High-Performance ‘Quantum Dot Mode-Locked Laser on silicon – Proven 4.1 Terabit-per-second transmission capacity – The Future of Telecommunications and Data Centers – UC Santa Barbara


Bowers Liu Optica

Ten years into the future.

That’s about how far UC Santa Barbara electrical and computer engineering professor John Bowers and his research team are reaching with the recent development of their mode-locked quantum dot lasers on silicon.

It’s technology that not only can massively increase the data transmission capacity of data centers, telecommunications companies and network hardware products to come, but do so with high stability, low noise and the energy efficiency of silicon photonics.

“The level of data traffic in the world is going up very, very fast,” said Bowers, co-author of a paper on the new technology in the journal Optica. Generally speaking, he explained, the transmission and data capacity of state-of-the-art telecommunications infrastructure must double roughly every two years to sustain high levels of performance. That means that even now, technology companies such as Intel and Cisco have to set their sights on the hardware of 2024 and beyond to stay competitive. quantum-dots-head-672x371

Enter the Bowers Group’s high-channel-count, 20 gigahertz, passively mode-locked quantum dot laser, directly grown — for the first time, to the group’s knowledge — on a silicon substrate. With a proven 4.1 terabit-per-second transmission capacity, it leaps an estimated full decade ahead from today’s best commercial standard for data transmission, which is currently reaching for 400 gigabits per second on Ethernet.

The technology is the latest high-performance candidate in an established technique called wavelength-division-multiplexing (WDM), which transmits numerous parallel signals over a single optical fiber using different wavelengths (colors). It has made possible the streaming and rapid data transfer we have come to rely on for our communications, entertainment and commerce.

The Bowers Group’s new technology takes advantage of several advances in telecommunications, photonics and materials with its quantum dot laser — a tiny, micron-sized light source — that can emit a broad range of light wavelengths over which data can be transmitted.

“We want more coherent wavelengths generated in one cheap light source,” said Songtao Liu, a postdoctoral researcher in the Bowers Group and lead author of the paper. “Quantum dots can offer you wide gain spectrum, and that’s why we can achieve a lot of channels.” Their quantum dot laser produces 64 channels, spaced at 20 GHz, and can be utilized as a transmitter to boost the system capacity.

The laser is passively ‘mode-locked’ — a technique that generates coherent optical ‘combs’ with fixed-channel spacing — to prevent noise from wavelength competition in the laser cavity and stabilize data transmission.

This technology represents a significant advance in the field of silicon electronic and photonic integrated circuits, in which the primary goal is to create components that use light (photons) and waveguides — unparalleled for data capacity and transmission speed as well as energy efficiency — alongside and even instead of electrons and wires. Silicon is a good material for the quality of light it can guide and preserve, and for the ease and low cost of its large-scale manufacture. However, it’s not so good for generating light.

“If you want to generate light efficiently, you want a direct band-gap semiconductor,” said Liu, referring to the ideal electronic structural property for light-emitting solids. “Silicon is an indirect band-gap semiconductor.” The Bowers Group’s quantum dot laser, grown on silicon molecule-by-molecule at UC Santa Barbara’s nanofabrication facilities, is a structure that takes advantage of the electronic properties of several semiconductor materials for performance and function (including their direct band-gaps), in addition to silicon’s own well-known optical and manufacturing benefits.

This quantum dot laser, and components like it, are expected to become the norm in telecommunications and data processing, as technology companies seek ways to improve their data capacity and transmission speeds.

“Data centers are now buying large amounts of silicon photonic transceivers,” Bowers pointed out. “And it went from nothing two years ago.”

Since Bowers a decade ago demonstrated the world’s first hybrid silicon laser (an effort in conjunction with Intel), the silicon photonics world has continued to create higher efficiency, higher performance technology while maintaining as small a footprint as possible, with an eye on mass production. The quantum dot laser on silicon, Bowers and Liu say, is state-of-the-art technology that delivers the superior performance that will be sought for future devices.

“We’re shooting far out there,” said Bowers, who holds the Fred Kavli Chair in Nanotechnology, “which is what university research should be doing.”

Research on this project was also conducted by Xinru Wu, Daehwan Jung, Justin Norman, MJ Kennedy, Hon K. Tsang and Arthur C. Gossard at UC Santa Barbara.

Story Source:

Materials provided by University of California – Santa BarbaraNote: Content may be edited for style and length.

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Los Alamos National Laboratory – Stable light from ‘squashed’ Quantum Dots provide viable alternative to presently employed nanoscale light sources used in the Commercialization of quantum-dot displays, TV’s and more …


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Novel colloidal quantum dots are formed of an emitting cadmium/selenium (Cd/Se) core enclosed into a compositionally graded CdxZn1-xSe shell wherein the fraction of zinc versus cadmium increases towards the dot’s periphery. Due to a …more

 

” The new colloidal processing techniques allow for preparation of virtually ideal quantum-dot emitters with nearly 100 percent emission quantum yields shown for a wide range of visible, infrared and ultraviolet wavelengths. These advances have been exploited in a variety of light-emission technologies, resulting in successful commercialization of quantum-dot displays and TV sets … “

Intentionally “squashing” colloidal quantum dots during chemical synthesis creates dots capable of stable, “blink-free” light emission that is fully comparable with the light produced by dots made with more complex processes. The squashed dots emit spectrally narrow light with a highly stable intensity and a non-fluctuating emission energy. New research at Los Alamos National Laboratory suggests that the strained colloidal quantum dots represent a viable alternative to presently employed nanoscale light sources, and they deserve exploration as single-particle, nanoscale light sources for optical “quantum” circuits, ultrasensitive sensors, and medical diagnostics.

squashed quantum dot morestableli

“In addition to exhibiting greatly improved performance over traditional produced , these new strained dots could offer unprecedented flexibility in manipulating their emission color, in combination with the unusually narrow, ‘subthermal’ linewidth,” said Victor Klimov, lead Los Alamos researcher on the project. “The squashed dots also show compatibility with virtually any substrate or embedding medium as well as various chemical and biological environments.”

