CdSe Nanoparticles Synthesized in Polymer Matrices

Materials Chemistry and Physics 97 (2006) 288–294
The processing of CdSe/Polymer nanocomposites
via solution organometallic chemistry
P.K. Khanna∗, Narendra Singh, Shobhit Charan, Sunil P. Lonkar,
A. Satyanarayana Reddy, Yogesh Patil, A. Kasi Viswanath
Nanomaterials Laboratory, Centre for Materials for Electronics Technology (C-MET), Panchwati, Off Pashan Road, Pune 411 008, India
Received 8 May 2005; received in revised form 26 July 2005; accepted 9 August 2005
Abstract
This paper presents in situ preparation of CdSe nanoparticles using poly(vinyl alcohol) [PVA] and polymethylmethacrylate [PMMA] as
matrices by use of organoselenium compound. Reaction of cadmium metal salt and 1,2,3-selenadiazole (the source of selenium) enabled
formation of CdSe nanoparticles. Use of selenadiazole in the present work with polymer is ﬁrst of its kind. The radical polymerization of
methycrylate monomer with benzoyl peroxide followed by reaction of respective reagents have been successfully employed to synthesize
CdSe/PMMA nanocomposite. Similarly, reaction between selenadiazole and cadmium metal salt in aq. PVA yielded polymer coated or mixed
CdSe nanoparticles. The UV–vis absorption spectra showed blue shift of about 200 nm with respect to band-gap energy of bulk CdSe, due to
size quantization effect in CdSe particles. An emission band was observed at 530 nm in photoluminescence spectrum (PL) of CdSe/PMMA.
IR spectra indicated shifts in the values of the polymer functional group due to nanoparticles. X-ray measurement of CdSe/Polymer nano-
composites showed broad pattern for cubic CdSe and particle size of CdSe was estimated to be <10 nm. TGA revealed gradual weight loss
between 200 and 400 ◦
C indicating increased thermal stability of the polymer.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Synthesis; Polymer; Semiconductor; Nanocomposite; XRD
1. Introduction
High quality nanocrystals of semiconductors are required
for their utilization in a range of application that include;
luminescent devices, biological markers, lasers and catalysis
[1]. There is currently a great interest in II–VI semiconduc-
tor particles, particularly organically capped soluble particles
of cadmium sulphide and selenide or embedded in poly-
meric matrices for their ready to use application in devices
[2–6]. Normally, organically-capped semiconductor particles
when re-dissolved in organic solvents, have limitations in
complete dissolution due to increased crystallinity during
the work-up of colloidal particles. This, thus restrict their
use in optoelectronics. The embedding of particles in poly-
mers has advantage as the loading of particles is typically
∗ Corresponding author. Tel.: +91 20 25899273; fax: +91 20 25898180.
E-mail addresses: pkk@cmetindia.org,
pawankhanna2002@yahoo.co.in (P.K. Khanna).
much less than the content of polymer (w/w) thus parti-
cles are well protected by thick surroundings. This leads to
well controlled size and reduced crystallinity of the parti-
cles. The solubility or dissolution of such polymeric matrix
based nanoparticles offer better possibility for their direct use
in devices. CdSe nanoparticles have been reported by several
methods [7–9]. Amongst the reported synthesis, the nanopar-
ticles of CdSe below 20 nm size (quantized particles) are most
effectively prepared by use of organometallic precursors via
solution chemistry. There are a number of reports on “single
source” precursors of cadmium and selenium for prepara-
tion of these nanoparticles but these methods are often quite
pains-taking [10,11]. Similarly, high quality CdSe nanopar-
ticles have been reported by use of dimethyl cadmium and
its reaction with trioctylphosphine selenide (TOPSe) but this
needs much higher temperature (about 300 ◦C). Though the
preparation of bulk cadmium selenide is rather simple [12],
the isolation of nanoparticles of mono-dispersed CdSe with a
narrow size distribution is highly challenging. For application
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2005.08.012

