Individual Single-walled Carbon Nanotubes Research Articles

Growth Termination and Multiple Nucleation of Single-Wall Carbon Nanotubes Evidenced by in Situ Transmission Electron Microscopy.

In order to controllably grow single-wall carbon nanotubes (SWCNTs), a better understanding of the growth processes and how they are influenced by external parameters such as catalyst and gaseous environment is required. Here, we present direct evidence of growth termination of individual SWCNTs and successive growth of additional SWCNTs on Co catalyst particles supported on MgO by means of environmental transmission electron microscopy. Such in situ observations reveal the plethora of solid carbon formations at the local scale while it is happening and thereby elucidate the multitude of configurations resulting from identical external synthesis conditions, which should be considered in the quest for controlled SWCNT growth. Using CO and a mixture of CO and H2 as carbon sources, we show that the growth of SWCNTs terminates with a reduced tube-catalyst adhesion strength. Two main reasons for the cessation are proposed: insufficient active carbon species and a certain amount of stress exerted at the tube-catalyst interface. Interestingly, it was observed that catalyst particles stayed active in terms of nucleating additional solid carbon structures after growth termination of the first SWCNT. These observations elucidate the importance of an in-depth understanding of the role of catalysts and carbon sources in the continued growth of SWCNTs. Furthermore, it serves as a guide for further control of carbon nanostructure synthesis via catalyst engineering and synthesis optimization.

(Invited) Single Carbon Nanotube Tracking Reveals Nanoscale Organization of the Extracellular Space in Live Brain Tissue

Sub-wavelength localizations of single molecules allows super-resolution imaging and a subtle probing of spatio temporal nano-environments in cells [1]. By using biocompatible luminescent single wall carbon nanotubes (SWCNTs) in the near-IR emission [2,3], here we show an approach to directly observe local structures and rheology of the extracellular space in a brain tissue using super-resolution imaging. Upon injection in live rat cerebroventricles [4], individual SWCNTs are followed for tens of minutes in acute slices as they diffuse inside the extracellular space. Because of the interplay between the nanotube geometry and the extracellular space local environment [5], we can extract information about the extracellular space dimension and local viscosity. We find a striking diversity of extracellular space dimensions down to 40 nm, and as well as of local viscosity values. Moreover, by chemically altering the brain extracellular matrix of the live animals before nanotube injection, we reveal that ECS rheology properties are affected, but that extracellular space alterations are local and inhomogeneous at nanoscale dimensions [6].

Quantum Optical Studies on Diazonium Doped Carbon Nanotubes

The intentional incorporation of impurities and defects can serve as a powerful tool for modification of the electronic and optical properties of host nanomaterials and enabling of new functionalities1. Introduction of Sp3 defects by covalent functionalization to single-walled carbon nanotubes (SWCNTs) generates new emission states with energies lower than the optical bandgap (E11) of SWCNTs by 100 to 300 meV2. With controllable emission wavelengths and high quantum yield, these defect states open up new opportunities for the applications of SWCNTs in photonics such as lasing, photon upconversion and single photon emission3. Recently, it has been shown that such doping sites are capable of emitting single photons at room-T, which opens the possibility of building room-T electrically-driven single photon emitters based on doped-SWCNTs. The realization of these devices needs the fine control of doping densities, to the point of introduction of solitary dopants on individual SWCNTs. It also requires the high emission quality from doping states, such as sharp linewidth of the emission spectra and non-blinking of the emission intensity. In this work, we showed a chemical approach capable of creating solitary dopants on SWCNTs of different diameters. Our method was based on the fine control of the doping of surfactant-suspended and poly[9,9-dioctylfluorenyl-2,7-diyl] (PFO) wrapped SWCNTs by different diazonium salts. Quantum optical studies were performed on the diazonium doped-SWCNTs. We first investigated the photoluminescence (PL) spectra of doped-SWCNTs at room temperature and low temperature (4K). Individual, bright and stable emission peaks were observed from doped SWCNTs at room-T. The linewidth of the PL peaks decreased to 200 mirco-eV at 4K. Next, Hanbury Brown-Twiss (HBT) experiment was performed to explore the single photon nature of the emissions. Strong photon ant-bunching behavior with purity above 99% were observed on the doping emission peaks in the range of 1100-1400 nm at RT(295k). [1] Piao, Yanmei. et al. Nature Chem. 2013, 5, 840. [2] Hartmann, Nicolai F. et al. Nanoscale. 2015, 7,20521. [3] Kim, Mijin. et al. J. Phys. Chem. C . 2016, 120, 11268. [4] Ma, Xuedan. et al. Nature Nanotech. 2015, 10, 671.

