Tunable Broadband NIR PL Emissions with (Nd3+/Er3+) Codoped TiO2 via Synergetic Energy Transfer

Tunable photoluminescence (PL) emissions in the near-infrared (NIR) region are the most advantageous qualities that benefit NIR light-dependent applications such as thermosensing, biosensing, solar cells, LED, bioimaging technologies, etc. Rare-earth trivalent (+3) doping is essential for the NIR light sources due to their unique transitions. Especially, multi-rare-earth doping is being highlighted for the effect of generating broad-band emissions. This work presents a concept of tunable NIR PL emission via codoping of (Er/Nd) into TiO2 at varied excitation wavelengths and stoichiometry ratios. We discovered the synergetic energy transfer (ET) mechanism among the Er3+ and Nd3+ energy states, which led to tuning the PL emissions over the NIR region to the mid-IR region. A broad-band PL emission from 850 to 1700 nm at an indirect excitation wavelength of 360 nm confirms the energy transfer (ET) occurring from TiO2 (host) to the lanthanide of Er3+ and Nd3+ (guests). In addition, the direct excitation (visible) excitation-dependent PL emission and their associated electron decay lifetime suggest the possibility of ET between Er3+ and Nd3+, which is evidenced by high energy transfer efficiency (ETE), whereas the ETE from Er3+ to Nd3+ is calculated to be 46% measured at the high intense PL emission of (1532 nm, Er: 4F(13/2) to I-15/2 ) under the excitation of 528 nm. These results are consistent with the enhanced QY values up to 90% for the (Er/Nd) codoped TiO2 compared to the single Er-doped TiO2. Similarly, ETE from Nd to Er is almost 48% attained at the emission of (1068 nm, Nd: 4F(3/2) to 4I(11/2))under 588 nm excitation and their changes in the QY values indicate the ET occurrence from Nd3+ to Er3+. Thus, it is hypothesized that the PL investigations support the synergetic ET between Nd3+ and Er3+ is responsible for the tunable PL emissions.