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year 2014

Topic: Polymers and their applications in the 21st century

1. Polymers

1. Definition of polymer polycondensation molecular

v By its definition, a polymer is a high molecular weight compound containing a sufficient amount of monomers or “monomeric units.

v In other words, polymers are linear chains consisting of a larger (N>1) number of identical units. For example, for synthetic polymers N ~ 102-104.

v As a rule, polymers are substances with a molecular weight of several thousand to several million.

2. First polymer production:

v In 1867, the Russian chemist Alexander Butlerov obtained the first polymer - previously unknown polyisobutylene.

v And in 1910, Sergei Lebedev, also a Russian chemist, synthesized the first sample of artificial rubber ((CH3)2C=CH2)n

3. Reactions for obtaining polymers - polycondensation and polymerization:

v Basically, all polymers are obtained by two methods - polycondensation and polymerization reactions.

v Molecules containing a multiple (more often double) bond enter into the polymerization reaction. Such reactions proceed by the addition mechanism, it all starts with the breaking of double bonds (reaction No. 1 - obtaining polyethylene):

v This type of reaction produces many polymers, including capron.

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year 2014

1. Classification of polymers:

2. Structure of polymers:

3. Application:

v Due to their valuable properties, polymers are used in engineering, textile industry, agriculture, and medicine. Automotive and shipbuilding, aircraft manufacturing and in everyday life (textiles and leather products, dishes, glue and varnishes, jewelry and other items).

v Based on macromolecular compounds, rubber, fibers, plastics, films and paints are produced.

2. Polymers. Application in the 21st century

v Science has not stood still for a long time, and during that period of time from the discovery of the polymer to the present day, a great many modifications of this amazing substance have been created. Some of the latest developments are the following three polymers, each with unique properties.

1. "Smart clay"

v The main component of such plasticine is polydimethylsiloxane - (C2H6OSi) n. This polymer combines several unusual properties. So, depending on different environmental conditions, it behaves differently: at rest, it spreads like a liquid, with a sharp mechanical impact it breaks into pieces like a solid body.

v “Smart Plasticine” was obtained by accident, its inventor mixed silicone oil with boric acid in the hope of obtaining a new kind of rubber, but the sticky mass turned out to be nothing like.

2. Hydrogel

v Hydrogels - are solid granules, a polymer substance capable of increasing in volume by more than ten times in a couple of hours. All that is needed is water, the granules will swell, become soft like wax, when the water evaporates, they will shrink and harden again. Such substances are called super-absorbents, they not only absorb a huge amount of water, the swollen polymer keeps it inside with its own molecules.

v When the solvent is absorbed by the polymer, the coils are stretched, i.e. in the initial state, the compressed polymer coil absorbs a solvent, such as water, and it is included inside the coil.

v This principle also underlies eco-soils, hydrogels used in agriculture. Usually, when watering plants, most of the water goes into the deeper layers of the soil. The hydrogel added to the soil does not allow it to flow through the fingers, even if the plant takes root through the granule, water will not pour out of it.

v Since the water molecule is embedded inside the polymer chains of the hydrogel, no water outflow is observed during the physical destruction of the hydrogel, and the system retains the same properties as before the destruction.

v The most striking example of the work of a super-absorbent is children's disposable diapers, even those who have not encountered them know how they work. The multi-layer construction contains the same liquid-absorbing polymer as a sponge. Hydrogel, a similar substance from a diaper, is also capable of performing more serious work, for example, in the oil industry.

v There have been serious problems in oil production for a long time. When pumping out, for every ton of "black gold" there are three tons of water. Huge amounts of money are spent on cleaning oil from excess liquid. For a long time, scientists have been looking for a way to separate oil from water before it enters the pipeline, the solution was found in the laboratory of Moscow State University.

v Polymer fluid is pumped into an oil well and behaves differently depending on whether the well passes through a water reservoir or through an oil reservoir.

v The principle of operation is quite simple. Once in the well, the polymer fluid reacts differently to oil and water, it does not react with “black gold”, but when the polymer meets water on its way, it immediately absorbs it. The swollen gel clogs the water layer and does not let it out. The expansion of the hydrogel creates additional pressure on the oil, which causes it to be squeezed out in a clean state.

3. "Smart medicine

v Some polymers have the ability to respond to changes in the external environment, so “smart plasticine” changes color depending on temperature. In cold water, it darkens noticeably, if transferred to room temperature water, it returns to its original color. When the temperature changes, the density of the coil changes, i.e. the lower the temperature, the smaller the volume of the coil, and thus, when the temperature drops, the dye is squeezed out, and when it is entrained, the dye is drawn into the coil, which leads to a change in color.

v The polymer squeezes out the paint like a sponge water, but what if the dye is replaced with a medicine, will the polymer be able to give out the right dose of the drug in a controlled manner? There is such a directed transport drug in a living organism, this problem, which is being solved and which needs to be solved, is being fought quite seriously.

v Most drugs are wasted. The tablet does not know how to find a diseased organ, having dissolved in the stomach, it will disperse throughout the body through the blood, no more than 10% of the drug will reach the right place. Ideally, the drug should go directly to the diseased organ and not cause side effects.

v “Smart polymers” can respond not only to temperature, they are sensitive to any change in the environment for which they will be programmed. We know that injury is accompanied by acidification; the environment becomes acidic, but this helium is made, so that when acidified, it shrinks a little and displaces the medicine that was injected into it.

v Based on the polymer gel, a unique medicine was created - wound healing hydrogels. The hydrogel consists of eight components that are mixed in distilled water in a certain sequence. On an industrial scale, each component is added at a certain time interval; during the reaction, these substances create a stable polymer structure, into which the drug is then added.

v The gel is a vehicle that contains a drug in microcapsules, it is also called “smart gel” - because, regardless of the people who use it, it searches for and finds lesions and provides assistance. As part of the hydrogel, not one but several drugs at once, once on the wound, the polymer gives them in turn, depending on what the body needs to anesthetize or start the healing process, the medicine is delivered to the wound gradually and for a long time, and then it can simply be washed off with water. Before this work, there was nothing like it in Russia.

v The shell of a capsule (tablet) works on the same principle, it is made of a special polymer, it is responsible not only for the delivery of medicines to their intended destination, but also for the release of a certain dose of medicine over a long period of time.

Bibliography

1. en.wikipedia.org

2. http://www.sigmapluss.ru/umniipolimer.php

3. http://www.kation-msk.ru/ru/press/article/15_8.html

4. http://xn--e1aogju.xn--p1ai/

5. http://www.km.ru/referats/7FA5CF33809646779974A80FDAD7A6CC

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Polar polymers are characterized by the presence of permanent dipoles in their structure. If the conformation of the polymer is rigidly fixed, the resulting moment of the molecule will be determined by whether the moments of the individual segments are added or subtracted. In general, polymer molecules are not in one fixed conformation, and the experimental value, rms dipole moment, is an average over many different conformations.

For polar polymers, the permittivity is determined not only by electronic but also by resonant and relaxation polarization. The characteristic time for the establishment of resonant polarization depends on temperature and is 10-13 -10-12 s. The relaxation polarization establishment time depends on temperature and varies by many orders of magnitude. Therefore, the permittivity of polar polymers decreases with frequency and depends in a complex way on temperature.

For polar polymers, which have a higher permittivity than nonpolar ones, the molar polarization decreases with increasing temperature. Relation (1.5) in this case is transformed to the form

where are the components of the tensor of the deformation polarizability of the molecule, is its constant (resulting) dipole moment of the molecule, is the Boltzmann constant, is the temperature. Equation (1.6) is often called the Debye equation for molar polarization.

The dipole moments of atomic groups essentially depend on the type of their chemical bond with the molecule in which they enter. The need to take into account the strong local interaction between the molecule and its environment and, as a consequence of this, the local

ordering, the introduction of the correlation coefficient was taken into account, defined as:

where is the number of nearest molecules in the system, γ is the angle between the molecule at the reference point and its nearest neighbor. Taking into account the correlation coefficient and some other improvements made by Fröhlich, the end result was the following equation (called the Fröhlich equation), which relates the macroscopic permittivity to the dipole moment of the molecule:

where is the refractive index of light in a given dielectric.

In all polar polymers, two types of relaxation losses are distinguished: dipole-segmental and dipole-group. The first type is due to the movement of large-scale segments of macromolecules, which can be represented as bending vibrations of the main molecular chain. The second type of loss is associated with the rotation of small polar groups contained in the side branches of the macromolecule. Several regions of dipole-group loss maxima (β, γ, δ) are observed when the polymer has polar groups with different mobility. Note that some mobility of polar groups is retained up to helium temperatures.

