Collapse of Single-Wall Carbon Nanotubes is Diameter Dependent

TLDR: The collapse pressure is found to be independent of the nanotube chirality, and a lower limit on the largest SWNT that remains inflated at atmospheric pressure is established.

Abstract: Introduction.—Since their identification in 1991, interest in carbon nanotubes has continued to grow, focusing on both their intrinsic properties and potential applications. The behavior of individual tubes has been explored via experiment [1‐3] and computer simulation, e.g., [4 ‐7], in both axial and bending geometries. Elastic properties have generally been found to be broadly consistent with the in-plane properties of graphite, but strengths have proved harder to assess, with simulation results consistently predicting higher values than have been observed experimentally, probably as a result of defects in the real materials. Carbon nanotubes have also been explored under hydrostatic pressure, using Raman spectroscopy [8‐13], x-ray diffraction [14,15], and neutron diffraction techniques [16]. Raman spectroscopy, in particular, has proved to be a very useful tool in the characterization of single-wall carbon nanotubes (SWNTs), revealing information about crystallinity, diameter, and even chirality. Under increasing hydrostatic pressure, the Raman peaks shift to higher frequencies, corresponding to a stiffening of the carbon framework. Some authors have noticed that the peak position of the tangential mode, corresponding to in-plane vibrations of adjacent carbon atoms in the graphene sheet, shifts linearly over two regimes with a change in gradient at a critical pressure of approximately 2 GPa, depending on the type of nanotube material used. A number of studies also reported the disappearance of the radial breathing mode (RBM) from the spectrum above that critical pressure. Similarly, x-ray results demonstrate the disappearance of scattering associated with the hexagonally close-packed lattice into which the SWNT bundles are organized [14]. Clearly, a structural phase transition occurs at this critical pressure, but the exact nature of this change has proved controversial. Most authors seem to favor a transition to a close-packed structure of hexagonally deformed nanotubes (‘‘polygonization’’), while others propose a complete flattening or ‘‘collapse’’ [17]. A previous TEM study of multiwall nanotubes (MWNT) found evidence for collapse to form ribbons, although the cause was unclear [18]. In a recent study [19], we used Raman spectroscopy to compare the behavior of bundles of single and a range of MWNTs to that of graphite, under hydrostatic pressure. The initial gradient of the peak shift could be explained entirely in geometric terms, using a continuum mechanics model and the relevant internal and external diameters. Above the critical pressure, the gradient was equal to that of the graphite, which exhibited no transition over the pressure range up to 10 GPa. We interpreted these results as evidence supporting the complete collapse of the hollow core of the nanostructures to produce materials resembling graphite in terms of density and hybridization. The peak shifts appeared to be completely reversible within experimental accuracy, as long as the maximum pressure was kept below 10 GPa. The SWNTs in this experiment were supplied by Tubes@Rice, and are generally considered to be predominantly either (10,10) nanotubes or other chiralities with similar diameters. The critical pressure for these nanotubes to collapse was found to be 2:1 0:2 GPa.