What is a QD TV, where to look for “quantum dots” and why they show better. Design of quantum dots. Quantum dot designs

LED, LCD, OLED, 4K, UHD... it would seem that the last thing the television industry needs right now is another technical acronym. But progress cannot be stopped, meet a couple more letters - QD (or Quantum Dot). Let me immediately note that the term “quantum dots” in physics has a broader meaning than is required for televisions. But in light of the current fashion for everything nanophysical, marketers of large corporations happily began to apply this difficult scientific concept. So I decided to figure out what kind of quantum dots these are and why everyone would want to buy a QD TV.

First, some science in a simplified form. " Quantum dot" is a semiconductor whose electrical properties depend on its size and shape (wiki). It must be so small that quantum size effects are pronounced. And these effects are regulated by the size of this very point, i.e. the energy of an emitted, for example, photon - in fact, the color - depends on the “dimensions”, if this word is applicable to such small objects.

In even more consumer language, these are tiny particles that will begin to glow in a certain spectrum if illuminated. If you apply them and “rub” them on thin film, then illuminate it, the film will begin to luminesce brightly. The essence of the technology is that the size of these dots is easy to control, which means achieving accurate color.

In fact, it is not so important what name large corporations choose, the main thing is what it should give to the consumer. And here the promise is quite simple - improved color rendition. To better understand how “quantum dots” will provide this, you need to remember the design of the LCD display.

Light under the crystal

An LCD TV (LCD) consists of three main parts: a white backlight, color filters (separating the light into red, blue and green colors) and liquid crystal matrix. The latter looks like a grid of tiny windows - pixels, which, in turn, consist of three subpixels (cells). Liquid crystals, like blinds, can block the light flow or, on the contrary, open completely; there are also intermediate states.

When the white light emitted by light emitting diodes (LEDs), today it is already difficult to find a TV with fluorescent lamps, as it was just a few years ago), passes, for example, through a pixel whose green and red cells are closed, then we see Blue colour. The degree of “participation” of each RGB pixel changes, and thus a color image is obtained.

As you understand, to ensure the color quality of the image, at least two things are required: accurate filter colors and the correct white backlight, preferably with a wide spectrum. It’s with the latter that LEDs have a problem.

Firstly, they are not actually white, in addition, they have a very narrow color spectrum. That is, the spectrum is wide white is achieved by additional coatings - there are several technologies, most often the so-called phosphor diodes with the addition of yellow are used. But this “quasi-white” color still falls short of the ideal. If you pass it through a prism (like in a physics lesson at school), it will not decompose into all the colors of the rainbow of equal intensity, as happens with sunlight. Red, for example, will appear much dimmer than green and blue.

Engineers, understandably, are trying to correct the situation and come up with workarounds. For example, you can lower the green and blue levels in the TV settings, but this will affect the overall brightness - the picture will become paler. So all manufacturers were looking for a source of white light, the decay of which would produce a uniform spectrum with colors of the same saturation. This is where quantum dots come to the rescue.

Quantum dots

Let me remind you that if we are talking about televisions, then “quantum dots” are microscopic crystals that luminesce when light hits them. They can “burn” in many different colors, it all depends on the size of the point. And given that scientists have now learned to control their sizes almost perfectly by changing the number of atoms they consist of, it is possible to obtain a glow of exactly the color that is needed. Quantum dots are also very stable - they do not change, which means that a dot designed to luminesce at a certain shade of red will remain that shade almost forever.

The engineers came up with the idea of ​​using the technology in the following way: a “quantum dot” coating is applied to a thin film, created to glow with a certain shade of red and green. And the LED is regular blue. And then someone will immediately guess: “everything is clear - there is a source of blue, and the dots will give green and red, which means we will get the same RGB model!” But no, technology works differently.

We must remember that “quantum dots” are located on one large sheet and they are not divided into subpixels, but simply mixed together. When a blue diode shines on the film, the dots emit red and green, as mentioned above, and only when all three of these colors are mixed do you get ideal source white light. And let me remind you that high-quality white light behind the matrix is ​​actually equal to the natural color rendering for the viewer’s eyes on the other side. At a minimum, because you don’t have to make corrections for loss or distortion of the spectrum.

It's still an LCD TV

The wide color gamut will be especially useful for new 4K TVs and 4:4:4 color subsampling, which awaits us in future standards. That's all well and good, but remember that quantum dots don't solve other problems with LCD TVs. For example, it is almost impossible to get perfect black, because liquid crystals (the same “blinds” that I wrote about above) are not able to completely block light. They can only “cover themselves”, but not close completely.

Quantum dots are designed to improve color reproduction, and this will significantly improve the impression of the picture. But this is not OLED technology or plasma, where the pixels are able to completely stop the flow of light. Nevertheless plasma TVs have retired and OLEDs are still too expensive for most consumers, so it's still good to know what manufacturers will be offering us soon the new kind LED TVs, which will show better.

How much does a “quantum TV” cost?

