Polymer coatings in a spin

Researchers at the US Department of Energy’s Los Alamos National Laboratory have found a way of building multiple thin layers of polymer coatings using centrifugal force.

Hsing-Lin Wang, a researcher in Los Alamos’ Bioscience Division has found an innovative method for building multiple thin layers of polymer coatings using centrifugal force.

The new method, developed by Wang and Jeanne M. Robinson of Los Alamos’ Chemistry Division, is said to drastically reduce thin-film production time and potentially improves the quality of thin-film devices.

Layers of extremely thin films of polymers are used in optical and electrical devices such as light-emitting diodes. For these applications, one or more layers of polymers are built up on a substrate such as glass, plastic or metal. The layers have specialised functions: Some conduct electrical impulses; others refract, reflect or produce light.

Traditionally, technicians produce multilayered thin film devices by dipping a substrate material into a polymer solution. Layers can be built on top of one another using this dipping technique, which works because each neighbouring polymer layer has an opposite electric charge than its neighbour and the layers stick to one another.

The dipping process takes several minutes or longer to produce a single layer, depending on the required thickness.

The new thin-film deposition method developed at Los Alamos by Wang and his colleagues can produce a single layer in just tenths of a second. Moreover, the new method gives technicians more control over the quality and thickness of the films.

Wang and his colleagues’ method works by spinning the substrate material on a turntable-type apparatus at up to 7,000 revolutions per minute.

Once the substrate is spinning, a technician adds a small portion of a polymer to the centre of the substrate. The polymer quickly spreads from the centre to the edges of the substrate material, producing a quality coating in only a fraction of a second.

The same technique can be used for subsequent layers of polymers. Wang and his colleagues have had good results when creating as many as 50 layers of polymers on top of one another, and they believe the technique will work for even more.

In addition to the improved speed in depositing films, Los Alamos’ spin-assembly method also can produce better quality films for some types of polymers.

With traditional dipping, a polymer solution can bleed into the layer it is supposed to coat because both substances are in contact with one another for a relatively long time. This blending of layers, known as interpenetration, can reduce the accuracy or performance of some thin-film devices.

Los Alamos’ spin assembly deposition method reduces contact time between polymers and helps ensure that layers remain separate and distinct.

Wang and colleagues looked at the film surfaces using atomic-force microscopy, which allows them to view minute features, and ellipsometry, which allows them to look at the thickness of individual film layers.

They found that the spin-assembly films had less interpenetration and better surface textures than similar films prepared by dipping.