Microencapsulation of moisture-sensitive CaS: Eu2+ particles with aluminum oxide

Article abstract of DOI:10.1149/1.3211959

Single-crystal, submicrometer-sized CaS:Eu luminescent particles were synthesized via a solvothermal route, and these moisture-sensitive particles were coated with aluminum oxide using atomic layer deposition (ALD). Photoluminescence (PL) spectra of coate

Full text of DOI:10.1149/1.3211959

Journal of The Electrochemical Society, 156 ͑11͒ J333-J337 ͑2009͒
J333
0013-4651/2009/156͑11͒/J333/5/$25.00 © The Electrochemical Society
Microencapsulation of Moisture-Sensitive CaS:Eu2+ Particles
with Aluminum Oxide
N. Avci,a,z J. Musschoot,b P. F. Smet,a K. Korthout,a A. Avci,c C. Detavernier,b,
*
and D. Poelmana
aLumiLab, bCOCOON, Department of Solid State Sciences, Ghent University, B-9000 Ghent, Belgium
cCenter for Microsystems Technology, Ghent University, B-9052 Ghent, Belgium
Single-crystal, submicrometer-sized CaS:Eu luminescent particles were synthesized via a solvothermal route, and these moisture-
sensitive particles were coated with aluminum oxide using atomic layer deposition ͑ALD͒. Photoluminescence ͑PL͒ spectra of
coated and uncoated particles were compared. They both showed a broad-band PL emission with a maximum of 650 nm.
Microencapsulation by aluminum oxide layers did not have a pronounced effect on the intensity of the emission. In situ lumines-
cence measurements during the accelerated aging ͑80°C, 80% relative humidity͒ of coated and uncoated CaS:Eu particles were
performed. While the uncoated phosphor was largely degraded within 30 h of aging, it was observed that a 20 nm thick aluminum
oxide coating dramatically increased the resistance of the luminescent material against moisture, showing the conformity of the
Al2O3 coating by the ALD process. Upon degradation, CaCO3 was formed, leading to Eu3+ emission as observed in cathodolu-
minescence. Finally, the use of these coated particles as a wavelength conversion material in light-emitting diodes was evaluated.
© 2009 The Electrochemical Society. ͓DOI: 10.1149/1.3211959͔ All rights reserved.
Manuscript submitted July 2, 2009; revised manuscript received July 31, 2009. Published September 8, 2009.
Among the rare-earth-doped materials, the family of binary and
state reaction utilizing a H2S atmosphere during heat-treatment9,20,21
or solvothermal synthesis.8,22 However, the use of H2S gas is not
recommended due to its toxicity.
ternary sulfides ͑showing different emission colors ranging from the
near UV to deep red͒ has a special place because they are promising
phosphors for general lighting, display applications,1,2 electrolumi-
nescent devices,3,4 and optical information storage.5,6 Divalent
europium-doped alkali earth sulfides, such as CaS:Eu2+ and
SrS:Eu2+ with intense light emissions at 650 and 620 nm,
respectively,7,8 are considered especially phosphor materials.9-11
However, the lack of stability with respect to water and other atmo-
spheric components impedes the usage of the alkaline earth sulfides
as phosphor hosts.9-15 When used in field emission displays, for
instance, the surface temperature of these resistive phosphors in-
creases. During operation, decomposition products, such as sulfur
dioxide and hydrogen sulfide gases, evolve from the surface and can
cause damage to the field emitters.16 This accelerates the dissocia-
tion and surface degradation under high coulomb charging.
Ca1−xSrxS:Eu phosphors are also currently used in white light-
emitting diodes ͑LEDs͒. Compared to the standard phosphor, yt-
trium aluminum garnet ͑YAG͒:Ce, the emission of Ca1−xSrxS:Eu
peaks at longer wavelengths, which allows the fabrication of LEDs
with lower color temperature and improved color rendering. To
maintain the emission color of such LEDs throughout their extended
lifetime, the stability of the phosphor material is of utmost impor-
tance.
