Infrared Study of Charge Carrier Confinement in Doped (6,5) Carbon Nanotubes

Abstract

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.

Impact of Dielectric Environment on Trion Emission from Single-Walled Carbon Nanotube Networks

“…For the commonly used dielectric PMMA, which also showed the least charge trapping and most balanced carrier mobilities, the positive and negative trion/exciton ratios were fairly similar. An explanation for these unexpected trends might be given by the model introduced by Eckstein et al 33,43 They proposed a hard segmentation of nanotubes by charge puddles to explain the emission properties of (electro)chemically doped (6,5) nanotubes. The assumption was that poorly screened counterions localize charge carriers and thus divide the nanotube into charge puddles, where trion emission originates from, and largely neutral nanotube segments where excitons are formed and reside.…”

“…An accessible and commonly used nanotube species for both spectroscopic − as well as charge transport studies − are (6,5) SWCNTs, which can be obtained in sufficient quantities through various separation techniques including polymer wrapping using poly­[(9,9-dioctylfluorenyl-2,7-diyl)- alt -(6,6′-(2,2′-bipyridine))] (PFO-BPy) . The relatively large band gap and narrow E 11 excitonic transition of (6,5) nanotubes around 1.24 eV enable easy detection even of further red-shifted trion emission.…”

Mid-Infrared Electrochromics Enabled by Intraband Modulation in Carbon Nanotube Networks

“…The position of the band edge can be extracted through fitting the linear trend of the absorbance near the band edge, and this is illustrated for single-layer devices in the unbiased, on and off states in Figure a with all spectra shown in Figure S9. This shift in transmission change in the MIR has been studied previously using chemical doping of nanotubes, noting that doping that influences the excitonic transitions correspondingly alters Drude-type intraband transitions in the MIR. The band edge positions for unbiased films are in the range of 2.9 (0.43 eV) and 3.5 μm (0.35 eV).…”

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

“…22,[29][30][31][32] Moreover, localization of excess charge carriers has also been instrumental for the interpretation of Breit-Wigner-Fano resonances in the IR spectra of doped SWNTs. 33,34 Here, we present findings in favor of inhomogeneous NR decay of excitons and trions in doped s-SWNTs. The discussion thus evolves around the role of charged impurity sites for the dynamics of excitons and of trions.…”