The new colloidal processing techniques allow for preparation of virtually ideal quantum-dot emitters with nearly 100 percent emission quantum yields shown for a wide range of visible, infrared and ultraviolet wavelengths. These advances have been exploited in a variety of light-emission technologies, resulting in successful commercialization of quantum-dot displays and TV sets.

The next frontier is exploration of  as single-particle, nanoscale light sources. Such future “single-dot” technologies would require particles with highly stable, nonfluctuating spectral characteristics. Recently, there has been considerable progress in eliminating random variations in emission intensity by protecting a small emitting core with an especially thick outer layer. However, these thick-shell structures still exhibit strong fluctuations in emission spectra.

los alamos xlosalamoslogo.png.pagespeed.ic.w4zn0ixzm6In a new publication in the journal Nature Materials, Los Alamos researchers demonstrated that spectral fluctuations in single-dot emission can be nearly completely suppressed by applying a new method of “strain engineering.” The key in this approach is to combine in a core/shell motif two semiconductors with directionally asymmetric lattice mismatch, which results in anisotropic compression of the emitting core.

This modifies the structures of electronic states of a  dot and thereby its  emitting properties. One implication of these changes is the realization of the regime of local charge neutrality of the emitting “exciton” state, which greatly reduces its coupling to lattice vibrations and fluctuating electrostatic environment, key to suppressing fluctuations in the emitted spectrum. An additional benefit of the modified electronic structures is dramatic narrowing of the  linewidth, which becomes smaller than the room-temperature thermal energy.

 Explore further: Sandwich structure of nanocrystals as quantum light source

More information: Young-Shin Park et al, Asymmetrically strained quantum dots with non-fluctuating single-dot emission spectra and subthermal room-temperature linewidths, Nature Materials (2018). DOI: 10.1038/s41563-018-0254-7

 

Quantum Dots and Lipid Rafts: Analytical Chemistry Solves a Nanoscale Mystery


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Article from Sustainable Nano

 

Remember all those great Black Friday deals on QLED televisions? You may not realize it, but they were all about nanotechnology!

The Q in QLED stands for quantum dots, which are not only being used to enhance the displays of TVs, but also are used in solar cells, medical imaging, and sensing.1-3 However, the disposal of these particles is not well regulated, leading to concern over their release into the environment. In the frenzy of holiday shopping, have you ever stopped to wonder what could happen if a quantum dot lands on the surface of a cell?

QLED

Figure 1. The “Q” in QLED TV stands for quantum dot (image by Samsung Newsroom)

As an analytical chemist, my mind is constantly blown by the suite of analytical tools that we have in the Center for Sustainable Nanotechnology to study really hard scientific questions like this one. I recently used two of these analytical tools, the atomic force microscope(AFM) and the quartz crystal microbalance (QCM) to tackle the tricky question about quantum dots on a cell surface. In our study, we looked at how quantum dots interact with supported lipid bilayers, which (as we explained in a previous blog post) we can use as a mimic of the cell membrane. The paper was called “Quaternary Amine-Terminated Quantum Dots Induce Structural Changes to Supported Lipid Bilayers.” 4

Q dots

Figure 2. The goal of this work was to understand the impact of quantum dots on supported lipid bilayers, which are a mimic for the outer membrane of cells. (image by Arielle Mensch)

 

Let me break down why this problem is so tricky and why it required really fancy tools to be able to study it. Everything we were studying was too small to be seen by eye or even using regular microscopes – the nanoparticles were about 6 nm and the lipid bilayers were about 4-5 nm – so we needed to use tools that allowed us to really zoom in to the nanoscale to get an idea of what was happening. Furthermore, the cell membrane of an organism is naturally wet, so we needed tools to allow us to work in liquids. Finally, the interactions between nanoparticles and membranes are dynamic, meaning they can change from moment to moment, so we really wanted to use tools that allowed us to monitor the interactions of the quantum dots and bilayers over time and not just take a single snapshot.

single image.png

Figure 3. Only capturing a single image doesn’t necessarily tell you everything you need to know about a situation… (image by Axel Naud)

 

With these requirements in mind, I set out to design a system that we could use to understand these interactions in liquid and in real time. We chose to work with supported lipid bilayers that contain something called phase-segregated domains, or “lipid rafts.” These lipid rafts are found in the cell membranes of different organisms, from plants to animals to bacteria, and are important for moving things in and out of the cell, which makes them very interesting to study. Furthermore, my collaborator, Dr. Eric Melby, previously showed that 4-nm, positively charged gold nanoparticles attached more  to supported lipid bilayers that had lipid rafts than those that didn’t (you can read more about his work here). This suggested that lipid rafts may play an important role in nanoparticle interactions with cell membranes, which was something I wanted to explore further with different types of nanoparticles, namely quantum dots.

 

lipid raft

Figure 4. We used supported lipid bilayers either with or without lipid rafts to understand the impact of quantum dots on these types of bilayers. (image adapted from Mensch et al.4 with permission from the American Chemical Society)

 

To start, I used Eric’s method of forming supported lipid bilayers either with or without lipid rafts using quartz crystal microbalance. As I’ve described previously, QCM is a very sensitive balance that uses a quartz crystal to measure changes in frequency, which we can use to figure out changes in mass. For example, if we add quantum dots to a bilayer formed on the quartz crystal and notice that the frequency starts to decrease, this tells us that the bilayer is getting heavier because the added quantum dots are sticking to it.

In my experiments, we saw that when we added quantum dots to bilayers with or without lipid rafts the frequency decreased over time (Figure 5). This told us that the quantum dots were attaching to our bilayers. Interestingly, when we rinsed the bilayer with buffer (to get rid of any loosely attached quantum dots), we first saw a decrease of mass (likely due to quantum dots leaving the bilayer) and then saw another increase in mass before the measurement leveled off. This was the first time that we had observed this type of change using QCM before. We hypothesized that this was due to the quantum dots causing some sort of restructuring of the bilayer, such as holes, multilayers, or a combination of events. But with QCM alone, we were unable to say for certain what was happening.