As the polarity of the polymer increases, the dielectric losses due to electrical conductivity increase. They are observed at high temperatures at low frequencies and increase exponentially with increasing temperature.

ties between conductors and circuit components in various electronic chips, allowing them to increase their speed.

Polyimides are considered in modern microelectronics as one of the most promising insulating materials. These polymers have good thermal, mechanical and electrical properties, which can be further improved if their dielectric constant is reduced. One of the simplest aromatic polyimides has the following structural formula:

In order to reduce the dielectric constant of polyimide, it was proposed to replace some of the hydrogen atoms with fluorine atoms, since the polarizability of the C–F bonds is less than that of the C–H bonds. The C–F bond is very polar, which, nevertheless, does not affect the permittivity at high frequencies, but can lead to an increase at low frequencies. However, polyimides are usually used at temperatures below the glass transition temperature, so orientational polarization is difficult and does not make a noticeable contribution in the operating frequency range. Moreover, the use of symmetric substitution helps to avoid the resulting dipole moment:

The use of fluorinated polyimides makes it possible to reduce the dielectric constant from 3.4 to 2.8.

Another way to reduce the dielectric constant is to increase the fraction of free volume1 in the polymer material. An increase in the free volume leads to a decrease in the number of polarizable groups per unit volume, thereby reducing the dielectric constant of the polymer. Estimates show that this method makes it possible to reduce the value of the permittivity by several tens of percent relative to the initial value.

In general, considering both methods, it can be concluded that when creating molecular structures with a low permittivity, the regulation of free volume is just as important as the choice of functional groups with low polarizability.

Along with the creation of polymeric dielectrics with a low dielectric constant, another problem has become urgent in recent years - the creation of thin-film polymeric dielectric materials with an ultrahigh value of the dielectric constant. They are supposed to be used as gate dielectric layers in organic field-effect transistors (OPTs). A number of specific requirements are imposed on the gate dielectrics of OPTs. These layers should have a high dielectric constant, low conductivity and losses, and their thickness should not exceed several hundred nanometers. Currently, thin layers of inorganic oxides, such as SiO2, Ta2 O5, Al2 O3, and a number of others, are widely used as gate dielectric layers in the manufacture of OPTs. The permittivity of these oxides is approximately 6 - 30 at a layer thickness of 5 to 500 nm.

1 The free volume in a polymer is the volume additional to that occupied by the atoms, based on their van der Waals radius, the volume.

The problem of transition from inorganic oxide to polymeric dielectric layers is associated with the need to simplify the manufacturing technology of OPTs, since the implementation of the “printer”1 technology for manufacturing OPTs with oxide dielectrics is difficult.

Polar polymer dielectrics should be considered as promising materials that can be used for these purposes. Of particular interest are polymer dielectrics whose molecules contain polar groups with a large dipole moment. A typical representative of this class of polymer dielectrics is polyvinyl alcohol cyan ether (CEPS). The structural formula of the CEPS monomer unit has the form

CEPS is characterized by one of the highest values ​​of dielectric constant among known polymeric materials. The ε value of this polymer at a frequency of about 103 Hz is

15, and tgδ does not exceed 0.1 - 0.15.

Such a significant permittivity of CEPS is due to the presence of highly polar nitrile (CN), car-

bonyl (C=O) and hydroxyl (OH) groups capable of orientation under the action of an external electric field (Fig. 1.12). With a favorable orientation of these groups, the maximum value of the dipole moment equal to 5.13 D is provided, but on average the total

1 The "printer" manufacturing technology of OPT is based on the inkjet printing method, as well as the printing method of microcontact printing and thermal transfer printing.

the dipole moment of the monomer unit (taking into account the correlation coefficient g = 0.84) is 3.63 D.

Rice. 1.12. A significant dipole moment of the CEPS monomer unit arises as a result of the orientation of the polar groups

Polymer dielectrics are widely used in various electronic devices. In organic electronics, they are most often used in the form of thin films; therefore, even at relatively low operating voltages, the electric field strength in them reaches significant values. Indeed, in a film 100 nm thick, when it is exposed to a voltage of 10 V, the average field strength is already 106 V/s, but in local regions of the polymer, for example, at the boundary of the amorphous or crystalline regions or at the electrode-polymer interface, it can significantly exceed this value. . Thus, the problems associated with the electrical strength of thin polymer films and their performance in a strong electric field are of paramount importance.

To date, it has been established that the electrical destruction of films is not a critical event that occurs when a certain field strength is reached. Their lifetime in an electric field (durability) decreases exponentially with an increase in its intensity. Electrical destruction by

polymer films can be viewed as a process consisting of two successive stages. At the first (preparatory) stage, the accumulation of damage to macromolecules initiated by the electric field occurs. The duration of this stage determines the durability of the film sample in an electric field (the time from the moment the voltage is applied to the polymer to breakdown). At the second (final) stage, the polymer dielectric loses its ability to resist the flow of high-density current, its sharp increase is observed, i.e., an electrical breakdown occurs.

The electrical strength of films of many polymers was studied at constant, alternating, and pulsed voltages. The studies performed show that the breakdown of thin films of polymers

personal types occurs in fields with an intensity of (2–6) 108 V/m.

This value practically does not differ from the field strength, in which, under conditions of limited partial discharges, thicker polymer films break through.

Important factors that largely determine the approaches used to consider the mechanism of electrical breakdown of thin-film polymer structures are the dependence of their electrical life on the field strength and the influence of the voltage rise rate and the electrode material on the breakdown strength.

The observed effect of the electric field strength on the durability and the rate of voltage rise on the breakdown strength seems to be a very important fact, since it can be considered as an indication that the electrical destruction of thin polymer films is indeed the result of a gradual accumulation of damage (changes) culminating in a breakdown. During this process, conditions are created under which, at a certain point in time, under the influence of a strong electric field, the polymer dielectric loses its "dielectric

properties” and is capable of passing significant currents, leading to its destruction (breakdown) due to heat release.

The degradation of a polymeric material in an electric field occurs due to the breaking of chemical bonds in polymer molecules, the release of energy during the recombination of charges, and heat release during the flow of a high-density current.

1.6. POLYMERS WITH OWN CONDUCTIVITY

The main difference between polymer dielectrics and polymers with intrinsic electronic conductivity is that the former do not contain conjugated chemical bonds that the latter do.

Among the variety of conducting polymers, in accordance with the classification proposed by A.V. Vannikov, based on the characteristics of the transport of charge carriers, the following groups can be conventionally distinguished.

1. Conductivity is determined by the transport of charge carriers along polymeric polyconjugated chains. Typical representatives of this group of polymers are oriented polyacetylene, polythiophene, polypyrrole.

2. Charge carriers move along polymer polyconjugated chains, but the total transport is determined by charge carrier jumps between polymer chains. This large group includes numerous derivatives of polyphenylene vinylene, polymethylphenylsilylene, and others. It should be noted here that intermolecular charge transfer greatly hinders transport, so the mobility of charge carriers in such polymers is significantly lower than the intramolecular mobility.

3. Localized transport centers are located in the main chain of the polymer, which does not have polyconjugation, for example, a polyimide containing triphenylamine or anthracene transport groups in the main chain.

4. Localized transport centers are side substituents of the polymer backbone. These include polyvinylcarbazole, polyepoxypropylcarbazole, polyvinyl anthracene, etc.

5. The last, most extensive group includes polymers doped with active low molecular weight compounds. In such compounds, it is the polymer matrix that, as a rule, determines physical-mechanical and spectral properties of the system.

The conduction mechanism of polymers belonging to groups 2–5 is hopping and is associated with the transfer of charge carriers through transport centers. By its nature and observed regularities, it is similar to the hopping mechanism of mobility. Depending on the nature of the polymer, the mobility in them can be electron or hole.

Hole transport is carried out through transport centers that have a minimum ionization potential. Usually these are aromatic amine groups or compounds. The hole transport is associated with the jump of an electron from the highest filled molecular orbital (HOMO level) of the neutral transport center to the molecular orbital of the neighboring positively charged transport center.

