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Luminescence and luminescence quenching of Eu2Mo4O15
Author’s Accepted Manuscript Luminescence and luminescence quenching of Eu2Mo4O15 Matas Janulevicius, Julija Grigorjevaite, Greta Merkininkaite, Simas Sakirzanovas, Arturas Katelnikovas www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(16)30303-9 http://dx.doi.org/10.1016/j.jlumin.2016.06.050 LUMIN14079
To appear in: Journal of Luminescence Received date: 7 March 2016 Revised date: 31 May 2016 Accepted date: 20 June 2016 Cite this article as: Matas Janulevicius, Julija Grigorjevaite, Greta Merkininkaite, Simas Sakirzanovas and Arturas Katelnikovas, Luminescence and luminescence quenching of Eu2Mo4O15, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.06.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Luminescence and luminescence quenching of Eu2Mo4O15 Matas Januleviciusa, Julija Grigorjevaitea, Greta Merkininkaitea, Simas Sakirzanovasb, Arturas Katelnikovasa,* a
Department of Analytical and Environmental Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania b
Department of Applied Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania *
Corresponding author: Tel: +370 697 23123; E-mail: [email protected]
Abstract A polycrystalline Eu2Mo4O15 phosphor sample was prepared by high temperature solid state reaction. Phase purity and morphological features of the phosphor were investigated by X-ray diffraction and scanning electron microscopy, respectively. Reflectance spectra showed that the optical band gap of Eu2Mo4O15 is 2.95 eV. Phosphor emits intensive red light when excited with 394 and 465 nm radiation. Temperature dependent emission and luminescence lifetime measurements revealed that external and internal quantum yields decrease at the same rate and that luminescence quenches due to photoionization. The calculated external quantum yields for 394 and 465 nm excitation were 7.8% and 53.5%, respectively.
Keywords Molybdate; thermal quenching; quantum yield; colour coordinate; luminous efficacy.
Introduction Inorganic materials based on molybdates form a large family of inorganic compounds. They are widely used in luminescent materials [1, 2], scintillators [3], electrochemical and electrochromic devices [4-6] and upconversion [7-9] due to their unique chemical and physical properties. Based on such broad application area the molybdates are intensively studied and new structures are reported every now and then [10]. Rare earth doped/substituted molybdates are of particular interest due to their potential application in solid state light sources based on near-UV LEDs[11-13] and biological imaging [14]. The absorption strength of spin and parity forbidden [Xe]4fn → [Xe]4fn transitions of rare earth ions usually is rather weak. However, these transitions become rather intensive in molybdate hosts. The absorption intensity of optical transitions at higher energies (< 400 nm) get especially strong due to admixing with charge 1
transfer (CT) band, which is allowed one. This phenomenon is exploited in manufacturing of very bright Eu3+ doped molybdate phosphors with extremely high quantum yields [15, 16]. The Eu2Mo4O15 compound reported in this work was obtained by chance when investigating a series of Y2Mo4O15:Eu3+ phosphors. The title compound crystallizes in different system if compared to Y2Mo4O15, therefore, their optical properties would be difficult to compare. The luminescence properties of Eu2Mo4O15 have not been reported so far. Thus, in this work we present a full investigation of Eu2Mo4O15 optical properties including temperature dependent emission and luminescence lifetime measurements.
