Energy transfer and luminescent properties of Eu3+, Tb3+, Eu3+-Yb3+and Tb3+-Yb3+ doped α-NaYF4 nanophosphors prepared by coprecipitation route

Abstract Cubic α-NaYF4 nanometer-sized crystals doped with Eu3+, Tb3+, Yb3+-Eu3+ or Yb3+-Tb3+ 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 singly- or co-doped nanophosphors exhibited strong red (Eu3+) or green (Tb3+) 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 Eu3+ doped samples, the Ω2 and Ω4 Judd-Ofelt intensity parameters were calculated. The concentration quenching of the Eu3+ or Tb3+ ion emissions in singly-doped or Yb3+ co-doped α-NaYF4 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.