Periodic Reporting for period 4 - TRIFECTs (Trions and sp3-Defects in Single-walled Carbon Nanotubes for Optoelectronics)
Reporting period: 2023-10-01 to 2024-07-31
Semiconducting single-walled carbon nanotubes (SWNTs) combine solution-processability, large carrier mobilities, and environmental stability with narrow-band light-emission in the near-infrared (800-1800 nm), e.g. for optoelectronic devices, telecommunication, sensing, 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 luminescent sp³-defects - play a fundamental role for 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 to use trions as probes for doping and charge transport in semiconducting SWNT networks, to understand and tune the interactions of sp³-defects with charge carriers in SWNTs, to modify and apply sp³ and other luminescent defects for enhanced light emission from SWNTs for optoelectronic devices and optical sensors, and to explore the spectroscopic properties of other low-dimensional materials (e.g. graphene nanoribbons) when doped.
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.
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.
The various newly developed techniques to create different luminescent sp³ and oxygen defects in carbon nanotubes controllably and on a large scale have allowed us to produce tailored SWNT dispersions with near-infrared emission of unprecedented photoluminescence quantum yields (i.e. brightness) and to integrate functional groups for sensing applications (e.g. ratiometric optical sensing of inorganic pyrophosphate). Our new and very straightforward method to quantify the number of luminescent defects per micrometer of SWNT using Raman spectroscopy enables – for the first time - the reliable comparison of functionalization methods between different labs. It also allows us to identify potential clustering of defects.
We now understand the impact of different luminescent defects on electroluminescence and charge transport in random SWNT networks, which also gives indirect insights into the role of intra- and inter-nanotube charge transport. We revealed the impact of the dielectric environment and temperature on trion emission and hence exciton and charge transport in optoelectronics devices based on SWNTs. The knowledge gained from SWNTs was applied to other low-dimensional materials, specifically solution-processed graphene nanoribbons (9-aGNR). We could show that against expectation they do not support the formation of trions but only polaronic states.
We now understand the impact of different luminescent defects on electroluminescence and charge transport in random SWNT networks, which also gives indirect insights into the role of intra- and inter-nanotube charge transport. We revealed the impact of the dielectric environment and temperature on trion emission and hence exciton and charge transport in optoelectronics devices based on SWNTs. The knowledge gained from SWNTs was applied to other low-dimensional materials, specifically solution-processed graphene nanoribbons (9-aGNR). We could show that against expectation they do not support the formation of trions but only polaronic states.