Pure hexagonal -NaGdF4 and Eu 3+ or Pr 3+-doped -NaGdF4 nanocrystals in shape of nanorods of 140 nm of length have been synthesized using a simple and rapid coprecipitation method. XRD analysis evidenced pure single phases exhibiting characteristic luminescence of Eu 3+ or Pr 3+ ions when excited under UV or blue radiations. Samples gave rise to intense orange-red emission and quite white emission in the case of respectively Eu 3+ or Pr 3+ doped samples. Optical properties are discussed in the frame of Judd-Ofelt theory and considering that an energy transfer occurs between active ions. Notably it was found that the quantum-cutting by a two-step energy transfer from Gd 3+ to Eu 3+ can improve the red emission of Eu 3+ ions under near-VUV excitation. Such quantum-cutting mechanism has been also considered in the case of Pr 3+ doped -NaGdF4. However, the efficiency of both processes in comparison with literature suggest that only one part of the energy in the excited states within Gd 3+ can be transferred to Eu 3+ for its red emission. In the case of Pr 3+ ions, the energy is more probably released through the self-trapped exciton emission certainly due to the lack of resonant VUV excitation.
Cubic α-NaYF 4 nanometer-sized crystals doped with Eu 3+ , Tb 3+ , Yb 3+-Eu 3+ or Yb 3+-Tb 3+ were synthesized by an original coprecipitation route. The obtained nanoparticles exhibited primary particles showing cubic shape with sizes ranging between 35 and 65 nm. The singlyor co-doped nanophosphors exhibited strong red (Eu 3+) or green (Tb 3+) fluorescence upon ultraviolet (UV) or near infrared (NIR) excitation, which resulted respectively from down-or up-conversion processes occurring in their structure. Spectroscopic properties were investigated on the basis of emission spectra as well as luminescence decays. From the emission spectra of Eu 3+ doped samples, the Ω 2 and Ω 4 Judd-Ofelt intensity parameters were calculated. The concentration quenching of the Eu 3+ or Tb 3+ ion emissions in singly-doped or Yb 3+ co-doped α-NaYF 4 were ascribed to resonant cross-relaxations. The main derived interaction between the active ions was evidenced as an electric dipole-dipole one through fitting the decays curves with the Inokuti-Hirayama model. The critical distances and energy transfer microparameters for the transfer processes were given showing very short range interaction. The dependence of integral up-conversion intensity on the NIR energy of the beam power was measured. The results indicated a two-photon process based on a cooperative energy transfer.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.