Covalent Defects Research Articles

(Invited) On-the-Fly Non-Adiabatic Dynamics Simulations of Single-Walled Carbon Nanotubes with Covalent Defects

Single-walled carbon nanotubes (SWCNTs) with covalent surface defects have been explored recently due to their promise for use in single-photon telecommunication emission and in spintronic applications. The dynamical evolution of excitons (the primary electronic excitations) in these systems has only been loosely explored using atomistic modeling due to the size-limitations of these large systems. We present computational modeling of non-radiative relaxation on a variety of SWCNT chiralities with single-defect functionalization schemes. Our modeling uses a trajectory surface hopping algorithm accounting for excitonic effects with a configuration interaction approach. We find a strong chirality and defect-composition dependence on the population relaxation (50 – 500 fs) between the nanotube band-gap excitation and the defect-associated, single-photon-emitting state, giving insight into the dynamic trapping nature of these localized excitonic states. Engineering fast population decay into the quasi-two-level sub-system with weak coupling to higher-energy states increases the effectiveness and controllability of these quantum light emitters.

On-the-Fly Nonadiabatic Dynamics Simulations of Single-Walled Carbon Nanotubes with Covalent Defects

Single-walled carbon nanotubes (SWCNTs) with covalent surface defects have been explored recently due to their promise for use in single-photon telecommunication emission and in spintronic applications. The all-atom dynamic evolution of electrostatically bound excitons (the primary electronic excitations) in these systems has only been loosely explored from a theoretical perspective due to the size limitations of these large systems (>500 atoms). In this work, we present computational modeling of nonradiative relaxation in a variety of SWCNT chiralities with single-defect functionalizations. Our excited-state dynamics modeling uses a trajectory surface hopping algorithm accounting for excitonic effects with a configuration interaction approach. We find a strong chirality and defect-composition dependence on the population relaxation (varying over 50-500 fs) between the primary nanotube band gap excitation E11 and the defect-associated, single-photon-emitting E11* state. These simulations give direct insight into the relaxation between the band-edge states and the localized excitonic state, in competition with dynamic trapping/detrapping processes observed in experiment. Engineering fast population decay into the quasi-two-level subsystem with weak coupling to higher-energy states increases the effectiveness and controllability of these quantum light emitters.

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

Solid-state single-photon sources are the essential building block for quantum photonics and quantum information technologies. The report here illustrates an advanced perspective of implanting coupled fluorescent quantum defects in single-wall carbon nanotubes (SWCNTs) through chemical reactions between the nanotube sidewall and the guanine nucleotides in single-stranded DNA (ssDNA) that coats the nanotube. Through low-temperature photoluminescence spectroscopy and photon correlation experiments, we have revealed that multiple guanine defects within a single ssDNA strand couple together to form a cumulative deep trapping potential supporting a localized quantum state that is capable of room-temperature single-photon emission. Helical wrapping of multiple ssDNA strands in single nanotubes allows a series of exciton localization sites to be created along the length of the SWCNTs. Such unique exciton trapping potential landscape gives rise to previously inaccessible intrinsic emission characteristics including coupling of multiple spatially separated quantum emitters. Our joint experimental and computational studies together reveal quantum emission properties from multiple spatially patterned covalent defects in SWCNTs, identify capture of the band-edge exciton as the underlying cause of the cross-correlated photon emission and open a distinctive prospect for tailoring chemically altered nanotubes as quantum light emitters.

Tailoring the Properties of Single-Wall Carbon Nanotube Samples through Structure-Selective Near-Infrared Photochemistry.

As-produced samples of single-wall carbon nanotubes (SWCNTs) contain many structural forms with characteristic properties. In particular, semiconducting SWCNTs display distinct near-IR optical absorption and emission peaks. We show that the selective irradiation of these absorption features can induce structure-specific functionalization in unsorted SWCNT samples. This approach is demonstrated with an ambient temperature photoreaction involving dissolved O2 and irradiation at 955, 985, and 1130 nm, causing preferential covalent reactions of (8,3), (6,5), and (7,6) SWCNTs, respectively. Treated samples showed permanent fluorescence quenching and absorption bleaching near the irradiation wavelength and an increase in the Raman D/G intensity ratio, indicating the formation of covalent defects. The reaction has a very low photochemical quantum yield and was observed for samples suspended in single-stranded DNA and in conventional surfactants that gave incomplete coverage of the nanotube surface. The approach of exploiting sharp nanotube near-IR transitions for structure-selective photochemistry provides a path to tailor SWCNT optical properties for several potential applications without the need for physical sorting.

Click Chemistry on Defects to Create Multifunctional Fluorescent Carbon Nanotubes

Single-walled carbon nanotubes (SWCNT) are versatile materials for imaging and sensing in the near infrared (nIR) range. Non-covalent adsorption with (bio)polymers such as DNA is a powerful approach to functionalize SWCNTs. This approach lead to a broad range of biosensors. However, covalent approaches would be beneficial for many applications because of higher stability and photophysical signal transduction. One route to covalent functionalization is reactions with diazonium salts that create covalent defects and a red-shifted peak in the nIR fluorescence spectrum attributed to a formerly dark exciton.Here, we used novel synthesized diazonium salts to create defects that can be easily further functionalized with click chemistry reactions (Thiol-en and azid-alkine). The modified SWCNTs were additionally non-covalently wrapped in biopolymers for solubilization and characterized via nIR spectroscopy and atomic force microscopy (AFM). Using this approach we covalently conjugated diverse molecules including small aliphatic molecules, peptides, DNA and nanobodies to the SWCNT backbone. Successful functionalization was verified by conjugation of organic visible fluorophores and single SWCNT colocalization. Additionally, these multifunctional SWCNTs were used as building block for biosensors that detect disease markers.In summary, we present a general approach to conjugate a broad range of molecules to SWCNTs using defect chemistry and demonstrate applications in biosensing.

(Invited) sp3 Defects of Single Walled Carbon Nanotubes: Photonic and Electronic Integration

Single walled carbon nanotubes with covalently functionalized sp3 defects have been rapidly emerging as a new class of transformational optical materials. Particularly, recent demonstration of room temperature, 1.55 µm single photon generation establish these covalent defects of SWCNTs as promising fundamental building blocks for defect driven quantum information technologies such as quantum key distribution. Defects also provide a significant boost in photoluminescence quantum yield (from <1% to ~28%) and emission stability, making envisioned applications of SWCNTs in optoelectronics, sensing, and imaging technologies more feasible. The key to full exploitation of these technological potentials lie in the ability to integrate these SWCNTs with defects into plasmonic/photonic cavities for enhancement of light matter interactions and into nano-electronic devices for electrical stimulation. Here in this talk I will report recent progresses we made together with our collaborators toward achieving this critical photonic and electronic integration. We have successfully integrated the SWCNTs with sp3 defects into 2D slab photonic crystals1 and metallo-dielectric antenna arrays.2 In case of coupling with 2D photonic crystal, we were able to demonstrate room temperature single photon generation with 3meV linewidth defined by the cavity quality factor and 50 fold enhancement in emission.1 We achieved narrowest low temperature emission linewidth of 35 µeV by coupling the defects with metallo-dielectric antennas.2 We were also successful in fabricating single SWCNT field effect transistors using dielectrophoresis and attained electroluminescence from the sp3 defects. Ishii, A.; He, X.; Hartmann, N. F.; Machiya, H.; Htoon, H.; Doorn, S. K.; Kato, Y. K. Enhanced Single-Photon Emission from Carbon-Nanotube Dopant States Coupled to Silicon Microcavities. Nano Lett. 18 , 3873-3878 (2018).Shayan, K.; He, X.; Luo, Y.; Rabut, C.; Li, X.; Hartmann, N.F.; Blackburn, J.L.; Doorn, S.K.; Htoon, H.; Strauf, S.; “Suppression of Pump-Induced Exciton Dephasing in Sidewall-Functionalized Single-Walled Carbon Nanotubes”, Nanoscale 10 , 12631 (2018).

Multifunctional NIR-Fluorescent Carbon Nanotubes for Imaging and Sensing By Using Defect Chemistry

Single-walled carbon nanotubes (SWCNT) are versatile materials for imaging and sensing in the near infrared (nIR) range. Non-covalent adsorption with (bio)polymers such as ssDNA is a powerful approach to functionalize SWCNTs. This approach lead to sensors that detect the release of biomolecules such as neurotransmitters from cells with high spatiotemporal resolution. However, covalent approaches would be beneficial for many applications because of higher stability and photophysical signal transduction. It has been reported that reactions with diazonium salts create covalent defects and a red-shifted peak in the nIR fluorescence spectrum attributed to a formerly dark exciton. Here, we use defect chemistry to covalently modify SWCNTs and at the same time change their fluorescence spectrum in a well-defined way. For that purpose, we synthesized a maleimide-carrying diazonium salt and used a light-catalyzed reaction to introduce defects. The maleimide group can be further functionalized using thiols in a thiol-en click reaction. The modified SWCNTs were additionally wrapped in DNA for solubilization and characterized via nIR spectroscopy and atomic force microscopy (AFM). Our results show that depending on the attached thiol compound these SWCNTs respond differently to small molecules such as the neurotransmitter dopamine. This approach was then used to optimise the selectivity profile and sensitivity for fluorescent sensing applications. Furthermore, we covalently conjugated antibodies to the SWCNTs and used them for nIR labeling in cells. In summary, we present a new approach to covalently functionalize SWCNTs using defect chemistry and demonstrate applications in nIR imaging and sensing.

