Research on nanoparticles (NPs) with red and near infra-red (NIR) emitting persistent luminescence is challenging, which impacts emergency lighting, radiation detection, medical imaging, and enhancement of solar cell devices. The ZnGa2O4 spinel material is a potential host for transition metals and rare-earth ions displaying different emission colors. Herein, doped ZnGa2O4 red emitting nanophosphors, in a near spherical shape, were synthesized via hydrothermal method at 180 °C for 4 h, 18 h, and 24 h with different doping concentrations (Cr3+/Eu3+). The crystallites were measured ~ 10 nm nm. The band gap of doped ZnGa2O4 nanomaterials varies from 4.2 eV to 4.4 eV. The excitation band’s position and shape are different, which consist of 220–300 nm, 310–360 nm, 370–480 nm, and 490–650 nm, attributing to a combination of the ZnGa2O4 host excitation, the O−Cr charge transfer band and the electron transitions of 4A2 → 4T1 (te2), 4A2 → 4T1 (t2e) and 4A2 → 4T2 of Cr3+. The ratio of the NIR emission intensity (700 nm, 2E → 4A2) to the blue (455 nm, of the matrix) increases with increasing Cr3+ concentrations to 0.010 mol.%, when only ZnGa2O4 is optically excited. Under the excitation of 254 nm, a broad band red emission at around 697 nm (with FWHM as 70 nm) was operative and synergistically increased when Eu3+ was added. Under UV irradiation, these nanomaterials showed enhanced red emission with increased exposure time, and long-time X-ray irradiation suggested afterglow. Cathodoluminescence suggests effective energy transfer to Cr3+ from host at around 703 nm, due to 3d intra-shell transition of Cr3, under electron irradiations, and the emission range 300–600 nm is under expressed by peaks of Cr3+ ions and/or Eu3+ ions, as visualized in optical luminescence.

Photo and ionized radiation-stimulated red emission in codoped ZnGa2O4 nanoparticles

ACS Spring 2022

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