Category Archives: Process Materials

China IC industry outlook


October 17, 2017

SEMI, the global industry association and provider of independent electronics market research, today announced its new China IC Industry Outlook Report, a comprehensive report for the electronics manufacturing supply chain. With an increasing presence in the global semiconductor manufacturing supply chain, the market opportunities in China are expanding dramatically.

China is the largest consumer of semiconductors in the world, but it currently relies mainly on semiconductor imports to drive its growth. Policies and investment funds are now in place to further advance the progress of indigenous suppliers in China throughout the entire semiconductor supply chain. This shift in policy and related initiatives have created widespread interest in the challenges and opportunities in China.

With at least 15 new fab projects underway or announced in China since 2017, spending on semiconductor fab equipment is forecast to surge to more than $12 billion, annually, by 2018. As a result, China is projected to be the top spending region in fab equipment by 2019, and is likely to approach record all-time levels for annual spending for a single region.

Figure 1

Figure 1

This report covers the full spectrum of the China IC industry within the context of the global semiconductor industry. With more than 60 charts, data tables, and industry maps from SEMI sources, the report reveals the history and the latest industry developments in China across vast geographical areas ranging from coastline cities to the less developed though emerging mid-western regions.

The China IC industry ecosystem outlook covers central and local government policies, public and private funding, the industry value chain from design to manufacturing and equipment to materials suppliers. Key players in each industry sector are highlighted and discussed, along with insights into China domestic companies with respect to their international peers, and potential supply implications from local equipment and material suppliers. The report specifically details semiconductor fab investment in China, as well as the supply chain for domestic equipment and material suppliers.

Figure 2

Figure 2

A new method that precisely measures the mysterious behavior and magnetic properties of electrons flowing across the surface of quantum materials could open a path to next-generation electronics.

Found at the heart of electronic devices, silicon-based semiconductors rely on the controlled electrical current responsible for powering electronics. These semiconductors can only access the electrons’ charge for energy, but electrons do more than carry a charge. They also have intrinsic angular momentum known as spin, which is a feature of quantum materials that, while elusive, can be manipulated to enhance electronic devices.

A team of scientists, led by An-Ping Li at the Department of Energy’s Oak Ridge National Laboratory, has developed an innovative microscopy technique to detect the spin of electrons in topological insulators, a new kind of quantum material that could be used in applications such as spintronics and quantum computing.

A new microscopy method developed by an ORNL-led team has four movable probing tips, is sensitive to the spin of moving electrons and produces high-resolution results. Using this approach, they observed the spin behavior of electrons on the surface of a quantum material. Credit: Saban Hus and An-Ping Li/Oak Ridge National Laboratory, U.S. Dept. of Energy

A new microscopy method developed by an ORNL-led team has four movable probing tips, is sensitive to the spin of moving electrons and produces high-resolution results. Using this approach, they observed the spin behavior of electrons on the surface of a quantum material. Credit: Saban Hus and An-Ping Li/Oak Ridge National Laboratory, U.S. Dept. of Energy

“The spin current, namely the total angular momentum of moving electrons, is a behavior in topological insulators that could not be accounted for until a spin-sensitive method was developed,” Li said.

Electronic devices continue to evolve rapidly and require more power packed into smaller components. This prompts the need for less costly, energy-efficient alternatives to charge-based electronics. A topological insulator carries electrical current along its surface, while deeper within the bulk material, it acts as an insulator. Electrons flowing across the material’s surface exhibit uniform spin directions, unlike in a semiconductor where electrons spin in varying directions.

“Charge-based devices are less energy efficient than spin-based ones,” said Li. “For spins to be useful, we need to control both their flow and orientation.”

To detect and better understand this quirky particle behavior, the team needed a method sensitive to the spin of moving electrons. Their new microscopy approach was tested on a single crystal of Bi2Te2Se, a material containing bismuth, tellurium and selenium. It measured how much voltage was produced along the material’s surface as the flow of electrons moved between specific points while sensing the voltage for each electron’s spin.