(Invited) Localized Covalent Defects on Carbon Nanotube Devices for Sensor Applications

Functionalization of carbon nanotube devices is essential to tailor their affinity with other molecules, which is especially critical for applications as sensors. Among functionalization approaches, covalent chemistry produces stable moieties on nanotubes sidewalls, but the chemistry is invasive and alters the electronic properties of the material. In this presentation, I will describe recent efforts to control covalent diazonium chemistry on carbon nanotubes and understand its impact on electrical conductance. In particular, I will contrast experiments between extended and localized defect coverage of adducts on the electrical response of devices, and present applications in single-molecule sensors.

Rigid, Conjugated Macrocycles for High Performance Organic Photodetectors.

Organic photodetectors (OPDs) are attractive for their high optical absorption coefficient, broad wavelength tunability, and compatibility with lightweight and flexible devices. Here we describe a new molecular design that enables high performance organic photodetectors. We use a rigid, conjugated macrocycle as the electron acceptor in devices to obtain high photocurrent and low dark current. We make a direct comparison between the devices made with the macrocyclic acceptor and an acyclic control molecule; we find that the superior performance of the macrocycle originates from its rigid, conjugated, and cyclic structure. The macrocycle's rigid structure reduces the number of charged defects originating from deformed sp2 carbons and covalent defects from photo/thermoactivation. With this molecular design, we are able to suppress dark current density while retaining high responsivity in an ultrasensitive nonfullerene OPD. Importantly, we achieve a detectivity of ∼1014 Jones at near zero bias voltage. This is without the need for extra carrier blocking layers commonly employed in fullerene-based devices. Our devices are comparable to the best fullerene-based photodetectors, and the sensitivity at low working voltages (<0.1 V) is a record for nonfullerene OPDs.

Covalent defects restrict supramolecular self-assembly of homopolypeptides: case study of β2-fibrils of poly-L-glutamic acid.

Poly-L-glutamic acid (PLGA) often serves as a model in studies on amyloid fibrils and conformational transitions in proteins, and as a precursor for synthetic biomaterials. Aggregation of PLGA chains and formation of amyloid-like fibrils was shown to continue on higher levels of superstructural self-assembly coinciding with the appearance of so-called β2-sheet conformation manifesting in dramatic redshift of infrared amide I′ band below 1600 cm−1. This spectral hallmark has been attributed to network of bifurcated hydrogen bonds coupling C = O and N-D (N-H) groups of the main chains to glutamate side chains. However, other authors reported that, under essentially identical conditions, PLGA forms the conventional in terms of infrared characteristics β1-sheet structure (exciton-split amide I′ band with peaks at ca. 1616 and 1683 cm−1). Here we attempt to shed light on this discrepancy by studying the effect of increasing concentration of intentionally induced defects in PLGA on the tendency to form β1/β2-type aggregates using infrared spectroscopy. We have employed carbodiimide-mediated covalent modification of Glu side chains with n-butylamine (NBA), as well as electrostatics-driven inclusion of polylysine chains, as two different ways to trigger structural defects in PLGA. Our study depicts a clear correlation between concentration of defects in PLGA and increasing tendency to depart from the β2-structure toward the one less demanding in terms of chemical uniformity of side chains: β1-structure. The varying predisposition to form β1- or β2-type aggregates assessed by infrared absorption was compared with the degree of morphological order observed in electron microscopy images. Our results are discussed in the context of latent covalent defects in homopolypeptides (especially with side chains capable of hydrogen-bonding) that could obscure their actual propensities to adopt different conformations, and limit applications in the field of synthetic biomaterials.

Effect of ionic and covalent defects on the properties of transparent carbon nanotube films

Transparent conducting films are rapidly emerging as one of the most promising applications of carbon nanotubes. In these less-than-perfect materials, understanding the types of defects and their effect on the transport and optical properties plays a significant role. In this contribution, we present examples of these effects, both beneficial and adverse.

Role of Covalent Defects on Phonon Softening in Metallic Carbon Nanotubes

We have examined how electrical characteristics and charging dependent Raman G-band phonon softening in individual metallic carbon nanotubes are influenced by covalent defects. In addition to decreasing electrical conductance with increasing on/off current ratio eventually leading to semiconducting behavior, adding covalent defects reduces the degree of softening and broadening of longitudinal optical (LO) phonon mode of the G-band near the charge neutrality point where the bands cross. On the other hand, the transverse optical (TO) mode softening is enhanced by defects. Implications on the interpretation of Raman G-band phonon softening and on utilizing Raman spectroscopy to examine covalent functionalization are discussed.