Quantum dots, the semiconducting nanocrystals enable efficient & cheaper displays, solar cells, and infrared cameras

Rajesh Uppal February 17, 2023 Nanotech, Photonics Comments Off on Quantum dots, the semiconducting nanocrystals enable efficient & cheaper displays, solar cells, and infrared cameras 1,856 Views

Driven by technology advances in solid-state optoelectronics, light-emitting diode (LED) lighting has been expanded to a wide range of applications in consumer electronics, automotive, architectural, and healthcare settings. Recently, there has been a drive to make lighting “smart” by incorporating internet of things (IoT) connectivity and machine learning-based artificial intelligence (AI).

Next-generation smart lighting is expected to be able to respond to personal mood or circadian rhythm in the daily ambient living environment while also rendering the colours of objects more accurately. In order to achieve these requirements, it is essential for smart lighting to have high colour rendering capability and wide colour controllability.

An emerging technology called Quantum Dot enhances flat-panel LED displays, commercial TVs and curved widescreen monitors, revealing many more colors and adding the necessary brightness to take full advantage of technologies like High Dynamic Range (HDR). Quantum dots are being extensively used in display devices as quantum dot displays offer high color resolution, better color purity, a highly immersive high-dynamic-range (HDR) experience, and higher energy efficiency compared to conventional displays.

A Quantum Dot is a human-made nanoparticle that has semiconductor properties. They’re tiny, ranging in size from two to 10 nanometers (1 nm = a billionth of a metre), with the size of the particle dictating the wavelength of light it emits, and therefore the color.

Quantum dots are also sometimes referred to as artificial atoms, a term that emphasizes that a quantum dot is a single object with bound, discrete electronic states, as is the case with naturally occurring atoms or molecules. Their properties are closely related to their minute dimensions as the charge bearers (i.e., electrons) in the dots are affected by the fact that they are “forced” to stay in a confined area, thereby presenting quantum phenomena. Quantum dots are particularly promising for optical applications due to their optical properties arising from the quantum confinement of electrons and holes. Their optical and electronic properties differ from those of larger particles, they are tunable, highly emissive and durable.

Quantum dots’ narrow emission spectra and wide excitation profiles enable them to change light into any color in the visible spectrum with greater efficiency. When Quantum Dots are hit with a light source, each dot emits a color of a specific bandwidth: Larger dots emit light that is skewed toward red, and progressively smaller dots emit light that is skewed more toward green. Due to several enhanced properties, quantum dots are being extensively used by display manufacturers. Many types of quantum dot will emit light of specific frequencies if electricity or light is applied to them, and these frequencies can be precisely tuned by changing the dots’ size, shape and material, giving rise to many applications.

Quantum dots exhibit properties that are intermediate between those of bulk semiconductors and those of discrete molecules. Their optoelectronic properties change as a function of both size and shape. Larger QDs (radius of 5–6 nm, for example) emit longer wavelengths resulting in emission colors such as orange or red. Smaller QDs (radius of 2–3 nm, for example) emit shorter wavelengths resulting in colors like blue and green, although the specific colors and sizes vary depending on the exact composition of the QD.

Optical Applications

Quantum dots are being analyzed for uses as diverse as LEDs, lasers, optical amplifiers, flat screen displays, memory and solar sensors. They are starting to become somewhat less expensive.

Quantum dots are nanocrystals that glow, a property that scientists have been working with to develop next-generation LEDs. When a quantum dot glows, it creates very pure light in a precise wavelength of red, blue or green. Conventional LEDs, found in our TV screens today, produce white light that is filtered to achieve desired colors, a process that leads to less bright and muddier colors.

Because of their highly tunable properties, QDs are of wide interest. Potential applications include transistors, solar cells, LEDs, diode lasers and second-harmonic generation, quantum computing, and medical imaging. Additionally, their small size allows for QDs to be suspended in solution which leads to possible uses in inkjet printing and spin-coating. Another technique where QDs have been used is Langmuir-Blodgett. These processing techniques result in less expensive and less time-consuming methods of semiconductor fabrication. One analyst predicts that sales of products engineered with quantum dots could reach $500 million within the next five years.

