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On the Ce3+ → Cr3+ energy transfer in Lu3Al5O12 garnets
Optical Materials 37 (2014) 204–210
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
On the Ce3+ ? Cr3+ energy transfer in Lu3Al5O12 garnets Eva Raudonyte a, Helga Bettentrup b, Dominik Uhlich b, Simas Sakirzanovas c, Olga Opuchovic d, Stasys Tautkus a, Arturas Katelnikovas a,⇑ a
Department of Analytical and Environmental Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania Tailorlux GmbH, Fraunhoferstr. 1, D-48161 Münster, Germany c Department of Applied Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania d Department of Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania b
a r t i c l e
i n f o
Article history: Received 14 April 2014 Received in revised form 25 May 2014 Accepted 26 May 2014 Available online 18 June 2014 Keywords: Sol–gel combustion Photoluminescence spectroscopy Energy transfer Quantum efficiency Luminous efficacy CIE colour point
a b s t r a c t Two series of Lu3Al5O12:Cr3+ and Lu3Al5O12:0.5% Ce3+,Cr3+ luminescent materials were prepared by a sol– gel combustion method. All samples were characterized by powder X-ray diffraction (XRD), infrared (IR) and photoluminescence (PL) measurements. Moreover, luminous efficacies (LE), CIE 1931 colour points, and quantum efficiencies (QE) were calculated and discussed. Luminescence measurements indicated that Ce3+ ions located at Lu3+ site transfers absorbed energy to Cr3+ ions located at Al3+ site. However, with increasing Cr3+ concentration the total light output of Lu3Al5O12:0.5% Ce3+,Cr3+ phosphors decrease. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Luminescent solar concentrators (LSC) have received much research interest worldwide during last few decades [1–4]. Generally speaking a LSC is a transparent polymer plate, which contains luminescent particles. Solar cells are connected to one or more edges of this polymer plate. The luminescent particles absorbs the visible spectrum solar radiation and emit the far-red or nearinfrared photons, which are waveguided via the total internal reflection to the solar cells at the edge of the polymer plate [5,6]. The advantage of LCS is that they can cover large areas without the need of large amounts of solar cells. Moreover, LSC does not require sun tracking system since it efficiently collect both diffused and direct light [7]. Usually, organic dies were employed in preparation in LSCs. However, the lack of stability of organic luminophores led to search of other materials for this application including quantum dots [8], rare-earth complexes [9], nanoclusters [1], quantum cutting phosphors [3], enhanced fluorescence by metal nanoparticles [4], etc. One of the recently suggested inorganic phosphors for LSCs was Ce3+,Cr3+ codoped Y3Al5O12 [6]. It is well know that in glasses Cr3+
ions possess broad absorption bands with peaks around 450 nm (4A2 ? 4T1) and 650 nm (4A2 ? 4T2) covering most of the solar spectrum [10]. In garnets, however, these bands are located at 430 and 600 nm, respectively, and emission originates from the 2 E ? 4A2 transition in the far-red spectral region. Since Cr3+ absorption arises from the forbidden d–d transitions its strength is not that high. On the other hand, Ce3+ absorption originates from the allowed [Xe]4f1 ? [Xe]5d1 transition, which in garnet type materials are located at 350 and 450 nm [11–13]. Moreover, in garnet type compounds Ce3+ emission is in the range of 450–700 nm and, therefore, partially overlaps with Cr3+ absorption band. This overlap leads to the Ce3+ ? Cr3+ energy transfer and stronger emission of Cr3+ in the desired far-red spectral region. YAG:Ce3+,Cr3+ luminescent materials were recently also investigated as LED phosphors with enhanced intensity in red spectral region [14]. In this paper, the luminescence properties, luminous efficacies, quantum efficiencies, and CIE 1931 colour points as a function of Cr3+ concentration in of Lu3Al5O12:Cr3+ and Lu3Al5O12:0.5% Ce3+, Cr3+ materials are studied.
2. Experimental ⇑ Corresponding author. Tel.: +370 60971428. E-mail address: [email protected] (A. Katelnikovas). http://dx.doi.org/10.1016/j.optmat.2014.05.025 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
Lu3Al5O12:Cr3+ and Lu3Al5O12:Ce3+,Cr3+ powder samples were prepared by a sol–gel combustion method employing
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tris(hydroxymethyl)-aminomethane (THMAM) as both complexing agent and fuel. During doping Al3+ was replaced by Cr3+, and Lu3+ was replaced by Ce3+. Ce3+ concentration was 0.5 mol%, and Cr3+ concentration was 0, 0.1, 1, 2, and 4 mol%. The gels were prepared using high purity Lu2O3 (99.99% Tailorlux), Al(NO3)39H2O (P98% Fluka), Cr(NO3)39H2O (99% Sigma–Aldrich), Ce(NO3)36H2O (99% Aldrich), and H2NC(CH2OH)3 (99% POCH). In the sol–gel combustion process lutetium oxide was firstly dissolved in hot diluted nitric acid. Subsequently, the resulting clear solution was evaporated till dryness in order to remove the excess or nitric acid. The dry residue was dissolved in deionized water and the required amounts of aluminium, chromium and/or cerium nitrates were added. The obtained solutions were stirred for 1 h at temperatures between 65 and 75 °C. Then THMAM at a molar ratio of 1:1 to all metal ions was added and the solution were stirred for an additional hour at the same temperature. After the concentrating final solutions by slow evaporation the sol turned into transparent gels. The temperature was raised to 250 °C and the self-maintaining gel combustion process has started accompanied with an evolution of a large amount of gasses. The obtained black-brown products were dried in the oven overnight at 150 °C and later ground to fine powders, which were firstly annealed at 1000 °C for 2 h in air to remove the residual carbon after the combustion process. Subsequently, obtained powders were sintered for 4 h at 1700 °C under CO atmosphere. The body colour of undoped Lu3Al5O12 was white. Ce3+ doped sample was yellow–green. The body colour of solely Cr3+ doped samples ranged from pale green to dark green depending on chromium concentration. The body colour of Ce3+/Cr3+ doped samples were dirty yellow. TG/DSC measurements of the precursor gels were recorded on a PerkinElmer STA 6000 Simultaneous Thermal Analyser. The heating rate was 10 °C/min. The sample weight was 2 mg. The atmosphere was synthetic air with flow rate of 20 mL/min. Powder XRD data were collected from 10° 6 2h 6 80° (step width 0.02° and integration time 0.5 s) using Ni-filtered Cu Ka radiation on a Rigaku MiniFlex II diffractometer working in Bragg–Brentano (h/2h) geometry. IR spectra were taken with PerkinElmer Frontier ATR-FTIR Spectrometer equipped with a liquid nitrogen cooled MCT detector. SEM images were taken with a FE-SEM Hitachi SU-70. Reflection spectra in the range of 250–800 nm were recorded on an Edinburgh Instruments FSL900 spectrometer equipped with a 450 W Xe arc lamp, a cooled (20 °C) single-photon counting photomultiplier (Hamamatsu R928) and a Teflon integration sphere. A BaSO4 sample (99% Sigma–Aldrich) was used as a reflectance standard. The excitation and emission slits were set to 2.5 and 0.4 nm, respectively. The step size was 1 nm and integration time 0.4 s. Excitation and emission spectra were recorded on an Edinburgh Instruments FSL900 spectrometer equipped with a 450 W Xe arc lamp, a cooled (20 °C) single-photon counting photomultiplier (Hamamatsu R928) and a lens optics for powder samples. The photoluminescence emission spectra were corrected by a correction file obtained from a tungsten incandescent lamp certified by the NPL (National Physic Laboratory, UK). When measuring emission spectra emission and excitation slits were set to 0.2 and 2.5 nm, respectively. When measuring excitation spectra emission and excitation slits were set to 1 and 0.5 nm, respectively. The step size was 1 nm and integration time 0.2 s. Quantum efficiencies were calculated according to the equation [15]
