ZnS coreshell nanoparticles

ZnS coreshell nanoparticles

Materials Chemistry and Physics 112 (2008) 789–792 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.e...

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Materials Chemistry and Physics 112 (2008) 789–792

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Synthesis and optical properties of CdS/ZnS coreshell nanoparticles B. Saraswathi Amma a , K. Manzoor b , K. Ramakrishna a , Manjunatha Pattabi a,∗ a b

Department of Materials Science, Mangalore University, Mangalagangotri 574199, India Amritha Institute of Medical Sciences, Cochin, India

a r t i c l e

i n f o

Article history: Received 17 August 2007 Received in revised form 19 June 2008 Accepted 21 June 2008 Keywords: Chalcogenides Chemical synthesis Optical properties Luminescence

a b s t r a c t Synthesis and optical properties of manganese (Mn2+ )-doped, polyvinyl pyrrolidone (PVP)–capped cadmium sulphide (CdS) nanoparticles coated with zinc sulphide (ZnS) are reported. Colloidal solution of Mn2+ -doped CdS nanoparticles capped with PVP is synthesized using methanol as solvent. PVP is used to control the particle size and to prevent agglomeration. Mn2+ doping is expected to help in increasing the CdS band edge photoluminescence (PL) emission. Addition of zinc nitrate and sodium sulphide alternately to the Mn2+ -doped, PVP-stabilized CdS colloid led to the formation of ZnS-coated CdS coreshell nanoparticles. Photoluminescence emission spectra recorded for (CdS–PVP)Mn nanoparticles showed two emission peaks, one at 416 nm and the other weaker peak at 586 nm which is attributed to Mn2+ emission. Intensity of Mn2+ peak increased with increase in the Mn2+ content. Mn2+ emission disappears when ZnS is coated over the CdS core, resulting in pure CdS band edge emission. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor nanocrystals capped with organic molecules can have a relatively large number of unpassivated surface sites as it is difficult to passivate both anionic and cationic surface sites simultaneously by these capping groups [1]. These unpassivated surface sites may act as non-radiative recombination centres which suppress their luminescence. In addition, organically capped nanocrystals have very long emission lifetimes and large Stoke’s shifts [2] whereas inorganically capped nanoparticles exhibit enhanced luminescence efficiencies [3] and shorter lifetimes [4]. The epitaxial growth of inorganic cap on the nanocrystals can eliminate both the anionic and cationic surface dangling bonds [1]. Inorganic passivation is, therefore, more effective than organic passivation for eliminating surface defect sites. It is reported that fluorescence intensity can be increased by covering the nanoparticle surface with Cd(OH)2 [5]. Zou et al. [2] have studied the effectiveness of various inorganic capping agents having different band gaps on the surface passivation of cadmium sulphide (CdS) nanoparticles. They have reported that it is possible to block the non-radiative 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 passivation because of its better

∗ Corresponding author. Fax: +91 824 2287367. E-mail address: [email protected] (M. Pattabi). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.06.043

charge and size compatibility 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 appreciable passivation resulting in enhancement of photoluminescence (PL) emission [6]. It is reported that growth of a CdS/ZnS graded shell on CdSe rods increased quantum efficiencies [7]. Synthesis of highly luminescent photostable coreshell nanocrystals of CdSe/CdS [1] and CdSe/ZnS [8] has been reported and the later has been used as fluorescent biological labels [9]. Doped semiconductor nanocrystals are extensively investigated to obtain basic information on impurity states in quantum dots and to examine their potential applications in novel light-emitting devices [8,10]. Yang et al. [11] have reported the synthesis of Mndoped CdS/ZnS coreshell nanocrystals with a room temperature Mn-emission quantum yield of 56%. Bhargava et al. [12] and Bhargava [13] have reported that doped semiconductor nanocrystals can yield high luminescence efficiencies. The doping of Mn2+ into II–VI semiconducting nanoparticles potentially gives rise to a new class of luminescent materials with a wide range of applications in displays, sensors and lasers. An appreciable enhancement in the PL property of ZnS-coated CdS nanoparticles in a polymer polycetyl-pvinylbenzyldimethylammonium chloride (PCVDAC) has also been reported [14]. We have earlier reported [15] that PVP helps to quench the surface defect-related emission in CdS, but with a slight reduction in the PL emission intensity. In the present study, the synthesis and characterization of Mn2+ -doped, PVP-capped CdS/ZnS coreshell nanocrystals is reported.


