Photoluminescence imaging of suspended single-walled carbon nanotubes

Emissive brightening in molecular graphene nanoribbons by twilight states

Carbon nanomaterials are expected to be bright and efficient emitters, but structural disorder, intermolecular interactions and the intrinsic presence of dark states suppress their photoluminescence. Here, we study synthetically-made graphene nanoribbons with atomically precise edges and which are designed to suppress intermolecular interactions to demonstrate strong photoluminescence in both solutions and thin films. The resulting high spectral resolution reveals strong vibron-electron coupling from the radial-breathing-like mode of the ribbons. In addition, their cove-edge structure produces inter-valley mixing, which brightens conventionally-dark states to generate hitherto-unrecognised twilight states as predicted by theory. The coupling of these states to the nanoribbon phonon modes affects absorption and emission differently, suggesting a complex interaction with both Herzberg-Teller and Franck- Condon coupling present. Detailed understanding of the fundamental electronic processes governing the optical response will help the tailored chemical design of nanocarbon optical devices, via gap tuning and side-chain functionalisation.

Optical coupling of individual air-suspended carbon nanotubes to silicon microcavities

Carbon nanotubes are a telecom band emitter compatible with silicon photonics, and when coupled to microcavities, they present opportunities for exploiting quantum electrodynamical effects. Microdisk resonators demonstrate the feasibility of integration into the silicon platform. Efficient coupling is achieved using photonic crystal air-mode nanobeam cavities. The molecular screening effect on nanotube emission allows for spectral tuning of the coupling. The Purcell effect of the coupled cavity-exciton system reveals near-unity radiative quantum efficiencies of the excitons in carbon nanotubes.

Chirality Dependence of Triplet Excitons in (6,5) and (7,5) Single-Wall Carbon Nanotubes Revealed by Optically Detected Magnetic Resonance

The excitonic structure of single-wall carbon nanotubes (SWCNTs) is chirality dependent and consists of multiple singlet and triplet excitons (TEs) of which only one singlet exciton (SE) is optically bright. In particular, the dark TEs have a large impact on the integration of SWCNTs in optoelectronic devices, where excitons are created electrically, such as in infrared light-emitting diodes, thereby strongly limiting their quantum efficiency. Here, we report the characterization of TEs in chirality-purified samples of (6,5) and (7,5) SWCNTs, either randomly oriented in a frozen solution or with in-plane preferential orientation in a film, by means of optically detected magnetic resonance (ODMR) spectroscopy. In both chiral structures, the nanotubes are shown to sustain three types of TEs. One TE exhibits axial symmetry with zero-field splitting (ZFS) parameters depending on SWCNT diameter, in good agreement with the tighter confinement expected in narrower-diameter nanotubes. The ZFS of this TE also depends on nanotube environment, pointing to slightly weaker confinement for surfactant-coated than for polymer-wrapped SWCNTs. A second TE type, with much smaller ZFS, does not show the same systematic trends with diameter and environment and has a less well-defined axial symmetry. This most likely corresponds to TEs trapped at defect sites at low temperature, as exemplified by comparing SWCNT samples from different origins and after different treatments. A third triplet has unresolved ZFS, implying it originates from weakly interacting spin pairs. Aside from the diameter dependence, ODMR thus provides insights in both the symmetry, confinement, and nature of TEs on semiconducting SWCNTs.

Multiwavelength Photothermal Imaging of Individual Single-Walled Carbon Nanotubes Suspended in a Solvent

Optical imaging of individual single-walled carbon nanotubes (SWCNTs) enables the characterization of heterogeneous SWCNT samples. However, previous measurement methods have targeted SWCNTs fixed on a substrate. In this study, absorption-contrast imaging of individual SWCNTs moving irregularly in a solvent was performed by simultaneous multiwavelength photothermal (PT) microscopy. Using this technique, heterogeneous samples containing semiconducting and metallic SWCNTs were characterized by absorption spectroscopy. The semiconducting and metallic SWCNTs were visualized in different colors in the obtained multiwavelength images due to their different absorption spectra. Statistical analysis of the multiwavelength signals revealed that semiconducting and metallic SWCNTs could be distinguished with more than 90% accuracy. Time-series PT imaging of the nanotube aggregates induced by salt addition was also conducted by performing single-nanotube measurements. Our study demonstrated that PT microscopy is a versatile technique for determining the composition and degree of aggregation of SWCNTs in liquid and polymeric media, which can promote the industrial application of such materials.

