Crystallize Nanoparticles by Precipitating Trace Polymeric Additives

Growing nanoparticle (NP) crystals has been pursued extensively using ligand chemistries such as DNA and supramolecules, controlled evaporation and patterned surfaces. Here, we show that a trace amount of polymeric impurities (<0.1 wt.%) leads to reproducible, rapid growth of high quality 3-D NP crystals in solution and on patterned substrates with high yield. The polymers preferentially precipitate on the NP surfaces inducing the formation of small NP clusters, which subsequently act as nuclei to initiate NP crystal growth in dilute solution. This precipitation-induced NP crystallization process is applicable for a range of polymers and the resultant 3-D NP crystals can be tuned by varying polymeric additives loading, solvent evaporation rate and NP size. Fundamentally, the present study elucidates how to balance cohesive energy density and NP diffusivity in the self-assembly to favor nuclei formation energetically and kinetic growth in dilute solutions. The results shown also opened up the process window to rapidly and reliably fabricate NP crystals over multiple length scales. Furthermore, the amount of these impurities needed to grow NP crystals (<0.1 %) reminds us the need to pay special attention to fine details to interpret experimental observations in nanoscience.