Down- and up-conversion luminescence of Tm3+/Ho3+ codoped LaOF nanoparticles.

Strong blue and green visible lights are obtained in Tm3+/Ho3+-codoped LaOF tetragonal nanocrystals (NCs) through down and up fluorescence conversions when the sample is excited selectively with ultraviolet and infrared photons, respectively. The excitation spectrum by detecting at 545 nm (Ho3+:5S2→5I8) reveals two peaks at 348 nm (Ho3+:5I8→5G3) and 355 nm (Tm3+:3H6→1D2), which indicates that the energy transfer process from Tm3+ to Ho3+ has occurred when Tm3+ is excited with 355 nm photons in Tm3+/Ho3+:LaOF. There is no energy transfer from Ho3+ to Tm3+ when Ho3+ is excited with 348 nm photons in codoped samples because the near resonant Ho3+ levels are significantly quenched by multiphonon relaxation. But enhanced green fluorescence of Ho3+ is obtained due to the ion situated at the low symmetry site in Ho3+/Tm3+:LaOF NCs. The intensity patterns of the up-conversion fluorescence would be fine-tuned by adjusting Tm3+ concentration and excitation wavelength. Spectroscopic analyses in down- and up-conversion processes indicate that Tm3+ ions possibly prefer to stay at the surface of NCs than Ho3+ ions. [ABSTRACT FROM AUTHOR]