Distribution of density in spherical colloidal particles by transmission electron microscopy

The variation in the distribution of density in colloidal spherical particles was evaluated by transmission electron microscopy (TEM), without utilizing a high-resolution mode, and quantified by image processing. The method eliminates the dependency of the image contrast on sample crystallinity. The advantage of such approach is manifested by the short time sample preparation, fast instrument tune-up, rapid image acquisition and analysis, all of which shorten the processing time. Colloid silver spheres and gold nanoparticles were examined and compared to amorphous silica and acrylate-methacrylate polymer spheres. The latter can be considered as ideal homogeneous model samples. Image files having *.dm3 extension, obtained from TEM, were processed with ImageJ software, and later analyzed with script written in Microsoft Visual C++. It is shown that the radial distribution of density of highly crystalline gold nanoparticles resembles the used models, while in larger polycrystalline silver spheres it differs significantly from the “ideal” case. Deviations from linearity for gold and silver were interpreted in terms of finite polydispersity and internal inhomogeneities. The described method made it possible to estimate rapidly and accurately the number of subunits needed to achieve properties of an equivalent sphere, without considering the crystalline nature of such particles.