Application of quantum dots. Quantum dot designs. Higher peak brightness

Good time days, Habrazhiteliki! I think many people have noticed that advertisements about displays based on quantum dot technology, the so-called QD – LED (QLED) displays, have begun to appear more and more often, despite the fact that at the moment this is just marketing. Similar to LED TV and Retina, this is a technology for creating LCD displays that uses quantum dot-based LEDs as backlight.

Your humble servant decided to figure out what quantum dots are and what they are used with.

Instead of introducing

Quantum dot- a fragment of a conductor or semiconductor, the charge carriers of which (electrons or holes) are limited in space in all three dimensions. The size of a quantum dot must be small enough for quantum effects to be significant. This is achieved if kinetic energy electron is noticeably greater than all other energy scales: first of all, greater than the temperature expressed in energy units. Quantum dots were first synthesized in the early 1980s by Alexei Ekimov in a glass matrix and by Louis E. Brous in colloidal solutions. The term "quantum dot" was coined by Mark Reed.

The energy spectrum of a quantum dot is discrete, and the distance between stationary energy levels of the charge carrier depends on the size of the quantum dot itself as - h/(2md^2), where:

1. h - reduced Planck constant;
2. d is the characteristic size of the point;
3. m is the effective mass of an electron at a point

In simple terms, a quantum dot is a semiconductor whose electrical characteristics depend on its size and shape.

For example, when an electron moves to a lower energy level, a photon is emitted; Since you can adjust the size of a quantum dot, you can also change the energy of the emitted photon, and therefore change the color of the light emitted by the quantum dot.

Types of Quantum Dots

There are two types:

• epitaxial quantum dots;
• colloidal quantum dots.

In fact, they are named after the methods used to obtain them. I will not talk about them in detail due to large quantity chemical terms (Google to help). I will only add that using colloidal synthesis it is possible to obtain nanocrystals coated with a layer of adsorbed surfactant molecules. Thus, they are soluble in organic solvents and, after modification, also in polar solvents.

Quantum dot design

Typically, a quantum dot is a semiconductor crystal in which quantum effects are realized. An electron in such a crystal feels like it is in a three-dimensional potential well and has many stationary energy levels. Accordingly, when moving from one level to another, a quantum dot can emit a photon. With all this, the transitions are easy to control by changing the dimensions of the crystal. It is also possible to transfer an electron to a high energy level and receive radiation from the transition between lower-lying levels and, as a result, we obtain luminescence. Actually, it was the observation of this phenomenon that served as the first observation of quantum dots.

Now about the displays

The history of full-fledged displays began in February 2011, when Samsung Electronics presented the development of a full-color display based on quantum QLED points. It was a 4-inch display controlled by an active matrix, i.e. Each color quantum dot pixel can be turned on and off by a thin film transistor.

To create a prototype, a layer of quantum dot solution is applied to a silicon circuit board and a solvent is sprayed on. Then a rubber stamp with a comb surface is pressed into the layer of quantum dots, separated and stamped onto glass or flexible plastic. This is how stripes of quantum dots are applied to a substrate. In color displays, each pixel contains a red, green or blue subpixel. Accordingly, these colors are used with different intensities to obtain the most more shades.

The next step in development was the publication of an article by scientists from the Indian Institute of Science in Bangalore. Where quantum dots were described that luminesce not only in orange, but also in the range from dark green to red.

Why is LCD worse?

The main difference between a QLED display and an LCD is that the latter can cover only 20-30% of the color range. Also, in QLED TVs there is no need to use a layer with light filters, since the crystals, when voltage is applied to them, always emit light with a clearly defined wavelength and, as a result, with the same color value.

There was also news about the sale of a computer display based on quantum dots in China. Unfortunately, I haven’t had a chance to check it with my own eyes, unlike on TV.

P.S. It is worth noting that the scope of application of quantum dots is not limited only to LED monitors; among other things, they can be used in field-effect transistors, photocells, laser diodes, and the possibility of using them in medicine and quantum computing is also being studied.

P.P.S. If we talk about my personal opinion, then I believe that they will not be popular for the next ten years, not because they are little known, but because the prices for these displays are sky-high, but I still want to hope that quantum the points will find their application in medicine, and will be used not only to increase profits, but also for good purposes.

Quantum dots are small crystals that emit light with precisely controlled color values. They significantly improve image quality without affecting the final cost of the devices.

