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A team of University of Wisconsin–Madison engineers has created the most functional flexible transistor in the world — and with it, a fast, simple and inexpensive fabrication process that’s easily scalable to the commercial level.

It’s an advance that could open the door to an increasingly interconnected world, enabling manufacturers to add “smart,” wireless capabilities to any number of large or small products or objects — like wearable sensors and computers for people and animals — that curve, bend, stretch and move.

Literal flexibility may bring the power of a new transistor developed at UW–Madison to digital devices that bend and move. PHOTO COURTESY OF JUNG-HUN SEO, UNIVERSITY AT BUFFALO, STATE UNIVERSITY OF NEW YORK

Literal flexibility may bring the power of a new transistor developed at UW–Madison to digital devices that bend and move. PHOTO COURTESY OF JUNG-HUN SEO, UNIVERSITY AT BUFFALO, STATE UNIVERSITY OF NEW YORK

Transistors are ubiquitous building blocks of modern electronics. The UW–Madison group’s advance is a twist on a two-decade-old industry standard: a BiCMOS (bipolar complementary metal oxide semiconductor) thin-film transistor, which combines two very different technologies — and speed, high current and low power dissipation in the form of heat and wasted energy — all on one surface.

As a result, these “mixed-signal” devices (with both analog and digital capabilities) deliver both brains and brawn and are the chip of choice for many of today’s portable electronic devices, including cellphones.

“The industry standard is very good,” says Zhenqiang (Jack) Ma, the Lynn H. Matthias Professor and Vilas Distinguished Achievement Professor in electrical and computer engineering at UW–Madison. “Now we can do the same things with our transistor — but it can bend.”

Ma is a world leader in high-frequency flexible electronics. He and his collaborators described their advance in the inaugural issue of the journal npj Flexible Electronics, published Sept. 27.

Making traditional BiCMOS flexible electronics is difficult, in part because the process takes several months and requires a multitude of delicate, high-temperature steps. Even a minor variation in temperature at any point could ruin all of the previous steps.

Ma and his collaborators fabricated their flexible electronics on a single-crystal silicon nanomembrane on a single bendable piece of plastic. The secret to their success is their unique process, which eliminates many steps and slashes both the time and cost of fabricating the transistors.

“In industry, they need to finish these in three months,” he says. “We finished it in a week.”

He says his group’s much simpler high-temperature process can scale to industry-level production right away.

“The key is that parameters are important,” he says. “One high-temperature step fixes everything — like glue. Now, we have more powerful mixed-signal tools. Basically, the idea is for flexible electronics to expand with this. The platform is getting bigger.”

His collaborators include Jung-Hun Seo of the University at Buffalo, State University of New York; Kan Zhang of UW–Madison; and Weidong Zhou of the University of Texas at Arlington.

A sea of spinning electrons


October 3, 2017

Picture two schools of fish swimming in clockwise and counterclockwise circles. It’s enough to make your head spin, and now scientists at Rutgers University-New Brunswick and the University of Florida have discovered the “chiral spin mode” – a sea of electrons spinning in opposing circles.

“We discovered a new collective spin mode that can be used to transport energy or information with very little energy dissipation, and it can be a platform for building novel electronic devices such as computers and processors,” said Girsh Blumberg, senior author of the study and a professor in the Department of Physics and Astronomy in Rutgers’ School of Arts and Sciences.

Collective chiral spin modes are propagating waves of electron spins that do not carry a charge current but modify the “spinning” directions of electrons. “Chiral” refers to entities, like your right and left hands, that are matching but asymmetrical and can’t be superimposed on their mirror image.

The study, led by Hsiang-Hsi (Sean) Kung, a graduate student in Blumberg’s Rutgers Laser Spectroscopy Lab, was published in Physical Review Letters. Kung used a custom-made, ultra-sensitive spectrometer to study a prototypical 3D topological insulator. A microscopic theoretical model that predicts the energy and temperature evolution of the chiral spin mode was developed by Saurabh Maiti and Professor Dmitrii Maslov at the University of Florida, strongly substantiating the experimental observation.

