Journal of Biomaterials and Nanobiotechnology, 2011, 2, 409-413
doi:10.4236/jbnb.2011.24050 Published Online October 2011 (
Copyright © 2011 SciRes. JBNB
Controlled Growth of CdS Nanocrystals:
Core/Shell viz Matrix
Prinsa Verma1,2*, Avinash C Pandey1
1Nanophosphor Application Center, Allahabad University, Allahabad, India; 2Satish Dhawan Space Center, India Space Research
Organization, Sriharikota, India.
Email: *
Received March 28th, 2011; revised May 29th, 2011; accepted July 25th, 2011.
The ability to precisely control the size of semiconductor nanocrystals can create an opportunity for producing functional
materials with new properties, which are of importance to applications such as Light emitting diodes, b io medical diagnosis,
solar cells, and spintronics. And size of nanoparticle can be controlled with efficient capping agent. For the same purpose
we reported, two types of capping, one will lead to nanomatrix and other to Shelled nanostructures. Enhancement in emis-
sion intensity observed with Shell nanostructures compare to matrix. PVP is used to control the particle size, to preven t ag -
glomeration and making thin films. A blue shift in energy level at the nanoscale is demonstrated by optical absorption. Elec-
tron microscopy studies with an SEM and TEM show a particle size of 10 nm and 15 nm. We also investigated the particle
size distribution of nanoparticles by small angle scattering (SAXS) study.
Keywords: Nanomatrix, Shell and Band Edge Emission
1. Introduction
Semiconductor quantum dots with diameters less than
10 nm have evoked great interest for intense investiga-
tions [1-4]. The number of atoms in such quantum dots
ranges from a few tens to hundreds of atoms. These
clusters of atoms exhibit the size effect with unique elec-
tronic and optical properties that depend upon the size of
the cluster. The surface to bulk atom ratio also changes
rapidly with size leading to drastic changes in thermal
and mechanical properties [5]. A 5 nm CdS cluster has
approximately 15% of the atoms on the surfaces. The
existence of this vast interface between the cluster and
the surrounding medium can have a profound effect on
the cluster properties [2]. As the size of CdSe particles
decrease from 10 nm to 1 nm, the percentage of surface
atoms increases from 20 to practically 100%. The surface
atoms usually have unsaturated bonds or dangling bonds.
These atoms have extra free energy and are more active
than those in the bulk [6]. The size dependent properties
appear when the radius of the particle is comparable to
the Bohr radius of the exciton in the bulk material. As
the particle size decreases the energy gap widens and the
absorption edge shifts towards higher energy side ac-
companied by the appearance of a strong excitonic peak.
By controlling the particle size the forbidden gap in the
semiconductor can be tuned and its optical and electronic
properties can be tailored so that they can be used in
many applications like photocatalysis, flat panel displays,
optoelectronic devices, molecular level quantum com-
puters, single electron devices etc [7].
For controlling particle size capping is required either
with organic or inorganic. A coating shell is built up around
the core materials to increase their chemical stability,
intensify their functions, improve their biocompatibility
compared with bare core materials, and confer specific
properties, such as optical, magnetic or mechanical
properties on the core materials [8,9]. Organic and inor-
ganic material can both used as shell material. Semi-
conductor nanocrystals capped with organic molecules
can have a relatively large number of unpassivated sur-
face sites as it is difficult to passivate both anionic and
cationic surface sites simultaneously by these capping
groups [10]. These unpassivated surface sites may act as
non-radiative recombination centres which suppress their
luminescence. In addition, organically capped nanocry-
stals have very long emission lifetimes and large Stoke
shifts [11] whereas inorganically capped nanoparticles
exhibit enhanced luminescence efficiencies [12] and
shorter lifetimes [13]. The epitaxial growth of inorganic
cap on the nanocrystals can eliminate both the anionic
Controlled Growth of CdS Nanocrystals: Core/Shell viz Matrix
and cationic surface dangling bonds [10]. Zou, et al. [11]
have studied the effectiveness of various inorganic cap-
ping agents having different band gaps on the surface
passivation of cadmium sulphide (CdS) nanoparticles.
They have reported that it is possible to block the nonra-
diative channels on the surface of these nanoparticles by
capping them with wider band gap inorganic materials
like Cd(OH)2 and zinc sulphide (ZnS). It is also reported
that ZnS is more effective than Cd(OH)2 in surface pas-
sivation because of its better charge and size compatibil-
ity with CdS, resulting in increased band edge emission.
