Direct imaging of dopant clustering in metal-oxide nanoparticles.

Dopant atoms are used to tailor the properties of materials. However, whether the desired effect is achieved through selective doping depends on the dopant distribution within the host material. The clustering of dopant atoms can have a deleterious effect on the achievable properties because a two-phase material is obtained instead of a homogeneous material. Thus, the examination of dopant fluctuations in nanodevices requires a reliable method to chemically probe individual atoms within the host material. This is particularly challenging in the case of functionalized nanoparticles where the characteristic length scale of the particles demands the use of a high-spatial-resolution and high-sensitivity technique. Here we demonstrate a chemically sensitive atomic resolution imaging technique which delivers direct site-specific information on the dopant distribution in nanoparticles. We employ electron energy-loss spectroscopy imaging in a scanning transmission electron microscope combined with multivariate statistical analysis to map the distribution of Ba dopant atoms in SrTiO(3) nanoparticles. Our results provide direct evidence for clustering of the Ba dopants in the SrTiO(3) nanoparticles outlining a possible explanation for the presence of polar nanoregions in the Ba:SrTiO(3) system. The results we present constitute the first example of site-specific atomic resolution spectroscopy of foreign atoms in doped nanoparticles and suggest a general strategy to ascertain the spatial distribution of impurity atoms in nanocrystals and hence improve the performance of nanoparticle-based devices.