Full Spectrum Raman Excitation Mapping Spectroscopy

A generalization of the Raman scattering (RS) spectrum, the Raman excitation map (REM) is a hyperspectral two-dimensional (2D) data set encoding vibrational spectra, electronic spectra and their coupling. Despite the great potential of REM for optical sensing and characterization with remarkable sensitivity and selectivity, the difficulty of obtaining maps and the length of time required to acquire them has been practically limiting. Here we show, with a simple setup using current optical equipment, that maps can be obtained much more rapidly than before (~ms to ~100 s now vs. ~1000 s to hours before) over a broad excitation range (here ~100 nm is demonstrated, with larger ranges straightforward to obtain), thus taking better advantage of scattering resonance. We obtain maps from different forms of carbon: graphite, graphene, purified single walled carbon nanotubes (SWCNTs) and chirality enriched SWCNTs. The relative speed and simplicity of the technique make REM a practical and sensitive tool for chemical analysis and materials characterization.

Laser-Induced Deposition of Carbon Nanotubes in Fiber Optic Tips of MMI Devices

The integration of carbon nanotubes (CNTs) into optical fibers allows the application of their unique properties in robust and versatile devices. Here, we present a laser-induced technique to obtain the deposition of CNTs onto the fiber optics tips of multimode interference (MMI) devices. An MMI device is constructed by splicing a section of no-core fiber (NCF) to a single-mode fiber (SMF). The tip of the MMI device is immersed into a liquid solution of CNTs and laser light is launched into the MMI device. CNTs solutions using water and methanol as solvents were tested. In addition, the use of a polymer dispersant polyvinylpyrrolidone (PVP) in the CNTs solutions was also studied. We found that the laser-induced deposition of CNTs performed in water-based solutions generates non-uniform deposits. On the other hand, the laser-induced deposition performed with methanol solutions generates uniform deposits over the fiber tip when no PVP is used and deposition at the center of the fiber when PVP is present in the CNTs solution. The results show the crucial role of the solvent on the spatial features of the laser-induced deposition process. Finally, we register and study the reflection spectra of the as-fabricated CNTs deposited MMI devices.

Advanced biomedical applications of carbon nanotube

With advances in nanotechnology, the applications of nanomaterial are developing widely and greatly. The characteristic properties of carbon nanotubes (CNTs) make them the most selective candidate for various multi-functional applications. The greater surface area of the CNTs in addition to the capability to manipulate the surfaces and dimensions has provided greater potential for this nanomaterial. The CNTs possess greater potential for applications in biomedicine due to their vital electrical, chemical, thermal, and mechanical properties. The unique properties of CNT are exploited for numerous applications in the biomedical field. They are useful in both therapeutic and diagnostic applications. They form novel carrier systems which are also capable of site-specific delivery of therapeutic agents. In addition, CNTs are of potential application in biosensing. Many recently reported advanced systems of CNT could be exploited for their immense potential in biomedicine in the future.

Multifunctional Carbon-Based Nanomaterials: Applications in Biomolecular Imaging and Therapy

Molecular imaging has been widely used not only as an important detection technology in the field of medical imaging for cancer diagnosis but also as a theranostic approach for cancer in recent years. Multifunctional carbon-based nanomaterials (MCBNs), characterized by unparalleled optical, electronic, and thermal properties, have attracted increasing interest and demonstrably hold the greatest promise in biomolecular imaging and therapy. As such, it should come as no surprise that MCBNs have already revealed a great deal of potential applications in biomedical areas, such as bioimaging, drug delivery, and tumor therapy. Carbon nanomaterials can be categorized as graphene, single-walled carbon nanotubes, mesoporous carbon, nanodiamonds, fullerenes, or carbon dots on the basis of their morphologies. In this article, reports of the use of MCBNs in various chemical conjugation/functionalization strategies, focusing on their applications in cancer molecular imaging and imaging-guided therapy, will be comprehensively summarized. MCBNs show the possibility to serve as optimal candidates for precise cancer biotheranostics.

Ultrafast, asymmetric charge transfer and slow charge recombination in porphyrin/CNT composites demonstrated by time-domain atomistic simulation

The versatile photochemical properties of porphyrin molecules make them excellent candidates for solar energy applications. Carbon nanotubes (CNTs) exhibit superior charge conductivity and have been combined with porphyrins to achieve efficient and ultrafast charge separation. Experiments show that the charge separated state lives less than 10 ps, which is too short for applications. Using real-time time-dependent tight binding density functional theory (DFTB) combined with non-adiabatic molecular dynamics (NAMD), we model photo-induced charge separation and recombination in two porphyrin/CNT composites. Having achieved excellent agreement with the experiment for the electron transfer from the porphyrins to the CNT, we demonstrate that hole transfer can be achieved upon CNT excitation, although in a less efficient way. By exciting the CNT one can extend light harvesting into lower energies of the solar spectrum and increase solar light conversion efficiency. We also show that the charge separated state can live over 1 ns. The two orders of magnitude difference from the experimental lifetime could arise due to the presence of defects or metallic tubes in the samples. The charge separated state is long-lived because the non-adiabatic electron-phonon coupling is very small, less than 1 meV, and the quantum coherence is short, 15-20 fs. The excited states in the isolated porphyrins and CNT live around 100 ps, in agreement with experiments as well. The porphyrin/CNT interaction occurs through the π-electron systems of the two species. The non-radiative relaxation is promoted by both high and low frequency phonons, with higher frequency phonons playing more important roles in electron relaxation than in hole relaxation. Low frequency phonons contribute significantly to the decay of the charge separated state, because they modulate the relative positions of the porphyrins and the CNT. The time-domain atomistic simulations provide a detailed understanding of the charge separation and recombination mechanisms, and generate valuable guidelines for the optimization of photovoltaic efficiency in modern nanoscale materials.

