You may have been asking yourself the question what is a quantum dot?
After all, the buzz surrounding quantum dots — especially in the TV and display industry — is loud right now.
And while this post is about what a quantum dot is and how they work and not about TV displays, television is something we’re all familiar with and most of us own at least one. This gives us a convenient and familiar way to explain some of the concepts behind them and answer the question: what is a quantum dot?
Toward that end, if you’re looking for a write up of various display technologies or recommendations on which TV to buy, sorry, this is not for you.
If you’re looking for information on quantum dots and want to learn more about them, this is for you.
As the point of this blog is to share information pertaining to electronics, I imagine these curious little dots may be of interest to some people, even if we can’t (yet) experiment with them at our workbench.
Enough said.
So, what is a quantum dot anyway?
What is a Quantum Dot?
You may also hear names like artificial atom, nanocrystal, quantum box, or just QD, among others. All these names are aliases and refer to quantum dots.
Quantum dots are semiconductor materials whose dimensions usually range from 1 nm to 10 nm, but can be bigger, growing to several micrometers depending on the application.
They can be made from many different semiconductors and in many geometrical shapes like cubes, spheres, and even pyramids.
Often, they are made from zinc selenide (ZnSe), cadmium selenide (CdSe), or indium phosphide (InP).
Quantum dots can even be grown in a beaker where colloidal nanocrystals are supplied in liquid suspension or in a plastic composite.
What’s colloidal mean?
A colloid is a mixture where one substance of microscopically dispersed insoluble particles is suspended throughout another substance.
Manufacturers also produce them using fancy-sounding techniques like molecular- beam epitaxy, chemical vapor deposition, and electron-beam lithography.
Figure 1 depicts quantum dots of different colors.
Figure 1: various quantum dots. Aren’t they pretty?
How Quantum Dots Work
When a quantum dot absorbs a photon of light, it generates an electron-hole pair which recombines and emits a new photon.
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You probably already know what an electron is and if you’ve studied semiconductors before you know what a hole is. If not, my post How Diodes Work – An Introduction can fill you in on holes (at least a little bit). Sorry for the pun, had to do it.
Anyway, a quantum dot’s energy level depends on its size (unlike other materials) which means the color of the photon a QD emits depends on its size. Bigger quantum dots emit longer wavelengths of light such as reds and oranges while smaller ones emit shorter wavelengths like green and violet.
This is due to the tight electron confinement within the material. Smaller dots emit more energetic wavelengths of light because confining the semiconductor excitation to a smaller volume requires more energy.
The ability to tune them makes them special (especially if you’re a display or TV manufacturer).
To make quantum dots of different sizes, manufacturers simply adjust the temperatures and timing of chemical reactions that make the dots.
Quantum dots coated with a semi-conductor of higher band gap are core-shell quantum dots, while those coated with multiple semiconductors of alternating higher and lower band gaps are called quantum well quantum dots.
These coatings improve the tunability and photoluminescence efficiency of the dot.
How Quantum Dots Work in TV Screens
Let’s explain QDs a bit further (and in a practical way) with a technology that most of us are familiar with: the TV.
Typical LCD screens are backlit by a gallium nitride (GaN) blue LED, which excites a special phosphor which in turns emits yellow light. Yellow and blue together make light that appears to be white but is also rich in yellow and blue wavelengths.
In fact, this is how the LED bulbs you may have in your home operate.
The problem is (for screens), this white light is weak in green and red parts of the spectra. Special filters built into LCD screens help with this, but the colors aren’t as pure as they should be because the filters are lossy. This explanation is a bit over-simplified, but the post is about quantum dots, not LCD screens.
Enter the dots.
Inserting quantum dots between the LCD backlight and those filters improves your picture by maximizing the amount of light that goes through at precise red, green, and blue wavelengths. And it saves energy in the process.
In fact, the blacklight itself is a good emitter of blue light, so the dots only need to focus on reds and greens.
And it is this blue backlight that excites the quantum dots, getting them to emit red and green photons of a narrower spectra than would be possible with traditional LCD screens.
So, the advantage quantum dots have over traditional LCD TVs is more vivid hues and color gamut.
With a quantum dot TV, very little light goes to waste. You get brighter, more-saturated, and more-accurate colors.
If you are TV shopping and interested in owning a quantum dot TV, look for markings like QD, QUHD, SUHD, and ULED. These are trade names that refer to quantum dot technology available today.
The Future of Quantum Dots
For some time now, there’s been rumors of printable, foldable, and even rollable screens flying around. While there are some screens available with limited flexibility, so far, the hype seems to remain unfulfilled.
Quantum dots will change this. In the next ten years, you’re likely to see screens that roll up and fold up. Printable QDs will make it possible to “paint” or wallpaper a TV screen on your wall.
In the next few years, screens using photo-emissive quantum dot technology should appear. Since the dots are an accurate light source in and of themselves, there is no need for the lossy color filters we talked about earlier. The blue LEDs will still be the backlight and will render blue colors on your screen. Gone will be the viewing angle issues we’ve come to expect from typical LCD screens. This will mark the beginning of the end for many LCD display technologies.
And coming up are other quantum dot technologies that promise to eliminate the backlight all together.
Electro-emissive quantum dot technology promises to eliminate both the filters and the backlight, plus sport a fast refresh rate (unlike LCD screens which are slow).
Finally, micro-LED screens with quantum dots will boast the same benefits as the electro-emissive versions, with more brightness and blacker blacks.
For a little preview of these coming technologies, see figures 2, 3, 4 and 5 from IEEE Spectrum.
Figure 2: current QD technology (a.k.a photo-enhanced QD technology) blends the dots with LCD technology.
Figure 3: photo-emissive QD technology will offer better viewing angles and eliminate lossy filters all together. Look for this technology in the next few years.
Figure 4: electro-emissive QD technology will boast an even better viewing angle and eliminate both the filters and the backlight.
Figure 5: micro-LED with QDs does all the above and will sport perfect blacks, which other QD technologies may struggle to do. It’s also said to be the brightest technology out there.
What is a Quantum Dot? More Than Just Television…
Quantum dots find their home in other things besides displays.
They show up in lasers, broadband LEDs, single photon sources, memory elements, photodetectors, solar cells, and more.
In the early days of quantum dot technology, cadmium — a toxic element, was often used.
Bringing the color quality of non-cadmium-based quantum dots to that of cadmium-based dots isn’t easy but they’re getting better all the time.
There’s also some excitement about printing QDs onto plastic and other flexible materials. Due to their small size and the fact that some are produced in a solution, they bear a close resemblance to printing inks.
Do you own a QD TV?
Are you aware of any kit, platform, or device that allows people to experiment with quantum dots at home?
Comment and tell us about it!
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References:
- Saleh, B.E.A. & Teich, M.C. Fundamentals of Photonics, 2nd Ed. Wiley, 2007. Print.
- Luo, Zhongsheng; Manders, Jesse & Yurek, Jeff. “Television’s Quantum-Dot Future.” IEEE Spectrum March 2018: 28-33 & 52-53. Print.
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Tom says
I don’t own a QD TV but have been toying with the idea of buying one. After reading your very informative post, I will probably wait a few years until the technology matures a bit more. Great explanation on what quantum dots are and how they work too.