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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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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-speed femtosecond laser plasmonic lithography of graphene oxide film

Posted By Graphene Council, Wednesday, May 27, 2020
Graphene analogues, such as graphene oxide (GO) and its reduced forms (rGO), are fascinating carbon materials due to the complementary properties endowed by the sp3-sp2 interconversion, revealing the substitutability and potential for industrialization of integrated graphene devices. Appropriate micro/nanostructural design of GO and rGO for controlling the energy band gap and surface chemical activity is important for developing strategic applications. The femtosecond laser plasmonic lithography (FPL) technology is a qualified candidate for generating the required structures due to its efficiency, high-quality, flexibility and controllability. However, as both the theoretical and experimental explorations of this method are still in their infancy, micro/nanoprocessing of graphene materials using FPL has not been realized. The feasibility of implementing the technique in practical applications is still questionable because most related studies only highlight the characteristics of the structure obtained from the processing but often ignore the complementary changes in the properties of the material itself.

In a new paper published in Light Science & Application, scientists from the State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, China, and co-workers presented a high-quality, efficient and large-area periodic micro/nanoripple manufacturing (~680 nm period) and photoreduction of GO films (~140 nm thickness) on a silicon substrate by using the FPL method. Interestingly, unlike most of the reported laser-induced periodic surface structures (LIPSS) in which the pattern alignment is perpendicular to the polarization of the incident light, they are found to have the extraordinary uniform distribution with orientation parallel to each other in this case. Such a phenomenon cannot be explained by the conventional theory of LIPSS, i.e., the interference between the incident light with TM mode and the excited surface plasmon (SP) wave. The analysis demonstrated that the laser-induced gradient reduction of GO film from its surface to the interior plays a key role, and it leads to an inhomogeneous slab with the maximum dielectric permittivity (DP) at the surface and a smaller DP in the interior that allows excitation of TE-mode surface plasmons (TE-SPs) and the subsequent uncommon interference. Due to the diverse physical mechanisms involved in the laser-rGO interaction, the LIPSS formation also exhibited unique characteristics such as strong robustness against a range of perturbations. Because the microprocessing contains no assistant operations, such as chemical etching, the properties of the graphene material are retained, which allows them for optoelectronic applications. As a matter of fact, through modulation of the photoreduction degree and structural design of the rGO surface, they realized the enhanced light absorption (~ 20%), thermal radiation (> 10°C) and anisotropic conductivities (anisotropy ratio ~ 0.46) from this film material. Based on it, they designed an on-chip, broadband photodetector with stable photoresponsivity (R ~ 0.7 mA W-1) even when exposed to light with the low power (0.1 mW). The authors of the paper summarize the significance of this work as follows:

"(1) The FPL technology is used for the first time to realize the preparation of high-quality, efficient and large-scale periodic micro/nanostructures on the surface of graphene materials; (2) The physical mechanisms of the laser-material interaction involved in FPL technology is further improved; (3) Both the structural characteristics and the properties of the processed material itself are taken into account in the application of photoelectric devices."

"Compared to laser direct writing adopting the same incident laser parameters, our FPL strategy takes only ~1/14000 of the time to process a centimetre-sized sample (1×1.2 cm2). At the same time, due to the possible nonlinear optical property, the FPL strategy induces an obvious 'self-repairing' phenomenon, which can effectively guarantee the processing quality. For example, we can prepare rGO-LIPSS films on different substrates and nondestructively transfer them onto other substrates."

"Our explanation of the experimental phenomena is markedly different from most of the principles at present. This will give us a clearer understanding of the relevant physical processes and lay a solid foundation for the further development of FPL technologies."

"The structured graphene materials by FPL technology present excellent photoelectric performance. The photoresponsivity is numerically comparable to the response of the samples obtained by other reduction methods (e.g., chemical and thermal) and is much larger than that of typical photoreduced ones. The anisotropy ratio is even larger than that of some natural anisotropic crystals. Our work combines the experimental exploration with the in-depth understanding of high-speed micro/nanopatterning of the regular rGO-LIPSS, which not only benefits fundamental physics but also facilitates the practical development of graphene analogues on the industrial scale. "

Tags:  Graphene  graphene oxide  photonics 

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Pretty as a peacock: The gemstone for the next generation of smart sensors

Posted By Graphene Council, Tuesday, May 19, 2020
An international team of scientists, led by the Universities of Surrey and Sussex, has developed colour-changing, flexible photonic crystals that could be used to develop sensors that warn when an earthquake might strike next.

The wearable, robust and low-cost sensors can respond sensitively to light, temperature, strain or other physical and chemical stimuli making them an extremely promising option for cost-effective smart visual sensing applications in a range of sectors including healthcare and food safety. 

In a study published by the journal Advanced Functional Materials, researchers outline a method to produce photonic crystals containing a minuscule amount of graphene resulting in a wide range of desirable qualities with outputs directly observable by the naked eye.

Intensely green under natural light, the extremely versatile sensors change colour to blue when stretched or turn transparent after being heated.

Dr. Izabela Jurewicz, Lecturer in Soft Matter Physics at the University of Surrey’s Faculty of Engineering and Physical Sciences, said “This work provides the first experimental demonstration of mechanically robust yet soft, free-standing and flexible, polymer-based opals containing solution-exfoliated pristine graphene. While these crystals are beautiful to look at, we’re also very excited about the huge impact they could make to people’s lives.”

Alan Dalton, Professor Of Experimental Physics at the University of Sussex’s School of Mathematical and Physical Sciences, said: ““Our research here has taken inspiration from the amazing biomimicry abilities in butterfly wings, peacock feathers and beetle shells where the colour comes from structure and not from pigments. Whereas nature has developed these materials over millions of years we are slowly catching up in a much shorter period.”