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.

Fluorescent Polymer-Single-Walled Carbon Nanotube Complexes with Charged and Noncharged Dendronized Perylene Bisimides for Bioimaging Studies

Fluorescent nanomaterials are expected to revolutionize medical diagnostic, imaging, and therapeutic tools due to their superior optical and structural properties. Their inefficient water solubility, cell permeability, biodistribution, and high toxicity, however, limit the full potential of their application. To overcome these obstacles, a water-soluble, fluorescent, cytocompatible polymer-single-walled carbon nanotube (SWNT) complex is introduced for bioimaging applications. The supramolecular complex consists of an alkylated polymer conjugated with neutral hydroxylated or charged sulfated dendronized perylene bisimides (PBIs) and SWNTs as a general immobilization platform. The polymer backbone solubilizes the SWNTs, decorates them with fluorescent PBIs, and strongly improves their cytocompatibility by wrapping around the SWNT scaffold. In photophysical measurements and biological in vitro studies, sulfated complexes exhibit superior optical properties, cellular uptake, and intracellular staining over their hydroxylated analogs. A toxicity assay confirms the highly improved cytocompatibility of the polymer-wrapped SWNTs toward surfactant-solubilized SWNTs. In microscopy studies the complexes allow for the direct imaging of the SWNTs' cellular uptake via the PBI and SWNT emission using the 1st and 2nd optical window for bioimaging. These findings render the polymer-SWNT complexes with nanometer size, dual fluorescence, multiple charges, and high cytocompatibility as valuable systems for a broad range of fluorescence bioimaging studies.

Temperature Distribution and Thermal Conductivity Measurements of Chirality-Assigned Single-Walled Carbon Nanotubes by Photoluminescence Imaging Spectroscopy

It is expected that single-walled carbon nanotubes (SWCNTs) have high thermal conductivity along the tube axis and that the thermal conductivities depend on their structure, such as length, diameter, chirality (n, m), and so forth. Although many experimental measurements of the thermal conductivity have been reported, the SWCNT structure was not characterized sufficiently. In particular, the chirality was not assigned, and it was not confirmed whether SWCNT was isolated or not (bundled with multiplicate SWCNTs). Therefore, measured values widely vary (10¹ to 10⁴ W/(m·K)) so far. Here, we measured the thermal conductivity of chirality-assigned SWCNTs, which were individually suspended, by using photoluminescence (PL) imaging spectroscopy. The temperature distribution along the tube axis was obtained, and the temperature dependence of the thermal conductivity was measured in a wide-temperature range (from 350 to 1000 K). For (9, 8) SWCNTs with 10-12 μm in length, the thermal conductivity was 1166 ± 243 W/(m·K) at 400 K. The proposed PL imaging spectroscopy enables to measure the thermal conductivity of SWCNTs with high precision and without any contacts, and it is an effective method in the temperature distribution measurements of nanomaterials.

Time-Resolved Photoluminescence Microscopy for the Analysis of Semiconductor-Based Paint Layers

In conservation, science semiconductors occur as the constituent matter of the so-called semiconductor pigments, produced following the Industrial Revolution and extensively used by modern painters. With recent research highlighting the occurrence of various degradation phenomena in semiconductor paints, it is clear that their detection by conventional optical fluorescence imaging and microscopy is limited by the complexity of historical painting materials. Here, we illustrate and prove the capabilities of time-resolved photoluminescence (TRPL) microscopy, equipped with both spectral and lifetime sensitivity at timescales ranging from nanoseconds to hundreds of microseconds, for the analysis of cross-sections of paint layers made of luminescent semiconductor pigments. The method is sensitive to heterogeneities within micro-samples and provides valuable information for the interpretation of the nature of the emissions in samples. A case study is presented on micro samples from a painting by Henri Matisse and serves to demonstrate how TRPL can be used to identify the semiconductor pigments zinc white and cadmium yellow, and to inform future investigations of the degradation of a cadmium yellow paint.

