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Graphene microbubbles make perfect lenses

Posted By Terrance Barkan, Monday, October 12, 2020
Tiny bubbles can solve large problems. Microbubbles -- around 1-50 micrometers in diameter -- have widespread applications. They're used for drug delivery, membrane cleaning, biofilm control, and water treatment. They've been applied as actuators in lab-on-a-chip devices for microfluidic mixing, ink-jet printing, and logic circuitry, and in photonics lithography and optical resonators. And they've contributed remarkably to biomedical imaging and applications like DNA trapping and manipulation.

Given the broad range of applications for microbubbles, many methods for generating them have been developed, including air stream compression to dissolve air into liquid, ultrasound to induce bubbles in water, and laser pulses to expose substrates immersed in liquids. However, these bubbles tend to be randomly dispersed in liquid and rather unstable.

According to Baohua Jia, professor and founding director of the Centre for Translational Atomaterials at Swinburne University of Technology, "For applications requiring precise bubble position and size, as well as high stability -- for example, in photonic applications like imaging and trapping -- creation of bubbles at accurate positions with controllable volume, curvature, and stability is essential." Jia explains that, for integration into biological or photonic platforms, it is highly desirable to have well controlled and stable microbubbles fabricated using a technique compatible with current processing technologies.

Balloons in graphene

Jia and fellow researchers from Swinburne University of Technology recently teamed up with researchers from National University of Singapore, Rutgers University, University of Melbourne, and Monash University, to develop a method to generate precisely controlled graphene microbubbles on a glass surface using laser pulses. Their report is published in the peer-reviewed, open-access journal, Advanced Photonics.

The group used graphene oxide materials, which consist of graphene film decorated with oxygen functional groups. Gases cannot penetrate through graphene oxide materials, so the researchers used laser to locally irradiate the graphene oxide film to generate gases to be encapsulated inside the film to form microbubbles -- like balloons. Han Lin, Senior Research Fellow at Swinburne University and first author on the paper, explains, "In this way, the positions of the microbubbles can be well controlled by the laser, and the microbubbles can be created and eliminated at will. In the meantime, the amount of gases can be controlled by the irradiating area and irradiating power. Therefore, high precision can be achieved."

Such a high-quality bubble can be used for advanced optoelectronic and micromechanical devices with high precision requirements.

The researchers found that the high uniformity of the graphene oxide films creates microbubbles with a perfect spherical curvature that can be used as concave reflective lenses. As a showcase, they used the concave reflective lenses to focus light. The team reports that the lens presents a high-quality focal spot in a very good shape and can be used as light source for microscopic imaging.

Lin explains that the reflective lenses are also able to focus light at different wavelengths at the same focal point without chromatic aberration. The team demonstrates the focusing of a ultrabroadband white light, covering visible to near-infrared range, with the same high performance, which is particularly useful in compact microscopy and spectroscopy.

Jia remarks that the research provides "a pathway for generating highly controlled microbubbles at will and integration of graphene microbubbles as dynamic and high precision nanophotonic components for miniaturized lab-on-a-chip devices, along with broad potential applications in high resolution spectroscopy and medical imaging."

Tags:  Baohua Jia  Graphene  graphene oxide  Han Lin  medical devices  Monash University  National University of Singapore  optoelectronics  Photonics  Rutgers University  Swinburne University of Technology  University of Melbourne 

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Intelligent nanomaterials for photonics

Posted By Terrance Barkan, Friday, October 9, 2020
At the latest since the Nobel Prize in Physics was awarded for research on graphene in 2010, 2D materials – nanosheets with atomic thickness – have been a hot topic in science.

This significant interest is due to their outstanding properties, which have enormous potential for a wide variety of applications. For instance, combined with optical fibres, 2D materials can enable novel applications in the areas of sensors, non-linear optics, and quantum technologies. However, combining these two components has so far been very laborious. Typically, the atomically thin layers had to be produced separately before being transferred by hand onto the optical fibre. Together with Australian colleagues, Jena researchers have now succeeded for the first time in growing 2D materials directly on optical fibres. This approach significantly facilitates manufacturing of such hybrids. The results of the study were reported recently in the renowned journal on materials science “Advanced Materials”.

