Coating Method Makes Molecule-Thick Layers

Kevin Robinson

A method for dip coating a substrate with organic polymers may increase the efficiency of light-emitting diodes (LEDs) and computer displays while reducing costs and environmental hazards.
The technique, being explored by a group of MIT researchers, uses the natural affinity of opposite electrical charges to make coatings that are as thin as a single layer of molecules.
The researchers, led by Michael Rubner, begin with a substrate, which most often has a natural, slightly negative electric charge. They dip the substrate into a bath of a positively charged polymer that has been dissolved in water. The polymer responds to the charge of the substrate by sticking to it tightly. For the next layer, the researchers switch back to negatively charged molecules, which stick to the positive surface.
Rubner said researchers have layered as many as 25 substances on a single substrate. Using computers and robot dipping mechanisms, the researchers can automate the process.

Thinner layers
Because the technique creates thinner layers than are possible with conventional spin coating techniques, it can be used to tailor an LED for efficiency. The method can lay down polymers to enhance the injection of the electrons and holes necessary for the LED to work. It also can coat substrates as large as a sheet of paper with few pinholes that could short out the device, Rubner added.
The researchers can also control the smoothness of each surface, putting down layers that are little rougher than the molecular surface itself.
In addition to creating LEDs, the technique can deposit a variety of coatings onto a surface, such as optical coatings to make lenses hydrophobic or hydrophilic. Other research groups are investigating using the method to create pH sensors, solar cells and waveguides.
The process uses no harsh solvents and is easy to set up for mass production, but it is slower than spin coating.