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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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A fast light detector made of two-dimensional materials

Posted By Graphene Council, Monday, February 17, 2020

Two research groups at ETH Zurich have joined forces to develop a novel light detector. It consists of two-dimensional layers of different materials that are coupled to a silicon optical waveguide. In the future, this approach can also be used to make LEDs and optical modulators.

Fast and highly efficient modulators as well as detectors for light are the core components of data transmission through fibre optic cables. In recent years, those building blocks for telecommunications based on existing optical materials have been constantly improved, but now it is getting increasingly difficult to achieve further improvements. That takes the combined forces of different specializations, as two research groups at ETH Zurich have now shown.

A group of scientists led by professors Jürg Leuthold of the Institute for Electromagnetic Fields and Lukas Novotny of the Institute for Photonics, together with colleagues at the National Institute for Material Science in Tsukuba (Japan), have developed an extremely fast and sensitive light detector based on the interplay between novel two-dimensional materials and nano-photonic optical waveguides. Their results were recently published in the scientific journal Nature Nanotechnology.

Two-dimensional materials

“In our detector we wanted to exploit the advantages of different materials whilst overcoming their individual constraints,” explains Nikolaus Flöry, a PhD student in Novotny’s group. “The best way of doing so is to fabricate a kind of artificial crystal – also known as heterostructure – from different layers that are each only a few atoms thick. Moreover, we were interested to know whether all the buzz about such two-dimensional materials for practical applications is actually justified.”

In two-dimensional materials, such as graphene, electrons only move in a plane rather than three spatial dimensions. This profoundly alters their transport properties, for instance when an electrical voltage is applied. While graphene is not the ideal choice for optics applications, compounds of transition metals such as molybdenum or tungsten and chalcogenes such as sulphur or tellurium (abbreviated as TMDC) are highly photosensitive and, on top of that, can be easily combined with silicon optical waveguides.

Interplay of different approaches

The expertise for the waveguides and high-speed optoelectronics came from the research group of Jürg Leuthold. Ping Ma, the group’s Senior Scientist, stresses that it was the interplay between the two approaches that made the new detector possible: “Understanding both the two-dimensional materials and the waveguides through which light is fed into the detector was of fundamental importance to our success. Together, we realized that two-dimensional materials are particularly suited to being combined with silicon waveguides. Our groups’ specializations complemented each other perfectly.”

The researchers had to find a way to make the ordinarily rather slow TMDC-based detectors faster. On the other hand, the detector had to be optimally coupled to the silicon structures used as an interface without sacrificing its high-speed performance.  

Speed through vertical structure
“We solved the speed problem by realizing a vertical heterostructure made of a TMDC – molybdenum ditelluride in our case – and graphene,” Flöry says. Differently from conventional detectors, in that way electrons excited by incoming light particles don’t need to first make their way through the bulk of the material before being measured. Instead, the two-dimensional layer of TMDC ensures that electrons can leave the material in a very short time either upwards or downwards.

The faster they leave, the larger is the bandwidth of the detector. The bandwidth indicates at what frequency data encoded in light pulses can be received. “We had hoped to get a few Gigahertz of bandwidth with our new technology – in the end, we actually reached 50 Gigahertz,” says Flöry. Up to now, bandwidths of less than a Gigahertz were possible with TMDC-based detectors.

Optimal light coupling, on the other hand, was achieved by integrating the detector into a nano-photonic optical waveguide. A so-called evanescent wave, which laterally protrudes from the waveguide, feeds the photons through a graphene layer (which has a low electrical resistance) into the molybdenum-ditelluride layer of the heterostructure. There, they excite electrons that are eventually detected as a current. The integrated waveguide design ensures that enough light is absorbed in that process.

Technology with multiple possibilities

The ETH researchers are convinced that with this combination of waveguides and heterostructures they can make not just light detectors, but also other optical elements such as light modulators, LEDs and lasers. “The possibilities are almost limitless,” Flöry and Ma enthuse about their discovery. “We just picked out the photodetector as an example of what can be done with this technology.”

