Structural and Phonon Properties of Bundled Single- and Double-Wall Carbon Nanotubes Under Pressure

In this work, we report a theoretical study of the structural and phonon properties of bundled single- (SWNTs) and double-walled carbon nanotubes (DWNTs) under hydrostatic compression. Our results confirm drastic changes in volume of SWNTs in a high-pressure regime as assigned by a phase transition from circular to collapsed phase which are strictly dependent on the tube diameter. For the DWNTs, those results show first a transformation to a polygonized shape of the outer tube and subsequently the simultaneous collapse of the outer and inner tube, at the onset of the inner tube ovalization. Before the DWNT collapse, phonon calculations reproduce the experimentally observed screening effect on the inner tube pressure-induced blue shift for both radial and tangential modes. Furthermore, the collapse of nanotube bundles induces a sudden redshift of tangential components in agreement with experimental reports. The G(z) band analysis of the SWNT collapsed tubes shows that the flattened regions of the tubes are at the origin of their G-band signal. This explains the observed graphite-type pressure evolution of the G band in the collapsed phase and provides in addition a means for the identification of collapsed tubes using Raman spectroscopy.