Energy structure and fluorescence of Eu
2¿
in ZnS:Eu nanoparticles
Wei Chen*
Centre for Chemical Physics and Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 3K7
and Department of Inorganic Chemistry 2, Chemical Centre, Lund University, P.O. Box 124, S-22100, Lund, Sweden
Jan-Olle Malm
Department of Inorganic Chemistry 2, Chemical Centre, Lund University, P.O. Box 124, S-22100, Lund, Sweden
Valery Zwiller
Department of Solid State Physics, Lund University, P.O. Box 118, S-22100 Lund, Sweden
Yining Huang
Centre for Chemical Physics and Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 3K7
Shuman Liu
Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912,
Beijing 100871, People’s Republic of China
Reine Wallenberg and Jan-Olov Bovin
Department of Inorganic Chemistry 2, Chemical Centre, Lund University, P.O. Box 124, S-22100, Lund, Sweden
Lars Samuelson
Department of Solid State Physics, Lund University, P.O. Box 118, S-22100 Lund, Sweden
Received 16 November 1999; revised manuscript received 23 December 1999
Eu
2+
-doped ZnS nanoparticles with an average size of around 3 nm were prepared, and an emission band
around 530 nm was observed. By heating in air at 150 °C, this emission decreased, while the typical sharp line
emission of Eu
3+
increased. This suggests that the emission around 530 nm is from intraion transition of Eu
2+
.
In bulk ZnS:Eu
2+
, no intraion transition of Eu
2+
was observed because the excited states of Eu
2+
are degen-
erate with the continuum of the ZnS conduction band. We show that the band gap in ZnS:Eu
2+
nanoparticles
opens up due to quantum confinement, such that the conduction band of ZnS is higher than the first excited
state of Eu
2+
, thus enabling the intraion transition of Eu
2+
to occur.
Doped semiconductor nanoparticles are interesting nano-
structured materials because their electronic and optical
properties, which are largely size dependent, may result in
practical applications such as high brightness displays. A
typical such material is ZnS:Mn nanoparticles which have
been widely studied.
1
Recently, the luminescence properties
of trivalent rare-earth RE ions Tb
3+
Ref. 1, Eu
3+
Ref.
2 in semiconductor nanoparticles have been reported. The
luminescence of Eu
2+
and Eu
3+
in xerogels
3,4
was also ex-
amined. However, to our knowledge, divalent RE ions doped
into semiconductor nanoparticles have received little atten-
tion.
Eu
2+
-doped ZnS is a promising phosphor. The advantage
of this phosphor over other Eu
2+
-doped sulphides CaS, SrS,
and BaS is that ZnS is more chemically stable. However, it
was reported that instead of luminescence from Eu
2+
, only
the Eu
2+
-bound exciton could be observed in bulk
ZnS:Eu
2+
, because the excited states of Eu
2+
are higher or
degenerate within the host conduction band.
5,6
In this paper,
we show that as the size of ZnS:Eu particles approaches a
critical dimension, emission from Eu
2+
becomes possible,
and we argue that this is due to the fact that the energy
structure of ZnS:Eu
2+
is modified by quantum size confine-
ment.
ZnS:Eu
2+
nanoparticles were prepared as follows: A four-
neck flask was charged with a solution containing 10-ml
methacrylic acid, 5-g citric acid, and 1000-ml ethanol
99.95%. The solution was stirred under N
2
for 2.5 h. A
second solution containing 8.009 g of Na
2
S and 200 ml of
ethanol, and a third solution containing 30.337 g of
ZnNO
3
2
6H
2
O, 0.114 g of EuCl
2
and 200 ml of ethanol
(Eu
2+
/Zn
2+
molar ratio 5:995 were prepared and added to
the first solution simultaneously via two different necks. Af-
ter the addition, the resulting solution was stirred constantly
under N
2
at 80 °C for 24 h. The p H value of the final solution
was 2.4. This relatively low p H value is required to prevent
the precipitation of unwanted Eu species from occurring. The
nanoparticles were extracted from the solution by centrifuga-
tion, and dried in vacuum at room temperature.
The nanocrystalline structure, size, and shape were ob-
served by x-ray powder diffraction XRD and high-
resolution transmission electron microscopy HRTEM. The
XRD pattern was recorded with an INEL diffractometer us-
ing a CPS 120 detector and a monochromatized Cu K 1(
=1.540 56 Å) radiation with -Si ( a =0.543 nm) as an in-
ternal standard. The particles were suspended in ethanol, and
brought onto a holey carbon covered copper grid for
PHYSICAL REVIEW B 15 APRIL 2000-II VOLUME 61, NUMBER 16
PRB 61 0163-1829/2000/6116/110214/$15.00 11 021 ©2000 The American Physical Society