Experimental The powder sample of Eu2Mo4O15 was prepared by high temperature solid state reaction. The stoichiometric amounts of Eu2O3 (99.99% Tailorlux), and MoO3 (99+% Acros Organics) were thoroughly blended in an agate mortar with some acetone as the grinding medium. The mixture of starting materials was dried, transferred to the porcelain crucible and annealed at 700 °C for 12 h in air. Powder XRD pattern was recorded in the range of 10° ≤ 2θ ≤ 80° (step width 0.02° and scanning speed 5 °/min.) using Ni-filtered Cu Kα radiation on a Rigaku MiniFlexII diffractometer working in Bragg-Brentano (θ/2θ) geometry. The SEM images were taken with Hitachi TM3000 scanning electron microscope. The accelerating voltage was 15 kV. Reflection spectra were recorded on an 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 Teflon coated integration sphere. Teflon was used as a reflectance standard. The excitation and emission slits were set to 3.00 and 0.50 nm respectively. Step width was 0.5 nm and integration time was 0.4 s. Excitation and emission spectra 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. The photoluminescence emission spectra were corrected by a correction file obtained from a tungsten incandescent lamp certified by NPL (National Physics Laboratory, UK). When measuring emission spectra (λex = 465 nm) excitation and emission slits were set to 0.6 and 0.25 nm respectively. When measuring excitation spectra (λem = 618 nm) 2
excitation and emission slits were set to 0.25 and 0.5 nm, respectively. The excitation spectra were corrected by a reference detector. In both cases step width was 0.25 nm and integration time was 0.2 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 kinetics studies were performed on the same FLS980 spectrometer. Xe μ-flash lamp was used as an excitation source. The excitation wavelength was 465 nm and emission was monitored at 618 nm. External quantum yields (EQY) were calculated by measuring emission spectrum of the Teflon sample in Teflon coated integration sphere under excitation of 394 and 465 nm. The emission spectra were recorded in the range of 380–800 nm and 450–800 nm, respectively. The same measurements were repeated for the phosphor sample. The EQY values were obtained employing the following formula [17]:
QY
I I
em , sample
abs,Teflon
where
I
I em,Teflon
I abs, sample
em , sample
and
I
100%
em , Teflon
Teflon, respectively. Likewise,
(1)
are integrated emission intensities of the phosphor sample and
I
abs, sample
I
and
abs, Teflon
are the integrated absorption of the
phosphor sample and Teflon, respectively. Quantum yield measurements were repeated five times for each excitation wavelength to gain some statistical data.
Results and discussion The Eu2Mo4O15 sample was obtained by chance when synthetizing the end sample of Y2Mo4O15:Eu3+ series. Y2Mo4O15 crystallizes in monoclinic lattice with P21/c (#14) space group [18]. However, the solubility of Eu3+ in Y2Mo4O15 seems to be limited to around 75% and complete replacement of Y3+ by Eu3+ ions yielded Eu2Mo4O15 with triclinic crystal structure and 3
P 1 (#2) space group. After the literature research we found that the title compound was reported as Eu6Mo12O45 by H. Naruke and T. Yamase [19]. The crystal structure of Eu2Mo4O15 is rather interesting. It comprises four [MoO4]2– tetrahedra, one [MoO5]4– square pyramid, one [MoO6]6– octahedron, and three different Eu sites each coordinated with eight oxygen ions. Two [MoO4]2– tetrahedra are isolated whereas the rest polyhedral undergo heavy condensation giving rise to a polymeric [Mo4O17]∞ chain running along the a-axis [19]. The recorded XRD pattern is given in Fig. 1 and matches well with a reference pattern of Eu2Mo4O15 (PDF-4+ (ICDD) 04-012-4672). One can note, however, that the integrated intensity ratio between sample and reference peaks is different, what could be explained by the preferred orientation of measured powder particles.
Fig. 1.
The morphological features of Eu2Mo4O15 were investigated by taking SEM images (Fig. 2). The obtained images reveal that powder is composed of various size crystallites fused together. This fusing suggests that the melting point of Eu2Mo4O15 is rather close to 700 °C temperature which was used during synthesis. The estimated average grain size of the powder is in the range of 15 – 20 µm. The observed small loose plate-like particles are likely to be the debris left after grinding of the synthesized powder.
Fig. 2.
