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

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 Cr3+/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+ -> Ln(3+) 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+ -> Ln(3+) 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.

Automated summary

This paper proposes co-doping with Cr3+/4+ as a strategy to enhance the near-infrared luminescence emission of Nd3+/Er3+-based luminescent thermometers and modulate the temperature sensitivity of the ratio of Nd3+ to Er3+ in YAG microcrystals. The study shows that increasing the Cr3+ concentration leads to enhanced emission intensity and significant improvement in temperature sensitivity.