Synthesis of nitrogen-doped single-walled carbon nanotubes and monitoring of doping by Raman spectroscopy

Nitrogen-doped single-walled carbon nanotubes （CNx-SWNTs） with tunable dopant concentrations were synthesized by chemical vapor deposition （CVD）, and their structure and elemental composition were characterized by using transmission electron microscopy （TEM） in combination with electron energy loss spectroscopy （EELS）. By comparing the Raman spectra of pristine and doped nanotubes, we observed the doping-induced Raman G band phonon stiffening and 2D band phonon softening, both of which reflect doping-induced renormalization of the electron and phonon energies in the nan- otubes and behave as expected in accord with the n-type doping effect. On the basis of first principles calculations of the distribution of delocalized carrier density in both the pristine and doped nanotubes, we show how the n-type doping occurs when nitrogen heteroatoms are substitutionally incorporated into the honeycomb tube-shell carbon lattice.

The Dynamics of the Nucleation, Growth and Termination of Single-Walled Carbon Nanotubes from in situ Raman Spectroscopy During Chemical Vapor Deposition

The dynamics of the chemical vapor deposition(CVD)of single-walled carbon nanotubes(SWNTs)is extracted experimentally using in situ Raman spectroscopy.Nanotubes are grown using a thinlm cobalt catalyst and an ethanol precursor in a miniature hot walled reactor with optical access.Raman spectra at room temperature and at the growth temperature are compared for two growth temperatures.The evolution of the G-band,D-band,and radial breathing mode(RBM)is tracked at the growth temperature with time resolution of a few seconds.There are three identifiable phases in the evolution of the Raman signal intensity:an initial exponential increasing phase,a linear growth phase,and a saturation phase.In situ optical spectroscopy thus enables the study of nucleation,steady growth,and deactivation processes to be investigated separately in real time.The evolution curves for all bands(G,D,and RBM),when scaled,collapse onto the same curve,to within experimental uncertainty.

Direct Observation of the Strong Interaction Between Carbon Nanotubes and Quartz Substrate

We present a chemical vapor deposition(CVD)method for the growth of uniform single-walled carbon nanotube(SWNT)arrays on a stable temperature(ST)-cut single crystal quartz substrate using a mixture of methanol and ethanol as carbon source.It is found that introducing methanol during the growth can improve the density and the length of the well-aligned SWNTs in the arrays as well as increase the SWNT/quartz interaction.Obvious“up-shifts”of G-band frequencies in the Raman spectra have been found for the aligned SWNTs.A well-designed control experiment shows that the G-band“up-shifts”originate from the strong interaction between SWNTs and the quartz substrate.It is believed that exploring this interaction will help to elucidate the growth mechanism;ultimately,this will help realize the promise of controlling the chirality of SWNTs.

Diameter dependent doping in horizontally aligned high-density N-doped SWNT arrays

We reported the growth of horizontally aligned nitrogen-doped single-walled carbon nanotubes (SWNTs) on quartz substrates. The synthesized SWNTs were comprehensively characterized at the single nanotube level. Owing to the highly aligned nature of the nanotubes, we were able to investigate the diameter dependent doping mechanism through systematic resonant Raman spectroscopy studies. Other than the formerly found narrowing effect by N-doping, we proposed that the nanotube diameter affects the introduction of N atoms into the carbon lattice in an elaborate way. The obtained doping level increased along with the nanotube diameter but lost the increasing trend when the diameter became larger and experienced a slight decrease after reaching the local peak value. These insights about the heteroatom doping into the carbon nanotubes could benefit the development of the carbon nanotube based functional materials and extend their application in a broad range of areas.