Pressure dependence of Raman modes in DWCNT filled with PbI2 semiconductor

Unpolarized Raman spectra of tangential modes on DWNTs filled with 1D nanocrystalline PbI2 semiconductor excited with 647 nm were studied at room temperature and elevated pressure up to 30 GPa. The tangential optical phonon modes of the carbon nanotubes are sensitive to the in plane stress and split into a contribution associated with the external and internal tube. Up to 11 GPa we find a pressure coefficient for the internal tube of 3.7 cm–1 GPa–1 and for the external tube of 6.3 cm–1 GPa–1. In addition, the tangential band of the external tubes broadens and decreases in amplitude. The corresponding Raman features of the internal tubes appear to be considerably less sensitive to pressure. In the range 11 to 15 GPa we observed a discontinuity in the slope (red shift) of the pressure dependence of the frequency of the tangential modes. This phase transition is associated to a possible structural distortion of the nanotube cross-section. When increasing the pressure furthermore up to 30 GPa the pressure coefficients for the tangential modes associated to the internal and external tubes are the same (10.6 cm–1 GPa–1). All findings lead to the conclusion that the outer tubes act as a protection shield fore the inner tubes (at least up to 11 GPa)