Download Physicochemical Stability of ZnS Quantum Dots Stabilized by Gum

University of Africa Journal of Science
University of Africa Journal of Science (U.A.J.S., Vol. 1, No.1, 90 -95)
Physicochemical Stability of ZnS Quantum Dots Stabilized by
Gum Arabic
Hatim Mohamed El-Khair Dirar*
Abstract
ZnS quantum dots synthesized as soft matter utilizing Gum Arabic as
stabilizer had exhibited quantum size effect, which is evidently observed from
the significant blue shift in absorption and emission peak. Strong
physicochemical stability has been attained and can be attributed to the perfect
surface passivation done by Gum Arabic.
Key words: ZnS Quantum Dots.
Introduction
The study of semiconductor quantum dots (QDs) has been advancing at rapid
pace. Much of interest in II-VI compound materials such as CdS [Eychuler et
al., 1991], CdSe [Huang et al., 1999] and ZnS [Hatim et al., 2002] stems from
the fact that their optical properties can be systematically tuned by variation of
their size. However optical properties of these QDs have been found to be
very sensitive to the physical and chemical properties of their surfaces
[Alivisatos,1996]. For example photoluminescence (PL) properties of bare
structure QDs showed a detectable surface emission, which is attributed to the
recombination of the carriers at the surface trap sites with different energies.
Surface defect states such as dangling bonds and
vacancies
are the
main sources of these trap states. These trap states can induce thermaland photo-instability when the QDs are subjected to thermal- and photooxidation, respectively. Upon subjecting CdS QDs to the thermal annealing
at different temperature, rapid decrease in the intensity of PL was observed as
the temperature increases [Eychuler et al., 1991].
*Department of Physics, School of Physics and Applied Physics, Faculty of Science and
Technology, Al-Nelain University, Khartoum, Sudan.
* Department of Physics, Faculty of Pure and Applied Science, International University of
Africa, Khartoum, Sudan.
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The increase of temperature enhances the nonradiative and radiative
recombination at band edge and trap states, respectively. Semiconductor
QDs coated with inorganic shells such as CdSe/CdS [Huang et al., 2000] and
CdSe/ZnS [Peng et al., 1997] had exhibited enhanced band edge emission
with highest quantum yields (>50%) [Huang et al., 2000]. QDs can also be
passivated with long chain organic surfactant such as trioctylphosphine oxide
(TOPO) [Browen Katari, 1994]. These capped QDs have room temperature
PL quantum yields as high as 10% with very long fluorescence lifetime and
often have some non-band-edge luminescence [Norris et al., 1994].
Slight changes in temperature can cause a large perturbation in optical
characteristics of the QDs system and thus limit their applications. Our
objective is to seek for a better way to improve the optical and thermal
characteristics of ZnS QDs, by means of stabilization techniques. In this work
we reported, for the first time, the enhancement of the band edge emission
from ZnS QDs stabilized by Gum Arabic (GA). Such capping leads to highly
stable robust structure ZnS QDs. As being depicted by the structural
characterization.
Materials and Methods
Synthesis of ZnS QDs:
The stabilized media are composed of an aqueous solution, well degassed and
adjusted to a proper pH. The precursors fro Zn+2 and S-2 ionic species were
chosen to be ZnNO3 and Na2S, respectively.
The absorption and emission spectra for the given samples were measured by
UV-Vis. 1200 and USB 2000 emission spectrophotometers, respectively.
Physicochemical stability test is done by subjecting the direct illumination as
well as oxidation for different time exposure and hence the stability has been
ensured by the optical characterization techniques.
Results and discussion
The absorption edge of ZnS QDs depicted in figure 1, had exhibited strong
blue shift band, with respect to the bulk ZnS, centered at nm. This shift is due
to the quantum size effect (Alivisatos, 1996), which is detectable at the
entirely measured size. The absence of a multiple transitions indicates the
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change of band gap from indirect to direct. Such changes can prevent
formation of assisted phonon and hence improving the quantum yield of ZnS
QDs [Alivisatos, 1996].
Absorption spectra of ZnS QDs capped by GA
Abs
S1
S2
S3
S4
250
300
350
400
450
500
Wavelength (nm)
Figure 1 shows the absorption spectra of ZnS QDs stabilized by Gum Arabic
The emission spectra depicted in figure 2 had exhibited very narrow blueshifted emission spectra, which are ranging from 330 to 380 nm. Such shifts
are due to the quantum confinement phenomena [Eychuler et al., 1991]. The
single peak obtained is attributed to the near band edge emission. The absence
of any red shifted emissions which are obtained in previous works and which
might be due to the surface defect states is a good indicator for the removal of
these defects[Hatim Mohamed et al., 2002]. Therefore, GA has proven to be a
most perfect stabilizer compared to others [Huang et al., 2000].
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Figure 2 shows the emission spectra of ZnS QDs stabilized by Gum Arabic.
Emission spectra from ZnS QDs shown in figure 3, represent the optical
characterization for the set of samples before and after being exposed in an
open and illuminated by sun light for 0, 15, 30 and 45 minutes exposure time.
The features of the spectra have shown that, the emission peaks were blueshifted, with respect to the bulk ZnS, with steady state intensities for a chosen
exposure time. The above spectral features indicate good photo-chemical
stability; such stability can be attributed to the complete surface passiviation,
and hence removal of defects caused by S vacancies act as radiative trap states
with energy closed to the bandedge [William and Allen, 1983].
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Photo-chemical Stability for ZnS Capped GA
Emission (a.u.)
S3
S3
S3
S3
300
350
400
0 min
15 min
30 min
45 min
450
500
550
Wavelength (nm)
Figure 3 indicates the Photo-chemical stability of ZnS QDs stabilized by Gum Arabic
Conclusion
ZnS quantum dots stabilized by GA had exhibited quantum size effect and
highest photo-chemical stability. Such improved physical properties can be
attributed to the perfect surface passiviation attained by Gum Arabic.
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