Revealing the nature of the Zn₂GeO₄ bluish-white emission in microwave-assisted hydrothermal synthesized nanorods

Recently, zinc germanate (Zn2GeO4, ZGO) has emerged as a material with significant potential for various applications due to its unique optical properties. Undoped, manganese (Mn) and and chromium (Cr)-doped ZGO were synthesized through microwave-assisted hydrothermal methods. The as-synthesized and thermal annealed materials were morphological and structurally characterized, and the optical properties of these willemite prismatic nanorods were thoroughly investigated. A room temperature (RT) bandgap energy close to 236 nm (∼5.25 eV) was obtained, which is slightly higher than the values reported so far in the literature. Furthermore, optically active absorption and luminescence bands from the ultraviolet to near-infrared were identified. All samples present intrinsic defect absorption with a maximum at 271 nm (∼4.58 eV) and a charge transfer Mn2+-O2- absorption band at 315 nm (∼3.94 eV). In addition, the so-called bluish-white structureless broad emission band is observed at RT at ca. 480 nm (∼2.58 eV) for all the analyzed samples. Our investigation indicates that this band is due to the overlap of two emitting centers: an intrinsic defect originating a blue luminescence (BL) and the 4T1→6A1 intraionic transition of Mn2+ leading to a green luminescence (GL), confirming Mn as a common contaminant in this matrix. For the Cr-doped samples, the thermal annealing treatment was seen to promote changes in the visible and near infrared (NIR) intraionic absorption bands. This enabled the identification of the presence of trivalent and tetravalent Cr ion charge states. Additionally, temperature-dependent photoluminescence measurements were carried out in the case of the as-synthesized ZGO:Mn, which is the sample with the highest GL intensity. It was found that the intensity of GL decreases with temperature (from 18 K to RT), with a thermal activation energy of 18 ± 2 meV for the nonradiative processes that compete with the observed luminescence. Moreover, persistent emission from the Mn2+ GL was recorded for at least 5 s and was attributed to multi-trapping/de-trapping processes occurring at different trap depths, which are responsible for the distinct decays observed.