Abstract
Divalent europium doped barium orthosilicate is a very important phosphor for the production of light emitting diodes (LEDs), generally associated to the green emission color of micron-sized crystals synthesized by means of solid-state reactions. This work presents the combustion synthesis as an energy and time-saving preparation method for very small nano-sized Ba2SiO4 particles, flexibly doped to acquire different emission energies. The size of the resulting spherical nanoparticles (NPs) of the green emitting Ba2SiO4:Eu2+ was estimated to about 35 nm applying the Scherrer equation and further characterized with aid of atomic force microscopy (AFM) as well as scanning electron microscopy (SEM). This phosphor is able to build homogeneous luminescent suspensions and was successfully down-sized without changing the optical properties in comparison to the bulk phosphors. Besides the X-ray diffraction (XRD) analysis and the different types of microscopy, the samples were characterized by luminescence spectroscopy. Undoped Ba2SiO4 NPs are not luminescent, but show characteristic red emission of the 5D0 → 7FJ (J = 0–4) electronic transitions when doped with Eu3+ ions. Moreover, these orthosilicate nanoparticles generate blue light at low temperatures due to impurity-trapped excitons, introduced by the partial substitution of the Ba2+ with Sr2+ ions in the Ba2SiO4 lattice causing a substantial distortion. A model for the temperature behavior of the defect luminescence as well as for their nature is provided, based on temperature-dependent luminescence spectra and lifetime measurements.
Highlights
Alkaline earth silicate phosphors activated with Eu2+ ions have been extensively studied over the past 40 years due to their excellent thermal stability, water resistance, non-sensitivity to air and extraordinary optical properties
Powder X-ray diffraction analysis was used for the confirmation of the crystal structure of the synthesized compounds
The post-synthesis annealing step is performed for improving the crystallinity and/or reducing the Eu3+ to Eu2+ ions
Summary
Alkaline earth silicate phosphors activated with Eu2+ ions have been extensively studied over the past 40 years due to their excellent thermal stability, water resistance, non-sensitivity to air and extraordinary optical properties. The phosphors reported within these works consist of micrometer-sized crystals These so-called bulk phosphors are usually prepared by means of solid state reactions, requiring high temperatures and long reaction time. In contrast to numerous papers dealing with bulk Ba2SiO4:Eu2+ materials very few publications about the production of submicron-sized Ba2SiO4:Eu2+ crystals are available This is remarkable in view of the importance of that phosphor. In these works, the crystal sizes are generally larger than 0.5 μm, prepared by applying laborious and time-consuming methods as sol-gel synthesis, spray pyrolysis and vapor phase techniques [8,9]. To the best of our knowledge, the production of barium orthosilicate nanoparticles by means of combustion synthesis is reported in this work for the first time. Undoped barium orthosilicate nanoparticles as well as those doped with Eu3+ and Sr2+ have never been produced or investigated before
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