carbon nanotubes. TUBALL is a revolutionary carbon nanotube for the tire industry. Carbon nanotubes: properties

Faculty of Physics

Department of Physics of Semiconductors and Optoelectronics

S. M. Plankina

"Carbon nanotubes"

Description of laboratory work for the course

"Materials and methods of nanotechnology"

Nizhny Novgorod 2006

The purpose of this work: to get acquainted with the properties, structure and technology of obtaining carbon nanotubes and to study their structure by the method of transmission electron microscopy.

1. Introduction

Until 1985, carbon was known to exist in nature in two allotropic states: a 3D form (diamond structure) and a layered 2D form (graphite structure). In graphite, each layer is formed from a grid of hexagons with the distance between nearest neighbors d c - c =0.142 nm. The layers are arranged in the ABAB... sequence (Fig. 1), where the I atoms lie directly above the atoms in adjacent planes, and the II atoms lie above the centers of hexagons in adjacent regions. The resulting crystallographic structure is shown in Fig. 1a, where a 1 and a 2 are unit vectors in the graphite plane, c is a unit vector perpendicular to the hexagonal plane. The distance between the planes in the grating is 0.337 nm.

Rice. 1. (a) Crystallographic structure of graphite. The lattice is defined by unit vectors a 1 , a 2 and c. (b) Corresponding Brillouin zone.

Because the distance between the layers is greater than the distance in hexes, graphite can be approximated as a 2D material. The calculation of the band structure shows the degeneracy of the bands at point K in the Brillouin zone (see Fig. 1b). This is of particular interest, due to the fact that the Fermi level crosses this degeneracy point, which characterizes this material as a semiconductor with a vanishing energy gap at T→0. If the calculations take into account interplanar interactions, then in the band structure there is a transition from a semiconductor to a semimetal due to the overlap of the energy bands.

In 1985, fullerenes were discovered by Harold Kroto and Richard Smalley - the 0D form, consisting of 60 carbon atoms. This discovery was awarded the 1996 Nobel Prize in Chemistry. In 1991, Iijima discovered a new 1D form of carbon - elongated tubular carbon formations called "nanotubes". The development by Kretschmer and Huffman of the technology for their production in macroscopic quantities marked the beginning of systematic studies of the surface structures of carbon. The main element of such structures is a graphite layer - a surface lined with regular five-six- and heptagons (pentagons, hexagons and heptagons) with carbon atoms located at the vertices. In the case of fullerenes, such a surface has a closed spherical or spheroidal shape (Fig. 2), each atom is associated with 3 neighbors and the bond is sp 2 . The most common C60 fullerene molecule consists of 20 hexagons and 12 pentagons. Its transverse size is 0.714 nm. Under certain conditions, C 60 molecules can be ordered and form a molecular crystal. Under certain conditions, at room temperature, C 60 molecules can be ordered and form reddish molecular crystals with a face-centered cubic lattice, the parameter of which is 1.41 nm.

Fig.2. Molecule C 60 .

2. Structure of carbon nanotubes

2.1 Chirality Angle and Diameter of Nanotubes

Carbon nanotubes are extended structures consisting of graphite layers rolled into a single-layer (SWNT) or multilayer (MWNT) tube. The known smallest nanotube diameter is 0.714 nm, which is the diameter of a C 60 fullerene molecule. The distance between the layers is almost always 0.34 nm, which corresponds to the distance between the layers in graphite. The length of such formations reaches tens of microns and exceeds their diameter by several orders of magnitude (Fig. 3). Nanotubes can be open or end in hemispheres resembling half a fullerene molecule.

The properties of a nanotube are determined by the angle of orientation of the graphite plane relative to the axis of the tube. Figure 3 shows two possible highly symmetrical structures of nanotubes - zigzag and armchair. But in practice, most nanotubes do not have such highly symmetrical shapes; in them, hexagons are twisted in a spiral around the axis of the pipe. These structures are called chiral.

Fig.3. Idealized models of single-walled nanotubes with zigzag (a) and armchair (b) orientations.

Rice. 4. Carbon nanotubes are formed by twisting graphite planes into a cylinder, connecting point A with A. The chirality angle is defined as q - (a). Armchair-type tube, with h = (4.4) - (b). Pitch P depends on the angle q - (c).

There are a limited number of schemes that can be used to build a nanotube from a graphite layer. Consider points A and A "in Fig. 4a. The vector connecting A and A" is defined as c h \u003d na 1 + ma 2, where n, m are real numbers, a 1, and 2 are unit vectors in the graphite plane. The tube is formed when the graphite layer is rolled up and the points A and A are connected. Then it is uniquely determined by the vector c h . Figure 5 shows the indexing scheme for the lattice vector c h .

The chirality indices of a single-layer tube uniquely determine its diameter:

where is the lattice constant. The relationship between indices and chirality angle is given by:

Fig.5. Lattice vector indexing scheme c h .

Zigzag nanotubes are determined by the angle Q =0° , which corresponds to the vector (n, m)= (n, 0). In them, the C-C bonds run parallel to the axis of the tube (Fig. 3, a).

The armchair structure is characterized by an angle Q = ± 30°, corresponding to the vector (n, m) = (2n, -n) or (n, n). This group of tubes will have C-C bonds perpendicular to the axis of the tube (Fig. 3b and 4b). The remaining combinations form tubes of the chiral type, with angles of 0°<<Q <30 о. Как видно из рис. 4с, шаг спирали Р зависит от угла Q .

2.2 Structure of multilayer nanotubes

Multilayer nanotubes differ from single-layer nanotubes in a much wider variety of shapes and configurations. The diversity of structures is manifested both in the longitudinal and transverse directions. Possible varieties of the transverse structure of multilayer nanotubes are shown in Figs. 6. A `Russian matryoshka' type structure (Fig. 6a) is a set of coaxially nested single-layer cylindrical nanotubes. Another variation of this structure, shown in Fig. 6b is a set of nested coaxial prisms. Finally, the last of the above structures (Fig. 6c) resembles a scroll. All the above structures are characterized by the value of the distance between adjacent graphite layers, which is close to the value of 0.34 nm, which is inherent in the distance between adjacent planes of crystalline graphite. The realization of one structure or another in a specific experimental situation depends on the conditions of nanotube synthesis.

Studies of multilayer nanotubes have shown that the distances between layers can vary from the standard value of 0.34 nm to twice the value of 0.68 nm. This indicates the presence of defects in nanotubes, when one of the layers is partially absent.

A significant portion of multiwalled nanotubes may have a polygonal cross-section such that flat surface areas are adjacent to high curvature surface areas that contain edges with a high degree of sp 3 hybridized carbon. These edges limit the surfaces composed of sp 2 -hybridized carbon and determine many of the properties of nanotubes.

Figure 6. Models of transverse structures of multilayer nanotubes (a) - "Russian matryoshka"; (b) hexagonal prism; (c) - scroll.

Another type of defects, often noted on the graphite surface of multilayer nanotubes, is associated with the incorporation into the surface, which consists mainly of hexagons, of a certain amount of pentagons or heptagons. The presence of such defects in the structure of nanotubes leads to a violation of their cylindrical shape, and the insertion of a pentagon causes a convex bend, while the insertion of a heptagon contributes to the appearance of a sharp elbow-shaped bend. Thus, such defects cause the appearance of bent and helical nanotubes, and the presence of helices with a constant pitch indicates a more or less regular arrangement of defects on the surface of the nanotube. It has been found that chair tubes can be connected to zigzag tubes by means of an elbow joint, including a pentagon on the outside of the elbow and a heptagon on its inside. As an example, in fig. 7 shows the connection of (5.5) chair pipe and (9.0) zigzag pipe.

Rice. 7. Illustration of the “elbow connection” between the (5.5) chair tube and the (9.0) zigzag tube. (a) Perspective drawing with pentagonal and hexagonal shaded rings, (b) structure projected onto the plane of symmetry of the elbow.

3. Methods for obtaining carbon nanotubes

3.1 Obtaining graphite in an arc discharge

The method is based on the formation of carbon nanotubes during thermal sputtering of a graphite electrode in an arc discharge plasma burning in a helium atmosphere. This method makes it possible to obtain nanotubes in an amount sufficient for a detailed study of their physicochemical properties.

The tube can be obtained from extended fragments of graphite, which are then twisted into a cylinder. The formation of extended fragments requires special conditions for heating graphite. The optimal conditions for obtaining nanotubes are realized in an arc discharge using electrolytic graphite as electrodes. On fig. Figure 8 shows a simplified scheme of the installation for the production of fullerenes and nanotubes.

Graphite sputtering is carried out by passing a current with a frequency of 60 Hz through the electrodes, the current value is from 100 to 200 A, the voltage is 10-20 V. By adjusting the spring tension, it is possible to ensure that the main part of the input power is released in the arc, and not in the graphite rod. The chamber is filled with helium at a pressure of 100 to 500 torr. The evaporation rate of graphite in this installation can reach 10 g/W. In this case, the surface of the copper casing, cooled by water, is covered with the graphite evaporation product, i.e. graphite soot. If the resulting powder is scraped off and kept for several hours in boiling toluene, a dark brown liquid is obtained. When it is evaporated in a rotating evaporator, a fine powder is obtained, its weight is not more than 10% of the weight of the original graphite soot, it contains up to 10% of fullerenes and nanotubes.

In the described method for obtaining nanotubes, helium plays the role of a buffer gas. Helium atoms carry away the energy released when the carbon fragments combine. Experience shows that the optimal helium pressure for obtaining fullerenes is in the range of 100 Torr, for obtaining nanotubes - in the range of 500 Torr.

Rice. 8. Scheme of the installation for the production of fullerenes and nanotubes. 1 - graphite electrodes; 2 - cooled copper bus; 3 - copper casing, 4 - springs.

Among the various products of thermal sputtering of graphite (fullerenes, nanoparticles, soot particles), a small part (several percent) is accounted for by multilayer nanotubes, which are partially attached to the cold surfaces of the installation, partially deposited on the surface along with soot.

