Dual luminescent nano-thermometry based on the selective excitation of optical centers in CaF2:Er3+ nanoparticles

We present a study about the feasibility of using the upconverted luminescence of CaF2:Er3+ nanoparticles (NPs), under near infrared excitation within the 4I15/2 → 4I11/2 absorption band, for thermal detection. In order to perform this study, it has been needed to investigate the upconversion processes as function of the Er3+ concentration and the excitation wavelength by using a tunable Ti:Sapphire laser. The results obtained indicate that, under this excitation scheme, the green (2H11/2:4S3/2) and red (4F9/2) emitting levels are populated through different multiphoton processes: Excited state absorption and energy transfer upconversion. Therefore, the ratio between the green and red emissions is also dependent on the Er3+ concentration and the excitation wavelength. Measurements based on site-selective emission/excitation, performed at room temperature, indicate that in these NPs there are at least two different optical centers whose emission bands can be isolated by an appropriate selection of the excitation wavelength. One these centers is compatible with the presence of isolated Er3+ ions, while the other one is tentatively related to the presence of clusters or aggregates. The upconverted luminescence of each optical center is analyzed as function of temperature by means of the ratiometric Fluorescence Intensity Ratio technique. The results indicate that both optical centers exhibit adequate thermal performances and relative sensitivities to be used in thermal sensing