Effect of the heat treatment on the spectroscopic properties of Er3+-Yb3+-doped transparent oxyfluoride nano-glass-ceramics

Er3+-doped and Er3+-Yb3+-co-doped transparent oxyfluoride glass-ceramics (GCs) containing LaF3 nano-crystals (NCs) have been prepared by the melting-quenching method and adequate thermal treatments. A detailed structural characterization, performed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM), confirms the precipitation of LaF3 NCs with a size in the 10-15 nm range. The crystal growth is fast in the very beginning, from a few min up to 3-5 h of treatment at 620 °C and then it slows down, up to 20 h. A SiO2 enriched viscous barrier is formed around the NCs, thus preventing further crystal growth. The incorporation of Er3+ and Yb3+ ions into the NCs is proved by HR-TEM and energy dispersive X-ray spectroscopy (EDXS) analysis. Photoluminescence measurements have been performed upon visible and near infrared excitation. The energy transfer between Yb3+ and Er3+ ions is evidenced by the enhancement of the near infrared and Up-Conversion (UC) emissions of Er3+ ions in the co-doped samples under 980 nm excitation. A moderate back transfer from Er3+ to Yb3+ ions has been also observed under near infrared excitation of Er3+ ions at 800 nm. Upconverted green ((2H11/2,4S3/2) → 4I15/2) and red (4F9/2 → 4I15/2) emissions were observed in all samples and attributed to a two photon process. The strong enhancement of the UC emission for the co-doped glass-ceramics samples further confirms the incorporation of the rare-earth ions in the LaF3 NCs with a lower maximum phonon energy environment which reduces multi-phonon relaxation rates. Different red-to-green emission ratios are obtained depending on the heat treatment conditions for the glass-ceramic samples.