Size-selective dispersion of dodecanethiol-coated nanocrystals in liquid and supercritical ethane by density tuning

Silver and gold nanocrystals, sterically stabilized with dodecanethiol ligands, were dispersed in supercritical ethane. Nanocrystal dispersibility was measured as a function of solvent density-with pressures and temperatures ranging from 138 to 414 bar and 25 degreesC to 65 degreesC, respectively-using UV/visible absorbance spectroscopy and transmission electron rnicroscopy (TEM). Dispersibility depends strongly on the nanocrystal size and solvent density. Higher density reduces the effective X interaction parameter for dodecane in ethane, which leads directly to increased steric repulsion between nanocrystals. Since the van der Waals attraction between the nanocrystal cores increases significantly with size, an increase in ethane density and thus solvent strength disperses larger nanocrystals with size selectivity. A reduction in ethane density precipitates the nanocrystals, again with size selectivity. Furthermore, the ethane density could be cycled to reversibly precipitate and disperse sterically stabilized nanocrystals selectively. Calculations of ligand tail solvation in terms of the cohesive energy densities of ethane and dodecane capture the essential physics of density tuning of the size of the dispersed nanocrystals.