Ultrafast Exciton Trapping at sp3 Quantum Defects in Carbon Nanotubes

Orientation Sensitive SEIRA Sensors Based on Single-Walled Carbon Nanotube Near Fields

Molecular vibrations that bear information about intrinsic properties of chemical compounds are challenging to detect at submonolayer densities. Surface-enhanced infrared absorption (SEIRA) spectroscopy has been proven to be a viable approach to enhance and detect weak vibration signals. Here, we report a SEIRA sensor based on mid-infrared surface plasmon resonances supported by single-walled carbon nanotubes (SWCNTs). Due to the 1D nature of SWCNTs, their plasmon modes are highly polarized with the electromagnetic fields spatially confined to nanometer scales. Leveraging these characteristics of SWCNTs, we observe a polarization selective coupling between their surface plasmons and vibrational modes of chemical bonds introduced onto their surfaces. A maximum modulation of ∼15% to the plasmon resonance peak is obtained for a submonolayer chemical group coverage. These findings suggest that SWCNTs may potentially serve as a highly sensitive SEIRA platform for revealing intricate information about molecular compositions and bond orientations.

[2 + 2] Cycloaddition Produces Divalent Organic Color-Centers with Reduced Heterogeneity in Single-Walled Carbon Nanotubes

Organic color centers (OCCs), generated by the covalent functionalization of single-walled carbon nanotubes, have been exploited for chemical sensing, bioimaging, and quantum technologies. However, monovalent OCCs can assume at least 6 different bonding configurations on the sp2 carbon lattice of a chiral nanotube, resulting in heterogeneous OCC photoluminescence emissions. Herein, we show that a heat-activated [2 + 2] cycloaddition reaction enables the synthesis of divalent OCCs with a reduced number of atomic bonding configurations. The chemistry occurs by simply mixing enophile molecules (e.g., methylmaleimide, maleic anhydride, and 4-cyclopentene-1,3-dione) with an ethylene glycol suspension of SWCNTs at elevated temperature (70-140 °C). Unlike monovalent OCC chemistries, we observe just three OCC emission peaks that can be assigned to the three possible bonding configurations of the divalent OCCs based on density functional theory calculations. Notably, these OCC photoluminescence peaks can be controlled by temperature to decrease the emission heterogeneity even further. This divalent chemistry provides a scalable way to synthesize OCCs with tightly controlled emissions for emerging applications.

Photoinduced Dynamics of a Single-Walled Carbon Nanotube with a sp3 Defect: The Importance of Excitonic Effects

Herein, we employed linear-response time-dependent functional theory nonadiabatic dynamic simulations to explore the photoinduced exciton dynamics of a chiral single-walled carbon nanotube CNT(6,5) covalently doped with a 4-nitrobenzyl group (CNT65-NO2). The results indicate that the introduction of a sp3 defect leads to the splitting of the degenerate VBM/VBM-1 and CBM/CBM+1 states. Both the VBM upshift and the CBM downshift are responsible for the experimentally observed redshifted E11* trapping state. The simulations reveal that the photoinduced exciton relaxation dynamics completes within 500 fs, which is consistent with the experimental work. On the other hand, we also conducted the nonadiabatic carrier (electron and hole) dynamic simulations, which completely ignore the excitonic effects. The comparison demonstrates that excitonic effects are indispensable. Deep analyses show that such effects induce several dark states, which play an important role in regulating the photoinduced dynamics of CNT65-NO2. The present work demonstrates the importance of including excitonic effects in simulating photoinduced processes of carbon nanotubes. In addition, it not only rationalizes previous experiments but also provides valuable insights that will help in the future rational design of novel covalently doped carbon nanotubes with superior photoluminescent properties.

Broken Symmetry Optical Transitions in (6,5) Single-Walled Carbon Nanotubes Containing sp3 Defects Revealed by First-Principles Theory

We present a first-principles many-body perturbation theory study of nitrophenyl-doped (6,5) single-walled nanotubes (SWCNTs) to understand how sp3 doping impacts the excitonic properties. sp3-doped SWCNTs are promising as a class of optoelectronic materials with bright tunable photoluminescence, long spin coherence, and single-photon emission (SPE), motivating the study of spin excitations. We predict that the dopant results in a single unpaired spin localized around the defect site, which induces multiple low-energy excitonic peaks. By comparing optical absorption and photoluminescence from experiment and theory, we identify the transitions responsible for the red-shifted, defect-induced E11* peak, which has demonstrated SPE for some dopants; the presence of this state is due to both the symmetry-breaking associated with the defect and the presence of the defect-induced in-gap state. Furthermore, we find an asymmetry between the contribution of the two spin channels, suggesting that this system has potential for spin-selective optical transitions.

