Enhanced photoluminescence and structural properties of (Cr,Zn) co-doped Al2O3 phosphors synthesized via solution combustion

Abstract

Chromium-doped alumina (Cr-doped Al₂O₃) is a versatile material utilized in various applications such as sensors, phosphors, and bioimaging. It possesses attractive properties such as sharp photoluminescence (PL), long PL lifetime and excellent photostability. Enhancing its PL intensity is crucial for expanding its applicability. One approach to achieve this is by incorporating additional elements into the sample matrix.  In this study, Cr-doped Al₂O₃ and (Cr,Zn)-co-doped Al₂O₃ samples were synthesized by solution combustion. X-ray diffraction (XRD) analysis showed that both samples displayed sharp, clear diffraction peaks consistent with the alpha-aluminum oxide (α- Al₂O₃) structure. Notably, the fundamental crystal structure did not change when Zn was introduced. The presence of Zn increased the crystallite size, suggesting that the zinc ions play a role in promoting the crystal growth and crystallization of Al₂O₃. Based on photoluminescence (PL) decay curves for (Cr,Zn)-co-doped Al₂O₃, the lifetime was estimated at around 3 ms. The energy band gap was calculated using UV-Vis spectroscopy data and was found to decrease from 4.31 eV to 4.23 eV after Zn addition, likely contributing to a 34% increase in PL intensity. This increment is attributed to the modification of the crystal field environment resulting from the addition of Zn²⁺ ions. A phosphor-converted light-emitting diode (pc-LED) was fabricated by depositing (Cr,Zn)-co-doped Al₂O₃ phosphor onto a 395 nm LED chip. The pc-LED with a 694 nm wavelength falls in the deep red spectrum, which is particularly beneficial for plant growth. The CCT of 2984 K suggests a warm light source, which could be useful for specific plant applications. However, the color purity of 24.6% indicates a mix of other wavelengths, which might affect efficiency depending on the targeted plant species.