Journal of The Electrochemical Society, 148 共10兲 H139-H142 共2001兲
H139
0013-4651/2001/148共10兲/H139/4/$7.00 © The Electrochemical Society, Inc.
Structure and Luminescence Properties of the ZnS:Cu, Al
Phosphor for Low Voltage Excitation
S. J. Lee,a,z J. E. Jang,a Y. W. Jin,b G. S. Park,c S. H. Park,b N. H. Kwon,a
Y. J. Park,b J. E. Jung,a N. S. Lee,b Ji-Beom Yoo,d J. H. You,e and J. M. Kima
a
Samsung Advanced Institute of Technology, The National Creative Research Initiatives Center for Electron
Emission Source, bDisplay Laboratory, Material and Device Sector, cAnalytical Engineering Laboratory,
Suwon, Korea
d
Department of Materials Engineering, Sungkyunkwan University, Suwon 440-746, Korea
e
Samsung SDI, Suwon, Korea
The ZnS:Cu, Al phosphor has been studied for low voltage excitation field emission devices. To improve the cathodoluminescence
共CL兲 brightness, the phosphor has been synthesized with iodide-type alkaline and alkali-earth metal as fluxes. CuI and AlF3 were
used as an activator and coactivator, respectively. The phosphor has been characterized in comparison with commercially available
cathode ray tubes 共CRTs兲 phosphor and conventionally prepared ones. Newly synthesized ZnS:Cu, Al phosphor prepared with
iodide-type fluxes has shown higher CL brightness than commercially available phosphor and conventionally prepared ones. The
phosphor structure has been analyzed using scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. From the analysis, the phosphor particles were composed of cubic and hexagonal phases. The cubic phase was much more
formed when iodide-type fluxes were used. This cubic phase helps the phosphor to have high brightness at low voltage operations.
© 2001 The Electrochemical Society. 关DOI: 10.1149/1.1396654兴 All rights reserved.
Manuscript submitted January 16, 2001; revised manuscript received April 26, 2001. Available electronically August 24, 2001.
Green light-emitting ZnS:Cu, Al1 phosphors have been applied to
a wide range of electron emission devices, in particular, to cathode
ray tubes 共CRTs兲 for a long time because of their excellent cathodoluminescent 共CL兲 characteristics. Commercially available CRT
phosphors are not suitable to low voltage excited field emission
devices 共FEDs兲, because they need high efficiency at low excitation
voltage, a high resistance to current saturation, and a high chemical,
thermal stability, and long lifetime at high current density.2
To develop the phosphor for low voltage excitation devices, it is
necessary to study new phosphors or to modify the commercially
available CRT phosphors. However, the latter has been studied more
actively than the former because of the difficulties of developing a
new material.
The methods to improve the phosphor properties are as follows:
select the high purity raw materials and fluxes, set up a firing condition, control the dopants concentration, modify the phosphor surface, and so on. Because Ni2⫹, Co2⫹, and Fe2⫹ metals are considered to be one of plausible causes of brightness decrease, purity is
very important in raw materials.3 The surface state of a phosphor
particle is very important, too. Because the penetration depth of
incident electron into phosphor particle is very small 共several
nanometers兲 compared with the particle size at low voltage 共⭐1
kV兲. Also, protective films are applied on the phosphor particle surface to increase the brightness and stability of phosphor.4
The ZnS:Cu, Al phosphor has been conventionally synthesized
using chloride-type fluxes or without flux. As an activator and coactivator, sulfide-type or nitrate-type chemicals were used.5,6 In this
paper, ZnS:Cu, Al phosphor has been synthesized using iodide-type
alkaline and alkali-earth metal as fluxes, a copper iodide as an activator, and aluminum fluoride as a coactivator for a low voltage
excitation phosphor.
Our ZnS:Cu, Al phosphor has been characterized in comparison
with commercial and conventionally prepared ones which were synthesized by us. The improvement of CL characteristics of the phosphors has been explained in terms of their morphologies and structures.
fluxes were homogeneously mixed with distilled H2O. The pastes
were dried in an oven at 100°C overnight and sieved. Then, the
dried powder was fired with 20 wt % sulfur in a reducing atmosphere (CO2 ⫹ CO) to prevent oxidation. Then, products were
sieved and washed with H2O to remove impurities.
To compare with our phosphor, two conventional phosphors have
been prepared and raw materials of three phosphors are shown in
Table I. Conventional phosphor 1 was prepared using CuSO4 and
Al共NO3兲3 for activator and coactivator, respectively, without flux
and fired at 1050°C for 40 min. Conventional phosphor 2 was prepared using ZnCl2 and NaCl as flux 1 and flux 2, respectively. An
activator and coactivator were the same as conventional phosphor 1
and fired at 950°C for 40 min. The conventional phosphor 1 and 2
have been synthesized on the basis of U.S. Pat. 3,704,232 and
4,925,593.
