October 16, 2007 – A team of scientists at IBM’s T.J. Watson Research Center say they have used Raman spectroscopy to measured the electron density in <2nm-dia. carbon nanotubes (CNT) by examining the interactions between electrons and phonons. The work will help better gauge how the CNTs release heat and impede electrical flow, thus measuring their suitability for use in future semiconductors.
More than a year ago IBM built an IC around a single carbon nanotube molecule, and have been improving performance of CNT transistors. But reproducibility has been a problem because the CNT’s electrical charge is influenced by environmental factors, so being able to measure local electron density changes is essential.
Their latest work, published online in the journal Nature Nanotechnology, is based on showing how Raman frequency in both metallic and semiconducting CNTs shifts in response to changes in charge density induced by an external gate field. The team monitored the color of the light scattered from a CNT and measured small changes in the color of the light corresponding to changes in electron density. Changes in the Raman spectra offer a way to probe local doping in CNTs, or charge carrier densities induced by environmental actions, they note. Behavior of the “G mode” (vibrational mode at 1580 cm-1), which shifts to higher frequency and narrows in linewidth in metallic carbon nanotubes at large fields, is analogous to that of graphene, they claim. But in semiconducting CNTs induced changes only shift the phonon frequency and do not affect linewidth.
The researchers claim they’ve devised a model to quantitatively explain the spectral changes, involving renormalization of the CNT phonon energy by the electron-phonon interaction as the carrier density in the CNT is changed. “These changes in the Raman spectra provide us with a powerful tool for probing local doping in carbon nanotubes in electronic device structures, or charge carrier densities induced by environmental interactions, on a length scale determined by the light diffraction limit,” they wrote.