Semiconducting nanostructures might be used to revolutionize electronics by replacing conventional silicon components and circuits. However, an obstacle to implementing the use of semiconducting single-wall carbon nanotubes is that metallic versions form unavoidably during the manufacturing process, contaminating the semiconducting nanotubes. But now, researchers have demonstrated a new imaging tool for rapidly screening these nanotubes that could hasten their use in electronic applications.

Developed by Ji-Xin Cheng, an associate professor of biomedical engineering and chemistry at Purdue University (West Lafayette, IN), the imaging technology uses a pulsing laser to deposit energy into the nanotubes, changing the nanotubes from a ground state to an excited state. Another laser, called a probe, senses the excited nanotubes and reveals the contrast between metallic and semiconductor tubes. Known as transient absorption, the technique measures the "metallicity" of the tubes. The detection method might be combined with another laser to zap the unwanted metallic nanotubes during manufacturing, leaving only the semiconducting tubes.

Working with nanomaterials for biomedical studies, researchers in Cheng's group were puzzled when they noticed that metallic nanoparticles and semiconducting nanowires transmitted and absorbed light differently after being exposed to the pulsing laser. Then researcher Chen Yang, a Purdue assistant professor of physical chemistry, suggested that this phenomenon could possibly be used to screen the nanotubes for nanoelectronic applications.

Measuring about 1 nm in diameter, roughly the length of 10 hydrogen atoms strung together, the nanotubes are far too small to be seen using a conventional light microscope. "They can be seen with an atomic force microscope, but this only tells you the morphology and surface features, not the metallic state of the nanotube," Cheng explains.The transient absorption imaging technique represents the only rapid method for telling the difference between the two types of nanotubes. A "label free" technique, the imaging method does not require that the nanotubes be marked with dyes, making it potentially practical for manufacturing applications, Cheng adds.