The body colour of Eu2Mo4O15 sample was yellowish suggesting that phosphor absorbs in the violet-blue spectral region. This assumption was confirmed by reflection spectra (Fig. 3a), which shows a broad absorption band from 250 to 420 nm. In order to calculate the optical band gap of the material the Kubelka-Munk function [20, 21] was applied to the reflection spectra. The estimated Eu2Mo4O15 optical band gap value is 2.95 eV (23810 cm–1). Above 440 nm the absorption lines originating from intraconfigurational [Xe]4f6 → [Xe]4f6 transitions of Eu3+ ions are clearly visible. The most intense sets of lines are located at 465, 535, and 590 nm and correspond to the
7
F0 → 5D2,
7
F1 → 5D1, and
7
F1 → 5D0 transitions, respectively. High
reflectivity at longer wavelengths indicate that the obtained phosphor is of high quality and possesses little defects. Fig. 3b shows excitation spectra at 77 K and room temperature (RT) for 618 nm emission. It is evident that temperature influences the excitation spectra a lot. Both excitation spectra were normalized for better comparison. The first thing that catches the eye is the width of the charge transfer (CT) band, which considerably shrinks at low temperature. This results in rise 4
of 7F0 → 5L6 transition intensity, which was suppressed at room temperature. The decrease of charge transfer energy (increase of the wavelength) at elevated temperatures is explained by increase of lattice vibrations, thus increasing the distance (reducing bond strength) between Eu3+ and O2– ions. It was also observed that the intensity of excitation lines originating from 7F1 and 7
F2 levels strongly decrease if the temperature of phosphor is lowered to 77 K. Reducing the
temperature leads to depopulation of 7F1 and 7F2 levels, therefore, only weak or no excitation intensity is observed [22]. The emission spectra of Eu2Mo4O15 phosphor under 465 nm excitation at RT and 77 K are depicted in Fig. 3c. Both spectra contain five sets of emission lines that originate from the 5
D0 → 7F0 (≈ 580 nm), 5D0 → 7F1 (585–603 nm), 5D0 → 7F2 (603–639 nm), 5D0 → 7F3 (639–
670 nm), and 5D0 → 7F4 (680–715 nm) transitions. Usually very sharp line due to 5D0 → 7F0 transition is rather broad for Eu2Mo4O15 what is in line with three different crystallographic sites of Eu3+. The emission line broadening is also observed at room temperature spectra if compared to the one taken at 77 K. This is also related to the higher lattice vibrations at RT leading to many non-equivalent Eu3+ sites emitting at slightly different wavelengths and, therefore, causing line broadening.
Fig. 3.
Fig. 4.
For further Eu2Mo4O15 phosphor investigation the temperature dependent emission spectra were recorded under 465 nm excitation. The obtained spectra are shown in Fig. 4a. and the respective normalized emission integrals are given in Fig. 4b. Emission intensity gradually decreased with the increasing temperature. However, the integral intensity increased up to 250 K and only then started severely decrease. The initial increase of integral intensity is related to the line broadening, which compensate the decrease of emission intensity to some extent. Nevertheless, at temperatures higher than 250 K the regular thermal quenching sets in and both emission and integral intensity decrease. Temperature dependent emission integrals were employed in TQ1/2 value estimation. The obtained experimental data were fit with Boltzmann sigmoidal function [15] and the obtained TQ1/2 value was 354 K (±7 K). TQ1/2 shows a temperature at which the phosphor loses half of its efficiency. The experimental data represented in Fig. 4b were also fit with a single barrier quenching model in order to obtain the activation energy (EA) of the thermal quenching process [11]: 5
I (T ) 1 I0 1 Be E A / kT
(2)
Here I(T) and I0 are temperature dependent emission integral and highest value of emission integral, respectively. EA is activation energy, k is Boltzmann constant (8.617342·10-5 eV/K) [23], and T is temperature in K. B is the quenching frequency factor. The calculated thermal quenching activation energy was 0.42 eV (±0.12 eV). The thermal quenching most likely occurs due to photoionization what is in line with decreasing optical band gap energy with increasing temperature. Temperature dependent decay curves of Eu2Mo4O15 phosphor were also measured and are shown in Fig. 4c. At all temperatures except 450 and 500 K a single exponential decay was observed. The decay curves corresponding to temperatures in the range of 77–300 K were very close to each other indicating that the internal quantum yield does not change in this temperature interval. However, when phosphor temperature exceeded 300 K the decay curves became steeper and steeper indicating shortening of the luminescence lifetimes. The decrease of luminescence lifetime values upon temperature increase can be