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Stability improvement of moisture sensitive CaS:Eu2+ micro-particles by coating with sol–gel alumina
Optical Materials 33 (2011) 1032–1035
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Stability improvement of moisture sensitive CaS:Eu2+ micro-particles by coating with sol–gel alumina Nursen Avci ⇑, Iolanda Cimieri, Philippe F. Smet, Dirk Poelman LumiLab, Dept. Solid State Sciences, Ghent University, Krijgslaan 281-S1, 9000 Gent, Belgium
a r t i c l e
i n f o
Article history: Received 2 June 2010 Received in revised form 28 July 2010 Accepted 31 July 2010 Available online 30 August 2010 Keywords: CaS:Eu2+ Alumina Sol–gel Protection layer
a b s t r a c t Here we report on the usage of alumina coating for the protection of moisture sensitive CaS:Eu2+ microparticles. Alumina sol was prepared using a water-free sol–gel technique and single crystal CaS:Eu2+ luminescent micro-particles were synthesized via solvothermal way. After deposition of the particles on a substrate, they were coated with an alumina layer and heat treated at 500 °C for 30 min. In addition to in situ measurements of accelerated ageing of the luminescent particles, photoluminescence (PL) spectra of coated and uncoated particles were compared. Coated particles showed a broad band PL emission with a maximum at 650 nm identical to that of uncoated particles. Coating with an alumina layer drastically increased the resistivity of the luminescent material against moisture. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Rare earth doped alkaline earth binary and ternary sulfides have a very special place in luminescent materials because of their relatively low synthesis temperature and broad emission spectra upon doping with europium and cerium [1,2]. Particularly to obtain red or orange emission CaS:Eu2+ and SrS:Eu2+ are considered suitable candidates [3–5]. Ca1x Srx S : Eu2þ phosphors with a strong absorption in the blue region are currently also used in white-light emitting diodes. Compared to the standard phosphor, YAG:Ce, the emission of Ca1x Srx S : Eu2þ peaks at longer wavelengths, which allows the fabrication of light emitting diodes (LEDs) with lower colour temperature and improved colour rendering [1]. In addition to wavelength converters in LEDs, alkaline earth sulfide phosphors are employed in different areas such as display applications [1], electroluminescent devices [6] and optical information storage [7]. Nevertheless, the lack of stability with respect to water and other atmospheric components hinders usage of the alkaline earth sulfides as phosphor hosts [1,3,8–12]. As an example, during the operation of LEDs, LED chips can reach relatively high temperatures of 450 K. Degradation of the alkaline earth sulfides is accelerated by temperature and decomposition products such as sulfur dioxide and hydrogen sulfide gases evolve from the surface. Therefore, the decomposition causes not only a decrease in the light intensity, but also a reduction in the reflectivity due to the chemical reaction between the released hydrogen sulfide gas and the sil-
⇑ Corresponding author. E-mail address: [email protected] (N. Avci). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.07.021
ver pad under the LED chip [3]. To maintain the emission colour and intensity of the LED throughout its extended lifetime, the stability of the phosphor material is of utmost importance. A number of encapsulation techniques have been utilized to improve the stability of sulfide phosphors [8,11–15]. In this case the properties of the encapsulant, usually in the form of an inert thin film, are as important as the coating technique. In order to improve the stability of sulfide by encapsulation with an inert film, the film should be thermally and chemically stable. In addition, it should be transparent to the excitation and emission light of the incorporated luminescent particles. The film should also homogeneously coat the particle surfaces. Oxides [8], organic materials [14] and nitrides [16] have been used as protection coating for sulfide phosphors. Alumina is one of the attractive materials in the oxide family. It has good thermal and chemical stability; moreover, its transparency window is very wide from ultra-violet to near infrared. Therefore alumina is a very promising material for protective coatings. A number of techniques have been used to synthesize alumina barrier layers namely atomic layer deposition (ALD), magnetron sputtering, chemical vapor deposition (CVD), and sol–gel. In these techniques, sol–gel is the most favorable one because this technique puts forward excellent control of stoichiometry, density and microstructure and short deposition time. Furthermore, using simple equipment and without the need for vacuum, large surface areas can be coated. Using a water based solution or an excess amount of water for hydrolysis and/or strong acid as a dispersing agent to prepare alumina may cause the degradation of moisture-sensitive material. This problem can be solved by using a non-aqueous sol–gel technique.
