Watching Nanoscale Octahedra Crystallize Joel D. Brock, Cornell University, DMR 0936384

1. Watching NanoscaleOctahedraCrystallizeJoel D. Brock, Cornell University, DMR 0936384 Intellectual Merits: While the assembly of spherical colloids into close-packed FCC superlattices is well understood as an entropic process in terms of the Kirkwood-Alder mechanism, much less is known about the crystallization of nanoparticles. In nanoparticles the ligand length of around 2 nm is on the same order of magnitude as the core diameters of typically 2-10 nm. Hence the ligands play a much more significant role in nanoparticle crystallization than in the larger colloids with core diameters of 100 nm to several microns. The outer ligand shell facilitates the formation of either FCC structures that minimize enthalpy, or BCC structures which maximize entropy. In recent experiments at the CHESS D1 end station, SUNY-Binghamton and CHESS scientists studied carefully the drying of a drop cast solution of Pt3Cu2nanoctahedra suspended in hexane using the D1 controlled vapor sample environment and grazing-incidence small-angle scattering (GISAXS). Kirkwood-Alder transition in the soft crystallization of Pt3Cu2 nanoscaleoctahedra. At first particles assemble from solution to an open bcc structure with still a lot of solvent in the lattice, the subsequent drying process is followed using in-situ GISAXS images (top left) J. Zhang, Z. Luo, B. Martens, Z. Quan, A. Kumbhar, N. Porter, Y. Wang, D. M. Smilgies, and J. Fang; "Reversible Kirkwood-Alder Transition Observed in Pt3Cu2 Nanoctahedron Assemblies under Controlled Solvent Annealing/Drying Conditions", Journal of the American Chemical Society 134 (34), 14043-14049 (2012) CHESS DMR-0936384 October 2012