Undoped CdSiO3 for contactless thermometer in the cryogenic range: pH-dependent intrinsic defects and emitting colour

Intrinsic photoluminescence (PL) of undoped CdSiO3 metasilicate samples were investigated with two main focuses. Firstly, a range of experimental strategies were employed aiming to identify the defect centres that generate the PL emission in this material. The second was related to how the sample preparation conditions can tune the PL emission for practical proposed devices. This work was divided in four parts, as follows: 1- Sample preparation: CdSiO3 was synthesized via sol-gel route varying the pH of the starting solution from 1.0 to 9.0. The final powders were calcined at 1000 ​°C for 3 ​h in open atmosphere; 2- Structural investigations: X-ray powder diffraction (XRD) were performed, associated with Rietveld refinement, in order to investigate the crystalline phases as a function of the pH of the starting solutions. Samples calcined using pH 1 and 3 displayed only the CdSiO3 metasilicate phase whereas for samples synthesized using pH values of 5, 7 and 9, a SiO2 secondary phase was present in low concentration. 3- PL measurements: The PL measurements were performed for excitation ranges from 4.5 to 7.0 ​eV (275–165 ​nm) and emission energies from 1.7 to 4.1 ​eV (730–302 ​nm), and in temperature ranges from 62 ​K up to room temperature. The behaviour of the PL emission, as a function of temperature and excitation energies, clearly pointed out that the intrinsic luminescence of CdSiO3 is composed by four emission channels (4 emission peaks) centred around 2.10 ​eV (peak 1), 2.66 ​eV (peak 2), 3.07 ​eV (peak 3) and 3.48 ​eV (peak 4). Also, it was possible to conclude, mainly due to the temperature effect on these four peaks, that peaks 1 and 2 are due to charges trapped in Cd and O vacancies and peaks 3 and 4 are due to self-trapped exciton (STE), generated after excitation and pinned at Cd vacancies and O vacancies, which link two Si tetrahedra. 4- Possible practical applications: One of the possible applications is related to the tuneability of the emission colour according to the pH of the starting solution. This parameter is crucial to the PL emission, since pH affects the ratio of hydrolysis and condensation of the Si precursor during the synthesis, resulting on different concentration of intrinsic defects which are involved in the luminescent process. This is the key aspect that makes the PL emission tunable. The second application arises from the temperature effect on the PL emission, that presents a variation of the fluorescence intensity ratio of about 2 orders of magnitude when the samples are cooled from room temperature down to 62 ​K. This quite interesting result indicates that CdSiO3 can be explored as a contactless fluorescence thermometer for cryogenic applications.