Can temperature be accurately sensed by red-green emission ratio in YNbO4: Ho3+/Yb3+ phosphor under 980 nm excitation?

Highlights: • YNbO₄: Ho³⁺/Yb³⁺ phosphor was synthesized via a solid-state reaction method. • Temperature had different effects on the green and the red UC luminescence processes. • The ratio I_R/I_G was sensitive to excitation power density and sample temperature. • The ratio I_R/I_G of Ho³⁺ on temperature may not be suitable for temperature sensing. Ho³⁺/Yb³⁺ co-doped YNbO₄ phosphor was synthesized via a high-temperature solid-state reaction technique. Excitation power density and temperature dependent up-conversion (UC) luminescence properties were systematically investigated. Under 980 nm excitation, intense green UC emissions accompanied with weak red and near infrared UC emissions were observed, which were attributed to the transitions of ⁵S₂/⁵F₄→⁵I₈, ⁵F₅→⁵I₈ and ⁵S₂/⁵F₄→⁵I₇, respectively. Varied population paths for generating the green and the red UC emissions were found from the analyses on the temperature and excitation power density dependent UC emissions. It was confirmed that 2-photon processes were responsible for both the green and the red UC emissions at room temperature. However, with an increase in temperature, the green UC emission still kept 2-photon process, and 1.5-photon process for the red UC emission occurred. In addition, the temperature dependent UC luminescence spectra displayed that the luminescence intensity ratio of the red to the green UC emissions was sensitive to both the excitation power density and the sample temperature. Thus, it was concluded that the dependence of the luminescence intensity ratio of the red to the green UC emissions of Ho³⁺ on temperature might not be suitable for accurate temperature sensing for YNbO₄: Ho³⁺/Yb³⁺ phosphor.