Electronic structure analysis and properties of Sr 2 CeO 4 grown by sol–gel method E. Talik a, *, L. Lipin ´ ska b , D. Skrzypek a , A. Skuta a,b , P. Zajdel a , A. Guzik a , H. Duda a a Institute of Physics, University of Silesia, ul. Uniwersytecka 4, 40-007 Katowice, Poland b Institute of Electronic Materials Technology, ul. Wo ´lczyn ´ska 133, 01-919 Warszawa, Poland 1. Introduction An unusual, inorganic phosphor, cerium strontium oxide Sr 2 CeO 4 , was discovered in 1998 by Danielson et al., by using the combinatorial synthesis from library consisting of over 25,000 chemically distinct compositions [1]. The luminescence of Sr 2 CeO 4 originates from a ligand-to-metal (from O 2 to Ce 4+ ) charge transfer [1]. It is quite different mechanism in comparison with the luminescence of compounds doped with rare earth ions, where the emission of the radiation is caused by the transition of electrons from the higher, excited states of the dopant. The emission spectrum of Sr 2 CeO 4 is wide and overlaps the visible range with the maximum approximately at 480 nm. This compound is considered as a material for a new generation of light sources – called SSL – solid state lighting, used in the technology of light emitting diodes LEDs. The huge advantages of systems based on LEDs, such as energy saving, the long time of life, white light with the regulated colour temperature etc. cause that they affect almost all markets of lighting – street, decorative, emergency or mobile. Sr 2 CeO 4 makes up the strong competition for luminescent oxide materials, in the peculiarity for materials doped with the rare earth ions. Further applications are displays with field emission FED or cathode ray tubes CRT [2–4]. The large investigative and applicable potential of Sr 2 CeO 4 caused that the multiple methods were engaged to produce it. Initially, the reaction in solid phase was applied [5]. In this method, due to slow diffusion, high temperatures approximately 1100 8C were required, what led to a partial decomposition of Sr 2 CeO 4 and deterioration of its spectroscopic proprieties. Wet chemical methods are superior over the synthesis in a solid phase. Mixing of components is done in a solution where diffusion runs very quickly and one can expect the homogeneity on the molecular level. Temperatures required on the individual stages of syntheses are considerably lower than applied in solid phase reaction. This makes possible obtaining less agglomerated powders with well shaped crystallites. Many chemical methods are described in the literature: the coprecipitation method [2], combustion [4], microwave-assisted hydrothermal synthesis [6], microemulsion method [7,8], pyrolysis [9] and the most often the sol–gel technique [3,10–14] for synthesis of this compound. Physical methods are also applied e.g. laser ablation methods [15], but they require advanced and sophisticated devices what makes them more expensive. In this work, the Sr 2 CeO 4 nanocrystals were synthetized by the modified sol–gel method. The objective of this work is to investigate the morphology, crystal and electronic structure, magnetic properties of the synthesized samples. 2. Experimental The nanocrystalline Sr 2 CeO 4 material was synthesized using Sr(NO 3 ) 2 and Ce(NO 3 ) 3 H 2 O as starting reagents. The suitable quantities of nitrates were dissolved in deionized water with the addition of the citric acid. The solution was warmed up to the temperature of 60 8C and continuously mixed for 4 h. Next, Materials Research Bulletin 47 (2012) 3107–3113 A R T I C L E I N F O Article history: Received 9 March 2012 Received in revised form 26 April 2012 Accepted 11 August 2012 Available online 17 August 2012 Keywords: A. Optical materials A. Oxides A. Sol–gel chemistry C. Electron microscopy D. Electronic structure A B S T R A C T Sr 2 CeO 4 is a very promising material due to its various industrial applications, e.g. for the construction of field emission displays and light emitting diodes. In this work, the phosphor was synthesized by the sol– gel method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) methods. They proved to be very good structural properties of the material and identified negligible impurities from the technological process. The decomposition of the surface of the nanocrystals was found but this did not decrease the spectral features of the compound. The analysis of the XPS O2p lines revealed contributions of two kinds of bonds: terminal and equatorial ones, in the ratio of 2/4. They are separated by 1.2 eV what is in agreement with the observed absorption spectra. A presence of the decomposed layers may produce asymmetric widening of the emission spectra towards a lower energy. ß 2012 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +48 32 359 1187; fax: +48 32 258 84 31. E-mail address: talik@us.edu.pl (E. Talik). Contents lists available at SciVerse ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.08.025