right|Схематичне зображення нанотрубки
'''Вуглецеві нанотрубки''' (, ,  ) — протяжні циліндричні структури діаметром від одного до декількох десятків нанометрів і завдовжки до декількох площин (графенів) і закінчуються зазвичай півсферичною головкою. 

Синоніми – волокнистий вуглець, каталітичний філаментарний вуглець, волокнистий піровуглець, нановолокна вуглецю. 

==Загальна інформація== 

НАНОТРУБКИ ВУГЛЕЦЮ – трубчасті наноутворення вуглецю. Виявлені у 1991 р. Бувають одно- і багатошарові. Відповідно діаметр цих трубок знаходиться у межах 0,4 – 500 нм, а довжина від 1 мкм до декількох десятків мікрометрів (при синтезі довгих волокон – і до десятків см). Утворюються при розкладанні вуглецьвмісних газів (СН<sub>4</sub>, С<sub>2</sub>Н<sub>4</sub>, С<sub>2</sub>Н<sub>2</sub>, СО, парів С<sub>6</sub>Н<sub>6</sub> і т.д.) на каталітично активних поверхнях металів (Fe, Co, Ni тощо) при т-рах 300 – 1500 °С. Н.в.  можуть набува-ти найрізноманітніших форм – від прямолінійних до скручених волокон (у т.ч. спіралей). Головна особливість цих вуглеце-вих наноструктур (як і фулеренів) – їх каркасна форма. Виявлені природні форми Н.в. (напр., у шунгітах), а також можуть продукуватися штучно. При цьому встановлено, що природні Н.в. утворюються при обробці вуглецьвмісних біологічних тканин особливим грибком – карбоксиметилцелюлофагом, який виявлений, зокрема, в карстових печерах Нової Зеландії і в Карелії (родовище шунгіту).

== Основні властивості ==

Н.в. дуже міцні як на розтяг, так і на згинання – модуль пружності вздовж осі трубки складає 7000 ГПа, тоді як для легова-ної сталі і найбільш пружного металу ітрію відповідно 200 і 520 ГПа. Прояв властивостей напівпровідника або металу в Н.в. залежить від їх геометричних параметрів і виду каталізатора.

=== Класифікація нанотрубок === 
300px|thumb|left|Для отримання нанотрубки (n, m), графітову площину треба розрізати по напрямах пунктирних ліній і скрутити уздовж напряму вектора R.
Як випливає з визначення, основна класифікація нанотрубок проводиться за способом згортання графітової площини. Цей спосіб згортання визначається двома числами ''n'' і ''m'', які задають розкладання напряму згортання на вектора трансляції графітових граток. Це проілюстровано на малюнку.

За значенням параметрів (''n'', ''m'') розрізняють:
* прямі (ахіральні) нанотрубки 
** «крісло» (armchair) n=m 
** зигзагоподібні (zigzag) m=0 або n=0 
* спіральні (хіральні) нанотрубки

Як неважко здогадатися, при дзеркальному відображенні (n, m) нанотрубка переходить в (m, n) нанотрубку, тому, трубка загального вигляду дзеркально несиметрична. Прямі ж нанотрубки або переходять в себе при дзеркальному відображенні (конфігурація «крісло»), або переходять в себе з точністю до повороту.

thumb|250px|3D model of three types of single-walled carbon nanotubes.

thumb|240px|This animation of a rotating carbon nanotube shows its 3D structure.

'''Carbon nanotubes''' (CNTs) are properties that make them potentially useful in many applications in architectural fields. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized.

Nanotubes are members of the buckyballs. The cylindrical nanotube usually has at least one end capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is in the order of a few nanometers (approximately 1/50,000th of the width of a human hair), while they can be up to several millimeters in length (single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).

The nature of the bonding of a nanotube is described by applied sp<sup>2</sup> bonds, similar to those of sp<sup>3</sub> bonds found in diamond, provides the molecules with their unique strength. Nanotubes naturally align themselves into "ropes" held together by Van der Waals forces. Under high pressure, nanotubes can merge together, trading some sp² bonds for sp³ bonds, giving the possibility of producing strong, unlimited-length wires through high-pressure nanotube linking.