Growth through a technologically relevant procedure
“We integrated transition metal dichalcogenides – a 2D material with excellent optical and photonic properties, which, for example, interacts strongly with light – into specially developed glass fibres,” explains Dr Falk Eilenberger of the Univer­sity of Jena and the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) in Germany. “Unlike in the past, we did not apply the half-nanometre-thick sheet manually, but grew it directly on the fibre,” says Eilenberger, a specialist in the field of nano­photonics. “This improvement means that the 2D material can be integrated into the fibre more easily and on a large scale. We were also able to show that the light in the glass fibre strongly interacts with its coating.” The step to a practical application for the intelligent nanomaterial thus created is no longer very far away.

The success has been achieved thanks to a growth process developed at the Institute of Physical Chemistry of the University of Jena, which overcomes previous hurdles. “By ana­lysing and controlling the growth parameters, we identified the conditions at which the 2D material can directly grow in the fibres,” says Jena 2D materials expert Prof. Andrey Turchanin, explaining the method based on chemical vapour deposition (CVD) techniques. Among other things, a temperature of over 700 degrees Celsius is necessary for the 2D material growth. 

Hybrid material platform
Despite this high temperature, the optical fibres can be used for the direct CVD growth: “The pure quartz glass that serves as the substrate withstands the high temperatures extremely well. It is heat-resistant up to 2,000 degrees Celsius,” says Prof. Markus A. Schmidt of the Leibniz Institute of Photonic Technology, who developed the fibres. “Their small diameter and flexibility enable a variety of applications,” adds Schmidt, who also holds an endowed professorship for fibre optics at the University of Jena.

The combination of 2D material and glass fibre has thus created an intelligent material platform that combines the best of both worlds. “Due to the functionalisation of the glass fibre with the 2D material, the interaction length between light and material has now been significantly increased,” says Dr Antony George, who is developing the manufacturing method for the novel 2D materials together with Turchanin.

Sensors and non-linear light converters
The team envisages potential applications for the newly developed materials system in two particular areas. Firstly, the materials combination is very promising for sensor technology. It could be used, for example, to detect low concentrations of gases. To this end, a green light sent through the fibre picks up information from the environment at the fibre areas functionalised with the 2D material. As external influences change the fluorescent properties of the 2D material, the light changes colour and returns to a measuring device as red light. Since the fibres are very fine, sensors based on this technology might also be suitable for applications in biotechnology or medicine.

Secondly, such a system could also be used as a non-linear light converter. Due to its non-linear properties, the hybrid optical fibre can be employed to convert a monochromatic laser light into white light for spectroscopy applications in biology and chemistry. The Jena researchers also envisage applications in the areas of quantum electronics and quantum commu­nication.

Exceptional interdisciplinary cooperation
The scientists involved in this development emphasise that the success of the project was primarily due to the exceptional interdisciplinary cooperation between various research institutes in Jena. Based on the Thuringian research group “2D-Sens” and the Collaborative Research Centre “Nonlinear Optics down to Atomic Scales” of Friedrich Schiller University, experts from the Institute of Applied Physics and Institute of Physical Chemistry of the University of Jena; the University’s Abbe Center of Photonics; the Fraunhofer Institute for Applied Optics and Precision Engineering IOF; and the Leibniz Institute of Photonic Technology are collaborating on this research, together with colleagues in Australia.

“We have brought diverse expertise to this project and we are delighted with the results achieved,” says Eilenberger. “We are convinced that the technology we have developed will further strengthen the state of Thuringia as an industrial centre with its focus on photonics and optoelectronics,” adds Turchanin. A patent applica­tion for the interdisciplinary team’s invention has recently been filed.

Tags:  2D materials  Andrey Turchanin  chemical vapour deposition  Falk Eilenberger  Fraunhofer Institute for Applied Optics and Precis  Graphene  Leibniz Institute of Photonic Technology  Markus A. Schmidt  nanomaterials  optical fibres  photonics  quantum technologies  Univer­sity of Jena 

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The most sensitive and fastest graphene microwave bolometer

Posted By Terrance Barkan, Friday, October 2, 2020
Bolometers are devices that measure the power of incident electromagnetic radiation thru the heating of materials, which exhibit a temperature-electric resistance dependence. These instruments are among the most sensitive detectors so far used for infrared radiation detection and are key tools for applications that range from advanced thermal imaging, night vision, infrared spectroscopy to observational astronomy, to name a few.

