Nanoparticle Self-Assembly by Drop Drying

A close-up look (through a microscope objective) at the air/liquid interface of a drying droplet containing gold nanoparticles (~6nm in diameter) in toluene. The paricles are coated with a shell of dodecanthiol molecules. Check out a schematic of how the the assembly process works here. As the droplet evaporates, the nanoparticles are swept to the surface of the drop and form 2D islands that eventually coalesce into a compact monolayer, covering the whole substrate (3mm x 4mm in our case).

Terry P. Bigioni, Xiao-Min Lin, Toan T. Nguyen, Eric Corwin, Thomas A. Witten, and Heinrich M. Jaeger, “Kinetically-Driven Self-Assembly of Highly-Ordered Nanocrystal Monolayers”, Nature Materials 5, 265-270 (2006).

The 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):

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Voronoi construction. 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 the paper, above. Islands near the perimeter were omitted in our analysis. An image of the final, almost dried monolayer covering the whole substrate (3mm x 4mm) is shown below. Note the small amount of damage at the lower right conrer, where the monolayer was disrupted by picking up the sample with a tweezer and the difference between the gold layer and the bare substrate is clearly seen. The pillow-like curvature comes from the fact that the excess dodecanethiol has not yet fully dried and thus there is a small amount of liquid still left under the monolayer. Note that the monolayer is NOT a single crystal, but that the highly ordered regions extend for many microns. For TEMs of this see the paper cited above.