The new method builds on a four-probe scanning tunneling microscope–an instrument that can pinpoint a material’s atomic activity with four movable probing tips–by adding a component to observe the spin behavior of electrons on the material’s surface. This approach not only includes spin sensitivity measurements. It also confines the current to a small area on the surface, which helps to keep electrons from escaping beneath the surface, providing high-resolution results.

“We successfully detected a voltage generated by the electron’s spin current,” said Li, who coauthored a paper published by Physical Review Letters that explains the method. “This work provides clear evidence of the spin current in topological insulators and opens a new avenue to study other quantum materials that could ultimately be applied in next-generation electronic devices.”

The same electrostatic charge that can make hair stand on end and attach balloons to clothing could be an efficient way to drive atomically thin electronic memory devices of the future, according to a new study led by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).

In a study published today in the journal Nature, scientists have found a way to reversibly change the atomic structure of a 2-D material by injecting, or “doping,” it with electrons. The process uses far less energy than current methods for changing the configuration of a material’s structure.

Schematic shows the configuration for structural phase transition on a molybdenum ditelluride monolayer (MoTe2, shown as yellow and blue spheres), which is anchored by a metal electrodes (top gate and ground). The ionic liquid covering the monolayer and electrodes enables a high density of electrons to populate the monolayer, leading to changes in the structural lattice from a hexagonal (2H) to monoclinic (1T') pattern. Credit: Ying Wang/Berkeley Lab

Schematic shows the configuration for structural phase transition on a molybdenum ditelluride monolayer (MoTe2, shown as yellow and blue spheres), which is anchored by a metal electrodes (top gate and ground). The ionic liquid covering the monolayer and electrodes enables a high density of electrons to populate the monolayer, leading to changes in the structural lattice from a hexagonal (2H) to monoclinic (1T’) pattern. Credit: Ying Wang/Berkeley Lab

“We show, for the first time, that it is possible to inject electrons to drive structural phase changes in materials,” said study principal investigator Xiang Zhang, senior faculty scientist at Berkeley Lab’s Materials Sciences Division and a professor at UC Berkeley. “By adding electrons into a material, the overall energy goes up and will tip off the balance, resulting in the atomic structure re-arranging to a new pattern that is more stable. Such electron doping-driven structural phase transitions at the 2-D limit is not only important in fundamental physics; it also opens the door for new electronic memory and low-power switching in the next generation of ultra-thin devices.”

Switching a material’s structural configuration from one phase to another is the fundamental, binary characteristic that underlies today’s digital circuitry. Electronic components capable of this phase transition have shrunk down to paper-thin sizes, but they are still considered to be bulk, 3-D layers by scientists. By comparison, 2-D monolayer materials are composed of a single layer of atoms or molecules whose thickness is 100,000 times as small as a human hair.

“The idea of electron doping to alter a material’s atomic structure is unique to 2-D materials, which are much more electrically tunable compared with 3-D bulk materials,” said study co-lead author Jun Xiao, a graduate student in Zhang’s lab.

The classic approach to driving the structural transition of materials involves heating to above 500 degrees Celsius. Such methods are energy-intensive and not feasible for practical applications. In addition, the excess heat can significantly reduce the life span of components in integrated circuits.

A number of research groups have also investigated the use of chemicals to alter the configuration of atoms in semiconductor materials, but that process is still difficult to control and has not been widely adopted by industry.

“Here we use electrostatic doping to control the atomic configuration of a two-dimensional material,” said study co-lead author Ying Wang, another graduate student in Zhang’s lab. “Compared to the use of chemicals, our method is reversible and free of impurities. It has greater potential for integration into the manufacturing of cell phones, computers and other electronic devices.”

The researchers used molybdenum ditelluride (MoTe2), a typical 2-D semiconductor, and coated it with an ionic liquid (DEME-TFSI), which has an ultra-high capacitance, or ability to store electric charges. The layer of ionic liquid allowed the researchers to inject the semiconductor with electrons at a density of a hundred trillion to a quadrillion per square centimeter. It is an electron density that is one to two orders higher in magnitude than what could be achieved in 3-D bulk materials, the researchers said.

