Single-walled Carbon Nanotubes Material Research Articles

Dopant-Modulated Conjugated Polymer Enrichment of Semiconducting SWCNTs.

Conjugated polymer extraction (CPE) is a low-cost, scalable process that can enrich single-walled carbon nanotube (SWCNT) materials in organic media. For other separation methods in aqueous phases, redox chemistry and/or pH control dramatically affect the sorting process of the SWCNTs. We have previously determined that the CPE process can be fine-tuned by adjusting the pH on the tube surface. Here, we systematically studied the effect of redox chemistry on the CPE process by adding organic p-/n-dopants. At a very strong p-/n-doping level, static repulsions dominated the interactions between the tubes and the CPE lost selectivity. When the doping level changed from a medium p-doping to a neutral state, the yield of CPE increased and the selectivity was compromised. We also observed chiral selectivity when a weak p-dopant was used. A photoluminescence excitation mapping under different titration conditions provided more insight into the doping level of the tubes relative to their diameters, chiralities, and redox potentials. We proposed a mechanism for the CPE process. The semiconducting and metallic tubes are separated because of their different solubilities, which are determined by the bundling energy between the tubes and are related to their doping level in polymer solutions.

Gate bias stress effect in single-walled carbon nanotubes field-effect-transistors

This paper deals with the effect of negative gate bias stress with different times on the Carbon Nanotube Field Effect Transistor (CNTFET) based gas sensors, obtained using Single-Walled Carbon Nanotubes (SWCNTs) mats as channel. The effects of Gate Bias Stress (GBS) on transistor performance are important; hence it is necessary to understand how our device behaves under bias stress. To perform this work, I-V characteristics, are taken by applying a constant gate voltage (Vgs) for an extended time. I-V characteristics after stress exhibited an identical shape to the one before stress with an important degradation of the current that flows through the transistors. A possible source of the Bias Stress Effect (BSE) is traps in the semiconductor itself which are spatially and energetically isolated due to the disorder of SWCNTs. This hypothesis will be confirmed by the saturation of the drain current shift ∆I after a prolonged period of time. All isolated trap states are saturated and only electrical traps still active and give us the original I-V characteristics.

Indexing the Quality of Single-Wall Carbon Nanotube Dispersions Using Absorption Spectra

We have found that absorption spectra of single-wall carbon nanotube (SWCNT) dispersions can be accurately represented as linear combinations of two underlying spectra. One, assigned to purified well-dispersed SWCNTs, is structured; the other, assigned to impurities and aggregates, is diffuse. To assess the quality of SWCNT dispersions, a small set of visible and short-wave IR spectra are first measured for a dispersed sample as it is purified by centrifugation. Those data are analyzed using a simple arithmetic process to estimate the pair of underlying component spectra. Then the spectra of other sample dispersions from the same SWCNT source are fit as linear combinations of the two component spectra, giving coefficients that provide a simple metric for sample quality (the S-index). We have validated this approach with measurements of fluorescence intensities, particle abundances, length distributions, and emissive aggregates in samples containing SWCNTs from three growth sources dispersed by high-shear mixing or tip sonication and then purified by moderate centrifugation, ultracentrifugation, or magnetic processing. Our absorption spectral analysis correlates closely with results from other characterization techniques. One unexpected outcome of our analyses is the estimate that no more than 7% by mass of raw SWCNT material used to prepare aqueous sodium deoxycholate suspensions becomes well-dispersed SWCNTs.

Passively Q-switched mode-locked low threshold Tm, Ho: LLF laser with an single walled carbon nanotubes saturable absorber

