In 2006, Marc Monthioux and Vladimir Kuznetsov published in an editorial in the magazine Carbon the story of the discovery of carbon nanotubes.[5]​ The identity of the discoverers is the subject of some controversy.[6]​ A large percentage of academic and popular literature attributes the discovery of nanometer-sized hollow tubes composed of graphitic carbon to Sumio Iijima, an NEC employee, in 1991. Iijima published an article which had a lot of impact and promoted research into the applications of carbon nanotubes. However, the first observations of carbon nanotubes date back much earlier: in 1952, Radushkevich and Lukyanovich published clear images of carbon tubes 50 nanometers in diameter in the Journal of Physical Chemistry Of Russia.[7] This article largely went unnoticed, due to its publication in Russian and the limited access of Western scientists to the Soviet press during the Cold War. Monthioux and Kuznetsov mentioned in their editorial:

In 1976, Morinobu Endo of the CNRS synthesized hollow tubes of rolled graphite sheets using a vapor phase growth technique. The first specimens observed would later be known as single-walled carbon nanotubes or SWNTs. Endo, in his initial study of carbon fibers, also observed a hollow, linearly extended tube with parallel faces of carbon layers near the core of the fiber.[8] This appears to be the first observation of multi-walled carbon nanotubes or MWCNTs. Mass-produced multi-walled carbon nanotubes today are obtained by a method with similarities to that developed by Endo, actually called the "Endo-process", in recognition of his early work and patents.[9] In 1979, John Abrahamson presented a paper at the 14th Biennial Carbon Conference at Pennsylvania State University that described carbon nanotubes as carbon fibers produced on carbon anodes during an arc discharge. Abrahamson characterized these fibers and proposed a hypothesis for their growth in a nitrogen atmosphere at low pressures.[10]​.

In 1981, a group of Soviet scientists published the results of the chemical and structural characterization of carbon nanoparticles produced by a thermocatalytic disproportionation of carbon monoxide. Using TEM images and Among them, a circular conformation (armchair nanotube) and a spiral helical arrangement (chiral tube).[11]​.

In 1987, Howard G. Tennent of Hyperion Catalysis was granted a US patent for the production of “cylindrical discrete carbon fibrils” with a fixed diameter between approximately 3.5 and 70 nm, a length two orders of magnitude greater than the diameter, and an outer region composed of essentially continuous multiple layers of ordered carbon atoms and a distinct inner core.

Iijima's 1991 discovery of multi-walled carbon nanotubes in the insoluble material resulting from burning graphite rods with an electric arc generated intense interest among the scientific community, as did Mintmire, Dunlap, and White's independent prediction that if single-walled carbon nanotubes could be made, they would exhibit remarkable conductive properties. Nanotube research was greatly accelerated following the discoveries by Iijima and Ichihashi at NEC and Bethune et al. at IBM of methods for producing single-walled carbon nanotubes by adding transition metal catalysts to carbon in an arc discharge. Tess and co-workers refined this catalytic method by vaporizing the carbon/transition metal combination in a high-temperature furnace, greatly improving the yield and purity of the nanotubes and subsequent characterization and application experiments. The arc discharge technique, known to produce buckminsterfullerene molecules in sufficient quantities to explore its properties,[12] therefore played an important role in the discoveries of single-walled and multi-walled nanotubes and extended the series of serendipitous discoveries related to fullerenes.

In 2020, the Keezhadi archaeological dig in Tamil Nadu, India, revealed ceramics about 2,500 years old whose coatings appear to contain carbon nanotubes. The strong mechanical properties of the nanotubes could have contributed to the preservation of the coatings for so many years.[13]​.

The structure of an ideal (infinitely long) single-walled carbon nanotube is that of a regular hexagonal network arranged on an infinite cylindrical surface, whose vertices are the positions of the carbon atoms. Since the length of the carbon-carbon bonds is more or less fixed, the diameter of the cylinder and the positions of the atoms on it are not arbitrary, but are restricted to certain values.[14]​.

To characterize the structure of nanotubes, a zigzag trace in a graphene-like hexagonal lattice is defined as a path "Path (graph theory)") that rotates 60°, alternating left and right, after traversing each bond. By convention, a walk that describes two turns to the left followed by two turns to the right every four steps is called an "armchair walk." In some carbon nanotubes, a closed zigzag path can be traced around the tube, and the tube is classified as a zigzag type or configuration, or simply as a "zigzag nanotube." If, on the other hand, the trace around the tube is a closed armchair path, it is said to be of the armchair type or an "armchair nanotube." Infinite zigzag or armchair nanotubes consist entirely of closed zigzag or armchair paths, connected to each other.

Zigzag and armchair configurations are not the only conformations that a single-walled nanotube can adopt; The structure of an infinitely long tube can be represented as imaginary sections formed by a cut parallel to its axis that passes through an atom A; The sections are superimposed on the structure formed by a flat graphene sheet so that the atoms and bonds coincide. The two halves of the A atom are located on opposite edges of the strip, above the A1 and A2 atoms of the graphene. The line from A1 to A2 corresponds to the circumference of the cylinder that the atom A passes through, and is perpendicular to the edges of the cut. In the graphene lattice, atoms can be divided into two classes, depending on the directions of their three bonds. In half of the atoms, their three bonds are oriented the same way, while in the other half the bonds are rotated 180°. The atoms A1 and A2, which correspond to the same atom A in the cylinder, must be of the same class. It follows that the circumference of the tube and the angle of the section depend on the lengths and directions of the lines connecting pairs of graphene atoms of the same class.

Let y be two linearly independent vectors connecting the graphene atom A1 to its two closest atoms with the same bond directions. That is, if the carbons arranged consecutively around a graphene cell are named C1 - C6, then it is the vector between C1 and C3 and the vector between C1 and C5. Then, for any other atom A2 with the same class as A1, the vector from A1 to A2 can be written as a linear combination, where and are integers. And, conversely, each pair of integers (,) defines a possible position of A2.[14]​.

Given and , you can reverse the theoretical operation and draw the vector on the graphene lattice, cut a strip along the lines perpendicular to through its ends A1 and A2, and roll the strip into a cylinder that joins the two points. If this construction is applied to a pair (k ,0), the result is a zigzag nanotube, with closed zigzag paths of 2 k atoms. If applied to a pair ( k, k ), we obtain an armchair tube, with closed armchair paths of 4 k atoms.

Contenido

La estructura del nanotubo no cambia si la tira se gira 60° en el sentido de las agujas del reloj alrededor de A1 antes de aplicar la reconstrucción hipotética descrita anteriormente. Tal rotación cambia el par (,) al par (,). De ello se deduce que muchas de las posibles posiciones de A2 con respecto a A1, es decir, muchos pares (,), corresponden a la misma disposición de átomos en el nanotubo. Ese es el caso, por ejemplo, de los seis pares (1,2), (-2,3), (-3,1), (-1,-2), (2,-3) y (3 ,-1). En particular, los pares ( k ,0) y (0, k ) describen la misma geometría del nanotubo. Esta redundancia se puede evitar considerando sólo pares (,) tales que 0 y 0; es decir, donde la dirección del vector se encuentre entre las de (inclusive) y (exclusive). Se puede verificar que todo nanotubo tiene exactamente un par (,) que cumple esas condiciones, al que se denomina «tipo de tubo».

Dos nanotubos pertenecen al mismo tipo si, y solo si, coincididen exactamente con el otro trans una rotación y traslación. En lugar del tipo (,), la estructura de un nanotubo de carbono se puede especificar con la longitud del vector (es decir, la circunferencia del nanotubo) y el ángulo entre las direcciones y , comprendido entre 0 (inclusive) y 60° (exclusive) en el sentido de las agujas del reloj. Si el diagrama se dibuja con horizontal, define la inclinación de la tira con respecto de la vertical.