The first QD TVs from Sony, Samsung and LG are promised to be shown at CES 2015 in January. However, China's TLC Multimedia is ahead of the curve, they have already released a 4K QD TV and say it is about to hit stores in China.

On this moment name exact cost TVs with new technology impossible, we are waiting for official statements. They wrote that QDs will cost three times less than OLEDs with similar functionality. In addition, the technology, as scientists say, is very inexpensive. Based on this, we can hope that Quantum Dot models will be widely available and simply replace conventional ones. However, I think that prices will still increase at first. As is usually the case with all new technologies.

Good day, 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

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 - ħ/(2md^2), where:

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

Types of Quantum Dots

• epitaxial quantum dots;
• colloidal quantum dots.

Quantum dot design

Now about the displays

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 were quantum dots described that not only luminesce? orange, but also in the range from dark green to red.

Why is LCD worse?

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.

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In order to receive general idea about the properties of material objects and the laws in accordance with which the macrocosm familiar to everyone “lives”, it is not at all necessary to complete higher education educational institution, because every day everyone faces their manifestations. Although in Lately The principle of similarity is increasingly being mentioned, the proponents of which argue that the micro and macro worlds are very similar, however, 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 word “photon” itself is of Greek origin and means “particle of light”. Therefore, it can be argued that when an electron changes its orbit, it absorbs (emits) a quantum of light. 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 smallest value 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

You can often find in textbooks 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 ( a full range of covers the range 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 a given material and exciton. It is worth noting that the color perception of light by the human eye is based on different

The most important object in the physics of low-dimensional semiconductor heterostructures are the so-called quasi-zero-dimensional systems or quantum dots. Give precise definition Quantum dots are quite difficult. This is due to the fact that in the physical literature, quantum dots refer to a wide class of quasi-zero-dimensional systems in which the effect of size quantization of the energy spectra of electrons, holes and excitons is manifested. This class primarily includes semiconductor crystals, in which all three spatial dimensions are on the order of the exciton Bohr radius in volumetric material. This definition assumes that the quantum dot is in a vacuum, gas or liquid environment, or is confined to some solid material other than the material from which it is made. In this case, the three-dimensional spatial limitation of elementary excitations in quantum dots is due to the presence of interfaces between various materials and environments, i.e., the existence of heteroboundaries. Such quantum dots are often called micro- or nanocrystals. However, this simple definition is not complete, since there are quantum dots for which there are no heterointerfaces in one or two dimensions. Despite this, the movement of electrons, holes or excitons in such quantum dots is spatially limited due to the presence of potential wells, which arise, for example, due to mechanical stresses or fluctuations in the thickness of semiconductor layers. In this sense, we can say that a quantum dot is any three-dimensional potential well filled with a semiconductor material, with characteristic dimensions of the order, in which the movement of electrons, holes and excitons is spatially limited in three dimensions.

Quantum dot manufacturing methods

Among the variety of different quantum dots, several main types can be distinguished, which are most often used in experimental studies and applications. First of all, these are nanocrystals in liquids, glasses and matrices of wide-gap dielectrics (Fig. 1). If they are grown in glass matrices, they usually have a spherical shape. It was in such a system, which consisted of CuCl quantum dots embedded in silicate glasses, that the effect of three-dimensional size quantization of excitons was first discovered when studying single-photon absorption. This work marked the beginning of the rapid development of the physics of quasi-zero-dimensional systems.

Fig.1.

Quantum dots in a crystalline dielectric matrix can be rectangular parallelepipeds, as is the case for CuCl-based quantum dots embedded in NaCl. Nanocrystals are also quantum dots grown in semiconductor matrices by droplet epitaxy.

To others important type Quantum dots are so-called self-organized quantum dots, which are manufactured by the Stranski-Krastanov method using the molecular beam epitaxy technique (Fig. 2). Their distinctive feature is that they are connected to each other through an ultrathin wettable layer, the material of which coincides with the material of the quantum dots. Thus, these quantum dots lack one of the heterointerfaces. This type, in principle, can include porous semiconductors, for example porous Si, as well as potential wells in thin semiconductor layers that arise due to fluctuations in the thickness of the layers.

Fig.2.

Fig.3. Structure with mechanical stress-induced InGaAs quantum dots. 1 - covering GaAs layer; 2 - self-organized InP quantum dots, which set mechanical stresses leading to the appearance of three-dimensional potential wells in the InGaAs layer; 3 and 6 - GaAs buffer layers; 4 - thin InGaAs quantum well, in which quantum dots induced by mechanical stress are formed; 5 - quantum dots; 7 - GaAs substrate. Dotted lines show mechanical stress profiles.

Quantum dots induced by mechanical stress can be classified as the third type (Fig. 3). They are formed in thin semiconductor layers due to mechanical stresses that arise due to mismatch of lattice constants of heterointerface materials. These mechanical stresses lead to the appearance of thin layer three-dimensional potential well for electrons, holes and excitons. From Fig. 3. It is clear that such quantum dots do not have heterointerfaces in two directions.