Electron transport occurs through transport centers characterized by maximum electron affinity. Most often, oxygen-containing groups act as such centers. An electron from the molecular orbital of the negatively charged center moves to the lowest free orbital (LUMO level) of the neighboring neutral transport center.

conductivity,

Structural formula Name

polyacetylene 10 4

polyphenylene 10 3

polypyrrole 10 3

polythiophene 10 3

polyaniline 10 2

Rice. 1.13. Structural formulas of conducting polymers

The electrical conductivity of polymers belonging to the first group is determined by the electrical conductivity of polymer chains. These polymers are polymers with high dark conductivity. Structural formulas and specific conductivity of some of them are presented in fig. 1.13.

-/a 0 /a

Rice. 1.14. Plot of dependence of energy on the wave vector of an electron in a monoatomic linear chain (a) and density of states g (E)

for this chain (b). States occupied by electrons at T = 0 are shaded

HISTORICAL FLASHBACK
Polymers with high electrical conductivity, pseudometallic and semiconductor properties were obtained as early as the 1960s. A classic example of this class of polymer is polyacetylene. Due to polyconjugated chemical bonds, its electrical conductivity can be changed over a wide range both during synthesis (by controlling the length of polymer chains) and under field influences (thermal, electromagnetic, ionizing radiation), leading to a corresponding change in either the primary structure of the polymer (structural rearrangement) , or to change the degree of its polymerization. Conducting polymers are widely used for the manufacture of electrodes for chemical current sources (polyanilines), automatic temperature controllers and voltage stabilizers (polyacrylonitriles), as capacitor electrolytes (salts of polypyrolles), etc. The discovery and study of the effect of photoconductivity in polynitriles, polyphthalocyanines, polyphenyls, and polyphenylenevinylenes led to the formation of photodetectors based on them, and the high “sensitivity” of the spectral characteristics of polymers to the initial structure and polymer component made it possible to create devices with a wide spectral range. True, for the sake of justice, it should be recognized that their luminescence quantum yield did not exceed a few percent. In the 1980s, as a result of studies of conducting polymers with a high degree of orientation of polymer chains in a bulk sample (which makes it possible to use the characteristics of a quasi-one-dimensional structure of macromolecules), polymeric quasi-crystalline materials with a high anisotropy of electrical characteristics were obtained. The mobility of charge carriers in them reached 5000–6000 cm2/V.s.
The variety of structures of polymer systems and the possibility of their modification provided researchers with the widest choice of material characteristics. This, of course, prompted them to try to implement active electronic devices based on polymeric materials. The work was carried out on the basis of the theory of semiconductor devices, the physical and technological principles of their formation, which were quite well developed by that time. At the same time, in polymer (molecular) systems, the energy states of carriers at the highest and lowest unoccupied levels of molecular orbitals acted as an analogue of the Fermi level, and the analogue of the doping process, leading to a change in the position of the Fermi level, was the operation of chemical substitution, causing a change in the ionization potential and electron affinity . By changing the primary structure of the polymer, one can set the levels of molecular orbitals and, consequently, the width of its band gap. Continuing the consideration of analogies, we point out that systems of linear polymers with conjugated bonds can be used as interconnects.
After a brief historical digression, let's consider the "promotion" of polymers in the world of modern electronic devices.

ORGANIC LEDS WITH HIGH BRIGHTNESS
Light-emitting diodes (LEDs) were the first electronic devices based on polymers. Now it can already be considered that the developments have practically reached the level that makes it possible to switch to the industrial production of organic LEDs, and the task of today is to create devices with a high luminosity. Numerous studies in this area in various ways led to the optimal design and technological option, which was called the "transparent organic light emitting diode" (Transparent Organic Light Emitting Diode - TOLED, Fig. 1). The principle of its operation is extremely simple and consists in the generation of radiation by polymer molecules under the action of an electric field as a result of the recombination of carriers in the electroluminescent layer. Structurally, the LED should be designed so that the transparent electrode, the hole transfer layer, the electroluminescent layer and the waveguide are as transparent as possible, and the electron transfer layer and the negative electrode provide maximum interference and specular reflection of radiation. To enhance the contribution of reflected radiation in some LED designs, the negative electrode is given an appropriate shape (for example, a concave parabolic mirror) and optical elements based on Fresnel lenses formed in the plastic layer are introduced.
Currently, new organic materials for LEDs are being actively studied. Thus, a conducting polymer with a maximum radiation intensity at a ratio of para- and metamodifications of 2:1 was obtained at the Fujitsu company based on a copolymer of para- and metabutadiene. A conductive thiophene-based polymer is used as the hole-injecting layer, which made it possible to reduce the operating voltage of the LED at high currents. The positive electrode made of Mg-In alloy is highly stable and provides a high level of electron injection. A negative indium tin oxide electrode is deposited on a glass substrate.
In the future, the company plans to use this organic LED with polysilicon thin-film field-effect transistors to create displays capable of reproducing a "live" image. These displays will have high brightness and wide viewing angles, and will cost significantly less than current AM LCDs.

DISPLAY SYSTEMS
Until recently, LEDs based on organic compounds were used only in cell phones and watches, since there were significant technological problems in maintaining the properties of LEDs when forming matrices. The development of “low temperature” technologies has removed this obstacle. The intensity of work and the range of tasks to be solved on the creation of flat indicators and displays based on organic materials are evidenced by research in the field of obtaining tunable color LEDs with a vertical structure (Princeton University) and a color organic EL display for wall-mounted TVs and mobile multimedia systems (Idemitsu Kosan company) , as well as to master the pilot production of polymer LEDs based on the technology of Cambridge Display Technology (Uniax) and the production of LCDs on plastic substrates (Ricon). The luminous efficiency of modern organic LEDs and information display devices based on them is 10–60 lm/W, the brightness of light emission reaches 50,000 cd/m2, and the service life is 10 thousand hours (at a brightness of 150 cd/m2).
The main achievement of the 90s was the development of organic blue LEDs, which made it possible to move on to the creation of full-color screens based on RGB triads. One of the main technological problems in this case is the damaging effect of technological processing during the formation of a set of LEDs (the first elements of the set are chemically affected when the second element is formed, and the first two elements are affected by the manufacturing process of the third element of the set). The presence of even minor chemical contamination (especially with alkali metals) can lead to a significant degradation of the properties of the electroluminescent material and cause significant changes in the luminescence intensity and spectral characteristics, and shorten the life of the device. Masking technology to protect the layers in the sequential production of triad elements inevitably leads to a limitation in the resolution of the display.
This problem has been successfully solved by switching to a maskless technology for manufacturing a three-dimensional rather than a planar structure. According to this technology, triad elements are made in the form of three- or four-sided pyramids, formed by pressing on a plastic substrate. On a certain face of all pyramids of the matrix, organic material is deposited by directional vapor deposition, providing radiation of one color. The substrate is then rotated through the appropriate angle (120° or 90°) and the material of the next glow color is deposited. On the fourth face, a layer of one of the colors of reduced brightness is formed, which allows expanding the color gamut reproduced by the display, as well as stabilizing the white balance during operation. This design provides an increase in resolution by almost three times. A polymer layer with pyramids and contact holes is deposited on top of an active matrix addressing thin-film field-effect transistor (TFT) circuit fabricated in conjunction with the bumps on a glass substrate. All components of the technological route have already been worked out, and the developers hope to produce fairly cheap displays with high performance.
Of undoubted interest are the developments of ultrathin organic displays. The Massachusetts Institute of Technology has developed a technology for producing displays on a plastic layer only 100 microns thick, which can be twisted into a roll with a radius of 5 mm without changing its properties. The image is formed in a layer of electrophoretic paste applied to the electrode grid on a flexible polymer substrate. The paste consists of microcapsules containing white (titanium dioxide - a standard component of conventional white) and black (a mixture of organic dyes) microparticles suspended in molten polyethylene. The shell of the capsules undergoes a special treatment to ensure its transparency. The average capsule size is about 50 microns. A grid of transparent electrodes is applied over the paste layer. When a voltage of one polarity is applied, the negatively charged white particles move to the top of the capsules and block the black particles. As a result, the capsule becomes white. When the polarity is reversed, the white particles move to the bottom of the capsule, and its color becomes black. The resolution of such a display determines the grid spacing of the electrodes, and already for the first samples it was comparable to the standard values ​​for laser printers. The power consumption of a display with a screen diagonal of 30 cm is 12 mW, the duration of information playback when the voltage is removed is not limited (until the new addressing). The image can be changed more than 107 times without performance degradation. On the basis of such a construct, it is possible to create “electronic paper”.
Xerox announced the preparation of copiers based on "electronic paper" - ultra-thin displays made using Gyricon technology, which involves the use of oil cavities with plastic spheres. When voltage is applied, the spheres are oriented relative to the surface with either the black or white side. Two AA batteries are enough to reproduce the image. Correction and updating of information is allowed. The only disadvantage of displays is the need to protect against electrical interference, in particular from static electricity. "Electronic paper", just like ordinary paper, is light, flexible, easy to read from any angle of view. In addition, it has such new properties as the ability to update information several thousand times and use an electronic pointer. According to Xerox specialists, the price of such paper will not exceed 25 cents per A4 sheet.

