Doping-Dependent Energy Transfer from Conjugated Polyelectrolytes to (6,5) Single-Walled Carbon Nanotubes

Abstract

Conjugated polymers exhibit strong interactions with single-walled carbon nanotubes (SWNTs). These enable the selective dispersion of specific semiconducting SWNTs in organic solvents and polymer-mediated energy transfer to the nanotubes followed by emission in the near-infrared. Conjugated polyelectrolytes with ionic side-chains can add further functionalities to these nanotube/polymer hybrids such as dispersibility in polar solvents (e.g., methanol) and self-doping. Here, we demonstrate and investigate energy transfer from a range of conjugated polymers to preselected (6,5) SWNTs with varying spectral overlap between the optical transitions of the polymer and nanotube. We find evidence for increased backbone planarization of the polymers wrapped around the nanotubes. Furthermore, ambient p-doping of hybrids of anionic conjugated polyelectrolytes and (6,5) SWNTs blocks energy transfer in contrast to cationic polyelectrolytes. By addition of a mild reducing agent, thus removing the p-doping, the energy transfer can be fully restored pointing toward an electron exchange mechanism. The p-doping of nanotube/polyelectrolyte hybrids in air and their doping-dependent emission and charge transport properties also become apparent in water-gated field-effect transistors based on such networks and might be useful for dual-signal sensing applications.

Spiropyran‐Functionalized Polymer–Carbon Nanotube Hybrids for Dynamic Optical Memory Devices and UV Sensors

“…This polymer exchange protocol was previously shown to enable efficient wrapping of (6,5) nanotubes with a variety of conjugated polymers. [ 20 ] Importantly, any remaining excess or unbound polymer is removed during a second filtration and washing step to obtain only SWNT/polymer hybrids in toluene. Note, the primary purpose of the functionalized wrapping polymer is to stabilize the nanotube dispersions sufficiently for processing and to ensure close proximity of the spiropyran moieties to the nanotubes with a controlled density.…”

“…This stabilization promotes further p‐doping by oxygen, leading to the observed exciton quenching and trion emission. Possibly, the local ionization potential of the (6,5) nanotubes (−4.9 eV) [ 20 ] is decreased (i.e., less negative) by the merocyanine dipoles, thus making them more likely to be p‐doped. Another feasible mechanism may involve splitting of excitons that are created during illumination, followed by a stabilization of the holes by the merocyanine dipoles and quenching of the electrons due to the ambient conditions (i.e., oxygen/water).…”

“…[ 9 b] UV illumination led to a significant decrease in carrier mobility probably due to charge scattering by the merocyanine dipoles, in agreement with reports on individual nanotubes. [ 16 a] While in the former case, the photochromic molecules were distributed randomly in the dielectric environment of the nanotubes, the targeted use of functionalized wrapping polymers [ 20 ] enables much more controlled interaction. It opens up many possibilities for tailored hybrid nanomaterials that combine the charge transport properties of nanotube networks and the tunable properties of polymers.…”

Charge Transfer from Photoexcited Semiconducting Single-Walled Carbon Nanotubes to Wide-Bandgap Wrapping Polymer

“… 13 − 15 When wrapped by typical semiconducting polymers, the narrow-bandgap nanotubes usually behave as acceptors for both charges and excitation. 16 − 21 …”

“… 22 , 23 , 25 The energy transfer observed in SWCNT/polymer composites also points to an electron-exchange mechanism. 11 , 16 However, only the first optical bandgap in semiconductors is usually taken into account in studies of the classical PCT mechanism. The electronic interplay between the complex band structure of SWCNTs and the wrapping polymer has not been clarified yet.…”