Recent Developments: Carbon Nanotubes SynthesisAbstractWith the perpetual demand of miniaturization of computer and electronic devices, it has become harder and harder for computer engineers to evade the laws of quantum dynamics. Scientists and engineers are now questioning the familiar metal wire even in its thinnest state to be suitable enough to wire the processors of tomorrow. Nanotechnologists and Applied Physicists attribute Carbon Nanotubes (CNTs) as the media for electrical transmission for tomorrow. The electrical and mechanical properties of the CNTs are such promising that some expect the advent of microprocessors aided by the CNT as soon as 2020 1. CNTs have also shown potential in structural and mechanical engineering. They have been quickly used in different industries to support the structures of airplanes and submarines.
Apart from being extraordinarily electrically and thermally conducting, CNTs are strongest, stiffest and the most tensile materials ever discovered, keeping their dimensions in mind 2. However, growing Nanotubes at an industrial level has proved to be difficult, time and time again. Scientists are currently experimenting on multiple ways to provide better the yields of nanotubes per batch and to improve the quality and the consistency of the nanotubes produced. This article explores in the recent developments that were made to better the sifting and sorting mechanisms of CNTs IntroductionFor the past twenty years, carbon nanotubes have sparked interest in a range of industries that are endeavoring to create the next generation of electrical products. Particularly, carbon Nanotubes have the potential and are set to replace the transistor, logic gates, interconnects and infrared emitters. There are two types of Nanotubes in production: the single-walled carbon nanotube (SWNT), and the multi-walled carbon nanotube. Visually an SWNT looks like an atom length thick graphite (i.e.
, graphene) sheet that has been rolled into a perfect cylinder. Multi-walled carbon nanotubes are two or more CNTs that are rolled into one another like a sleeve. In either case the radius and the twist angle called the chiral angle of the CNT is of the utmost importance, as many electrical, mechanical and optical parameters of the nanotube eventually depend on that. A control is required to keep the formation of the Nanotube within the specific production parameters. This twist is sometimes described as the chirality or helicity of the nanotube and is represented as the chiral vector (n,m).
A miniscule change in this chiral vector, even on the atomic level may imply a staggering variation in the mechanical and optoelectronic behavior of the nanotube. It is noted that in an uncontrolled production, a random distribution of SWNT helicities riddles the entire batch and one-third of the SWNTs will behave as metals while the remaining two-thirds will act as semiconductors at room temperature. Moreover, varying diameters of the nanotubes exponentially change the electronic and optical properties of the SWNTs.
Synthetic production methods have lacked appropriate regulation over SWNT structures, leading to non-homogenous batches with sporadic properties. Additionally these methods yield CNTs that are afflicted with impurities, graphitic carbons, and metallic catalyst particles lodged with in the structures. This variance in the CNTs per batch, inhibit the industrial use of the SWNTs and prevent this carbon nanostructure to take over the electronic manufacturing sector, since commercial applications require consistent reproducible batches of CNTs every time. In this article, we discuss the recent advances toward the eventual objective of producing relatively homogenous carbon nanotube materials.
This article firstly discusses on how to sift, sort and purify batches made by synthetic methods according to their respective physical and electronic properties. The second part of the article discusses how to grow carbon nanotubes with prerequisite specifications for an ever demanding electronic industry. Post-Synthetic SortingPost-Synthetic sorting schemes for SWNTs usually subject the batch to chemical baths that interact with every type of CNT is a differentiating manner. These chemicals are introduced to the batch once a batch is collectively tested to detect what type of SWNTs are present in the batch in the first place, so that the sifting and the sorting mechanisms are more efficient. The type of Post-Synthetic chemical baths also depends on the type of SWNTs that are required by the manufacturer. The collective test of the batch could be done by Raman Spectroscopy by which a laser light is shown to a batch of nanomaterial and the resulting wavelengths detected can dictate the type of chemical structures of the nanotube present in the batch. Further, the batch can be processed by using chromatography, ultracentrifugation, or electrophoresis to sort the random batch according to the electrical, dimensional and optical properties of the CNTs.Selective ChemistryVarious organic and non-organic chemistries exhibit to discriminate SWNTs in solution as a function of electronic type, diameter, chiral angle, and chiral handedness.
