Tag Archives: letter-leds-tech

Graphene quantum dots created at Rice University grab onto graphene platelets like barnacles attach themselves to the hull of a boat. But these dots enhance the properties of the mothership, making them better than platinum catalysts for certain reactions within fuel cells.

The Rice lab of chemist James Tour created dots known as GQDs from coal last year and have now combined these nanoscale dots with microscopic sheets of graphene, the one-atom-thick form of carbon, to create a hybrid that could greatly cut the cost of generating energy with fuel cells.

The research is the subject of a new paper in the American Chemical Society journal ACS Nano.

The lab discovered boiling down a solution of GQDs and graphene oxide sheets (exfoliated from common graphite) combined them into self-assembling nanoscale platelets that could then be treated with nitrogen and boron. The hybrid material combined the advantages of each component: an abundance of edges where chemical reactions take place and excellent conductivity between GQDs provided by the graphene base. The boron and nitrogen collectively add more catalytically active sites to the material than either element would add alone.

“The GQDs add to the system an enormous amount of edge, which permits the chemistry of oxygen reduction, one of the two needed reactions for operation in a fuel cell,” Tour said. “The graphene provides the conductive matrix required. So it’s a superb hybridization.”

The Tour lab’s material outperformed commercial platinum/carbon hybrids commonly found in fuel cells. The material showed an oxygen reduction reaction of about 15 millivolts more in positive onset potential – the start of the reaction – and 70 percent larger current density than platinum-based catalysts.

The materials required to make the flake-like hybrids are much cheaper, too, Tour said. “The efficiency is better than platinum in terms of oxygen reduction, permitting one to sidestep the most prohibitive hurdle in fuel-cell generation — the cost of the precious metal,” he said.

Rice graduate student Huilong Fei is the paper’s lead author. Co-authors are graduate students Ruquan Ye, Gonglan Ye, Yongji Gong, Zhiwei Peng and Errol Samuel; research technician Xiujun Fan; and Pulickel Ajayan, the Benjamin M. and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials Science and of chemistry and chair of the Department of Materials Science and NanoEngineering, all of Rice.

Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of materials science and nanoengineering and of computer science.

The Office of Naval Research Multidisciplinary University Research Initiative (MURI) program, the Air Force Office of Scientific Research and its MURI program supported the research.

Quantum Materials Corp today announced that it is scaling volume production of photoactive quantum dots for use in next-generation photovoltaic solar power technologies. While offering numerous advantages for solar power generation, the high cost and difficulty of producing large quantities of quantum dots with which to develop thin film solar cells has until now kept them from commercial utilization and acceptance. The company is also seeking partners for pilot thin-film quantum dot solar cell factories with Quantum Materials’ automated quantum dot production system supplying the material necessary to support daily runs of continuous roll-to-roll thin film production.

The environment for solar advances is reaching an investment capital ‘tipping point’ as highlighted by the recent pledge by the Rockefellers, who made their vast fortune on oil, to divest a total of $50 billion from fossil fuel investments and focus on supporting alternative energy solutions. They have joined some 650 individuals and 180 institutions, including 50 new foundations, which hold over $50 billion in total assets, that have pledged to divest from supporting fossil fuels over five years since the divestment movement launched three years ago.

“Cost-effective volume production of photoactive quantum dots will create the foundation for invigorating capital investment and adoption of solar energy technologies as thin-film photovoltaics drive down relative cost-per-watt ratios,” said Quantum Materials Founder and CEO Stephen Squires. “Without subsidies solar energy has not been an attractive investment for business and residential markets and as a result adoption in countries that have not aggressively subsidized solar, like the United States, has been stillborn since 2008. With our recent patent award and patent acquisitions we now have the IP protection underlying our high volume photoactive quantum dots production processes that will drive significant cost reductions for thin film solar cells and increase non-subsidized solar energy adoption.”

