Nanocrystal stabilization, synthesis and assembly using supercritical fluids
Supercritical and compressed solvents provide a unique medium for nanocrystal synthesis and assembly as their tunable solvation strength and favorable wetting characteristics have the potential to overcome current processing limitations. Here we examine nanocrystal dispersibility, separation, synthesis and organization with compressed solvents. Gold and silver nanocrystals were dispersed in carbon dioxide and ethane by using the appropriate capping ligands. Larger nanocrystals, which exhibit stronger core attractions, required better solvent conditions (higher densities) than smaller nanocrystals in order to form a dispersion. Lowering the solvent density precipitated the largest nanocrystals demonstrating density tunable colloidal separations in supercritical fluids. Silver, iridium and platinum nanocrystals were synthesized in supercritical CO2 by reducing a miscible organometallic precursor. By reducing the precursor in the presence of a thiol, particle growth was quenched and the nanocrystals could be collected, cleaned and redispersed in compatible solvents. Tuning solvent density and ligand type allowed the nanocrystal growth mechanism to be controlled from a mix of coagulation and condensation at conditions of strong steric stabilization, leading to small monodisperse particles, to coagulation at poor stabilization conditions, leading to large polydisperse particles. Superlattice formation was examined by assembling gold nanocrystals from liquid carbon dioxide. The resulting structures varied from disorganized liquids at fast evaporation rates to hexatic states with highly ordered regions at slower evaporation rates. Comparison with a computer simulated reference state showed that the crystallization kinetics were slower than diffusion limited, likely due to ensemble rearrangement during the late stages of assembly. Finally, gold and indium manganese arsenide nanocrystal dispersions were drop-cast from volatile solvents under humid conditions to form macroporous nanocrystal thin films. The porous structures were templated by condensed water droplets as a result of solvent evaporation. Prevention of droplet coalescence by interfacially active nanocrystals, which adsorbed onto the surface of the droplets, led to the formation of highly ordered pore structures.