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Raman spectroscopy study of heat-treated and boron-doped double wall carbon nanotubes

We performed Raman spectroscopy experiments on undoped and boron-doped double walled carbon nanotubes DWNTs that exhibit the “coalescence inducing mode” as these DWNTs are heat treated to temperatures between 1200 ° C and 2000 ° C. The fact that boron doping promotes DWNT coalescence at lower temperatures allowed us to study in greater detail the behavior of first- and second-order Raman modes as a function of temperature with regard to the coalescence process. Furthermore, by using various excitation laser energies we probed DWNTs with different metallic M and semiconducting S inner and outer tubes. We find that regardless of their M and S configurations, the smaller diameter nanotubes disappear at a faster rate than their larger diameter counterparts as the heat treatment temperature is increased. We also observe that the frequency of the G band is mostly determined by the diameter of the semiconducting layer of those DWNTs that are in resonance with the laser excitation energy. Finally, we explain the contributions to the G band from the inner and outer layers of a DWNT.