Generation of single-walled carbon nanotubes from alcohol and generation mechanism by molecular dynamics simulations

Recent advances in high-purity and high-yield catalytic chemical vapor deposition (CVD) generation of single-walled carbon nanotubes (SWNTs) from alcohol are comprehensively presented and discussed on the basis of results obtained from both experimental and numerical investigations. We have uniquely adopted alcohol as a carbon feedstock, and this has resulted in high-quality, low-temperature synthesis of SWNTs. This technique can produce SWNTs even at a very low temperature of 550 degrees C, which is about 300 degrees C lower than the conventional CVD methods in which methane or acetylene is typically used. We demonstrate the excellence of the proposed alcohol catalytic CVD method for high-yield production of SWNTs when Fe-Co on USY-zeolite powder was used as a catalyst. At optimum CVD conditions, a SWNT yield of more than 40 wt % was achieved over the weight of the catalytic powder within the reaction time of 120 min. In addition to the advantages for mass production, this method is also suitable for the direct synthesis of high-quality SWNTs on Si and quartz substrates when combined with the newly developed liquid-based "dip-coat" technique to mount catalytic metals on the surface of substrates. This method allows easy and costless loading of catalytic metals without the need for any support or underlayer materials that were usually required in previous studies for the generation of a sufficient quantity of SWNTs on an Si surface. Finally, the result of molecular dynamics simulation for the SWNT growth process is presented to obtain a fundamental insight into the initial growth mechanism on the catalytic particles.     

Ultralow feeding gas flow guiding growth of large-scale horizontally aligned single-walled carbon nanotube arrays

On the basis of the rational analysis about the fluidic property of the system, an ultralow gas flow chemical vapor deposition (CVD) strategy was designed to prepare large-scale horizontally aligned ultralong single-walled carbon nanotube (SWNT) arrays. SWNT arrays could be well obtained under extremely low feeding flow of 1.5 sccm in a 1 in. quartz tube reactor. It was confirmed that the tubes grew floatingly and could cross microtrenches or climb over micro-obstacles in ultraslow gas flow. SWNTs arrays also could be formed no matter the substrate was placed vertically or upside down. The growth mechanism was discussed. Both the buoyancy effect induced by gas temperature/density difference and gas flow stability played dominant roles. More attractively, simultaneous batch-scale preparation of SWNT arrays was realized by the ultralow gas flow strategy. This new strategy turns to be more abstemious, efficient, promising, and flexible compared with the high gas flow rate fast-heating CVD processes.     

Synthesis-condition dependence of carbon nanotube growth by alcohol catalytic chemical vapor deposition method

We report the dependence of growth yield of single-walled carbon nanotubes (SWNTs) on heat-treatment time and catalyst film thickness by the alcohol catalytic chemical vapor deposition method. Three types of heat-treatment, synthesis of 30 min, synthesis of 30 min after annealing of 30 min, and synthesis of 60 min, were investigated. Thickness of Co catalyst film was varied from 1 to 10 nm. In the case of thinner Co film less than 3 nm, long synthesis time of 60 min is favorable for the effective SWNT growth, because of the small amount of Co catalyst. In the case of thicker Co film more than 3 nm, an amount of grown SWNTs by 30 min synthesis after 30 min annealing and by 60 min synthesis was much higher than that by 30 min synthesis without annealing, showing that total heat-treatment time of 60 min is important for the SWNT growth. Results suggest that the conversion from the thicker film of Co to nano-particle which acts as catalyst takes place during the first 30 min.     

Growth of single-walled carbon nanotube arrays from samarium on substrates

In this letter, it is reported for the first time that samarium is an effective catalyst for single-walled carbon nanotubes(SWNTs) growth via a chemical vapor deposition(CVD) process. Horizontally superlong well-oriented SWNT arrays can be generated under suitable conditions by using ethanol as carbon source. The single-wall structure was characterized by scanning electron microscopy, Raman spectroscopy and atomic force microscopy. The results show that the SWNTs from samarium have better conductivity and better structural uniformity with less defects. This rare earth metal element provides not only an alternative catalyst for SWNTs growth but also a possible way to generate high percentage of superlong semiconducting SWNT arrays for various applications of nanoelectronic device.     

