High thermal sensitivity and the selectable upconversion color of Ln, Yb:Y6O5F8 nanotubes

Abstract

Yb³⁺-sensitized, Ln³⁺(Er³⁺, Pr³⁺)-doped Y₆O₅F₈ micron-sized bundles of highly crystalline individual nanotubes have been prepared through hydrothermal syntheses at 185 °C. The inhomogeneous broadening observed in their optical spectra is associated with the large distribution of crystal fields around Y³⁺(Ln³⁺) sites in the orthorhombic Pbcm Vernier-type Y₆O₅F₈ host. Based on ratiometric analyses of the thermal evolution of intensities of near-infrared NIR (∼978 nm)-excited green upconversion emissions corresponding to ²H_11/2, ⁴S_3/2 → ⁴I_15/2 Er³⁺ transitions, the temperature sensing behaviour of Er, Yb:Y₆O₅F₈ was studied. This thermal sensor exhibits a very high sensitivity S = 0.0060 K⁻¹ at physiological temperatures (22–50 °C), which surpasses the S value found for Er, Yb:β-NaYF₄ at these temperatures, and a maximum S = 0.0082 K⁻¹ at ∼225 °C. Also under NIR diode laser excitation, the color of the upconverted light from codoped Pr, Er, Yb:Y₆O₅F₈ nanotubes can be selected by the control of the Pr³⁺ concentration and by the excitation regime and power density. Samples with low Pr³⁺ concentration emit green light, and the selection between bluish-green light and white light has been demonstrated with high Pr³⁺ concentration (2 mol%), under pulsed or continuous wave excitation, respectively.