Nitrogen
Doping Mechanism in Small Diameter Single-Walled
Carbon Nanotubes: Impact on Electronic Properties and Growth Selectivity
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Abstract
Nitrogen doping in carbon nanostructures
has attracted interest
for more than a decade, and recent implementation of such structures
in energy conversion systems has boosted the interest even more. Despite
numerous studies, the structural conformation and stability of nitrogen
functionalities in small diameter single-walled carbon nanotubes (SWNTs),
and the impact of these functionalities on the electronic and mechanical
properties of the SWNTs, are incomplete. Here we report a detailed
study on nitrogen doping in SWNTs with diameters in the range of 0.8–1.0
nm, with well-defined chirality. We show that the introduction of
nitrogen in the carbon framework significantly alters the stability
of certain tubes, opening for the possibility to selectively grow
nitrogen-doped SWNTs with certain chirality and diameter. At low nitrogen
concentration, pyridinic functionalities are readily incorporated
and the tubular structure is well pertained. At higher concentrations,
pyrrolic functionalities are formed, which leads to significant structural
deformation of the nanotubes and hence a stop in growth of crystalline
SWNTs. Raman spectroscopy is an important tool to understand guest
atom doping and electronic charge transfer in SWNTs. By correlating
the influence of defined nitrogen functionalities on the electronic
properties of SWNTs with different chirality, we make precise interpretation
of experimental Raman data. We show that the previous interpretation
of the double-resonance G′-peak in many aspects is wrong and
instead can be well-correlated to the type of nitrogen doping of SWNTs
originating from the p- or n-doping nature of the nitrogen incorporation.
Our results are supported by experimental and theoretical data