Covering the phosphor surface with a coating layer is a technique
is often applied to improve the alkaline earth sulfide stability; in this
case the properties of the inert thin film and the coating technique
are very important.11,14,17,18 The choice of the matrix material assur-
ing the stability of the sulfide is not trivial. It should be thermally
and chemically stable, and it should also be transparent to the exci-
tation ͑when used as an LED phosphor͒ and emission light of the
incorporated particles. The coating layer should homogeneously
cover the phosphor surface area. Alternatively, small particles can be
protected against moisture by microencapsulation, where all par-
ticles are homogeneously coated with a thin protective coating indi-
vidually. Such coatings have been applied for powder electrolumi-
nescent phosphors19 and are usually deposited by wet chemical
means. Apart from this, preparation of well crystalline alkaline earth
sulfides with a stoichiometric balance is equally important for im-
proving the stability of the phosphor. Some techniques to synthesize
alkaline earth sulfides are found in the literature, such as a solid-
Atomic layer deposition ͑ALD͒ is a technique based on the
pulsed exposure of a surface to a metal-containing precursor and a
reactive gas. The precursor reacts in a self-limiting manner with the
surface until a monolayer of molecules is chemisorbed. The gas
pulse leads to the removal of the ligands around the metal in the
precursor and the formation of a compound film. By varying the
number of pulses, ALD enables a well-controlled deposition of nan-
ometer thin, conformal pinhole-free films on complex substrates
with uniformity over large areas. The deposition temperature is low
compared to traditional chemical vapor deposition. The ALD tech-
nique and its potential applications in semiconductor devices and
nanotechnology have been discussed in more detail elsewhere.23-25
Recently, ALD technique has been used to encapsulate moisture-
26
sensitive materials such as ZnS:Cu and organic thin films.27,28 To
obtain an efficient barrier layer via the ALD technique, Fan et al.
claimed that a coating thickness of at least about 90 nm is required
for ZnS:Cu,26 while it was reported that a 30 nm thick coating is
sufficient for organic thin films.27,28
In the present work, we describe the preparation of films consist-
ing of CaS:Eu2+ luminescent particles synthesized via a solvother-
mal route with the addition of an aluminum oxide layer using ALD.
The chemical stability of the uncoated and coated phosphor layers
under accelerated aging conditions ͓80°C and 80% relative humidity
͑RH͔͒ is compared, and the photoluminescent properties of coated
phosphor layers are investigated. Finally, the results are compared to
other recently reported encapsulation methods for sulfides.
Experimental
Single-crystal CaS:Eu2+ particles were synthesized via a solvo-
thermal synthesis method, as described in detail elsewhere.8 Typical
particle sizes were between 0.25 and 0.75 ␮m, with clearly defined
crystal faces. After synthesis, a suspension of CaS:Eu2+ particles in
ethanol was prepared. Phosphor layers were obtained by dripping
the suspension on a silicon substrate and drying it in air at 40°C.
This process was reiterated many times up to the complete coverage
of the substrates with CaS:Eu particles. No annealing treatment was
performed.
The deposition of the ALD alumina coating was performed in a
home-built system.29 The reactor was constantly evacuated with a
turbomolecular pump and heated to 95°C. The base pressure before
deposition was about 10−4 Pa. Trimethylaluminum ͑TMA, 97%,
Sigma Aldrich͒ and deionized water were used as the aluminum
precursor and reactive gas, respectively.27 The TMA and H2O con-
*
Electrochemical Society Active Member.
z E-mail: nursen.avci@ugent.be
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Journal of The Electrochemical Society, 156 ͑11͒ J333-J337 ͑2009͒
Figure 1. XRD spectra of a 100 nm thick aluminum oxide film on a quartz
substrate, of the uncoated CaS:Eu particles, and the 20 nm thick aluminum
oxide film coated CaS:Eu before and after 57 h of accelerated aging. The
lattice planes of CaS ͑data from ICDD file no. 65-7852͒ are indicated. Lines
represent the peak positions of CaCO3 ͑data from ICDD file no. 72-0506͒.