Single-walled nanotubes are formed when a small admixture of Fe, Co, Ni, Cd is added to the anode (i.e., by adding catalysts). In addition, SWNTs are obtained by oxidizing multiwalled nanotubes. For the purpose of oxidation, multilayer nanotubes are treated with oxygen at moderate heating, or with boiling nitric acid, in the latter case five-membered graphite rings are removed, leading to the opening of the ends of the tubes. Oxidation allows the upper layers to be removed from the multilayer tube and its ends to be exposed. Since the reactivity of nanoparticles is higher than that of nanotubes, the fraction of nanotubes in the remaining part of it increases with significant destruction of the carbon product as a result of oxidation.

3.2 Laser evaporation method

An alternative to growing nanotubes in an arc discharge is the laser evaporation method. In this method, SWNTs are mainly synthesized by evaporating a mixture of carbon and transition metals with a laser beam from a target consisting of an alloy of a metal with graphite. Compared with the arc discharge method, direct evaporation allows more detailed control of growth conditions, long operations, and production of nanotubes with higher yields and better quality. The fundamental principles underlying the production of SWNTs by laser evaporation are the same as in the arc discharge method: carbon atoms begin to accumulate and form a compound at the location of the metal catalyst particles. In the setup (Fig. 9), the scanning laser beam was focused into a 6–7 mm spot on a target containing graphite metal. The target was placed in a tube filled (at elevated pressure) with argon and heated to 1200°C. The soot that was formed during laser evaporation was carried away by the argon flow from the high temperature zone and deposited on a water-cooled copper collector located at the outlet of the tube.

Rice. 9. Scheme of the laser ablation setup.

3.3 Chemical vapor deposition

Plasma chemical vapor deposition (CVD) is based on the fact that a gaseous source of carbon (most often methane, acetylene or carbon monoxide) is exposed to some high-energy source (plasma or resistively heated coil) in order to split the molecule into a reactive active atomic carbon. Next, it is sputtered over a heated substrate coated with a catalyst (usually these are transition metals of the first period Fe, Co, Ni, etc.), on which carbon is deposited. Nanotubes are formed only under strictly observed parameters. Accurate reproduction of the direction of growth of nanotubes and their positioning at the nanometer level can be achieved only when they are obtained by the catalytic CVD method. Precise control over the diameter of nanotubes and their growth rate is possible. Depending on the diameter of the catalyst particles, only SWCNTs or MWNTs can grow. In practice, this property is widely used in the technology of creating probes for scanning probe microscopy. By setting the position of the catalyst at the end of the silicon needle of the cantilever, it is possible to grow a nanotube, which will significantly improve the reproducibility of the characteristics and resolution of the microscope, both during scanning and during lithographic operations.

Typically, the synthesis of nanotubes by the PDT method occurs in two stages: the preparation of a catalyst and the actual growth of nanotubes. The catalyst is deposited by sputtering the transition metal onto the substrate surface, and then, using chemical etching or annealing, the formation of catalyst particles is initiated, on which nanotubes subsequently grow (Fig. 10). The temperature during the synthesis of nanotubes varies from 600 to 900 °C.

Among the many methods of PQO, one should note the method of catalytic pyrolysis of hydrocarbons (Fig. 10), in which it is possible to implement flexible and separate control of the conditions for the formation of nanotubes.

Iron is usually used as a catalyst, which is formed in a reducing medium from various iron compounds (iron (III) chloride, iron (III) salicylate or iron pentacarbonyl). A mixture of iron salts with a hydrocarbon (benzene) is sprayed into the reaction chamber either with a directed flow of argon or using an ultrasonic sprayer. The resulting aerosol with an argon flow enters the quartz reactor. In the zone of the preheating furnace, the aerosol flow is heated to a temperature of ~250 °C, the hydrocarbon evaporates, and the process of decomposition of the metal-containing salt begins. Further, the aerosol enters the zone of the pyrolysis furnace, the temperature in which is 900 °C. At this temperature, the formation of micro- and nanosized catalyst particles, hydrocarbon pyrolysis, and the formation of various carbon structures, including nanotubes, occur on metal particles and reactor walls. Then the gas flow, moving through the reaction tube, enters the cooling zone. The pyrolysis products are deposited at the end of the pyrolysis zone on a water-cooled copper rod.

Rice. 10. Scheme of the installation of catalytic pyrolysis of hydrocarbons.

4. Properties of carbon nanotubes

Carbon nanotubes combine the properties of molecules and solids and are considered by some researchers as an intermediate state of matter. The results of the first studies of carbon nanotubes indicate their unusual properties. Some properties of single-walled nanotubes are given in Table. one.

The electrical properties of SWNTs are largely determined by their chirality. Numerous theoretical calculations give a general rule for determining the type of SWCNT conductivity:

tubes with (n, n) are always metallic;

tubes with n – m= 3j, where j is not a zero integer, are semiconductors with a small band gap; and all the rest are semiconductors with a large bandgap.

In fact, the band theory for n – m = 3j tubes gives a metallic type of conductivity, but a small gap opens when the plane is curved in the case of nonzero j. Nanotubes of the armchair type (n, n) in the one-electron representation remain metallic regardless of the surface curvature, which is due to their symmetry. As the tube radius R increases, the band gap for semiconductors with large and small widths decreases according to the law 1/R and 1/R2, respectively. Thus, for the majority of experimentally observed nanotubes, the gap with a small width, which is determined by the curvature effect, will be so small that in practical applications all tubes with n – m = 3j at room temperature are considered to be metallic.

Table 1

Properties	Single-walled nanotubes	Comparison with known data
characteristic size	Diameter 0.6 to 1.8 nm	Limit of electron lithography 7 nm
Density	1.33-1.4 g/cm3	aluminum density
Tensile strength		The strongest steel alloy breaks at 2 GPa
Elasticity	Flexible to any angle	Carbon metals and fibers break at grain boundaries
current density	Estimates give up to 1 G A / cm 2	Copper wires burn out when
Auto emission	Activated at 1-3 V at a distance of 1 µm	Molybdenum needles require 50 - 100 volts and are short lived
Thermal conductivity	Predict up to 6000 W/mK	Pure diamond has 3320 W/mK
temperature stability	Up to 2800°C in vacuum and 750°C in air	Metallization in schemes melts at 600 - 1000°С
		Gold 10$/g

The high mechanical strength of carbon nanotubes in combination with their electrical conductivity makes it possible to use them as a probe in scanning probe microscopes, which increases the resolution of devices of this kind by several orders of magnitude and puts them on a par with such a unique device as a field ion microscope.

Nanotubes have high emission characteristics; the current density of autoelectronic emission at a voltage of about 500 V reaches values ​​of the order of 0.1 A. cm -2 at room temperature. This opens up the possibility of creating a new generation of displays based on them.

Nanotubes with an open end exhibit a capillary effect and are able to draw in molten metals and other liquid substances. Realization of this property of nanotubes opens up the prospect of creating conductive filaments with a diameter of about a nanometer.

The use of nanotubes in chemical technology seems very promising, which is associated, on the one hand, with their high specific surface area and chemical stability, and, on the other hand, with the possibility of attaching various radicals to the surface of nanotubes, which can later serve as either catalytic centers or nuclei. for various chemical transformations. The formation of repeatedly twisted randomly oriented helical structures by nanotubes leads to the appearance of a significant number of nanometer-sized cavities inside the nanotube material, accessible for the penetration of liquids or gases from outside. As a result, the specific surface area of ​​a material composed of nanotubes is close to the corresponding value for an individual nanotube. This value in the case of a single-layer nanotube is about 600 m 2 g -1 . Such a high value of the specific surface area of ​​nanotubes opens up the possibility of their use as a porous material in filters, in chemical technology devices, etc.

Currently, various options for the use of carbon nanotubes in gas sensors have been proposed, which are actively used in ecology, energy, medicine, and agriculture. Gas sensors based on the change in thermoelectric power or resistance during the adsorption of molecules of various gases on the surface of nanotubes have been created.

5. Application of nanotubes in electronics

Although the technological applications of nanotubes based on their high specific surface are of considerable applied interest, the most attractive are those directions of using nanotubes that are associated with developments in various fields of modern electronics. Such properties of a nanotube as its small size, which varies considerably depending on the synthesis conditions, electrical conductivity, mechanical strength, and chemical stability, make it possible to consider a nanotube as the basis for future microelectronic elements.

The introduction of a single-layer nanotube as a defect in the ideal structure of a pentagon-heptagon pair (as in Fig. 7) changes its chirality and, as a consequence, its electronic properties. If we consider the (8.0)/(7.1) structure, then it follows from the calculations that the tube with chirality (8.0) is a semiconductor with a band gap of 1.2 eV, while a tube with chirality (7 ,1) is a semimetal. Thus, this bent nanotube should be a metal-semiconductor molecular transition and can be used to create a rectifying diode - one of the main elements of electronic circuits.

Similarly, as a result of the introduction of a defect, semiconductor-semiconductor heterojunctions with different values ​​of the band gap can be obtained. Thus, nanotubes with embedded defects can form the basis of a semiconductor element of record-breaking small dimensions. The problem of introducing a defect into the ideal structure of a single-walled nanotube presents certain technical difficulties, but it can be expected that as a result of the development of the recently created technology for obtaining single-walled nanotubes with a certain chirality, this problem will be successfully solved.

Based on carbon nanotubes, it was possible to create a transistor, which in its properties exceeds similar circuits made of silicon, which is currently the main component in the manufacture of semiconductor microcircuits. Source and drain platinum electrodes were formed onto the surface of a p- or n-type silicon substrate preliminarily coated with a 120-nm SiO2 layer, and single-layer nanotubes were deposited from the solution (Fig. 11).

Fig.11. Field-effect transistor on a semiconductor nanotube. The nanotube lies on a non-conducting (quartz) substrate in contact with two ultra-thin wires; a silicon layer (a) is used as the third electrode (gate); dependence of the conductivity in the circuit on the gate potential (b) 3 .

The task

1. Get acquainted with the properties, structure and technology of obtaining carbon nanotubes.

2. Prepare material containing carbon nanotubes for examination by transmission electron microscopy.

3. Obtain a focused image of nanotubes at various magnifications. At the highest possible resolution, estimate the size (length and diameter) of the proposed nanotubes. Make a conclusion about the nature of nanotubes (single-layer or multilayer) and the observed defects.

test questions

1. Electronic structure of carbon materials. Structure of single-layer nanotubes. Structure of multilayer nanotubes.

2. Properties of carbon nanotubes.

3. Main parameters determining the electrical properties of nanotubes. General rule for determining the type of conductivity of a single-walled nanotube.