R R ðISample Þdk ðIBlack Þdk 1 RRef R QE ¼ QERef R 1 RSample ðIRef Þdk ðIBlack Þdk
R
emission integral of the sample, (IBlack)dk is the emission integral R of the black standard, (IRef)dk is the emission integral of the reference material, and RRef and RSample are the reflection values at 435 nm of reference material and sample, respectively. The black standard (Flock Paper #55, Edmund Optics) was used to eliminate the dark count rate of the detector. For QE determination of all phosphor samples were excited at 435 nm. The error of the quantum efficiency calculations has been found to be ±5%. All measurements were performed at room temperature and ambient pressure in air. 3. Results and discussion 3.1. Preparation and structural properties The TG/DSC curves of the Lu–Al–O gel after the combustion process are depicted in Fig. 1. The TG curve indicate that the mass loss occurs in two main steps, similar that was observed in Ca–Lu–Al– Si–O system reported in our earlier work [15]. The first step (peak at 495 °C) yielded in a weight loss of about 17% and can be attributed to burning of the organic residues that remained after the combustion process. This is in good agreement with DSC curve, which shows exothermal processes occurring in this temperature range. The second weight loss step of around 6% occurs in the temperature range 850–1000 °C and can be assigned to the decomposition of metal carbonates or oxycarbonates. This is supported by the DSC curve showing endothermic processes occurring in this temperature range. Moreover, the full decomposition of lutetium carbonate was reported to be around 800 °C [16] what is rather similar to the results obtained in our study. The TG/DSC curves of the gels of samples containing Cr and/or Ce showed similar behaviour. The powder XRD patterns of as prepared gel, and gels annealed at 1000 °C and 1700 °C are shown in Fig. 2. The reference pattern of Lu3Al5O12 (PDF4+ (ICDD) 04-001-9996) [17] was also added for comparison. It is obvious that during the combustion process no crystallization takes place. However, annealing the gel at 1000 °C yields the single phase lutetium aluminium garnet. The XRD peaks are rather broad indicating that the formed crystallites are small. The size of crystallites at this stage was calculated by employing Scherrer equation [18]
t¼
0:9k B cos hB
ð2Þ
where t is the mean crystallite size, k the X-ray wavelength, B the line broadening at half maximum intensity (FWHM) (in radians),
ð1Þ
where QERef is the quantum efficiency (88%) (kEx = 435 nm) of the R reference material (YAG:Ce3+, U728, Philips), (ISample)dk is the
Fig. 1. TG/DSC curves of the Lu–Al–O precursor gel obtained after the combustion process.
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Fig. 2. Powder XRD patterns of as prepared Lu–Al–O gel, gel annealed at 1000 °C, and at 1700 °C, and Lu3Al5O12 crystal [17].
and hB the Bragg angle. The mean crystallite size was found to be about 20 nm and was independent on chromium and/or cerium concentration. Raising the sintering temperature to 1700 °C resulted in much narrower peaks due to increase of particle size of the material. The same pattern was observed for samples doped with chromium and/or cerium demonstrating that the dopant ions does not affect the crystal structure formation (at least up to investigated concentrations). Fig. 3 depicts the FTIR spectra of Lu–Al–O gel after the combustion process and sintered at 1000 and 1700 °C. The spectrum of gel after combustion contains two broad and intense absorption bands
Fig. 3. IR spectra of as prepared Lu–Al–O gel, gel annealed at 1000 °C, and at 1700 °C.
located at 1700–1300 cm1 and 1000–500 cm1. The first band can be attributed to the carbonates [19], which formed during combustion process of the gel. Similar results were observed by Leleckaite et al. [20], where yttrium aluminium garnet powders were synthesized by aqueous sol–gel technique employing 1,2-ethandiole as complexing agent. The second band can be assigned to the characteristic metal–oxygen vibrations. The spectra of Lu3Al5O12 samples annealed at 1000 and 1700 °C lacks the first band indicating that the metal carbonates that formed during the combustion process are completely decomposed at these temperatures. Moreover, the broad band observed at 1000–500 cm1 split into several sharp bands when the gel was sintered either at 1000 or 1700 °C. Such split is typical for the garnet type compounds and these results goes hand in hand with the powder XRD data presented in Fig. 2, showing the single phase Lu3Al5O12 garnets formed at those temperatures. The morphology features of the Lu3Al5O12 and Lu2.985Ce0.015 Al4.8Cr0.2O12 gels and gels annealed at 1000 and 1700 °C were inspected by taking SEM images, which are shown in Fig. 4. The as-synthesized gels of both undoped (Fig. 4a) and doped (Fig. 4d) samples possess similar sponge like morphology. The presence of large and small pores are clearly visible. These pores likely formed due to escaping gasses during the combustion process. Sintering of the aforementioned gels at 1000 °C have not induced much changes in particle morphology (Fig. 4b and e). The sponge like structure is still present, however, with some larger holes inside. These holes are the result of escaping gasses, which formed during the burning of organic residues that were left after combustion process. The increase of sintering temperature to 1700 °C resulted in the particle growth, what is in line with the peak narrowing in the XRD patterns at elevated temperatures. Powders sintered at 1700 °C consist of well-shaped and highly agglomerated particles with the size of 1–3 lm (Fig. 4c and f). No noticeable difference between the morphologies of undoped and doped powders was observed. 3.2. Reflection spectra The reflection spectra of Cr3+ and Ce3+/Cr3+ doped samples are shown in Fig. 5a and b, respectively. The undoped Lu3Al5O12 powder showed high reflectance at longer wavelengths indicating high brilliance of the prepared sample. The spectra of samples containing Cr3+ possessed two broad and strong absorption bands with the maxima at 420 and 580 nm. These bands can be attributed to the optical transitions from the ground state 4A2 to the excited states 4 T1 and 4T2, respectively [21,22]. As expected, the increase of Cr3+ concentration resulted in stronger absorption. However, the increase of chromium concentration has also resulted in decrease of reflectance at longer wavelengths what caused the greyishing of the samples. This can be attributed to the formation of defects. There are many forms of defects (surface, antisite, Schottky, etc.) that are present in the powder materials. Moreover, the increase of Cr3+ concentration might also lead to formation of Cr3+ pair centres, which by various charge compensation schemes may facilitate defect formation. However, at this stage it is not clear which defects are causing greyishing of powders and this issue needs a separate study. The reflection spectra of Ce3+/Cr3+ doped samples also contained typical Ce3+ two absorption bands in garnet type materials. The absorption originates from the ground state 2F5/2 of Ce3+ to the two lowest crystal field components of the 5d orbital. It is also worth to note that very weak absorption line from spinforbidden 4A2 ? 2E transition [22] of Cr3+ was also observed in reflection spectra of both Cr3+ and Ce3+/Cr3+ doped samples. Since the 2E level is nearly independent on the crystal field strength the emission from it occurs at the same wavelength as observed in reflectance spectra (zero-phonon lines) [22,23].