Chirality and mirror symmetry

A nanotube is chiral if it is of type (,) with and ; Its enantiomer (mirror image) is of the type (,). The only types of nanotubes that are achiral are zigzag configuration tubes (k,0) and armchair configuration tubes (k,k). Since the enantiomers have the same structure, only the types (,) with and can be considered. The angle between and , which can take values ​​between 0 and 30°, is called the "chiral angle" of the nanotube.

Circumference and diameter

From y you can calculate the circumference, which is the length of the vector:.

The diameter of the tube is:.

These formulas are only approximations, especially for small y where the bonds are strained; They also do not take into account the thickness of the wall.

The angle between y and the circumference are related to the indices and by:.

where is the clockwise angle between the horizontal and the coordinate vector. In turn, given and , the type can be obtained using the formulas:.

which must result in whole numbers.

Diameter

If the indices and have extremely low values, the structure of the molecule is not tubular and in many cases it is not even stable. For example, the structure defined by the pair (1,0), of the zigzag type, is simply a chain of carbons; Such a molecule actually exists: it is called carbyne and shares some properties with nanotubes—such as orbital hybridization and high tensile strength, among others—but it does not have a hollow space inside and may not be able to be synthesized as a condensed phase. The pair (2,0) would theoretically produce a chain of four fused rings; and (1,1), the armchair type would consist of a chain of four biconnected rings. These structures have not been observed experimentally.

The thinnest carbon nanotube observed is the armchair structure with (2,2) type, which has a diameter of 0.3 nm. This nanotube was grown inside a multi-walled carbon nanotube. The assignment of the nanotube type was carried out using a combination of high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and density functional theory (DFT) methods.[15]​.

The thinnest single-walled carbon nanotube isolated as a free-standing molecule is approximately 0.43 nm in diameter. It has been suggested that it may be of type (5,1) or (4,2).

Length

The longest carbon nanotubes observed as of 2013 were about 0.5 m long. These nanotubes were grown on silicon substrates using an improved chemical vapor deposition (CVD) method and form electrically uniform arrays of single-walled carbon nanotubes.

The shortest carbon nanotube is considered to be the organic compound cycloparaphenylene, synthesized in 2008 by Ramesh Jasti.[19] Since then, other small molecules made up of carbon nanotubes have been synthesized.[20]​.

Density

The highest density nanotubes were obtained in 2013, by cultivation on a titanium-coated copper surface coated with the cocatalysts cobalt and molybdenum at temperatures lower than the typical 450 °C, these tubes reached an average length of 380 nm and a density of 1.6 g cm. The material exhibited ohmic conductivity, with a minimum resistance of ~22 kΩ.[21][22]​.

Multi-walled nanotubes

Multi-walled nanotubes consist of several concentric graphene tubes. The distance between layers in multi-wall tubes is roughly the distance between the graphene layers in graphite, approximately 3.4 Å. Multi-walled nanotubes can adopt two different structures, characterized by different models. The "roll (manuscript)" scroll model describes a single layer rolled over itself. In the "matrioshka" model, the most common, the graphene layers are arranged in concentric cylinders; for example, a single-walled nanotube (0.8) inside a larger nanotube (0.17). The component tubes can be metallic or superconducting. Due to statistical probability issues and restrictions on the diameter of the concentric tubes, the entire structure is a metal with a bandgap equal to zero.[23]​.

Double-walled carbon nanotubes are especially interesting, because their morphology and properties are similar to those of single-walled nanotubes, but they are more resistant to chemical attacks;[24]​ this property is important for adding functional groups to the surface of the nanotubes to give them different properties. In single-walled tubes, this process destroys double bonds, leaves "holes" in the nanotube and causes unwanted modifications to its electrical and mechanical properties. In double-walled nanotubes, only the outer layer is affected. The large scale (gram) synthesis of double-walled nanotubes using the CCVD technique was introduced in p 2003 from the selective reduction of oxide solutions into methane and hydrogen.[25]​.

The telescopic movement capacity of the inner layers of multi-walled nanotubes,[26]​ together with their unique mechanical properties,[27]​ could lead to their use as moving arms in nanomechanical devices. The retraction force that occurs in telescopic movement is caused by the Lennard-Jones interaction between the walls of the nanotube, and its value is approximately 1.5 nN.[28]​.

Unions and crossings

Junctions between two or more nanotubes have been the subject of theoretical discussions,[29][30]​ and are quite frequently observed in samples prepared by arc discharge as well as by chemical vapor deposition. Lambin et al. analyzed the theoretical electronic properties of such junctions and pointed out that a connection between a metallic tube and a semiconductor tube would represent a nanoscale heterojunction, which could form part of an electronic circuit based on nanotubes.[31]​.

The junctions between nanotubes and graphene have been studied both theoretically[32]​ and experimentally,[33]​ and form the basis of pillared graphene, in which parallel graphene sheets are separated by short nanotubes.[34]​ Pillared graphene represents a class of three-dimensional carbon nanotube architectures.

Recently, several studies have highlighted the possibility of using carbon nanotubes as building blocks for macroscopic (greater than 100 nm) three-dimensional carbon devices. Lalwani et al. published a new radical-initiated thermal cross-linking method to fabricate macroscopic porous carbon structures using single- and multi-walled carbon nanotubes as building blocks.[35] These structures possess pores of macro-micro- and nanoscopic dimensions, and the porosity can be tailored for specific applications. They can be used as devices for energy storage, supercapacitors, field emission transistors, high-performance catalysis, photovoltaic and biomedical devices, implants and sensors.[36][37]​.

Other morphologies

Carbon nanobuds or nanobuds are a material created in the 2000s, which combines carbon nanotubes and fullerenes. They are made up of spherical molecules, similar to fullerenes, covalently attached to the outer side walls of a carbon nanotube, like buds or "buds" on a branch. This hybrid material combines useful properties of fullerenes and carbon nanotubes. They are exceptionally good field emitters.[38]​ In composite materials, attached fullerene molecules can function as molecular anchors that prevent sliding of the nanotubes, thus improving the mechanical properties of the composite.

Carbon sheaths are another material that combines fullerenes with carbon nanotubes. In this case, the nanotube encapsulates the fullerene molecules inside, like peas in a pod.[39]​[40]​ They have interesting magnetic properties under certain heating and irradiation conditions. They can also be used as an oscillator in certain investigations.[41][42]​.

A nanotorus is a carbon nanotube folded on itself in the shape of a torus "Torus (geometry)") (shape of 8). Nanotori have many unique properties, such as very high magnetic moments for specific radii.[43]​ The magnetic moment, thermal stability, and other properties vary greatly depending on the radius of the torus and the radius of the tube.[43]​[44]​.

Carbon nanotubes can also be combined with graphene sheets grown along the side walls of multi-walled nanotubes. Sheet density can vary as a function of deposition conditions—for example, temperature and time—from less than a dozen graphene layers to a thicker, more graphite-like structure.[45]​ Deposition of a high density of graphene sheets along aligned CNTs can significantly increase the total loading capacity per unit nominal area compared to other carbon nanostructures.[46]​.

Carbon nanotubes can form a quasi-monodimensional structure similar in shape to a stack of cups (cup-stacked nanotubes or CSCNT); Unlike regular nanotubes and other carbon nanostructures, which normally behave as conductors, CSCNTs are semiconductors due to the stacking of graphene layers.[47]​.

