News Release

Wide-range luminescent thermometers? – Quite a challenge!

Peer-Reviewed Publication

Light Publishing Center, Changchun Institute of Optics, Fine Mechanics And Physics, CAS

An overview of the discussion sessions in this review

image: (a) Thermodynamic foundation for optimum performance of an excited three-level Boltzmann-based luminescent thermometer; (b) Indirect upconversion excitation scheme for Gd3+ by energy transfer from the 4f15d1-related electronic states of Pr3+; (c-d) Assessment of the optimum performance of a wide-range multilevel luminescent Boltzmann thermometer based on Gd3+ in YAl3(BO3)4:Pr3+,Gd3+: (c) relative sensitivities Sr, (d) temperature uncertainty σT. view more 

Credit: by Dechao Yu, Huaiyong Li, Dawei Zhang, Qinyuan Zhang, Andries Meijerink and Markus Suta

Luminescent thermometers become increasingly important for remote, non-invasive temperature sensing at especially small lateral dimensions, at which other, contact-based detection principles only work in a limited way or even not at all. Typical application sectors are temperature recording on catalytic surfaces or in biomedical processes.

        Especially the lanthanoid ions have emerged as leading luminescent ions in this area due to their rich electronic structure with selected radiatively emitting excited levels having energy separations in the order of typical thermal energies. In this case, the intensity ratio of the related emission peaks from two thermally coupled excited levels from the 4fn configuration follows Boltzmann’s law. This is appealing for applications as the calibration model is simple, can be linearized, and the fitting parameters identified with independently measurable quantities.

        Unfortunately, Boltzmann thermometers only offer high measurement precision in a limited temperature range that depends on the employed energy gap. A simple thermodynamic solution to overcome this obstacle and to widen of the dynamic range of the luminescent thermometer is the usage of more than two excited states.

        Aiming at high-temperature luminescence thermometry introduces several additional practical issues, however. A prominent issue is the appearance of a sloping background due to black-body radiation. As the intensity of thermal radiation scales with the fourth power of temperature and the maximum of the related emission spectrum shifts towards the visible range at higher temperatures, an ideal wide-range luminescent thermometer should emit in the ultraviolet (UV) range.

        Gd3+ (4f7) fulfills all these requirements for effective wide-range luminescence thermometry but is unfortunately a weak UV absorber. Thus, we came up with an alternative concept. Since the blue InGaN LED emitting at around 450 nm has undergone a tremendous optimization that allows to reach high incident power densities with blue LEDs very easily nowadays, we used Pr3+ (4f2) as a sensitizer for Gd3+. Pr3+ absorbs at 450 nm and under sufficiently high irradiance, this ion is excited into 4f15d1-based states with energies in the UV range, like the excited levels of Gd3+. This allows to excite Gd3+ by means of a cheap 450 nm light source.

        This conceptually different approach also limits the choice for a potential host compound. It should be stable over a wide range of temperatures, preferentially without any phase transition. In addition, the 5d orbitals are much more sensitive to the locally surrounding ligand field, which requires a host that shifts the energy of the excited 4f15d1 configuration of Pr3+ to a similar range of the excited levels of Gd3+. Finally, the host structure should intrinsically keep the doped Gd3+ ions far apart. This allows to incorporate a higher fraction of Gd3+ ions to increase the absorption strength while avoiding energy migration processes that could lead to concentration quenching of the UV luminescence of Gd3+. The huntite-type crystallizing borate YAl3(BO3)4 matches all these requirements and is an ideally suited host compound to test these requirements.

        Upon inclusion of all these design principles, we were able to demonstrate that YAl3(BO3)4:Pr3+,Gd3+ can be used as an effective luminescent thermometer with constantly low relative measurement uncertainty (< 0.1%) in the range between 30 K and 800 K. The current work therefore represents the first example of a wide-range luminescent Boltzmann thermometer that also incorporates practical concerns in its design.


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