Our phosphor was prepared using ZnI2 and NaI as flux 1 and flux
2. CuI and AlF3 were used as activator and coactivator, respectively.
The amount of ZnI2 and NaI added were 0.002 to about 2.0 and
Experimental
Phosphor synthesis.—Figure 1 shows the phosphor synthesis
process. We used predetermined purity 共⭓4 N兲 raw materials to
prepare ZnS:Cu, Al phosphors. ZnS, activator, coactivator, and
z
E-mail: [email protected]
Figure 1. The process of ZnS:Cu, Al phosphor synthesis.
Journal of The Electrochemical Society, 148 共10兲 H139-H142 共2001兲
H140
Table I. Raw materials of ZnS:Cu, Al phosphor prepared by a
conventional method and using iodide-type fluxes.
Host
activator
coactivator
Flux 1
Flux 2
By conventional
phosphor 1
By conventional
phosphor 2
By iodide-type
fluxes
ZnS
CuSO4
Al共NO3兲3
-
ZnS
CuSO4
Al共NO3兲3
ZnCl2
NaCl
ZnS
CuI
AlF3
ZnI2
NaI
0.005 to about 2.0 wt %, respectively. Especially CuI was used as a
form of ammonia complex. Then, the phosphor was fired at 950°C
for 40 min. For all of the above-mentioned phosphors, ZnS host
material was the same.
Analysis.—Phosphor shapes and structures were observed by
field emission scanning electron microscopy 共SEM兲 共S4500 II, Hitachi兲 and X-ray powder diffraction 共XRD兲 共ADP 1700, Philips兲.
High-resolution transmission electron microscopy 共HRTEM兲
共H9000, Hitachi兲 was performed to investigate the microstructure of
phosphors. Reflective CL was measured at accelerating voltages
from 500 to 7000 V in vacuum of 1 ⫻ 10⫺7 Torr.
Figure 3. XRD patterns of each phosphor; the range of 2␪ ⫽ 25-34°.
lot of growth axes, and round shaped particles are created in the
chloride flux system. The particle growth rate is relatively slow and
particles grow one direction in the iodide flux system. Therefore,
microcrystal particles are created.
Results and Discussion
Figure 2a and b are the SEM image of conventional phosphor 1
and conventional phosphor 2, respectively. Figure 2c is the SEM
image of the phosphor synthesized with iodide-type fluxes. The average particle size of three phosphors is about 1.0-3.0 ␮m.
In general, the shapes of phosphor particle are microcrystals,
round crystals, and mixtures of these. The particle shape of phosphor is affected by kinds and amount of fluxes, firing temperature,
and firing time.7 Although the mean particle size of three phosphors
are similar, the particle shapes of three phosphors are a little bit
different. The particle shape of conventional phosphor 1 is mainly a
round crystal. The particle shape of conventional phosphor 2 is usually a round crystal and mixed with some microcrystal particles.
Microcrystal particles are bounded by a circle. Our phosphor synthesized using iodide-type fluxes has mainly microcrystal shaped
particles.
In the case of these phosphors, the particle shapes are especially
affected by the kinds of fluxes and the particle growth rate. The
particle growth rate is faster in the chloride flux system 共ZnCl2,
NaCl兲 than in the iodide flux system 共ZnI2, NaI兲. The particle has a
Figure 2. SEM images of 共a兲 and 共b兲 ZnS:Cu, Al prepared by conventional
methods and 共c兲 ZnS:Cu, Al synthesized with iodide-type fluxes.
Figure 4. TEM images of phosphors prepared 共a兲 by the conventional
method 1, 共b兲 by the conventional method 2, and 共c兲 with iodide-type fluxes.
Arrows indicate distinguished areas.
Journal of The Electrochemical Society, 148 共10兲 H139-H142 共2001兲
Figure 3 shows the XRD patterns of conventional phosphor 1,
conventional phosphor 2, and the phosphor synthesized with iodidetype fluxes. The peaks are composed of both hexagonal phase and
cubic phase and the lattice parameter almost does not change in all
phosphors. But the degree of crystallinity shows some difference
among the phosphors. In the case of the phosphor synthesized with
the iodide-type fluxes, the peaks of cubic phase increase; this means
the peaks of hexagonal phase decrease compared with conventionally prepared ones.
Detailed structures of the phosphor were investigated by TEM.
Figure 4 shows the thin cross sections of three phosphor particles.