Quantum dot LED - new screen technology Conventional LCD TVs are capable of transmitting only 20-30% of the color range perceived by the human eye. The image on an OLED screen is more realistic, but this technology is not suitable for mass production of large displays. But recently it has been replaced by a new one that provides the ability to display accurate color values. It's about about so-called quantum dots. At the beginning of 2013, Sony introduced the first TV based on quantum dots (Quantum dot LED, QLED). This year, other device models will be launched into mass production, and they will cost the same as regular LCD TVs and significantly less than OLED solutions. What is the difference between displays produced using new technology, from standard LCD screens?

LCD TVs do not have pure colors

Liquid crystal displays are made up of five layers: the starting point is white light emitted by LEDs and passed through several filters. Polarizing filters located at the front and rear, in combination with liquid crystals, regulate the transmitted light flux, lowering or increasing the brightness. This is possible thanks to pixel transistors, which affect how much light passes through the filters (red, green, blue). The combination of colors of these three subpixels, on which filters are applied, ultimately gives a certain color value of the pixel. Mixing colors is not a problem, but it is impossible to get pure red, green or blue this way. The reason here lies in filters that transmit not just one wave of a certain length, but a whole bunch of waves of different lengths. For example, orange light also passes through a red filter.

The LED lights up when voltage is applied to it. This causes electrons to move from the N-type material to the P-type material. N-type material contains atoms with an excess number of electrons. P-type material contains atoms that lack electrons. When excess electrons enter the latter, they release energy in the form of light. In a conventional semiconductor crystal, this is typically white light produced by many different wavelengths. The reason for this is that electrons can be in different energy levels. Therefore, the emitted photons have different energies, which is expressed in different wavelengths of radiation.

Quantum dots - stable light

In QLED displays, the light source is quantum dots - crystals several nanometers in size. In this case, there is no need for a layer with light filters, since when voltage is applied to them, the crystals always emit light with a clearly defined wavelength, and therefore color value - the energy zone is reduced to one energy level. This effect is explained by the tiny size of a quantum dot, in which an electron, like in an atom, is able to move only in a limited space. As in an atom, the electron of a quantum dot can only occupy strictly defined energy levels. Due to the fact that these energy levels also depend on the material, it becomes possible to specifically tune the optical properties of quantum dots. For example, to obtain red color, crystals from an alloy of cadmium, zinc and selenium (CdZnSe), the size of which is about 10–12 nm, are used. Cadmium and selenium alloy suitable for yellow, green and blue colors, the latter can also be obtained by using nanocrystals from a compound of zinc and sulfur with a size of 2–3 nm.

Due to the fact that mass production of blue crystals is associated with great difficulties and costs, the TV presented by Sony is not a “pure” QLED TV based on quantum dots. At the back of the displays produced by QD Vision is a layer of blue LEDs, the light of which passes through a layer of red and green nanocrystals. As a result, they essentially replace the currently common light filters. Thanks to this, the color gamut increases by 50% compared to conventional LCD TVs, but does not reach the level of a “pure” QLED screen. The latter, in addition to a wider color gamut, have another advantage: they save energy, since there is no need for a layer with light filters. Thanks to this, the front part of the screen in QLED TVs also receives more light than in conventional TVs, which transmit only about 5% of the luminous flux.

Quantum dots in HDTV

Our eyes are capable of seeing more colors than HDTVs can display. Displays based on quantum dots can change this situation. Quantum dots are tiny particles a few nanometers in diameter that emit light at one specific wavelength and always with the same color value. If we talk about the light filters used in modern TVs, they provide only blurry colors.

Screens without filters

In modern TVs, the white light of LED lamps (backlight) becomes colored thanks to light filters. In a quantum dot display (QLED), color is produced directly at the light source. The brightness control systems using liquid crystals and polarization have not undergone any changes.

Light cells in comparison

In LEDs, electrons move from an N-type material to a P-type material, releasing energy in the form white light with different wavelengths. The filter generates the desired color. In QLED TVs, nanocrystals emit light with a specific wavelength, and therefore color.

Wider color gamut

Quantum dot displays are capable of displaying more natural colors (red, green, blue) than traditional TVs, covering a wider color range that is closest to our color perception.

Size and material determine color

When an electron (e) connects with a quantum dot, energy is released in the form of photons (P). Using various materials and by changing the size of nanocrystals, it is possible to influence the amount of this energy and, as a consequence, the length of the light wave.

In order to receive general idea about properties material items and the laws in accordance with which the macroworld familiar to everyone “lives”, it is not at all necessary to graduate from a higher educational institution, because every day everyone is faced with their manifestations. Although recently the principle of similarity has been increasingly mentioned, its proponents claim that the micro and macro worlds are very similar, nevertheless, there is still a difference. This is especially noticeable with very small sizes of bodies and objects. Quantum dots, sometimes called nanodots, are one of these cases.

