Materials Chemistry and Physics 113 (2009) 749–755 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Structural, optical and magnetic properties of nanocrystalline Zn 0.9 Co 0.1 O-based diluted magnetic semiconductors Y. Kalyana Lakshmi a , K. Srinivas a , B. Sreedhar b , M. Manivel Raja c , M. Vithal d , P. Venugopal Reddy a,∗ a Department of Physics, Osmania University, Hyderabad 500007, India b I&PC Division, Indian Institute of Chemical Technology, Hyderabad 500007, India c Defence Material Research Laboratory, Hyderabad 500058, India d Department of Chemistry, Osmania University, Hyderabad 500007, India article info Article history: Received 1 December 2007 Received in revised form 28 May 2008 Accepted 3 August 2008 PACS: 75.50.Pp 74.25.Gz 75.30.Hx 75.60.Ej Keywords: Diluted magnetic semiconductor Nanostructures Optical properties Magnetic properties abstract With a view to understand the influence of nano size on various properties of cobalt-doped ZnO-based diluted magnetic semiconductors, a series of materials were prepared by the citrate gel route. The phase and morphology studies have been carried out by X-ray diffraction and transmission electron microscopy, respectively. All the samples of the present investigation are found to have hexagonal wurtzite structure and crystallite sizes are found to vary from 25 nm to 65 nm. From the optical absorption measurements it has been observed that upon doping with cobalt, the energy band gap is found to shift towards lower energy side (red shift) while it shifts towards higher energy side (blue shift) when the crystallite size is increased continuously. It has been observed from the XPS results that oxidation state of Cobalt is +2 and that the difference in binding energies of Co 2p 3/2 and Co 2p 1/2 is found to increase continuously with increasing crystallite size. Finally, all the samples are found to exhibit room temperature ferromagnetism and the specific magnetization decreases with increasing crystallite size. Published by Elsevier B.V. 1. Introduction Diluted Magnetic semiconductors (DMS) have recently attracted a great deal of attention due to possibility of manipulating charge and spin degrees of freedom. They exhibit unique mag- netic, magneto-optical and magneto-electrical effects and can be exploited as spintronic devices [1]. Efforts are going on to meet some of the deficiencies of these materials such as intrin- sic ferromagnetism, high ferromagnetic Curie-temperature, large magnetization and a precise controllable spin properties, etc. In search of a new DMSs, lead the attention towards wide band gap oxide-based DMSs such as Zn 1-x TM x O, Ti 1-x TM x O 2 , Sn 1-x TM x O 2 and Hf 1-x TM x O 2 (TM: Co, Ni, Mn, Fe) [2–5], etc., because of their stable ferromagnetism near or above room temperature. Among oxide-based DMSs, ZnO doped with small amount of Co 2+ have attracted considerably. Moreover, ZnO has a large exciton binding energy of 60 meV, with wide band gap energy (∼3.3 eV) resulting in efficient excitonic emission at room temperature as well as suitable in making transparent devices. ∗ Corresponding author. Tel.: +91 40 27682287; fax: +91 40 27090020. E-mail addresses: pvreddy@osmania.ac.in, paduruvenugopalreddy@gmail.com (P.V. Reddy). It is well known that nano size has great influence on the performance of several material systems. Moreover, the role of dimensionality in shaping the spin-polarized electronic struc- ture of nanocrystalline DMSs is important in understanding their ferromagnetic behavior [6–12]. Apart from this, nanocrystalline Zn 0.9 Co 0.1 O may be exploited for ferro-fluids, magnetic recording, and biomedical applications because quantum confinement may result in intriguing magnetic properties. As the structural, mag- netic and electronic properties of Zn 0.9 Co 0.1 O are sensitive to the nano size of the materials, an effort has been made to understand the influence of nano size on their structural, optical and ferromag- netic behavior and the results of such an investigation are presented here. 2. Experimental procedure The nanocrystalline Zn0.9Co0.1O samples were prepared using citrate gel route by taking the corresponding starting materials as nitrates. More details are given in an earlier publication [9]. In this method, first the nitrates were converted into citrates and by adding suitable chemicals, pH was adjusted to 6.5. After getting a solution on slow evaporation, a gelating reagent ethylene glycol was added and heated at about 180 ◦ C to get a gel. After a few more steps dry fluffy porous mass (precursor), was calcined at 250 ◦ C for 3 h. Finally, the samples in the form of pellets were sintered at 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C temperatures for 3 h in order to obtain the materials with varying crystallite size. 0254-0584/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.matchemphys.2008.08.021