Photoluminescence imaging of solitary dopant sites in covalently doped single-wall carbon nanotubes

Quantum Defect Sensitization via Phase-Changing Supercharged Antibody Fragments

Quantum defects in single-walled carbon nanotubes promote exciton localization, which enables potential applications in biodevices and quantum light sources. However, the effects of local electric fields on the emissive energy states of quantum defects and how they can be controlled are unexplored. Here, we investigate quantum defect sensitization by engineering an intrinsically disordered protein to undergo a phase change at a quantum defect site. We designed a supercharged single-chain antibody fragment (scFv) to enable a full ligand-induced folding transition from an intrinsically disordered state to a compact folded state in the presence of a cytokine. The supercharged scFv was conjugated to a quantum defect to induce a substantial local electric change upon ligand binding. Employing the detection of a proinflammatory biomarker, interleukin-6, as a representative model system, supercharged scFv-coupled quantum defects exhibited robust fluorescence wavelength shifts concomitant with the protein folding transition. Quantum chemical simulations suggest that the quantum defects amplify the optical response to the localization of charges produced upon the antigen-induced folding of the proteins, which is difficult to achieve in unmodified nanotubes. These findings portend new approaches to modulate quantum defect emission for biomarker sensing and protein biophysics and to engineer proteins to modulate binding signal transduction.

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.

Chemical Patterning on Nanocarbons: Functionality Typewriting

Nanocarbon materials have become extraordinarily compelling for their significant potential in the cutting-edge science and technology. These materials exhibit exceptional physicochemical properties due to their distinctive low-dimensional structures and tailored surface characteristics. An attractive direction at the forefront of this field involves the spatially resolved chemical functionalization of a diverse range of nanocarbons, encompassing carbon nanotubes, graphene, and a myriad of derivative structures. In tandem with the technological leaps in lithography, these endeavors have fostered the creation of a novel class of nanocarbon materials with finely tunable physical and chemical attributes, and programmable multi-functionalities, paving the way for new applications in fields such as nanoelectronics, sensing, photonics, and quantum technologies. Our review examines the swift and dynamic advancements in nanocarbon chemical patterning. Key breakthroughs and future opportunities are highlighted. This review not only provides an in-depth understanding of this fast-paced field but also helps to catalyze the rational design of advanced next-generation nanocarbon-based materials and devices.

Stochastic Formation of Quantum Defects in Carbon Nanotubes

Small perturbations in the structure of materials significantly affect their properties. One example is single wall carbon nanotubes (SWCNTs), which exhibit chirality-dependent near-infrared (NIR) fluorescence. They can be modified with quantum defects through the reaction with diazonium salts, and the number or distribution of these defects determines their photophysics. However, the presence of multiple chiralities in typical SWCNT samples complicates the identification of defect-related emission features. Here, we show that quantum defects do not affect aqueous two-phase extraction (ATPE) of different SWCNT chiralities into different phases, which suggests low numbers of defects. For bulk samples, the bandgap emission (E11) of monochiral (6,5)-SWCNTs decreases, and the defect-related emission feature (E11*) increases with diazonium salt concentration and represents a proxy for the defect number. The high purity of monochiral samples from ATPE allows us to image NIR fluorescence contributions (E11 = 986 nm and E11* = 1140 nm) on the single SWCNT level. Interestingly, we observe a stochastic (Poisson) distribution of quantum defects. SWCNTs have most likely one to three defects (for low to high (bulk) quantum defect densities). Additionally, we verify this number by following single reaction events that appear as discrete steps in the temporal fluorescence traces. We thereby count single reactions via NIR imaging and demonstrate that stochasticity plays a crucial role in the optical properties of SWCNTs. These results show that there can be a large discrepancy between ensemble and single particle experiments/properties of nanomaterials.

Functionalization of Carbon Nanotubes Surface by Aryl Groups: A Review

The review is devoted to the methods of introducing aryl functional groups to the CNT surface. Arylated nanotubes are characterized by extended solubility, and are widely used in photoelectronics, semiconductor technology, and bioelectrocatalysis. The main emphasis is on arylation methods according to the radical mechanism, such as the Gomberg-Bachmann and Billups reactions, and the decomposition of peroxides. At the same time, less common approaches are also considered. For each of the described reactions, a mechanism is presented in the context of the effect on the properties of functionalized nanotubes and their application. As a result, this will allow us to choose the optimal modification method for specific practical tasks.

Synthetic control over the binding configuration of luminescent sp3-defects in single-walled carbon nanotubes

The controlled functionalization of single-walled carbon nanotubes with luminescent sp³-defects has created the potential to employ them as quantum-light sources in the near-infrared. For that, it is crucial to control their spectral diversity. The emission wavelength is determined by the binding configuration of the defects rather than the molecular structure of the attached groups. However, current functionalization methods produce a variety of binding configurations and thus emission wavelengths. We introduce a simple reaction protocol for the creation of only one type of luminescent defect in polymer-sorted (6,5) nanotubes, which is more red-shifted and exhibits longer photoluminescence lifetimes than the commonly obtained binding configurations. We demonstrate single-photon emission at room temperature and expand this functionalization to other polymer-wrapped nanotubes with emission further in the near-infrared. As the selectivity of the reaction with various aniline derivatives depends on the presence of an organic base we propose nucleophilic addition as the reaction mechanism.