The recent issuance of Quantum Materials’ Republic of China (Taiwan) patent – ‘Hybrid Organic Solar Cells with Photoactive Semiconductor Nanoparticles Enclosed in Surface Modifiers’ – combined with other recent patents acquired from Bayer AG provide Quantum Materials the robust intellectual property protection needed to bring their optimized photoactive quantum dot solar cell materials to market. The Bayer patents incorporate broad descriptions of materials, fundamental design of quantum dot solar cells and processes for manufacturing them and enhance Quantum Material’s technology portfolio in printing Quantum Dot displays, solar cells and other printed electronic devices by gravure or high-speed roll-to-roll. The company is seeking partners with which to utilize proprietary materials and processes to drive next generation thin-film solutions.

Flexible LEDs


September 24, 2014

Flexible light-emitting diode (LED) displays and solar cells crafted with inorganic compound semiconductor micro-rods are moving one step closer to reality, thanks to graphene and the work of a team of researchers in Korea.

Currently, most flexible electronics and optoelectronics devices are fabricated using organic materials. But inorganic compound semiconductors such as gallium nitride (GaN) can provide plenty of advantages over organic materials for use in these devices — including superior optical, electrical and mechanical properties.

One major obstacle that has so far prevented the use of inorganic compound semiconductors in these types of applications was the difficulty of growing them on flexible substrates.

In the journal APL Materials, from AIP Publishing, a team of Seoul National University (SNU) researchers led by Professor Gyu-Chul Yi describes their work growing GaN micro-rods on graphene to create transferrable LEDs and enable the fabrication of bendable and stretchable devices.

“GaN microstructures and nanostructures are garnering attention within the research community as light-emitting devices because of their variable-color light emission and high-density integration properties,” explained Yi. “When combined with graphene substrates, these microstructures also show excellent tolerance for mechanical deformation.”

Why choose graphene for substrates? Ultrathin graphene films consist of weakly bonded layers of hexagonally arranged carbon atoms held together by strong covalent bonds. This makes graphene an ideal substrate “because it provides the desired flexibility with excellent mechanical strength — and it’s also chemically and physically stable at temperatures in excess of 1,000°C,” said Yi.

It’s important to note that for the GaN micro-rod growth, the very stable and inactive surface of graphene offers a small number of nucleation sites for GaN growth, which would enhance three-dimensional island growth of GaN micro-rods on graphene.

To create the actual GaN microstructure LEDs on the graphene substrates, the team uses a catalyst-free metal-organic chemical vapor deposition (MOCVD) process they developed back in 2002.

“Among the technique’s key criteria, it’s necessary to maintain high crystallinity, control over doping, formation of heterostructures and quantum structures, and vertically aligned growth onto underlying substrates,” Yi says.

When the team put the bendability and reliability of GaN micro-rod LEDs fabricated on graphene to the test, they found that “the resulting flexible LEDs showed intense electroluminescence (EL) and were reliable — there was no significant degradation in optical performance after 1,000 bending cycles,” noted Kunook Chung, the article’s lead author and a graduate student in SNU’s Physics Department.

This represents a tremendous breakthrough for next-generation electronics and optoelectronics devices — enabling the use of large-scale and low-cost manufacturing processes.

“By taking advantage of larger-sized graphene films, hybrid heterostructures can be used to fabricate various electronics and optoelectronics devices such as flexible and wearable LED displays for commercial use,” said Yi.

The National Science Foundation (NSF) and Semiconductor Research Corporation (SRC) today announced nine research awards to 10 universities totaling nearly $4 million under a joint program focused on Secure, Trustworthy, Assured and Resilient Semiconductors and Systems (STARSS).

The awards support research at the circuit, architecture and system levels on new strategies, methods and tools to decrease the likelihood of unintended behavior or access; increase resistance and resilience to tampering; and improve the ability to provide authentication throughout the supply chain and in the field.

“The processes and tools used to design and manufacture semiconductors ensure that the resulting product does what it is supposed to do. However, a key question that must also be addressed is whether the product does anything else, such as behaving in ways that are unintended or malicious,” said Keith Marzullo, division director of NSF’s Computer and Network Systems Division, which leads the NSF/SRC partnership on STARSS. “Through this partnership with SRC, we are pleased to focus on hardware and systems security research addressing this challenge and to provide a unique opportunity to facilitate the transition of this research into practical use.”

