Controlled Growth of CdS Nanocrystals: Core/Shell viz Matrix

doi:10.4236/jbnb.2011.24050

Paper Menu >>

Journal Menu >>

Journal of Biomaterials and Nanobiotechnology, 2011, 2, 409-413

doi:10.4236/jbnb.2011.24050 Published Online October 2011 (http://www.SciRP.org/journal/jbnb)

409

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: *[email protected]

Received March 28th, 2011; revised May 29th, 2011; accepted July 25th, 2011.

ABSTRACT

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

410

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-

tometer.

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

Controlled Growth of CdS Nanocrystals: Core/Shell viz Matrix411

(a)

(b)

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

[18]:



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

H/PC.

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

Controlled Growth of CdS Nanocrystals: Core/Shell viz Matrix

412

(a)

(b)

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.

REFERENCES

[1] N. Herron, Y. Wang and H. Eckert, “Synthesis and Char-

acterization of Surface-Capped, Size-Quantized CdS

Clusters. Chemical Control of Cluster Size,” Journal of

the American Chemical Society, Vol. 112, No. 4, 1990,

pp. 1322-1326. doi:10.1021/ja00160a004

[2] Y. Wang and N. Herron, “Nanometer-Sized Semicon-

ductor Clusters: Materials Synthesis, Quantum Size Ef-

fects, and Photophysical Properties,” Journal of Physical

Chemistry, Vol. 95, 1991, pp. 525 -532.

doi:10.1021/j100155a009

[3] L. E. Brus, “A Simple Model for the Ionization Potential,

Electron Affinity, and Aqueous Redox Potentials of

Small Semiconductor Crystallites,’’ Journal of Physical

Chemistry, Vol. 79, No. 11, 1983, pp. 5566-5571.

doi:10.1063/1.445676

[4] M. Nirmal and L. Brus, “Luminescence Photophysics in

Semiconductor Nanocrystals,” Accounts of Chemical Re-

search, Vol. 32, No. 5, 1999, pp. 407-414.

doi:10.1021/ar9700320

[5] M. Kundu, A. A. Khosravi, S. K. Kulkarni and P. Singh,

“Synthesis and Study of Organically Capped Ultra Small

Clusters of Cadmium Sulphide,” Journal of Materials

Science, Vol. 32, No. 1, 1997, pp. 245-258.

doi:10.1023/A:1018520425765

[6] X. B. Chen, Y. B. Lou and C. Burda, “Spectroscopic

Controlled Growth of CdS Nanocrystals: Core/Shell viz Matrix

413

Characterization of II-VI Core-Shell Nanoparticles CdSe/

CdS,” International Journal of Nanotechnology, Vol. 1,

No. 1-2, 2004, pp. 105-118.

[7] S. K. Kulkarni, U. Winkler, N. Deshmukh, P. H. Borse, R.

Fink and E. Umbach, “Investigations on Chemically

Capped CdS, ZnS and ZnCdS Nanoparticles,” Applied

Surface Science, Vol. 169-170, No. 1-2, 2001, pp.

438-446. doi:10.1016/S0169-4332(00)00700-5

[8] B. Saraswathi-Amma, K. Manzoor, K. Ramakrishna and

M. Pattabi, “Synthesis and Optical Properties of CdS/ZnS

Coreshell Nnanoparticles,” Materials Chemistry and

Physics, Vol. 112, No. 3, 2008, pp. 789-792.

doi:10.1016/j.matchemphys.2008.06.043

[9] Y. D. Wuab and L. S. Wang, “A Novel Sonochemical

Synthesis and Nanostructured Assembly of Polyvi-

nylpyrrolidone-Capped CdS Colloidal Nanoparticles,”

Journal of Non-Crystalline Solids, Vol. 354, No. 26, 2008,

pp. 2993-3000. doi:10.1016/j.jnoncrysol.2007.12.005

[10] X. G. Peng, M. C. Schlamp, A. V. Kadavanich and A. P.

Alivisatos, “Epitaxial Growth of Highly Luminescent

CdSe/CdS Core/Shell Nanocrystals with Photostability

and Electronic Accessibility,” Journal of the American

Chemical Society, Vol. 119, No. 30, 1997, pp. 7019-7029.

doi:10.1021/ja970754m

[11] B. S. Zou, R. B. Little, J. P. Wang and M. A. El-Sayed,

“Effect of Different Capping Environments on the Opti-

cal Properties of CdS Nanoparticles in Reverse Micelles,”

International Journal of Quantum Chemistry, Vol. 72, No.

4, 1999, pp. 439-450.

doi:10.1002/(SICI)1097-461X(1999)72:4<439::AID-QU

A27>3.0.CO;2-Q

[12] M. A. Hines and P. Guyot-Sionnest, “Synthesis and Char-

acterization of Strongly Luminescing ZnS-Capped CdSe

Nanocrystals,” Journal of Physical Chemistry, Vol. 100,

No. 2, 1996, pp. 468- 471. doi:10.1021/jp9530562

[13] A. Eychmuller, A. Hasselbarth, L. Katsikas and H.

Weller, “Fluorescence Mechanism of Highly Monodis-

perse Q-Sized CdS Colloids,” Journal of Luminescence,

Vol. 48-49, No. 2, 1991, pp. 745-749.

doi:10.1016/0022-2313(91)90232-K

[14] H. Yang and H. P. Holloway, “Enhanced Photolumines-

cence from CdS:Mn/ZnS Core/Shell Quantum Dots,” Ap-

plied Physics Letters, Vol. 82, No. 12, 2003, pp. 4633-

4843. doi:10.1063/1.1563305

[15] P. Verma and A. C. Pandey, “Organic Capping-Effect and

Mechanism in Mn-Doped CdS Nanocomposites,” Phy-

sica B: Condensed Matter, Vol. 405, No. 5, 2010, pp.

1253-1257. doi:10.1016/j.physb.2009.11.060

[16] X. C. Wu, A. M. Bittner and K. Kern, “Synthesis, Photo-

luminescence and Adsorption of CdS/Dendrimer Nano-

composites,” Journal of Physical Chemistry B, Vol. 109,

No. 1, 2005, pp. 230-239.

doi:10.1021/jp046072k

[17] A. Z. Malik, P. O’Brien and N. Revaprasadu, “Synthesis

of TOPO-Capped Mn-doped ZnS and CdS Quantum

Dots,” Journal of Material Chemistry, Vol. 11, No. 9,

2001, pp. 2382-2386.

doi:10.1039/b102709n

[18] A. Guinier and G. Fournet, “Small Angle Scattering of X-

rays,” Journal of Polymer Chemistry, Vol. 19, No. 93,

1956, p. 594 .

[19] L. E. Alexander, “X-ray Diffraction Methods in Polymer

Science,” Journal of Polymer Science Part B: Polymer

Letters, Vol. 9, No. 8, 1970, pp. 635-636.

[20] S. Sapra, J. Nanda, D. D. Sarma, F. A. El-Al and G. Ho-

des, “Blue Emission from Cysteine Ester Passivated Cad-

mium Sulfide Nanoclusters,” Chemical Communications,

Vol. 21, 2001, pp. 2188-2189.

doi:10.1039/b106420g