Effect of the nanoparticle size on thermometric properties of a single-band ratiometric luminescent thermometer in NaYF4:Nd3+

Due to the fact that temperature is one of the key physical quantities determining the occurrence of physical phenomena, chemical reactions or biological processes, one of the important issues to be solved is the precise determination of temperature. In order to achieve this even under demanding conditions, a lot of attention has recently been focused on methods of remote temperature readout, especially on luminescence thermometry. One of the recent and very interesting methods, the so-called single-band ratiometric luminescence thermometry, is based on the phenomenon of absorption from an excited state and allows for temperature determination based on thermally induced changes in the intensity of a single emission band under two excitation conditions, i.e., by absorption from the ground state and from the excited state. However, the complexity of the latter process in particular requires a number of studies leading to an understanding of the factors affecting the excited level occupancy, which will enable the development of highly sensitive luminescent thermometers based on this method. One of the important aspects is to investigate the influence of the size of the material and the associated occurrence of surface effects, as considered in this work. NaYF4:2%Nd3+ nanocrystals with grain sizes varying from 15 nm to 200 nm were analyzed. A regular trend of widening the useful temperature range with increasing nanocrystallite size was observed. Very high relative sensitivities were obtained, which in the nanocrystals of the largest size analyzed were as high as 19.1% K-1 at 198 K.