Scientists at the University of California, Los Angeles, report in the November issue of Nature Materials that exposing polymer nanofibers to a camera flash enables the fabrication of patterned films as well as of novel composite materials. The phenomenon may open the door to the development of new classes of chemical and biological sensors and separation membranes.Such "flash welding" is an effect of nonradiative relaxation in nanoscale structures. Phonons generated by the absorption of the incident light are scattered by the ends of a fiber, trapping them within it. As a result, the internal temperature of polymer nanofibers exposed to a camera flash rises rapidly, causing them to melt and fuse into a uniform film with different material properties from the unexposed fibers.The scientists demonstrated that flash welding powders of polyaniline nanofibers through a copper photomask yielded 70.7-µm-sq regions of cross-linked fibers separated by unaffected nanofibers. They also showed that the effect could be used to embed 1-µm-diameter polystyrene beads in a cross-linked polyaniline matrix by flash welding a mixture of the fibers and beads, producing a nanocomposite with new optical properties.