5. Fields of application of carbon nanotubes.

6. Methods for obtaining nanotubes: the method of thermal decomposition of graphite in an arc discharge, the method of laser evaporation of graphite, the method of chemical vapor deposition.

Literature

1. Harris, P. Carbon nanotubes and related structures. New materials of the XXI century. / P. Harris - M.: Technosfera, 2003.-336 p.

2. Eletsky, A. V. Carbon nanotubes / A. V. Eletsky // Successes in physical sciences. - 1997.- T 167, No. 9 - S. 945 - 972

3. Bobrinetsky, I. I. Formation and study of the electrophysical properties of planar structures based on carbon nanotubes. Dissertation for the degree of candidate of technical sciences// II Bobrinetsky. – Moscow, 2004.-145 p.

Bernaerts D. et al./ in Physics and Chemistry of fullerenes and Derivaties (Eds H. Kusmany et al.) – Singapore, World Scientific. – 1995. – P.551

Thes A. et al. / Science. - 1996. - 273 - P. 483

Wind, S. J. Vertical scaling of carbon nanotube field-effect transistors using top gate electrodes / S. J. Wind, Appenzeller J., Martel R., Derycke and Avouris P. // Appl. Phys. Lett. - 2002.- 80. P.3817.

Tans S.J., Devoret M.H., Dai H. // Nature.1997. V.386. P.474-477.

Introduction

Even 15-20 years ago, many did not even think about the possible replacement of silicon. Few could have imagined that already at the beginning of the twenty-first century, a real “nanometer race” would begin between semiconductor companies. Gradual rapprochement with the nanoworld makes you wonder what will happen next? Will the famous Moore's law be continued? After all, with the transition to more subtle production standards, developers face more and more complex tasks. Many experts are generally inclined to believe that in a dozen or two years, silicon will approach a physically insurmountable limit, when it will no longer be possible to create thinner silicon structures.

Judging by recent research, one of the most likely (but far from the only) candidates for the position of "silicon substitutes" are carbon-based materials - carbon nanotubes and graphene - which, presumably, can become the basis of future nanoelectronics. We wanted to talk about them in this article. Rather, we will talk more about nanotubes, since they were obtained earlier and are better studied. There are far fewer developments related to graphene, but this does not detract from its dignity. Some researchers believe that graphene is a more promising material than carbon nanotubes, so we will also say a few words about it today. Moreover, some recent achievements of researchers give a little optimism.

In fact, it is very difficult to cover all the achievements in these rapidly developing areas within the framework of one article, so we will focus only on the key events of recent months. The purpose of the article is to briefly acquaint readers with the most important and most interesting recent achievements in the field of "carbon" nanoelectronics and promising areas of its application. For those who are interested, finding a lot of more detailed information on this topic should not be difficult (especially with knowledge of English).

carbon nanotubes

After the addition of another one (fullerenes) to the traditional three allotropic forms of carbon (graphite, diamond and carbine), over the next few years, a flurry of reports rained down from research laboratories about the discovery and study of various carbon-based structures with interesting properties, such as nanotubes, nanorings, ultrafine materials, etc.

First of all, we are interested in carbon nanotubes - hollow elongated cylindrical structures with a diameter of the order of a few to tens of nanometers (the length of traditional nanotubes is calculated in microns, although structures with a length of the order of millimeters and even centimeters are already being obtained in laboratories). These nanostructures can be represented as follows: we simply take a strip of a graphite plane and roll it into a cylinder. Of course, this is just a figurative representation. In reality, it is not possible to directly obtain a graphite plane and twist it “into a tube”. Methods for obtaining carbon nanotubes are a rather complex and voluminous technical problem, and their consideration is beyond the scope of this article.

Carbon nanotubes are characterized by a wide variety of shapes. For example, they can be single-walled or multi-walled (single-layer or multi-layered), straight or helical, long or short, etc. Importantly, nanotubes have proven to be remarkably strong in tension and bending. Under the action of high mechanical stresses, nanotubes do not tear or break, but their structure is simply rearranged. By the way, since we are talking about the strength of nanotubes, it is interesting to note one of the latest studies on the nature of this property.

Rice University researchers led by Boris Jacobson have found that carbon nanotubes behave like "smart self-healing structures" (the study was published February 16, 2007 in the journal Physical Review Letters). Thus, under critical mechanical stress and deformations caused by temperature changes or radioactive radiation, nanotubes are able to "repair" themselves. It turns out that in addition to 6-carbon cells, nanotubes also contain five- and seven-atom clusters. These 5/7-atom cells exhibit unusual behavior, cycling along the surface of the carbon nanotube like steamboats on the sea. When damage occurs at the site of the defect, these cells take part in the "wound healing", redistributing energy.

In addition, nanotubes exhibit many unexpected electrical, magnetic, and optical properties, which have already become the objects of a number of studies. A feature of carbon nanotubes is their electrical conductivity, which turned out to be higher than that of all known conductors. They also have excellent thermal conductivity, are chemically stable and, most interestingly, can acquire semiconducting properties. In terms of electronic properties, carbon nanotubes can behave like metals or like semiconductors, which is determined by the orientation of the carbon polygons relative to the tube axis.

Nanotubes tend to adhere tightly to each other, forming sets consisting of metal and semiconductor nanotubes. Until now, a difficult task is the synthesis of an array of only semiconductor nanotubes or the separation (separation) of semiconductor nanotubes from metal ones. We will get acquainted with the latest methods of solving this problem further.

Graphene

Graphene, compared to carbon nanotubes, was obtained much later. Perhaps this explains the fact that so far we hear about graphene in the news much less often than about carbon nanotubes, since it is less studied. But this does not detract from its merits. By the way, a couple of weeks ago, graphene was in the spotlight in scientific circles, thanks to a new development by researchers. But more on that later, but now a little history.

In October 2004, the BBC News information resource reported that Professor Andre Geim and his colleagues from the University of Manchester (UK), together with the group of Dr. Novoselov (Chernogolovka, Russia), managed to obtain a material one carbon atom thick. Called graphene, it is a two-dimensional planar carbon molecule one atom thick. For the first time in the world, it was possible to separate an atomic layer from a graphite crystal.

At the same time, Game and his team proposed the so-called graphene-based ballistic transistor. Graphene will make it possible to create transistors and other semiconductor devices with very small dimensions (of the order of several nanometers). Reducing the length of the transistor channel leads to a change in its properties. In the nanoworld, the role of quantum effects is increasing. Electrons move through the channel like a de Broglie wave, and this reduces the number of collisions and, accordingly, increases the energy efficiency of the transistor.

Graphene can be thought of as an "unfolded" carbon nanotube. The increased mobility of electrons makes it one of the most promising materials for nanoelectronics. Since less than three years have passed since the receipt of graphene, its properties have not yet been studied very well. But the first interesting results of the experiments are already there.

Latest carbon advances

Since we first got acquainted with carbon nanotubes (chronologically, they were the first to be obtained), in this part of the article we will also start with them. Probably, you may have the following question: if carbon nanotubes are so good and promising, why haven't they been introduced into mass production yet?

One of the main problems was already mentioned at the beginning of the article. A method for synthesizing an array consisting only of nanotubes with certain properties, shape and dimensions, which could be introduced into mass production, has not yet been created. More attention is paid to the sorting of the "mixed" array, consisting of nanotubes with semiconductor and metallic properties (sorting by length and diameter is no less important). Here it is appropriate to recall one of the first developments in this area, which belongs to IBM, after which we will move on to the latest achievements.

An April 2001 paper, "Engineering Carbon Nanotubes and Nanotube Circuits Using Electrical Breakdown," reports that IBM researchers have for the first time built a transistor based on carbon nanotubes 1 nanometer in diameter and a few microns long. Attention was focused on the fact that they managed to find a way to make such production mass in the future.

Scientists at IBM developed a method that allowed them to destroy all metal nanotubes while leaving semiconductor ones intact. In the first step, an array of nanotubes is placed on a silicon dioxide substrate. Next, electrodes are formed on top of the nanotubes. The silicon substrate plays the role of the bottom electrode and facilitates the blocking of semiconductor nanotubes. Then the overvoltage is applied. As a result, "unprotected" nanotubes with metallic properties are destroyed, while semiconductor ones remain intact.

But this is all just in words, but in reality the process itself looks much more complicated. Plans were reported to bring the development to perfection in 3-4 years (i.e., by 2004/2005), but, as we can see, there have been no reports of the introduction of this technology yet.

Now let's move on to the present, namely, the end of last autumn. Then the Technology Review website reported on a new method for sorting carbon nanotubes, which was developed by researchers at Northwestern University (Northwestern University). In addition to separation based on conductive properties, this method also allows nanotubes to be sorted by their diameter.

It is curious that initially the goal was to sort only by diameter, and the ability to sort by electrical conductivity was a surprise to the researchers themselves. Professor of Chemistry at the University of Montreal (Montreal, Canada) Richard Martel noted that the new sorting method can be called a major breakthrough in this area.

The new sorting method is based on ultracentrifugation, which involves the rotation of the material at high speeds up to 64 thousand revolutions per minute. Before that, a surfactant is applied to the array of nanotubes, which, after ultracentrifugation, is distributed unevenly in accordance with the diameter and electrical conductivity of the nanotubes. One of those who closely familiarized with the new method, University of Florida (University of Florida at Gainesville) professor Andrew Rinzler said that the proposed sorting method will allow to obtain an array with a concentration of semiconductor tubes of 99% and higher.

The new technology has already been used for experimental purposes. Using sorted semiconductor nanotubes, transistors with a relatively simple structure have been created that can be used to control pixels in monitor and TV panels.

By the way, unlike the IBM method, when metal nanotubes were simply destroyed, researchers at Northwestern University can also obtain metal nanotubes using ultracentrifugation, which can also be used in electronic devices. For example, they can be used as transparent electrodes in some types of displays and organic solar cells.

