Size- and concentration-dependent Eu2+/Eu3+ mixed luminescent characteristics of rare-earth-doped CaF2 nanoparticles and their monolithic epoxy nanocomposites

CaF2 is a promising luminescent host material for a wide range of applications, due to its high transparency, non-hygroscopy, low refractive index, low density, low backscattering efficiency, and abundance in nature. Here we report the photoluminescence (PL) and cathodoluminescence (CL) characteristics of rare-earth-doped CaF2 nanoparticles and their composite monoliths with epoxy with visible emissions. The CaF2 nanoparticles of ca. 36, 108 and 305 nm were prepared by a facile solution-mixing process and subsequent hydrothermal treatment. As the particle size decreases, the luminescence intensity increases for single-doped particles, while oppositely decreases for codoped particles because of energy transfer. Further, Eu2+- and Tb3+-codoped CaF2 nanoparticles exhibit red emission, providing an efficient path for red emission through an energy transfer by Tb3+ bridging without direct Eu3+ doping. The effects of size and concentration are found to be pronounced in CL than PL, as the charge transfer between Eu2+ and Eu3+ reduces the concentration quenching in CL. The luminescence emission characteristics of epoxy/CaF2 monoliths are different from those of particles due to the interactions with the epoxy molecules, and thus the energy transfer is hindered in PL while not in CL, enabling them for high-energy irradiation applications.