E. van der Kolk, P. Dorenbos, K. Krämer, D. Biner and H. U. Güdel
High-resolution luminescence spectroscopy study of down-conversion routes in NaGdF₄:Nd³⁺ and NaGdF₄:Tm³⁺ using synchrotron radiation
Phys. Rev. B 77, 125110/1-7 (2008)                 

Abstract: Down-conversion in lanthanide doped luminescent materials is a promising route to significantly enhance the energy efficiency of silicon solar cells, plasma display panels, or mercury-free lighting tubes because it results in the emission of two photons for each absorbed higher energy photon. The Gd³⁺/Eu³⁺ ion couple shows down-conversion of vacuum-ultraviolet light into visible light with an efficiency close to 190%. The low absorption strength of the ⁶G_7/2 levels of Gd³⁺ (the starting point of the down-conversion process), however, prevents efficient excitation of the down-conversion process and therefore application. We have performed a high resolution luminescence spectroscopy study, using synchrotron radiation, in order to investigate the possibility to use the strong 4f5d absorption transitions of Nd³⁺ and Tm³⁺ to sensitize the high energy ⁶G_7/2 level of Gd³⁺ in the phosphors NaGdF₄:2%Nd³⁺ and NaGdF₄:2%Tm³⁺. Tm³⁺ appears to be an efficient sensitizer of the ⁶G_7/2 state of Gd³⁺. It was also found that sensitization is followed by two successive energy transfer processes exciting two Tm³⁺ ions in the ³H₄ state which results in the emission of two infrared photons for one absorbed vacuum-ultraviolet photon. Nd³⁺ is not a good sensitizer of the ⁶G_7/2 state in NaGdF₄. Instead Nd³⁺ efficiently transfers its energy by cross relaxation to the lower energy ⁶D_J states of Gd³⁺ but leaving the Nd³⁺ ion excited in the ⁴F_3/2 state. Successive energy transfer from Gd³⁺ back to Nd³⁺ excites a second Nd³⁺ ion in the ⁴F_3/2 state. Also, in this case, two infrared photons can be emitted for one absorbed vacuum-ultraviolet photon.