Photoluminescence of Sm²⁺ / Sm³⁺ doped Al<inf>2</inf>O<inf>3</inf> coatings formed by plasma electrolytic oxidation of aluminum

Abstract

Sm doped Al2O3 coatings are formed by plasma electrolytic oxidation of aluminum substrate in supporting electrolyte with addition of various concentrations of Sm2O3 particles. Photoluminescence (PL) emission spectra of Sm doped Al2O3 coatings exhibit three distinct regions: the first region is related to Al2O3 PL band with a maximum positioned at about 410 nm, the second region in the range from 540 nm to 670 nm features sharp emission bands related to f–f transitions of Sm3+ ions from excited level 4G5/2 to lover levels 6HJ (J = 5/2, 7/2, 9/2), while the third region in the range from 670 nm to 750 nm is characterized by sharp emission bands corresponding to 5D0 → 7FJ (J = 0, 1, 2) transitions of Sm2+ ions. PL excitation spectrum monitored at 410 nm exhibits maximum at about 260 nm, while PL excitation spectrum monitored at the wavelength of the most intense PL of Sm3+ ions (transition 4G5/2 → 6H5/2 at 599 nm) can be divided into two regions. The broad band region from 240 nm to 300 nm with a maximum at about 250 nm is associated with the electron transfer transition from 2p orbital of O2- ions to 4 f orbital of Sm3+ ions, while the series of peaks ranging from 300 nm to 550 nm corresponds to direct excitation of the Sm3+ ground state 6H5/2 into higher levels. PL excitation spectrum obtained by monitoring the 688 nm (5D0 → 7F0) emission from Sm2+ ions shows three broad bands centered around 285 nm, 340 nm, and 480 nm, which arise from the 4f6 → 4f55d1 transitions of Sm2+ ions. Evolution of PL emission spectra excited at 250 nm indicates that at the beginning of PEO all incorporated samarium ions are present as Sm2+, while prolonged PEO process leads to oxidation of Sm2+ ions and their conversion to Sm3+ ions.