In the near future, the scientists want to use their findings and investigate other two-dimensional materials. About a hundred of them are known to date, which gives countless possible combinations for novel heterostructures. Moreover, they want to exploit other physical effects, such as plasmons, in order to improve the performance of their device even further.

Tags:  2D materials  ETH Zurich  Graphene  Jürg Leuthold  LED  Lukas Novotny  Nikolaus Flöry  optoelectronics 

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Physicist obtain atomically thin molybdenum disulfide films on large-area substrates

Posted By Graphene Council, Thursday, January 23, 2020
Researchers from the Moscow Institute of Physics and Technology have managed to grow atomically thin films of molybdenum disulfide spanning up to several tens of square centimeters. It was demonstrated that the material's structure can be modified by varying the synthesis temperature. The films, which are of interest to electronics and optoelectronics, were obtained at 900-1,000 degrees Celsius. The findings were published in the journal ACS Applied Nano Materials.

Two-dimensional materials are attracting considerable interest due to their unique properties stemming from their structure and quantum mechanical restrictions. The family of 2D materials includes metals, semimetals, semiconductors, and insulators. Graphene, which is perhaps the most famous 2D material, is a monolayer of carbon atoms. It has the highest charge-carrier mobility recorded to date. However, graphene has no band gap under standard conditions, and that limits its applications.

Unlike graphene, the optimal width of the bandgap in molybdenum disulfide (MoS2) makes it suitable for use in electronic devices. Each MoS2 layer has a sandwich structure, with a layer of molybdenum squeezed between two layers of sulfur atoms. Two-dimensional van der Waals heterostructures, which combine different 2D materials, show great promise as well. In fact, they are already widely used in energy-related applications and catalysis. Wafer-scale (large-area) synthesis of 2D molybdenum disulfide shows the potential for breakthrough advances in the creation of transparent and flexible electronic devices, optical communication for next-generation computers, as well as in other fields of electronics and optoelectronics.

"The method we came up with to synthesize MoS2 involves two steps. First, a film of MoO3 is grown using the atomic layer deposition technique, which offers precise atomic layer thickness and allows conformal coating of all surfaces. And MoO3 can easily be obtained on wafers of up to 300 millimeters in diameter. Next, the film is heat-treated in sulfur vapor. As a result, the oxygen atoms in MoO3 are replaced by sulfur atoms, and MoS2 is formed. We have already learned to grow atomically thin MoS2 films on an area of up to several tens of square centimeters," explains Andrey Markeev, the head of MIPT's Atomic Layer Deposition Lab.

The researchers determined that the structure of the film depends on the sulfurization temperature. The films sulfurized at 500 ? contain crystalline grains, a few nanometers each, embedded in an amorphous matrix. At 700 ?, these crystallites are about 10-20 nm across and the S-Mo-S layers are oriented perpendicular to the surface. As a result, the surface has numerous dangling bonds. Such structure demonstrates high catalytic activity in many reactions, including the hydrogen evolution reaction. For MoS2 to be used in electronics, the S-Mo-S layers have to be parallel to the surface, which is achieved at sulfurization temperatures of 900-1,000 ?. The resulting films are as thin as 1.3 nm, or two molecular layers, and have a commercially significant (i.e., large enough) area.

The MoS2 films synthesized under optimal conditions were introduced into metal-dielectric-semiconductor prototype structures, which are based on ferroelectric hafnium oxide and model a field-effect transistor. The MoS2 film in these structures served as a semiconductor channel. Its conductivity was controlled by switching the polarization direction of the ferroelectric layer. When in contact with MoS2, the La:(HfO2-ZrO2) material, which was earlier developed in the MIPT lab, was found to have a residual polarization of approximately 18 microcoulombs per square centimeter. With a switching endurance of 5 million cycles, it topped the previous world record of 100,000 cycles for silicon channels.

Tags:  2D materials  ACS Applied Nano Materials  Andrey Markeev  Graphene  Moscow Institute of Physics and Technology  Optoelectronics  Semiconductors 

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Chemists have managed to stabilize the 'capricious' phosphorus

Posted By Graphene Council, Tuesday, January 21, 2020
An international team of Russian, Swedish and Ukrainian scientists has identified an effective strategy to improve the stability of two-dimensional black phosphorus, which is a promising material for use in optoelectronics.

