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
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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

 

Ultra-bright Ultra-fast light emission ‘Nano-crystals’ (Quantum Dots) Applications the for Displays and Super-Computers


Extremely bright and fast light emission Nano-Crystals

An international team of researchers from ETH Zurich, IBM Research Zurich, Empa and four American research institutions have found the explanation for why a class of nanocrystals that has been intensively studied in recent years shines in such incredibly bright colours.

The nanocrystals contain caesium lead halide compounds that are arranged in a perovskite lattice structure.

Three years ago, Maksym Kovalenko, a professor at ETH Zurich and Empa, succeeded in creating nanocrystals – or quantum dots, as they are also known – from this semiconductor material. “These tiny crystals have proved to be extremely bright and fast emitting light sources, brighter and faster than any other type of quantum dot studied so far,” says Kovalenko.

By varying the composition of the chemical elements and the size of the nanoparticles, he also succeeded in producing a variety of nanocrystals that light up in the colours of the whole visible spectrum. These quantum dots are thus also being treated as components for future light-emitting diodes and displays.

A caesium lead bromide nanocrystal under the electron microscope (crystal width: 14 nanometres). Individual atoms are visible as points. (Image: ETH Zurich / Empa / Maksym Kovalenko)

In a study published in the most recent edition of the scientific journal Nature (“Bright triplet excitons in caesium lead halide perovskites”), the international research team examined these nanocrystals individually and in great detail. The scientists were able to confirm that the nanocrystals emit light extremely quickly.

Previously-studied quantum dots typically emit light around 20 nanoseconds after being excited when at room temperature, which is already very quick. “However, caesium lead halide quantum dots emit light at room temperature after just one nanosecond,” explains Michael Becker, first author of the study. He is a doctoral student at ETH Zurich and is carrying out his doctoral project at IBM Research.

Electron-hole pair in an excited energy state

Understanding why caesium lead halide quantum dots are not only fast but also very bright entails diving into the world of individual atoms, light particles (photons) and electrons. “You can use a photon to excite semiconductor nanocrystals so that an electron leaves its original place in the crystal lattice, leaving behind a hole,” explains David Norris, Professor of Materials Engineering at ETH Zurich.

The result is an electron-hole pair in an excited energy state. If the electron-hole pair reverts to its energy ground state, light is emitted.

Under certain conditions, different excited energy states are possible; in many materials, the most likely of these states is called a dark one. “In such a dark state, the electron hole pair cannot revert to its energy ground state immediately and therefore the light emission is suppressed and occurs delayed. This limits the brightness”, says Rainer Mahrt, a scientist at IBM Research.

A sample with several green glowing perovskite quantum dots excited by a blue laser. (Image: IBM Research / Thilo Stöferle)

No dark state

The researchers were able to show that the caesium lead halide quantum dots differ from other quantum dots: their most likely excited energy state is not a dark state. Excited electron-hole pairs are much more likely to find themselves in a state in which they can emit light immediately. “This is the reason that they shine so brightly,” says Norris.

The researchers came to this conclusion using their new experimental data and with the help of theoretical work led by Alexander Efros, a theoretical physicist at the Naval Research Laboratory in Washington. He is a pioneer in quantum dot research and, 35 years ago, was among the first scientists to explain how traditional semiconductor quantum dots function.

Great news for data transmission

As the examined caesium lead halide quantum dots are not only bright but also inexpensive to produce they could be applied in television displays, with efforts being undertaken by several companies, in Switzerland and world-wide. “Also, as these quantum dots can rapidly emit photons, they are of particular interest for use in optical communication within data centres and supercomputers, where fast, small and efficient components are central,” says Mahrt.

Another future application could be the optical simulation of quantum systems which is of great importance to fundamental research and materials science.

ETH professor Norris is also interested in using the new knowledge for the development of new materials. “As we now understand why these quantum dots are so bright, we can also think about engineering other materials with similar or even better properties,” he says.

Source: By Fabio Bergamin, ETH Zurich

NREL, University of Washington Scientists Elevate Quantum Dot Solar Cell World Record to 13.4 Percent



Researchers at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) established a new world efficiency record for quantum dot solar cells, at 13.4 percent.

Colloidal quantum dots are electronic materials and because of their astonishingly small size (typically 3-20 nanometers in dimension) they possess fascinating optical properties. 


Quantum dot solar cells emerged in 2010 as the newest technology on an NREL chart that tracks research efforts to convert sunlight to electricity with increasing efficiency. 