Employing single walled carbon nanotubes (SWCNT) grown by the vertical growth method as a saturable absorber for the initiation of the pulse generation, and designing a low threshold resonant cavity, we demonstrate a stable passively Q-switched mode-locked (QML) Tm, Ho:LiLuF4 solid-state laser with low threshold for the first time. With wavelength tunable Ti:sapphire solid laser operating at 785 nm as a pumping source, continuous-wave (CW) absorbed pump thresholds of 52, 59 and 62 mW are obtained by using 1.5%, 3% and 5% output coupled mirrors respectively. In this case, the maximum output powers are 645, 828 and 940 mW respectively, whose corresponding slope deficiencies are 31.02%, 39.16% and 43.78%, respectively. When the SWCNT-SAs is inserted in the cavity, the cavity loss is further increased, so the laser threshold is improved. Employing the 1.5% output mirror, a laser threshold is obtained to be as low as 85 mW, but the maximum laser output power is only 70 mW, corresponding slope efficiency is 3.42%; employing the 3% output coupling mirror, the laser threshold is obtained to be as low as 99 mW, the maximum output power is 154 mW, and the corresponding slope efficiency is 8.47%; employing the 5% output mirror, owing to the loss in the cavity being too large, the QML operation cannot be achieved. The output power of the 3% output mirror is twice higher than that of the 1.5% output mirror, but the laser threshold difference is only 14 mW. With a comprehensive analysis, we use the 3% output mirror. In this case, a stable QML operation with a threshold of 250 mW is obtained. When the absorption pump power is 1.85 W, the maximum output power is 154 mW with a typical Q-switched pulse envelope width of 300 s, which is corresponding to a 178.6 MHz of the mode-locked frequency. The modulation depth in Q-switching envelope is close to 100%. According to the definition of the rise time and considering the symmetric shape of the mode locked pulse, we could assume the duration of the pulse to be approximately 1.25 times more than the rise time of the pulse. So the width of the mode locked pulse is estimated to be about 663 ps. The results show that the SWCNT is a promising SA for QML solid-state laser with the 2 m wavelength. In the later stage, we increase the pump power, optimize the quality of the SWCNT material, and compensate for the dispersion in the cavity. It is expected to achieve a stable continuous mode-locking operation, and obtain a femtosecond mode-locked ultrashort pulse output. The mode-locked mid-infrared pulses have a lot of potential applications such as ultrafast molecule spectroscopy, the generation of mid-IR pulse, laser radar, atmospheric environment monitoring, etc.

Perspectives of Single-Wall Carbon Nanotube Production in the Arc Discharge Process

<p>Single-wall carbon nanotubes (SWNTs) promise wide applications in many technical fields. As a result purified SWNT material is sold now on the West market at more than $1000 per 1 gram. Thus developing an effective technology for SWNTs production rises to a very important scientific problem. The perspectives of three existing methods providing raw material in the technology of SWNT production have been analyzed. They are i) pulsed laser evaporation of graphite/metal composites, ii) evaporation of graphite electrodes with metal content in the arc discharge process, and iii) catalytic decomposition of the mixture of CO and metal carbonyl catalyst precursor. The observed dynamics of SWNT market points to replacing the laser method of SWNTs production by the arc process. The conclusion has been made that the technology based on the arc process will be the major one for the fabrication of purified SWNTs at least for the next five years. A reliable estimation of a low price limit of SWNTs was derived from a comparison of two technologies based on the arc discharge process: the first one is the production of SWNTs and the second one is the production of a fullerene mixture С<sub>60</sub> + С<sub>70</sub>. The main conclusion was made that the price of purified SWNTs should always be more by 2-3 times the price of fullerene mixture. The parameters of a lab-scale technology for the production of purified SWNTs are listed. A large-scale application of the developed technology is expected to reduce the price of purified SWNTs by approximately ten times. The methods now employed for the characterization of products containing SWNTs are briefly observed. It is concluded that electron microscopy, thermogravimetric analysis, absorption and Raman spectroscopy, measurement of the specific surface aria, optical microscopy – each in separation is not enough for extensive characterization of a sample containing SWNTs, and all these methods should be used together.</p>

Calculating Mechanics Characteristics of Single-Walled Carbon Nanotube Materials by Finite Element Method

In this paper, we establish the mechanical property analysis of Single-walled Carbon Nanotubes (SWCNTs) modified beam element model based on the molecular structural mechanics method. Then we study the mechanical properties of their radial direction characteristics using the finite element software Abaqus. The model simulated the different bending stiffness with rectangular section beam elements C-C chemical force field. When the graphene curled into arbitrary chirality of SWCNTs spatial structure, the adjacent beam position will change the moment of inertia of the section of the beam. Compared with the original beam element model and the calculation results, we found that the established model largely reduced the overestimate of the original model of mechanical properties on the radial direction of the SWCNTs. At the same time, compared with other methods available in the literature results and the experimental data, the results can be in good agreement.