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In this paper, we explain the modification of moisture sensitive CaS : Eu2þ particles synthesized via a solvothermal route by coating with alumina prepared via a non-aqueous sol–gel technique. In addition to the investigation of the effect of coating on photoluminescence properties of the phosphor, the comparison between moisture resistance of uncoated and coated particles is done under accelerated ageing conditions (80 °C and 80% relative humidity). We also examine the stability of uncoated and coated particles after heat treatment at 500 °C. 2. Experimental The synthesis of single crystal CaS : Eu2þ particles via a solvothermal synthesis method was described in detail elsewhere [5]. After the synthesis, CaS : Eu2þ particles were kept in ethanol as suspension, to prevent degradation caused by humidity. Typical particle sizes were between 0.25 and 0.75 lm, with clearly defined crystal faces. Phosphor layers were obtained by dripping the suspension on a silicon substrate and drying in air at 40 °C. This process was reiterated a number of times until complete coverage of the substrates with CaS : Eu2þ particles was obtained. To assess the influence of the heat treatment (as required during the formation of the Al2 O3 coating), an uncoated phosphor layer was also subjected to an heat treatment at 500 °C for 30 min in air. Alumina sol was prepared using a non-aqueous sol–gel technique. The sol was synthesized using aluminium sec-butoxide ðAl½OðCH3 ÞCHC2 H5 3 Þ (Alfa Aesar, 95%) as precursor, n-butyl alcohol (n-BuOH) (Alfa Aesar, 99.4%) as solvent and acetylacetone (AcAcH) (Alfa Aesar, 99%) as chelating agent. The molar ratio between acetylacetone and aluminium sec-butoxide was 1. The concentration of the sols was adjusted to 0.5 M. Initially, solvent and acetylacetone were mixed, and then aluminium sec-butoxide was added to this mixture. Eventually the solution was stirred for 6 h at 30 °C. After the stirring, a transparent and very stable sol was obtained. In order to check the necessity of AcAcH in the solution, sols were also prepared without AcAcH. The deposition of the alumina coating on CaS : Eu2þ particles was performed by spin coating at 800 rpm for 20 s. After coating, the coated phosphor layer was heat treated at 500 °C for 30 min in air to remove the organic groups. In the remainder of the article, ‘coating’ refers to both the spin coating and the subsequent heat treatment. As alumina layers without AcAcH did not protect the particles, all further layers were prepared using AcAcH. X-ray diffraction (XRD, Bruker D8-Discovery, CuKa radiation) measurements were employed to acquire information about the crystal structure of uncoated and coated particles. Photoluminescence emission and excitation spectrum were recorded with a FS920 luminescence spectrometer (Edinburgh Instruments). The stability of the phosphor layers against moisture was inspected with an in situ photoluminescence measurement during accelerated ageing of the uncoated and coated samples at 80 °C and 80% relative humidity. PL intensities of the sample in a humidity chamber (CST, CS-40/200) were measured with an HR2000+ luminescence spectrometer (ocean optics). Samples were excited by a LED with an emission wavelength of 440 nm. Using a Bif400-UV/ VIS Y-shaped optical fiber cable (ocean optics) excitation light from the LED to the sample and emission light from the sample to the spectrometer were separated. 3. Results and discussion Initially, decomposition of CaS : Eu2þ particles was examined in two conditions. In the first one, the uncoated sample was kept in a humidity chamber at 80 °C and 80% relative humidity and in the second one the uncoated sample was heat treated at 500 °C.
Fig. 1. XRD spectra of the uncoated CaS : Eu2þ particles, before any treatment, after 75 h of accelerated ageing and after heat treatment. The lattice planes of CaS (data from ICDD file No. 65-7852) are indicated. Lines represent the peak positions of CaSO4 (data from ICDD file No. 37-1496). Triangles indicate the peak positions of CaCO3 (data from ICDD file No. 72-0506).
Fig. 1 shows the XRD results of uncoated CaS : Eu2þ particles before any treatment, after 75 h of accelerated ageing and after heat treatment. The heat treated sample was not subjected to any ageing. The as-deposited particles were single phase cubic CaS. The peaks related to CaS disappear after the accelerated ageing process and only CaCO3 peaks with small intensity are observed. This is evidence of the decomposition of CaS to CaCO3 in the presence of CO2 and H2 O at lower temperature [17]. After the heat treatment at 500 °C with a heating rate of 10 °C/min for 30 min, the differential intensity of the diffraction peaks from CaS reduced by about 60% with the formation of CaSO4 (Fig. 1). There are two factors which are critical for the decomposition process [10]. The first one is the particle size, the decomposition of small particles is easier than bigger ones and the second one is heating rate, using slower heating rate causes more decomposition. Since the particles (between 0.25 lm and 0.75 lm) were rather small compared to earlier studies [12,14], it was not surprising to have a strong decomposition for this sample. PL excitation and emission spectra of CaS : Eu2þ before (dashed lines) and after (solid lines) heat treatment at 500 °C for 30 min are shown in Fig. 2. The excitation and emission spectra were recorded at an emission