Triangles indicate the peak positions of CaSO4·0.64H2O ͑data from ICDD
file no. 36-0617͒.
tainers were stored at room temperature and connected to the reactor
via heated lines. Using needle valves, the effective pressure of TMA
was set to 0.1 Pa and the pressure of H2O to 0.3 Pa. Gas pulses were
delivered by opening computer-controlled pneumatic valves. The
samples were placed on a resistive heating element, resulting in a
deposition temperature of 200°C. One ALD cycle consisted of 2 s of
TMA, 5 s pumping, 5 s H2O vapor, and 10 s pumping. About 0.1 nm
of aluminum oxide was deposited for one cycle. To investigate the
crystal structure and transparency of the aluminum oxide films, 20
and 100 nm thick aluminum oxide films were deposited on quartz
substrate ͑Heraeus͒. Aluminum oxide thin films with thicknesses of
10 and 20 nm were grown on CaS:Eu. No postdeposition treatments
were performed.
Scanning electron microscopy ͑SEM, FEI Quanta 200F͒ was
used to characterize the morphology of the particles and coating
layers. The chemical composition of the samples was investigated
by energy dispersive X-ray analysis ͑EDX, EDAX Genesis 4000͒,
and the cathodoluminescence ͑CL͒ was measured by electron-beam
excitation inside the scanning electron microscope using a fiber-
coupled QE65000 luminescence spectrometer ͑Ocean Optics͒. X-ray
diffraction ͑XRD, Bruker D8-Discovery͒ measurements were em-
ployed to acquire information about the crystal structure of the alu-
minum oxide films on quartz and coated particles. PL emission and
excitation were measured with an FS920 luminescence spectrometer
͑Edinburgh Instruments͒. Transparency measurements of the alumi-
num oxide thin films on the quartz substrate were executed by a
Cary 500 UV/visible/near-infrared spectrophotometer. The chemical
stability of the phosphor layers was inspected with an in situ PL
measurement during the accelerated aging of the uncoated and
coated samples at 80°C and 80% RH. PL intensities of the sample in
the humidity chamber ͑Clima Temperatur Systeme, CS-40/200͒
were measured with an HR2000+ luminescence spectrometer
͑Ocean Optics͒. Samples were excited by an LED with an emission
wavelength of 440 nm. Using a IF400-UV/VIS Y-shaped optical
fiber cable ͑Ocean Optics͒, excitation light from the LED to the
sample and emission light from the sample to the spectrometer were
separated.
Figure 2. ͑a͒ PL excitation and emission spectra of ͑b͒ coated and ͑c͒ un-
coated CaS:Eu recorded at an emission wavelength of 650 nm for ͑a͒ and at
excitation wavelengths of 275 and 450 nm for ͑b͒ and ͑c͒, respectively.
although traces of partially hydrated CaSO4 were observed. The
sulfate formation probably originated from the reaction of the CaS
phosphor with water used as a reactive gas in the ALD coating
process. This indicates that the CaS:Eu particles are essentially un-
affected by the coating. There are no traces of aluminum oxide on
the XRD pattern after coating because the deposition temperature is
too low to obtain crystallized aluminum oxide.30,31 The absence of
XRD peaks was confirmed on a 100 nm thick Al2O3 thin film de-
posited directly on a quartz substrate ͑Fig. 1͒.
Results and Discussion
For the CaS:Eu particles coated with aluminum oxide, it was
observed that traces of CaCO3 were formed after aging for 57 h. The
peak positions of the compound are indicated as lines in Fig. 1. This
is evidence of the decomposition of CaS to CaCO3 due to the pres-
ence of CO2 and H2O by the following reaction32
Figure 1 presents the XRD results of the uncoated CaS:Eu par-
ticles, after coating with a 20 nm thick aluminum oxide film, and
after 57 h of accelerated aging of the coated CaS:Eu phosphor layer.