Even though they have proven to be excellent sensors for this specific range of radiation, the challenge lies in attaining high sensitivity, fast response time and strong light absorption, which not always are accomplished all together. Many studies have been conducted to obtain these higher-sensitivity bolometers by searching to reduce the size of the detector and thus increase the thermal response, and in doing so, they have found that graphene seems to be an excellent candidate for this.

If we focus on the infrared range, several experiments have demonstrated that if you take a sheet of graphene and place it in between two layers of superconducting material to create a Josephson junction, you can obtain a single photon detector device. At low temperatures, and in the absence of photons, a superconducting current flows through the device. When a single infrared photon passes through the detector, the heat it generates is enough to warm up the graphene, which alters the Josephson junction such that no superconducting current can flow. So you can actually detect the photons that are passing through the device by measuring the current. This can be done basically because graphene has an almost negligible electronic heat capacity. This means that, contrary to materials that retain heat like water, in the case of graphene a single low-energy photon can heat the detector enough to block the superconducting current, and then dissipate quickly, allowing the detector to rapidly reset, and thus achieving very fast time responses and high sensitivities.

Trying to take a step further and move to higher wavelengths, in a recent study published in Nature, a team of scientists which includes ICFO researcher Dmitri Efetov, together with colleagues from Harvard University, Raytheon BBN Technologies, MIT, and the National Institute for Material Sciences, has been able to develop a graphene-based bolometer that can detect microwave photons at extremely high sensitivities and with fast time responses.

Just like with the infrared range, the team took a sheet of graphene and placed it in between two layers of superconducting material to create a Josephson junction. This time, they went an entirely new route and attached a microwave resonator to generate the microwave photons and by passing these photons through the device, were able to reach an unprecedented detection levels. In particular, they were able to detect single photons with a much lower energy resolution, equivalent to that of a single 32 Ghz photon, and achieve detection readouts 100.000 times faster than the fastest nanowire bolometers constructed so far.

The results achieved in this study mean a major breakthrough in the field of bolometers. Not only has graphene proven to be an ideal material for infrared sensing and imaging, but it has also proven to span to higher wavelengths, reaching the microwave, where it has also shown to attain extremely high sensitivities and ultra-fast read out times.

As Prof. at ICFO Dmitri Efetov comments "such achievements were thought impossible with traditional materials, and graphene did the trick again. This open entirely new avenues for quantum sensors for quantum computation and quantum communication".

Tags:  Dmitri Efetov  Graphene  ICFO  photonics  quantum materials  Sensors 

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Physicists 'trick' photons into behaving like electrons using a 'synthetic' magnetic field

Posted By Graphene Council, Thursday, September 17, 2020
Scientists have discovered an elegant way of manipulating light using a "synthetic" Lorentz force -- which in nature is responsible for many fascinating phenomena including the Aurora Borealis.

A team of theoretical physicists from the University of Exeter has pioneered a new technique to create tuneable artificial magnetic fields, which enable photons to mimic the dynamics of charged particles in real magnetic fields.

The team believe the new research, published in leading journal Nature Photonics, could have important implications for future photonic devices as it provides a novel way of manipulating light below the diffraction limit.

When charged particles, like electrons, pass through a magnetic field they feel a Lorentz force due to their electric charge, which curves their trajectory around the magnetic field lines.

This Lorentz force is responsible for many fascinating phenomena, ranging from the beautiful Northern Lights, to the famous quantum-Hall effect whose discovery was awarded the Nobel Prize.

However, because photons do not carry an electric charge, they cannot be straightforwardly controlled using real magnetic fields since they do not experience a Lorentz force; a severe limitation that is dictated by the fundamental laws of physics.

The research team have shown that it is possible to create artificial magnetic fields for light by distorting honeycomb metasurfaces -- ultra-thin 2D surfaces that are engineered to have structure on a scale much smaller than the wavelength of light.

The Exeter team were inspired by a remarkable discovery ten years ago, where it was shown that electrons propagating through a strained graphene membrane behave as if they were subjected to a large magnetic field.

The major drawback with this strain engineering approach is that to tune the artificial magnetic field one is required to modify the strain pattern with precision, which is extremely challenging, if not impossible, to do with photonic structures.

The Exeter physicists have proposed an elegant solution to overcome this fundamental lack of tunability.