 

QCM.png

Figure 5. By QCM we saw that the quantum dots attached to the lipid bilayers. However, interesting frequency shifts after the rinse suggested that something more complicated was going on with these interactions. (image adapted from Mensch et al.4 with permission from the American Chemical Society)

 

Because we were uncertain what impact the quantum dots were having on the structure of the bilayer, we decided to use another analytical technique to get an actual picture of what was happening. This time we used atomic force microscopy (AFM). This technique allows us to study these interactions in liquid and over time, which if you remember were two very key factors to this work. I’ve described AFM in detail previously here, but briefly AFM works by using a very sharp tip that is attached at the end of a cantilever. We line up a laser to the end of this tip, which reflects off the tip onto a sensitive detector. As the tip scans across the sample, the laser light will move up or down on the detector depending on the height of the sample. From these changes in the laser’s position, we’re able to determine how tall features of the sample are.

 

AFM.png

Figure 6. Atomic force microscopy allows us to visualize the interaction of quantum dots and supported lipid bilayers in liquid and in real time. (image by Arielle Mensch)

 

For the first part of our AFM experiment, we formed lipid bilayers with lipid rafts. These rafts are about 1 nm taller than the other part of the bilayer. You can see this in Figure 7, where the brighter regions of the bilayer are the lipid rafts.

 

AFM lipid rafts

Figure 7. Lipid rafts within a supported lipid bilayer are ~1 nm taller than the surrounding regions of the bilayer. The 2-micrometer scale bar equals 2,000 nanometers, and the axis on the left shows you how the brightness of each region corresponds to a height in nanometers. (image adapted from Mensch et al.4 with permission from the American Chemical Society)

 

To investigate the impact of quantum dots on these bilayers, we added quantum dots to the bilayers and collected AFM images over time. This allowed us to monitor the changes to the structure of the bilayers. Figure 8 shows what we found – and it was pretty neat!

 

afm images.png

Figure 8. Sequence of AFM images showing the disappearance of lipid rafts over 15 min. The blue arrows are pointing to the lipid rafts or disappearance of lipid rafts in the images. The axis on the right shows you how the brightness of each region corresponds to a height in nanometers. (image adapted from Mensch et al.4with permission from the American Chemical Society))

 

When we added quantum dots to the lipid bilayers, the lipid rafts shrank and eventually disappeared! It only took about 15 minutes for them to completely disappear. You can see this by following the blue arrows in Figure 8. The other bright regions in the images are quantum dots binding to the bilayers and inducing structural changes (increasing the height in these regions or burrowing into the bilayer). These two changes are consistent with the mass changes we saw using QCM. We believe that the lipid rafts collapse because of an increase in energy due to the addition of the quantum dots. Basically, it is easier for the lipid rafts to mix together with the other components in the bilayer rather than stay separated.

 

Schematic

Figure 9. Schematic showing how positively charged quantum dots can cause the collapse of lipid rafts in supported lipid bilayers. (image adapted from Mensch et al.1 with permission from the American Chemical Society)

 

So, you might be wondering what all of this means. Well, to summarize, we found that positively charged quantum dots attach to supported lipid bilayers either with or without lipid rafts present. They also cause restructuring of the bilayers. In particular, when lipid rafts were present, the quantum dots actually caused the collapse of these important cell membrane components. Lipid rafts are found in the cell membranes of many different organisms, so this could have important implications in figuring out how nanoparticles affect different organisms.

But like with all good studies, there are still many more questions to explore! For this study we used supported lipid bilayers, but it would be really interesting to look at lipid rafts naturally within the cell membranes of actual organisms to see if we see the same effects. Furthermore, we can consider different types of nanoparticles with different surface coatings and see if that changes the results. So, the next time I see a QLED TV at the store, I’ll be sure to admire its beautiful colors, but I’ll also be thinking about my next research project.


ADDITIONAL RESOURCES


REFERENCES

  1. Martynenko, I. V.; Litvin, A. P.; Purcell-Milton, F.; Baranov, A. V.; Fedorov, A. V.; Gun’ko, Y. K. Application of semiconductor quantum dots in bioimaging and biosensing. Materials Chemistry B, 2017, 5, 6701−6727. doi: 10.1039/C7TB01425B
  2. Rühle, S.; Shalom, M.; Zaban, A. Quantum-dot-sensitized solar cells. ChemPhysChem2010, 11, 2290−304. doi: 10.1002/cphc.201000069
  3. Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H. Quantum dot bioconjugates for imaging, labelling and sensing. Nature Materials, 2005, 4, 436-446. doi: 10.1038/nmat1390
  4. Mensch, A.C., Buchman, J.T., Haynes, C.L., Pedersen, J.A., Hamers, R.J. Quaternary amine-terminated quantum dots induce structural changes to supported lipid bilayers. Langmuir, 2018, 34, 12369-12378. DOI: 10.1021/acs.langmuir.8b02047

Quantum Dots leader completes deal to manufacture NextGen Cadmium Free QD’s in Asia


A leading US quantum dot and nanomaterials manufacturer has announced a licensing and manufacturing deal in Assam, India.

The company, Quantum Materials Corp (QMC), has a range of products which can be used to make anything from superior Ultra High Definition television displays to ultra-thin solar cells and more efficient batteries.

The agreement will not only lead to significant job opportunities in the locality of Assam, but is also a major step in deploying QMC’s extraordinary technologies in the region.

There is the opportunity to adopt next-generation solar photovoltaic technology in the area, after the implementation of recent tariffs on imported photovoltaics into India.

QMC’s cadmium-free quantum dots offer a less hazardous and eco-friendlier alternative for producers and consumers, providing them with the color benefit without the risks of toxicity or liability.

The incorporation of cadmium in quantum dots has restricted their adoption, keeping manufacturers from leveraging the benefits of the technology. Restriction of Hazardous Substances regulations currently state that 1,000 parts per million (ppm) cadmium can be used, however this exception will soon expire and only 100 ppm of cadmium will be acceptable. In 2015, the European Parliament banned the continued use of cadmium in display and lighting devices.

img_0866Read More: What are quantum dots? The Science and Applications

Furthermore, controls and regulations are growing in Asia, with China implementing new laws of its own.