The uncoated particles were single phase cubic CaS. The intensity
and the position of the CaS peaks remained the same after coating,
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Journal of The Electrochemical Society, 156 ͑11͒ J333-J337 ͑2009͒
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Figure 3. Transmission spectra of ͑a͒ 20 and ͑b͒ 100 nm thick aluminum
oxide films on a quartz substrate.
CaS + H2O + CO2 → CaCO3 + H2S
Figure 2a presents the PL excitation spectra of the coated ͑20 nm
Al2O3͒ and uncoated CaS:Eu samples. Measurements were carried
out at room temperature and probed at an emission wavelength of
650 nm. While the direct excitation from the 4f7͑8S7/2͒ ground state
to the 4f65d ͑T2g͒ excited state is responsible for the low energy
͑high wavelength͒ side excitation band peaking at 450–500 nm, the
high energy ͑low wavelength͒ side excitation band between 250 and
300 nm is due to the combination of the CaS bandgap transition and
the excitation to the 4f65d ͑Eg͒ state in Eu2+ 33 As shown in Fig. 2a,
.
the aluminum oxide layer has a negligible influence on the excita-
tion spectra.
In the decomposition of CaS:Eu to CaSO4:Eu, an emission peak
at 385 nm is expected.34,35 The PL emission spectrum of the un-
coated and 20 nm thick aluminum oxide layer coated phosphor lay-
ers at an excitation wavelength of 275 nm and room temperature is
presented in Fig. 2b. Although after the coating there is a trace of
CaSO4 in the XRD spectra, the fraction of the decomposition of
CaS:Eu to CaSO4:Eu is too low to affect the emission spectrum
͑Fig. 2b͒.
Figure 4. SEM images of ͑a͒ uncoated and ͑b͒ aluminum oxide coated ͑20
nm͒ CaS:Eu particles.
Figure 2c shows the PL emission spectrum of the uncoated and
aluminum oxide coated ͑20 nm͒ phosphor layers. Measurements
were carried out at room temperature at an excitation wavelength of
450 nm. As clearly shown in Fig. 2c, there is no shift on the emis-
sion spectra after the coating, and there is no significant difference
between the PL intensity of the coated and uncoated samples. This
emphasizes that there is no formation of other luminescent centers
nor defect luminescence, and the spectrum due to CaS:Eu remains
intact, with the emission from the 4f65d ͑T2g͒ excited state of Eu2+
Figure 4 shows the SEM images of the CaS phosphor layer be-
fore and after coating with a 20 nm thick aluminum oxide film. As
seen in the figure, the coated phosphor surface is smooth and clean.
The elemental composition of the surface region of the uncoated and
coated samples was examined with EDX. The EDX results show
that the ratio between the atomic concentration of S and Ca is the
same before and after coating, indicating that no major loss of sulfur
͑due to the release of H2S͒ occurs during the coating process. The
concentration of Ca and S does not drop to zero upon full coverage
of the particles with aluminum oxide because of the large informa-
tion depth of EDX compared to the oxide film thickness.
Figure 5 shows a more detailed SEM image of the CaS:Eu par-
ticles coated with 20 nm of aluminum oxide, along with EDX maps
for the Al K ͑1.5 keV͒ and Ca K ͑3.7 keV͒ characteristic X-rays
obtained at an electron-beam energy of 10 keV. The information
depth was estimated using Casino Monte Carlo simulations.39 In a
CaS thin film, 90% of the detected X-rays emanate from the top 700
nm. For the large CaS:Eu particle in the center of Fig. 5, only one
aluminum oxide layer is measured, yielding a low but detectable Al
signal in the corresponding EDX map. For the other much smaller
particles, the electron beam excites at least the aluminum oxide
layer on both sides of each particle, while characteristic X-rays are
also generated in the underlying coated particles. As a consequence,
to the 4f7͑8S7/2͒ ground state of Eu2+ 33 In both cases, the maximum
.
of the spectra is located at 650 nm, in line with the results in Ref. 7.
