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Oxygen vs. nanochip


September 25, 2018

For the first time ever, an international team of scientists from NUST MISIS, the Hungarian Academy of Sciences, the University of Namur (Belgium), and Korea Research Institute for Standards & Science has managed to trace in details the structural changes of two-dimensional molybdenum disulfide under long-term environmental impact. The new data narrows the scope of its potential application in microelectronics and at the same time opens up new prospects for the use of two-dimensional materials as catalysts. The research results have been published in the international scientific journal Nature Chemistry.

Pavel Sorokin, head of the research team and leading researcher at the NUST MISIS Laboratory of Inorganic Nanomaterials. Credit: Sergey Gnuskov/NUST MISIS

Molybdenum disulfide (MoS2) is considered a promising basis for a variety of ultra-small electronic devices such as high-frequency detectors, rectifiers, and transistors, so research teams around the world are actively studying its two-dimensional format, nanofilm. However, the new research conducted by NUST MISIS scientists has demonstrated that when this two-dimensional material is significantly oxidized in air, it turns into another connection.

Any electronic device using MoS2, without proper protection would simply stop working relatively quickly. To potentially use MoS2 in microelectronics, the devices would have to be encapsulated.

«For the first time ever, we have managed to experimentally prove that a single-layer molybdenum disulfide strongly degrades under environmental conditions, oxidizing and turning into a solid solution MoS2-xOx,. The functions of a two-dimensional semiconductor without defects and losses can be implemented with molybdenum diselenides, another material with a similar structure», said Pavel Sorokin, head of the research team and leading researcher at the NUST MISIS Laboratory of Inorganic Nanomaterials.

In the experiments, two-dimensional layers of molybdenum disulfide obtained as a result of the stratification of molybdenum disulfide crystals by ultrasound, were maintained in environmental conditions at normal room temperature and lighting for long periods (more than a year and a half), during which scientists observed the changes in the structure of its surface.

«Thanks to the use of tunneling microscopy, we were able to track the structural changes of crystals of two-dimensional sulfur disulfide at the atomic level during long-term exposure to environmental conditions. We have discovered that the material previously considered stable is actually subject to spontaneous oxidation, but at the same time, the original crystal structure of MoS2 monolayers retains formations of MoS2-xOx solid solutions. Our simulations have allowed us to propose a mechanism of forming such solid solutions, and the results of the theoretical calculations are in complete agreement with our experimental measurements» – said Zakhar Popov, one of the co-authors of the study and a senior researcher at the NUST MISIS Laboratory of Inorganic Nanomaterials.

«The study’s second key discovery is the new material that the monolayer of the molybdenum disulfide turns into is a two-dimensional crystal of a solid solution MoS2-xOx, which is an effective catalyst for electromechanical processes», concluded Sorokin.

The Semiconductor Industry Association (SIA), in collaboration with the Semiconductor Research Corporation (SRC), today announced the winners of its 2018 University Research Awards: Dr. Judy Hoyt, professor of electrical engineering and computer science at the Massachusetts Institute of Technology (MIT), and Dr. Naresh Shanbhag, professor of electrical and computer engineering at the University of Illinois at Urbana-Champaign. Professors Hoyt and Shanbhag will receive the awards in conjunction with the SIA Annual Award Dinner on Nov. 29, 2018 in San Jose, Calif.

“Research is the lifeblood of innovation, spurring new technologies that drive growth in the semiconductor industry and throughout the U.S. economy,” said John Neuffer, president and CEO of SIA, which represents U.S. leadership in semiconductor manufacturing, design, and research. “Throughout their distinguished careers, Professors Hoyt and Shanbhag have advanced groundbreaking scientific research, driven breakthroughs in semiconductor technology, and helped strengthen America’s global technological leadership. We are pleased to recognize Dr. Hoyt and Dr. Shanbhag for their tremendous accomplishments.”

Neuffer also highlighted the importance of government investments in semiconductor research funded through agencies such as the National Science Foundation, the National Institute of Standards and Technology, the U.S. Department of Energy, and the Defense Department’s Defense Advanced Research Projects Agency. He expressed SIA’s readiness to work with the Trump administration and Congress to prioritize these investments in scientific research.

“The University Research Award was established to recognize lifetime achievements in semiconductor research by university faculty,” said Ken Hansen, president & CEO of SRC. “Drs. Shanbhag and Hoyt have repeatedly advanced the state-of-the-art semiconductor design and technology in their respective fields. These esteemed professors’ influence on their students has produced new leaders and contributors in the semiconductor industry. The research output from universities tackling industry relevant challenges plays an integral role in next-generation innovations. It is with great appreciation and admiration that the entire SRC team congratulates Dr. Shanbhag and Dr. Hoyt.”

Dr. Hoyt will receive the honor for excellence in semiconductor technology research. She is being recognized for her contributions in pioneering development of strained Si MOSFET devices. Dr. Hoyt’s work helped to break the 10nm barrier and is broadly adopted by companies such as Intel, TSMC, IBM, and others. From 1988-1999, Dr. Hoyt was a senior research scientist in electrical engineering at Stanford University. In January 2000, she joined the faculty at MIT in the Department of Electrical Engineering and Computer Science. She currently serves as associate director within the Microsystems Technology Laboratories (MTL). Dr. Hoyt received a Ph.D. in Applied Physics from Stanford University.

Dr. Shanbhag will receive the award for excellence in semiconductor design research. Specifically, he is being honored for pioneering an Information-Theoretic approach for computing by fusing Claude Shannon’s theory for communications with Turing machines. After designing DSL chip-sets at AT&T Bell Laboratories (1993-1995), he joined the faculty at the University of Illinois at Urbana-Champaign in the Department of Electrical and Computer Engineering where he now holds the Jack S. Kilby Professorship. He co-founded Intersymbol Communications, Inc., and served as CTO (2000-2007), bringing electronic dispersion compensation chip-sets for OC-192 ultra long-haul fiber optic links. In January 2013, Dr. Shanbhag became the founding director of the Systems On Nanoscale Information fabriCs (SONIC) Center, a five-year, multi-university center funded by DARPA and SRC. Dr. Shanbhag received a Ph.D. from the University of Minnesota in Electrical Engineering.

The ability of metallic or semiconducting materials to absorb, reflect and act upon light is of primary importance to scientists developing optoelectronics – electronic devices that interact with light to perform tasks. Rice University scientists have now produced a method to determine the properties of atom-thin materials that promise to refine the modulation and manipulation of light.

Rice University researchers modeled two-dimensional materials to quantify how they react to light. They calculated how the atom-thick materials in single or stacked layers would transmit, absorb and reflect light. The graphs above measure the maximum absorbance of several of the 55 materials tested. Credit: Yakobson Research Group/Rice University

Two-dimensional materials have been a hot research topic since graphene, a flat lattice of carbon atoms, was identified in 2001. Since then, scientists have raced to develop, either in theory or in the lab, novel 2D materials with a range of optical, electronic and physical properties.

Until now, they have lacked a comprehensive guide to the optical properties those materials offer as ultrathin reflectors, transmitters or absorbers.

The Rice lab of materials theorist Boris Yakobson took up the challenge. Yakobson and his co-authors, graduate student and lead author Sunny Gupta, postdoctoral researcher Sharmila Shirodkar and research scientist Alex Kutana, used state-of-the-art theoretical methods to compute the maximum optical properties of 55 2D materials.

“The important thing now that we understand the protocol is that we can use it to analyze any 2D material,” Gupta said. “This is a big computational effort, but now it’s possible to evaluate any material at a deeper quantitative level.”

