Finite length and solvent analysis effects on the squash mode of single walled carbon nanotubes

Nanotube diameters (d) are usually characterized using the radial breathing mode d–1; the squash mode frequency (f) however is predicted to vary as d–2. We demonstrate using the MM+ forcefield that for lengths <9 nm the symmetric squash mode (SSM) and asymmetric squash mode (ASM) ((10,0) SWNT (single wall carbon nanotubes)) are non-degenerate with Δf ≤ 55 cm–1. In solution, the SWNT-water interaction upshifts the ASM by 20 cm–1 and the SSM by 10 cm–1. Such asymmetries could be used to simultaneously characterize the length and diameter of short nanotubes for applications including nanoresonators and biomedical probes.