Preparation and characterization of polymer/CdS nanostructured photoluminescent films

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Chemistry Journal of Moldova. General, Industrial and Ecological Chemistry. 2014, 9(2), 74-79
POLYMER NANOCOMPOSITE BASED ON STYRENE WITH BUTYL
METHACRYLATE AND INORGANIC SEMICONDUCTOR CdS
Mihail Iovua, Mihai Enachescub, Ion Culeaca*, Victor Verlana, Stefan Robuc,
Dionezie Bojinb, Iurie Nistora, Ion Cojocarua
aInstitute of Applied Physics of Academy of Sciences of Moldova, 5, Academiei str., Chisinau MD 2028, Republic of Moldova
bCSSNT, University Politehnica of Bucharest, 313, Splaiul Independentei, sector 6, Bucharest, RO-060042, Romania
cMoldova State University, 60, Mateevici str., Chisinau MD 2009, Republic of Moldova
*e-mail: ionculeac@gmail.com
Abstract. We present experimental results on copolymer-based nanocomposite made of styrene with butyl
methacrylate (1:1) and inorganic semiconductor CdS. Thin fi lm composite samples have been characterized by UV-
Vis absorption and photoluminescent spectroscopy, as well as by transmission electron microscopy. Transmission
electron microscope examination confi rms a relatively narrow distribution of CdS nanoclusters in the SBMA matrix,
which covers the range 2-10 nm. On the other side, the average CdS particles size estimated from the position of fi rst
excitonic peak in the UV-Vis absorption spectrum was found to be 2.8 nm and 4.4 nm for two samples with different
duration of thermal treatment, which is in good agreement with PL experimental data. The PL spectrum for CdS
nanocrystals is dominated by near-band-edge emission. The relatively narrow line width (40-45 nm) of the main PL
band suggests the nanoparticles having narrow size distribution. On the other side, relatively low PL emission from
surface trap states at longer wavelengths were observed in the region 500-750 nm indicating on recombination on
defects.
Keywords: nanocomposite, polymer matrix, photoluminescence, exciton.
Introduction
Nanocomposite (NC) materials belong to one of the most dynamic domain of research and development, and
they occupy an important place is the fi eld of nanotechnology [1-6]. Nanocomposite materials made of a polymer matrix
and an inorganic semiconductor have become a prominent area of research and technology because of their attractive
properties [4-6]. NCs offer a variety of new possibilities beyond those of conventional materials as well as compared
to NCs constituent components. The basic feature of nanoscale materials resides in the possibility of tuning of their
physical and chemical properties through varying the size of incorporated nanoparticles. This actually means obtaining
different properties within the same chemical composition just by changing the size of nanoparticles [7-10].
Among existing variety of nanocomposite materials polymer based NCs attract a lot of research efforts because
of many advantages, compared to conventional materials. These advantages refer to their relatively simple technology,
low cost, easy tuning, etc. Polymer NCs can be obtained in the form of thin fi lms, bulk, or fi ber samples, etc., by
relatively simple technological methods, among them drop-wise deposition, spin coating, extrusion, etc. Polymer
based NC materials with inorganic semiconductors comprise a polymer material as a matrix and incorporated inorganic
semiconductor nanoparticles as fi llers [11-12]. The role of the polymer matrix in nanocomposites is to assemble the
nanoparticles into clusters, avoiding agglomeration, inducing ordering and orientation in self-assembling structures, etc.
As one of the most important II-VI group semiconductors CdS nanoparticles have received great attention
because of their attractive properties and potential for application in photonics and optoelectronics [14-16]. In the
present work we report preparation and characterization of photoluminescent polymer-inorganic nanocomposite thin
fi lms based on styrene-butylmethacrylate copolymer (SBMA) (1:1) and inorganic semiconductor CdS.