Their work, which appears this month in the American Chemical Society journal ACS Nano, details the monolayers’ transmittance, absorbance and reflectance, properties they collectively dubbed TAR. At the nanoscale, light can interact with materials in unique ways, prompting electron-photon interactions or triggering plasmons that absorb light at one frequency and emit it in another.

Manipulating 2D materials lets researchers design ever smaller devices like sensors or light-driven circuits. But first it helps to know how sensitive a material is to a particular wavelength of light, from infrared to visible colors to ultraviolet.

“Generally, the common wisdom is that 2D materials are so thin that they should appear to be essentially transparent, with negligible reflection and absorption,” Yakobson said. “Surprisingly, we found that each material has an expressive optical signature, with a large portion of light of a particular color (wavelength) being absorbed or reflected.”

The co-authors anticipate photodetecting and modulating devices and polarizing filters are possible applications for 2D materials that have directionally dependent optical properties. “Multilayer coatings could provide good protection from radiation or light, like from lasers,” Shirodkar said. “In the latter case, heterostructured (multilayered) films — coatings of complementary materials — may be needed. Greater intensities of light could produce nonlinear effects, and accounting for those will certainly require further research.”

The researchers modeled 2D stacks as well as single layers. “Stacks can broaden the spectral range or bring about new functionality, like polarizers,” Kutana said. “We can think about using stacked heterostructure patterns to store information or even for cryptography.”

Among their results, the researchers verified that stacks of graphene and borophene are highly reflective of mid-infrared light. Their most striking discovery was that a material made of more than 100 single-atom layers of boron — which would still be only about 40 nanometers thick — would reflect more than 99 percent of light from the infrared to ultraviolet, outperforming doped graphene and bulk silver.

There’s a side benefit that fits with Yakobson’s artistic sensibility as well. “Now that we know the optical properties of all these materials – the colors they reflect and transmit when hit with light – we can think about making Tiffany-style stained-glass windows on the nanoscale,” he said. “That would be fantastic!”

Developing materials suitable for use in optoelectronic devices is currently a very active area of research. The search for materials for use in photoelectric conversion elements has to be carried out in parallel with developing the optimal film formation process for each material, and this can take a few years for just one material. Until now there has been a trade-off, balancing electronic properties and material morphology. Researchers at Osaka University have developed a two-step process that can produce materials with good morphological properties in addition to excellent photoresistor performance. Their findings were published in the Journal of Physical Chemistry Letters.

The powder sample is insoluble, therefore fabrication of devices using wet processes is not possible. Credit: Osaka University

Bismuth sulfide, Bi2S3, belongs to a class of materials known as metal chalcogenides, which show significant promise owing to their optical and electronic properties. However, the performance of Bi2S3-based photoresponsive devices is dependent on the method used to process the film, and many of the reported approaches are hampered by low film crystallinity. Even when high crystallinity is achieved, the nature of the grains can have a negative effect on performance, therefore films with low surface roughness and large grain size are desirable.

“We searched more than 200 materials using a unique, ultra high-speed screening method that can evaluate performance, even when only powdered samples are available,” study corresponding author Akinori Saeki says. “We found that bismuth sulfide, which is inexpensive and less toxic than conventional inorganic solar cell materials, can be processed in a way that does not compromise its excellent photoelectrical properties.”

The technique used produces a 2D layered film in two treatment steps; solution spin-coating followed by crystallization. The photo response performance of the resulting film showed improvements of 6-100 times compared with those of films prepared using other processing methods. Owing to the non-toxic and abundant nature of bismuth and sulfur, the findings are expected to influence the development of commercial optoelectronic devices including solar cells.

“We demonstrated a facile processing technique that does not compromise material performance,” lead author Ryosuke Nishikubo says. “We believe that solution-processable bismuth-based semiconductors are viable alternatives to commercially available inorganic solar cells and show promise for widespread future use. The fact that they are non-toxic also sets them apart from other alternative optoelectronic materials, such as lead halide perovskites.”

