Doping of semiconductor nanostructures is essential for their application in electronic as well as photonic devices. Recent studies have focused on the assessment of electrochemical gate doping (1,2) or chemical doping (3) of semiconducting single-wall carbon nanotubes (SWNTs) by absorption and photoluminescence spectroscopy. Moderate doping levels on the order of 0.5 holes·nm^-1 were found to lead to a redistribution of oscillator strength from the first subband exciton (S₁) to the trion (2).

Here, we report on an investigation of trion dynamics by femtosecond pump-probe transient absorption experiments (VIS pump/NIR probe and NIR pump/NIR probe) in moderately doped (6,5)-SWNT films. SWNTs were charged by gate-doping or by interaction with redox-active gold chloride. We find that optical excitation of energetically lower lying trionic states leads to a delayed rise of the first subband exciton’s S₁ photobleach (PB) signature on the ps time-scale. However, the amplitude of the S₁ PB signal is surprisingly high considering that the energy separation of trion and exciton exceeds thermal energies by nearly an order of magnitude, making uphill thermal population transfer highly unlikely. Extremely weak photoluminescence from the S₁ exciton following trion excitation in hole-doped films indicates that energy transfer from the trion to the first subband exciton is indeed negligible (≲ 10^-3). The photobleach is thus essentially due to ground state depletion and reflects the ps-dynamics of the trion decay. Excitation energy and temperature dependent transient absorption experiments reveal that this process is thermally activated.

(1) J. Park et al., J. Am. Chem. Soc., 134, 14461 (2012).

(2) H. Hartleb, F. Späth and T. Hertel, ACS Nano, 9, 10461 (2015).

(3) R. Matsunaga, K. Matsuda and Y. Kanemitsu, Phys. Rev. Lett., 106, 037404 (2011).