Experimental
The technology of preparation of the nanocomposite thin fi lms made of styrene with butylmethacrylate (SBMA)
(1:1) and inorganic semiconductor CdS was described elsewhere [17-18]. At the fi rst step a solution of cadmium nitrate
Cd(NO3)2 was mixed with a solution of thiocarbamide, SC(NH2)2, taken in equimolar ratio. In this way we obtain a
complex compound Cd(NO3)2× SC(NH2)2, well soluble in distilled water. The SBMA copolymer, which is used as a
matrix, was dissolved in an organic solvent, partially hydrophilic (e.g. dimethylformamide). In this way a copolymer
solution with concentration 10 g of polymer in 100 ml solvent was prepared. At the next step an appropriate quantity of
Cd(NO3)2× SC(NH2)2 related to CdS concentration of 20 and 40 mass% relative to SBMA copolymer was added in the
solution. All this composition was mixed by vigorous stirring. For preparation of the nanocomposite thin fi lms the fi nal
solution was cast onto a clean, fl at glass plate, and heated at 100 ºC during 30-60 min.
The composite layers were obtained on clean glass substrates for measuring of optical transmission and
photoluminescence. The thickness of the thin fi lm layers was in the range 5-15 μm. Nanocomposite thin fi lms were
characterized by measuring UV-Vis transmission and photoluminescent (PL) spectra, and by transmission electron
microscopy (TEM EM 410). Optical transmission spectra of the samples were registered in the range 400–800 nm on a

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M. Iovu et al. / Chem. J. Mold. 2014, 9(2), 74-79
Specord UV-Vis spectrophotometer or M40 photospectrometer. PL spectra of nanocomposite thin fi lms were measured
at room temperature under excitation of a laser beam 337 or 405 nm using a MDR-23 monochromator and a Hamamatsu
photomultiplier module H9319-12 operating in a photon counting regime.
Results and discussion
Examination of prepared thin fi lms suggests that they contain clusters of CdS of different sizes. This can be
seen on nanocomposite thin fi lms SMBA + 40%CdS deposited on glass substrate (Figure 1). Transmission electron
microscope images indicate on a relatively narrow distribution of the CdS nanoclusters in the SBMA matrix, which
covers the range ~ 2-10 nm (Figure 1a). The small nanoparticle diameter correlates with the strong shift of the PL
maximum at ~ 383 nm. On the other hand, in the case of larger thermal treatment time one can observe a less dispersed
but larger nanoclasters diameter (Figure 1b). From the image in Figure 1b the nanoparticles size can be estimated to be in
the range ~20-30 nm. The UV-Vis absorption spectra of nanocomposite thin-fi lm samples deposited on glass substrates
are presented in Figure 2. The position of the main excitonic peak one can clearly distinguish on these spectra. These
peaks are situated at 381 nm (2.97 eV) and 429 nm (3.28 eV) respectively.
(a) (b)
Figure 1. TEM image of the nanocomposite thin fi lms SBMA+40%CdS obtained at different
thermal treating time 30 (a) and 60 (b) min.
