Article pubs.acs.org/JPCC
Correlation between Cr3+ Luminescence and Oxygen Vacancy Disorder in Strontium Titanate under MeV Ion Irradiation M. L. Crespillo,*,† J. T. Graham,‡,† F. Agulló-López,§ Y. Zhang,∥ and W. J. Weber*,†,∥ †
Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States Department of Mining and Nuclear Engineering, Missouri University of Science and Technology, Rolla, Missouri 65409, United States § Centro de Microanálisis de Materiales, CMAM-UAM, Cantoblanco, Madrid 28049, Spain ∥ Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡
ABSTRACT: Strontium titanate (SrTiO3), a model system with a strongly correlated electronic structure, has attracted much attention recently because of its outstanding physicochemical properties and considerable potentials for technological applications. The capability to control oxygen vacancy profiles and their effect on valence states of cations will increase significantly the functionality of devices based on transition metal oxides. This work presents new insights into the near-infrared luminescence emission of Cr3+ centers in stoichiometric SrTiO3 induced using 3 MeV protons at temperatures of 100 K, 170 K, and room temperature. The study covers a wide spectral range, including near-infrared, visible, and near-UV regions. Our main purpose is to investigate the role of the oxygen vacancies introduced by energetic charged particles on the shape and yield of induced luminescence spectra, in particular to explore the interplay between the Cr3+ luminescence at 1.55 eV and oxygen disorder. A clear correlation is found between the decay of the Cr luminescence yield during irradiation and the growth of a band at 2.0 eV, well-resolved below 170 K, which has been very recently attributed to d−d transitions of electrons self-trapped as Ti3+ in the close vicinity of oxygen vacancies. This correlation suggests irradiation-induced oxidation of the Cr3+ (Cr3+ → Crn+, n > 3) via trapping of irradiation-induced holes, while the partner electrons are self-trapped as Ti3+. These new results provide effective guidelines for further understanding the electronic and photocatalytic behavior of STO:Cr3+. dependent on the charge state of chromium.18−21 One should remark that the charge state is strongly influenced by the location of the Fermi level in the STO crystal, which can be modified through impurity doping, thermochemical treatments, and irradiation. Luminescence is a main, and often unique, optical technique to investigate the properties of transition metal impurities in dielectric materials and their relation to the lattice structure. Photoluminescence (PL), as a key optical technique, is widely used to identify and characterize excited states of defect centers.22 The luminescence behavior of Cr in STO under a variety of excitation sources has been extensively investigated.23−28 The reported experiments include excitation by electrons (cathodoluminescence, CL), UV, X-rays (radioluminescence, RL), and laser pulses. In particular, using photoluminescence (PL) excitation above the band gap (3.27 eV), a well-resolved near-infrared (NIR) band at 1.55 eV (often called the B-band), together with a set of vibronic side-bands,
1. INTRODUCTION Strontium titanate (SrTiO3, STO) is an oxide crystal with cubic perovskite structure at room temperature (RT) that exhibits a paraelectric−ferroelectric transition at ≈35 K and another one at around 100 K, corresponding to a cubic-tetragonal transition.1 It shows a rich physicochemical behavior, including a variety of electrical properties, that modify the response of the crystal from insulating to semiconducting, metallic, and even superconducting. Moreover, it manifests intense photocatalytic activity for hydrogen generation from water decomposition.2 This latter property is closely related to doping with transition metal impurities, such as chromium, as well as to thermal treatments and ion irradiation that introduce oxygen vacancies.3−10 In particular, chromium doping appears to play a key role in all these processes and allows for a reversible insulator-to-conductor transition or resistive memory ef fect.11−13 It is generally accepted that the Cr dopant ions in STO substitute for Ti4+ and occupy an octahedral site as Cr4+14 but shift to Cr3+ under electronic excitation. The possibility of other valence states, such as Cr6+, as well as the occupancy of Sr sites has been also discussed,15−17 mostly in connection with the photocatalytic response of STO, since this activity is very © 2017 American Chemical Society
Received: May 7, 2017 Revised: August 24, 2017 Published: August 25, 2017 19758