Covalent functionalization of single-walled carbon nanotubes with luminescent sp³-defects generally produces a variety of binding configurations and emission wavelengths. The authors propose a base-mediated nucleophilic functionalization approach to selectively achieve configurations for E₁₁* and E₁₁*^- or purely E₁₁*^- defect emission.

Photoluminescence Dynamics Defined by Exciton Trapping Potential of Coupled Defect States in DNA-Functionalized Carbon Nanotubes

Chemical reactions between semiconducting single-wall carbon nanotubes (SWCNTs) and single-stranded DNA (ssDNA) achieve spatially patterned covalent functionalization sites and create coupled fluorescent quantum defects on the nanotube surface, tailoring SWCNT photophysics for applications such as single-photon emitters in quantum information technologies. The evaluation of relaxation dynamics of photoluminescence (PL) from those coupled quantum defects is essential for understanding the nanotube electronic structure and beneficial to the design of quantum light emitters. Here, we measured the PL decay for ssDNA-functionalized SWCNTs as a function of the guanine content of the ssDNA oligo that dictates the red-shifting of their PL emission peaks relative to the band-edge exciton. We then correlate the observed dependence of PL decay dynamics on energy red-shifts to the exciton potential energy landscape, which is modeled using first-principles approaches based upon the morphology of ssDNA-altered SWCNTs obtained by atomic force microscopy (AFM) imaging. Our simulations illustrate that the multiple guanine defects introduced within a single ssDNA strand strongly interact to create a deep exciton trapping well, acting as a single hybrid trap. The emission decay from the distinctive trapping potential landscape is found to be biexponential for ssDNA-modified SWCNTs. We attributed the fast time component of the biexponential PL decay to the redistribution of exciton population among the lowest energy bright states and a manifold of dark states emerging from the coupling of multiple guanine defects. The long lifetime component in the biexponential decay, on the other hand, is attributed to the redistribution of exciton population among different exciton trapping sites that arise from the binding of multiple ssDNA strands along the nanotube axis. AFM measurements indicate that those trapping sites are separated on average by ∼8 nm along the nanotube axis.

Controlling Defect-State Photophysics in Covalently Functionalized Single-Walled Carbon Nanotubes

ConspectusSingle-walled carbon nanotubes (SWCNTs) show promise as light sources for modern fiber optical communications due to their emission wavelengths tunable via chirality and diameter dependency. However, the emission quantum yields are relatively low owing to the existence of low-lying dark electronic states and fast excitonic diffusion leading to carrier quenching at defects. Covalent functionalization of SWCNTs addresses this problem by brightening their infrared emission. Namely, introduction of sp³-hybridized defects makes the lowest energy transitions optically active for some defect geometries and enables further control of their optical properties. Such functionalized SWCNTs are currently the only material exhibiting room-temperature single photon emission at telecom relevant infrared wavelengths. While this fluorescence is strong and has the right wavelength, functionalization introduces a variety of emission peaks resulting in spectrally broad inhomogeneous photoluminescence that prohibits the use of SWCNTs in practical applications. Consequently, there is a strong need to control the emission diversity in order to render these materials useful for applications. Recent experimental and computational work has attributed the emissive diversity to the presence of multiple localized defect geometries each resulting in distinct emission energy. This Account outlines methods by which the morphology of the defect in functionalized SWCNTs can be controlled to reduce emissive diversity and to tune the fluorescence wavelengths. The chirality-dependent trends of emission energies with respect to individual defect morphologies are explored. It is demonstrated that defect geometries originating from functionalization of SWCNT carbon atoms along bonds with strong π-orbital mismatch are favorable. Furthermore, the effect of controlling the defect itself through use of different chemical groups is also discussed. Such tunability is enabled due to the formation of specific defect geometries in close proximity to other existing defects. This takes advantage of the changes in π-orbital mismatch enforced by existing defects and the resulting changes in reactivities toward formation of specific defect morphologies. Furthermore, the trends in emissive energies are highly dependent on the value of mod(n-m,3) for an (n,m) tube chirality. These powerful concepts allow for a targeted formation of defects that emit at desired energies based on SWCNT single chirality enriched samples. Finally, the impact of functionalization with specific types of defects that enforce certain defect geometries due to steric constraints in bond lengths and angles to the SWCNT are discussed. We further relate to a similar effect that is present in systems where high density of surface defects is formed due to high reactant concentration. The outlined strategies suggested by simulations are instrumental in guiding experimental efforts toward the generation of functionalized SWCNTs with tunable emission energies.

Carbon Nanotube Photoluminescence Modulation by Local Chemical and Supramolecular Chemical Functionalization

ConspectusCarbon nanotubes (CNTs) have been central materials in nanoscience and nanotechnologies. Single-walled CNTs (SWCNTs) consisting of a cylindrical graphene show a metallic (met) or semiconducting (sc) property depending on their rolling up manner (chirality). The sc-SWCNTs show characteristic chirality-dependent optical properties of their absorption and photoluminescence (PL) in the near-infrared (NIR) region. These are derived from their highly π-conjugated structures having semiconducting crystalline sp² carbon networks with defined nanoarchitectures that afford a strong quantum confinement and weak dielectric screening. Consequently, photoirradiation of the SWCNTs produces a stable and mobile exciton (excited electron-hole pair) even at room temperature, and the exciton properties dominate such optical phenomena in the SWCNTs. However, the mobile excitons decrease the PL efficiency due to nonradiative relaxation including collision with tube edges and relaxation to lower-lying dark states. A breakthrough regarding the efficient use of the mobile exciton for PL has recently been achieved by local chemical functionalization of the SWCNTs, in which the chemical reactions introduce local defects of oxygen and sp³ carbon atoms in the tube structures. The defect doping creates new emissive doped sites that have narrower band gaps and trap the mobile excitons, which provides locally functionalized SWCNTs (lf-SWCNTs). As a result, the localized exciton produces E₁₁* PL with red-shifted wavelengths and enhanced PL quantum yields compared to the original E₁₁ PL of the nonmodified SWCNTs.In this Account, we describe recently revealed fundamental properties of the lf-SWCNTs based on the analyses by photophysics, theoretical calculations, and electrochemistry combined with in situ PL spectroscopy. The new insight allows us to expand the wavelength regions of the NIR E₁₁* PL derived from the localized exciton, in which upconversion generates a higher energy PL through thermal activation and proximal doped site formation using bis-aryldiazonium modifiers provides a much lower energy PL than typical E₁₁* PL. Moreover, owing to the chemical reaction-dominant doping process, the molecular structure design of modifiers succeeds in producing functionalized lf-SWCNTs; namely, molecular functions are incorporated into the doped sites for their PL modulation. The wavelength changes/switching in the E₁₁* PL selectively occurs by a supramolecular approach using molecular recognition and imine chemistry. Therefore, the local chemical functionalization of the SWCNTs is a key to designing the properties and creating their new functions of the lf-SWCNTs. Fundamental understanding of the doped site properties of the lf-SWCNTs and molecularly driven approaches for exciton and defect engineering would unveil the intrinsic natures of these materials, which is crucial for elevating the SWCNT-based nanotechnologies to the next stage. The resulting materials are of interest in the fields of high performance NIR-II imaging and sensing for bio/medical analyses and single-photon emitters in quantum information technology.

Photolithographic Patterning of Organic Color-Centers

Organic color-centers (OCCs) have emerged as promising single-photon emitters for solid-state quantum technologies, chemically specific sensing, and near-infrared bioimaging. However, these quantum light sources are currently synthesized in bulk solution, lacking the spatial control required for on-chip integration. The ability to pattern OCCs on solid substrates with high spatial precision and molecularly defined structure is essential to interface electronics and advance their quantum applications. Herein, a lithographic generation of OCCs on solid-state semiconducting single-walled carbon nanotube films at spatially defined locations is presented. By using light-driven diazoether chemistry, it is possible to directly pattern p-nitroaryl OCCs, which demonstrate chemically specific spectral signatures at programmed positions as confirmed by Raman mapping and hyperspectral photoluminescence imaging. This light-driven technique enables the fabrication of OCC arrays on solid films that fluoresce in the shortwave infrared and presents an important step toward the direct writing of quantum emitters and other functionalities at the molecular level.

Mod(n-m,3) Dependence of Defect-State Emission Bands in Aryl-Functionalized Carbon Nanotubes

Molecularly functionalized single-walled carbon nanotubes (SWCNTs) are potentially useful for fiber optical applications due to their room temperature single-photon emission capacity at telecommunication wavelengths. Several distinct defect geometries are generated upon covalent functionalization. While it has been shown that the defect geometry controls electron localization around the defect site, thereby changing the electronic structure and generating new optically bright red-shifted emission bands, the reasons for such localization remain unexplained. Our joint experimental and computational studies of functionalized SWCNTs with various chiralities show that the value of mod(n-m,3) in an (n,m) chiral nanotube plays a key role in the relative ordering of defect-dependent emission energies. This dependence is linked to the complex nodal characteristics of electronic wave function extending along specific bonds in the tube, which justifies the defect-geometry dependent exciton localization. This insight helps to uncover the essential structural motifs allowing tuning the redshifts of emission energies in functionalized SWCNTs.