related to the increased probability of nonradiative processes, which decrease lifetime of the excited state [24]. The temperature dependent decay curves were fit employing a single exponential decay function (except for 450 and 500 K, which were fit with bi-exponential decay function) and the obtained luminescence lifetime (τ1/e) values are represented in Fig. 4d. The lifetime values like emission integrals remain rather constant up to 250 K and only then start sharply decreasing. The TQ1/2 value calculated from temperature dependent luminescence lifetime data was 362 K (±5 K). This is the same as the value achieved from emission integral data if the calculation errors are taken into account. The emission integrals are proportional to the external quantum yield, whereas the luminescence lifetimes are proportional to the internal quantum yield (IQY). The relation between the both can be written as [15]:
EQY IQY esc
(3)
where ηesc is the escape efficiency of emitted photons from the phosphor particle. Since emission integrals and luminescence lifetimes change identically with increasing temperature it can be concluded that EQY = IQY. In this case it is also clear that ηesc must be close to unity. The external quantum yield at room temperature calculated for 465 nm (7F0 → 5D2) excitation was 53.5% (±1.5%). Such relatively high quantum yield of this fully concentrated compound can be explained by efficient shielding of Eu3+ ions from each other by molybdate units and by the fact that the distance between Eu3+ ions is rather large (3.680 – 6.014 Å) [19]. 6
On the other hand, the EQY for 394 nm (7F0 → 5L6) excitation was considerably lower – only 7.8% (±0.4%). This phenomenon can be explained by the fact that 5L6 level is already inside the band gap, thus lots of excitation energy is lost. Finally, the CIE 1931 colour coordinates and luminous efficacy (LE) values were calculated from room temperature emission spectra (λex = 465 nm). The colour coordinate of Eu2Mo4O15 phosphor is (0.6622; 0.3375) and LE = 208 lm/Wopt. The colour coordinates of Eu2Mo4O15 and Y2Mo4O15:Eu3+ (0.662; 0.334) [25] are shown in a fragment of CIE 1931 colour space diagram depicted in inset of Fig. 3c. It is evident that colour point of Eu2Mo4O15 is directly on the edge of colour space diagram what shows superior colour saturation if compared to the Y2Mo4O15:Eu3+. The LE values are relatively lower if compared to the other Eu3+ doped molybdates [15]. This is due to the intensive emission at around 700 nm where the human eye sensitivity is low.
Conclusions All in all, Eu2Mo4O15 is a very interesting red emitting phosphor possessing moderate quantum yield, which likely could still be improved to some extent by choosing different synthesis method, temperature, or particle size/shape optimization. Unfortunately, it suffers from the severe thermal quenching at temperatures above 300 K, what would prevent it from high temperature applications. However, due to the unique emission spectra this phosphor could find applications in the areas where the quantum yield is not of the primary importance, for instance luminescent markers and so on.
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[8] A.M. Kaczmarek, R. Van Deun, Chem. Soc. Rev. 42 (2013) 8835-8848. [9] H.N. Luitel, R. Chand, T. Torikai, M. Yada, T. Watari, RSC Adv. 5 (2015) 17034-17040. [10] I. Hartenbach, H.A. Hoppe, K. Kazmierczak, T. Schleid, Dalton Trans. 43 (2014) 1401614021. [11] F. Baur, F. Glocker, T. Justel, J. Mater. Chem. C 3 (2015) 2054-2064. [12] L. Hou, S. Cui, Z. Fu, Z. Wu, X. Fu, J.H. Jeong, Dalton Trans. 43 (2014) 5382-5392. [13] S. Cui, Y. Zhu, W. Xu, P. Zhou, L. Xia, X. Chen, H. Song, W. Han, Dalton Trans. 43 (2014) 13293-13298. [14] M. Yang, Y. Liang, Q. Gui, B. Zhao, D. Jin, M. Lin, L. Yan, H. You, L. Dai, Y. Liu, Sci. Rep. 5 (2015) 11844. [15] A. Katelnikovas, J. Plewa, S. Sakirzanovas, D. Dutczak, D. Enseling, F. Baur, H. Winkler, A. Kareiva, T. Jüstel, J. Mater. Chem. 22 (2012) 22126-22134. [16] F. Baur, T. Justel, Aust. J. Chem. 68 (2015) 1727-1734. [17] F. Baur, A. Katelnikovas, S. Sakirzanovas, R. Petry, T. Justel, Z. Naturforsch. B 69 (2014) 183-192. [18] H. Wang, J. Peng, X.J. Li, Y.M. Hao, D.F. Chen, Z. Hu, Physica B 388 (2007) 278-284. [19] H. Naruke, T. Yamase, Inorg. Chem. 41 (2002) 6514-6520. [20] P. Kubelka, J. Opt. Soc. Am. 38 (1948) 448-457. [21] H. Deng, Z. Zhao, J. Wang, Z. Hei, M. Li, H.M. Noh, J.H. Jeong, R. Yu, J. Solid State Chem. 228 (2015) 110-116. [22] K. Binnemans, Bull. Soc. Chim. Belg. 105 (1996) 793-798. [23] CRC Handbook of Chemistry and Physics, 90th Edition (CD-ROM Version 2010), CRC Press/Taylor and Francis, Boca Raton, FL. [24] W.M. Yen, S. Shionoya, H. Yamamoto, Fundamentals of Phosphors, CRC Press, Boca Raton, 2007. [25] H. Deng, N. Xue, Z. Hei, M. He, T. Wang, N. Xie, R. Yu, Optical Materials Express 5 (2015) 490-496.