Particularly, fluorene-based chemistry has yielded metal-semiconductor separation 19 and diameter selectivity. 20–22 Flavin mononucleotides also have been found to form helical assemblies around SWNTs that depend strongly on the SWNT chiral angle 23. Similarly, geometrically constrained polyaromatic amphiphiles adsorb differentially as a function of SWNT chiral angle,24 while oligo-acenes provide diameter separation 25. Custom designed diporphyrins 26–27 and, more recently, monoporphyrins 28 have been particularly effective at discriminating SWNTs as a function of chiral handedness, thus yielding optically active SWNT samples.While the above-mentioned organic chemistries possess many advantages, including effectiveness in the manufacture of SWNT thin-film transistors, 30 the most impressive chiral selectivity has been achieved using DNA. A DNA strand is negatively charged, so in an aqueous solution it is able to wrap itself around positively charged Nanotubes that have the same twist as that of the DNA. Thus the Nanostructure is skimmed off by using a positive electrode that attracts the negatively charged DNA strand. Due to its ability to efficiently disperse SWNTs in aqueous solution, DNA has been widely used in SWNT sorting techniques, including ultracentrifugation 31-32 and chromatography 33-36.
Although these early efforts identified some dependence of SWNT sorting effectiveness on the DNA sequence, a systematic search for DNA sequences that yield single-chirality sensitivity has been reported only recently. In particular, 20 distinct DNA sequences of different twist and negatively charged wrapping have been discovered to sort Nanotubes of a specific chirality. 36While this DNA-based approach provides exceptional chiral purity, these samples have not yet been subjected to rigorous testing in electronic or optoelectronic applications. Density-Gradient UltracentrifugationDensity-gradient ultracentrifugation (DGU) is a bio inspired sorting technique that allows SWNTs to be separated by their buoyant density.
29,37 In DGU, SWNTs are loaded into a density gradient that is intentionally formed in a centrifuge tube. In the presence of a centrifugal field, the SWNTs experience a driving force that induces motion toward their respective isopycnic points (i.e., the location where the buoyant density of the SWNT matches the local density of the gradient). Once the SWNTs have layered in the gradient according to their buoyant density, established fractionation schemes are employed to extract the density-sorted SWNTs. Recent work also has demonstrated the compatibility of SWNT DGU with electrolytes, perylene surfactants, covalent functionalization, sucrose gradients,41-45 and organic solvents.
By operating in the transient regime, DGU has further been employed for length fractionation of SWNTs. Ultimately, the flexibility of DGU has been exemplified by its recent application to a variety of other nanomaterial, including double-walled carbon nanotubes, 48-52 MWNTs,51 ultra-short SWNT capsules,52 single-walled carbon nanohorns, gold nanocrystals, and graphene. Agarose GelDrawing inspiration from biochemistry, where agarose which is a type of marine algae gels are commonly used in bio separation techniques, recent work has focused on the use of agarose gels for SWNT sorting. Agarose gels have since been engaged in a diverse range of SWNT sorting schemes, including “freeze and squeeze,” centrifugation, diffusion, and permeation. The freeze and squeeze procedure—in which an agarose gel containing SWNTs and SDS is frozen, thawed, and squeezed—possesses the distinct attribute of experimental simplicity. While the resulting SWNT purity (95% semiconducting, 70% metallic) is less competitive than many other sorting techniques, agarose gel–sorted semiconducting SWNTs have been successfully employed in thin-film FETs.
