Time-resolved Raman and luminescence spectroscopy of synthetic REE-doped hydroxylapatites and natural apatites
Résumé
Using continuous and time-resolved spectroscopy, we investigate Raman and luminescence signals from synthetic hydroxylapatites doped with trivalent REE including Dy 3+ , Eu 3+ , Nd 3+ , and Sm 3+ , as well as REE in natural apatites, with laser excitations at 532 nm and 785 nm. We demonstrate that time-resolved spectroscopy is an extremely efficient method to tone down the luminescence from Raman spectra or, alternatively, to investigate the luminescence signal without the interference from the Raman contribution. Time-resolved luminescence spectroscopy is found to be a powerful technique for generating specific high-quality luminescence spectra for the REE emission activators in apatites by using appropriate combinations of delay and gate width for the time synchronization of the laser pulse and ICCD detector. This allows for the unambiguous detection and identification of the activators by avoiding the overlapping of various emission signals in the luminescence spectra. This is particularly useful in the case of natural samples, which often have several activators for luminescence. In the case of synthetic REE-doped apatites, a quenching process for luminescence due to activator concentration is seen for Eu 3+ and Sm 3+ , i.e. the higher the concentration, the shorter the luminescence decay time. The interpretation of luminescence decay time in natural apatites is promising but more complex because of energy transfers between the various luminescence activators present in the crystal lattice. Luminescence is a powerful technique for detecting the presence of REE in apatites down to ppm levels, though quantifying the concentration is still a challenge.
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