Correlation between Cr³⁺ Luminescence and Oxygen Vacancy Disorder in Strontium Titanate under MeV Ion Irradiation

Abstract

Strontium titanate (SrTiO 3 ), 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 Cr 3+ centers in stoichiometric SrTiO 3 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 Cr 3+ 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 Ti 3+ in the close vicinity of oxygen vacancies. This correlation suggests irradiation-induced oxidation of the Cr 3+ (Cr 3+ → Cr n+ , n > 3) via trapping of irradiation-induced holes, while the partner electrons are self-trapped as Ti 3+ . These new results provide effective guidelines for further understanding the electronic and photocatalytic behavior of STO:Cr 3+ .

Recent Advances on Carrier and Exciton Self-Trapping in Strontium Titanate: Understanding the Luminescence Emissions

“…Ionoluminescence from STO samples showing a well-resolved extrinsic emission associated with chromium impurities [5] has been obtained at 100 K under 3 MeV H irradiation. The impurity initially presenting the +4 valence state captures electrons during irradiation and converts to Cr 3+ , yielding a well-known and intense emission band with a vibronic structure centered at around 1.55 eV (zero-phonon line).…”

“…These processes are consistent with charge interplay between oxygen vacancies and Cr impurities, as illustrated in Figure 9. This correlation can be interpreted as a balance between the concentration of electron-polarons (Ti 3+ ) trapped at the oxygen vacancies, responsible for the red emission, and the decrease of the Cr 3+ population associated with the trapping of the hole partners by the Cr 4+ [5].…”

“…Strontium titanate (SrTiO 3 ), designated from here on as STO, is a very useful multifunctional ceramic material exhibiting remarkable and fascinating physical and chemical properties. This paradigmatic example of oxide perovskites has a high potential for electrical, optoelectronic, photocatalytic, and magnetic applications, and constitutes a basis for the so-called complex oxide-based microelectronics [1][2][3][4][5]. Moreover, STO is a model transition metal oxide material with a simple crystal structure that serves as a reference to understand the behavior of all other oxide perovskites, such as BaTiO 3 , KTaO 3 , and PbTiO 3 .…”

“…More relevant for our review are the studies based on the monitoring of oxygen vacancy defects and Ti 3+ states at the surfaces and its impact on the luminescence behavior [20,21]. However, it is important to mention that the luminescence spectra and behavior observed under different excitation sources from UV light [10,17,[22][23][24][25][26][27][28][29][30] to X-rays [16], electrons [31], and ion beams [5,32], having very different penetration depths, show a similar pattern apparently associated to a bulk response. In spite of these considerations, we believe that surface luminescence is beyond the scope of our review and deserves independent attention.…”

“…Time resolved PL spectra observed at 4.5 K are shown as inset, (b) Temperature dependence of weighted mean lifetimes <τ> deduced from Equation (4) in[22], (c) Temperature dependence of the integrated PL intensity. Solid line represents fitting curve according to Equation(5) in[22] with activation energy ∆E = 43 meV. (a.u.…”