Diameter Distribution Of Nanotubes Research Articles

Residence time effect on single-walled carbon nanotube synthesis in an aerosol CVD reactor

• Residence time in aerosol CVD was tuned while nanotube nucleation maintained. • Carbon nanotube length and I G /I D ratio increase with a higher residence time. • The increase in length and defectiveness drops equivalent sheet resistance. • Effect of vortices, diffusion, and thermophoresis on nanotube yield was shown. • SWCNT/HAuCl 4 thin films with equivalent sheet resistance of 51 Ω/□ were obtained. We examine an effect of residence time on the growth of single-walled carbon nanotubes (SWCNTs) in an aerosol (floating catalyst) chemical vapor deposition (CVD) process using CO as a carbon source and ferrocene as a catalyst precursor. The key feature of aerosol CVD reactors, namely stabilization of fine nanoparticles by an extreme dilution, limits the method for catalyst evolution studies. We show an approach to examine the role of the residence time while maintaining all the processes preceding the nanotube growth (catalyst formation, nanotube nucleation). Using the diameter distribution of nanotubes as a fingerprint of the SWCNT nucleation stage, we have proven the latter to be unaffected by changes in the residence time. Using SEM observations, we have revealed a quite intuitive but inspiring correlation between the carbon nanotube length and residence time. We have also found a decrease in the SWCNT yield caused by the drop in the aerosol concentration, which could be attributed to the gas-phase losses as well as the shift in the catalyst activation degree. Nevertheless, the trends observed allowed us to reach a tenfold decrease in an equivalent sheet resistance (sheet resistance of a film with 90% transmittance at 550 nm) by a threefold increase in the residence time at optimal CO 2 concentration. SWCNT films with the equivalent sheet resistance as low as 245 Ω/□ and 51 Ω/□ (for pristine and doped carbon nanotubes, correspondingly) ensure the promising future of the proposed strategy for optimization of both aerosol CVD reactors and SWCNT-based films for optoelectronic applications.

Enhancement of photocurrent response for self-ordered Nb2O5 nanotubes synthesized at room temperature

The major challenge regarding new photocatalysts is to obtain a semiconductor with high and stable photocurrent from inexpensive and abundant materials. Niobium oxide meets some of these requirements, but its photocurrent is usually very low. Therefore, this study introduces the synthesis and characterization of self-organized Nb2O5 nanotubes (Nb2O5Nt) presenting a high photocurrent density of 0.04 mA cm−2 under incidence of artificial sunlight, of 100 mW cm−2, and potential of 1.5 V without further surface modification. The best catalyst was obtained through anodization of a Nb foil at 90 V for 2 h in a one-compartment and two-electrodes cell at room temperature, using glycerol, H2O, and NH4F as the electrolyte, followed by annealing at 450 °C for 60 min. The water concentration was the most important factor to obtain the desired morphology with a good distribution of nanotube diameters, high UV absorption, low bang gap (3.2 eV) compared to the observed for the Nb2O5 semiconductor (3.4 eV), and great photocurrent density. Such properties turn the Nb2O5Nt into a promising catalyst for direct use in photochemical reactions or as a good platform yet to be combined in heterojunctions or applied for doping and decoration.

Charge Transport in Mixed Semiconducting Carbon Nanotube Networks with Tailored Mixing Ratios.

The ability to prepare uniform and dense networks of purely semiconducting single-walled carbon nanotubes (SWNTs) has enabled the design of various (opto-)electronic devices, especially field-effect transistors (FETs) with high carrier mobilities. Further optimization of these SWNT networks is desired to surpass established solution-processable semiconductors. The average diameter and diameter distribution of nanotubes in a dense network were found to influence the overall charge carrier mobility; e.g., networks with a broad range of SWNT diameters show inferior transport properties. Here, we investigate charge transport in FETs with nanotube networks comprising polymer-sorted small diameter (6,5) SWNTs (0.76 nm) and large diameter plasma torch SWNTs (1.17-1.55 nm) in defined mixing ratios. All transistors show balanced ambipolar transport with high on/off current ratios and negligible hysteresis. While the range of bandgaps in these networks creates a highly uneven energy landscape for charge carrier hopping, the extracted hole and electron mobilities vary nonlinearly with the network composition from the lowest mobility (15 cm2 V-1 s-1) for only (6,5) SWNT to the highest mobility (30 cm2 V-1 s-1) for only plasma torch SWNTs. A comparison to numerically simulated network mobilities shows that a superposition of thermally activated hopping across SWNT-SWNT junctions and diameter-dependent intratube transport is required to reproduce the experimental data. These results also emphasize the need for monochiral large diameter nanotubes for maximum carrier mobilities in random networks.

