An investigation into the optimal growth of single-walled carbon nanotubes (SWNTs) in vertical arrays, or carpets, is presented utilizing atomic hydrogen catalyst activation with hot filament chemical vapor deposition. Using acetylene decomposition over Fe catalyst, we study the effect of oxidant-assisted growth using O2, CO2, and H2O. Whereas trace amounts of O2 result in the lack of any catalytic activity, CO2 and H2O are found to dramatically enhance the catalyst lifetime. On the basis of the saturation effect of oxidant concentration for both CO2 and H2O, we present this as being due to catalyst stabilization from surface hydroxyl groups, with H2O having the most dominant effect upon carpet growth. Utilizing water-assisted growth, this process is further optimized to yield high-quality single-walled carbon nanotubes. High temperature growth (∼775 °C) yields the highest-quality SWNTs, whereas controllable growth of double- and few-walled nanotubes can also be achieved at lower temperatures (550−600 °C). Finally, ultralong carpets are demonstrated by utilizing the optimal SWNT growth conditions under an enhanced carbon flux environment.

A new two-step purification method of carbon nanotubes (CNTs) involving a microwave treatment followed by a gas-phase chlorination process is reported. The significant advantage of this method over conventional cleaning carbon nanotubes procedures is that under microwave treatment in air, the carbon shells that encase the residual metal catalyst particles are removed and the metallic iron is exposed and subsequently oxidized making it accessible for chemical removal. The products from microwave and chlorine treatment have been characterized by TG/DTA, SEM, TEM, EDX, XPS, and Raman spectroscopy. The oxidation state of the iron residue is observed to change from Fe(0) to Fe(II)/Fe(III) after microwave treatment and atmospheric exposure. The effects of the duration and number of microwave exposures has been investigated. This rapid and effective microwave step favours the subsequent chlorination treatment enabling a more effective cleaning procedure to take place, yielding higher purity single- and multi-walled CNTs.

Potassium vapor was used to longitudinally split vertically aligned multiwalled carbon nanotubes carpets (VA-CNTs). The resulting structures have a carpet of partially split MWCNTs and graphene nanoribbons (GNRs). The split structures were characterized by scanning electron microscopy, transmission electron microscopy, atomic force microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. When compared to the original VA-CNTs carpet, the split VA-CNTs carpet has enhanced electrochemical performance with better specific capacitance in a supercapacitor. Furthermore, the split VA-CNTs carpet has excellent cyclability as a supercapacitor electrode material. There is a measured maximum power density of 103 kW/kg at an energy density of 5.2 Wh/kg and a maximum energy density of 9.4 Wh/kg. The superior electrochemical performances of the split VA-CNTs can be attributed to the increased surface area for ion accessibility after splitting, and the lasting conductivity of the structure with their vertical conductive paths based on the preserved GNR alignment.Keywords: energy density; graphene nanoribbon carpets; power density; specific capacitance; split VA-CNTs; supercapacitor; vertically aligned multiwalled carbon nanotubes

The diameter dependence of the collapse of single- and double-walled carbon nanotubes to two- and four-walled graphene nanoribbons with closed edges (CExGNRs) has been experimentally determined and compared to theory. TEM and AFM were used to characterize nanotubes grown from preformed 4.0 nm diameter aluminum–iron oxide particles. Experimental data indicate that the energy equivalence point (the diameter at which the energy of a round and fully collapsed nanotube is the same) is 2.6 and 4.0 nm for single- and double-walled carbon nanotubes, respectively. Molecular dynamics simulations predict similar energy equivalence diameters with the use of ε = 54 meV/pair to calculate the carbon–carbon van der Waals interaction.Keywords: aluminum−iron oxide nanoparticles; carbon−carbon van der Waals interaction; collapsed nanotube; double-walled carbon nanotubes; graphene nanoribbon; molecular dynamics simulation; single-walled carbon nanotubes

Despite the many processes developed for carbon nanotube synthesis, few if any of these control the carbon nanotube diameter and length simultaneously. Here, we report a process whereby we synthesize vertically aligned carbon nanotube arrays (VA-CNT) using water-assisted chemical vapor deposition from solution processed premade and near-monodisperse iron oxide nanoparticles. Utilizing a dendrimer-assisted iron oxide nanoparticle monolayer deposition technique, the synthesis of high quality VA-CNTs is observed with a surprising degree of walls uniformity and diameters that correlate closely with the catalyst particle size. Specifically, we utilize 8.3 and 15.4 nm nanoparticle sizes to grow uniform, large diameter VA-CNTs. We observe control of the VA-CNT diameter and number of walls based on the nanoparticle size, with the 8.3 nm nanoparticles growing over 90% four-walled CNTs. Additionally, there is a sparse population of VA-CNTs with large diameters and few walls that tend to flatten into nanostructures resembling paired-layer graphene nanoribbons.Keywords: catalyst; catalyst support; nanoparticles; vertically aligned carbon nanotubes;

We demonstrate the fabrication of solid-state dielectric energy storage materials from self-assembled, aligned single-walled carbon nanotube arrays (VA-SWNTs). The arrays are transferred as intact structures to a conductive substrate and the nanotubes are conformally coated with a thin metal-oxide dielectric and a conductive counter-electrode layer using atomic layer deposition. Experimental results yield values in agreement with those obtained through capacitive modeling using Al2O3 dielectric coatings (C > 20 mF/cm3), and the solid-state dielectric architecture enables the operation of these devices at substantially higher frequencies than conventional electrolyte-based capacitor designs. Furthermore, modeling of supercapacitor architectures utilizing other dielectric layers suggests the ability to achieve energy densities above 10 W h/kg while still exhibiting power densities comparable to conventional solid-state capacitor devices. This device design efficiently converts the high surface area available in the conductive VA-SWNT electrode to space for energy storage while boasting a robust solid-state material framework that is versatile for use in a range of conditions not practical with current energy storage technology.

We demonstrate the water-assisted supergrowth of vertically aligned single-walled carbon–nitrogen nanotubes (SWNNTs) using a simple liquid/gas-phase precursor system. In situ characterization of gas-phase nitrogen-containing precursors and their correlation to growth identifies HCN as the most active precursor for SWNNT growth, analogous to C2H2 for single-walled carbon nanotubes (SWNTs). Utilizing Raman spectroscopy, combined with XPS and in situ mass spectrometry during growth, we demonstrate the ability to probe N atoms at low concentrations (10–5 at. % N) in the SWNNT. Additionally, we demonstrate sensitivity of SWNNT optical transitions to N-doping through absorbance measurements, which appear to be a sensitive fingerprint for SWNNT doping. Finally, we demonstrate the fabrication of SWNT/SWNNT heterojunctions in the self-assembled carpet morphology that can be printed to arbitrary host substrates and facilitate potential emerging applications for this material. This work brings together new aspects regarding the growth, characterization, and materials processing that can yield advanced material architectures involving electronically tuned SWNNT array networks.Keywords: carbon nanotubes; carpets; chemical vapor deposition; HCN; heterojunction; nitrogen doping; Raman spectroscopy

Self-assembled monolayers (SAMs) of iron oxide nanoparticles have been prepared using carboxylic-acid-terminated dendrimers. The iron-containing SAM was used as the catalyst for growth of vertical arrays of carbon nanotubes (CNTs). This approach has the potential for producing diameter controlled CNTs from premade catalyst nanoparticles as well as large scale production of CNTs by chemical vapor deposition.Keywords: catalyst; monolayer; nanoparticles; nanotubes; self-assembly

A procedure for vertically aligned carbon nanotube (VA-CNT) production has been developed through liquid-phase deposition of alumoxanes (aluminum oxide hydroxides, boehmite) as a catalyst support. Through a simple spin-coating of alumoxane nanoparticles, uniform centimer-square thin film surfaces were coated and used as supports for subsequent deposition of metal catalyst. Uniform VA-CNTs are observed to grow from this film following deposition of both conventional evaporated Fe catalyst, as well as premade Fe nanoparticles drop-dried from the liquid phase. The quality and uniformity of the VA-CNTs are comparable to growth from conventional evaporated layers of Al2O3. The combined use of alumoxane and Fe nanoparticles to coat surfaces represents an inexpensive and scalable approach to large-scale VA-CNT production that makes chemical vapor deposition significantly more competitive when compared to other CNT production techniques.Keywords: alumoxane; catalyst support; chemical vapor deposition; vertically aligned carbon nanotubes

The present study demonstrates the viability of the reductive attachment step of the single walled carbon nanotube (SWCNT) lengthening process in which long SWCNTs are grown from short nanotube seeds. Aryl sulfonate sidewall-functionalized, carboxylate end-functionalized SWCNTs are attached to an inorganic cluster pro-catalyst (FeMoC) via ligand exchange. The SWCNT–FeMoC complex was electrodeposited onto highly ordered pyrolytic graphite (HOPG), heated and exposed to etching conditions. Pre- and post-treatment AFM imaging shows that controlled reductive etching of the SWCNTs is attainable at a variety of pressures and temperatures in hot surface/cold gas and hot surface/hot gas systems.

A scalable and facile approach is demonstrated where as-grown patterns of well-aligned structures composed of single-walled carbon nanotubes (SWNT) synthesized via water-assisted chemical vapor deposition (CVD) can be transferred, or printed, to any host surface in a single dry, room-temperature step using the growth substrate as a stamp. We demonstrate compatibility of this process with multiple transfers for large-scale device and specifically tailored pattern fabrication. Utilizing this transfer approach, anisotropic optical properties of the SWNT films are probed via polarized absorption, Raman, and photoluminescence spectroscopies. Using a simple model to describe optical transitions in the large SWNT species present in the aligned samples, polarized absorption data are demonstrated as an effective tool for accurate assignment of the diameter distribution from broad absorption features located in the infrared. This can be performed on either well-aligned samples or unaligned doped samples, allowing simple and rapid feedback of the SWNT diameter distribution that can be challenging and time-consuming to obtain in other optical methods. Furthermore, we discuss challenges in accurately characterizing alignment in structures of long versus short carbon nanotubes through optical techniques, where SWNT length makes a difference in the information obtained in such measurements. This work provides new insight to the efficient transfer and optical properties of an emerging class of long, large diameter SWNT species typically produced in the CVD process.Keywords: carbon nanotubes; carpets; optical absorption; Raman spectroscopy

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.

An efficient technique using hydrazine (N2H4) vapor as an agent for the rapid reduction of high-density layers of catalytic nanoparticles is demonstrated. With as little as 10 mTorr hydrazine bled into a thermal chemical vapor deposition (CVD) apparatus, efficient reduction of metal-oxide catalyst particles is achieved more rapidly than when using atomic hydrogen as the reducing agent. Postreduction catalyst imaging emphasizes the differences in nanoparticle formation under different reduction environments, with the most uniform and compact catalyst size distribution observed following hydrazine exposure. Low-temperature reduction studies suggest that as little as 15 s N2H4 exposure at temperatures of 350 °C can yield a reduced catalyst layer preceding the synthesis of dense, aligned arrays of single-walled carbon nanotubes (SWNT) with uniform height. This work demonstrates a simple route toward scalable, vapor transport reduction of metal-oxide catalyst relevant to a number of catalytic applications, including the synthesis and selective synthesis of aligned SWNT arrays.Keywords: carbon nanotubes; catalysis; chemical vapor deposition; hydrazine

A novel process is demonstrated whereby dense arrays of single-walled carbon nanotubes (SWNT) are grown directly at the interface of a carbon material or carbon fiber. This growth process combines the concepts of SWNT tip growth and alumina-supported SWNT base growth to yield what we refer to as “odako” growth. In odako growth, an alumina flake detaches from the carbon surface and supports catalytic growth of dense SWNT arrays at the tip, leaving a direct interface between the carbon surface and the dense SWNT arrays. In addition to being a new and novel form of SWNT array growth, this technique provides a route toward future development of many important applications for dense aligned SWNT arrays.

We present a robust method for synthesis of aligned, single-walled carbon nanotube (CNT) “flying carpets” from nanostructured alumina flakes. Roll-to-roll e-beam deposition is utilized to produce the flakes, and hot filament chemical vapor deposition is utilized to grow dense, aligned carbon nanotubes from the flakes with remarkably high CNT yields. The flakes are captured inside a mesh cage and freely suspended in the gas flow during growth. Optical characterization indicates the presence of high quality, small diameter single-walled carbon nanotubes.

Utilizing aligned carbon nanotube arrays grown from chemical vapor deposition, we present a highly scalable route toward the formation of ribbons and ultrathin transparent films directly from vertically aligned single-walled carbon nanotube arrays (SWNT carpets). To “lay-over” the aligned nanotubes to form a film, we use a roller which acts to compress the film and preserve the alignment of nanotubes within the film. As we demonstrate, we can control the nanotube-catalyst interaction, leading to highly efficient transfer of the film to virtually any host substrate by following growth with a controlled H2O vapor etch. In addition, we demonstrate our ability to grow carpets on patterned substrates leading to upright carpet lines, which can be rolled over to form transparent films composed of ultralong carbon nanotubes. This work demonstrates a highly scalable technique to form homogeneous, transparent films of aligned SWNTs that can be ultralong with absolutely no need for liquid phase SWNT processing.Keywords: carbon nanotubes; chemical vapor deposition; transparent films

Vertically aligned carbon nanotubes are grown from Al2O3-supported Fe−Mo catalyst in a hot filament chemical vapor deposition apparatus. We compare the effect of carbon nanotube growth on deposition of 0.5 and 1 nm thick Fe catalyst layers before and after deposition of 0.1 and 0.2 nm thick layers of Mo. We observe that the order of deposition plays a role in the height of the nanotube arrays, especially evident during growth at elevated reaction pressures where carbon flux is higher. We investigate the role of temperature and pressure on features of the nanotube arrays such as height, alignment, quality, volumetric density, and diameter distribution for each of the catalyst thicknesses and for each case of Fe/Mo and Mo/Fe. We compare our results to those obtained from carpets grown from pure Fe catalyst, and observe that a Mo cocatalyst can be advantageous regardless of how it is deposited. However, we find that the order of deposition plays a key role in the temperature and pressure range in which optimal single-walled carbon nanotube growth occurs.