Room temperature ferromagnetism in thiol-capped CdSe and CdSe:Cu nanoparticles Shashi B. Singh, Mukta V. Limaye, Sadgopal K. Date, Sulabha K. Kulkarni * DST Unit on Nanoscience, Department of Physics, University of Pune, Pune 411 007, India article info Article history: Received 25 June 2008 In final form 10 September 2008 Available online 13 September 2008 abstract Observations of ferromagnetism at room temperature in semiconductor nanoparticles would make them suitable candidates in spintronics devices. It has been found that the pure and copper doped CdSe nano- particles (2 nm) exhibit ferromagnetism at room temperature. Magnetization studies revealed a well- defined hysteresis loop with saturation magnetization value of 2.6 memu/gm and coercivity value of 198 Oe for the pure CdSe nanoparticles at room temperature. The saturation magnetization value was found to increase at lower doping concentration of copper and then decrease consistently with increase in the concentration. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction There is a tremendous interest in the behavior (physical and chemical properties) of semiconductor nanoparticles (NPs) on the basis of quantum confinement effects. Band gap tuning by chang- ing the particle size and consequent band edge luminescence over the entire visible range has been used in optoelectronic devices, bio labeling, etc. [1–3]. Further, it has been anticipated that the transi- tion metal (TM) doped semiconductor NPs will be useful candi- dates for spintronics applications [3–8]. A large number of theoretical as well as experimental results are also available in the literature on nitrides and oxides of semiconductors with con- flicting results on room temperature ferromagnetism [8–17]. Sun- deresan et al. [15] have reported the ferromagnetism at room temperature in some oxide nanoparticles like CeO 2 , Al 2 O 3 , ZnO, In 2 O 3 , and SnO 2 . Seehra et al. [18] have recently observed the fer- romagnetism in trioctylphosphine oxide (TOPO)-capped CdSe quantum dots. They have shown that smaller the quantum dot more is the coercivity and the saturation magnetization. We report here room temperature ferromagnetic behavior (well-defined hysteresis loop observed) of chemically synthesized thiol-capped pure and Cu doped CdSe nanoparticles. Although, the use of trioctylphosphine oxide–trioctylphosphine (TOPO– TOP) in the synthesis produces good quality CdSe nanoparticles [1,18,19], the green chemistry root of synthesizing CdSe nanopar- ticles is desired which produces reasonably good nanoparticles [2,20]. Since Cu metal and all its secondary phases are not ferro- magnetic, the observation of the loop itself is really intriguing. To explain the mechanism behind it, we have performed some sys- tematic analyses of the samples using X-ray diffraction (XRD), transmission electron microscope (TEM), vibrating sample magne- tometer (VSM), and Fourier transform infrared (FTIR) techniques, and the results are discussed in this Letter. 2. Experimental 2.1. Synthesis of undoped and doped CdSe nanoparticles In order to synthesize pure and Cu doped CdSe nanoparticles, a simple low temperature chemical technique [21] was followed with less toxic chemicals as compared to those in the TOPO–TOP synthesis. In this processing route of Cd 1x Cu x Se nanoparticles, re- quired amounts of CuCl 2 2H 2 O(x mmol) and (CH 3 COO) 2 Cd2H 2 O (2.2  x mmol) were dissolved in 150 mL of N,N-dimethyl formam- ide (DMF) to obtain different doping concentrations. Mercap- toethanol was used as a capping agent and added to the above solution, and subsequently, the separately prepared solution of so- dium selenite (Na 2 SeO 3 ) in distilled water was added drop by drop. The resulting solution was refluxed for 3 h. The precipitate then obtained was washed repeatedly by methanol and dried under vacuum. It was found that the actual percentages of Cu doping were 1.8%, 4.5%, 7.1%, and 17.2% for nominal doping of 2%, 5%, 10%, and 15%, respectively. 2.2. Characterization of nanoparticles The presence of any undesired (magnetic or otherwise) impu- rity and atomic weight percentage (At. wt.%) was checked using X-ray energy dispersive spectrometer (EDS) on JEOL JSM 6360A. X-ray diffraction (XRD) analysis was performed using Bruker D-8 Advance X-ray diffractometer using Cu K a (1.54 Å) X-ray source. TEM images were recorded using Tecnai 20 G2 electron micro- scope operated at 100 KeV. The samples for the TEM analysis were 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.09.020 * Corresponding author. Fax: +91 20 25691684. E-mail address: skk@physics.unipune.ernet.in (S.K. Kulkarni). Chemical Physics Letters 464 (2008) 208–210 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett