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 6G7/2
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 6G7/2 level of Gd3+ in the phosphors NaGdF4:2%Nd3+
and NaGdF4:2%Tm3+. Tm3+ appears to be an efficient
sensitizer of the 6G7/2 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 6G7/2
state in NaGdF4. Instead Nd3+ efficiently transfers its energy
by cross relaxation to the lower energy 6DJ
states of Gd3+ but leaving the Nd3+ ion excited in the
4F3/2 state. Successive energy transfer
from Gd3+ back to Nd3+ excites a second Nd3+ ion
in the 4F3/2 state. Also, in this case, two infrared
photons can be emitted for one absorbed vacuum-ultraviolet photon.