ORGANIC THIN FILM TRANSISTORS
In the manufacture of displays, the joint formation of TFTs by traditional technology and organic LEDs is difficult due to high-temperature processes that cause degradation of the properties of organic materials. TFTs based on organic materials can be fabricated at lower temperatures and, at the same time, cheap plastic substrates can be used instead of expensive glass ones, which will significantly reduce the cost of the entire product. The development of organic TFT technology opens up great opportunities for creating ultra-light and ultra-flat displays with high flexibility and strength. Solving the technological issues of obtaining TFTs based on organic materials will make it possible to manufacture all display elements using similar technological processes, which will reduce production costs and reduce the heterogeneity of the equipment used. According to their characteristics, modern organic TFTs are not inferior to standard ones on amorphous silicon films. The typical structure of organic TPT is shown in Fig.2.
A prototype TFT on pentacene with a gate length and width of 5 and 500 µm, respectively, and a gate dielectric thickness of 140 nm had a threshold voltage of 10 V and a saturation drift mobility of 1.7 cm2/V.s (a record result for organic transistors). To reduce the leakage current between individual TFTs, a specific Corbino topology is used, in which the source electrode forms a closed ring around the TFT active region, in the center of which the drain electrode is located. With this design, the gate controls all the current flowing from the drain to the source, which provides an on-to-off current ratio of ~108, as well as low leakage currents (the off-state current is close to the noise level).
Thus, it can be stated that the technological problem of forming information display devices completely from organic materials has already been solved today.

OPTOELECTRONICS AND LASER TECHNOLOGY
Advances in the creation of organic LEDs and information display systems also stimulate the development of devices with electrical excitation based on organic polymers, one of the most promising materials for the manufacture of new types of optoelectronic integrated circuits. The main advantages of such ICs are their low cost and rather simple technology, suitable for mastering mass production. Research in this area is carried out by many firms in the USA, Germany, Austria and Italy. And today, industrial polymer fibers are already used in standard hybrid optoelectronic circuits.
More than a dozen polymers with semiconductor properties suitable for laser generation in the entire visible range have been studied. Of particular interest to developers are conjugated polymers with side chains, since it is the side chains that determine the width of the energy band, i.e. radiation wavelength. Due to the high extinction of the generated radiation (films as thin as 0.1 µm absorb 90% of the radiation), the weak dependence of the photoluminescence quantum efficiency on the amount of active polymer in the resonator, and the large energy shift between the absorption and emission spectra (which makes it easy to achieve population inversion), conjugated polymers even at small thicknesses, they are suitable for the formation of the laser active medium. The high solubility of conjugated polymers with side chains in common organic solvents greatly simplifies the technology of depositing and forming the necessary layered film topological structures, including the traditional methods of photolithography well developed in microelectronics.
One of the most serious problems in fabricating devices with electrical excitation on polymer films is the high density of the threshold generation current (~1 kAChcm2). It is solved by introducing a distributed feedback and a distributed Bragg reflector (DRB) in order to increase the quality factor of the resonator. ROB performs the function of a resonator mirror. It is formed by alternating polymer layers of various thicknesses with low and high values ​​of the refractive index. Since the length of the resonator varies depending on the wavelength of the radiation, an ROB with a similar structure can support multimode generation.
An example of the successful use of polymers in laser technology is the first electrically excited organic-material laser from Lucent Technologies, which is suitable for industrial production. It is made on crystals of tetracene, the molecules of which contain four benzene rings. The field structure (a channel 25 µm wide and 200–400 µm long) was created on tetracene layers 1–10 µm thick, obtained by vapor deposition on a dielectric substrate in an inert gas flow. A layer of aluminum oxide with a thickness of 0.15 μm was used as a dielectric, and the control electrodes were made of aluminum-doped zinc oxide. The structure is a planar multimode waveguide with a total internal loss of ~100 cm-2. The laser resonator was formed by cleavage of a tetracene crystal with the formation of facets with a reflection coefficient of ~8%. At a high density of the injection current in the resonator, channeling of radiation at a wavelength of 575.7 nm was observed with amplification during operation in the multimode mode. At room temperature, the laser operated in a pulsed mode, and at 200 K, in a continuous-wave mode. With a decrease in reflection losses due to the introduction of distributed feedback and ROB, operation in continuous mode and at room temperature is possible. The advantage of the laser is the possibility of frequency tuning, since the emission spectrum of tetracene is quite wide.
Lasers based on organic materials are much cheaper than semiconductor ones, and a wide choice of materials makes it possible to cover a significant spectral range. It is safe to predict that such lasers will find wide application in optical memory and laser printers in the near future.

INDUSTRIAL DEVELOPMENT OF POLYMER TECHNOLOGY
Despite all the heterogeneity of the polymeric materials used, most of the operations for creating devices and structural elements are similar in structure and can be largely unified. These operations, first of all, include the processes of deposition (deposition) of polymer layers and the processes of shaping. It has already been indicated above that for most thin film and thick film materials, well-established processes of vapor deposition, screen printing and lithography (for soluble compositions) can be used.
A revolutionary approach to the development of technology for the mass production of electronic devices based on organic films was demonstrated by the Californian company Rolltronics. According to its technology, called roll-to-roll (from coil to coil), a large coil with flexible plastic is used in the conveyor production cycle, which plays the role of the substrate of the future device (Fig. 3). The length of the plastic tape is more than 300 m, and the width can exceed 1 m. Sequential application and formation of layers is carried out in specialized processing chambers that ensure the implementation of the entire technological cycle. The developers believe that they will be able to form structures at temperatures no higher than 100–125°C, which will allow the use of most modern polymeric materials.
Together with Iowa Thin Film Technologies, Rolltronics planned to commission a roll-to-roll production line by the end of 2001. The main element of future designs, a thin-film transistor, was chosen as a "pen test". In addition to TFTs, the firm intends to manufacture memory circuits, power devices and display elements, as well as all components of electronic books and electronic paper. The roll-to-roll technology is suitable for forming flat screens, LED lighting and information panels, solar cells, optoelectronic devices and semiconductor lasers. Representatives of the company call this technology a breakthrough into the future, emphasizing its extremely high efficiency and productivity, which will allow the transition to mass production of new types of electronic devices and dramatically reduce their cost.

DEVELOPMENT PROSPECTS
The physical principles used and the technology of "polymer electronics" are the first natural step towards molecular electronics. This is explained by the fact that, in contrast to classical solid-state electronics, where the properties of a crystalline body are considered and active structures are formed in its volume, in the case of using polymers, it is necessary to take into account the properties of molecules. In the transition to true molecular electronics, when individual molecules already act as an active element, the main task is to choose a technological method for point (local) impact on a molecule and change its primary chemical structure. Naturally, if a technological tool is not capable of locally modifying the initial molecular system at the atomic level, methods of its self-building and self-regulation should be developed, as happens in nature in the life cycle of viruses and bacteriophages. In the first approximation, these include the Langmuir-Blodgett method for obtaining monolayer films or the method of self-assembled monolayers of oligomers on a metal substrate (Self-assembled monolayers - SAM). These methods can be conditionally, by analogy with the technology of solid-state devices, attributed to "single-layer" epitaxy.
One of the options for the transition to molecular electronics is “hybrid” technology, when “molecular elements” are used using methods of classical electronics. An example of such a combined technology is the constructive use of carbon nanotubes proposed by IBM to create transistors that are 500 times smaller than modern silicon devices. In addition, in the absence of oxygen, they are able to withstand heating up to 1000°C.
Modern means of modification and control of atomic structures - atomic force microscopy (AFM) and scanning tunneling microscopy (STM) - can meet the technological requirements at the atomic level. But, unfortunately, both AFM and STM are sequential methods with not very high performance, and in the near future they will be used only as a laboratory tool. Nevertheless, it was with the help of AFM and STM that molecular electronics devices were first successfully created. These methods also make it possible to solve the most difficult problem of assembling molecular electronic devices - the formation of contacts. Theoretical models of AFM and STM methods of structure formation and measurements are still being developed, and many more discoveries can be expected here. However, the implementation of molecular electronics methods suitable for industrial development is a matter of the future.