Probing Trions at Chemically Tailored Trapping Defects

Trions, charged excitons that are reminiscent of hydrogen and positronium ions, have been intensively studied for energy harvesting, light-emitting diodes, lasing, and quantum computing applications because of their inherent connection with electron spin and dark excitons. However, these quasi-particles are typically present as a minority species at room temperature making it difficult for quantitative experimental measurements. Here, we show that by chemically engineering the well depth of sp3 quantum defects through a series of alkyl functional groups covalently attached to semiconducting carbon nanotube hosts, trions can be efficiently generated and localized at the trapping chemical defects. The exciton-electron binding energy of the trapped trion approaches 119 meV, which more than doubles that of "free" trions in the same host material (54 meV) and other nanoscale systems (2-45 meV). Magnetoluminescence spectroscopy suggests the absence of dark states in the energetic vicinity of trapped trions. Unexpectedly, the trapped trions are approximately 7.3-fold brighter than the brightest previously reported and 16 times as bright as native nanotube excitons, with a photoluminescence lifetime that is more than 100 times larger than that of free trions. These intriguing observations are understood by an efficient conversion of dark excitons to bright trions at the defect sites. This work makes trions synthetically accessible and uncovers the rich photophysics of these tricarrier quasi-particles, which may find broad implications in bioimaging, chemical sensing, energy harvesting, and light emitting in the short-wave infrared.

New Light on Molecule-Nanotube Hybrids

Optoelectronics benefits from outstanding new nanomaterials that provide emission and detection in the visible and near-infrared range, photoswitches, two level systems for single photon emission, etc. Among these, carbon nanotubes are envisioned as game changers despite difficult handling and control over chirality burdening their use. However, recent breakthroughs on hybrid carbon nanotubes have established nanotubes as pioneers for a new family of building blocks for optics and quantum optics. Functionalization of carbon nanotubes with molecules or polymers not only preserves the nanotube properties from the environment, but also promotes new performance abilities to the resulting hybrids. Photoluminescence and Raman signals are enhanced in the hybrids, which questions the nature of the electronic coupling between nanotube and molecules. Furthermore, coupling to optical cavities dramatically enhances single photon emission, which operates up to room temperature. This new light on nanotube hybrids shows their potential to push optoelectronics a step forward.

Brightening of Long, Polymer-Wrapped Carbon Nanotubes by sp3 Functionalization in Organic Solvents

The functionalization of semiconducting single-walled carbon nanotubes (SWNTs) with sp3 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 currently 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 sp3 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.

One-Pot, Large-Scale Synthesis of Organic Color Center-Tailored Semiconducting Carbon Nanotubes

Organic color center-tailored semiconducting single-walled carbon nanotubes are a rising family of synthetic quantum emitters that display bright defect photoluminescence molecularly tunable for imaging, sensing, and quantum information processing. A major advance in this area would be the development of a high-yield synthetic route that is capable of producing these materials well exceeding the current μg/mL scale. Here, we demonstrate that adding a chlorosulfonic acid solution of raw carbon nanotubes, sodium nitrite, and an aniline derivative into water readily leads to the synthesis of organic color center-tailored nanotubes. This unexpectedly simple one-pot reaction is highly scalable (yielding hundreds of milligrams of materials in a single run), efficient (reaction completes in seconds), and versatile (achieved the synthesis of organic color centers previously unattainable). The implanted organic color centers can be easily tailored by choosing from the more than 40 aniline derivatives that are commercially available, including many fluoroaniline and aminobenzoic acid derivatives, and that are difficult to convert into diazonium salts. We found this chemistry works for all the nanotube chiralities investigated. The synthesized materials are neat solids that can be directly dispersed in either water or an organic solvent by a surfactant or polymer depending on the specific application. The nanotube products can also be further sorted into single chirality-enriched fractions with defect-specific photoluminescence that is tunable over ∼1100 to ∼1550 nm. This one-pot chemistry thus provides a highly scalable synthesis of organic color centers for many potential applications that require large quantities of materials.

Creating fluorescent quantum defects in carbon nanotubes using hypochlorite and light

Covalent doping of single-walled carbon nanotubes (SWCNTs) can modify their optical properties, enabling applications as single-photon emitters and bio-imaging agents. We report here a simple, quick, and controllable method for preparing oxygen-doped SWCNTs with desirable emission spectra. Aqueous nanotube dispersions are treated at room temperature with NaClO (bleach) and then UV-irradiated for less than one minute to achieve optimized O-doping. The doping efficiency is controlled by varying surfactant concentration and type, NaClO concentration, and irradiation dose. Photochemical action spectra indicate that doping involves reaction of SWCNT sidewalls with oxygen atoms formed by photolysis of ClO^- ions. Variance spectroscopy of products reveals that most individual nanotubes in optimally treated samples show both pristine and doped emission. A continuous flow reactor is described that allows efficient preparation of milligram quantities of O-doped SWCNTs. Finally, we demonstrate a bio-imaging application that gives high contrast short-wavelength infrared fluorescence images of vasculature and lymphatic structures in mice injected with only ~100 ng of the doped nanotubes.

