Photon Correlation Spectroscopy of Luminescent Quantum Defects in Carbon Nanotubes

Correlation Measurements for Carbon Nanotubes with Quantum Defects

Single-photon sources are essential building blocks for the development of photonic quantum technology. Regarding potential practical application, an on-demand electrically driven quantum-light emitter on a chip is notably crucial for photonic integrated circuits. Here, we propose functionalized single-walled carbon nanotube field-effect transistors as a promising solid-state quantum-light source by demonstrating photon antibunching behavior via electrical excitation. The sp3 quantum defects were formed on the surface of (7, 5) carbon nanotubes by 3,5-dichlorophenyl functionalization, and individual carbon nanotubes were wired to graphene electrode pairs. Filtered electroluminescent defect-state emission at 77 K was coupled into a Hanbury Brown and Twiss experiment setup, and single-photon emission was observed by performing second-order correlation function measurements. We discuss the dependence of the intensity correlation measurement on electrical power and emission wavelength, highlighting the challenges of performing such measurements while simultaneously analyzing acquired data. Our results indicate a route toward room-temperature electrically triggered single-photon emission.

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.

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.

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.

Signatures of Chemical Dopants in Simulated Resonance Raman Spectroscopy of Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) with organic sp² or sp³ hybridization defects allow the robust tunability of many optoelectronic properties in these topologically interesting quasi-one-dimensional materials. Recent resonant Raman experiments have illuminated new features in the intermediate-frequency region upon functionalization that change with the degree of functionalization as well as with interactions between defect sites. In this Letter, we report ab initio simulated near-resonant Raman spectroscopy results for pristine and chemically functionalized SWCNT models and find new features concomitant with experimental observations. We are able to assign the character of these features by varying the frequency of the external Raman laser frequency near the defect-induced E₁₁* optical transition using a perturbative treatment of the electronic structure of the system. The obtained insights establish relationships between the nanotube atomistic structure and Raman spectra facilitating further exploration of SWCNTs with tunable optical properties tuned by chemical functionalization.

Coupling between Emissive Defects on Carbon Nanotubes: Modeling Insights

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) with organic molecules results in red-shifted emissive states associated with sp³-defects in the tube lattice, which facilitate their improved optical functionality, including single-photon emission. The energy of the defect-based electronic excitations (excitons) depends on the molecular adducts, the configuration of the defect, and concentration of defects. Here we model the interactions between two sp³-defects placed at various distances in the (6,5) SWCNT using time-dependent density functional theory. Calculations reveal that these interactions conform to the effective model of J-aggregates for well-spaced defects (>2 nm), leading to a red-shifted and optically allowed (bright) lowest energy exciton. H-aggregate behavior is not observed for any defect orientations, which is beneficial for emission. The splitting between the lowest energy bright and optically forbidden (dark) excitons and the pristine excitonic band are controlled by the single-defect configurations and their axial separation. These findings enable a synthetic design strategy for SWCNTs with tunable near-infrared emission.

Quantum Light Emission from Coupled Defect States in DNA-Functionalized Carbon Nanotubes

Solid-state single-photon sources are essential building blocks for quantum photonics and quantum information technologies. This study demonstrates promising single-photon emission from quantum defects generated in single-wall carbon nanotubes (SWCNTs) by covalent reaction with guanine nucleotides in their single-stranded DNA coatings. Low-temperature photoluminescence spectroscopy and photon-correlation measurements on individual guanine-functionalized SWCNTs (GF-SWCNTs) indicate that multiple, closely spaced guanine defect sites within a single ssDNA strand collectively form an exciton trapping potential that supports a localized quantum state capable of room-temperature single-photon emission. In addition, exciton traps from adjacent ssDNA strands are weakly coupled to give cross-correlations between their separate photon emissions. Theoretical modeling identifies coupling mechanism as a capture of band-edge excitons. Because the spatial pattern of nanotube functionalization sites can be readily controlled by selecting ssDNA base sequences, GF-SWCNTs should become a versatile family of quantum light emitters with engineered properties.

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.

Interaction of Luminescent Defects in Carbon Nanotubes with Covalently Attached Stable Organic Radicals

The functionalization of single-walled carbon nanotubes (SWCNTs) with luminescent sp³ defects has greatly improved their performance in applications such as quantum light sources and bioimaging. Here, we report the covalent functionalization of purified semiconducting SWCNTs with stable organic radicals (perchlorotriphenylmethyl, PTM) carrying a net spin. This model system allows us to use the near-infrared photoluminescence arising from the defect-localized exciton as a highly sensitive probe for the short-range interaction between the PTM radical and the SWCNT. Our results point toward an increased triplet exciton population due to radical-enhanced intersystem crossing, which could provide access to the elusive triplet manifold in SWCNTs. Furthermore, this simple synthetic route to spin-labeled defects could enable magnetic resonance studies complementary to in vivo fluorescence imaging with functionalized SWCNTs and facilitate the scalable fabrication of spintronic devices with magnetically switchable charge transport.

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.

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.