The initial lead sulfide quantum dot solar cells had an efficiency of 2.9 percent. Since then, improvements have pushed that number into double digits for lead sulfide reaching a record of 12 percent set last year by the University of Toronto. 

The improvement from the initial efficiency to the previous record came from better understanding of the connectivity between individual quantum dots, better overall device structures and reducing defects in quantum dots.


 NREL scientists Joey Luther and Erin Sanehira are part of a team that has helped NREL set an efficiency record of 13.4% for a quantum dot solar cell.

The latest development in quantum dot solar cells comes from a completely different quantum dot material. The new quantum dot leader is cesium lead triiodide (CsPbI3), and is within the recently emerging family of halide perovskite materials. 

In quantum dot form, CsPbI3 produces an exceptionally large voltage (about 1.2 volts) at open circuit.

“This voltage, coupled with the material’s bandgap, makes them an ideal candidate for the top layer in a multijunction solar cell,” said Joseph Luther, a senior scientist and project leader in the Chemical Materials and Nanoscience team at NREL. 

The top cell must be highly efficient but transparent at longer wavelengths to allow that portion of sunlight to reach lower layers. 
Tandem cells can deliver a higher efficiency than conventional silicon solar panels that dominate today’s solar market.

This latest advance, titled “Enhanced mobility CsPbI3 quantum dot arrays for record-efficiency, high-voltage photovoltaic cells,” is published in Science Advances. The paper was co-authored by Erin Sanehira, Ashley Marshall, Jeffrey Christians, Steven Harvey, Peter Ciesielski, Lance Wheeler, Philip Schulz, and Matthew Beard, all from NREL; and Lih Lin from the University of Washington.

The multijunction approach is often used for space applications where high efficiency is more critical than the cost to make a solar module. 
The quantum dot perovskite materials developed by Luther and the NREL/University of Washington team could be paired with cheap thin-film perovskite materials to achieve similar high efficiency as demonstrated for space solar cells, but built at even lower costs than silicon technology–making them an ideal technology for both terrestrial and space applications.

“Often, the materials used in space and rooftop applications are totally different. It is exciting to see possible configurations that could be used for both situations,” said Erin Sanehira a doctoral student at the University of Washington who conducted research at NREL.

The NREL research was funded by DOE’s Office of Science, while Sanehira and Lin acknowledge a NASA space technology fellowship.

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.

Light-activated Nanoparticles (Quantum Dots) can supercharge current antibiotics


QDs and Antibiotics CU 171004142650_1_540x360CU Boulder researcher Colleen Courtney (left) speaks with Assistant Professor Anushree Chatterjee (right) inside a lab in the BioFrontiers Institute.
Credit: University of Colorado Boulder

Light-activated nanoparticles, also known as quantum dots, can provide a crucial boost in effectiveness for antibiotic treatments used to combat drug-resistant superbugs such as E. coliand Salmonella, new University of Colorado Boulder research shows.

Multi-drug resistant pathogens, which evolve their defenses faster than new antibiotic treatments can be developed to treat them, cost the United States an estimated $20 billion in direct healthcare costs and an additional $35 billion in lost productivity in 2013.

CU Boulder researchers, however, were able to re-potentiate existing antibiotics for certain clinical isolate infections by introducing nano-engineered quantum dots, which can be deployed selectively and activated or de-activated using specific wavelengths of light.

Rather than attacking the infecting bacteria conventionally, the dots release superoxide, a chemical species that interferes with the bacteria’s metabolic and cellular processes, triggering a fight response that makes it more susceptible to the original antibiotic.

“We’ve developed a one-two knockout punch,” said Prashant Nagpal, an assistant professor in CU Boulder’s Department of Chemical and Biological Engineering (CHBE) and the co-lead author of the study. “The bacteria’s natural fight reaction [to the dots] actually leaves it more vulnerable.”

The findings, which were published today in the journal Science Advances, show that the dots reduced the effective antibiotic resistance of the clinical isolate infections by a factor of 1,000 without producing adverse side effects.

“We are thinking more like the bug,” said Anushree Chatterjee, an assistant professor in CHBE and the co-lead author of the study. “This is a novel strategy that plays against the infection’s normal strength and catalyzes the antibiotic instead.”

While other previous antibiotic treatments have proven too indiscriminate in their attack, the quantum dots have the advantage of being able to work selectively on an intracellular level. Salmonella, for example, can grow and reproduce inside host cells. The dots, however, are small enough to slip inside and help clear the infection from within.