Scaling up single-wall carbon nanotube laser annealing: effect on electrical resistance and hydrogen adsorption

Electrical resistance and hydrogen adsorption of laser-annealed single-wall carbon nanotube mats.

Covalently Functionalized SWCNTs as Tailored p-Type Dopants for Perovskite Solar Cells.

The covalent functionalization of (7,6)-enriched single-walled carbon nanotubes (SWCNTs) with oligophenylenevinylene (OPV) moieties terminating with a dimethylamino group is proposed as an efficient way to enhance the affinity of CNTs with spiro-MeOTAD in perovskite-based solar cells. The evidence of SWCNTs functionalization and the degree of OPV substitution on SWCNTs are established from TGA, XPS, TEM, and Raman techniques. Our tailored doping materials afford photovoltaic performances in line with conventional Li-doped spiro-MeOTAD, showing at the same time a significantly improved chemical stability of the perovskite component over time. Furthermore, the comparison of the photovoltaic performances with those obtained with nonfunctionalized SWCNTs suggest that the presence of the organic appends ensures highly reproducible PV performances. These results demonstrate the suitability of this functionalized SWCNT material as a valid doping agent for spiro-MeOTAD, representing a viable alternative to the conventional Li salt.

Semiconducting response in single‐walled carbon nanotubes filled with cadmium chloride

AbstractHere, we report on filling of single‐walled carbon nanotubes (SWCNTs) with cadmium chloride by a capillary filling method. Changes in the electronic properties of the filled nanotubes were traced by multifrequency Raman spectroscopy using seven excitation laser wavelengths. A detailed analysis of the Raman response of pristine and filled nanotubes was conducted. The fitting of the Raman spectra of either type of SWCNT material allowed comparing the modifications in the electronic structure of the filled SWCNTs with certain diameters. The encapsulated cadmium chloride led to dominating semiconducting and suppressed metallic characteristics in the spectroscopic response of the filled SWCNTs.

Towards nanoreliability of sensors incorporating interfaces between single-walled carbon nanotubes and metals: molecular dynamics simulations and in situ experiments using electron microscopy

In this paper we present results of our recent efforts to understand the mechanical interface behaviour of single-walled carbon nanotubes (SWCNTs) embedded in metal matrices. We conducted experimental pull-out tests of SWCNTs embedded in Pd and found maximum forces in the range F ≈ (10 to 65) nN. These values are in good agreement with forces obtained from molecular dynamics simulations taking into account surface functional groups (SFGs) covalently linked to the SWCNT material. The dominant failure mode in experiment is a SWCNT rupture, which can be explained with the presence of SFGs. For further in depth investigations, we present a tensile actuation test system based on a thermal actuator to perform pull-out tests inside a transmission electron microscope with the objective to obtain in situ images of SWCNT–metal interfaces under mechanical loads at the atomic scale. First experiments confirmed the presence of suspended thin metal electrodes to embed SWCNTs. These suspended thin metal electrodes are electron transparent at the designated SWCNT locations. Actuator movements were evaluated by digital image correlation and we observed systematic actuator movements. Although significant image drifts occured during actuation, we achieved atomic resolution of the metal electrode and stable movement in the focal plane of the electron microscope.

Melt mixed SWCNT-polypropylene composites with very low electrical percolation

Abstract Singlewalled carbon nanotube material of the type TUBALL™ (OCSiAl) was used to prepare composites with polypropylene by melt mixing using a conical twin screw micro-compounder. The compression moulded composites showed electrical percolation between 0.075 and 0.1 wt % and achieved volume resistivity values lower than 1 kOhm-cm already at 0.8 wt % loading. Light microscopy and scanning electron microscopy revealed good distribution and dispersion into small diameter bundles as well as retained high nanotube length. In connection with the very low percolation threshold this indicates that the SWCNT material shows an exceptionally good dispersibility which may be due to relatively high nanotube diameters with a mean value of 1.6 nm. In tensile tests already 0.1 wt % nanotube additions resulted in slight increase in Young’s modulus and maximum stress. Tuball™ SWCNT material seems to be very promising for conductivity enhancement.