Processing materials for device applications without compromising their electronic properties is important for making materials commercially relevant. The reported process has been used to successfully prepare other metal sulfide semiconductors such as lead sulfide, demonstrating the versatility of the approach.

When 80 microns is enough


September 17, 2018

Should you care that scientists can control a baffling current? Their research results could someday affect your daily living.

Physicists have managed to send and control a spin current across longer distances than ever before – and in a material that was previously considered unsuitable for the task.

We’ll return to what that strange sentence really means. But a spin current is a current that is kept going without relying on a simultaneous current of electrical charges.

“We’ve transferred spins more than 80 microns in an antiferromagnet,” says Arne Brataas, a professor at the Norwegian University of Science and Technology’s (NTNU) Department of Physics, and head of the university’s recently launched Center for Quantum Spintronics (QuSpin).

Spin current is initiated with an electric field at one end of the material, an antiferromagnet. The spin in the antiferromagnet alternates direction (yellow and blue arrows). The signal spreads as a wave (green arrows) through the antiferromagnet. At the other end of the material, the spin current is transferred to an electric current again. Credit: Illustration: Kolbjørn Skarpnes/NTNU

Eighty microns – a mere 8/100 000th of a metre – is that so impressive?

“We’re not exactly sending signals to the other side of the city. But this is far in the world of nanoelectronics,” says Brataas.

Nanoelectronics forms the basis for all the smart technology we surround ourselves with.

Right about now you can start doing your happy dance. That’s because 80 microns is getting to be a great enough distance to matter to people besides the scientists who are interested in knowledge for its own sake.

QuSpin has been collaborating with international physicists, including several in Germany and the Netherlands. The results are so intriguing that they are being published in the latest issue of the journal Nature.

So what is spintronics?

The technology of the future may depend on spintronics. If you don’t know what it is, you might as well learn about it. But you can also jump ahead to the next section if you just want to learn about its practical uses.

Atoms have several parts. Electrons are the negatively charged particles, as many of us learned in science class.

But electrons don’t only have a charge, they also have spin, an apparent internal rotation.

The spin has a direction, which is the basis of magnetism. A ferromagnetic material has a preference to align the spins in one particular direction. These materials are the magnets that you put on the refrigerator door.

Antiferromagnetic materials are also magnetic, but you don’t notice their magnetic quality. The atoms in the material alternate between spins in opposite directions. These alterations effectively zero out the total spin so that the material itself does not have a magnetic moment. These materials don’t work on your refrigerator door.

How spins are organized can therefore have very clear consequences for how a material behaves. The spin can be exploited.

Magnetism can transfer signals

Brataas and his colleagues aren’t currently focusing so much on practical uses, so that’s up to us to do.

Today’s technology transmits signals in a computer’s microchips by means of an electric charge. In an electrical current, electrons and spin both flow through the material.

But in the future a spin current will be able do parts of the same job, using magnetism to transmit the signals instead, without electrons passing through with the current.

What are the benefits of spin currents?

Well, for one, a spin current can sometimes flow more readily than an electrical charge current, since only the spin moves and not the electrons. This results in less energy loss in transmitting the signal.

A spin current does not generate a lot of heat. As the transistors on microchips have become ever smaller, overheating has become a growing problem, causing microchips to melt. Using spin current means smaller transistors can be used -another practical feature as new electronic gizmos pop up everywhere.

Spin current can also be controlled much more quickly. So all the new gizmos can be a lot faster.

Controlling spin current

These results would not be nearly as exciting if physicists couldn’t control the spin current at the same time. But they can.

Physicists can start the spin current by applying an electric field at one end of the material. The signal flows through the material without the electrons moving from one end to the other.

Physicists can also do the opposite at the other end, and transfer the spin current to an electrical current.