Purcell-enhanced quantum yield from carbon nanotube excitons coupled to plasmonic nanocavities

Single-walled carbon nanotubes (SWCNTs) are promising absorbers and emitters to enable novel photonic applications and devices but are also known to suffer from low optical quantum yields. Here we demonstrate SWCNT excitons coupled to plasmonic nanocavity arrays reaching deeply into the Purcell regime with Purcell factors (F_P) up to F_P = 180 (average F_P = 57), Purcell-enhanced quantum yields of 62% (average 42%), and a photon emission rate of 15 MHz into the first lens. The cavity coupling is quasi-deterministic since the photophysical properties of every SWCNT are enhanced by at least one order of magnitude. Furthermore, the measured ultra-narrow exciton linewidth (18 μeV) reaches the radiative lifetime limit, which is promising towards generation of transform-limited single photons. To demonstrate utility beyond quantum light sources we show that nanocavity-coupled SWCNTs perform as single-molecule thermometers detecting plasmonically induced heat at cryogenic temperatures in a unique interplay of excitons, phonons, and plasmons at the nanoscale.

Single-walled carbon nanotubes offer exciting optoelectronic applications but generally suffer from low quantum yields. Here, Luo et al. demonstrate that coupling nanotubes to plasmonic antennas can lead to large Purcell enhancement and corresponding increase in quantum yield as well as plasmonic thermometry at the single molecule level.

Carbon Nanotubes as Fluorescent Labels for Surface Plasmon Resonance-Assisted Fluoroimmunoassay

The photoluminescence properties of carbon nanotubes (CNTs), including the large Stokes shift and the absence of fluorescent photobleaching, can be used as a fluorescent label in biological measurements. In this study, the performance of CNTs as a fluorescent label for surface plasmon resonance (SPR)-assisted fluoroimmunoassay is evaluated. The fluorescence of (8, 3) CNTs with an excitation wavelength of 670 nm and an emission wavelength of 970 nm is observed using a sensor chip equipped with a prism-integrated microfluidic channel to excite the SPR. The minimum detectable concentration of a CNT dispersed in water using a visible camera is 0.25 μg/mL, which is equivalent to 2 × 10¹⁰ tubes/mL. The target analyte detection using the CNT fluorescent labels is theoretically investigated by evaluating the detectable number of CNTs in a detection volume. Assuming detection of virus particles which are bound with 100 CNT labels, the minimum number of detectable virus particles is calculated to be 900. The result indicates that CNTs are effective fluorescent labels for SPR-assisted fluoroimmunoassay.

Progress Towards Applications of Carbon Nanotube Photoluminescence

In the fifteen years following the discovery of single-walled carbon nanotube (SWCNT) photoluminescence, investigators have made significant progress in their understanding of the phenomenon and towards the development of applications. The intrinsic potential of semiconducting carbon nanotubes - a family of bright, photostable near infrared (NIR) fluorophores (900-2100 nm) with tunable properties, has motivated their use as optical probes and sensors. In this perspective, we highlight the advances made in the synthesis, processing, modification, separation, and metrology of carbon nanotubes in the context of applications of their photoluminescence.

A Low-Energy Electron Beam Does Not Damage Single-Walled Carbon Nanotubes and Graphene

Scanning electron microscopy (SEM) is a principal tool for studying nanomaterials, including carbon nanotubes and graphene. Imaging carbon nanomaterials by SEM, however, increases the disorder mode (D-mode) in their Raman spectra. Early studies, which relied on ambiguous ensemble measurements, claimed that the D-mode indicates damage to the specimens by a low-energy electron beam (e-beam). This claim has been accepted by the nanomaterials community for more than a decade without thorough examination. Here we demonstrate that a low-energy e-beam does not damage carbon nanomaterials. By performing measurements on single nanotubes, we independently examined the following factors: (1) the e-beam irradiation itself, (2) the e-beam-deposited hydrocarbon, and (3) the amorphous carbon deposited during synthesis of the material. We concluded that the e-beam-induced D-mode of both carbon nanotubes and graphene originates solely from the irradiated amorphous carbon and not from the e-beam itself or the hydrocarbon. The results of this study should help minimize potential ambiguities for researchers imaging a broad range of nanomaterials by electron microscopy.

Quantum dot-like excitonic behavior in individual single walled-carbon nanotubes

Semiconducting single-walled carbon nanotubes are one-dimensional materials with great prospects for applications such as optoelectronic and quantum information devices. Yet, their optical performance is hindered by low fluorescent yield. Highly mobile excitons interacting with quenching sites are attributed to be one of the main non-radiative decay mechanisms that shortens the exciton lifetime. In this paper we report on time-integrated photoluminescence measurements on individual polymer wrapped semiconducting carbon nanotubes. An ultra narrow linewidth we observed demonstrates intrinsic exciton dynamics. Furthermore, we identify a state filling effect in individual carbon nanotubes at cryogenic temperatures as previously observed in quantum dots. We propose that each of the CNTs is segmented into a chain of zero-dimensional states confined by a varying local potential along the CNT, determined by local environmental factors such as the amount of polymer wrapping. Spectral diffusion is also observed, which is consistent with the tunneling of excitons between these confined states.