Alkane Filled Single-Wall Carbon Nanotubes: Endohedral Volume Control for Improved Nanotube Properties

Liquid-phase processing of single-wall carbon nanotubes (SWCNTs) generally results in the exposure of their core to the environment (opening) due to the energy input necessary for purification and solubilization, and this often affects the SWCNT properties [1-3]. For instance, for aqueous processing, SWCNTs are routinely getting filled with water, which leads to significant redshifts to, and inhomogeneous broadening of, the electronic transitions of the SWCNTs, as well as a substantial decrease of their fluorescence quantum efficiency [1]. Selection of (remaining) empty (end-capped) SWCNTs to avoid these detrimental effects is possible by means of ultracentrifugation, but is a natively low yield process [4]. In this work, adverse filling is prevented by intentionally prefilling the SWCNTs with alkanes, serving as a passive and highly homogeneous spacer. In contrast to water and other encountered fillants, the alkane core is of low dielectric nature and therefore only weakly affects the electronic structure of the SWCNTs, and hence effectively simulates empty core conditions. This yields highly resolved optical spectra with strongly shifted peak positions compared to water filled SWCNTs (see Figure 1), and fluorescence efficiencies approaching those of empty SWCNTs. It is demonstrated that this can be realized using a wide variety of linear as well as cyclic alkanes, in combination with various SWCNT materials having very different diameter distributions.

(Invited) Thermoelectric Properties of Semiconducting Single-Walled Carbon Nanotube Networks

Nanostructured organic semiconductors (OSCs), including single-walled carbon nanotubes (SWCNTs), offer a number of intriguing technological characteristics for thermoelectric applications, such as earth-abundant raw materials, low-cost deposition, and flexible form factors. We will initially present a series of experiments focused on understanding the thermoelectric performance of enriched semiconducting SWCNT networks dispersed in a wide bandgap semiconducting polymer matrix, followed by more recent work aimed at understanding the role that the semiconducting polymer plays in the observed transport properties. Rational choice of the starting SWCNT material and the semiconducting polymer allows us to sensitively tune the s-SWCNT diameter and band gap distributions within the composites. Consistent with theoretical calculations that consider the density of electronic states in individual s-SWCNTs, we observe a distinct dependence of the thermopower and thermoelectric power factor on the bandgap (or diameter) of the carbon nanotubes. We have measured large thermopower values (as high as ~2,500 µV/K for s-SWCNT networks with very low electrical conductivity) and impressive thermoelectric power factor as large as ~350 µW/m·K2. By varying the carrier density injected into the s-SWCNT networks by a stable charge-transfer dopant, we are able to probe the relationship between the electrical conductivity and Seebeck coefficient (thermopower) in the s-SWCNT networks as a function of the carrier density and position of the Fermi energy. For carbon nanotubes prepared by high-pressure carbon monoxide (HiPCO) conversion, as we tune the carrier density, we are able to maintain a thermopower above 100 µV/K over almost the entire range of hole densities, corresponding to conductivities up to 40,000 S/m, resulting in a thermoelectric power factor of >300 µW/m·K2. These studies suggest that the low dimensionality of the SWCNTs has a stronger impact on the electrical conductivity than the thermopower, implying that they are less strongly coupled in these systems than is observed for compound inorganic semiconductors. By modifying an approach that allows us to strip the dispersing polymer from the s-SWCNTs we were able to demonstrate that the polymer appears to play no role in modifying the barriers to electrical transport present at tube-tube junctions, and simply controls the extent of nanotube bundling and thereby the surface area available to the charge-transfer dopant. This study also indicates that densification of the s-SWCNT network results in a two-fold enhancement of the thermoelectric power factor, suggesting that careful control of the amount and nature of the matrix material is required for high-performance s-SWCNT thermoelectric materials. Finally, we present a data from sensitive transport measurement technique, based on a microfabricated silicon nitride thermal isolation platform, to probe transport in the s-SWCNT networks, showing that the thermoelectric figure of merit (zT) is positively correlated with the measurement temperature, increasing by a factor of ~2.5 from 300 K to 350 K. These observations demonstrate the ability to exert exquisite control of the thermoelectric performance by controlling the composition of the s-SWCNT network and tuning the carrier density (i.e., Fermi energy), and touts SWCNTs as an avenue for realizing thermally stable room temperature thermoelectric devices fashioned from inexpensive and abundant organic constituents.

