Influence of Length of Alkyl Chains Used To Passivate Silver Sulfide Nanoparticles on Two- and Three-Dimensional Self-Organization

Silver sulfide Ag2S nanocrystals have been synthesized in reverse micelles and coated with thioalkyl chains with different lengths. With short chains (≤C8) and very long ones (≥C16) the particles cannot be extracted from micelles. For short alkyl chains, this is due to disorder at the interface. For longer chains it is attributed to the highly hydrophobic character of the alkyl chains. For alkyl chains varying from C8 to C14, the long-range order of 2D self-assembly depends on the number of particles deposited on the support; at low concentration, rings are formed and then disappear by increasing the number of particles. When the thiol alkyl chain length is increased, an increase in the average distance between two adjacent particles is observed. It is correlated with the change in the chain conformation and defects, which depend on the chain length. When particles are self-organized in 2D, the distance between adjacent particles centers varies linearly with the particle radius. This indicates that the interparticle distance does not change with the particle size and varies with the length of the alkyl chain. For short chains (≤C6), the gauche defects prevent attachment to the surface, and self-organization in 2D is prevented. Hence, by using short chains, the distance between particles cannot be markedly reduced. By the increase of the particle concentration, 3D superlattices are obtained. The best faceted and largest fcc crystals are formed with the largest particles and densest chain packing. When the chain length is too long (C14) end-gauche defects occur, which show an edge−edge core spacing smaller than one chain length in the all trans zigzag conformation. This prevents the 3D growth.