The size of nanoparticles has been evaluated from the position of the fi rst excitonic peak from the data in
Figure 2 through the empirical relation [19-21]:
)29.13()102352.9()109557.1()106521.6( 22438 uuu 
OOO
D, (1)
where D (nm) is the diameter of the nanoparticles and λ (nm) is the wavelength of the fi rst excitonic peak of the
corresponding sample. In this way we can evaluate the diameter of the nanoparticles as 2.8 nm and 4.4 nm. Such
nanoparticles size implies a strong confi nement of the charge carriers, while the confi nement energies of the electron
and hole are much larger than the energy of Coulomb interaction [22,24]. The position of the fi rst excitonic energy can
be estimated from the relation, which connects the size of the nanocrystal and the excitonic energy E [21-23]:
)
4
8.1
(
1
)
11
(
20
2
**2
22
r
he
g
e
R
mmR
E
E
HSH
S
 !, (2)
where Eg is the energy gap of bulk CdS, R is the size of the nanoparticle, me
* and mh
* are the effective masses of the
electron and hole. The corresponding values for the effective masses are as reported elsewhere [24-26]: 19.0
0
*
m
me and
8.0
0
*

m
m
h
; and the dielectric constant εr = 5.7; ε0 is the permittivity of free space and e is the electron charge. If we take
the particles diameter as determined from the absorption spectra in Figure 2 as D1= 2.8 nm and D2 = 4.4 nm, then the
calculated exciton energies are respectively 2.72 eV and 3.36 eV. On the other side the positions of the main excitonic
peak determined from the absorption spectra in Figure 2 are 2.97 eV (curve 1) and 3.28 eV (curve 2) respectively. These
values are higher than the energy gap Eg of the bulk CdS (2.42 eV at 300 K [24-26]) which indicates on a blue shift of
the absorption edge. The increase of the band gap is determined by the quantum size effect of these small crystallites,
and the calculated diameter of the nanocrystals (2.8 nm and 4.4 nm) is comparative to the excitonic Bohr radius ~ 3 nm
of CdS [21-23].

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Figure 2. Absorbance spectrum of SBMA+40%CdS nanocopmposite samples
with different nanoparticles size: (1) 2.8 nm and (2) 4.4 nm.
Figure 3 represents the PL spectra of nanocomposites SBMA/CdS measured in two different samples SBMA/
CdS characterized by different nanoparticles size [27]. Nanocomposite samples SBMA/CdS exhibit a luminescent
signal in the range 350-600 nm, with the position of PL maximums varying in dependence of the dimensions of the
CdS nanoparticles in polymer matrix. Variation of the thermal treatment time leads to variation of the size of CdS
nanoparticles and shifts the maximum of the photoluminescence peak in the high energy region. The PL emission peaks
of the nanocomposites SBMA/CdS are found to be located at 383 nm and 463 nm respectively (samples 2.3 and 2.1 in
Figure 3a). The excitonic energy for the corresponding nanocomposite samples (Figure 2) corresponds to 2.97 and 3.28
eV respectively. These values indicate that the luminescence peaks are Stokes-shifted from their band gap energy. The
full width at half maximum (FWHM) of the PL peak in Figure 3a represents 45 (2.3) and 40 nm (2.1). The full width
of ~ 40 nm suggests the size dispersion of the nanoparticles around the mean diameter value is relatively narrow. The
broadening of the PL spectra for the sample 2.3 of approximately ~ 45 nm can be attributed basically to nanoparticle
size dispersion and less to the presence of charges on the surface of the nanoparticles [24-27]. The PL maximums can be
attributed to direct transitions from the conduction band to valence band.
350 400 450 500 550 600
0
500
1000
1500
2000
2500
2.3
2.1
x3.5
I, a.u.
, nm
350 400 450 500 550 600
0
500
1000
1500
2000
2500
3000
Model: Gauss
xc1 379.8
w1 45.5
A1 90578
xc2 411
w2 77.3
A2 88284
I, a.u.
, nm
(a) (b)
Figure 3. PL spectra of NC SBMA+40% CdS measured at room temperature under excitation 337 nm:
(a) sample 2.3 – nanoparticles size 2.4 nm; sample 2.1 – nanoparticles size 4.2 nm;
(b) deconvolution of PL spectra for sample 2.3.
In addition to blue-violet PL main bands nanocomposite samples exhibit several weaker red peaks positioned
at 562 nm, 599 and 639 nm (Figure 4). While the PL maxima in the visible range can be attributed to direct transitions
from the conduction band to valence band, the PL emission in the red range can be associated to the transitions from the
donor levels to the valence band of CdS [25-27].
M. Iovu et al. / Chem. J. Mold. 2014, 9(2), 74-79

77
600 650 700 750 800
0
500
1000
1500
2000
2500
Model: Gauss
xc 671
w 100
A 253965
672
I, a.u.