The blue and red cones show the energy and momentum of surface electrons in a 3D topological insulator. The spin structure is shown in the blue and red arrows at the top and bottom, respectively. Light promotes electrons from the blue cone into the red cone, with the spin direction flipping. The orderly spinning leads to the chiral spin mode observed in this study. Credit: Hsiang-Hsi (Sean) Kung/Rutgers University-New Brunswick

The blue and red cones show the energy and momentum of surface electrons in a 3D topological insulator. The spin structure is shown in the blue and red arrows at the top and bottom, respectively. Light promotes electrons from the blue cone into the red cone, with the spin direction flipping. The orderly spinning leads to the chiral spin mode observed in this study.
Credit: Hsiang-Hsi (Sean) Kung/Rutgers University-New Brunswick

In a vacuum, electrons are simple, boring elementary particles. But in solids, the collective behavior of many electrons interacting with each other and the underlying platform may result in phenomena that lead to new applications in superconductivity, magnetism and piezoelectricity (voltage generated via materials placed under pressure), to name a few. Condensed matter science, which focuses on solids, liquids and other concentrated forms of matter, seeks to reveal new phenomena in new materials.

Silicon-based electronics, such as computer chips and computers, are one of the most important inventions in human history. But silicon leads to significant energy loss when scaled down. One alternative is to harness the spins of electrons to transport information through extremely thin wires, which in theory would slash energy loss.

The newly discovered “chiral spin mode” stems from the sea of electrons on the surface of “3D topological insulators.” These special insulators have nonmagnetic, insulating material with robust metallic surfaces, and the electrons are confined so they move only on 2D surfaces.

Most importantly, the electrons’ spinning axes are level and perpendicular to their velocity. Chiral spin modes emerge naturally from the surface of such insulating materials, but they were never observed before due to crystalline defects. The experimental observation in the current study was made possible following the development of ultra-clean crystals by Rutgers doctoral student Xueyun Wang and Board of Governors Professor Sang-Wook Cheong in the Rutgers Center for Emergent Materials.

The discovery paves new paths for building next generation low-loss electronic devices.

Applied Energy Systems (AES), provider of high and ultra high purity gas systems, services, and solutions – including design, manufacturing, testing, installation, and expert field service – is showcasing the capabilities of its SEMI-GAS® Xturion™ Blixer™ to support various processes that require forming gas mixtures. The Blixer™ provides a cost-effective alternative to purchasing expensive pre-mixed gas cylinders by enabling operators to blend their own mixtures on-site in their facility.

The ultra high purity gas mixing blender is used by customers across a diverse range of industries to uniformly mix H2 and N2 concentrations in customizable ratios that meet their distinct process requirements. Mixtures can be adjusted in real-time via the system’s GigaGuard™ PLC Controller, which features a 9” Siemens color touchscreen for intuitive operation, allowing the user to fine-tune formulations on demand. This makes the system particularly appealing for high volume applications, eliminating the need to stock a variety of pre-mixed forming gas concentrations, decreasing the frequency of cylinder change-outs, reducing tool downtime, increasing productivity, and ultimately providing the end user with a significant cost savings.

The Blixer™ system is designed to provide a continuous flow of precise gas blends and includes a static mixing tube and surge/mixing tank to address dynamic flow changes and effectively maintain mix tolerances. It is also equipped with a Thermal Conductivity Hydrogen Gas Analyzer, featuring auto-calibration capability and a low flow alarm, to ensure +/- 1% blending accuracy. Its PLC Controller includes Ethernet connectivity to allow for seamless integration with a facility’s Monitoring System, and the system’s hydrogen hazardous gas detector and automatic shutdown feature alert operators during undesirable system conditions.

