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A multivalued optical memory composed of 2D materials

Posted By Graphene Council, Friday, September 11, 2020
The National Institute for Materials Science (NIMS) has developed a memory device capable of storing multiple values using both optical and voltage input values. This device composed of layered two-dimensional materials is able to optically control the amount of charge stored in these layers. This technology may be used to significantly increase the capacity of memory devices and applied to the development of various optoelectronic devices. The research was published in Advanced Functional Materials ("Laser-assisted multilevel non-volatile memory device based on 2D van-der-Waals fewlayer-ReS2/h-BN/Graphene Heterostructures").

Memory devices used to store information (e.g., flash memory) play an indispensable role in today’s information society. The recording density of these devices has substantially increased in the past 20 years. In anticipation of widespread adoption of IoT technologies in the near future, it is desirable to accelerate the development of higher speed, larger capacity memory devices.

However, the current approach to increasing memory capacity and energy efficiency through silicon microfabrication is about to reach its limits. Development of memory devices with different working principles therefore has been awaited.

To meet expected technology needs, this research group has developed a transistor memory device composed of layered two-dimensional materials, including rhenium disulfide (ReS2) – a semiconductor – serving as a channel transistor, hexagonal boron nitride (h-BN) used as an insulating tunnel layer and graphene functioning as a floating gate.

This device records data by storing charge carriers in the floating gate in a manner similar to conventional flash memory. Hole-electron pairs in the ReS2 layer are prone to excitation when irradiated with light. The number of these pairs can be regulated by changing the intensity of the light.

The group succeeded in creating a mechanism that allows the amount of charge in the graphene layer to gradually decrease as the exited electrons once again couple with the holes in this layer. This success enabled the device to operate as a multivalued memory capable of efficiently controlling the amount of stored charge in stages through the combined use of light and voltage.

Moreover, this device can operate energy efficiently by minimizing electric current leakage—an achievement made possible by layering two-dimensional materials, thereby smoothening the interfaces between them at an atomic level.

This technology may be used to significantly increase the capacity and energy efficiency of memory devices. It also may be applied to the development of various optoelectronic devices, including optical logic circuits and highly sensitive photosensors capable of controlling the amount of charge stored in them through combined use of light and voltage.

This project was carried out by a research group consisting of Yutaka Wakayama (Leader of the Quantum Device Engineering Group (QDEG), International Center for Materials Nanoarchitectonics (MANA), NIMS), Bablu Mukherjee (Postdoctoral Researcher, QDEG, MANA, NIMS) and Shu Nakaharai (Principal Researcher, QDEG, MANA, NIMS).

This study was conducted in conjunction with another project entitled “Development of a ultra-sensitive photosensor using two-dimensional atomic film layers” funded by the Grant-in-Aid for JSPS Fellows.

Tags:  2D materials  Bablu Mukherjee  Graphene  International Center for Materials Nanoarchitecton  optoelectronics  Semiconductor  Sensors  Shu Nakaharai  The National Institute for Materials Science  Yutaka Wakayama 

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A torchbearer for the future of applied materials and optoelectronics — Professor Chen Hsiang

Posted By Graphene Council, Wednesday, September 9, 2020
For those unfamiliar with the terms “applied materials” and “optoelectronic engineering,” a few keywords such as “semiconductors” and “sensors” should jolt one’s memory. The importance of this cutting-edge field can be illustrated by examining recent Nobel Prize winners and their research.

First, three Japanese researchers were jointly awarded the 2014 Nobel Prize for Physics "for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources," for they held the key to the elusive blue LED — Gallium nitride (GaN).

Another pair of laureates, both of Russian heritage, were awarded in 2010 "for groundbreaking experiments regarding the two-dimensional material graphene." Graphene is a newly discovered form of carbon that is prized by manufacturers of touchscreens, light panels, and solar cells for its superior transparency and heat-conducting properties.