Figure captions: Fig. 5. XRD pattern of Eu2Mo4O15 and a reference pattern of Eu2Mo4O15. Fig. 6. SEM images of Eu2Mo4O15 under magnification of (a) 1.0 k and (b) 2.5 k. Fig. 7. Reflection (a), excitation at RT and 77 K (b), and emission at RT and 77 K (c) spectra of Eu2Mo4O15. Inset in (c) shows a fraction of CIE 1931 colour space diagram with colour coordinates with Eu2Mo4O15 and Y2Mo4O15:Eu3+. Fig. 8. Temperature dependent emission spectra of Eu2Mo4O15 (a) and TQ1/2 estimation (b). Temperature dependent decay curves of Eu2Mo4O15 (c) and TQ1/2 estimation (d).
8
9
10
11
PII: DOI: Reference:
S0022-2313(16)30303-9 http://dx.doi.org/10.1016/j.jlumin.2016.06.050 LUMIN14079
To appear in: Journal of Luminescence Received date: 7 March 2016 Revised date: 31 May 2016 Accepted date: 20 June 2016 Cite this article as: Matas Janulevicius, Julija Grigorjevaite, Greta Merkininkaite, Simas Sakirzanovas and Arturas Katelnikovas, Luminescence and luminescence quenching of Eu2Mo4O15, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.06.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Luminescence and luminescence quenching of Eu2Mo4O15 Matas Januleviciusa, Julija Grigorjevaitea, Greta Merkininkaitea, Simas Sakirzanovasb, Arturas Katelnikovasa,* a
Department of Analytical and Environmental Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania b
Department of Applied Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania *
Corresponding author: Tel: +370 697 23123; E-mail: [email protected]
Abstract A polycrystalline Eu2Mo4O15 phosphor sample was prepared by high temperature solid state reaction. Phase purity and morphological features of the phosphor were investigated by X-ray diffraction and scanning electron microscopy, respectively. Reflectance spectra showed that the optical band gap of Eu2Mo4O15 is 2.95 eV. Phosphor emits intensive red light when excited with 394 and 465 nm radiation. Temperature dependent emission and luminescence lifetime measurements revealed that external and internal quantum yields decrease at the same rate and that luminescence quenches due to photoionization. The calculated external quantum yields for 394 and 465 nm excitation were 7.8% and 53.5%, respectively.
Keywords Molybdate; thermal quenching; quantum yield; colour coordinate; luminous efficacy.
Introduction Inorganic materials based on molybdates form a large family of inorganic compounds. They are widely used in luminescent materials [1, 2], scintillators [3], electrochemical and electrochromic devices [4-6] and upconversion [7-9] due to their unique chemical and physical properties. Based on such broad application area the molybdates are intensively studied and new structures are reported every now and then [10]. Rare earth doped/substituted molybdates are of particular interest due to their potential application in solid state light sources based on near-UV LEDs[11-13] and biological imaging [14]. The absorption strength of spin and parity forbidden [Xe]4fn → [Xe]4fn transitions of rare earth ions usually is rather weak. However, these transitions become rather intensive in molybdate hosts. The absorption intensity of optical transitions at higher energies (< 400 nm) get especially strong due to admixing with charge 1
transfer (CT) band, which is allowed one. This phenomenon is exploited in manufacturing of very bright Eu3+ doped molybdate phosphors with extremely high quantum yields [15, 16]. The Eu2Mo4O15 compound reported in this work was obtained by chance when investigating a series of Y2Mo4O15:Eu3+ phosphors. The title compound crystallizes in different system if compared to Y2Mo4O15, therefore, their optical properties would be difficult to compare. The luminescence properties of Eu2Mo4O15 have not been reported so far. Thus, in this work we present a full investigation of Eu2Mo4O15 optical properties including temperature dependent emission and luminescence lifetime measurements.