Chirality-Selective Photoluminescence Enhancement of ssDNA-Wrapped Single-Walled Carbon Nanotubes Modified with Gold Nanoparticles

In this work, a convenient method to enhance the photoluminescence (PL) of single-walled carbon nanotubes (SWNTs) in aqueous solutions is provided. Dispersing by single-stranded DNA (ssDNA) and modifying with gold nanoparticles (AuNPs), about tenfold PL enhancement of the SWNTs is observed. More importantly, the selective PL enhancement is achieved for some particular chiralities of interest over all other chiralities, by using certain specific ssDNA sequences that are reported to recognize these particular chiralities. By forming AuNP-DNA-SWNT nanohybrids, ssDNA serves as superior molecular spacers that on one hand protect SWNT from direct contacting with AuNP and causing PL quench, and on the other hand attract the AuNP in close proximity to the SWNT to enhance its PL. This PL enhancement method can be utilized for the PL analysis of SWNTs in aqueous solutions, for biomedical imaging, and may serve as a prescreening method for the recognition and separation of single chirality SWNTs by ssDNA.

Carbon nanomaterials: multi-functional agents for biomedical fluorescence and Raman imaging

Carbon based nanomaterials have emerged over the last few years as important agents for biomedical fluorescence and Raman imaging applications. These spectroscopic techniques utilize either fluorescently labelled carbon nanomaterials or the intrinsic photophysical properties of the carbon nanomaterial. In this review article we present the utilization and performance of several classes of carbon nanomaterials, namely carbon nanotubes, carbon nanohorns, carbon nanoonions, nanodiamonds and different graphene derivatives, which are currently employed for in vitro as well as in vivo imaging in biology and medicine. A variety of different approaches, imaging agents and techniques are examined and the specific properties of the various carbon based imaging agents are discussed. Some theranostic carbon nanomaterials, which combine diagnostic features (i.e. imaging) with cell specific targeting and therapeutic approaches (i.e. drug delivery or photothermal therapy), are also included in this overview.

A Comparison of Photocurrent Mechanisms in Quasi-Metallic and Semiconducting Carbon Nanotube pn-Junctions

We present a comparative study of quasi-metallic (Eg ∼ 100 meV) and semiconducting (Eg ∼ 1 eV) suspended carbon nanotube pn-junctions introduced by electrostatic gating. While the built-in fields of the quasi-metallic carbon nanotubes (CNTs) are 1-2 orders of magnitude smaller than those of the semiconducting CNTs, their photocurrent is 2 orders of magnitude higher than the corresponding semiconducting CNT devices under the same experimental conditions. Here, the large exciton binding energy in semiconducting nanotubes (∼400 meV) makes it difficult for excitons to dissociate into free carriers that can contribute to an externally measured photocurent. As such, semiconducting nanotubes require a phonon to assist in the exciton dissociation process, in order to produce a finite photocurrent, while quasi-metallic nanotubes do not. The quasi-metallic nanotubes have much lower exciton binding energies (∼50 meV) as well as a continuum of electronic states to decay into and, therefore, do not require the absorption of a phonon in order to dissociate, making it much easier for these excitons to produce a photocurrent. We performed detailed simulations of the band energies in quasi-metallic and semiconducting nanotube devices in order to obtain the electric field profiles along the lengths of the nanotubes. These simulations predict maximum built-in electric field strengths of 2.3 V/μm for semiconducting and 0.032-0.22 V/μm for quasi-metallic nanotubes under the applied gate voltages used in this study.

Optoelectronic Switching of a Carbon Nanotube Chiral Junction Imaged with Nanometer Spatial Resolution

Chiral junctions of carbon nanotubes have the potential of serving as optically or electrically controllable switches. To investigate optoelectronic tuning of a chiral junction, we stamp carbon nanotubes onto a transparent gold surface and locate a tube with a semiconducting-metallic junction. We image topography, laser absorption at 532 nm, and measure I-V curves of the junction with nanometer spatial resolution. The bandgaps on both sides of the junction depend on the applied tip field (Stark effect), so the semiconducting-metallic nature of the junction can be tuned by varying the electric field from the STM tip. Although absolute field values can only be estimated because of the unknown tip geometry, the bandgap shifts are larger than expected from the tip field alone, so optical rectification of the laser and carrier generation by the laser must also affect the bandgap switching of the chiral junction.

Photoluminescence enhancement of aligned arrays of single-walled carbon nanotubes by polymer transfer

The photoluminescence of as-grown, aligned single-walled carbon nanotubes (SWNTs) on quartz is strongly quenched and barely detectable. Here we show that transferring these SWNTs to another substrate such as clean quartz or glass increases their emission efficiency by up to two orders of magnitude. By statistical analysis of large nanotube arrays we show at what point of the transfer process the emission enhancement occurs and how it depends on the receiving substrate and the employed transfer polymer. We find that hydrophobic polystyrene (PS) as the transfer polymer results in higher photoluminescence enhancement than the more hydrophilic poly(methyl methacrylate) (PMMA). Possible mechanisms for this enhancement such as strain relief, disruption of the strong interaction of SWNTs with the substrate and localized emissive states are discussed.

