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Polarized luminescence of Anistropic LaPo4 Eu Nanocrystal Polymorphs

by Anne-Marie - published on

Among the various types of phosphors, lanthanides perform the most informative luminescence spectra identifying the chemical structure of the host material. However, there is a disconnection between the profound understanding of the lanthanide optics established in 20th century and the current research trend focusing on the lighting and probing applications using lanthanide-doped nanostructures. It is thus an essential task to rationalize the correlation between the optical signals and the material’s structure in the newly accessed nanoscale regime.

In this article published on JACS (doi:10.1021/jacs.8b03983), we address the tracking of the structural evolution of LaPO4 nanocrystals by observing the luminescence of doped Eu3+ ions. We analyze systematically the emission polarization corresponding to the Eu3+ site symmetry state being modified during the crystal phase transition. Further, we demonstrate the superior sensitivity of our spectroscopic method revealing the initiation of the phase transition occurring at the non-trivially low temperature, while such a minor change was not detectable by the X-ray diffraction analysis. Although in the past lanthanide dopants were used to investigate bulk material structures, the varying luminescence during the phase transition has not been systematically studied so far. Especially, the luminescence polarization of lanthanides has been an almost forgotten characteristic, which can indeed provide abundant information on material’s fine structure. We also recently introduced an application of this property as a novel orientation probe (Nature Nanotechnology 12, 2017, 914-919).

Figure. (a) XRD patterns, (b) photoluminescence spectra (λex = 394.5 nm), and (c) SEM images of the aligned LaPO4:Eu (5%) nanorod films for different final annealing temperatures (scale bars 200 nm).

Our result integrating the spectroscopic observation with the nanocrystal polymorphism suggests to utilize lanthanide dopants as a probe for the structural evolution of various phase changing materials. This would enable precise analysis of the delicate and often inhomogeneous structures of nanomaterials bearing the emphasized effects lead by the small size and high surface area. Also, this spectroscopic way can be easily combined with the in-situ experiments for small amount of samples (ex. single nanoparticle microscopy), that are not compatible with the heavy X-ray instruments.

Polarized Luminescence of Anisotropic LaPO4:Eu Nanocrystal Polymorphs, E. Chaudan, J. Kim, S. Tusseau-Nenez, P. Goldner, O. L. Malta, J. Peretti, and T. Gacoin, Journal of American Chemical Society, published online 3 July 2018. DOI: 10.1021/jacs.8b03983