Cadmium-catalyzed surface growth of single-walled carbon nanotubes with high efficiency

We demonstrate that cadmium (Cd) can catalyze the growth of single-walled carbon nanotubes (SWNTs) with high efficiency. The Cd nanocatalysts, prepared using a diblock copolymer templating method, were uniformly spaced over a large deposition area with an average diameter of 1.9 nm and narrow size distribution. By using the normal-heating and fast-heating method, random and horizontally aligned arrays of SWNTs can be generated. The density of the SWNTs can be altered by the chemical vapor deposition conditions. The morphology and microstructure of the SWNTs characterized by scanning electron microscopy, Raman spectroscopy, atomic force microscopy, and high-resolution transmission electron microscopy revealed that the grown nanotubes do not have carbonaceous particles and have good crystallinity. In addition, after careful check with superlong nanotubes 735 out of 790 nanotubes were found to be deposited with Ag (93%) and only 7% SWNTs without Ag deposition. While for superlong SWNT arrays from Fe, 32% long SWNTs without Ag deposition was found, the high percentage of SWNTs with Ag deposition from Cd indicates that the SWNTs have better conductivity and better structural uniformity with less defects.     

Synthesis-condition dependence of carbon nanotube growth by alcohol catalytic chemical vapor deposition method

We report the dependence of growth yield of single-walled carbon nanotubes (SWNTs) on heat-treatment time and catalyst film thickness by the alcohol catalytic chemical vapor deposition method. Three types of heat-treatment, synthesis of 30 min, synthesis of 30 min after annealing of 30 min, and synthesis of 60 min, were investigated. Thickness of Co catalyst film was varied from 1 to 10 nm. In the case of thinner Co film less than 3 nm, long synthesis time of 60 min is favorable for the effective SWNT growth, because of the small amount of Co catalyst. In the case of thicker Co film more than 3 nm, an amount of grown SWNTs by 30 min synthesis after 30 min annealing and by 60 min synthesis was much higher than that by 30 min synthesis without annealing, showing that total heat-treatment time of 60 min is important for the SWNT growth. Results suggest that the conversion from the thicker film of Co to nano-particle which acts as catalyst takes place during the first 30 min.     

Guided growth of large-scale, horizontally aligned arrays of single-walled carbon nanotubes and their use in thin-film transistors

A convenient process for generating large-scale, horizontally aligned arrays of pristine, single-walled carbon nanotubes (SWNTs) is described. The approach uses guided growth, by chemical vapor deposition (CVD), of SWNTs on miscut single-crystal quartz substrates. Studies of the growth reveal important relationships between the density and alignment of the tubes, the CVD conditions, and the morphology of the quartz. Electrodes and dielectrics patterned on top of these arrays yield thin-film transistors that use the SWNTs as effective thin-film semiconductors. The ability to build high-performance devices of this type suggests significant promise for large-scale aligned arrays of SWNTs in electronics, sensors, and other applications.     

Catalyst-free growth of oriented single-walled carbon nanotubes on mica by ethanol chemical vapor deposition

In this letter, it is reported for the first time that single-walled carbon nanotubes (SWNTs) can grow on mica substrate without additional catalyst by chemical vapor deposition (CVD) using ethanol as carbon source. The single-wall structure was characterized by Raman spectra and AFM (Atomic Force Microscopy) measurements. The growth of carbon nanotubes on mica surface contributes to the small amount of iron oxide in bare mica. The uniform dispersion and nanosized Fe particles formed from the reduction of iron oxide favor for the growth of SWNTs. Horizontally aligned superlong SWNTs arrays can be successfully generated on the mica surface, which is proved to be guided by the gas flow and under “kite growth mechanism”. The mica is a machinable material which can be easily cut and made a narrow slit on, thus the nanotubes can traverse the slit which can be in millimeter scale and long suspended SWNTs can be generated. This will provide an opportunity to manipulate individual SWNT for various purposes.     