Companies are seeking to use quantum dots as the basis for the next generation of flat panel displays. Seth Coe-Sullivan is chief technology officer and co-founder of the young company, which is seeking to use quantum dots as the basis for the next generation of flat panel displays. The company says that its quantum dots already exceed the capabilities of LCDs and even the stringent colorimetry standards set by the National Television Standards Committee. Its green quantum dot LED has achieved external quantum efficiencies of 0.81 percent, efficiencies of 2.7 lm/W at a brightness of 140 cd/m2. Its red quantum dot LED has an external quantum efficiency of 3.1 percent and efficiencies of 3.4 lm/W, measured at a brightness of 210 cd/m2. A blue quantum dot LED has been produced, but its capabilities have not been announced.

They can be modified and manipulated into increasingly complex arrays and incorporated into resins and polymers. Quantum Dots are usually applied to a sheet of film that sits as a layer in that “sandwich” in front of the LED backlight that’s used to illuminate an LCD. The light passes through the LCD display stack, with the Quantum Dot color filter layer enhancing and enabling the LCD to reveal a wider and more saturated range of colors than would otherwise be possible.

Until now, blue-glowing quantum dots, which are crucial for creating a full range of color, have proved particularly challenging for researchers to develop. However, University of Toronto (U of T) researcher Dr. Yitong Dong and collaborators have made a huge leap in blue quantum dot fluorescence, results they recently published in Nature Nanotechnology. “The idea is that if you have a blue LED, you have everything. We can always down convert the light from blue to green and red,” says Dong. “Let’s say you have green, then you cannot use this lower-energy light to make blue.” The team’s breakthrough has led to quantum dots that produce green light at an external quantum efficiency (EQE) of 22% and blue at 12.3%. The theoretical maximum efficiency is not far off at 25%, and this is the first blue perovskite LED reported as achieving an EQE higher than 10%.

CLS techniques, particularly GIWAXS on the HXMA beamline, allowed the researchers to verify the structures achieved in their quantum dot films. This validated their results and helped clarify what the structural changes achieve in terms of LED performance. The first challenge was uniformity, important to ensuring a clear blue color and to prevent the LED from moving towards producing green light. “We used a special synthetic approach to achieve a very uniform assembly, so every single particle has the same size and shape. The overall film is nearly perfect and maintains the blue emission conditions all the way through,” says Dong.

Next, the team needed to tackle the charge injection needed to excite the dots into luminescence. Since the crystals are not very stable, they need stabilizing molecules to act as scaffolding and support them. These are typically long molecule chains, with up to 18 carbon-non-conductive molecules at the surface, making it hard to get the energy to produce light. “We used a special surface structure to stabilize the quantum dot. Compared to the films made with long chain molecules capped quantum dots, our film has 100 times higher conductivity, sometimes even 1000 times higher.” This remarkable performance is a key benchmark in bringing these nanocrystal LEDs to market. However, stability remains an issue and quantum dot LEDs suffer from short lifetimes. Dong is excited about the potential for the field and adds, “I like photons, these are interesting materials, and, well, these glowing crystals are just beautiful.”

By choosing quantum dots that are between three and 30 nanometres in diameter, the Cambridge University team achieved correlated colour temperature (CCT) of between 2243 K (reddish) to 9207 K (bright, midday sun) which far outshines the 2200 K–6500 K CCT range achieved by LEDs. CCT is the lighting colour equivalent of weather forecast “feels like” temperature ranges.

Although quantum dots as light sources have been investigated for almost 30 years, this innovative approach is scalable to large area lighting systems and can be made with a printing process. “This is a world-first: a fully optimised, high-performance quantum-dot-based smart white lighting system,” says Professor Jong Min Kim from Cambridge’s Department of Engineering who also co-led the research. It’s the “first milestone toward the full exploitation of quantum-dot-based smart white lighting for daily applications”.

