Ultrafast dynamics of coherent phonon and photoexcited carriers in carbon nanotubes and nanotube-protein complex systems
Carbon nanotubes (CNTs) are one of the most intriguing nano-materials, which are useful for molecular devices, such as a chemical sensor and a nano-machine capable for medical science. One potential way to realize the nano-machine by the use of CNTs, which can work in biological systems, e.g., a drug delivery system, is to use them in solution with proteins. Therefore, CNT devices will be used in various pH environments in biological systems, e.g., the typical pH of human arterial blood is ~7.4, that is weakly alkaline. Since the chemical reactivity of CNTs is dominated by the fundamental physical processes on their surface, investigation of dynamics at surfaces, such as charge transfer, exciton-plasmon coupling, and electron-phonon energy transfer, is crucial to prepare suitable environments for CNT devices.

We have used a femtosecond pump-probe impulsive Raman technique to explore the ultrafast dynamics of micelle suspended single walled carbon nanotubes (SWNTs) in various pH environments. The structures of coherent phonon spectra of the radial breathing modes (RBMs) exhibit significant pH dependence, to which we attribute the effect of the protonation at the surface of SWNTs, resulting in the modification of electronic properties of semiconductor SWNTs. Analysis of the time-domain data using a time-frequency transformation uncovers also a second transient longitudinal breathing mode, which vanishes after 1 ps of the photoexcitation.

This spectroscopic technique is more sensitive to various environments of CNTs than conventional Raman spectroscopy because the transient snap shot of the CNT motion can be monitored. Thus, we have explored the ultrafast vibrational dynamics of coherent RBMs in real time using the femtosecond transmission technique based on impulsive Raman scattering. The two dominant RBMs were observed at 6.4 and 7.2 THz, whose spectral intensities depended on the pH value. The enhancement of the lower RBM (6.4 THz) at the higher pH values could be attributed to the change in the electronic structure of SWNT from p-doped to nondoped nature by the deprotonation effect. Dephasing time of the lower RBM extracted from the time-domain datum exhibited pH dependence, which was explained by the electron-phonon coupling through deformation potential interaction, while that of the higher RBM did not depend on the pH value, implying existence of the electron-phonon decoupling in a few picoseconds. Analysis of the time-domain data by both the continuous wavelet transform and the Gabor transform methods revealed the new peak at $\approx$ 4.4 THz in the transient response within 1 ps, which appeared to be the signature of the longitudinal breathing mode of SWNT.

*Collaboration with Prof. K. Shiraki.

transient response from CNT
Fig.1 Time-resolved transmission for SWNT/SDS sample at pH = 3. Coherent RBMs are observed just after the transient electronic response at t = 0 ps. Pump-probe setup around the quart cell is schematically shown in the inset.
PRB 80, 245428 (2009).

Fig. 2. (a) The CWT chronogram for pH = 3. (b) Enlarged GT spectra around 4 THz for pH = 3, in which the broad band appears at ~4.4 THz. The solid curve in (b) is the fit to the GT spectra with a Gaussian function.
PRB 80, 245428 (2009).