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Fig. 4. SEM images of undoped Lu3Al5O12 (a) gel and powders sintered at (b) 1000 °C and (c) 1700 °C; Lu2.985Ce0.015Al4.8Cr0.2O12 (0.5 mol% of Ce3+ and 4 mol% of Cr3+) (d) gels and powders sintered at (e) 1000 °C and (f) 1700 °C.
Fig. 5. Reflection spectra of (a) Cr3+ doped Lu3Al5O12 and (b) Ce3+/Cr3+ doped Lu3Al5O12 powders. The reflection spectrum of YAG:Ce3+ standard U728 is given for the reference.
3.3. Excitation and emission spectra Similar to reflection spectra the 4A2 ? 4T2, 4A2 ? 4T1(4F), and A2 ? 4T1(4P) transitions of Cr3+ dominate in the excitation spectra of Lu3Al5O12:Cr3+ (Fig. 6a). The intensity of excitation bands
4
increased when samples were doped up to 1% of Cr3+. Exceeding this concentration of chromium ions resulted in gradual reduction of intensity of excitation bands. The excitation spectra of 0.1% and 1% Cr3+ doped samples were recorded for the 695 nm emission since this line was the strongest one in the emission spectra
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The excitation and emission spectra of Ce3+/Cr3+ doped samples are given in Fig. 7a and b, respectively. Excitation spectra were recorded for the strongest emission line at 687 nm in emission spectra. The excitation spectra of 0.5% Ce3+ doped Lu3Al5O12 sample was recorded monitoring two emission wavelengths, namely, 560 and 687 nm. Monitoring emission at 560 nm leads to typical excitation spectra of Ce3+ ions in garnet type materials with high intensity bands peaking at 345 and 445 nm. However, monitoring emission at 687 nm gives very weak excitation bands. This is in line with the emission spectrum of solely Ce3+ doped lutetium aluminium garnet shown in Fig. 7b since there is nearly no emission at this wavelength. The situation changes when Cr3+ ions are introduced to the host matrix. Addition of as little as 0.1% of Cr3+ results in high intensity excitation bands of Ce3+ in the spectra thus confirming the Ce3+ ? Cr3+ energy transfer. The highest intensity was obtained for 0.5% Cr3+/1% Cr3+ doped sample. The increased Cr3+ concentration resulted in gradual decrease of intensity. The emission spectra of Lu3Al5O12:0.5% Ce3+ consisted of broad and asymmetric band ranging from 460 to 750 nm with the maximum at 510 nm and the shoulder at 550 nm. This asymmetry arises from the strong overlap of the two bands originating in the transitions from the lowest crystal-field component of the [Xe]5d1 configuration to the spin-orbit split sublevels 2F5/2 and 2F7/2 of the [Xe]4f1 configuration of Ce3+ ions [15,29]. The emission intensity of cerium doped sample was around 80% of the standard. Introduction of 0.1% Cr3+ into the host matrix decreased the Ce3+ emission more
Fig. 6. Excitation (a) and emission (b) spectra of Lu3Al5O12:Cr3+ samples. The excitation and emission spectra of YAG:Ce3+ standard U728 are given for the intensity comparison.
(Fig. 6b). However, when samples were doped with 2% and 4% of Cr3+ the 687 nm line became dominant in the emission spectra. Therefore, the excitation spectra of these two samples were recorded for this emission wavelength. It was observed that the change of monitored emission from 695 to 687 nm resulted in excitation band shift to longer wavelengths by 30 nm. The spectra of these bands match well with the ones observed in the reflection spectra. After some literature research it was concluded that emission band at 695 nm belongs to Cr3+ ions in the Al2O3 host matrix [24–26]. Even though no impurity phases were observed in the powder XRD patterns, the PL measurements are more sensitive and, therefore, the Al2O3 impurity was detected. The emission spectra (Fig. 6b) of Lu3Al5O12:Cr3+ powders were recorded upon 435 nm excitation. The spectra contain typical Cr3+ emission bands and lines in garnet type materials [6,14,27]. The line emission at 687 and 695 nm originate from the 2E ? 4A2 transition of Cr3+ ions in Lu3Al5O12 and Al2O3, respectively. The other emission bands around 687 nm line can be attributed to the vibronic sidebands [24,28]. The most intensive emission was observed for the samples doped with 1% and 2% of Cr3+. Further increase of dopant concentration significantly reduced emission intensity likely due to concentration quenching. The emission spectrum of standard material YAG:Ce3+ (U728, Philips) under the same conditions was also recorded in order to compare the emission intensities. It is obvious that Cr3+ emission intensity in Lu3Al5O12 host matrix is comparable to that of YAG:Ce3+, but does not exceed it. However, it is also clear that the total light output is much higher for the cerium doped garnet since its emission band is significantly broader.
Fig. 7. Excitation (a) and emission (b) spectra of Lu3Al5O12:Ce3+,Cr3+ samples. The excitation and emission spectra of YAG:Ce3+ standard U728 are given for the intensity comparison.
E. Raudonyte et al. / Optical Materials 37 (2014) 204–210
than twice. With further increase of chromium concentration the intensity of Ce3+ emission band progressively decreased and almost vanished when Cr3+ concentration reached 4%. The strongest chromium emission in 0.5% Ce3+/Cr3+ doped samples was found in specimen with 1% Cr3+. By comparing Figs. 6b and 7b it is evident that chromium emission is twice stronger in the samples containing Ce3+ ions. Thus, it can be concluded that cerium ions act as sensitizer to chromium ions. The comparison of Ce3+ and Cr3+ emission integrals as a function of chromium concentration in 0.5% Ce3+/Cr3+ doped samples is given in Fig. 8. Emission spectra of both ions overlap, therefore, the emission integrals of Ce3+ and Cr3+ were calculated in the range of 460–650 nm and 650–900 nm, respectively. It was also calculated that only 3% of total Ce3+ emission is in the range of 650– 00 nm and around 4% of total Cr3+ emission is in the range of 460–650 nm. Therefore, the resulting calculation error is also included in Fig. 8. Strong quenching of cerium emission with increasing chromium concentration is observed what is in line with results represented in Fig. 7b. Chromium emission reaches maximum at concentration of 1% and also rapidly decreases with R R increasing Cr3+ content. The ratio I(Cr3+)/ I(Ce3+) is slightly less than 1 for the sample doped with 0.1% Cr3+. This indicates that more photons are coming out from Ce3+ ions. However, at higher concentrations Cr3+ emission starts to dominate what again is in line with Fig. 7b.
209
Fig. 9. Quantum efficiencies of Lu3Al5O12:Cr3+ and Lu3Al5O12:Ce3+,Cr3+ powder samples.