Las propiedades mecánicas, eléctricas, ópticas y térmicas de los nanotubos de carbono de pared simple guardan más relación con la estructura del nanotubo —tipificada por los índices (,)—, que con otras propiedades geométricas: diferentes tipos de nanotubos, pueden tener propiedades radicalmente distintas. La estructura de bandas para un nanotubo de tipo (,) puede calcularse fácilmente.[48]​ En 1999, Hiromichi Kataura") introdujo un gráfico basado en estos cálculos para explicar resultados experimentales. Los gráficos de Kataura relacionan el diámetro del tubo con las bandas de energía y su forma oscilante ilustra la fuerte dependencia con (,).[49]​ Por ejemplo, los nanotubos de tipos (10,1) y (8,3) tienen un diámetro muy parecido, pero el primero se asemeja a un metal y el segundo es un semiconductor.

Mechanical properties

Carbon nanotubes are the strongest and stiffest molecules yet discovered in terms of tensile strength and elastic modulus. These mechanical properties come from the robustness of the covalent sp bonds formed between the individual carbon atoms. In 2000, a value of 63 GPa was obtained for the breaking stress of a multi-walled carbon nanotube;[2] this means that a wire with a cross section of 1 mm could support a weight of 6422 kg. Other studies, including one carried out in 2008, revealed that isolated nanotubes have a tensile strength of about 100 GPa, consistent with the predictions of quantum and atomistic models.[50] Given the low density of carbon nanotubes - between 1.3 and 1.4 g/cm -,[51] their specific strength of up to 48,000 kN m kg is the highest of known materials; For comparison, the equivalent value for high carbon steel is 154 kN·m·kg.

Although the resistance of single nanotubes is extremely high, the weak interactions between adjacent tubes lead to a significant reduction in the effective resistance of multi-walled carbon nanotubes and nanotube bundles, which can reach only a few GPa.[52]​ Irradiation by high-energy electrons, which creates connections between the surface and the inner tubes, increases the resistance of these materials to around 60 GPa for multi-walled carbon nanotubes,[50]​ and about 17 GPa for double-walled nanotube bundles.[52]​.

On the other hand, nanotubes do not show as much resistance to compression: due to their hollow structure and the high ratio between their length and width, they tend to collapse under compression, torsion or bending forces.[53]​ They are also quite soft in the radial direction and even Van der Waals forces between two adjacent nanotubes can deform them. Several groups have performed nanoindentations with an atomic force microscope and used contact atomic force microscopy to measure the radial elasticity of multi- and single-walled carbon nanotubes, respectively; The Young's modulus obtained is of the order of several GPa.

Electrical properties

Unlike graphene, which is a two-dimensional semimetal, carbon nanotubes can behave like metals or semiconductors along the tubular axis depending on their structure. A nanotube of type (, ) is a metal if; if it is a multiple of 3 and , the nanotube is a quasi-metal, with a very small bandgap; otherwise, the nanotube is a moderate semiconductor.[54] Thus, all armchair nanotubes are metallic, and (6,4), (9,1), etc. nanotubes are metallic. are semiconductors.[55] Carbon nanotubes are not semimetallic because the degenerate point (the point where the π [bonding] band meets the π* [anti-bonding] band, at which the energy goes to zero) moves slightly away from the K point in the Brillouin zone due to the curvature of the tube surface, which causes hybridization between the σ* and antibonding bands. π* and the modification of the band dispersion.

There are exceptions to the rule to determine whether a nanotube behaves as a semiconductor or metal; Specifically, the curvature of small diameter nanotubes influences the electrical properties: for example, a type (5.0) nanotube, which should be a semiconductor according to the rule, is actually a metal. Likewise, small diameter zigzag and chiral nanotubes have a bandgap and do not behave like metals, although this exception does not apply to armchair nanotubes.[56]​

In theory, metallic nanotubes can conduct a current density of 4 × 10 A/cm, which exceeds the conductivity of metals such as copper by three orders of magnitude.[57] There is interest in the use of carbon nanotubes in the interconnections of integrated circuits and components to improve the conductivity of composite materials. Many groups are attempting to commercialize conductive wire composed of nanotubes, despite the challenges presented by current saturation under voltage,[58]​ and the high resistivity of the junctions between nanotubes and impurities, which lowers the conductivity of the wires considerably with respect to that of individual nanotubes by several orders of magnitude.

Due to the cross section of the order of nanometers, electrons propagate only along the axis of the nanotubes, which is why they are called "one-dimensional conductors." The maximum electrical conductance of a single-walled carbon nanotube is , where is the quantum unit of conductance.[59]​.

The importance of the π electron system in the electronic properties of graphene causes different effects in the doping "Doping (semiconductors)") of carbon nanotubes and crystalline semiconductors from the same group of the periodic table—such as, for example, silicon. The replacement of carbon atoms in the nanotube wall with boron or nitrogen dopants is usually carried out by chemical vapor deposition and produces p-type and n-type materials respectively, as occurs in silicon. However, some nonsubstitutive (intercalated or adsorbed) dopants, such as alkali metals and electron-rich metallocenes, result in n-type conduction, because they donate electrons to the π-electron system of the nanotube. In contrast, π-electron acceptors such as FeCl or electron-deficient metallocenes function as p-type dopants because they remove π-electrons from the top of the valence band.

Optical properties

Carbon nanotubes emit infrared radiation under electrical excitation and exhibit photoconductivity. These optical properties allow their use as light-emitting diodes (LED)[67]​[68]​ and photodetectors[69]​ The unique optical property of carbon nanotubes is not efficiency, which is still relatively low, but rather the close selectivity in the wavelength of light emission and detection and the possibility of its fine adjustment by modifying the structure of the nanotubes. Other applications based on optical characteristics are bolometric[70]​ and optoelectronic memory devices.[71]​ Nanotube fluorescence has been investigated for use in detectors and sensors in the field of biomedicine.[72][73][74]​.

Thermal properties

Carbon nanotubes are very good thermal conductors in the direction of the axis, exhibiting a property known as "ballistic conduction"; At the same time, they are good insulators in the radial direction. Measurements show that the thermal conductivity of a single-walled nanotube at room temperature is approximately 3500 W m K along its axis;[75] In comparison, a metal considered a good thermal conductor, such as copper, transmits 385 W m K. In the radial direction, the thermal conductivity of the nanotube is around 1.52 W·m·K,[76]​ similar to that of soil.

In macroscopic assemblies of nanotubes, such as films or fibers, up to 1500 W·m·K have been measured.[77]​ Networks composed of nanotubes have different values ​​of thermal conductivity, from the level of thermal insulation with a thermal conductivity of 0.1 W·m·K to high values.[78]​ The specific value depends on the presence of impurities, misalignments and other factors. Carbon nanotubes are estimated to be stable up to a temperature of 2800 °C in a vacuum and around 750 °C in air.[79] In contrast, the metal wires in a microchip melt at temperatures between 600 and 1000 °C.

Crystallographic defects strongly influence the thermal properties of the tube. Such defects lead to phonon scattering, which in turn increases the phonon relaxation rate and reduces the mean free path and thermal conductivity of the nanotube structures. Phonon transport simulations indicate that substitution defects, such as nitrogen or boron, mainly lead to scattering of high-frequency optical phonons. However, larger scale defects, such as Stone–Wales defects, cause phonon scattering over a wide range of frequencies and cause a further reduction in thermal conductivity.[80] The thermal properties of nanotubes can also be modified by encapsulating metals or gases inside them.

There are several techniques to produce nanotubes in considerable quantities, such as arc discharge, laser ablation, chemical vapor deposition, and high-pressure carbon monoxide decomposition (HiPCO). Arc discharge and laser ablation are batch processes, chemical vapor deposition can be used for both batch and continuous processes,[81]​[82]​ and HiPCO is a continuous gas phase process.[83]​ Most of these processes take place in vacuum or, in methods that use a gas, in the presence of it.