Through spectroscopic analysis, the researchers determined that the injection of electrons changed the atoms’ arrangement of the molybdenum ditelluride from a hexagonal shape to one that is monoclinic, which has more of a slanted cuboid shape. Once the electrons were retracted, the crystal structure returned to its original hexagonal pattern, showing that the phase transition is reversible. Moreover, these two types of atom arrangements have very different symmetries, providing a large contrast for applications in optical components.

“Such an atomically thin device could have dual functions, serving simultaneously as optical or electrical transistors, and hence broaden the functionalities of the electronics used in our daily lives,” said Wang.

Using a simple layer-by-layer coating technique, researchers from the U.S. and Korea have developed a paper-based flexible supercapacitor that could be used to help power wearable devices. The device uses metallic nanoparticles to coat cellulose fibers in the paper, creating supercapacitor electrodes with high energy and power densities – and the best performance so far in a textile-based supercapacitor.

By implanting conductive and charge storage materials in the paper, the technique creates large surface areas that function as current collectors and nanoparticle reservoirs for the electrodes. Testing shows that devices fabricated with the technique can be folded thousands of times without affecting conductivity.

“This type of flexible energy storage device could provide unique opportunities for connectivity among wearable and internet of things devices,” said Seung Woo Lee, an assistant professor in the Woodruff School of Mechanical Engineering at the Georgia Institute of Technology. “We could support an evolution of the most advanced portable electronics. We also have an opportunity to combine this supercapacitor with energy-harvesting devices that could power biomedical sensors, consumer and military electronics, and similar applications.”

The research, done with collaborators at Korea University, was supported by the National Research Foundation of Korea and reported September 14 in the journal Nature Communications.

Yongmin Ko, Minseong Kwon, Wan Ki Bae, Byeongyong Lee, Seung Woo Lee & Jinhan Cho, “Flexible supercapacitor electrodes based on real metal-like cellulose papers,” (Nature Communications, 2017) http://dx.doi.org/10.1038/s41467-017-00550-3

Yongmin Ko, Minseong Kwon, Wan Ki Bae, Byeongyong Lee, Seung Woo Lee & Jinhan Cho, “Flexible supercapacitor electrodes based on real metal-like cellulose papers,” (Nature Communications, 2017) http://dx.doi.org/10.1038/s41467-017-00550-3

Energy storage devices are generally judged on three properties: their energy density, power density and cycling stability. Supercapacitors often have high power density, but low energy density – the amount of energy that can be stored – compared to batteries, which often have the opposite attributes. In developing their new technique, Lee and collaborator Jinhan Cho from the Department of Chemical and Biological Engineering at Korea University set out to boost energy density of the supercapacitors while maintaining their high power output.

They began by dipping paper samples into a beaker of solution containing an amine surfactant material designed to bind the gold nanoparticles to the paper. Next they dipped the paper into a solution containing gold nanoparticles. Because the fibers are porous, the surfactants and nanoparticles enter the fibers and become strongly attached, creating a conformal coating on each fiber.

By repeating the dipping steps, the researchers created a conductive paper on which they added alternating layers of metal oxide energy storage materials such as manganese oxide. The ligand-mediated layer-by-layer approach helped minimize the contact resistance between neighboring metal and/or metal oxide nanoparticles. Using the simple process done at room temperatures, the layers can be built up to provide the desired electrical properties.

“It’s basically a very simple process,” Lee said. “The layer-by-layer process, which we did in alternating beakers, provides a good conformal coating on the cellulose fibers. We can fold the resulting metallized paper and otherwise flex it without damage to the conductivity.”

Though the research involved small samples of paper, the solution-based technique could likely be scaled up using larger tanks or even a spray-on technique. “There should be no limitation on the size of the samples that we could produce,” Lee said. “We just need to establish the optimal layer thickness that provides good conductivity while minimizing the use of the nanoparticles to optimize the tradeoff between cost and performance.”