(Invited) In-Situ Raman Observation of an Isolated Single-Walled Carbon Nanotube to Monitor Highly Localized Plasmon-Induced Reaction

Structural and electrical properties of an individual single-walled carbon nanotube (SWNT) was characterized by in-situ electrochemical surface-enhanced Raman scattering (EC-SERS) measurement. An isolated SWNT was supported on a well-defined Au nanodimer structure, which possesses a localized surface plasmon resonance (LSPR) field at the nanogap. Intense SERS spectra were measured for the SWNT at the gap region. The electronic structural change in an isolated SWNT by photo-indueed oxidation reactions was monitored as the function of the time and the light intensity. The results indicate that EC-SERS technique is a powerful tool for detailed analysis of the local density of the states under the excitation of SWNT by LSPR.

Rheological alteration of erythrocytes exposed to carbon nanotubes.

Single-walled carbon nanotubes (SWNTs) have been increasingly used in a variety of biomedical applications, such as in vivo delivery of drugs and tumor imaging. Potential exposure of SWNTs to human red blood cells (RBCs) may cause serious toxicity including alteration of mechanical properties of cells. The present study investigated the cellular response to exposure of SWNTs with measuring rheological characteristics of RBCs, including hemolysis, deformability, aggregation, and morphological changes. RBCs were exposed to two different dispersion-state samples (i.e. individual SWNTs and bundled SWNTs) in chitosan hydroxyphenyl acetamide (CHPA) solutions. The concentrations of SWNTs were carefully chosen to avoid any hemorheological alterations due to hemolysis. Rheological characteristics were measured using microfluidic-laser diffractometry and aggregometry. Our results show that the bundled SWNTs had higher hemolytic activity than did the individual SWNTs. RBC aggregation apparently decreased as the concentration of SWNTs or incubation time increased. Additionally, bundled SWNTs caused significant alterations in the shape and fusion of RBCs. In conclusion, bundled SWNTs were found to be more toxic than individual SWNTs. These results provide important insights into the interactions between RBCs and SWNTs and will facilitate assessment of the risk of nanomaterial toxicity of blood.

Stable optical trapping and sensitive characterization of nanostructures using standing-wave Raman tweezers

Optical manipulation and label-free characterization of nanoscale structures open up new possibilities for assembly and control of nanodevices and biomolecules. Optical tweezers integrated with Raman spectroscopy allows analyzing a single trapped particle, but is generally less effective for individual nanoparticles. The main challenge is the weak gradient force on nanoparticles that is insufficient to overcome the destabilizing effect of scattering force and Brownian motion. Here, we present standing-wave Raman tweezers for stable trapping and sensitive characterization of single isolated nanostructures with a low laser power by combining a standing-wave optical trap with confocal Raman spectroscopy. This scheme has stronger intensity gradients and balanced scattering forces, and thus can be used to analyze many nanoparticles that cannot be measured with single-beam Raman tweezers, including individual single-walled carbon nanotubes (SWCNT), graphene flakes, biological particles, SERS-active metal nanoparticles, and high-refractive semiconductor nanoparticles. This would enable sorting and characterization of specific SWCNTs and other nanoparticles based on their increased Raman fingerprints.

Carbon Nanotube-Quantum Dot Nanohybrids: Coupling with Single-Particle Control in Aqueous Solution.

A strategy is reported for the controlled assembly of organic-inorganic heterostructures consisting of individual single-walled carbon nanotubes (SWCNTs) selectively coupled to single semiconductor quantum dots (QDs). The assembly in aqueous solution was controlled towards the formation of monofunctionalized SWCNT-QD structures. Photoluminescence studies in solution, and on surfaces at the single nanohybrid level, showed evidence of electronic coupling between the two nanostructures. The ability to covalently couple heterostructures with single particle control is crucial for the design of novel QD-based optoelectronic and light-energy conversion devices.

