Aversatile and simple new technique for patterning submicrometer features in a conjugated polymer using an infrared laser is described in the August 10 issue of Nano Letters (DOI: 10.1021/nl2011593; p. 3128). While the potential for low-cost processing is a key advantage of using organic semiconductor materials in the place of conventional silicon electronics, many patterning techniques used at present such as contact printing and dip-pen lithography are either time-consuming or expensive. In their recent article, R. Kaner from the University of California–Los Angeles, G. Wallace of the University of Wollongong, Australia, and co-workers demonstrate how a laser in a commercial disk drive can be used to write features with tunable conductivity onto polyaniline-coated discs.
The team made use of a commercially available program normally used to create images on a compact disc, by activating a dye coating with a 788 nm laser. By replacing this coating with the well-known conductive polymer polyaniline, features could instead be written into the polymer by thermally inducing cross-linking at the irradiated regions. The high photothermal conversion efficiency and poor thermal conductivity of polyaniline allows the laser to induce highly localized melting of the polymer fibers into visibly welded regions as thin as 1 μm. Chemical cross-linking between neighboring fibers is proposed to occur through the formation of heterocycles containing two nitrogen atoms from adjacent chains, and the accompanying rearrangement of carbon double bonds can be clearly observed by the change in infrared spectra.
A scanning electron micrograph of a polyaniline film with lines of cross-linked polymer formed by an infrared laser. Reproduced with permission from Nano Lett. (DOI: 10.1021/nl2011593; p. 3128). © 2011 American Chemical Society.
The program’s facility for creating grayscale images by varying the intensity of the laser could also be neatly translated into controlling the conductivity of the polyaniline features. At maximum laser intensity, the irradiated regions were rendered electrically insulating by the cross-linking, but as the intensity was lowered, a range of intermediate resistances from insulating to metallic could be achieved. Corresponding changes in the emission spectrum and reflectivity of the polymer film also allowed the team to create high-resolution color images, which could be varied in a myriad of ways by altering the polymer’s oxidation state or through the addition of chemical substituents.
The ability to produce thin, uniform dielectric regions within a conductive polymer film makes this maskless lithographic technique a particularly attractive route to flexible batteries, supercapacitors, or interdigitated electrodes. As a one-step process performed entirely in the solid state, it serves as a promising demonstration of the potentially low-cost large-scale methods available to organic electronics, said the researchers.