NSF’s involvement in STARSS is part of its Secure and Trustworthy Cyberspace (SaTC) portfolio, which in August announced nearly $75 million in cybersecurity awards.

The STARRS program expands SRC’s Trustworthy and Secure Semiconductors and Systems (T3S) program, engaging 10 universities across the U.S. Initial T3S industry participants are Freescale, Intel Corporation and Mentor Graphics. NSF is the first federal partner.

“The goal of SRC’s T3S initiative is to develop cost-effective strategies and tools for the design and manufacture of chips and systems that are reliable, trustworthy and secure,” said Celia Merzbacher, SRC Vice President for Innovative Partnerships. “This includes designing for security and assurance at the outset so as to build in resistance and resilience to attack or tampering. The research enabled by the STARSS program with NSF is a cornerstone of this overall effort.”

SRC is a university-research consortium for semiconductors and related technologies.

A number of trends are motivating industry and government to support research in hardware and system security. The design and manufacture of semiconductor circuits and systems requires many steps and involves the work of hundreds of engineers — typically distributed across multiple locations and organizations worldwide. Moreover, a typical microprocessor is likely to include dozens of design modules from various sources. Designers at each level need assurance that the components being incorporated can be trusted in order for the final system to be trustworthy.

Today, the design and manufacture of semiconductor circuits and systems includes extensive verification and testing to ensure the final product does what it is intended to do. Similar approaches are needed to provide assurance that the product is authentic and does not allow unwanted functionality, access or control. This includes strategies, tools and methods at all stages, from architecture through manufacture  and throughout the lifecycle of the product.

The first round of awards made through the STARSS program will support nine research projects with diverse areas of focus. They are:

·      “Combating integrated circuit counterfeiting using secure chip odometers” – Carnegie Mellon University researchers will design and implement secure chip odometers to provide integrated circuits (ICs) with both a secure gauge of use/age and an authentication of provenance to detect counterfeit ICs;

·      “Intellectual Property (IP) Trust-A comprehensive framework for IP integrity validation”- Case Western Reserve University and University of Florida researchers will develop a comprehensive and scalable framework for IP trust analysis and verification by evaluating IPs of diverse types and forms and develop threat models, taxonomy and instances of IP trust/integrity issues.

·      “Design of low-cost, memory-based security primitives and techniques for high-volume products” – University of Connecticut researchers will develop metrics and algorithms to make static RAM physical “unclonable” functions that are substantially more reliable at extreme operating conditions and aging, and extend this to dynamic RAM and Flash;

·      “Trojan detection and diagnosis in mixed-signal systems using on-the-fly learned, pre-computed and side channel tests” – Georgia Institute of Technology researchers will leverage knowledge of state of the art mixed-signal/analog/radio frequency for detection of Trojans in generic mixed-signal systems;

·      “Metric and CAD for differential power analysis (DPA) resistance” – Iowa State University researchers will investigate statistical metrics and design techniques to measure and defend against DPA attacks;

·      “Design of secure and anti-counterfeit integrated circuits” – University of Minnesota researchers will develop hierarchical approaches for authentication and obfuscation of chips;

·      “Hardware authentication through high-capacity, physical unclonable functions (PUF)-based secret key generation and lattice coding” – University of Texas at Austin researchers will develop strong machine-learning resistant PUFs, capable of producing high-entropy outputs, and a new lattice-based stability algorithm for high-capacity secret key generation; and

·      “Fault-attack awareness using microprocessor enhancements” – Virginia Institute of Technology and State University researchers will develop a collection of hardware techniques for microprocessor architectures to detect fault injection attacks, and to mitigate fault analysis through an appropriate response in software.

·      “Invariant carrying machine for hardware assurance” – Northwestern University researchers will develop techniques for improving the reliability and trustworthiness of hardware systems via an Invariant-Carrying Machine approach.