“We have found the Blixer™ to be especially beneficial to customers using forming gas mixtures because it gives them flexibility to custom-blend H2/N2 concentrations in the exact ratios they desire—instead of investing in expensive pre-mixed cylinders that still may not be precisely mixed to their unique process requirements,” said Greg Havrilla, Technical Inside Sales Engineer for AES. “The system’s value spans industries. We’ve seen it support laser-based technology development, semiconductor fabrication, electrically-powered vehicle manufacturing, sustainable energy solutions, and a variety of industrial manufacturing applications. Its flexibility is reflected in its ability to satisfy a range of process-driven demands.”

AES-SEMI-GAS-Xturion-Blixer-System

Graphene is a sheet of carbon that is only one atom thick, and it has drawn worldwide attention as a new material. A research group from Kumamoto University, Japan has discovered that pressure can be generated by simply stacking graphene oxide nanosheets, a material that closely resembles graphene. They also found that the pressure can be increased by reducing the interlayer distance through heat treatment. It is an innovative approach for applying high pressure without using an enormous amount of energy.

The 2010 Nobel Prize in Physics was awarded to two scientists, Andre Geim and Konstantin Novoselov, for groundbreaking graphene experiments. The carbon material is very thin, strong, flexible, and has high electrical conductivity. Oxidized graphene nanosheets have many oxygen functional groups at the front and back of graphene, and previous research has shown that if several layers of oxidized graphene nanosheets are heat treated, the interlayer distance shrinks as oxygen functional groups are eliminated.

This led the researchers at Kumamoto University, Japan to consider that reducing the interlayer distance of graphene oxide nanosheets, could allow it to be used as a compressor that applies pressure to a substance sandwiched between the sheets. To measure pressure between nanosheets, they used molecular materials that change the electrical state of metal ions in response to pressure (spin crossover phenomenon). They observed an electrical state change of iron nanoparticles by sandwiching the material and measuring the spin crossover phenomenon between graphene oxide nanosheets.

As the interlayer distance becomes smaller, the pressure between layers rises. This means that the pressure value can be adjusted by the heat treatment temperature. The maximum pressure the researchers measured was 38 x 106 Pa (101,300 Pa at atmospheric pressure, or about 375 atm). Moreover, they found that pressure does not occur unless the nanosheets are properly stacked.

“There are several examples of special materials that cause compression by just sandwiching or wrapping, similar to our results here,” said Assistant Professor Ryo Ohtani of Kumamoto University, who led the study. “But, as far as we know, this graphene nanosheet is the first example in the world with the ability to adjust applied pressure by simply changing the heat treatment temperature. We expect that this “nano-compressor” will lead to new developments from fields such as material chemistry or physics. Particularly since this technique produces high pressures that normally cannot be obtained without adding a large amount of energy.”

Quantum dots are nanometre-sized semiconductor particles with potential applications in solar cells and electronics. Scientists from the University of Groningen and their colleagues from ETH Zürich have now discovered how to increase the efficiency of charge conductivity in lead-sulphur quantum dots. Their results will be published in the journal Science Advances on 29 September.

Quantum dots are clusters of some 1,000 atoms which act as one large ‘super-atom’. The dots, which are synthesized as colloids, i.e. suspended in a liquid like a sort of paint, can be organized into thin films with simple solution-based processing techniques. These thin films can turn light into electricity. However, scientists have discovered that the electronic properties are a bottleneck. ‘Especially the conduction of holes, the positive counterpart to negatively charged electrons’, explains Daniel Balazs, PhD student in the Photophysics and Optoelectronics group of Prof. Maria A. Loi at the University of Groningen Zernike Institute for Advanced Materials.

Stoichiometry

Loi’s group works with lead-sulphide quantum dots. When light produces an electron-hole pair in these dots, the electron and hole do not move with the same efficiency through the assembly of dots. When the transport of either is limited, the holes and electrons can easily recombine, which reduces the efficiency of light-to-energy conversion. Balazs therefore set out to improve the poor hole conductance in the quantum dots and to find a toolkit to make this class of materials tunable and multifunctional.