== Discovery ==

A 2006 editorial written by Marc Monthioux and Vladimir Kuznetsov in the journal ''Carbon'' has described the interesting and often misstated origin of the carbon nanotube. A large percentage of academic and popular literature attributes the discovery of hollow, nanometer sized tubes composed of graphitic carbon to Sumio Iijima of NEC in 1991.<ref name="carbon"></ref>

In 1952 L. V. Radushkevich and V. M. Lukyanovich published clear images of 50 nanometer diameter tubes made of carbon in the Soviet ''Journal of Physical Chemistry''.<ref>}}</ref> This discovery was largely unnoticed, as the article was published in the Russian language, and Western scientists' access to Soviet press was limited during the Cold War. It is likely that carbon nanotubes were produced before this date, but the invention of the transmission electron microscope allowed the direct visualization of these structures.

Carbon nanotubes have been produced and observed under a variety of conditions prior to 1991. A paper by Oberlin, Endo, and Koyama published in 1976 clearly showed hollow carbon fibres with nanometer-scale diameters using a vapour-growth technique.<ref></ref> Additionally, the authors show a TEM image of a nanotube consisting of a single wall of graphene. Later, Endo has referred to this image as a single-walled nanotube.<ref></ref>

Furthermore, in 1979, John Abrahamson presented evidence of carbon nanotubes at the 14th Biennial Conference of Carbon at Penn State University. The conference paper described carbon nanotubes as carbon fibers which were produced on carbon anodes during arc discharge. A characterization of these fibres was given as well as hypotheses for their growth in a nitrogen atmosphere at low pressures.<ref></ref>

In 1981 a group of Soviet scientists published the results of chemical and structural characterization of carbon nanoparticles produced by a thermocatalytical disproportionation of carbon monoxide. Using TEM images and XRD patterns, the authors suggested that their “carbon multi-layer tubular crystals” were formed by rolling graphene layers into cylinders. Additionally, they speculated that during rolling graphene layers into a cylinder, many different arrangements of graphene hexagonal nets are possible. They suggested two possibilities of such arrangements: circular arrangement (armchair nanotube) and a spiral, helical arrangement (chiral tube).<ref>Izvestiya Akademii Nauk SSSR, Metals. 1982, #3, p.12-17 [in Russian]</ref>

In 1987, Howard G. Tennent of Hyperion Catalysis was issued a U.S. patent for the production of "cylindrical discrete carbon fibrils" with a "constant diameter between about 3.5 and about 70&nbsp;nanometers…, length 10² times the diameter, and an outer region of multiple essentially continuous layers of ordered carbon atoms and a distinct inner core…."<ref></ref>

Iijima's discovery of carbon nanotubes in the insoluble material of arc-burned graphite rods<ref></ref> created the buzz that is now associated with carbon nanotubes. Nanotube research accelerated greatly following the independent discoveries<ref></ref><ref></ref> by Bethune at IBM<ref></ref> and Iijima at NEC of ''single-walled'' carbon nanotubes and methods to specifically produce them by adding transition-metal catalysts to the carbon in an arc discharge.
The arc discharge technique was well-known to produce the famed Buckminster fullerene on a preparative scale,<ref name="Kratschmer-C60"></ref> and these results appeared to extend the run of accidental discoveries relating to fullerenes. The original observation of fullerenes in mass spectrometry was not anticipated,<ref></ref> and the first mass-production technique by Krätschmer and Huffman was used for several years before realising that it produced fullerenes.<ref name="Kratschmer-C60" />

The discovery of nanotubes remains a contentious issue, especially because several scientists involved in the research could be likely candidates for the Nobel Prize. Many believe that Iijima's report in 1991 is of particular importance because it brought carbon nanotubes into the awareness of the scientific community as a whole. See the reference for a review of the history of the discovery of carbon nanotubes.<ref name="carbon" />

In 2000, Professor Tang Zikang and Wang Ning successfully created the smallest stable carbon nanotubes in the world, measuring at just 0.4 nanometres in diameter. <ref></ref>

== Types of carbon nanotubes and related structures ==
=== Single-walled ===

<gallery perrow="4">
image:Carbon_nanotube_armchair_povray.PNG | armchair (n, n)
image:Carbon_nanorim_armchair_povray.PNG | the chiral vector is bent, while the translation vector stays straight
image:Carbon_nanoribbon_povray.PNG | graphene nanoribbon
image:Carbon_nanorim_zigzag_povray.PNG | the chiral vector is bent, while the translation vector stays straight
image:Carbon_nanotube_zigzag_povray.PNG | zigzag (n,0)
image:Carbon_nanotube_chiral_povray.PNG | chiral (n, m)
image:Carbon_nanorim_chiral_povray.PNG | n and m can be counted at the end of the tube
image:Carbon_nanoribbon_chiral_povray.PNG | graphene nanoribbon
</gallery>

thumb|300px|The (''n'',''m'') nanotube naming scheme can be thought of as a vector ('''C'''<sub>h</sub>) in an infinite graphene sheet that describes how to "roll up" the graphene sheet to make the nanotube. '''T''' denotes the tube axis, and '''a'''<sub>1</sub> and '''a'''<sub>2</sub> are the unit vectors of graphene in real space.

Most single-walled nanotubes (SWNT) have a diameter of close to 1&nbsp;nanometer, with a tube length that can be many thousands of times longer. The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a seamless cylinder. The way the graphene sheet is wrapped is represented by a pair of indices (''n'',''m'') called the chiral vector. The integers ''n'' and ''m'' denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. If ''m''=0, the nanotubes are called "zigzag". If ''n''=''m'', the nanotubes are called "armchair". Otherwise, they are called "chiral".

Single-walled nanotubes are a very important variety of carbon nanotube because they exhibit important electric properties that are not shared by the multi-walled carbon nanotube (MWNT) variants. Single-walled nanotubes are the most likely candidate for miniaturizing electronics beyond the micro electromechanical scale that is currently the basis of modern electronics. The most basic building block of these systems is the electric wire, and SWNTs can be excellent conductors.<ref></ref> One useful application of SWNTs is in the development of the first intramolecular field effect transistors (FETs). The production of the first intramolecular logic gate using SWNT FETs has recently become possible as well.<ref></ref> To create a logic gate you must have both a p-FET and an n-FET. Because SWNTs are p-FETs when exposed to oxygen and n-FETs when unexposed to oxygen, it is possible to protect half of a SWNT from oxygen exposure, while exposing the other half to oxygen. This results in a single SWNT that acts as a NOT logic gate with both p and n-type FETs within the same molecule.

Single-walled nanotubes are still very expensive to produce, around $1500 per gram as of 2000, and the development of more affordable synthesis techniques is vital to the future of carbon nanotechnology. If cheaper means of synthesis cannot be discovered, it would make it financially impossible to apply this technology to commercial-scale applications.<ref name="nanotubes for electronics"></ref> Several suppliers offer as-produced arc discharge SWNTs for ~$50–100 per gram as of 2007.<ref></ref><ref></ref>

=== Multi-walled ===

Multi-walled nanotubes (MWNT) consist of multiple layers of graphite rolled in on themselves to form a tube shape. There are two models which can be used to describe the structures of multi-walled nanotubes. In the ''Russian Doll'' model, sheets of graphite are arranged in concentric cylinders, e.g. a (0,8) single-walled nanotube (SWNT) within a larger (0,10) single-walled nanotube. In the ''Parchment'' model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled up newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.3 Å (330 pm).

The special place of double-walled carbon nanotubes (DWNT) must be emphasized here because they combine very similar morphology and properties as compared to SWNT, while improving significantly their resistance to chemicals. This is especially important when functionalisation is required (this means grafting of chemical functions at the surface of the nanotubes) to add new properties to the CNT. In the case of SWNT, covalent functionalisation will break some C=C double bonds, leaving "holes" in the structure on the nanotube and thus modifying both its mechanical and electrical properties. In the case of DWNT, only the outer wall is modified. DWNT synthesis on the gram-scale was first proposed in 2003<ref></ref> by the CCVD technique, from the selective reduction of oxides solid solutions in methane and hydrogen.

=== Fullerite ===

incompressible nanotube forms, hardness. However, due to the way that nanotubes intertwine, P-SWNTs don't have the corresponding crystal lattice that makes it possible to cut diamonds neatly. This same structure results in a less brittle material, as any impact that the structure sustains is spread out throughout the material.

=== Torus ===

A nanotorus is a theoretically described carbon nanotube bent into a torus (doughnut shape). Nanotori have many unique properties, such as magnetic moments 1000 times larger than previously expected for certain specific radii.<ref name="nanotori"></ref> Properties such as magnetic moment, thermal stability, etc. vary widely depending on radius of the torus and radius of the tube.<ref name="nanotori" /><ref></ref>

=== Nanobud ===

thumb|right|250px|A stable [[Carbon nanobud|nanobud structure]] Carbon nanobuds are a newly discovered material combining two previously discovered allotropes of carbon: carbon nanotubes and fullerenes. In this new material fullerene-like "buds" are covalently bonded to the outer sidewalls of the underlying carbon nanotube. This hybrid material has useful properties of both fullerenes and carbon nanotubes. In particular, they have been found to be exceptionally good field emitters.

===Nanoflower===
The first nanoflower was created in Japan and was actually the accidental outcome of an experiment on nanotubes.

== Properties ==
=== Strength ===

Carbon nanotubes are the strongest and stiffest materials on earth, in terms of gigapascals (GPa). (This, for illustration, translates into ability to endure weight of 6300 kg on a cable with cross-section of 1 mm<sup>2</sup>.) Since carbon nanotubes have a low density for a solid of 1.3-1.4&nbsp;g·cm<sup>&minus;3</sup>,<ref name="nanotubes for electronics"/> its specific strength of up to 48,000&nbsp;kN·m·kg<sup>&minus;1</sup> is the best of known materials, compared to high-carbon steel's 154&nbsp;kN·m·kg<sup>&minus;1</sup>.

Under excessive tensile strain, the tubes will undergo plastic deformation, which means the deformation is permanent. This deformation begins at strains of approximately 5% and can increase the maximum strain the tube undergoes before fracture by releasing strain energy.

CNTs are not nearly as strong under compression. Because of their hollow structure and high aspect ratio, they tend to undergo buckling when placed under compressive, torsional or bending stress.

{| style="font-size: 95%; margin: 1em auto 1em auto; text-align: left;" class="wikitable" border="0"
|+ Comparison of Mechanical Properties<ref>http://www.weizmann.ac.il/wagner/COURSES/Reading%20material%20(papers)/Encyclopedy_of_polymer_science_2003.pdf | accessdate = 2007-12-08 </ref><ref>[http://www.assda.asn.au/index.php Australian Stainless Steel Development Association (ASSDA) - Home]</ref><ref></ref><ref></ref><ref></ref><ref></ref><ref></ref>
|-
! Material
! Young's Modulus (TPa)
! Tensile Strength (GPa)
! Elongation at Break (%)
|-
| SWNT
| align="center"| '''~'''1 (from 1 to 5)
| align="center"| 13-53<sup>E</sup>
| align="center"| 16
|-
| Armchair SWNT
| align="center"| 0.94<sup>T</sup>
| align="center"| 126.2<sup>T</sup>
| align="center"| 23.1
|-
| Zigzag SWNT
| align="center"| 0.94<sup>T</sup>
| align="center"| 94.5<sup>T</sup>
| align="center"| 15.6-17.5
|-
| Chiral SWNT
| align="center"| 0.92
|
|
|-
| MWNT
| align="center"| 0.8-0.9<sup>E</sup>
| align="center"| 150
|
|-
| Stainless Steel
| align="center"| '''~'''0.2
| align="center"| '''~'''0.65-1
| align="center"| 15-50
|-
| Kevlar
| align="center"| '''~'''0.15
| align="center"| '''~'''3.5
| align="center"| '''~'''2
|-
| Kevlar<sup>T</sup>
| align="center"| 0.25
| align="center"| 29.6
|
|}

<sup>E</sup>Experimental observation

<sup>T</sup>Theoretical prediction

=== Kinetic ===

Multi-walled nanotubes, multiple concentric nanotubes precisely nested within one another, exhibit a striking telescoping property whereby an inner nanotube core may slide, almost without friction, within its outer nanotube shell thus creating an atomically perfect linear or rotational bearing.
This is one of the first true examples of motor<ref>[http://www.berkeley.edu/news/media/releases/2003/07/23_motor.shtml 07.23.2003 - Physicists build world's smallest motor using nanotubes and etched silicon<!-- Bot generated title -->]</ref>.
Future applications such as a gigahertz mechanical oscillator are also envisaged.

=== Electrical ===

Because of the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. For a given (''n'',''m'') nanotube, if ''n'' − ''m'' is a multiple of 3, then the nanotube is metallic, otherwise the nanotube is a semiconductor. Thus all armchair (''n''=''m'') nanotubes are metallic, and nanotubes (5,0), (6,4), (9,1), etc. are semiconducting. In theory, metallic nanotubes can have an electrical current density more than 1,000 times greater than metals such as silver and copper.

=== Thermal ===

All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as "ballistic conduction," but good insulators laterally to the tube axis. It is predicted that carbon nanotubes will be able to transmit up to 6000 watts per meter per kelvin at room temperature; compare this to copper, a metal well-known for its good thermal conductivity, which only transmits 385 W·m<sup>&minus;1</sup>·K<sup>&minus;1</sup>. The temperature stability of carbon nanotubes is estimated to be up to 2800 degrees Celsius in vacuum and about 750 degrees Celsius in air.

=== Defects ===

As with any material, the existence of defects affects the material properties. Defects can occur in the form of atomic vacancies. High levels of such defects can lower the tensile strength by up to 85%. Another form of defect that may occur in carbon nanotubes is known as the Stone Wales defect, which creates a pentagon and heptagon pair by rearrangement of the bonds. Because of the very small structure of CNTs, the tensile strength of the tube is dependent on the weakest segment of it in a similar manner to a chain, where a defect in a single link diminishes the strength of the entire chain.

The tube's electrical properties are also affected by the presence of defects. A common result is the lowered conductivity through the defective region of the tube. Some defect formation in armchair-type tubes (which can conduct electricity) can cause the region surrounding that defect to become semiconducting. Furthermore single monoatomic vacancies induce magnetic properties.

The tube's thermal properties are heavily affected by defects. Such defects lead to phonon scattering, which in turn increases the relaxation rate of the phonons. This reduces the mean free path, and reduces the thermal conductivity of nanotube structures.  Phonon transport simulations indicate that substitutional defects such as nitrogen or boron will primarily 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, leading to a greater reduction in thermal conductivity<ref></ref>.

=== One-Dimensional Transport ===

Due to their nanoscale dimensions, electron transport in carbon nanotubes will take place through quantum effects and will only propagate along the axis of the tube. Because of this special transport property, carbon nanotubes are frequently referred to as “one-dimensional” in scientific articles.

=== Toxicity ===
Effects of nanotubes may lead to cancer, studies say<ref>http://www.washingtonpost.com/wp-dyn/content/article/2008/05/20/AR2008052001331.html?hpid=sec-health#</ref>
Determining the toxicity of carbon nanotubes has been one of the most pressing questions in Nanotechnology. Results from various scientific tests on cells have so far proven confusing, with some results indicating it to be highly toxic and others showing no signs of toxicity.<ref></ref> The best current explanation for these apparently contradictory results is that no two production processes make exactly the same type of CNTs.  Some contain measureable levels of impurities such as cobalt and nickel which have documented toxicity and which may be the true causes of the effects.   Additionally CNTs have a range of physical and chemical properties (e.g., surface area, zeta potential) that are not often controlled for in toxicology studies.  A study led by Alexandra Porter from the University of Cambridge shows once CNTs are inside the cell, they accumulate in the cytoplasm and cause cell death, but without rigorous CNT/impurity characterization it is difficult to interpret this result or determine its significance. <ref></ref> A more recent study reveals that carbon nanotubes, when injected in the lungs of mice, are incapable of being disposed of by specialized cells in the lung as these tubes are too large for the cells to engulf, thus leading to constitutive inflammation, a hallmark, precancerous symptom. However, the doses required to achieve this response are also considered high or even extreme. <ref>Nature Nanotechnology, Advance Publication - doi:10.1038/nnano.2008.111 - http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2008.111.html</ref>

== Synthesis ==

Techniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, high pressure carbon monoxide (HiPCO), and chemical vapor deposition (CVD). Most of these processes take place in vacuum or with process gases. CVD growth of CNTs can take place in vacuum or at atmospheric pressure. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth processes are making CNTs more commercially viable.

It is now thought by some that the catalysts or methods involved in forging damascus steel (a forging technique lost to time) may provide vital hints for manufacturing nanotubes cheaply, after they were recently discovered to be a component of that ancient sword metal.<ref></ref><ref></ref>