfer from 3P0 to 1D2 levels occurs and only red emission is observed [8–10]. Based on such versatility of emission spectra, Pr3+ doped compounds found application in X-ray detectors, field emission displays (FEDs) and excimer discharge lamps; moreover, novel fast scintillators for positron emission tomographs (PET) could be obtained employing fast [Xe]4f15d1 → [Xe]4f2 luminescence of Pr3+ ions [7,11–16].
⁎
Corresponding author. E-mail address: [email protected] (A. Katelnikovas).
http://dx.doi.org/10.1016/j.jlumin.2017.06.014 Received 25 March 2017; Received in revised form 9 May 2017; Accepted 6 June 2017 Available online 07 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Recently, Pr3+ doped molybdates gained lots of attention due to their excellent mechanical properties, good physical, chemical, thermal stability and optical properties [17]. Pr3+ doped molybdates, therefore, are considered for application in optical amplifiers, white LEDs and displays as red phosphors [18–21]. In this work, the luminescence properties of Y2Mo4O15:Pr3+ powders were investigated as a function of Pr3+ concentration and temperature. These materials showed good colour saturation and high luminous efficacies. Moreover, it turned out that emission spectra can be significantly tuned by selecting the appropriate Pr3+ concentration and/or temperature. This feature is very desirable considering the application in luminescent security pigments. 2. Experimental Y2Mo4O15:Pr3+ powder samples were prepared by a typical solidstate reaction method from high purity Pr6O11 (99.9% Acros Organics), MoO3 (99+% Acros Organics) and Y2O3 (99.99% Tailorlux). The stoichiometric amounts of yttrium, praseodymium, and molybdenum oxides were weighed to the nearest 0.0001 g, placed to the agate mortar and thoroughly mixed using a small amount of acetone as grinding media. The obtained blend was transferred to a porcelain crucible and annealed at 700 °C for 12 h in air [22]. During doping Y3+ was replaced by Pr3+, with concentrations ranging from 0.1% to 30%. Powder XRD analysis has been carried out employing a Rigaku MiniFlexII diffractometer working in the Bragg–Brentano (θ/2θ)
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focusing geometry. The data were collected within 2θ angle from 10° to 80° (step width 0.02° and scanning speed 5°/min) using Ni-filtered Cu Kα radiation. SEM images of phosphor powders were taken with a FE-SEM Hitachi SU-70. The accelerating voltage was 2 kV. Reflection spectra in the range of 250–800 nm were recorded on an Edinburgh Instruments FSL980 spectrometer equipped with a 450 W Xe arc lamp, double excitation and emission monochromators, a cooled (−20 °C) single-photon counting photomultiplier (Hamamatsu R928) and a Teflon coated integration sphere. Teflon was also used as a reflectance standard. The excitation and emission slits were set to 3.0 and 0.15 nm, respectively. Step width was 0.5 nm and integration time 0.2 s. Excitation and emission spectra for powder samples were recorded on the Edinburgh Instruments FLS980 spectrometer equipped with double excitation and emission monochromators, 450 W Xe arc lamp, a cooled (−20 °C) single-photon counting photomultiplier (Hamamatsu R928) and mirror optics for powder samples. When measuring excitation spectra (λem = 619 nm) excitation and emission slits were set to 0.5 and 1 nm, respectively. The excitation spectra were corrected by a reference detector. When measuring emission spectra (λex = 448 nm) excitation and emission slits were set to 5 and 0.25 nm respectively. The photoluminescence emission spectra were corrected by a correction file obtained from a tungsten incandescent lamp certified by NPL (National Physics Laboratory, UK). In both cases step width was 0.5 nm and integration time was 0.4 s. For thermal quenching (TQ) measurements a cryostat “MicrostatN” from the Oxford Instruments had been applied to the present spectrometer. Liquid nitrogen was used as a cooling agent. The measurements were performed at 77 K and at 100–500 K in 50 K intervals. Temperature stabilization time was 90 s and temperature tolerance was set to ± 5 K. During the measurements dried nitrogen was flushed over the cryostat window to avoid the condensation of water at low temperatures on the surface of the window. The photoluminescence decay curves were measured on the FLS980 spectrometer. A Xe μ-flash lamp μF920 was used as an excitation source. Excitation wavelength was 448 or 583 nm and emission was monitored at 619 or 609 nm.
Fig. 1. XRD patterns of undoped (a), 25% Pr3+ doped (b), and 30% Pr3+ doped (c) Y2Mo4O15:Pr3+ samples. The reference pattern of Y2Mo4O15 is given for comparison (d).
are well formed and with rather broad size distribution in the range of 1–10 µm. No substantial changes in particle size and morphology was observed when Pr3+ was introduced into the lattice. The body colour of undoped Y2Mo4O15 sample is white, indicating that it does not absorb radiation in the visible spectral region. This goes hand in hand with the reflection spectra depicted in Fig. 3, which indicate only strong absorption of the Y2Mo4O15 host in the range of 250–400 nm. Doped samples gradually gained greenish tint upon increasing Pr3+ concentration due to increasing absorption of Pr3+ ions in the blue and red spectral regions. Three absorption lines lying in the blue spectral region can be assigned to the 3H4 → 3P2 (ca. 450 nm), 3H4 → 3P1 + 1I6 (ca. 475 nm), and 3H4 → 3P0 (ca. 490 nm) transitions of Pr3+ ions. The collection of absorption lines in the red spectral region, in turn, can be assigned to the 3H4 → 1D2 (ca. 595 nm) transition. Fig. 3 also demonstrate that synthesized powders are of high quality since the reflectance at longer wavelengths are close to unity [27]. The optical band gap of the undoped sample was calculated from the absorption spectrum, which was obtained from reflection spectrum by applying Kubelka-Munk function [28]. The obtained optical band gap value for Y2Mo4O15 sample was 3.71 eV (334 nm). It is also obvious that the UV absorption band of Pr3+ doped sample is shifted to the longer wavelengths (lower energies) what might be attributed to the Pr3+ [Xe]4f2 → [Xe]4f15d1 absorption that coincides with the band gap absorption and causes the small shift towards lower energies. Fig. 4a shows normalized excitation spectra of 0.5% and 10% Pr3+ doped samples for 609 nm emission. Recorded spectra contain a broad band in the range of 250–370 nm and two sets of lines in the range of 480–510 nm and 575–610 nm. These results are consistent with the reflection spectra shown in Fig. 3. The broad band can be attributed to the host matrix absorption, whereas lines in the blue and red spectral regions can be assigned to the 3H4 → 3PJ + 1I6 and 3H4 → 1D2 transitions, respectively. It is interesting to note that samples are more efficiently excited through host matrix absorption band when Pr3+ concentration increases. The normalized emission spectra of Y2Mo4O15:Pr3+ phosphors doped with 0.5% and 10% Pr3+ under 448 nm excitation are depicted in Fig. 4b. The spectra contain seven sets of emission lines that originate from electronic transitions 3PJ → 3H4 (ca. 490 nm), 3PJ → 3H5 (ca.
