Quantum dots (QD) are very small semiconductor particles, that have a radius of a few nanometres (1 nm = a billionth of a metre). They are a central theme in nanotechnology. 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 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. Their optical and electronic properties differ from those of larger particles, they are tunable, highly emissive and durable. The emitted color can be tuned simply by changing the quantum dot’s diameter or by tweaking its chemical composition. They can be modified and manipulated into increasingly complex arrays and incorporated into resins and polymers.
Quantum dots are particularly promising for optical applications due to their optical properties arising from the quantum confinement of electrons and holes. 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.
Quantum dots have also been suggested as implementations of qubits for quantum information processing. “When a QD is provided with energy,” explains Prof. Felici, “via luminous radition or electric impluses, for example, the system first shifts to an excited state and then loses energy by emitting light or photons. By correctly exciting a QD, it can be primed to emit a single photon for each excitation impulse. This ability is extremely interesting, especially in view of the use of single photons as “quantum bits” or qubits in computing and quantum information protocols. Electrical engineering professor Supriyo Bandyopadhyay is attempting to make quantum computers with quantum dots.”The process is difficult” said Bandyopadhyay, “but the payoff is tremendous.”
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.
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.
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.”
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.”
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 (Fig. 1).
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.