At the nanoscale, the ZnGa<sub>2</sub>O<sub>4</sub> spinel doped with chromium (III) is an interesting material for <i>in vivo</i> optical imaging due to its bright red persistent luminescence after UV and visible excitation. Moreover its persistent luminescent properties can be improved with the incorporation of bismuth (III) as a co-dopant without any structure changes. The nanoparticles are synthesized by soft chemistry using microwave heating in aqueous media. These very small sized nanophosphors (around 10 nm) present interesting long lasting persistent luminescence after annealing at 1000°C and they can be excited both under UV and under visible LED excitation. In this work we try to understand the mechanisms of the persistent luminescent properties of such nanomaterials. Thermoluminescence is performed to investigate trapping and detrapping processes as well as trap distribution. The chromium local environment is studied by Electron Paramagnetic Resonance. <sup>71</sup>Ga Nuclear Magnetic Resonance is used to get information on the gallium ions repartition (tetrahedral or octahedral site) in the structure. Comparison of optical properties versus local structure increases the understanding of the persistent luminescence mechanism and gives insights to the new modalities for their use as nanoprobes for <i>in vivo</i> imaging.
We investigated the blue excitable persistent luminescence properties in the Ce<sup>3+</sup>-doped garnet ceramics with the composition of Y<sub>3</sub>Al<sub>5-x</sub>Ga<sub>x</sub>O<sub>12</sub>:Ce<sup>3+</sup> (x=0, 1, 2, 3, 3.5, 4). The persistent luminescence was observed in the sample with x=3 and 3.5 by the blue excitation. In these materials, the energy gap between the lowest 5d<sub>1</sub> excited level of Ce<sup>3+</sup> and the conduction band is much closer compared with x=0, 1, 2 samples. As a result, the efficient electron transfer to the electron trap occurs through the conduction band by the blue excitation in the x=3 and 3.5 samples. The thermoluminescence (TL) was observed in all the samples by UV excitation and the TL peaks were shifted to lower energy with increasing Ga content. The decreases of the threshold energy of photoionization and the electron trap depth with increasing Ga content can be caused by lowering conduction band. Therefore, we demonstrated that the persistent luminescence properties, such as storagetable wavelength and persistent decay profile, are controlled by changing Ga content. We also discovered that the persistent luminescence intensity and duration time were improved by co-doping with metal ions.