3. Results and discussion The formula of Y2Mo4O15 compound does not show its real structure. It could be better expressed as Y2[MoO4]2[Mo2O7] [23]. This compound is isostructural with Ho2[MoO4]2[Mo2O7], which in 1988 was reported by V.A. Efremov et al. [24] Y2Mo4O15 compound adopts monoclinic Bravais lattice with the space group P21/c (#14), Z=2. The unit cell consists of isolated [MoO4]2- and [Mo2O7]2- units and Y3+ ions filling the voids. There is one crystallographic site for yttrium ions and they are coordinated by seven oxygen ions [23,25]. It was assumed that Pr3+ ions occupy Y3+ sites due to identical charge and similar ionic radii (r(Pr3+) = 0.99 Å for CN = 6, and r(Y3+) = 0.90 Å for CN = 6) of both ions [26]. The XRD patterns of undoped, 25% and 30% Pr3+ doped samples are given in Fig. 1. The reference pattern of Y2Mo4O15 (PDF4+ (ICDD) 04–008–9148) is also given for comparison. The XRD patterns of samples up to 25% Pr3+ match well with reference pattern indicating that single phase compounds are obtained. However, when Pr3+ concentration was increased to 30% and above the additional peaks appeared in the patterns implying the formation of secondary phases. These peaks are marked with arrows in Fig. 1c and their intensity increased with further increase of Pr3+ concentration. All these findings suggest that the solubility limit of Pr3+ in Y2Mo4O15 lattice is around 25%. The morphology features of obtained Y2Mo4O15 and Y2Mo4O15:25% Pr3+ powders we inspected by taking high resolution SEM images, which are given in Fig. 2. The obtained particles of undoped compound 526
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Fig. 2. SEM images of Y2Mo4O15 (a) and Y2Mo4O15:25%Pr3+ (b).
individual line to a particular transition. In order to at least partly do that we have also recorded emission spectra under 583 nm excitation (3H4 → 1D2 absorption transition). In this case, lines observed in the emission spectra originate solely from the excited 1D2 level. The obtained emission spectra for the samples doped with 0.5% and 10% Pr3+ are given in Fig. S1. The emission spectra of Y2Mo4O15:0.5%Pr3+ (Fig. S1a) are virtually the same regardless the excitation wavelength (448 or 583 nm). The only difference in these spectra the emerging small peak at ca. 619 nm when samples were excited with 448 nm radiation. This suggests that this line originate from the 3PJ → 3H6 transition. Of course the other transitions originating from the 3PJ levels were also absent in the emission spectrum when sample was excited with 583 nm radiation. The changes in emission spectra of 10% Pr3+ doped sample under these excitation wavelengths were substantial. Virtually no emission lines from 1D2 → 3H5 (ca. 700 nm) and 1D2 → 3H6 (ca. 800 nm) transitions were observed in emission spectrum recorded for 448 nm excitation, thus it could be assumed that the same is also true for the 1D2 → 3H4 transition. This leads to the conclusion that the emission lines in the range of 580–640 nm could be attributed to the 3PJ → 3H6 transition. Moreover, the time resolved emission spectra (TRES) were also recorded in order to support these findings. The TRES in the time range of 1–7.5 μs are given in Fig. S2. These measurements showed that faster spin-allowed 3PJ → 3H6 transition yields the emission line at ca. 619 nm. Moreover, the emission from 3PJ → 3H6 transition is over in less than 7.5 μs and only the lines originating from the slower spinforbidden 1D2 → 3H4 transition remain after 7.5 μs. Emission spectra of 0.5% and 10% Pr3+ doped samples are significantly different. The one recorded for 0.5% Pr3+ specimen is dominated by intensive 1D2 → 3HJ transitions. Since the samples were
Fig. 3. Reflection spectra of Y2Mo4O15 and Y2Mo4O15:25%Pr3+.
535 nm), 3PJ → 3H6 and 1D2 → 3H4 (ca. 590–630 nm), 3PJ → 3F2 (ca. 650 nm), 1D2 → 3H5 (ca. 700 nm), 3PJ → 3F4 (ca. 740 nm), and 1D2 → 3 H6 (ca. 800 nm) of Pr3+ ions [29]. The strongest emission intensity was obtained for the sample doped with 0.5% Pr3+. This sample showed 2.8 ± 0.5% quantum yield. The quantum yield of the other samples was not possible to measure due to too low absorption and/or emission what yielded unreliable data. Further increase of Pr3+ concentration has resulted in significant decrease of emission intensity due to concentration quenching. Concentration quenching occurring at low Pr3+ concentrations is a well-known problem even though efficient luminescence in some heavily Pr3+ doped compounds is reported every so often [30]. The emission lines originating from 3PJ → 3H6 and 1D2 → 3H4 transitions are overlapping, thus it is very difficult to assign every
Fig. 4. (a) Excitation (λem = 609 nm) and (b) emission (λex = 448 nm) spectra of samples doped with 0.5% and 10% Pr3+.
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Fig. 5. PL decay curves of Y2Mo4O15:Pr3+ samples as a function of Pr3+ concentration. (a) λex = 448 nm and λem = 619 nm, (b) λex = 583 nm and λem = 609 nm. Inset table shows calculated decay values. IRF denote Instrument Response Function.
