Single-walled carbon nanotubes (SWCNTs) are one-dimensional allotropes of carbon atoms with sp2 hybridization prepared by rolling graphene sheets up to adopt a tubular morphology. In general, one, two or more rolls of graphene sheets are known as single- (SWCNTs), double- (DWCVTs) and multi-walled graphene nanotubes (MWCNTs), respectively. Technically, single-walled carbon nanotubes have found applications in various fields of science, technology, new-age nanoelectroic devices and industry due to their remarkable and unique chemical and physical properties. With regard to the few techniques presented so far, carbon nanotubes are obtained based on the so-called physical top-down synthesis method 1. Carbon nanotubes are considered as the building blocks of molecular electrical circuits and they not only have the potential to be employed as interconnectors between active molecular elements in a given device, but are also capable of serving as the device element itself.
Properties of Carbon Nanotubes
Since their invention in 1991, carbon nanotubes have been drawing attention thanks to their structural and physicochemical properties. Carbon nanotubes in general, show unique thermal and chemical stabilities, higher tensile strength and perfect transport conductivity. Even though unmodified carbon nanotubes possess favorable potential in many fields, functionalized and modified carbon nanotubes have shown to adopt excellent biomedical and industrial applications. Single-walled carbon nanotubes diameter and helicity are described by the roll-up vector connecting equivalent based on their crystalline structure. Their electronic-band structure calculations determine whether a given single-walled carbon nanotube is a semiconductor or a metal. It is always instructive to consider the electronic properties of graphite in order to understand the single-walled carbon nanotubes electronic properties in a carbon-carbon sp2 hybridization. Graphite is known to be a zero-gap or semimetal semiconductor with their conduction and valence band degenerate at six points. Therefore, a particular piece of the graphene sheet with its two-dimensional (2D) nature rolls up to create the one-dimensional (1D) single-walled carbon nanotube inhering the mother graphite’s properties in an amplified quality behaving like a metal or a semiconductor based on the crystalline lattice geometry. Regarding this, almost two-third of SWCNTs are semiconductors with one-third being metallic agent 2.
Synthesis and Preparation of Single-Walled Carbon Nanotubes
Over the past twenty years, there have been a lot of attempts to introduce an efficient method to produce carbon nanotubes in industrial scales. Among the common techniques, laser ablation, arc discharge and chemical vapor deposition (CVD) have appeared to result in products with mixed and varying properties which can limit their compatibility and applicability. Considering all the troubles and problems of mass and industrial production of carbon nanotubes, annual single-walled carbon nanotube production as estimated to be some seven tons per year, while multi-walled carbon nanotubes are expected to hit 300 tons in a year. The problem is the synthesis methods mentioned above yield a mixture of single- double- and multi-walled carbon nanotubes with various physical properties and diameters. This is enough motif for researchers from around the world to put as much effort to suggest methods with high product selectivity.
Figure: Single Walled Carbon Nanotube (SWCNT)
Generally, the single-walled carbon nanotubes synthesis methods are categorized as substrate-free growth techniques and substrate-bound growth technique. The substrate-free carbon nanotube growth comprises a series of large-scale industrial efforts because of the simplicity in mass production with a continuous process compared to the batch process. Primarily, there are two techniques based on the substrate free growth process namely laser ablation and arc discharge, while the substrate-bound growth process is mostly tailored to some specialized fields in science and electronics. The substrate-bound technique involves the use of a substrate that maintains carbon nanotubes during the growth process. The growth process in substrate-bound techniques involves base-growth in which carbon nanotubes growth takes place when the interface of the growth substrate is in contact with the catalyst nanoparticles layer. The other method in substrate-bound category is tip-growth where the catalyst nanoparticles lift off the substrate and then are supported during the growth process 3.
Applications of Single-Walled Carbon Nanotubes
Carbon nanotubes are synthesized in a variety of morphologies such as short, long, open, closed, single, double and multi-walled types based on a particular application. Each morphology demands a special production cost and are produced in larger quantities for commercial purposes. Surprisingly, single-walled carbon nanotubes have found a lot of applications as catalysts, flat panel displays, polymer additives, nanopores for STM, electron field emitters as elements for cathode ray lighting and in energy conversion, anodes in biomedicine, lithium batteries, gas discharge tubes for telecommunication networks, atomic force microscopy (AFM), hydrogen storage, nanotube composites, shielding for electromagnetic radiation, coatings and fillings, nanolithography, drug delivery, sensor designing semiconductors , field emission, nanoelectrodes and supercapacitors. Prior to use, biomedical applications of single-walled carbon nanotubes for example, have to include functionalization and modification in order to make them compatible to pharmacological and toxicological standards and precautions. Since carbon nanotubes ae insoluble in aqueous solutions, their surface has to be functionalized with proper hydrophilic groups to make them soluble.
Figure 2: Tem images a, showing a 5 nm projected diameter carbon nanotube that is bent like letter “z” but still not broken; b, showing SWNT bundles with some wavy parts.
In field emissions applications, carbon nanotubes are well-known for their field emitting properties due to their high electrical conductivity and considerable sharp tips. The tip sharpness quality in field emission determines how they process emission at low voltages which is a significant property for designing and manufacturing of electrical devices with low power. Carbon nanotubes are capable of carrying huge amount of electrical current density while it remains extremely stable. These properties make single-walled carbon nanotubes practical candidates to be employed in field-emission flat-plane display screens. Carbon nanotube-based displays are equipped with a separate electron gun for any single pixel to achieve the highest quality. In energy sector, carbon nanotubes have specific and important properties needed for materials with applications in capacitors and batteries and electrodes. Carbon nanotubes’ perfect electrical conductivity, linear geometry and incredible high surface area make them excellent agents to be applied as electrolytes. They are also used in tire industries within the polymers, ceramics, conductive adhesives and connectors and nano porous filters.
Single-walled carbon nanotubes (SWCNTs) as well as double- and multi-walled carbon nanotubes with the inherent amplified properties of graphite have been causing huge and incredible impacts on the outcomes of scientific studies results, industrial products and electronic due to their physical and chemical properties along with their flexibility towards medication and functionalization for more and more compatibilities. The SWCTs surprising tubular morphology with the diameter to length ratio of 1 to 1000 exhibit distinct physicochemical characteristics compared to those of MWCNTs and DWCNTs and are extensively applied in miniaturizing electronics and as conductors.