Abstract
Samples of 0.01% and 0.3% Tm3+-doped β-NaYF4 show upconverted UV luminescence at 27 660 cm-1 (361 nm) after blue excitation at 21 140 cm-1 (473 nm). Contradictory upconversion mechanisms in the literature are reviewed and two of them are investigated in detail. Their agreement with emission and two-color excitation experiments is examined and compared. Decay curves are analyzed using the Inokuti-Hirayama model, an average rate equation model, and a microscopic rate equation model that includes the correct extent of energy transfer. Energy migration is found to be negligible in these samples, and hence the average rate equation model fails to correctly describe the decay curves. The microscopic rate equation model accurately fits the experimental data and reveals the strength and multipolarity of various interactions. This microscopic model is able to determine the most likely upconversion mechanism.
Highlights
Upconversion (UC) is the absorption of two or more low-energy photons and the subsequent emission of a high-energy photon.[1]
Two Tm3+–Tm3+ energy transfer processes are responsible for the upconverted UV luminescence in b-NaYF4:Tm3+ after 3H6 - 1G4 blue excitation at 21 140 cmÀ1: the cross-relaxation step 3H6 + 1G4 - 3H5 + 3H4 and the energy transfer upconversion step 3H4 + 1G4 - 3F4 + 1D2
The alternative mechanism consisting of the energy transfer upconversion step 1G4 + 1G4 - 3F3 + 1D2 and non-radiative relaxation 3F3 - 3H4 cannot explain all experimental data
Summary
Upconversion (UC) is the absorption of two or more low-energy photons and the subsequent emission of a high-energy photon.[1] Several trivalent lanthanide ions are commonly used for this purpose, for example singly doped or codoped into insulator lattices. Tm3+ ions can emit light at different wavelengths from the NIR to the UV and show upconversion in singly doped samples under NIR, red, and blue excitation.[2,3,4,5] Tm3+ codoped with Yb3+ shows strong blue and UV emission under 980 nm excitation.[6,7,8]. The detailed mechanism of singly doped Tm3+ upconversion under different excitation energies has been studied in several crystalline and vitreous environments. These processes may be responsible for the efficient blue and UV emission in samples codoped with Yb3+.7
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