Doping by chemical or physical means is key for the development of future semiconductor technologies. Ideally, charge carriers should be able to move freely in a homogeneous environment. Here, we report on evidence suggesting that excess carriers in electrochemically p-doped semiconducting single-wall carbon nanotubes (s-SWNTs) become localized, most likely due to poorly screened Coulomb interactions with counterions in the Helmholtz layer. A quantitative analysis of blue-shift, broadening, and asymmetry of the first exciton absorption band also reveals that doping leads to hard segmentation of s-SWNTs with intrinsic undoped segments being separated by randomly distributed charge puddles approximately 4 nm in width. Light absorption in these doped segments is associated with the formation of trions, spatially separated from neutral excitons. Acceleration of exciton decay in doped samples is governed by diffusive exciton transport to, and nonradiative decay at charge puddles within 3.2 ps in moderately doped s-SWNTs. The results suggest that conventional band-filling in s-SWNTs breaks down due to inhomogeneous electrochemical doping.

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]

Doping of semiconductors, such as single-wall carbon nanotubes (SWNTs), by the introduction or removal of charges can be used for modification of their electronic and optical structure. This offers unique opportunities for the fabrication of versatile materials with new properties. However, despite a broad body of work,1−3 the control and spectroscopic assessment of the doping of SWNTs is still being developed4,5 in order to be able to reproducibly tailor semiconductor properties. Here, we have studied the photophysical properties of chemically doped SWNTs by absorption and photoluminescence spectroscopy. Polychiral samples of purified HiPCO soot were charged by oxidation with gold chloride. Thanks to very well-resolved and narrow exciton bands in these samples, the trionic signatures induced by charges could be resolved for up to five distinct nanotube species. An investigation of spectral changes in absorption measurements as a function of gold chloride concentration reveals that tubes with smaller bandgaps are doped at lower gold chloride concentrations. The trion binding energy with respect to the dipole-allowed first sub-band exciton transition was also found to increase systematically with decreasing diameter in agreement with previous findings.1,2 In addition, we find that SWNT oxidation potentials decrease nearly linearly with increasing nanotube diameter. Lastly, a surprisingly abrupt change of exciton oscillator strengths at redox potentials near the nanotube band edge provides evidence for band gap renormalization. References 1. R. Matsunaga, K. Matsuda, Y. Kanemitsu, Phys. Rev. Lett. 2011, 106, 037404. 2. J. S. Park, Y. Hirana, S. Mouri, Y. Miyauchi, N. Nakashima, K. Matsuda, JACS 2012, 134, 14461−14466. 3. S. M. Santos, B. Yuma, S. Berciaud, J. Shaver, M. Gallart, P. Gilliot, L. Cognet, B. Lounis, Phys. Rev. Lett. 2011, 107, 187401. 4. H. Hartleb, F. Späth, T. Hertel, ACS Nano 2015, 9, 10461−10470. 5. M. Yoshida, A. Popert, Y. K. Kato, Phys. Rev. B 2016, 93, 041402.

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