CONCLUSION
All of the above shows that electronics is on the verge of a "polymer" revolution. In the next three to five years, it will be possible to "print" electronics as wallpaper. Such plastic "wallpapers" will be used to create full-color screens and displays, solar batteries and white LED lighting panels, electronic paper and much more. New electronic products based on polymeric materials, which will appear in the next decade, will revolutionize the operating conditions of electronic equipment, expand the possibilities of information technology, and create the prerequisites for the transition to new principles of organization, education, life and entertainment. The task of Russian electronics is not to "miss" this breakthrough and to get involved in the development of polymer electronics in a worthy manner.

Literature
Laser Focus World, 2001, v.37, no.3, p. 41–44.
Semiconductor International, 2000, v.23, no. 8, p.46.
Semiconductor International, 2001, v.24, no.6, p.50.
Semiconductor International, 2001, v.24, no. 8, p.40.
Solid State Technology, 2000, v.43, no. 3, p. 63–77.
Photonics Spectra 2000, v.34, no.5, p.44.
Journal of American Chemical Society, 2000, v.122, no. 2, p. 339–347.
Foreign electronic technology, 2000, issue 1, p. 66–72.

Article for the competition "bio/mol/text": Scientists have long dreamed of turning animals and plants into cyborgs controlled by electrical signals, and they are trying to do it in a variety of ways. So, about 10 years ago, a new scientific field appeared - organic bioelectronics - in which electrically conductive polymers act as intermediaries between living beings and computers. Remote control of rose leaf color, artificial neuron and pain point treatment - the first results of this triple alliance are already impressive.

Nomination Sponsor - .

The general sponsor of the competition, according to our crowdfunding, was an entrepreneur Konstantin Sinyushin, for which he has a huge human respect!

The Audience Choice Award was sponsored by Atlas.

The sponsor of the publication of this article is Andrey Alexandrovich Kiselev.

All living organisms are a bit robots or computers. Only instead of the usual electricity - electrons running through the wires to the outlet and back - we are controlled by nerve impulses, streams of charged molecules called ions. And the “buttons” in live electrical circuits are pressed not by fingers, but by special substances - neurotransmitters. When their concentration exceeds a certain limit, a chain of biochemical reactions begins in the cell membranes of neurons, which ends with the excitation of a nerve impulse.

Now scientists are trying to “marry” the computers inside us with familiar silicon microcircuits: brain-computer interfaces already know how to recognize the activity of nerve cells and convert them into meaningful commands for electronics. So, using the power of thought, you can play simple games, move a robotic prosthetic arm, or even control a quadrocopter. However, all these devices still suffer from errors and inaccuracies - it is not easy to cross electronic and ionic currents in one device.

"Translators" from the living language to the language of microcircuits can be electrically conductive polymers that conduct both types of current simultaneously (Fig. 1). Discovered in the 70s of the last century, these materials were actively studied by many scientists: they were used to make transistors, solar cells, organic light emitting diodes (OLED) and other organic electronic devices.

Figure 1. Schematic representation of organic ( on right) and inorganic ( left) semiconductors in contact with an electrolyte. The sizes of charged ions are much larger than the distances between atoms in inorganic semiconductors, and therefore ionic conduction in these materials is impossible. At the same time, the characteristic sizes of voids between chains of macromolecules of conjugated polymers are comparable with the sizes of hydrated ions, and therefore ionic conductivity is possible in this class of compounds.

Now the advantages of electrically conductive polymers - flexibility, simplicity and variability of synthesis, as well as biocompatibility and ionic conductivity - are trying to use organic bioelectronics - a very young field of materials science, which already has something to boast of.

Diagnosis from the inside

The operation of many brain-computer interfaces is based on EEG recording: a cap with electrodes is fixed on a person’s head, in which, under the influence of ionic currents flowing in the brain, their own electronic currents arise. In a 2013 paper, scientists from France proposed using organic electrochemical transistors for the same purpose.

Ordinary semiconductor transistors are the main components of all electrical logic circuits, a kind of electronic buttons with three contacts. The relatively large current flowing in them from one pin to the other can be controlled by a small signal (much less current or voltage in the case of a FET) applied to the third pin. By assembling many transistors in one circuit, it is possible to amplify, attenuate and transform any electrical signals or, in other words, process information.

Organic transistors work similarly, with which researchers have recorded epileptic activity in live laboratory mice. The third control pin in this transistor was made of a conductive polymer and injected directly into the brains of rodents. The polymer changed its structure (and, as a result, its conductivity) along with fluctuations in the electrical activity of nerve cells, and as a result, even small characteristic changes in ion currents in the “cyborg” brain led to noticeable changes in the current flowing from the input contact of the transistor to the output (Fig. 2). ).

Figure 2. in vivo registration of the electrical activity of the brain using organic transistors. pink the color shows the dependence taken with the help of an organic electrochemical transistor, blue- plastic electrode, black- metal electrode. Please note that the last two electrodes register an electrical signal by potential jumps, and the transistor - by current jumps in an electrically conductive channel.

In their experiment, the French showed that organic transistors make it possible to record the electrical activity of the brain much more accurately than their modern inorganic counterparts. In the experiments of other scientific groups, organic transistors are successfully used to take an ECG or, for example, determine the concentration of lactic acid, glucose and other biomolecules.

plastic neurons

Today, neurological and psychiatric diseases are treated mainly with the help of drugs, but it can be very difficult to choose their dosage, deliver the drug precisely to certain cells and at the same time take into account its side effects on various processes in the body. A large team of Swedish scientists from several institutes proposed to solve these problems using the same electrically conductive polymers, or rather, using another organic bioelectronics device - an organic electronic ion pump capable of pumping ions from one medium to another.

In their work, the researchers studied laboratory rats, in which they first caused neuropathic pain (its cause is not an external stimulus, but the disrupted work of the neurons themselves), and then treated it with the help of a point injection of a neurotransmitter GABA (gamma-aminobutyric acid), which reduces irritation of the central nervous system. A miniature organic pump (about 12 cm long and 6 mm in diameter) was injected into the spinal cord of rats, and its reservoir was filled with GABA (Fig. 3). With the application of an external electrical voltage, GABA molecules began to exit through four ion-conducting polymer channels into the intercellular space (video 1).

Figure 3. Implantable organic electrochemical pump. A - a photograph of the device, B - a schematic representation of the device, on the left - an electrical contact, in the center - a reservoir with GABA, on the right - excretory channels. The total length of the device is 120 mm, the tank diameter is 6 mm. C - four organic electrochemical outlets are located at the points where the branches of the sciatic nerve enter the spinal cord.

Video 1. Organoelectronic ion pump

As a result, pain disappeared in rats (this was checked using a tactile test: elastic threads of various stiffness were brought to the paws of rats and it was monitored starting from what pressure the animal would withdraw the paw), and no side effects were observed. With all other methods of treating neuropathic pain with GABA, the drug is injected into the spinal cord at a high dose, which is distributed throughout the nervous system and, in addition to suppressing pain, leads to walking disorders, lethargy, and other side effects.

Parallel to this work, the same group of researchers made the first polymer-based artificial neuron. In it, the ion pump was combined with biosensors sensitive to glutamic acid(the most common excitatory neurotransmitter) and acetylcholine(a neurotransmitter that transmits a signal from neurons to muscle tissue). For example, in one of the experiments, a “plastic” neuron monitored the level of glutamate in a Petri dish, and when a certain threshold was exceeded, a current was excited in it, which opened the reservoir of an ion pump that released acetylcholine into the environment.

The work of an artificial neuron is very similar to how real ones function: a nerve impulse is excited in one of them and runs through the entire cell to the place of contact with another neuron, glutamic acid is released there, which, as it were, presses a button and excites the next neuron (Fig. 4) . So, along the chain of neurons, the impulse reaches the muscle cell, which is already excited not by glutamic acid, but by acetylcholine. The plastic neuron created by the Swedes may well repeat these actions and transmit signals to other cells. In the experiment, these were SH-SY5Y neuroblastoma cells, whose activation was monitored by characteristic increases in the concentration of ions upon binding of acetylcholine receptors.

Figure 4. The scheme for converting a chemical signal into an electrical one and back in an artificial polymer neuron is identical to the operation scheme of a living neuron. Biosensor ( represented in green) responds to an increase in the concentration of one neurotransmitter ( orange dots), which generates an electron flow that excites an organic electrochemical pump ( represented in blue) releasing another neurotransmitter ( blue dots).