Investigation of diffusion length distribution on polycrystalline silicon wafers via photoluminescence methods

Characterization of the diffusion length of solar cells in space has been widely studied using various methods, but few studies have focused on a fast, simple way to obtain the quantified diffusion length distribution on a silicon wafer. In this work, we present two different facile methods of doing this by fitting photoluminescence images taken in two different wavelength ranges or from different sides. These methods, which are based on measuring the ratio of two photoluminescence images, yield absolute values of the diffusion length and are less sensitive to the inhomogeneity of the incident laser beam. A theoretical simulation and experimental demonstration of this method are presented. The diffusion length distributions on a polycrystalline silicon wafer obtained by the two methods show good agreement.

Bright Fraction of Single-Walled Carbon Nanotubes through Correlated Fluorescence and Topography Measurements

Correlated measurements of fluorescence and topography were performed for individual single-walled carbon nanotubes (SWNTs) on quartz using epifluorescence confocal microscopy and atomic force microscopy (AFM). Surprisingly, only ~11% of all SWNTs in DNA-wrapped samples were found to be highly emissive on quartz, suggesting that the ensemble fluorescence quantum yield is low because only a small population of SWNTs fluoresces strongly. Qualitatively similar conclusions were obtained from control studies using a sodium cholate surfactant system. To accommodate AFM measurements, excess surfactant was removed from the substrate. Though individual SWNTs on nonrinsed and rinsed surfaces displayed differences in fluorescence intensities and line widths, arising from the influence of the local environment on individual SWNT optical measurements, photoluminescence data from both samples displayed consistent trends.

A short-wavelength infrared emitting multimodal probe for non-invasive visualization of phagocyte cell migration in living mice

For the non-invasive visualization of cell migration in deep tissues, we synthesized a short-wavelength infrared (SWIR) emitting multimodal probe that contains PbS/CdS quantum dots, rhodamine 6G and iron oxide nanoparticles. This probe enables multimodal (SWIR fluorescence/magnetic resonance) imaging of phagocyte cell migration in living mice.

Theranostic applications of carbon nanomaterials in cancer: Focus on imaging and cargo delivery

Carbon based nanomaterials have attracted significant attention over the past decades due to their unique physical properties, versatile functionalization chemistry, and biological compatibility. In this review, we will summarize the current state-of-the-art applications of carbon nanomaterials in cancer imaging and drug delivery/therapy. The carbon nanomaterials will be categorized into fullerenes, nanotubes, nanohorns, nanodiamonds, nanodots and graphene derivatives based on their morphologies. The chemical conjugation/functionalization strategies of each category will be introduced before focusing on their applications in cancer imaging (fluorescence/bioluminescence, magnetic resonance (MR), positron emission tomography (PET), single-photon emission computed tomography (SPECT), photoacoustic, Raman imaging, etc.) and cargo (chemo/gene/therapy) delivery. The advantages and limitations of each category and the potential clinical utilization of these carbon nanomaterials will be discussed. Multifunctional carbon nanoplatforms have the potential to serve as optimal candidates for image-guided delivery vectors for cancer.

Gate-controlled generation of optical pulse trains using individual carbon nanotubes

In single-walled carbon nanotubes, electron–hole pairs form tightly bound excitons because of limited screening. These excitons display a variety of interactions and processes that could be exploited for applications in nanoscale photonics and optoelectronics. Here we report on optical pulse-train generation from individual air-suspended carbon nanotubes under an application of square-wave gate voltages. Electrostatically induced carrier accumulation quenches photoluminescence, while a voltage sign reversal purges those carriers, resetting the nanotubes to become luminescent temporarily. Frequency-domain measurements reveal photoluminescence recovery with characteristic frequencies that increase with excitation laser power, showing that photoexcited carriers provide a self-limiting mechanism for pulsed emission. Time-resolved measurements directly confirm the presence of an optical pulse train synchronized to the gate voltage signal, and flexible control over pulse timing and duration is also demonstrated. These results identify an unconventional route for optical pulse generation and electrical-to-optical signal conversion, opening up new prospects for controlling light at the nanoscale.

The photocurrent and luminescence of carbon nanotubes is governed by excitonic processes with diverse uses in nano-photonics. Here, Jiang et al. generate optical pulses from individual air-suspended carbon nanotubes under an application of square-wave gate voltages with control over pulse timing and duration.

Redox properties of a single (7,5)single-walled carbon nanotube determined by an in situ photoluminescence spectroelectrochemical method

The determination of electronic states of single-walled carbon nanotubes (SWNTs) has been a central issue in science and nanotechnology of carbon nanotubes. We here describe the oxidation and reduction potentials of a single SWNT determined by in situ photoluminescence (PL) spectroelectrochemical measurements. By PL imaging and single SWNT PL spectroscopy, the stepwise quenching behavior of the PL from a single (7,5)SWNT was detected as the outer-applied potentials increased. Based on the analysis of the obtained potential-dependent PL plots using the Nernst equation, the oxidation and reduction potentials of the (7,5) tube are successfully determined as 0.41 V and -0.38 V vs. Ag/AgCl, respectively, which shift from those of the bulk (7,5)SWNTs. We further observed a PL blueshift and narrowing of the line width as the external-applied potential to the single SWNT increases. The present results are important for understanding the electronic properties of a single (n,m)SWNT and its applications.