They have managed to do this at temperatures approaching room temperature. Granted, 200 degrees Kelvin – or -73.15 degrees Celsius – is a bit chilly for a room, but it falls within the range of naturally occurring temperatures on Earth. The researchers expect that they will be able do this experiment at a more comfortable room temperature pretty soon as well.

The research group used the antiferromagnet hematite, an iron oxide (Fe2O3), in their experiments.

The results this time are clearly just a step along the way. The research team will continue to test other materials and look at how these materials respond to different types of influences.

High risk, but important

NTNU’s QuSpin was awarded Norwegian Centre of Excellence (SFF) status last year, a highly regarded recognition. The centre was created to combine theory with experimental physics in the spintronics field and can already show world-leading results.

“The centre focuses on high risk projects of major importance in many different directions,” says Brataas.

The status that SFF confers provides more stable research funding, since QuSpin is guaranteed support for ten years. The funding facilitates high-risk research that can fail too. And they do, all the time.

Many of their experiments do not match the theories or vice versa, and that is important in its own way. But some experiments are spot on, and they can have particularly great significance.

SUNY Polytechnic Institute (SUNY Poly) announced today that Professor of Nanobioscience Dr. Nate Cady has been awarded $500,000 in funding from the National Science Foundation to develop advanced computing systems based on a novel approach to the creation of non-volatile memory architecture. This research, which will also support student opportunities, aims to advance today’s typical computing model, in which processing and memory are separate, by bringing them together to make the entire process faster and more energy efficient.

“I am proud to congratulate Professor Cady on this National Science Foundation (NSF) award which is focused on enabling advanced computing capabilities, and notably, has important implications for advances in artificial intelligence,” said SUNY Poly Interim President Dr. Grace Wang. “The NSF’s selection of Dr. Cady’s research for this funding exemplifies the quality and impact of SUNY Poly’s research where our faculty and students leverage our world-class high-tech resources, explore new frontiers, and develop critical technologies for our society.”

The research will enable the design of a scalable computing infrastructure that uses nanoscale non-volatile memory (NVM) devices for both storage and computation. One of the current limits to computing speed is the result of current personal computing architecture, which separates the processor and memory and leads to a cap on data throughput, known as the “von Neumann bottleneck.” By combining storage and computation on the same device, the project circumvents this barrier and creates scalable solutions for extreme-scale computing—computing that is up to one thousand times more capable than current comparable computing—based on wires that cross each other to form memory cells at every intersection. This more powerful capability is made possible because each memory cell, acting like a synapse of the human brain, can be switched on or off, similar to the 1’s and 0’s of current computing, but it can also store many other values between the on or off states, increasing the amount of information that a given memory cell can store exponentially.

“This grant showcases the incredible potential of our faculty to tackle real-world problems with high-tech solutions that stem from the SUNY Poly’s advanced labs, cleanrooms, and capabilities. This news is especially exciting for a number of our graduate students who will be able to focus on this promising research area where they will be at the cutting-edge,” said SUNY Poly Interim Provost Dr. Steven Schneider.

“Dr. Cady’s research is a powerful example of the kind of expertise that SUNY Poly’s faculty possess as our innovation-centered ecosystem provides us with unique opportunities to move the technologies of the future forward,” said SUNY Poly Interim Dean of the College of Nanoscale Sciences; Empire Innovation Professor of Nanoscale Science; and Executive Director, Center for Nanoscale Metrology Dr. Alain Diebold.

“I look forward to advancing this non-volatile memory research at SUNY Poly, using the institution’s cutting-edge fabrication facilities in order to address current computing bottlenecks that slow computing capability and waste energy,” said Dr. Cady. “This grant will drive the development of computing and memory infrastructure that will be evaluated using high-performance simulations and experimental benchmarking within our state-of-the-art laboratory at SUNY Poly where we are eager to develop the architecture that can help revolutionize processing and memory capabilities for next-gen computers.”

