Spectrochimica Acta Part A 72 (2009) 285–290 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa Structural and optical investigation of semiconductor CdSe/CdS core–shell quantum dot thin films A.B. Sharma b , Sudhir Kumar Sharma a,b,∗ , M. Sharma b , R.K. Pandey b , D.S. Reddy c a Department of Instrumentation, Indian Institute of Science, Bangalore, Karanataka 560012, India b Department of Physics, Barkatullah University, Bhopal, M.P. 462026, India c Department of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea article info Article history: Received 28 May 2008 Received in revised form 2 September 2008 Accepted 26 September 2008 Keywords: Semiconductor quantum dots X-ray diffraction studies (XRD) Transmission electron microscopy (TEM) Optical absorption spectroscopy abstract Highly luminescent CdSe/CdS core–shell nanocrystals have been assembled on indium tin oxide (ITO) coated glass substrates using a wet synthesis route. The physical properties of the quantum dots (QD) have been investigated using X-ray diffraction, transmission electron microscopy and optical absorption spec- troscopy techniques. These quantum dots showed a strong enhancement in the near band edge absorption. The in situ luminescence behavior has been interpreted in the light of the quantum confinement effect and induced strain in the core–shell structure. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Semiconductor quantum dots have been conventionally sta- bilized using a protective layer of organic surfactants such as Tri-n-octyl-phosphine oxide (TOPO) [1–4], alkane thiol [5–7], mer- captoethanol [8] etc. These stabilizing ligands are effective in controlling particle agglomeration and ripening, but luminescence yield is very poor. Capping of inorganic shell materials with a wider band gap has been shown significant improvement in the lumines- cence quantum yield, enhancement in photo stability but decrease the fluorescence lifetime [9–12]. Synthesis of CdSe/CdS core–shell nano-particles in aqueous medium using wet-chemical synthesis was studied by Hao et al. [13]. They found a strong narrow band edge luminescence, indicating successful passivation by removing sur- face defects. Large red shift in band edge emission was found by Liu et al. [14] in CdSe/CdS core–shell nano-clusters synthesized in aque- ous medium. The role of shell thickness in stimulated emission and photo-induced absorption in CdSe core–shell nano-rods were stud- ied by Creti et al. [15]. For device application, the development of core–shell quantum dot structures as a thin film on the conducting substrates can be more beneficial. We have directed our attention to explore such a possibility of the core–shell development by the wet synthesis. ∗ Corresponding author. Tel.: +91 80 22932890; fax: +91 80 22931056. E-mail address: drsudhirsharma@gmail.com (S.K. Sharma). 2. Experimental details The synthesis of core-shell quantum dots (QDs) in aqueous solu- tion is really very difficult. Addition of Cd and S precursors into the aqueous solution of CdSe QDs usually resulted in homoge- neous nucleation of new CdS QDs rather than formation of CdS shell around CdSe. In order to achieve high-quality CdSe-CdS QDs under wet aerobic conditions a simple route using sodium sulfide as the source of sulfur was designed to produce core–shell CdSe- CdS QDs, in which ammonia was the reducing reagent and TOPO as a capping ligand. The CdSe quantum dots sample CK17 used as core was synthesized at room temperature in an aqueous reaction matrix containing sodium seleno-sulphite, cadmium chloride and tri-ethanolamine in 1:3:1.4 molar ratios at pH 11. The size of the CdSe quantum dots were controlled by adding the surfactant like mercaptoethanol in the bath. The ratio of surfactant to selenium used in this synthesis was 1:2.42 approximately. Moreover, CdSe quantum dots prepared by above discussed procedure were washed thoroughly in double distilled water several times and preserved in a methanol solution. The growth of the CdS shell was accomplished by using cad- mium chloride (CdCl 2 ) and thiourea in molar ratio 1:7.5 in an aqueous reaction matrix. The pH control of pH 9.0 of the reaction matrix was maintained by adjusting the amount of ammonia (NH 3 ) drops in the bath. A predetermined quantity of this solution was injected into a suspension of CdSe quantum dots at room tempera- ture and the solution was vigorously stirred. Growth of the CdS shell was allowed to proceed for a predetermined duration. A precleaned 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.09.031