B. Saraswathi Amma et al. / Materials Chemistry and Physics 112 (2008) 789–792

2. Experimental Cadmium nitrate and sodium sulphide solutions in methanol (3.2 mM) were taken as precursors for the synthesis of the CdS core. Manganese acetate was used as the Mn2+ source to dope the CdS particles and polyvinyl pyrrolidone (PVP) as the stabilizer. 25 ml of cadmium nitrate solution was taken in a conical flask and solutions of manganese acetate (2 ml of 2 g wt.% or 5 g wt.%) and PVP (10 ml of 0.5 g wt.%) in methanol were added to this and stirred. 15 ml of sodium sulphide was then added with continuous stirring until a yellow solution of (CdS–PVP)Mn colloid formed. A solution of (CdS–PVP) was also prepared without adding manganese acetate for comparison. 2.5 ml of zinc nitrate in methanol (2 g wt.%) was added drop-wise to (CdS–PVP)Mn solution, maintaining the reaction temperature at 75 ◦ C, followed by the addition of 10 ml sodium sulphide solution. The percentage of zinc was varied in the precursor solution to vary the ZnS content in the coreshell. The % ZnS given refers to the Zn concentration in the precursor solution. Sequential addition of Zn2+ and S2− ions to the (CdS–PVP)Mn core solution formed coreshell structures. Optical absorption studies were carried out on the nanoparticle colloids using a UV-Vis-NIR scanning spectrophotometer (SHIMADZU UV-3101PC) with solvent methanol as reference solution. Photoluminescence studies were also carried out on the colloidal solution using a JASCO-FP-6500 spectrafluorometer. A thin film formed by spreading the solution over a glass plate was used for X-ray diffraction (XRD) studies using a BRUKER D8 Advance X-ray Diffractometer with Cu K␣ radiation (1.54 Å). Transmission electron micrographs (TEM) were recorded using a Philips CM12 transmission electron microscope.

3. Results and discussion Fig. 1 is the optical absorption spectra of (CdS–PVP), (CdS–PVP)Mn and (CdS–PVP)Mn/ZnS coreshell nanoparticles. A red shift from the (CdS–PVP) optical absorption edge is observed for the Mn-doped CdS nanoparticles. With the addition of ZnS, the absorption edge shifts to a wavelength which is lower than that of the (CdS–PVP). 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]. Blue shift in the absorption edge from the corresponding bulk value (Fig. 1), implies the quantum confinement effect of the CdS nanoscale particles [17]. Lu et al. [14] have reported different optical absorption spectra for the mixed system of CdS and ZnS and CdS/ZnS coreshell structure. Apart from the formation of coreshell structures, isolated ZnS nanoparticles at the surface of CdS nanoparticles and Znx Cd1−x S (0 < x < 1) alloys may also form, during the preparation of such binary compounds. Controlling the experimental parameters can avoid unwanted structures [18]. The absence of an exciton peak or another steep edge in the UV region of the optical absorption spectra excludes the possibility of formation of isolated ZnS nanoparticles. The absorption edge of (CdS–PVP)Mn/ZnS shifts to lower wavelengths from that of CdS–PVP alone as seen in Fig. 1. But with increasing ZnS content in the sample, there is a regular

Fig. 1. Optical absorption spectra for (CdS–PVP), (CdS–PVP)Mn and (CdS–PVP) Mn/ZnS coreshell nanoparticles.

Fig. 2. Optical absorption spectra for (CdS–PVP)Mn/ZnS at various concentrations of ZnS.

red shift of the band edge due to partial leakage of exciton into the ZnS layer (Fig. 2). However, the absorption edges are still at lower wavelengths compared to CdS–PVP alone. Peng et al. [1] and Dabbousi et al. [8] have also observed similar behaviour for CdSe/CdS and CdSe/ZnS coreshell structures, respectively. The red shift of

Fig. 3. Photoluminescence emission spectra for (CdS–PVP)Mn and (CdS–PVP) Mn/ZnS coreshell nanoparticles.

