  The study of the G-mode pressure coefficients of carbon nanotubes, reflecting
the stiff sp2 bond pressure dependence, is essential to the understanding of
their extraordinary mechanical properties as well as fundamental mechanics.
However, it is hindered by the availability of carbon nanotubes samples only as
bundles or isolated with surfactants. Octadecylamine functionalized carbon
nanotubes are mostly of a single diameter and can be stably dispersed in 1,
2-dichloroethane and chloroform without surfactants. Here we perform high
pressure Raman spectroscopy on these tubes and obtain their experimental G-mode
pressure coefficients for individual tubes and bundles. The G-plus pressure
coefficient for bundles is only about half of that for individual tubes in 1,
2-dichloroethane and is about two-thirds in chloroform. The G-minus pressure
coefficient for bundles is about one-third of G-plus in 1, 2-dichloroethane and
about the same in chloroform. These results for the first time provide
unambiguous experimental evidence of the significant effect of bundling on
carbon nanotubes' G-mode pressure coefficients, identifying it as one of the
major reasons for the lack of consensus on what the values should be in the
literature.