Laser ablation consists of vaporizing a graphite target by irradiating a laser pulse in a high-temperature reactor and in the presence of an inert gas. Nanotubes form when vaporized graphite comes into contact with the cold surface and condenses on the walls of the reactor. This process usually has a typical yield of 70% by weight and produces monolayer nanotubes with a diameter that can be controlled by varying the temperature inside the reactor.

Arc discharge was the first technique with which it was possible to produce carbon nanotubes in appreciable quantities. The method consists of passing an electric current of hundreds of amperes between two graphite electrodes in an atmosphere of inert gas at low pressure. The subsequent discharge sublimes the carbon in the electrodes and fullerenes and carbon nanotubes are generated. The typical yield, using this technique, is of the order of 30% by weight and the products obtained are both monolayer and multilayer nanotubes of a typical length of about 50 microns "Micrometer (unit length)"). It can be combined with the oxidation purification method developed by Ebbesen in 1994,[84]​ which consists of heating the extracted fullerite after discharge to 1000 K, in an oxygen atmosphere for 30 minutes.

Chemical vapor deposition is widely used, since it has a very high performance and provides a certain degree of control over the diameter, length and morphology of the nonatubes. With the help of particle catalysts, large quantities of nanotubes can be synthesized using this technique, and several nanotube and nanotube fiber factories already exist in the world. A problem associated with this method is the high variability in the characteristics of the nanotubes.[85]​.

The HiPCO process has the potential to produce more commercially viable carbon nanotubes. [86]​ With this method, high purity single-walled carbon nanotubes are obtained in greater quantities. The HiPCO reactor operates at temperatures of 900-1100 °C and a pressure of 30-50 bar.[87]​ It uses carbon monoxide as a carbon source and iron pentacarbonyl or nickel tetracarbonyl as a catalyst. These catalysts provide a nucleation site for nanotubes to grow,[83]​ while chemical vapor deposition can use cheaper iron-based catalysts, such as ferrocene.

Thermal vapor deposition is also used to obtain vertically aligned carbon nanotube arrays. In this process, a substrate (quartz, silicon, stainless steel, carbon fibers, etc.) is coated with a layer of catalytic metal, usually iron, deposited by sputtering with a thickness of 1 to 5 nm. Often, the substrate is first prepared with a 10–50 nm layer of alumina, which imparts controllable wetting and good interfacial properties. To initiate the growth of nanotubes, two gases are mixed in the reactor. A process gas—such as ammonia, nitrogen, hydrogen, etc.—and another gas used as a carbon source—such as acetylene, ethylene, ethanol, methane, etc.—When the substrate is heated to the growth temperature (600-850°C), the continuous iron film breaks up into small islands where carbon nanotubes form. The diameter of the tube depends on the size of the island, which in turn is determined by the thickness of the catalyst layer. The time the metal island can remain at the growth temperature is limited, as they are mobile and can fuse into a smaller number of larger islands. Annealing at the growth temperature reduces the nanotube density.

If a plasma "Plasma (state of matter)") is generated by applying an intense electric field, during the growth process (known as plasma-enhanced chemical vapor deposition), then the growth of the nanotube will follow the direction of the electric field.

The prepared carbon nanotubes usually contain impurities, such as other carbon allotropes (amorphous carbon, fullerene, etc.) or the metal used as a catalyst.[88]​[89]​ These impurities must be removed to be able to use the nanotubes in various applications.[90]​.

Carbon nanotubes are known to disperse weakly in many solvents, such as water, as a consequence of strong intermolecular p-p interactions. This makes it difficult to process in industrial applications. To improve their stability and solubility in water, several techniques have been developed consisting of the modification of the surface of nanotubes.[4] Chemical methods, such as covalent functionalization, can be based on the oxidation of nanotubes with strong acids, such as sulfuric acid, nitric acid or a mixture of both to fix the carboxylic groups on the surface of the nanotubes as a final product or for subsequent modification by esterification or amination.[91]​ Free radical grafting is a promising technique that uses alkyl or aryl peroxides, substituted anilines and diazonium salts as starting agents.[92]​.

Free radical grafting of macromolecules onto the surface of the nanotubes results in improved solubility compared to common acid treatments, because the large functional molecules facilitate the dispersion of the nanotubes in a variety of solvents, even with a low degree of functionalization. In 2017, covalent functionalization of multi-walled carbon nanotubes was observed using nails. The nanotubes are functionalized in a vessel by a free radical grafting reaction and are then dispersed in water producing a highly stable nanfluidic aqueous suspension.[93]​.

There are many metrology standards and reference materials available for carbon nanotubes.[94]​.

For single-walled carbon nanotubes, the ISO has established a method based on optical absorption spectroscopy to measure the diameter, purity and fraction of metallic nanotubes,[95]​ as well as protocols to characterize the morphology and elemental composition that use transmission electron microscopy and scanning electron microscopy respectively, together with X-ray spectrometry analysis.[96][97]​.

It is NIST standard SRM 2483 is a single-walled carbon nanotube powder used as a reference material for elemental analysis. It has been characterized by thermogravimetric analysis, neutron activation analysis, neutron activation analysis, inductively coupled plasma mass spectroscopy, resonant Raman scattering, visible near-infrared fluorescence spectroscopy and absorption spectroscopy, scanning electron microscopy, and transmission electron microscopy.[98][99] The National Research Council of Canada also offers a certified reference material SWCNT-1 for characterized by neutron activation analysis and inductively coupled plasma mass spectroscopy.[94]​[100]​ NIST RM 8281 is a mixture of single-walled carbon nanotubes of three different lengths.[98]​ [101]​.

For multi-walled carbon nanotubes, ISO/TR 10929 identifies the basic properties and impurity content,[102]​ while ISO/TS 11888 describes morphological characterization using scanning electron microscopy, transmission electron microscopy, viscometry and light scattering "Scattering (physics)").[103]​ ISO/TS 10798 is also valid for multi-walled carbon nanotubes.[97]​.

Carbon nanotubes are used in industry and consumer goods. They consist of highly absorbent black battery components, polymers, and paint. There are other applications in development, such as field effect transistors for electronic components, high resistance fabrics, biosensors for biomedical and agricultural uses, among others.

Some examples of the use of carbon nanotubes are listed below:

Among the applications in development the following can be highlighted:

Scientific studies indicate that nanoparticles represent a greater health risk than other forms of materials due to the greater surface area per unit of mass. Increasing length and diameter of nanotubes correlates with increased toxicity [123]​ and pathological alterations in the lung.[124]​ The biological interactions of nanotubes are not well understood and the field is open to continued toxicological studies. It is often difficult to determine the causal factors for a given effect, and since carbon is relatively biologically inert, the toxicity attributed to carbon nanotubes could be due in part to residual contamination from the metal catalyst. As an example, the multi-walled nanotube known as Mitsui-7 has been reliably shown to be carcinogenic, but the reason is unclear.[125]​ On the other hand, although carbon nanotubes cause lung inflammation and toxicity in mice, exposure to aerosols generated by sanding polymer-coated multi-wall nanotube composites, representative of the product as found in commonly used materials, does not result in such toxicity.[126]​.

Agencies such as the US National Institute for Occupational Safety and Health (NIOSH) or the Swedish Occupational Environment Agency (Arbetsmiljöverket) have published bulletins detailing the potential hazards and recommended exposure limit for carbon fibers and nanotubes.[127]​[128]​ NIOSH has determined non-regulatory recommended exposure limits (RELs) of 1 μg/m for nanotubes. of carbon and carbon nanofibers as the breathable mass in an average of 8 hours. [129]​ As of October 2016, single-walled carbon nanotubes have been registered under the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) regulations of the European Union.[130]​.

