Asymmetric Dimers Can be Formed by Dewetting Half-Shells of Gold Deposited on the Surfaces of Spherical Oxide Colloids

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Original entry: Lidiya Mishchenko, APPHY 226, Spring 2009

Reference: Yu Lu, Hui Xiong, Xuchuan Jiang, Younan Xia, Mara Prentiss, and George M. Whitesides J. Am. Chem. Soc., 2003, 125 (42), 12724-12725 [[1]]

Keywords: Dewetting, Surface energy, Colloid

Abstract: Asymmetric dimers were formed by depositing a gold film on oxide colloidal particles and annealing. Dewetting of the gold surface caused the formation of gold beads on these nanoparticles.

Soft Matter Example:

SEM and TEM images of the colloidal particles with gold beads formed after annealing

This paper addresses the very interesting phenomenon of dewetting of gold from colloids. Actually the surface chemistry of gold is such that one always needs an "adhesion layer" in order to not have the gold flake off. In this case it was a Ti/W alloy adhesion layer. They began by making gold half shells on top of each colloid by sputtering them with gold.

The gold beaded up during annealing on the colloidal particles at the edge where it was "thinnest". This is an interesting effect that is probably relevant to all kinds of dewetting phenomena.

The other relevant soft matter issue that this paper addressed was that of "surface energy". Attempting to anneal the gold in air, Ar, and H2 gas, they saw differences in dewetting. While with air and Ar gas, the morphology of the gold nanoparticles was smooth and crystalline, "the use of H2 led to the formation of gold microcrystals with poorly defined shapes and nonuniform sizes." This was supposed to be due to the change in surface energy of the colloids as the environment was changed. I suppose it is possible that the hydrogen gas somehow adsorbed to the colloidal surface (or the adhesion layer) and made the surface higher in energy (more similar to gold) or simply non-uniform. Both of these scenarios I would imagine could cause non-uniform dewetting.

A final remark that was interesting was that when the colloids used were too big (more than 2 microns in diameter), "the gold half shells tended to break into several small pieces on top of each silica bead upon heating." This is also an interesting dewetting phenomenon and something that should be considered when dewetting on a surface is utilized experimentally.

Note: These particles are reminiscent of "janus" particles: [[2]]