Selective Growth As discussed in the previous section, significant progress has been made in the post-synthetic separation of carbon nanotubes. However, since these post- synthetic processes are often time consuming and involve solution-phase processing that might cause contamination or degradation, it is desirable to develop gas-phase selective growth or etching methods that are compatible with conventional semiconductor processing. In particular, this section describes progress toward the production of semiconducting SWNT films via direct synthesis or post-synthetic chemical etching that does not involve the SWNT solvation step. The semiconducting SWNT films should be well-aligned and uniform over the entire wafer for optimal FET performance. Indeed, significant progress has been made along both directions: Chemical vapor deposition (CVD) methods with and without plasma enhancement have been used for preferential production of SWNTs, with a high percentage of semiconducting nanotubes (~90%) or even SWNTs with a specific chirality distribution; and post-synthetic chemical etching has been demonstrated for the selective removal of undesired metallic nanotubes from SWNT thin films. More recently, it has been shown that horizontally well-aligned semiconducting SWNTs can be directly grown with high uniformity over large areas, representing a significant advance in the selective production of SWNTs for semiconductor electronics.
Overall, this section provides a tutorial review of recent literature concerning the preferential growth of semiconducting SWNTs and selective etching of metallic SWNTs. Preferential Growth of Semiconducting Carbon NanotubesArguably, the first indication that SWNTs could be grown selectively was the observation of a high percentage of (6,5) and (7,5) SWNTs among semiconducting nanotubes grown from Co/Mo catalysts (CoMoCAT), where (6,5) and (7,5) are the (n,m) components of the nanotube chiral vector. In this early work, researchers used two-dimensional fluorescence spectroscopy to identify the chiralities of all the semiconducting nanotubes in the sample. They discovered that CoMoCAT samples show two dominant structures: (6,5) and (7,5), which together account for 57% of the semiconducting nanotubes. Assuming that metallic nanotubes comprise one-third of the total, then the (6,5) and (7,5) structures represent 38% of all SWNTs in the CoMoCAT sample.
Similar chirality selective growth also has been observed in other catalyst/precursor systems, including growth on Co–MCM-41 cobalt impregnated mesoporous molecular sieve catalysts using CO as the precursor and ethanol growth at low pressure and low temperature.The preferential growth of semiconducting SWNTs was first observed in low-temperature plasma-enhanced CVD (PECVD) experiments.76 In particular, when the growth temperature was reduced to 600°C, it was observed that the PECVD method preferentially yields semiconducting nanotubes with purities approaching 90%. The SWNT samples grown using the methods discussed previously are random films or powder samples, which are not in the desired geometry for electronic applications. Powder samples still need to be purified and deposited in a controlled manner onto a substrate for device fabrication and the random nanotube films contain a large number of nanotube- nanotube junctions and overlapping nanotubes that can degrade the performance of nanotube devices.
Control of nanotube alignment has been a longstanding challenge for researchers in this field. Ideally, selective growth should be able to yield two types of aligned nanotube samples: vertically aligned nanotube forests and horizontally aligned nanotube arrays. Although the growth of vertically aligned MWNTs has been known for some time, the synthesis of vertically aligned SWNTs was achieved only recently. Furthermore, the selective growth of semiconducting vertically aligned SWNTs has since been reported. In these experiments, semiconducting vertically aligned SWNT forests were achieved using combined PECVD and rapid heating, with a low-pressure (30 mTorr) C2H2 flow as the carbon source.
Using Raman spectroscopy, the authors estimated semiconducting purities of ~96%. This purity estimation was corroborated by FETs that demonstrated high on/off ratios. Even though vertically aligned SWNTs can be directly used for certain electronic devices, horizontally aligned nanotube arrays are more suitable and desirable in most cases due to straightforward integration with existing Si technology. The growth of horizontally aligned nanotubes was first achieved using an externally applied electric field.