P.K. Khanna et al. / Materials Chemistry and Physics 97 (2006) 288–294 289
in optoelectronics, normally the polymeric matrices are the
most desirous as the film of the nano particles/polymer com-
posite can be cast easily and thus offer direct use in devices.
The re-dispersed nanoparticles when loaded in polymer by
dissolution of the as-synthesized particles because solubility
(in terms of re-dispersion) of nanoparticles can not be guar-
anteed. This thus leads to difficulties in knowing the concen-
tration of nanoparticles within the polymer. However, if the
nanoparticles are synthesized in polymeric reaction medium,
such issues can be tackled with much ease. In addition,
multi-stage loading of the particles can be avoided. The most
important requirement of course, is the use of suitable pre-
cursors, i.e. the precursors that can be easily converted to end
product during the reaction. Organometallic precursors are
known to offer these advantages. Release of selenium upon
thermolysis of 1,2,3-selenadiazoles has been well studied and
widely utilized in synthetic chemistry [13,14]. Recently, we
reported that the reaction of cycloalkeno-1,2,3-selendiazole
with metal salts, can lead to formation of fine powders of
CdSe and silver selenide [12,15–17]. Our initial efforts on
use of 1,2,3-selenadiazole as a source of selenium for semi-
conductor particles prompted us to test its suitability for
trappingtheseparticlesinpolymers.Inthepresentstudy,PVA
and PMMA is employed as surfactant-cum-stabilizer-cum-
matrix for CdSe nanoparticles after their in situ synthesis.
This, we believe, is a novel thought for generating nanopar-
ticles in polymer. Therefore, in the current work, reaction
of cadmium acetate with polymer followed by its reaction
with cycloocteno-1,2,3-selenadiazole has been successfully
attempted to generate a one-pot novel method for synthesis
of CdSe nanoparticles in the film and powder form trapped
in polymer.
2. Experimental
2.1. General
Cycloocteno-1,2,3-selenadiazole (4,5,6,7,8,9-hexahydro-
cycloocta-1,2,3-selenadiazole) was prepared by literature
method [13]. UV–vis spectra were recorded in DMF on a
Hitachi 3210 UV–vis spectrophotometer. Scanning electron
microscopy (SEM)/EDAX was done on a Philips XL-30
instrument. Powder X-ray diffraction patterns were mea-
sured using Cu K␣ radiation (λ = 1.5406 ˚A) on a Mini Flex
Rigaku Philips Model. The thermo gravimetric analysis of
the nanocomposite was carried out on a Perkin-Elmer Model
no. 7. The photoluminescence (PL) measurement were done
at Shimadzu centre at the University of Pune.
2.2. In situ synthesis of CdSe nanocrystals in poly(vinyl
alcohol)
PVA (1g) was taken in two-necked round bottom flask
in DMF and stirred at room temperature under nitrogen
atmosphere for 1 h. To it were added, cadmium acetate and
octeno-1,2,3-selenadiazole (in DMF) in 1:1 ratio at room
temperature. The reaction mixture was heated to 140 ± 5 ◦C.
The initial orange colloidal solution was obtained at about
80 ◦C, which indicated formation of nanosized CdSe by mon-
itoring the reaction by absorption spectroscopy. The reaction
was continued for a couple of hours. After cooling it down
to room temperature, it was centrifuged and washed with
toluene, methanol and finally with diethyl ether to obtain a
light brown powder.
2.3. In situ synthesis of CdSe nanoparticles in
polymethylmethacrylate [PMMA]
Polymethyl methacrylate (PMMA) 10 mL stock solution
(10 g/50 mL in DMF stored as stock solution under N2 atmo-
sphere) was taken in a round bottom Schlenk flask and the
volumewasincreasedtoabout30 mL.ThePMMAsolutionin
DMF was degassed and flushed with N2. To it, both cadmium
acetate (0.266 g, dissolved in 10 mL DMF) and cycloocteno-
1,2,3-selenadiazole (0.215 g dissolved in 10 mL DMF) were
added through a syringe. The reaction mixture was stirred
at 80–100 ◦C for 8 h under N2 atmosphere. The colour of
reaction mixture became slightly brown in about 2 h. The
reaction was continued for additional 5–6 h to obtain a sharp
absorption band. The heating was continued for another 2 h
to complete the reaction. Subsequently a brown transparent
film was cast by vacuum evaporation technique.
3. Results and discussion
Selenadiazole can be thermally converted to either (1)
cyclooctyne via extrusion of Se atom, or (2) 1,4-diselenin
due to dimerization of an intermediate [13]. The extrusion
lead to generation of highly reactive selenium, and this has
been exploited in the present work. In our earlier efforts, we
found that once the selenium is heated in any high boiling
solvent such as DMF and ethylene glycol, the released Se
immediately reacts with the metal ion present in the reac-
tion vessel producing bulk powders. In order to isolate CdSe
nanocrystals, we designed our experiments in such a way that
the released selenium should not react in the first place with
the metal ion.
Continued heating of selenadiazole in dipolar aprotic sol-
vent such as DMF containing PVA and cadmium salt resulted
in reddish colloidal solution, which by centrifugation, and
organic washings afforded brown colored powder of CdSe.
Schemes 1–3 shows a schematic mechanism of release of
selenium from selenadiazole and formation of CdSe/PVA
nanocomposite via reaction of either Cd-salt or Se with
hydrogen ions generated by the solvent. Similarly, the reac-
tion of Cd-salt with methylmethacrylate monomer in pres-
ence of a catalyst for polymerization, resulted in an orange
to red suspension. It is opined that the initial reaction of cad-
mium acetate with acrylate monomer may result in formation
of cadmium acrylate intermediate that subsequently polymer-