3.4. Quantum efficiencies and chromaticity diagrams Quantum efficiencies of all prepared specimens were calculated according to Eq. (1) and are shown in Fig. 9. In Lu3Al5O12:Cr3+ series the highest efficiency 48% and 41% was obtained for the samples doped with 1% and 2% of Cr3+, respectively. Sample doped with 4% of Cr3+ showed only 16% of efficiency indicating strong concentration quenching. Lu3Al5O12:0.5% Ce3+ possessed 75% quantum yield. However, after the addition of 0.1% Cr3+ this value has dropped to 58% and continued decreasing with increasing chromium concentration. Sample doped with 4% Cr3+ showed only 6% of quantum yield, which is roughly 3 times lower than sample without Ce3+. Quantum yield calculations clearly shows that increasing Cr3+ concentration efficiently quenches the total efficiency of the phosphor. Fig. 10 shows a fragment of the CIE 1931 chromaticity diagram with the colour points of Lu3Al5O12:Cr3+ and Lu3Al5O12:0.5% Ce3+, Cr3+ specimens as a function of Cr3+ concentration. The colour points of the former are located in the red region and does not change with increasing Cr3+ concentration. This goes hand in hand
Fig. 10. A fragment of the CIE 1931 chromaticity diagram with colour points of Lu3Al5O12:Cr3+ and Lu3Al5O12:Ce3+,Cr3+ powder samples. The exact colour point values and the corresponding luminous efficacy values are given in the inset table.
with the relatively constant luminous efficacy values. However, the colour points of the latter shows strong dependence on chromium concentration as could be expected from Fig. 7b. The strong shift from yellowish-green to yellow–green region is observed with increasing Cr content. This shift was accompanied by the decrease of luminous efficacy values what can be explained by smaller overlap of human eye sensitivity curve and emission spectra. 4. Conclusions
R R Fig. 8. Integrated emission intensities of Ce3+ ( I(Ce3+)) and Cr3+ ( I(Cr3+)) together R R 3+ 3+ 3+ with the ratio of ( I(Cr ))/( I(Ce )) as a function of Cr concentration in Lu2.985Ce0.015Al5O12:Cr3+.
In this study the photoluminescence properties of Lu3Al5O12: Cr3+ and Lu3Al5O12:0.5% Ce3+,Cr3+ luminescent materials and Ce3+ ? Cr3+ energy transfer were investigated and discussed. In this system Cr3+ ions can be excited in the visible spectral region from 340 to 650 nm yielding the far-red emission at 687 nm. Codoping with Ce3+ ions resulted in increased Cr3+ emission intensity thus confirming the presence of Ce3+ ? Cr3+ energy transfer. Unfortunately, the overall quantum yield of Ce3+,Cr3+ doped specimens decreased with increasing Cr3+ concentration, indicating that some quenching processes also occurs between these two ions. The efficiency of such phosphors might be increased by choosing the
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garnet structure compounds, whose Ce3+ emission spectra overlaps better with the absorption spectra of Cr3+ ions. Few of such compounds could be Y3Al3MgSiO12:Ce3+ (kem = 575 nm) [29], Y3AlMg2 Si2O12:Ce3+ (kem = 600 nm) [30] and Lu2CaMg2(Si,Ge)3O12:Ce3+ (kem = 620 nm) [31]. Acknowledgement The study was funded from the European Community’s social foundation under Grant Agreement No. VP1-3.1-ŠMM-08-K-01004/KS-120000-1756. References [1] Y. Zhao, R.R. Lunt, Adv. Energy Mater. 3 (2013) 1143–1148. [2] W.G.J.H.M.V. Sark, K.W.J. Barnham, L.H. Slooff, A.J. Chatten, A. Büchtemann, A. Meyer, S.J. McCormack, R. Koole, D.J. Farrell, R. Bose, E.E. Bende, A.R. Burgers, T. Budel, J. Quilitz, M. Kennedy, T. Meyer, C.D.M. Donegá, A. Meijerink, D. Vanmaekelbergh, Opt. Express 16 (2008) 21773–21792. [3] B.M. van der Ende, L. Aarts, A. Meijerink, Adv. Mater. (Weinheim, Ger.) 21 (2009) 3073–3077. [4] M.G. Debije, P.P.C. Verbunt, Adv. Energy Mater. 2 (2012) 12–35. [5] L.H. Slooff, E.E. Bende, A.R. Burgers, T. Budel, M. Pravettoni, R.P. Kenny, E.D. Dunlop, A. Büchtemann, Phys. Status Solidi RRL 2 (2008) 257–259. [6] L.M. Shao, X.P. Jing, J. Lumin. 131 (2011) 1216–1221. [7] A. Goetzberger, W. Greube, Appl. Phys. 14 (1977) 123–139. [8] M.K. Mishra, A. Mandal, J. Saha, G. De, Opt. Mater. 35 (2013) 2604–2612. [9] M.M. Nolasco, P.M. Vaz, V.T. Freitas, P.P. Lima, P.S. Andre, R.A.S. Ferreira, P.D. Vaz, P. Ribeiro-Claro, L.D. Carlos, J. Mater. Chem. A 1 (2013) 7339–7350. [10] R. Reisfeld, J. Less-Common Met. 93 (1983) 243–251. [11] V. Bachmann, C. Ronda, A. Meijerink, Chem. Mater. 21 (2009) 2077–2084. [12] J.M. Ogieglo, Luminescence and Energy Transfer in Garnet Scintillators Ph.D. Thesis, University of Utrecht, Utrecht, The Netherlands, 2012.
[13] J.M. Ogieglo, A. Katelnikovas, A. Zych, T. Jüstel, A. Meijerink, C.R. Ronda, J. Phys. Chem. A 117 (2013) 2479–2484. [14] W.D. Wang, J.K. Tang, S.T. Hsu, J. Wang, B.P. Sullivan, Chem. Phys. Lett. 457 (2008) 103–105. [15] A. Katelnikovas, J. Plewa, D. Dutczak, S. Möller, D. Enseling, H. Winkler, A. Kareiva, T. Jüstel, Opt. Mater. 34 (2012) 1195–1201. [16] S. Liu, R.J. Ma, Acta Chem. Scand. 51 (1997) 893–895. [17] A.M. Antyukhov, N.V. Moiseev, S.A. Smirnova, T.M. Lysenko, Inorg. Mater. 26 (1990) 1443–1447. [18] A.R. West, Solid State Chemistry and its Applications, John Wiley & Sons, Chichester, 1984. [19] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, fourth ed., Wiley, New York, 1986. [20] A. Leleckaite, A. Kareiva, H. Bettentrup, T. Jüstel, H.J. Meyer, Z. Anorg, Allg. Chem. 631 (2005) 2987–2993. [21] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994. [22] W.M. Yen, S. Shionoya, H. Yamamoto, Fundamentals of Phosphors, CRC Press, Boca Raton, 2007. [23] N.M. Avram, M.G. Brik, Optical Properties of 3d-Ions in Crystals: Spectroscopy and Crystal Field Analysis, first ed., Springer, New York, 2013. [24] K.M. Kinsman, J. McKittrick, E. Sluzky, K. Hesse, J. Am. Ceram. Soc. 77 (1994) 2866–2872. [25] V.N. Snytnikov, V.O. Stoyanovskii, T.V. Larina, O.P. Krivoruchko, V.A. Ushakov, V.N. Parmon, Kinet. Catal. 49 (2008) 291–298. [26] M. Kirm, E. Feldbach, A. Kotlov, P. Liblik, A. Lushchik, M. Oja, E. Palcevskis, Radiat. Meas. 45 (2010) 618–620. [27] J. Kvapil, J. Kvapil, B. Perner, Czech. J. Phys. B 37 (1987) 1277–1287. [28] B.K. Sevast’yanov, D.T. Sviridov, V.P. Orekhova, L.B. Pasternak, R.K. Sviridova, T.F. Veremeichik, Sov. J. Quantum Electron. 2 (1973) 339–343. [29] A. Katelnikovas, H. Winkler, A. Kareiva, T. Jüstel, Opt. Mater. 33 (2011) 992– 995. [30] A. Katelnikovas, H. Bettentrup, D. Uhlich, S. Sakirzanovas, T. Jüstel, A. Kareiva, J. Lumin. 129 (2009) 1356–1361. [31] A.A. Setlur, W.J. Heward, Y. Gao, A.M. Srivastava, R.G. Chandran, M.V. Shankar, Chem. Mater. 18 (2006) 3314–3322.