‘The root of the problem is the lead-sulphur stoichiometry’, he explains. In quantum dots, nearly half the atoms are on the surface of the super-atom. In the lead-sulphur system, lead atoms preferentially fill the outer part, which means a ratio of lead to sulphur of 1:3 rather than 1:1. This excess of lead makes this quantum dot a better conductor of electrons than holes.

Thin films

In bulk material, transport is generally improved by ‘doping‘ the material: adding small amounts of impurities. However, attempts to add sulphur to the quantum dots have failed so far. But now Balazs and Loi have found a way to do this and thus increase hole mobility without affecting electron mobility.

Many groups have tried to combine the addition of sulphur with other production steps. However, this caused many problems, such as disrupting the assembly of the dots in the thin film. Instead, Balazs first produced ordered thin films and then added activated sulphur. Sulphur atoms were thus successfully added to the surface of the quantum dots, without affecting the other properties of the film. ‘A careful analysis of the chemical and physical processes during the assembly of quantum dot thin films and the addition of extra sulphur were what was needed to get this result. That’s why our group, with the cooperation of our chemistry colleagues from Zürich, was successful in the end.’

Devices

Loi’s team is now able to add different amounts of sulphur, which enables them to tune the electric properties of the super-atom assemblies. ‘We now know that we can improve the efficiency of quantum dot solar cells above the current record of 11%. The next step is to show that this method can also make other types of functional devices such as thermoelectric devices.’ It underlines the unique properties of quantum dots: they act as one atom with specific electric properties. ‘And now we can assemble them and can engineer their electrical properties as we wish. That is something which is impossible with bulk materials and it opens new perspectives for electronic and optoelectronic devices.’

GLOBALFOUNDRIES is now delivering in volume its 14nm High Performance (HP) technology that will enable IBM’s next-generation of processors for server systems. The jointly developed 14HP process is specifically designed to deliver the ultra-high performance and data-processing capacity IBM needs to support its cloud, commerce, and enterprise solutions in the era of big data and cognitive computing. IBM announced general availability of the IBM Z on September 13.

14HP is the industry’s only technology to integrate a three-dimensional FinFET transistor architecture on a silicon-on-insulator (SOI) substrate. Featuring a 17-layer metal stack and more than eight billion transistors per chip, the technology leverages embedded DRAM and other innovative features to deliver higher performance, reduced energy, and better area scaling over previous generations to address a wide range of deep computing workloads.

The 14HP technology powers the processors that run IBM’s latest z14 mainframes. The underlying semiconductor process allows IBM customers to enable massive transaction scale of high-volume workloads, apply machine learning to their most valuable data, and rapidly derive actionable insights to enable intelligent decisions—all while delivering pervasive encryption that provides the ultimate in data protection.

“GlobalFoundries has been a strategic partner in the development of a custom semiconductor technology to enable the aggressive requirements of the processors for our newest server systems,” said Ross Mauril, general manager, IBM Z. “We are excited to bring this 14HP technology to our IBM Z product line.”

“GF and IBM together have an unmatched heritage of developing and manufacturing ultra-high performance SOI chips,” said Mike Cadigan, senior vice president of global sales and business development at GF. “This new generation of 14HP processors is another example of the close collaboration between our engineering teams to meet the demands of a new generation of server systems.”

“The 14HP technology leverages the proven 14nm FinFET high-volume experience of our Fab 8 facility in Saratoga County, N.Y.,” said Tom Caulfield, senior vice president and general manager of GF’s Fab 8. “We are in high volume production with a broad set of customer designs across a range of applications. Our mature and diverse manufacturing capability will enable IBM to bring its latest processor designs to market to service their broad customer base.”

Perovskite solar cells (PSCs) can offer high light-conversion efficiency with low manufacturing costs. But to be commercially viable, perovskite films must also be durable and not degrade under solar light over time. EPFL scientists have now greatly improved the operational stability of PSCs, retaining more than 95% of their initial efficiencies of over 20% under full sunlight illumination at 60oC for more than 1000 hours. The breakthrough, which marks the highest stability for perovskite solar cells, is published in Science.

