Optical properties of CdS nanoparticles and the energy transfer from CdS nanoparticles to Rhodamine 6G Paramita Saha Chowdhury a , Pratik Sen b , Amitava Patra a, * a Sol-Gel Division, Central Glass and Ceramic Research Institute, 196 Raja S.C. Mullic Road, Jadavpur, Kolkata 700 032, India b Physical Chemistry Department, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Received 10 June 2005; in final form 20 July 2005 Available online 19 August 2005 Abstract In the present work, we report the synthesis of colloidal CdS nanoparticles with different sizes and the band gaps of these samples exhibit a systematic dependence on the size of particles. We also study the photophysical properties of CdS nanoparticles and the energy transfer from CdS nanoparticles to Rhodamine 6G dyes by steady state and time resolved photoluminescence spectroscopy. The experimental results show that the relaxation of the excited state of CdS nanoparticles is composed of three different compo- nents. The calculated energy transfer efficiency from CdS nanoparticles to dye is 43% for 3.0 nm CdS nanoparticles. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction A burst of research activities has been emerged in the synthesis and characterization of semiconductor nano- particles for size dependent optical properties, because they find wide applications in photonic [1,2]. When the dimensions of nanocrystalline particles approach the exciton Bohr radius, a blue shift in energy is observed due to the quantum confinement phenomenon. The effective mass model is commonly used to study the size dependence of optical properties of quantum dots (QD) system [3]. The tunability of the properties of nanopar- ticles by controlling their size may provide an advantage in formulating new composite materials with optimized properties for various applications [4]. Recently, there has been a growing interest in the study of nanocrystals (NCs) in biomolecules such as peptides, proteins and DNA for biophotonic applications [5–8]. The under- standing of the Forster resonance energy transfer from NCs to dyes is one of the most important topics now- a-day. The efficiency of FRET depends on the inverse sixth power of the distance of separations between donor and acceptor molecules. Only few studies have been published on energy transfer from NCs to polymer, dyes [9–11]. Dyes are commonly used in biological sys- tem for Forster resonance energy transfer (FRET) study. It is now well established that NCs have several advantages over dyes in FRET studies in biological sys- tem. FRET is a process where the excitation energy of a donor is nonradiatively transferred to a nearby accep- tor. For this process, it is necessary to significant overlap between the emission and absorption spectrum of the donor and acceptor respectively. In addition, the broad emission spectrum of the donor with its long red tail can often overlap significantly with the emission spectrum of the acceptor. No report has been found on the study of energy transfer from CdS nanoparticle to Rhodamine 6G by time resolved spectroscopy, to our knowledge. There is plenty of scope for understanding the energy transfers from CdS nanocrystals to Rhodamine 6G dye by time resolved spectroscopy. In this Letter, we report the synthesis of water soluble CdS nanoparticles 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.07.088 * Corresponding author. Fax: +91 33 24730957. E-mail address: apatra@cgcri.res.in (A. Patra). www.elsevier.com/locate/cplett Chemical Physics Letters 413 (2005) 311–314