Single Crystal CaS:Eu and SrS:Eu Luminescent Particles
Obtained by Solvothermal Synthesis
J. E. Van Haecke,
z
P. F. Smet, K. De Keyser, and D. Poelman
LumiLab, Department of Solid State Sciences, Ghent University, B-9000 Ghent, Belgium
Single crystal CaS:Eu and SrS:Eu luminescent particles were synthesized via a solvothermal route at relatively low temperature
200°C. The as-obtained suspensions were strongly photoluminescent PL, pointing at good Eu incorporation. The phosphors
showed a broad PL emission band with an emission peak at 663 and 623 nm for CaS:Eu and SrS:Eu, respectively. The synthesis
method meets the growing interest in small monodisperse particles. The composition and morphology of the particles was
evaluated with transmission electron microscopy, scanning electron microscopy-energy dispersive X-ray and electron backscatter
diffraction, particles were found to be mostly monocrystalline. X-ray diffraction showed a cubic structure with space group Fm3m.
To control the growth of the crystallites, thioglycerol was added as capping agent, narrowing the size distribution and facilitating
the growth of the single crystals.
© 2007 The Electrochemical Society. DOI: 10.1149/1.2756974 All rights reserved.
Manuscript submitted March 30, 2007; revised manuscript received May 21, 2007. Available electronically July 25, 2007.
Traditionally, alkaline earth sulfide phosphors have been pre-
pared by sulfurization of alkaline earth oxides or carbonates under a
flow of H
2
S or CS
2
, or by reducing sulfates with H
2
or H
2
S.
1,2
These reactions require high processing temperatures 800°C to
allow adequate diffusion of atoms or ions. Such experimental con-
ditions lead almost invariably to thermodynamically stable phases
with simple lattice structures of high density and high symmetry,
3,4
giving little control in material stochiometry. Most of these reactions
have to be performed in an oxygen-free environment or involve
toxic agents such as H
2
S.
Furthermore, the requirements for phosphors have become more
stringent, as smaller and smaller particles are being required. Phos-
phor particles, used in ink jet printers, are required to have a diam-
eter in the range of 1 m or less in order to be suspended in an ink
formulation.
5
In order to achieve higher resolution in computer
monitors, smaller phosphor particles are often needed. Biological
assays require monodispersity as well as small particle size
phosphors.
6
Current synthesis methods for sulfide phosphor particles
result in bulk particles with an average size of the order of several
micrometers. These traditional methods do not meet the challenges
presented in the production of small and tailored phosphor particles.
A solvothermal synthesis method
7,8
offers several advantages
compared to these traditional preparation methods. The reaction is
carried out in a high-pressure autoclave. There, the solubility of the
solid reactants is increased, which speeds up the reaction to generate
particles. Diffusion and growth control are obtained by using a suit-
able solvent. The major advantage of this approach is that the reac-
tion can take place at relatively low synthesis temperature. The
method is quite environmentally friendly as no toxic gases are used
and the required energy input is significantly lower compared to
solid-state reactions. Adding capping agents offers the ability to tune
the size and shape of the materials, which is the first corner stone in
nanotechnology, not only to improve current materials, but also to
explore possible, size-dependent novel physical properties.
9
The
method allows the synthesis of small sulfide particles,
7,8
subject of
the present study.
Alkaline earth sulfide phosphors, for example, CaS and SrS
doped phosphors, are employed in a wide variety of lighting and
display applications, such as field emission display,
10
wavelength
converters in light emitting diodes LEDs for solid-state lighting
11
and most importantly inorganic electroluminescent devices.
12-14
In
particular, CaS:Eu and SrS:Eu have been studied for a long time as
materials for inorganic luminescence.
2,4,15
Recently, the presence of
IR up-conversion and optical storage in Sm co-doped CaS:Eu and
SrS:Eu has sparked new interest in these materials.
16,17
Application of CaS:Eu and SrS:Eu in phosphor converted LEDs
is promising, if their chemical and thermal stability can be
enhanced.
18
If one obtains monodisperse particles with a limited
size, encapsulation of the particles against hydrolysis will become
much easier. Sun et al. prepared CaS:Eu nanoparticles through a wet
chemical process in ethanol.
19
These nanoparticles had a very low
fluorescent intensity because of their extremely poor crystallinity. To
obtain fluorescent material, annealing at temperatures above 700°C
was necessary. Also the CaS:Eu nanoparticles prepared by an
alkoxide method reported by Sawada et al.
20
did not show any pho-
toluminescent emission. Heating at 700°C in N
2
was necessary to
incorporate Eu
2+
into the CaS lattice. Wang et al. prepared undoped
CaS and SrS nanoparticles with solvothermal synthesis
7
and pointed
out the major advantages of this synthesis method. This synthesis
was also successfully applied to prepare CaS nanocrystallites doped
with Bi, Ag, and Pb.
21
It is interesting to note that the growth of
macroscopic single crystals of CaS or SrS is far from obvious. Only
one synthesis method, a floating hot zone technique employing a
xenon arc image furnace, has been reported.
22
In this work, we apply the solvothermal synthesis as proposed by
Wang et al.
7
to prepare strongly luminescent monodisperse
micrometer- and submicrometer-sized CaS:Eu and SrS:Eu single
crystals. Both the luminescent and structural characteristics of these
phosphor materials were studied thoroughly, as well as the influence
of a capping agent thioglycerol. The morphology and composition
of the particles was evaluated.
Experimental
CaS:Eu and SrS:Eu particles were prepared with a solvothermal
synthesis method.
7
The starting materials were anhydrous SrCl
2
99.5% Alfa Aesar or CaCl
2
·2H
2
O 99% Alfa Aesar, EuCl
3
·nH
2
O
99.9% and sulfur powder. Ethylenediamine C
2
H
8
N
2
99% Alfa
Aesar, having a critical temperature of 319.9°C and a critical pres-
sure of 62.1 bar, was used as solvent. SrCl
2
, CaCl
2
·2H
2
O and
EuCl
3
·xH
2
O were dried at 170°C for 2 h under a nitrogen flow and
ethylenediamine was cooled to 2°C prior to use. An appropriate
amount of MCl
2
M = Ca, Sr, EuCl
3
and sulfur powder in 15%
excess were added into a Teflon-lined autoclave Autoclave France
Eze Seal and cooled ethylenediamine was added. The autoclave
was maintained at 200°C for 12 h corresponding to an ethylenedi-
amine vapor pressure of 8.3 bar and then cooled to room tempera-
ture naturally about 4 h. A synthesis temperature of 200°C gives a
good compromise between the size of the crystallites and the yield
of the product.
The strongly coordinating ethylenediamine is an excellent sol-
vent for the solvothermal synthesis of metal chalcogenides.
9
Alka-
line earth chlorides have reasonable solubility in ethylenediamine.
The polarity and the ligand of the solvent can successfully control
the growth of the crystallites.
9
Ethylenediamine binds to Ca
2+
or
Sr
2+
to form a relatively stable compound. As the temperature is
z
E-mail: jo.vanhaecke@ugent.be
Journal of The Electrochemical Society, 154 9 J278-J282 2007
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J278