Challenges of stability

Conventional silicon solar cells have reached a point of maturation, with efficiencies plateauing around 25% and problems of high-cost manufacturing, heavyweight, and rigidity has remained largely unresolved. On the contrary, a relatively new photovoltaic technology based on perovskite solar cells has already achieved more than 22% efficiency.

Given the vast chemical versatility, and the low-cost processability of perovskite materials, the PSCs hold the promise to lead the future of photovoltaic technology by offering cheap, light weight and highly efficient solar cells. But until now, only highly expensive, prototype organic hole-transporting materials (HTMs,selectively transporting positive charges in a solar cell) have been able to achieve power-conversion efficiencies over 20%. And by virtue of their ingredients, these hole-transporting materials adversely affect the long-term operational stability of the PSC.

Therefore, investigating cheap and stable hole transporters that produce equally high efficiencies is in great demand to enable large-scale deployment of perovskite solar cells. Among various inorganic HTMs, cuprous thiocyanate (CuSCN) stands out as a stable, efficient and cheap candidate ($0.5/gr versus $500 /gr for the commonly used spiro-OMeTAD). But previous attempts to use CuSCN as a hole transporter in perovskite solar cells have yielded only moderately stabilized efficiencies and poor device stability, due to problems associated with depositing a high-quality CuSCN layer atop of the perovskite film, as wells as the chemical instability of the CuSCN layer when integrated into a perovskite solar cell.

A stable solution

Now, researchers at Michael Grätzel’s lab at EPFL, in a project led by postdocs Neha Arora and M. Ibrahim Dar, have introduced two new concepts that overcome the major shortcomings of CuSCN-based perovskite solar cells. First, they developed a simple dynamic solution-based method for depositing highly conformal, 60-nm thick CuSCN layers that allows the fabrication of perovskite solar cells with stabilized power-conversion efficiencies exceeding 20%. This is comparable to the efficiencies of the best performing, state-of-the-art spiro-OMeTAD-based perovskite solar cells.

Second, the scientists introduced a thin spacer layer of reduced graphene oxide between the CuSCN and a gold layer. This innovation allowed the perovskite solar cells to achieve excellent operational stability, retaining over 95% of their initial efficiency while operating at a maximum power point for 1000 hours under full-sun illumination at 60 °C. This surpasses even the stability of organic HTM-based perovskite solar cells that are heavily researched and have recently dominated the field.

The researchers also discovered that the instability of the perovskite devices originates from the degradation of CuSCN/gold contact during the solar cell’s operation.

“This is a major breakthrough in perovskite solar-cell research and will pave the way for large-scale commercial deployment of this very promising new photovoltaic technology,” says Michael Grätzel. “It will benefit the numerous scientists in the field that have been intensively searching for a material that could replace the currently used, prohibitively expensive organic hole-transporters,” adds M. Ibrahim Dar.

OEM Group announced today a Post-Dice Clean solution on the proven Cintillio™ Batch Spray platform following plasma and laser dicing methods. Designed specifically to remove residue and particles left behind from these dicing methods, OEM Group’s Cintillio™ SST (Spray Solvent Tool) and Cintillio™ Eco-Clean systems utilize their patented Enhanced Spray Technology (EST) to deliver process improvement through uniform media flow with a nozzle-per-wafer concept ensuring uniform flow and increased rinse efficiency.

After wafers are singulated prior to “pick and place,” the conventional method of cleaning is by water rinsing; however, some singulation methods, particularly plasma and laser, may leave behind residues that water cannot clean. Slag, polymers, and other residues impede device performance and may cause corrosion or affect downstream processes. The Cintillio™ post-dice clean process successfully removes these residues to maintain final device performance. Chris Forgey, CTO for OEM Group says, “We’re pleased to leverage our patented Ozone process specifically for post dice clean applications, delivering value and superior process capability for this specific application.”

