Broadband near-Infrared Sensitization of Visible Upconversion Luminescence in V3+ and Mo3+ Co-Doped Cs2NaYCl6

Abstract

Crystals of Cs2NaYCl6 singly doped with Mo3+ or V3+ and a series of V3+:Mo3+ codoped Cs2NaYCl6 crystals were studied by low-temperature optical absorption and luminescence spectroscopy with particular focus on their near-infrared (NIR) to visible (vis) photon upconversion properties. In the codoped crystals 15 K Mo3+ 2T2g → 4A2g upconversion luminescence around 14300 cm-1 is not only observed after direct excitation into the weak and narrow Mo3+ 2Eg and 2T1g ground-state absorptions around 9700 cm-1, but also after excitation into the broad V3+ 3T1g → 3T2g absorption band between 10000 and 12500 cm-1. This is because the excitation energy is transferred from V3+ 1T2g to Mo3+ 2Eg, and this process is studied as a function of the relative V3+ and Mo3+ dopant concentrations. The Mo3+ 2T2g higher excited-state luminescence is not affected by the V3+ codopants, and this is because neither of the Mo3+ 2T2g emissions spectrally overlaps with V3+ absorption. The upconversion mechanisms which are active in the V3+:Mo3+ codoped Cs2NaYCl6 crystals at 15 K are identified on the basis of one- and two-color upconversion luminescence excitation spectroscopy. V3+ is found not only to act as a sensitizer for Mo3+ 2Eg, but also to participate in a nonradiative energy transfer upconversion (ETU) process. At 15 K, there is a strong spectral overlap of V3 + 3T1g → 3T2g ground-state absorption (GSA) with Mo3+ 2Eg → 4T2g excited-state absorption (ESA), leading to efficient GSA/ESA upconversion in 0.8% V3+:0.3% Mo3+:Cs2NaYCl6 at 15 K. Excitation with the broadband NIR output of a tungsten lamp induces more than an order of magnitude more 15 K vis upconversion luminescence in 0.8% V3+:0.3% Mo3+:Cs2NaYCl6 than in purely 0.3% Mo3+ doped Cs2NaYCl6.