Chen Hsiang, chair of National Chi Nan University’s Department of Applied Materials and Optoelectronic Engineering, began his academic journey in the field of electrical engineering before delving into photonics and nanomaterials. His alma mater, National Taiwan University’s fiercely competitive Department of Electrical Engineering, has been the top choice for Taiwanese students taking the university entrance exam for the past decades.

After rigorous training through NTU's undergraduate and graduate programs from 1991 to 1997, Chen left Taipei to pursue a doctoral degree at the University of California, Irvine from 2005 to 2008. It was here, in a dimly lighted campus laboratory, that he first caught a glimpse of the imperfections within the GaN transistors of that era. He proceeded to dedicate his thesis to this discovery, and graduated with both a Ph. D. degree and a book offer from a German publisher.

“At that time, researching GaN transistors was a new field,” the distinguished professor explains. “These high-power transistors are used in cellular towers, satellites, and even in outer space, but [the design then] lacked stability and contained structural flaws that could be rectified by optoelectronics.”

The materials used in his doctoral studies were procured from an American arms manufacturer that crafted F51 fighter jets and is now known as Northrop Grumman, a global aerospace and defense technology company. Unable to secure a source for such transistors upon returning to Taiwan, Chen turned his attention to the more readily available zinc oxide (ZnO) nanoparticles.

Described by the professor as “structurally identical” to the hexagonal columnar basalt found on Taiwan's Penghu Islands, crystalized ZnO particles are actually a million times larger in terms of mass. This stretch of surface is extremely advantageous in making light, portable nano-sensors that can be used to reliably measure carbon monoxide levels or ultraviolent rays.

Chen compares the process — that of introducing nanomaterials to zinc oxide to create completely new ZoN nanostructures — to “changing the toppings on a subway sandwich” to refine the properties of the end product differently each time.

Respected among his peers as a well-trained engineer who has never ceased his research efforts, Chen maintains a steady publishing average of 8 articles per year in international science master journals listed on the Science Citation Index (SCI).

This track record is matched by only a handful of NCNU faculty members, however Chen humbly redirects the compliment instead to acknowledge the collective hard work of the optoelectronics department’s instructors and student researchers.

He interjects: “There is a student who is working on those fresh perovskite [solar] cells, heard it was similar to Intel’s research.”

Chen took up the post of departmental chair last year upon completing a sabbatical and visiting at the research lab of Yale's acclaimed Professor of Technological Innovation Jung Han (韓仲). Apart from livening up his department’s recruitment and teaching process, he is also leading the way for more case studies, hands-on experiments, and industry knowledge such as the latest breakthroughs in technology and applications.

One of his recent lectures was on optical tweezers invented by the 2018 Nobel Prize in Physics team that grab atoms, molecules, and DNA with laser beam fingers; the lasers push small particles towards the center of the beam and hold them there.

The professor dutifully recites the tremendous employment opportunities that come with a bachelor's degree in the field: Taiwan Semiconductor Manufacturing Company (TSMC), United Microelectronics Corporation (UMC), Micron Technology, and Epistar. Other graduates opt for further studies at institutions such as Carnegie Mellon and Duke.

Two recent graduates are now serving as research engineers at TSMC, he says, drawing attention to the importance of deep familiarity with both the compositional and modular properties of semiconductors. “Having a background in manufacturing and sensor-testing semiconductors, as well as knowledge of the physics and materials used, will open up a lot of doors in both the electronics industry and the optoelectronics field.”

Academic-industry cooperation on a community level is another passion of Chen's, in which he seeks to deepen exchanges and partnerships with local LED firms and solar cell makers such as those based at Nantou's science park. “Local businesses are in need of highly skilled labor, graduates are in need of employment; we are here to create networks,” he explains.

In recent years, NCNU has been an avid participant in several programs supported by the Ministry of Education's Center for University Social Responsibility. These include cross-fertilizing Taiwan's agricultural powerhouse with optoelectronics, and now Nantou’s water bamboo and passion fruits are grown with the aid of LED lights.

Moreover, NCNU researchers are currently identifying the best wavelength, intensity, and duration for specific cultivars based on their innate growth cycle and biological characteristics.

