Nanotube ringsWe have found a way to convert ropes of straight nanotubes into nanotube rings. The rings are composed of many layers of single-walled nanotubes, and have a radius of typically 0.7 micron. Coiling has been observed in proteins and other biomolecules, where hydrogen bonding is thought to provide the main force for coiling. Carbon nanotubes however present a novel behavior where coiling involves only van der Waals forces. The rings, which we can position on metal electrodes, allow us to study novel electric transport phenomena. Shown below is an AFM micrograph of a one micron-diameter ring (the purple circle) placed over gold electrodes (the light blue objects).
SEM (Scanning Electron Microscope) image of nanotube rings on a silicon substrate. The image is magnified 8000 times.
The nanotubes used to form the rings are extremely small; their diameter is only 1.4 nm. They are 1-dimensional conductors and at low temperatures, quantum interference phenomena dominate electrical transport through the tubes. The ring geometry allows us to observe such quantum effects in these 1D conductors. We find clear evidence of weak localization, which enables us to determine the electron's phase coherence length, which can be as large as 0.5 mm at 3 K.
The plot below shows the resistance of a ring as a function of the magnetic field applied through it. The resistance is maximum at zero field because electrons propagating around the ring in opposite direction interfere with each other constructively at the point of injection, forming a standing wave. Applying a magnetic field breaks time-reversal invariance, and the contructive interference is lost.
Nanoscale science and technology group at the IBM T. J. Watson Research Center, Yorktown Heights, New York.
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