excited to the 3PJ levels, it is evident, that substantial amount of these levels relaxes to 1D2 level, from which luminescence occurs. The energy difference between 3PJ and 1D2 levels is in the range of 4000–5000 cm−1, thus the relaxation is likely caused by multiphonon relaxation, since Y2Mo4O15 host possesses rather high phonon energies (ca. 950 cm−1) [23]. However, with increasing Pr3+ concentration the intensity of 1D2 → 3H4 transition started decreasing and virtually vanished when Pr3+ content reached 10%. This phenomenon is likely caused by cross-relaxation process from 1D2 level, what of course become more probable with increasing Pr3+ concentration. Such cross relaxation mechanisms can be written as: 1D2 + 3H4 → 1G4 + 3F4 or 1 D2 + 3H4 → 3F4 + 1G4 [31]. The recorded PL decay curves of Y2Mo4O15:Pr3+ samples as a function of Pr3+ concentration are given in Fig. 5. The decay curves for 448 nm excitation and 619 nm emission are given in Fig. 5a, whereas decay curves for 583 nm excitation and 609 nm emission are represented in Fig. 5b. Excitation at 448 nm yields emission from both 3PJ → 3H6 and 1D2 → 3H4 transitions, meanwhile only 1D2 → 3H4 transition emission is observed if samples are excited with 583 nm. The curves get steeper with increasing Pr3+ concentration indicating fast decrease of PL decay lifetimes and internal efficiency. In order to calculate the PL decay lifetimes decay curves were fit using triexponential fit function:
I (t ) = A + B1 e−t / τ1 + B2 e−t / τ2 + B3 e−t / τ3
Different situation was observed for 10% Pr3+ doped sample, where low temperature emission spectra are dominated by 3PJ → 3H4, 3PJ → 3 H6, 3PJ → 3F2, and 3PJ → 3F4 transitions. Contrary to the temperature dependent emission spectra of 0.5% Pr3+ counterpart, almost no emission from 1D2 level was observed. It is likely, that multiphonon relaxation from 3PJ to 1D2 levels for 10% Pr3+ doped sample also increases with increasing temperature but the sample undergo severe cross-relaxation and the 1D2 level is depopulated in non-radiative pathway, and only emission from 3PJ levels are seen in the spectra. This goes line in line with the decrease of 1D2 → 3H4 emission intensity with increasing Pr3+ concentration observed at room temperature measurements (Fig. 4b). The temperature dependent integrated emission values (normalized to 77 K) of 0.5% (Fig. 6c) and 10% Pr3+ (Fig. 7b) doped samples were used to determine the activation energy – Ea of thermal quenching process and temperature at which phosphor loses half of its luminescence intensity, denoted as TQ1/2. The mentioned data were fit with Fermi-Dirac distribution function [33]:
I (T ) =
I0 1+B×e
−Ea kT
(2)
Here I(T) and I0 are luminescence intensity at a given temperature T and zero Kelvin, respectively. B is frequency factor and k is Boltzmann constant (8.617342·10−5 eV/K) [34]. The obtained Ea values for 0.5% and 10% Pr3+ doped compounds were 0.19 ± 0.02 eV and 0.086 ± 0.012 eV, respectively. Expressing T for I(T)/I0 = 1/2 from Eq. (2) yields Eq. (3) which gives TQ1/2 values:
(1)
Here I(t) is PL intensity at a given time t; A is background; B1, B2, B3 are constants; and τ1, τ2, τ3, are PL decay lifetimes. The obtained values are given in the inset table of Figs. 5a and b, which also includes calculated average PL lifetime values τ . The slightly higher PL lifetime values were obtained when samples were excited at 583 nm (Fig. 5b) what could be related to the absence of faster emission from 3PJ levels. The decrease of PL lifetime values with increasing Pr3+ concentration is likely caused by increasing probability of cross relaxation processes, such as 3P0 + 3 H4 → 3H6 + 1D2 [32]. However, it is still not clear why samples showed three kinds of luminescence decay and this matter needs further investigation. In order to evaluate the Y2Mo4O15:Pr3+ luminescent materials performance at elevated temperature, the temperature dependent emission spectra were recorded in the range of 77–500 K. Such emission spectra of two representative samples doped with 0.5% and 10% Pr3+ are shown in Figs. 6a and 7a, respectively. The most intensive lines in low temperature emission spectra of 0.5% Pr3+ doped samples originate from 1D2 → 3H4, 3PJ → 3F2, 1D2 → 3H5, and 1D2 → 3H6, transitions. The temperature increase leads to decrease of emission intensity, which to some extent is compensated by emission line broadening at least to ca. 300 K. Further increase of the temperature leads to the severe reduction in emission intensity as a result of thermal quenching (see Fig. 6c). Besides the overall decrease of emission intensity with increasing temperature, the increased emission from the higher vibronic states of 1D2 level was also observed as shown in the inset of Fig. 6a.
TQ1/2 =
−Ea k × ln(1/ B )
(3) 3+
The calculated TQ1/2 values for 0.5% and 10% Pr doped samples were 400 ± 7 K and 238 ± 9 K, respectively. These values are consistent with Ea values; i.e. higher quenching temperature requires more energy. The temperature dependent measurements confirm, that heavily Pr3+ doped samples suffer from severe thermal quenching. The temperature dependent photoluminescence decay curves of Y2Mo4O15:0.5%Pr3+ sample were also recorded and are shown in Fig. 6b. The curves become steeper with increasing temperature indicating that the rate of non-radiative transitions from 3PJ levels increases. The calculated temperature dependent average decay time values are plotted in Fig. 6c and the profile is very similar for the one obtained from normalized emission integrals of the same sample. Unfortunately, we were not able to measure temperature dependent photoluminescence decay curves for the Y2Mo4O15:10%Pr3+ sample, since the PL decay was already too fast for the μ-flash lamp. Fig. 8a shows the Pr3+ concentration dependent colour points of Y2Mo4O15:Pr3+ samples in the CIE 1931 colour space diagram. The colour coordinates of samples with low Pr3+ content (up to 1%) are in 528
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Fig. 6. Temperature dependent emission spectra (a), decay curves (b) and Ea, TQ1/2 estimation for Y2Mo4O15:0.5%Pr3+ (c). The inset of section (a) shows normalized (at 609 nm) emission spectra. The IRF in section (b) denote Instrument response function. Please note that the error bars for the lifetime values in section (c) are smaller than the symbol.
human eye sensitivity curve (with maximum at 555 nm and the highest LE value of 683 lm/Wopt). The calculated LE values as a function of Pr3+ concentration and as a function of temperature of 0.5% and 10% Pr3+ doped samples are given in Fig. 8d. The increasing Pr3+ concentration leads to decreasing LE values due to raising emission intensity in the cyan (3PJ → 3H4 transition) and especially in the deep red (3PJ → 3F4 transition) spectral region where human eye is very insensitive [36]. The LE values of 0.5% Pr3+ doped sample increases with raising temperature up to 300 K, and then start sharply decreasing. The increase can be attributed to the already discussed relative increase of the 1D2 → 3H4 transition in the orange spectral region where human eye is more sensitive if compared to the red. The decreasing LE values above 300 K could be explained by the emission line broadening and increasing relative intensity in the red spectral region. The LE values of 10% Pr3+ doped sample increased with increasing temperature what is associated with virtually stable relative emission from 3PJ → 3H4 transition (cyan region), increasing relative intensity of 3PJ → 3H5 transition (green region) and decreasing relative emission intensities of the transitions in the red spectral region. All this leads to the increase of LE values with increasing temperature.
the red area and close to the edge of the CIE 1931 chromaticity diagram. This shows high colour saturation. However, further increase of Pr3+ concentration resulted in colour coordinate shift towards the centre of diagram and 25% Pr3+ doped sample showed emission whose colour coordinate was in the white area straight on the black body locus (BBL) with the colour temperature of ca. 3400 K. The present observation is in good agreement with Pr3+ concentration dependent emission (Fig. 4b) where part of the red emission intensity decreased and intensity of cyan and green emission increased with increasing Pr3+ concentration. The temperature dependent colour coordinates of 0.5% and 10% Pr3+ doped samples were also calculated and are depicted in Fig. 8b and c, respectively. The colour coordinates of the former are close to the edge of the colour diagram and shifts linearly from red to orange spectral region with increasing temperature. This is explained by the relative increase of emission from higher 1D2 vibronic levels at elevated temperatures what was already discussed above. The temperature dependent colour coordinates of 10% Pr3+ doped sample, however, showed a completely different behaviour; being slightly distant form the edge of the chromaticity diagram they shift to the orange spectral region and move closer to the edge of the diagram as the temperature increases. Besides the colour coordinates, the luminous efficacy (LE) values were also calculated from emission spectra. Luminous efficacy is a parameter describing how bright the radiation is perceived by the average human eye and are calculated by using the following equation [35]:
LE (lm / Wopt ) = 683 (lm / Wopt ) ×
∫ I (λ ) V (λ ) dλ ∫ I (λ ) dλ
4. Conclusions In summary, single phase Y2Mo4O15:Pr3+ luminescent materials with Pr3+ concentration up to 25% were prepared by solid state reaction at relatively low temperature. The optical properties of prepared powders were investigated as a function of Pr3+ concentration and temperature. The luminescence measurements revealed that emission spectra of synthesized powders can be tuned by adjusting the Pr3+ concentration and/or temperature. It also turned out that the increase of Pr3+ concentration leads to severe concentration quenching due to cross relaxation. Nevertheless, samples with higher Pr3+ concentration
(4)
Here I(λ) is the emission spectrum of the phosphor and V(λ) is the
Fig. 7. Temperature dependent emission spectra (a) and Ea, TQ1/2 estimation for Y2Mo4O15:10%Pr3+ (b).
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Fig. 8. Fragments of CIE 1931 colour space diagram with Pr3+ concentration dependent colour coordinates (a), temperature dependent colour coordinates of 0.5% Pr3+ (b) and 10% Pr3+ (c) doped samples. Section (d) shows luminous efficacy values as a function of Pr3+ concentration and as a function of temperature for 0.5% and 10% Pr3+ doped specimens. [15] A. Zych, C.D. Donega, A. Meijerink, Fast d-f emission in Ce3+, Pr3+ and Nd3+ activated RbCl, Opt. Mater. 33 (2011) 347–354. [16] M. Nikl, P. Bohacek, A. Vedda, M. Fasoli, J. Pejchal, A. Beitlerova, M. Fraternali, M. Livan, Luminescence and scintillation characteristics of heavily Pr3+-doped PbWO4 single crystals, J. Appl. Phys. 104 (2008) 093514. [17] V.I. Lukanin, A.Y. Karasik, Two-photon interband absorption coefficients in tungstate and molybdate crystals, Opt. Commun. 336 (2015) 207–212. [18] X. Yang, F. Huang, Z. Huang, F. Cao, J. Zhang, Small-size and monodispersed redemitting Pr3+ doped barium molybdate nanocrystals with ultrahigh color purity, RSC Adv. 6 (2016) 65311–65314. [19] B. Xu, J. Liu, C. Song, H. Luo, Y. Jie Peng, X. Yu, Synthesis and Tunable Luminescent Properties of Red Phosphor Li1−mAgmLa0.99−nYnPr0.01(MoO4)2 with Blue Excitation for White LEDs, J. Am. Ceram. Soc. 95 (2012) 250–256. [20] Y. Guan, T. Tsuboi, Y. Huang, W. Huang, Broadband infrared emission of Pr3+doped BaGd2(MoO4)4 for optical amplifier, J. Appl. Phys. 115 (2014) 213104. [21] Y. Zhou, J. Liu, X. Yang, X. Yu, J. Zhuang, A promising deep red phosphor AgLaMo2O8:Pr3+ with blue excitation for white LED application, J. Electrochem. Soc. 157 (2010) H278–H280. [22] H. Deng, Z. Zhao, J. Wang, Z. Hei, M. Li, H.M. Noh, J.H. Jeong, R. Yu, Photoluminescence properties of a new orange–red emitting Sm3+-doped Y2Mo4O15 phosphor, J. Solid State Chem. 228 (2015) 110–116. [23] M. Janulevicius, P. Marmokas, M. Misevicius, J. Grigorjevaite, L. Mikoliunaite, S. Sakirzanovas, A. Katelnikovas, Luminescence and luminescence quenching of highly efficient Y2Mo4O15:Eu3+ phosphors and ceramics, Sci. Rep. 6 (2016) 26098. [24] V.A. Efremov, N.N. Davydova, L.Z. Gokhman, A.A. Evdokimov, V.K. Trunov, Crystal structure of Ho2Mo4O15 holmium tetramolybdate, Zh. Neorg. Khim. 33 (1988) 3005–3010. [25] H. Deng, N. Xue, Z. Hei, M. He, T. Wang, N. Xie, R. Yu, Close-relationship between the luminescence and structural characteristics in efficient nano-phosphor Y2Mo4O15:Eu3+, Opt. Mater. Express 5 (2015) 490–496. [26] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A32 (1976) 751–767. [27] J. Grigorjevaite, A. Katelnikovas, Luminescence and luminescence quenching of K2Bi(PO4)(MoO4):Eu3+ phosphors with efficiencies close to unity, ACS Appl. Mater. Interfaces 8 (2016) 31772–31782. [28] P. Kubelka, New contributions to the optics of intensely light-scattering materials. Part I, J. Opt. Soc. Am. 38 (1948) 448–457. [29] W.T. Carnall, H. Crosswhite, H.M. Crosswhite, Energy Level Structure and Transition Probabilities in the Spectra of the Trivalent Lanthanides in LaF3, Argonne National Laboratory Report, Lemont, IL, 1977. [30] B. Herden, A. Meijerink, F.T. Rabouw, M. Haase, T. Jüstel, On the efficient luminescence of β-Na(La1−xPrx)F4, J. Lumin. 146 (2014) 302–306. [31] L. Del Longo, M. Ferrari, E. Zanghellini, M. Bettinelli, J.A. Capobianco, M. Montagna, F. Rossi, Optical spectroscopy of zinc borate glass activated by Pr3+ ions, J. Non-Cryst. Solids 231 (1998) 178–188. [32] R. Naccache, F. Vetrone, A. Speghini, M. Bettinelli, J.A. Capobianco, Cross-relaxation and upconversion processes in Pr3+ singly doped and Pr3+/Yb3+ codoped nanocrystalline Gd3Ga5O12: the sensitizer/activator relationship, J. Phys. Chem. C 112 (2008) 7750–7756. [33] M. Muller, T. Justel, Energy transfer and unusual decay behaviour of BaCa2Si3O9:Eu2+,Mn2+ phosphor, Dalton Trans. 44 (2015) 10368–10376. [34] CRC Handbook of Chemistry and Physics, 90th edition, CRC Press/Taylor and Francis, Boca Raton, FL, 2010. [35] P.F. Smet, A.B. Parmentier, D. Poelman, Selecting conversion phosphors for white light-emitting diodes, J. Electrochem. Soc. 158 (2011) R37–R54. [36] E.F. Schubert, Light-emitting Diodes, Cambridge University Press, Cambridge, UK; New York, 2003.
possess rather unique emission spectra and could be applied, for instance, in luminescent security markers, even though some efficiency would be sacrificed. Acknowledgements The authors gratefully thank Danas Sakalauskas (Vilnius University) for taking SEM images. The authors would also like to thank the reviewers for their valuable comments and suggestions. This research was funded by a grant (No. S-LZ-17-4/LSS-6100002007) from the Research Council of Lithuania. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2017.06.014. References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994. [2] J.L. Sommerdijk, A. Bril, A.Wd Jager, Luminescence of Pr3+ activated fluorides, J. Lumin. 9 (1974) 288–296. [3] J.L. Sommerdijk, A. Bril, A.Wd Jager, Two photon luminescence with ultraviolet excitation of trivalent praseodymium, J. Lumin. 8 (1974) 341–343. [4] W.W. Piper, J.A. DeLuca, F.S. Ham, Cascade fluorescent decay in Pr3+-doped fluorides: achievement of a quantum yield greater than unity for emission of visible light, J. Lumin. 8 (1974) 344–348. [5] A.M. Srivastava, W.W. Beers, Luminescence of Pr3+ in SrAl12O19: observation of two photon luminescence in oxide lattice, J. Lumin. 71 (1997) 285–290. [6] B. Zhou, L. Tao, Y.H. Tsang, W. Jin, E.Y.-B. Pun, Superbroadband near-IR photoluminescence from Pr3+-doped fluorotellurite glasses, Opt. Express 20 (2012) 3803–3813. [7] A.M. Srivastava, Aspects of Pr3+ luminescence in solids (Part B), J. Lumin. 169 (2016) 445–449. [8] P. Boutinaud, E. Pinel, M. Oubaha, R. Mahiou, E. Cavalli, M. Bettinelli, Making red emitting phosphors with Pr3+, Opt. Mater. 28 (2006) 9–13. [9] E. Pinel, P. Boutinaud, R. Mahiou, Using a structural criterion for the selection of red-emitting oxide-based compounds containing Pr3+, J. Alloy. Compd. 374 (2004) 165–168. [10] A. Katelnikovas, H. Bettentrup, D. Dutczak, A. Kareiva, T. Jüstel, On the correlation between the composition of Pr3+ doped garnet type materials and their photoluminescence properties, J. Lumin. 131 (2011) 2754–2761. [11] T. Jüstel, W. Mayr, O. Mastenbroek, Low-pressure gas discharge lamp comprising a UV-B phosphor, US20070247052 A1. [12] J.M.A. Caiut, S. Lechevallier, J. Dexpert-Ghys, B. Caillier, P. Guillot, UVC emitting phosphors obtained by spray pyrolysis, J. Lumin. 131 (2011) 628–632. [13] A.M. Srivastava, Inter- and intraconfigurational optical transitions of the Pr3+ ion for application in lighting and scintillator technologies, J. Lumin. 129 (2009) 1419–1421. [14] J.L. Yuan, J. Wang, D.B. Xiong, J.T. Zhao, Y.B. Fu, G.B. Zhang, C.S. Shi, Potential PDP phosphors with strong absorption around 172 nm: rare earth doped NaLaP2O7 and NaGdP2O7, J. Lumin. 126 (2007) 717–722.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and optical properties of Y2Mo4O15 doped by Pr3+ a
b
Indre Mackeviciute , Ausra Linkeviciute , Arturas Katelnikovas a b
a,⁎
MARK
Faculty of Chemistry and Geosciences, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania State Research Institute, Centre for Physical Sciences and Technology, Sauletekio Avenue 3, LT-10257 Vilnius, Lithuania
A R T I C L E I N F O
A B S T R A C T
Keywords: Solid state synthesis Praseodymium Photoluminescence Optical band gap Thermal quenching
A set of Y2Mo4O15 compounds doped with 0.1–30% Pr3+ was synthesized by conventional high temperature solid state synthesis. The structural, morphological and optical characteristics of the compounds were investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) analysis, thermal quenching (TQ), photoluminescence (PL) and fluorescence lifetime techniques. It turned out that Pr3+ emission spectra are very sensitive to the concentration and temperature in the Y2Mo4O15 host matrix. Samples showed mainly red and deep red luminescence when excited with blue radiation. A severe concentration quenching was observed in samples with higher Pr3+ content due to cross relaxation. Y2Mo4O15:Pr3+ luminescent materials possess rather unique emission spectra, which can be tuned by adopting the appropriate Pr3+ concentration and/or temperature. Thus these materials could be used as luminescent security markers where such properties are desirable.
1. Introduction Pr3+ is one of the most unique rare earth ions that can luminesce in a very broad spectrum ranging from deep UV through the visible to the infrared spectral range [1]. The type of luminescence spectrum of Pr3+ doped inorganic compounds is governed by the covalent character of the respective bonds between activator and the neighbouring anions, and the crystal field splitting. For instance, if the chosen host matrix has low crystal field strength and wide enough optical band gap, then all crystal field states of [Xe]4f15d1 configuration are located above the 1S0 state and efficiently populate it leading to efficient line emission in the UV region. Besides, in these cases the photon cascade emission is also sometimes observed, leading to external quantum yields larger than 100% [2–5]. If the stronger crystal field strength shifts the lowest crystal-field component of the [Xe]4f15d1 configuration below 1S0 state, then broad emission bands to the terminal 3HJ and 3F2 levels are observed in the UV region. The direct excitation of 3PJ, 1D2 and other levels of the [Xe]4f2 configuration yields characteristic sharp emission lines from cyan spectral region to infrared [6,7]. Moreover, in some compounds an efficient energy transfer from 3P0 to 1D2 levels occurs and only red emission is observed [8–10]. Based on such versatility of emission spectra, Pr3+ doped compounds found application in X-ray detectors, field emission displays (FEDs) and excimer discharge lamps; moreover, novel fast scintillators for positron emission tomographs (PET) could be obtained employing fast [Xe]4f15d1 → [Xe]4f2 luminescence of Pr3+ ions [7,11–16].