Dr. Cady’s research will support SUNY Poly graduate students who will be able to obtain hands-on experience developing the computing/memory structures. The materials for this project will be developed, demonstrated, and then integrated with traditional complementary metal-oxide-semiconductor (CMOS) computer chips as part of a larger production, which will utilize SUNY Poly’s 200mm and 300mm state-of-the-art fabrication facilities. The University of Central Florida is receiving its own funds for collaborative research related to this effort.

Computing using multiple parallel flows of current through data stored in nanoscale “crossbars” is often fast and more energy-efficient, but the design of such crossbars is highly unintuitive for human designers. More specifically, this project explores formal methods for more efficiently conducting Boolean searches and using artificial intelligence techniques such as best-first search, in addition to automatically synthesizing non-volatile memory crossbar designs from specifications written in a high-level programming language.

A new technique makes it possible to obtain an individual fingerprint of the current-carrying edge states occurring in novel materials such as topological insulators or 2D materials. Physicists of the University of Basel present the new method together with American scientists in Nature Communications.

Measured tunneling current and its dependence on the two applied magnetic fields: The fans of red/yellow curves each correspond to a fingerprint of the conducting edge states. Each individual curve separately shows one of the edge states. Credit: University of Basel, Department of Physics

While insulators do not conduct electrical currents, some special materials exhibit peculiar electrical properties: though not conducting through their bulk, their surfaces and edges may support electrical currents due to quantum mechanical effects, and do so even without causing losses.

Such so-called topological insulators have attracted great interest in recent years due to their remarkable properties. In particular, their robust edge states are very promising since they could lead to great technological advances.

Currents flowing only along the edges

Similar effects as the edge states of such topological insulators also appear when a two-dimensional metal is exposed to a strong magnetic field at low temperatures. When the so-called quantum Hall effect is realized, current is thought to flow only at the edges, where several conducting channels are formed.

Probing individual edge states

Until now, it was not possible to address the numerous current carrying states individually or to determine their positions separately. The new technique now makes it possible to obtain an exact fingerprint of the current carrying edge states with nanometer resolution.

This is reported by researchers of the Department of Physics and the Swiss Nanoscience Institute of the University of Basel in collaboration with colleagues of the University of California, Los Angeles, as well as of Harvard and Princeton University, USA.

In order to measure the fingerprint of the conducting edge states, the physicists lead by Prof. Dominik Zumbühl have further developed a technique based on tunneling spectroscopy.

They have used a gallium arsenide nanowire located at the sample edge which runs in parallel to the edge states under investigation. In this configuration, electrons may jump (tunnel) back and forth between a specific edge state and the nanowire as long as the energies in both systems coincide. Using an additional magnetic field, the scientists control the momentum of tunneling electrons and can address individual edge states. From the measured tunneling currents, the position and evolution of each edge state may be obtained with nanometer precision.

Tracking the evolution

This new technique is very versatile and can also be used to study dynamically evolving systems. Upon increasing the magnetic field, the number of edge states is reduced, and their distribution is modified. For the first time, the scientists were able to watch the full edge state evolution starting from their formation at very low magnetic fields.

With increasing magnetic field, the edge states are first compressed towards the sample boundary until eventually, they move towards the inside of the sample and then disappear completely. Analytical and numerical models developed by the research team agree very well with the experimental data.

“This new technique is not only very useful to study the quantum Hall edge states,” Dominik Zumbühl comments the results of the international collaboration. “It might also be employed to investigate new exotic materials such as topological insulators, graphene or other 2D materials.”

Sandwiching two-dimensional materials used in nanoelectronic devices between their three-dimensional silicon bases and an ultrathin layer of aluminum oxide can significantly reduce the risk of component failure due to overheating, according to a new study published in the journal of Advanced Materials led by researchers at the University of Illinois at Chicago College of Engineering.

An experimental transistor using silicon oxide for the base, carbide for the 2D material and aluminum oxide for the encapsulating material. Credit: (Image: Zahra Hemmat).

