Near-infrared to visible and ultraviolet upconversion in TiO₂ thin films modified with Er and Yb

Upconversion as a modification strategy to enhance the utilization of sunlight in titanium dioxide photoanodes with an internal upconverter was investigated. TiO₂ thin films containing an Er activator and Yb sensitizer were deposited in the magnetron sputtering process on conducting glass, amorphous silica, and silicon. Scanning electron microscopy, energy dispersive spectroscopy, grazing incidence X-ray diffraction, and X-ray absorption spectroscopy allowed assessment of the thin film composition, structure, and microstructure. Optical and photoluminescence properties were measured by means of spectrophotometry and spectrofluorometry. Changing the content of Er³⁺ (1, 2, 10 at%) and Yb³⁺ (1, 10 at%) ions allowed us to achieve thin film upconverters with a crystallized and amorphous host. Upon 980 nm laser excitation Er³⁺ exhibits upconversion with the main emission in green (²H_11/2 → ⁴I_15/2, λ _em ≈ 525 nm) and weak emission in red (⁴F_9/2 → ⁴I_15/2, λ _em ≈ 660 nm). For a thin film with a higher ytterbium content (10 at%) a significant increase in red emission and upconversion from NIR to UV was observed. The average decay times of green emission for TiO₂:Er and TiO₂:Er,Yb thin films were calculated based on time-resolved emission measurements.