Controlling the optical properties of carbon nanotubes with organic colour-centre quantum defects

Previously unwelcome, defects are emerging as a new frontier of research, providing a molecular focal point to study the coupling of electrons, excitons, phonons and spin in low-dimensional materials. This opportunity is particularly intriguing in semiconducting single-walled carbon nanotubes, in which covalently bonding organic functional groups to the sp 2 carbon lattice can produce tunable sp 3 quantum defects that fluoresce brightly in the shortwave IR, emitting pure single photons at room temperature. These novel physical properties have made such synthetic defects, or 'organic colour centres', exciting new systems for chemistry, physics, materials science, engineering and quantum technologies. This Review examines progress in this emerging field and presents a unified description of this new family of quantum emitters, as well as providing an outlook of the rapidly expanding research and applications of synthetic defects.

Intrinsic limits of defect-state photoluminescence dynamics in functionalized carbon nanotubes

Defect states introduced to single wall carbon nanotubes (SWCNTs) by covalent functionalization give rise to novel photophysics and are showing promise as sources of room-temperature quantum emission of interest for quantum information technologies. Evaluation of their ultimate potential for such needs requires a knowledge of intrinsic dynamic and coherence behaviors. Here we probe population relaxation and dephasing time (T1 and T2, respectively) of defect states following deposition of functionalized SWCNTs on polystyrene substrates that are subjected to an isopropanol rinse to remove surfactant. Low-temperature (4 K) photo-luminescence linewidths (∼100 μeV) following surfactant removal are a factor of ten narrower than those for unrinsed SWCNTs. Measured recombination lifetimes, on the order of 1.5 ns, compare well with those estimated from DFT calculations, indicating that the intrinsic radiatively-limited lifetime is approached following this sample treatment. Dephasing times evaluated directly through an interferometric approach compare closely to those established by photoluminescence linewidths. Dephasing times as high as 12 ps are found; a factor of up to 6 times greater than those evaluated for band-edge exciton states. Such enhancement of dephasing and photoluminescence lifetime behavior is a direct consequence of exciton localization at the SWCNT defect sites.

Mapping Structure-Property Relationships of Organic Color Centers

Organic color centers are an emergent class of quantum emitters that hold vast potential for applications in bioimaging, chemical sensing, and quantum information processing. Here, we show that these synthetic color centers follow interesting structure-property relationships through comparative spectral studies of 14 purified single-walled carbon nanotube chiralities and 30 different functional groups that vary in electron-withdrawing capability and bonding configurations. The defect emission is tunable by as much as 400 meV in the near-infrared as a function of host structure and the chemical nature of the color centers. However, the emission energy is nearly free from chiral angle and family patterns of the nanotube host (although this strongly depends on the nanotube diameter), suggesting that a trapped exciton at the organic color centers to some degree electronically decouples from the one-dimensional semiconductor host. Our findings provide important insights for designing and controlling this new family of synthetic color centers.

Solvent- and Wavelength-Dependent Photoluminescence Relaxation Dynamics of Carbon Nanotube sp3 Defect States

Photoluminescent sp³ defect states introduced to single wall carbon nanotubes (SWCNTs) through low-level covalent functionalization create new photophysical behaviors and functionality as a result of defect sites acting as exciton traps. Evaluation of relaxation dynamics in varying dielectric environments can aid in advancing a more complete description of defect-state relaxation pathways and electronic structure. Here, we exploit helical wrapping polymers as a route to suspending (6,5) SWCNTs covalently functionalized with 4-methoxybenzene in solvent systems including H₂O, D₂O, methanol, dimethylformamide, tetrahydrofuran, and toluene, spanning a range of dielectric constants from 80 to 3. Defect-state photoluminescence decays were measured as a function of emission wavelength and solvent environment. Emission decays are biexponential, with short lifetime components on the order of 65 ps and long components ranging from around 100 to 350 ps. Both short and long decay components increase as emission wavelength increases, while only the long lifetime component shows a solvent dependence. We demonstrate that the wavelength dependence is a consequence of thermal detrapping of defect-state excitons to produce mobile E₁₁ excitons, providing an important mechanism for loss of defect-state population. Deeper trap states (i.e., those emitting at longer wavelengths) result in a decreased rate for thermal loss. The solvent-independent behavior of the short lifetime component is consistent with its assignment as the characteristic time for redistribution of exciton population between bright and dark defect states. The solvent dependence of the long lifetime component is shown to be consistent with relaxation via an electronic to vibrational energy transfer mechanism, in which energy is resonantly lost to solvent vibrations in a complementary mechanism to multiphonon decay processes.

Carbon nanotubes as emerging quantum-light sources

Progress in quantum computing and quantum cryptography requires efficient, electrically triggered, single-photon sources at room temperature in the telecom wavelengths. It has been long known that semiconducting single-wall carbon nanotubes (SWCNTs) display strong excitonic binding and emit light over a broad range of wavelengths, but their use has been hampered by a low quantum yield and a high sensitivity to spectral diffusion and blinking. In this Perspective, we discuss recent advances in the mastering of SWCNT optical properties by chemistry, electrical contacting and resonator coupling towards advancing their use as quantum light sources. We describe the latest results in terms of single-photon purity, generation efficiency and indistinguishability. Finally, we consider the main fundamental challenges stemming from the unique properties of SWCNTs and the most promising roads for SWCNT-based chip integrated quantum photonic sources.