We will not delve into other problems that prevent the introduction of nanotubes, such as technological difficulties in integrating into serial electronic devices, as well as significant energy losses at the junctions of metal with nanotubes, which is due to the high contact resistance. Most likely, the disclosure of these serious topics will seem of little interest and too complicated for the average reader, moreover, it may take several pages.

As for graphene, let's start reviewing the achievements in this area in the spring of last year. In April 2006, the Science Express journal published a fundamental study of the properties of graphene, conducted by a group of scientists from the Georgia Institute of Technology (GIT, USA) and the French National Center for Scientific Research (Centre National de la Recherche Scientifique).

The first important thesis of the work is that graphene-based electronic circuits can be produced by traditional equipment used in the semiconductor industry. GIT Institute Professor Walt de Heer summarized the success of the study as follows: “We have shown that we can create graphene material, “cut” graphene structures, and that graphene has excellent electrical properties. This material is characterized by high electron mobility.”

Many scientists and researchers themselves say that they laid the foundation (base) of graphene electronics. It is noted that carbon nanotubes are only the first step towards the world of nanoelectronics. In the future of electronics, Walt de Heer and his colleagues see exactly graphene. It is noteworthy that the research is supported by Intel, and it does not throw money down the drain.

Let us now briefly describe the method for obtaining graphene and graphene microcircuits proposed by Walt de Heer and his colleagues. By heating a silicon carbide substrate in a high vacuum, the scientists force the silicon atoms to leave the substrate, leaving only a thin layer of carbon atoms (graphene). In the next step, they apply a photoresist material (photoresist) and use traditional electron beam lithography to etch the desired "patterns", that is, they use commonly used manufacturing technologies today. This is a significant advantage of graphene over nanotubes.

As a result, scientists were able to etch 80-nm nanostructures. In this way, a graphene field-effect transistor was created. A serious drawback can be called the large leakage currents of the created device, although scientists at that time were not upset at all. They believed that at the initial stage this is quite normal. In addition, a fully functional quantum interference device has been created that can be used to control electronic waves.

Since the spring of last year, there have been no high-profile achievements like the April development. At least they did not appear on the pages of Internet sites. But February of this year was marked by several events at once and again made me think about “graphene prospects”.

At the beginning of last month, AMO (AMO nanoelectronics group) presented its development as part of the ALEGRA project. AMO engineers have managed to create a graphene top-gated transistor, which makes their structure similar to modern silicon field-effect transistors (MOSFETs). Interestingly, the graphene transistor was created using traditional CMOS manufacturing technology.

Unlike MOSFETs (MOS - Metal Oxide Semiconductor), graphene transistors created by AMO engineers are characterized by higher electron mobility and switching speed. Unfortunately, at the moment the details of the development are not disclosed. The first details will be published in April this year in the IEEE Electron Device Letters.

Now let's move on to another "fresh" development - a graphene transistor that works as a single-electron semiconductor device. Interestingly, the creators of this device are already known to us Professor Geim, Russian scientist Konstantin Novoselov and others.

This transistor has regions where the electrical charge becomes quantized. In this case, the effect of the Coulomb blockade is observed (during the transition of the electron, a voltage appears that prevents the movement of the following particles, it repels fellow particles with its charge. This phenomenon was called the Coulomb blockade. Due to the blockade, the next electron will pass only when the previous one moves away from the transition. Thus , the particles will be able to "jump" only at certain intervals). As a result, only one electron can pass through the channel of the transistor, which is only a few nanometers wide. That is, it becomes possible to control semiconductor devices with just one electron.

The ability to control individual electrons opens up new opportunities for the creators of electronic circuits. As a result, the gate voltage can be significantly reduced. Devices based on single-electron graphene transistors will be characterized by high sensitivity and excellent speed performance. Of course, the dimensions will also decrease by an order of magnitude. Importantly, a serious problem, characteristic of the Walt de Heer prototype graphene transistor, has been overcome - high leakage currents.

It should be noted that single-electron devices have already been created using traditional silicon. But the problem is that most of them can only work at very low temperatures (although there are already samples that work at room temperature, but they are much larger than graphene transistors). The brainchild of Geim and his colleagues can easily work at room temperature.

Prospects for the use of carbon nanomaterials

Most likely, this part of the article will be the most interesting to readers. After all, theory is one thing, and the embodiment of the achievements of science in real devices useful to a person, even prototypes, should be of interest to the consumer. Generally speaking, the possible scope of application of carbon nanotubes and graphene is quite diverse, but we are primarily interested in the world of electronics. I would like to note right away that graphene is a “younger” carbon material and is still only at the beginning of the research path, therefore, in this part of the article, the focus will be on devices and technologies based on carbon nanotubes.

Displays

The use of carbon nanotubes in displays is closely related to the FED (Field Emission Display) technology, which was developed by the French company LETI and was first introduced back in 1991. Unlike a CRT, which uses up to three so-called "hot" cathodes, FED displays initially used a matrix of many "cold" cathodes. As it turned out, too high a reject rate made FED displays uncompetitive. In addition, in 1997-1998 there was a trend towards a significant reduction in the cost of liquid crystal panels, which, as it seemed then, left no chance for FED technology.

The brainchild of LETI received a "second wind" by the end of the last century, when the first studies of FED displays appeared, in which it was proposed to use arrays of carbon nanotubes as cathodes. A number of major manufacturers have shown interest in carbon nanotube displays, including Samsung, Motorola, Fujitsu, Canon, Toshiba, Philips, LG, Hitachi, Pioneer and others, well-known to everyone. In the illustration you see one of the options for implementing FED displays on carbon nanotubes SDNT (small diameter carbon nanotubes, carbon nanotubes of small diameter).

It is noted that FED-displays on carbon nanotubes can compete with modern panels with a large diagonal and in the future will seriously compete primarily with plasma panels (they now dominate the sector with super-large diagonals). Most importantly, carbon nanotubes will significantly reduce the cost of manufacturing FED displays.

From the latest news in the world of nanotube FED displays, it is worth recalling the recent announcement by Motorola that its developments are almost ready to leave the walls of research laboratories and move into mass production. Interestingly, Motorola does not plan to build its own factories for the production of nanotube displays and is currently in licensing negotiations with several manufacturers. Motorola's head of research and development, James Jaskie, noted that two Asian companies are already building factories to produce carbon nanotube displays. So nanotube displays are not so distant future, and it is time to take them seriously.

One of the big challenges faced by Motorola's engineers was to create a low-temperature method for producing carbon nanotubes on a substrate (so as not to melt the glass substrate). And this technological barrier has already been overcome. It also reports the successful completion of the development of methods for sorting nanotubes, which for many companies operating in this industry has become an "insurmountable obstacle."

Director of DiplaySearch Steve Jurichich (Steve Jurichich) believes that it is too early to rejoice at Motorola. After all, there is still a conquest of the market ahead, where the place "under the sun" has already been taken by manufacturers of liquid crystal and plasma panels. Do not forget about other promising technologies, such as OLED (organic light-emitting diode displays), QD-LED (quantum-dot LED, a kind of LED displays using the so-called quantum dots, developed by the American company QD Vision). Besides, Samsung Electronics and a joint project to introduce nanotube displays from Canon and Toshiba (by the way, they are planning to start deliveries of the first nanotube displays by the end of this year) may become tough competition for Motorola in the future.

Carbon nanotubes have found application not only in FED displays. Researchers at the Regroupement Quebecois sur les Materiaux de Pointe laboratory (Quebec, Canada) proposed using a material based on single-walled carbon nanotubes as electrodes for OLED displays. According to the Nano Technology World website, the new technology will allow the creation of very thin electronic paper. Due to the high strength of the nanotubes and the extremely thin electrode array, OLED displays can be very flexible and also have a high degree of transparency.

Memory

Before starting a story about the most interesting "carbon" developments in the field of memory, I would like to note that research on information storage technologies in general is one of the most actively developing areas at the present time. The recent Consumer Electronic Show (Las Vegas) and Hanover CeBIT showed that interest in various drives and data storage systems does not subside over time, but only increases. And this is not surprising. Just think: according to the analytical organization IDC, in 2006, about 161 billion gigabytes of information (161 exabytes) were generated, which is ten times more than in previous years!

During the past 2006, one could only marvel at the inventive ideas of scientists. What have we just not seen: memory on gold nanoparticles, and memory based on superconductors, and even memory ... on viruses and bacteria! Recently, more and more often in the news such non-volatile memory technologies as MRAM, FRAM, PRAM and others are mentioned, which are no longer only “paper” exhibits or demonstration prototypes, but quite efficient devices. So carbon nanotube-based memory technologies are only a small part of the research on information storage.

Perhaps, we will begin our story about "nanotube" memory with the developments of Nantero, which has already become quite famous in its field. It all started back in 2001, when large investments were attracted to the young company, which made it possible to start active development of a new type of non-volatile NRAM memory based on carbon nanotubes. Last year we saw some serious developments by Nantero. In April 2006, the company announced the creation of an NRAM-type memory switch manufactured to the 22nm standard. In addition to proprietary Nantero developments, existing manufacturing technologies were involved in the creation of a new device. In May of the same year, its carbon nanotube device technology was successfully integrated into CMOS manufacturing at LSI Logic Corporation (at ON Semiconductor's factory).

At the end of 2006, a significant event took place. Nantero announced that it has overcome all major technological barriers to mass production of carbon nanotube chips using traditional equipment. A method has been developed for depositing nanotubes on a silicon substrate using such a well-known method as spin-coating, after which lithography and etching, traditional for semiconductor production, are used. One of the advantages of NRAM-memory is high read / write speeds.

However, we will not delve into the technical subtleties. I will only note that such achievements give Nantero every reason to count on success. If the company's engineers manage to bring the development to its logical end and the production of NRAM chips will not be very expensive (and the possibility of using existing equipment gives us the right to hope for this), then we will witness the emergence of a new formidable weapon in the memory market, which can seriously squeeze the existing types of memory, including SRAM, DRAM, NAND, NOR, etc.

As in many other areas of science and technology, carbon nanotube memory research is carried out not only by commercial companies such as Nantero, but also by the laboratories of the world's leading educational institutions. Among the interesting works devoted to "carbon" memory, I would like to note the development of the staff of the Hong Kong Polytechnic University (Hong-Kong Polytechnic University), published in April last year on the pages of the online publication Applied Physics Letters.