Researchers from the University of Alabama in Huntsville and the University of Oklahoma have found a new way to control the properties of quantum dots, those tiny chunks of semiconductor material that glow different colors depending on their size. Quantum dots, which are so small they start to exhibit atom-like quantum properties, have a wide range of potential applications, from sensors, light-emitting diodes, and solar cells, to fluorescent tags for biomedical imaging and qubits in quantum computing.

This image shows the experimental set-up researchers used to analyze the behavior of quantum dots placed on metal oxides. A laser illuminated the quantum dots to make them glow and a spectrometer was used to analyze the light they emitted. Credit: Seyed Sadeghi/ University of Alabama, Huntsville

This image shows the experimental set-up researchers used to analyze the behavior of quantum dots placed on metal oxides. A laser illuminated the quantum dots to make them glow and a spectrometer was used to analyze the light they emitted. Credit: Seyed Sadeghi/ University of Alabama, Huntsville

A key property of quantum dots that makes them so useful is their fluorescence. Scientists can “tune” quantum dots to emit a specific color of light by adjusting their size — small dots glow blue and large dots glow red. However, the dots’ ability to glow can change over time with exposure to light and air.

Seyed Sadeghi, a physicist at the University of Alabama in Huntsville, wondered if it would be possible to better control how quantum dots react to their environment. His team had previously found that placing quantum dots of a certain type on nanometer-thin layers of chromium and aluminum oxides significantly altered the dots’ behavior: the aluminum oxide increased their emission efficiency, while the chromium oxide increased the dots’ degradation rate when exposed to air. The researchers decided to extend their investigations to quantum dots with different structures.

Quantum dots come in a variety of shapes, sizes, and materials. For Sadeghi and his colleagues’ most recent studies, published in the Journal of Applied Physics, from AIP Publishing, the researchers probed the behavior of four different types of commercially available quantum dots. Some of the quantum dots had protective shells, while others did not. Additionally, some of the dots had cores made of binary materials (two types of semiconductors), while others had ternary material cores (three types of semiconductors). All of the quantum dots had been manufactured by chemical synthesis.

The researchers found that ultrathin aluminum oxide could make quantum dots glow brighter and that the effect was much more significant for quantum dots without protective shells. They also found that while quantum dots with both binary and ternary cores shrink after reacting with the oxygen in air, ternary core dots placed on aluminum oxide glowed brighter despite the shrinkage. This observation surprised the researchers, Sadeghi said, and while they don’t yet have an explanation for the difference, they are continuing to study it.

“The results of these studies can serve to enhance emission efficiency of quantum dots, which is an important feature for many applications such as light emitting devices, sensors, detectors, photovoltaic devices, and the investigation of a wide range of quantum and nano-scale physical phenomena,” Sadeghi said. Quantum dots have already helped increase the efficiencies of many optical devices, he noted, and the further development and application of quantum dots’ unique properties, including in the fields of biological imaging and medicine, continues to be a prime focus of scientific study. As a next step in their own research, Sadeghi and his colleagues plan to investigate how metal oxides might affect the behavior of quantum dots when they are close to metallic nanoparticles.

Intematix Corporation, a manufacturer of phosphor solutions for LED lighting, has collaborated with SABIC’s Innovative Plastics business to create the ChromaLit Linear – a LED offering developed for the lighting industry.  By using Intematix’s remote phosphor technology and SABIC’s LEXAN LUX resins, lighting customers can now achieve the energy efficiency and reliability benefits of LEDs, while also experiencing increased optical efficiency and better light uniformity-a critical factor for commercial environments.

“SABIC is excited to have worked with Intematix to design a solution that successfully addresses a historic challenge with LED lighting used in commercial applications. In addition to being more efficient, the new LED system can be both extruded and injection molded,”  said Venugopal Koka, Director of Electrical Industrial and Lighting Marketing for SABIC’s Innovative Plastics business.