Professor Gehan Amaratunga from Cambridge’s Department of Engineering, who co-led the research, says this increases the potential for a positive impact on human health and well-being, as they can be fine-tuned to respond to individual mood and circadian rhythms.

Solar Technology Breakthrough: World Record Quantum Dot Solar Cell Efficiency

The University of Queensland UQ researchers set a world record for the conversion of solar energy to electricity via the use of tiny nanoparticles called ‘quantum dots’, which pass electrons between one another and generate electrical current when exposed to solar energy in a solar cell device. The development represents a significant step towards making the technology commercially-viable and supporting global renewable energy targets. “Eventually it could play a major part in meeting the United Nations’ goal to increase the share of renewable energy in the global energy mix.”

The mixed caesium and formamidinium lead triiodide perovskite system (Cs1−xFAxPbI3) in the form of quantum dots (QDs) offers a pathway towards stable perovskite-based photovoltaics and optoelectronics. However, it remains challenging to synthesize such multinary QDs with desirable properties for high-performance QD solar cells (QDSCs).

Professor Wang’s team set the world record for quantum dot solar cell efficiency by developing a unique surface engineering strategy. Overcoming previous challenges around the fact that the surface of quantum dots tend to be rough and unstable – making them less efficient at converting solar into electrical current. Professor Lianzhou Wang, who led the breakthrough, said conventional solar technologies used rigid, expensive materials.

They further demonstrated that the QD devices exhibit substantially enhanced photostability compared with their thin-film counterparts because of suppressed phase segregation, and they retain 94% of the original power conversion efficiency (PCE) under continuous 1-sun illumination for 600 h.

“This new generation of quantum dots is compatible with more affordable and large-scale printable technologies,” Professor Wang said. “The near 25 percent improvement in efficiency we have achieved over the previous world record is important. It is effectively the difference between quantum dot solar cell technology being an exciting ‘prospect’ and being commercially viable.” “The new class of quantum dots the University has developed are flexible and printable,” he said. “This opens up a huge range of potential applications, including the possibility to use it as a transparent skin to power cars, planes, homes, and wearable technology.

Quantum dots could enable cheaper infrared cameras for self-driving cars, consumer electronics

Today’s infrared cameras are made by successively laying down multiple layers of semiconductors — a tricky and error-prone process that makes them too expensive to go into most consumer electronics. Guyot-Sionnest’s lab instead turned to quantum dots — tiny nanoparticles just a few nanometers in size. (One nanometer is how much your fingernails grow per second.) At that scale they have odd properties that change depending on their size, which scientists can control by tuning the particle to the right size. In this case, quantum dots can be tuned to pick up wavelengths of infrared light.

This ‘tunability’ is important for cameras, because they need to pick up different parts of the infrared spectrum. “Collecting multiple wavelengths within the infrared gives you more spectral information — it’s like adding color to black-and-white TV,” Tang explained. “Short-wave gives you textural and chemical composition information; mid-wave gives you temperature.” They tweaked the quantum dots so that they had a formula to detect short-wave infrared and one for mid-wave infrared. Then they laid both together on top of a silicon wafer.

The resulting camera performs extremely well and is much easier to produce. “It’s a very simple process,” Tang said. “You take a beaker, inject a solution, inject a second solution, wait five to 10 minutes, and you have a new solution that can be easily fabricated into a functional device.” “Traditional methods to make infrared cameras are very expensive, both in materials and time, but this method is much faster and offers excellent performance,” said postdoctoral researcher Xin Tang, the first author on a study which appeared in Feb. 2020 in Nature Photonics

There are many potential uses for inexpensive infrared cameras, the scientists said, including autonomous vehicles, which rely on sensors to scan the road and surroundings. Infrared can detect heat signatures from living beings and see through fog or haze, so car engineers would love to include them, but the cost is prohibitive. They would come in handy for scientists, too. “If I wanted to buy an infrared detector for my laboratory today, it would cost me $25,000 or more,” Guyot-Sionnest said. “But they would be very useful in many disciplines. For example, proteins give off signals in infrared, which a biologist would love to easily track.”