How do a new generation of Taiwanese scholars prepare themselves for this field? To this, Chen replies with the 3 keystones of optoelectronics — light, display, and energy source.

NCNU's curriculum prides itself on providing in-depth understanding of the characteristics of the materials used, as well as the parameters for reading photonic and gaseous levels. This field is a gateway to electrical engineering, chemistry, physics, optoelectronics, and many more fascinating areas of study, so why not take the chance to learn more about semiconductors to broaden one's scientific knowledge and employability?

Professor Chen's rich scientific sensibilities have further cemented the credibility of NCNU's Department of Applied Materials and Optoelectronic Engineering. The reward for developing engaging research projects and experiment-based training? Exceeding recruitment expectations during the time of the coronavirus — full classrooms that the devoted Chen sees as a divine deliverance of grace.

Tags:  Chen Hsiang  Graphene  LED  nanomaterials  National Chi Nan University  optoelectronics  Semiconductors 

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Fast electrical modulation of nanoscale erbium-graphene systems towards quantum technology applications

Posted By Graphene Council, Thursday, August 27, 2020
Quantum technologies promise to revolutionize information technology and communications by taking advantage of some peculiar aspects of quantum physics, such as quantum state superposition and entanglement. Research is moving forward in different directions with the goal of building optimal devices for quantum information processing, secure communication, and high-precision sensing.

Systems based on rare-earth ions, such as erbium, are very relevant to this quest, in particular because they typically have very long decoherence times, which means that quantum states persist longer than in other systems.  Furthermore, erbium emits light at a wavelength of 1.5 micrometers, one of the main bands for optical communications systems. Hybrid systems containing nanoscale rare-earth components may prove highly versatile and useful to meet the needs of various (quantum) optoelectronic applications.

A team of researchers including Dr Klaas-Jan Tielrooij, leader of the Ultrafast Dynamics in Nanoscale Systems group at the ICN2, and scientists from the Institute of Photonic Sciences (ICFO) and the Institut de Recherche de Chimie Paris (IRCP) have combined a 10 nm thin film of an erbium-doped oxide crystal with monolayer graphene. This hybrid system exhibits extremely strong emitter-environment interactions due to the physical closeness of the emitters to graphene, and the strong dipole-dipole coupling to Dirac electrons.

Their study, recently published in Nature Communications, showed that a large fraction of excited erbium ions decays more than a thousand times faster than normal due to the presence of graphene. This implies that more than 99.9% of the energy flows from these excited emitters to graphene through near-field interactions – where the near-field is the region of the electromagnetic field closest to the object that emits the radiation; in this specific case, it means at a distance from the emitter much smaller than the wavelength of the emitted light. The energy that is transferred from excited emitters to graphene leads to either electron-hole pair generation or plasmon launching (see illustration) in graphene, depending on the Fermi energy of graphene.

Moreover, as reported in the paper, the authors were able to efficiently control the near-field interactions of this hybrid system and to modulate them dynamically by applying a small electrical voltage of just a few volts. This is possible because the gate voltage allows for tuning graphene’s Fermi energy over a large range. The emitter-environment interactions were controlled with high modulation frequencies — up to 300 kHz, which is three orders of magnitude higher than the emitter’s normal radiative decay rate.

This fast dynamic modulation can lead to interesting phenomena and applications, such as the emission of single photons with controlled waveform and quantum entanglement generation by collective plasmon emission. The development of hybrid systems enabling fast control over the near-field interactions, as this erbium-graphene platform, also provides an interesting tool to manipulate quantum states in nanoscale solid state devices by means of conventional electronics. Further studies on these structures will certainly open the way to wider applications in optoelectronic, plasmonic and quantum technologies.

Tags:  Graphene  ICN2  Klaas-Jan Tielrooij  optoelectronics  quantum materials 

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A highly light-absorbent and tunable material

Posted By Graphene Council, Friday, August 14, 2020
By layering different two-dimensional materials, physicists at the University of Basel have created a novel structure with the ability to absorb almost all light of a selected wavelength. The achievement relies on a double layer of molybdenum disulfide. The new structure’s particular properties make it a candidate for applications in optical components or as a source of individual photons, which play a key role in quantum research. The results were published in the scientific journal Nature Nanotechnology.

