Material properties of IR-to-IR down-converting Er and Nd-Doped ZrO₂ nanopowders

Specially tuned luminescent lanthanide-doped IR-to-IR down-converting nanopowders are utilized as taggants to tag and track explosives and valuables. Zirconia (ZrO₂)-based oxides doped with an Er or Nd dopant offer a cheaper alternative to fluoride taggants. Maximum IR-to-IR down-conversion intensity was achieved with low concentrations (1 mol%) of Er or Nd dopant. Er-doped ZrO₂ powders possessed strong emission bands from 1444 nm to 1600 nm. Nd-doped ZrO₂ powders possessed strong emission bands from 1055 nm to 1151 nm as well as 1323 nm–1473 nm. No visible light emission was detected for any of the doped compositions. For all doped compositions, increasing the phase fraction of monoclinic structure substantially increased the luminescent intensities and is hypothesized to be a result of decreased lattice symmetry and oxygen vacancy concentration decreasing the number of non-radiative pathways for energy transfer. Our results suggest that Er and Nd-doped ZrO₂ represents an excellent candidate for an IR-IR taggant system because it possesses a wide array of unique emission spectra which may be tailored by controlling dopant chemistry, dopant concentration, and annealing temperature.