Later, laminar gas flow in the CVD chamber and single-crystal substrates also were employed to guide the horizontal growth. Among these methods, directed growth on single-crystal substrates has shown the most promise for producing large-area, well-aligned, uniform arrays of SWNTs that can be directly used for FET fabrication. Most recently, the simultaneous demonstration of horizontal alignment and high-purity semiconducting SWNTs (>98%) has been achieved for a precise set of growth conditions on quartz substrates. The researchers discovered that mixed methanol and ethanol precursors yielded selective growth of semiconducting SWNTs only on ST-cut quartz substrates. The researchers also directly fabricated FET devices with high on/off ratios. Even though the mechanism is still not fully clear, the empirical results represent an important milestone toward the application of SWNTs in electronic technology.Selective Etching of Metallic Carbon NanotubesAlternative approaches to the selective growth of all semiconducting nanotubes include selective etching of metallic nanotubes 44and/or manipulation of the electronic properties of SWNTs through covalent chemical functionalization.44,45 In either case, the desired final product is an SWNT film with semiconducting character suitable for electronic devices.
IBM reported the first successful selective removal of metallic nanotubes from a carbon nanotube film. This approach has been used to permanently modify MWNTs117 and SWNT ropes using current-induced electrical breakdown to eliminate either individual shells one at a time in MWNTs or to selectively remove metallic nanotubes in an SWNT bundle. However, this method requires that the nanotubes be connected to metal electrodes and that a third electrode is available to apply a gate potential in order to minimize electrical conduction in semiconducting SWNTs and thus achieve the selective breakdown of the metallic SWNTs.
Even though devices with high performance can be fabricated in this manner, it is difficult to scale up this method for the fabrication of a large number of devices. Subsequently, selective chemical modification of metallic nanotubes was developed by various groups to achieve similar results. For example, diazonium salts have been selectively reacted with metallic nanotubes, reducing their electrical conductivity, in an SWNT film in order to achieve high on/off ratios in thin-film FETs.45,46,47 Similarly, gas phase chemical reactions have been identified that selectively react with metallic nanotubes.
This latter work has shown that the selective reaction is sensitive to the diameter of the nanotubes. Overall, the covalent functionalization of metallic nanotubes has advantages and disadvantages compared to competing methods. Advantageously, these chemical reactions can be simultaneously applied to large areas, which enable process scalability.
On the other hand, even though covalent functionalization has demonstrated selectivity toward metallic nanotubes, some semiconducting nanotubes are affected, leading to degradation in device performance. Additionally, some chemical reactions, such as the reaction with diazonium salts, are reversible at elevated temperature, causing long-term stability issues in electronic devices. Recently, an alternative approach has been reported that significantly lowers the conductivity of metallic nanotubes to achieve low off-currents while maintaining sufficiently high carrier mobilities for improved device performance. Specifically, researchers showed that both device parameters can be concurrently manipulated by controlling the degree of functionalization. The approach is based on a controlled cyclo-addition reaction of HiPco SWNTs with fluorinated polyolefins, yielding a network of SWNTs that can then be dispersed in an organic solvent. The resulting semiconducting SWNT inks then are used to form percolating networks from which high-mobility devices are fabricated without further nanotube separation. ConclusionIn summary, substantial progress has been made in recent years toward the ultimate goal of producing homogeneous carbon nanotube materials suitable for high-performance electronic and optoelectronic applications. While the post synthetic sorting and selective growth approach outlined in this article often are viewed to be competing approaches, several complementary and cooperative opportunities exist.
For example, since the purity and yield of the output of any post-synthetic sorting scheme depends on the initial quality of the raw material, advances in selective growth immediately imply improved output from post-synthetic sorting techniques. Similarly, efforts to epitaxially grow SWNTs from seed material will benefit from the high-purity samples generated by nano – tube-sorting approaches. With high purity thin-film SWNT transistors already yielding mobilities of 100 cm2/Vs, on/off ratios of 49, high frequencies of 80 GHz, infrared electroluminescence, and mechanical flexibility, homogenous carbon nanotube materials are poised to affect a variety of electronic and optoelectronic technologies.
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