QMC signed the License and Development Agreement with Amtronics CC to allow for the establishment of large scale, low cost quantum dot production for the development and future commercial manufacture of: ultra-high definition display panels, solid state lighting LEDs and quantum dot driven thin-film solar cells.

The Agreement provides Amtronics CC with the right to manufacture quantum dots and thin-film quantum dot solar cells for commercial supply in India, as well as the right to use the QDXTM trademark and technical data to support its marketing initiatives. Under the terms of the Agreement, QMC receives an immediate upfront license fee of US$1,000,000 in addition to technology development funding, scheduled milestone payments and royalties on all quantum dots/solar cells produced.

The 12,000 square feet nanotech-focused facility is being established as the anchor project within the recently announced Electronics Manufacturing Cluster in the Guwahati Tech City.

“We are extremely pleased to partner with Amtronics CC and Amtron as they establish the necessary infrastructure to support large scale thin-film, quantum dot based solar cell production in Assam India using QMC patented technologies” explained Stephen B. Squires, President and CEO of Quantum Materials Corp.

“India’s recent implementation of tariffs applied to imported solar photovoltaics creates an ideal opportunity to establish QMC’s next generation thin-film photovoltaics for broad adoption in the region. I am highly confident that our technologies will help India fulfill its goal to deploy low cost renewables as a significant step toward energy independence”

Dr. George Anthony Balchin, Managing Director of Amtronics CC added, “We are pleased to be involved and provide the initial US $20,000,000 in funding for this enterprise and are anxious to see these extraordinary technologies deployed in a region that will benefit from both the end product as well as the significant potential for job creation.

The initial capital infusion will be used to build out the facility, purchase all the production and process equipment, including the micro reactors, train the staff and provide the initial working capital. It is very rare and rewarding to be involved with a project that is the culmination of a group of like-minded individuals striving for a common goal that has so much potential to enhance the lives of so many.”

Commenting further QMC CEO Squires stated: “As India represents one of the largest renewable energy and consumer electronics markets in the world, our partnership with Amtronics CC is an important step in expanding the value of the QMC franchise globally. This partnership will allow us to address global challenges such as rising energy costs, energy security, increasing power consumption and environmental quality on a more rapid basis.”

Eco-friendly nanoparticles for artificial photosynthesis – Indium-based quantum dots produce clean hydrogen fuel from water and sunlight for a sustainable Energy Source


Synthetic Photo Synthesis id51165
Researchers at the University of Zurich have developed a nanoparticle type for novel use in artificial photosynthesis by adding zinc sulfide on the surface of indium-based quantum dots. These quantum dots produce clean hydrogen fuel from water and sunlight – a sustainable source of energy. They introduce new eco-friendly and powerful materials to solar photocatalysis.
Quantum dots are true all-rounders. These material structures, which are only a few nanometers in size, display a similar behavior to that of molecules or atoms, and their form, size and number of electrons can be modulated systematically. This means that their electrical and optical characteristics can be customized for a number of target areas, such as new display technologies, biomedical applications as well as photovoltaics and photocatalysis.

Fuel production using sunlight and water

Another current line of application-oriented research aims to generate hydrogen directly from water and solar light. Hydrogen, a clean and efficient energy source, can be converted into forms of fuel that are used widely, including methanol and gasoline. The most promising types of quantum dots previously used in energy research contain cadmium, which has been banned from many commodities due to its toxicity.
The team of Greta Patzke, Professor at the Department of Chemistry of the University of Zurich, and scientists from Southwest Petroleum University in Chengdu and the Chinese Academy of Sciences have now developed a new type of nanomaterials without toxic components for photocatalysis (Nature Communications“Efficient Photocatalytic Hydrogen Evolution with Ligand Engineered All-Inorganic InP and InP/ZnS Colloidal Quantum Dots”).

Indium-containing core with a thin layer of zinc sulfide

The three-nanometer particles consist of a core of indium phosphide with a very thin surrounding layer of zinc sulfide and sulfide ligands.
Schematic representation of photocatalytic hydrogen production with InP/ZnS quantum dots in a typical assay
Schematic representation of photocatalytic hydrogen production with InP/ZnS quantum dots in a typical assay. (Image: Shan Yu) (click on image to enlarge)
“Compared to the quantum dots that contain cadmium, the new composites are not only environmentally friendly, but also highly efficient when it comes to producing hydrogen from light and water,” explains Greta Patzke.
Sulfide ligands on the quantum dot surface were found to facilitate the crucial steps involved in light-driven chemical reactions, namely the efficient separation of charge carriers and their rapid transfer to the nanoparticle surface.

Great potential for eco-friendly applications

The newly developed cadmium-free nanomaterials have the potential to serve as a more eco-friendly alternative for a variety of commercial fields.
“The water-soluble and biocompatible indium-based quantum dots can in the future also be tested in terms of biomass conversion to hydrogen. Or they could be developed into low-toxic biosensors or non-linear optical materials, for example,” adds Greta Patzke.
She will continue to focus on the development of catalysts for artificial photosynthesis within the University Research Priority Program LightChEC. This interdisciplinary research program aims to develop new molecules, materials and processes for the direct storage of solar light energy in chemical bonds.
Source: University of Zurich

Using one quantum dot to sense changes in another: Applications for developing advanced electronic and photonic devices


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Scanning electron micrograph of InAs self-assembled quantum dot transistor device. Credit: Osaka University

Quantum dots are nanometer-sized boxes that have attracted much scientific interest for use in nanotechnology because their properties obey quantum mechanics and are requisites to developing advanced electronic and photonic devices.

Quantum dots that self-assemble during their formation are particularly attractive as tunable light emitters in nanoelectronic devices and for studying quantum physics because of their quantized transport behavior. It is important to develop a way to measure the charge in a single self-assembled quantum dot to achieve quantum information processing; however, this is difficult because the metal electrodes needed for the measurement can screen out the very small charge of the quantum dot.

Researchers at Osaka University have recently developed the first device based on two self-assembled quantum dots that can measure the single-electron charge of one quantum dot using a second as a sensor.

The device was fabricated using two indium arsenide (InAs)  connected to electrodes that were deliberately narrowed to minimize the undesirable screening effect.

