Development of SiO2- CaF2 Based Oxyfluoride Glass-Ceramic for Up- Conversion Study

Abstract

Up-conversion (UC) is a non-linear optical process based on a two-photon mechanism which includes the energy conversion from the lower energy region (Near-infrared) to the higher energy (Visible) region. UC process has been used efficiently for various optical applications like solar cells, imaging, bio-sensing, and gene therapy. For a successful UC, the primary requirement is the presence of a low phonon host matrix in which rare- earth elements need to be embedded. Lanthanides like ytterbium (Yb3+) and Erbium (Er3+) are highly recommended for the UC process because of their simple ladder pattern energy level schemes. They exhibit luminescence through f-f transitions and high resistance to photo bleaching. Fluorides like PbF2, NaYF4, LaF3, and CaF2 are the most commonly used host for UC. However, the fluoride low phonon matrix has a problem of weathering that reduces the fluorescence efficiency. Embedded low phonon host in oxide glass matrix can solve this problem and increase the chemical stability. CaF2 has been chosen as a low phonon host in the present work, and silica has been chosen as an oxide matrix. Instead of making a core-shell particle that involves a complicated procedure, a simple sol-gel technique has been attempted for SiO2-CaF2 glass-ceramic nanoparticle synthesis. The concentration of CaF2 has been varied from 5 mol% to 25 mol%. Er3+ has been chosen as an activator, and Yb3+ has been chosen as a sensitizer. The amount of Yb3+ and Er3+ has also been varied to observe the effect of concentration on luminescence property. Further, in the same matrix, Na2O has been added to create non-bridging oxygen in the matrix. An increase in non-bridging oxygen results high refractive index and its effect on luminescence property has also been examined. All the glass-ceramic samples have been prepared by the sol-gel method. The effect of CaF2 addition and the Er3+/Yb3+ doping on the crystallization behavior has also been examined through the Differential thermal analysis (DTA) technique. The samples were heat-treated based on the DTA measurement to develop the CaF2 phase. Their morphology has been examined through the transmission electron microscope (TEM). All samples have been tested for their luminescence characteristics by exciting it by 980 nm laser. The target was set to obtained both green and red emissions. Most of the samples have shown both green and red luminescence. All the prepared samples' cell viability has been studied, which indicated more than 90% cell compatibility. The protein adsorption study of rare-earth-doped Up Conversion Nano Particles (UCNPs) revealed a lower adsorption rate due to surface charge repulsion. Therefore, the proposed synthesized nano glass- ceramic can be used for bio-imaging applications.