Benchmark study of the length dependent thermal conductivity of individual suspended, pristine SWCNTs.

The thermal conductivity of individual suspended single-walled carbon nanotubes (SWCNTs) has been theoretically predicated to increase with length but this has never been verified experimentally. This then leads to the question of whether the thermal conductivity saturates to a finite constant value in ultra-long SWCNTs. This paper reports on experimental measurements of the thermal conductivity of individual suspended SWCNTs as a function of the characteristic thermal transport length using the same individual suspended SWCNT sample. Interestingly, at around 360 K, the thermal conductivity first increases with increasing characteristic length and then saturates to a finite constant value at a characteristic length of ∼10 μm. These experimental results provide a fundamental understanding of the phonon transport characteristics in suspended, pristine SWCNTs.

Fingerprinting seamless single-walled carbon nanotube junctions via the migration of encapsulated N2molecules from bottom to top: are arrays of VA-SWNTs continuous?

Structure control such as diameter changes along single-walled carbon nanotubes (SWNTs) can be achieved in arrays of vertically aligned (VA-) SWNTs by switching the feedstock during growth. The local nature of the macroscopic transition from one diameter to another is then questioned as one can either envisage seamless transitions or discontinuous individual SWNTs. Here, we demonstrate that encapsulated molecules can serve as markers to doubtlessly identify seamless interconnections in macroscopic samples. A migration of nitrogen molecules inside the continuous SWNTs is observed using bulk scale measurements on double-layered SWNT arrays synthesized from different carbon/nitrogen feedstocks. The existence of N2 molecules at the top of the SWNT arrays proves that there are continuous SWNTs throughout the double layered arrays with different diameters.

Intramolecular p-i-n junction photovoltaic device based on selectively doped carbon nanotubes

Photovoltaic devices show promising applications in detection and energy fields as well as in next-generation optoelectronic circuits. The use of the ideal photosensitive material and device design are critical for achieving a high-performance photovoltaic device. Here, an intramolecular p-i-n junction photovoltaic device based on selectively doped carbon nanotubes is investigated. In this kind of device, the opposite ends of an individual single-walled carbon nanotube (SWCNT) channel are doped selectively by triethyloxonium hexachloroantimonate (OA) and polyethylene imine (PEI) to obtain stable p- and n- type SWCNT segments respectively, while the middle segment of the SWCNT is kept intrinsic, causing the formation of an intra-tube p-i-n junction for the efficient separation of photogenerated electron-hole pairs. The optical-absorption and electrical testing demonstrate that the OA and PEI can dope the SWCNTs into the stable p- and n- types, respectively. In the dark, the prepared p-i-n junction device behaves as a diode with a high rectification ratio >103 that can be tuned by the gate voltage. Under a 1550-nm monochromatic illumination, the device exhibits a good photovoltaic effect with a large open-circuit voltage of 0.41V and an external power conversion efficiency of ~4.2%. The quantum efficiency of the device is estimated to be as high as ~73%.

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

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

In situ study of single-walled carbon nanotube growth in an environmental scanning electron microscope

Monitoring individual single-walled carbon nanotubes (SWCNTs) during their growth is a highly sought-after goal in view of understanding the processes involved in the nucleation, elongation and termination which ultimately control the diameter and chiral selectivity. Here, we report on the first truly in situ observations of SWCNT growth in an environmental scanning electron microscope (ESEM). The CNT growth from lithographically patterned catalysts was investigated as a function of the catalyst type (Fe, Co or Ni), temperature, type of precursor (ethanol or acetylene), gas phase composition and pressure, and pretreatment conditions, and we report on the most appropriate conditions for SWCNT growth in ESEM conditions. We show that this approach allows the observation at the submicron scale of the different steps of the nanotube synthesis including the catalyst reduction, the growth and percolation of the nanotube network, and the deposition of individual nanotubes grown in the gas phase on the substrate. Despite these obvious advantages, we identified a few limitations which will need to be tackled for fully taking advantage of the approach, for instance for monitoring the growth of individual SWCNTs by ESEM, including the short lifetime of the catalyst nanoparticles, the preference for kite growth (by opposition to surface growth) and the influence of the electron beam on the nanotube growth.

