Optical properties of nanocrystalline titanium oxide R. Plugaru National Institute for Research and Development in Microtechnologies, P.O. Box 38-160, 023573 Bucharest, Romania Available online 11 April 2008 Abstract Optical emission properties of nanocrystalline TiO 2 powder and of polycrystalline TiO 2 obtained by powder sintering have been studied by cathodoluminescence in the scanning electron microscope. Radiative transitions in nanocrystalline TiO 2 powder were associated with shallow traps induced by surface oxygen vacancies. In polycrystalline TiO 2 the presence of shallow traps diminishes and the radiative transitions involve energy levels induced by Ti 3+ ions. A red shift of the infrared emission band of titanium ions appears to be determined by oxygen vacancies affecting the band gap energy. © 2008 Elsevier B.V. All rights reserved. Keywords: Titanium oxide; Oxygen vacancies; Radiative transitions; Optical properties; Cathodoluminescence 1. Introduction Recent applications of titanium oxide in photocatalytic processes and nanoelectronics have motivated extensive theoretical and experimental studies on the electronic structure of this wide band gap semiconductor [1–3]. Of great interest is band gap tailoring because of its effects on enhancement of optical absorption in visible range and increase of photo- catalytic decomposition rate [4,5]. On the other hand, the high k property of TiO 2 as well as magneto-doping processes were investigated for applications as gate dielectric material and electron spin based nanodevices [6,7]. All these applications require a profound understanding of quantum processes in which both oxygen vacancies (OV) and titanium ions in various electronic states are involved. Although an impressive number of publications have been devoted to the analysis of OV formation and behavior, an accurate explication of the relationship between defects structure and electronic structure still is not resolved. For instance, there is no broad agreement on whether the intrinsic doping leads to energy levels formation inside the band gap, or the energy states change the band gap width [8,9]. The present study contributes to the investigation of optical emission spectra and electronic bands of TiO 2 nanocrystalline powder and of polycrystalline TiO 2 obtained from this powder by a sintering process. The experimental results are discussed in relationship with the presence of shallow traps associated with surface OV and energy levels induced in the band gap by Ti 3+ ions. 2. Experimental details Nanocrystalline titanium dioxide (TiO 2 ) powder with mixed anatase and rutile structure (provided by Sigma-Aldrich) was used as starting material in the present study. The powder was compressed disks, with 7 mm diameter and 2 mm thickness. These samples were annealed in argon and in air atmosphere at 1100 °C for 30 min, then the polycrystalline material was subjected to treatments: i) in oxygen, performed at 800 °C for 30 min and 120 min and ii) in forming gas, (40% hydrogen) at 750 °C for 30 min. The optical emission properties were investigated by cathodoluminescence in the scanning electron microscope (CL-SEM). The CL emission in visible and near infrared spectral region was detected in a Hitachi S 2500 SEM and a Leica 440 SEM microscope equipped with Hamamatsu R-928 photomultiplier for visible light and cooled ADC germanium detector for infrared spectral region. The CL spectra were recorded using an Oriel 78215 computer controlled mono- chromator or a CCD camera with a built in spectrograph (Hamamatsu PMA-11). The measurements were performed in the temperature range 90–300 K, using an accelerating voltage of 20 kV. The morphology of the polycrystalline surfaces was investigated in the secondary electron mode (SE) of the SEM. Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 8179 – 8183 E-mail address: rodica.plugaru@imt.ro. www.elsevier.com/locate/tsf 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.04.039