⁎
Corresponding author. E-mail address: [email protected] (A. Katelnikovas).
http://dx.doi.org/10.1016/j.jlumin.2017.06.014 Received 25 March 2017; Received in revised form 9 May 2017; Accepted 6 June 2017 Available online 07 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Recently, Pr3+ doped molybdates gained lots of attention due to their excellent mechanical properties, good physical, chemical, thermal stability and optical properties [17]. Pr3+ doped molybdates, therefore, are considered for application in optical amplifiers, white LEDs and displays as red phosphors [18–21]. In this work, the luminescence properties of Y2Mo4O15:Pr3+ powders were investigated as a function of Pr3+ concentration and temperature. These materials showed good colour saturation and high luminous efficacies. Moreover, it turned out that emission spectra can be significantly tuned by selecting the appropriate Pr3+ concentration and/or temperature. This feature is very desirable considering the application in luminescent security pigments. 2. Experimental Y2Mo4O15:Pr3+ powder samples were prepared by a typical solidstate reaction method from high purity Pr6O11 (99.9% Acros Organics), MoO3 (99+% Acros Organics) and Y2O3 (99.99% Tailorlux). The stoichiometric amounts of yttrium, praseodymium, and molybdenum oxides were weighed to the nearest 0.0001 g, placed to the agate mortar and thoroughly mixed using a small amount of acetone as grinding media. The obtained blend was transferred to a porcelain crucible and annealed at 700 °C for 12 h in air [22]. During doping Y3+ was replaced by Pr3+, with concentrations ranging from 0.1% to 30%. Powder XRD analysis has been carried out employing a Rigaku MiniFlexII diffractometer working in the Bragg–Brentano (θ/2θ)
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focusing geometry. The data were collected within 2θ angle from 10° to 80° (step width 0.02° and scanning speed 5°/min) using Ni-filtered Cu Kα radiation. SEM images of phosphor powders were taken with a FE-SEM Hitachi SU-70. The accelerating voltage was 2 kV. Reflection spectra in the range of 250–800 nm were recorded on an Edinburgh Instruments FSL980 spectrometer equipped with a 450 W Xe arc lamp, double excitation and emission monochromators, a cooled (−20 °C) single-photon counting photomultiplier (Hamamatsu R928) and a Teflon coated integration sphere. Teflon was also used as a reflectance standard. The excitation and emission slits were set to 3.0 and 0.15 nm, respectively. Step width was 0.5 nm and integration time 0.2 s. Excitation and emission spectra for powder samples were recorded on the Edinburgh Instruments FLS980 spectrometer equipped with double excitation and emission monochromators, 450 W Xe arc lamp, a cooled (−20 °C) single-photon counting photomultiplier (Hamamatsu R928) and mirror optics for powder samples. When measuring excitation spectra (λem = 619 nm) excitation and emission slits were set to 0.5 and 1 nm, respectively. The excitation spectra were corrected by a reference detector. When measuring emission spectra (λex = 448 nm) excitation and emission slits were set to 5 and 0.25 nm respectively. The photoluminescence emission spectra were corrected by a correction file obtained from a tungsten incandescent lamp certified by NPL (National Physics Laboratory, UK). In both cases step width was 0.5 nm and integration time was 0.4 s. For thermal quenching (TQ) measurements a cryostat “MicrostatN” from the Oxford Instruments had been applied to the present spectrometer. Liquid nitrogen was used as a cooling agent. The measurements were performed at 77 K and at 100–500 K in 50 K intervals. Temperature stabilization time was 90 s and temperature tolerance was set to ± 5 K. During the measurements dried nitrogen was flushed over the cryostat window to avoid the condensation of water at low temperatures on the surface of the window. The photoluminescence decay curves were measured on the FLS980 spectrometer. A Xe μ-flash lamp μF920 was used as an excitation source. Excitation wavelength was 448 or 583 nm and emission was monitored at 619 or 609 nm.
Fig. 1. XRD patterns of undoped (a), 25% Pr3+ doped (b), and 30% Pr3+ doped (c) Y2Mo4O15:Pr3+ samples. The reference pattern of Y2Mo4O15 is given for comparison (d).
are well formed and with rather broad size distribution in the range of 1–10 µm. No substantial changes in particle size and morphology was observed when Pr3+ was introduced into the lattice. The body colour of undoped Y2Mo4O15 sample is white, indicating that it does not absorb radiation in the visible spectral region. This goes hand in hand with the reflection spectra depicted in Fig. 3, which indicate only strong absorption of the Y2Mo4O15 host in the range of 250–400 nm. Doped samples gradually gained greenish tint upon increasing Pr3+ concentration due to increasing absorption of Pr3+ ions in the blue and red spectral regions. Three absorption lines lying in the blue spectral region can be assigned to the 3H4 → 3P2 (ca. 450 nm), 3H4 → 3P1 + 1I6 (ca. 475 nm), and 3H4 → 3P0 (ca. 490 nm) transitions of Pr3+ ions. The collection of absorption lines in the red spectral region, in turn, can be assigned to the 3H4 → 1D2 (ca. 595 nm) transition. Fig. 3 also demonstrate that synthesized powders are of high quality since the reflectance at longer wavelengths are close to unity [27]. The optical band gap of the undoped sample was calculated from the absorption spectrum, which was obtained from reflection spectrum by applying Kubelka-Munk function [28]. The obtained optical band gap value for Y2Mo4O15 sample was 3.71 eV (334 nm). It is also obvious that the UV absorption band of Pr3+ doped sample is shifted to the longer wavelengths (lower energies) what might be attributed to the Pr3+ [Xe]4f2 → [Xe]4f15d1 absorption that coincides with the band gap absorption and causes the small shift towards lower energies. Fig. 4a shows normalized excitation spectra of 0.5% and 10% Pr3+ doped samples for 609 nm emission. Recorded spectra contain a broad band in the range of 250–370 nm and two sets of lines in the range of 480–510 nm and 575–610 nm. These results are consistent with the reflection spectra shown in Fig. 3. The broad band can be attributed to the host matrix absorption, whereas lines in the blue and red spectral regions can be assigned to the 3H4 → 3PJ + 1I6 and 3H4 → 1D2 transitions, respectively. It is interesting to note that samples are more efficiently excited through host matrix absorption band when Pr3+ concentration increases. The normalized emission spectra of Y2Mo4O15:Pr3+ phosphors doped with 0.5% and 10% Pr3+ under 448 nm excitation are depicted in Fig. 4b. The spectra contain seven sets of emission lines that originate from electronic transitions 3PJ → 3H4 (ca. 490 nm), 3PJ → 3H5 (ca.
3. Results and discussion The formula of Y2Mo4O15 compound does not show its real structure. It could be better expressed as Y2[MoO4]2[Mo2O7] [23]. This compound is isostructural with Ho2[MoO4]2[Mo2O7], which in 1988 was reported by V.A. Efremov et al. [24] Y2Mo4O15 compound adopts monoclinic Bravais lattice with the space group P21/c (#14), Z=2. The unit cell consists of isolated [MoO4]2- and [Mo2O7]2- units and Y3+ ions filling the voids. There is one crystallographic site for yttrium ions and they are coordinated by seven oxygen ions [23,25]. It was assumed that Pr3+ ions occupy Y3+ sites due to identical charge and similar ionic radii (r(Pr3+) = 0.99 Å for CN = 6, and r(Y3+) = 0.90 Å for CN = 6) of both ions [26]. The XRD patterns of undoped, 25% and 30% Pr3+ doped samples are given in Fig. 1. The reference pattern of Y2Mo4O15 (PDF4+ (ICDD) 04–008–9148) is also given for comparison. The XRD patterns of samples up to 25% Pr3+ match well with reference pattern indicating that single phase compounds are obtained. However, when Pr3+ concentration was increased to 30% and above the additional peaks appeared in the patterns implying the formation of secondary phases. These peaks are marked with arrows in Fig. 1c and their intensity increased with further increase of Pr3+ concentration. All these findings suggest that the solubility limit of Pr3+ in Y2Mo4O15 lattice is around 25%. The morphology features of obtained Y2Mo4O15 and Y2Mo4O15:25% Pr3+ powders we inspected by taking high resolution SEM images, which are given in Fig. 2. The obtained particles of undoped compound 526
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Fig. 2. SEM images of Y2Mo4O15 (a) and Y2Mo4O15:25%Pr3+ (b).
