High pressure study of double wall carbon nanotubes

Abstract: High pressure Raman studies were performed in the High Pressure
Spectroscopy Laboratory at Luleå University of Technology on Double Wall
Carbon Nanotubes (DWCNTs). Laser excitation energies used were 1.96 eV and
2.33 eV. High pressure on the sample was achieved using a Membrane Diamond
Anvil Cell (MDAC). Pressure dependence of the tube’s Raman peaks were
investigated in the pressure range 0 to approximately 10 GPa. Both radial
(RBM) and tangential (G band) modes were investigated during pressure
increase. The overall trend of the intensity of the Raman signals is that
it decreases when pressure is applied. This behavior is most prominent for
the metallic outer tubes. The outer tubes are more affected by pressure
than the inner tubes, which are proven by their higher derivative of the
Raman peak pressure dependence.

Investigating the metallic outer tubes under pressure reveals that some
anomaly in the pressure dependence around 5 GPa occur for both RBM and G
band. Results show that the Raman peaks of those tubes stop shifting to
higher frequencies at approximately 5 GPa and start to downshift until they
reach a more or less constant value at elevated pressures. Intensity
decrease in this pressure region is also prominent.

In order to polymerize DWCNTs a heating system was designed and
manufactured. The design was chosen with respect to high temperature
treatment of the sample in a MDAC. Since the cell becomes very hot,
equipment and laboratory personnel have to be protected and temperatures at
critical points have to be measured. Stainless steel was chosen as the main
material since it has poor heat conductive properties.