Remote phosphor is a lighting system technology that uses a phosphor component separated from the blue LED energy source. The independent phosphor emits light when excited by blue light. When the phosphor has been separated from the energy source it results in better lighting uniformity and consistency. Intematix selected SABIC’s LEXAN LUX transparent, diffusion and reflective grades for their ChromaLit Linear remote phosphor offering for their superior efficiency and flexible processing capabilities. In addition, the LEXAN LUX base material provides a UL94 flame rating of V0.

Linear light sources are in widespread use for illuminating commercial and industrial applications worldwide. Office lighting and other commercial applications have been challenging for white LEDs because of the need to diffuse the point sources, reducing system efficacy.  The ChromaLit Linear product delivers naturally uniform, high-quality light with conversion efficacy of up to 215 lumens per radiant watt or up to 163 lumens per system watt when used with the most efficient blue LEDs available.

“We expect a whole new set of valuable remote phosphor solutions to emerge as we draw upon SABIC’s world-class expertise in advanced thermoplastics,” said Mark Swoboda, Chief Executive Officer for Intematix. “Our experience has demonstrated that bringing our two companies’ innovations together results in ground-breaking products that accelerate market adoption of LED-based lighting systems.”

Following Intematix’s successful commercialization of the ChromaLit Linear, SABIC’s Innovative Plastics business and Intematix will continue to collaborate on new technology developments to help enable lighting OEMs to take advantage of expanded remote phosphor solutions for solid-state lighting (SSL). The continued collaboration will allow Intematix and SABIC to further combine their complementary expertise driving the development of new applications, technologies and materials which can hasten and optimize the commercialization of new LED products, platforms and lighting applications.

Cree, Inc. has announced that its C2M, 1200V, 80mOhm SiC MOSFETs have been selected by Sanix Corporation, Japan, to be designed into their new 9.9kW three-phase solar inverters for use in the construction of commercial photovoltaic systems in the fast-growing Japanese solar energy market.

“Through this partnership with Cree and their SiC technology, Sanix is able to capture more market share in the competitive Japan solar market,” said Hiroshi Soga, general manager, Sanix Incorporated. “Cree’s silicon carbide MOSFETs were critical for Sanix to meet our efficiency and thermal design targets. SiC switches reduced losses in our inverter electronics by more than 30 percent versus the silicon super-junction MOSFETs we were considering. In addition to providing a large efficiency gain, Cree’s latest generation C2M SiC MOSFETs were priced competitively, making it possible to replace lower voltage, less rugged, and less efficient silicon MOSFETs.”

Utilized in the primary power conversion stage of the solar inverter, Cree’s 1200V C2M0080120D MOSFETs feature faster switching characteristics and up to one-third the switching losses of comparably-rated 900V silicon super-junction MOSFETs. By significantly reducing switching losses, Cree’s SiC MOSFETs enable lower total system energy losses, higher frequency switching, and cooler operating temperatures. These benefits improve conversion efficiency and reduce the system’s size, weight, complexity, and thermal management requirements. At the system level, performance is improved, cost is decreased, and lifetime of the inverter is extended.

“Cree is extremely pleased that Sanix has chosen to specify our C2M, 1200V SiC MOSFET technology in its new 9.9kW solar inverters. Cree SiC power devices can provide significant advantages with regard to PV inverter efficiency, reliability, and cost, and will provide Sanix with a critical competitive advantage as they continue to expand their share of the Japanese solar market,” said Cengiz Balkas, general manager and vice president, Cree Power and RF.

Demonstrated to achieve up to three times the power density of typical silicon technology, Cree’s C2M family of SiC MOSFETs are available in 1200V and 1700V, ranging from 1Ω to 25 mΩ. C2M MOSFETs have been designed into a range of industrial power applications since their March 2013 market introduction and continue to experience increasing demand. Cree is currently delivering production volumes of SiC MOSFETs to Sanix and other PV inverter manufacturers, as well as to makers of industrial power supplies, auxiliary power converters, battery chargers, and motor drives.