wavelength of 650 nm and at an excitation wavelength of 450 nm, respectively in Fig. 2a. The high energy side excitation band between 250 and 300 nm is a combination of the CaS band gap transition and the excitation to the 4f 6 5d ðEg Þ state in Eu2þ (Fig. 2a). A Decrease of the CaS : Eu2þ particle size due to the partial decomposition results in a relative increase of the intensity of the excitation peak from the CaS band gap because of the different absorption lengths for both processes. This changing of the relative ratio between these two peaks results in shifting of the high energy side band in the excitation spectra of CaS : Eu2þ (Fig. 2a) [18]. After the heat treatment the intensity of the emission peak at 650 nm reduces by 80% nevertheless there is no shift in the emission spectra. PL emission spectra recorded at an excitation wavelength of 275 nm of CaS : Eu2þ before (dashed line) and after (solid line) heat treatment at 500 °C for 30 min are shown in Fig. 2b. The small emission peak at 385 nm formed after heat treatment indicates the decomposition of CaS : Eu2þ to CaSO4 : Eu2þ [19]. The broad peak at 500 nm could not be identified. As seen in Fig. 2b, after the heat treatment there is a deformation in the emission peak at 650 nm. This may suggest the formation of other luminescent centres such as Eu3þ [19]. No PL signal could be obtained after 75 h of accelerated ageing at 80 °C and 80% relative humidity, showing full decomposition. After the observation of the (partial) decomposition of uncoated particles during heat treatment at 500 °C and accelerated ageing at
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Fig. 2. (a) PL Excitation and emission spectra of CaS : Eu2þ before (dashed lines) and after (solid lines) heat treatment at 500 °C for 30 min. Excitation spectra were recorded at an emission wavelength of 650 nm and emission spectra were recorded at an excitation wavelength of 450 nm. (b) PL emission spectra of CaS : Eu2þ before (dashed line) and after (solid line) heat treatment at 500 °C for 30 min recorded at an excitation wavelength of 275 nm.
After the ageing, the diffraction pattern remained the same as after the coating (Fig. 3). This emphasizes that alumina layer hinders the oxidation of CaS particles during ageing. The PL excitation and emission spectra of CaS : Eu2þ before (dashed lines) and after (solid lines) coating are shown in Fig. 4. The excitation and emission spectra were recorded at an emission wavelength of 650 nm and at an excitation wavelength of 450 nm, respectively (Fig. 4a). In addition to the blue shifting at higher energy side in the excitation spectra after coating, there is a decrease in the intensity of the excitation peak at lower energy side. These facts imply that the phosphor particles become smaller through decomposition, as the relative reduction in absorption length can alter the shape of the excitation spectrum close to the bandgap. Although water was not used for the synthesis of the alumina sol, water was produced by the condensation reaction and this may be the reason for the partial decomposition of CaS : Eu2þ . The intensity of the emission peak at 650 nm (Fig. 4a) reduces to half after coating nevertheless there is no shift or deformation in the peak. Fig. 4b shows the PL emission spectra of uncoated (dashed line) and coated (solid line) CaS : Eu2þ at an excitation wavelength of 275 nm. A small peak at 385 nm was observed after coating (solid line, Fig. 4b), this peak is an evidence of a small amount of CaSO4 in the phosphor. When the emission spectra of heat treated uncoated CaS : Eu2þ particles (Fig. 2a) and coated particles (Fig. 4a) are compared, it can be seen that the emission intensity of uncoated particles decreases to 20% although the coated particles were heat treated at the same temperature as uncoated one, that of coated particles decreases only to 50%. This highlights that the alumina layer prevents the particle from decomposition during the heat treatment.
80 °C and 80% relative humidity, the effect of the alumina coating on the properties of CaS : Eu2þ phosphor was investigated. After coating of the CaS : Eu2þ particles with an alumina layer, the coated particles were heat treated at 500 °C with a heating rate of 10 °C/ min for 30 min to remove the organic groups from the layer. In order to examine how good the alumina film performs as protection layer the coated CaS : Eu2þ was aged (80 °C and 80% relative humidity). The XRD spectra of uncoated and coated CaS : Eu2þ particles before and after 100 h of accelerated ageing are presented in Fig. 3. The intensity of the (200) peak of CaS reduced to 70% after coating and it remained the same after 100 h of accelerated ageing. However traces of CaSO4 were observed, both before and after ageing.
Fig. 3. XRD spectra of of the uncoated CaS : Eu2þ particles, the CaS : Eu2þ particles coated with alumina film before and after 100 h of accelerated ageing. The lattice planes of CaS (data from ICDD file No. 65-7852) are indicated. Lines represent the peak positions of CaSO4 (data from ICDD file No. 37-0184).
Fig. 4. (a) PL Excitation and emission spectra of uncoated (dashed lines) and coated (solid lines) CaS : Eu2þ . Excitation spectra were recorded at an emission wavelength of 650 nm and emission spectra were recorded at an excitation wavelength of 450 nm. (b) PL emission spectra of uncoated (dashed lines) and coated (solid lines) CaS : Eu2þ recorded at an excitation wavelength of 275 nm.