290 P.K. Khanna et al. / Materials Chemistry and Physics 97 (2006) 288–294
Scheme 1. Mechanism for release of Se from selenadiazole.
Scheme 2. Proposed mechanism for formation of CdSe.
izes and reacts with the selenadiazole to generate polymer
embedded CdSe (Scheme 4).
UV–vis spectrum of CdSe/PVA and CdSe/PMMA reac-
tion solutions prepared following above methodology are
shown in Fig. 1(i) and (ii). The solution absorption spectrum
of CdSe/PVA nano-composite in DMF showed a sharp band
at about 550 nm (band gap 2.25 eV) suggesting a blue shift
in absorption wavelength with respect to bulk CdSe and the
pattern indicate that the particles have narrow size distribu-
tion in solution (Fig. 1(i)) however, the absorption spectrum
of CdSe/PMMA showed band at about 510 nm, indicating
that the PMMA is better than PVA for controlling particle
growth of CdSe (Fig. 1(ii)). The absorption bands of solution
and the film do not differ in their position thus indicating that
even after slight heating (for casting film) the particle size
distribution remain un-disturbed. This could be very useful
when considered for application of such composites in opto-
electronics. The band gap of bulk CdSe is reported to be
712 nm (1.74 eV) [18]. The present measurements show that
there is a blue shift of about 150–200 nm in comparison to the
band gap energy of bulk CdSe. The blue shifts in the absorp-
tion spectrum of semiconductors is observed due to quantum
confinement effect leading to increased energy-gap between
the highest occupied molecular orbitals (HOMO) and lowest
un-occupied molecular orbitals (LUMO) in semiconductors.
The absorption measurements thus show that octeno-1,2,3-
selenadiazole is a good source of selenium for preparation of
CdSe nanoparticles in PVA and PMMA.
The photoluminescence spectrum (Fig. 2) of
CdSe/PMMA film that was cast from the solution whose
UV–vis absorption spectrum is shown in Fig. 1(ii), gave a
peak at 530 nm at an excitation wavelength of 450 nm. The
absorption of this composite solution and film showed a band
at 510 nm thus a Stoke’s shift of about 25 nm is observed
in comparison to absorption band. Such Stokes shifting of
emission wavelength is a common feature of semiconductor
quantum dots. The bathochromic shift with reference to
UV–vis absorption value suggests that the emission arises
from re-combination from shallow trap states [11].
In the FTIR spectrum of pure PVA showed a band at
ν1375 cm−1 was normally observed due to the coupling
of O H vibrations at ν1420 cm−1 with the C H wagging
vibrations. The FTIR spectrum of CdSe/PVA nanocompos-
ite (Fig. 3(i)) show an increase in the transmittance and a
Scheme 3. Proposed mechanism of an in situ synthesis of CdSe nanoparticles in PVA.

Scheme 4. Proposed mechanism of an in situ synthesis of CdSe nanoparticles and polymerization of MMA monomer (represented in unit chain of polymer).
Fig. 1. UV–vis of (i) CdSe/PVA solution and (ii) CdSe/PMMA solution (a) and film (b).
Fig. 2. Photoluminescence spectrum of CdSe/PMMA film.
slight shift (about 20–25 cm−1) of the band at ν1375 cm−1,
whichindicatesthedecouplingbetweenO HandC Hvibra-
tions due to bonding interaction between O H and CdSe
nanoparticles. FTIR spectrum of PVA coated CdSe shows all
relevant peaks for PVA. The characterisitic peaks for –OH
is found to be at ν3375 cm−1 which is slightly shifted from
its original value of about ν3350 cm−1 in PVA alone. Similar
observation and findings have been reported for silver/PVA
nanocomposite and this could considered relevant to the IR
spectrum of CdSe/PVA [19]. Similarly, the IR measurement
of CdSe/PMMA (Fig. 3 (ii)) showed that the peaks have
become broad and these are shifted in their wave numbers
with respect to their original value in PMMA. The IR spec-
trum in transmission mode showed very broad peaks however
when the same was recorded in absorption mode, all peaks
due to PMMA were found to be present with slight broaden-