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.

Zero dark leakage current single-walled carbon nanotube diodes

We describe measurements of diode leakage currents in p-n and p-i-n diodes formed along individual single-walled carbon nanotubes (SWNTs) in the ranges well below the direct detection limit. When cooled, these diodes exhibit leakage currents down in the range of 10−25 A or equivalent to 1 electron/19 days. To verify our measurement of such low leakage currents, we use the photovoltaic property under varying temperatures to extract the dark diode leakage currents. Since nanotubes are sensitive in the near IR spectrum, these diodes show promise as zero dark current noise, near-infrared detectors.

Scanning Electrochemical Microscopy of Carbon Nanomaterials and Graphite.

Carbon materials are tremendously important as electrode materials in both fundamental and applied electrochemistry. Recently, significant attention has been given not only to traditional carbon materials, but also to carbon nanomaterials for various electrochemical applications in energy conversion and storage as well as sensing. Importantly, many of these applications require fast electron-transfer (ET) reactions between a carbon surface and a redox-active molecule in solution. It, however, has not been well understood how heterogeneous ET kinetics at a carbon/solution interface is affected by the electronic structure, defect, and contamination of the carbon surface. Problematically, it is highly challenging to measure the intrinsic electrochemical reactivity of a carbon surface, which is readily passivated by adventitious organic contaminants. This Account summarizes our recent studies of carbon nanomaterials and graphite by scanning electrochemical microscopy (SECM) not only to reveal the fast ET kinetics of simple ferrocene derivatives at their graphitic surfaces, but also to obtain mechanistic insights into their extraordinary electrochemical reactivity. Specifically, we implemented new principles and technologies to reliably and reproducibly enable nanoscale SECM measurements. We took advantage of a new SECM imaging principle to resolve the high reactivity of the sidewall of individual single walled carbon nanotubes. In addition, we developed SECM-based nanogap voltammetry to find that monolayer graphene grown by chemical vapor deposition yields an unprecedentedly high standard ET rate constant, k(0), of ≥25 cm/s, which was >1000 times higher than that reported in the literature. Remarkably, the nonideal asymmetry of paired nanogap voltammograms revealed that the high reactivity of graphitic surfaces is compromised by their contamination with airborne hydrocarbons. Most recently, we protected the clean surface of highly oriented pyrolytic graphite from the airborne contaminants during its exfoliation and handling by forming a water adlayer to obtain a reliable k(0) value of ≥12 cm/s from symmetric pairs of nanogap voltammograms. We envision that SECM of clean graphitic surfaces will enable us to reliably address not only effects of their electronic structures on their electrochemical reactivity, but also the activity of carbon-based or carbon-supported electrocatalysts for fuel cells and batteries.

Inkjet printed carbon nanotubes in short channel field effect transistors: influence of nanotube distortion and gate insulator interface modification

We report on the effects of mechanical distortion and gate insulator-semiconductor interface modification on the electronic transport characteristics of inkjet printed short channel length single walled carbon nanotube (SWCNT) transistors with Al2O3 top gate insulators. In these transistors, which are typically ambipolar, the average nanotube length is greater than the source-drain (S/D) spacing resulting in individual SWCNTs spanning the entire channel length. Mechanical distortion of the nanotubes due to bending near source and drain contacts when they are not recessed is found to suppress electron transport and transform the ambipolar transistors into p-type devices. Inclusion of printed interfacial polymer layers such as poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) between the SWCNTs and Al2O3 also results in p-type doping and reductions in electron transport. We discuss mechanisms responsible for these effects.

