Down- and up-conversion photoluminescence of ZrO<inf>2</inf>:Ho³⁺ and ZrO<inf>2</inf>:Ho³⁺/Yb³⁺ coatings formed by plasma electrolytic oxidation

Abstract

This paper discusses ZrO2:Ho3+ and ZrO2:Ho3+/Yb3+ coating formation by the plasma electrolytic oxidation (PEO) of zirconium in alkaline electrolyte containing Ho2O3 and Ho2O3/Yb2O3 particles, respectively. The morphology, chemical and phase composition, as well as down- and up-conversion photoluminescence (PL) of the coatings, were investigated. Down-conversion PL emission spectra of ZrO2:Ho3+ excited by 280 nm radiation are composed of broad PL band associated with ZrO2 host and bands corresponding to f–f transitions of Ho3+. The main PL emission bands of Ho3+ at around 540 nm and 550 nm are assigned to 5F4→5I8 and 5S2→5I8 transitions, respectively. The down-conversion PL intensity of ZrO2:Ho3+ excited by 450 nm radiation (transition 5I8→5G6) strongly depends on the concentration of Ho3+ incorporated into ZrO2 coatings. That is, it depends on the duration of the PEO process and concentration of Ho2O3 particles in electrolyte. PL intensity increases with rising Ho3+ concentration in ZrO2 coatings, reaching a maximum Ho:Zr ratio of about 18⋅10−3 and shows concentration quenching for higher values. Excitation of ZrO2:Ho3+/Yb3+ coatings by 980 nm laser diode results in the appearance of a strong green (5F4,5S2→5I8), weaker red (5F5→5I8), and NIR (5F4,5S2→5I7) up-conversion emissions. An increase of the PL intensity with increasing Yb3+ concentration indicates an efficient transfer of energy from excited Yb3+ ions to Ho3+ ions.