Electron Microscopy Study of Silver Nanoparticles Obtained by Thermal Evaporation

Metallic nanoparticles are promising objects of study, since their properties greatly differ from the properties of bulk material. In analyzing nanoparticles, it is important to investigate their size, stability, structural features, and spatial arrangement. In this study, initial and annealed silver nanoparticles from ∼2 to 10 nm in size formed on a carbon substrate by vacuum thermal evaporation are investigated by high resolution transmission electron microscopy and their shape and structure are classified. The examined nanoparticle types include faceted ellipsoid ones with a polycrystalline structure, coarse ones with a single-crystal structure and twins, icosahedral and decahedral ones with multiple twinning, and fine single-crystal nanoparticles smaller than 3.5 nm. It is established that, after annealing, the total number of nanoparticles decreases by a factor of ~1.3, the number of fine nanoparticles almost halves, and the fraction of nanoparticles with icosahedral and decahedral cross sections increases by a factor of ~1.5. It is shown that nanoparticles smaller than 5 nm become unstable already after a few seconds of exposure to high-energy electrons. For fine single-crystal nanoparticles smaller than 3.5 nm, the average crystal-lattice parameter is found by precise determination of the centers of atomic columns in their images and calculation of the local distances between atoms located in the mutually perpendicular (200) and (022) planes. It is shown that, in such nanoparticles both before and after annealing, there are no noticeable crystal-structure distortions and their lattice parameter is similar to the value characteristic of bulk silver.