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.

Film-forming agents are an important part of personal care products. They provide a durable, flexible and continuous covering to keratinous substrates such as skin, hair or nails in addition to aesthetic benefits. While these films must be durable, they must also be thin enough to maintain a natural appearance and feel. For specified benefits, it is important that film-formers provide a continuous layer, which can be challenging when thin films crack or chip.

In relation, Daeyeon Lee, PhD, and Jacob Prosser, along with fellow researchers from the University of Pennsylvania, were investigating the manipulation of nanoparticle films through spin-coating conditions when they discovered a method to prevent cracks in films for solar cell coatings, and possibly for cosmetics.

Nano Spin Coating

Nanoparticle films are necessary for a number of industries, yet they often crack when dried. Currently there are no valuable solutions for this problem, therefore Lee’s team utilized spin coating to manipulate and control the thickness of nanoparticle films, adjusting conditions such as the rotational rate and suspension properties. The films were created by suspending silica or titania nanoparticles in water, spreading these mixtures over a surface, and rapidly spinning them as the water evaporated. This created an even coating of the nanoparticle material. Spin coating had been previously used to generate nanoparticle and polymer films, but cracks often appeared during deposition.

“We began by determining if the existing polymer model could be directly and immediately applied. [The spin- coated silica nanoparticles] displayed a distinct feature that allowed us to group them into two separate types. This feature was cracks. There were spin-coated films that contained fully interconnected crack networks, and films that did not; the apparent difference being correlated to a certain thickness threshold,” explained Lee. The team compared these findings to the polymer model and found similarities in the spin-coating conditions in addition to some unexplainable differences.

Before examining the thickness threshold, the team’s research veered toward the use of spin coating to fabricate Bragg reflectors (used in solar cells), which brightly reflect certain colors of light. The team wanted to stack nine layers to achieve high light reflection, but cracks began to form as they reached six layers.