Luminescence properties of co-doped Eu³⁺, Bi³⁺ Lu₂O₃/polyvinylpyrrolidone films

The effect of the polyvinylpyrrolidone (PVP, MW = 130,000) molar content, PVP/Lu = 1, 2.5, 4 and 5; on the photoluminescent and structural properties of sol-gel derived Lu₂O₃:Eu³⁺, Bi³⁺ has been analyzed. Thin hybrid films were deposited by means of the dip-coating technique on silica quartz substrates. Films deposited at 700 °C presented a cubic structure, with non-preferential orientation, even with the presence of PVP. The photoluminescence (PL) spectrum and Comission Internationale de l'éclairage (CIE) chromaticity diagram of films revealed a reddish Eu³⁺ emission at 612 nm (⁵D₀ → ⁷F₂) with an excitation at 320 nm of the Bi ions (6s² → 6s6p), showing a highly-effective energy transfer process form Bi³⁺ to Eu³⁺ luminescent centers. On the other hand, the color temperature of the samples is strongly dependent on the PVP content, as a consequence of the observed difference on the branching ratios of ⁵D₀ → ⁷F_J transitions of europium ions. Lifetime studies present two different behaviors for the thin films: A non-exponential nature for the lower PVP contents, and a simple exponential nature for the highest PVP one, showing that the PVP tends to promote a better dissolution of segregates and, therefore, increases the lifetime of the Eu³⁺ emission.