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this into account, in combination with the excellent resistance against of both the ALD and sol–gel coated particles, ALD is the better technique to prepare a protection layer for sulfides as it gives a higher luminescence efficiency. A disadvantage is it being more expensive as coating technique compared to sol–gel techniques. Hence, sol–gel based inorganic coatings seem an interesting route for the protection of luminescent sulfides, on condition that the degradation during the necessary heat treatment can be reduced. Acknowledgements
Fig. 5. PL intensity of uncoated (line A) sample and sample coated with an alumina film (line B) as a function of time at 80 °C and 80% relative humidity.
This research was carried out under the Interuniversity attraction poles programme IAP/VI-17 (INANOMAT) financed by the Belgian State, Federal science policy office. We gratefully acknowledge Bjorn Vandecasteele for help with in situ accelerated ageing measurements. References
Fig. 5 shows the in situ measurement of the photoluminescence intensity during accelerated ageing of the uncoated and coated samples at 80 °C and 80% relative humidity. Line A represents the degradation of an uncoated sample and line B is related to the degradation of samples coated with an alumina layer. As seen in Fig. 5, during the first 30 h the PL emission of uncoated CaS : Eu2þ almost entirely disappears. After 75 h, the PL intensity of uncoated phosphor was not detectable anymore, while the PL intensity of the coated sample was around 95% of its initial value. 4. Conclusions CaS : Eu2þ -based materials are good candidates as wavelength converting material in LEDs for general lighting if their stability can be enhanced. Phosphor layers were prepared using submicron-sized single crystalline CaS : Eu2þ synthesized via a solvothermal route. The phosphor layers were coated with alumina prepared via a non-aqueous sol–gel technique to improve their stability. The experimental results show that an alumina film strongly enhances the resistance of the phosphor layer against moisture and high temperature in spite of the partial degradation of the sample surface during the coating process. In our previous report we coated the moisture sensitive particles using ALD aluminium oxide [8]. Unlike the sol–gel technique it does not require a post-deposition heat treatment of the particles. Therefore degradation of the particles during the coating process was almost negligible. Taking
[1] P.F. Smet, I. Moreels, Z. Hens, D. Poelman, Materials 3 (2010) 2834–2883. [2] P.F. Smet, K. Korthout, J.E. Van Haecke, D. Poelman, Mater. Sci. Eng. B 146 (2008) 264–268. [3] H.H. Shin, B.Y. Kim, B.Y. Han, J.S. Yoo, Jpn. J. Appl. Phys. 47 (2008) 3524–3526. [4] P.F. Smet, D. Poelman, J. Phys. D: Appl. Phys. 42 (2009) 095306. [5] J.E. Van Haecke, P.F. Smet, K. De Keyser, D. Poelman, J. Electrochem. Soc. 154 (2007) J278–J282. [6] D. Poelman, R. Vercaemst, R.L. Van Meirhaeghe, W.H. Laflere, F. Cardon, J. Lumin. 75 (1997) 175–181. [7] M. Weidner, A. Osvet, G. Schierning, M. Batentschuk, A. Winnacker, J. Appl. Phys. 100 (2006) 073701. [8] N. Avci, J. Musschoot, P.F. Smet, K. Korthout, A. Avci, C. Detavernier, D. Poelman, J. Electrochem. Soc. 156 (2009) J333–J337. [9] J.W. Brightwell, B. Ray, P. Sephton, I.V.F. Viney, J. Cryst. Growth 86 (1988) 634– 638. [10] J. Madarász, T. Leskelä, J.N.L. Rautanen, J. Mater. Chem. 6 (1996) 781–787. [11] C.F. Guo, B.L. Chu, M.M. Wu, Q. Su, J. Lumin. 105 (2003) 121–126. [12] C.F. Guo, B.L. Chu, Q. Su, Appl. Surf. Sci. 225 (2004) 198–203. [13] M. Suchea, S. Christoulakis, E. Androulidaki, M. Koudoumas, Mater. Sci. Eng. B 150 (2008) 130–134. [14] S.H. Yoo, C.K. Kim, J. Electrochem. Soc. 156 (2009) 170–173. [15] I.W. Park, J.H. Kim, J.S. Yoo, H.H. Shin, C.K. Kim, C.K. Choi, J. Electrochem. Soc. 155 (2008) 132–135. [16] J. Lin, Y. Huang, Y. Bando, C.C. Tang, D. Golberg, Chem. Commun. 43 (2009) 6631–6633. [17] M.W. Brooks, S. Lynn, Ind. Eng. Chem. Res. 36 (1997) 4236–4242. [18] N. Avci, P.F. Smet, K. De Buysscher, I. Van Driessche, S. Hoste, D. Poelman, in: 14th International Workshop on Inorganic and Organic Electroluminescence 2008 International Conference on the Science and Technology of Emissive Displays and Lighting, Rome, Italy, 2008, pp. 209–212. [19] R.R. Patil, P.L. Muthal, S.M. Dhopte, V.K. Kondawar, S.V. Moharil, J. Lumin. 126 (2007) 571–574.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Stability improvement of moisture sensitive CaS:Eu2+ micro-particles by coating with sol–gel alumina Nursen Avci ⇑, Iolanda Cimieri, Philippe F. Smet, Dirk Poelman LumiLab, Dept. Solid State Sciences, Ghent University, Krijgslaan 281-S1, 9000 Gent, Belgium
