The role of Cr3+ and Cr4+ in emission brightness enhancement and sensitivity improvement of NIR-emitting Nd3+/Er3+ ratiometric luminescent thermometers

Abstract

Despite the numerous advantages of lanthanide based luminescent thermometers, a major limitation is the limited brightness associated with the low absorption cross section of the 4f–4f transitions. In this paper, co-doping with Cr^3+/4+ is proposed as a strategy to enhance the near-infrared luminescence emission of Nd³⁺/Er³⁺-based luminescent thermometers, as well as to modulate the temperature sensitivity of the ratio of Nd³⁺ to Er³⁺ in YAG microcrystals. It was found that an increase in the Cr³⁺ concentration leads to the enhancement of the YAG:Nd³⁺,Er³⁺ emission intensity via the Cr³⁺ → Ln³⁺ sensitization process. The quantitative analysis of the absolute brightness indicates the 30-fold enhancement of their near infrared emission upon broad white light excitation for 20% Cr³⁺ with respect to the non co-doped with the Cr³⁺ counterpart. In addition, the phonon-assisted nature of the simultaneously occurring Cr³⁺ → Ln³⁺ and Nd³⁺ → Cr⁴⁺ → Er³⁺ energy transfers leads to a 7-fold increase of the relative temperature sensitivity of the ratio of Nd³⁺ to Er³⁺ emission. In conclusion, co-doping with transition metals can enhance simultaneously the luminescence brightness and the temperature sensitivity of NIR luminescent thermometers, holding promise for sensitive and robust remote temperature sensing at physiological temperatures.