Less less

Let's remember classic device atom, for example hydrogen. It includes a nucleus, which, due to the presence of a positively charged proton in it, has a plus, that is, +1 (since hydrogen is the first element in the periodic table). Accordingly, at a certain distance from the nucleus there is an electron (-1), forming an electron shell. Obviously, if you increase the value, this will entail the addition of new electrons (remember: in general, the atom is electrically neutral).

The distance between each electron and the nucleus is determined by the energy levels of the negatively charged particles. Each orbit is constant; the overall configuration of the particles determines the material. Electrons can jump from one orbit to another, absorbing or releasing energy through photons of one frequency or another. The most distant orbits contain electrons with the maximum energy level. Interestingly, the photon itself exhibits a dual nature, being defined simultaneously as a massless particle and electromagnetic radiation.

The very word "photon" Greek origin, it means "particle of light". Therefore, it can be argued that when an electron changes its orbit, it absorbs (emits) a quantum of light. IN in this case It is appropriate to explain the meaning of another word - “quantum”. In fact, there is nothing complicated. The word comes from the Latin “quantum”, which literally translates as the smallest value of any physical quantity (here radiation). Let us explain with an example what a quantum is: if, when measuring weight, the smallest indivisible quantity was a milligram, then it could be called that. This is how a seemingly complex term is simply explained.

Quantum Dots Explained

Often in textbooks you can find the following definition for a nanodot - this is an extremely small particle of any material, the dimensions of which are comparable to the emitted wavelength of an electron (the full spectrum covers the limit from 1 to 10 nanometers). Inside it, the value of a single negative charge carrier is less than outside, so the electron is limited in its movements.

However, the term "quantum dots" can be explained differently. An electron that has absorbed a photon “rises” to a higher energy level, and in its place a “shortage” is formed - a so-called hole. Accordingly, if an electron has a -1 charge, then a hole has a +1 charge. Trying to return to its previous stable state, the electron emits a photon. The connection of charge carriers “-” and “+” in this case is called an exciton and in physics is understood as a particle. Its size depends on the level of absorbed energy (higher orbit). Quantum dots are precisely these particles. The frequency of energy emitted by an electron directly depends on the particle size of this material and exciton. It is worth noting that the color perception of light by the human eye is based on different

"Nanotechnology" is a word with complicated history and the context in the Russian language, unfortunately, is slightly discredited. However, if we ignore the ironic socio-economic overtones, we can state that in recent years nanotechnology has begun to evolve from a scientific and theoretical concept into forms that in the foreseeable future can become real commercial products and enter our lives.

A great example of this is quantum dots. Technologies using semiconductor nanoparticles are gradually finding applications in completely different fields: medicine, printing, photovoltaics, electronics - some of the products still exist at the prototype level, in some places the technology has been partially implemented, and some are already in practical use.

So what is a “quantum dot” and what is it eaten with?

A quantum dot is a nanocrystal of inorganic semiconductor material (silicon, indium phosphide, cadmium selenide). “Nano” means measured in parts per billion, and the sizes of such crystals range from 2 to 10 nanometers. Because of their small size, electrons in nanoparticles behave very differently from those in bulk semiconductors.

The energy spectrum of a quantum dot is heterogeneous; it has separate energy levels for an electron (a negatively charged particle) and a hole. A hole in semiconductors is an unfilled valence bond, a carrier of positive charge numerically equal to an electron, it appears when the bond between the nucleus and the electron is broken.

If conditions are created under which the charge carrier in the crystal moves from level to level, then during this transition a photon is emitted. By changing the particle size, you can control the absorption frequency and wavelength of this radiation. In practice, this means that depending on the particle size of the dot, when irradiated, they will glow in different colors.

The ability to control the wavelength of radiation through the particle size makes it possible to obtain stable substances from quantum dots that convert the energy they absorb into light radiation - photostable phosphors.

Solutions based on quantum dots are superior to traditional organic and inorganic phosphors in a number of parameters that are important for those practical applications that require precise, tunable luminescence.

Advantages of quantum dots:

• Photostable, retain fluorescent properties for several years.
• High resistance to photofading: 100 – 1000 times higher than that of organic fluorophores.
• High quantum yield of fluorescence – up to 90%.
• Wide excitation spectrum: from UV to IR (400 – 200 nm).
• High color purity due to high fluorescence peaks (25-40 nm).
• High resistance to chemical degradation.

Another advantage, especially for printing, is that quantum dots can be used to make sols - highly dispersed colloidal systems with a liquid medium in which they are distributed. fine particles. This means that they can be used to produce solutions suitable for inkjet printing.