Among their many potential applications are:

Time-temperature indicators (TTI) for intelligent packaging – The sensors are able to give a visual indication if perishables, such as food or pharmaceuticals, have experienced undesirable time-temperature histories. The crystals are extremely sensitive to even a small rise in temperature between 20 and 100 degrees C.

Finger print analysis - Their pressure-responsive shape-memory characteristics are attractive for biometric and anti-counterfeiting applications. Pressing the crystals with a bare finger can reveal fingerprints with high precision showing well-defined ridges from the skin.

Bio-sensing – The photonic crystals can be used as tissue scaffolds for understanding human biology and disease. If functionalised with biomolecules could act as highly sensitive point-of-care testing devices for respiratory viruses offering inexpensive, reliable, user-friendly biosensing systems.

Bio/health monitoring – The sensors mechanochromic response allows for their application as body sensors which could help improve technique in sports players.

Healthcare safety – Scientists suggest the sensors could be used in a wrist band which changes colour to indicate to patients if their healthcare practitioner has washed their hands before entering an examination room.

The research draws on the Materials Physics Group’s (University of Sussex) expertise in the liquid processing of two-dimensional nanomaterials, Soft Matter Group's (University of Surrey) experience in polymer colloids and combines it with expertise at the Advanced Technology Institute in optical modelling of complex materials. Both universities are working with the Sussex-based company Advanced Materials Development (AMD) Ltd to commercialise the technology.

Joseph Keddie, Professor of Soft Matter Physics at the University of Surrey, said: “Polymer particles are used to manufacture everyday objects such as inks and paints. In this research, we were able finely distribute graphene at distances comparable to the wavelengths of visible light and showed how adding tiny amounts of the two-dimensional wonder-material leads to emerging new capabilities.” 

John Lee, CEO of Advanced Materials Development (AMD) Ltd, said: “Given the versatility of these crystals, this method represents a simple, inexpensive and scalable approach to produce multi-functional graphene infused synthetic opals and opens up exciting applications for novel nanomaterial-based photonics. We are very excited to be able to bring it to market in near future.”

Tags:  2D materials  Advanced Materials Development  Alan Dalton  Graphene  Izabela Jurewicz  John Lee  Joseph Keddie  nanomaterials  photonics  Universities of Surrey  Universities of Sussex 

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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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'White graphene' aerogel material creates pleasant laser light

Posted By Graphene Council, Friday, March 20, 2020
With a porosity of 99.99 %, it consists practically only of air, making it one of the lightest materials in the world: Aerobornitride is the name of the material developed by an international research team led by Kiel University. The scientists assume that they have thereby created a central basis for bringing laser light into a broad application range.

Based on a boron-nitrogen compound, they developed a special three-dimensional nanostructure that scatters light very strongly and hardly absorbs it. Irradiated with a laser, the material emits uniform lighting, which, depending on the type of laser, is much more efficient and powerful than LED light. Thus, lamps for car headlights, projectors or room lighting with laser light could become smaller and brighter in the future.

The research team presents their results in Nature Communications ("Conversionless efficient and broadband laser light diffusers for high brightness illumination applications").

More light in the smallest space

In research and industry, laser light has long been considered the “next generation” of light sources that could even exceed the efficiency of LEDs (light-emitting diode).

“For very bright or a lot of light, you need a large number of LEDs and thus space. But the same amount of light could also be obtained with a single laser diode that is one-thousandth smaller,” Dr. Fabian Schütt emphasizes the potential.

The materials scientist from the working group "Functional Nanomaterials" at Kiel University is the first author of the study, which involves other researchers from Germany, England, Italy, Denmark and South Korea.

Powerful small light sources allow numerous applications. The first test applications, such as in car headlights, are already available, but laser lamps have not yet become widely accepted. On the one hand, this is due to the intense, directed light of the laser diodes. On the other hand, the light consists of only one wavelength, so it is monochromatic. This leads to an unpleasant flickering when a laser beam hits a surface and is reflected there.

Porous structure scatters the light extremely strongly

“Previous developments to laser light normally work with phosphors. However, they produce a relatively cold light, are not stable in the long term and are not very efficient,” says Professor Rainer Adelung, head of the working group.

The research team in Kiel is taking a different approach: They developed a highly scattering nanostructure of hexagonal boron nitride, also known as "white graphene", which absorbs almost no light. The structure consists of a filigree network of countless fine hollow microtubes. When a laser beam hits these, it is extremely scattered inside the network structure, creating a homogeneous light source.

"Our material acts more or less like an artificial fog that produces a uniform, pleasant light output," explains Schütt. The strong scattering also contributes to the fact that the disturbing flickering is no longer visible to the human eye.

The nanostructure not only ensures that the material withstands the intense laser light, but can also scatter different wavelengths. Red, green and blue laser light can be mixed in order to create specific color effects in addition to normal white - for example, for use in innovative room lighting. Here, extremely lightweight laser diodes could lead to completely new design concepts in the future.

"However, in order to compete with LEDs in the future, the efficiency of laser diodes must be improved as well," says Schütt. The research team is now looking for industrial partners to take the step from the laboratory to application.

Wide range of applications for aeromaterials

Meanwhile the researchers from Kiel can use their method to develop highly porous nanostructures for different materials, besides boron nitride also graphene or graphite. In this way, more and more new, lightweight materials, so-called “aeromaterials”, are created, which allow particularly innovative applications. For example, the scientists are currently doing research in collaboration with companies and other universities to develop self-cleaning air filters for aircraft.

Tags:  Aerobornitride  Fabian Schütt  Graphene  Kiel University  nanomaterials  photonics  Rainer Adelung 

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