​Quantum dot: What it is and what it may potentially do

Quantum dot (QD) materials have been commercialised for use in displays by Samsung to increase colour clarity and energy efficiency for TVs, but the 'nanocrystals' may be applied in diverse industries in the future, ranging from sensors for IoT, biomedicine, energy harvesting, and security.

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Quantum dot material can potentially greatly enhance various products in existing industries. (Photo: Cho Mu-Hyun)

The human imagination is limitless. Internet of Things (IoT), driverless cars, drones, virtual reality (VR) and long-sustaining batteries are hot topics in the tech sphere, and though some parts of those visions have indeed manifested themselves to the real world, mostly in the form of compact devices and viable prototypes in the lab, the reality is that many of them remain only as concepts.

Limitations in software and hardware, and the time it takes for the public to absorb those concepts or ideas are some of the obvious hurdles. Then there is the most fundamental limitation of all: Materials.

On the material end, graphene is gaining both huge interest and research funds as a potential candidate for wide use. Semiconductor devices (that is, memory chips, processors and the like), which enjoyed the long-blessing from the properties of silicon, are faced with the end of Moore's Law, and alternative materials have been discussed alongside the changing of processes.

Nanocrystals

Though quantum dot, or QD, has been mentioned since the 1980s, it was professors CB Murray, DJ Norris, and MG Bawendi at Massachusetts Institute of Technology (MIT), who published a paper in 1993 entitled Synthesis and Characterization of Nearly Monodisperse CdE(E = Sulfur, Selenium, Tellurium) Semiconductor Nanocrystallites, which showed it could possibly be made and used. The team successfully synthesized 5.1 nanometre crystals, and the paper has since been cited over 8,000 times, which, in academic circles, elevates it to "rock star" status.

QD are nanoscale -- nanometre is one billionth of a meter -- semiconductor devices, called nanocrystals, that can confine electrons and electron holes in 3D. They can be synthesized in different processes using different materials. Because of the ability to tune its size and its optical and electronic properties, which can be engineered depending on what your aim of use is, potentially, it can be applied to a wide variety of industries using different materials.

At the biyearly ninth Internal Conference for Quantum Dots at Ramada Plaza Jeju Hotel, Jeju, South Korea, academics and industry experts from 30 countries shared over 400 separate researches and new ways to synthesize them. "On a personal note, it is good to see progress," said professor Bawendi, principal investigator of Bawendi Group at the MIT Department of Chemistry, in a separate interview in Seoul. "25 years ago, there were just a few of us. Now there is a huge conference. It is now described as a community."

Optical and electrical QD can be used to process light to light, light to electricity, electricity to light, and electricity to electricity. "In terms of application, obviously there is downshifting for colour applications, but also energy conversion, bio imaging, as an interesting material for basic research. It's a different material than molecular materials that people are used to. Its properties are unique enough that we are trying it in a variety of applications. Sometimes it makes sense, and sometimes it doesn't."

From Displays to "QLED"

Commercial use has only recently emerged, first in visual displays. By placing a QD film atop a Liquid Crystal Display (LCD), less light emission from the backlight produces better colours. Sony first launched it to the world with its QD-layered LCD TV in 2013, but the product was shelved due to its use of cadmium, deemed hazardous to humans by the European Union. Samsung, having launched a concept model for the tech in 2011, took the baton and launched its flagship SUHD TVs in 2015, the first commercial QD LCD TVs that do not use cadmium, and again this year.

Chinese firms are using it for TVs and monitors, albeit with cadmium however, and the market is only expected to expand going forward.

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Will QLED replace LCD and OLED in the future? (Photo: Cho Mu-Hyun)

Colour is produced by what size QD is tuned to -- the larger it is, the larger the wavelength, which becomes redder. Smaller is shorter, which produces blue. Because blue requires smaller QD, it has proven a more consistent challenge then other colours. For more on QD on TVs, read CNET's explanation here.

Many pin the technology against LG's OLED, or organic light-emitting diode, and spin the story as a winner-loser, right or wrong choice between the two technologies, but this is a trite generalisation.

For instance, in smartphones, Samsung uses OLED -- and Apple will too reportedly starting next year -- and in turn, LG uses LCD for theirs. The correct observation is that different technologies are deemed optimized for different categories by companies, depending on their own current priorities, past and future investment road maps, and outlooks.

At least in theory, the potential in QD goes beyond just enhancing existing LCDs. Instead of having QD as a film on the LCD, if QD diodes that can emit light themselves without a backlight but by just inducing them with electricity, the so-called "QLED," or quantum dot light-emitting diode is possible. The advantage will be an end-product that is just as thin as OLED and energy efficient with even better colours.

Silicon, used in semiconductors, is a relatively heavy mass chemical, so using QD material that is lighter with higher energy density to replace them can potentially make devices lighter, though this is a far more unlikely scenario to happen any time soon.

Sensors

QD materials can greatly enhance sensors, and image sensors are a crucial element in smartphones. Sensors are also a vital component in the IoT and driverless cars. Having QD on sensors can greatly enhance their sensitivity. Having a layer of QD sprayed on them will allow them to read even when there is little heat or light, or even in the infrared.

