  Controlling doping is essential for a successful integration of semiconductor
materials into device technologies. However, the assessment of doping levels
and the distribution of charge carriers in carbon nanotubes or other nanoscale
semiconductor materials is often either limited to a qualitative attribution of
being 'high' or 'low' or it is entirely absent. Here, we describe efforts
toward a quantitative characterization of doping in redox- or electrochemically
doped semiconducting carbon nanotubes (s-SWNTs) using VIS-NIR absorption
spectroscopy. We discuss how carrier densities up to about 0.5 $\rm nm^{-1}$
can be quantified with a sensitivity of roughly one charge per $10^4$ carbon
atoms assuming in-homogeneous or homogeneous carrier distributions.