Fig. 4. Photoluminescence emission spectra for (CdS–PVP)Mn at two concentrations of Mn.

B. Saraswathi Amma et al. / Materials Chemistry and Physics 112 (2008) 789–792

Fig. 5. Photoluminescence emission spectra for (CdS–PVP)Mn/ZnS at two typical concentrations of ZnS.

band gaps rules out the existence of Znx Cd1−x S alloys, as the band gap would then blue shift with increasing Zn2+ content for such alloys. The PL spectra for (CdS–PVP)Mn and (CdS–PVP)Mn/ZnS are shown in Fig. 3 at an excitation wavelength of 370 nm along with the optical absorption spectra. The PL spectra for (CdS–PVP)Mn exhibit the near band edge emission due to CdS nanoparticles at 416 nm and a weak emission at 586 nm. The latter is attributed to Mn emission as suggested by Malik et al. [17] for Mn2+ -doped


Fig. 6. X-ray Diffractogram of (CdS–PVP)Mn/ZnS coreshell nanoparticles with 8% of ZnS.

CdS nanoparticles. In the PL spectra for (CdS–PVP)Mn/ZnS coreshell nanoparticles, emission peak at 586 nm is no longer present, which implies surface passivation by ZnS coating. Shift in the PL emission peak from 416 to 438 nm is observed after the formation of ZnS layer similar to the red shift observed in the PL emission peak for CdSe/ZnS coreshell particles by Dabbousi et al. [8]. Fig. 4 is the PL spectra for (CdS–PVP)Mn with two concentrations of Mn2+ . Photoluminescence emission peak at 586 nm increased

Fig. 7. (a) Transmission electron micrograph of (CdS–PVP)Mn nanoparticles and (b) the corresponding histogram.

Fig. 8. (a) Transmission electron micrograph of (CdS–PVP)Mn/ZnS coreshell nanoparticles and (b) the corresponding histogram.


B. Saraswathi Amma et al. / Materials Chemistry and Physics 112 (2008) 789–792

with an increase in Mn2+ content, which indicates that the peak at 586 nm is due to Mn2+ doping. The PL emission peak seen in Fig. 4 at 620 nm is attributed to deep trap emission due to surface defects [19,20]. Both the peaks disappear after the ZnS shell formation. Fig. 5 shows the increase in PL intensity with increase in ZnS content. It is reported that for a mixed system of CdS and ZnS, the PL spectrum drops in intensity with increasing ZnS concentration and for a CdS/ZnS coreshell system, the PL intensity increases with increase in ZnS concentration [14]. The increased PL intensity with increasing ZnS concentration (Fig. 5) provides further evidence for the formation of coreshell nanoparticles rather than a mixture of CdS and ZnS nanoparticles. Fig. 6 is the X-ray diffractogram of (CdS–PVP)Mn/ZnS coreshell particles and the diffraction peaks seen at 2 values of 26.9◦ (1 1 1), 43.9◦ (2 2 0) and 52.0◦ (3 1 1) are similar to our earlier observation for cubic CdS–PVP particles [15] and also to ZnS-coated CdS particles reported by Youn et al. [21]. Separate peaks corresponding to Mn or ZnS are not observed. The presence of a small foreign inclusion [18] or a shell layer over the core does not affect the XRD peaks of the core particles [1,7,22]. Figs. 7(a) and 8(a) show the TEM pictures of (CdS–PVP)Mn and (CdS–PVP)Mn/ZnS coreshell nanoparticles, respectively. It can be seen from the figures that the particles, which look aggregated or clustered together in the (CdS–PVP)Mn sample (Fig. 7(a)) become well defined after the ZnS shell formation over the core, i.e. for (CdS–PVP)Mn/ZnS (Fig. 8(a)). Corresponding histograms are shown in Figs. 7(b) and 8(b). The particle size of ZnS-coated samples is comparatively larger (∼5 nm) than the uncoated ones (∼4 nm). Audinet et al. [23] have reported an increase in particle size of ZnScoated CdS nanoparticles by a factor of two after the introduction of Zn2+ ions. 4. Conclusions The following conclusions may be drawn from the present studies of (CdS–PVP)Mn/ZnS coreshell nanocomposites: • The absence of exciton peak or a second absorption edge in the UV region as well as the red shift in the absorption spectra with an increase in zinc sulphide content indicates the formation of (CdS–PVP)Mn/ZnS coreshell structures.