a r t i c l e
i n f o
Article history: Received 2 June 2010 Received in revised form 28 July 2010 Accepted 31 July 2010 Available online 30 August 2010 Keywords: CaS:Eu2+ Alumina Sol–gel Protection layer
a b s t r a c t Here we report on the usage of alumina coating for the protection of moisture sensitive CaS:Eu2+ microparticles. Alumina sol was prepared using a water-free sol–gel technique and single crystal CaS:Eu2+ luminescent micro-particles were synthesized via solvothermal way. After deposition of the particles on a substrate, they were coated with an alumina layer and heat treated at 500 °C for 30 min. In addition to in situ measurements of accelerated ageing of the luminescent particles, photoluminescence (PL) spectra of coated and uncoated particles were compared. Coated particles showed a broad band PL emission with a maximum at 650 nm identical to that of uncoated particles. Coating with an alumina layer drastically increased the resistivity of the luminescent material against moisture. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Rare earth doped alkaline earth binary and ternary sulfides have a very special place in luminescent materials because of their relatively low synthesis temperature and broad emission spectra upon doping with europium and cerium [1,2]. Particularly to obtain red or orange emission CaS:Eu2+ and SrS:Eu2+ are considered suitable candidates [3–5]. Ca1x Srx S : Eu2þ phosphors with a strong absorption in the blue region are currently also used in white-light emitting diodes. Compared to the standard phosphor, YAG:Ce, the emission of Ca1x Srx S : Eu2þ peaks at longer wavelengths, which allows the fabrication of light emitting diodes (LEDs) with lower colour temperature and improved colour rendering [1]. In addition to wavelength converters in LEDs, alkaline earth sulfide phosphors are employed in different areas such as display applications [1], electroluminescent devices [6] and optical information storage [7]. Nevertheless, the lack of stability with respect to water and other atmospheric components hinders usage of the alkaline earth sulfides as phosphor hosts [1,3,8–12]. As an example, during the operation of LEDs, LED chips can reach relatively high temperatures of 450 K. Degradation of the alkaline earth sulfides is accelerated by temperature and decomposition products such as sulfur dioxide and hydrogen sulfide gases evolve from the surface. Therefore, the decomposition causes not only a decrease in the light intensity, but also a reduction in the reflectivity due to the chemical reaction between the released hydrogen sulfide gas and the sil-
⇑ Corresponding author. E-mail address: [email protected] (N. Avci). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.07.021
ver pad under the LED chip [3]. To maintain the emission colour and intensity of the LED throughout its extended lifetime, the stability of the phosphor material is of utmost importance. A number of encapsulation techniques have been utilized to improve the stability of sulfide phosphors [8,11–15]. In this case the properties of the encapsulant, usually in the form of an inert thin film, are as important as the coating technique. In order to improve the stability of sulfide by encapsulation with an inert film, the film should be thermally and chemically stable. In addition, it should be transparent to the excitation and emission light of the incorporated luminescent particles. The film should also homogeneously coat the particle surfaces. Oxides [8], organic materials [14] and nitrides [16] have been used as protection coating for sulfide phosphors. Alumina is one of the attractive materials in the oxide family. It has good thermal and chemical stability; moreover, its transparency window is very wide from ultra-violet to near infrared. Therefore alumina is a very promising material for protective coatings. A number of techniques have been used to synthesize alumina barrier layers namely atomic layer deposition (ALD), magnetron sputtering, chemical vapor deposition (CVD), and sol–gel. In these techniques, sol–gel is the most favorable one because this technique puts forward excellent control of stoichiometry, density and microstructure and short deposition time. Furthermore, using simple equipment and without the need for vacuum, large surface areas can be coated. Using a water based solution or an excess amount of water for hydrolysis and/or strong acid as a dispersing agent to prepare alumina may cause the degradation of moisture-sensitive material. This problem can be solved by using a non-aqueous sol–gel technique.