individual line to a particular transition. In order to at least partly do that we have also recorded emission spectra under 583 nm excitation (3H4 → 1D2 absorption transition). In this case, lines observed in the emission spectra originate solely from the excited 1D2 level. The obtained emission spectra for the samples doped with 0.5% and 10% Pr3+ are given in Fig. S1. The emission spectra of Y2Mo4O15:0.5%Pr3+ (Fig. S1a) are virtually the same regardless the excitation wavelength (448 or 583 nm). The only difference in these spectra the emerging small peak at ca. 619 nm when samples were excited with 448 nm radiation. This suggests that this line originate from the 3PJ → 3H6 transition. Of course the other transitions originating from the 3PJ levels were also absent in the emission spectrum when sample was excited with 583 nm radiation. The changes in emission spectra of 10% Pr3+ doped sample under these excitation wavelengths were substantial. Virtually no emission lines from 1D2 → 3H5 (ca. 700 nm) and 1D2 → 3H6 (ca. 800 nm) transitions were observed in emission spectrum recorded for 448 nm excitation, thus it could be assumed that the same is also true for the 1D2 → 3H4 transition. This leads to the conclusion that the emission lines in the range of 580–640 nm could be attributed to the 3PJ → 3H6 transition. Moreover, the time resolved emission spectra (TRES) were also recorded in order to support these findings. The TRES in the time range of 1–7.5 μs are given in Fig. S2. These measurements showed that faster spin-allowed 3PJ → 3H6 transition yields the emission line at ca. 619 nm. Moreover, the emission from 3PJ → 3H6 transition is over in less than 7.5 μs and only the lines originating from the slower spinforbidden 1D2 → 3H4 transition remain after 7.5 μs. Emission spectra of 0.5% and 10% Pr3+ doped samples are significantly different. The one recorded for 0.5% Pr3+ specimen is dominated by intensive 1D2 → 3HJ transitions. Since the samples were
Fig. 3. Reflection spectra of Y2Mo4O15 and Y2Mo4O15:25%Pr3+.
535 nm), 3PJ → 3H6 and 1D2 → 3H4 (ca. 590–630 nm), 3PJ → 3F2 (ca. 650 nm), 1D2 → 3H5 (ca. 700 nm), 3PJ → 3F4 (ca. 740 nm), and 1D2 → 3 H6 (ca. 800 nm) of Pr3+ ions [29]. The strongest emission intensity was obtained for the sample doped with 0.5% Pr3+. This sample showed 2.8 ± 0.5% quantum yield. The quantum yield of the other samples was not possible to measure due to too low absorption and/or emission what yielded unreliable data. Further increase of Pr3+ concentration has resulted in significant decrease of emission intensity due to concentration quenching. Concentration quenching occurring at low Pr3+ concentrations is a well-known problem even though efficient luminescence in some heavily Pr3+ doped compounds is reported every so often [30]. The emission lines originating from 3PJ → 3H6 and 1D2 → 3H4 transitions are overlapping, thus it is very difficult to assign every
Fig. 4. (a) Excitation (λem = 609 nm) and (b) emission (λex = 448 nm) spectra of samples doped with 0.5% and 10% Pr3+.
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Fig. 5. PL decay curves of Y2Mo4O15:Pr3+ samples as a function of Pr3+ concentration. (a) λex = 448 nm and λem = 619 nm, (b) λex = 583 nm and λem = 609 nm. Inset table shows calculated decay values. IRF denote Instrument Response Function.
excited to the 3PJ levels, it is evident, that substantial amount of these levels relaxes to 1D2 level, from which luminescence occurs. The energy difference between 3PJ and 1D2 levels is in the range of 4000–5000 cm−1, thus the relaxation is likely caused by multiphonon relaxation, since Y2Mo4O15 host possesses rather high phonon energies (ca. 950 cm−1) [23]. However, with increasing Pr3+ concentration the intensity of 1D2 → 3H4 transition started decreasing and virtually vanished when Pr3+ content reached 10%. This phenomenon is likely caused by cross-relaxation process from 1D2 level, what of course become more probable with increasing Pr3+ concentration. Such cross relaxation mechanisms can be written as: 1D2 + 3H4 → 1G4 + 3F4 or 1 D2 + 3H4 → 3F4 + 1G4 [31]. The recorded PL decay curves of Y2Mo4O15:Pr3+ samples as a function of Pr3+ concentration are given in Fig. 5. The decay curves for 448 nm excitation and 619 nm emission are given in Fig. 5a, whereas decay curves for 583 nm excitation and 609 nm emission are represented in Fig. 5b. Excitation at 448 nm yields emission from both 3PJ → 3H6 and 1D2 → 3H4 transitions, meanwhile only 1D2 → 3H4 transition emission is observed if samples are excited with 583 nm. The curves get steeper with increasing Pr3+ concentration indicating fast decrease of PL decay lifetimes and internal efficiency. In order to calculate the PL decay lifetimes decay curves were fit using triexponential fit function:
I (t ) = A + B1 e−t / τ1 + B2 e−t / τ2 + B3 e−t / τ3
Different situation was observed for 10% Pr3+ doped sample, where low temperature emission spectra are dominated by 3PJ → 3H4, 3PJ → 3 H6, 3PJ → 3F2, and 3PJ → 3F4 transitions. Contrary to the temperature dependent emission spectra of 0.5% Pr3+ counterpart, almost no emission from 1D2 level was observed. It is likely, that multiphonon relaxation from 3PJ to 1D2 levels for 10% Pr3+ doped sample also increases with increasing temperature but the sample undergo severe cross-relaxation and the 1D2 level is depopulated in non-radiative pathway, and only emission from 3PJ levels are seen in the spectra. This goes line in line with the decrease of 1D2 → 3H4 emission intensity with increasing Pr3+ concentration observed at room temperature measurements (Fig. 4b). The temperature dependent integrated emission values (normalized to 77 K) of 0.5% (Fig. 6c) and 10% Pr3+ (Fig. 7b) doped samples were used to determine the activation energy – Ea of thermal quenching process and temperature at which phosphor loses half of its luminescence intensity, denoted as TQ1/2. The mentioned data were fit with Fermi-Dirac distribution function [33]:
I (T ) =
I0 1+B×e
−Ea kT
(2)
Here I(T) and I0 are luminescence intensity at a given temperature T and zero Kelvin, respectively. B is frequency factor and k is Boltzmann constant (8.617342·10−5 eV/K) [34]. The obtained Ea values for 0.5% and 10% Pr3+ doped compounds were 0.19 ± 0.02 eV and 0.086 ± 0.012 eV, respectively. Expressing T for I(T)/I0 = 1/2 from Eq. (2) yields Eq. (3) which gives TQ1/2 values:
(1)
Here I(t) is PL intensity at a given time t; A is background; B1, B2, B3 are constants; and τ1, τ2, τ3, are PL decay lifetimes. The obtained values are given in the inset table of Figs. 5a and b, which also includes calculated average PL lifetime values τ . The slightly higher PL lifetime values were obtained when samples were excited at 583 nm (Fig. 5b) what could be related to the absence of faster emission from 3PJ levels. The decrease of PL lifetime values with increasing Pr3+ concentration is likely caused by increasing probability of cross relaxation processes, such as 3P0 + 3 H4 → 3H6 + 1D2 [32]. However, it is still not clear why samples showed three kinds of luminescence decay and this matter needs further investigation. In order to evaluate the Y2Mo4O15:Pr3+ luminescent materials performance at elevated temperature, the temperature dependent emission spectra were recorded in the range of 77–500 K. Such emission spectra of two representative samples doped with 0.5% and 10% Pr3+ are shown in Figs. 6a and 7a, respectively. The most intensive lines in low temperature emission spectra of 0.5% Pr3+ doped samples originate from 1D2 → 3H4, 3PJ → 3F2, 1D2 → 3H5, and 1D2 → 3H6, transitions. The temperature increase leads to decrease of emission intensity, which to some extent is compensated by emission line broadening at least to ca. 300 K. Further increase of the temperature leads to the severe reduction in emission intensity as a result of thermal quenching (see Fig. 6c). Besides the overall decrease of emission intensity with increasing temperature, the increased emission from the higher vibronic states of 1D2 level was also observed as shown in the inset of Fig. 6a.