Unlike many similar developments that function only at very low temperatures, the device created by physicists Jiyan Dai and Lu (X. B. Lu) can also work at room temperature. The non-volatile memory created by Hong Kong researchers is not as fast as Nantero's NRAM, so the prospect of removing DRAM from the throne is likely to fail. But as a potential replacement for traditional flash memory, it can be considered.

In order to understand in general terms the principle of the functioning of this memory, it is enough to look at the illustration below (b). Carbon nanotubes (CNT, carbon nanotubes) play the role of a charge storage (memory) layer. They seem to be sandwiched between two layers of HfAlO (consisting of hafnium, aluminum and oxygen), which play the role of a control gate and an oxide layer. The entire structure is placed on a silicon substrate.

A rather original solution was proposed by Korean scientists Jon Won Kang (Jeong Won Kang) and Kin Yan (Qing Jiang). They managed to develop a memory based on the so-called telescopic nanotubes. The principle underlying the new development was discovered back in 2002 and was described in the work "Multiwalled Carbon Nanotubes as Gigahertz Oscillators". Its authors managed to establish that a nanotube with another nanotube of smaller diameter embedded in it forms an oscillator, reaching a frequency of oscillations of the order of gigahertz.

The high sliding speed of nanotubes nested in other nanotubes determines the speed of a new type of memory. Yong Won Kang and Kin Yan argue that their design can be used not only as flash memory, but also as high-speed RAM. The principle of memory operation is easy to understand based on the figure.

As you can see, a pair of nested nanotubes is placed between two electrodes. When a charge is applied to one of the electrodes, the inner nanotube moves in one direction or another under the action of van der Waals forces. This development has one significant drawback: a sample of such memory can only work at very low temperatures. However, scientists are confident that these problems are temporary and can be overcome in the next stages of research.

Quite naturally, many developments will remain stillborn. After all, a prototype working in the laboratory is one thing, but on the way to the commercialization of technology there are always many difficulties, and not only purely technical, but also material ones. In any case, the existing works inspire a certain optimism and are quite informative.

Processors

Now let's dream about what kind of carbon future processors can have. The giants of the processor industry are actively looking for new ways to extend Gordon Moore's law, and every year it becomes more and more difficult for them. The reduction in the size of semiconductor elements and the huge density of their placement on a crystal each time poses a very difficult task of reducing leakage currents. The main directions for solving such problems are the search for new materials for use in semiconductor devices and changing their very structure.

As you probably know, recently IBM and Intel almost simultaneously announced the use of new materials to create transistors that will be used in the next generation of processors. Hafnium-based materials with a high dielectric constant (high-k) have been proposed as a gate dielectric instead of silicon dioxide. When creating a gate electrode, silicon will be displaced by metal alloys.

As we can see, even today there is a gradual replacement of silicon and materials based on it with more promising compounds. Many companies have been thinking about replacing silicon for a long time. One of the largest sponsors of research projects in the field of carbon nanotubes and graphene are IBM and Intel.

At the end of March last year, a group of researchers from IBM and two universities in Florida and New York announced the creation of the first complete electronic integrated circuit based on just one carbon nanotube. This scheme is five times thinner than the diameter of a human hair and can only be observed through a powerful electron microscope.

IBM researchers have been able to achieve speeds nearly a million times faster than previously achieved with multiple nanotube circuits. Although these speeds are still slower than today's silicon chips, IBM scientists are confident that new nanotechnological processes will eventually unleash the enormous potential of carbon nanotube electronics.

According to Professor Joerg Appenzeller, the nanotube-based ring oscillator created by the researchers is an excellent tool for studying the characteristics of carbon electronic elements. A ring oscillator is a circuit in which chip manufacturers typically test the feasibility of new manufacturing processes or materials. This schema helps predict how new technologies will behave in finished products.

For a relatively long time, Intel has been conducting its research on the possible use of carbon nanotubes in processors. Recall that Intel is not indifferent to nanotubes, forced the recent Symposium for the American Vacuum Society, which actively discussed the latest achievements of the company in this area.

By the way, a prototype chip has already been developed, where carbon nanotubes are used as interconnects. As is known. the transition to more precise standards entails an increase in the electrical resistance of the connecting conductors In the late 90s, microcircuit manufacturers switched to using copper conductors instead of aluminum ones. But already in recent years, even copper has ceased to satisfy processor manufacturers, and gradually they are preparing a replacement for it.

One of the promising directions is the use of carbon nanotubes. By the way, as we mentioned at the beginning of the article, carbon nanotubes not only have better conductivity compared to metals, but can also play the role of semiconductors. Thus, it looks like a real possibility in the future to completely replace silicon in processors and other microcircuits and create chips made entirely of carbon nanotubes.

On the other hand, it is also too early to “bury” silicon. First, the complete displacement of silicon by carbon nanotubes in microcircuits is unlikely to occur in the next decade. And this is noted by the authors of successful developments themselves. Secondly, silicon also has prospects. In addition to carbon nanotubes, silicon also has a chance to secure a future in nanoelectronics - in the form of silicon nanowires, nanotubes, nanodots and other structures, which are also the subject of study in many research laboratories.

Afterword

In conclusion, I would like to add that this article managed to cover only a very small part of what is currently happening in the field of carbon nanoelectronics. Bright minds continue to invent sophisticated technologies, some of which may become the foundation of the electronics of the future. Some tend to believe that nanorobots, transparent displays, televisions that can be twisted into a thin tube, and other amazing devices remain fiction and will become reality only in the very distant future. But a number of amazing studies already today make us think that all this is not such a distant prospect.

In addition, in addition to the carbon nanotubes and graphene discussed in this article, amazing discoveries are taking place in molecular electronics. Curious research is being carried out in the field of communication between the biological and silicon worlds. There are many prospects for the development of the computer industry. And no one will probably undertake to predict what will happen in 10-15 years. One thing is clear: there are still many exciting discoveries and amazing devices ahead of us.

Sources of information used when writing the article

• [email protected] ()
• PhysOrg.com ()))
• IBM Research()
• K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov. "Electric Field Effect in Atomically Thin Carbon Films"
• K. S. Novoselov, D. Jiang, F. Schedin, V. V. Khotkevich, S. V. Morozov, and A.K. Geim "Two-dimensional atomic crystals"
• Quanshui Zheng, Qing Jiang. "Multiwalled Carbon Nanotubes as Gigahertz Oscillators"

An ideal nanotube is a graphene plane rolled into a cylinder, that is, a surface lined with regular hexagons, at the tops of which carbon atoms are located. The result of such an operation depends on the orientation angle of the graphene plane with respect to the nanotube axis. The orientation angle, in turn, sets the chirality of the nanotube, which determines, in particular, its electrical characteristics.

The chirality indices of a single-layer nanotube (m, n) uniquely determine its diameter D. This relationship has the following form:

D = 3 d 0 π ⋅ m 2 + n 2 + mn (\displaystyle D=(\frac ((\sqrt (3))d_(0))(\pi ))\cdot (\sqrt (m^(2 )+n^(2)+mn))),

where d 0 (\displaystyle d_(0))= 0.142 nm - the distance between adjacent carbon atoms in the graphite plane. The relationship between the chirality indices (m, n) and the angle α is given by:

sin ⁡ α = m 3 2 m 2 + n 2 + mn (\displaystyle \sin (\alpha )=(\frac (m(\sqrt (3)))(2(\sqrt (m^(2)+n ^(2)+mn))))).

Among the various possible directions of nanotube folding, there are those for which the alignment of the hexagon (m, n) with the origin does not require distortion of its structure. These directions correspond, in particular, to the angles α = 30° (armchair configuration) and α = 0° (zigzag configuration). These configurations correspond to the chiralities (n, n) and (0, n), respectively.

Single wall nanotubes

The structure of single-walled nanotubes observed experimentally differs in many respects from the idealized picture presented above. First of all, this concerns the tops of the nanotube, the shape of which, as follows from observations, is far from the ideal hemisphere.

A special place among single-walled nanotubes is occupied by the so-called armchair nanotubes or nanotubes with chirality [10, 10]. In nanotubes of this type, two of the C–C bonds that make up each six-membered ring are oriented parallel to the longitudinal axis of the tube. Nanotubes with a similar structure should have a purely metallic structure.

Single-walled nanotubes are used in lithium-ion batteries, carbon fiber materials, and the automotive industry. In lead-acid batteries, the addition of single-walled nanotubes significantly increases the number of recharge cycles. For single-walled carbon nanotubes, the strength factor is 50 (\displaystyle 50) GPa, and steel 1 (\displaystyle 1) GPa .

Multiwalled nanotubes

The implementation of one or another structure of multiwalled nanotubes in a specific experimental situation depends on the synthesis conditions. An analysis of the available experimental data indicates that the most typical structure of multi-walled nanotubes is a structure with sections of the “Russian doll” and “papier-mâché” type alternately located along the length. In this case, smaller "tubes" are sequentially nested in larger tubes. Such a model is supported, for example, by the facts on the intercalation of potassium or ferric chloride into the "intertube" space and the formation of structures of the "bead" type.

Discovery history

There are many theoretical works on the prediction of this allotropic form carbon. In the work, the chemist Jones (Dedalus) speculated about coiled tubes of graphite. In the work of L. A. Chernozatonsky et al., published in the same year as the work of Iijima, carbon nanotubes were obtained and described, and M. Yu. nanotubes in g., but also suggested their great elasticity.

For the first time, the possibility of forming nanoparticles in the form of tubes was discovered for carbon. At present, similar structures have been obtained from boron nitride, silicon carbide, transition metal oxides, and some other compounds. The diameter of nanotubes varies from one to several tens of nanometers, and the length reaches several microns.