Effect of CNT length and structural density on viscoelasticity of buckypaper: A coarse-grained molecular dynamics study

Carbon nanotube (CNT) buckypapers, having exceptional mechanical and electrical properties, have been reported to demonstrate frequency-invariant and temperature-invariant viscoelastic properties. In an attempt to provide an in-depth insight into the viscoelasticity of buckypapers with (5, 5) single-walled CNTs (SWCNTs), we perform coarse-grained non-equilibrium molecular dynamics simulations to investigate the effects of oscillatory shear strain amplitude, buckypaper's density, and length of individual SWCNTs on the viscoelastic properties. SWCNT buckypapers exhibit linear viscoelasticity over shear strain amplitude from 0.03 to 0.05. Higher density SWCNT buckypapers can result in larger dynamic stiffness and a higher loss factor of up to ∼0.29. In the frequency-independent regime (≤1 GHz), increasing the length of individual SWCNTs causes a very slight decrease of elastic properties and has minor influence on the viscous mode. Thus, this study provides deep insight into the viscoelasticity of (5, 5) SWCNT buckypapers and demonstrates controllability of the excellent energy dissipation potential of buckypapers, and can thus help us design new energy dissipation devices from carbon nanomaterials.

Terahertz Spectroscopy of Individual Single-Walled Carbon Nanotubes as a Probe of Luttinger Liquid Physics.

Luttinger liquid theory predicts that collective electron excitations due to strong electron-electron interactions in a one-dimensional (1D) system will result in a modification of the collective charge-propagation velocity. By utilizing a circuit model for an individual metallic single-walled carbon nanotube as a nanotransmission line, it has been shown that the frequency-dependent terahertz impedance of a carbon nanotube can probe this expected 1D Luttinger liquid behavior. We excite terahertz standing-wave resonances on individual antenna-coupled metallic single-walled carbon nanotubes. The terahertz signal is rectified using the nanotube contact nonlinearity, allowing for a low-frequency readout of the coupled terahertz current. The charge velocity on the nanotube is determined from the terahertz spectral response. Our measurements show that a carbon nanotube can behave as a Luttinger liquid system with charge-propagation velocities that are faster than the Fermi velocity. Understanding what determines the charge velocity in low-dimensional conductors is important for the development of next generation nanodevices.

Quantitative Conductive Atomic Force Microscopy on Single-Walled Carbon Nanotube-Based Polymer Composites.

Conductive atomic force microscopy (C-AFM) is a valuable technique for correlating the electrical properties of a material with its topographic features and for identifying and characterizing conductive pathways in polymer composites. However, aspects such as compatibility between tip material and sample, contact force and area between the tip and the sample, tip degradation and environmental conditions render quantifying the results quite challenging. This study aims at finding the suitable conditions for C-AFM to generate reliable, reproducible, and quantitative current maps that can be used to calculate the resistance in each point of a single-walled carbon nanotube (SWCNT) network, nonimpregnated as well as impregnated with a polymer. The results obtained emphasize the technique's limitation at the macroscale as the resistance of these highly conductive samples cannot be distinguished from the tip-sample contact resistance. Quantitative C-AFM measurements on thin composite sections of 150-350 nm enable the separation of sample and tip-sample contact resistance, but also indicate that these sections are not representative for the overall SWCNT network. Nevertheless, the technique was successfully used to characterize the local electrical properties of the composite material, such as sample homogeneity and resistance range of individual SWCNT clusters, at the nano- and microscale.

Cavity-enhanced Raman microscopy of individual carbon nanotubes.

Raman spectroscopy reveals chemically specific information and provides label-free insight into the molecular world. However, the signals are intrinsically weak and call for enhancement techniques. Here, we demonstrate Purcell enhancement of Raman scattering in a tunable high-finesse microcavity, and utilize it for molecular diagnostics by combined Raman and absorption imaging. Studying individual single-wall carbon nanotubes, we identify crucial structural parameters such as nanotube radius, electronic structure and extinction cross-section. We observe a 320-times enhanced Raman scattering spectral density and an effective Purcell factor of 6.2, together with a collection efficiency of 60%. Potential for significantly higher enhancement, quantitative signals, inherent spectral filtering and absence of intrinsic background in cavity-vacuum stimulated Raman scattering render the technique a promising tool for molecular imaging. Furthermore, cavity-enhanced Raman transitions involving localized excitons could potentially be used for gaining quantum control over nanomechanical motion and open a route for molecular cavity optomechanics.