Influence of the non-thermally coupled three-photon band on the performance of Y₂O₃: Yb³⁺/Er³⁺ single-particle nanothermometers

Nanoscale temperature sensing is increasingly being explored to study physical-chemical processes at the nanoscale. Since direct contact thermometers, like thermistors, are usually not suitable for high spatial resolution applications, luminescence thermometry raises as an alternative. In this case, a nanoparticle can be used as a probe and the temperature measurement is done by analyzing the luminescence emission. One of the most exploited candidates as luminescence nanothermometer probes are dielectric nanoparticles doped with lanthanide ions (Ln³⁺). They offer a high photostability and the possibility of using non-cytotoxic host matrices, targeting biological applications. In a typical approach, one can measure the temperature from Ln³⁺-based systems by recording their emission luminescence spectrum and computing the Luminescence Intensity Ratio between two so-called thermally coupled levels [1], which should follow the Boltzmann distribution. Another great advantage of using Ln³⁺-doped systems is to exploit the upconversion (UC) process, being possible to excite the thermally coupled levels with light of lower energy. In this sense, codoped Yb³⁺/Er³⁺ systems are among the most efficient ones. In such cases, the electrons in the ground state of the Yb³⁺ ions can be excited with a laser near 980 nm. Depending on the Yb³⁺ and Er³⁺ ions' proximity, the energy can be efficiently transferred from Yb³⁺ to Er³⁺ ions. Consecutive energy transfer (ET) steps can thus lead to the UC phenomena, populating higher-lying electronic states. In Er³⁺ ions, the thermally coupled levels, ²H₁₁/₂ and ⁴S₃/₂ need a two-step ET of 977 nm excitation. They decay radiatively to the ground state (⁴I₁₅/₂) resulting in luminescent bands in the green spectral region, with central wavelengths of ∼525 nm and ∼550 nm, respectively. Depending on the excitation power density, higher-order multiphoton absorption can also happen, therefore populating states above the thermally coupled ones.