Structural properties

• elastic properties; defects when the critical load is exceeded:
  • in most cases, they represent a destroyed cell-hexagon of the grid - with the formation of a pentagon or septagon in its place. It follows from the specific features of graphene that defective nanotubes will be distorted in a similar way, that is, with the appearance of bulges (at 5) and saddle surfaces (at 7). Of greatest interest in this case is the combination of these distortions, especially those located opposite each other (the Stone-Wales defect) - this reduces the strength of the nanotube, but forms a stable distortion in its structure that changes the properties of the latter: in other words, a permanent bend is formed in the nanotube.
• open and closed nanotubes

Electronic properties of nanotubes

Electronic properties of the graphite plane

• Reciprocal lattice, first Brillouin zone

All K points of the first Brillouin zone are separated from each other by the translation vector of the reciprocal lattice, so they are all actually equivalent. Similarly, all points of K" are equivalent.

• Spectrum in the strong-coupling approximation (See Graphene for more details)

• Dirac points (See Graphene for details)

• Behavior of the spectrum upon application of a longitudinal magnetic field

Accounting for the interaction of electrons

• Bosonization
• Luttinger's liquid
• Experimental status

Superconductivity in nanotubes

Excitons and biexcitons in nanotubes

An exciton (Latin excito - “I excite”) is a hydrogen-like quasi-particle, which is an electronic excitation in a dielectric or semiconductor, migrating through the crystal and not associated with the transfer of electric charge and mass.

Although an exciton consists of an electron and a hole, it should be considered an independent elementary (non-reducible) particle in cases where the interaction energy of an electron and a hole is of the same order as the energy of their motion, and the interaction energy between two excitons is small compared to the energy of each of them. An exciton can be considered an elementary quasi-particle in those phenomena in which it acts as a whole formation that is not subjected to influences capable of destroying it.

A biexciton is a bound state of two excitons. It is, in fact, an exciton molecule.

For the first time, the idea of ​​the possibility of forming an exciton molecule and some of its properties were described independently by S. A. Moskalenko and M. A. Lampert.

The formation of a biexciton manifests itself in the optical absorption spectra in the form of discrete bands converging toward the short-wavelength side according to a hydrogen-like law. It follows from such a structure of the spectra that the formation of not only the ground, but also excited states of biexcitons is possible.

The stability of a biexciton should depend on the binding energy of the exciton itself, the ratio of the effective masses of electrons and holes, and their anisotropy.

The biexciton formation energy is less than twice the exciton energy by the value of the biexciton binding energy.

Optical properties of nanotubes

Memristor properties of nanotubes

However, the yield of CNTs remained low. The introduction of small additions of nickel and cobalt (0.5 at.%) into graphite made it possible to increase the yield of CNTs up to 70–90%. From that moment, a new stage began in the concept of the mechanism of nanotube formation. It became obvious that the metal is a growth catalyst. Thus, the first works appeared on the production of nanotubes by a low-temperature method - the method of catalytic pyrolysis of hydrocarbons (CVD), where particles of an iron group metal were used as a catalyst. One of the options for the installation for the production of nanotubes and nanofibers by the CVD method is a reactor into which an inert carrier gas is supplied, which carries the catalyst and hydrocarbon to the high temperature zone.

Simplified, the CNT growth mechanism is as follows. The carbon formed during the thermal decomposition of the hydrocarbon dissolves in the metal nanoparticle. When a high concentration of carbon in the particle is reached, on one of the faces of the catalyst particle, an energetically favorable "isolation" of excess carbon occurs in the form of a distorted semi-fullerene cap. This is how a nanotube is born. The decomposed carbon continues to enter the catalyst particle, and in order to release the excess of its concentration in the melt, it must be constantly disposed of. The rising hemisphere (semifullerene) from the surface of the melt carries with it the dissolved excess carbon, whose atoms outside the melt form a C-C bond, which is a cylindrical frame-nanotube.

The melting temperature of a particle in a nanosized state depends on its radius. The smaller the radius, the lower the melting point due to the Gibbs-Thompson effect. Therefore, iron nanoparticles with a size of about 10 nm are in a molten state below 600°C. To date, low-temperature synthesis of CNTs has been carried out by the catalytic pyrolysis of acetylene in the presence of Fe particles at 550°C. Reducing the synthesis temperature also has negative consequences. At lower temperatures, CNTs with a large diameter (about 100 nm) and a strongly defective structure such as "bamboo" or "nested nanocones" are obtained. The resulting materials consist only of carbon, but they do not even come close to the extraordinary characteristics (for example, Young's modulus) observed in single-walled carbon nanotubes obtained by laser ablation or electric arc synthesis.

CVD is a more controllable method that allows one to control the growth location and geometrical parameters of carbon tubes on any type of substrate. In order to obtain an array of CNTs on the surface of a substrate, catalyst particles are first formed on the surface by condensing an extremely small amount of it. Formation of the catalyst is possible using chemical deposition from a solution containing a catalyst, thermal evaporation, ion beam sputtering, or magnetron sputtering. Insignificant variations in the amount of condensed matter per unit surface area cause a significant change in the size and number of catalytic nanoparticles and, therefore, leads to the formation of CNTs that differ in diameter and height in different areas of the substrate. Controlled growth of CNTs is possible if a Ct-Me-N alloy is used as a catalyst, where Ct (catalyst) is selected from the group Ni, Co, Fe, Pd; Me (binder metal) - selected from the group Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Re; N (nitrogen). The attractiveness of this process of CNT growth on films of alloys of a catalytic metal with metals of groups V-VII of the Periodic Table of Elements lies in a wide range of factors for controlling the process, which makes it possible to control the parameters of CNT arrays, such as height, density, and diameter. When alloy films are used, CNT growth is possible on thin films of various thicknesses and conductivity. All this makes it possible to integrate this process into integrated technologies.

Fibers from carbon tubes

For the practical application of CNTs, a method is currently being sought to create extended fibers based on them, which, in turn, can be woven into a stranded wire. It has already been possible to create extended fibers from carbon nanotubes, which have high electrical conductivity and strength superior to steel.

Toxicity of nanotubes

Experimental results in recent years have shown that long multi-walled carbon nanotubes (MNTs) can elicit a response similar to that of asbestos fibers. People employed in the extraction and processing of asbestos are several times more likely to develop tumors and lung cancer than the general population. The carcinogenicity of fibers of different types of asbestos is very different and depends on the diameter and type of fibers. Due to their low weight and size, carbon nanotubes penetrate the respiratory tract along with air. As a result, they concentrate in the pleura. Small particles and short nanotubes exit through pores in the chest wall (diameter 3-8 µm), while long nanotubes can be trapped and cause pathological changes over time.

Comparative experiments on the addition of single-walled carbon nanotubes (SWCNT) to mouse food showed no noticeable reaction of the latter in the case of nanotubes with a length of the order of microns. Whereas the use of shortened SWNTs with a length of 200-500 nm led to the "impression" of needle nanotubes into the walls of the stomach.

Purification from catalysts

Nanoscale metal catalysts are important components of many efficient methods for the synthesis of CNTs, and in particular for CVD processes. They also allow, to some extent, to control the structure and chirality of the resulting CNTs. During synthesis, catalysts can convert carbonaceous compounds into tubular carbon, while they themselves typically become partially encapsulated by graphitized layers of carbon. Thus, they can become part of the resulting CNT product. Such metallic impurities can be problematic for many CNT applications. Catalysts such as nickel, cobalt or yttrium can cause toxicological problems, for example. While non-encapsulated catalysts are relatively easy to wash out with mineral acids, encapsulated catalysts require preliminary oxidative treatment to open the coating shell of the catalysts. Efficient removal of catalysts, especially encapsulated ones, while maintaining the CNT structure is a complex and time-consuming procedure. Many CNT purification options have already been studied and individually optimized for the quality of the CNTs used. A new approach to the purification of CNTs, which makes it possible to simultaneously open and evaporate encapsulated metal catalysts, is the extremely rapid heating of CNTs and their impurities in thermal plasma.

Notes

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Ministry of Education and Science of the Russian Federation

Federal State Institution of Higher Professional Education

Russian University of Chemical Technology D. I. Mendeleev

Faculty of Petroleum Chemistry and Polymeric Materials

Department of Chemical Technology of Carbon Materials

PRACTICE REPORT

on the topic CARBON NANOTUBES AND NANOVOLKS

Completed by: Marinin S. D.

Checked by: Doctor of Chemical Sciences, Bukharkina T.V.

Moscow, 2013

Introduction

The field of nanotechnology is considered worldwide as a key topic for the technologies of the 21st century. The possibilities of their versatile application in such areas of the economy as the production of semiconductors, medicine, sensor technology, ecology, automotive, building materials, biotechnology, chemistry, aviation and aerospace, mechanical engineering and the textile industry, carry a huge potential for growth. The use of nanotechnology products will save on raw materials and energy consumption, reduce emissions into the atmosphere and thus contribute to the sustainable development of the economy.

Developments in the field of nanotechnologies are carried out by a new interdisciplinary field - nanoscience, one of the areas of which is nanochemistry. Nanochemistry arose at the turn of the century, when it seemed that everything in chemistry was already open, everything was clear, and all that remained was to use the acquired knowledge for the benefit of society.

Chemists have always known and well understood the importance of atoms and molecules as the basic building blocks of a huge chemical foundation. At the same time, the development of new research methods, such as electron microscopy, highly selective mass spectroscopy, in combination with special sample preparation methods, made it possible to obtain information about particles containing a small number of atoms, less than a hundred.

These particles, about 1 nm in size (10-9 m is just a millimeter divided by a million), have unusual, hard-to-predict chemical properties.

The most famous and understandable for most people are the following nanostructures such as fullerenes, graphene, carbon nanotubes and nanofibers. They all consist of carbon atoms bonded to each other, but their shape varies significantly. Graphene is a plane, monolayer, "veil" of carbon atoms in SP 2 hybridization. Fullerenes are closed polygons, somewhat reminiscent of a soccer ball. Nanotubes are cylindrical hollow volumetric bodies. Nanofibers can be cones, cylinders, bowls. In my work, I will try to highlight exactly nanotubes and nanofibers.

