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]
Electronic
degrees of freedom and their coupling to lattice vibrations
in semiconductors can be strongly modified by doping. Accordingly,
the addition of surplus charge carriers to chirality-mixed carbon
nanotube samples has previously been found to give rise to a Drude-type
plasmon feature as well as Fano-type antiresonances in the far- to
mid-infrared spectral range (FIR/MIR). Here we investigate the FIR/MIR
response of redox chemically doped semiconducting (6,5) carbon nanotubes
(s-SWNTs). We find that, contrary to expectations, the Drude-type
plasmon shifts to lower wavenumbers with increasing
doping level. By means of Monte Carlo simulations of the optical response,
we attribute this behavior to the confinement of excess charge carriers
at low doping levels and their progressive delocalization when approaching
degenerate doping. The coupling of vibrational modes to intraband
excitations in the doped s-SWNTs can be probed via a double-resonance
process similar to that responsible for the Raman D-band. The resulting
Fano antiresonances shed new light onto the character and coupling
of electronic and vibrational degrees of freedom in these one-dimensional
semiconductors.
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