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In this paper, we explain the modification of moisture sensitive CaS : Eu2þ particles synthesized via a solvothermal route by coating with alumina prepared via a non-aqueous sol–gel technique. In addition to the investigation of the effect of coating on photoluminescence properties of the phosphor, the comparison between moisture resistance of uncoated and coated particles is done under accelerated ageing conditions (80 °C and 80% relative humidity). We also examine the stability of uncoated and coated particles after heat treatment at 500 °C. 2. Experimental The synthesis of single crystal CaS : Eu2þ particles via a solvothermal synthesis method was described in detail elsewhere [5]. After the synthesis, CaS : Eu2þ particles were kept in ethanol as suspension, to prevent degradation caused by humidity. Typical particle sizes were between 0.25 and 0.75 lm, with clearly defined crystal faces. Phosphor layers were obtained by dripping the suspension on a silicon substrate and drying in air at 40 °C. This process was reiterated a number of times until complete coverage of the substrates with CaS : Eu2þ particles was obtained. To assess the influence of the heat treatment (as required during the formation of the Al2 O3 coating), an uncoated phosphor layer was also subjected to an heat treatment at 500 °C for 30 min in air. Alumina sol was prepared using a non-aqueous sol–gel technique. The sol was synthesized using aluminium sec-butoxide ðAl½OðCH3 ÞCHC2 H5 3 Þ (Alfa Aesar, 95%) as precursor, n-butyl alcohol (n-BuOH) (Alfa Aesar, 99.4%) as solvent and acetylacetone (AcAcH) (Alfa Aesar, 99%) as chelating agent. The molar ratio between acetylacetone and aluminium sec-butoxide was 1. The concentration of the sols was adjusted to 0.5 M. Initially, solvent and acetylacetone were mixed, and then aluminium sec-butoxide was added to this mixture. Eventually the solution was stirred for 6 h at 30 °C. After the stirring, a transparent and very stable sol was obtained. In order to check the necessity of AcAcH in the solution, sols were also prepared without AcAcH. The deposition of the alumina coating on CaS : Eu2þ particles was performed by spin coating at 800 rpm for 20 s. After coating, the coated phosphor layer was heat treated at 500 °C for 30 min in air to remove the organic groups. In the remainder of the article, ‘coating’ refers to both the spin coating and the subsequent heat treatment. As alumina layers without AcAcH did not protect the particles, all further layers were prepared using AcAcH. X-ray diffraction (XRD, Bruker D8-Discovery, CuKa radiation) measurements were employed to acquire information about the crystal structure of uncoated and coated particles. Photoluminescence emission and excitation spectrum were recorded with a FS920 luminescence spectrometer (Edinburgh Instruments). The stability of the phosphor layers against moisture was inspected with an in situ photoluminescence measurement during accelerated ageing of the uncoated and coated samples at 80 °C and 80% relative humidity. PL intensities of the sample in a humidity chamber (CST, CS-40/200) were measured with an HR2000+ luminescence spectrometer (ocean optics). Samples were excited by a LED with an emission wavelength of 440 nm. Using a Bif400-UV/ VIS Y-shaped optical fiber cable (ocean optics) excitation light from the LED to the sample and emission light from the sample to the spectrometer were separated. 3. Results and discussion Initially, decomposition of CaS : Eu2þ particles was examined in two conditions. In the first one, the uncoated sample was kept in a humidity chamber at 80 °C and 80% relative humidity and in the second one the uncoated sample was heat treated at 500 °C.
Fig. 1. XRD spectra of the uncoated CaS : Eu2þ particles, before any treatment, after 75 h of accelerated ageing and after heat treatment. The lattice planes of CaS (data from ICDD file No. 65-7852) are indicated. Lines represent the peak positions of CaSO4 (data from ICDD file No. 37-1496). Triangles indicate the peak positions of CaCO3 (data from ICDD file No. 72-0506).
Fig. 1 shows the XRD results of uncoated CaS : Eu2þ particles before any treatment, after 75 h of accelerated ageing and after heat treatment. The heat treated sample was not subjected to any ageing. The as-deposited particles were single phase cubic CaS. The peaks related to CaS disappear after the accelerated ageing process and only CaCO3 peaks with small intensity are observed. This is evidence of the decomposition of CaS to CaCO3 in the presence of CO2 and H2 O at lower temperature [17]. After the heat treatment at 500 °C with a heating rate of 10 °C/min for 30 min, the differential intensity of the diffraction peaks from CaS reduced by about 60% with the formation of CaSO4 (Fig. 1). There are two factors which are critical for the decomposition process [10]. The first one is the particle size, the decomposition of small particles is easier than bigger ones and the second one is heating rate, using slower heating rate causes more decomposition. Since the particles (between 0.25 lm and 0.75 lm) were rather small compared to earlier studies [12,14], it was not surprising to have a strong decomposition for this sample. PL excitation and emission spectra of CaS : Eu2þ before (dashed lines) and after (solid lines) heat treatment at 500 °C for 30 min are shown in Fig. 2. The excitation and emission spectra were recorded at an emission