Direct observation of the strong interaction between carbon nanotubes and quartz substrate

We present a chemical vapor deposition (CVD) method for the growth of uniform single-walled carbon nanotube (SWNT) arrays on a stable temperature (ST)-cut single crystal quartz substrate using a mixture of methanol and ethanol as carbon source. It is found that introducing methanol during the growth can improve the density and the length of the well-aligned SWNTs in the arrays as well as increase the SWNT/quartz interaction. Obvious “up-shifts” of G-band frequencies in the Raman spectra have been found for the aligned SWNTs. A welldesigned control experiment shows that the G-band “up-shifts” originate from the strong interaction between SWNTs and the quartz substrate. It is believed that exploring this interaction will help to elucidate the growth mechanism; ultimately, this will help realize the promise of controlling the chirality of SWNTs.     

Efficient growth of horizontally aligned single-walled carbon nanotubes by chemical vapor deposition over MgO-supported bimetallic co-based catalysts

A series of supported cobalt-based catalysts (Co-X, where X = Mn, V, W, Gd, Mo) has been investigated for the growth of single-walled carbon nanotubes via catalytic decomposition of CH4. At 850 degrees C, Mn, W and Gd promoted Co-catalysts produce SWNTs while Mo and V do not yield any SWNTs. Furthermore, Co-Gd catalysts produce high quality SWNTs with very few defects, and also narrow diameter and chirality distributions (approximately 1 nm). The growth of horizontally aligned SWNTs (approximately 1 mm long) using the Co-Gd catalyst is also demonstrated.     

Unexpected hole transfer leads to high efficiency single-walled carbon nanotube hybrid photovoltaic

We report surprisingly efficient photocurrent generation at individual single-walled carbon nanotube (SWNT) /poly(3-hexylthiophene-2,5-diyl) (P3HT) junctions. Contrary to previous prediction, both semiconducting SWNTs (s-SWNTs) and metallic SWNTs (m-SWNTs) function as efficient hole acceptors. By active tuning of SWNTs' Fermi level, we confirm that P3HT p-dopes both s-SWNT and m-SWNT, and the work function difference between the nanotube and P3HT leads to a built-in voltage driving the efficient exciton dissociation and hole transfer. We further demonstrate square millimeter scale SWNT/P3HT bilayer photovoltaics using horizontally aligned SWNT arrays. Importantly, the devices exhibit greater than 90% effective external quantum efficiency. These key findings will not only enhance our knowledge of photocurrent generation at nanoscale interfaces, but also make selective omission of m-SWNT redundant, promising carbon nanomaterial-based, low-cost, high-efficiency hybrid photovoltaics.     

Designing Catalysts for Chirality‐Selective Synthesis of Single‐Walled Carbon Nanotubes: Past Success and Future Opportunity

A major obstacle for the applications of single‐walled carbon nanotubes (SWNTs) in electronic devices is their structural diversity, ending in SWNTs with diverse electrical properties. Catalytic chemical vapor deposition has shown great promise in directly synthesizing high‐quality SWNTs with a high selectivity to specific chirality (n, m). During the growth process, the tube–catalyst interface plays crucial roles in regulating the SWNT nucleation thermodynamics and growth kinetics, ultimately governing the SWNT chirality distribution. Starting with the introduction of SWNT growth modes, this review seeks to extend the knowledge about chirality‐selective synthesis by clarifying the energetically favored SWNT cap nucleation and the threshold step for SWNT growth, which describes how the tube–catalyst interface affects both the nucleus energy and the new carbon atom incorporation. Such understandings are subsequently applied to interpret the (n, m) specific growth achieved on a variety of templates, such as SWNT segments or predefined molecular seeds, transition metal (Fe, Co and Ni)‐containing catalysts at low reaction temperatures, W‐based alloy catalysts, and metal carbides at relatively high reaction temperatures. The up to date achievements on chirality‐controlled synthesis of SWNTs is summarized and the remaining major challenges existing in the SWNT synthesis field are discussed.     