The researchers demonstrated that their self-assembly technique improves several aspects of the paper supercapacitor, including its areal performance, an important factor for measuring flexible energy-storage electrodes. The maximum power and energy density of the metallic paper-based supercapacitors are estimated to be 15.1 mW/cm2 and 267.3 uW/cm2, respectively, substantially outperforming conventional paper or textile supercapacitors.

The next steps will include testing the technique on flexible fabrics, and developing flexible batteries that could work with the supercapacitors. The researchers used gold nanoparticles because they are easy to work with, but plan to test less expensive metals such as silver and copper to reduce the cost.

During his Ph.D. work, Lee developed the layer-by-layer self-assembly process for energy storage using different materials. With his Korean collaborators, he saw a new opportunity to apply that to flexible and wearable devices with nanoparticles.

“We have nanoscale control over the coating applied to the paper,” he added. “If we increase the number of layers, the performance continues to increase. And it’s all based on ordinary paper.”

In addition to those already mentioned, the research team included Yongmin Ko and Minseong Kwon from Korea University, Wan Ki Bae from the Photoelectronic Hybrids Research Center at the Korea Institute of Science and Technology, and Byeongyong Lee from Georgia Tech.

Physicists at the University of California, Riverside have developed a photodetector – a device that senses light – by combining two distinct inorganic materials and producing quantum mechanical processes that could revolutionize the way solar energy is collected.

Photodetectors are almost ubiquitous, found in cameras, cell phones, remote controls, solar cells, and even the panels of space shuttles. Measuring just microns across, these tiny devices convert light into electrons, whose subsequent movement generates an electronic signal. Increasing the efficiency of light-to-electricity conversion has been one of the primary aims in photodetector construction since their invention.

Lab researchers stacked two atomic layers of tungsten diselenide (WSe2) on a single atomic layer of molybdenum diselenide (MoSe2). Such stacking results in properties vastly different from those of the parent layers, allowing for customized electronic engineering at the tiniest possible scale.

This image shows an energy diagram of the WSe2-MoSe2 device. When a photon (1) strikes the WSe2 layer, it knocks loose an electron (2), freeing it to conduct through the WSe2 (3). At the junction between the two materials, the electron drops down into MoSe2 (4). The energy given off in the drop catapults a second electron from the WSe2 (5) into the MoSe2 (6), where both electrons are free to move and generate electricity. Credit: University Communications, UC Riverside.

This image shows an energy diagram of the WSe2-MoSe2 device. When a photon (1) strikes the WSe2 layer, it knocks loose an electron (2), freeing it to conduct through the WSe2 (3). At the junction between the two materials, the electron drops down into MoSe2 (4). The energy given off in the drop catapults a second electron from the WSe2 (5) into the MoSe2 (6), where both electrons are free to move and generate electricity. Credit: University Communications, UC Riverside.

Within atoms, electrons live in states that determine their energy level. When electrons move from one state to another, they either acquire or lose energy. Above a certain energy level, electrons can move freely. An electron moving into a lower energy state can transfer enough energy to knock loose another electron.

UC Riverside physicists observed that when a photon strikes the WSe2 layer, it knocks loose an electron, freeing it to conduct through the WSe2. At the junction between WSe2 and MoSe2, the electron drops down into MoSe2. The energy given off then catapults a second electron from the WSe2 into the MoSe2, where both electrons become free to move and generate electricity.

“We are seeing a new phenomenon occurring,” said Nathaniel M. Gabor, an assistant professor of physics, who led the research team. “Normally, when an electron jumps between energy states, it wastes energy. In our experiment, the waste energy instead creates another electron, doubling its efficiency. Understanding such processes, together with improved designs that push beyond the theoretical efficiency limits, will have a broad significance with regard to designing new ultra-efficient photovoltaic devices.”

Study results appear today in Nature Nanotechnology.

“The electron in WSe2 that is initially energized by the photon has an energy that is low with respect to WSe2,” said Fatemeh Barati, a graduate student in Gabor’s Quantum Materials Optoelectronics lab and the co-first author of the research paper. “With the application of a small electric field, it transfers to MoSe2, where its energy, with respect to this new material, is high. Meaning, it can now lose energy. This energy is dissipated as kinetic energy that dislodges the additional electron from WSe2.”

