Spectroscopic evidence of preferential excitation of interfacial EuIII by interfacial energy transfer process on core@shell nanoparticles

Abstract

The hierarchical core@shell nanostructure in upconversion nanoparticle (UCNP) has allowed high intensities of upconversion emission by interfacial energy transfer (IET) mechanism, which can also account for high downshifting emission intensities observed for core@shell nanostructures. It is well accepted that the emission of active ions, sensitized by IET is originated from ions located into narrow core@shell interface, which has been shown by physically separation of donor and acceptor ions in the core@shell nanostructure. Herein we have spectroscopic proof of the preferential sensitization of the ions into the core@shell interface by IET. We have analyzed the excitation and emission spectra profiles of the EuIII sensitized by GdIII ions on hierarchically structured β-NaYF4 nanoparticles. In this work, EuIII emission occurs through a downshifting mechanism that is like the last step of the energy transfer pathway from GdIII to active ions in nanostructured UCNPs as part of a known UC-IET processes. Photophysical results indicate that IET sensitizes mainly the EuIII ions located at the core@shell interface, where these ions experience a variety of distorted chemical environments. Such a non-symmetrical electrical fields results in an inomogeneous broadening of 5D0 → 7F2 transition emission band. The preferential, but not unique, sensitization of EuIII ions by energy transfer from GdIII ions located at distorted interface may shine light on the upconversion emission bands intensities observed in several works describing the IET in UCNPs and spectrocopically confirms that GdIII -mediated emission occurs spatially in a narrow core@shell interfacial region.