In 2006, Marc Monthioux and Vladimir Kuznetsov published in an editorial in the magazine Carbon the story of the discovery of carbon nanotubes.[5]​ The identity of the discoverers is the subject of some controversy.[6]​ A large percentage of academic and popular literature attributes the discovery of nanometer-sized hollow tubes composed of graphitic carbon to Sumio Iijima, an NEC employee, in 1991. Iijima published an article which had a lot of impact and promoted research into the applications of carbon nanotubes. However, the first observations of carbon nanotubes date back much earlier: in 1952, Radushkevich and Lukyanovich published clear images of carbon tubes 50 nanometers in diameter in the Journal of Physical Chemistry Of Russia.[7] This article largely went unnoticed, due to its publication in Russian and the limited access of Western scientists to the Soviet press during the Cold War. Monthioux and Kuznetsov mentioned in their editorial:

In 1976, Morinobu Endo of the CNRS synthesized hollow tubes of rolled graphite sheets using a vapor phase growth technique. The first specimens observed would later be known as single-walled carbon nanotubes or SWNTs. Endo, in his initial study of carbon fibers, also observed a hollow, linearly extended tube with parallel faces of carbon layers near the core of the fiber.[8] This appears to be the first observation of multi-walled carbon nanotubes or MWCNTs. Mass-produced multi-walled carbon nanotubes today are obtained by a method with similarities to that developed by Endo, actually called the "Endo-process", in recognition of his early work and patents.[9] In 1979, John Abrahamson presented a paper at the 14th Biennial Carbon Conference at Pennsylvania State University that described carbon nanotubes as carbon fibers produced on carbon anodes during an arc discharge. Abrahamson characterized these fibers and proposed a hypothesis for their growth in a nitrogen atmosphere at low pressures.[10]​.

In 1981, a group of Soviet scientists published the results of the chemical and structural characterization of carbon nanoparticles produced by a thermocatalytic disproportionation of carbon monoxide. Using TEM images and Among them, a circular conformation (armchair nanotube) and a spiral helical arrangement (chiral tube).[11]​.

In 1987, Howard G. Tennent of Hyperion Catalysis was granted a US patent for the production of “cylindrical discrete carbon fibrils” with a fixed diameter between approximately 3.5 and 70 nm, a length two orders of magnitude greater than the diameter, and an outer region composed of essentially continuous multiple layers of ordered carbon atoms and a distinct inner core.

Iijima's 1991 discovery of multi-walled carbon nanotubes in the insoluble material resulting from burning graphite rods with an electric arc generated intense interest among the scientific community, as did Mintmire, Dunlap, and White's independent prediction that if single-walled carbon nanotubes could be made, they would exhibit remarkable conductive properties. Nanotube research was greatly accelerated following the discoveries by Iijima and Ichihashi at NEC and Bethune et al. at IBM of methods for producing single-walled carbon nanotubes by adding transition metal catalysts to carbon in an arc discharge. Tess and co-workers refined this catalytic method by vaporizing the carbon/transition metal combination in a high-temperature furnace, greatly improving the yield and purity of the nanotubes and subsequent characterization and application experiments. The arc discharge technique, known to produce buckminsterfullerene molecules in sufficient quantities to explore its properties,[12] therefore played an important role in the discoveries of single-walled and multi-walled nanotubes and extended the series of serendipitous discoveries related to fullerenes.

In 2020, the Keezhadi archaeological dig in Tamil Nadu, India, revealed ceramics about 2,500 years old whose coatings appear to contain carbon nanotubes. The strong mechanical properties of the nanotubes could have contributed to the preservation of the coatings for so many years.[13]​.

The structure of an ideal (infinitely long) single-walled carbon nanotube is that of a regular hexagonal network arranged on an infinite cylindrical surface, whose vertices are the positions of the carbon atoms. Since the length of the carbon-carbon bonds is more or less fixed, the diameter of the cylinder and the positions of the atoms on it are not arbitrary, but are restricted to certain values.[14]​.

To characterize the structure of nanotubes, a zigzag trace in a graphene-like hexagonal lattice is defined as a path "Path (graph theory)") that rotates 60°, alternating left and right, after traversing each bond. By convention, a walk that describes two turns to the left followed by two turns to the right every four steps is called an "armchair walk." In some carbon nanotubes, a closed zigzag path can be traced around the tube, and the tube is classified as a zigzag type or configuration, or simply as a "zigzag nanotube." If, on the other hand, the trace around the tube is a closed armchair path, it is said to be of the armchair type or an "armchair nanotube." Infinite zigzag or armchair nanotubes consist entirely of closed zigzag or armchair paths, connected to each other.

Zigzag and armchair configurations are not the only conformations that a single-walled nanotube can adopt; The structure of an infinitely long tube can be represented as imaginary sections formed by a cut parallel to its axis that passes through an atom A; The sections are superimposed on the structure formed by a flat graphene sheet so that the atoms and bonds coincide. The two halves of the A atom are located on opposite edges of the strip, above the A1 and A2 atoms of the graphene. The line from A1 to A2 corresponds to the circumference of the cylinder that the atom A passes through, and is perpendicular to the edges of the cut. In the graphene lattice, atoms can be divided into two classes, depending on the directions of their three bonds. In half of the atoms, their three bonds are oriented the same way, while in the other half the bonds are rotated 180°. The atoms A1 and A2, which correspond to the same atom A in the cylinder, must be of the same class. It follows that the circumference of the tube and the angle of the section depend on the lengths and directions of the lines connecting pairs of graphene atoms of the same class.

Let y be two linearly independent vectors connecting the graphene atom A1 to its two closest atoms with the same bond directions. That is, if the carbons arranged consecutively around a graphene cell are named C1 - C6, then it is the vector between C1 and C3 and the vector between C1 and C5. Then, for any other atom A2 with the same class as A1, the vector from A1 to A2 can be written as a linear combination, where and are integers. And, conversely, each pair of integers (,) defines a possible position of A2.[14]​.

Given and , you can reverse the theoretical operation and draw the vector on the graphene lattice, cut a strip along the lines perpendicular to through its ends A1 and A2, and roll the strip into a cylinder that joins the two points. If this construction is applied to a pair (k ,0), the result is a zigzag nanotube, with closed zigzag paths of 2 k atoms. If applied to a pair ( k, k ), we obtain an armchair tube, with closed armchair paths of 4 k atoms.

Contenido

La estructura del nanotubo no cambia si la tira se gira 60° en el sentido de las agujas del reloj alrededor de A1 antes de aplicar la reconstrucción hipotética descrita anteriormente. Tal rotación cambia el par (,) al par (,). De ello se deduce que muchas de las posibles posiciones de A2 con respecto a A1, es decir, muchos pares (,), corresponden a la misma disposición de átomos en el nanotubo. Ese es el caso, por ejemplo, de los seis pares (1,2), (-2,3), (-3,1), (-1,-2), (2,-3) y (3 ,-1). En particular, los pares ( k ,0) y (0, k ) describen la misma geometría del nanotubo. Esta redundancia se puede evitar considerando sólo pares (,) tales que 0 y 0; es decir, donde la dirección del vector se encuentre entre las de (inclusive) y (exclusive). Se puede verificar que todo nanotubo tiene exactamente un par (,) que cumple esas condiciones, al que se denomina «tipo de tubo».

Dos nanotubos pertenecen al mismo tipo si, y solo si, coincididen exactamente con el otro trans una rotación y traslación. En lugar del tipo (,), la estructura de un nanotubo de carbono se puede especificar con la longitud del vector (es decir, la circunferencia del nanotubo) y el ángulo entre las direcciones y , comprendido entre 0 (inclusive) y 60° (exclusive) en el sentido de las agujas del reloj. Si el diagrama se dibuja con horizontal, define la inclinación de la tira con respecto de la vertical.

