Upconversion spectroscopy and properties of NaYF4 doped with Er3+, Tm3+ and/or Yb3+

Abstract A spectroscopic investigation of NaYF 4 powders doped with several different concentrations of Er 3 + , Tm 3 + and/or Yb 3 + is described. Rare earth-doped NaYF 4 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 NaYF 4 is presented here for the first time, demonstrating that the dominant phonon modes in NaYF 4 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 NaYF 4 samples. Excitation spectroscopy up to ∼ 70000 cm - 1 in an NaErF 4 sample reveals a multitude of Er 3 + 4f excitations, including the illustrious 2 F(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 Er 3 + , Yb 3 + codoped NaYF 4 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 + F 5 / 2 2 → F 7 / 2 2 , Er 3 + F 9 / 2 4 → I 15 / 2 4 and Er 3 + S 3 / 2 4 → I 15 / 2 4 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 Er 3 + ions.