Seoul Semiconductor announced the release of a new LED light engine with Acrich 3 technology on August 27, 2014. The new light engine is comprised of an LED module with Acrich MJT 5050 series LED, Acrich3 IC technology, and a heat sink and secondary optics. The Acrich light engine does not require a complex AC/DC converter and can be operated directly from the AC mains which simplifies designs, reduces component count and improves on the reliability of the luminaire. This new 30W Acrich light engine delivers a typical luminous flux of 3000 lumens at 120VAC operation at 5000K corresponding to a typical efficiency of 100 lumens per watt. When operated in a power compensation mode the Acrich3 technology can adapt to variations in the line-voltage as great as 20% and still deliver power-level regulation within 5% to ensure uniformity of the light output.

The new Acrich3 solution enables smart lighting control systems where the Acrich3 technology can interface through a wide variety of wireless networks such as IEEE 802.15.4, WiFi, and Bluetooth to control dimming, and further optimize on energy savings. This IP67 light engine is available in various color temperatures and beam patterns.

Seoul Semiconductor Executive Vice President of Lighting sales division, Jay Kim has stated that, “The payback period for streetlights can be significantly reduced with this new Acrich light engine. By eliminating AC/DC converters in streetlights, maintenance costs can be lowered and reliability can be improved without compromising on price, quality and energy savings to dramatically improve the lighting experience for customers. This product will enable the market to come up with the next generation of high quality, energy efficient and competitively priced LED lamps.”

Read more: Seoul Semiconductor moves in to the Top 5 global LED manufacturers

SemiLEDs Corporation, a global provider of vertical LED technology solutions, today announced sampling and volume availability of the first in its new Enhanced FlipChip, or EF, LED series.

The series launches with the EF-B40, a blue 40-mil flip chip that simplifies the packaging and integration process by eliminating wire-bonding while increasing both lumen-density and decreasing the lumen-per-dollar value proposition while enabling packagers to use standard surface mount assembly techniques.

“SemiLEDs’ unique flip chip approach combines a sapphire front surface and proprietary back side architecture that provides the electrical contacts exclusively on the bottom of the chip, making it fully compatible with chip-on-board (COB) surface mount processes,” Mark Tuttle, General Manager for SemiLEDs Optoelectronics Co., Ltd., explained. “Eliminating wire-bonds also lowers the profile of the chips, and allows them to be placed more closely together, which results in higher lumen-density and reduces the complexity of the optics. The EF series is an ideal platform for COB assemblies, or really for any approach that calls for either secondary optic design or high-density mounting.”

Flip chip construction presents what was originally the bottom sapphire layer in a horizontal LED structure as the top surface of the chip. By “flipping” the chip in this manner, the electrical pads become part of the bottom of the device rather than running bonding wires from the top surface of the chip down to the package or board. Not only are delicate areas of the chip protected by the clear sapphire layer, but by eliminating wire bonds, arguably the most failure-prone part of any LED assembly, both reliability and overall design flexibility of the packaged device are increased. In addition, individual chips may be more closely mounted, opening the door to more densely packed arrays of LED chips. The nearly continuous light emitting surface, unbroken by gaps, bonding wires, or top electrodes, can greatly simplify the mounting and mixing requirements of the optics, producing smooth lighting effects. In addition, in a flip chip structure, the heat-generating junction is positioned adjacent to the substrate, increasing thermal conductivity and allowing improved device performance at high currents.

The EF-B40 is available in wavelengths from 445 to 460nm, with outputs of up to 300 lumens at 1A as a packaged emitter. The SAC compatible chips are offered with standard Au bonding pads, or are available with an AuSn option to further reduce thermal resistance and add to system reliability. A 140-degree viewing angle makes the EF ideal for general and commercial lighting, while the lowered profile addresses the application needs of LED backlight, smartphone flash or LED projector.

SemiLEDs’ EF series of LED chips are RoHS compliant with production quantities available now.

Scientists have developed what they believe is the thinnest-possible semiconductor, a new class of nanoscale materials made in sheets only three atoms thick.

As seen under an optical microscope, the heterostructures have a triangular shape. The two different monolayer semiconductors can be recognized through their different colors.

