Terbium Ion-Mediated Energy Transfer in WO<sub>3</sub>:Tb<sup>3+</sup> and Eu<sup>3+</sup> Phosphors for UV-Sensitized White Light Emission
Chandresh Kumar Rastogi, Shilendra Kumar Sharma, Sayantan Sasmal, Raj Ganesh S. Pala, Jitendra Kumar, Sri Sivakumar
Abstract
Incorporation of suitable lanthanide (Ln3+) ions into semiconducting WO3 can be useful to produce host-sensitized luminescence for solid-state lighting applications. Codoping of another Ln3+ ion can assist in transferring energy from the host to the activator Ln3+ ion to produce bright luminescence depending upon the electronic structure of the doped system. As a case study, Eu3+ and Tb3+ ion–doped WO3 phosphors (WO3:Tb3+xEu3+y, x = 0–0.05 and y = 0–0.20) have been prepared over a wide range of doping concentrations to investigate the role of the Tb3+ ion as a sensitizer and realize host-sensitized emission from Eu3+ ions. The steady-state and time-resolved photoluminescence (TRPL) data for WO3:Tb3+xEu3+y (x = 0–0.05 and y = 0–0.10) samples confirm that Tb3+ ions assist in excitation of Eu3+ ions via sequential energy transfers from the host to Tb3+ ions followed by Tb3+ to Eu3+ ions. The energy transfer process is controlled by optimizing their doping concentrations, and a single-phase white-light-emitting phosphor with a composition WO3:Tb3+0.05Eu3+0.0005 has been developed. The electronic band structures and projected density of state plots for the WO3:Tb3+0.03125Eu3+00.03125 system obtained using density functional theory (DFT)-based simulations confirm the formation of impurity states due to Eu3+ and Tb3+ ions within the forbidden gap of WO3. Based on the TRPL and DFT data, it is confirmed that the Tb3+ ions act as a bridge between the conduction band edge of WO3 and excited states of Eu3+ ions to transfer energy and facilitate characteristic emission from europium species.