Cooperative luminescence from low temperature synthesized α-Al₂O₃: Yb³⁺ phosphor by using solution combustion

Ytterbium (Yb³⁺) doped aluminium oxide (Al₂O₃) powder phosphor was successfully synthesized by solution combustion method. The structure, vibrational bending modes, particle morphology, chemical composition, and photoluminescent properties were analyzed using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and photoluminescent (PL) spectroscopy respectively. The XRD patterns confirmed that Al₂O₃ crystallized in its stable polymorphic hexagonal α-Al₂O₃ phase with space group R3c. The average crystallite size estimated from Debye-Scherrer equation was 29 nm. The FTIR confirmed the Al-O vibrations associated with α-Al₂O₃ and the Yb-O vibrations. The SEM data showed that the powders were made up of particles with different shapes but well defined boundaries. Furthermore, the SEM images show that the Yb³⁺ ions resided on the grain boundaries. When the powders were excited using a 325 nm He-Cd laser, the emission was observed in the near infrared (NIR) at 975 nm due to the ²F_5/2→²F_7/2 transition of Yb³⁺. However, the bluish green emission with a maxima at ~ 480 nm was observed as a result of cooperative luminescence of Yb³⁺ when the powders were excited in the NIR with an excitation wavelength of 980 nm. Cooperative energy transfer (CET) mechanism producing NIR emission for the 325 nm laser excited Al₂O₃:Yb³⁺ powders is presented and discussed.