Application areas of quantum dots:

Protection of documents and products from falsification: securities, banknotes, identity cards, stamps, seals, certificates, certificates, plastic cards, trademarks. A multicolor coding system based on quantum dots can be commercially in demand for color marking of products in the food, pharmaceutical, chemical industries, jewelry, and works of art.

Due to the fact that the liquid base can be water-based or UV-curable, using ink with quantum dots you can mark almost any object - for paper and other absorbent bases - water-based ink, and for non-absorbent ones (glass, wood, metal, synthetic polymers , composites) – UV ink.

Marker in medical and biological research. Due to the fact that biological markers, DNA and RNA fragments that react to certain type cells, they can be used as a contrast in biological studies and diagnosis of cancer in the early stages, when the tumor is not yet detected standard methods diagnostics

The use of quantum dots as fluorescent labels for studying tumor cells in vitro is one of the most promising and rapidly developing areas of application of quantum dots in biomedicine.

The mass implementation of this technology is hampered only by the question of the safety of using contrasts with quantum dots in invivo studies, since most of They are made from very toxic materials, and their sizes are so small that they easily penetrate any body barriers.

Quantum dot displays: QLED – the technology for creating LCD displays with LED backlight using quantum dots has already been tested by leading electronics manufacturers. The use of this technology makes it possible to reduce display energy consumption, increase luminous flux compared to LED screens by 25-30%, richer colors, clear color rendition, color depth, and the ability to make screens ultra-thin and flexible.

The prototype of the first display using this technology was presented by Samsung in February 2011, and the first computer display was released by Philips.

It uses quantum dots to produce red and green colors from the emission spectrum of blue LEDs, which ensures color rendition close to natural. In 2013, Sony released a QLED screen that works on the same principle. Currently, this technology for producing large screens is not widely used due to the high cost of production.

Quantum dot laser. A laser whose working medium is quantum dots in the emitting region has a number of advantages compared to traditional semiconductor lasers based on quantum wells. They have better characteristics in terms of frequency band, noise intensity, they are less sensitive to temperature changes.

Due to the fact that changing the composition and size of a quantum dot makes it possible to control the active medium of such a laser, it has become possible to work at wavelengths that were previously inaccessible. This technology is actively used in practice in medicine; with its help, a laser scalpel was created.

Energy

Several models of thin-film solar cells have also been developed based on quantum dots. They are based on the following principle of operation: photons of light strike a photovoltaic material containing quantum dots, stimulating the appearance of a pair of electron and hole, the energy of which is equal to or exceeds the minimum energy required for an electron of a given semiconductor in order to move from a bound state to a free one. By changing the size of the material's nanocrystals, it is possible to vary the “energy performance” of the photovoltaic material.

Based on this principle, several original working prototypes of various types of solar panels have already been created.

In 2011, researchers at the University of Notre Dame proposed a titanium dioxide-based “solar paint” that, when applied, could turn any object into a solar cell. It has a fairly low efficiency (only 1%), but it is cheap to produce and can be produced in large volumes.

In 2014, Scientists from the Massachusetts Institute of Technology presented a method for manufacturing solar cells from ultra-thin layers of quantum dots, the efficiency of their development is 9%, and the main know-how lies in the technology of combining quantum dots into a film.

In 2015, the Laboratory of the Center for Advanced Technologies of Solar Photovoltaics in Los Alamos proposed its project of window-solar panels with an efficiency of 3.2%, consisting of a transparent luminescent quantum concentrator, which can occupy a fairly large area, and compact solar photocells.

But researchers from the American National Renewable Energy Laboratory (NREL), in search of the optimal combination of metals to produce a cell with maximum quantum efficiency, created a real performance record holder - the internal and external quantum efficiency of their battery in tests was 114% and 130%, respectively.

These parameters are not the efficiency of the battery, which now shows a relatively small percentage - only 4.5%, however, optimizing the collection of photo stream was not the key goal of the study, which consisted only in selecting the most effective combination of elements. However, it is worth noting that prior to the NREL experiment, no battery had demonstrated a quantum efficiency greater than 100%.

As we see, the potential areas of practical application of quantum dots are wide and varied, theoretical developments are being carried out in several directions at once. Mass introduction of them into various fields is hampered by a number of limitations: the high cost of producing the dots themselves, their toxicity, imperfection and economic inexpediency of the production technology itself.

In the very near future, a color coding and ink marking system based on quantum dots may become widespread. Realizing that this market niche has not yet been occupied, but is promising and knowledge-intensive, the IQDEMY company, as one of the research tasks of its chemical laboratory (Novosibirsk), has identified the development of an optimal formulation of UV-curable ink and water-based ink containing quantum dots.

