YAG:Ce nanoparticle lightsources.

We investigate the luminescence properties of 10 nm yttrium aluminum garnet (YAG) nanoparticles doped with Ce ions at 0.2%, 4% and 13% that are designed as active probes for scanning near-field optical microscopy. They are produced by a physical method without any subsequent treatment, which is imposed by the desired application. The structural analysis reveals the amorphous nature of the particles, which we relate to some compositional defects as indicated by the elemental analysis. The optimum emission is obtained with a doping level of 4%. The emission of the YAG nanoparticles doped at 0.2% is strongly perturbed by the crystalline disorder whereas the 13% doped particles hardly exhibit any luminescence. In the latter case, the presence of Ce(4+) ions is confirmed, indicating that the Ce concentration is too high to be incorporated efficiently in YAG nanoparticles in the trivalent state. By a unique procedure combining cathodoluminescence and Rutherford backscattering spectrometry, we demonstrate that the enhancement of the particle luminescence yield is not proportional to the doping concentration, the emission enhancement being larger than the Ce concentration increase. Time-resolved photoluminescence reveals the presence of quenching centres likely related to the crystalline disorder as well as the presence of two distinct Ce ion populations. Eventually, nano-cathodoluminescence indicates that the emission and therefore the distribution of the doping Ce ions and of the defects are homogeneous.