Charlie-Ray Mann, the lead scientist and author of the study, explains: "These metasurfaces, support hybrid light-matter excitations, called polaritons, which are trapped on the metasurface.

"They are then deflected by the distortions in the metasurface in a similar way to how magnetic fields deflect charged particles.

"By exploiting the hybrid nature of the polaritons, we show that you can tune the artificial magnetic field by modifying the real electromagnetic environment surrounding the metasurface."

For the study, the researchers embedded the metasurface between two mirrors -- known as a photonic cavity -- and show that one can tune the artificial magnetic field by changing only the width of the photonic cavity, thereby removing the need to modify the distortion in the metasurface.

Charlie added: "We have even demonstrated that you can switch off the artificial magnetic field entirely at a critical cavity width, without having to remove the distortion in the metasurface, something that is impossible to do in graphene or any system that emulates graphene.

"Using this mechanism you can bend the trajectory of the polaritons using a tunable Lorentz-like force and also observe Landau quantization of the polariton cyclotron orbits, in direct analogy with what happens to charged particles in real magnetic fields.

"Moreover, we have shown that you can drastically reconfigure the polariton Landau level spectrum by simply changing the cavity width."

Dr Eros Mariani, the lead supervisor of the study, said: "Being able to emulate phenomena with photons that are usually thought to be exclusive to charged particles is fascinating from a fundamental point of view, but it could also have important implications for photonics applications.

"We're excited to see where this discovery leads, as it poses many intriguing questions which can be explored in many different experimental platforms across the electromagnetic spectrum."

Tags:  2D materials  Charlie-Ray Mann  Eros Mariani  Graphene  photonics  University of Exeter 

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Trapping and controlling light at the interface of atomically thin nanomaterials

Posted By Graphene Council, Friday, August 21, 2020
Light can partake in peculiar phenomena at the nanoscale. Exploring these phenomena can unlock sophisticated applications and provide useful insights into the interactions between light waves and other materials.

In a recent study, scientists at Cornell University propose a novel method by which nanoscale light can be manipulated and transported. These special modes of light transport are known to arise at finely tuned interfaces between slightly different nanomaterials. Minwoo Jung, lead researcher on this study, illustrates this concept through a simple analogy: "A floating tube has a hole in the middle, but a normal balloon doesn't. No matter how you squeeze the round balloon, it cannot be reshaped like a donut-at least not without popping the balloon, re-knitting the rubber, and re-injecting the air. Thus, a tube and a balloon are distinct in their topology because they are not connected through a smooth deformation."

Jung further explains that physicists have been interested in gluing two topologically distinct materials side by side so that one of them acts like a balloon and the other like a tube. This means that, at their interface, a process that connects these two materials must occur, much like the poking/popping/re-knitting/re-injecting from a balloon to a tube. Under the right conditions, this process can give rise to a strong channel for transmitting energy or information along the interface. Because this process can be applied to light (which acts as a carrier of energy or information), this branch of physics is called topological photonics.

Jung and his team combined the fascinating concept of topological photonics with an innovative technique that traps light in an atomically thin material. This method brought together two rapidly emerging fields in applied and fundamental physics: graphene nanolight and topological photonics. Jung says, "Graphene is a promising platform for storing and controlling nanoscale light and could be key in the development of on-chip and ultracompact nanophotonic devices, such as waveguides and cavities."

The research team ran simulations involving a graphene sheet layered on a nanopatterned material that functions as a metagate. This honeycomb-like metagate consists of a solid layer of material with holes of different sizes, centered at the vertices of the hexagons. The varying radii of these holes affect the way in which the photons pass through the material. The scientists found that strategically "gluing" together two different metagates creates a topological effect that confines photons at their interface in a predictable, controllable manner.

Different choices of metagate designs demonstrate the dimensional hierarchy of the device's topology. Specifically, depending on the metagate geometry, nanolight can be made to flow along one-dimensional edges of the topological interface or can be topologically stored at zero-dimensional (point-like) vertices. Moreover, the metagate allows for on-and-off electric switching of these waveguides or cavities. Such battery-operated topological effects can benefit the technological adoption of topological photonics in practical devices.

