Emission properties of (SrTiO 3 –TiO 2 ):Pr 3+ eutectic with self-organized fractal microstructure M. Kaczkan a,⇑ , D.A. Pawlak b , S. Turczyn ´ ski b , M. Malinowski a a Institute of Microelectronics and Optoelectronic, Koszykowa 75, 00-662 Warsaw, Poland b Institute of Electronic Materials Technology, Wólczyn ´ska 133, 01-919 Warsaw, Poland article info Article history: Available online 6 July 2011 Keywords: Eutectic Pr 3+ Praseodymium SrTiO 3 –TiO 2 Luminescence abstract An investigation of spectroscopic properties of (SrTiO 3 –TiO 2 ):Pr 3+ eutectic and, for comparison, of bulk SrTiO 3 :Pr 3+ and TiO 2 :Pr 3+ crystals is presented. Luminescence spectra have been measured under both 450 nm and 350 nm excitation wavelength. For UV excitation they are characterized by a dominant red luminescence corresponding to transition from the 1 D 2 level of Pr 3+ ions. The mechanism responsible for quenching of blue (from 3 P 0 state) and intensification of red luminescence is proposed to be ther- mally-induced radiationless relaxation involving a low-lying Pr 3+ –Ti 4+ intervalence charge transfer state. Measured decay constants of 1 D 2 excited state of Pr 3+ are compared with values obtained for other pra- seodymium doped titanate hosts. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction In recent years in the field of photonics, concepts of new mate- rials appeared such as photonic crystals and metamaterials that exhibit unusual, not observed in the constituent materials, electro- magnetic properties (e.g. negative refractive index, artificial mag- netism and cloaking) [1,2]. Self-organized eutectics, because of their specific structure with precipitate/particle size ranging from hundreds of micrometers to tens of nanometers, seems to be prom- ising novel materials for applications in optoelectronics area [3,4]. SrTiO 3 –TiO 2 eutectic is both bulk and a microstructured mate- rial with TiO 2 and SrTiO 3 as the component phases, as shown in Fig. 1b [5]. TiO 2 in the form of rutile is a wide bandgap semicon- ductor (energy gap 3.2 eV), while SrTiO 3 is a well-known incipi- ent ferroelectric and has been reported to also show metallic-like behavior [6]. In this work luminescence properties of Pr 3+ doped SrTiO 3 –TiO 2 eutectic and, for comparison, SrTiO 3 and TiO 2 bulk single crystals are presented. Praseodymium ion, emitting in blue, green 1 and red spectral range, is continuously considered as one of the most prom- ising activators. The red emitting Pr 3+ doped ATiO 3 compounds (A = Ca, Sr, Ba) are known as red phosphors with high quantum effi- ciency and chromatic coordinates (x= 0.68, y= 0.31) very close to ‘‘ideal red’’ [7] and have been investigated for their applications in display devices [8]. 2. Materials and experiment The eutectics were grown by the micro-pulling-down method (l-PD), with different pulling rates (0.15, 0.45, 1, 5, 10, and 15 mm/min) in the form of rods 3 mm in diameter and a few cen- timeters in length. Presented results concern samples obtained with 5 mm/min pulling rates. The as-grown eutectic bi-crystals have a metallic appearance and luster, probably due to high amount of oxygen vacancies. In Fig. 1a the samples cut from the crystal rods perpendicu- larly to the growth direction and polished are presented. The eutectic is a bulk material as seen in Fig. 1a and it is also a microstructured material, as shown in Fig. 1b. Presence of two phases SrTiO 3 and TiO 2 (rutile) in well-defined orientations was confirmed using X-ray diffraction (XRD) method. As it has been shown in [5] the SrTiO 3 phase grows along h110i(Pm3m space group) and the TiO 2 phase grows along h001i(P42/mnm space group) crystallographic orientation. The microstructure contain- ing both large and small particles exhibits a fractal character. Energy dispersive spectroscopy (EDS) reveals that the matrix consists of a SrTiO 3 phase (light color in Fig. 1), while the pat- tern-forming phase is formed from the TiO 2 phase (in the form of rutile). More details of structural properties of obtained micro structures have been previously reported [5]. Bulk SrTiO 3 :Pr 3+ and TiO 2 :Pr 3+ crystals, also investigated in this work, were grown using the same l-PD method. Continuous wave emission and excitation spectra and decay curves were obtained by using PTI spectrofluorimeter with xenon lamps as excitation sources. Low temperature measurements were performed using closed-cycle liquid He cryostat. 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.04.041 ⇑ Corresponding author. Tel.: +48 22 234 5047; fax: +48 22 628 8740. E-mail address: m.kaczkan@elka.pw.edu.pl (M. Kaczkan). 1 For interpretation of color in Figs. 3 and 4, the reader is referred to the web version of this article. Optical Materials 33 (2011) 1519–1524 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat