Carbon nanotubes (CNTs) are cylindrical nanostructures composed entirely of carbon atoms. These tubes are seamless cylinders of graphene, the single-atom-thick sheet of carbon, rolled up into a tube. A carbon nanotube’s diameter is typically in the nanometer range, about one hundred thousandth the width of a human hair, while its length can extend to several millimeters. The modern study of carbon nanotubes was significantly accelerated by the work of Sumio Iijima in 1991.
Atomic Structure and Classification
The fundamental building block of a carbon nanotube is a sheet of graphene, where carbon atoms are arranged in a repeating hexagonal lattice. The precise way this sheet is rolled determines the tube’s specific structure and its physical and electrical characteristics. CNTs are primarily grouped into two classifications based on their number of layers.
Single-Walled Carbon Nanotubes (SWNTs) consist of just one rolled-up layer of graphene, resulting in a tube with a diameter typically between 0.5 and 2.0 nanometers. These are the simplest and often the most structurally perfect form of CNT. Multi-Walled Carbon Nanotubes (MWNTs) are more complex, composed of multiple concentric tubes nested within one another. While MWNTs are generally thicker and more robust than SWNTs, their properties can be more complicated due to the interactions between the different layers.
The exact angle at which the graphene sheet is rolled, referred to as its chirality, determines the tube’s atomic arrangement along its circumference. This structural twist is defined by two indices, and it dictates whether the nanotube exhibits a zigzag, armchair, or chiral structure.
Defining Physical and Electrical Properties
Their mechanical strength is extraordinary, resulting from the strong covalent bonds between the carbon atoms within the seamless lattice. Carbon nanotubes are considered one of the strongest and stiffest materials known, possessing a tensile strength significantly greater than that of steel while maintaining a low density. This high strength is coupled with remarkable flexibility, allowing CNTs to be bent considerably without fracturing.
Depending on the roll-up angle, a CNT can behave either as a metal, conducting electricity with almost no resistance, or as a semiconductor, where its conductivity can be switched on and off. Metallic CNTs can theoretically carry an electrical current density over a thousand times greater than copper wiring.
Carbon nanotubes are highly efficient conductors of heat along their length. The rigidity of the carbon-carbon bonds effectively transmits thermal vibrations, leading to excellent thermal conductivity. This property makes them highly effective at dissipating heat, which is advantageous for thermal management systems in miniaturized electronics.
Primary Manufacturing Techniques
The three main methods used to synthesize CNTs are arc discharge, laser ablation, and chemical vapor deposition (CVD). These methods offer varying degrees of control over the quality, quantity, and type of nanotube produced.
Arc Discharge
The arc discharge method involves creating a high-current electric arc between two graphite electrodes in an inert gas atmosphere, such as helium. The intense heat vaporizes the carbon from one electrode, which then cools and condenses on the other, forming nanotubes. This process is known for producing high-quality CNTs with minimal structural defects.
Laser Ablation
Laser ablation uses a high-powered pulsed laser to strike a graphite target mixed with metal catalysts, such as nickel and cobalt. The target is placed inside a high-temperature reactor where the vaporized carbon forms nanotubes as it cools. This technique offers a high yield and is particularly effective at producing high-purity single-walled nanotubes, although it is generally more expensive than other methods.
Chemical Vapor Deposition (CVD)
CVD is the most common method for industrial-scale production because of its scalability and cost-effectiveness. This process introduces a carbon-containing gas, such as methane or acetylene, into a reactor containing a substrate coated with metal catalyst nanoparticles. The gas decomposes, and the carbon atoms deposit onto the catalyst particles, self-assembling to grow the nanotube structures. CVD provides the best control over the final properties and allows for the continuous growth of CNTs.
Current and Emerging Applications
In materials science, CNTs are used to create advanced composites, acting as a reinforcing agent to produce stronger and lighter materials. This has led to their incorporation into high-performance products, such as aerospace components, specialized sporting equipment like bicycle frames and tennis rackets, and lightweight structural adhesives.
In the electronics field, the ability of CNTs to be either metallic or semiconducting makes them attractive for next-generation devices. They are being explored as replacements for traditional silicon in smaller, faster transistors and as conductive inks for flexible and transparent electronic displays.
Researchers are investigating the potential of CNTs as sophisticated drug delivery systems, where they can be chemically modified to target specific cells, such as those in tumors. CNTs are also used in the development of highly sensitive biosensors for the noninvasive monitoring of biological markers in the body.
In energy storage, CNTs improve the performance of advanced batteries and supercapacitors. Their high electrical conductivity and large surface area allow them to rapidly gather and transport charge, which is crucial for increasing charging speeds and overall energy density. CNTs provide mechanical support to battery electrodes during repeated charging and discharging cycles, helping to extend the lifespan of devices.