Near-infrared to visible and ultraviolet upconversion in TiO 2 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. TiO 2 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 Er 3+ (1, 2, 10 at%) and Yb 3+ (1, 10 at%) ions allowed us to achieve thin film upconverters with a crystallized and amorphous host. Upon 980 nm laser excitation Er 3+ exhibits upconversion with the main emission in green ( 2 H 11/2 → 4 I 15/2 , λ em ≈ 525 nm) and weak emission in red ( 4 F 9/2 → 4 I 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 TiO 2 :Er and TiO 2 :Er,Yb thin films were calculated based on time-resolved emission measurements.