The most effective mechanism of fluorination has been revealed. In addition to increased stability compared to previously proposed structures, the materials predicted by the researchers showed high antioxidative stability. The main results of the work have been presented in The Journal of Physical Chemistry Letters.

Black phosphorus is obtained from white phosphorus under conditions of high pressure and elevated temperature. The material has a layered structure and resembles graphite in appearance and properties. However, unlike graphite, it is a good semiconductor.

"Phosphorene is a monolayer of black phosphorus with interesting physical properties (high anisotropic electrical and thermal conductivity, flexible band gap variability depending on the number of layers), which makes it a promising material for use in various fields of optoelectronics (transistors, inverters, flexible electronics, solar panels). Unfortunately, one of its main problems is instability in the environment. Unlike its volumetric analogue, which is almost immune to external conditions, phosphorene quickly begins to attach oxygen from the air and degrades within a few hours. As one of the strategies for improving the stability of phosphorene, mechanism of fluorination was proposed. Over the past five years, scientists have proposed several theoretically possible options for such a "coupling". An experiment was conducted that showed a significant increase in the stability of phosphorus in ambient conditions after fluorination. However, the features of the obtained material structure remained unexplained.

Using various theoretical approaches, my colleagues and I showed that the previously proposed structures of "stabilized" phosphorus were actually unstable. It is known that phosphorus is able to form compounds with 3 or 5 fluorine atoms. Our calculations also confirmed that the characteristic coordination of the phosphorus atom in the PF system is 3 or 5. By sequential addition of atoms, it was possible to identify the most effective and really working mechanism by which fluorine atoms should attach to the surface of phosphorene. Thus, we have determined the type of structures that are likely to have been obtained by our predecessors in the above-mentioned experiment," -- said Artem Kuklin, a research fellow of SibFU.

Scientists note that the materials formed by the predicted mechanism are really stable and have increased antioxidant ability (that is, they are not quickly degradable) and their electronic properties, which do not differ much from the properties of pure phosphorus, provide the possibility of their practical application in optoelectronic devices, i.e. transistors, solar panels, flexible electronics, LEDs, photosensors, biomedical devices, optical devices for storing and transmitting information, etc.

Tags:  Artem Kuklin  Graphene  optoelectronics  photonics  Semiconductor  Siberian Federal University 

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Energy levels in electrons of 2D materials are mapped for the first time

Posted By Graphene Council, Thursday, January 9, 2020
Researchers based at the National Graphene Institute at The University of Manchester have developed an innovative measurement method that allows, for the first time, the mapping of the energy levels of electrons in the conduction band of semiconducting 2D materials.

Writing in Nature Communications, a team led by Dr Roman Gorbachev reports the first precise mapping of the conduction band of 2D indium selenide (InSe) using resonant tunnelling spectroscopy, to access the previously unexplored part of the electronic structure. They observed multiple subbands for both electrons and holes and tracked their evolution with the number of atomic layers in InSe.

Many emerging technologies rely on novel semiconductor structures, where the motion of electrons is restricted in one or more directions. Such confinement is in the nature of 2D materials and it is responsible for many of their new and exciting properties.

For instance, the colour of the emitted light shifts towards shorter wavelengths as they get thinner, analogous to quantum dots changing colour when their size is varied. As another consequence, the allowed energy available for the electrons in such materials, called conduction and valence bands, split into multiple subbands.

We hope this study will pave the way for exploration of intersubband transitions and lead to development of prototype optoelectronic devices with tuneable emission in the challenging terahertz range, Dr Roman Gorbachev.

Optical transitions between such subbands present a large potential for real-life applications as they provide optically active in terahertz and far-infrared ranges, which can be employed for security and communication technologies as light emitters or detectors.

Dr Roman Gorbachev said: “We hope this study will pave the way for exploration of intersubband transitions and lead to development of prototype optoelectronic devices with tuneable emission in the challenging terahertz range.”

Tags:  2D materials  Graphene  optoelectronics  Roman Gorbachev  Semiconductor  University of Manchester 

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