Jung's team is optimistic that the synergistic combination of graphene nanolight and topological photonics will spur advances in relevant research areas, like optics, material sciences, and solid-state physics. Their graphene-based material system is simple, efficient, and suitable for nanophotonic applications: a step forward in harnessing the full potential of light.

Tags:  Cornell University  Graphene  Minwoo Jung  photonics 

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Building better electron sources with graphene

Posted By Graphene Council, Thursday, July 2, 2020
Photocathodes that produce electron beams for electron microscopes and advanced accelerators can be refreshed and rebuilt repeatedly without opening the devices that rely on them, provided the electron emitting materials are deposited on single-atom-thick layers of carbon known as graphene, according to a new study published in the journal Applied Physics Letters.

“The machines that rely on these electron emitters typically operate under high vacuum,” said Los Alamos National Laboratory physicist Hisato Yamaguchi. “By choosing graphene over materials like silicon or molybdenum, which tend to degrade during use, we can clean the substrate and redeposit electron-emitting materials without opening the vacuum. This can dramatically reduce downtime and labor involved in replacing photocathodes.”

The researchers studied photocathodes made of cesium potassium antimonide, which efficiently emit electrons when illuminated with high-power, green laser light. The photocathode efficiency falls with use, and the photocathodes must be either replaced or renewed with the electron-emitting material baked off and replaced in situ. When the researchers renewed photocathodes on substrates of silicon or molybdenum, which are common materials for such devices, the photocathode performance degraded with each cycle. Following the same procedure with graphene serving as the substrate resulted in uniformly high electron emission, time and time again.

The researchers proposed that the resilience of photocathodes deposited on graphene surfaces was due to weaker binding between the emitter atoms and the underlying carbon layer. Numerical calculations based on the material properties of the emitters and graphene were consistent with the hypothesis.

The authors concluded their study by stating, “Our results provide a foundation for graphene-based, reusable substrates for high [quantum efficiency] semiconductor photocathodes.”

Tags:  Graphene  Hisato Yamaguchi  Los Alamos National Laboratory  Photonics 

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Physicists obtain molecular 'fingerprints' using plasmons

Posted By Graphene Council, Friday, June 26, 2020
Scientists from the Center for Photonics and 2D Materials of the Moscow Institute of Physics and Technology (MIPT), the University of Oviedo, Donostia International Physics Center, and CIC nanoGUNE have proposed a new way to study the properties of individual organic molecules and nanolayers of molecules. The approach, described in Nanophotonics, relies on V-shaped graphene-metal film structures.

Nondestructive analysis of molecules via infrared spectroscopy is vital in many situations in organic and inorganic chemistry: for controlling gas concentrations, detecting polymer degradation, measuring alcohol content in the blood, etc. However, this simple method is not applicable to small numbers of molecules in a nanovolume. In their recent study, researchers from Russia and Spain propose a way to address this.

A key notion underlying the new technique is that of a plasmon. Broadly defined, it refers to an electron oscillation coupled to an electromagnetic wave. Propagating together, the two can be viewed as a quasiparticle.

The study considered plasmons in a wedge-shaped structure several dozen nanometers in size. One side of the wedge is a one-atom-thick layer of carbon atoms, known as graphene. It accommodates plasmons propagating along the sheet, with oscillating charges in the form of Dirac electrons or holes. The other side of the V-shaped structure is a gold or other electrically conductive metal film that runs nearly parallel to the graphene sheet. The space in between is filled with a tapering layer of dielectric material -- for example, boron nitride -- that is 2 nanometers thick at its narrowest (fig. 1).

Such a setup enables plasmon localization, or focusing. This refers to a process that converts regular plasmons into shorter-wavelength ones, called acoustic. As a plasmon propagates along graphene, its field is forced into progressively smaller spaces in the tapering wedge. As a result, the wavelength becomes many times smaller and the field amplitude in the region between the metal and graphene gets amplified. In that manner, a regular plasmon gradually transforms into an acoustic one.

"It was previously known that polaritons and wave modes undergo such compression in tapering waveguides. We set out to examine this process specifically for graphene, but then went on to consider the possible applications of the graphene-metal system in terms of producing molecular spectra," said paper co-author Kirill Voronin from the MIPT Laboratory of Nanooptics and Plasmonics.