“These super-resistant bugs already exist right now, especially in hospitals,” said Nagpal. “It’s just a matter of not contracting them. But they are one mutation away from becoming much more widespread infections.”

Overall, Chatterjee said, the most important advantage of the quantum dot technology is that it offers clinicians an adaptable multifaceted approach to fighting infections that are already straining the limits of current treatments.

“Disease works much faster than we do,” she said. “Medicine needs to evolve as well.”

Going forward, the researchers envision quantum dots as a kind of platform technology that can be scaled and modified to combat a wide range of infections and potentially expand to other therapeutic applications.

Story Source:

Materials provided by University of Colorado at Boulder. Original written by Trent Knoss. Note: Content may be edited for style and length.


Journal Reference:

  1. Colleen M. Courtney, Samuel M. Goodman, Toni A. Nagy, Max Levy, Pallavi Bhusal, Nancy E. Madinger, Corrella S. Detweiler, Prashant Nagpal, and Anushree Chatterjee. Potentiating antibiotics in drug-resistant clinical isolates via stimuli-activated superoxide generationScience Advances, 04 Oct 2017 DOI: 10.1126/sciadv.1701776

UC Berkeley Labs: A Semiconductor That Can Beat the Heat



Berkeley Lab, UC Berkeley scientists discover unique thermoelectric properties in cesium tin iodide

JULY 31, 2017

A newly discovered collective rattling effect in a type of crystalline semiconductor blocks most heat transfer while preserving high electrical conductivity – a rare pairing that scientists say could reduce heat buildup in electronic devices and turbine engines, among other possible applications.

A team led by scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) discovered these exotic traits in a class of materials known as halide perovskites, which are also considered promising candidates for next-generation solar panels, nanoscale lasers, electronic cooling, and electronic displays.

These interrelated thermal and electrical (or “thermoelectric”) properties were found in nanoscale wires of cesium tin iodide (CsSnI3). The material was observed to have one of the lowest levels of heat conductivity among materials with a continuous crystalline structure.


Image – Rattling structures of halide perovskites: cesium tin iodide (left) and cesium lead iodide (right). (Credit: Berkeley Lab/UC Berkeley)

This so-called single-crystal material can also be more easily produced in large quantities than typical thermoelectric materials, such as silicon-germanium, researchers said.

“Its properties originate from the crystal structure itself. It’s an atomic sort of phenomenon,” said Woochul Lee, a postdoctoral researcher at Berkeley Lab who was the lead author of the study, published the week of July 31 in the Proceedings of the National Academy of Sciences journal. These are the first published results relating to the thermoelectric performance of this single crystal material.

Researchers earlier thought that the material’s thermal properties were the product of “caged” atoms rattling around within the material’s crystalline structure, as had been observed in some other materials. Such rattling can serve to disrupt heat transfer in a material.

“We initially thought it was atoms of cesium, a heavy element, moving around in the material,” said Peidong Yang, a senior faculty scientist at Berkeley Lab’s Materials Sciences Division who led the study.

Jeffrey Grossman, a researcher at the Massachusetts Institute of Technology, then performed some theory work and computerized simulations that helped to explain what the team had observed. 

Researchers also used Berkeley Lab’s Molecular Foundry, which specializes in nanoscale research, in the study.

“We believe there is essentially a rattling mechanism, not just with the cesium. It’s the overall structure that’s rattling; it’s a collective rattling,” Yang said. “The rattling mechanism is associated with the crystal structure itself,” and is not the product of a collection of tiny crystal cages. “It is group atomic motion,” he added.

Within the material’s crystal structure, the distance between atoms is shrinking and growing in a collective way that prevents heat from easily flowing through.

But because the material is composed of an orderly, single-crystal structure, electrical current can still flow through it despite this collective rattling. Picture its electrical conductivity is like a submarine traveling smoothly in calm underwater currents, while its thermal conductivity is like a sailboat tossed about in heavy seas at the surface.

Yang said two major applications for thermoelectric materials are in cooling, and in converting heat into electrical current. For this particular cesium tin iodide material, cooling applications such as a coating to help cool electronic camera sensors may be easier to achieve than heat-to-electrical conversion, he said.

A challenge is that the material is highly reactive to air and water, so it requires a protective coating or encapsulation to function in a device.

Cesium tin iodide was first discovered as a semiconductor material decades ago, and only in recent years has it been rediscovered for its other unique traits, Yang said. “It turns out to be an amazing gold mine of physical properties,” he noted.


