Luminescence properties of Yb:Er:KYsub(3)Fsub(10) nanophosphor and thermal treatment effects

Abstract

In this work, we present the spectroscopic properties of KY3F10 nanocrystals activated with erbium and codoped with ytterbium ions. The most important processes that lead to the erbium upconversion of green and red emissions of Er3+ were identified. A time-resolved luminescence spectroscopy technique was employed to measure the luminescence decays of 4S3/2 and 4F9/2 excited levels of Er3+ and to determine the upconversion processes and the luminescence efficiencies of erbium in the visible. Analysis of the luminescence kinetics in Yb:Er:KY3F10 shows a rapid upconversion (Up1) for the green emission with a time constant of 0.31 μs after pulsed laser excitation at 972 nm for as synthesized nanocrystals, which is faster than the time constant measured for the bulk crystal (23 μs). In addition, it is observed a second upconversion process (non-resonant) (Up2) responsible for the red emission (Er3+), which competes with Up1 process. However, the luminescence efficiency of the green emission (4S3/2) is observed to be very low (1.6%) for the as synthesized nanocrystal (25 °C). Nevertheless, it increases with the nanopowder heat treatment reaching an efficiency of 99% (T = 550 °C) relative to the bulk crystal. Similar luminescence behavior was observed for the 4F9/2 level (Er3+) that emits red emission. X-ray diffraction analysis of nanopowder by Rietveld method reveled that the mean crystallite size remains unchanged (8.3–12.3 nm) after thermal treatments with T ∼ 400 °C, while the 4S3/2 luminescence efficiency strongly increases to 20%. The luminescence dynamics indicates that Er3+ ions distribution plays a determinant role in the luminescence efficiency of green and red emissions of Er3+ besides also the strong influence on the upconversions processes. The observed luminescence effect is caused by the non-uniform Er3+ (and Yb3+) ions distribution due to the nanocrystal grown, which introduces a concentration gradient that increases towards the nanoparticle surface. This concentration effect produces strong (Er × Er) cross-relaxations depleting the excited states populations of 4S3/2 and 4F9/2 levels and their luminescence efficiencies in KY3F10 nanocrystals. The concentration gradient is very accentuated in the as synthesized nanocrystal and gradually decreases with the thermal treatments where the dopant ions can migrate through the lattice towards the nanocrystal’s interior to get a more uniform and random distribution, which is reached after heat treatment to T = 550 °C.