Many of today’s silicon-based electronic components contain 2D materials such as graphene. Incorporating 2D materials like graphene — which is composed of a single-atom-thick layer of carbon atoms — into these components allows them to be several orders of magnitude smaller than if they were made with conventional, 3D materials. In addition, 2D materials also enable other unique functionalities. But nanoelectronic components with 2D materials have an Achilles’ heel — they are prone to overheating. This is because of poor heat conductance from 2D materials to the silicon base.

“In the field of nanoelectronics, the poor heat dissipation of 2D materials has been a bottleneck to fully realizing their potential in enabling the manufacture of ever-smaller electronics while maintaining functionality,” said Amin Salehi-Khojin, associate professor of mechanical and industrial engineering in UIC’s College of Engineering.

One of the reasons 2D materials can’t efficiently transfer heat to silicon is that the interactions between the 2D materials and silicon in components like transistors are rather weak.

“Bonds between the 2D materials and the silicon substrate are not very strong, so when heat builds up in the 2D material, it creates hot spots causing overheat and device failure,” explained Zahra Hemmat, a graduate student in the UIC College of Engineering and co-first author of the paper.

In order to enhance the connection between the 2D material and the silicon base to improve heat conductance away from the 2D material into the silicon, engineers have experimented with adding an additional ultra-thin layer of material on top of the 2D layer — in effect creating a “nano-sandwich” with the silicon base and ultrathin material as the “bread.”

“By adding another ‘encapsulating’ layer on top of the 2D material, we have been able to double the energy transfer between the 2D material and the silicon base,” Salehi-Khojin said.

Salehi-Khojin and his colleagues created an experimental transistor using silicon oxide for the base, carbide for the 2D material and aluminum oxide for the encapsulating material. At room temperature, the researchers saw that the conductance of heat from the carbide to the silicon base was twice as high with the addition of the aluminum oxide layer versus without it.

“While our transistor is an experimental model, it proves that by adding an additional, encapsulating layer to these 2D nanoelectronics, we can significantly increase heat transfer to the silicon base, which will go a long way towards preserving functionality of these components by reducing the likelihood that they burn out,” said Salehi-Khojin. “Our next steps will include testing out different encapsulating layers to see if we can further improve heat transfer.”

Scientists have developed a photoelectrode that can harvest 85 percent of visible light in a 30 nanometers-thin semiconductor layer between gold layers, converting light energy 11 times more efficiently than previous methods.

In the pursuit of realizing a sustainable society, there is an ever-increasing demand to develop revolutionary solar cells or artificial photosynthesis systems that utilize visible light energy from the sun while using as few materials as possible.

The research team, led by Professor Hiroaki Misawa of the Research Institute for Electronic Science at Hokkaido University, has been aiming to develop a photoelectrode that can harvest visible light across a wide spectral range by using gold nanoparticles loaded on a semiconductor. But merely applying a layer of gold nanoparticles did not lead to a sufficient amount of light absorption, because they took in light with only a narrow spectral range.

Left: The newly developed photoelectrode, a sandwich of semiconductor layer (TiO2) between gold film (Au film) and gold nanoparticles (Au NPs). The gold nanoparticles were partially inlaid onto the surface of the titanium dioxide thin-film to enhance light absorption. Right: The photoelectrode (Au-NP/TiO2/Au-film) with 7nm of inlaid depth traps light making it nontransparent (top). An Au-NP/TiO2 structure without the Au film are shown for comparison (bottom). Credit: Misawa H. et al., Nature Nanotechnology, July 30, 2018

In the study published in Nature Nanotechnology, the research team sandwiched a semiconductor, a 30-nanometer titanium dioxide thin-film, between a 100-nanometer gold film and gold nanoparticles to enhance light absorption. When the system is irradiated by light from the gold nanoparticle side, the gold film worked as a mirror, trapping the light in a cavity between two gold layers and helping the nanoparticles absorb more light.