SEM images of suspended micro-island devices. Individual AIHP NW is suspended between two membranes. (Credit: Berkeley Lab/UC Berkeley)

To measure the thermal conductivity of the material, researchers bridged two islands of an anchoring material with a cesium tin iodide nanowire. The nanowire was connected at either end to micro-islands that functioned as both a heater and a thermometer. 
Researchers heated one of the islands and precisely measured how the nanowire transported heat to the other island.

They also performed scanning electron microscopy to precisely measure the dimensions of the nanowire. They used these dimensions to provide an exacting measure of the material’s thermal conductivity. The team repeated the experiment with several different nanowire materials and multiple nanowire samples to compare thermoelectric properties and verify the thermal conductivity measurements.

“A next step is to alloy this (cesium tin iodide) material,” Lee said. “This may improve the thermoelectric properties.”

Also, just as computer chip manufacturers implant a succession of elements into silicon wafers to improve their electronic properties – a process known as “doping” – scientists hope to use similar techniques to more fully exploit the thermoelectric traits of this semiconductor material. This is relatively unexplored territory for this class of materials, Yang said.

The research team also included other scientists from Berkeley Lab’s Materials Sciences Division and the Molecular Foundry, the Kavli Energy NanoScience Institute at UC Berkeley and Berkeley Lab, and UC Berkeley’s Department of Chemistry.

The Molecular Foundry is a DOE Office of Science User Facility that provides free access to state-of-the-art equipment and multidisciplinary expertise in nanoscale science to visiting scientists from all over the world.

This work was supported by the Department of Energy’s Office of Basic Energy Sciences.
More information about Peidong Yang’s research group: http://nanowires.berkeley.edu/.

###
Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel Prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit http://www.lbl.gov.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Researchers develop blue-, yellow-, and red-emitting Graphene Quantum Dots


Graphene QD's China id47457_1

PL spectra of GQDs (a), PEI1800 GQDs (b), and PEI600 GQDs (c) at different excitation wavelengths. Inset: photograph of aqueous solution of these three GQDs under room light (left) and 365 nm UV irradiation lamp (right). UV-vis absorption spectra (d) of GQDs, PEI1800 GQDs, and PEI600 GQDs dispersed in water. (© ACS) 

Graphene quantum dots (GQDs) show great potential in the fields of photoelectronics, photovoltaics, biosensing, and bioimaging owing to their unique photoluminescence (PL) properties, including excellent biocompatibility, low toxicity, and high stability against photobleaching and photoblinking.

However, further development of GQDs is limited by their synthetic methodology and unclear PL mechanism. Therefore, it is urgent to find efficient and universal methods for the synthesis of GQDs with high stability, controllable surface properties, and tunable PL emission wavelength.In new work reported in ACS Applied Materials & Interfaces (“Red, Yellow, and Blue Luminescence by Graphene Quantum Dots: Syntheses, Mechanism, and Cellular Imaging”), researchers in China have synthesized PL-tunable GQDs with blue, yellow, and red emission colors by coating with polyethyleneimine (PEI) of different molecular weights.

photoluminescence spectra of graphene quantum dotsPL spectra of GQDs (a), PEI1800 GQDs (b), and PEI600 GQDs (c) at different excitation wavelengths. Inset: photograph of aqueous solution of these three GQDs under room light (left) and 365 nm UV irradiation lamp (right). UV-vis absorption spectra (d) of GQDs, PEI1800 GQDs, and PEI600 GQDs dispersed in water. (© ACS) (click on image to enlarge)

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(continued from above)

The team employed TEM, AFM, XRD, FTIR, XPS, DLS, and zeta potential to characterize the structures of the as-prepared GQDs and they stufied the PL mechanism by theoretical calculations.The average sizes of uncoated yellow-emitting GQDs, blue-emitting PEI1800 GQDs, and red-emitting PEI600 GQDs were 2.37, 6.05, and 57.31 nm, respectively. The yellow-emitting and blue-emitting GQDs were monolayer structures, whereas the red-emitting GQDs were multilayer structures. The red-emitting GQDs possessed a big PEI cage with multiple GQDs inside, whereas the blue-emitting PEI-coated GQDs had a single GQD core.The scientists found that carboxyl groups were changed to amide groups on the surface of GQDs and that this amidation reaction was crucial for PL change. By analyzing the molecular orbital and charge density, it was found that amide bonds decreased the conjugation and increased the energy gap thus inducing the blue shift of the PL.For the red-emitting GQDs, the conjugation area was enlarged by the interaction of GQDs in the PEI cage; thus, the PL peak exhibited a red shift.Remarkably, as the team points out, all GQDs exhibited good stability at high ionic strength and resisted photobleaching. Cell viability after treatment with the as-prepared GQDs indicated that GQDs had quite low cytotoxicity.”The GQDs could be used for bioimaging and are expected to be widely applied in multicolor imaging and bioanalysis applications,” the authors cocnlude their report. “We hope that this work will inspire the design of even better GQDs with tunable PL properties.”

