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Carbon nanotubes (CNTs) are cylindrically shaped carbon sheets, typically existing as the allotrope graphite. Carbon nanotubes are generally grouped on several factors that influence their properties such as shape, arrangement of carbon atoms,and whether the CNTs are single-walled or multi-walled [1]. As a result of the strong carbon-to-carbon bonding, CNTs exhibit unique properties in material sciences and electronics that are finding numerous applications in science and consumer products [2].

Carbon nanotubes are currently being researched as an emerging technology in numerous fields such as electronics, optics, materials uses, and medicine. The intricate production process of carbon nanotubes are a barrier to entry into many mass market applications and new methods are being explored to allow development of nanotubes as well as customization to meet demands of the aforemention industries. Development of “ultra-long” carbon nanotubes with extremely high length-to-diameter ratios is being researched due to their potential to greatly increase the applications of CNTs across their many fields of use.


Configuration

As with many allotropes of carbon, graphite utilizes sp2 carbon bonds, meaning every atom of carbon is bonded with three other carbon atoms. Given a single sheet of carbon atoms, one can manipulate the material to achieve slight variations of cylindrical shape, length, and diameter. A set of vectors denoted by (n,m) represent how a graphite sheet is manipulated to achieve the tubular shape. For example, a larger vector value will yield a large diameter where instead a lower value could give the tube an angular structure with a larger length. For mechanical and electrical properties of CNTs, it is these aspects that can alter how the nanotubes behave. [3]

Graphene, a single-atom graphite sheet, can be manipulated into a single tube-like structure referred to as a single-walled carbon nanotubes. Alternatively, a series of increasingly smaller nanotubes contained within one another that makes for parallel layers of graphene are known as multi-walled carbon nanotubes. (Insert public domain photograph of single and multi-walled carbon nanotubes)

Properties

Many of the unique properties commonly associated with carbon nanotubes can be attributed to their symmetrical and rounded form a well as the strength of the sp2 carbon-to-carbon bonds. Sp2 bonds are among the strongest atomics bonds in chemistry due to the configuration of valence electrons and the energy levels of carbon atoms[4]. Aligned nanotubes, or CNTs that are parallell with one another, can lead to some electrical or mechanical outcomes that are not otherwise possible [5].

Mechanical

At their peak, carbon nanotubes are shown to have 5 times the stiffness of steel and 50 times the tensile strength. Strength of CNTs is generally determined by the density of the shape of the bonding pattern, diameter of the nanotube, and whether or not the CNTs are aligned. However, when placed under a sufficiently high strain, carbon nanotubes are shown to undergo a permanent breakdown at the molecular level. "Ultra-long" carbon nanotubes, with lengths of around several centimeters, provide the ability to weave many strands of aligned single-walled carbon nanotubes together, resulting in dense nanotube strands with several times the strength and resistance [6].

Electrical

The uniformity of carbon bonds means carbon nanotubes have potential to have different electrical conductivities. Carbon Nanotubes are typically either semiconductors or conductors as conducive as metals such as copper [7]. The relationship between the array of carbon atoms (n, m) is the key factor in determining whether a nanotubes is conductor or semiconductor.

Multi-walled carbon nanotubes are being investigated as superconducting material due to their symmetry combined with cascaded nanotubes, superconductivity can be achieved by “linking” the different layers of the nanotubes. That being said, the temperature of operation for superconductivity is around 14K. While a temperature this low is impractical for mainstream uses, it is relatively higher than other superconducting materials [8]. Single walled nanotubes may also exhibit superconductivity but have not been demonstrated to do so at the nanotube lengths of their multi-walled counterparts [9].

Applications

Electronics

Due to their versatile electrical properties, carbon nanotubes are being utilized as a material that can lead to highly improved electronics in many regards. When functioning as a semiconductor, carbon nanotubes can be applied towards developing field effect transistors (FETs) on the molecular scale that operate properly at room temperature, a function not all electronics have. Nano-scale transistors made with CNTs have paved the way for immensly small logic circuits that are smaller than most macoscopic transistors utilized in electronics. Many staples of digital logic circuits can be fabricated using several single nanotube circuits such as inverters and logical NOR gates [10]. Research is being conducted to explore the use of CNTs as memory units. While some success has been made in the field, CNTs tend to dissipate the charge after several hours.

Carbon nanotubes can be imbuedin a thin solid as a thin film. This film has many versatile characteristics of carbon nanotubes such as mechanical strength and high electrical conductiance. Nanofilaments typically have very high molecular surface areas but without the stregth or conductivity of CNTs. However, CNT films combine these many traits to form a more advanced hybrid of nanofilms and CNTs. In applications such as capacitances, where surface area increases the amout of energy stored, carbon nanotube films provide alternatives or possible to progress technology at the microscopic scale. In addition to energy storage, CNT films are expanding to implementation in a number of fields including electrodes in batteries, photovoltaic cells, and light-emitting diodes.