This second lecture took us deeper into the field of synthesizing nano- or meso-objects with original or even complex shapes (Figure 1). In particular, we focused on metals (M) and noble metals (gold, silver, palladium), simple metal oxides (MxOy, SiO2, Fe2O3) and metal chalcogenides (MxXy with M = Cd, Ag, and X = S, Se). After evaluating and comparing various synthesis methodologies based on metal salt reduction, decomposition of organometallic precursors, hydrolytic or non-hydrolytic condensation, cation conversion or exchange methods (oxidation, galvanic displacement, nanoscale Kirkendal effect), we focused on the synthesis of more complex homo- or heterostructures based for the most part on one or more heteronucleations associated in some cases with the presence of a favorable epitaxial relationship. Analysis of this topic highlights the importance of regulating the degree of solution supersaturation and controlling the diffusion of reagents. This can be achieved by selecting the nature of the precursors and the reaction medium (aqueous, organic, oxygenated or inert atmosphere), controlling the concentrations, the mixing mode, the reaction time, the temperature and the heating mode, whether conventional or microwave. On the other hand, hybridization of growing mineral objects by the presence of organic substances (ligands, surfactants,...) makes it possible to regulate the degree of supersaturation by complex formation, to control the size of objects formed by the presence of ligands, organic adsorbates, to differentiate face growth modes and generate original morphologies by differential adsorption, to modify the adsorption-desorption equilibria of growing clusters thus avoiding irreversible aggregation. In the second part, we discussed the principles of construction of a number of objects of diverse chemical nature, on several examples that can be grouped into three families:
1) nanocrystals with simple core-crown structures (metal-silica, metal-metal, hollow core and metal or oxide crown) ;
2) nanocrystals with more complex structures based on several noble metals (rattle, hollow box, nano-cages, shields, windows, etc.) or a core with several crowns of different compositions;
3) heterostructured mesocrystals in the form of octopods (CdS, CdSe), symmetrical dumbbells (Gold-CdS), matchsticks (Gold-CdS) or rods composed of superlattices (CdS-Ag2S).
We have illustrated this subject extensively by presenting numerous examples of nanocrystals obtained by these processes, and their physical properties, which are of particular interest in photonics. If we confine ourselves to compositions based on noble metals, metal chalcogenides and certain simple oxides, we can conclude that today's chemists know how to control the relative stability of the various polymorphs, the positioning of the different components in heterostructures, the preferred crystallographic direction of growth and therefore the shape, anisotropy and spatial composition of a good number of nano-objects. Within this framework, alas still too limited, we can say that these nanobricks are sculpted to the nearest atom.
