Many-particle excitations in non-covalently doped single-walled carbon nanotubes

Timofei V Eremin,Petr A Obraztsov,Vladimir A Velikanov,Tatiana V Shubina,Elena D Obraztsova

doi:10.1038/s41598-019-50333-7

Abstract

Doping of single-walled carbon nanotubes leads to the formation of new energy levels which are able to participate in optical processes. Here, we investigate (6,5)-single walled carbon nanotubes doped in a solution of hydrochloric acid using optical absorption, photoluminescence, and pump-probe transient absorption techniques. We find that, beyond a certain level of doping, the optical spectra of such nanotubes exhibit the spectral features related to two doping-induced levels, which we assign to a localized exciton X and a trion T, appearing in addition to an ordinary exciton {E}_{1}. We evaluate the formation and relaxation kinetics of respective states and demonstrate that the kinetics difference between E1 and X energy levels perfectly matches the kinetics of the state T. This original finding evidences the formation of trions through nonradiative relaxation via the X level, rather than via a direct optical excitation from the ground energy state of nanotubes.

Highlights

Doping of single-walled carbon nanotubes leads to the formation of new energy levels which are able to participate in optical processes
We provide new physical insight on the energy structure of single-walled carbon nanotubes (SWNTs) doped with hydrochloric acid and on the physical nature and behaviour of the corresponding many-particle excitations in such modified nanomaterial
We observed two doping-induced energy levels X and T in hydrochloric acid (HCl)-doped [6,5]-SWNTs, which we cautiously ascribe to the exciton, localized on the physisorbed H + ion, and to the trion, respectively

Summary

Introduction

Doping of single-walled carbon nanotubes leads to the formation of new energy levels which are able to participate in optical processes. The first experimental confirmation of this prediction was done by Matsunaga et al, who observed new peaks in absorption and PL spectra of SWNTs after p-doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and hydrochloric acid (HCl). The first experimental confirmation of this prediction was done by Matsunaga et al, who observed new peaks in absorption and PL spectra of SWNTs after p-doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and hydrochloric acid (HCl)5 These new peaks were assigned to the optical transitions between the ground (non-excited) energy state of the nanotubes and a new doping-induced energy state, located approximately 100–200 meV (depending on the nanotube diameter) below the bright exciton, and attributed to the positive trion. Later, applying doping methods such as gate-doping, electrochemical doping, chemical doping with F4TCNQ and HCl, several groups reported new features in linear absorption and PL spectra and attributed them to a direct optical excitation and a radiative decay of trions, respectively. Compared to the main exciton peak, such new peaks exhibit increased intensity with increased doping concentration, while the total absorbance and luminescence signal decreases

Methods

Results

Conclusion