Quantifying Doping Levels in Carbon Nanotubes by Optical Spectroscopy

Abstract

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 nm −1 can be quantified with a sensitivity of roughly one charge per 10 4 carbon atoms assuming in-homogeneous or homogeneous carrier distributions. arXiv:1909.05181v1 [cond-mat.mtrl-sci]

Charge Transfer from Photoexcited Semiconducting Single-Walled Carbon Nanotubes to Wide-Bandgap Wrapping Polymer

“…The (6,5) SWCNT polaron can be created in steady state by two methods, namely redox-chemical doping 22 , 25 , 55 − 57 and electrochemical doping. 6 , 58 , 59 We carried out the redox-chemical hole-doping of the Hybrid dispersion with NOBF 4 (+1.00 V vs Fc/Fc + in CH 2 Cl 2 ) 60 as a one-electron oxidant.…”

“…The (6,5) SWCNTs hole-polaron features a bleached and blue-shifted E 00 → E 11 transition and an additional absorption band covering 1030−1200 nm, the line shape of which strongly depends on the charge carrier level. 25,55,[57][58][59]63 The electronic absorption transitions of oxidized nanotubes are explained by the electron depletion of the top of the valence band, which results in an increase in the E 00 → E 11 transition energy and leads to additional electronic transitions. 25,59 It is noteworthy that studies by electron paramagnetic resonance (EPR) spectroscopy reveal that the unpaired electrons in lightly reduced SWCNTs are relatively free and fast-relaxing.…”

Diffusive and Unimolecular Nonradiative Decay of Excited States in Doped Carbon Nanotubes

“…The first subband X 1 exciton band at 1.239 eV looses practically all of its oscillator strength while becoming broader, more asymmetric and shifting to higher energies by about 70 meV before disappearing into a broad and featureless absorption background at the highest doping levels. 22,35 Changes in the second subband range are considerably more subtle. The exciton X 2 at 2.158 eV appears to split into two features, one of which undergoes similar blue-shift as the X 1 band, while the other seems to acquire most of the oscillator strength and shifts to slightly lower energies.…”

“…This interpretation is based on the notion that excitons which are more strongly confined by closely spaced impurities are also more strongly blue shifted in absorption. 35 At the same time these excitons also decay more rapidly due to their proximity to quenching impurities, thus becoming practically imperceptible in PL spectra. The result is that PL spectra are dominated by emission from weakly confined excitons that are not blue-shifted, need more time to diffuse to a quenching site and thus have a higher probability for radiative decay.…”

“…22 According to this view, excitons between closely spaced impurities will thus experience a greater confinement-induced blue shift while at the same time being more strongly coupled to NR decay channels at the charged impurity sites. 22,35,38 The doping-induced changes of absorption and emission spectra are summarized in Figure 2. This includes normalized data sets for the change of absorbance at the energy of the second subband exciton, the change of first subband exciton and trion oscillator strengths, and the change of their PL signals.…”