, nm
550 600 650 700 750 800
0
500
1000
1500
2000
2500
638.8
599.4
562
Model: Gauss
xc1 562.4
w1 53
A1 59581
xc2 599
w2 29.9
A2 12971
xc3 638.8
w3 120
A3 284734
I
, a.u.
, nm
(a) (b)
Figure 4. PL spectra of nanocomposite samples SBMA/CdS under excitation of light beam 405 nm:
(a) SBMA+20%CdS (sample 1.1); (b) SBMA+40%CdS (sample 2.1).
Figure 5 illustrates the scheme of optical absorption in CdS nanoparticles incorporated in the copolymer
matrix SBMA. In the case of confi nement the absorption spectrum of the nanocrystallies exhibits a quasi discrete
character versus the photon energy and the density of states in quantum dots peaks at certain energies. As a result of the
confi nement the energetic QDs gap increases compared to bulk semiconductor. The nanocystallite energetic spectrum
behaves like an isolate hydrogen atom with the energetic levels 1s, 1p, 1d, etc.
Figure 5. A simplifi ed scheme of QDs energy levels and their absorption bands
in the SBMA/CdS nanocomposite.
When a photon is absorbed by the CdS semiconductor an electron is excited from the valence into the
conduction band, and a positively-charged hole is created in the valence band. When the excitation energy corresponds
to optical transition 1s(h) - 1s(e) one or two electrons can appear on the energetic level 1s(e) (Figure 5). In the later case
the level 1s(e) is split into two sublevels 1S1/2(e) and 1S3/2(e) with anti-parallel spins. In this way, as a result of optical
excitation an exciton is created – a bound state of electron and hole, which are attracted to each other by the electrostatic
Coulomb force. The onset of the optical absorption can be considered as the threshold of optical absorption of the CdS
nanocrystals. At each absorption event an exciton appears, consequently because of many different energetic levels there
are many different groups of excitons. The smallest excitonic energy can be determined from the relation (2), and this
energy can be attributed to the absorption threshold of confi ned nanoparticle.
PL mechanism is illustrated in Figure 6. Under UV radiation the electrons from the holes levels 1s, 1p and 1d
are excited to the electrons levels 1s, 1p and 1d. When the photon energy is absorbed in CdS material excitons appear.
In the case of annihilation of these excitons a photon is emitted, according to the transitions 1s(e) - 1s(h), 1p(e)- 1s(h),
1d(e) - 1s(h), etc. In the case of CdS incorporated in the copolymer SBMA matrix, additional to the transitions described
above, there are other transitions with energy transfer from the singlet and triplet levels in the copolymer to the energy
levels of CdS nanocrystal 1s(e), 1p(e), 1d(e) (Figure 6). Because of the dispersion of the size of CdS nanoparticles in the
matrix of SBMA the magnitude of FWHM is a bit larger. Besides, there are PL bands shifted to red, and which can be
related to the defects within the bulk of CdS as well as on the surface of nanocrystalites.
M. Iovu et al. / Chem. J. Mold. 2014, 9(2), 74-79

78
Figure 6. Illustration of the mechanism of energy transfer and photoluminescence in NC.
Conclusions
Copolymer-based nanocomposites made of styrene with butyl methacrylate and inorganic semiconductor CdS
have been investigated. Nanocomposite samples with the concentration of CdS semiconductor 20 and 40% have been
studied. The average CdS particle size estimated from TEM correlates with the particle size estimated from the UV-Vis
absorption spectrum and was found to be in the range 2-10 nm. The nanocomposite samples exhibit a major PL band in
the visible range 350-500 and a weak PL signal in the range 600-800 nm.
Acknowledgement
This research was supported by the Academy of Sciences of Moldova (research grants 11.817.05.03A,
13.820.05.15/RoF and 14.819.02.20A).
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