Insights into the Matrix Emission of Sol–Gel-Derived LaAlO₃ Nanoparticles Under Synchrotron UV Excitation

Wide-bandgap mixed-oxide nanoparticles are attractive materials for light emission, sensing, and energy applications. The balance between radiative (light-emitting) and non-radiative (heat-dissipating) processes is key for making efficient luminescent materials. Lanthanum aluminate perovskite LaAlO₃ is a promising host among such materials because it has a wide band gap, high thermal and chemical stability, and a flexible perovskite structure. Luminescence properties of undoped LaAlO₃ sol–gel nanoparticles as well as those of nanoparticles doped with Gd and Tm ions were investigated using synchrotron light at the DESY PETRA III Beamline P66. Emission spectra of the matrix revealed a wide band in the 350–650 nm spectral range. The peak of these bands is located around 420 nm at room temperature. Decreasing the temperature leads to the appearance of an additional long wavelength component at 460 nm. The maxima of excitation spectra of the matrix emission is located at 220 nm, which is near the bandgap edge. These measurements were interpreted in regard to reflection and transmission measurements of a commercial LaAlO₃ single crystal. It was found that the matrix emission is very weak for the single crystal, whereas for nanoparticles, it is much more intense. Analysis of luminescence decay curves showed that emission lifetimes of the matrix emission are typical for excitonic emission, in good agreement with the location of its excitation band being near the fundamental absorption edge. Further, the 420 and 460 nm emission bands are characterized by different temperature behaviors that can be used for luminescence thermometry. The obtained experimental results allowed us to show that mixed oxide perovskites are good hosts for rare-earth doping, their matrix emission has an excitonic nature, and to discover that LaAlO₃ nanoparticles have potential for optical sensing applications.