This page contains details about Kinetically-Driven Self Assembly of Highly-Ordered Nanoparticle Monolayers (this is also the title of an accompanying paper by Terry P. Bigioni, Xiao-Min Lin, Toan T. Nguyen, Eric I. Corwin, Thomas A. Witten and Heinrich M. Jaeger, Nature Materials 5, 265–270 (2006). pdf file).

Below: A quicktime movie showing the self-assembly of nanoparticles into two-dimesional islands (light-colored shapes) at the top surface of a droplet of colloidal solution.The field of view is 300µm. This video shows an example of raw microscopy footage taken during the drying process. Due to the receding liquid-air interface and the droplet curvature, the microscope is continuously refocused as portions of the field of view drift out of focus. The particles are 6nm diameter Au nanocrystals coated with dodecanethiol ligands. The solvent is toluene. A small amount of excess dodecanethiol has been added to the solution. The strong scattering of visible light by the nanocrystals allows for direct observation of monolayer islands by ordinary light microscopy once the islands' lateral extent exceeds about 1 micron.

By analyzing a sequence of successive video frames, individual islands can be tracked and their growth laws established. To this end, the island areas are determined by thresholding the video image and counting the associated pixels, and the Voronoi cells are established for each island. An example of a video frame analyzed in this manner is shown below (field of view = 300µm):

The maximal particle collection area for each island was identified by constructing Voronoi cells around the centers of mass of all islands in the frame. The numbered cells are from the data set in figure 3(c) of our paper, above. Islands near the perimeter were omitted in our analysis.

What happens when the monolayers are self-assembled without excess dodecanethiol? Without excess thiol, particles are not localized on the liquid-air interface and compact monolayers do not form; rather, the results are lace-like deposits similar to those observed by others (Refs 3-6, 9 in our paper, above) . The image below shows what happens when conditions are not adjusted correctly, specifically when the self-assembly occurs under solvent evaporation conditions below the critical flux boundary. (a) An optical image of the corner of a drying drop on a Si3N4 substrate. A golden, fulgent “coffee-stain” deposit is visible at the edge of the substrate with a redcolored band of concentrated solution along the perimeter. Aggregates are visible in the interior. (b-d) Transmission electron microscopy (TEM) images show some examples of the different structures formed in this regime, strongly resembling those previously observed.