The structure of (Au) bipyramids, very promising for their specific plasmonic properties, is fully analyzed for gradual stages of growth. From combined use of HRTEM (high resolution transmission electronic microscopy) in specific conditions and SAED (selected area electronic diffraction) analysis, the atomic structure of these elongated objects of different sites is extracted (from 2 to 60 nm). In the case of silver(I)assisted growth from citrate capped seeds, a clear pentatwinned structure, made by the superimposition of specific pairs of crystallographic zones is found for the different size nanoparticles representing the different steps of the growing bipyramids. The experimental results were analyzed in the framework of a recent atomistic approach developed for metal environment interactions to account for the stability of multitwinned nanorods or bipyramids in a complex environment. Efficient in describing the multitwinned nanoparticles and the drastic effects of environment on the stabilization of elongated multitwinned shapes, this model supports the experimental features of bipyramids and formation from citrate seeds as a function of the nature of the surface chemical ligands: citrate seeds in a growth solution prepared without silver(I) yields the formation of a few pentatwinned nanorods, while the silver(I)-assisted growth process mainly leads to pentatwinned bipyramids. The conservation of a bulk pentatwinned structure inherited from Isotropic decahedral (i-Dh) seeds all along the growth is retrieved by this approach for two different environments.
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