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Trions and sp3-Defects in Single-walled Carbon Nanotubes for Optoelectronics

Periodic Reporting for period 2 - TRIFECTs (Trions and sp3-Defects in Single-walled Carbon Nanotubes for Optoelectronics)

Reporting period: 2020-10-01 to 2022-03-31

Semiconducting single-walled carbon nanotubes (SWNTs) combine solution-processability, large carrier mobilities, narrow emission linewidths and environmental stability for optoelectronic devices with light-emission in the near-infrared (800-1800 nm), e.g. for optical data communication and bio-imaging when sorted by (n,m) species. The recent availability of highly pure, monochiral semiconducting SWNTs as bulk materials allows us to employ and further tailor their charge transport and light emission properties and thus enables their application in real-world devices. Two emissive species - charged excitons (trions) and bright sp³-defects - play a fundamental role for tailored SWNT luminescence. Both show red-shifted, narrow and enhanced emission that could be used in optoelectronic devices, imaging in the second biological window and single-photon emission for secure telecommunication. Trions and emissive defects are not limited to SWNTs and hence these concepts could be transferred and applied to other low-dimensional semiconductors. The goals of this project are to understand and use trions and trion-polaritons for light emission and polaritonic charge transport in optical cavities, to understand and tune the interactions of sp³-defects with charges and trions in SWNTs, to modify and apply sp³-defects for enhanced light emission from SWNTs in optoelectronic devices, and to explore trions in other new low-dimensional materials (e.g. graphene nanoribbons and novel monolayered semiconductors).
Within the first half of this project we have developed a new synthetic route that allows us to controllably create sp³ defects in polymer-wrapped monochiral (6,5) single-walled carbon nanotubes (SWNTs) in organic solvents with aryldiazonium salts using a phase transfer agent. This new method can be scaled up and thus allows us to investigate the optical properties of functionalized SWNTs in unprecedented detail and integrate dense films of them in optoelectronic devices. We also found a reaction scheme that creates an even more red-shifted sp³ defect emission in semiconducting SWNTs using 2-haloanilines and a strong organic base. We could show that nanotubes with these defects exhibit single-photon emission at room temperature with over 90% purity. The combination of both functionalization techniques provides us with a toolbox for more advanced functionalization but also with enough material for device fabrication. For example, dense films of functionalized (6,5) SWNTs could be used for strong-light matter coupling in Fabry-Perot cavities and the radiative pumping of exciton-polaritons via these defects was demonstrated. Field-effect transistors with networks of SWNTs with different concentrations of sp³ defects were used to study their impact on charge transport. Although the defects lowered both hole and electron mobilities, the transistors still worked and emitted more red-shifted light from the defects in the near infrared. Furthermore, we have applied and developed spectroscopic techniques such as charge modulation absorption and fluorescence spectroscopy to study charge transport and - in particular - trions in operating SWNT network field-effect transistors. Using the developed techniques to functionalize nanotubes, we created sp³ defects with a stable organic radical (PTM) attached to them, which had a direct impact on the emission efficiency and fluorescence lifetime of the defects. These radical-substituted defects enhance the triplet population of the nanotubes via radical-enhanced intersystem crossing. Finally, low temperature spectroscopy setups (optical cryostat, 4 K) were built and applied to investigate the temperature-dependent optical properties of trions and sp³ defects in carbon nanotubes and other low-dimensional semiconductors.
The newly developed techniques to create different sp³ defects in carbon nanotubes on a large scale will allow us to attach more complex functional groups and investigate their impact on emission (including response to external stimuli) and charge transport in more detail. The impact of dielectric environment, electric and magnetic fields as well as optical cavities on trion emission in devices will be studied to draw up guidelines for more efficient and tunable optoelectronic devices operating in the near-infrared. The knowledge gained from the first part of the project will be applied to other low-dimensional materials such as graphene nanoribbons and other 2D semiconductors to explore their potential as absorbers/emitters with and without doping or defects in the near-infrared.
Artistic illustration of single photon emission from sp³ defects