Resistive sensing of gaseous nitrogen dioxide using a dispersion of single-walled carbon nanotubes in an ionic liquid

Single-walled carbon nanotubes (SWCNTs) were dispersed in an imidazolium-based ionic liquid (IL) and investigated in terms of structural quality, surface functionalization and inter-CNT force. Analysis by field emission electron microscopy and transmission electron microscopy shows the IL layer to coat the SWNTs, and FTIR and Raman spectroscopy confirm strong binding of the ILs to the SWNTs. Two kinds of resistive sensors were fabricated, one by drop casting of IL-wrapped SWCNTs, the other by conventional dispersion of SWCNTs. Good response and recovery to NO2 is achieved with the IL-wrapped SWCNTs material upon UV-light exposure, which is needed because decrease the desorption energy barrier to increase the gas molecule desorption. NO2 can be detected in the 1–20ppm concentration range. The sensor is not interfered by humidity due to the hydrophobic tail of PF6 (ionic liquid) that makes our sensor highly resistant to moisture.

Mechanical characterization of suspended strips of meshed single-walled carbon nanotubes

A thin film of single walled carbon nanotube (SWCNT) mesh has good potential to integrate the existing electromechanical functions with flexible devices. In this paper, SWCNT mats are transferred to a patterned polymer SU-8 substrate using a wet contact print method, forming a suspended bridge over a groove in the substrate. The front edge of a tipless AFM cantilever loads the suspension at the centerline, causing it to deform into a V-shape by mixed bending and stretching. The mechanical response of load versus AFM displacement is fitted to a linear elastic model to extract the average elastic modulus. Reversible loading-unloading shows little or no permanent damage due to mechanical loads.

Enhanced Electrical Conductivity in Extruded Single-Wall Carbon Nanotube Wires from Modified Coagulation Parameters and Mechanical Processing.

Single-wall carbon nanotubes (SWCNTs) synthesized via laser vaporization have been dispersed using chlorosulfonic acid (CSA) and extruded under varying coagulation conditions to fabricate multifunctional wires. The use of high purity SWCNT material based upon established purification methods yields wires with highly aligned nanoscale morphology and an over 4× improvement in electrical conductivity over as-produced SWCNT material. A series of eight liquids have been evaluated for use as a coagulant bath, and each coagulant yielded unique wire morphology based on its interaction with the SWCNT-CSA dispersion. In particular, dimethylacetamide as a coagulant bath is shown to fabricate highly uniform SWCNT wires, and acetone coagulant baths result in the highest specific conductivity and tensile strength. A 2× improvement in specific conductivity has been measured for SWCNT wires following tensioning induced both during extrusion via increased coagulant bath depth and during solvent evaporation via mechanical strain, over that of as-extruded wires from shallower coagulant baths. Overall, combination of the optimized coagulation parameters has yielded acid-doped wires with the highest reported room temperature electrical conductivities to date of 4.1-5.0 MS/m and tensile strengths of 210-250 MPa. Such improvements in bulk electrical conductivity can impact the adoption of metal-free, multifunctional SWCNT materials for advanced cabling architectures.