DOI: 10.1021/acs.jpcc.7b04352 J. Phys. Chem. C 2017, 121, 19758−19766
Article
The Journal of Physical Chemistry C
2. EXPERIMENTAL DETAILS High-purity stoichiometric STO (001) single crystals, provided by MTI Corporation (Richmond, CA), were used in this study. The crystals were not intentionally doped, and the Cr concentration (Cr is lower than 0.5 ppm wt) was lower than in previous papers, such as those by Stokowski et al.24 and Kim et al.26 Due to the relevance of possible charge transfer processes between transition metal impurities on the light emission features observed in our work, Table 1 lists the
has been observed in STO. It has been associated with spinforbidden optical transitions 2Eg → 4A2g of Cr3+ centers incorporated in the crystal during growth.23−26 The vibronic side-bands, dominant on the low-energy side, have been attributed to the contribution of lattice modes and possibly a local mode. The emission yield of the NIR bands has been shown to increase with temperature between 50 and 100 K and then decrease above such temperature up to RT.29 The decrease in this temperature range has been attributed to nonradiative decay processes, in accordance with the temperature dependence of the fluorescence lifetime of Cr3+ in STO. The yield at or near RT is very low, making the PL difficult to observe. It is worthwhile mentioning that the photoconductivity of the material presents a quite similar excitation spectrum to that for the PL.29 Although the 1.55 eV fluorescence of STO has been attributed to electronic transitions of a Cr3+ ion, there is little work currently available for excitation with ion beams (ionoluminescence, IL). This technique provides remarkable advantages over those using other excitation sources.30−33 First, the very high electronic excitation rate (electronic stopping power, Se) for energetic ions offers a higher sensitivity to detect low Cr concentrations. Moreover, in comparison to monochromatic light (laser) sources, IL presents a very broad excitation energy range, involving in-gap as well as conduction and valence band levels. On the other hand, the IL data may serve to better portray the features and dynamics of the irradiation-induced structural damage. Finally, the irradiationinduced lattice defects, mainly oxygen vacancies, allow real-time investigation of their synergistic interaction with chromium dopants. Consequentially, the IL results will provide a more complete picture of the Cr3+ luminescence behavior in perovskites and may provide new insights into their electronic structure and photocatalytic response. Novel luminescence response of SrTiO3:Cr to 3 MeV proton irradiation is investigated in relation to the generated oxygen disorder. Irradiations have been performed at temperatures of 100 K, 170 K, and RT (i.e., on the cubic phase of the crystal), and the spectra cover a wide wavelength range from 1.3 to 3.3 eV. Under 3 MeV proton irradiation, isolated vacancies and interstitials are expected resulting from either electronic excitation or elastic ion-atom scattering (nuclear energy loss, Sn). The paper presented here is in the line with previous luminescence experiments on SrTiO334−39 and essentially follows the same methodology used in our previous work.39 In this reference, a well-resolved emission band peaked at 2.0 eV has been observed at 100 K and is partially resolved at 170 K. It has been attributed to electrons self-trapped (polarons) as Ti3+ adjacent to an oxygen vacancy,27−29,34−39 and it is, therefore, considered to be an indicator of the generated oxygen disorder. In the present work, a close correlation has been found between the evolution of this band and the Cr3+ luminescence yield. Specifically, a noticeable decay of the NIR Cr luminescence on increasing fluence has been correlated with the growth of the 2.0 eV luminescence emission, which should correspond to the formation of oxygen vacancies during irradiation. Thus, the two luminescence emissions correspond to the two final fates of the e−h (electron−hole) pairs excited by the irradiation: holes trapped at Cr3+ (leading to Crn+), whereas the electron partners become self-trapped at oxygen vacancies. We expect that these new data provide a more complete understanding of the role of oxygen vacancies on the electronic structure and luminescence behavior of STO.
Table 1. List of Impurities Concentration of STO (100) Single Crystals from MTI Corporation (Purity: 99.99742%) According to the Manufacturer Analyses element
NRM (ppm wt)
Li B Na Mg Al Si P S Cl K V Cr Fe Co Ni Nb Ba Pb