"QD can be applied to a variety of different sensors. It can be one that transfers light into electric signals. For instance, for driverless cars, photo detectors that can sense without the need of light can be potentially made," said Chang Hyuk, head of Material Research Center at Samsung Advanced Institute of Technology (SAIT).

A Samsung spokesman said that the company was not working on QD-applied sensors and that Chang was speaking only of possibilities.

Image sensors in smartphones, and commercial cameras, for instance, currently only read light visible to the human eye. Experts at the conference said there is really no hurdle to apply it so commercial viability must be the key. If QD becomes cheap enough, consumers may see even better, affordable super-cameras in their smartphones.

Increasing sensitivity for sensors in drones and driverless cars are of paramount importance due to rising concerns for their safety.

Biomedicine & Bio-imaging

QD use for biomedicine and bio-imaging is garnering much attention and is being tested on animals in labs worldwide. Because organic materials tend not to be fluorescent very well -- which is needed for bio-imaging -- alternative inorganic materials are being researched, including QD, said Bawendi.

"Right now we are more and more going into infrared in bio-imaging. Organic molecules tend not to fluoresce very well inside the body. Property of tissue is very interesting, because they are auto-fluorescent. The intrinsic fluorescent tissues overpower those used to image them," said the professor. "QD is one material that can potentially overcome that."

A possible, sophisticated scenario use can be this: Professor Lyudmila Turyanska of the University of Nottingham, who researches graphene-QD hybrid applications, said QD sensors can be used for imaging of near-infrared emission inside the body, which will greatly help doctors observe its movement once it is ingested. In theory, being able to track the QD sensor will allow for correct tumour diagnosis.

Most cancers reappear due to incorrect measurement of affected tissues that leaves residues after operation. When applying radiation during therapy, inevitably healthy cells are damaged around it as well. A super QD sensor-computer can potentially safely travel into the body, correctly detect the affected tissue, and target it specifically.

A team at Pohang University of Science and Technology (POSTECH) of Korea is spraying QD conjugates at live animal colon tissue as well as samples from humans.

Professors at the conference cautioned against any deployment soon, however. The biggest difficulty is QD being an inorganic material that is not supposed to be in the body and poses, especially to health agencies such as FTA, potential dangers. "For animals there is a potential for it to be useful. For humans, I say let's not be too optimistic. For biological there is a role to play, for therapeutic, I am not sure," said Bawendi.

Energy Harvesting

QD's ability to convert light into electricity, therefore energy, is also an area that gets much interest from academics. An interesting concept is to have QD materials sprayed on windows of buildings that can possibly save daytime energy consumption.

There are two ways of achieving this: To have something that absorbs the light and re-emits it to different solar cells on the sides. Then the light is filtered as it goes in through the solar cells and converted to energy, explained Bawendi. Another way is to have a solar cell that is partly transparent that absorbs infrared and UV on the windows.

Both can be done with QD. But because it can also be done with other materials, it is still debatable whether this application is viable and no business case has been made, said the professor. "You never know, as the material gets better."

Security Printing

A novel way to use QD is ink security, or security printing. By adding conductive materials, such as QD, into inks, governments can reduce production of counterfeit paper money and businesses can protect their sensitive documents. They can use optical readers to discern whether an item is fake or not.

Of course, silver and copper are indeed currently used for security printing, but the problem is that the techniques can be easily overcome by criminals. Having specially made QD materials and an accompanying reader for specific purposes will greatly decrease the likelihood of counterfeits.

The conductive ink market is huge, and may be worth $3.36 billion by 2018. It could prove a catalyst for the QD market to grow if adoption rate improves over the years.

Future

Though interesting as they are, there are many limitations for many of the above concepts to be commercialised any time soon. The low yield rate from synthesis forces QD materials to be commercially expensive. The stability of the produced material is also a barrier.

Avoiding hazardous materials such as cadmium and lead is a serious one, due to the overall rise of global concern over the environment. "In terms of material, there is definitely a need for [it to be] perceived to be less toxic," said Bawendi.

Another challenge is the quickening of life cycles of commercial products that far exceed that of the materials.

"The development speeds of devices are superfast. In contrast, in materials it is relatively slow. Chip processing powers are doubled per year while for batteries, for the past 20 years, capacity has only tripled," said Chang. "So we have to think of how to enhance the speed of development for the material themselves [to match those of devices as much as possible]."

"So what we do now is work, from the start, with device developers and make a roadmap together. We take into account what properties the device is looking for and try to make materials according to that need."

The South Korean tech giant seriously pushed QD research starting in 1995. Most research used cadmium -- which is still the most efficient chemical to make QD -- but the company decided to avoid the material in 2010 due to increased environmental concerns.

SAIT employs around 1,100 research staff, with less than 30 percent of them working on inorganic nanomaterials, which include QD, and one of five core research areas. The other four are organic semi-conducting materials, battery materials, optical film, and metabolic engineering. Any one of those research areas may lead to a novel material for the public, that like QD, can enhance our lives.

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Professor Bawendi gives the lowdown on quantum dot to ZDNet in Seoul. (Photo: Cho Mu-Hyun)

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