Growth of a wide band gap semiconductor ZnS on the
surface of a narrower band gap semiconductor CdS,
forming CdS/ZnS coreshell nanoparticles, leads to ap-
preciable passivation resulting in enhancement of pho-
toluminescence (PL) emission [14].
We reported synthesis of Mn2+ doped PVP-CdS nano-
matrix and CdS/ZnS core/shell nanostructures through
wet chemical approach. We capped CdS with both or-
ganic and inorganic capping agents. As organic is re-
sponsible for stroke’s shift where as inorganic gives bet-
ter enhanced luminescence efficiencies so we can choose
capping as per our requirement. After shell formation by
ZnS, monodisperse CdS nanoparticles were prepared,
which exhibited significantly enhanced luminescence
and high chemical stability. PVP role as a capping agent
is already discussed in our earlier paper [15].
2. Experiment Procedures
2.1. Sample Preparation
All the reactants and solvents were analytical grade. Cad-
mium acetate dihydrate Cd(CH3COO)2·2H2O (98%),
manganese acetate Mn(CH3COO)2 (98%), sodium sul-
fide (Na2S), zinc acetate Zn(CH3COO)2 (99.99%), were
purchased from Aldrich and used to synthesize or passi-
vate the CdS:Mn nanocrystals. Each (Cd2+, Zn2+, Mn2+)
and S2– containing standard aqueous solution was pre-
pared by dissolving Cd(CH3COO)2·2H2O, Zn(CH3COOH)2,
Mn(CH3CO O)2, and Na2S in water. The concentrations
of Cd2, Zn2+ and S2- in water were 0.1 M and the ratio of
Mn2+ to Cd2+ was fixed to 2 mol%.
Here we are going to report two synthesis processes.
In a typical preparation process 1.5 ml of 0.1M
Mn(CH3CO O)2 and 0.05 g PVP was dissolved in 25 ml
of 0.1M Cd(CH3COO)2·2H2O aqueous solution followed
by 15 ml of 0.1M sodium sulphide with continuous stir-
ring until a yellow solution of PVP capped CdS nanoma-
trix were formed. Where as in other set 2) 1.5 ml of 0.1M
Mn(CH3COO) 2 and 0.05 g PVP was dissolved in 25 ml
of 0.1M Cd(CH3COO)2·2H2O aqueous solution, then
15 ml of 0.1M sodium sulphide is added followed by
addition of 25 ml of 0.1M Zn(CH3COOH)2 with con-
tinuous stirring for favourable synthesis of Core/Shell
nanostructurs. Sequential addition of S2 and Zn2+ ions to
the (CdS)Mn core solution formed core/shell structures.
The obtained yellow solution was stored at roomtempe-
rature for measurements of optical absorption and pho-
toluminescence properties. Core/shell is a result of reac-
tion between Zn(CH3COOH)2 and excess Na2S, which
will lead to deposition of ZnS as a shell on the formed
core CdS nanoparticle.
2.2. Influence of Organic Capping
Two coordinating groups, nitrogen and carboxyl are pre-
sent in Poly Vinyl Pyrrolidone (PVP). Oxygen present in
PVP molecule makes coordinate bond with the Cd ions
whereas the lone pair of electrons on nitrogen in pyr-
rolidone is conjugated with the adjacent carbonyl group
and remaining oxygen in carboxylate makes coordinate
bond with the Mn ions. In PVP-CdS samples, we expect
a similar bonding at the nanoparticles, where in C=O-Cd+2
and C=O-Mn2+ bonds which can give rise to overlapping
of molecular orbitals of PVP with atomic orbitals of metal
ions. PVP as a capping agent plays a significant role not
only to increase the stability but also for the effective
doping of Mn into the CdS nanophosphors, which can be
attributed to the formation of coordinate bonding groups
between lone pair of oxygen atoms and those of metallic
atoms Cd and Mn in the CdS nanocomposite [15].
2.3. Characteristics Measurements
Transmission electron microscopic (TEM) photographs
were taken on Technai 30 G2 S-Twin. Ultraviolet–visi-
ble (UV-Vis) measured on Perkin–Elmer Mc Pherson
2035, Small angle scattering pattern were obtained by a
Rigaku D/max-2200 PC diffractometer operated at 40
kV/20 mA and Photoluminescence spectrophotometer
were measured on a Perkin–Elmer LS 55 spectropho-
3. Result and Discussion
3.1. TEM Analysis
Sample preparation for TEM-Samples in mg were dis-
solved in ethanol and sonicated for 2 hrs then one drop of
this sample were taken on carbon coated copper grid, and
it was dried in oven for 24 hrs at 50˚C. Magnification is
200 KeV.
Figures 1(a) and (b) shows a typical TEM images of
the CdS/ZnS core/shell nanoparticles and PVP-CdS na-
nomatrix. Nanomatrix which are shown in Figure 1(b) was
estimated to be 15 nm but they showed non uniform dis-
tribution. Where as core/shell CdS/ZnS nanoparticles are
well dispersed and estimated to be 10 nm (Figure 1(a)).