Experimental The powder sample of Eu2Mo4O15 was prepared by high temperature solid state reaction. The stoichiometric amounts of Eu2O3 (99.99% Tailorlux), and MoO3 (99+% Acros Organics) were thoroughly blended in an agate mortar with some acetone as the grinding medium. The mixture of starting materials was dried, transferred to the porcelain crucible and annealed at 700 °C for 12 h in air. Powder XRD pattern was recorded in the range of 10° ≤ 2θ ≤ 80° (step width 0.02° and scanning speed 5 °/min.) using Ni-filtered Cu Kα radiation on a Rigaku MiniFlexII diffractometer working in Bragg-Brentano (θ/2θ) geometry. The SEM images were taken with Hitachi TM3000 scanning electron microscope. The accelerating voltage was 15 kV. Reflection spectra were recorded on an 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 Teflon coated integration sphere. Teflon was used as a reflectance standard. The excitation and emission slits were set to 3.00 and 0.50 nm respectively. Step width was 0.5 nm and integration time was 0.4 s. Excitation and emission spectra 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. The photoluminescence emission spectra were corrected by a correction file obtained from a tungsten incandescent lamp certified by NPL (National Physics Laboratory, UK). When measuring emission spectra (λex = 465 nm) excitation and emission slits were set to 0.6 and 0.25 nm respectively. When measuring excitation spectra (λem = 618 nm) 2
excitation and emission slits were set to 0.25 and 0.5 nm, respectively. The excitation spectra were corrected by a reference detector. In both cases step width was 0.25 nm and integration time was 0.2 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 kinetics studies were performed on the same FLS980 spectrometer. Xe μ-flash lamp was used as an excitation source. The excitation wavelength was 465 nm and emission was monitored at 618 nm. External quantum yields (EQY) were calculated by measuring emission spectrum of the Teflon sample in Teflon coated integration sphere under excitation of 394 and 465 nm. The emission spectra were recorded in the range of 380–800 nm and 450–800 nm, respectively. The same measurements were repeated for the phosphor sample. The EQY values were obtained employing the following formula [17]:
QY
I I
em , sample
abs,Teflon
where
I
I em,Teflon
I abs, sample
em , sample
and
I
100%
em , Teflon
Teflon, respectively. Likewise,
(1)
are integrated emission intensities of the phosphor sample and
I
abs, sample
I
and
abs, Teflon
are the integrated absorption of the
phosphor sample and Teflon, respectively. Quantum yield measurements were repeated five times for each excitation wavelength to gain some statistical data.
Results and discussion The Eu2Mo4O15 sample was obtained by chance when synthetizing the end sample of Y2Mo4O15:Eu3+ series. Y2Mo4O15 crystallizes in monoclinic lattice with P21/c (#14) space group [18]. However, the solubility of Eu3+ in Y2Mo4O15 seems to be limited to around 75% and complete replacement of Y3+ by Eu3+ ions yielded Eu2Mo4O15 with triclinic crystal structure and 3
P 1 (#2) space group. After the literature research we found that the title compound was reported as Eu6Mo12O45 by H. Naruke and T. Yamase [19]. The crystal structure of Eu2Mo4O15 is rather interesting. It comprises four [MoO4]2– tetrahedra, one [MoO5]4– square pyramid, one [MoO6]6– octahedron, and three different Eu sites each coordinated with eight oxygen ions. Two [MoO4]2– tetrahedra are isolated whereas the rest polyhedral undergo heavy condensation giving rise to a polymeric [Mo4O17]∞ chain running along the a-axis [19]. The recorded XRD pattern is given in Fig. 1 and matches well with a reference pattern of Eu2Mo4O15 (PDF-4+ (ICDD) 04-012-4672). One can note, however, that the integrated intensity ratio between sample and reference peaks is different, what could be explained by the preferred orientation of measured powder particles.
Fig. 1.
