Spin-coated Layers for Nano Film Strength

By: Katie Anderson, Cosmetics & Toiletries magazine

Posted: March 5, 2013, from the March 2013 issue of Cosmetics & Toiletries.

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“Seeing that we were able to stack multiple layers one after the other with spin coating was something quite interesting to us, and different from what others had observed with some polymer based films. Also, seeing that we could make a composite film having a thickness greater than the lowest critical cracking thickness of the two [nanoparticle films] in the composite suggested that multicoating may play a role in preventing crack formation,” added Lee. This inspired the team to apply a multi-coating approach to the silica and titania nanoparticle films.

A Layered Approach

Before creating multiple layers of the nanoparticle films, the team first evaluated whether they would disintegrate when re-exposed to the solvent (water). After re-submerging, drying and measuring the films up to 2 1/2 months later, the team found neither the silica nor the titania nanoparticle film was disintegrated, which meant layering was possible. Upon multicoating the silica and titania films, the team was elated to see a thick film result.

The next step was to address cracking. To determine if the multicoating method could be used to avoid cracking during deposition, the researchers successfully built a film having a thickness greater than a single spin-coated layer before it cracked. The team increased the film thickness to determine if there was a threshold at which the film would crack, and there was. Multicoated films were then made with different coating thicknesses, and the final film thickness producing cracks was observed. The thickness of the individual layers altered when the final film would crack in an inverse manner.

“Thinner coating thicknesses gave larger final film thicknesses before cracking would occur,” said Lee. The team then made a multicoated film that was thicker than what a single coated film cracks at, and deposited a single coated film of equal thickness on top. “If the film would crack and crack similar to a single coated film having a total thickness the same as both films taken together, this would indicate that the applied stresses in multicoated films might be different during fabrication, as compared to single coated films,” explained Lee. Conversely, no crack would mean that the strength of multicoated films was greater than for the single coated films. Interestingly, the multicoated film layers on the bottom did not crack, whereas cracks in the film propagated down through the top single coated layer, showing the film had grown stronger during the multicoating process somehow. The researchers hypothesized that the bonds preventing the nanoparticle films from cracking were oxygen bridges.

“We believe that these bonds are forming and giving increased strength or adhesion to the particle network in the nanoparticle films, which is enough to prevent the particles from being torn apart by the stresses imposed during drying of the film that cause cracks,” explained Lee. The team believes multicoating can be used to avoid cracks in other metal oxide nanoparticle films, i.e., alumina, zirconia and ceria, due to their similarity in structure. The team is also investigating whether this multicoating method can be used with other film deposition techniques to prevent crack formation in various colloidal films.

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