Ultrafast vibrational motion of carbon nanotubes in different pH environments

Abstract

We have used a femtosecond pump-probe impulsive Raman technique to explore the ultrafast dynamics of micelle suspended single-walled carbon nanotubes ͑SWNTs͒ in various pH environments. The structures of coherent phonon spectra of the radial breathing modes 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. DOI: 10.1103/PhysRevB.80.245428 PACS number͑s͒: 78.47.JϪ, 78.67.Ch, 63.22.Gh, 63.20.kd Carbon nanotubes ͑CNTs͒ are one of the most intriguing nanomaterials, which are useful for molecular devices, such as a chemical sensor 1 and a nanomachine capable for medical science. 2 One potential way to realize the nanomachine 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. 2 Therefore, CNT devices will be used in various pH environments in biological systems, e.g., the typical pH of human arterial blood is Ϸ7.4 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, 3 excitonplasmon coupling, 4 and electron-phonon energy transfer, 5 is crucial to prepare suitable environments for CNT devices.Single-walled carbon nanotubes ͑SWNTs͒ possess simplest structure of CNTs and it has metallic or semiconducting conductivity, depending on the arrangement of carbon atoms, referred as roll-up vectors. 6 Recently frequency-domain spectroscopy has revealed the diameter-selective Raman scattering from radial breathing modes ͑RBMs͒ in SWNT,7,8 where the frequency of the RBMs depended on the inverse of the diameter. 3 Moreover, Strano et al. 9 have investigated the change in the Raman spectra of RBM induced by the protonation and showed that the protons change electronic structure of SWNT, i.e., protonation is band-gap selective and consequently the condition of resonant Raman scattering was modified, depending on the pH values. Coherent phonon spectroscopy is another potential method to study phonon dynamics in time-domain. Up to date, there have been only several studies on ultrafast phenomena in CNTs, 10-14 and early time stage dynamics of photoexcitation of electronic system and subsequent phonon excitation and relaxation was not been well explored. Motivated by the observation of the early time stage dynamics of electron-phonon coupling, Lüer et al. 15 have recently investigated the electron-phonon coupling by means of coherent phonon spectroscopy with 10 fs time resolution, where they focused their attention to the high-frequency optical modes rather than the RBMs.In this paper, the coherent oscillation of the RBMs in SWNTs has been measured in various pH environments to show that the subpic...

Theory of coherent phonons in carbon nanotubes and graphene nanoribbons

“…The coherent phonon dynamics in micelle suspended nanotubes can also probe details of the interaction of the SWNTs with the environment. Makino et al [49] studied the dynamics of coherent RBM phonons in micelle suspended SWNTs in femtosecond pump-probe impulsive Raman experiments and found a strong environmental pH dependence in the CP spectra which they attributed to protonation at the SWNT surface. Using pulse shaping techniques to create a train of pump pulses resonant with the CP period, one can generate and detect coherent phonons in nanotubes of a specific chirality in an ensemble sample [50].…”

Exciton diffusion in semiconducting single-walled carbon nanotubes studied by transient absorption microscopy

“…Time-resolved photoluminescence measurements have also shown decay times of several 10 ps. [21][22][23][24][25][26] Other aspects of exciton dynamics have also been studied, including exciton-exciton annihilation, 27 exciton dephasing, 28 exciton-phonon interactions, [29][30][31][32] multiple exciton generation, 33,34 intraexciton transition, 35 and exciton nonlinearities.…”

Ultrafast spectroscopy of coherent phonon in carbon nanotubes using sub-5-fs visible pulses

“…In the previous papers [55,[47][48][49][50][51][52][53][54][55], based on the DER analysis, the modulation was explained in terms of a sinusoidal electronic energy modulation due to an RBM-induced change in the diameter of a CNT [55,[47][48][49][50][51][52][53][54][55]. In the following, the mechanism involved in the DER-type contribution is more fully discussed.…”

“…CP dynamics in SWNTs have been studied by several groups using the impulsive excitation method [46][47][48][49][50][51][52][53][54][55]. However, there are still some important concerns which need to be addressed, especially the mechanism of CP generation, which is related to coupling between phonon modes and the electronic structure.…”