Diameter controlled chemical vapor deposition synthesis of single-walled carbon nanotubes

In this study, we systematically investigated the influence of catalyst preparation procedures on the mean diameter of single-walled carbon nanotubes (SWNTs) synthesized by the alcohol catalytic chemical vapor deposition (ACCVD) process. It was found that the SWNT diameter is dependent upon both reduction temperature and time, with lower reduction temperature and/or shorter reduction time resulting in smaller diameter SWNTs. The morphology of the SWNTs also changed from vertically aligned to randomly oriented when the reduction temperature was below 500 degrees C. We also found that introducing a small amount of water during the catalyst reduction stage significantly decreased the mean diameter of the SWNTs. Lastly, we report on the use of a new binary catalyst system in which rhodium was combined with cobalt. This new Co/Rh combination produced SWNTs of smaller diameter than the conventional Co/Mo catalyst.     

Ultralong aligned single-walled carbon nanotubes on flexible fluorphlogopite mica for strain sensors

Single-walled carbon nanotubes (SWNTs) are expected to be an ideal candidate for making highly efficient strain sensing devices owing to their unique mechanical, electronic, and electromechanical properties. Here we present the use of fluorphlogopite mica (F-mica) as a flexible, high-temperature-bearing and mechanically robust substrate for the direct growth of horizontally aligned ultra-long SWNT arrays by chemical vapor deposition (CVD), which in turn enables the straightforward, facile, and cost-effective fabrication of macro-scale SWNT-array-based strain sensors. Strain sensing tests of the SWNT-array devices demonstrated fairly good strain sensitivity with high ON-state current density.      

Ultrahigh density modulation of aligned single-walled carbon nanotube arrays

Controlling the densities of aligned single-walled carbon nanotube arrays (SWNTs) on ST-cut quartz is a critical step in various applications of these materials. However the growth mechanism for tuning SWNT density using the chemical vapor deposition (CVD) method is still not well understood, preventing the development of efficient ways to obtain the desired results. Here we report a general “periodic” approach that achieves ultrahigh density modulation of SWNT arrays on ST-cut quartz substrates—with densities increased by up to ∼60 times compared with conventional methods using the same catalyst densities—by varying the CH₄ gas “off” time. This approach is applicable to a wide range of initial catalyst densities, substrates, catalyst types and growth conditions. We propose a general mechanism for the catalyst size-dependent nucleation of SWNTs associated with different free carbon concentrations, which explains all the observations. Moreover, the validity of the model is supported by systematic experiments involving the variation of key parameters in the “periodic” CVD approach.      

Controlled growth of single-walled carbon nanotubes for unique nanodevices

The controlled growth of bent and horizontally aligned single-walled carbon nanotubes (SWNTs) is demonstrated in this study. The bent SWNTs growth is attributed to the interaction between van der Waals force with substrate and aerodynamic force from gas flow. The curvature of bent SWNTs can be tailored by adjusting the angle between gas flow and step-edge direction. Electrical characterization shows that the one-dimensional resistivity of bent SWNTs is correlated with the curvature, which is due to strain induced energy bandgap variation. Additionally, a downshift of 10 cm(-1) in G-band is found at curved part by Raman analysis, which may be resulted from the bending induced carbon-carbon bond variation. In addition, horizontally aligned SWNTs and crossbar SWNTs were demonstrated. To prove the possibility of integrating the SWNTs having controllable morphology in carbon nanotube based electronics, an inverter with a gain of 2 was built on an individual horizontally aligned carbon nanotube.     

Wafer scale synthesis of dense aligned arrays of single-walled carbon nanotubes

Here we present an easy one-step approach to pattern uniform catalyst lines for the growth of dense, aligned parallel arrays of single-walled carbon nanotubes (SWNTs) on quartz wafers by using photolithography or polydimethylsiloxane (PDMS) stamp microcontact printing (μCP). By directly doping an FeCl₃/methanol solution into Shipley 1827 photoresist or polyvinylpyrrolidone (PVP), various catalyst lines can be well-patterned on a wafer scale. In addition, during the chemical vapor deposition (CVD) growth of SWNTs the polymer layers play a very important role in the formation of mono-dispersed nanoparticles. This universal and efficient method for the patterning growth of SWNTs arrays on a surface is compatible with the microelectronics industry, thus enabling of the fabrication highly integrated circuits of SWNTs.     