“The two  dots in the device showed significant capacitive coupling,” says Haruki Kiyama. “As a result, the single-electron charging of one dot was detected as a change in the current of the other dot.”

The current response of the sensor quantum dot depended on the number of electrons in the target dot. Hence the device can be used for real-time detection of single-electron tunneling in a quantum dot. The tunneling events of single electrons in and out of the target quantum dot were detected as switching between high and low current states in the sensor quantum dot. Detection of such tunneling events is important for the measurement of single spins towards electron spin qubits.

“Sensing single charges in self-assembled quantum dots is exciting for a number of reasons,” explains Akira Oiwa. “The ability to achieve electrical readout of single electron states can be combined with photonics and used in quantum communications. In addition, our device concept can be extended to different materials and systems to study the physics of self-assembled quantum dots.”

Two quantum dots are better than one: Using one dot to sense changes in another
Real-time traces of the charge sensor quantum dot (QD1) current. Changes in the charge sensor current indicate the increase and decrease of electron number in the adjacent quantum dot (QD2). Credit: Osaka University

An electronic device using self-assembled quantum dots to detect single-electron events is a novel strategy for increasing our understanding of the physics of quantum dots and to aid the development of advanced nanoelectronics and quantum computing.

 Explore further: Simultaneous detection of multiple spin states in a single quantum dot

More information: Haruki Kiyama et al, Single-electron charge sensing in self-assembled quantum dots, Scientific Reports (2018). DOI: 10.1038/s41598-018-31268-x

 

Quantum dots could aid in fight against Parkinson’s


A large team of researchers with members from several institutions in the U.S., Korea and Japan has found that injecting quantum dots into the bloodstreams of mice led to a reduction in fibrils associated with Parkinson’s disease. In their paper published in the journal Nature Nanotechnology, the group describes their studies of the impact of quantum dots made of graphene on synuclein and what they found.

Quantum dots are particles that exist at the nanoscale and are made of semiconducting materials. Because they exhibit quantum properties, scientists have been conducting experiments to learn more about changes they cause to organisms when embedded in their cells. In this new effort, the researchers became interested in the idea of embedding quantum dots in synuclein cells.

Synucleins make up a group or family of proteins and are typically found in neural tissue.

One type, an alpha-synuclein, has been found to be associated with the formation of fibrils as part of the development of Parkinson’s disease. To see how such a protein might react when exposed to quantum dots, the researchers combined the two in a petri dish and watched what happened. They found that the quantum dots became bound to the protein, and in so doing, prevented it from clumping into fibrils. They also found that doing so after fibrils had already formed caused them to come apart. Impressed with their findings, the team pushed their research further.

Noting that quantum dots are small enough to pass through the blood/brain barrier, they injected quantum dots into mice with induced Parkinson’s disease and monitored them for several months. They report that after six months, the mice showed improvements in symptoms.

Read A Related Article

Quantum dots in brain could treat Parkinson’s and Alzheimer’s diseases

The researchers suggest that quantum dots might have a similar impact on multiple ailments where fibrilization occurs, noting that another team had found that injecting them into Alzheimer’s mouse models produced similar results.

It is still not known if injecting similar or different types of quantum dots into human patients might have the same effect, they note. Nor is it known if doing so would have any undesirable side effects. Still, the researchers are optimistic about the idea of using quantum dots for treatment of such diseases and because of that, have initiated plans for testing with other animals—and down the road they are looking at the possibility of conducting clinical trials in humans.

Source

The future of photonics using quantum dots – researchers are trying to integrate photonics into silicon devices.


QDots for Photonics 180327141726_1_540x360One type of laser that’s particularly suited for quantum dots is a mode-locked laser, which passively generates ultrashort pulses less than one picosecond in duration.
Credit: Peter Allen

The future of photonics using quantum dots

Thousands of miles of fiber-optic cables crisscross the globe and package everything from financial data to cat videos into light. But when the signal arrives at your local data center, it runs into a silicon bottleneck. Instead of light, computers run on electrons moving through silicon-based chips — which, despite huge advances, are still less efficient than photonics.

To break through this bottleneck, 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, atom like structures called quantum dots.

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.”

Story Source:

Materials provided by American Institute of PhysicsNote: Content may be edited for style and length.


Journal Reference:

  1. S. A. Kazazis, E. Papadomanolaki, M. Androulidaki, M. Kayambaki, E. Iliopoulos. Optical properties of InGaN thin films in the entire composition rangeJournal of Applied Physics, 2018; 123 (12): 125101 DOI: 10.1063/1.5020988

Oak Ridge National Laboratory – Demystifying Quantum Dot Conundrums – A ‘Titan’ Task at Lawrence Berkeley National Laboratory


QDs Demystified colloidal_nanoparticle-329x300

Complete atomistic model of the colloidal lead sulfide nanoparticle, also known as a quantum dot, passivated with oleic acid, oleyl amine and hydroxyl ligands.

Berkeley researchers use Titan to seek solutions to decades-old nanocrystal mysteries

Since their discovery over two decades ago Rice – Smalley – Curly Institute – “Buckyballs” ] they have gone relatively unnoticed until recently. Now they are showing up in our TVs, smart phones, solar panels, and even our bodies. For something so small, we are beginning to see them everywhere.

Nanocrystals (NCs) are making a significant impact in materials sciences, resulting in better, more energy-efficient products, and in many cases at lower costs. This is because small is different; in other words, at nanoscale the relevant material properties and phenomena behave in interesting and useful ways—differently from how they behave at a scale, say, visible to the human eye.

The benefits of NC technologies are well known; however, on the atomic level, many of the characteristics and behaviors that make them so beneficial remain a mystery.

But thanks to the high-performance computing resources at the Oak Ridge Leadership Computing Facility (OLCF), they’re becoming less mysterious with the help of the Titan supercomputer, a Cray XK7 capable of 27 petaflops, or 27 quadrillion calculations per second. After completing a 3-year project, researchers from Lawrence Berkeley National Laboratory (LBNL) recently published two articles in the journal Science, revealing new insights into the NC atomic structure.

“Understanding more of the fundamental physics in different nanosystems, like how they grow, how the electrons behave, or how different molecules affect the system, allows us to utilize some control,” said lead investigator Lin-Wang Wang, a senior staff scientist at LBL. “And if we can control things like the crystal’s size and shape, we will be able to further advance their usefulness.”

The crystal soup concoction

When thinking about NCs, mind the scale—nanoscale is crazy small.

Picture a strand of hair: 1 nanometer is approximately 100,000 times smaller. Furthermore, each atom inside the crystals—arranged in a repeating, orderly 3D pattern—is 10 times smaller still. The structures Wang and his researchers deal with are typically about 5 nanometers, units with about 2,000 atoms each, called quantum dots (QDs).

It sounds intimidating, but surprisingly, synthesizing QDs is relatively simple; controlling their structure, however, is not.

“We call it a chemical cocktail,” Wang said. “Traditionally, laboratory experimentalists add ingredients like salt and acid [precursors] to organic solvents, or ‘soup,’ which, done under the right conditions, will cause the crystals to grow.”

Once that process is performed, within a matter of minutes a new crystal is formed. But saying “new crystal,” however, isn’t entirely accurate. NCs have the innate ability to regenerate. That is, if an NC is cut into smaller pieces, those smaller pieces will in fact grow back to their original shape during synthesis. Another way of describing it would be to say that once an NC reaches approximately 5 nanometers, growth in some cases abruptly stops.

But why do they do that? How do they do that? What’s happening on the NC surface that we can’t see? Are other molecules playing a role we don’t know about?

“At the atomic level, these were questions no one really had any answers to,” Wang said. “So the details of the atomic structure during passivation have for many, many years been like a black box.”

And according to him, a better understanding of what goes on during passivation is the key to opening that box.

“Imagine it like this,” said Danylo Zherebetskyy, a postdoctoral researcher in Wang’s group. “During passivation, organic molecules act like builders delivering bricks [atoms] to the particles’ surfaces. The molecules form extensions from the crystal’s center, almost like hairs that guide the atoms where to land, growing the crystal brick by brick.”

Those hairs are known as ligands, and not only are they responsible for growth, but they also influence how QDs interact with light, electrical charges, and other materials. And learning how to control ligand passivation, Zherebetskyy said, is critical in advancing nanotech applications.

Dissecting dots

rice QD finetuneThrough an allocation from the OLCF’s Innovative and Novel Computational Impact on Theory and Experiment program, Wang and his team sought to answer those questions via a series of simulations, using Titan to calculate the surface energies and passivation patterns of lead sulfide and platinum nanocrystals.

Whereas laboratory experiments and various imaging methods could offer only hypotheses, for the first time Titan gave researchers precise predictions about NC surface development. Using the density functional theory codes VASP, LS3DF, and PEtot—quantum mechanics-based applications used to calculate the electronic and structural properties of molecules in many-body systems—researchers were able to develop an accurate, testable model, revealing an up-close examination of how each atom arranges itself within the system and how each of those molecules binds with other elements during passivation.

Because NC growth happens layer by layer, outward from the center, understanding the strange, abrupt stoppage in growth required the team to work from the outside in by slicing off sections of the NC surfaces. This process creates additional surfaces, each having different molecular arrangements and energies.

Wang explained: “For simplicity’s sake, let’s just look at two different surfaces. According to the old theory, based on principles in thermodynamics, the surface with the higher energy should grow the most because a higher energy means it’s more active, giving us an energetics picture. But what we find is just the opposite.”

As it turns out, the real secret to NC growth, Titan found, is ligand mobility.

“It takes a certain level of energy to displace a ligand on the surface, and that energy defines the ligand’s mobility,” Zherebetskyy said. “And the more energy it costs to displace the ligand, the more immobile it becomes.”

For growth to happen, he said, once a builder lays a brick, that ligand has to move out of the way so another atom can land next to it. Titan’s simulations made it clear, showing surfaces with lower energies facilitating the process, whereas higher energies actually create such a strong bond between the NC surface and the ligand molecules that passivation becomes completely blocked. Hence, NC growth abruptly stops.

“People have suspected this to be the case,” Wang said, “but until Titan, they never had the evidence to confirm that kinetic processes play a more important role than previously believed.”

The not-so-mysterious molecule

“I talk about size changing or controlling the properties, but the shape can also change the properties,” Wang said. “Different shapes will have different electronic structures and therefore will have different surfaces. Likewise, different surfaces require different methods of passivation.

“For almost 20 years we have been synthesizing inorganic [lead sulfide] quantum dots. But nevertheless, on the atomic level, no one has been able to figure out how the molecules attach to the particles’ surfaces.”

The reason, he explained, is that during passivation, a myriad of chemical reactions are taking place, making it impossible for physical testing or even advanced microscopy methods to identify every actor involved.

This was evidenced when the team placed a lead sulfide NC particle—recognized for its natural symmetry and its ability to form distinct facets—passivated with Oleic acid under Titan’s microscope to focus on unmasking unknown agents. By simulating the experiment, researchers can easily add or remove various molecules such as hydrogen and oxygen to observe their effects on passivation.

Consequently, because hydrogen and oxygen are two key ingredients of the Earth’s most valuable resource, it wasn’t entirely surprising when Titan revealed water to be the masked culprit behind the scenes of so many molecular mysteries. It was thought not possible because part of the synthesis process requires heating the precursor to 110°C (230°F), presumably eliminating any trace of water.

They found that water molecules occur as a byproduct of decomposition during crystal synthesis. Titan showed that water molecules, previously thought to exist only as a free-floating molecule within the soup, actually were a vital component in surface passivation.

“In the past, different experiments have revealed different features about surface passivation, from the amount of ligand molecules to the lead and sulfur atom ratios, but no one had ever put them together to agree with a single atomic model,” Wang said. “This is the first time all those observations actually fit.”

Truth be told

Berkeley_Lab_Logo_Small 082016Titan’s findings subsequently aided the team in reproducing their experiments in the lab, giving them the physical confirmation needed to disprove several controversial theories.

The team produced large amounts of data by parallelizing their codes to take advantage of a significant number of Titan’s approximately 300,000 cores. Generated data sets were efficiently managed by routing them through the OLCF’s High Performance Storage System, a tape-based archiving tool.