Along with the patented Enhanced Spray Technology (EST), both platforms adapt wafer carriers and rotors to hold multiple “diced wafer-on-tape-on-frame” substrates, delivering greater throughput, reduced chemical utilization, space efficient footprint, and excellent overall performance. According to OEM Group Applications Lab Manager, Joshua Levinson, Ph.D., “Any device manufacturer who performs back-end processing of wafers and who employs wafer singulation to create diced substrates will benefit from our solutions. Batch processing also reduces the number of cleaning tools required in a fab and lowers overall cost of ownership, waste generation, and DI water usage.”

With global headquarters in metro Phoenix, Arizona and additional sites throughout the North America, Europe, Japan and Asia, OEM Group, LLC is a semiconductor capital equipment manufacturer and innovator in new and remanufactured 75mm–200mm tools and services.

Solar-Tectic LLC (“ST”) announced today that a patent application for a method of making III-V thin-film tandem solar cells with high performance has been allowed by the US Patent and Trademark Office. The patent, the first ever for a thin III-V layer on crystalline silicon thin-film, covers group III-V elements such as Gallium Arsenide (GaAs), and Indium Gallium Phosphide (InGaP), for the top layer, as well as all inorganic materials, including, silicon, germanium, etc., for the bottom layer.  Group III-V compounds such as Gallium Arsenide (GaAs) are proven photovoltaic materials with high efficiencies but until now have been cost prohibitive because high quality III-V material such as GaAs is expensive. Moreover, the cost of substrates on which to grow III-V materials, such as germanium, which is known to be an ideal material, has kept the technology from market entry. In the breakthrough technology here, ultra-thin films of III-V materials and silicon (or germanium) replace expensive, thicker wafers thereby lowering the costs dramatically. The inventor is Ashok Chaudhari, CEO of Solar-Tectic LLC.

III-V tandem (or multi-junction) cells built on wafers such as silicon are currently being developed in labs, with high efficiencies of around ~30%.  The highest dual-junction cell efficiency (32.8%) came from a tandem cell that stacked a layer of gallium arsenide (GaAs) atop crystalline silicon. Manufacturing costs are expensive especially if a germanium wafer is used as the bottom material in the two layer tandem structure.  In order to compete with low cost silicon wafer technology which is 90% of the global solar panel market, efficiencies must not only be as high as silicon wafers or greater (21.7% and 26.7% are lab records for poly- and monocrystalline silicon wafer cells, respectively), but manufacturing costs must also be lower. This is achievable in the Solar-Tectic LLC patented technology, which uses common industrial manufacturing processes and at low temperature. There is no wafer involved which saves material and energy; instead a thin film allows for precise control of growth parameters. A glass substrate instead of wafer also allows for a bifacial cell design for increased efficiency. A cost effective ~30% efficient III-V tandem solar cell in today’s market would revolutionize the solar energy industry by dramatically reducing the balance of system (BoS) costs, and thereby reduce the need for fossil fuel generated electricity. Silicon wafer technology based on polycrystalline or monocrystalline silicon could become obsolete.

Importantly, the entire patented process for the Solar-Tectic LLC III-V tandem cell can be environmentally friendly since non-toxic metals can be used to deposit the crystalline thin-film materials for both the bottom layer in the tandem configuration as well as in the top, III-V, layer.

The technology also has great promise for LED manufacturing using for example Gallium Nitride.

A “Tandem Series” of solar cell technologies has been launched by Solar-Tectic LLC, which includes a variety of different proven semiconductor photovoltaic materials for the top layer on silicon and/or germanium bottom layers. Recently patents for a tin perovskite and germanium perovskite thin-film tandem solar cell were also granted.

The ITC ruling on September 22 means that it is likely that tariffs will be imposed on crystalline silicon wafers sold in the US. These tariffs will not apply to thin-film solar cell technology, such as ST’s.

Band gaps, made to order


September 28, 2017

Control is a constant challenge for materials scientists, who are always seeking the perfect material — and the perfect way of treating it — to induce exactly the right electronic or optical activity required for a given application.