Viscoelasticity of Single-Walled Carbon Nanotubes in Unsaturated Polyester Resin: Effects of Purity and Chirality Distribution

The recent commercialization of single-walled carbon nanotubes (SWNT) with controlled chirality distributions has created new opportunities for producing SWNT materials with tailored electrical properties. However, there has been relatively little research on understanding the effects of chirality distribution and SWNT purity on the two main determinants of nanocomposite mechanical properties: dispersion microstructure and nanotube–resin interactions. To establish a framework for comparing dispersion and interactions, we investigated the viscoelasticity of four SouthWest NanoTechnologies’ SWNT products: a low and high purity semiconducting grade and a low and high purity metallic grade. Optical microscopy of the dispersions did not show any significant differences between the SWNT types. However, analysis of the dispersions’ viscoelastic properties revealed the difference in dispersion microstructure and the relative strength of SWNT–resin interactions. While all four products had a similar percolation th...

Hybrid Theranostic Platform for Second Near-IR Window Light Triggered Selective Two-Photon Imaging and Photothermal Killing of Targeted Melanoma Cells.

Despite advances in the medical field, even in the 21st century cancer is one of the leading causes of death for men and women in the world. Since the second near-infrared (NIR) biological window light between 950 and 1350 nm offers highly efficient tissue penetration, the current article reports the development of hybrid theranostic platform using anti-GD2 antibody attached gold nanoparticle (GNP) conjugated, single-wall carbon nanotube (SWCNT) for second near-IR light triggered selective imaging and efficient photothermal therapy of human melanoma cancer cell. Reported results demonstrate that due to strong plasmon-coupling, two-photon luminescence (TPL) intensity from theranostic GNP attached SWCNT materials is 6 orders of magnitude higher than GNP or SWCNT alone. Experimental and FDTD simulation data indicate that the huge enhancement of TPL intensity is mainly due to strong resonance enhancement coupled with the stronger electric field enhancement. Due to plasmon coupling, the theranostic material serves as a local nanoantennae to enhance the photothermal capability via strong optical energy absorption. Reported data show that theranostic SWCNT can be used for selective two-photon imaging of melanoma UACC903 cell using 1100 nm light. Photothermal killing experiment with 1.0 W/cm(2) 980 nm laser light demonstrates that 100% of melanoma UACC903 cells can be killed using theranostic SWCNT bind melanoma cells after just 8 min of exposure. These results demonstrate that due to plasmon coupling, the theranostic GNP attached SWCNT material serves as a two-photon imaging and photothermal source for cancer cells in biological window II.

Bench-top aqueous two-phase extraction of isolated individual single-walled carbon nanotubes

Isolation and purification of single-walled carbon nanotubes (SWCNTs) are prerequisites for their implementation in various applications. In this work, we present a fast (∼5 min), low-cost, and easily scalable bench-top approach to the extraction of high-quality isolated SWCNTs from bundles and impurities in an aqueous dispersion. The extraction procedure, based on aqueous two-phase (ATP) separation, is widely applicable to any SWCNT source (tested on samples up to 1.7 nm in diameter) and independent of defect density, purity, diameter, and length. The extracted dispersions demonstrate that the removal of large aggregates, small bundles, and impurities is comparable to that by density gradient ultracentrifugation, but without the need for high-end instrumentation. Raman and fluorescence-excitation spectroscopy, single-nanotube fluorescence imaging, atomic force and transmission electron microscopy, and thermogravimetric analysis all confirm the high purity of the isolated SWCNTs. By predispersing the SWCNTs without sonication (only gentle stirring), full-length, pristine SWCNTs can be isolated (tested up to 20 μm). Hence, this simple ATP method will find immediate application in the generation of SWCNT materials for all levels of nanotube research and applications, from fundamental studies to high-performance devices.

Intraexciton Transitions Observed in High Stability Doped Single-Wall Carbon Nanotube Films and Solutions

Normally hidden excited state transitions were observed in large diameter single-walled carbon nanotubes for the first time by controlling the degree of doping. With increased doping, the normally observed photoinduced bleach (PB) signal associated with the creation of excitons switched to a photoinduced absorption (PA). A delay in the onset of the PA signal made it clear that the absorption was due to an intraexcitonic transition. The study was afforded by the development of novel methods that allowed samples to be reproducibly and stably prepared in specific states. Our results further the understanding of the photophysics of large diameter single-walled carbon nanotube materials, which are promising for a variety of optoelectronic applications.