Tiny Loudspeakers

THIN FILMS of carbon nanotubes can function as loudspeakers when they're fed with sound frequency electric currents (Nano Lett., DOI: 10.1021/nl802750z). Only a few tens of nanometers thick, the speakers are transparent, flexible, and stretchable, and they can be tailored into any shape and size, according to the Chinese scientists who created them.

Unlike most speakers, the nanotube-based devices have no magnets or moving parts. They're prepared by first growing carbon nanotubes that are 10 nm in diameter like grass on a 4-inch silicon wafer. These nanotubes are then converted into a continuous film up to 10 cm wide and 60 m long—enough to make 500 10-cm² speakers. Two electrodes are attached to the thin film so that by simply applying a sinusoidal voltage across them, sound is emitted via the thermoacoustic effect.

The nanospeakers "could open up new applications of and approaches to manufacturing loudspeakers and other acoustic devices," note Tsinghua University's Kaili Jiang and Shoushan Fan, who spearheaded the research. They add that the speakers can be mounted on virtually any surface, including walls, ceilings, windows, flags, and clothes.

John A. Rogers, a materials science professor at the University of Illinois, Urbana-Champaign, calls the nanotube speakers ingenious. "These results cap off a year of remarkable progress in carbon nanotube technology," he says. "Only a couple of years ago, research in carbon nanotubes was still dominated by studies of individual, handcrafted devices. Although it's still hard to know when carbon nanotubes will be competitive against entrenched electronic materials, at least now we can begin to make realistic assessments."