Quantum dot lasers

As the demand of data is growing and semiconductor microelectronics is unable to keep pace with the requirements, researchers have turned to photonics to remove silicon bottleneck. Photonics provide many advantages of higher speed and low power requirements. Therefore researchers are trying to integrate photonics into silicon devices. They’ve been developing lasers — a crucial component of photonic circuits — that work seamlessly on silicon. In a paper appearing this week in APL Photonics, from AIP Publishing, researchers from the University of California, Santa Barbara write that the future of silicon-based lasers may be in tiny, atomlike structures called quantum dots.

Quantum dot based semiconductor lasers are superior to other lasers, due to the discrete density of states, low threshold current and temperature dependence, high optical gain and quantum efficiency and high modulation speed. Semiconductor lasers are the most important lasers that are used in cable television signals, telephone and image communications, computer networks and interconnections, CD-ROM drivers, reading barcodes, laser printers, optical integrated circuits, telecommunications, signal processing, and a large number of medical and military applications.

Such lasers could save a lot of energy. Replacing the electronic components that connect devices with photonic components could cut energy use by 20 to 75 percent, Justin Norman, a graduate student at UC Santa Barbara, said. “It’s a substantial cut to global energy consumption just by having a way to integrate lasers and photonic circuits with silicon.” Silicon, however, does not have the right properties for lasers. Researchers have instead turned to a class of materials from Groups III and V of the periodic table because these materials can be integrated with silicon.

Initially, the researchers struggled to find a functional integration method, but ultimately ended up using quantum dots because they can be grown directly on silicon, Norman said. Quantum dots are semiconductor particles only a few nanometers wide — small enough that they behave like individual atoms. When driven with electrical current, electrons and positively charged holes become confined in the dots and recombine to emit light — a property that can be exploited to make lasers.

The researchers made their III-V quantum-dot lasers using a technique called molecular beam epitaxy. They deposit the III-V material onto the silicon substrate, and its atoms self-assemble into a crystalline structure. But the crystal structure of silicon differs from III-V materials, leading to defects that allow electrons and holes to escape, degrading performance. Fortunately, because quantum dots are packed together at high densities — more than 50 billion dots per square centimeter — they capture electrons and holes before the particles are lost.

These lasers have many other advantages, Norman said. For example, quantum dots are more stable in photonic circuits because they have localized atomlike energy states. They can also run on less power because they don’t need as much electric current. Moreover, they can operate at higher temperatures and be scaled down to smaller sizes.

In just the last year, researchers have made considerable progress thanks to advances in material growth, Norman said. Now, the lasers operate at 35 degrees Celsius without much degradation and the researchers report that the lifetime could be up to 10 million hours.

They are now testing lasers that can operate at 60 to 80 degrees Celsius, the more typical temperature range of a data center or supercomputer. They’re also working on designing epitaxial waveguides and other photonic components, Norman said. “Suddenly,” he said, “we’ve made so much progress that things are looking a little more near term.”

High-Purity Single-Photon Emitter Operating in the 1.5μm Band enables Secure Quantum Key Distribution at World-Record Distance of 120 km

Institute for Nano Quantum Information Electronics (Director: Professor Yasuhiko Arakawa), the University of Tokyo, in collaboration with Fujitsu Laboratories Ltd. and NEC Corporation, announced that they have achieved quantum key distribution at a world-record distance of 120 km using a system with a single-photon emitter.

They employed a high-purity quantum dot single-photon emitter operating in the 1.5μm band, which reduces the occurrence of simultaneous multi-photon emissions, one of the major limiting factors for long-distance QKD, to one in a million. Therefore they avoided the security vulnerability of attenuated pseudo single-photon emitter that has high probability of generating unwanted multiple photons. They also avoided the complexity of alternate method of artificially mixing optical pulses with different intensities (decoy states) to avoid eavesdropping.

Their Single-Photon Emitter was developed by illuminating (exciting) a quantum dot placed in a so-called “optical horn structure”. The wavelength of the excitation pulse is tuned to the appropriate energy level of a quantum dot. If the time duration of the excitation pulse is long, there is a greater chance of two or more photons being emitted per each excitation.