Environmental Electrometry with Luminescent Carbon Nanotubes

We demonstrate that localized excitons in luminescent carbon nanotubes can be utilized to study electrostatic fluctuations in the nanotube environment with sensitivity down to the elementary charge. By monitoring the temporal evolution of the cryogenic photoluminescence from individual carbon nanotubes grown on silicon oxide and hexagonal boron nitride, we characterize the dynamics of charge trap defects for both dielectric supports. We find a one order of magnitude reduction in the photoluminescence spectral wandering for nanotubes on extended atomically flat terraces of hexagonal boron nitride. For nanotubes on hexagonal boron nitride with pronounced spectral fluctuations, our analysis suggests proximity to terrace ridges where charge fluctuators agglomerate to exhibit areal densities exceeding those of silicon oxide. Our results establish carbon nanotubes as sensitive probes of environmental charge fluctuations and highlight their potential for applications in electrometric nanodevices with all-optical readout.

Suppression of exciton dephasing in sidewall-functionalized carbon nanotubes embedded into metallo-dielectric antennas

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) is a promising route to enhance the quantum yield of exciton emission and can lead to single-photon emission at room temperature. However, the spectral linewidth of the defect-related E11* emission remains rather broad. Here, we systematically investigate the low-temperature exciton emission of individual SWCNTs that have been dispersed with sodium-deoxycholate (DOC) and polyfluorene (PFO-BPy), are grown by laser vaporization (LV) or by CoMoCat techniques and are functionalized with oxygen as well as 3,5-dichlorobenzene groups. The E11 excitons in oxygen-functionalized SWCNTs remain rather broad with up to 10 meV linewidth while exciton emission from 3,5-dichlorobenzene functionalized SWCNTs is found to be about one order of magnitude narrower. In all cases, wrapping with PFO-BPy provides significantly better protection against pump induced dephasing compared to DOC. To further study the influence of exciton localization on pump-induced dephasing, we have embedded the functionalized SWCNTs into metallo-dielectric antenna cavities to maximize light collection. We show that 0D excitons attributed to the E11* emission of 3,5-dichlorobenzene quantum defects of LV-grown SWCNTs can display near resolution-limited linewidths down to 35 μeV. Interestingly, these 0D excitons give rise to a 3-fold suppressed pump-induced exciton dephasing compared to the E11 excitons in the same SWCNT. These findings provide a foundation to build a unified description of the emergence of novel optical behavior from the interplay of covalently introduced defects, dispersants, and exciton confinement in SWCNTs and might further lead to the realization of indistinguishable photons from carbon nanotubes.

Ultrashort Carbon Nanotubes That Fluoresce Brightly in the Near-Infrared

The intrinsic near-infrared photoluminescence observed in long single-walled carbon nanotubes is known to be quenched in ultrashort nanotubes due to their tiny size as compared to the exciton diffusion length in these materials (>100 nm). Here, we show that intense photoluminescence can be created in ultrashort nanotubes (∼40 nm length) upon incorporation of exciton-trapping sp³ defect sites. Using super-resolution photoluminescence imaging at <25 nm resolution, we directly show the preferential localization of excitons at the nanotube ends, which separate by less than 40 nm and behave as independent emitters. This unexpected observation opens the possibility to synthesize fluorescent ultrashort nanotubes-a goal that has been long thought impossible-for bioimaging applications, where bright near-infrared photoluminescence and small size are highly desirable, and for quantum information science, where high quality and well-controlled near-infrared single photon emitters are needed.

Enhanced Single-Photon Emission from Carbon-Nanotube Dopant States Coupled to Silicon Microcavities

Single-walled carbon nanotubes are a promising material as quantum light sources at room temperature and as nanoscale light sources for integrated photonic circuits on silicon. Here, we show that the integration of dopant states in carbon nanotubes and silicon microcavities can provide bright and high-purity single-photon emitters on a silicon photonics platform at room temperature. We perform photoluminescence spectroscopy and observe the enhancement of emission from the dopant states by a factor of ∼50, and cavity-enhanced radiative decay is confirmed using time-resolved measurements, in which a ∼30% decrease of emission lifetime is observed. The statistics of photons emitted from the cavity-coupled dopant states are investigated by photon-correlation measurements, and high-purity single photon generation is observed. The excitation power dependence of photon emission statistics shows that the degree of photon antibunching can be kept high even when the excitation power increases, while the single-photon emission rate can be increased to ∼1.7 × 10⁷ Hz.