“Without Titan our calculations would have taken forever, if not been outright impossible,” Zherebetskyy said.

“Our ability to simulate research has become a very powerful technique,” Wang added. “If you cannot do a simulation to get your result, then you probably still don’t fully understand your problem.”

Thanks to Titan and the many resources offered at the OLCF, a US Department of Energy Office of ScienceUser Facility, the team was able to combine computations with various methods of physical testing to produce groundbreaking results in the advancement of new nanomaterials studies.

“I think these projects are just the beginning,” Wang said. “We now have a complete model. So that, I think, will open the door for future investigations into the surface states for more realistic calculations for the QD.”

—Jeremy Rumsey

Oak Ridge National Laboratory is supported by the US Department of Energy’s Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

MIT: Novel methods of synthesizing quantum dot materials – promising materials for high performance in electronic and optical devices


QD 3-novelmethodsThese images show scanning electron micrographs of the researchers’ sample quantum dot films. The dark spots are the individual quantum dots, each about 5 nanometers in diameter. Images a and b show the consistent size and alignment of the …more

For quantum dot (QD) materials to perform well in devices such as solar cells, the nanoscale crystals in them need to pack together tightly so that electrons can hop easily from one dot to the next and flow out as current. MIT researchers have now made QD films in which the dots vary by just one atom in diameter and are organized into solid lattices with unprecedented order. Subsequent processing pulls the QDs in the film closer together, further easing the electrons’ pathway. Tests using an ultrafast laser confirm that the energy levels of vacancies in adjacent QDs are so similar that hopping electrons don’t get stuck in low-energy dots along the way.

Taken together, the results suggest a new direction for ongoing efforts to develop these promising materials for high performance in electronic and optical devices.

In recent decades, much research attention has focused on electronic materials made of , which are tiny crystals of semiconducting materials a few nanometers in diameter. After three decades of research, QDs are now being used in TV displays, where they emit bright light in vivid colors that can be fine-tuned by changing the sizes of the nanoparticles. But many opportunities remain for taking advantage of these remarkable materials.

“QDs are a really promising underlying materials technology for  applications,” says William Tisdale, the ARCO Career Development Professor in Energy Studies and an associate professor of chemical engineering.

QD materials pique his interest for several reasons. QDs are easily synthesized in a solvent at low temperatures using standard procedures. The QD-bearing solvent can then be deposited on a surface—small or large, rigid or flexible—and as it dries, the QDs are left behind as a solid. Best of all, the electronic and optical properties of that solid can be controlled by tuning the QDs.

“With QDs, you have all these degrees of freedom,” says Tisdale. “You can change their composition, size, shape, and surface chemistry to fabricate a material that’s tailored for your application.”

The ability to adjust electron behavior to suit specific devices is of particular interest. For example, in solar photovoltaics (PVs), electrons should pick up energy from sunlight and then move rapidly through the material and out as current before they lose their excess energy. In light-emitting diodes (LEDs), high-energy “excited” electrons should relax on cue, emitting their extra energy as light.

With thermoelectric (TE) devices, QD materials could be a game-changer. When TE materials are hotter on one side than the other, they generate electricity. So TE devices could turn waste heat in car engines, industrial equipment, and other sources into power—without combustion or moving parts. The TE effect has been known for a century, but devices using TE materials have remained inefficient. The problem: While those materials conduct electricity well, they also conduct heat well, so the temperatures of the two ends of a device quickly equalize. In most materials, measures to decrease heat flow also decrease electron flow.

“With QDs, we can control those two properties separately,” says Tisdale. “So we can simultaneously engineer our material so it’s good at transferring electrical charge but bad at transporting heat.”

Making good arrays

One challenge in working with QDs has been to make particles that are all the same size and shape. During QD synthesis, quadrillions of nanocrystals are deposited onto a surface, where they self-assemble in an orderly fashion as they dry. If the individual QDs aren’t all exactly the same, they can’t pack together tightly, and electrons won’t move easily from one nanocrystal to the next.

Three years ago, a team in Tisdale’s lab led by Mark Weidman Ph.D. ’16 demonstrated a way to reduce that structural disorder. In a series of experiments with lead-sulfide QDs, team members found that carefully selecting the ratio between the lead and sulfur in the starting materials would produce QDs of uniform size.

“As those nanocrystals dry, they self-assemble into a beautifully ordered arrangement we call a superlattice,” Tisdale says.

Novel methods of synthesizing quantum dot materials
As shown in these schematics, at the center of a quantum dot is a core of a semiconducting material. Radiating outward from that core are arms, or ligands, of an organic material. The ligands keep the quantum dots in solution from sticking …more

Scattering electron microscope images of those superlattices taken from several angles show lined-up, 5-nanometer-diameter nanocrystals throughout the samples and confirm the long-range ordering of the QDs.

For a closer examination of their materials, Weidman performed a series of X-ray scattering experiments at the National Synchrotron Light Source at Brookhaven National Laboratory. Data from those experiments showed both how the QDs are positioned relative to one another and how they’re oriented, that is, whether they’re all facing the same way. The results confirmed that QDs in the superlattices are well ordered and essentially all the same.

“On average, the difference in diameter between one nanocrystal and another was less than the size of one more atom added to the surface,” says Tisdale. “So these QDs have unprecedented monodispersity, and they exhibit structural behavior that we hadn’t seen previously because no one could make QDs this monodisperse.”

Controlling electron hopping

The researchers next focused on how to tailor their monodisperse QD materials for efficient transfer of electrical current. “In a PV or TE device made of QDs, the electrons need to be able to hop effortlessly from one dot to the next and then do that many thousands of times as they make their way to the metal electrode,” Tisdale explains.

One way to influence hopping is by controlling the spacing from one QD to the next. A single QD consists of a core of semiconducting material—in this work, lead sulfide—with chemically bound arms, or ligands, made of organic (carbon-containing) molecules radiating outward. The ligands play a critical role—without them, as the QDs form in solution, they’d stick together and drop out as a solid clump. Once the QD layer is dry, the ligands end up as solid spacers that determine how far apart the nanocrystals are.