wavelength of 650 nm and at an excitation wavelength of 450 nm, respectively in Fig. 2a. The high energy side excitation band between 250 and 300 nm is a combination of the CaS band gap transition and the excitation to the 4f 6 5d ðEg Þ state in Eu2þ (Fig. 2a). A Decrease of the CaS : Eu2þ particle size due to the partial decomposition results in a relative increase of the intensity of the excitation peak from the CaS band gap because of the different absorption lengths for both processes. This changing of the relative ratio between these two peaks results in shifting of the high energy side band in the excitation spectra of CaS : Eu2þ (Fig. 2a) [18]. After the heat treatment the intensity of the emission peak at 650 nm reduces by 80% nevertheless there is no shift in the emission spectra. PL emission spectra recorded at an excitation wavelength of 275 nm of CaS : Eu2þ before (dashed line) and after (solid line) heat treatment at 500 °C for 30 min are shown in Fig. 2b. The small emission peak at 385 nm formed after heat treatment indicates the decomposition of CaS : Eu2þ to CaSO4 : Eu2þ [19]. The broad peak at 500 nm could not be identified. As seen in Fig. 2b, after the heat treatment there is a deformation in the emission peak at 650 nm. This may suggest the formation of other luminescent centres such as Eu3þ [19]. No PL signal could be obtained after 75 h of accelerated ageing at 80 °C and 80% relative humidity, showing full decomposition. After the observation of the (partial) decomposition of uncoated particles during heat treatment at 500 °C and accelerated ageing at
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Fig. 2. (a) PL Excitation and emission spectra of CaS : Eu2þ before (dashed lines) and after (solid lines) heat treatment at 500 °C for 30 min. Excitation spectra were recorded at an emission wavelength of 650 nm and emission spectra were recorded at an excitation wavelength of 450 nm. (b) PL emission spectra of CaS : Eu2þ before (dashed line) and after (solid line) heat treatment at 500 °C for 30 min recorded at an excitation wavelength of 275 nm.
After the ageing, the diffraction pattern remained the same as after the coating (Fig. 3). This emphasizes that alumina layer hinders the oxidation of CaS particles during ageing. The PL excitation and emission spectra of CaS : Eu2þ before (dashed lines) and after (solid lines) coating are shown in Fig. 4. The excitation and emission spectra were recorded at an emission wavelength of 650 nm and at an excitation wavelength of 450 nm, respectively (Fig. 4a). In addition to the blue shifting at higher energy side in the excitation spectra after coating, there is a decrease in the intensity of the excitation peak at lower energy side. These facts imply that the phosphor particles become smaller through decomposition, as the relative reduction in absorption length can alter the shape of the excitation spectrum close to the bandgap. Although water was not used for the synthesis of the alumina sol, water was produced by the condensation reaction and this may be the reason for the partial decomposition of CaS : Eu2þ . The intensity of the emission peak at 650 nm (Fig. 4a) reduces to half after coating nevertheless there is no shift or deformation in the peak. Fig. 4b shows the PL emission spectra of uncoated (dashed line) and coated (solid line) CaS : Eu2þ at an excitation wavelength of 275 nm. A small peak at 385 nm was observed after coating (solid line, Fig. 4b), this peak is an evidence of a small amount of CaSO4 in the phosphor. When the emission spectra of heat treated uncoated CaS : Eu2þ particles (Fig. 2a) and coated particles (Fig. 4a) are compared, it can be seen that the emission intensity of uncoated particles decreases to 20% although the coated particles were heat treated at the same temperature as uncoated one, that of coated particles decreases only to 50%. This highlights that the alumina layer prevents the particle from decomposition during the heat treatment.
80 °C and 80% relative humidity, the effect of the alumina coating on the properties of CaS : Eu2þ phosphor was investigated. After coating of the CaS : Eu2þ particles with an alumina layer, the coated particles were heat treated at 500 °C with a heating rate of 10 °C/ min for 30 min to remove the organic groups from the layer. In order to examine how good the alumina film performs as protection layer the coated CaS : Eu2þ was aged (80 °C and 80% relative humidity). The XRD spectra of uncoated and coated CaS : Eu2þ particles before and after 100 h of accelerated ageing are presented in Fig. 3. The intensity of the (200) peak of CaS reduced to 70% after coating and it remained the same after 100 h of accelerated ageing. However traces of CaSO4 were observed, both before and after ageing.
Fig. 3. XRD spectra of of the uncoated CaS : Eu2þ particles, the CaS : Eu2þ particles coated with alumina film before and after 100 h of accelerated ageing. The lattice planes of CaS (data from ICDD file No. 65-7852) are indicated. Lines represent the peak positions of CaSO4 (data from ICDD file No. 37-0184).
Fig. 4. (a) PL Excitation and emission spectra of uncoated (dashed lines) and coated (solid lines) CaS : Eu2þ . Excitation spectra were recorded at an emission wavelength of 650 nm and emission spectra were recorded at an excitation wavelength of 450 nm. (b) PL emission spectra of uncoated (dashed lines) and coated (solid lines) CaS : Eu2þ recorded at an excitation wavelength of 275 nm.