TQ1/2 =
−Ea k × ln(1/ B )
(3) 3+
The calculated TQ1/2 values for 0.5% and 10% Pr doped samples were 400 ± 7 K and 238 ± 9 K, respectively. These values are consistent with Ea values; i.e. higher quenching temperature requires more energy. The temperature dependent measurements confirm, that heavily Pr3+ doped samples suffer from severe thermal quenching. The temperature dependent photoluminescence decay curves of Y2Mo4O15:0.5%Pr3+ sample were also recorded and are shown in Fig. 6b. The curves become steeper with increasing temperature indicating that the rate of non-radiative transitions from 3PJ levels increases. The calculated temperature dependent average decay time values are plotted in Fig. 6c and the profile is very similar for the one obtained from normalized emission integrals of the same sample. Unfortunately, we were not able to measure temperature dependent photoluminescence decay curves for the Y2Mo4O15:10%Pr3+ sample, since the PL decay was already too fast for the μ-flash lamp. Fig. 8a shows the Pr3+ concentration dependent colour points of Y2Mo4O15:Pr3+ samples in the CIE 1931 colour space diagram. The colour coordinates of samples with low Pr3+ content (up to 1%) are in 528
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Fig. 6. Temperature dependent emission spectra (a), decay curves (b) and Ea, TQ1/2 estimation for Y2Mo4O15:0.5%Pr3+ (c). The inset of section (a) shows normalized (at 609 nm) emission spectra. The IRF in section (b) denote Instrument response function. Please note that the error bars for the lifetime values in section (c) are smaller than the symbol.
human eye sensitivity curve (with maximum at 555 nm and the highest LE value of 683 lm/Wopt). The calculated LE values as a function of Pr3+ concentration and as a function of temperature of 0.5% and 10% Pr3+ doped samples are given in Fig. 8d. The increasing Pr3+ concentration leads to decreasing LE values due to raising emission intensity in the cyan (3PJ → 3H4 transition) and especially in the deep red (3PJ → 3F4 transition) spectral region where human eye is very insensitive [36]. The LE values of 0.5% Pr3+ doped sample increases with raising temperature up to 300 K, and then start sharply decreasing. The increase can be attributed to the already discussed relative increase of the 1D2 → 3H4 transition in the orange spectral region where human eye is more sensitive if compared to the red. The decreasing LE values above 300 K could be explained by the emission line broadening and increasing relative intensity in the red spectral region. The LE values of 10% Pr3+ doped sample increased with increasing temperature what is associated with virtually stable relative emission from 3PJ → 3H4 transition (cyan region), increasing relative intensity of 3PJ → 3H5 transition (green region) and decreasing relative emission intensities of the transitions in the red spectral region. All this leads to the increase of LE values with increasing temperature.
the red area and close to the edge of the CIE 1931 chromaticity diagram. This shows high colour saturation. However, further increase of Pr3+ concentration resulted in colour coordinate shift towards the centre of diagram and 25% Pr3+ doped sample showed emission whose colour coordinate was in the white area straight on the black body locus (BBL) with the colour temperature of ca. 3400 K. The present observation is in good agreement with Pr3+ concentration dependent emission (Fig. 4b) where part of the red emission intensity decreased and intensity of cyan and green emission increased with increasing Pr3+ concentration. The temperature dependent colour coordinates of 0.5% and 10% Pr3+ doped samples were also calculated and are depicted in Fig. 8b and c, respectively. The colour coordinates of the former are close to the edge of the colour diagram and shifts linearly from red to orange spectral region with increasing temperature. This is explained by the relative increase of emission from higher 1D2 vibronic levels at elevated temperatures what was already discussed above. The temperature dependent colour coordinates of 10% Pr3+ doped sample, however, showed a completely different behaviour; being slightly distant form the edge of the chromaticity diagram they shift to the orange spectral region and move closer to the edge of the diagram as the temperature increases. Besides the colour coordinates, the luminous efficacy (LE) values were also calculated from emission spectra. Luminous efficacy is a parameter describing how bright the radiation is perceived by the average human eye and are calculated by using the following equation [35]:
LE (lm / Wopt ) = 683 (lm / Wopt ) ×
∫ I (λ ) V (λ ) dλ ∫ I (λ ) dλ
4. Conclusions In summary, single phase Y2Mo4O15:Pr3+ luminescent materials with Pr3+ concentration up to 25% were prepared by solid state reaction at relatively low temperature. The optical properties of prepared powders were investigated as a function of Pr3+ concentration and temperature. The luminescence measurements revealed that emission spectra of synthesized powders can be tuned by adjusting the Pr3+ concentration and/or temperature. It also turned out that the increase of Pr3+ concentration leads to severe concentration quenching due to cross relaxation. Nevertheless, samples with higher Pr3+ concentration
(4)
Here I(λ) is the emission spectrum of the phosphor and V(λ) is the
Fig. 7. Temperature dependent emission spectra (a) and Ea, TQ1/2 estimation for Y2Mo4O15:10%Pr3+ (b).
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