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On the Ce3+ ? Cr3+ energy transfer in Lu3Al5O12 garnets Eva Raudonyte a, Helga Bettentrup b, Dominik Uhlich b, Simas Sakirzanovas c, Olga Opuchovic d, Stasys Tautkus a, Arturas Katelnikovas a,⇑ a
Department of Analytical and Environmental Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania Tailorlux GmbH, Fraunhoferstr. 1, D-48161 Münster, Germany c Department of Applied Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania d Department of Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania b
a r t i c l e
i n f o
Article history: Received 14 April 2014 Received in revised form 25 May 2014 Accepted 26 May 2014 Available online 18 June 2014 Keywords: Sol–gel combustion Photoluminescence spectroscopy Energy transfer Quantum efficiency Luminous efficacy CIE colour point
a b s t r a c t Two series of Lu3Al5O12:Cr3+ and Lu3Al5O12:0.5% Ce3+,Cr3+ luminescent materials were prepared by a sol– gel combustion method. All samples were characterized by powder X-ray diffraction (XRD), infrared (IR) and photoluminescence (PL) measurements. Moreover, luminous efficacies (LE), CIE 1931 colour points, and quantum efficiencies (QE) were calculated and discussed. Luminescence measurements indicated that Ce3+ ions located at Lu3+ site transfers absorbed energy to Cr3+ ions located at Al3+ site. However, with increasing Cr3+ concentration the total light output of Lu3Al5O12:0.5% Ce3+,Cr3+ phosphors decrease. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Luminescent solar concentrators (LSC) have received much research interest worldwide during last few decades [1–4]. Generally speaking a LSC is a transparent polymer plate, which contains luminescent particles. Solar cells are connected to one or more edges of this polymer plate. The luminescent particles absorbs the visible spectrum solar radiation and emit the far-red or nearinfrared photons, which are waveguided via the total internal reflection to the solar cells at the edge of the polymer plate [5,6]. The advantage of LCS is that they can cover large areas without the need of large amounts of solar cells. Moreover, LSC does not require sun tracking system since it efficiently collect both diffused and direct light [7]. Usually, organic dies were employed in preparation in LSCs. However, the lack of stability of organic luminophores led to search of other materials for this application including quantum dots [8], rare-earth complexes [9], nanoclusters [1], quantum cutting phosphors [3], enhanced fluorescence by metal nanoparticles [4], etc. One of the recently suggested inorganic phosphors for LSCs was Ce3+,Cr3+ codoped Y3Al5O12 [6]. It is well know that in glasses Cr3+
ions possess broad absorption bands with peaks around 450 nm (4A2 ? 4T1) and 650 nm (4A2 ? 4T2) covering most of the solar spectrum [10]. In garnets, however, these bands are located at 430 and 600 nm, respectively, and emission originates from the 2 E ? 4A2 transition in the far-red spectral region. Since Cr3+ absorption arises from the forbidden d–d transitions its strength is not that high. On the other hand, Ce3+ absorption originates from the allowed [Xe]4f1 ? [Xe]5d1 transition, which in garnet type materials are located at 350 and 450 nm [11–13]. Moreover, in garnet type compounds Ce3+ emission is in the range of 450–700 nm and, therefore, partially overlaps with Cr3+ absorption band. This overlap leads to the Ce3+ ? Cr3+ energy transfer and stronger emission of Cr3+ in the desired far-red spectral region. YAG:Ce3+,Cr3+ luminescent materials were recently also investigated as LED phosphors with enhanced intensity in red spectral region [14]. In this paper, the luminescence properties, luminous efficacies, quantum efficiencies, and CIE 1931 colour points as a function of Cr3+ concentration in of Lu3Al5O12:Cr3+ and Lu3Al5O12:0.5% Ce3+, Cr3+ materials are studied.
2. Experimental ⇑ Corresponding author. Tel.: +370 60971428. E-mail address: [email protected] (A. Katelnikovas). http://dx.doi.org/10.1016/j.optmat.2014.05.025 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
Lu3Al5O12:Cr3+ and Lu3Al5O12:Ce3+,Cr3+ powder samples were prepared by a sol–gel combustion method employing
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tris(hydroxymethyl)-aminomethane (THMAM) as both complexing agent and fuel. During doping Al3+ was replaced by Cr3+, and Lu3+ was replaced by Ce3+. Ce3+ concentration was 0.5 mol%, and Cr3+ concentration was 0, 0.1, 1, 2, and 4 mol%. The gels were prepared using high purity Lu2O3 (99.99% Tailorlux), Al(NO3)39H2O (P98% Fluka), Cr(NO3)39H2O (99% Sigma–Aldrich), Ce(NO3)36H2O (99% Aldrich), and H2NC(CH2OH)3 (99% POCH). In the sol–gel combustion process lutetium oxide was firstly dissolved in hot diluted nitric acid. Subsequently, the resulting clear solution was evaporated till dryness in order to remove the excess or nitric acid. The dry residue was dissolved in deionized water and the required amounts of aluminium, chromium and/or cerium nitrates were added. The obtained solutions were stirred for 1 h at temperatures between 65 and 75 °C. Then THMAM at a molar ratio of 1:1 to all metal ions was added and the solution were stirred for an additional hour at the same temperature. After the concentrating final solutions by slow evaporation the sol turned into transparent gels. The temperature was raised to 250 °C and the self-maintaining gel combustion process has started accompanied with an evolution of a large amount of gasses. The obtained black-brown products were dried in the oven overnight at 150 °C and later ground to fine powders, which were firstly annealed at 1000 °C for 2 h in air to remove the residual carbon after the combustion process. Subsequently, obtained powders were sintered for 4 h at 1700 °C under CO atmosphere. The body colour of undoped Lu3Al5O12 was white. Ce3+ doped sample was yellow–green. The body colour of solely Cr3+ doped samples ranged from pale green to dark green depending on chromium concentration. The body colour of Ce3+/Cr3+ doped samples were dirty yellow. TG/DSC measurements of the precursor gels were recorded on a PerkinElmer STA 6000 Simultaneous Thermal Analyser. The heating rate was 10 °C/min. The sample weight was 2 mg. The atmosphere was synthetic air with flow rate of 20 mL/min. Powder XRD data were collected from 10° 6 2h 6 80° (step width 0.02° and integration time 0.5 s) using Ni-filtered Cu Ka radiation on a Rigaku MiniFlex II diffractometer working in Bragg–Brentano (h/2h) geometry. IR spectra were taken with PerkinElmer Frontier ATR-FTIR Spectrometer equipped with a liquid nitrogen cooled MCT detector. SEM images were taken with a FE-SEM Hitachi SU-70. Reflection spectra in the range of 250–800 nm were recorded on an Edinburgh Instruments FSL900 spectrometer equipped with a 450 W Xe arc lamp, a cooled (20 °C) single-photon counting photomultiplier (Hamamatsu R928) and a Teflon integration sphere. A BaSO4 sample (99% Sigma–Aldrich) was used as a reflectance standard. The excitation and emission slits were set to 2.5 and 0.4 nm, respectively. The step size was 1 nm and integration time 0.4 s. Excitation and emission spectra were recorded on an Edinburgh Instruments FSL900 spectrometer equipped with a 450 W Xe arc lamp, a cooled (20 °C) single-photon counting photomultiplier (Hamamatsu R928) and a lens optics for powder samples. The photoluminescence emission spectra were corrected by a correction file obtained from a tungsten incandescent lamp certified by the NPL (National Physic Laboratory, UK). When measuring emission spectra emission and excitation slits were set to 0.2 and 2.5 nm, respectively. When measuring excitation spectra emission and excitation slits were set to 1 and 0.5 nm, respectively. The step size was 1 nm and integration time 0.2 s. Quantum efficiencies were calculated according to the equation [15]
