Photochemical Creation of Fluorescent Quantum Defects in Semiconducting Carbon Nanotube Hosts

[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.

Recent progress in controlling the photoluminescence properties of single-walled carbon nanotubes by oxidation and alkylation

The functionalization of single-walled carbon nanotubes (SWCNTs) has received considerable attention in the last decade since highly efficient near-infrared photoluminescence (PL) has been observed to be red-shifted compared with the intrinsic PL peak of pristine SWCNTs. The PL wavelength has been manipulated using arylation reactions with aryldiazonium salts and aryl halides. Additionally, simple oxidation and alkylation reactions have proven effective in extensively adjusting the PL wavelength, with the resulting PL efficiency varying based on the chosen reaction techniques and molecular structures. This review discusses the latest developments in tailoring the PL attributes of SWCNTs by oxidation and alkylation processes. (6,5) SWCNTs exhibit intrinsic emission at 980 nm, and the PL wavelength can be controlled in the range of 1100-1320 nm by chemical modification. In addition, recent developments in chiral separation techniques have increased our understanding of the control of the PL wavelength, extending to the selection of excitation and emission wavelengths, by chemical modification of SWCNTs with different chiral indices.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

Covalent conjugation of proteins onto fluorescent single-walled carbon nanotubes for biological and medical applications

Single-walled carbon nanotubes (SWCNTs) have optical properties that are conducive for biological applications such as sensing, delivery, and imaging. These applications necessitate the immobilization of macromolecules that can serve as therapeutic drugs, molecular templates, or modulators of surface interactions. Although previous studies have focused on non-covalent immobilization strategies, recent advances have introduced covalent functional handles that can preserve or even enhance the SWCNT optical properties. This review presents an overview of covalent sidewall modifications of SWCNTs, with a focus on the latest generation of "sp³ defect" modifications. We summarize and compare the reaction conditions and the reported products of these sp³ chemistries. We further review the underlying photophysics governing SWCNT fluorescence and apply these principles to the fluorescence emitted from these covalently modified SWCNTs. Finally, we provide an outlook on additional chemistries that could be applied to covalently conjugate proteins to these chemically modified, fluorescent SWCNTs. We review the advantages of these approaches, emerging opportunities for further improvement, as well as their implications for enabling new technologies.

Quantum Coupling of Two Atomic Defects in a Carbon Nanotube Semiconductor

Chemical defects can create organic color centers in the graphitic lattice of single-walled carbon nanotubes. However, the underlying physics remains somewhat of a mystery. Here we show that two sp³ atomic defects can interact with each other in a way reminiscent of atoms bonding to form molecules. Each defect creates an atom-like mid-gap state within the band gap of the nanotube semiconductor. Two such defects, when brought close to each other, interact to form a split pair of orbitals akin to two hydrogen atoms covalently bonding to form a H₂ molecule. This unexpected finding may help in understanding the nature of atomic defects in solids and provide a fresh perspective to the engineering of these color centers.

Can armchair nanotubes host organic color centers?

We use time-dependent density functional theory to investigate the possibility of hosting organic color centers in (6, 6) armchair single-walled carbon nanotubes, which are known to be metallic. Our calculations show that in short segments of (6, 6) nanotubes∼5nm in length there is a dipole-allowed singlet transition related to the quantum confinement of charge carriers in the smaller segments. The introduction ofsp3defects to the surface of (6, 6) nanotubes results in new dipole-allowed excited states. Some of these states are redshifted from the native confinement state of the defect-free (6, 6) segments; this is similar behavior to what is observed withsp3defects to exciton transitions in semiconducting carbon nanotubes. This result suggests the possibility of electrically wiring organic color centers directly through armchair carbon nanotube hosts.

Formation of organic color centers in air-suspended carbon nanotubes using vapor-phase reaction

Organic color centers in single-walled carbon nanotubes have demonstrated exceptional ability to generate single photons at room temperature in the telecom range. Combining the color centers with pristine air-suspended nanotubes would be desirable for improved performance, but all current synthetic methods occur in solution which makes them incompatible. Here we demonstrate the formation of color centers in air-suspended nanotubes using a vapor-phase reaction. Functionalization is directly verified by photoluminescence spectroscopy, with unambiguous statistics from more than a few thousand individual nanotubes. The color centers show strong diameter-dependent emission, which can be explained with a model for chemical reactivity considering strain along the tube curvature. We also estimate the defect density by comparing the experiments with simulations based on a one-dimensional exciton diffusion equation. Our results highlight the influence of the nanotube structure on vapor-phase reactivity and emission properties, providing guidelines for the development of high-performance near-infrared quantum light sources.