The team tested its idea on a molecule known as CBP, which is used in pharmaceutics and organic light emitting diodes. It is characterized by a prominent absorption peak at a wavelength of 6.9 micrometers. The study looked at the response of a layer of molecules, which was placed in the thin part of the wedge, between the metal and graphene. The molecular layer was as thin as 2 nanometers, or three orders of magnitude smaller than the wavelength of the laser exciting plasmons. Measuring such a low absorption of the molecules would be impossible using conventional spectroscopy.

In the setup proposed by the physicists, however, the field is localized in a much tighter space, enabling the team to focus on the sample so well as to register a response from several molecules or even a single large molecule such as DNA.

There are different ways to excite plasmons in graphene. The most efficient technique relies on a scattering-type scanning near-field microscope. Its needle is positioned close to graphene and irradiated with a focused light beam. Since the needle point is very small, it can excite waves with a very large wave vector -- and a small wavelength. Plasmons excited away from the tapered end of the wedge travel along graphene toward the molecules that are to be analyzed. After interacting with the molecules, the plasmons are reflected at the tapered end of the wedge and then scattered by the same needle that initially excited them, which thus doubles as a detector.

"We calculated the reflection coefficient, that is, the ratio of the reflected plasmon intensity to the intensity of the original laser radiation. The reflection coefficient clearly depends on frequency, and the maximum frequency coincides with the absorption peak of the molecules. It becomes apparent that the absorption is very weak -- about several percent -- in the case of regular graphene plasmons. When it comes to acoustic plasmons, the reflection coefficient is tens of percent lower. This means that the radiation is strongly absorbed in the small layer of molecules," adds the paper's co-author and MIPT visiting professor Alexey Nikitin, a researcher at Donostia International Physics Center, Spain.

After certain improvements to the technological processes involved, the scheme proposed by the Russian and Spanish researchers can be used as the basis for creating actual devices. According to the team, they would mainly be useful for investigating the properties of poorly studied organic compounds and for detecting known ones.

Tags:  2D materials  boron nitride  Graphene  Kirill Voronin  Moscow Institute of Physics and Technology  Photonics  University of Oviedo 

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Engineers advance insights on black phosphorus as a material for future ultra-low power flexible electronics

Posted By Graphene Council, Thursday, June 18, 2020
Black phosphorus is a crystalline material that is attracting growing research interest from semiconductor device engineers, chemists and material scientists to create high-quality atomically thin films.

From the perspective of a 2D layered material, black phosphorus shows promise for applications in next-generation flexible electronics that could enable advances in semiconductors, medical imaging, night vision and optical communication networks.

As a prospective graphene and silicon substitute, it has outstanding properties like tunable bandgap, which graphene lacks. A bandgap, an energy band in which no electron states can exist, is essential for creating the on/off flow of electrons that are needed in digital logic and for the generation of photons for LEDs and lasers.

Unfortunately, black phosphorus is hard to make and hard to keep. It degrades quickly when exposed to air. Why this happens and the exact mechanisms by which it happens—whether oxygen or moisture in the air degrade or both—remain a topic of active debate in the research community.

Vanderbilt engineering researchers have shown for the first time that the reaction of black phosphorus to oxygen can be observed at the atomic scale using in situ-transmission electron microscopy (TEM).  See YouTube videos.

The results are reported in their paper—Visualizing Oxidation Mechanisms in Few-Layered Black Phosphorus via In Situ Transmission Electron Microscopy—in the American Chemical Society’s Applied Materials & Interfaces journal.

“In research, a lot of times different and often contradictory hypothesis exist in the scientific community. However, the ability to observe a reaction at atomic resolution in real-time offers much needed clarity to propel advances. We are using the insights from our in-situ TEM experiments at atomic resolution in our lab to develop novel synthesis and preservation methods for black phosphorus,” said Piran Kidambi, assistant professor of chemical and biomolecular engineering.

“Current approaches have looked at encapsulating it with an oxide or polymer layer without really understanding why or how the oxidation proceeds,” said Andrew E. Naclerio, second year graduate student in the Department of Chemical and Biomolecular Engineering and the paper’s first author.

“Most understanding of black phosphorus oxidation has been based on results from spectroscopic probes,” said Kidambi, Naclerio’s adviser. In collaboration with Dmitri Zakharov, staff scientist at Brookhaven National Laboratory in Upton, New York, the team used environmental transmission electron microscopy (ETEM), which provides real time in-situ observation of structural  information on a sample and reaction at atomic resolution.

