Transparent nanophosphor films with high quantum efficiency through cold compaction
A facile method to improve the transparency, mechanical stability and quantum yield (QY) of luminescent nanoparticle films is presented. Porous layers of the crystalline Y2O3:Eu nanophosphor with an average particle size of 24 nm were produced by flame spray pyrolysis. The coatings were sandwiched between two rigid substrates and exposed to cold isostatic pressing (CIP) at 900 MPa. The second substrate could be removed afterwards without damage. Compaction increased the particle packing density up to 60 vol% and nearly eliminated light scattering in the films, thus making them transparent throughout the visible range. At the same time, the luminescence decay time constant decreased from 3.3 to 1.4 ms confirming an increase in the internal QY of the nanophosphor from 0.31 to 0.60. A good match between the experimental data with the nanocrystal cavity model of radiative decay of photoluminescence was demonstrated. The increase of the external brightness of the coatings was limited to 28% (for thin coatings it even decreased) due to the onset of light trapping by multiple internal reflection. Deliberate introduction of scatterers on the surface of the film allowed the extracted intensity to increase by at least 70%, thus reaching 55% of the maximum brightness of a commercial micrometer-sized Y2O3:Eu phosphor powder. The CIP-processed coatings possessing a final thickness between 1 and 12 μm behaved as smooth crack-free solid films with excellent mechanical stability. The proposed method of cold compaction offers an advantage of rapid processing avoiding a high-temperature post-treatment for all applications of transparent phosphor films and other optical coatings.