Organic color centers in single-walled carbon nanotubes can act as single-photon sources in the telecom range. Here the authors report the functionalization of air-suspended nanotubes through a vapor-phase photochemical reaction, demonstrating a further tailoring of quantum emitter materials.

Reconfiguring Organic Color Centers on the sp2 Carbon Lattice of Single-Walled Carbon Nanotubes

Organic color centers (OCCs) are atomic defects that can be synthetically created in single-walled carbon nanotube hosts to enable the emission of shortwave infrared single photons at room temperature. However, all known chemistries developed thus far to generate these quantum defects produce a variety of bonding configurations, posing a formidable challenge to the synthesis of identical, uniformly emitting color centers. Herein, we show that laser irradiation of the nanotube host can locally reconfigure the chemical bonding of aryl OCCs on (6,5) nanotubes to significantly reduce their spectral inhomogeneity. After irradiation the defect emission narrows in distribution by ∼26% to yield a single photoluminescence peak. We use hyperspectral photoluminescence imaging to follow this structural transformation on the single nanotube level. Density functional theory calculations corroborate our experimental observations, suggesting that the OCCs convert from kinetic structures to the more thermodynamically stable configuration. This approach may enable localized tuning and creation of identical OCCs for emerging applications in bioimaging, molecular sensing, and quantum information sciences.

Nanomaterials for Quantum Information Science and Engineering

Quantum information science and engineering (QISE)-which entails the use of quantum mechanical states for information processing, communications, and sensing-and the area of nanoscience and nanotechnology have dominated condensed matter physics and materials science research in the 21st century. Solid-state devices for QISE have, to this point, predominantly been designed with bulk materials as their constituents. This review considers how nanomaterials (i.e., materials with intrinsic quantum confinement) may offer inherent advantages over conventional materials for QISE. The materials challenges for specific types of qubits, along with how emerging nanomaterials may overcome these challenges, are identified. Challenges for and progress toward nanomaterials-based quantum devices are condidered. The overall aim of the review is to help close the gap between the nanotechnology and quantum information communities and inspire research that will lead to next-generation quantum devices for scalable and practical quantum applications.

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.

[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.

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.

Fluorescent sp3 Defect-Tailored Carbon Nanotubes Enable NIR-II Single Particle Imaging in Live Brain Slices at Ultra-Low Excitation Doses

Cellular and tissue imaging in the second near-infrared window (NIR-II, ~1000–1350 nm) is advantageous for in vivo studies because of low light extinction by biological constituents at these wavelengths. However, deep tissue imaging at the single molecule sensitivity has not been achieved in the NIR-II window due to lack of suitable bio-probes. Single-walled carbon nanotubes have emerged as promising near-infrared luminescent molecular bio-probes; yet, their inefficient photoluminescence (quantum yield ~1%) drives requirements for sizeable excitation doses (~1–10 kW/cm²) that are significantly blue-shifted from the NIR-II region (<850 nm) and may thus ultimately compromise live tissue. Here, we show that single nanotube imaging can be achieved in live brain tissue using ultralow excitation doses (~0.1 kW/cm²), an order of magnitude lower than those currently used. To accomplish this, we synthesized fluorescent sp³-defect tailored (6,5) carbon nanotubes which, when excited at their first order excitonic transition (~985 nm) fluoresce brightly at ~1160 nm. The biocompatibility of these functionalized nanotubes, which are wrapped by encapsulation agent (phospholipid-polyethylene glycol), is demonstrated using standard cytotoxicity assays. Single molecule photophysical studies of these biocompatible nanotubes allowed us to identify the optimal luminescence properties in the context of biological imaging.

Sonochemical reaction to control the near-infrared photoluminescence properties of single-walled carbon nanotubes

The effect of ultrasonic irradiation on the optical properties of single-walled carbon nanotubes (SWNTs) was investigated. Upon sonication in D2O in the presence of sodium dodecylbenzene sulfonate (SDBS) under air, red-shifted photoluminescence (PL) peaks at ∼1043 and ∼1118 nm were observed from the aqueous suspensions of (6,4) and (6,5)SWNTs, accompanied by a decrease in the intensity of the intrinsic PL peaks. Upon sonication with SDBS under an Ar atmosphere, the rate of spectral change increased with the sonication time and new PL peaks emerged at 1043, 1118, and 1221 nm. Meanwhile, upon the addition of 1-butanol, the PL peaks emerged only at 1043 nm and 1118 nm, while the emergence of the peak at 1221 nm was inhibited. On the other hand, a suspension with highly dispersed SWNTs was obtained upon sonication in the presence of sodium cholate without any change in the intrinsic optical properties of SWNTs. These experimental results reveal that the PL characteristics of SWNTs can be controlled by controlling the sonication conditions such as the type of surfactant used, the concentration of SWNTs, reaction environment, and the presence of an inhibitor such as 1-butanol.