The first printing samples received are impressive and open up further prospects for the practical development of this technology:

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COURSE WORK

in the discipline "Biomedical transducers and sensor systems"

Quantum dots and biosensors based on them

Introduction. 3

Quantum dots. General information. 5

Classification of quantum dots. 6

Photoluminescent quantum dots. 9

Obtaining quantum dots. eleven

Biosensors using quantum dots. Prospects for their use in clinical diagnostics. 13

Conclusion. 15

Bibliography. 16

Introduction.

Quantum dots (QDs) are isolated nanoobjects whose properties differ significantly from the properties of bulk material of the same composition. It should be immediately noted that quantum dots are more mathematical model, rather than real objects. And this is due to the impossibility of forming completely separate structures - small particles always interact with the environment, being in a liquid medium or a solid matrix.

To understand what quantum dots are and understand them electronic structure, imagine an ancient Greek amphitheater. Now imagine that an exciting performance is unfolding on stage, and the audience is filled with people who have come to watch the actors play. So it turns out that the behavior of people in the theater is in many ways similar to the behavior of quantum dot (QD) electrons. During the performance, the actors move around the arena without going into the audience, and the spectators themselves watch the action from their seats and do not go down to the stage. The arena is the lower filled levels of the quantum dot, and the spectator rows are the excited electronic levels with higher energy. In this case, just as a viewer can be in any row of the hall, an electron can occupy any energy level of a quantum dot, but cannot be located between them. When buying tickets for the performance at the box office, everyone tried to get the most best places- as close to the stage as possible. Indeed, who would want to sit in the last row, where you can’t see the actor’s face even with binoculars! Therefore, when the audience is seated before the start of the performance, all the lower rows of the hall are filled, just as in the stationary state of the CT, which has the lowest energy, the lower energy levels are completely occupied by electrons. However, during the performance, one of the spectators may leave his seat, for example, because the music on stage is playing too loudly or he just got caught by an unpleasant neighbor, and move to the free top row. This is how, in a quantum dot, an electron, under the influence of an external influence, is forced to move to a higher energy level that is not occupied by other electrons, leading to the formation of an excited state of a quantum dot. You are probably wondering what happens to that empty space on the energy level where the electron used to be - the so-called hole? It turns out that, through charge interactions, the electron remains connected to it and can go back at any moment, just as a spectator who has moved can always change his mind and return to the place indicated on his ticket. An electron-hole pair is called an “exciton” from the English word “excited”, which means “excited”. Migration between energy levels CT, similar to the ascent or descent of one of the spectators, is accompanied by a change in the energy of the electron, which corresponds to the absorption or emission of a quantum of light (photon) when the electron moves to a higher or higher level, respectively. low level. The behavior of electrons in a quantum dot described above leads to a discrete energy spectrum that is uncharacteristic of macro-objects, for which QDs are often called artificial atoms in which electron levels are discrete.

The strength (energy) of the connection between a hole and an electron determines the exciton radius, which is a characteristic value for each substance. If the particle size is smaller than the exciton radius, then the exciton is limited in space by its size, and the corresponding binding energy changes significantly compared to the bulk substance (see “quantum-size effect”). It is not difficult to guess that if the exciton energy changes, then the energy of the photon emitted by the system when the excited electron moves to its original place also changes. Thus, by obtaining monodisperse colloidal solutions of nanoparticles of various sizes, it is possible to control the energies of transitions in a wide range of the optical spectrum.

Quantum dots. General information.

The first quantum dots were metal nanoparticles, which were synthesized in ancient Egypt for coloring various glasses (by the way, ruby stars Kremlin were obtained using a similar technology), although more traditional and widely known QDs are GaN semiconductor particles grown on substrates and colloidal solutions of CdSe nanocrystals. IN currently There are many ways to obtain quantum dots, for example, they can be “cut” out of thin layers of semiconductor “heterostructures” using “nanolithography”, or they can be spontaneously formed in the form of nano-sized inclusions of structures of one type of semiconductor material in the matrix of another. Using the “molecular beam epitaxy” method, with a significant difference in the parameters of the unit cell of the substrate and the deposited layer, it is possible to achieve the growth of pyramidal quantum dots on the substrate, for the study of the properties of which Academician Zh.I. Alferov was awarded Nobel Prize. By controlling the conditions of the synthesis processes, it is theoretically possible to obtain quantum dots of certain sizes with specified properties.