Solvent Isotope Effects on the Creation of Fluorescent Quantum Defects in Carbon Nanotubes by Aryl Diazonium Chemistry

The integration of aryl diazonium and carbon nanotube chemistries has offered rich and versatile tools for creating nanomaterials of unique optical and electronic properties in a controllable fashion. The diazonium reaction with single-wall carbon nanotubes (SWCNTs) is known to proceed through a radical or carbocation mechanism in aqueous solutions, with deuterated water (D2O) being the frequently used solvent. Here, we show strong water solvent isotope effects on the aryl diazonium reaction with SWCNTs for creating fluorescent quantum defects using water (H2O) and D2O. We found a deduced reaction constant of ∼18.2 times larger value in D2O than in H2O, potentially due to their different chemical properties. We also observed the generation of new defect photoluminescence over a broad concentration range of diazonium reactants in H2O, as opposed to a narrow window of reaction conditions in D2O under UV excitation. Without UV light, the physical adsorption of diazonium on the surface of SWCNTs led to the fluorescence quenching of nanotubes. These findings provide important insights into the aryl diazonium chemistry with carbon nanotubes for creating promising material platforms for optical sensing, imaging, and quantum communication technologies.

Over Two-Fold Photoluminescence Enhancement from Single-Walled Carbon Nanotubes Induced by Oxygen Doping

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) is a promising way to improve their photoluminescent (PL) brightness and thus make them applicable as a base material for infrared light emitters. We report as high as over two-fold enhancement of the SWCNT PL brightness by using oxygen doping via the UV photodissociation of hypochlorite ions. By analyzing the temporal evolution of the PL and Raman spectra of SWCNTs in the course of the doping process, we conclude that the enhancement of SWCNTs PL brightness depends on the homogeneity of induced quantum defects distribution over the SWCNT surface.

Non-negligible roles of charge transfer excitons in ultrafast excitation energy transfer dynamics of a double-walled carbon nanotube

Herein, we employed a developed linear response time dependent density functional theory-based nonadiabatic dynamics simulation method that explicitly takes into account the excitonic effects to investigate photoinduced excitation energy transfer dynamics of a double-walled carbon nanotube (CNT) model with different excitation energies. The E₁₁ excitation of the outer CNT will generate a local excitation (LE) |out*〉 exciton due to its low energy, which does not induce any charge separation. In contrast, the E₁₁ excitation of the inner CNT can generate four kinds of excitons with the LE exciton |in*〉 dominates. In the 500-fs dynamics simulation, the LE exciton |in*〉 and charge transfer (CT) excitons |out^-in⁺〉 and |out⁺in^-〉 are all gradually converted to the |out*〉 exciton, corresponding to a photoinduced excitation energy transfer, which is consistent with experimental studies. Finally, when the excitation energy is close to the E₂₂ state of the outer CNT (∼1.05 eV), a mixed population of different excitons, with the |out*〉 exciton dominated, is generated. Then, photoinduced energy transfer from the outer to inner CNTs occurs in the first 50 fs, which is followed by an inner to outer excitation energy transfer that is completed in 400 fs. The present work not only sheds important light on the mechanistic details of wavelength-dependent excitation energy transfer of a double-walled CNT model but also demonstrates the roles and importance of CT excitons in photoinduced excitation energy transfer. It also emphasized that explicitly including the excitonic effects in electronic structure calculations and nonadiabatic dynamics simulations is significant for correct understanding/rational design of optoelectronic properties of periodically extended systems.

Room Temperature Lasing from Semiconducting Single-Walled Carbon Nanotubes

Miniaturized near-infrared semiconductor lasers that are able to generate coherent light with low energy consumption have widespread applications in fields such as optical interconnects, neuromorphic computing, and deep-tissue optogenetics. With optical transitions at near-infrared wavelengths, diameter-tunable electronic structures, and superlative optoelectronic properties, semiconducting single-walled carbon nanotubes (SWCNTs) are promising candidates for nanolaser applications. However, despite significant efforts in this direction and recent progress toward enhancing spontaneous emission from SWCNTs through Purcell effects, SWCNT-based excitonic lasers have not yet been demonstrated. Leveraging an optimized cavity-emitter integration scheme enabled by a self-assembly process, here we couple SWCNT emission to the whispering gallery modes supported by polymer microspheres, resulting in room temperature excitonic lasing with an average lasing threshold of 4.5 kW/cm². The high photostability of SWCNTs allows stable lasing for prolonged duration with minimal degradation. This experimental realization of excitonic lasing from SWCNTs, combined with their versatile electronic and optical properties that can be further controlled by chemical modification, offers far-reaching opportunities for tunable near-infrared nanolasers that are applicable for optical signal processing, in vivo biosensing, and optoelectronic devices.

