Structural and optical studies of CdTe nanocrystals
embedded into polymer host
N. TOUKAa,*, B. BOUDINEb, O. HALIMIb, M. SEBAISb
University of BOUIRA, Faculty of Sciences, BOUIRA 10000, Algeria
b
Laboratory of Crystallography, Department of Physics, University of Constantine 1, Road Ain El bey 25000, Algeria
a
The organic amorphous matrix of polystyrene (PS) is transparent in the ultraviolet-visible area and so it allows the study of optical
properties of CdTe nanocrystals in this field. The thin films of PS/CdTe nanocomposite were prepared using a colloidal solution and
were deposited by spin coating technique on glass substrate. A size distribution of particles in solid matrix was observed in XRD/SEM
which were supported by UV/VIS optical absorption spectra. The investigation of samples by X-ray shows diffraction peak of CdTe
semiconductor with cubic zinc blend structure. The optical absorption spectra shows a large size distribution of the CdTe crystallites and
a shift of the band gap to the high energies compared to one of CdTe bulk crystal. The photoluminescence spectrum shows a wide
emission band at 650 nm attributed to the radiative band to band transitions in the CdTe nanocrystallites. These results confirm the
dispersion of CdTe nanocrystallites in the amorphous polymer PS and reveal optical activity of the fabricated PS/CdTe nanocomposite.
Keywords: Nanocrystals CdTe, Polystyrene (PS), Quantum confinement, Photoluminescence.
1. Introduction
II-VI classes of semiconductor Nano Crystal are
extensively
studied
for
their
optoelectronic,
photochemical, and nonlinear optical properties [1, 2, 3].
Semiconductor nano crystals, like CdTe, show size
dependent optical properties and are very important in
basic nano science research, colloidal science, biomedical
labeling, light emitting diodes, solar cells, and lasers [4-5].
In general, semiconductor nano crystals show novel
optical and electronic properties when they have size
comparable to, or smaller than, the dimensions of the
exciton within their corresponding bulk material. This
allows us to create unique properties for the nano crystal
by engineering the size and composition and the chemical
functionality of their surrounding medium. Organic–
inorganic hybrid nanostructures have attracted increasing
attention in the fabrication of various devices, ranging
from optoelectronics to gas sensors [6–7]. Especially,
hybrid systems composed of inorganic nanoparticles
embedded in polymer matrix have been considered as
available nanostructures for next generation devices since
they could possess the advantages of mixing the organic
materials, such as flexibility and light weight, and the
inorganic materials such as heat and chemical resistances.
Moreover, the incorporation of functional inorganic
materials in polymer matrix is expected as smart materials
because polymers are capable of fabricating multifunctional wearable devices. We report in this work the
synthesis, the structural characterization, and optical
properties of CdTe-NCs dispersed into a transparent
polystyrene matrix by colloidal solution and deposited on
a glass substrate using spin coating technique. Structural
characterization is achieved though XRD and SEM, while
the optical properties are determined using optical
absorption and photoluminescence.
2. Experimental
Organic–inorganic hybrid nanostructures PS/CdTeNCs were prepared by colloidal solution. A host solution
was prepared by dissolving amorphous polymer
Polystyrene (PS) in chloroform (CHCl3) with a
concentration of 0.02 g/ml. This solution was stirred at
50°C for 2 hours. A guest solution was prepared with 0.20
g of mechanically crushed NC’s CdTe powder dispersed in
10 ml of chloroform. Finally, both solutions were mixed
and homogenized by magnetic agitation for 24 h. Glass
slabs used as a substrate were degreased, rinsed
thoroughly with distilled water. Thin films were deposited
by spin coating technique performed under various
conditions depending on the viscosity of the mixture. The
rotational speed ranged from 300 to 2000 rpm in order to
control the thickness. All experiments were performed at
room temperature and ambient pressure.
X-ray diffraction patterns of the films were measured
using an X-ray diffractometer model D8 Advance, Bruker
with Ni filtered Cu radiation generated at 40 kV and 30
mA (CuKα =1.542 A) as the X-ray source. The absorption
spectrum of the thin films was recorded using a UVvisible/NIR spectrophotometer (Perking Elmer, model
Lambda 19). Photoluminescence data were obtained by
exciting the sample at room temperature with a 350 nm
radiation generated by an Argon laser.
3.1. X-ray diffraction analysis
The X-ray diffraction of the Nanocomposite CdTe-PS
(Fig. 1) is shown in Figure 1. XRD pattern exhibits
polycrystalline nature and a major diffraction peak is
observed at 2θ = 23° which corresponds to the cubic (111)
orientation. The presence of the predominant peak at 2θ =
23° suggests that the NC’s CdTe dispersed in PS is of zinc
blende structure with a preferential orientation along the
(111) plane [8].
The average radius of the dispersed particles in the PS
matrix was estimated using the Scherrer formula (E 1) [9]
in which the crystallites are assumed to possess a spherical
shape.
2R 
0.89
  cos 
The distinct lowest energy features in the spectra
indicate a narrow size distribution and high crystallinity of
the NCs, in agreement with the results of SEM where we
also notice the good dispersion of NC’s of CdTe in the
matrix (Fig. 3).
627nm
Optical density (a. u)
3. Results and discussion
(1)
600
660
720
780
wavelength (nm)
R is particle radius, λ corresponds to CuKα tube
wavelength emission, θ is the diffracted angle and ∆θ the
full width at half maximum (FWHW) in radians of the
peak. The calculated average value of the size of the
particles was about 16 nm justifying hence the nanometric
size of the particles.
Fig. 2. Absorption spectra of CdTe NCs-PS composite
thin film.
Intensity (a. u)
23°
(111)
Fig. 3. SEM images of the CdTe NC’s dispersed in PS matrix.
3. 4. Photoluminescence analysis
30
60
90
2 theta (Degrees)
Fig. 1. X-ray diffraction patterns of CdTe – PS
nanocomposite thin film.
3. 2. Absorption optical analysis
Polystyrene polymer was used as host matrix because
its transparency in the visible wavelength range. Fig. 2
shows the optical absorption spectrum of the
nanocomposite NCs CdTe-PS thin film. The significant
blue shift of the lowest energy absorption peak at 627 nm
(1.97 eV) from the bandgap energy of the bulk CdTe 1.43 eV [10] evidences strong confinement of carriers in
these NCs. Based on the models reported in literature
[9,11], the average diameter of the NCs dispersed in PS
matrix was estimated at 3.5 nm.
Photoluminescence (PL) of the nanocomposite PS/
NCs-CdTe were carried out at room temperature (300 K)
using 514.5 nm exciting radiation available from a Hg-Xe
lamp. PL spectra exhibits intense emission bands (Fig. 4)
at 627 nm with the full width at half maximum (FWHW)
of 110 nm was ascribed to the interband exciton
recombination of the NC’s CdTe.
[11] N. Touka, B. Boudine, O. Halimi, M. Sebais,
Optoelectronics and Advanced Materials – Rapid
Communications 6, 583 (2012).
15000
Intensity PL (u.a)
_________________________
*Corresponding author: Nassimtouka@yahoo.fr
10000
5000
0
1,2
1,5
1,8
2,1
2,4
2,7
3,0
Energy eV)
Fig. 4. Photoluminescence spectra of CdTe
NCs – PS composite.
4. Conclusions
The preparation of CdTe NCs using colloidal solution
and their good incorporation in a polymer matrix is
reported and supported by complementary investigations.
The investigation of samples by X-ray shows diffraction
peak of CdTe-NCs compound with cubic zinc blend
structure. These CdTe-NCs exhibit both a blue shift and
discrete energy states at low crystallites size. A strong
quantum confinement was established. Photoluminescence
spectra of the CdTe NCs – Polymer exhibits intense
emission bands due to HOMO – LUMO transition. These
results confirm the dispersion of CdTe nanocrystals in the
amorphous polymer PS and reveal optical activity of the
fabricated CdTe/PS nanocomposite in the visible area.
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