NPG Asia Materials research highlight | doi:

Carbon nanotubes: Feel the vibe

Carbon nanotubes have a range of appealing properties, such as superior electronic conductivity, and are of significant interest for many applications. The electron dynamics in carbon nanotubes can in principle be studied through the observation of vibrational (phonon) properties; however, experimental techniques have so far failed to unravel phonon dynamics at short time-scales. Katsuyoshi Ikeda and Kohei Uosaki from Hokkaido University in Japan¹ have now developed a broadband coherent Raman spectroscopy technique that provides access to short time-scales for the direct and simultaneous measurement of the relaxation time of several excited phonon modes.

In carbon nanotubes, the coupling between electrons and vibrational excitations (phonons) is strong. The ‘G phonons’ in particular are strongly related to electron dynamics. However, important parameters such as the lifetime of G phonons remain a subject of debate. Here, the researchers studied phonon excitations in carbon nanotubes using their broadband coherent Raman scattering microscopy technique, which involves the use of a laser pulse with broad wavelength distribution to excite vibrations in the carbon nanotubes. “It was progress in the generation of broadband laser pulses from a photonic crystal fiber that enabled these experiments,” says Ikeda.

After a defined delay following the broadband excitation, a second narrowband laser pulse for stimulated Raman scattering measurements then probes the number of coherent phonons persisting in the excited vibrational state (Fig. 1). By adjusting the delay between these two pulses, time-resolved phonon spectra can be measured with an accuracy of less than one picosecond. Such fine temporal resolution makes it possible to capture the evolution of a broad phonon spectrum.

Using this technique, the researchers were able to observe several phonon excitations in the carbon nanotubes, including the important G phonons. “It is remarkable that such a high-frequency phonon such as the G phonon can be observed by this broadband technique,” says Ikeda.

The measured lifetime of the G phonons at room temperature was around 1.1 picoseconds. Next, it will be interesting to see how this property changes under different experimental conditions, such as temperature. Detailed measurements of the behavior of G phonons for individual carbon nanotubes are expected to lead to a deeper understating of their electron transport behavior.

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

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