The particle seems to be divided into two regions. One is cubic
phase and the other is hexagonal phase. Arrows indicate cubic
phase. In Fig. 4c, the cubic phase is remarkably broader in the
phosphor prepared using iodide fluxes than that in phosphors prepared by conventional methods.
Fast Fourier transformation 共FFT兲 of a HRTEM image was performed to analyze a magnified structure of each region. Figure 5a
shows a FFT study of conventional phosphor 1. Figure 5b is
HRTEM image of the part that is indicated by circle in Fig. 5a. This
part divides into three by a square, i.e., 共I兲, 共II兲, and 共III兲. The FFT
results of 共I兲, 共II兲, and 共III兲 parts are Fig. 5c, d, and e. From the FFT
study, the 共I兲 and 共III兲 parts are composed of the cubic and the
hexagonal phase, respectively, and the 共II兲 part that is the border of
cubic and hexagonal phase is a mixture of the two phases. Consequently, SEM, XRD, TEM, and HRTEM studies show coincided
results. The cubic phase of ZnS was more efficiently made by iodide
fluxes than by chloride fluxes. This cubic phase can exert CL brightness increase in low voltage excitation.
In order to relate phosphor structure with output light intensity,
we compared the phosphor brightness. Figure 6 shows CL brightness curve for the phosphors from 500 to 7000 V. The current density from electron gun was about 0.2 mA/cm2 at 1 kV. The
ZnS:Cu, Al phosphor, which was prepared with iodide-type fluxes,
shows the highest brightness among the phosphors synthesized by
conventional methods and commercial phosphor at all accelerating
voltages. The inset in Fig. 6 shows the CL brightness of the phosphors from 500 to 1000 V.
Figure 5. 共a兲 TEM image of the phosphor and 共b兲 HRTEM image of an
indicated area by a circle in a. Also, FFT diffraction patterns of 共c兲 I, 共d兲 II,
and 共e兲 III areas indicated by squares in b.
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Figure 6. CL brightness for the phosphors at various accelerating voltages:
commercial available for CRTs, prepared by conventional methods, and
synthesized with iodide-type fluxes. Inset is the CL brightness below
1 kV.
The CL brightness of a phosphor is affected by the phosphor
structure. The number of donor-acceptor pairs 共D-A pairs兲 relate
with lattice constants of ZnS 共hexagonal ZnS: a ⫽ 3.820 Å,
b ⫽ 6.260 Å, and cubic ZnS: a ⫽ 5.4 Å兲. If Cu⫹2 and Al⫹2 are
placed within an appreciable distance for the interaction, the D-A
pairs form in the cubic ZnS lattice site more effectively than they do
in hexagonal lattice site. More D-A pairs occur in the cubic phase
than in the hexagonal phase due to a larger lattice constant of the
cubic phase. The number f (N) of D-A pairs at given separation
distance 共r兲 is distributed with f (N) ⫽ 1/(2␲␴) 1/2 • exp关 ⫺ (E
⫺E(r)2)/2␴ 兴 , f (N) is the number of D-A pairs, and E g , E D , and E A
are bandgap energy, donor level energy, and accept level energy,
respectively, where E ⫽ E g ⫺(E D ⫹ E A). 8 The iodide-type fluxes
build up the cubic phase more effectively, and as a result, enhance
the CL brightness of phosphor.
Figure 7 shows the CL spectra of the samples. In the case of the
phosphor prepared with iodide fluxes, the CL spectrum peak shifts
to a long wavelength. The short lattice distance of the pair recombination centers contribute to the increase of the CL intensities of
the phosphor prepared with iodide fluxes.
Figure 7. The CL spectrum of the prepared phosphors.
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Journal of The Electrochemical Society, 148 共10兲 H139-H142 共2001兲
Also, absorption transition of the ZnS:Cu,Al phosphor before
excitation is more useful in the case of cubic ZnS. The reason of this
is that the bandgap energy is lower in cubic ZnS 共3.6 eV兲 than
in hexagonal ZnS 共3.8 eV兲.9
We believe that by adopting a new flux system we can modify
the crystal structure of the ZnS:Cu, Al phosphor. This result means
that the modified phosphor has high brightness without changing a
color coordinate.
Conclusion
The ZnS:Cu, Al phosphor has been synthesized with iodide-type
alkaline and alkali-earth metal as fluxes for low voltage excitation
FEDs. The phosphor has much more cubic structure when iodidetype materials are used. The photon energies of CL are changed and
the cubic phase exerts a beneficial influence upon the brightness of
the phosphor.
Acknowledgments
This work was supported by a national strategic fundamental
research program for creative research development sponsored by
the Korean Ministry of Science and Technology.
Samsung Advanced Institute of Technology assisted in meeting the publication costs of this article.
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