Films in a Flash

NANOMATERIALS

Graduate student Jiaxing Huang was fooling around with his digital camera in the lab one evening when he made an unexpected discovery. He took a flash photo of a vial of polyaniline nanofibers and heard a pop. "So I did it again with an even closer flash and then smoke emanated from the powder along with the odor of burning plastic," he recounts.

Huang soon discovered that by flashing a mat of dark green polyaniline nanofibers on a substrate, he could fuse the fibers into a smooth, golden, continuous film of cross-linked polyaniline.

Huang and his thesis adviser, chemistry professor Richard B. Kaner of the University of California, Los Angeles, report their "flash welding" process in Nature Materials [published online Oct. 24, http://dx.doi.org/10.1038/nmat1242].

In an accompanying commentary, chemists Dan Li and Younan Xia of the University of Washington, Seattle, write that the discovery "provides a versatile new technique for processing polymers into potentially useful structures."

Other groups previously reported that an ordinary camera flash can make single-walled carbon nanotubes and silicon nanowires burst into flames. When these nanostructures absorb light, local hot spots develop that may lead to ignition.

Polyaniline, Kaner and Huang explain, converts most of the energy it absorbs from light into heat. But the heat may be diffused by the melting of the nanofibers, preventing complete structural breakdown or combustion, they suggest.

The UCLA chemists point out that flash welding appears to be a convenient way to make an asymmetric film--that is, one having a dense skin on top and a porous layer underneath. Asymmetric films have found use in many applications, including separation membranes and chemical sensors and actuators. Such films, though, are usually made using relatively time-consuming processes and flash welding might prove faster.

A key advantage of flash welding, the chemists note, is its ability to form patterned films. By flash-illuminating a mat of nanofibers through a patterned mask, the pattern can be transferred to the mat, just as in photolithography. This technique "will be especially useful for organic micro- or nanodevices," the researchers suggest.

The heat generated by a camera flash also can be used to weld polyaniline to another polymer, offering a rapid and clean way to make polymer-polymer blends and composites, according to the UCLA team. For example, Huang prepared a composite film by flash welding a mixture of polyaniline nanofibers and polystyrene microspheres. Particles of polytetrafluoroethylene also can be embedded in a polyaniline matrix in this way, the researchers report.

Preliminary tests on polyaniline derivatives and other conducting polymers such as polypyrrole and polythiophene indicate that they, too, can be flash welded when nanostructures smaller than 100 nm are present.

Huang, now a postdoc at UC Berkeley, says, "I wouldn't be surprised at all to start hearing 'Let's go flash our nanomaterial' in nanoscience labs."