Upconversion photoluminescence of Ho$^{3+}$-Yb$^{3+}$ 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 light-emission 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.