“This is one of the few microscopes in the United States and the world with the capability to perform atomic resolution imaging while introducing gases and heating,” Kidambi said. The collaboration grew from a peer-reviewed user proposal and is funded by Department of Energy (DOE).

“Some insights we obtained were that the reaction proceeds via the formation of an amorphous layer that subsequently evaporates. Different crystallographic edges lead to varying degrees of etching and this agrees well will with theoretical calculations,” Kidambi said.

The collaboration for theoretical calculations with two of the paper’s authors, researchers  Jeevesh Kumar and Mayank Shrivastava at the Indian Institute of Science in Bangalore, was formed at a conference where Kidambi was invited to give a talk.

The team aim to synthesize atomically thin films of black phosphorus using chemical vapor deposition, and insights on oxidation can be used to develop effective passivation techniques.

The Kidambi Research Group in the School of Engineering’s Department of Chemical and Biomolecular Engineering is affiliated with the Vanderbilt Institute of Nanoscale Science and Engineering (VINSE), the Interdisciplinary Materials Science Program and the Vanderbilt University Data Science Institute.

Tags:  Andrew E. Naclerio  Electronics  Graphene  photonics  Piran Kidambi  Vanderbilt Institute of Nanoscale Science and Engi 

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Applying 'magic angle' twistronics to manipulate the flow of light

Posted By Graphene Council, Friday, June 12, 2020
Monash researchers are part of an international collaboration applying 'twistronics' concepts (the science of layering and twisting 2D materials to control their electrical properties) to manipulate the flow of light in extreme ways.

The findings, published today in the journal Nature, hold the promise for leapfrog advances in a variety of light-driven technologies, including nano-imaging devices; high-speed, low-energy optical computers; and biosensors.

This is the first application of Moire physics and twistronics to the light-based technologies, photonics and polaritonics, opening unique opportunities for extreme photonic dispersion engineering and robust control of polaritons on 2D materials.

APPLYING TWISTRONICS TO PHOTONS

The team took inspiration from the recent discovery of superconductivity in a pair of stacked graphene layers that were rotated to the 'magic twist angle' of 1.1 degrees.

In this stacked, misaligned configuration, electrons flow with no resistance, while separately, each of the two graphene layers shows no special electrical properties.

The discovery has shown how the careful control of rotational symmetries can unveil unexpected material responses.

The research team was led by Andrea Alù at the Advanced Science Research Center at the Graduate Center, CUNY, Cheng-Wei Qiu at National University of Singapore and Qiaoliang Bao formerly at Monash University.

The team discovered that an analogous principle can be applied to manipulate light in highly unusual ways. At a specific rotation angle between two ultrathin layers of molybdenum-trioxide, the researchers were able to prevent optical diffraction and enable robust light propagation in a tightly focused beam at desired wavelengths.

Typically, light radiated from a small emitter placed over a flat surface expands away in circles very much like the waves excited by a stone that falls into a pond. In their experiments, the researchers stacked two thin sheets of molybdenum-trioxide and rotated one of the layers with respect to the other. When the materials were excited by a tiny optical emitter, they observed widely controllable light waves over the surface as the rotation angle was varied. In particular, they showed that at the photonic 'magical twist angle' the configured bilayer supports robust, diffraction-free light propagation in tightly focused channel beams over a wide range of wavelengths.

"While photons - the quanta of light - have very different physical properties than electrons, we have been intrigued by the emerging discovery of twistronics, and have been wondering if twisted two-dimensional materials may also provide unusual transport properties for light, to benefit photon-based technologies," said Andrea Alù.

"To unveil this phenomenon, we used thin layers of molybdenum trioxide. By stacking two of such layers on top of each other and controlling their relative rotation, we have observed dramatic control of the light guiding properties. At the photonic magic angle, light does not diffract, and it propagates very confined along straight lines. This is an ideal feature for nanoscience and photonic technologies."

"Our experiments were far beyond our expectations," said Dr Qingdong Ou, who led the experimental component of the study at Monash University. "By stacking 'with a twist' two thin slabs of a natural 2D material, we can manipulate infrared light propagation, most intriguingly, in a highly collimated style."

"Our study shows that twistronics for photons can open truly exciting opportunities for light-based technologies, and we are excited to continue exploring these opportunities," said National University of Singapore graduate student Guangwei Hu, who led the theoretical component.