From electronic roses to the greenest energy

Research on mice, rats and other laboratory animals must be coordinated with ethics commissions, and therefore the most daring experiments in organic bioelectronics are easier to put on plants. So, at the end of 2015, the same Swedish group made the first cyborg rose. True, she still doesn’t know how to do anything spectacular - neither open up at the touch of a button on the control panel, nor change her color depending on the humidity of the environment, nor capture the world, but the researchers still managed to do something interesting.

In the first experiment, a cut rose was placed in water with an electrically conductive polymer dissolved, which rose up the stem and formed a conductive channel in the rose. Next, the scientists brought electrical contacts to the ends of the channel and inserted a control electrode into the handle - a gold wire coated with a conductive polymer. So a kind of organic transistor was going to be inside the rose. At the same time, several control electrodes could be connected to one channel at once and a simple logic circuit could be made, through which the current flows only when certain control voltages are applied to both gold wires.

In the second experiment, an aqueous solution of another electrically conductive polymer, which can change color when an external voltage is applied, was pumped into rose leaves using a syringe. Electrodes were brought to the leaf, the current was turned on and - voila: the veins of the leaf acquired a bluish-green tint. It was the polymer injected into them that turned from colorless to blue (video 2). At the same time, when the tension was removed, the leaf again became a healthy green color.

So scientists have shown that with the help of simple techniques inside plants, you can create simple electronic circuits. In the future, this will allow us to control their physiology and, for example, achieve higher yields without genetic modifications, or even make tiny power plants using the energy of photosynthesis. Of course, it sounds too expensive for now, but someday organic bioelectronics technologies will make it possible to point-by-point control of each plant, and not the entire population at once.

Bioelectronic future

The first experiments have shown that organic bioelectronic devices are quite capable of receiving, transmitting and processing bioelectrical signals. What's next? Now they have learned how to make polymeric materials biocompatible and biodegradable, and therefore any living organism can literally be stuffed with chips based on them. All that remains is to teach them how to wirelessly transmit information, and inside the human body it will be possible to create a local network of sensors that constantly monitor various medical indicators such as glucose levels, heart rate and electrical activity of selected neurons, and then transmit their signals to implanted medical robots based on the same ionic sensors. pumps so that they start to deal with the problem.

If you don’t like the idea of ​​becoming such a cyborg at all, you can simply swallow a pill with a built-in flexible microcircuit - by acidity, temperature and concentration of various substances, it will calculate exactly where to release the medicine, and, having done a good deed, it will simply be digested inside us like some piece of sugar.

Introduction

In 1965, at the dawn of the computer age, Gordon Moore, director of research at Fairchild Semiconductors, predicted that the number of transistors on a chip would double every year. It's been 35 years and Moore's Law is still in effect. True, over time, the practice of microelectronic production made a slight amendment to it: today it is believed that the doubling of the number of transistors occurs every 18 months. This growth slowdown is caused by the complexity of microchip architecture. And yet, for silicon technology, Moore's prediction cannot hold forever.

But there is another, fundamental limitation on "Moore's law". The increase in the density of elements on the chip is achieved by reducing their size. Even today, the distance between processor elements can be 0.13x10 -6 meters (the so-called 0.13-micron technology). When the size of transistors and the distance between them reach several tens of nanometers, the so-called size effects will come into effect - physical phenomena that completely disrupt the operation of traditional silicon devices. In addition, with a decrease in the thickness of the dielectric in field-effect transistors, the probability of electrons passing through it increases, which also prevents the normal operation of devices.

Another way to improve performance is to use other semiconductors instead of silicon, such as gallium arsenide (GaAs). Due to the higher mobility of electrons in this material, it is possible to increase the speed of devices by an order of magnitude. However, technologies based on gallium arsenide are much more complicated than silicon ones. Therefore, although considerable funds have been invested in the study of GaAs over the past two decades, integrated circuits based on it are used mainly in the military field. Here, their high cost is offset by low power consumption, high speed and radiation resistance. However, the development of devices based on GaAs remains subject to limitations due to both fundamental physical principles and manufacturing technology.

That is why today specialists in various fields of science and technology are looking for alternative ways of further development of microelectronics. One way to solve the problem is offered by molecular electronics.

MOLECULAR ELECTRONICS - FUTURE TECHNOLOGY.

The possibility of using molecular materials and individual molecules as active elements of electronics has long attracted the attention of researchers in various fields of science. However, only recently, when the boundaries of the potential possibilities of semiconductor technology have become practically tangible, the interest in the molecular ideology of constructing the basic elements of electronics has moved into the mainstream of active and targeted research, which today has become one of the most important and promising scientific and technical areas of electronics.

Further prospects for the development of electronics are associated with the creation of devices that use quantum phenomena, in which the account already goes to units of electrons. Recently, theoretical and experimental studies of artificially created low-dimensional structures have been widely carried out; quantum layers, wires and dots. It is expected that the specific quantum phenomena observed in these systems can form the basis for the creation of a fundamentally new type of electronic devices.

The transition to the quantum level is undoubtedly a new, important stage in the development of electronics, since allows you to go to work with almost single electrons and create memory elements in which one electron can correspond to one bit of information. However, the creation of artificial quantum structures is a very difficult technological task. Recently, it has become obvious that the implementation of such structures is associated with great technological difficulties even when creating single elements, and insurmountable difficulties arise when creating chips with multi-million elements. The way out of this situation, according to many researchers, is the transition to a new technology - molecular electronics.

The fundamental possibility of using individual molecules as active elements of microelectronics was expressed by Feynman back in 1957. Later, he showed that quantum mechanical laws are not an obstacle to the creation of atomic-sized electronic devices, as long as the information recording density does not exceed 1 bit/atom. However, only with the advent of the works of Carter and Aviram began to talk about molecular electronics as a new interdisciplinary field, including physics, chemistry, microelectronics and computer science, and aimed at transferring microelectronics to a new element base - molecular electronic devices.

This definitely suggests an analogy with the history of the development of precision time devices, which have gone from mechanical chronometers using various types of pendulums, through quartz clocks based on solid state resonances, and finally, today the most accurate clocks use intramolecular effects in ammonia molecules, etc. . Electronics is developing in a similar way, having gone from mechanical electromagnetic relays and vacuum tubes to solid-state transistors and microcircuits, and today it has come to the threshold beyond which lies the field of molecular technology.

It is no coincidence that the main attention was focused on molecular systems. First, a molecule is an ideal quantum structure consisting of individual atoms, the movement of electrons along which is determined by quantum chemical laws and is the natural limit of miniaturization. Another, no less important feature of molecular technology is that the creation of such quantum structures is greatly facilitated by the fact that their creation is based on the principle of self-assembly. The ability of atoms and molecules under certain conditions to spontaneously combine into predetermined molecular formations is a means of organizing microscopic quantum structures; operation with molecules predetermines the way of their creation. It is the synthesis of a molecular system that is the first act of self-assembly of the corresponding devices. This achieves the identity of the assembled ensembles and, accordingly, the identity of the dimensions of the elements and, thereby, the reliability and efficiency of quantum processes and the functioning of molecular devices.

From the very beginning of the development of the molecular approach in microelectronics, the question of the physical principles of the functioning of molecular electronic devices remained open. Therefore, the main efforts were focused on their search, with the main attention being paid to single molecules or molecular ensembles. Despite a large number of works in this direction, the practical implementation of molecular devices is far from complete. One of the reasons for this is that, especially in the initial period of the formation of molecular electronics, a strong emphasis was placed on the work of individual molecules, the search and creation of bistable molecules that mimic trigger properties. Of course, this approach is very attractive in terms of miniaturization, but it leaves little chance that molecular electronic devices can be created in the near future.

The development of a new approach in microelectronics requires the solution of a number of problems in three main areas: the development of physical principles for the functioning of electronic devices; synthesis of new molecules capable of storing, transmitting and transforming information; development of methods for organizing molecules into a supramolecular ensemble or a molecular electronic device.

Currently, an intensive search is underway for the concepts of the development of molecular electronics and the physical principles of functioning, and the foundations for constructing basic elements are being developed. Molecular electronics is becoming a new interdisciplinary field of science that combines solid state physics, molecular physics, organic and inorganic chemistry and aims to transfer electronic devices to a new element base. To solve the set tasks and concentrate the efforts of researchers working in various fields of knowledge, centers of molecular electronics, joint laboratories are being created in all industrialized countries, international conferences and seminars are held.