Quantum dots are available both as cores and as core-shell heterostructures. Due to their small size, QDs have properties different from bulk semiconductors. Spatial restriction of the movement of charge carriers leads to a quantum-size effect, expressed in the discrete structure of electronic levels, which is why QDs are sometimes called “artificial atoms.”

Depending on the size and chemical composition Quantum dots exhibit photoluminescence in the visible and near-infrared ranges. Due to their high size uniformity (more than 95%), the proposed nanocrystals have narrow emission spectra (fluorescence peak half-width 20-30 nm), which ensures phenomenal color purity.

Quantum dots can be supplied as solutions in non-polar organic solvents such as hexane, toluene, chloroform, or as dry powders.

QDs are still a “young” object of research, but the broad prospects for their use in the design of lasers and displays of a new generation are already quite obvious. The optical properties of QDs are used in the most unexpected areas of science, which require tunable luminescent properties of the material; for example, in medical research, it is possible to “illuminate” diseased tissues with their help.

Classification of quantum dots.

Colloidal synthesis of quantum dots offers wide possibilities both in obtaining quantum dots based on various semiconductor materials and quantum dots with different geometries (shapes). Of no less importance is the possibility of synthesizing quantum dots composed of different semiconductors. Colloidal quantum dots will be characterized by composition, size, and shape.

1. Quantum dot composition (semiconductor material)

First of all, quantum dots are of practical interest as luminescent materials. The main requirements for semiconductor materials on the basis of which quantum dots are synthesized are the following. First of all, this is the direct-gap nature of the band spectrum - it ensures effective luminescence, and secondly, the low effective mass of charge carriers - the manifestation of quantum-size effects in a fairly wide range of sizes (of course, by the standards of nanocrystals). The following classes of semiconductor materials can be distinguished. Wide-gap semiconductors (oxides ZnO, TiO2) - ultraviolet range. Mid-band semiconductors (A2B6, for example cadmium chalcogenides, A3B5) - visible range.

Ranges of changes in the effective band gap of quantum dots at

changing size from 3 to 10 nm.

The figure shows the possibility of varying the effective band gap for the most common semiconductor materials in the form of nanocrystals with a size in the range of 3-10 nm. From a practical point of view, important optical ranges are visible 400-750 nm, near IR 800-900 nm - blood transparency window, 1300-1550 nm - telecommunications range

1. Quantum Dot Shape

In addition to composition and size, their shape will have a serious impact on the properties of quantum dots.

- Spherical(directly quantum dots) - most of the quantum dots. At the moment they have the greatest practical application. The easiest to manufacture.

- Ellipsoidal(nanorods) - nanocrystals elongated along one direction.

Ellipticity coefficient 2-10. The indicated boundaries are arbitrary. From a practical point of view, this class of quantum dots is used as sources of polarized radiation. At high ellipticity coefficients >50, this type of nanocrystals is often called nanowires.

- Nanocrystals with complex geometry(eg tetrapods). A sufficient variety of shapes can be synthesized - cubic, asterisks, etc., as well as branched structures. From a practical point of view, tetrapods could find applications as molecular switches. At the moment they are of largely academic interest.

1. Multicomponent quantum dots

Colloidal chemistry methods make it possible to synthesize multicomponent quantum dots from semiconductors with different characteristics, primarily with different band gaps. This classification is in many ways similar to that traditionally used in semiconductors.

Doped Quantum Dots

As a rule, the amount of introduced impurity is small (1-10 atoms per quantum dot with an average number of atoms in a quantum dot of 300-1000). Electronic structure In this case, the quantum dot does not change; the interaction between the impurity atom and the excited state of the quantum dot is of a dipole nature and is reduced to the transfer of excitation. The main alloying impurities are manganese, copper (luminescence in the visible range).

Quantum dots based on solid solutions.

For quantum dots, the formation of solid solutions of semiconductors is possible if the mutual solubility of materials in the bulk state is observed. As in the case of bulk semiconductors, the formation of solid solutions leads to a modification of the energy spectrum - the effective characteristics are a superposition of the values ​​for individual semiconductors. This approach allows you to change the effective band gap at a fixed size - providing another way to control the characteristics of quantum dots.

Quantum dots based on heterojunctions.

This approach is implemented in quantum dots of the core-shell type (the core is made of one semiconductor, the shell is made of another). In general, it involves the formation of contact between two parts from different semiconductors. By analogy with classical theory heterojunctions, two types of core-shell quantum dots can be distinguished.

Photoluminescent quantum dots.

