Size-induced transition-temperature reduction in nanoparticles of ZnS
S. B. Qadri, E. F. Skelton, D. Hsu, A. D. Dinsmore, J. Yang, H. F. Gray, and B. R. Ratna
U.S. Naval Research Laboratory, Washington, DC 20375-5320
Received 9 April 1999; revised manuscript received 16 June 1999
X-ray-diffraction studies of nanometer-sized particles of zinc sulfide show a significant reduction in the
zinc-blende-to-wurtzite phase transition temperature, as compared to the bulk value. Five nanoparticle samples
were annealed in vacuum at temperatures increasing from room temperature 23 °C to 500 °C. Post-anneal
analyses revealed an increase of crystallite size, accompanied by a partial transformation from the cubic,
zinc-blende structure to the hexagonal, wurtzite structure at temperatures as low as 400 °C. This is significantly
less than accepted bulk transition temperature of 1020 °C. The particles also show some lattice distortion with
decreasing particle size and there is a monotonic reduction in the specific volume of about 2.3% as the particle
size decreases from about 240 to about 30 Å. S0163-18299904837-7
ZnS is an important material for a variety of applications
such as electrolumiscent devices, solar cells, and other opto-
electronic devices. Nanometer-sized semiconductor particles
have attracted much attention because of their novel elec-
tronic and optical properties originating from quantum
confinement.
1
Recently, nanometer-sized particles of PbS,
CdS, and CdSe showed distortion from the lattice of bulk
materials and the presence of strain with reduced particle
size.
2–4
Although a variety of polytypes were observed for
bulk ZnS by x-ray investigations, they are all related to two
basic structures: the cubic zinc-blende structure 3C and the
hexagonal wurtzite structure 2H.
5
The most stable form of
zinc sulfide is the cubic structure and in the bulk it trans-
forms to wurtzite structure at 1020 °C. The bulk zinc sulfide
melts at a temperature of 1650 °C. Recently Goldstein et al.
reported that nanoparticles of CdS melt at a substantially
reduced temperature.
6
In this paper, we report structural studies of nanoparticles
of zinc sulfide at various annealing temperatures under
vacuum conditions. The starting particle size of zinc sulfide
was 2.8 nm. These particles were synthesized using a tech-
nique in which the bicontinuous cubic phase exhibited by
some lipids and surfactants is used as a matrix to provide a
uniform nanometer-sized reaction chamber for the formation
of nanoparticles.
7
High-resolution transmission electron mi-
croscopic TEM studies showed that the particles are highly
monodispersed (std. dev.7%). High-resolution TEM im-
ages show that the particles are monocrystalline and indicate
a small anisotropy in shape.
X-ray-diffraction scans were taken on a Rigaku diffracto-
meter using Cu K radiation from a rotating anode x-ray
generator operating at 50 kV and 200 mA. The as-made
nanocrystalline ZnS sample was divided into four portions
for annealing in vacuum at four different temperatures. A
zirconium oxide crucible with a diameter of 1.25 cm and
height of 1.9 cm was filled with the sample and placed into a
resistive heater made of Pt foil, mounted on a removable
vacuum flange. A Pt-Pt 10% Rh thermocouple was inserted
into the middle of the powder sample to measure the tem-
perature. In several hours, the sample chamber was initially
pumped down to a vacuum of 3 10
-7
Torr using a 6-inch
liquid-nitrogen-trapped oil diffusion pump. The sample was
then heated at a rate of 20°/m, until the final desired tem-
perature was reached. During the heating, considerable out-
gassing was observed. For short times, the pressure in the
vacuum chamber was as high as 1 10
-5
Torr, but within a
few minutes after reaching the final temperature, the pressure
would stabilize to the high 10
-7
Torr region. After 45 min of
annealing at the desired temperature, the sample was cooled
to room temperature at a rate of about 15–20°/min. This
procedure was carried out for four samples separately an-
nealed at four temperatures, 300° 2°, 350° 2°, 400°
PHYSICAL REVIEW B
CONDENSED MATTER AND MATERIALS PHYSICS
THIRD SERIES, VOLUME 60, NUMBER 13 1 OCTOBER 1999-I
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