Correction Scheme for Comparison of Computed and Experimental Optical Transition Energies in Functionalized Single-Walled Carbon Nanotubes

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) introduces red-shifted emission features in the near-infrared spectral range due to exciton localization around the defect site. Such chemical modifications increase their potential use as near-infrared emitters and single-photon sources in telecommunications applications. Density functional theory (DFT) studies using finite-length tube models have been used to calculate their optical transition energies. Predicted energies are typically blue-shifted compared to experiment due to methodology errors including imprecise self-interaction corrections in the density functional and finite-size basis sets. Furthermore, artificial quantum confinement in finite models cannot be corrected by a constant-energy shift since they depend on the degree of exciton localization. Herein, we present a method that corrects the emission energies predicted by time-dependent DFT. Confinement and methodology errors are separately estimated using experimental data for unmodified tubes. Corrected emission energies are in remarkable agreement with experiment, suggesting the value of this straightforward method toward predicting and interpreting the optical features of functionalized SWCNTs.

Low-Temperature Single Carbon Nanotube Spectroscopy of sp3 Quantum Defects

Aiming to unravel the relationship between chemical configuration and electronic structure of sp³ defects of aryl-functionalized (6,5) single-walled carbon nanotubes (SWCNTs), we perform low-temperature single nanotube photoluminescence (PL) spectroscopy studies and correlate our observations with quantum chemistry simulations. We observe sharp emission peaks from individual defect sites that are spread over an extremely broad, 1000-1350 nm, spectral range. Our simulations allow us to attribute this spectral diversity to the occurrence of six chemically and energetically distinct defect states resulting from topological variation in the chemical binding configuration of the monovalent aryl groups. Both PL emission efficiency and spectral line width of the defect states are strongly influenced by the local dielectric environment. Wrapping the SWCNT with a polyfluorene polymer provides the best isolation from the environment and yields the brightest emission with near-resolution limited spectral line width of 270 μeV, as well as spectrally resolved emission wings associated with localized acoustic phonons. Pump-dependent studies further revealed that the defect states are capable of emitting single, sharp, isolated PL peaks over 3 orders of magnitude increase in pump power, a key characteristic of two-level systems and an important prerequisite for single-photon emission with high purity. These findings point to the tremendous potential of sp³ defects in development of room temperature quantum light sources capable of operating at telecommunication wavelengths as the emission of the defect states can readily be extended to this range via use of larger diameter SWCNTs.

Substituent effects on the redox states of locally functionalized single-walled carbon nanotubes revealed by in situ photoluminescence spectroelectrochemistry

Single-walled carbon nanotubes (SWNTs) with local chemical modification have been recognized as a novel near infrared (NIR) photoluminescent nanomaterial due to the emergence of a new red-shifted photoluminescence (PL) with enhanced quantum yields. As a characteristic feature of the locally functionalized SWNTs (lf-SWNTs), PL wavelength changes occur with the structural dependence of the substituent structures in the modified aryl groups, showing up to a 60 nm peak shift according to an electronic property difference of the aryl groups. Up to now, however, the structural effect on the electronic states of the lf-SWNTs has been discussed only on the basis of theoretical calculations due to the very limited amount of modifications. Herein, we describe the successfully-determined electronic states of the aryl-modified lf-SWNTs with different substituents (Ar-X SWNTs) using an in situ PL spectroelectrochemical method based on electrochemical quenching of the PL intensities analyzed by the Nernst equation. In particular, we reveal that the local functionalization of (6,5)SWNTs induced potential changes in the energy levels of the HOMO and the LUMO by -23 to -38 meV and +20 to +22 meV, respectively, compared to those of the pristine SWNTs, which generates exciton trapping sites with narrower band gaps. Moreover, the HOMO levels of the Ar-X SWNTs specifically shift in a negative potential direction by 15 meV according to an enhancement of the electron-accepting property of the substituents in the aryl groups that corresponds to an increase in the Hammet substituent constants, suggesting the importance of the dipole effect from the aryl groups on the lf-SWNTs to the level shift of the frontier orbitals. Our method is a promising way to characterize the electronic features of the lf-SWNTs.

Multi-exciton emission from solitary dopant states of carbon nanotubes

By separating the photons from slow and fast decays of single and multi-exciton states in a time gated 2^nd order photon correlation experiment, we show that solitary oxygen dopant states of single-walled carbon nanotubes (SWCNTs) allow emission of photon pairs with efficiencies as high as 44% of single exciton emission. Our pump dependent time resolved photoluminescence (PL) studies further reveal diffusion-limited exciton-exciton annihilation as the key process that limits the emission of multi-excitons at high pump fluences. We further postulate that creation of additional permanent exciton quenching sites occurring under intense laser irradiation leads to permanent PL quenching. With this work, we bring out multi-excitonic processes of solitary dopant states as a new area to be explored for potential applications in lasing and entangled photon generation.

