Carbon nanotubes are proving useful for sensing applications because they produce a strong Raman signal and make a detectable sound when hit with a laser. Nanotubes fluoresce as well, but not brightly. However, researchers have discovered that extremely short carbon nanotubes behave as quantum dots do and emit brighter fluorescence than bundled nanotubes.These atomic force microscopy images show carbon nanotubes of various lengths. Researchers studied very short ones and found that they behave as quantum dots do. Reprinted with permission of the American Chemical Society.Xiaoming Sun at Beijing University of Chemical Technology and Hongjie Dai’s group at Stanford University in California made the discovery. They used ultrasonic waves to cut single-walled carbon nanotubes of the semiconducting variety into various lengths. Because cutting the nanotubes might unroll the tubes into sheets, they mixed the carbon with a phospholipid-based surfactant that promoted tube formation.They separated the shortest nanotubes from the mixture of various lengths using a density-gradient centrifugation protocol that they developed specifically for this purpose. They created the density gradient by adding three layers of increasing concentrations of iodixanol diluted in water to centrifuge test tubes containing the carbon nanotubes. Then they ran the centrifuge at 50,000 rpm and at a force of ∼300,000 g. After removing 100-μl fractions from the centrifuge test tubes, they used atomic force microscopy to characterize the optical properties of the fraction containing the shortest nanotubes, which measured 7.5 nm on average.The investigators used a spectrometer made by Varian Inc. of Palo Alto, Calif. to measure the absorption of the extremely short carbon nanotubes across the wavelength range of 550 to 1500 nm. They also characterized the intensity of the fluorescence from the carbon nanotubes. To do this, they excited the nanotubes across a range of 550 to 840 nm with an Osram xenon short arc lamp and a Lot-Oriel monochromator. They measured the resulting fluorescence emission across the 900- to 1500-nm range with an Acton spectrometer and a Princeton Instruments InGaAs array detector. The fluorescence spectra were compared with the excitation energy in electron volts as measured by a Lot-Oriel silicon photodetector.The researchers discovered that short, unbundled nanotubes fluoresced most brightly, and they found that all of the absorption and emission peaks in the ultrashort carbon nanotubes had shifted to the blue end of the spectrum by up to approximately 30 meV in terms of required excitation energy. Mathematical modeling showed that these blueshifts were consistent with quantum confinement along the tube length. Therefore, the researchers concluded that they had created carbon-nanotube quantum dots with emission spectra tunable by the length of the nanotubes.Journal of the American Chemical Society, May 21, 2008, pp. 6551-6558.