Now, and apparently, and in the near future, it is difficult to talk about the creation of molecular electronic devices operating on the basis of the functioning of single molecules, but we can really talk about the use of molecular systems in which intramolecular effects have a macroscopic manifestation. Such materials can be called "intelligent materials". The stage of creating "intelligent materials", i.e. the stage of functional molecular electronics, a natural and necessary period in the development of electronics, is a definite stage in the transition from semiconductor to molecular technology. But it is possible that this period will be longer than we now think. It seems more realistic, especially at the early stages of the development of molecular electronics, to use the macroscopic properties of molecular systems, which would be determined by structural reorganizations occurring at the level of individual molecular ensembles. The physical principle of functioning of such electronic devices should remove dimensional restrictions, at least up to the size of large molecular formations. From the point of view of electronics and the potential possibility of docking molecular devices with their semiconductor counterparts, it would be preferable to deal with molecular systems that change their electronic conductivity under external influences, primarily under the influence of an electric field.

The ideas of molecular electronics are not reduced to a simple replacement of a semiconductor transistor with a molecular one, although this particular problem will also be solved. The main goal, however, is to create complex molecular systems that simultaneously implement several different effects that perform a complex task. It is natural, first of all, to include the task of creating a universal memory element as the most important part of any information-computing device among the tasks of this type. It seems very obvious that the potential of molecular electronics will be revealed to a greater extent by creating neural networks consisting of neurons and electroactive synapses connecting them. The creation by means of molecular electronics of artificial neurons, various types of sensors included in a single network, will open the way to the realization of all the potentialities inherent in the neurocomputer ideology, will allow the creation of a fundamentally new type of information and computing systems and come close to solving the problem of creating artificial intelligence.

Bacteriorhodopsin: structure and functions.

Molecular electronics is defined as encoding (recording), processing and recognition (reading) of information at the molecular and macromolecular levels. The main advantage of the molecular approximation lies in the possibility of molecular design and production of devices "from the bottom up", i.e. atom by atom or fragment by fragment, the parameters of the devices are determined by organic synthesis and genetic engineering methods. Two well-recognized advantages of molecular electronics are a significant reduction in device size and gate propagation delays.

Bioelectronics, which is a branch of molecular electronics, explores the possibility of using biopolymers as modules controlled by light or electrical impulses in computer and optical systems. The main requirement for likely candidates among a large family of biopolymers is that they must reversibly change their structure in response to some physical impact and generate at least two discrete states that differ in easily measurable physical characteristics (for example, spectral parameters).

In this regard, proteins are of considerable interest, the main function of which is associated with the transformation of light energy into chemical energy in various photosynthetic systems. The most likely candidate among them is a light-dependent proton pump - bacteriorhodopsin (BR) from a halophilic microorganism Halobacterium salinarum(previously Halobacterium halobium), discovered in 1971.

Bacteriorhodopsin, a retinal-containing proton transport generator, is a transmembrane protein of 248 amino acids with a molecular weight of 26 kDa, penetrating the membrane in the form of seven a- spirals; The N- and C-terminals of the polypeptide chain are located on opposite sides of the cytoplasmic membrane: the N-terminus faces outward, and the C-terminus faces inside the cell (Fig. 1, 2).

Fig.1. BR model in the elements of the secondary structure. The amino acids have been isolated
involved in proton transport: aspartic acid residues in circles,
squared arginine residue. With Lys-216 (K-216) a Schiff base (SB) is formed.
The arrow shows the direction of proton transport.

Chromophore BR - protonated retinal aldimine with a The -amino group of the Lys-216 residue is located in the hydrophobic part of the molecule. After the absorption of a light quantum during the photocycle, retinal isomerizes from all-E to 13Z-shape. The protein microenvironment of the chromophore can be considered as a receptor with substrate specificity for all-E /13Z-retinal, which catalyzes this isomerization at room temperature. In addition, some amino acids are responsible for the suppression of isomerizations other than all-E /13Z, for example from all-E- to 7Z-, 9Z-, 11Z-retinal. The rest of the polypeptide chain provides a proton transport channel or shields the photochromic internal group from environmental influences.

The mutual topography of the secondary structure elements formed by the BR polypeptide chain after the absorption of a light quantum by the chromophore molecule changes, resulting in the formation of a channel for the transmembrane transfer of protons from the cytoplasm to the external environment. However, the molecular mechanism of light-dependent transport is still unknown.

Fig.2. Schematic model of the three-dimensional (spatial) structure of BR Seven a -helices form a chromophore cavity and a transmembrane proton transfer channel.

BR is contained in the cell membrane H. salinarum- halophilic archaebacteria that lives and breeds in salt marshes and lakes, where the concentration of NaCl can exceed 4 M, which is 6 times higher than in sea water (~ 0.6 M). This unique protein is in many ways similar to the visual protein rhodopsin, although their physiological functions are different. While visual rhodopsin acts as the primary photoreceptor that provides dark vision to most vertebrates, the physiological role of BR is to enable halobacteria to act as facultative anaerobes when the partial pressure of oxygen in the environment is low. The protein functions as a light-dependent proton pump, which ensures the formation of an electrochemical gradient of protons on the surface of the cell membrane, which, in turn, serves to store energy. The primary work done by the gradient is the synthesis of ATP through anaerobic (photosynthetic) phosphorylation and, in this case, is a classic example of Mitchell's chemiosmotic hypothesis of oxidative phosphorylation. When there is no light and the partial pressure of oxygen is high, the bacteria revert to aerobic oxidative phosphorylation.
Cells H. salinarum also contain two so-called sensory rhodopsins (SR I and SR II), which provide positive and negative phototaxis. Different wavelengths are read by CP I and CP II as detector molecules, which causes a cascade of signals that control the flagellar motor of the bacterium. With the help of this elementary process of light perception, microorganisms independently move into the light of a suitable spectral composition. In addition, cells have halorhodopsin (GH), which is a light-dependent pump of Cl - ions. Its main function is to transport chloride ions into the cell, which are constantly lost by the bacterium, moving in the direction from the inside to the outside under the action of the electric field created by the BR. The mechanism of action of GR is unclear. It is assumed that Cl - binds to the positively charged quaternary nitrogen of the protonated Schiff base, and the isomerization of retinal from all-E to the 13Z-form causes the movement of this nitrogen with the Cl ion attached to it - from the input to the output Cl - - conducting path.

Fig.3. A section of the purple membrane (top view).

BR is localized in areas of cell membranes H. salinarum in the form of purple membranes (PM), forming two-dimensional crystals with a hexagonal lattice. These areas contain the protein itself, some lipids, carotenoids and water (Fig. 3). They are usually oval or round in shape with an average diameter of about 0.5 µm and contain about 25% lipids and 75% protein. PM are resistant to sunlight, exposure to oxygen, temperatures over 80ºC (in water) and up to 140ºC (dry), pH from 0 to 12, high ionic strength (3 M NaCl), action of most proteases, sensitive to mixtures of polar organic solvents with water, but are resistant to non-polar solvents such as hexane. Of great practical importance is the existing possibility of embedding PM in polymer matrices without loss of photochemical properties.

Light-induced proton transport is accompanied by a number of cyclic spectral changes in BR, the totality of which is called the photocycle (Fig. 4). Thirty years of research has led to a fairly detailed understanding of the photocycle, but the details of proton transport are still being studied.

The photochemical cycle of BR consists of individual intermediates, which can be identified both by absorption maxima and by the kinetics of formation and decay. Figure 4 shows a simplified model of the BR photocycle.

Fig.4. Photocycle BR.

The photochemical and thermal stages are shown as thick and thin arrows, respectively. Vertical symbols indicate all-E-conformation of retinal (intermediates B and O), oblique symbols - to the 13Z-conformation. In the dark, BR turns into a 1:1 mixture D and B, this mixture is called dark-adapted BR. When the BR is illuminated, light adaptation occurs, i.e. transition to the ground state B. From there, the photocycle begins, which leads to the transport of a proton across the membrane. During the transition L to M lasting approximately 40 μs, the Schiff base is deprotonated and Asp85 becomes protonated. From there, the proton goes to the outside of the extracellular part of the proton channel. During the transition M to N aldimine is reprotonated. The Asp96 residue acts as a proton donor. Asp96 is reprotonated through the cytoplasmic proton hemichannel. While all transformations between intermediates are reversible, the transition from M I to M II is believed to be the major irreversible step in the photocycle. During this transition, the nitrogen of the Schiff base becomes inaccessible to the extracellular part of the proton channel, but only to the cytoplasmic half-channel, which is associated with conformational changes in the protein molecule.