Templating of self-alignment patterns of anisotropic gold nanoparticles on ordered SWNT macrostructures

We report a simple and versatile technique for oriented assembly of gold nanorods on aligned single-walled carbon nanotube (SWNT) macrostructures, such as thin nanotube films and nanotube fibers. The deposition and assembly is accomplished via drop drying of dilute gold nanorod suspensions on SWNT macrostructures under ambient conditions. Guided by anisotropic interactions, gold nanorods, and polygonal platelets spontaneously align with SWNTs, resulting in macroscopic arrays of locally ordered nanorods supported on aligned SWNT substrates. SEM reveals that the scalar order parameter of rods relative to the local average SWNT alignment is 0.7 for rods on SWNT films and 0.9 for rods on SWNT fibers. This self-alignment is enabled by anisotropic gold nanoparticle-SWNT interactions and is observed for a wide range of nanoparticles, including nanorods with aspect ratios ranging from 2-35, thin gold triangular and other polygonal platelets. The plasmonic properties of aligned gold nanorods together with superior electronic, chemical and mechanical properties of SWNTs make these hybrid nanocomposites valuable for the design of self-assembled multifunctional optoelectronic materials and optical metamaterials.     

Printed multilayer superstructures of aligned single-walled carbon nanotubes for electronic applications

We developed means to form multilayer superstructures of large collections of single-walled carbon nanotubes (SWNTs) configured in horizontally aligned arrays, random networks, and complex geometries of arrays and networks on a wide range of substrates. The approach involves guided growth of SWNTs on crystalline and amorphous substrates followed by sequential, multiple step transfer of the resulting collections of tubes to target substrates, such as high-k thin dielectrics on silicon wafers, transparent plates of glass, cylindrical tubes and other curved surfaces, and thin, flexible sheets of plastic. Electrical measurements on dense, bilayer superstructures, including crossbars, random networks, and aligned arrays on networks of SWNTs reveal some important characteristics of representative systems. These and other layouts of SWNTs might find applications not only in electronics but also in areas such as optoelectronics, sensors, nanomechanical systems, and microfluidics.     

Size engineering of metal nanoparticles to diameter-specified growth of single-walled carbon nanotubes with horizontal alignment on quartz

The electronic, physical and optical properties of single-walled carbon nanotubes (SWNTs) are governed by their diameter and chirality, and thus much research has been focused on controlling the diameter and chirality of SWNTs. To date, control of the catalyst particle size has been thought to be one of the most promising approaches to control the diameter or chirality of SWNTs owing to the correlation between catalyst particle size and tube diameter.In this study, we demonstrate the size engineering of catalytic nanoparticles for the controlled growth of diameter-specified and horizontally aligned SWNTs on quartz substrates. Uniformly sized iron nanoparticles derived from ferritin molecules were used as a catalyst, and their size was intentionally decreased via thermal heat treatment at 900?°C under atmospheric Ar ambient. ST-cut quartz wafers were used as growth substrates in order to elucidate the effect of the size of the nanoparticles on the tube diameter and the effect of catalyst size on the degree of parallel alignment on the quartz substrates. SWNTs grown by chemical vapor deposition using methane as feedstock exhibited a high degree of horizontal alignment when the particle density was low enough to produce individual SWNTs without bundling. Annealing for 60?min at 900?°C produced a reduction of nanoparticle diameter from 2.6 to 1.8?nm and a decrease in the mean tube diameter from 1.2 to 0.8?nm, respectively. Raman spectroscopy results corroborated the observation that prolonged heat treatment of nanoparticles yields thinner tubes with narrower size distributions. The results of this work suggest that straightforward thermal annealing can be a facile way to obtain uniform-sized SWNTs as well as catalytic nanoparticles.