Electronic structure and chemical nature of oxygen dopant states in carbon nanotubes

We performed low temperature photoluminescence (PL) studies on individual oxygen-doped single-walled carbon nanotubes (SWCNTs) and correlated our observations to electronic structure simulations. Our experiment reveals multiple sharp asymmetric emission peaks at energies 50-300 meV red-shifted from that of the E11 bright exciton peak. Our simulation suggests an association of these peaks with deep trap states tied to different specific chemical adducts. In addition, oxygen doping is also observed to split the E11 exciton into two or more states with an energy splitting <40 meV. We attribute these states to dark states that are brightened through defect-induced symmetry breaking. While the wave functions of these brightened states are delocalized, those of the deep-trap states are strongly localized and pinned to the dopants. These findings are consistent with our experimental observation of asymmetric broadening of the deep trap emission peaks, which can result from interaction between pinned excitons and one-dimensional phonons. Exciton pinning also increases the sensitivity of the deep traps to the local dielectric environment, leading to a large inhomogeneous broadening. Observations of multiple spectral features on single nanotubes indicate the possibility of different chemical adducts coexisting on a given nanotube.

Diffusion-assisted photoexcitation transfer in coupled semiconducting carbon nanotube thin films

We utilize femtosecond transient absorption spectroscopy to study dynamics of photoexcitation migration in films of semiconducting single-wall carbon nanotubes. Films of nanotubes in close contact enable energy migration such as needed in photovoltaic and electroluminescent devices. Two types of films composed of nanotube fibers are utilized in this study: densely packed and very porous. By comparing exciton kinetics in these films, we characterize excitation transfer between carbon nanotubes inside fibers versus between fibers. We find that intrafiber transfer takes place in both types of films, whereas interfiber transfer is greatly suppressed in the porous one. Using films with different nanotube composition, we are able to test several models of exciton transfer. The data are inconsistent with models that rely on through-space interfiber energy transfer. A model that fits the experimental results postulates that interfiber transfer occurs only at intersections between fibers, and the excitons reach the intersections by diffusing along the long-axis of the tubes. We find that time constants for the inter- and intrafiber transfers are 0.2-0.4 and 7 ps, respectively. In total, hopping between fibers accounts for about 60% of all exciton downhill transfer prior to 4 ps in the dense film. The results are discussed with regards to transmission electron micrographs of the films. This study provides a rigorous analysis of the photophysics in this new class of promising materials for photovoltaics and other technologies.

Spectroscopic investigation of electrochemically charged individual (6,5) single-walled carbon nanotubes

Individual single-walled carbon nanotubes (SWNTs) of (6,5) chirality were investigated by means of optical spectroscopy while their charge state was controlled electrochemically. The photoluminescence of the SWNTs was found to be quenched at positive and negative potentials, where the onset and offset varied for each individual SWNT. We propose that differences in the local environment of the individual SWNT lead to a shift of the Fermi energy, resulting in a distribution of the oxidation and reduction potentials. The exciton emission energy was found to correlate with the oxidation and reduction potential. Further proof of a correlation was found by deliberately doping individual SWNTs and monitoring their photoluminescence spectral shift.

Synthesis and optical properties of emission-tunable PbS/CdS core–shell quantum dots for in vivo fluorescence imaging in the second near-infrared window

This paper describes the synthesis and optical properties of PbS/CdS quantum dots for in vivo fluorescence imaging.

Ultrathin gold nanowire-functionalized carbon nanotubes for hybrid molecular sensing

Carbon nanotubes (CNTs) have shown great potential as sensing component in the electrochemical field effect transistor and optical sensors, because of their extraordinary one-dimensional electronic structure, thermal conductivity, and tunable and stable near-infrared emission. However, the insolubility of CNTs due to strong van der Waals interactions limits their use in the field of nanotechnology. In this study, we demonstrate that noncovalent ultrathin gold nanowires functionalized multiwalled carbon nanotube (GNW-CNT) hybrid sensing agents show highly efficient and selective immune molecular sensing in electrochemical and near-infrared photoacoustic imaging methods. A detection limit of 0.01 ng/mL for the alpha-fetoprotein (AFP) antigen with high selectivity is shown. The extraordinary optical absorption, thermal, and electric conductivity of hybrid GNW-CNTs presented in this study could be an effective tactic to integrate imaging, sensing, and treatment functionalities.

