Probing high-pressure properties of single-wall carbon nanotubes through fullerene encapsulation

The high pressure behavior of bundled $1.35\pm 0.1\mathrm{nm}$ diameter single wall carbon nanotubes (SWNT) filled with ${\mathrm{C}}_{70}$ fullerenes (usually called peapods) has been investigated by Raman spectroscopy and compared with the corresponding behavior of the nonfilled SWNT. We show experimentally that two reversible pressure-induced transitions take place in the compressed bundle SWNT. The first transition, in the $2–2.5\mathrm{GPa}$ range, is in good correspondence with predictions of the thermodynamic instability of the nanotube circular cross section for the studied tube diameter. An interaction between the fullerenes and the tube walls is then observed at about $3.5\mathrm{GPa}$, which evidences a progressive deformation of the tube cross section. The second transition takes place at pressures between 10 and $30\mathrm{GPa}$, and is evidenced by two effects by a strong frequency downshift of the Raman transverse modes and the concomitant disappearance of the fullerenes Raman modes in peapods. The pressure at which the second transition takes place is strongly dependent on the nature of the pressure transmitting medium. We also report irreversible effects at high pressure as the shortening of the tubes, the formation of nanostructures and the disappearance of the ${\mathrm{C}}_{70}$ Raman signal in some cases. Transmission electron microscopy studies are also reported supporting these transformations.

Figure 1

Transmission electron microscopy images of the initial sample of peapods with different enlarging. It can be seen in (a) and (b) that the tubes are organized in bundles. From (c) and (d), we estimate the diameter of these tubes, close to $1.5\mathrm{nm}$, and their filling factor, higher than 80%.

Figure 2

Pressure evolution of the Raman spectra of ${\mathrm{C}}_{70}$ peapods when paraffin oil is used as pressure transmitting medium. (a) RBM signal up to its attenuation. (b) Spectral region between 350 and $1280{\mathrm{cm}}^{-1}$ dominated by the ${\mathrm{C}}_{70}$ modes. The lower central panel shows the peak positions corresponding to the contribution to the signal of (t) nanotubes and (f) ${\mathrm{C}}_{70}$ fullerenes at ambient pressure. (c) TM signal. The upper spectra shown in the three panels were measured after pressure release. The boxes under the lower spectra give the magnification of the different spectra at ambient pressure.

Figure 3

Pressure evolution of the Raman spectra between 350 and $1280{\mathrm{cm}}^{-1}$ of the ${\mathrm{C}}_{70}$ peapods when a 4:1 methanol:ethanol mixture is used as pressure transmitting medium. As in Fig. 2, the lower plot shows the peak positions corresponding to the contribution to the signal of (t) nanotubes and (f) ${\mathrm{C}}_{70}$ fullerenes at ambient pressure.

Figure 4

Pressure variation of the RBM linewidth of the ${\mathrm{C}}_{70}$ peapods in oil (filled triangles), alcohol (filled circles), and argon (filled stars), and empty nanotubes in oil (hollow triangles). Dashed lines are linear fits after the transition.

Figure 5

Pressure evolution of the Raman mode frequencies for the encapsulated ${\mathrm{C}}_{70}$ fullerenes with (a) oil or (b) alcohol (circles) and argon (stars) as pressure transmitting medium. The mode intensities are normalized to the highest of each pressure. The obtained relative intensities are displayed in gray level. The vertical dashed lines mark the observed transitions (see text). Linear fits are included in (a) for modes having an important change in their pressure slopes upon the first transition. These fits are kept in (b) for comparison.

Figure 6

Proposed pressure evolution of the nanotube shape in nanotube [from (a) to (c)] and peapods [from (d) to (f)]. Arrows correspond to the two observable transitions in each system at low pressure [$\left(\mathrm{a}\right)\to \left(\mathrm{b}\right)$ and $\left(\mathrm{d}\right)\to \left(\mathrm{e}\right)$] and at high pressure [$\left(\mathrm{b}\right)\to \left(\mathrm{c}\right)$ and $\left(\mathrm{e}\right)\to \left(\mathrm{f}\right)$]. (a)–(e) show the proposed cross sections: [(a) and (d)] circular, [(b) and (e)] elliptical or polygonized, and (c) racetrack or peanut type. In (f) is shown a longitudinal cut of the peapod after the second transition. See text for discussion.

Figure 7

Measured pressure evolution of the frequency of the TM maximum intensity for the peapods (filled symbols) and the empty nanotubes (hollow symbols) in oil (triangles), alcohol (circles), and argon (stars). The solid lines are guides for the eye for increasing pressures; the dashed lines represent the approximative paths during the decompression. The vertical dashed lines mark the transitions where the TM frequencies suddenly downshift. The data for empty nanotube in argon (hollow stars) are obtained from Ref. 23.

Figure 8

Recovered spectra of peapods at ambient conditions after different pressure treatments (16, 34, and $36\mathrm{GPa}$) when the pressure transmitting medium is oil.

Figure 9

Recovered spectra of peapods at ambient conditions after a pressure cycle up to $36\mathrm{GPa}$ when using oil, alcohol, or argon as pressure transmitting medium.

Figure 10

Transmission electron microscopy images of the sample of peapods after pressure treatment. The nanotube bundles are shorter (broken) after a pressure treatment at (a) $22\mathrm{GPa}$ in oil and (b) $36\mathrm{GPa}$ in alcohol. Additional nanostructures are also formed after a $36\mathrm{GPa}$ treatment in alcohol, like these (c) multiwalled nanocones and (d) polygonized nano-onions.