Chirality and mirror symmetry

A nanotube is chiral if it is of type (,) with and ; Its enantiomer (mirror image) is of the type (,). The only types of nanotubes that are achiral are zigzag configuration tubes (k,0) and armchair configuration tubes (k,k). Since the enantiomers have the same structure, only the types (,) with and can be considered. The angle between and , which can take values ​​between 0 and 30°, is called the "chiral angle" of the nanotube.

Circumference and diameter

From y you can calculate the circumference, which is the length of the vector:.

The diameter of the tube is:.

These formulas are only approximations, especially for small y where the bonds are strained; They also do not take into account the thickness of the wall.

The angle between y and the circumference are related to the indices and by:.

where is the clockwise angle between the horizontal and the coordinate vector. In turn, given and , the type can be obtained using the formulas:.

which must result in whole numbers.

Diameter

If the indices and have extremely low values, the structure of the molecule is not tubular and in many cases it is not even stable. For example, the structure defined by the pair (1,0), of the zigzag type, is simply a chain of carbons; Such a molecule actually exists: it is called carbyne and shares some properties with nanotubes—such as orbital hybridization and high tensile strength, among others—but it does not have a hollow space inside and may not be able to be synthesized as a condensed phase. The pair (2,0) would theoretically produce a chain of four fused rings; and (1,1), the armchair type would consist of a chain of four biconnected rings. These structures have not been observed experimentally.

The thinnest carbon nanotube observed is the armchair structure with (2,2) type, which has a diameter of 0.3 nm. This nanotube was grown inside a multi-walled carbon nanotube. The assignment of the nanotube type was carried out using a combination of high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and density functional theory (DFT) methods.[15]​.

The thinnest single-walled carbon nanotube isolated as a free-standing molecule is approximately 0.43 nm in diameter. It has been suggested that it may be of type (5,1) or (4,2).

Length

The longest carbon nanotubes observed as of 2013 were about 0.5 m long. These nanotubes were grown on silicon substrates using an improved chemical vapor deposition (CVD) method and form electrically uniform arrays of single-walled carbon nanotubes.

The shortest carbon nanotube is considered to be the organic compound cycloparaphenylene, synthesized in 2008 by Ramesh Jasti.[19] Since then, other small molecules made up of carbon nanotubes have been synthesized.[20]​.

Density

The highest density nanotubes were obtained in 2013, by cultivation on a titanium-coated copper surface coated with the cocatalysts cobalt and molybdenum at temperatures lower than the typical 450 °C, these tubes reached an average length of 380 nm and a density of 1.6 g cm. The material exhibited ohmic conductivity, with a minimum resistance of ~22 kΩ.[21][22]​.

Multi-walled nanotubes

Multi-walled nanotubes consist of several concentric graphene tubes. The distance between layers in multi-wall tubes is roughly the distance between the graphene layers in graphite, approximately 3.4 Å. Multi-walled nanotubes can adopt two different structures, characterized by different models. The "roll (manuscript)" scroll model describes a single layer rolled over itself. In the "matrioshka" model, the most common, the graphene layers are arranged in concentric cylinders; for example, a single-walled nanotube (0.8) inside a larger nanotube (0.17). The component tubes can be metallic or superconducting. Due to statistical probability issues and restrictions on the diameter of the concentric tubes, the entire structure is a metal with a bandgap equal to zero.[23]​.

Double-walled carbon nanotubes are especially interesting, because their morphology and properties are similar to those of single-walled nanotubes, but they are more resistant to chemical attacks;[24]​ this property is important for adding functional groups to the surface of the nanotubes to give them different properties. In single-walled tubes, this process destroys double bonds, leaves "holes" in the nanotube and causes unwanted modifications to its electrical and mechanical properties. In double-walled nanotubes, only the outer layer is affected. The large scale (gram) synthesis of double-walled nanotubes using the CCVD technique was introduced in p 2003 from the selective reduction of oxide solutions into methane and hydrogen.[25]​.

The telescopic movement capacity of the inner layers of multi-walled nanotubes,[26]​ together with their unique mechanical properties,[27]​ could lead to their use as moving arms in nanomechanical devices. The retraction force that occurs in telescopic movement is caused by the Lennard-Jones interaction between the walls of the nanotube, and its value is approximately 1.5 nN.[28]​.

Unions and crossings

Junctions between two or more nanotubes have been the subject of theoretical discussions,[29][30]​ and are quite frequently observed in samples prepared by arc discharge as well as by chemical vapor deposition. Lambin et al. analyzed the theoretical electronic properties of such junctions and pointed out that a connection between a metallic tube and a semiconductor tube would represent a nanoscale heterojunction, which could form part of an electronic circuit based on nanotubes.[31]​.

The junctions between nanotubes and graphene have been studied both theoretically[32]​ and experimentally,[33]​ and form the basis of pillared graphene, in which parallel graphene sheets are separated by short nanotubes.[34]​ Pillared graphene represents a class of three-dimensional carbon nanotube architectures.

Recently, several studies have highlighted the possibility of using carbon nanotubes as building blocks for macroscopic (greater than 100 nm) three-dimensional carbon devices. Lalwani et al. published a new radical-initiated thermal cross-linking method to fabricate macroscopic porous carbon structures using single- and multi-walled carbon nanotubes as building blocks.[35] These structures possess pores of macro-micro- and nanoscopic dimensions, and the porosity can be tailored for specific applications. They can be used as devices for energy storage, supercapacitors, field emission transistors, high-performance catalysis, photovoltaic and biomedical devices, implants and sensors.[36][37]​.

Other morphologies

Carbon nanobuds or nanobuds are a material created in the 2000s, which combines carbon nanotubes and fullerenes. They are made up of spherical molecules, similar to fullerenes, covalently attached to the outer side walls of a carbon nanotube, like buds or "buds" on a branch. This hybrid material combines useful properties of fullerenes and carbon nanotubes. They are exceptionally good field emitters.[38]​ In composite materials, attached fullerene molecules can function as molecular anchors that prevent sliding of the nanotubes, thus improving the mechanical properties of the composite.

Carbon sheaths are another material that combines fullerenes with carbon nanotubes. In this case, the nanotube encapsulates the fullerene molecules inside, like peas in a pod.[39]​[40]​ They have interesting magnetic properties under certain heating and irradiation conditions. They can also be used as an oscillator in certain investigations.[41][42]​.

A nanotorus is a carbon nanotube folded on itself in the shape of a torus "Torus (geometry)") (shape of 8). Nanotori have many unique properties, such as very high magnetic moments for specific radii.[43]​ The magnetic moment, thermal stability, and other properties vary greatly depending on the radius of the torus and the radius of the tube.[43]​[44]​.

Carbon nanotubes can also be combined with graphene sheets grown along the side walls of multi-walled nanotubes. Sheet density can vary as a function of deposition conditions—for example, temperature and time—from less than a dozen graphene layers to a thicker, more graphite-like structure.[45]​ Deposition of a high density of graphene sheets along aligned CNTs can significantly increase the total loading capacity per unit nominal area compared to other carbon nanostructures.[46]​.

Carbon nanotubes can form a quasi-monodimensional structure similar in shape to a stack of cups (cup-stacked nanotubes or CSCNT); Unlike regular nanotubes and other carbon nanostructures, which normally behave as conductors, CSCNTs are semiconductors due to the stacking of graphene layers.[47]​.