Novel two-dimensional materials are currently a hot research topic around the world. Of special interest are van der Waals heterostructures, which are made up of individual layers of different materials held together by van der Waals forces. The interactions between the different layers can give the resulting material entirely new properties.

Double layer unlocks crucial properties
There are already van der Waals heterostructures that absorb up to 100 percent of light. Single-layers of molybdenum disulfide offer absorption capacities in this range. When light is absorbed, an electron vacates its original position in the valence band, leaving behind a positively charged hole. The electron moves to a higher energy level, known as the conduction band, where it can move freely.

The resulting hole and the electron are attracted to each other in accordance with Coulomb’s law, giving rise to bound electron-hole pairs that remain stable at room temperature. However, with single-layer molybdenum disulfide there is no way to control which light wavelengths are absorbed. “It is only when a second layer of molybdenum disulfide is added that we get tunability, an essential property for application purposes,” explains Professor Richard Warburton of the University of Basel’s Department of Physics and Swiss Nanoscience Institute.

Absorption and tunability
Working in close collaboration with researchers in France, Warburton and his team have succeeded in creating such a structure. The physicists used a double layer of molybdenum disulfide sandwiched between an insulator and the electrical conductor graphene on each side.

“If we apply a voltage to the outer graphene layers, this generates an electric field that affects the absorption properties of the two molybdenum disulfide layers,” explains Nadine Leisgang, a doctoral student in Warburton’s team and lead author of the study. “By adjusting the voltage applied, we can select the wavelengths at which the electron-hole pairs are formed in these layers.”

“This research could pave the way for a new approach to developing optoelectronic devices such as modulators,” adds Richard Warburton. Modulators are used to selectively change a signal’s amplitude. Another potential application is generating individual photons, with important implications for quantum technology. 

Tags:  2D materials  Graphene  optoelectronics  Richard Warburton  University of Basel 

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Vapor fix lifts up perovskite crystal performance

Posted By Graphene Council, Saturday, June 20, 2020
A simple and noninvasive treatment could become a prime post-crystallization process to optimize the optoelectronic properties of hybrid perovskite solar cell materials.

In this treatment devised by KAUST, bromine vapors penetrate the surface of as-synthesized perovskite crystals to reach their deep-lying layers, removing surface and bulk defects generated during crystal growth.

Lead-containing hybrid perovskites, such as methylammonium lead tribromide (MAPbBr3), present unique charge transport properties and easy processability in solution. These make them attractive as potential low-cost alternatives to traditional silicon-based light-harvesting solar cell materials. However, approaches that use solution processing to crystalize them tend to leave contaminants, such as oxygen and amorphous carbon. These approaches also produce halide vacancies that create lead cations, which can trap electrons to form metallic lead and restrict charge transport.

Various chemical treatments can reduce these defects, but most tinker with the composition of the precursor solution to optimize thin film and crystal formation. However, the researchers from the KAUST Solar Center sought something simpler.

"We were interested in developing a facile recipe that could be applied once crystal formation was complete," says Ahmad Kirmani, now a postdoc at the National Renewable Energy Laboratory, U.S., who conducted the study under the supervision of Aram Amassian and Omar Mohammed.

Co-author Ahmed Mansour, now a postdoc at Helmholtz-Zentrum Berlin, Germany, describes how the researchers chose a bromine vapor treatment because they had previously observed the improved conductivity of graphene when exposed to bromine. "Bromine is a volatile liquid at room temperature and readily evaporates without the need for any external source of energy," Mansour says.

The researchers suspended MAPbBr3 crystals in a bromine-vapor-saturated environment and monitored the effects of bromine exposure on material properties.

