We have developed several new and very effective synthetic methods that allow us to controllably create different luminescent sp³ and oxygen defects in (6,5) and other semiconducting single-walled carbon nanotubes (SWNTs) both in organic solvents (polymer-wrapped) and in aqueous dispersions. These new methods were scaled up and allowed us to investigate the optical properties of functionalized SWNTs in unprecedented detail and integrate dense films of them in optoelectronic devices. For example, we demonstrated and explored a new reaction scheme that creates even more red-shifted sp³ defect emission from semiconducting SWNTs showing single-photon emission at room temperature with over 90% purity. New methods for the introduction of luminescent oxygen defects in SWNTs with benign reactants have led to unusually high fluorescence quantum yields (> 3%) for nanotubes dispersed in water with biocompatible surfactants. With these oxygen defects, even very short (50 nm) SWNTs could still be used for efficient and high-resolution near-infrared fluorescence imaging (e.g. of biological tissue).
In order to properly quantify the number of introduced luminescent defects for different functionalization schemes we developed and independently validated an analytical method that only requires standard Raman spectroscopy and hence enables reliable comparison between different functionalization techniques and different labs.
The combination of different functionalization techniques provided 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 were used for strong-light matter coupling in Fabry-Perot cavities and radiative pumping of exciton-polaritons via these defects was observed. Field-effect transistors with networks of SWNTs with different densities and different types of sp³ defects were used to study the impact of defects on charge transport and electroluminescence. Although the defects lowered both hole and electron mobilities, the transistors still showed good ambipolar transport and emitted near-infrared light from the defects reaching the telecommunication O-band wavelength range.
Using the developed techniques to functionalize nanotubes, we created sp³ defects with a stable organic radical (PTM) attached to them, leading to enhanced triplet population via radical-enhanced intersystem crossing. Other attached functional groups (aryl alkynes) enabled ratiometric fluorescence sensing of the biomarker inorganic pyrophosphate (via copper ion displacement) in the second biological window.
Furthermore, we have applied and developed spectroscopic techniques such as near-infrared charge modulation absorption and fluorescence spectroscopy to study charge transport and - in particular - trions in operating SWNT network field-effect transistors with different dielectric environments. Positive and negative trions showed an unexpected sensitivity to specific charge traps of the surrounding dielectric, which makes them excellent optical probes for such trap states in optoelectronic devices. 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, providing for instance a direct and independent quantification of defect densities in SWNTs. Graphene nanoribbons were synthesized and their optical properties were characterized after chemical and electrochemical doping. In contrast to theoretical predictions, only polaron and no trion signatures were observed.
These results have been disseminated in more than 20 peer-reviewed and open access publications, numerous conference and workshop presentations by team members and a pending patent application.