Las propiedades mecánicas, eléctricas, ópticas y térmicas de los nanotubos de carbono de pared simple guardan más relación con la estructura del nanotubo —tipificada por los índices (,)—, que con otras propiedades geométricas: diferentes tipos de nanotubos, pueden tener propiedades radicalmente distintas. La estructura de bandas para un nanotubo de tipo (,) puede calcularse fácilmente.[48]​ En 1999, Hiromichi Kataura") introdujo un gráfico basado en estos cálculos para explicar resultados experimentales. Los gráficos de Kataura relacionan el diámetro del tubo con las bandas de energía y su forma oscilante ilustra la fuerte dependencia con (,).[49]​ Por ejemplo, los nanotubos de tipos (10,1) y (8,3) tienen un diámetro muy parecido, pero el primero se asemeja a un metal y el segundo es un semiconductor.

Mechanical properties

Carbon nanotubes are the strongest and stiffest molecules yet discovered in terms of tensile strength and elastic modulus. These mechanical properties come from the robustness of the covalent sp bonds formed between the individual carbon atoms. In 2000, a value of 63 GPa was obtained for the breaking stress of a multi-walled carbon nanotube;[2] this means that a wire with a cross section of 1 mm could support a weight of 6422 kg. Other studies, including one carried out in 2008, revealed that isolated nanotubes have a tensile strength of about 100 GPa, consistent with the predictions of quantum and atomistic models.[50] Given the low density of carbon nanotubes - between 1.3 and 1.4 g/cm -,[51] their specific strength of up to 48,000 kN m kg is the highest of known materials; For comparison, the equivalent value for high carbon steel is 154 kN·m·kg.

Although the resistance of single nanotubes is extremely high, the weak interactions between adjacent tubes lead to a significant reduction in the effective resistance of multi-walled carbon nanotubes and nanotube bundles, which can reach only a few GPa.[52]​ Irradiation by high-energy electrons, which creates connections between the surface and the inner tubes, increases the resistance of these materials to around 60 GPa for multi-walled carbon nanotubes,[50]​ and about 17 GPa for double-walled nanotube bundles.[52]​.

On the other hand, nanotubes do not show as much resistance to compression: due to their hollow structure and the high ratio between their length and width, they tend to collapse under compression, torsion or bending forces.[53]​ They are also quite soft in the radial direction and even Van der Waals forces between two adjacent nanotubes can deform them. Several groups have performed nanoindentations with an atomic force microscope and used contact atomic force microscopy to measure the radial elasticity of multi- and single-walled carbon nanotubes, respectively; The Young's modulus obtained is of the order of several GPa.

Electrical properties

Unlike graphene, which is a two-dimensional semimetal, carbon nanotubes can behave like metals or semiconductors along the tubular axis depending on their structure. A nanotube of type (, ) is a metal if; if it is a multiple of 3 and , the nanotube is a quasi-metal, with a very small bandgap; otherwise, the nanotube is a moderate semiconductor.[54] Thus, all armchair nanotubes are metallic, and (6,4), (9,1), etc. nanotubes are metallic. are semiconductors.[55] Carbon nanotubes are not semimetallic because the degenerate point (the point where the π [bonding] band meets the π* [anti-bonding] band, at which the energy goes to zero) moves slightly away from the K point in the Brillouin zone due to the curvature of the tube surface, which causes hybridization between the σ* and antibonding bands. π* and the modification of the band dispersion.

There are exceptions to the rule to determine whether a nanotube behaves as a semiconductor or metal; Specifically, the curvature of small diameter nanotubes influences the electrical properties: for example, a type (5.0) nanotube, which should be a semiconductor according to the rule, is actually a metal. Likewise, small diameter zigzag and chiral nanotubes have a bandgap and do not behave like metals, although this exception does not apply to armchair nanotubes.[56]​

In theory, metallic nanotubes can conduct a current density of 4 × 10 A/cm, which exceeds the conductivity of metals such as copper by three orders of magnitude.[57] There is interest in the use of carbon nanotubes in the interconnections of integrated circuits and components to improve the conductivity of composite materials. Many groups are attempting to commercialize conductive wire composed of nanotubes, despite the challenges presented by current saturation under voltage,[58]​ and the high resistivity of the junctions between nanotubes and impurities, which lowers the conductivity of the wires considerably with respect to that of individual nanotubes by several orders of magnitude.

Due to the cross section of the order of nanometers, electrons propagate only along the axis of the nanotubes, which is why they are called "one-dimensional conductors." The maximum electrical conductance of a single-walled carbon nanotube is , where is the quantum unit of conductance.[59]​.

The importance of the π electron system in the electronic properties of graphene causes different effects in the doping "Doping (semiconductors)") of carbon nanotubes and crystalline semiconductors from the same group of the periodic table—such as, for example, silicon. The replacement of carbon atoms in the nanotube wall with boron or nitrogen dopants is usually carried out by chemical vapor deposition and produces p-type and n-type materials respectively, as occurs in silicon. However, some nonsubstitutive (intercalated or adsorbed) dopants, such as alkali metals and electron-rich metallocenes, result in n-type conduction, because they donate electrons to the π-electron system of the nanotube. In contrast, π-electron acceptors such as FeCl or electron-deficient metallocenes function as p-type dopants because they remove π-electrons from the top of the valence band.

Optical properties

Carbon nanotubes emit infrared radiation under electrical excitation and exhibit photoconductivity. These optical properties allow their use as light-emitting diodes (LED)[67]​[68]​ and photodetectors[69]​ The unique optical property of carbon nanotubes is not efficiency, which is still relatively low, but rather the close selectivity in the wavelength of light emission and detection and the possibility of its fine adjustment by modifying the structure of the nanotubes. Other applications based on optical characteristics are bolometric[70]​ and optoelectronic memory devices.[71]​ Nanotube fluorescence has been investigated for use in detectors and sensors in the field of biomedicine.[72][73][74]​.

Thermal properties

Carbon nanotubes are very good thermal conductors in the direction of the axis, exhibiting a property known as "ballistic conduction"; At the same time, they are good insulators in the radial direction. Measurements show that the thermal conductivity of a single-walled nanotube at room temperature is approximately 3500 W m K along its axis;[75] In comparison, a metal considered a good thermal conductor, such as copper, transmits 385 W m K. In the radial direction, the thermal conductivity of the nanotube is around 1.52 W·m·K,[76]​ similar to that of soil.

In macroscopic assemblies of nanotubes, such as films or fibers, up to 1500 W·m·K have been measured.[77]​ Networks composed of nanotubes have different values ​​of thermal conductivity, from the level of thermal insulation with a thermal conductivity of 0.1 W·m·K to high values.[78]​ The specific value depends on the presence of impurities, misalignments and other factors. Carbon nanotubes are estimated to be stable up to a temperature of 2800 °C in a vacuum and around 750 °C in air.[79] In contrast, the metal wires in a microchip melt at temperatures between 600 and 1000 °C.

Crystallographic defects strongly influence the thermal properties of the tube. Such defects lead to phonon scattering, which in turn increases the phonon relaxation rate and reduces the mean free path and thermal conductivity of the nanotube structures. Phonon transport simulations indicate that substitution defects, such as nitrogen or boron, mainly lead to scattering of high-frequency optical phonons. However, larger scale defects, such as Stone–Wales defects, cause phonon scattering over a wide range of frequencies and cause a further reduction in thermal conductivity.[80] The thermal properties of nanotubes can also be modified by encapsulating metals or gases inside them.

There are several techniques to produce nanotubes in considerable quantities, such as arc discharge, laser ablation, chemical vapor deposition, and high-pressure carbon monoxide decomposition (HiPCO). Arc discharge and laser ablation are batch processes, chemical vapor deposition can be used for both batch and continuous processes,[81]​[82]​ and HiPCO is a continuous gas phase process.[83]​ Most of these processes take place in vacuum or, in methods that use a gas, in the presence of it.