This time, however, using dispersion-compensation technology, the temporal width of the illuminating light was compressed, so as to obtain shorter excitation pulses. By doing so, the probability of emitting multiple photons per one pulse was reduced to one in a million, resulting in the successful creation of a high-purity single-photon emitter having the world’s highest performance. The setup of a QKD system was greatly simplified, so that the high level of security guaranteed by the laws of quantum mechanics can be attained.

Quantum dot is a nanometer-sized semiconductor crystal that can confine an electron in three dimensions. When an electron is confined in this nanocrystal, the electron density of states is completely discrete.

“This research has demonstrated that the odds of being able to generate a single photon can be doubled by using a relatively simple technique – and this technique can be scaled up to ultimately generate single photons with 100% probability.”

Military Applications

Another quantum dot project that receives military funding involves University of Nebraska-Lincoln professors Paul Snyder, Sina Balkir, Ned Ianno, Frazer Williams and Bandyopadhyay. This specialized group is hard at work on the creation of special military “bees.” These man-made, bee-sized aerial vehicles are not your typical pollinators, gathering military intelligence instead of honey.

For the bee project, the engineering team has worked together on a device known as a cellular neural network that uses quantum dots to process complicated visual information into a useful form. These futuristic mini-machines will be able to travel into enemy territory and gather visual information as well as function as the proverbial “fly on the wall” by eavesdropping on top-secret discussions.”As small, powerful computers with wings, these bees have to be somewhat intelligent,” said Ianno, also an electrical engineering professor.

Others imagine them being sprinkled over hostile terrain, where they would cling to enemies like invisible parasites, allowing them to be tracked and identified. Oregon-based Voxtel makes a product, “NightMarks,” based on tiny nanocrystal quantum dots that can be used for “tagging, tracking and locating” targets. These nanocrystal quantum dots can be hidden in clear liquids and seen only through a sensor like night-vision goggles. Therefore if these can be applied to suspicious person then he could be tracked using night vision goggles from far distane without him noticing. One US DOD contract asks for “covert microtaggants composed of nanocrystals” visible through sensors like night-vision goggles to “enable war fighters the ability to track entities buried in urban clutter.”

A satellite defense system based on quantum dot technology

Raytheon Company has developed a counter measure system using quantum dots to protect space assets such as satellites from missile attacks. They have developed a decoy consisting of quantum dots of different sizes and shapes that are engineered to emit radiation having a radiation profile similar to that of the asset.

This decoy is found to be more accurate in mimicking the radiation profile of the asset, thereby diverting the anti-satellite weapons from the target more efficiently than the existing conventional counter measure systems which use pyrolytic materials like Magnesium-Viton-Teflon (MVT) flares to generate intense radiation by means of an exothermic reaction. Such conventional counter measure systems are usually limited by the availability of oxygen in the space environment (for exothermic reactions) and they are inadequate in many aspects as new devices like Long Wavelength Infrared sensors (LWIR) are now able to easily differentiate between the decoy and the target spacecraft.

Protection systems for the growing number of satellites and other space assets against hostile threats and collisions should have the ability to precisely replicate the spectral signature of the target to act as an effective decoy. Quantum dots are well known for their size-dependent electro-optical properties, and by controlling the geometrical size, shape and the strength of the confinement potential, they can be easily tuned to emit radiation at desired wavelengths.

These quantum dots can be engineered to produce any kind of spectral signature by mixing different kinds (InSb, PbTe, HgTe, HgSe, CdTe, CdSe and CdS) and sizes in the required ratio so as to mimic the exact spectral signature of the target.

The researchers propose that such a mixture of quantum dots, when dispersed like a cloud in space, may act as an efficient decoy, making even the advanced sensor devices unable to differentiate between the target and the decoy. The cloud of quantum dots can be created either by exploding a small pack of quantum dots suspended in an inert gas (Argon/Helium) or by just spraying them from a storage tank.

