  We investigate the luminescence properties of 10 nm 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 defect 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 Ce4+ 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 particles 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 ions populations. Eventually, nano-cathodoluminescence
indicates that the emission and therefore the distribution of the doping Ce
ions and of the defects are homogeneous.