Probing Carrier Dynamics in sp3-Functionalized Single-Walled Carbon Nanotubes with Time-Resolved Terahertz Spectroscopy

The controlled introduction of covalent sp³ defects into semiconducting single-walled carbon nanotubes (SWCNTs) gives rise to exciton localization and red-shifted near-infrared luminescence. The single-photon emission characteristics of these functionalized SWCNTs make them interesting candidates for electrically driven quantum light sources. However, the impact of sp³ defects on the carrier dynamics and charge transport in carbon nanotubes remains an open question. Here, we use ultrafast, time-resolved optical-pump terahertz-probe spectroscopy as a direct and quantitative technique to investigate the microscopic and temperature-dependent charge transport properties of pristine and functionalized (6,5) SWCNTs in dispersions and thin films. We find that sp³ functionalization increases charge carrier scattering, thus reducing the intra-nanotube carrier mobility. In combination with electrical measurements of SWCNT network field-effect transistors, these data enable us to distinguish between contributions of intra-nanotube band transport, sp³ defect scattering and inter-nanotube carrier hopping to the overall charge transport properties of nanotube networks.

[2π + 2π] Photocycloaddition of Enones to Single-Walled Carbon Nanotubes Creates Fluorescent Quantum Defects

Single-walled carbon nanotubes (SWCNTs) have been widely applied in biomedical fields such as drug delivery, biosensing, bioimaging, and tissue engineering. Understanding their reactivity with biomolecules is important for these applications. We describe here a photoinduced cycloaddition reaction between enones and SWCNTs. By creating covalent and tunable sp³ defects in the sp² carbon lattice of SWCNTs through [2π + 2π] photocycloaddition, a bright red-shifted photoluminescence was gradually generated. The photocycloaddition functionalization was demonstrated with various organic molecules bearing an enone functional group, including biologically important oxygenated lipid metabolites. The mechanism of this reaction was studied empirically and using computational methods. Density functional theory calculations were employed to elucidate the identity of the reaction product and understand the origin of different substrate reactivities. The results of this study can enable engineering of the optical and electronic properties of semiconducting SWCNTs and provide understanding into their interactions with the lipid biocorona.

sp3-Functionalization of Single-Walled Carbon Nanotubes Creates Localized Spins

Chemical functionalization-introduced sp³ quantum defects in single-walled carbon nanotubes (SWCNTs) have shown compelling optical properties for their potential applications in quantum information science and bioimaging. Here, we utilize temperature- and power-dependent electron spin resonance measurements to study the fundamental spin properties of SWCNTs functionalized with well-controlled densities of sp³ quantum defects. Signatures of isolated spins that are highly localized at the sp³ defect sites are observed, which we further confirm with density functional theory calculations. Applying temperature-dependent line width analysis and power-saturation measurements, we estimate the spin-lattice relaxation time T₁ and spin dephasing time T₂ to be around 9 μs and 40 ns, respectively. These findings of the localized spin states that are associated with the sp³ quantum defects not only deepen our understanding of the molecular structures of the quantum defects but could also have strong implications for their applications in quantum information science.

Photoexcited Aromatic Reactants Give Multicolor Carbon Nanotube Fluorescence from Quantum Defects

Covalent functionalization of single-wall carbon nanotubes (SWCNTs) can be valuable for modifying their electronic properties and creating fluorescent quantum defects. We report here a previously unreported category of such reactions involving interactions of photoexcited aromatic compounds with SWCNT sidewalls. When aqueous suspensions of SWCNTs are exposed to organic aromatic compounds and then irradiated by UV light, fluorescent defects are formed in the nanotubes at rates that depend on the aromatic ring substituents. In reactions with aniline or iodoaniline, strong spectral sidebands appear within 1 min. Total SWCNT photoluminescence can be enhanced by a factor as large as ∼5. Notably, emission spectra of reacted SWCNTs depend on the presence or absence of dissolved oxygen during the reaction. For (6,5) SWCNTs, treatment when oxygen is present gives an additional emission band red-shifted by 160 meV from the pristine position, whereas treatment without oxygen leads to two additional emission bands red-shifted by 140 and 270 meV. Variance spectroscopy shows the presence of individual "multicolor" nanotubes with three distinct emission bands (pristine plus two shifted). The facile generation of dual fluorescent quantum defects in SWCNTs provides emission closer to standard telecom wavelengths, advancing the prospects for applications as single-photon sources in quantum information processing.