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this into account, in combination with the excellent resistance against of both the ALD and sol–gel coated particles, ALD is the better technique to prepare a protection layer for sulfides as it gives a higher luminescence efficiency. A disadvantage is it being more expensive as coating technique compared to sol–gel techniques. Hence, sol–gel based inorganic coatings seem an interesting route for the protection of luminescent sulfides, on condition that the degradation during the necessary heat treatment can be reduced. Acknowledgements
Fig. 5. PL intensity of uncoated (line A) sample and sample coated with an alumina film (line B) as a function of time at 80 °C and 80% relative humidity.
This research was carried out under the Interuniversity attraction poles programme IAP/VI-17 (INANOMAT) financed by the Belgian State, Federal science policy office. We gratefully acknowledge Bjorn Vandecasteele for help with in situ accelerated ageing measurements. References
Fig. 5 shows the in situ measurement of the photoluminescence intensity during accelerated ageing of the uncoated and coated samples at 80 °C and 80% relative humidity. Line A represents the degradation of an uncoated sample and line B is related to the degradation of samples coated with an alumina layer. As seen in Fig. 5, during the first 30 h the PL emission of uncoated CaS : Eu2þ almost entirely disappears. After 75 h, the PL intensity of uncoated phosphor was not detectable anymore, while the PL intensity of the coated sample was around 95% of its initial value. 4. Conclusions CaS : Eu2þ -based materials are good candidates as wavelength converting material in LEDs for general lighting if their stability can be enhanced. Phosphor layers were prepared using submicron-sized single crystalline CaS : Eu2þ synthesized via a solvothermal route. The phosphor layers were coated with alumina prepared via a non-aqueous sol–gel technique to improve their stability. The experimental results show that an alumina film strongly enhances the resistance of the phosphor layer against moisture and high temperature in spite of the partial degradation of the sample surface during the coating process. In our previous report we coated the moisture sensitive particles using ALD aluminium oxide [8]. Unlike the sol–gel technique it does not require a post-deposition heat treatment of the particles. Therefore degradation of the particles during the coating process was almost negligible. Taking
[1] P.F. Smet, I. Moreels, Z. Hens, D. Poelman, Materials 3 (2010) 2834–2883. [2] P.F. Smet, K. Korthout, J.E. Van Haecke, D. Poelman, Mater. Sci. Eng. B 146 (2008) 264–268. [3] H.H. Shin, B.Y. Kim, B.Y. Han, J.S. Yoo, Jpn. J. Appl. Phys. 47 (2008) 3524–3526. [4] P.F. Smet, D. Poelman, J. Phys. D: Appl. Phys. 42 (2009) 095306. [5] J.E. Van Haecke, P.F. Smet, K. De Keyser, D. Poelman, J. Electrochem. Soc. 154 (2007) J278–J282. [6] D. Poelman, R. Vercaemst, R.L. Van Meirhaeghe, W.H. Laflere, F. Cardon, J. Lumin. 75 (1997) 175–181. [7] M. Weidner, A. Osvet, G. Schierning, M. Batentschuk, A. Winnacker, J. Appl. Phys. 100 (2006) 073701. [8] N. Avci, J. Musschoot, P.F. Smet, K. Korthout, A. Avci, C. Detavernier, D. Poelman, J. Electrochem. Soc. 156 (2009) J333–J337. [9] J.W. Brightwell, B. Ray, P. Sephton, I.V.F. Viney, J. Cryst. Growth 86 (1988) 634– 638. [10] J. Madarász, T. Leskelä, J.N.L. Rautanen, J. Mater. Chem. 6 (1996) 781–787. [11] C.F. Guo, B.L. Chu, M.M. Wu, Q. Su, J. Lumin. 105 (2003) 121–126. [12] C.F. Guo, B.L. Chu, Q. Su, Appl. Surf. Sci. 225 (2004) 198–203. [13] M. Suchea, S. Christoulakis, E. Androulidaki, M. Koudoumas, Mater. Sci. Eng. B 150 (2008) 130–134. [14] S.H. Yoo, C.K. Kim, J. Electrochem. Soc. 156 (2009) 170–173. [15] I.W. Park, J.H. Kim, J.S. Yoo, H.H. Shin, C.K. Kim, C.K. Choi, J. Electrochem. Soc. 155 (2008) 132–135. [16] J. Lin, Y. Huang, Y. Bando, C.C. Tang, D. Golberg, Chem. Commun. 43 (2009) 6631–6633. [17] M.W. Brooks, S. Lynn, Ind. Eng. Chem. Res. 36 (1997) 4236–4242. [18] N. Avci, P.F. Smet, K. De Buysscher, I. Van Driessche, S. Hoste, D. Poelman, in: 14th International Workshop on Inorganic and Organic Electroluminescence 2008 International Conference on the Science and Technology of Emissive Displays and Lighting, Rome, Italy, 2008, pp. 209–212. [19] R.R. Patil, P.L. Muthal, S.M. Dhopte, V.K. Kondawar, S.V. Moharil, J. Lumin. 126 (2007) 571–574.