One key challenge to modulating activity in a semiconductor is controlling its band gap. When a material is excited with energy, say, a light pulse, the wider its band gap, the shorter the wavelength of the light it emits. The narrower the band gap, the longer the wavelength.

As electronics and the devices that incorporate them — smartphones, laptops and the like — have become smaller and smaller, the semiconductor transistors that power them have shrunk to the point of being not much larger than an atom. They can’t get much smaller. To overcome this limitation, researchers are seeking ways to harness the unique characteristics of nanoscale atomic cluster arrays — known as quantum dot superlattices — for building next generation electronics such as large-scale quantum information systems. In the quantum realm, precision is even more important.

New research conducted by UC Santa Barbara’s Department of Electrical and Computer Engineering reveals a major advance in precision superlattices materials. The findings by Professor Kaustav Banerjee, his Ph.D. students Xuejun Xie, Jiahao Kang and Wei Cao, postdoctoral fellow Jae Hwan Chu and collaborators at Rice University appear in the journal Nature Scientific Reports.

Their team’s research uses a focused electron beam to fabricate a large-scale quantum dot superlattice on which each quantum dot has a specific pre-determined size positioned at a precise location on an atomically thin sheet of two-dimensional (2-D) semiconductor molybdenum disulphide (MoS2). When the focused electron beam interacts with the MoS2 monolayer, it turns that area — which is on the order of a nanometer in diameter — from semiconducting to metallic. The quantum dots can be placed less than four nanometers apart, so that they become an artificial crystal — essentially a new 2-D material where the band gap can be specified to order, from 1.8 to 1.4 electron volts (eV).

This is the first time that scientists have created a large-area 2-D superlattice — nanoscale atomic clusters in an ordered grid — on an atomically thin material on which both the size and location of quantum dots are precisely controlled. The process not only creates several quantum dots, but can also be applied directly to large-scale fabrication of 2-D quantum dot superlattices. “We can, therefore, change the overall properties of the 2-D crystal,” Banerjee said.

Each quantum dot acts as a quantum well, where electron-hole activity occurs, and all of the dots in the grid are close enough to each other to ensure interactions. The researchers can vary the spacing and size of the dots to vary the band gap, which determines the wavelength of light it emits.

“Using this technique, we can engineer the band gap to match the application,” Banerjee said. Quantum dot superlattices have been widely investigated for creating materials with tunable band gaps but all were made using “bottom-up” methods in which atoms naturally and spontaneously combine to form a macro-object. But those methods make it inherently difficult to design the lattice structure as desired and, thus, to achieve optimal performance.

As an example, depending on conditions, combining carbon atoms yields only two results in the bulk (or 3-D) form: graphite or diamond. These cannot be ‘tuned’ and so cannot make anything in between. But when atoms can be precisely positioned, the material can be designed with desired characteristics.

“Our approach overcomes the problems of randomness and proximity, enabling control of the band gap and all the other characteristics you might want the material to have — with a high level of precision,” Xie said. “This is a new way to make materials, and it will have many uses, particularly in quantum computing and communication applications. The dots on the superlattice are so close to each other that the electrons are coupled, an important requirement for quantum computing.”

The quantum dot is theoretically an artificial “atom.” The developed technique makes such design and “tuning” possible by enabling top-down control of the size and the position of the artificial atoms at large scale.

To demonstrate the level of control achieved, the authors produced an image of “UCSB” spelled out in a grid of quantum dots. By using different doses from the electron beam, they were able to cause different areas of the university’s initials to light up at different wavelengths.

“When you change the dose of the electron beam, you can change the size of the quantum dot in the local region, and once you do that, you can control the band gap of the 2-D material,” Banerjee explained. “If you say you want a band gap of 1.6 eV, I can give it to you. If you want 1.5 eV, I can do that, too, starting with the same material.”

This demonstration of tunable direct band gap could usher a new generation of light-emitting devices for photonics applications.