Primary Story Contributed by Micheal Berger Nanowerk

Quantum Dot Transistor Simulates Synaptic Responses and Functions of Neurons


 

QD Transistor id47090

This research demonstrates a nanoscaled memdevice able to act as an electronic analogue of tipping buckets that allows reducing the dimensionality and complexity of a sensing problem by transforming it into a counting problem. The device offers a well adjustable, tunable, and reliable periodic reset that is controlled by the amounts of transferred quantum dot charges per gate voltage sweep. When subjected to periodic voltage sweeps, the quantum dot (bucket) may require up to several sweeps before a rapid full discharge occurs thus displaying period doubling, period tripling, and so on between self-governing reset operations. (© ACS)

A transistor that simulates some of the functions of neurons has been invented based on experiments and models developed by researchers at the Federal University of São Carlos (UFSCar) in São Paulo State, Brazil, Würzburg University in Germany, and the University of South Carolina in the United States.The device, which has micrometric as well as nanometric parts, can see light, count, and store information in its own structure, dispensing with the need for a complementary memory unit.It is described in an article in the journal Nano Letters (“Nanoscale tipping bucket effect in a quantum dot transistor-based counter”

“In this article, we show that transistors based on quantum dots can perform complex operations directly in memory. This can lead to the development of new kinds of device and computer circuit in which memory units are combined with logical processing units, economizing space, time, and power consumption,” said Victor Lopez Richard, a professor in UFSCar’s Physics Department and one of the coordinators of the study.
The transistor was produced by a technique called epitaxial growth, which consists of coating a crystal substrate with thin film. On this microscopic substrate, nanoscopic droplets of indium arsenide act as quantum dots, confining electrons in quantized states. Memory functionality is derived from the dynamics of electrical charging and discharging of the quantum dots, creating current patterns with periodicities that are modulated by the voltage applied to the transistor’s gates or the light absorbed by the quantum dots.
“The key feature of our device is its intrinsic memory stored as an electric charge inside the quantum dots,” Richard said. “The challenge is to control the dynamics of these charges so that the transistor can manifest different states. Its functionality consists of the ability to count, memorize, and perform the simple arithmetic operations normally done by calculators, but using incomparably less space, time, and power.”
According to Richard, the transistor is not likely to be used in quantum computing because this requires other quantum effects. However, it could lead to the development of a platform for use in equipment such as counters or calculators, with memory intrinsically linked to the transistor itself and all functions available in the same system at the nanometric scale, with no need for a separate space for storage.
“Moreover, you could say the transistor can see light because quantum dots are sensitive to photons,” Richard said, “and just like electric voltage, the dynamics of the charging and discharging of quantum dots can be controlled via the absorption of photons, simulating synaptic responses and some functions of neurons.”
Further research will be necessary before the transistor can be used as a technological resource. For now, it works only at extremely low temperatures – approximately 4 Kelvin, the temperature of liquid helium.
“Our goal is to make it functional at higher temperatures and even at room temperature. To do that, we’ll have to find a way to separate the electronic spaces of the system sufficiently to prevent them from being affected by temperature. We need more refined control of synthesis and material growth techniques in order to fine-tune the charging and discharging channels. And the states stored in the quantum dots have to be quantized,” Richard said.
Source: Fundação de Amparo à Pesquisa do Estado de São Paulo

Read more: Quantum dot transistor simulates functions of neurons

Advanced (SWIR) Quantum Dots Offer Solution for Tagging and Imaging the Biological Processes in LIVE Animals


nanocrystalsFluorescent quantum dots are valuable tools used to tag and image biological processes in live animals. However, precise fluorescent imaging at the cellular and molecular levels has not been possible because of non-specific fluorescence and light scattering by surrounding tissues.

Now researchers have created short wave infrared (SWIR) quantum dots that resolve many of these problems. The system was used in live mice to image working organs, take metabolic measurements, and track microvascular blood flow in normal brain and brain tumors.