3.2. UV-Vis Spectra
Pure PVP solution exhibited nearly no absorption in the
opyright © 2011 SciRes. JBNB
Controlled Growth of CdS Nanocrystals: Core/Shell viz Matrix411
Figure 1. TEM images of (a) CdS/ZnS core/shell (b) PVP-
CdS matrix.
selected region. The generation of CdS nanoparticles
could also be identified from both the color change and
the UV-Vis spectrum of the as-prepared products. For
bulk CdS and ZnS, the absorption edges are at 500 and
335 nm, corresponding to energy gaps 2.5 and 3.7 eV,
respectively [16]. The obtained yellow solution showed
an absorption onset at about ~415 nm in nanomatrix and
~400 nm in core/shell (Figure 2), blue shift in the ab-
sorption edge from the corresponding bulk value, implies
the quantum confinement effect of the CdS nanoscale
particles [17].
3.3. Particle Size Distribution from SAXS Study
Figure 3 shows the small-angle x-ray scattering pattern
from PVP-ZnS nanomatrix and CdS/ZnS core/shell. The
scattering intensity I (s) from the nanoparticles follows
the Guinier small-angle scattering intensity equation
exp 3
IsI MnsR
Here Ie, M, n, s and R0 are scattering intensity per elec-
tron, the number of grain, the number of electron per
grain, scattering vector and inertial radius respectively.
The particle size distributions of the nanoparticles could
be calculated by simply plotting tangents on the curve of
Figure 2. Uv-Vis absorption spectrum of (a) PVP-CdS ma-
trix (b) CdS/ZnS core/shell.
Figure 3. SAXS pattern of CdS nanoparticle s.
the graph log I-s2 ( Fankuchen Method ) [19]. Figures
4(a) and (b) shows Guinier plot of scattered intensity
from the CdS/ZnS core/shell and PVP-CdS nanomatrix.
Particle size distribution of CdS/ZnS core/shell by analy-
sis of Guinier plot of scattered intensity is 81.3 wt% par-
ticles of 10 nm size, 14.5 wt% particles of 19.81 nm and
4.2 wt% of 57.9 nm. Particle size distribution of PVP-
CdS nanomatrix by analysis of Guinier plot of scattered
intensity is 73.3 wt% particles of 15 nm size, 22.9 wt%
particles of 25.39 nm and 3.8 wt% of 35.86 nm. This
particle size distribution calculation was done by grain
size analysis program provided by Rigaku D/Max-2200
3.4. PL Spectra
Figure 5 shows photoluminescence spectra of the PVP-
CdS nanomatrix and CdS/ZnS core/shell nanoparticles.
CdS/ZnS core/shell nanoparticles shows enhanced emis-
sion peak compared with PVP-CdS matrix. The strong
Copyright © 2011 SciRes. JBNB
Controlled Growth of CdS Nanocrystals: Core/Shell viz Matrix
Figure 4. Guinier plot of scattered intensity from (a) CdS/
ZnS core/shell (b) P VP- CdS nanomatr ix.
Figure 5. Photoluminescence spectra (a) CdS/ZnS core/shell
(b) PVP-CdS matrix.
luminescence demonstrated the influence of surface im-
provement. The sharpness of the emission peak showed
the monodispersion of the sample [20], as also observed
from TEM.
4. Conclusions
Our approach is to synthesis material for device purpose,
having controlled growth, enhanced luminescence. The
results of the studies carried out on the effect of organic
and inorganic capping over CdS nanoparticles are pre-
sented in this article. One will lead to core/shell struc-
tures and other to matrix by simple wet chemical method.
Complete coverage of the CdS core with a wider energy
gap ZnS, enhanced the PL performance of the CdS core.
We also investigated the particle size distribution of CdS
nanoparticles by small angle scattering (SAXS) study.
TEM, SAXS, UV and optical characterization showed
that nanoparticles showed perfect surface passivation,
regular shape and well defined or nearly monodispersed
nanoparticles after the inorganic shell formation over the
CdS core compared to organically capped CdS. Efficient
capping can be achieved by the proper choice of the cap-
ping agent for a given application.
5. Acknowledgements
The authors would like to thank all members of Nano-
phosphor Application Center, Allahabad University.
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