The researchers propose a counter measure system employing quantum dots to divert the missile attack from a spacecraft or a satellite. The space asset that is to be protected is monitored for threats by a ground tracking system which sends early warnings about a detected missile to the space control centers which in turn can activate the guardian satellite (a nanosatellite of less than 100 kg) orbiting in the co-orbit of the space asset. Upon activation the guardian satellite releases the cloud of quantum dots so as to trick the missile into seeking the decoy instead of the protected asset.

Global Quantum Dot Display Market

The quantum dot display market is forecast to be valued at over US$ 4 Bn in 2022, experiencing a Y-o-Y increase of 30% as compared to 2021. Future projections indicate that the market will surge over 17x to reach US$ 69.79 Bn by 2032.

Modern displays not only offer higher brightness and contrast ratios but are also available in higher resolutions and consume less power than conventional displays. Display manufacturers are developing displays with improved specifications to strengthen their position in the market.

Quantum dot displays are well suited for displays due to properties such as high luminous efficacy, high brightness, and low power consumption. The use of inorganic materials prevents burn-ins, which are common in other types of displays.

Growing demand in downstream markets for energy-efficient display panels such as televisions, smartphones, tablets, personal computers and other electronics has forced OEMs and display manufacturers to build state-of-the-art displays capable of delivering maximum energy efficiency and increasing color quality. The numerous benefits of using quantum dot displays will be a major factor driving market growth.

The quantum dot display technology is witnessing impressive penetration in the television display industry, expected to generate an absolute dollar opportunity worth US$ 7.2 Bn.

Cadmium is the most efficient raw material that can be used for the manufacturing of quantum dots. However, heavy metal contamination leads to environmental pollution and has adverse effects on human health. Using heavy metals in household products can lead to major health problems.

Stringent regulations governing the use of cadmium in quantum dots displays will limit their adoption and prevent manufacturers from reaping the benefits of this technology. Cadmium used in display devices has been prohibited in several regions. As a result, quantum dot display manufacturers are transitioning to cadmium-free technology.

However, many companies and research associations are producing alternatives for cadmium-based quantum dots. Elements such as indium and zinc are suitable substitutes for cadmium for the development of quantum dots. Soon, cadmium-free quantum dots are expected to be widely available and are expected to dominate the market as a result of their safety, strong performance, and environmental benefits.

North America dominates the global quantum dots market, owing primarily to the high demand for QLEDs and early adoption of the technology. Furthermore, the population’s increased awareness of health and fitness drives the quantum dots market size.

Furthermore, the presence of technology providers, as well as increasing investments by government agencies and aerospace & defense in research and development of machine learning technology, influence market growth. A market share worth 40% is projected for the North American market.

The global quantum dot display market across APAC is estimated to grow at a CAGR of over 32% over the forecast period. Increasing demand for energy-efficient technologies are the primary factors for increased demand for quantum dot displays in countries like China and India.

The adoption of quantum dot displays is ever increasing with the growing expenditure on electronic gadgets. APAC, in the last few years has made remarkable progress to become a powerhouse in the electronics accessories industry.

The quantum dot display market is consolidated with a few prominent manufacturers namely Samsung Electronics Co. Ltd., LG Display Co. Ltd., Sony Corporation, Sharp Corporation and CSOT (China Star Optoelectronics Technology), acquiring a share of more than 70%. Numerous approaches, such as expansion, introduction of new products, joint ventures, agreements, alliances and acquisitions, are being employed by top competitors. Some of the other developments in the market are:

In October 2019, Samsung invested US$ 11 Billion in the world’s first dedicated quantum dot display manufacturing facility. Beginning in early 2021, the new production unit is set to operate with a monthly capacity of 30,000 quantum dot display substrates.
In January 2019, Hewlett-Packard announced the launch of world’s first quantum dot display on glass. The company also launched its new range of “Pavilion 27” monitors with quantum dot display.
In February 2019, Nanosys demonstrated its revolutionary quantum efficiency for heavy metal-free red, green, and blue QDEL systems, and successfully printed displays in their laboratories using ink jet technology. This is a major advance in quantum dot engineering of the next decade.