The morphological features of Eu2Mo4O15 were investigated by taking SEM images (Fig. 2). The obtained images reveal that powder is composed of various size crystallites fused together. This fusing suggests that the melting point of Eu2Mo4O15 is rather close to 700 °C temperature which was used during synthesis. The estimated average grain size of the powder is in the range of 15 – 20 µm. The observed small loose plate-like particles are likely to be the debris left after grinding of the synthesized powder.
Fig. 2.
The body colour of Eu2Mo4O15 sample was yellowish suggesting that phosphor absorbs in the violet-blue spectral region. This assumption was confirmed by reflection spectra (Fig. 3a), which shows a broad absorption band from 250 to 420 nm. In order to calculate the optical band gap of the material the Kubelka-Munk function [20, 21] was applied to the reflection spectra. The estimated Eu2Mo4O15 optical band gap value is 2.95 eV (23810 cm–1). Above 440 nm the absorption lines originating from intraconfigurational [Xe]4f6 → [Xe]4f6 transitions of Eu3+ ions are clearly visible. The most intense sets of lines are located at 465, 535, and 590 nm and correspond to the
7
F0 → 5D2,
7
F1 → 5D1, and
7
F1 → 5D0 transitions, respectively. High
reflectivity at longer wavelengths indicate that the obtained phosphor is of high quality and possesses little defects. Fig. 3b shows excitation spectra at 77 K and room temperature (RT) for 618 nm emission. It is evident that temperature influences the excitation spectra a lot. Both excitation spectra were normalized for better comparison. The first thing that catches the eye is the width of the charge transfer (CT) band, which considerably shrinks at low temperature. This results in rise 4
of 7F0 → 5L6 transition intensity, which was suppressed at room temperature. The decrease of charge transfer energy (increase of the wavelength) at elevated temperatures is explained by increase of lattice vibrations, thus increasing the distance (reducing bond strength) between Eu3+ and O2– ions. It was also observed that the intensity of excitation lines originating from 7F1 and 7
F2 levels strongly decrease if the temperature of phosphor is lowered to 77 K. Reducing the
temperature leads to depopulation of 7F1 and 7F2 levels, therefore, only weak or no excitation intensity is observed [22]. The emission spectra of Eu2Mo4O15 phosphor under 465 nm excitation at RT and 77 K are depicted in Fig. 3c. Both spectra contain five sets of emission lines that originate from the 5
D0 → 7F0 (≈ 580 nm), 5D0 → 7F1 (585–603 nm), 5D0 → 7F2 (603–639 nm), 5D0 → 7F3 (639–
670 nm), and 5D0 → 7F4 (680–715 nm) transitions. Usually very sharp line due to 5D0 → 7F0 transition is rather broad for Eu2Mo4O15 what is in line with three different crystallographic sites of Eu3+. The emission line broadening is also observed at room temperature spectra if compared to the one taken at 77 K. This is also related to the higher lattice vibrations at RT leading to many non-equivalent Eu3+ sites emitting at slightly different wavelengths and, therefore, causing line broadening.
Fig. 3.
Fig. 4.
For further Eu2Mo4O15 phosphor investigation the temperature dependent emission spectra were recorded under 465 nm excitation. The obtained spectra are shown in Fig. 4a. and the respective normalized emission integrals are given in Fig. 4b. Emission intensity gradually decreased with the increasing temperature. However, the integral intensity increased up to 250 K and only then started severely decrease. The initial increase of integral intensity is related to the line broadening, which compensate the decrease of emission intensity to some extent. Nevertheless, at temperatures higher than 250 K the regular thermal quenching sets in and both emission and integral intensity decrease. Temperature dependent emission integrals were employed in TQ1/2 value estimation. The obtained experimental data were fit with Boltzmann sigmoidal function [15] and the obtained TQ1/2 value was 354 K (±7 K). TQ1/2 shows a temperature at which the phosphor loses half of its efficiency. The experimental data represented in Fig. 4b were also fit with a single barrier quenching model in order to obtain the activation energy (EA) of the thermal quenching process [11]: 5
I (T ) 1 I0 1 Be E A / kT
(2)