The physicochemical properties of the intermediates are characterized by the wavelength of their absorption maxima and the value of the specific molar extinction coefficient. The protonation of SB and the configuration of the retinylidene residue affect the magnitude of the absorption maxima. During the BR photocycle, several temperature-dependent conformational changes occur in the protein, so the formation of most intermediates can be suppressed by cooling.

In addition to the main photocycle, there are two states that can be artificially induced. In intermediates P and Q retinal conformation 9Z. This is achieved after photochemical excitation all-E-retinal when Asp85 is protonated at the same time. This can be achieved in wild-type BR at low pH or deionization (formation of so-called blue membranes), but these preparations are not stable. An alternative approach is to replace Asp85 with an amino acid having a different pKa value that remains uncharged at the pH of interest, or to completely remove the carboxyl group by site-directed mutagenesis. The stability of such mutant blue membranes is higher.

The unique properties of bacteriorhodopsin provide a wide range of technical applications in which it can be used, however, only optical ones are currently commercially feasible, since their integration into modern technical systems is the simplest.

Optical applications are based on the use of BR films - polymer matrices of various compositions with protein molecules included in them. For the first time in the world, such films based on the wild-type BR were obtained and studied in our country within the framework of the "Rhodopsin" project; In the 1980s, the effectiveness and prospects of using such materials, called "Biochrome", as photochromic materials and a medium for holographic recording were demonstrated.

Of great interest is the possibility of varying the photochemical properties of BR films:
a) replacement of the natural chromophore with a modified one;
b) chemical (physico-chemical) influences;
c) point substitutions of certain amino acid residues by genetic engineering methods.

Such modified materials may have valuable specific properties, which will predetermine their use as the element base of a biocomputer.

thinking molecule

In recent years, scientists in many countries have returned to the old and simple idea of ​​a "chemical" computer in which calculations are performed by individual molecules. Over the past year, researchers from several laboratories at once have been able to obtain brilliant results in this area that promise to radically change the situation.

Scientists have achieved great success in working with pseudorotoxan molecules (they are shown in Fig. 1).

They managed to fit such a molecule, which has the shape of a ring, onto an axis - a linear molecule. In order to prevent the ring from jumping off the axis, large molecular fragments are attached to its ends, playing the role of "nuts" (various donor groups were used in this capacity). When reacting with an acid (H+) or a base (B), the ring can slide from one end of the axis to the other, "switching" the chemical state. It's funny that, in principle, at the molecular level, a mechanical device is recreated, very similar to the connection of rods and wheels in the first, most primitive, computing devices of the 17th century (however, if you wish, you can also see the simplest clerical abacus in this molecular structure, with one knuckle on each twig).

This elegant chemical switch molecule was studied back in the early 90s, however, for the practical implementation of the idea, it was still necessary to come up with methods for combining and controlling arrays of these minimicrodiodes. Having created a monolayer of similarly oriented molecules of this type on the surface of the metal (this very difficult task was solved using the latest nanotechnological self-assembly methods), scientists deposited the thinnest layer of gold on it and have already created primitive prototypes of logic gates on this basis.

A few months later, a joint group of Mark Read and James Tour (from Yale and Rice Universities) demonstrated another class of switch molecules to the public. The results were so impressive that the magazine "Scientific American" (June 2000) even put on the cover of the announcement "The Birth of Molecular Electronics" (I would like to add - finally!). As one of the authors wrote with restrained pride: "We have created a molecule with a variable electrical conductivity, which can accumulate electrons on our command, that is, to work as a storage device."

First of all, James Tour, using a special technique, synthesized a molecular chain of benzene-1,4-dithiolate units 14 nanometers long. Groups were introduced into it that capture electrons if the molecule is "under tension." The most difficult problem, which was also overcome, was that the switch must be a reversible chemical process. For a molecule to work as a memory element, it must be taught not only to capture electrons, but to hold them only for a given time. Strictly speaking, this is precisely the main achievement of Reed and Tour with colleagues.
An electrochemical (in the strictest and most literal sense of the term!) switch is shown in fig. 2 (left side). It is a chain of three benzene rings, to the central of which NO 2 and NH 2 groups are attached from opposite sides (highlighted in color in the figure). Such an asymmetric molecular configuration creates an electron cloud of a complex shape, resulting in a surprisingly beautiful and fundamentally important physical effect for solving the problem: when a field is applied, the molecule twists, its resistance changes, and it begins to pass current (right side of the figure). When the field is removed, the molecule spins in the opposite direction and returns to its original state. A switch based on this principle is a linear chain of about 1,000 nitrobenzenethiol molecules located between two metal contacts. Moreover, measurements using tunneling microscopy (a fragment of a molecular chain was soldered between ultrathin needle-shaped gold electrodes; the experimental geometry is shown in Fig. 3) made it possible to obtain the operating parameters of the switch, which can rightly be called the molecular current-voltage characteristic and molecular conductivity (Fig. .four). The conductance curve (which, by the way, turned out to be very close to the calculated one) has a clearly pronounced dip. This makes it possible to transfer sections of the molecule from a conducting state to a non-conducting state, and vice versa, by a simple change in the applied voltage. Formally and actually obtained (the chemist, of course, prefer the term "synthesized") a molecular triode. Indeed, this can be considered the first stage in the creation of molecular electronics.

Conclusion

Although the theoretical foundations of moletronics have already been sufficiently well developed and prototypes of almost all elements of logical circuits have been created, however, significant difficulties arise in the way of actually building a molecular computer. The outwardly obvious possibility of using individual molecules as logical elements of electronic devices turns out to be very problematic due to the specific properties of molecular systems and the requirements for logical elements.

First of all, the logical element must have high reliability of operation when a control action is applied. If we consider the optical connection between the elements, then in the system one molecule - one photon, the reliability of switching will be low due to the relatively low probability of the transition of the molecule to an excited state. One can try to overcome this difficulty by simultaneously using a large number of quanta. But this contradicts another important requirement: the efficiency of signal conversion by a separate element should be close to unity, that is, the average reaction power should be commensurate with the average impact power. Otherwise, when elements are combined into a chain, the probability of their operation will decrease as they move away from the beginning of the chain. In addition, the element must unambiguously switch to the required state and remain in it for a sufficiently long time - until the next impact. For relatively simple molecules, this requirement is usually not satisfied: if the transition to an excited state can be controlled, then the reverse transition can occur spontaneously.

However, not everything is so bad. The use of large organic molecules or their complexes makes it possible, in principle, to circumvent the enumerated difficulties. For example, in some proteins the efficiency of electron-optical conversion is close to unity. In addition, for large bioorganic molecules, the lifetime of the excited state reaches tens of seconds.

But even if a single molecular computing element does not have the reliability of its silicon predecessors, the efficient operation of the future computer can be achieved by combining the principles of moletronics and parallel computing used in supercomputers. To do this, you need to make several identical molecular logic elements work in parallel. Then the incorrect operation of one of them will not lead to a noticeable failure in the calculations. A modern massively parallel supercomputer with many hundreds of processors can maintain high performance even if 75% of them fail. Almost all living systems use the principle of parallelism. Therefore, the imperfection of organisms at the level of individual cells or genes does not prevent them from functioning effectively.

Today in the world there are more than a dozen scientific and technological centers involved in the development of molecular electronics devices. Annual conferences bring together hundreds of experts in this field.

Great interest in moletronics is caused not only by the prospects of building a computer, but also by the wide possibilities for the development of new technologies. Due to the high sensitivity of molecular electronic devices to light, they can be used to create efficient solar energy converters, simulate the process of photosynthesis, and develop a new class of image detectors, the principle of which will resemble the work of the human eye. Molecular devices can also be used as selective sensors, responding only to certain types of molecules. Such sensors are necessary in ecology, industry, and medicine. A sensor made of organic molecules is much easier to implant into a human body in order to monitor its condition.

Solving the problems facing molecular electronics requires the efforts of a wide range of scientists working in the field of academic knowledge from colloidal chemistry and biology to theoretical physics, as well as in the field of high technologies. In addition, significant financial investments are required.

It is also necessary to train new highly qualified personnel for work in this complex area, which lies at the intersection of sciences. But, apparently, in 10-15 years it will play a significant role in science and technology.

List of material used

According to the network Internet , articles:

1. Goncharova E., Bachelor of Biotechnology;

2. Zaitsev V., Shishlova A., Department of Physics, Lomonosov Moscow State University M. V. Lomonosov;

3. Krieger Yu., Ph.D. n.