They were pleasantly surprised to find that bromine vapors suppressed metallic lead on the surface as well as in the bulk of the crystals, Mohammed says. "This meant that we could access the bulk properties of these crystals, such as their electrical conductivity," he adds. Prolonged bromine exposure produced a dramatic 10,000-fold enhancement in bulk electrical conductivity and a 50-fold increase in carrier mobility. Further assessment revealed that perovskite crystallization leaves behind voids and imperfections, which allows bromine to diffuse and permeate through the crystals.

Each of the former team members is currently exploring more applications for their treatment, such as for improving the power conversion efficiency of solar cells containing perovskite thin films as absorbers or for single-crystal devices--such as transistors, photodetectors and radiation detectors--that require excellent carrier mobility and intrinsic optoelectronic properties.

Tags:  Ahmad Kirmani  Graphene  KAUST  Omar Mohammed  optoelectronics 

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Particles trapped in twisted materials and quantum fingerprints identified

Posted By Graphene Council, Tuesday, June 2, 2020
This key paper has identified how to trap interlayer excitons (IXs) and identified their quantum fingerprints. The IXs are trapped by the interaction of two sheets of atoms, made of different transition metal dichalcogenides (TMDs), which are stacked together with a small twist to form a moiré pattern.

For the less quantum-literate, or more fashion-minded, moiré interference patterns emerge whenever two similar but slightly offset templates are combined – such as silk fabric that has been subjected to heat and pressure to give it a rippled appearance. In the Quantum Photonics Lab, led by Professor Gerardot, the moiré patterns affect the key properties of atomic heterostructures to create a new quantum material.

Two-dimensional (2D) materials, such as graphene or TMDs, can form a variety of heterostructures held together by weak van der Waals (vdW) forces, endowing scientists with a rich toolbox for engineering their optoelectronic properties. VdW multilayers may also form moiré patterns – a periodic variation of the alignment between corresponding atoms in adjacent layers – by twisting the sheets by a relative angle and/or combining materials with different lattice constants.

"Interlayer excitons trapped in atomic moiré patterns hold great promise...and investigations on their fundamental properties are crucial for future developments", Prof. Brian Gerardot.

In addition, peculiar features derive from the 2D nature of the TMD layers including a phenomenon named spin-valley-layer locking, which open up potential connections to the larger fields of spintronics and valleytronics that are of interest for next-generation optoelectronic devices.

Professor Gerardot explains the significance of his findings: “Interlayer excitons trapped in atomic moiré patterns hold great promise for the design of quantum materials based on van der Waals heterostructures, and investigations on their fundamental properties are crucial for future developments in the field.”

The scientific community still seeks strategies to verify the nature of the trapping sites and understand the role of sample imperfections. A combination of experimental methods could be employed to clarify the role of atomic reconstruction, strain and other defects, correlating optical measurements and non-invasive microscopy techniques.

The Quantum Photonics Lab is designing fully tuneable electronic devices, based on the twisted quantum materials, to fully understand how the moiré can interact with each other and be exploited for quantum optics applications.

Within a field particularly rich in opportunities, science moves at an impressive pace and many breakthroughs can be expected.

Tags:  Brian Gerardot  Graphene  optoelectronics  Photonics  quantum material 

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High-quality boron nitride grown at atmospheric pressure

Posted By Graphene Council, Wednesday, April 22, 2020
Graphene Flagship researchers at RWTH Aachen University, Germany and ONERA-CNRS, France, in collaboration with researchers at the Peter Grunberg Institute, Germany, the University of Versailles, France, and Kansas State University, US, have reported a significant step forward in growing monoisotopic hexagonal boron nitride at atmospheric pressure for the production of large and very high-quality crystals.

Hexagonal boron nitride (hBN) is the unsung hero of graphene-based devices. Much progress over the last decade was enabled by the realisation that 'sandwiching' graphene between two hBN crystals can significantly improve the quality and performance of the resulting devices. This finding paved the way to a series of exciting developments, including the discoveries of exotic effects such as magic-angle superconductivity and proof-of-concept demonstrations of sensors with unrivalled sensitivity.

