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We have examined the 2D self-assembly of CdSe nanocrystals at the liquid/solid interface on a graphite surface using atomic force microscopy (AFM) to explore fundamental questions of nanocrystal-solvent phase diagrams and the interaction potential between two solvated nanocrystals. These studies are essential to our goal of understanding how to create complex thin films containing nanocrystals. These nearly monodispersed 4 nm diameter CdSe nanocrystals have a monolayer of trioctyl phosphine oxide (TOPO) outside the inorganic core, and thus are soluble in organic solvents. When a drop of nanocrystal dispersion dries on a flat graphite surface, nanoparticles nucleate and self assemble into various structures. The final patterns are determined by the microscopic interparticle and particle/solvent interactions. As the concentration at the interface increases, the nanoparticles first behave like a two-dimensional lattice gas and then nucleate, grow and coalesce. This self assembly of CdSe nanocrystals is seen to be strikingly different in hexane and chloroform solution. In chloroform, the disk-like coalescence leads to smaller disks than in hexane, and these disks coalesce into more ribbon-like structures than in hexane, with a width of 80 nm. This suggests that nanoparticle/nanoparticle interactions are weaker in chloroform than in hexane. The observed bicontinuous patterns imply that aggregation occurs by fluid-fluid spinodal nucleation and subsequent coarsening. The 2D nanocrystal phase diagram is closer to the classical three phase (solid-liquid-gas) diagram of van der Waals particles, rather than the two phase diagrams of hard spheres or adhesive particles. The drying process acts as a quench of the reduced temperature, below the critical temperature into the unstable spinodal region, because of an increased van der Waals interaction in a 2D layer. The aggregated nanocrystals on graphite appear to represent a liquid nanocrystal phase. test
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