Sunday, February 17, 2019

Quantum dots, QLED displays beat OLED and friends

Quantum dots are beautiful physically as well as spiritually

Do you have a QLED television? Did you recently spend some time in TV shops to look for the TV sets – and their underlying physics – that impresses you most?

After a Chinese lunch today, I looked more carefully at the descriptions of the TVs in an electronics shops than I did in recent 5 years or more. It looks like the TVs were brighter and more amazing than just half a year ago when I had to buy a new TV. Modern flat displays are obviously a highly representative class of the practical applications of quantum mechanics – and this union isn't sufficiently celebrated.



The displays generally include LCDs – the final "D" is usually a "display" – where "LC" stands for "liquid crystals". Those are compounds in between crystals and liquids which have the ability to manipulate the light that goes through, depending on the electric fields in them. So the funny glasses from your old calculators are an example. They don't really emit light but they become opaque or transparent – and in a polarized way – depending on the attached voltages. That's how you see something.



While old calculators have these dark LCDs (some of the bulky ones also had active displays producing red light – otherwise digits of the same "digital shape"), two decades ago, we started to have LED displays instead of the CRT displays (cathode-ray tubes, I will not discuss this pre-history at all, I think that none of us is crying that CRTs are gone). Those are active semiconductor "light emitting diodes" – here the "D" means something else. So some semiconductors create messy light when electricity gets through. You shouldn't confuse those with LED-backlit LCDs where the source of the light are LED diodes in the background that are always on, and they may be shielded by liquid crystals.

A type of the liquid crystals (LCDs) are TFTs, thin film transistors, and subclasses are OLED and AMOLED. Here the semiconductors are organic which I find messy, like food, and these things literally get spoiled as food, although fortunately less quickly. ;-) "O" stands for "organic" and "AM" is an "active matrix". Intuitively, I don't have much trust in proteins, organic stuff, and similar biophysical mud.

Then we used to have plasma TVs that have mostly lost the market battle against cheap LCDs and more expensive OLEDs. As the name indicates, there is really "plasma" there, i.e. an ionized gas (in some small balloons, hopefully), and it emits light depending on the electric conditions. It had the wonderful black background and OK colors. But it ate lots of electricity and the pixels could have been killed easily. And I instinctively don't understand how "plasma" may be quite safe for health. Maybe you do but I don't. So while I dislike environmentalists, I also have some trouble with a plasma TV because it looks like a collection of millions of tiny thermonuclear reactors that occasionally explode. ;-)

They were wonderful but rest in peace, plasma TVs, good riddance. However, today, I was simply amazed by the QLED TVs. Samsung seems to have a near monopoly in QLEDs. Their models were also curved "to slightly surround the viewer" (opposite curvature than the very old TVs) which I consider a good (small) idea. The amount of light was amazing – while not insanely exaggerated like in those plasma TVs. But it was the "pure colors" that have stunned me most. I just immediately thought: this is a red, green, and blue color of a quality that I have never seen on a TV before.

Like every human, I am only sensitive to three colorful bands, basically "red", "green", and "blue" bands, using our three types of color sensitive cells. So we can't distinguish some wavelengths of light from a carefully fine-tuned mixture of red, green, and blue. But in the QLED case, I could almost say: holy cow, these are so pure RGB colors that they must be monochromatic. And indeed, they almost were. You know, the other displays always make all colors slightly grey. They can't avoid mixing a little bit of green and red when they want to show blue – or any permutation of this statement. But QLED are capable of mixing the minimum of the other light because they can emit an almost monochromatic light.

The spectral width of the pure red, green, and blue light emitted from QLED displays is just "a few percent" or some "dozens of nanometers" at most – out of the 400-800 nanometers that is the wavelength of the (violet-to-red) visible light. So it's almost monochromatic. How is it created? It's created from quantum dots, those are the objects that give "Q" to the QLEDs. Yes, it's the quantum.

Quantum dots are "artificial physical systems" whose size is a few nanometers (2-10 nm in Samsung's case). Note that the hydrogen atom radius is about 0.1 nanometers. So quantum dots are just "dozens of times" larger than an atom. And this comment isn't just a good way to compare the sizes. They work in a similar way. The whole point of the quantum dot is that it is an artifically produced "analogy" of an atom. By the generalized atom, I mean a physical system where the electrons (or holes) may be confined in discrete energy eigenstates which are analogous to the discrete energy eigenstates of electrons inside an atom. You can calculate those by solving some Schrödinger's equation for the electron.

Now, because the quantum dot is about "dozens of times" larger than an atom, the corresponding momentum is "a dozen of times" smaller than the electron's momentum in an atom which results in some "dozens of times" lower energies or frequencies of the emitted light. That's exactly appropriate to get the frequencies down, from some nearby ultraviolet frequencies of the basic transitions of the hydrogen atom to the visible spectrum.

Depending on how you exactly produce the quantum dot (and incredibly enough, Samsung may produce whole cheap plates with millions of quantum dots of the proper size on them), you may obtain "artificial atoms" whose simplest transitions produce monochromatic light that is red or green or anything you like. My understanding is that Samsung likes a blue quantum dot source of light in the background whose energy may be reshaped by quantum dots into their characteristic green, red (or no) light. So the blue light plays a somewhat privileged role, effectively replacing the white light used by the LED-backlit displays. At any rate, it looks wonderful.

Now I read that the QLED displays are somewhat thicker and therefore heavier than the likes of OLED. But please, Samsung or someone else – ideally Nokia LOL – produce some QLED tablets and smartphones for us. I want this plasma-TV-like brightness and contrast combined with the incredibly pure, monochromatic colors. I don't care about the thin design and low weight so much. These attributes are greatly overestimated, I think. I believe that the producers often overstate how much the buyers care about their tablets' and smartphones' thin physique (a similar comment applies to Miss Czechia and similar contests, too, but I should use a more respectful word for the buyers). On top of that, there would be lots of applications of these displays where the thin dimensions clearly don't matter much. Like tablets used in restaurants and shops for various purposes.

QLED also has poorer viewing angles – you need to sit in front of the screen perpendicularly. That's a flip side of a lower power consumption per brightness.

People who find OLED to look prettier than QLED must have a different personality than your humble correspondent. QLED is colorful, pure, long-lived (because the nanocrystals in quantum dots may be anorganic), while OLED is just some messy and stinky grey spoiled eggs. ;-)

Quantum dots – artificial atoms – are also a promising path to build a quantum computer. As I said, they're artificial atoms i.e. playgrounds that show the undergraduate quantum mechanics – the problem of an electron in an external potential – very nicely while the form of the potential may be largely engineered by the multi-nanometer design of the quantum dot.

I think that even the old transistor could have been invented "randomly" by people who didn't quite understand quantum mechanics – and there was some "pre-inventor" in the 1920s who actually "pre-invented" transistors but didn't understand quantum mechanics. But the proper full-blown inventors of transistors did know what they were doing. They did understand quantum mechanics. And the need to understand what's going on was increasing as people were adding these wonderful LCD, LED, TFT, OLED, AMOLED, QLED things. It would be extremely hard to invent and build such things if you didn't understand quantum mechanics, I think. Even the very idea that a thing like a "quantum dot" could emit monochromatic light would be something that inventors ignorant of quantum mechanics wouldn't think of for thousands of years, do you agree?

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