Visible upconverted and NIR luminescence properties of Er3+ and Er3+/Yb3+-doped Lu3Sc2Ga3O12 nanocrystals

Abstract

Trivalent erbium and ytterbium embedded in Lu3Sc2Ga3O12 nanocrystals in the garnet phase were synthesised employing the citrate sol-gel method. The crystalline phase and size, grain size, and effect of ion concentration and laser pump power on the photoluminescence properties of lanthanide ions were systematically characterised. XRD results showed that the present phosphor was in the cubic garnet phase with an average crystallite size of 27 nm and a lattice constant of 12.325 Å. Morphology images revealed the particles to be nearly spherical and agglomerated. Partial energy level diagrams have been constructed using diffused reflectance and excitation spectra. Stokes and anti-Stokes photoluminescence spectra were observed under 980 nm laser excitation. The near-infrared (NIR) photons (980 nm) were successfully converted into visible radiation. Upconverted luminescence spectra consisted of two strong peaks located in the green and red regions, corresponding to the 2H11/2,4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of the Er3+ ion. The intensity ratio of red to green emission bands increased with increasing Er3+ or Yb3+ concentrations as well as with the pump power of laser. Power-dependent photoluminescence spectra revealed that there were two photons involved in the upconversion mechanism. The decay profiles of the thermally coupled 2H11/2,4S3/2 levels of Er3+ showed a non-exponential nature accompanied by a shortening of lifetime as the concentration increased. CIE colour co-ordinates were located in the green region and shifted to the yellow region with increasing Yb3+ ion concentration. NIR emission spectra exhibited an intense peak at around 1535 nm, attributed to the 4I13/2 → 4I15/2 transition of Er3+ ion, the intensity of which was observed to increase with the incorporation of Yb3+ ions up to 5 mol%, confirming the existence of Yb3+-to-Er3+ energy transfer processes. Therefore, the present phosphors can be used for photovoltaic cells, solid-state light emitting diodes, and display devices.