Variations in the Raman peak shift as a function of hydrostatic pressure for various carbon nanostructures: A simple geometric effect

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Abstract
We have investigated pressure-induced Raman peak shifts for various carbon nanostructures with distinct differences in the degree of structural order. The high-frequency tangential vibrational modes of the hollow nanostructures, as well as those of graphite crystals and a macroscopic carbon fiber used as reference materials, were observed to shift to higher wave numbers. The hollow nanostructures and the carbon fiber displayed two distinct pressure regimes with transition pressures between 0.75 and 2.2 GPa, whereas the graphite crystals showed a linear pressure dependence up to hydrostatic pressures of 5 GPa. The observed peak shifts were reversible for all hollow nanostructures and graphite. Although the pressure-induced Raman peak shift in the low pressure regime could be used to identify the elastic properties of the macroscopic carbon fiber, a theoretical model shows that the observed deviations in the pressure coefficients of the hollow nanostructures in this regime can be explained entirely on the basis of geometric effects. The close match of all Raman peak shifts in the high pressure regime indicates a reversible flattening of the nanostructures at the transition point.