Until now, the most widely used hBN crystals came from the National Institute of Material Science in Tsukuba, Japan. These crystals are grown using a process at high temperatures (over 1500°C) and extremely high pressures (over 40,000 times atmospheric pressure). "The pioneering contribution by the Japanase researchers Taniguchi and Watanabe to graphene research is invaluable", begins Christoph Stampfer from Graphene Flagship Partner RWTH Aachen University, Germany. "They provide hundreds of labs around the world with ultra-pure hBN at no charge. Without their contribution, a lot of what we are doing today would not be possible."

However, this hBN growth method comes with some limitations. Among them is the small crystal size, which is limited to a few 100 µm, and the complexity of the growth process. This is suitable for fundamental research, but beyond this, a method with better scalability is needed. Now Graphene Flagship researchers tested hBN crystals grown with a new methodology that works at atmospheric pressure, developed by a team of researchers led by James Edgar at Kansas State University, US. This new approach shows great promise for more demanding research and production.

"I was very excited when Edgar proposed that we test the quality of his hBN", says Stampfer. "His growth method could be suitable for large-scale production". The method for growing hBN at atmospheric pressure is indeed much simpler and cheaper than previous alternatives and allows for the isotopic concentration to be controlled.

"The hBN crystals we received were the largest I have ever seen, and they were all based either on isotopically pure boron-10 or boron-11" says Jens Sonntag, a graduate student at Graphene Flagship Partner RWTH Aachen University. Sonntag tested the quality of the flakes first using confocal Raman spectroscopy. In addition, Graphene Flagship partners in ONERA-CNRS, France, led by Annick Loiseau, carried out advanced luminescence measurements. Both measurements indicated high isotope purity and high crystal quality.

However, the strongest evidence for the high hBN qualitycame from transport measurements performed on devices containing graphene sandwiched between monoisotopic hBN. They showed equivalent performance to a state-of-the-art device based on hBN from Japan, with better performance in some areas.

"This is a clear indication of the extremely high quality of these hBN crystals," says Stampfer. "This is great news for the whole graphene community, because it shows that it is, in principle, possible to produce high quality hBN on a large scale, bringing us one step closer to real applications based on high-performance graphene electronics and optoelectronics. Furthermore, the possibility of controlling the isotopic concentration of the crystals opens the door to experiments that were not possible before."

Mar García-Hernández, Work Package Leader for Enabling Materials, adds: "Free-standing graphene, being the thinnest material known, exhibits a large surface area and, therefore, is extremely sensitive to its surrounding environment, which, in turn, results in substantial degradation of its exceptional properties. However, there is a clear strategy to avoid these deleterious effects: encapsulating graphene between two protective layers."

García-Hernández continues: "When graphene is encapsulated by hBN, it reveals its intrinsic properties. This makes hBN an essential material to integrate graphene into current technologies and demonstrates the importance of devising new scalable synthetic routes for large-scale hBN production. This work not only provides a new and simpler path to produce high-quality hBN crystals on a large scale, but it also enables the production of monoisotopic material, which further reduces the degradation of graphene when encapsulated by two layers."

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "This is a nice example of collaboration between the EU and the US, which we fostered via numerous bilateral workshops. Devising alternative approaches to produce high-quality hBN crystals is crucial to enable us to exploit the ultimate properties of graphene in opto-electronics applications. Furthermore, this work will lead to significant progress in fundamental research."

Tags:  Andrea C. Ferrari  Christoph Stampfer  Graphene  Graphene Flagship  Hexagonal boron nitride  Mar García-Hernández  ONERA-CNRS  optoelectronics  RWTH Aachen University  Sensors 

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AI finds 2D materials in the blink of an eye

Posted By Graphene Council, Thursday, April 2, 2020
Researchers at the Institute of Industrial Science, a part of The University of Tokyo, demonstrated a novel artificial intelligence system that can find and label 2D materials in microscope images in the blink of an eye. This work can help shorten the time required for 2D material-based electronics to be ready for consumer devices.

