Inkjet fabrication of highly efficient luminescent Eu-doped ZrO₂ nanostructures

We have demonstrated for the first time an inkjet fabrication of highly efficient luminescent structures based on Eu-doped ZrO₂ nanocrystals (3.4 ± 0.3 nm), with a refractive index close to the one of the bulk materials. The nanoparticles were synthesised using a nonhydrolytic method in benzyl alcohol where the particles were post treated using acetic acid, leading to the formation of a stable colloid. It was shown that the non-polar methyl group of the acetic acid is responsible for its penetration through the hydrophobic layer all the way through to the surface of the ZrO₂, leading to the cleavage of the Zr-OCH₂C₆H₅ bond and the formation of surface acetate species and a concomitant decomposition of the zirconia superlattice. Hereby we show a new and efficient universal ink production through a multi-step process - starting from solvothermal synthesis, dispersion of nanocrystals in water, and adaptation of the rheological parameters of the resulting sols. Eventually, we were able to obtain inks that we used for the production of optical coatings, monolayer luminescent-protected holography and anti-counterfeiting printing. These structures, obtained at room temperature through inkjet printing, present dense xerogel structures with high optical transparency, a high refractive index and more efficient luminescence compared with the non-homogeneous structures produced as a mixture of rare-earth elements and nanocrystals.