Carbon nanomaterials for electronics, optoelectronics, photovoltaics, and sensing

In the last three decades, zero-dimensional, one-dimensional, and two-dimensional carbon nanomaterials (i.e., fullerenes, carbon nanotubes, and graphene, respectively) have attracted significant attention from the scientific community due to their unique electronic, optical, thermal, mechanical, and chemical properties. While early work showed that these properties could enable high performance in selected applications, issues surrounding structural inhomogeneity and imprecise assembly have impeded robust and reliable implementation of carbon nanomaterials in widespread technologies. However, with recent advances in synthesis, sorting, and assembly techniques, carbon nanomaterials are experiencing renewed interest as the basis of numerous scalable technologies. Here, we present an extensive review of carbon nanomaterials in electronic, optoelectronic, photovoltaic, and sensing devices with a particular focus on the latest examples based on the highest purity samples. Specific attention is devoted to each class of carbon nanomaterial, thereby allowing comparative analysis of the suitability of fullerenes, carbon nanotubes, and graphene for each application area. In this manner, this article will provide guidance to future application developers and also articulate the remaining research challenges confronting this field.

Mapping charge transport by electroluminescence in chirality-selected carbon nanotube networks

We demonstrate random network single-walled carbon nanotube (SWNT) field-effect transistors (FETs) in bottom contact/top gate geometry with only five different semiconducting nanotube species that were selected by dispersion with poly(9,9-dioctylfluorene) in toluene. These FETs are highly ambipolar with balanced hole and electron mobilities and emit near-infrared light with narrow peak widths (<40 meV) and good efficiency. We spatially resolve the electroluminescence from the channel region during a gate voltage sweep and can thus trace charge transport paths through the SWNT thin film. A shift of emission intensity to large diameter nanotubes and gate-voltage-dependent photoluminescence quenching of the different nanotube species indicates excitation transfer within the network and preferential charge accumulation on small band gap nanotubes. Apart from applications as near-infrared emitters with selectable emission wavelengths and narrow line widths, these devices will help to understand and model charge transport in realistic carbon nanotube networks.

Optical properties of graphene nanoribbons encapsulated in single-walled carbon nanotubes

We report the photoluminescence (PL) from graphene nanoribbons (GNRs) encapsulated in single-walled carbon nanotubes (SWCNTs). New PL spectral features originating from GNRs have been detected in the visible spectral range. PL peaks from GNRs have resonant character, and their positions depend on the ribbon geometrical structure in accordance with the theoretical predictions. GNRs were synthesized using confined polymerization and fusion of coronene molecules. GNR@SWCNTs material demonstrates a bright photoluminescence both in infrared (IR) and visible regions. The photoluminescence excitation mapping in the near-IR spectral range has revealed the geometry-dependent shifts of the SWCNT peaks (up to 11 meV in excitation and emission) after the process of polymerization of coronene molecules inside the nanotubes. This behavior has been attributed to the strain of SWCNTs induced by insertion of the coronene molecules.

Bright, long-lived and coherent excitons in carbon nanotube quantum dots

Carbon nanotubes exhibit a wealth of unique physical properties. By virtue of their exceptionally low mass and extreme stiffness they provide ultrahigh-quality mechanical resonances, promise long electron spin coherence times in a nuclear-spin free lattice for quantum information processing and spintronics, and feature unprecedented tunability of optical transitions for optoelectronic applications. Excitons in semiconducting single-walled carbon nanotubes could facilitate the upconversion of spin, mechanical or hybrid spin-mechanical degrees of freedom to optical frequencies for efficient manipulation and detection. However, successful implementation of such schemes with carbon nanotubes has been impeded by rapid exciton decoherence at non-radiative quenching sites, environmental dephasing and emission intermittence. Here we demonstrate that these limitations may be overcome by exciton localization in suspended carbon nanotubes. For excitons localized in nanotube quantum dots we found narrow optical lines free of spectral wandering, radiative exciton lifetimes and effectively suppressed blinking. Our findings identify the great potential of localized excitons for efficient and spectrally precise interfacing of photons, phonons and spins in novel carbon nanotube-based quantum devices.

Disorder limited exciton transport in colloidal single-wall carbon nanotubes

We present measurements of S(1) exciton transport in (6,5) carbon nanotubes at room temperature in a colloidal environment. Exciton diffusion lengths associated with end quenching paired with photoluminescence lifetimes provide a direct basis for determining a median diffusion constant of approximately 7.5 cm(2)s(-1). Our experimental results are compared to model diffusion constants calculated using a realistic exciton dispersion accounting for a logarithmic correction due to the exchange self-energy and a nonequilibrium distribution between bright and dark excitons. The intrinsic diffusion constant associated with acoustic phonon scattering is too large to explain the observed diffusion length, and as such, we attribute the observed transport to disorder-limited diffusional transport associated with the dynamics of the colloidal interface. In this model an effective surface potential limits the exciton mean free path to the same size as that of the exciton wave function, defined by the strength of the electron-hole Coulomb interaction.