Optical Voltammetry of Polymer-Encapsulated Single-Walled Carbon Nanotubes

The semiconducting single-walled carbon nanotube (SWCNT), noncovalently wrapped by a polymeric monolayer, is a nanoscale semiconductor-electrolyte interface under investigation for sensing, photonics, and photovoltaic applications. SWCNT complexes are routinely observed to sensitize various electrochemical/redox phenomena, even in the absence of an external field. While the photoluminescence response to gate voltage depends on the redox potential of the nanotube, analogous optical voltammetry of functionalized carbon nanotubes could be conducted in suspension without applying voltage but by varying the solution conditions as well as the chemistry of the encapsulating polymer. Steady-state photoluminescence, absorbance, and in situ measurements of O2/H2O reactivity show correlation with the pH/pK a-dependent reactivity of π-rich coatings. The nanotube emission responses suggest that the presence of photogenerated potential may explain the observed coating electrochemical reactivity. This work finds that electronic and chemical interactions of the nanotube with the encapsulating polymer may play a critical role in applications that depend on radiative recombination, such as optical sensing.

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.

Chirality enriched carbon nanotubes with tunable wrapping via corona phase exchange purification (CPEP)

Single-walled carbon nanotubes (SWCNTs) have unique photophysical properties and serve as building blocks for biosensors, functional materials and devices. For many applications it is crucial to use chirality-pure SWCNTs, which requires sophisticated processes. Purification procedures such as wrapping by certain polymers, phase separation, density gradient centrifugation or gel chromatography have been developed and yield distinct SWCNT species wrapped by a specific polymer or surfactant. However, many applications require a different organic functionalization (corona) around the SWCNTs instead of the one used for the purification process. Here, we present a novel efficient and straightforward process to gain chirality pure SWCNTs with tunable functionalization. Our approach uses polyfluorene (PFO) polymers to enrich certain chiralities but the polymer is removed again and finally exchanged to any desired organic phase. We demonstrate this concept by dispersing SWCNTs in poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6'-{2,2'-bipyridine})] (PFO-BPy), which is known to preferentially solubilize (6,5)-SWCNTs. Then PFO-BPy is removed and recycled, while letting the SWCNTs adsorb/agglomerate on sodium chloride (NaCl) crystals, which act as a toluene-stable but water-soluble filler material. In the last step these purified SWCNTs are redispersed in different polymers, surfactants and ssDNA. This corona phase exchange purification (CPEP) approach was also extended to other PFO variants to enrich and functionalize (7,5)-SWCNTs. CPEP purified and functionalized SWCNTs display monodisperse nIR spectra, which are important for fundamental studies and applications that rely on spectral changes. We show this advantage for SWCNT-based nIR fluorescent sensors for the neurotransmitter dopamine and red-shifted sp³ defect peaks . In summary, CPEP makes use of PFO polymers for chirality enrichment but provides access to chirality enriched SWCNTs functionalized in any desired polymer, surfactant or biopolymer.

Single-defect spectroscopy in the shortwave infrared

Chemical defects that fluoresce in the shortwave infrared open exciting opportunities in deep-penetration bioimaging, chemically specific sensing, and quantum technologies. However, the atomic size of defects and the high noise of infrared detectors have posed significant challenges to the studies of these unique emitters. Here we demonstrate high throughput single-defect spectroscopy in the shortwave infrared capable of quantitatively and spectrally resolving chemical defects at the single defect level. By cooling an InGaAs detector array down to -190 °C and implementing a nondestructive readout scheme, we are able to capture low light fluorescent events in the shortwave infrared with a signal-to-noise ratio improved by more than three orders-of-magnitude. As a demonstration, we show it is possible to resolve individual chemical defects in carbon nanotube semiconductors, simultaneously collecting a full spectrum for each defect within the entire field of view at the single defect limit.

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.

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.