ChemMatCARS Nuggets - Materials Science

Reversible Wrinkling in Atom-Thick Gold Layers

Accomplishment
Gold nanoparticles decorated with thiol ligands can form a 2D nanocrystal monolayer when spread on the surface of water. When squeezed hard horizontally, the monolayer wrinkles into tri-layer folds. If excess thiol molecules are mixed into the solution before it is spread on the water, the wrinkles will unfold when the layer is allowed to expand.

Optical microscopy and synchrotron x-ray radiation were used to investigate in details the formation of the nanocrystal monolayer. Optical microscopy reveals the morphology of the thin film in micrometer scales while the radiation method yields information on the packing symmetry of the spheres and the thickness of the layers in nanometer and subnanometer scales.

Impact
Monolayers of 2D nanospheres such as those studied have interesting electrical and mechanical properties. Because of these properties, monolayer films could serve as sensing devices. In such devices, reversibility of compression-related changes could be a technological advantage. In addition, the wrinkling behavior could shed light on other types of thin molecular layers, such as cell membranes.

(a) A monolayer of gold nanospheres immediately after spreading. Barrier is at left and pressure sensor is at right. (b) Appearance of a typical gold nanoparticle monolayer. The monolayer is characterized by its uniform blue color.

(1) Under 2D pressure from a barrier, a monolayer of thiol-coated gold nanoparticles particles floating on water has folded over itself like wrinkled paper (darkest areas).
(2) As the barrier moves away, reducing the 2D pressure, the wrinkles smooth out. (Barrier moves up from top edge of picture.)
(3) The reversibility of the wrinkles depends on the concentration of the excess thiol molecules in the nanosphere solution.

Principal Investigators: David G. Schultz (University of Chicago), Xiao-Min Lin (Argonne National Laboratory), Dongxu Li, Jeff Gebhardt, Mati Meron, P. James Viccaro, and Binhua Lin (all University of Chicago)
Published in: D.G. Schultz, et al., "Structure, Wrinkling, and Reversibility of Langmuir Monolayers of Gold Nanoparticles," J. Phys. Chem. B 110, 24522 (2006). DOI: 10.1021/jp063820s

September 2007

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