Solitary Oxygen Dopant Emission from Carbon Nanotubes Modified by Dielectric Metasurfaces

All-dielectric metasurfaces made from arrays of high index nanoresonators supporting strong magnetic dipole modes have emerged as a low-loss alternative to plasmonic metasurfaces. Here we use oxygen-doped single-walled carbon nanotubes (SWCNTs) as quantum emitters and couple them to silicon metasurfaces to study effects of the magnetic dipole modes of the constituent nanoresonators on the photoluminescence (PL) of individual SWCNTs. We find that when in resonance, the magnetic mode of the silicon nanoresonators can lead to a moderate average PL enhancement of 0.8-4.0 of the SWCNTs, accompanied by an average increase in the radiative decay rate by a factor of 1.5-3.0. More interestingly, single dopant polarization experiments show an anomalous photoluminescence polarization rotation by coupling individual SWCNTs to silicon nanoresonators. Numerical simulations indicate that this is caused by modification of near-field polarization distribution at certain areas in the proximity of the silicon nanoresonators at the excitation wavelength, thus presenting an approach to control emission polarization. These findings indicate silicon nanoresonators as potential building blocks of quantum photonic circuits capable of manipulating PL intensity and polarization of single photon sources.

Near infrared photoluminescence modulation by defect site design using aryl isomers in locally functionalized single-walled carbon nanotubes

Unexpected near infrared photoluminescence of locally-functionalized single-walled carbon nanotubes upon introducing isomeric structures in the defect sites is reported.

Photoluminescence Imaging of Polyfluorene Surface Structures on Semiconducting Carbon Nanotubes: Implications for Thin Film Exciton Transport

Single-walled carbon nanotubes (SWCNTs) have potential to act as light-harvesting elements in thin film photovoltaic devices, but performance is in part limited by the efficiency of exciton diffusion processes within the films. Factors contributing to exciton transport can include film morphology encompassing nanotube orientation, connectivity, and interaction geometry. Such factors are often defined by nanotube surface structures that are not yet well understood. Here, we present the results of a combined pump-probe and photoluminescence imaging study of polyfluorene (PFO)-wrapped (6,5) and (7,5) SWCNTs that provide additional insight into the role played by polymer structures in defining exciton transport. Pump-probe measurements suggest exciton transport occurs over larger length scales in films composed of PFO-wrapped (7,5) SWCNTs, compared to those prepared from PFO-bpy-wrapped (6,5) SWCNTs. To explore the role the difference in polymer structure may play as a possible origin of differing transport behaviors, we performed a photoluminescence imaging study of individual polymer-wrapped (6,5) and (7,5) SWCNTs. The PFO-bpy-wrapped (6,5) SWCNTs showed more uniform intensity distributions along their lengths, in contrast to the PFO-wrapped (7,5) SWCNTs, which showed irregular, discontinuous intensity distributions. These differences likely originate from differences in surface coverage and suggest the PFO wrapping on (7,5) nanotubes produces a more open surface structure than is available with the PFO-bpy wrapping of (6,5) nanotubes. The open structure likely leads to improved intertube coupling that enhances exciton transport within the (7,5) films, consistent with the results of our pump-probe measurements.

Photoluminescence Dynamics of Aryl sp(3) Defect States in Single-Walled Carbon Nanotubes

Photoluminescent defect states introduced by sp(3) functionalization of semiconducting carbon nanotubes are rapidly emerging as important routes for boosting emission quantum yields and introducing new functionality. Knowledge of the relaxation dynamics of these states is required for understanding how functionalizing agents (molecular dopants) may be designed to access specific behaviors. We measure photoluminescence (PL) decay dynamics of sp(3) defect states introduced by aryl functionalization of the carbon nanotube surface. Results are given for five different nanotube chiralities, each doped with a range of aryl functionality. We find that the PL decays of these sp(3) defect states are biexponential, with both components relaxing on time scales of ∼100 ps. Exciton trapping at defects is found to increases PL lifetimes by a factor of 5-10, in comparison to those for the free exciton. A significant chirality dependence is observed in the decay times, ranging from 77 ps for (7,5) nanotubes to >600 ps for (5,4) structures. The strong correlation of time constants with emission energy indicates relaxation occurs via multiphonon decay processes, with close agreement to theoretical expectations. Variation of the aryl dopant further modulates decay times by 10-15%. The aryl defects also affect PL lifetimes of the free E11 exciton. Shortening of the E11 bright state lifetime as defect density increases provides further confirmation that defects act as exciton traps. A similar shortening of the E11 dark exciton lifetime is found as defect density increases, providing strong experimental evidence that dark excitons are also trapped at such defect sites.

Optical Excitation of Carbon Nanotubes Drives Localized Diazonium Reactions

Covalent chemistries have been widely used to modify carbon nanomaterials; however, they typically lack the precision and efficiency required to directly engineer their optical and electronic properties. Here, we show, for the first time, that visible light which is tuned into resonance with carbon nanotubes can be used to drive their functionalization by aryldiazonium salts. The optical excitation accelerates the reaction rate 154-fold (±13) and makes it possible to significantly improve the efficiency of covalent bonding to the sp(2) carbon lattice. Control experiments suggest that the reaction is dominated by a localized photothermal effect. This light-driven reaction paves the way for precise nanochemistry that can directly tailor carbon nanomaterials at the optical and electronic levels.