Structure of nanotubes and nanofibers

What are carbon nanotubes? Carbon nanotubes are a carbon material, which is a cylindrical structure with a diameter of several nanometers, consisting of graphite planes rolled into a tube. The graphite plane is a continuous hexagonal grid with carbon atoms at the vertices of the hexagons. Carbon nanotubes can vary in length, diameter, chirality (symmetries of the rolled graphite plane), and number of layers. Chirality<#"280" src="doc_zip1.jpg" />

Single-walled nanotubes. Single-walled carbon nanotubes (SWCNTs) are a subspecies of carbon nanofibers with a structure formed by folding graphene into a cylinder with its sides joined without a seam. Rolling graphene into a cylinder without a seam is only possible in a finite number of ways, differing in the direction of the two-dimensional vector that connects two equivalent points on graphene that coincide when it is rolled into a cylinder. This vector is called the chirality vector single-layer carbon nanotube. Thus, single-walled carbon nanotubes differ in diameter and chirality. The diameter of single-walled nanotubes, according to experimental data, varies from ~ 0.7 nm to ~ 3-4 nm. The length of a single-walled nanotube can reach 4 cm. There are three forms of SWCNTs: achiral "chair" type (two sides of each hexagon are oriented perpendicular to the CNT axis), achiral "zigzag" type (two sides of each hexagon are oriented parallel to the CNT axis), and chiral or helical (each the side of the hexagon is located to the CNT axis at an angle other than 0 and 90 º ). Thus, achiral CNTs of the “armchair” type are characterized by indices (n, n), of the “zigzag” type - (n, 0), chiral - (n, m).

Multiwalled nanotubes. Multilayer carbon nanotubes (MWCNTs) are a subspecies of carbon nanofibers with a structure formed by several nested single-layer carbon nanotubes (see Fig. 2). The outer diameter of multiwalled nanotubes varies over a wide range from a few nanometers to tens of nanometers.

The number of layers in an MWCNT is most often no more than 10, but in some cases it reaches several tens.

Sometimes, among multilayer nanotubes, two-layer nanotubes are singled out as a special type. The “Russian dolls” type structure is a set of coaxially nested cylindrical tubes. Another type of this structure is a set of nested coaxial prisms. Finally, the last of these structures resembles a scroll (scroll). For all structures in Fig. characteristic value of the distance between adjacent graphene layers, close to the value of 0.34 nm, inherent in the distance between adjacent planes of crystalline graphite<#"128" src="doc_zip3.jpg" />

Russian Matryoshka Roll Papier-mache

Carbon nanofibers (CNFs) are a class of materials in which curved graphene layers or nanocones are folded into a one-dimensional filament whose internal structure can be characterized by an angle? between the graphene layers and the fiber axis. One common distinction is between the two main fiber types: Herringbone, with densely packed conical graphene layers and large α, and Bamboo, with cylindrical cup-like graphene layers and small α, which are more like multiwalled carbon nanotubes.<#"228" src="doc_zip4.jpg" />

a - nanofiber "coin column";

b - "Christmas tree structure" nanofiber (stack of cones, "fish bone");

c - nanofiber "stack of cups" ("lamp shades");

d - nanotube "Russian matryoshka";

e - bamboo-shaped nanofiber;

e - nanofiber with spherical sections;

g - nanofiber with polyhedral sections

The isolation of carbon nanotubes as a separate subspecies is due to the fact that their properties differ markedly for the better from the properties of other types of carbon nanofibers. This is explained by the fact that the graphene layer, which forms the nanotube wall along its entire length, has high tensile strength, thermal and electrical conductivity. In contrast to this, transitions from one graphene layer to another occur in carbon nanofibers moving along the wall. The presence of interlayer contacts and high defectiveness of the structure of nanofibers significantly impairs their physical characteristics.

History

It is difficult to talk about the history of nanotubes and nanofibers separately, because these products often accompany each other during synthesis. One of the first data on the production of carbon nanofibers is probably an 1889 patent for the production of tubular forms of carbon formed during the pyrolysis of a mixture of CH4 and H2 in an iron crucible by Hughes and Chambers. They used a mixture of methane and hydrogen to grow carbon filaments by pyrolysis of the gas, followed by carbon precipitation. It became possible to talk about obtaining these fibers for sure much later, when it became possible to study their structure using an electron microscope. The first observation of carbon nanofibers using electron microscopy was made in the early 1950s by Soviet scientists Radushkevich and Lukyanovich, who published an article in the Soviet Journal of Physical Chemistry showing hollow graphitic fibers of carbon that were 50 nanometers in diameter. In the early 1970s, Japanese researchers Koyama and Endo succeeded in producing carbon fibers by vapor deposition (VGCF) with a diameter of 1 µm and a length of more than 1 mm. Later, in the early 1980s, Tibbets in the USA and Benissad in France continued to improve the carbon fiber (VGCF) process. In the USA, more in-depth research into the synthesis and properties of these materials for practical applications was carried out by R. Terry K. Baker and was motivated by the need to suppress the growth of carbon nanofibers due to persistent problems caused by material accumulation in various commercial processes, especially in the field of oil refining. . The first attempt to commercialize carbon fibers grown from the gas phase was made by the Japanese company Nikosso in 1991 under the brand name Grasker, in the same year Ijima published his famous article reporting the discovery of carbon nanotubes.<#"justify">Receipt

Currently, syntheses based on pyrolysis of hydrocarbons and sublimation and desublimation of graphite are mainly used.

Sublimation-desublimation of graphitecan be implemented in several ways:

• arc method,
• radiant heating (use of solar concentrators or laser radiation),
• laser-thermal,
• heating with an electron or ion beam,
• plasma sublimation,
• resistive heating.

Many of these options have their own variations. The hierarchy of some variants of the electric arc method is shown in the diagram:

At present, the most common method is thermal sputtering of graphite electrodes in arc discharge plasma. The synthesis process is carried out in a chamber filled with helium at a pressure of about 500 mm Hg. Art. During plasma combustion, intense thermal evaporation of the anode occurs, while a deposit is formed on the end surface of the cathode, in which carbon nanotubes are formed. The maximum number of nanotubes is formed when the plasma current is minimal and its density is about 100 A/cm2. In experimental setups, the voltage between the electrodes is about 15–25 V, the discharge current is several tens of amperes, and the distance between the ends of the graphite electrodes is 1–2 mm. During the synthesis process, about 90% of the mass of the anode is deposited on the cathode. The resulting numerous nanotubes have a length of about 40 μm. They grow on the cathode perpendicular to the flat surface of its end and are collected into cylindrical beams about 50 μm in diameter.

Nanotube bundles regularly coat the cathode surface, forming a honeycomb structure. The content of nanotubes in the carbon deposit is about 60%. To separate the components, the resulting precipitate is placed in methanol and sonicated. The result is a suspension which, after the addition of water, is subjected to separation in a centrifuge. Large particles adhere to the walls of the centrifuge, while the nanotubes remain floating in suspension. Then the nanotubes are washed in nitric acid and dried in a gaseous flow of oxygen and hydrogen in a ratio of 1:4 at a temperature of 750 0C for 5 minutes. As a result of such processing, a light porous material is obtained, consisting of numerous nanotubes with an average diameter of 20 nm and a length of 10 μm. So far, the maximum nanofiber length achieved is 1 cm.

Pyrolysis of hydrocarbons

In terms of the choice of initial reagents and methods of conducting processes, this group has a significantly larger number of options than the methods of sublimation and desublimation of graphite. It provides more precise control over the process of CNT formation, is more suitable for large-scale production and allows the production of not only carbon nanomaterials themselves, but also certain structures on substrates, macroscopic fibers consisting of nanotubes, as well as composite materials, in particular, modified with carbon CNTs. carbon fibers and carbon paper, ceramic composites. Using the recently developed nanospheric lithography, it was possible to obtain photonic crystals from CNTs. In this way, it is possible to isolate CNTs of a certain diameter and length.

The advantages of the pyrolytic method, in addition, include the possibility of its implementation for matrix synthesis, for example, using porous alumina membranes or molecular sieves. Using aluminum oxide, it is possible to obtain branched CNTs and CNT membranes. The main disadvantages of the matrix method are the high cost of many matrices, their small size, and the need to use active reagents and harsh conditions for dissolving the matrices.

The pyrolysis of three hydrocarbons, methane, acetylene, and benzene, as well as the thermal decomposition (disproportionation) of CO are most often used for the synthesis of CNTs and CNFs. Methane, like carbon monoxide, is not prone to decomposition at low temperatures (non-catalytic decomposition of methane begins at ~900 about C), which makes it possible to synthesize SWCNTs with a relatively small amount of amorphous carbon impurities. Carbon monoxide does not decompose at low temperatures for another reason: kinetic. The difference in the behavior of various substances is visible in Fig. 94.

The advantages of methane over other hydrocarbons and carbon monoxide include the fact that its pyrolysis with the formation of CNTs or CNFs is combined with the release of H 2and can be used in existing H2 production .

Catalysts

The catalysts for the formation of CNTs and CNFs are Fe, Co, and Ni; promoters, which are introduced in smaller amounts, are mainly Mo, W or Cr (less often - V, Mn, Pt and Pd), catalyst carriers are non-volatile oxides and hydroxides of metals (Mg, Ca, Al, La, Si, Ti, Zr) , solid solutions, some salts and minerals (carbonates, spinels, perovskites, hydrotalcite, natural clays, diatomites), molecular sieves (in particular, zeolites), silica gel, airgel, aluminum gel, porous Si and amorphous C. At the same time, V, Cr, Mo, W, Mn and, probably, some other metals under pyrolysis conditions are in the form of compounds - oxides, carbides, metallates, etc.

Noble metals (Pd, Ru, PdSe), alloys (mischmetal, permalloy, nichrome, monel, stainless steel, Co-V, Fe-Cr, Fe-Sn, Fe-Ni-Cr, Fe-Ni- C, Co-Fe-Ni, hard alloy Co-WC, etc.), CoSi 2and CoGe 2, LaNi 5, MmNi 5(Mm - mischmetal), alloys of Zr and other hydride-forming metals. On the contrary, Au and Ag inhibit the formation of CNTs.

Catalysts can be deposited on silicon coated with a thin oxide film, on germanium, some types of glass, and substrates made of other materials.

