Brightening of Long, Polymer-Wrapped Carbon Nanotubes by sp3^{3} Functionalization in Organic Solvents

The functionalization of semiconducting single-walled carbon nanotubes (SWNTs) with sp$^{3}$ defects that act as luminescent exciton traps is a powerful means to enhance their photoluminescence quantum yield (PLQY) and to add optical properties. However, the synthetic methods employed to introduce these defects are so far limited to aqueous dispersions of surfactant-coated SWNTs, often with short tube lengths, residual metallic nanotubes and poor film formation properties. In contrast to that, dispersions of polymer-wrapped SWNTs in organic solvents feature unrivaled purity, higher PLQY and are easily processed into thin films for device applications. Here, we introduce a simple and scalable phase-transfer method to solubilize diazonium salts in organic nonhalogenated solvents for the controlled reaction with polymer-wrapped SWNTs to create luminescent aryl defects. Absolute PLQY measurements are applied to reliably quantify the defect-induced brightening. The optimization of defect density and trap depth results in PLQYs of up to 4 % with 90 % of photons emitted through the defect channel. We further reveal the strong impact of initial SWNT quality and length on the relative brightening by sp$^{3}$ defects. The efficient and simple production of large quantities of defect-tailored polymer-sorted SWNTs enables aerosol-jet printing and spin-coating of thin films with bright and nearly reabsorption-free defect emission, which are desired for carbon nanotube-based near-infrared light-emitting devices

Strong Light-Matter Coupling with Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) are a promising material for strongly coupled optoelectronic devices, due to their outstanding electrical properties in combination with their narrowband excitonic absorption and emission in the near-infrared. The rich SWCNT photophysics allow to study the interaction of exciton-polaritons with a range of other quasi-particles, such as phonons and biexcitons, as well as with synthetic, luminescent sp3 defects at room temperature. However, the ultimate goal of polariton condensation has not been achived with SWCNT exciton-polaritons so far, and hence understanding their specific polariton population mechanism with respect to their unique photophysical properties is crucial. Here, time-dependent fluorescence and transmission measurements are used to track the exciton-polariton population in strongly coupled metalclad microcavities, identify the dominant relaxation pathways and transitions, use luminescent sp3 defects to increase the polariton population by radiative pumping, and manipulate the SWCNT absorption edge by strong coupling in hybrid organic photodiodes. By investigating the fluorescence decay of SWCNT exciton-polaritons, it is shown, that the dominant population mechanism in this system is radiative pumping. To overcome the thusly imposed limitation of the polariton population by the low SWCNT photoluminescence quantum yield, the SWCNTs are functionalized with luminescent sp3 defects, leading to a population increase up to 10-fold for highly emissive detunings (photon fractions > 90%). By changing the substituents and the binding pattern, tuning of the defect emission could be further employed to access application-relevant near-infrared wavelengths and improve the conditions for polariton condensation. Furthermore, the SWCNT exciton-polariton dynamics are studied in the ultrafast regime by transient transmission spectroscopy. The results reveal a polariton-mediated biexciton transition, that is threefold more efficient than in weakly coupled SWCNTs. The polariton to biexciton transition under off-resonant polariton excitation also indicates fast population transfer from dark to bright polaritons beyond the exciton and photon dephasing times. The efficient biexciton transition of strongly coupled SWCNTs may enable to study correlated many-body states at room temperature, that are predicted for excitonic molecules in strongly coupled high quality cavities. Lastly, strongly coupled SWCNT hybrid organic photodiodes are presented, demonstrating how exciton-polaritons enable light-detection far beyond the intrinsic SWCNT absorption edge. For equal external quantum efficiency, photocarrier generation was observed 200 nm further into the near-infrared as compared to previously reported strongly coupled photodiodes. Thus, representing the first step towards efficient and tuneable polariton-mediated photocurrent generation by SWCNT hybrid organic photodiodes at application-relevant wavelengths

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

As narrow optical bandgap materials, semiconducting single-walled carbon nanotubes (SWCNTs) are rarely regarded as charge donors in photoinduced charge-transfer (PCT) reactions. However, the unique band structure and unusual exciton dynamics of SWCNTs add more possibilities to the classical PCT mechanism. In this work, we demonstrate PCT from photoexcited semiconducting (6,5) SWCNTs to a wide-bandgap wrapping poly-[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(6,6′)-(2,2′-bipyridine)] (PFO–BPy) via femtosecond transient absorption spectroscopy. By monitoring the spectral dynamics of the SWCNT polaron, we show that charge transfer from photoexcited SWCNTs to PFO–BPy can be driven not only by the energetically favorable E$_{33}$ transition but also by the energetically unfavorable E$_{22}$ excitation under high pump fluence. This unusual PCT from narrow-bandgap SWCNTs toward a wide-bandgap polymer originates from the up-converted high-energy excitonic state (E$_{33}$ or higher) that is promoted by the Auger recombination of excitons and charge carriers in SWCNTs. These insights provide new pathways for charge separation in SWCNT-based photodetectors and photovoltaic cells