Of particular interest are photoluminescent quantum dots, in which the absorption of a photon produces electron-hole pairs, and the recombination of electrons and holes causes fluorescence. Such quantum dots have a narrow and symmetrical fluorescence peak, the position of which is determined by their size. Thus, depending on their size and composition, QDs can fluoresce in the UV, visible, or IR regions of the spectrum.

Quantum dots based on cadmium chalcogenides fluoresce in different colors depending on their size

For example, quantum dots ZnS, CdS And ZnSe fluoresce in the UV region, CdSe And CdTe in the visible, and PbS, PbSe And PbTe in the near IR region (700-3000 nm). In addition, from the above compounds it is possible to create heterostructures, the optical properties of which may differ from those of the original compounds. The most popular is to build a shell of a wider-gap semiconductor onto a core from a narrow-gap semiconductor, for example, onto a core CdSe grow a shell from ZnS :

Model of the structure of a quantum dot consisting of a CdSe core coated with an epitaxial shell of ZnS (sphalerite structural type)

This technique makes it possible to significantly increase the stability of QDs to oxidation, as well as significantly increase the quantum yield of fluorescence by reducing the number of defects on the surface of the core. A distinctive property of QDs is a continuous absorption spectrum (fluorescence excitation) over a wide range of wavelengths, which also depends on the size of the QD. This makes it possible to simultaneously excite different quantum dots at the same wavelength. In addition, QDs have higher brightness and better photostability compared to traditional fluorophores.

Such unique optical properties of quantum dots open up broad prospects for their use as optical sensors, fluorescent markers, photosensitizers in medicine, as well as for the manufacture of photodetectors in the IR region, high-efficiency solar cells, subminiature LEDs, white light sources, single-electron transistors and nonlinear -optical devices.

Obtaining quantum dots

There are two main methods for producing quantum dots: colloidal synthesis, carried out by mixing precursors “in a flask,” and epitaxy, i.e. oriented growth of crystals on the surface of the substrate.

The first method (colloidal synthesis) is implemented in several variants: at high or room temperature, in an inert atmosphere in organic solvents, or in aqueous solution, with or without organometallic precursors, with or without molecular clusters to facilitate nucleation. High-temperature chemical synthesis is also used, carried out in an inert atmosphere by heating inorganometallic precursors dissolved in high-boiling organic solvents. This makes it possible to obtain quantum dots of uniform size with a high fluorescence quantum yield.

As a result of colloidal synthesis, nanocrystals are obtained covered with a layer of adsorbed surfactant molecules:

Schematic illustration of a core-shell colloidal quantum dot with a hydrophobic surface. The core of a narrow-gap semiconductor (for example, CdSe) is shown in orange, the shell of a wide-gap semiconductor (for example, ZnS) is shown in red, and the organic shell of surfactant molecules is shown in black.

Thanks to the hydrophobic organic shell, colloidal quantum dots can be dissolved in any non-polar solvents, and, with appropriate modification, in water and alcohols. Another advantage of colloidal synthesis is the possibility of obtaining quantum dots in sub-kilogram quantities.

The second method (epitaxy) - the formation of nanostructures on the surface of another material, as a rule, involves the use of unique and expensive equipment and, in addition, leads to the production of quantum dots “tied” to the matrix. The epitaxy method is difficult to scale to the industrial level, which makes it less attractive for mass production of quantum dots.

Biosensors using quantum dots. Prospects for their use in clinical diagnostics.

Quantum dot - a very small physical object, the size of which is smaller than the Bohr exciton radius, which leads to the occurrence of quantum effects, for example, strong fluorescence.

The advantage of quantum dots is that they can be excited by a single radiation source. Depending on their diameter, they shine with different light, and quantum dots of all colors are excited by one source.

At the Institute of Bioorganic Chemistry named after. Academicians M.M. Shemyakin and Yu.A. Ovchinnikov RAS produces quantum dots in the form of colloidal nanocrystals, which allows them to be used as fluorescent labels. They are very bright, even with a regular microscope you can see individual nanocrystals. In addition, they are photoresistant—they can glow for a long time when exposed to high-power density radiation.

Another advantage of quantum dots is that, depending on the material from which they are made, it is possible to obtain fluorescence in the infrared range where biological tissues are most transparent. Moreover, their fluorescence efficiency is incomparable with any other fluorophores, which allows them to be used for visualization various entities in biological tissues.

Using the example of diagnosing an autoimmune disease - systemic sclerosis (scleroderma) - the possibility of quantum dots in clinical proteomics was demonstrated. Diagnosis is based on recording autoimmune antibodies.

In autoimmune diseases, the body's own proteins begin to affect their own biological objects (cell walls, etc.), which causes severe pathology. At the same time, autoimmune antibodies appear in biological fluids, which they took advantage of to carry out diagnostics and detect autoantibodies.