In existing solar panels models, one photon can at most generate one electron. In the prototype the researchers developed, one photon can generate two electrons or more through a process called electron multiplication.

The researchers explained that in ultrasmall materials, electrons behave like waves. Though it is unintuitive at large scales, the process of generating two electrons from one photon is perfectly allowable at extremely small length scales. When a material, such as WSe2 or MoSe2, gets thinned down to dimensions nearing the electron’s wavelength, the material’s properties begin to change in inexplicable, unpredictable, and mysterious ways.

“It’s like a wave stuck between walls closing in,” Gabor said. “Quantum mechanically, this changes all the scales. The combination of two different ultra small materials gives rise to an entirely new multiplication process. Two plus two equals five.”

“Ideally, in a solar cell we would want light coming in to turn into several electrons,” said Max Grossnickle, also a graduate student in Gabor’s lab and the research paper’s co-first author. “Our paper shows that this is possible.”

Barati noted that more electrons could be generated also by increasing the temperature of the device.

“We saw a doubling of electrons in our device at 340 degrees Kelvin (150 F), which is slightly above room temperature,” she said. “Few materials show this phenomenon around room temperature. As we increase this temperature, we should see more than a doubling of electrons.”

Electron multiplication in conventional photocell devices typically requires applied voltages of 10-100 volts. To observe the doubling of electrons, the researchers used only 1.2 volts, the typical voltage supplied by an AA battery.

“Such low voltage operation, and therefore low power consumption, may herald a revolutionary direction in photodetector and solar cell material design,” Grossnickle said.

He explained that the efficiency of a photovoltaic device is governed by a simple competition: light energy is either converted into waste heat or useful electronic power.

“Ultrathin materials may tip the balance in this competition by simultaneously limiting heat generation, while increasing electronic power,” he said.

Gabor explained that the quantum mechanical phenomenon his team observed in their device is similar to what occurs when cosmic rays, coming into contact with the Earth’s atmosphere with high kinetic energy, produce an array of new particles.

He speculated that the team’s findings could find applications in unforeseen ways.

“These materials, being only an atom thick, are nearly transparent,” he said. “It’s conceivable that one day we might see them included in paint or in solar cells incorporated into windows. Because these materials are flexible, we can envision their application in wearable photovoltaics, with the materials being integrated into the fabric. We could have, say, a suit that generates power – energy-harvesting technology that would be essentially invisible.”

WIN Semiconductors Corp (TPEx:3105), the world’s largest pure-play compound semiconductor foundry, has released an optimized version of its 0.25µm gallium nitride technology, NP25, that provides superior DC and RF transistor performance. NP25 is a 0.25µm-gate GaN-on-SiC process, and offers users the flexibility to produce both fully integrated amplifier products as well as custom discrete transistors. In production since 2014, the optimized 0.25µm process offers enhanced RF performance with fast switching time, higher gain and increased power added efficiency for demanding power applications through Ku-band

Optimized NP25 transistors exhibit more ideal DC and RF IV characteristics and provide 2 dB higher maximum stable gain. Increased gain leads directly to higher power density and PAE under a range of tuning and bias conditions. This performance-optimized process is fully qualified and supported with a comprehensive design kit and transistor models.

The WIN NP25 technology is fabricated on 4-inch silicon carbide substrates and operates at a drain bias of 28 volts. At 10GHz, NP25 provides saturated output power of 5 watts/mm with 19 dB linear gain and over 65% power added efficiency. These performance metrics make the NP25 process well suited for a variety of high power, broad bandwidth and linear transmit functions in the radar, satellite communications, and wireless infrastructure markets.

As microchips become ever smaller and therefore faster, the shrinking size of their copper interconnects leads to increased electrical resistivity at the nanoscale. Finding a solution to this impending technical bottleneck is a major problem for the semiconductor industry.