“Quantum dots are small (nanoscale) particles that can be engineered to emit light at different wavelengths,” explains Behrouz Shabestari, Ph.D., director of the Optical Imaging Program at NIH’s National Institute of Biomedical Imaging and Bioengineering, which co-funded the research. “When they are injected into a live animal, the emitted fluorescent light can be seen with special cameras. By engineering the dots to bind to specific tissues of interest, researchers can use them to study biological processes in real-time.” qdot_tech_note_graph
An international group of investigators led by Moungi G. Bawendi, Ph.D., the Lester Wolfe Professor in Chemistry at the Massachusetts Institute of Technology, collaborated to create what Bawendi calls the “next-generation,” of quantum dots.
Said Bawendi, “We took advantage of the special qualities of short wave infrared light, which is essentially the ability to give a clear bright signal emitted from the tissue of interest that is not blocked or scattered by the surrounding tissues. The system allows us to view biological processes in living, moving animals with great clarity and detail.”
The work is described in the April issue of the journal Nature Biomedical Engineering (“Next-generation in vivo optical imaging with short-wave infrared quantum dots”).
experimental set-up with composite SWIR quantum dots injected into the circulation and then imaged through a cranial window in the mouse brain
The top outlines the experimental set-up with composite SWIR quantum dots injected into the circulation and then imaged through a cranial window in the mouse brain. The bottom shows the resulting fluorescent image with healthy arteries in red, veins in blue, and the disorganized blood vessels of a brain tumor in green.

Engineering SWIR quantum dots to target tissues of interest

While the inner core of a SWIR quantum dot (SWIR-QDs) generates the unique fluorescent properties of short wave infrared light, the other critical component of the dot is the outer surface, which must be engineered to target a tissue of interest. The researchers call this “functionalization,” which means making them useful for studying specific tissues and biological processes. Bawendi and colleagues engineered three distinct types of SWIR quantum dots to demonstrate their use in studying different biological processes.
The first type of SWIR-QDs were engineered with phospholipid micelle surface coatings. Micelles are small particles that have a hydrophilic (water-loving) outer shell and a hydrophobic (water repelling) inner layer. The micelle-embedded SWIR-QDs dissolved and circulated through the bloodstream for an extended period, allowing the researchers to study heart and respiration rates in awake mice.
The advantage of these SWIR-QDs is the ability to image physiological processes that occur too rapidly to be detected by common imaging methods such as MRI or PET. This ability would allow unobtrusive monitoring of animals in their normal environment for changes in heartbeat and breathing rates during various exercise tests or in response to drug candidates for conditions such as cardiac arrhythmia.
The second type of SWIR-QDs created were embedded in chylomicrons. Chylomicrons are lipoprotein particles that consist of triglycerides, phospholipids, cholesterol, and proteins and are known to transport dietary lipids from the intestines to other locations.

These SWIR-QDs were used to study the movement and metabolism of lipids between brown adipose tissue, blood, and liver in real-time. The researchers explained that lipid-coated SWIR-QDs could be used to assess the immediate effects of medications designed to affect lipid metabolism—for example, to increase the liver’s uptake of lipids from the bloodstream of an individual with high cholesterol.

SWIR quantum dot imaging
a) Experimental set-up with lipid micelle SWIR quantum dots injected into the circulation and whole body scan with SWIR camera. b) Resulting fluorescent image shows the accumulation of the lipid micelle SWIR quantum dots in the liver (blue circle) and heart (red circle).
The third type of SWIR-QDs were composites, containing multiples QDs, and coated with PEG, which allows them to dissolve in blood. This third type was used to measure blood flow in the vasculature of the mouse brain by tracking individual SWIR-QD composite particles as they moved through the blood vessels. The researchers could view the dramatic differences between blood flow in healthy vasculature and in vessels at the margin of a brain tumor.
These SWIR-QDs would make it possible to measure blood flow in the brain before and after a stroke, and changes in response to experimental stroke medications.
“In addition to the ability to test much-needed new medications to treat stroke, the potential application to difficult-to-treat tumors is one that we are also very excited about,” said Bawendi. “We can potentially use SWIR-QDs to study how the blood flow pattern in a tumor changes over time. We could monitor disease progression or regression in response to drug treatment.

This opens a new way to assess experimental treatments for both stroke and brain cancer that have not been possible with other imaging methods.”

Source: National Institute of Biomedical Imaging and Bioengineering

Read more: Advanced quantum dots shed bright light on biological processes