Two-dimensional materials offer an exciting new platform for the creation of electronic devices, such as transistors and light-emitting diodes. The family of crystals that can be made just one atom thick include metals, semiconductors, and insulators. Many of these are stable under ambient conditions, and their properties often different significantly from those of their 3D counterparts. Even stacking a few layers together can alter the electronic characteristics to make them suitable for next-generation batteries, smartphone screens, detectors, and solar cells. And perhaps even more amazing: you can make some yourself using office supplies. The 2010 Nobel Prize in Physics was awarded for the realization that atomically thin "graphene" can be obtained by exfoliating piece of pencil lead, graphite, with a piece of sticky scotch tape.

So, what keeps you from making your own electronic devices at work between meetings? Unfortunately, the atomically thin 2D crystals have low fabrication yields and their optical contrasts comprise a very broad range, and finding them under a microscope is a tedious job.

Now, a team led by The University of Tokyo has succeeded in automating this task using machine learning. The used many labeled examples with various lighting to train the computer to detect the outline and thickness of the flakes without having to fine tune the microscope parameters. "By using machine learning instead of conventional rule-based detection algorithms, our system was robust to changing conditions," says first author Satoru Masubuchi.

The method is generalizable to many other 2D materials, sometimes without needing any addition data. In fact, the algorithm was able to detect tungsten diselenide and molybdenum diselenide flakes just by being trained with tungsten ditelluride examples. With the ability to determine, in less than 200 milliseconds, the location and thickness of the exfoliated samples, the system can be integrated with a motorized optical microscope.

"The automated searching and cataloging of 2D materials will allow researchers to test a large number of samples simply by exfoliating and running the automated algorithm," senior author Tomoki Machida says. "This will greatly speed the development cycle of new electronic devices based on 2D materials, as well as advance the study of superconductivity and ferromagnetism in 2D, where there is no long-range order."

Tags:  2D materials  Graphene  optoelectronics  Satoru Masubuchi  The University of Tokyo  Tomoki Machida 

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Ultrathin graphene film offers new concept for solar energy

Posted By Graphene Council, Tuesday, March 31, 2020

Researchers at Swinburne, the University of Sydney and Australian National University have collaborated to develop a solar absorbing, ultrathin film with unique properties that has great potential for use in solar thermal energy harvesting.

The 90 nanometre material is 1000 times finer than a human hair and is able to rapidly heat up to 160°C under natural sunlight in an open environment.

This new graphene-based material also opens new avenues in:

- thermophotovoltaics (the direct conversion of heat to electricity)
- solar seawater desalination
- infrared light source and heater
- optical components: modulators and interconnects for communication devices
- photodetectors
- colourful display

It could even lead to the development of ‘invisible cloaking technology’ through developing large-scale thin films enclosing the objects to be ‘hidden’.

The researchers have developed a 2.5cm x 5cm working prototype to demonstrate the photo-thermal performance of the graphene-based metamaterial absorber.

They have also proposed a scalable manufacture strategy to fabricate the proposed graphene-based absorber at low cost.

“This is among many graphene innovations in our group,” says Professor Baohua Jia, Research Leader, Nanophotonic Solar Technology, in Swinburne’s Centre for Micro-Photonics.

“In this work, the reduced graphene oxide layer and grating structures were coated with a solution and fabricated by a laser nanofabrication method, respectively, which are both scalable and low cost.”

‌‌“Our cost-effective and scalable graphene absorber is promising for integrated, large-scale applications that require polarisation-independent, angle insensitive and broad bandwidth absorption, such as energy-harvesting, thermal emitters, optical interconnects, photodetectors and optical modulators,” says first author of this research paper, Dr Han Lin, Senior Research Fellow in Swinburne’s Centre for Micro-Photonics.

“Fabrication on a flexible substrate and the robustness stemming from graphene make it suitable for industrial use,” Dr Keng-Te Lin, another author, added.

"The physical effect causing this outstanding absorption in such a thin layer is quite general and thereby opens up a lot of exciting applications,” says Dr Bjorn Sturmberg, who completed his PhD in physics at the University of Sydney in 2016 and now holds a position at the Australian National University.

“The result shows what can be achieved through collaboration between different universities, in this case with the University of Sydney and Swinburne, each bringing in their own expertise to discover new science and applications for our science,” says Professor Martijn de Sterke, Director of the Institute of Photonics and Optical Science.