"Following our previous discovery published in Nature in 2018, we found that biaxial van der Waals semiconductors like α-MoO3 and V2O5 represent an emerging family of material supporting exotic polaritonic behaviors," said A/Prof Qiaoliang Bao, "These natural-born hyperbolic materials offer an unprecedented platform for controlling the flow of energy at the nanoscale."

DEVELOPMENT OF TWISTRONICS AND MAGIC ANGLES IN GRAPHENE

Novel electronic properties in 'misaligned' graphene sheets was first predicted by National University of Singapore Professor (and FLEET Partner Investigator) Antonio Castro Neto in 2007, and the 'magic angle' of 1.1 degrees was theorised by FLEET PI (University of Texas in Austin) in 2011.

Superconductivity in twisted graphene was experimentally demonstrated by Pablo Jarillo-Herrero (MIT) in 2018.

THE STUDY

Topological polaritons and photonic magic angles in twisted α-MoO3 bi-layers was published in Nature today, 11 June 2020 (DOI 10.1038/s41586-020-2359-9 ).

As well as support from the Australian Research Council, support was also provided by the US Air Force Office of Scientific Research, >Vannevar Bush Fellowship, Office of Naval Research, and National Science Foundation, as well as Singapore's Agency for Science Technology and Research (A*STAR), and China's National Natural Science Foundation.

Layering and twisting of 2D materials was performed at Monash University (Department of Materials Science and Engineering), while the topological polaritons was observed and characterised at the Melbourne Centre for Nanofabrication (MCN), the Victorian Node of the Australian National Fabrication Facility (ANFF).

PHOTONICS AT FLEET

Experimental physicist Dr Qingdong Ou is a research fellow now working with Prof Michael Fuhrer at Monash University to study nano-device fabrication based on 2D materials, within FLEET's Enabling technology B.

Qingdong seeks to minimise energy losses in light-matter interactions, aiming to realise ultra-low energy consumption in 2D-material-based photonic and optoelectronic devices. He also studies highly-confined low-loss polaritons in 2D materials using near-field optical nano-imaging within FLEET's Research theme 2.

FLEET is an Australian Research Council Centre of Excellence developing a new generation of ultra-low energy electronics.

Tags:  2D materials  Andrea Alù  CUNY  Electronics  Graphene  Monash University  photonics  Qingdong Ou 

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A scalable manufacturing-technology for highly sensitive photodetectors on flexible substrates

Posted By Graphene Council, Thursday, June 4, 2020
Researchers from AMO GmbH and RWTH Aachen University have successfully demonstrated high-responsivity molybdenum disulfide (MoS2) photodetectors on flexible substrates, realized with a scalable manufacturing technology. The work has been recently published in the journal ACS Photonics, and it is the result of a cooperation with the University of Siegen, Raith B.V., AIXTRON SE, and the University of Wuppertal.

Molybdenum disulfide is a two-dimensional material that is ideally suited for realizing flexible high-sensitivity photodetectors. However, most of the devices demonstrated so far are based on MoS2 crystals of only a few micrometers in size, obtained in a complex process, poorly compatible with an industrial-scale implementation.

In their recent work, Schneider and co-workers have demonstrated an approach scalable to large-volume production of high-performance phototedectors, starting from MoS2 deposited on sapphire wafers using Metal Organic Vapor Phase Epitaxy (MOVPE). The excellent cooperation between AMO, RWTH, and AIXTRON has allowed optimizing the tools for material-growth (a commercial AIXTRON Planetary Reactor), as well as the transfer processes and the technology for realizing highly-sensitive photodetectors on flexible substrates.

This work is an important step towards real-life applications of 2D materials for flexible electronics in the areas of the Internet of Things and medical devices. In particular, “blue light hazard” – a possible risk related to certain modern light sources – can be efficiently detected by the present sensor concept. The research work was funded by the European Union (QUEFORMAL, 829035) and Graphene Flagship (785219, 881603), European regional funds (HEA2D, NW-1-1-036), the German Research Foundation (MOSTFLEX, 407080863) and the German Ministry of Education and Research (NeuroTec, 16ES1134).

Tags:  AIXTRON  AMO GmbH  Graphene  Photonics  RWTH Aachen University 

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