Nowadays, applications of nanomaterials increasingly demand for monodisperse nanoparticles (NPs), since their defined properties form the basis for a wide range of applications. Gold NPs are of special interest due to their fascinating optical properties related to localized surface plasmons. This plasmonic behavior is highly responsive to extrinsic factors (physical-chemical environment, temperature, interactions of particles, stabilizing agent, etc.) and their morphology (size, shape, quality of surface). We present a three-step seed mediated synthesis for highly monodisperse single crystalline Au NPs stabilized by hexadecylpyridinium chloride (CPC) in variable sizes (24-90 nm) with an adjustable ratio of the cubic and octahedral facets.  This synthesis thus circumvents the common reproducibility drawbacks of CTAB-dependent syntheses.  The effect of the additive KBr and reducing agent ascorbic acid on shape and growth kinetics was systematically investigated and an explanation for the switching of the Au NPs morphology from cubic to octahedral is provided. The plasmonic response of individual particles and their assemblies is demonstrated using spatially resolved EELS to show the optical behavior of the particles, which is far superior to that of lithographically fabricated Au nanostructures.  These particles may then be used to assemble mesocrystals. Such superstructures of crystallographically aligned NPs may lead to strong directional and additional novel collective properties that cannot occur in any other structure of the same size range. 
 F. Kirner et al., DOI: 10.1039/d0tc01748e. R. G. Rayavarapu et al., DOI: 10.1021/la100166f. M. Mayer et al., DOI: 10.1002/anie.201708398. E. V. Sturm et al., DOI: 10.1039/c6cs00208k.