The PL emission and excitation spectra of the uncoated and coated
samples suggest that coating with a 20 nm thick aluminum oxide
layer via ALD does not affect the PL properties of the sample. The
transmission spectra of the aluminum oxide films on the quartz sub-
strate ͑Fig. 3͒ support this conclusion. Lines ͑a͒ and ͑b͒ represent the
transmission spectra of 20 and 100 nm thick aluminum oxide films
on the quartz substrate, respectively. Interference fringes are seen in
the transmission spectrum of the 100 nm thick aluminum oxide film.
The minima and maxima are at the expected positions for an alumi-
num oxide film with a thickness of 100 nm.36 Figure 3 shows that
there is no significant absorption between 200 and 800 nm, allowing
for both an efficient excitation and emission of the CaS:Eu particles
after coating with aluminum oxide. This is indeed what one would
expect, due to the high transparency of aluminum oxide down to the
vacuum UV, because aluminum oxide has a bandgap of around 9
eV.37,38
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J336
Journal of The Electrochemical Society, 156 ͑11͒ J333-J337 ͑2009͒
sents the degradation of an uncoated sample, and lines ͑B͒ and ͑C͒
are related to the degradation of samples coated with 10 and 20 nm
thick aluminum oxides, respectively. As shown in Fig. 6, the PL
intensity of the uncoated CaS:Eu has dropped to around 35% after
20 h, and the normalized PL intensities of the samples coated with
10 and 20 nm thick layers are around 55 and 95% relative to the
starting PL intensity. While the normalized PL intensities of the
uncoated and 10 nm thick film-coated samples reduce to 10 and 5%,
respectively, the normalized PL intensity of a sample coated with 20
nm Al2O3 is still around 90% after 40 h. Clearly, a thickness of 10
nm is insufficient for long-term protection of the CaS phosphor
layer. Even after 57 h, the PL normalized intensity of a sample
coated with a 20 nm thick film is around 85%. Therefore, we have
obtained a conformal aluminum oxide coating on the particles,
which is a distinct advantage of ALD techniques over physical vapor
deposition techniques. The latter only offer line-of-sight deposition
and do not enable protection of the particles homogeneously on all
sides.40,41
Figure 6 indicates that even after 70 h, the normalized PL inten-
sity of the sample coated with a 10 nm thick film is still 5%. The
remaining intensity emphasizes that degradation products ͑CaCO3 in
our case͒ cover the outside of the CaS particles, thus slowing down
degradation of CaS.32,42,43 The formation of CaCO3:Eu was also
confirmed by evaluation of the CL of the degraded phosphor layers.
Figure 7 shows the CL emission intensity of part of a degraded,
uncoated phosphor layer. The CL emission intensity strongly varies
over the phosphor layer, with distinct and mainly larger particles
still showing strong emission. The CL spectrum in these areas is
dominated by the CaS:Eu2+ emission, while the emission in weakly
emitting areas mostly consists of Eu3+ emission lines ͑Fig. 6͒. In the
latter areas, a decreased sulfur and an increased carbon concentra-
tion were measured by EDX. In CaCO3, europium can be in both the
divalent and trivalent states, although the divalent state is only ob-
tained under specific reducing conditions.44
Figure 5. SEM images of aluminum oxide coated ͑20 nm͒ CaS:Eu particles
͑top͒ and elemental maps of Al and Ca obtained with EDX at 10 keV ͑bot-
tom͒.
the EDX signal of aluminum is much stronger in the EDX mapping
for these particles, as expected ͑Fig. 5͒. This shows that a conformal
coating is obtained, as can be expected for this type of ALD. This is
in contrast to line-of-sight deposition techniques, which would yield
an almost uniform Al signal in the EDX mapping.
XRD, PL emission and excitation spectra, and SEM and EDX
analysis indicate that coating with aluminum oxide by ALD tech-
nique does not create luminescent centers nor does it change the
composition of the host material ͑CaS͒. We then focused on the
protective properties of the aluminum oxide layer on the CaS:Eu
phosphor layers against moisture. Figure 6 shows the in situ mea-
surement of the PL intensity during accelerated aging of the un-
coated and coated samples at 80°C and 80% RH. Line ͑A͒ repre-
Park et al. and Yoo and Kim. recently reported on the encapsu-
lation of CaS:Eu powder with SiO2 nanocrystals using a polymer
binder.11,45 During accelerated aging tests at high temperatures and
high humidity levels, it was shown that the stability against moisture
was significantly improved upon encapsulation. They used CaS:Eu
powder with a relatively large particle size ͑4.8 ␮m͒,45 in contrast
to the particles used in this work ͑0.5 ␮m on average͒. Although the
solvothermal synthesis method can be modified to yield particles in
the 10 ␮m range, we deliberately chose to use smaller particles in
this work to have a much larger surface-to-volume ratio. In this way,
the encapsulating layer should be of high quality and uniformity to
protect the particles, which is the case upon deposition of a 20 nm
thick Al2O3 film. For practical use of these particles ͑or the similar
46
compounds Ca1−xSrxS:Eu with tuned emission spectrum͒ as a
wavelength converter in LEDs, optimization of the particle size and
the Al2O3 layer thickness should be performed. As all components
of the resulting composite layer are inorganic, its thermal stability is
excellent, in contrast to that of organically encapsulated systems.
Conclusion
The chemical instability of alkaline earth sulfides is a big handi-
cap for the market penetration of these materials. Enhancement of
the stability of sulfide phosphors would increase their application
area, and CaS:Eu-based materials could reliably be used as wave-
length converting materials in LEDs for general lighting.11,14,17
A
higher stability of the CaS:Eu phosphor layer lowers the color varia-
tion in a white LED during its lifetime. Submicrometer-sized single-
crystalline CaS:Eu was synthesized using a solvothermal route, and
phosphor layers of these particles were prepared. To improve the
stability of the phosphor layers, they were coated with aluminum
oxide via ALD. The experimental results show that water as a reac-
tant causes a small degradation on the sample surface but the oxide
layer has no effect on the PL properties of CaS:Eu. Future work will
Figure 6. PL intensities of the ͑A͒ uncoated sample and samples coated with
͑B͒ 10 and ͑C͒ 20 nm thick aluminum oxide film samples as a function of
time at 80°C and 80% RH.
47
aim at other reactants such as ozone ͑O3͒
to avoid the present
limited degradation during the deposition of the aluminum oxide.
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Journal of The Electrochemical Society, 156 ͑11͒ J333-J337 ͑2009͒
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Figure 7. SEM image ͑left͒ and CL inten-
sity map ͑middle͒ of an uncoated CaS:Eu
phosphor layer, which was aged for 40 h.
CL emission spectra ͑right͒ for the regions
indicated.
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The conformal coating of the particles strongly enhances the resis-
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Acknowledgments
22. P. F. Smet and D. Poelman, J. Phys. D: Appl. Phys., 42, 095306 ͑2009͒.
23. J. Dendooven, D. Deduytsche, J. Musschoot, R. L. Vanmeirhaeghe, and C. Detav-
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One of the authors ͑P.F.S.͒ is a postdoctoral research fellow of
Fonds Wetenschappelijk Onderzoek-Vlaanderen. This research was
carried out under the Interuniversity Attraction Poles program IAP/
VI-17 ͑INANOMAT͒ financed by the Belgian State Federal Science
Policy Office. The authors gratefully acknowledge Bjorn Vande-
casteele for his help with the in situ accelerated aging measure-
ments.
24. M. Ylilammi, Thin Solid Films, 279, 124 ͑1996͒.
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