The origin of room-temperature self-trapped-exciton emission in LiF nanoparticles

Abstract

In this contribution, we extend the current understanding of energy-trapping mechanisms in wide-band-gap inorganic crystals via the formation and decay of the self-trapped exciton (STE) using pure and copper-doped lithium fluoride (LiF) nanoparticles. The study aims at filling in some of the knowledge gaps underpinning the search for enhanced dosimetric performance, enabling novel 3D imaging techniques based on transparent materials exhibiting intrinsic scintillation phenomena. Here, LiF stands out as the lightest and highest-band-gap dielectric. We use a combination of luminescence-spectroscopy techniques at low and elevated temperatures to confirm the role of an STE triplet state as the radiative recombination center driving ionizing-radiation-induced luminescence phenomena centered at 325 nm, namely radioluminescence and optically stimulated luminescence (OSL). We demonstrate that the role of copper doping as a luminescence enhancer is based on the trapping of the self-trapped hole at room temperature, which would otherwise become mobile allowing for hopping of electronic excitation in the lattice at temperatures above 150 K. Finally, we show results from optimization studies aiming at establishing the best synthesis parameters in terms of OSL yield and discuss in more detail the role of the nanoparticle surface, possible energy trapping, and recombination pathways.