There are a number of antibodies to scleroderma. The diagnostic capabilities of quantum dots were demonstrated using the example of two antibodies. Antigens to autoantibodies were applied to the surface of polymer microspheres containing quantum dots of a given color (each antigen had its own microsphere color). The testing mixture contained, in addition to microspheres, also secondary antibodies associated with a signal fluorophore. Next, a sample was added to the mixture, and if it contained the desired autoantibody, a complex was formed in the mixture microsphere - autoantibody - signal fluorophore.

Essentially, the autoantibody was a linker that linked a microsphere of a certain color to a signal fluorophore. These microspheres were then analyzed using flow cytometry. The appearance of a simultaneous signal from the microsphere and the signal fluorophore is evidence that binding has occurred and a complex has formed on the surface of the microsphere, including secondary antibodies with the signal fluorophore. At this moment, the microsphere crystals and the signal fluorophore, which was associated with the secondary antibody, actually shone.

The simultaneous appearance of both signals indicates that the mixture contains a detectable target - an autoantibody, which is a marker of the disease. This is a classic “sandwich” registration method, when there are two recognition molecules, i.e. The possibility of simultaneous analysis of several markers has been demonstrated, which is the basis for high reliability of diagnosis and the possibility of creating drugs that can detect the disease at an early stage.

Use as biotags.

The creation of fluorescent labels based on quantum dots is very promising. The following advantages of quantum dots over organic dyes can be distinguished: the ability to control the luminescence wavelength, high extinction coefficient, solubility in a wide range of solvents, stability of luminescence to the environment, high photostability. We can also note the possibility of chemical (or, moreover, biological) modification of the surface of quantum dots, allowing selective binding to biological objects. The right picture shows the staining of cell elements using water-soluble quantum dots that luminesce in the visible range. The left figure shows an example of using the non-destructive optical tomography method. The photograph was taken in the near-infrared range using quantum dots with luminescence in the range of 800-900 nm (the transparency window of warm-blooded blood) introduced into a mouse.

Fig.21. Using quantum dots as biotags.

Conclusion.

Currently, medical applications using quantum dots are still limited, due to the fact that the effect of nanoparticles on human health has not been sufficiently studied. However, their use in the diagnosis of dangerous diseases seems very promising; in particular, a method of immunofluorescence analysis has been developed on their basis. And in the treatment of oncological diseases, for example, the method of so-called photodynamic therapy is already used. Nanoparticles are injected into the tumor, then they are irradiated, and then this energy is transferred from them to oxygen, which goes into an excited state and “burns out” the tumor from the inside.

Biologists say it is easy to design quantum dots that respond at any wavelength, such as the near-infrared spectrum. Then it will be possible to find tumors hidden deep inside the body.

In addition, certain nanoparticles can give a characteristic response in magnetic resonance imaging.

The researchers' future plans look even more tempting. New quantum dots connected to a set of biomolecules will not only find and indicate a tumor, but also deliver new generations of drugs precisely to the site.

It is possible that this particular application of nanotechnology will be the closest to practical and mass implementation that we have seen in laboratories in recent years.

Another direction is optoelectronics and new types of LEDs - economical, miniature, bright. The advantages of quantum dots are used here, such as their high photostability (which guarantees long-term operation of devices created on their basis) and the ability to provide any color (with an accuracy of one or two nanometers on the wavelength scale) and any color temperature (from 2 degrees Kelvin up to 10 thousand and above). In the future, LEDs can be used to make displays for monitors - very thin, flexible, with high image contrast.

Bibliography.

1.http://www.nanometer.ru/2007/06/06/quantum_dots_2650.html

1. Tananaev P.N., Dorofeev S.G., Vasiliev R.B., Kuznetsova T.A.. Preparation of CdSe nanocrystals doped with copper // Inorganic materials. 2009. T. 45. No. 4. P. 393-398.
2. Oleynikov V.A., Sukhanova A.V., Nabiev I.R. Fluorescent semiconductor nanocrystals

in biology and medicine // Nano. - 2007. - P. 160 173.

1. Snee P.T., Somers R.C., Gautham N., Zimmer J.P., Bawendi M.G., Nocera D.G. A Ratiometric CdSe/ZnS Nanocrystal pH Sensor // J. Am. Chem. Soc.. - 2006. - V. 128. P. 13320 13321.
2. Kulbachinsky V. A. Semiconductor quantum dots // Soros educational journal. - 2001. - T. 7. - No. 4. - pp. 98 - 104.

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