Time-resolved Raman and luminescence spectroscopy of synthetic REE-doped hydroxylapatites and natural apatites

Abstract

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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“…Phosphates such as apatite and merrillite have characteristic yet distinct Raman spectra. Their Raman spectra have been described in terrestrial and extraterrestrial samples 27,61–64 . The symmetric stretching vibration mode of PO 4 3− provides the most intense Raman peak between 950 and 990 cm −1 , labeled as ν 1 .…”

“…The total intensities of the luminescence peaks can then be represented against the gate delay, see Figure S5B. The lifetime is then calculated as the slope at origin of the regression line going through the experimental points, giving a value of 0.56 ms. Fau et al 27 revealed a linear correlation between lifetime and Eu concentration inside synthetic Eu‐doped apatites. Here we obtain a lower value compared to the lifetime in apatite.…”

“…All samples were also analyzed using a custom-built, TRR and TRL spectrometer as described by Fau et al 24,27 The laser is a nanosecond (1.2 ns FWHM, 1 mJ per pulse) pulsed, diode-pumped solid-state laser operating at 532 nm with a 10-to 2000-Hz repetition rate. The laser is slightly defocused at the sample surface through a microscope objective (MPlan Apo Mitutoyo 20X, NA = 0.42) yielding an analytical footprint of $90 μm to control the laser irradiance at the sample surface (Fau et al 24 ; see below).…”

“…Their Raman spectra have been described in terrestrial and extraterrestrial samples. 27,[61][62][63][64] The symmetric stretching vibration mode of PO 4 3À provides the most intense Raman peak between 950 and 990 cm À1 , labeled as ν 1 . Apatite has a single ν 1 band at 964 cm À1 , and merrillite has a doublet of peaks around 955 and 975 cm À1 .…”

“…The presence of other elements at trace level like Mn 2+ or REE 3+ inside the crystal lattice by substitution in the Ca sites can produce luminescence bands in the Raman spectra, induced by laser excitation. 27 Examples of such luminescence activators are Er 3+ (540-550 nm), Dy 3+ (590 nm), Eu 3+ (570-580 nm), and Sm 3+ (560-570 and 600 nm). Mn 2+ is characterized by a very broad emission centered at 595 nm generating a strong background in Raman spectra.…”