R R ðISample Þdk ðIBlack Þdk 1 RRef R QE ¼ QERef R 1 RSample ðIRef Þdk ðIBlack Þdk
R
emission integral of the sample, (IBlack)dk is the emission integral R of the black standard, (IRef)dk is the emission integral of the reference material, and RRef and RSample are the reflection values at 435 nm of reference material and sample, respectively. The black standard (Flock Paper #55, Edmund Optics) was used to eliminate the dark count rate of the detector. For QE determination of all phosphor samples were excited at 435 nm. The error of the quantum efficiency calculations has been found to be ±5%. All measurements were performed at room temperature and ambient pressure in air. 3. Results and discussion 3.1. Preparation and structural properties The TG/DSC curves of the Lu–Al–O gel after the combustion process are depicted in Fig. 1. The TG curve indicate that the mass loss occurs in two main steps, similar that was observed in Ca–Lu–Al– Si–O system reported in our earlier work [15]. The first step (peak at 495 °C) yielded in a weight loss of about 17% and can be attributed to burning of the organic residues that remained after the combustion process. This is in good agreement with DSC curve, which shows exothermal processes occurring in this temperature range. The second weight loss step of around 6% occurs in the temperature range 850–1000 °C and can be assigned to the decomposition of metal carbonates or oxycarbonates. This is supported by the DSC curve showing endothermic processes occurring in this temperature range. Moreover, the full decomposition of lutetium carbonate was reported to be around 800 °C [16] what is rather similar to the results obtained in our study. The TG/DSC curves of the gels of samples containing Cr and/or Ce showed similar behaviour. The powder XRD patterns of as prepared gel, and gels annealed at 1000 °C and 1700 °C are shown in Fig. 2. The reference pattern of Lu3Al5O12 (PDF4+ (ICDD) 04-001-9996) [17] was also added for comparison. It is obvious that during the combustion process no crystallization takes place. However, annealing the gel at 1000 °C yields the single phase lutetium aluminium garnet. The XRD peaks are rather broad indicating that the formed crystallites are small. The size of crystallites at this stage was calculated by employing Scherrer equation [18]
t¼
0:9k B cos hB
ð2Þ
where t is the mean crystallite size, k the X-ray wavelength, B the line broadening at half maximum intensity (FWHM) (in radians),
ð1Þ
where QERef is the quantum efficiency (88%) (kEx = 435 nm) of the R reference material (YAG:Ce3+, U728, Philips), (ISample)dk is the
Fig. 1. TG/DSC curves of the Lu–Al–O precursor gel obtained after the combustion process.
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Fig. 2. Powder XRD patterns of as prepared Lu–Al–O gel, gel annealed at 1000 °C, and at 1700 °C, and Lu3Al5O12 crystal [17].
and hB the Bragg angle. The mean crystallite size was found to be about 20 nm and was independent on chromium and/or cerium concentration. Raising the sintering temperature to 1700 °C resulted in much narrower peaks due to increase of particle size of the material. The same pattern was observed for samples doped with chromium and/or cerium demonstrating that the dopant ions does not affect the crystal structure formation (at least up to investigated concentrations). Fig. 3 depicts the FTIR spectra of Lu–Al–O gel after the combustion process and sintered at 1000 and 1700 °C. The spectrum of gel after combustion contains two broad and intense absorption bands
Fig. 3. IR spectra of as prepared Lu–Al–O gel, gel annealed at 1000 °C, and at 1700 °C.
located at 1700–1300 cm1 and 1000–500 cm1. The first band can be attributed to the carbonates [19], which formed during combustion process of the gel. Similar results were observed by Leleckaite et al. [20], where yttrium aluminium garnet powders were synthesized by aqueous sol–gel technique employing 1,2-ethandiole as complexing agent. The second band can be assigned to the characteristic metal–oxygen vibrations. The spectra of Lu3Al5O12 samples annealed at 1000 and 1700 °C lacks the first band indicating that the metal carbonates that formed during the combustion process are completely decomposed at these temperatures. Moreover, the broad band observed at 1000–500 cm1 split into several sharp bands when the gel was sintered either at 1000 or 1700 °C. Such split is typical for the garnet type compounds and these results goes hand in hand with the powder XRD data presented in Fig. 2, showing the single phase Lu3Al5O12 garnets formed at those temperatures. The morphology features of the Lu3Al5O12 and Lu2.985Ce0.015 Al4.8Cr0.2O12 gels and gels annealed at 1000 and 1700 °C were inspected by taking SEM images, which are shown in Fig. 4. The as-synthesized gels of both undoped (Fig. 4a) and doped (Fig. 4d) samples possess similar sponge like morphology. The presence of large and small pores are clearly visible. These pores likely formed due to escaping gasses during the combustion process. Sintering of the aforementioned gels at 1000 °C have not induced much changes in particle morphology (Fig. 4b and e). The sponge like structure is still present, however, with some larger holes inside. These holes are the result of escaping gasses, which formed during the burning of organic residues that were left after combustion process. The increase of sintering temperature to 1700 °C resulted in the particle growth, what is in line with the peak narrowing in the XRD patterns at elevated temperatures. Powders sintered at 1700 °C consist of well-shaped and highly agglomerated particles with the size of 1–3 lm (Fig. 4c and f). No noticeable difference between the morphologies of undoped and doped powders was observed. 3.2. Reflection spectra The reflection spectra of Cr3+ and Ce3+/Cr3+ doped samples are shown in Fig. 5a and b, respectively. The undoped Lu3Al5O12 powder showed high reflectance at longer wavelengths indicating high brilliance of the prepared sample. The spectra of samples containing Cr3+ possessed two broad and strong absorption bands with the maxima at 420 and 580 nm. These bands can be attributed to the optical transitions from the ground state 4A2 to the excited states 4 T1 and 4T2, respectively [21,22]. As expected, the increase of Cr3+ concentration resulted in stronger absorption. However, the increase of chromium concentration has also resulted in decrease of reflectance at longer wavelengths what caused the greyishing of the samples. This can be attributed to the formation of defects. There are many forms of defects (surface, antisite, Schottky, etc.) that are present in the powder materials. Moreover, the increase of Cr3+ concentration might also lead to formation of Cr3+ pair centres, which by various charge compensation schemes may facilitate defect formation. However, at this stage it is not clear which defects are causing greyishing of powders and this issue needs a separate study. The reflection spectra of Ce3+/Cr3+ doped samples also contained typical Ce3+ two absorption bands in garnet type materials. The absorption originates from the ground state 2F5/2 of Ce3+ to the two lowest crystal field components of the 5d orbital. It is also worth to note that very weak absorption line from spinforbidden 4A2 ? 2E transition [22] of Cr3+ was also observed in reflection spectra of both Cr3+ and Ce3+/Cr3+ doped samples. Since the 2E level is nearly independent on the crystal field strength the emission from it occurs at the same wavelength as observed in reflectance spectra (zero-phonon lines) [22,23].
