J. F. Suyver, J. Grimm, M. K. van Veen, D. Biner, K. W. Krämer and H. U. Güdel
Upconversion spectroscopy and properties of NaYF₄ doped with Er³⁺, Tm³⁺ and/or Yb³⁺
J. Lumin. 117, 1-12 (2006)                 

Abstract: A spectroscopic investigation of NaYF₄ powders doped with several different concentrations of Er³⁺, Tm³⁺ and/or Yb³⁺ is described. Rare earth-doped NaYF₄ is known to be a very efficient near-infrared to visible upconverter. The overview emission spectra for all samples are presented and from these the upconversion efficiency is calculated. Raman spectroscopy of undoped NaYF₄ is presented here for the first time, demonstrating that the dominant phonon modes in NaYF₄ lie between 300 and 400 cm^–1. The fact that these phonon modes are also the optically active ones is further demonstrated by temperature-dependent excitation spectroscopy. These surprisingly low-energy phonon modes explain the extraordinarily high upconversion efficiency of the rare earth-doped NaYF₄ samples. Excitation spectroscopy up to 70000 cm^–1 in an NaErF₄ sample reveals a multitude of Er³⁺ 4f excitations, including the illustrious ²F(2)_5/2 one that has not been observed in excitation spectroscopy before. From the low-temperature power-dependence of the emission intensities for an Er³⁺, Yb³⁺ codoped NaYF₄ sample, it is concluded that the dominant upconversion mechanism at low temperature is a different one than at room temperature. From direct excitation, the lifetimes of the Yb^{3+ 2}F_5/2 ²F_7/2, Er^{3+ 4}F_9/2 ⁴I_15/2 and Er^{3+ 4}S_3/2 ^çI_15/2 emissions are determined as a function of temperature for all samples. At elevated temperatures, a significant decrease in the green lifetime is observed, which is correlated to a simultaneous quenching in the luminescence intensity. This quenching is ascribed to cross-relaxation between two nearby Er³⁺ ions.