**4. Conclusions**

In this work, the impact of the host material composition on the temperature-dependent luminescent properties of vanadium-doped nanocrystalline garnets was investigated. It was demonstrated that the incorporation of Ga3+ ions into the Y3Al5−<sup>x</sup>GaxO12:V structure enables modification of the emission color of the phosphor by the stabilization of the vanadium ions on the V4<sup>+</sup> oxidation state. Taking advantage from the fact that V4<sup>+</sup> ions, due to their similar ionic radii, mainly occupy the octahedral site of Ga3+ ions, the enlargement of their amount leads to the increase of their emission intensity. Moreover, a growing number of V4<sup>+</sup> ions cause a shortening of the average V5<sup>+</sup>–V4<sup>+</sup> distance facilitating interionic energy transfer between them. Conducted studies regarding the influence of temperature on the emission intensities of the vanadium ions at different oxidation states reveal that the most susceptible to thermal quenching is the V5<sup>+</sup> emission intensity. On the other hand, due to the V5<sup>+</sup> → V4<sup>+</sup> energy transfer, the V4<sup>+</sup> emission intensity increases with temperature. The higher the amount of Ga3+ ions in the host, the more evident the enhancement of V4<sup>+</sup> emission intensity and the lower the threshold temperature above which this enhancement occurs. Taking advantage form the fact of opposite temperature dependence of V5<sup>+</sup> and V4<sup>+</sup> emission intensities, their ratio was used to create the bandshape luminescent thermometer, to the best of our knowledge, the highest relative sensitivity of V-based luminescent thermometers up to date Smax, 2.64%/◦C, 2.56%/◦C and 2.49%/◦C for Y3Al2Ga3O12 (at 0 ◦C), Y3AlGa4O12 (at −20 ◦C) and Y3Ga5O12 (at −100 ◦C), respectively. With an increase of the Ga3+ concentration, the value of the relative sensitivity, as well as the temperature at which *Smax* was observed, can be modified. Additionally, it was found that the higher the contamination of Ga3+ ions, the more evident the change of the chromatic coordinates of emitted light with temperature changes in a −150 ◦C–300 ◦C temperature range. As was proven in this manuscript, the introduction of the Ga3+ ions in the garne<sup>t</sup> host enables modification of the

performance of nanocrystalline luminescent thermometer like: its usable temperature range, maximal value of the relative sensitivity, as well as the temperature at which maximal sensitivity can be obtained. The dominant effect, which is responsible for described modification of the luminescent properties of V doped luminescent thermometers, is the increase of the V5<sup>+</sup> → V4<sup>+</sup> energy transfer probability associated with the growing number of the crystallographic sites that can be occupied by the V4<sup>+</sup> ions. This shortens the average distance between the interaction ions, facilitating energy transfer process.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/10/1375/s1, Figure S1: (**a**), (**b**), (**c**) The luminescence decay profile of <sup>V</sup>5+, V4<sup>+</sup> and V3<sup>+</sup> ions for different Ga3+, respectively; Figure S2: Emission spectra of V-doped nanocrystals recorded in the range of −150 ◦C–300 ◦C.

**Author Contributions:** Formal analysis, K.K. and L.M.; Investigation, K.K. and L.M.; Methodology, K.K. and K.L.; Writing—original draft, K.K. and L.M.; Writing—review and editing, L.M.

**Funding:** The "High sensitive thermal imaging for biomedical and microelectronic application" project is carried out within the First Team programme of the Foundation for Polish Science, co-financed by the European Union under the European Regional Development Fund.

**Conflicts of Interest:** The authors declare no conflict of interest.
