Raman signature to identify the structural transition of single-wall carbon nanotubes under high pressure

Raman spectra of single-walled carbon nanotubes (SWNTs) with diameters of 0.6–1.3 nm have been studied under high pressure. A “plateau” in the pressure dependence of the $G$-band frequencies was observed in all experiments, both with and without pressure transmission medium. Near the onset of the $G$-band plateau, the corresponding radial breathing mode (RBM) lines become very weak. A strong broadening of the full width at half maximum of the RBMs just before the onset of the $G$-band plateau suggests that a structural transition starts in the SWNTs. Raman spectra from SWNTs released from different pressures also indicate that a significant structural transition occurs during the $G$-band plateau process. Simulations of the structural changes and the corresponding Raman modes of a nanotube under compression show a behavior similar to the experimental observations. Based on the experimental results and the theoretical simulation, a detailed model is suggested for the structural transition of SWNTs, corresponding to the experimentally obtained Raman results in the high-pressure domain.

Figure 1

RBMs of the nanotubes measured using three excitation wavelengths of 514.5, 632.8, and 830 nm. For each RBM peak the corresponding diameter (in nanometers) is indicated.

Figure 2

(Color online) (a) Raman spectra in the RBM and $G$-band regions for SWNTs, measured with 830 nm laser excitation using liquid argon as PTM. (b) The frequencies of two RBMs (circles) and the most intense $G$-band peak (squares) near $1594{\text{cm}}^{-1}$ as a function of pressure. (c) The intensities of the two dominant RBMs as functions of pressure. (d) Raman spectra of SWNTs at 0.87, 8.51, and 11.65 GPa, $G$-band intensity is normalized. (e) The linewidths of the two dominant RBMs (at 236 and $304{\text{cm}}^{-1}$) as functions of pressure.

Figure 3

(a) The Raman spectra of SWNTs measured with 830 nm laser excitation without PTM. (b) The frequencies of RBMs (circle) and the $G$ band (square) (originally at $1594{\text{cm}}^{-1}$) as functions of pressure. (c) The intensities of three dominant RBMs (236, 270, and $304{\text{cm}}^{-1}$) as a functions of pressure. (d) The linewidths of three RBMs (at 236, 270, and $304{\text{cm}}^{-1}$) as functions of pressure.

Figure 4

(Color online) The frequency of the $G$ band ($1594{\text{cm}}^{-1}$ mode) as a function of applied pressure with a PTM (triangles) or without a PTM (squares); inset shows the intensity change in the dominant RBMs with increasing pressure (using a PTM). The excitation wavelength is 514.5 nm.

Figure 5

The Raman spectra of SWNTs released from different pressure (a) in the experiments with a PTM, measured with 830 nm laser excitation, and (b) in the experiments without a PTM, measured with 633 nm laser excitation.

Figure 6

(a) Sketch of a (5,5) FSWNT model (upper panel), and the system energies of deformed nanotubes with different optimized structural shapes. The distance ($x$ axis) is the shortest distance between two C atoms located opposite each other on different sides of the axis of the FSWNT (marked with black atoms). (b) Calculated relative intensities and Raman shifts for the same nanotube.