A standard ligand material used in QD synthesis is . Given the length of an oleic acid ligand, the QDs in the dry superlattice end up about 2.6 nanometers apart—and that’s a problem.

“That may sound like a small distance, but it’s not,” says Tisdale. “It’s way too big for a hopping electron to get across.”

Using shorter ligands in the starting solution would reduce that distance, but they wouldn’t keep the QDs from sticking together when they’re in solution. “So we needed to swap out the long oleic acid ligands in our solid materials for something shorter” after the film formed, Tisdale says.

To achieve that replacement, the researchers use a process called ligand exchange. First, they prepare a mixture of a shorter ligand and an organic solvent that will dissolve oleic acid but not the lead sulfide QDs. They then submerge the QD film in that mixture for 24 hours. During that time, the oleic acid ligands dissolve, and the new, shorter ligands take their place, pulling the QDs closer together. The solvent and oleic acid are then rinsed off.

Tests with various ligands confirmed their impact on interparticle spacing. Depending on the length of the selected ligand, the researchers could reduce that spacing from the original 2.6 nanometers with oleic acid all the way down to 0.4 nanometers. However, while the resulting films have beautifully ordered regions—perfect for fundamental studies—inserting the shorter ligands tends to generate cracks as the overall volume of the QD sample shrinks.

Energetic alignment of nanocrystals

One result of that work came as a surprise: Ligands known to yield high performance in lead-sulfide-based solar cells didn’t produce the shortest interparticle spacing in their tests.

Novel methods of synthesizing quantum dot materials
These graphs show electron energy measurements in a standard quantum dot film (top) and in a film made from monodisperse quantum dots (bottom). In each graph, the data points show energy measurements at initial excitation — indicated by the …more

“Reducing that spacing to get good conductivity is necessary,” says Tisdale. “But there may be other aspects of our QD material that we need to optimize to facilitate electron transfer.”

One possibility is a mismatch between the energy levels of the electrons in adjacent QDs. In any material, electrons exist at only two energy levels—a low ground state and a high excited state. If an electron in a QD film receives extra energy—say, from incoming sunlight—it can jump up to its excited state and move through the material until it finds a low-energy opening left behind by another traveling electron. It then drops down to its ground state, releasing its excess energy as heat or light.

In solid crystals, those two energy levels are a fixed characteristic of the material itself. But in QDs, they vary with particle size. Make a QD smaller and the energy level of its excited electrons increases. Again, variability in QD size can create problems. Once excited, a high-energy electron in a small QD will hop from dot to dot—until it comes to a large, low-energy QD.

“Excited electrons like going downhill more than they like going uphill, so they tend to hang out on the low-energy dots,” says Tisdale. “If there’s then a high-energy dot in the way, it takes them a long time to get past that bottleneck.”

So the greater mismatch between energy levels—called energetic disorder—the worse the electron mobility. To measure the impact of energetic disorder on electron flow in their samples, Rachel Gilmore Ph.D. ’17 and her collaborators used a technique called pump-probe spectroscopy—as far as they know, the first time this method has been used to study electron hopping in QDs.

QDs in an excited state absorb light differently than do those in the ground state, so shining light through a material and taking an absorption spectrum provides a measure of the electronic states in it. But in QD materials, electron hopping events can occur within picoseconds—10-12 of a second—which is faster than any electrical detector can measure.

The researchers therefore set up a special experiment using an ultrafast laser, whose beam is made up of quick pulses occurring at 100,000 per second. Their setup subdivides the laser beam such that a single pulse is split into a pump pulse that excites a sample and—after a delay measured in femtoseconds (10-15 seconds)—a corresponding probe pulse that measures the sample’s energy state after the delay. By gradually increasing the delay between the pump and probe pulses, they gather absorption spectra that show how much electron transfer has occurred and how quickly the excited electrons drop back to their ground state.

Using this technique, they measured electron energy in a QD sample with standard dot-to-dot variability and in one of the monodisperse samples. In the sample with standard variability, the excited electrons lose much of their excess energy within 3 nanoseconds. In the monodisperse sample, little energy is lost in the same time period—an indication that the energy levels of the QDs are all about the same.

By combining their spectroscopy results with computer simulations of the electron transport process, the researchers extracted electron hopping times ranging from 80 picoseconds for their smallest quantum dots to over 1 nanosecond for the largest ones. And they concluded that their QD materials are at the theoretical limit of how little energetic disorder is possible. Indeed, any difference in energy between neighboring QDs isn’t a problem. At room temperature, energy levels are always vibrating a bit, and those fluctuations are larger than the small differences from one QD to the next.

“So at some instant, random kicks in energy from the environment will cause the  of the QDs to line up, and the electron will do a quick hop,” says Tisdale.

The way forward

With energetic disorder no longer a concern, Tisdale concludes that further progress in making commercially viable QD  will require better ways of dealing with structural disorder. He and his team tested several methods of performing ligand exchange in solid samples, and none produced films with consistent QD size and spacing over large areas without cracks. As a result, he now believes that efforts to optimize that process “may not take us where we need to go.”

What’s needed instead is a way to put short ligands on the QDs when they’re in solution and then let them self-assemble into the desired structure.

“There are some emerging strategies for solution-phase ligand exchange,” he says. “If they’re successfully developed and combined with monodisperse QDs, we should be able to produce beautifully ordered, large-area structures well suited for devices such as solar cells, LEDs, and thermoelectric systems.”

 Explore further: Extremely bright and fast light emission

More information: Rachel H. Gilmore et al. Charge Carrier Hopping Dynamics in Homogeneously Broadened PbS Quantum Dot Solids, Nano Letters (2017). DOI: 10.1021/acs.nanolett.6b04201

Mark C. Weidman et al. Monodisperse, Air-Stable PbS Nanocrystals via Precursor Stoichiometry Control, ACS Nano (2014). DOI: 10.1021/nn5018654

Mark C. Weidman et al. Interparticle Spacing and Structural Ordering in Superlattice PbS Nanocrystal Solids Undergoing Ligand Exchange, Chemistry of Materials (2014). DOI: 10.1021/cm503626s