Quantum Defects in Fluorescent Carbon Nanotubes for Sensing and Mechanistic Studies

Single wall carbon nanotubes (SWCNT) fluoresce in the near infrared (NIR) and have been assembled with biopolymers such as DNA to form highly sensitive molecular sensors. They change their fluorescence when they interact with analytes. Despite the progress in engineering of these sensors the underlying mechanisms are still not understood. Here, we identify processes and rate constants that explain the photophysical signal transduction by exploiting sp3 quantum defects in the sp2 carbon lattice of SWCNTs. As a model system we use ssDNA coated (6,5)-SWCNTs, which increase their NIR emission (E11, 990 nm) up to + 250 % in response to the important neurotransmitter dopamine. In contrast, SWCNTs coated with DNA but with a low number of NO2-Aryl sp3 quantum defects decrease both their E11 (-35%) and defect related E11* emission (- 50%) at 1130 nm. Consequently, the interaction with the analyte does not change the radiative exciton decay pathway alone. Furthermore, the fluorescence response of pristine SWCNTs increases with SWCNT length, suggesting that exciton diffusion is affected. The quantum yield of pristine (6,5)-SWCNTs increases in response to the analyte from 0.6 % to 1.3 % and points to a change in non-radiative rate constants. These experimental results are explained by a Monte Carlo simulation of exciton diffusion, which supports a change of two non-radiative decay pathways together with an increase of exciton diffusion (3 rate constant model). The combination of such SWCNTs with defects and without defects enables the assembly of ratiometric sensors with opposing responses at different wavelengths. In summary, we demonstrate how perturbation of a system with quantum defects reveals the photophysical mechanism and reverses optical responses.</div

Spectroscopic near-infrared photodetectors enabled by strong light-matter coupling in (6,5) single-walled carbon nanotubes

Special Issue: Polariton Chemistry: Molecules in Cavities and Plasmonic Media. Funding: The authors gratefully acknowledge funding by the Volkswagen Foundation within project No. 93404. A.M. acknowledges further funding through an individual fellowship of the Deutsche Forschungsgemeinschaft (No. 404587082).Strong light-matter coupling leads to the formation of mixed exciton-polariton states, allowing for a rigorous manipulation of the absorption and emission of excitonic materials. Here, we demonstrate the realization of this promising concept in organic photodetectors. By hybridizing the E11 exciton of semiconducting (6,5) single-walled carbon nanotubes (SWNTs) with near-infrared cavity photons, we create spectrally tunable polariton states within a photodiode. In turn, we are able to red-shift the detection peak which coincides with the lower polariton band. Our photodiodes comprise a metal cavity to mediate strong coupling between light and SWNTs and utilize P3HT and PC70BM as electron donor and acceptor, respectively. The diodes are formed either via mixing of SWNTs, P3HT and PC70BM to create a bulk heterojunction or by sequential processing of layers to form flat heterojunctions. The resulting near-infrared sensors show tunable, efficient exciton harvesting in an application-relevant wavelength range between 1000 nm and 1300 nm, with optical simulations showing a possible extension beyond 1500 nm.Publisher PDFPeer reviewe

Spectroscopic near-infrared photodetectors enabled by strong light-matter coupling in (6,5) single-walled carbon nanotubes (dataset)

Strong light-matter coupling leads to the formation of mixed exciton-polariton states, allowing for a rigorous manipulation of the absorption and emission of excitonic materials. Here, we demonstrate the realization of this promising concept in organic photodetectors. By hybridizing the E11 exciton of semiconducting (6,5) single-walled carbon nanotubes (SWNTs) with near-infrared cavity photons, we create spectrally tunable polariton states within a photodiode. In turn, we are able to red-shift the detection peak which coincides with the lower polariton band. Our photodiodes comprise a metal cavity to mediate strong coupling between light and SWNTs and utilize P3HT and PC70BM as electron donor and acceptor, respectively. The diodes are formed either via mixing of SWNTs, P3HT and PC70BM to create a bulk heterojunction or by sequential processing of layers to form flat heterojunctions. The resulting near-infrared sensors show tunable, efficient exciton harvesting in an application-relevant wavelength range between 1000 nm and 1300 nm, with optical simulations showing a possible extension beyond 1500 nm