Porous silicon obtained by electrochemical etching of single-crystal silicon in a solution of a certain composition is considered to be an ideal catalyst carrier. Porous silicon may contain micropores (< 2 нм), мезопоры и макропоры (>100 nm). To obtain catalysts, traditional methods are used:

• mixing (rarely sintering) of powders;
• deposition or electrochemical deposition of metals on a substrate, followed by the transformation of a continuous thin film into islands of nanosize (layer-by-layer deposition of several metals is also used;
• chemical vapor deposition;
• dipping the substrate into the solution;
• applying a suspension of catalyst particles to a substrate;
• applying the solution to a rotating substrate;
• impregnation of inert powders with salts;
• coprecipitation of oxides or hydroxides;
• ion exchange;
• colloidal methods (sol-gel process, reverse micelles method);
• thermal decomposition of salts;
• combustion of metal nitrates.

In addition to the two groups described above, a large number of other methods for obtaining CNTs have been developed. They can be classified according to the carbon sources used. The starting compounds are: graphite and other forms of solid carbon, organic compounds, inorganic compounds, organometallic compounds. Graphite can be converted into CNTs in several ways: by intense ball milling followed by high-temperature annealing; electrolysis of molten salts; splitting into separate graphene sheets and subsequent spontaneous twisting of these sheets. Amorphous carbon can be converted into CNTs when processed under hydrothermal conditions. CNTs were obtained from carbon black (soot) by high-temperature transformation with or without catalysts, as well as by interaction with water vapor under pressure. Nanotubular structures are contained in products of vacuum annealing (1000 about C) films of diamond-like carbon in the presence of a catalyst. Finally, the catalytic high-temperature transformation of fullerite C 60or its treatment under hydrothermal conditions also leads to the formation of CNTs.

Carbon nanotubes exist in nature. A group of Mexican researchers found them in oil samples taken from a depth of 5.6 km (Velasco-Santos, 2003). The CNT diameter ranged from several nanometers to tens of nanometers, and the length reached 2 μm. Some of them were filled with various nanoparticles.

Purification of carbon nanotubes

None of the common methods for obtaining CNTs makes it possible to isolate them in their pure form. Impurities to NT can be fullerenes, amorphous carbon, graphitized particles, catalyst particles.

There are three groups of CNT cleaning methods:

1. destructive,
2. non-destructive,
3. combined.

Destructive methods use chemical reactions, which can be oxidative or reductive, and are based on differences in the reactivity of different carbon forms. For oxidation, either solutions of oxidizing agents or gaseous reagents are used; for reduction, hydrogen is used. The methods make it possible to isolate high-purity CNTs, but are associated with the loss of tubes.

Non-destructive methods include extraction, flocculation and selective precipitation, cross-flow microfiltration, exclusion chromatography, electrophoresis, selective reaction with organic polymers. As a rule, these methods are inefficient and inefficient.

Properties of carbon nanotubes

Mechanical. Nanotubes, as was said, are an extremely strong material, both in tension and in bending. Moreover, under the action of mechanical stresses exceeding the critical ones, nanotubes do not "break", but are rearranged. Based on the high strength properties of nanotubes, it can be argued that they are the best material for a space elevator tether at the moment. As the results of experiments and numerical simulation show, the Young's modulus of a single-layer nanotube reaches values ​​of the order of 1-5 TPa, which is an order of magnitude greater than that of steel. The graph below shows a comparison between a single-walled nanotube and high-strength steel.

The cable of the space elevator is estimated to withstand a mechanical stress of 62.5 GPa

Tensile diagram (dependence of mechanical stress ? from relative elongation?)

To demonstrate the significant difference between currently the strongest materials and carbon nanotubes, let's do the following thought experiment. Imagine that, as it was assumed earlier, a certain wedge-shaped homogeneous structure consisting of the most durable materials to date will serve as a cable for a space elevator, then the diameter of the cable at GEO (geostationary Earth orbit) will be about 2 km and will narrow to 1 mm at the surface Earth. In this case, the total mass will be 60 * 1010 tons. If carbon nanotubes were used as the material, then the diameter of the cable at GEO was 0.26 mm and 0.15 mm at the Earth's surface, and therefore the total mass was 9.2 tons. As can be seen from the above facts, carbon nanofiber is exactly the material that is needed to build a cable, the actual diameter of which will be about 0.75 m, in order to withstand also the electromagnetic system used to propel the space elevator car.

Electrical. Due to the small size of carbon nanotubes, only in 1996 was it possible to directly measure their electrical resistivity using a four-prong method.

Gold stripes were deposited on a polished silicon oxide surface in a vacuum. Nanotubes 2–3 µm long were deposited between them. Then, four tungsten conductors 80 nm thick were deposited on one of the nanotubes chosen for measurement. Each of the tungsten conductors had contact with one of the gold strips. The distance between contacts on the nanotube was from 0.3 to 1 μm. The results of direct measurements showed that the resistivity of nanotubes can vary within a significant range - from 5.1 * 10 -6up to 0.8 ohm/cm. The minimum resistivity is an order of magnitude lower than that of graphite. Most of the nanotubes have metallic conductivity, while the smaller part exhibits the properties of a semiconductor with a band gap of 0.1 to 0.3 eV.

French and Russian researchers (from IPTM RAS, Chernogolovka) discovered another property of nanotubes, which is superconductivity. They measured the current-voltage characteristics of an individual single-walled nanotube with a diameter of ~1 nm, rolled into a bundle of a large number of single-walled nanotubes, as well as individual multilayer nanotubes. A superconducting current at a temperature close to 4K was observed between two superconducting metal contacts. The features of charge transfer in a nanotube essentially differ from those that are inherent in ordinary, three-dimensional conductors and, apparently, are explained by the one-dimensional nature of the transfer.

Also, de Girom from the University of Lausanne (Switzerland) discovered an interesting property: a sharp (about two orders of magnitude) change in conductivity with a small, by 5-10o, bending of a single-layer nanotube. This property can expand the scope of nanotubes. On the one hand, the nanotube turns out to be a ready-made highly sensitive converter of mechanical vibrations into an electrical signal and vice versa (in fact, it is a telephone receiver a few microns long and about a nanometer in diameter), and, on the other hand, it is a practically ready-made sensor of the smallest deformations. Such a sensor could be used in devices that monitor the state of mechanical components and parts on which the safety of people depends, for example, passengers of trains and aircraft, personnel of nuclear and thermal power plants, etc.

Capillary. Experiments have shown that an open nanotube has capillary properties. To open a nanotube, it is necessary to remove the upper part - the cap. One way to remove is to anneal nanotubes at a temperature of 850 0C for several hours in a stream of carbon dioxide. As a result of oxidation, about 10% of all nanotubes are open. Another way to destroy the closed ends of nanotubes is exposure to concentrated nitric acid for 4.5 hours at a temperature of 2400 C. As a result of this treatment, 80% of the nanotubes become open.

The first studies of capillary phenomena showed that a liquid penetrates into the nanotube channel if its surface tension is not higher than 200 mN/m. Therefore, to introduce any substances into nanotubes, solvents with a low surface tension are used. For example, concentrated nitric acid, the surface tension of which is low (43 mN/m), is used to introduce certain metals into the nanotube channel. Then annealing is carried out at 4000 C for 4 hours in a hydrogen atmosphere, which leads to the reduction of the metal. In this way, nanotubes containing nickel, cobalt, and iron were obtained.

Along with metals, carbon nanotubes can be filled with gaseous substances, such as molecular hydrogen. This ability is of practical importance, because it opens up the possibility of safe storage of hydrogen, which can be used as an environmentally friendly fuel in internal combustion engines. Also, scientists were able to place a whole chain of fullerenes with gadolinium atoms already embedded in them (see Fig. 5).

Rice. 5. Inside C60 inside a single-walled nanotube

Capillary effects and filling of nanotubes

nanotube carbon pyrolysis electric arc

Soon after the discovery of carbon nanotubes, the attention of researchers was attracted by the possibility of filling nanotubes with various substances, which is not only of scientific interest, but also of great importance for applied problems, since a nanotube filled with a conducting, semiconducting, or superconducting material can be considered as the smallest of all known nanotubes. present time elements of microelectronics. Scientific interest in this problem is associated with the possibility of obtaining an experimentally substantiated answer to the question: at what minimum sizes do capillary phenomena retain their features inherent in macroscopic objects? For the first time, this problem was considered in the problem of the retraction of an HP molecule inside nanotubes under the action of polarization forces. It was shown that the capillary phenomena leading to the drawing in of liquids that wet the inner surface of the tube into the capillary retain their nature upon transition to nanometer-diameter tubes.

Capillary phenomena in carbon nanotubes were first experimentally carried out in a work where the effect of capillary retraction of molten lead into nanotubes was observed. In this experiment, an electric arc intended for the synthesis of nanotubes was ignited between electrodes with a diameter of 0.8 and a length of 15 cm at a voltage of 30 V and a current of 180–200 A. A layer of material 3–4 cm high formed on the cathode surface as a result of thermal destruction of the anode surface was removed from the chamber and kept for 5 h at T = 850°C in a flow of carbon dioxide. This operation, as a result of which the sample lost about 10% of the mass, contributed to the purification of the sample from particles of amorphous graphite and the discovery of nanotubes in the precipitate. The central part of the precipitate containing nanotubes was placed in ethanol and sonicated. The oxidation product dispersed in chloroform was applied to a carbon tape with holes for observation with an electron microscope. As observations showed, the tubes that were not subjected to processing had a seamless structure, heads of the correct shape and a diameter of 0.8 to 10 nm. As a result of oxidation, about 10% of the nanotubes turned out to have damaged caps, and some of the layers near the top were torn off. A sample containing nanotubes intended for observation was filled in vacuum with drops of molten lead, which were obtained by irradiating a metal surface with an electron beam. In this case, lead droplets 1 to 15 nm in size were observed on the outer surface of the nanotubes. The nanotubes were annealed in air at Т = 400°С (above the melting point of lead) for 30 min. As the results of observations made with the help of an electron microscope show, after annealing some of the nanotubes turned out to be filled with a solid material. A similar effect of filling nanotubes was observed upon irradiation of the heads of tubes opened as a result of annealing with a powerful electron beam. With a sufficiently strong irradiation, the material near the open end of the tube melts and penetrates inside. The presence of lead inside the tubes was established by X-ray diffraction and electron spectroscopy. The diameter of the thinnest lead wire was 1.5 nm. According to the results of observations, the number of filled nanotubes did not exceed 1%.

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