Unveiling the Evolutions of Nanotube Diameter Distribution during the Growth of Single-Walled Carbon Nanotubes.

In situ and ex situ Raman measurements were used to study the dynamics of the populations of single-walled carbon nanotubes (SWCNTs) during their catalytic growth by chemical vapor deposition. Our study reveals that the nanotube diameter distribution strongly evolves during SWCNT growth but in dissimilar ways depending on the growth conditions. We notably show that high selectivity can be obtained using short or moderate growth times. High-resolution transmission electron microscopy observations support that Ostwald ripening is the key process driving these seemingly contradictory results by regulating the size distribution and lifetime of the active catalyst particles. Ostwald ripening appears as the main termination mechanism for the smallest diameter tubes, whereas carbon poisoning dominates for the largest ones. By unveiling the key concept of dynamic competition between nanotube growth and catalyst ripening, we show that time can be used as an active parameter to control the growth selectivity of carbon nanotubes and other 1D systems.

(Invited) Transport and Emission Properties of Polymer-Sorted Monochiral and Mixed Carbon Nanotube Networks

The selective dispersion of semiconducting carbon nanotubes (s-SWNT) by wrapping with conjugated polymers has enabled their application in electronic and optoelectronic devices such as field-effect transistors and light-emitting field-effect transistors. Here, we show that the dispersion process can be easily scaled-up while maintaining good photoluminescence yield (2%) and nanotube length (>1 µm) to obtain concentrated dispersions and ultimately thin (<10 nm) or thick films (>100 nm) of s-SWNT with excellent optical and electrical properties. We demonstrate ambipolar charge transport and near-infrared light-emission in field-effect transistors based on monochiral and mixed networks of s-SWNT and investigate the influence of the nanotube diameter distribution on charge transport within the network depending on the applied gate voltage and thus charge carrier density. Voltage-dependent photoluminescence and electroluminescence spectra provide insight into the charge distribution among the different nanotubes. For very high charge carrier densities (e.g. by electrolyte-gating) we observe red-shifted trion emission in addition to exciton emission. Furthermore, monochiral s-SWNT films are interesting materials for strong light-matter interaction due to their small Stokes shift and large exciton binding energy. Here, we demonstrate exciton-polariton formation in suitable cavities and emission enhancement in periodic feedback structures.

Ferrocene-ethanol-mist CVD Grown SWCNT Films as Transparent Electrodes

The floating catalyst CVD using ferrocene–ethanol mist was successfully used to deposit single-walled carbon nanotube (SWCNT) films from the gaseous phase onto a Si substrate and a membrane filter. The home-built vertical mist-CVD system was used without the use of H2 or CO gases. The tiny mist was generated using a high-frequency ultrasonic vibration. The effects of various parameters including furnace temperature, ferrocene/ethanol ratio, the deposition position in the reactor, flow rate of carrier gas, and deposition time on the SWCNTs formation were investigated using SEM, TEM and Raman spectroscopy. The furnace temperature and the flow rate of carrier gas were found to determine the diameter and crystallinity of nanotubes. The ferrocene concentration in ethanol influenced the diameter distribution of nanotubes and the amount of impurity particles in the materials. The collected SWCNT films on a filter were directly transferred to a laminate sheet by using a hot press laminator. The properties of SWCNTs were investigated by UV-Vis spectrophotometer and four-point probe measurement. The high sheet resistance was observed for the as-transferred films. However, the initial results show that a sheet resistance of 10 kΩ/cm2 and an 80% transmittance at the wavelength of 550nm after nitric acid treatment.

Incorporation of boron and nitrogen in carbon nanomaterials and its influence on their structure and opto-electronical properties

The presence of hexagonal boron nitride in the initial C:BN:Ni:Y 2O 3 mixture in the arc-discharge process leads to the modification of carbon nanostructures and to the creation of B x C y N z entities. BN-incorporation into these carbon nanostructures (single-wall nanotubes and carbon flakes) has been analyzed by high resolution transmission electron microscopy and electron energy loss spectroscopy. The optical absorption spectroscopy measurements have revealed a monotonous increase of the bandgap value of the synthesized nanotubes with the increase of the content of BN phase in the initial mixture of the synthesis compounds. Furthermore, a narrowing of the nanotube diameter distribution in favor of small diameters takes place in the presence of BN during the synthesis process.

Spectroscopic Characteristics of Differently Produced Single‐Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWNTs) synthesized with different methods are investigated by using multiple characterization techniques, including Raman scattering, optical absorption, and X-ray absorption near edge structure, along with X-ray photoemission by following the total valence bands and C 1s core-level spectra. Four different SWNT materials (produced by arc discharge, HiPco, laser ablation, and CoMoCat methods) contain nanotubes with diameters ranging from 0.7 to 2.8 nm. The diameter distribution and the composition of metallic and semiconducting tubes of the SWNT materials are strongly affected by the synthesis method. Similar sp(2) hybridization of carbon in the oxygenated SWNT structure can be found, but different surface functionalities are introduced while the tubes are processed. All the SWNTs demonstrate stronger plasmon resonance excitations and lower electron binding energy than graphite and multiwalled carbon nanotubes. These SWNT materials also exhibit different valence-band X-ray photoemission features, which are considerably affected by the nanotube diameter distribution and metallic/semiconducting composition.

Density Gradient Ultracentrifugation of Arc Produced Single-Wall Carbon Nanotubes

A great breakthrough in separation of single-wall carbon nanotubes (SWNTs) by a density gradient ultracentrifugation during last years has allowed to obtain fractions of separated metal or semiconductor SWNTs. Another important property of this separation method is the possibility to obtain fractions with the desired positions of absorption peaks. For such fractions a quantum efficiency is significantly increased in case of using them as a non-linear optical material for formation of ultrafast beam modulators. Arc SWNTs demonstrate a saturable absorption in a wide spectral range (from 1.3 μm up to 1.95 μm). The density gradient ultracentrifugation allows to obtain the fractions with a narrow nanotube diameter distribution. This also increases the efficiency of SWNT-based saturable absorbers. In this work the density gradient ultracentrifugation technique has been used and optimized for SWNT separation. The aqueous suspensions of arc nanotubes have been treated. Different combinations and concentrations of surfactants (sodium dodecyl sulphate (SDS) and sodium cholate (SC)) were used for the gradient and the sample layer formation. A gradual shift of the absorption peaks over the layers has been obtained. This shift is very important because it opens a way to adjust the absorption peak position to the exact laser wavelength. The fractions with a prevalence of metallic SWNTs (up to 97%) have been obtained.

Loosening the DNA wrapping around single-walled carbon nanotubes by increasing thestrand length

In this study, we discuss the influence of DNA strand length on DNA wrapping ofsingle-walled carbon nanotubes under high-shear sonication and find that different strandlength results in changed DNA–nanotube interaction, which is sensitively probed by theupshift extent of the Raman radial breathing mode bands of nanotubes due to DNAwrapping. The difference in the interaction between nanotubes and DNA strands of variouslength results in apparently different degrees of wrapping compactness, revealed by atomicforce microscopy observations, and nanotube selectivity in wrapping, indicated by bothRaman and photoluminescence spectroscopy results. The above findings can be utilized toprecisely control the nanotube diameter distribution and modulate the physicochemicalproperties of the nanotube wrapped by DNA without any direct functionalization ofnanotubes. This finding is of considerable interest from both theoretical and practicalstandpoints.

Recycling Ultrathin Catalyst Layers for Multiple Single-Walled Carbon Nanotube Array Regrowth Cycles and Selectivity in Catalyst Activation

We utilize chemical vapor deposition to demonstrate the reactivation of catalyst for multiple regrowths of vertically aligned carbon nanotube arrays (carpets) in the context of water-assisted supergrowth. The carpets are transferred from the growth substrate, and the catalyst and substrate are reactivated by annealing in air. Annealing parameters control the length and diameter distribution of nanotubes in the regrown carpets due to active catalyst termination followed by a size-dependent process of iron carbide particle reoxidation. The lack of achieving indefinite regrowth from a 0.5 nm thick Fe layer can be attributed to factors such as catalyst dynamics taking place during the growth and regrowth processes.

The Role of the Substrate Surface Morphology and Water in Growth of Vertically Aligned Single-Walled Carbon Nanotubes

Growth of high quality, vertically aligned single-walled carbon nanotubes (carpets) is achieved using a rapid insertion hot filament chemical vapor deposition (HF-CVD) technique. The effect of the substrate morphology on growth is explored by comparing carpets grown on epitaxially polished MgO substrates to those grown on "as-cut", macroscopically rough MgO substrates. Depending on the substrate morphology, we observe differences in both the overall carpet morphology as well as the diameter distribution of nanotubes grown in the carpet based on optical measurements. In addition, we explore the role of water in the growth of carpets on MgO and the conventional Al2O3 coated Si substrates. We find that the addition of a small amount of water is beneficial to the growth rates of the SWNT carpets, enhancing the growth rates by up to eight times.

Influence of the Catalyst Hydrogen Pretreatment on the Growth of Vertically Aligned Nitrogen-Doped Carbon Nanotubes

Thin vertically aligned N-doped nanotubes with narrow layered nanotube diameter distribution and a defined doping level have been synthesized by chemical vapor deposition. Multilayered catalysts and a pure acetonitrile C/N feedstock have been employed. The observed nitrogen content was up to 6% with a predominant sp2 bonding configuration in relation to other nitrogen incorporated forms. Depending on the process parameters, we were able to tune the nitrogen content and the formation ratio of small nanotubes to bamboo nanotubes. The use of multilayer catalyst allows a large temperature window in which substitutionally doped structures form in comparison to other catalysts.

High quality double wall carbon nanotubes with a defined diameter distribution by chemical vapor deposition from alcohol

We have synthesized double wall carbon nanotubes (DWNTs) with few defects and little amorphous carbon by hot wall chemical vapor deposition (CVD) of alcohol. Catalysts for the DWNT growth were made from cobalt and molybdenum acetates. Scanning electron microscopy, transmission electron microscopy, multi frequency resonance Raman spectroscopy and optical absorption spectroscopy were used for characterization of the product with regard to DWNT yield, the nanotube diameter distribution, defect concentration and amorphous carbon content. Base pressures lower than 1 × 10 −5 mbar in the CVD reactor considerably suppress defects in the DWNTs. Optimized growth conditions for DWNT formation are presented.

The effects of nitrogen and boron doping on the optical emission and diameters of single-walled carbon nanotubes

Using TEM, absorption and photoluminescence-excitation spectroscopy we have shown that nitrogen and nitrogen/boron doping of single-walled carbon nanotubes produce significant changes in both the optical properties and the diameter distribution of nanotubes produced by the arc-discharge method. Smaller diameter tubes are preferentially formed in the presence of boron. In addition the presence of nitrogen is found to significantly affect the emission properties of the nanotube ensemble, causing a shift in the dominant emission to lower energies, possibly due to changes in the bundling structure of the nanotubes in solution, but only very small changes are observed in the emission energies for individual nanotubes.

In situassessment of carbon nanotube diameter distribution with photoelectron spectroscopy

In situ UV-photoelectron spectroscopy (He I and He II) was performed on multiwalled carbon nanotubes (CNTs) with clearly differentiated diameter distributions. A significant dependence of valence- and conduction-band characteristics on the mean CNT diameter was observed, which was determined by high-resolution TEM and micro-Raman spectroscopy. The decrease of relative intensity of the $\ensuremath{\pi}$ states at $\ensuremath{-}3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the He II experiments, indicative of increasing rehybridization between orbitals, was directly correlated with decreasing mean diameters. Furthermore, a progressive broadening of the unoccupied ${\ensuremath{\sigma}}^{*}$ band at $7.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ was found in the He I spectra.

Magnetic separation of Fe catalyst from single-walled carbon nanotubes in an aqueous surfactant solution

We report an efficient technique to separate ferromagnetic catalyst particles from an aqueous surfactant solution of single-walled carbon nanotubes (SWNTs) by the use of a 1.3 T permanent magnet. High resolution transmission electron microscopy (HRTEM) demonstrates that SWNTs are coated with a surfactant layer that stabilises the aqueous dispersions of SWNTs. The residual quantities of Fe catalyst (∼3%) can be effectively removed from a colloid solution of SWNTs in a magnetic field while absorbance spectra of the initial and purified solutions show that the nanotube diameter distribution remains unchanged.

Enhanced Raman microprobe imaging of single-wall carbon nanotubes

We explore Raman microprobe capabilities of visualizing single-wall carbonnanotubes (SWCNTs). Although this technique is limited to the micron scale, wedemonstrate that images of individual SWCNTs, bundles, or their agglomerates canbe generated by mapping Raman active elementary excitations. We measuredthe Raman response from carbon vibrations in SWCNTs excited by confocalscanning of a focused laser beam. Carbon vibrations reveal key characteristics ofSWCNTs such as the nanotube diameter distribution (radial breathing modes (RBM),100–300 cm−1), the presence of defects and functional groups (D-mode,1300–1350 cm−1), strain and oxidation states of SWCNTs, as well as the metallic orsemiconducting character of the tubes encoded in the lineshape of the G-modes at1520–1600 cm−1. In addition, SWCNTs are highly anisotropic scatterers. The Raman response from aSWCNT is maximal for incident light polarization parallel to the tube axis and vanishingfor perpendicular directions. We show that the SWCNT bundle shape or direction can bedetermined, with some limitations, from a set of Raman images taken for two orthogonaldirections of the incident light polarization.

Comparative studies on acid and thermal based selective purification of HiPCO produced single-walled carbon nanotubes

We report a comparative study on gas and liquid phase oxidative treatments of SWNTs, using a HiPCO produced raw material with a broad distribution of nanotube diameters. The effects of these two processes were correlated using absorption and Raman spectroscopy (radial breathing and G modes) and TEM. We conclude that liquid phase oxidation is a continuous diameter-selective process eliminating narrower SWNTs through oxidizing sidewalls of SWNTs. Gas phase oxidation is a more discrete process requiring high activation energy for diameter-selective purification. Unlike liquid phase oxidation the gas phase process preferentially oxidizes SWNTs without introducing sidewall defects.

High-rate flame synthesis of vertically aligned carbon nanotubes using electric field control

The electric field controlled synthesis of carbon nanomaterials on a Ni-based catalytic support positioned at the fuel side of the opposed flow oxy-flame is studied experimentally. Carbon nanomaterials formed on the probe surface are comparatively analyzed for two characteristic operational modes: a grounded probe mode and a floating probe mode. In a grounded mode a number of various carbon nanostructures are formed depending on the probe location in flame. Observed nanoforms include multi-walled carbon nanotubes (MWNTs), MWNT bundles, helically coiled tubular nanofibers, and ribbon-like coiled nanofibers with rectangular cross-section. The presence of various carbon nanoforms is attributed to the space variation of flame parameters, namely flame temperature and concentration of chemical species. It is found that the presence of an electric potential (floating mode operation) provides the ability to control the nanostructure morphology and synthesis rate. A thick layer (35–40 μm) of vertically aligned carbon nanotubes (VACNTs) is found to be formed on the probe surface in the floating potential mode. This layer is characterized by high uniformity and narrow distribution of nanotube diameters. Overall, the electric field control method demonstrates stabilization of the structure in a wide flame region while growth rate remains dependent on flame location.