Quantum Dots Connect New Display Technology

QD Vision's John Ho holds a tube of the finished polymerized quantum dot material that promises to revolutionize the way we see televised color.

ALEXANDRIA, Va. Even though quantum dot technology goes back the 1980s, it's not exactly something you'll find in every household just yet. However, if this year's Consumer Electronics Show is any indication, the terminology will soon be just as ubiquitous as LED, LCD CFL, CD ROM and countless other technologies that have moved directly out of the lab into big box stores, and ultimately into the homes and hands of consumers.

The show signs shouting: UHD, curved screens, and bendable displays that have appeared on the show floor in the past few years are now being joined by placards proclaiming: powered by quantum dots, quantum dot enabled, and the like. As the demand for bigger screens, sharper pictures and better color continues, quantum dots seemed destined to play larger and larger roles.

WHAT IS A QUANTUM DOT?

A quick answer is that it's a very tiny semiconductor crystal; one that's so small it exhibits behavior described by quantum mechanics.

As explained by John Ho, advanced development manager at QD Vision, a Lexington, Mass., company that manufactures large quantities of such crystals:

Quantum dots are very simply nanocrystals that are made out of semiconductors, and are invisible to the naked eye because of their size, said Ho. And they can emit light very, very pure light. They don't scatter light and would be transparent if you put them between sheets of plastic.

They are anywhere from two to 20 nanometers [across]. When you have those sizes, you actually have quantum mechanic effects, like a spring in a box. If you shrink the box, the spring is compressed and you have more energy in the spring same thing in quantum dots. If you shrink the diameter of that quantum dot, the energy goes higher, which means that the light it emits becomes bluer. If you expand the diameter the color goes toward longer wavelengths.

Ho observed that it's possible to use the same semiconductor material to create crystals that emit virtually any color by simply changing the size of the crystals.

One of several quantum dot-powered displays shown at the 2015 CES

That's the benefit of quantum dots, he said. You can actually tune that wavelength.

A second benefit comes from the fact that, while quantum dots are very narrowband emitters, they are broadband absorbers of light. When excited by high-energy ultraviolet or blue wavelengths, in falling back to their ground state, they give off the desired color as set by their physical size.

They convert with almost 100 percent efficiency to the prescribed wavelengths of red or green, depending on their size, said Ho. The spectrum they emit is very pure. /span>

COLOR TV DISPLAY ADVANTAGES

It's this very pure (narrowband) emission that makes quantum dot technology valuable in color television displays. In conventional LCD flat screens, a source of light is required behind the liquid crystal light valve. In some displays, light comes from a cold-cathode fluorescent emitter.

In more modern panels, the source of illumination has been from white LEDs. However, in terms of providing the colors needed to replicate the desired NTSC color gamut, such light sources leave a lot to be desired. The overall effect is shrinkage of the displayed gamut away from the chromaticity values established by CRT phosphor characteristics early in color television's history.

The reason the gamut is so small is that you have a blue LED with a yttrium aluminum garnet phosphor on top to create a white LED, said Ho. The resulting white spectrum is peaked around 540-550 nanometers, but is very broad. So you have a blue peak [along with] peaked up green or yellow. LCDs have filters for the backlight in the front panel and use [these] color filters to specify blue, green and red channels that are narrow so you can get pure primaries. However, filters aren't perfect; if you have a very broad emitter, you have a lot more leakage between the blue and the green and the green and the red channels. When you start mixing these colors the gamut shrinks. /span>

According to Ho, to widen the color gamut of the display, you need to use an illumination source that has very precisely defined color emission one providing very narrowband sources of red, blue and green. Quantum dots lend themselves to this application very nicely.

To really extend the gamut back out, you want to start in the backlight with very narrow emitters, said Ho. The blue LED light source is acting as a narrow blue emitter and the green and red quantum dots are absorbing that and then re-emitting a very narrow green and red. This is the light that's going through the front panel. The filters there don't have to work as hard because there's not as much leakage. You recover your pure green, pure red and pure blue. That allows you to get more than 100 percent of the NTSC gamut. /span>

EXCEEDING NTSC

The color television gamut as specified by the NTSC in 1953 versus what's realizable with OLED and quantum dot display technologies. (Color IQ is a QD Vision trademark for quantum dot technology.)

According to Ho, there is a push in display technology to match the color gamut described by Michael Pointer, a researcher in color vision who developed a gamut of all the colors in nature that can be recognized by the human visual system. /span>

Pointer's gamut represents all the naturally occurring colors in the world, said Ho. A gentleman named Pointer went around measuring things with a spectrometer and noticed many fell outside of the sRGB gamut standard [a color space developed by HP and Microsoft in 1996 for use in connection with color displays, printers and the Internet] an

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