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Fig. 4. SEM images of undoped Lu3Al5O12 (a) gel and powders sintered at (b) 1000 °C and (c) 1700 °C; Lu2.985Ce0.015Al4.8Cr0.2O12 (0.5 mol% of Ce3+ and 4 mol% of Cr3+) (d) gels and powders sintered at (e) 1000 °C and (f) 1700 °C.
Fig. 5. Reflection spectra of (a) Cr3+ doped Lu3Al5O12 and (b) Ce3+/Cr3+ doped Lu3Al5O12 powders. The reflection spectrum of YAG:Ce3+ standard U728 is given for the reference.
3.3. Excitation and emission spectra Similar to reflection spectra the 4A2 ? 4T2, 4A2 ? 4T1(4F), and A2 ? 4T1(4P) transitions of Cr3+ dominate in the excitation spectra of Lu3Al5O12:Cr3+ (Fig. 6a). The intensity of excitation bands
4
increased when samples were doped up to 1% of Cr3+. Exceeding this concentration of chromium ions resulted in gradual reduction of intensity of excitation bands. The excitation spectra of 0.1% and 1% Cr3+ doped samples were recorded for the 695 nm emission since this line was the strongest one in the emission spectra
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The excitation and emission spectra of Ce3+/Cr3+ doped samples are given in Fig. 7a and b, respectively. Excitation spectra were recorded for the strongest emission line at 687 nm in emission spectra. The excitation spectra of 0.5% Ce3+ doped Lu3Al5O12 sample was recorded monitoring two emission wavelengths, namely, 560 and 687 nm. Monitoring emission at 560 nm leads to typical excitation spectra of Ce3+ ions in garnet type materials with high intensity bands peaking at 345 and 445 nm. However, monitoring emission at 687 nm gives very weak excitation bands. This is in line with the emission spectrum of solely Ce3+ doped lutetium aluminium garnet shown in Fig. 7b since there is nearly no emission at this wavelength. The situation changes when Cr3+ ions are introduced to the host matrix. Addition of as little as 0.1% of Cr3+ results in high intensity excitation bands of Ce3+ in the spectra thus confirming the Ce3+ ? Cr3+ energy transfer. The highest intensity was obtained for 0.5% Cr3+/1% Cr3+ doped sample. The increased Cr3+ concentration resulted in gradual decrease of intensity. The emission spectra of Lu3Al5O12:0.5% Ce3+ consisted of broad and asymmetric band ranging from 460 to 750 nm with the maximum at 510 nm and the shoulder at 550 nm. This asymmetry arises from the strong overlap of the two bands originating in the transitions from the lowest crystal-field component of the [Xe]5d1 configuration to the spin-orbit split sublevels 2F5/2 and 2F7/2 of the [Xe]4f1 configuration of Ce3+ ions [15,29]. The emission intensity of cerium doped sample was around 80% of the standard. Introduction of 0.1% Cr3+ into the host matrix decreased the Ce3+ emission more
Fig. 6. Excitation (a) and emission (b) spectra of Lu3Al5O12:Cr3+ samples. The excitation and emission spectra of YAG:Ce3+ standard U728 are given for the intensity comparison.
(Fig. 6b). However, when samples were doped with 2% and 4% of Cr3+ the 687 nm line became dominant in the emission spectra. Therefore, the excitation spectra of these two samples were recorded for this emission wavelength. It was observed that the change of monitored emission from 695 to 687 nm resulted in excitation band shift to longer wavelengths by 30 nm. The spectra of these bands match well with the ones observed in the reflection spectra. After some literature research it was concluded that emission band at 695 nm belongs to Cr3+ ions in the Al2O3 host matrix [24–26]. Even though no impurity phases were observed in the powder XRD patterns, the PL measurements are more sensitive and, therefore, the Al2O3 impurity was detected. The emission spectra (Fig. 6b) of Lu3Al5O12:Cr3+ powders were recorded upon 435 nm excitation. The spectra contain typical Cr3+ emission bands and lines in garnet type materials [6,14,27]. The line emission at 687 and 695 nm originate from the 2E ? 4A2 transition of Cr3+ ions in Lu3Al5O12 and Al2O3, respectively. The other emission bands around 687 nm line can be attributed to the vibronic sidebands [24,28]. The most intensive emission was observed for the samples doped with 1% and 2% of Cr3+. Further increase of dopant concentration significantly reduced emission intensity likely due to concentration quenching. The emission spectrum of standard material YAG:Ce3+ (U728, Philips) under the same conditions was also recorded in order to compare the emission intensities. It is obvious that Cr3+ emission intensity in Lu3Al5O12 host matrix is comparable to that of YAG:Ce3+, but does not exceed it. However, it is also clear that the total light output is much higher for the cerium doped garnet since its emission band is significantly broader.
Fig. 7. Excitation (a) and emission (b) spectra of Lu3Al5O12:Ce3+,Cr3+ samples. The excitation and emission spectra of YAG:Ce3+ standard U728 are given for the intensity comparison.
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than twice. With further increase of chromium concentration the intensity of Ce3+ emission band progressively decreased and almost vanished when Cr3+ concentration reached 4%. The strongest chromium emission in 0.5% Ce3+/Cr3+ doped samples was found in specimen with 1% Cr3+. By comparing Figs. 6b and 7b it is evident that chromium emission is twice stronger in the samples containing Ce3+ ions. Thus, it can be concluded that cerium ions act as sensitizer to chromium ions. The comparison of Ce3+ and Cr3+ emission integrals as a function of chromium concentration in 0.5% Ce3+/Cr3+ doped samples is given in Fig. 8. Emission spectra of both ions overlap, therefore, the emission integrals of Ce3+ and Cr3+ were calculated in the range of 460–650 nm and 650–900 nm, respectively. It was also calculated that only 3% of total Ce3+ emission is in the range of 650– 00 nm and around 4% of total Cr3+ emission is in the range of 460–650 nm. Therefore, the resulting calculation error is also included in Fig. 8. Strong quenching of cerium emission with increasing chromium concentration is observed what is in line with results represented in Fig. 7b. Chromium emission reaches maximum at concentration of 1% and also rapidly decreases with R R increasing Cr3+ content. The ratio I(Cr3+)/ I(Ce3+) is slightly less than 1 for the sample doped with 0.1% Cr3+. This indicates that more photons are coming out from Ce3+ ions. However, at higher concentrations Cr3+ emission starts to dominate what again is in line with Fig. 7b.
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Fig. 9. Quantum efficiencies of Lu3Al5O12:Cr3+ and Lu3Al5O12:Ce3+,Cr3+ powder samples.
