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Interparticle Interactions in Self Assembly

In one study we are examining the 2D self assembly of CdSe nanocrystals at the liquid/solid interface on graphite surfaces using atomic force microscopy (AFM), in order 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. The thermodynamics and kinetics of this self assembly are currently being investigated.

This is a disk composed of ~400 nanocrystals formed by homogeneous nucleation and growth when a hexane solution dries on the graphite surface. The nanocrystal assembly shows hexagonal close packing.

In solvents, these disks are able to move as a unit on the graphite surface. Here, a snapshot of disk coalescence to form short chains is shown. Some disks consist of a monolayer, while some of them have a second layer on top (seen on the central bright region). These disks have an average diameter of 170 nm, and are composed of about one thousand individual CdSe nanocrystals.

When a drop of chloroform solution of CdSe nanocrystals dries on graphite, the nanocrystals form smaller disks compared to those formed in hexane. Here again we see the coalescence of disks, but their average diameter is only 80 nm.
In chloroform these disks can coalesce into diffuse structures. These ribbon-like structures still have a width of 80 nm. It is clear that particle/particle interactions are weaker in chloroform than in hexane.

These observed bicontinuous patterns, studied as a function of surface coverage and drying time, 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 sphere or adhesive particles. We believe that 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. This effort is being coordinated with the MRSEC effort in copolymer directed nanocrystal self-assembly.