Upconversion photoluminescence of Ho3+-Yb3+ doped barium titanate nanocrystallites: Optical tools for structural phase detection and temperature probing

Abstract

Authors have explored the photo-physical properties of Ho 3+-Yb 3+ doped Batio 3 nanocrystals and proposed an intuitive method to probe temperature and crystal phase structure of the matrix. Structural phase change of doped crystals was analyzed in terms of their X-ray diffraction, and it was confirmed through second harmonic generation. We give insights on upconversion of energy of lightemission in Ho 3+-Yb 3+ : Batio 3 nanocrystals upon a 980 nm laser-light excitation and subsequently, the excited state dynamics were studied with the help of dependence of upconversion luminescence on excitation power and measuring-temperature. to understand the nature of occupancies of the Ho 3+ ions at the ti-and Ba-sites, we performed site-selective, time-resolved spectroscopic measurements at various crystal phases. Based on the lifetime analysis, it is inferred that the Ho 3+ ions are present at two types of sites in barium titanate lattice. One of those is the 6-coordinated Ti-site of low symmetry, while the other one is the 12-coordinated Ba-site of higher symmetry. The upconversion emission of the nanocrystals are found to be temperature-sensitive (12 to 300 K), indicating possible use as a self-referenced temperature probe. An analysis of the temperature dependent emissions from 5 f 4 and 5 S 2 levels of Ho 3+ ions, gives a maximum value of temperature sensitivity ~ 0.0095 K −1 at 12 K. Furthermore, we observe a sharp change in the luminescence intensity at ~180 K due to a ferroelectric phase change of the sample. the correlation of upconversion luminescence with the results of X-ray diffraction and second harmonic generation at different crystal phases implies that the frequency upconversion may be used as a probe of structural change of the lattice. Ferroelectric titanates of a perovskite ABO 3 structure (where A and B are usually divalent and tetravalent cations, respectively) are widely used in many applications due to their excellent dielectric, magnetic and electro-optic properties 1-4. In general, a perfect perovskite has a cubic unit cell of a simple crystal structure of a CaTiO 3mineral perovskite. Here, the tolerance factor (T f) governs the nature of distortions from its ideal shape, e.g., a rhombohedral or an orthorhombic GdFeO 3-type crystal structure is observed on effectively small T f ≤ 1, while polytypic structures turn up at T f > 1. The wide range of physical properties of ABO 3-type oxides are mainly dependent on the relative sizes of the ions, electronic configuration of the ions, dopants and substituents 5,6. For example, Pandey et al. have investigated structure and electronic/magnetic properties of such oxides BiFeO 3 − PbTiO 3 , BiFeO 3-BaTiO 3 , etc. in this series 7,8. Small ABO 3 crystallites are particularly used in liquefied petroleum gas sensing and humidity sensing 9-12. Further, barium titanate (BaTiO 3) is an important nonlinear optical material which has numerous applications in light modulation, optical memory storage, optical switching and electro-optic phase modulator 13-16. Additio...

Holmium(iii) molecular nanomagnets for optical thermometry exploring the luminescence re-absorption effect

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