Here I(T) and I0 are temperature dependent emission integral and highest value of emission integral, respectively. EA is activation energy, k is Boltzmann constant (8.617342·10-5 eV/K) [23], and T is temperature in K. B is the quenching frequency factor. The calculated thermal quenching activation energy was 0.42 eV (±0.12 eV). The thermal quenching most likely occurs due to photoionization what is in line with decreasing optical band gap energy with increasing temperature. Temperature dependent decay curves of Eu2Mo4O15 phosphor were also measured and are shown in Fig. 4c. At all temperatures except 450 and 500 K a single exponential decay was observed. The decay curves corresponding to temperatures in the range of 77–300 K were very close to each other indicating that the internal quantum yield does not change in this temperature interval. However, when phosphor temperature exceeded 300 K the decay curves became steeper and steeper indicating shortening of the luminescence lifetimes. The decrease of luminescence lifetime values upon temperature increase can be related to the increased probability of nonradiative processes, which decrease lifetime of the excited state [24]. The temperature dependent decay curves were fit employing a single exponential decay function (except for 450 and 500 K, which were fit with bi-exponential decay function) and the obtained luminescence lifetime (τ1/e) values are represented in Fig. 4d. The lifetime values like emission integrals remain rather constant up to 250 K and only then start sharply decreasing. The TQ1/2 value calculated from temperature dependent luminescence lifetime data was 362 K (±5 K). This is the same as the value achieved from emission integral data if the calculation errors are taken into account. The emission integrals are proportional to the external quantum yield, whereas the luminescence lifetimes are proportional to the internal quantum yield (IQY). The relation between the both can be written as [15]:
EQY IQY esc
(3)
where ηesc is the escape efficiency of emitted photons from the phosphor particle. Since emission integrals and luminescence lifetimes change identically with increasing temperature it can be concluded that EQY = IQY. In this case it is also clear that ηesc must be close to unity. The external quantum yield at room temperature calculated for 465 nm (7F0 → 5D2) excitation was 53.5% (±1.5%). Such relatively high quantum yield of this fully concentrated compound can be explained by efficient shielding of Eu3+ ions from each other by molybdate units and by the fact that the distance between Eu3+ ions is rather large (3.680 – 6.014 Å) [19]. 6
On the other hand, the EQY for 394 nm (7F0 → 5L6) excitation was considerably lower – only 7.8% (±0.4%). This phenomenon can be explained by the fact that 5L6 level is already inside the band gap, thus lots of excitation energy is lost. Finally, the CIE 1931 colour coordinates and luminous efficacy (LE) values were calculated from room temperature emission spectra (λex = 465 nm). The colour coordinate of Eu2Mo4O15 phosphor is (0.6622; 0.3375) and LE = 208 lm/Wopt. The colour coordinates of Eu2Mo4O15 and Y2Mo4O15:Eu3+ (0.662; 0.334) [25] are shown in a fragment of CIE 1931 colour space diagram depicted in inset of Fig. 3c. It is evident that colour point of Eu2Mo4O15 is directly on the edge of colour space diagram what shows superior colour saturation if compared to the Y2Mo4O15:Eu3+. The LE values are relatively lower if compared to the other Eu3+ doped molybdates [15]. This is due to the intensive emission at around 700 nm where the human eye sensitivity is low.
Conclusions All in all, Eu2Mo4O15 is a very interesting red emitting phosphor possessing moderate quantum yield, which likely could still be improved to some extent by choosing different synthesis method, temperature, or particle size/shape optimization. Unfortunately, it suffers from the severe thermal quenching at temperatures above 300 K, what would prevent it from high temperature applications. However, due to the unique emission spectra this phosphor could find applications in the areas where the quantum yield is not of the primary importance, for instance luminescent markers and so on.
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Figure captions: Fig. 5. XRD pattern of Eu2Mo4O15 and a reference pattern of Eu2Mo4O15. Fig. 6. SEM images of Eu2Mo4O15 under magnification of (a) 1.0 k and (b) 2.5 k. Fig. 7. Reflection (a), excitation at RT and 77 K (b), and emission at RT and 77 K (c) spectra of Eu2Mo4O15. Inset in (c) shows a fraction of CIE 1931 colour space diagram with colour coordinates with Eu2Mo4O15 and Y2Mo4O15:Eu3+. Fig. 8. Temperature dependent emission spectra of Eu2Mo4O15 (a) and TQ1/2 estimation (b). Temperature dependent decay curves of Eu2Mo4O15 (c) and TQ1/2 estimation (d).
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