Luminescence properties of lanthanide and ytterbium lanthanide titanate thin films grown by atomic layer deposition

Lanthanide based luminescent materials are highly suitable as down conversion materials in combination with a UV-absorbing host material. The authors have used TiO₂ as the UV-absorbing host material and investigated the energy transfer between TiO₂ and 11 different lanthanide ions, Ln³⁺ (Ln = La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb) in thin films grown by atomic layer deposition. They have also investigated the possibility to improve the overall energy transfer from TiO₂ to Yb³⁺ with a second Ln³⁺, in order to enhance down conversion. The films were grown at a substrate temperature of 300 °C, using the Ln(thd)₃/O₃ (thd = 2,2,6,6-tetramethyl-3,5-heptanedione) and TiCl₄/H₂O precursor pairs. The focus of the work is to explore the energy transfer from TiO₂ to Ln³⁺ ions, and the energy transfer between Ln³⁺ and Yb³⁺ ions, which could lead to efficient down conversion. The samples have been characterized by x-ray diffraction, x-ray fluorescence, spectroscopic ellipsometry, and photoluminescence. All films were amorphous as deposited, and the samples have been annealed at 600, 800, and 1000 °C in order to investigate the correlation between the crystallinity and luminescence. The lanthanum titanium oxide samples showed a weak and broad emission centered at 540 nm, which was absent in all the other samples, indicating energy transfer from TiO₂ to Ln³⁺ in all other lanthanide samples. In the amorphous phase, all samples, apart from La, Tb, and Tm, showed a typical f-f emission when excited by a 325 nm HeCd laser. None of the samples showed any luminescence after annealing at 1000 °C due to the formation of Ln₂Ti₂O₇. Samples containing Nd, Sm, and Eu show a change in emission spectrum when annealed at 800 °C compared to the as-deposited samples, indicating that the smaller lanthanides crystallize in a different manner than the larger lanthanides. Energy transfer from Ln³⁺ to Yb³⁺ was observed neither in the amorphous or annealed samples. On the other hand, Yb³⁺ was found to be an efficient sensitizer for Ho³⁺'s 1200 nm emission.