• The PL intensity of (CdS–PVP)Mn/ZnS is more than that of (CdS–PVP)Mn nanoparticles. • Pure band edge emission is observed in the coreshell structure, indicating effective surface passivation by ZnS layer on the CdS core. • Well defined and nearly monodispersed coreshell nanoparticles are formed as seen from TEM. Acknowledgements The authors thank the Department of Science and Technology, Government of India for the XRD facility and Dr. P.K. Nair, IIT, Madras for the TEM pictures. Saraswathi Amma B. thanks the University Grants Commission, Government of India for a teacher fellowship. References [1] X.G. Peng, M.C. Schlamp, A.V. Kadavanich, A.P. Alivisatos, J. Am. Chem. Soc. 119 (1997) 7019. [2] B.S. Zou, R.B. Little, J.P. Wang, M.A. El-Sayed, Int. J. Quantum Chem. 72 (1999) 439. [3] M.A. Hines, P. Guyot-Sionnest, J. Phys. Chem. 100 (1996) 468. [4] A. Eychmuller, A. Hasselbarth, L. Katsikas, H. Weller, J. Lumin. 48–49 (1991) 745. [5] L. Spanhel, H. Weller, A. Fujtik, A. Henglein, Ber. Bunsenges. Phys. Chem. 91 (1987) 88. [6] H. Yang, H.P. Holloway, Appl. Phys. Lett. 82 (2003) 1965. [7] L. Manna, E.C. Scher, L.S. Li, A.P. Alivisatos, J. Am. Chem. Soc. 124 (2002) 7136. [8] B.O. Dabbousi, J. Rodriguez-Viejo, F.V. Mikulec, J.R. Heine, H. Mattoussi, R. Ober, K.F. Jensen, M.G. Bawendi, J. Phys. Chem. B 101 (1997) 9463. [9] J.K. Jaiswal, H. Mattoussi, J.M. Mauro, S.M. Simon, Nat. Biotechnol. 21 (2003) 47. [10] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [11] Y. Yang, O. Chen, A. Angerhofer, Y.C. Cao, J. Am. Chem. Soc. 128 (2006) 12428. [12] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. [13] R.N. Bhargava, J. Lumin. 70 (1996) 85. [14] S.Y. Lu, M.L. Wu, H.L. Chen, J. Appl. Phys. 93 (2003) 5789. [15] M. Pattabi, B. Saraswathi Amma, K. Manzoor, Mater. Res. Bull. 42 (2007) 828. [16] X.C. Wu, A.M. Bittner, K. Kern, J. Phys. Chem. B 109 (2005) 230. [17] A.Z. Malik, P. O’Brien, N. Revaprasadu, J. Mater. Chem. 11 (2001) 2382. [18] A.R. Kortan, R. Hull, R.L. Opila, M.G. Bawendi, M.L. Steigerwald, P.J. Carroll, L.E. Brus, J. Am. Chem. Soc. 112 (1990) 1327. [19] N. Herron, Y. Wang, H. Eckert, J. Am. Chem. Soc. 112 (1990) 13226. [20] K. Murakoshi, H. Hosokawa, M. Saitoh, Yugiwada, T. Sakata, H. Mori, M. Satoh, S. Yanagida, J. Chem. Soc. Faraday Trans. 94 (1998) 579. [21] H.C. Youn, B. Subhash, J.H. Fendler, J. Phys. Chem. 92 (1988) 6320. [22] H. Yang, P.H. Holloway, C. Cunnigham, K.S. Schanze, J. Chem. Phys. 121 (2004) 10233. [23] L. Audinet, C. Ricolleau, M. Gandais, T. Gacoin, J.P. Boilot, P.A. Buffat, Philos. Mag. A 79 (1999) 2379.