Estudo da dopagem em nanotubos de carbono por espectroscopia Raman ressonante.

Abstract

Doping of single wall carbon nanotubes with boron, nitrogen, phosphorous and sulfur was studied by resonance Raman spectroscopy. The doping atoms were inserted in the tube walls during synthesis, by using chemical vapor deposition, laser ablation andarc discharge methods. The analysis of the Raman spectra shows that the insertion of dopants generates tubes with smaller diameter for boron and phosphorous doped samples. No significant change in the diameter distribution of nitrogen doped samples was observed, despite of different results found in the literature, where N induces the growth of smallerdiameter tubes. Analysis of defect induced Raman bands shows that doping does not damage the overall structure of the tubes, except in the case of sulfur, whose doped samples contain a low percentage of nanotubes when compared with another carbon nanomaterials. Doping occurs naturally by adsorption of environmental atoms and molecules, or by defects caused during the growth of several structures. Even at small levels, doping is relevant in microscopic and nanometric systems. It is crucial then to understand the properties of these systems. But when the doping levels are below the detection limit of most of the characterization techniques, such as electron diffraction or electron loss spectroscopy, it is hard to systematize the doping effects. In this work, it is shown thatthe analysis of the second order Raman spectrum (G0 band) is capable of identifying doping at small levels in single wall carbon nanotubes, via renormalization of the electron and phonon energies near the doping sites. The p/n-type of doping can also be determined by this technique.The insertion of phosphorous into the nanotube lattice causes distinct changes in the electron and phonon structure when compared with nitrogen doping. Nitrogen and phosphorous are electron donors to nanotubes, but the later is too big to fit in the carbon hexagonal lattice of the nanotubes, and the charge transfer has a localized nature. Hereit is shown that the G0 band is also sensitive to these differences between nitrogen and phosphorous doping, opening a new branch to the characterization of specific dopants in carbon nanotubes.