The University of Washington researchers have demonstrated that two of these single-layer semiconductor materials can be connected in an atomically seamless fashion known as a heterojunction. This result could be the basis for next-generation flexible and transparent computing, better light-emitting diodes, or LEDs, and solar technologies.

As seen under an optical microscope, the heterostructures have a triangular shape. The two different monolayer semiconductors can be recognized through their different colors. Photo credit: U of Washington

As seen under an optical microscope, the heterostructures have a triangular shape. The two different monolayer semiconductors can be recognized through their different colors. Photo credit: U of Washington

“Heterojunctions are fundamental elements of electronic and photonic devices,” said senior author Xiaodong Xu, a UW assistant professor of materials science and engineering and of physics. “Our experimental demonstration of such junctions between two-dimensional materials should enable new kinds of transistors, LEDs, nanolasers, and solar cells to be developed for highly integrated electronic and optical circuits within a single atomic plane.”

The research was published online this week in Nature Materials.

The researchers discovered that two flat semiconductor materials can be connected edge-to-edge with crystalline perfection. They worked with two single-layer, or monolayer, materials – molybdenum diselenide and tungsten diselenide – that have very similar structures, which was key to creating the composite two-dimensional semiconductor.

Collaborators from the electron microscopy center at the University of Warwick in England found that all the atoms in both materials formed a single honeycomb lattice structure, without any distortions or discontinuities. This provides the strongest possible link between two single-layer materials, necessary for flexible devices. Within the same family of materials it is feasible that researchers could bond other pairs together in the same way.

A high-resolution scanning transmission electron microscopy (STEM) image shows the lattice structure of the heterojunctions in atomic precision. Photo credit: U of Warwick

A high-resolution scanning transmission electron microscopy (STEM) image shows the lattice structure of the heterojunctions in atomic precision. Photo credit: U of Warwick

The researchers created the junctions in a small furnace at the UW. First, they inserted a powder mixture of the two materials into a chamber heated to 900 degrees Celsius (1,652 F). Hydrogen gas was then passed through the chamber and the evaporated atoms from one of the materials were carried toward a cooler region of the tube and deposited as single-layer crystals in the shape of triangles.

After a while, evaporated atoms from the second material then attached to the edges of the triangle to create a seamless semiconducting heterojunction.

“This is a scalable technique,” said Sanfeng Wu, a UW doctoral student in physics and one of the lead authors. “Because the materials have different properties, they evaporate and separate at different times automatically. The second material forms around the first triangle that just previously formed. That’s why these lattices are so beautifully connected.”

With a larger furnace, it would be possible to mass-produce sheets of these semiconductor heterostructures, the researchers said. On a small scale, it takes about five minutes to grow the crystals, with up to two hours of heating and cooling time.

“We are very excited about the new science and engineering opportunities provided by these novel structures,” said senior author David Cobden, a UW professor of physics. “In the future, combinations of two-dimensional materials may be integrated together in this way to form all kinds of interesting electronic structures such as in-plane quantum wells and quantum wires, superlattices, fully functioning transistors, and even complete electronic circuits.”

This photoluminescence intensity map shows a typical piece of the lateral heterostructures. The junction region produces an enhanced light emission, indicating its application potential in optoelectronics. Photo credit: U of Washington

This photoluminescence intensity map shows a typical piece of the lateral heterostructures. The junction region produces an enhanced light emission, indicating its application potential in optoelectronics. Photo credit: U of Washington

The researchers have already demonstrated that the junction interacts with light much more strongly than the rest of the monolayer, which is encouraging for optoelectric and photonic applications like solar cells.

Other co-authors are Chunming Huang and Pasqual Rivera of UW physics; Ana Sanchez, Richard Beanland and Jonathan Peters at the University of Warwick; Jason Ross of UW materials science and engineering; and Wang Yao, a theoretical physicist of the University of Hong Kong.

This research was funded by the U.S. Department of Energy, the UW’s Clean Energy Institute, the Research Grant Council of Hong Kong, the University Grants Committee of Hong Kong, the Croucher Foundation, the Science City Research Alliance and the Higher Education Funding Council for England’s Strategic Development Fund.