3.4. Quantum efficiencies and chromaticity diagrams Quantum efficiencies of all prepared specimens were calculated according to Eq. (1) and are shown in Fig. 9. In Lu3Al5O12:Cr3+ series the highest efficiency 48% and 41% was obtained for the samples doped with 1% and 2% of Cr3+, respectively. Sample doped with 4% of Cr3+ showed only 16% of efficiency indicating strong concentration quenching. Lu3Al5O12:0.5% Ce3+ possessed 75% quantum yield. However, after the addition of 0.1% Cr3+ this value has dropped to 58% and continued decreasing with increasing chromium concentration. Sample doped with 4% Cr3+ showed only 6% of quantum yield, which is roughly 3 times lower than sample without Ce3+. Quantum yield calculations clearly shows that increasing Cr3+ concentration efficiently quenches the total efficiency of the phosphor. Fig. 10 shows a fragment of the CIE 1931 chromaticity diagram with the colour points of Lu3Al5O12:Cr3+ and Lu3Al5O12:0.5% Ce3+, Cr3+ specimens as a function of Cr3+ concentration. The colour points of the former are located in the red region and does not change with increasing Cr3+ concentration. This goes hand in hand
Fig. 10. A fragment of the CIE 1931 chromaticity diagram with colour points of Lu3Al5O12:Cr3+ and Lu3Al5O12:Ce3+,Cr3+ powder samples. The exact colour point values and the corresponding luminous efficacy values are given in the inset table.
with the relatively constant luminous efficacy values. However, the colour points of the latter shows strong dependence on chromium concentration as could be expected from Fig. 7b. The strong shift from yellowish-green to yellow–green region is observed with increasing Cr content. This shift was accompanied by the decrease of luminous efficacy values what can be explained by smaller overlap of human eye sensitivity curve and emission spectra. 4. Conclusions
R R Fig. 8. Integrated emission intensities of Ce3+ ( I(Ce3+)) and Cr3+ ( I(Cr3+)) together R R 3+ 3+ 3+ with the ratio of ( I(Cr ))/( I(Ce )) as a function of Cr concentration in Lu2.985Ce0.015Al5O12:Cr3+.
In this study the photoluminescence properties of Lu3Al5O12: Cr3+ and Lu3Al5O12:0.5% Ce3+,Cr3+ luminescent materials and Ce3+ ? Cr3+ energy transfer were investigated and discussed. In this system Cr3+ ions can be excited in the visible spectral region from 340 to 650 nm yielding the far-red emission at 687 nm. Codoping with Ce3+ ions resulted in increased Cr3+ emission intensity thus confirming the presence of Ce3+ ? Cr3+ energy transfer. Unfortunately, the overall quantum yield of Ce3+,Cr3+ doped specimens decreased with increasing Cr3+ concentration, indicating that some quenching processes also occurs between these two ions. The efficiency of such phosphors might be increased by choosing the
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garnet structure compounds, whose Ce3+ emission spectra overlaps better with the absorption spectra of Cr3+ ions. Few of such compounds could be Y3Al3MgSiO12:Ce3+ (kem = 575 nm) [29], Y3AlMg2 Si2O12:Ce3+ (kem = 600 nm) [30] and Lu2CaMg2(Si,Ge)3O12:Ce3+ (kem = 620 nm) [31]. Acknowledgement The study was funded from the European Community’s social foundation under Grant Agreement No. VP1-3.1-ŠMM-08-K-01004/KS-120000-1756. References [1] Y. Zhao, R.R. Lunt, Adv. Energy Mater. 3 (2013) 1143–1148. [2] W.G.J.H.M.V. Sark, K.W.J. Barnham, L.H. Slooff, A.J. Chatten, A. Büchtemann, A. Meyer, S.J. McCormack, R. Koole, D.J. Farrell, R. Bose, E.E. Bende, A.R. Burgers, T. Budel, J. Quilitz, M. Kennedy, T. Meyer, C.D.M. Donegá, A. Meijerink, D. Vanmaekelbergh, Opt. Express 16 (2008) 21773–21792. [3] B.M. van der Ende, L. Aarts, A. Meijerink, Adv. Mater. (Weinheim, Ger.) 21 (2009) 3073–3077. [4] M.G. Debije, P.P.C. Verbunt, Adv. Energy Mater. 2 (2012) 12–35. [5] L.H. Slooff, E.E. Bende, A.R. Burgers, T. Budel, M. Pravettoni, R.P. Kenny, E.D. Dunlop, A. Büchtemann, Phys. Status Solidi RRL 2 (2008) 257–259. [6] L.M. Shao, X.P. Jing, J. Lumin. 131 (2011) 1216–1221. [7] A. Goetzberger, W. Greube, Appl. Phys. 14 (1977) 123–139. [8] M.K. Mishra, A. Mandal, J. Saha, G. De, Opt. Mater. 35 (2013) 2604–2612. [9] M.M. Nolasco, P.M. Vaz, V.T. Freitas, P.P. Lima, P.S. Andre, R.A.S. Ferreira, P.D. Vaz, P. Ribeiro-Claro, L.D. Carlos, J. Mater. Chem. A 1 (2013) 7339–7350. [10] R. Reisfeld, J. Less-Common Met. 93 (1983) 243–251. [11] V. Bachmann, C. Ronda, A. Meijerink, Chem. Mater. 21 (2009) 2077–2084. [12] J.M. Ogieglo, Luminescence and Energy Transfer in Garnet Scintillators Ph.D. Thesis, University of Utrecht, Utrecht, The Netherlands, 2012.
[13] J.M. Ogieglo, A. Katelnikovas, A. Zych, T. Jüstel, A. Meijerink, C.R. Ronda, J. Phys. Chem. A 117 (2013) 2479–2484. [14] W.D. Wang, J.K. Tang, S.T. Hsu, J. Wang, B.P. Sullivan, Chem. Phys. Lett. 457 (2008) 103–105. [15] A. Katelnikovas, J. Plewa, D. Dutczak, S. Möller, D. Enseling, H. Winkler, A. Kareiva, T. Jüstel, Opt. Mater. 34 (2012) 1195–1201. [16] S. Liu, R.J. Ma, Acta Chem. Scand. 51 (1997) 893–895. [17] A.M. Antyukhov, N.V. Moiseev, S.A. Smirnova, T.M. Lysenko, Inorg. Mater. 26 (1990) 1443–1447. [18] A.R. West, Solid State Chemistry and its Applications, John Wiley & Sons, Chichester, 1984. [19] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, fourth ed., Wiley, New York, 1986. [20] A. Leleckaite, A. Kareiva, H. Bettentrup, T. Jüstel, H.J. Meyer, Z. Anorg, Allg. Chem. 631 (2005) 2987–2993. [21] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994. [22] W.M. Yen, S. Shionoya, H. Yamamoto, Fundamentals of Phosphors, CRC Press, Boca Raton, 2007. [23] N.M. Avram, M.G. Brik, Optical Properties of 3d-Ions in Crystals: Spectroscopy and Crystal Field Analysis, first ed., Springer, New York, 2013. [24] K.M. Kinsman, J. McKittrick, E. Sluzky, K. Hesse, J. Am. Ceram. Soc. 77 (1994) 2866–2872. [25] V.N. Snytnikov, V.O. Stoyanovskii, T.V. Larina, O.P. Krivoruchko, V.A. Ushakov, V.N. Parmon, Kinet. Catal. 49 (2008) 291–298. [26] M. Kirm, E. Feldbach, A. Kotlov, P. Liblik, A. Lushchik, M. Oja, E. Palcevskis, Radiat. Meas. 45 (2010) 618–620. [27] J. Kvapil, J. Kvapil, B. Perner, Czech. J. Phys. B 37 (1987) 1277–1287. [28] B.K. Sevast’yanov, D.T. Sviridov, V.P. Orekhova, L.B. Pasternak, R.K. Sviridova, T.F. Veremeichik, Sov. J. Quantum Electron. 2 (1973) 339–343. [29] A. Katelnikovas, H. Winkler, A. Kareiva, T. Jüstel, Opt. Mater. 33 (2011) 992– 995. [30] A. Katelnikovas, H. Bettentrup, D. Uhlich, S. Sakirzanovas, T. Jüstel, A. Kareiva, J. Lumin. 129 (2009) 1356–1361. [31] A.A. Setlur, W.J. Heward, Y. Gao, A.M. Srivastava, R.G. Chandran, M.V. Shankar, Chem. Mater. 18 (2006) 3314–3322.