Laser ablation consists of vaporizing a graphite target by irradiating a laser pulse in a high-temperature reactor and in the presence of an inert gas. Nanotubes form when vaporized graphite comes into contact with the cold surface and condenses on the walls of the reactor. This process usually has a typical yield of 70% by weight and produces monolayer nanotubes with a diameter that can be controlled by varying the temperature inside the reactor.

Arc discharge was the first technique with which it was possible to produce carbon nanotubes in appreciable quantities. The method consists of passing an electric current of hundreds of amperes between two graphite electrodes in an atmosphere of inert gas at low pressure. The subsequent discharge sublimes the carbon in the electrodes and fullerenes and carbon nanotubes are generated. The typical yield, using this technique, is of the order of 30% by weight and the products obtained are both monolayer and multilayer nanotubes of a typical length of about 50 microns "Micrometer (unit length)"). It can be combined with the oxidation purification method developed by Ebbesen in 1994,[84]​ which consists of heating the extracted fullerite after discharge to 1000 K, in an oxygen atmosphere for 30 minutes.

Chemical vapor deposition is widely used, since it has a very high performance and provides a certain degree of control over the diameter, length and morphology of the nonatubes. With the help of particle catalysts, large quantities of nanotubes can be synthesized using this technique, and several nanotube and nanotube fiber factories already exist in the world. A problem associated with this method is the high variability in the characteristics of the nanotubes.[85]​.

The HiPCO process has the potential to produce more commercially viable carbon nanotubes. [86]​ With this method, high purity single-walled carbon nanotubes are obtained in greater quantities. The HiPCO reactor operates at temperatures of 900-1100 °C and a pressure of 30-50 bar.[87]​ It uses carbon monoxide as a carbon source and iron pentacarbonyl or nickel tetracarbonyl as a catalyst. These catalysts provide a nucleation site for nanotubes to grow,[83]​ while chemical vapor deposition can use cheaper iron-based catalysts, such as ferrocene.

Thermal vapor deposition is also used to obtain vertically aligned carbon nanotube arrays. In this process, a substrate (quartz, silicon, stainless steel, carbon fibers, etc.) is coated with a layer of catalytic metal, usually iron, deposited by sputtering with a thickness of 1 to 5 nm. Often, the substrate is first prepared with a 10–50 nm layer of alumina, which imparts controllable wetting and good interfacial properties. To initiate the growth of nanotubes, two gases are mixed in the reactor. A process gas—such as ammonia, nitrogen, hydrogen, etc.—and another gas used as a carbon source—such as acetylene, ethylene, ethanol, methane, etc.—When the substrate is heated to the growth temperature (600-850°C), the continuous iron film breaks up into small islands where carbon nanotubes form. The diameter of the tube depends on the size of the island, which in turn is determined by the thickness of the catalyst layer. The time the metal island can remain at the growth temperature is limited, as they are mobile and can fuse into a smaller number of larger islands. Annealing at the growth temperature reduces the nanotube density.

If a plasma "Plasma (state of matter)") is generated by applying an intense electric field, during the growth process (known as plasma-enhanced chemical vapor deposition), then the growth of the nanotube will follow the direction of the electric field.

The prepared carbon nanotubes usually contain impurities, such as other carbon allotropes (amorphous carbon, fullerene, etc.) or the metal used as a catalyst.[88]​[89]​ These impurities must be removed to be able to use the nanotubes in various applications.[90]​.

Carbon nanotubes are known to disperse weakly in many solvents, such as water, as a consequence of strong intermolecular p-p interactions. This makes it difficult to process in industrial applications. To improve their stability and solubility in water, several techniques have been developed consisting of the modification of the surface of nanotubes.[4] Chemical methods, such as covalent functionalization, can be based on the oxidation of nanotubes with strong acids, such as sulfuric acid, nitric acid or a mixture of both to fix the carboxylic groups on the surface of the nanotubes as a final product or for subsequent modification by esterification or amination.[91]​ Free radical grafting is a promising technique that uses alkyl or aryl peroxides, substituted anilines and diazonium salts as starting agents.[92]​.

Free radical grafting of macromolecules onto the surface of the nanotubes results in improved solubility compared to common acid treatments, because the large functional molecules facilitate the dispersion of the nanotubes in a variety of solvents, even with a low degree of functionalization. In 2017, covalent functionalization of multi-walled carbon nanotubes was observed using nails. The nanotubes are functionalized in a vessel by a free radical grafting reaction and are then dispersed in water producing a highly stable nanfluidic aqueous suspension.[93]​.

There are many metrology standards and reference materials available for carbon nanotubes.[94]​.

For single-walled carbon nanotubes, the ISO has established a method based on optical absorption spectroscopy to measure the diameter, purity and fraction of metallic nanotubes,[95]​ as well as protocols to characterize the morphology and elemental composition that use transmission electron microscopy and scanning electron microscopy respectively, together with X-ray spectrometry analysis.[96][97]​.

It is NIST standard SRM 2483 is a single-walled carbon nanotube powder used as a reference material for elemental analysis. It has been characterized by thermogravimetric analysis, neutron activation analysis, neutron activation analysis, inductively coupled plasma mass spectroscopy, resonant Raman scattering, visible near-infrared fluorescence spectroscopy and absorption spectroscopy, scanning electron microscopy, and transmission electron microscopy.[98][99] The National Research Council of Canada also offers a certified reference material SWCNT-1 for characterized by neutron activation analysis and inductively coupled plasma mass spectroscopy.[94]​[100]​ NIST RM 8281 is a mixture of single-walled carbon nanotubes of three different lengths.[98]​ [101]​.

For multi-walled carbon nanotubes, ISO/TR 10929 identifies the basic properties and impurity content,[102]​ while ISO/TS 11888 describes morphological characterization using scanning electron microscopy, transmission electron microscopy, viscometry and light scattering "Scattering (physics)").[103]​ ISO/TS 10798 is also valid for multi-walled carbon nanotubes.[97]​.

Carbon nanotubes are used in industry and consumer goods. They consist of highly absorbent black battery components, polymers, and paint. There are other applications in development, such as field effect transistors for electronic components, high resistance fabrics, biosensors for biomedical and agricultural uses, among others.

Some examples of the use of carbon nanotubes are listed below:

Among the applications in development the following can be highlighted:

Scientific studies indicate that nanoparticles represent a greater health risk than other forms of materials due to the greater surface area per unit of mass. Increasing length and diameter of nanotubes correlates with increased toxicity [123]​ and pathological alterations in the lung.[124]​ The biological interactions of nanotubes are not well understood and the field is open to continued toxicological studies. It is often difficult to determine the causal factors for a given effect, and since carbon is relatively biologically inert, the toxicity attributed to carbon nanotubes could be due in part to residual contamination from the metal catalyst. As an example, the multi-walled nanotube known as Mitsui-7 has been reliably shown to be carcinogenic, but the reason is unclear.[125]​ On the other hand, although carbon nanotubes cause lung inflammation and toxicity in mice, exposure to aerosols generated by sanding polymer-coated multi-wall nanotube composites, representative of the product as found in commonly used materials, does not result in such toxicity.[126]​.

Agencies such as the US National Institute for Occupational Safety and Health (NIOSH) or the Swedish Occupational Environment Agency (Arbetsmiljöverket) have published bulletins detailing the potential hazards and recommended exposure limit for carbon fibers and nanotubes.[127]​[128]​ NIOSH has determined non-regulatory recommended exposure limits (RELs) of 1 μg/m for nanotubes. of carbon and carbon nanofibers as the breathable mass in an average of 8 hours. [129]​ As of October 2016, single-walled carbon nanotubes have been registered under the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) regulations of the European Union.[130]​.