To their surprise, more than 85 percent of all visible light was harvested by the photoelectrode, which was far more efficient than previous methods. Gold nanoparticles are known to exhibit a phenomenon called localized plasmon resonance which absorbs a certain wavelength of light. “Our photoelectrode successfully created a new condition in which plasmon and visible light trapped in the titanium oxide layer strongly interact, allowing light with a broad range of wavelengths to be absorbed by gold nanoparticles,” says Hiroaki Misawa.

When gold nanoparticles absorb light, the additional energy triggers electron excitation in the gold, which transfers electrons to the semiconductor. “The light energy conversion efficiency is 11 times higher than those without light-trapping functions,” Misawa explained. The boosted efficiency also led to an enhanced water splitting: the electrons reduced hydrogen ions to hydrogen, while the remaining electron holes oxidized water to produce oxygen — a promising process to yield clean energy.

“Using very small amounts of material, this photoelectrode enables an efficient conversion of sunlight into renewable energy, further contributing to the realization of a sustainable society,” the researchers concluded.

Schottky diode is composed of a metal in contact with a semiconductor. Despite its simple construction, Schottky diode is a tremendously useful component and is omnipresent in modern electronics. Schottky diode fabricated using two-dimensional (2D) materials have attracted major research spotlight in recent years due to their great promises in practical applications such as transistors, rectifiers, radio frequency generators, logic gates, solar cells, chemical sensors, photodetectors, flexible electronics and so on.

The understanding of 2D material-based Schottky diode is, however, plagued by multiple mysteries. Several theoretical models have co-existed in the literatures and a model is often selected a priori without rigorous justifications. It is not uncommon to see a model, whose underlying physics fundamentally contradicts with the physical properties of 2D materials, being deployed to analyse a 2D material Schottky diode.

Reporting in Physical Review Letters, researchers from the Singapore University of Technology and Design (SUTD) have made a major step forward in resolving the mysteries surrounding 2D material Schottky diode. By employing a rigorous theoretical analysis, they developed a new theory to describe different variants of 2D-material-based Schottky diodes under a unifying framework. The new theory lays down a foundation that helps to unite prior contrasting models, thus resolving a major confusion in 2D material electronics.

Schematic drawing of a 2D-material-based lateral (left) and vertical (right) Schottky diode. For broad classes of 2D materials, the current-temperature relation can be universally described by a scaling exponent of 3/2 and 1, respectively, for lateral and vertical Schottky diodes. Credit: Singapore University of Technology and Design

“A particularly remarkable finding is that the electrical current flowing across a 2D material Schottky diode follows a one-size-fits-all universal scaling law for many types of 2D materials,” said first-author Dr. Yee Sin Ang from SUTD.

Universal scaling law is highly valuable in physics since it provides a practical “Swiss knife” for uncovering the inner workings of a physical system. Universal scaling law has appeared in many branches of physics, such as semiconductor, superconductor, fluid dynamics, mechanical fractures, and even in complex systems such as animal life span, election results, transportation and city growth.

The universal scaling law discovered by SUTD researchers dictates how electrical current varies with temperature and is widely applicable to broad classes of 2D systems including semiconductor quantum well, graphene, silicene, germanene, stanene, transition metal dichalcogenides and the thin-films of topological solids.

“The simple mathematical form of the scaling law is particularly useful for applied scientists and engineers in developing novel 2D material electronics,” said co-author Prof. Hui Ying Yang from SUTD.

The scaling laws discovered by SUTD researchers provide a simple tool for the extraction of Schottky barrier height – a physical quantity critically important for performance optimisation of 2D material electronics.

“The new theory has far reaching impact in solid state physics,” said co-author and principal investigator of this research, Prof. Lay Kee Ang from SUTD, “It signals the breakdown of classic diode equation widely used for traditional materials over the past 60 years, and shall improve our understanding on how to design better 2D material electronics.”