“Through our collaboration we came up with a very innovative and successful result.

“We have essentially developed a new class of optical material, the properties of which can be tuned for multiple uses.”

Tags:  Australian National University  Baohua Jia  Bjorn Sturmberg  Graphene  Keng-Te Lin  Martijn de Sterke  optoelectronics  photonics  Swinburne University of Technology 

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Graphene, perovskites, and silicon – an ideal tandem for efficient solar cells

Posted By Graphene Council, Wednesday, March 4, 2020

Graphene Flagship researchers at the University of Rome Tor Vergata, the Italian Institute of Technology (IIT) and its spin-off, Graphene Flagship Associate Member BeDimensional, in cooperation with ENEA have successfully combined graphene with tandem perovskite-silicon solar cells to achieve efficiencies of up to 26.3%. Moreover, they envisioned a new manufacturing method that, thanks to the versatility of graphene, allows to reduce production costs and could lead to the production of large-area solar panels. Graphene-based tandem solar cells almost double the efficiency of pure silicon.

Laws of physics limit the maximum efficiency of silicon solar cells to 32%. For that reason, scientists have spent decades trying to come up with other alternatives, such as III-V and perovskites. However, the latter present several manufacturing challenges, and scaling up the production of solar panels is a key step towards success. With 'tandem cells', scientists  had previously combined the advantages of both silicon and perovskites – however stability, efficiency and large-scale manufacturing still seemed like a far-fledged dream.

But then graphene came into play – and it could be a game changer. Graphene Flagship researchers identified its potential for energy harvesting, and in fact have dedicated two different industry-oriented 'Spearhead Projects' to dig into the possibilities of graphene-based solar cells. This new paper published in Joule – a reference journal in the field of energy research – is yet another proof that graphene and related layered materials will enable the commercialisation of more efficient and cost-effective large area solar panels.

Aldo di Carlo, lead author and researcher in Graphene Flagship partner University of Rome Tor Vergata, explains: "Our new approach to manufacture graphene-enabled tandem solar cells provides a double advantage. First, it can be applied to enhance all the different types of perovskite solar cells currently available, including those processed at high temperatures. But more importantly, we can incorporate our graphene using the widespread 'solution manufacturing methods', key to further deploy our technologies industrially and deliver large-surface, graphene-enabled solar panels."

Francesco Bonaccorso, co-author, co-founder of Graphene Flagship spin-off BeDimensional, says: "This innovative approach proposed in the context of the Graphene Flagship is the first step toward the development of tandem solar cells delivering an efficiency higher than the limit of single junction silicon devices. Layered materials will be pivotal in reaching this target.".

Emmanuel Kymakis, Graphene Flagship Energy Generation Work Package Leader, says: "There are some compatibility issues that have to be tackled before the full exploitation of the perovskite-Si tandem PVs concept. This pioneering work demonstrates that the integration of GRMs inks with on-demand morphology and tuneable optoelectronic properties in a tandem structure, can lead to high-throughput industrial manufacturing. Graphene and related materials improve the performance, stability and scalability of these devices.

The stacked silicon-perovskite configuration will act as the foundation of the new Graphene Flagship Spearhead Project GRAPES, in which a pilot line fabrication of graphene-based perovskite-silicon tandem solar cells will take place, paving the way towards breaking the 30% efficiency barrier and a significant decrease on the Levelized Cost of Energy."

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "The application of graphene and related materials to solar energy generation was recognized as a strategic priority since the start of the Graphene Flagship. The first graphene-based solar farm is being set up this year. These new results underpin our effort for the following 3 years to produce panels defining the state of the art. This also shows how the work of the Graphene Flagship strongly aligns with the UN's Sustainable Development Goals."

Tags:  Aldo di Carlo  Andrea C. Ferrari  Emmanuel Kymakis  Francesco Bonaccorso  Graphene  Graphene Flagship  optoelectronics  solar cells  University of Rome Tor Vergata 

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