Fig. 3. FTIR spectra (i) of CdSe/PVA powder (Transmittance mode) and (ii) of CdSe/PMMA film (absorbance mode).
Fig. 4. XRD of CdSe/PVA nanocomposite powder.
ing and the peak at ν1723 cm−1 is assigned to the ester group
of the PMMA. This peak is slightly shifted in the compos-
ite than the peak reported for PMMA alone at ν1728 cm−1
[20]. This indicate that the composite may have some chem-
ical bonding between the polymer and the surface of CdSe
particles.
Fig. 4 (XRD) shows peaks characteristic of cubic CdSe
[21]. All peaks of Bragg’s scattering in XRD of PVA/CdSe
nanocmposite are observed. The reflections at 1 1 1, 2 0 0,
2 2 0, 3 1 1, 2 2 2 are broad pattern due to size quantization
effect. It is well reported that a broad XRD pattern is sup-
portive of small particle size [21]. This is due to the fact
that during the reduction in particle size, the crystallinity of
the semiconductor particle is marginally lost and therefore
Bragg’s reflections are poor in such small particles. Calcula-
tion of particles size by use of Scherrer equation resulted in
a size of about <5 nm (Table 1a).
Table 1
Calculation of particle size and lattice spacing from XRD patterns of PVA/CdSe and PMMA/CdSe powders (from Figs. 4 and 5)
2θ (degrees) θ (degrees) Crystal planes Full width at half maximum (FWHM) aParticle size (nm) bLattice spacing ( ˚A)
(a) PVA/CdSe
25.66 12.80 1 1 1 3.118 2.6 3.476
42.08 21.04 2 2 0 2.203 3.8 2.144
49.67 24.80 3 1 1 1.734 5.0 1.835
(b) PMMA/CdSe
25.58 12.79 1 1 1 2.30 7.9 3.484
42.02 21.01 2 2 0 1.28 6.6 2.147
49.68 24.84 3 1 1 2.06 4.2 1.833
a Calculation done by Scherrer Equation, i.e. t = 0.9λ/β cos θ.
b Calculated by, i.e. d = n λ/(2sin θ) where n = 1.
Fig. 5. XRD of CdSe/PMMA nanocomposite powder.
Fig. 6. XRD of CdSe/PMMA nanocomposite film.
Figs. 5 and 6 show XRD pattern of the CdSe/PMMA pow-
der and film, respectively. It is seen from the figures that
in case of powder where the concentration of CdSe with
respect to polymer is about 20% (w/w), the XRD pattern
is very clear with peaks characteristic of cubic CdSe. All
peaks of Bragg’s scattering in CdSe/PMMA nanocmposite

are observed with broadening and the particles size calcu-
lation by Scherrer’s equation resulted in crystallite size of
about <10 nm (Table 1b). This is again due to size quantiza-
tion effect.
The reflections are further poorer in the XRD pattern of the
film that only gave a very broad pattern typically of amor-
phous characteristics (Fig. 6). As the peak at 2 theta at 42
could be seen in the figure, it suggested that the film has
some crytallinity.
TGA (Fig. 7) analysis showed a two-stage decomposi-
tion pattern for the CdSe/PVA nano-composite. Initial weight
loss at about 256 ◦C can be considered due to decomposi-
tion of PVA. The second-stage weight loss at about 820 ◦C
may be due to possible initial decomposition of CdSe. A
total of about 55% wt. loss was observed when the analy-
sis was done between room temperature to 900 ◦C at a rate
of 10 ◦C min−1 under nitrogen atmosphere. Similarly, TGA
(Fig. 8) of CdSe/PMMA analysis also showed a two-stage
decomposition pattern as against a three-step decomposition
pattern for PMMA alone. This indicated that the thermal sta-
bility of CdSe/PMMA nano-composite has been altered. The
analysis curve shows that the decomposition starts at about
200 ◦C and continues till 400 ◦C.
Fig. 7. TGA of PVA/CdSe film.
The CdSe/Polymer nanocomposites were analyzed by
scanning electron microscopy, which revealed that in case
of CdSe/PMMA film, tiny particles are spread across the
matrix (Fig. 9). There appears to be un-evenness in the film
morphology. This could be due to crystalline nature of the
particles present in the polymer. It was not possible to obtain
good quality SEM of CdSe/PVA due to charging of the
composite during analysis. Nonetheless, it is seen from the
picture (Fig. 9b) that the spherical particles are present in
Fig. 8. Thermogram of (a) PMMA and (b) CdSe/PMMA nanocomposite film.
Fig. 9. SEM pictures of (a) CdSe/PMMA and (b) CdSe/PVA nanocomposites.

the matrix. The quality of the film that was obtained from
PMMA appears to be better than PVA and this further sug-
gests that PMMA is a better matrix in the present work than
PVA.
4. Conclusions
It is shown in the present work that cycloocteno-1,2,3-
seplenadiazolecanbesuccessfullyusedassourceofselenium
in synthesis of CdSe nanoparticles embedded in polymers.
The optical and other characterization shows that the band-
gap energy of CdSe is increased and that the final nanocom-
posite has cubic phase of CdSe. TGA suggest that the thermal
stability of the polymers has increased due to presence of
nanoparticles.
Acknowledgments
We thank Executive Director, C-MET for encouragement
and permission and DST (Govt. of India) for financial support
through grant no. SP/S1/H-34/99-Part-II. P.K.K. thanks, Mrs.
Rina Gorte, for initial experimental assistance.
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