The formation process of CdSe-Au binary nanoparticle superlattices (BNSLs) was investigated to elucidate the optimal conditions for self-assembly. Nanoparticle mixtures were prepared under suitable and variable solvent evaporation conditions and the products analyzed by TEM. 1-Dodecanethiol was found to be critical for superlattice formation, prior to the onset of short-range nanoparticle−nanoparticle interactions that likely dictate structure, highlighting the multiple roles of this organic polar molecule as both a ligand that coats the particle during synthesis and then as a high boiling point/crystallization solvent for the separate step of BNSL formation. The influence of 1-dodecanethiol was investigated under various concentrations to determine optimized conditions for guided assembly, for which relatively large (>1 µm2) areas of superlattices could be routinely formed. Depending on appropriate selection of the radius ratio, AB or AB5 binary structures of CdSe-Au nanoparticles were generated. The opt...