One promising possibility involves reducing the resistivity size effect by altering the crystalline orientation of interconnect materials. A pair of researchers from Rensselaer Polytechnic Institute conducted electron transport measurements in epitaxial single-crystal layers of tungsten (W) as one such potential interconnect solution. They performed first-principles simulations, finding a definite orientation-dependent effect. The anisotropic resistivity effect they found was most marked between layers with two particular orientations of the lattice structure, namely W(001) and W(110). The work is published this week in the Journal of Applied Physics, from AIP Publishing.

The measured resistivity of epitaxial tungsten layers with (001) and (011) crystal orientation vs thickness d. The tungsten Fermi surface is color coded according to the wave vector dependent Fermi velocity vf. At small thickness, where surface scattering dominates, W(011) is nearly twice as conductive as W(001). Transport simulations indicate that this is due to the anisotropy in the Fermi surface. These results indicate how narrow wires in future computer chips can be made two times more conductive, effectively reducing the required electric power by 50 percent. Credit: Daniel Gall, Rensselaer Polytechnic Institute

The measured resistivity of epitaxial tungsten layers with (001) and (011) crystal orientation vs thickness d. The tungsten Fermi surface is color coded according to the wave vector dependent Fermi velocity vf. At small thickness, where surface scattering dominates, W(011) is nearly twice as conductive as W(001). Transport simulations indicate that this is due to the anisotropy in the Fermi surface. These results indicate how narrow wires in future computer chips can be made two times more conductive, effectively reducing the required electric power by 50 percent. Credit: Daniel Gall, Rensselaer Polytechnic Institute

Author Pengyuan Zheng noted that both the 2013 and 2015 International Technology Roadmap for Semiconductors (ITRS) called for new materials to replace copper as interconnect material to limit resistance increase at reduced scale and minimize both power consumption and signal delay.

In their study, Zheng and co-author Daniel Gall chose tungsten because of its asymmetric Fermi surface — its electron energy structure. This made it a good candidate to demonstrate the anisotropic resistivity effect at the small scales of interest. “The bulk material is completely isotropic, so the resistivity is the same in all directions,” Gall said. “But if we have thin films, then the resistivity varies considerably.”

To test the most promising orientations, the researchers grew epitaxial W(001) and W(110) films on substrates and conducted resistivity measurements of both while immersed in liquid nitrogen at 77 Kelvin (about -196 degrees Celsius) and at room temperature, or 295 Kelvin. “We had roughly a factor of 2 difference in the resistivity between the 001 oriented tungsten and 110 oriented tungsten,” Gall said, but they found considerably smaller resistivity in the W(011) layers.

Although the measured anisotropic resistance effect was in good agreement with what they expected from calculations, the effective mean free path — the average distance electrons can move before scattering against a boundary — in the thin film experiments was much larger than the theoretical value for bulk tungsten.

“An electron travels through a wire on a diagonal, it hits a surface, gets scattered, and then continues traveling until it hits something else, maybe the other side of the wire or a lattice vibration,” Gall said. “But this model looks wrong for small wires.”

The experimenters believe this may be explained by quantum mechanical processes of the electrons that arise at these limited scales. Electrons may be simultaneously touching both sides of the wire or experiencing increased electron-phonon (lattice vibrations) coupling as the layer thickness decreases, phenomena that could affect the search for another metal to replace copper interconnects.

“The envisioned conductivity advantages of rhodium, iridium, and nickel may be smaller than predicted,” said Zheng. Findings like these will prove increasingly important as quantum mechanical scales become more commonplace for the demands of interconnects.

The research team is continuing to explore the anisotropic size effect in other metals with nonspherical Fermi surfaces, such as molybdenum. They found that the orientation of the surface relative to the layer orientation and transport direction is vital, as it determines the actual increase in resistivity at these reduced dimensions.

“The results presented in this paper clearly demonstrate that the correct choice of crystalline orientation has the potential to reduce nanowire resistance,” said Zheng. The importance of the work extends beyond current nanoelectronics to new and developing technologies, including transparent flexible conductors, thermoelectrics and memristors that can potentially store information. “It’s the problem that defines what you can do in the next technology,” Gall said.

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.

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.”

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.