Category Archives: Packaging Materials

Materials that are hybrid constructions (combining organic and inorganic precursors) and quasi-two-dimensional (with malleable and highly compactable molecular structures) are on the rise in several technological applications, such as the fabrication of ever-smaller optoelectronic devices.

An article published in the journal Physical Review B describes a study in this field resulting from the doctoral research of Diana Meneses Gustin and Luís Cabral, both supervised by Victor Lopez Richard, a professor at the Federal University of São Carlos (UFSCar) in Brazil. Cabral was co-supervised by Juarez Lopes Ferreira da Silva, a professor at the University of São Paulo’s São Carlos Chemistry Institute (IQSC-USP). Gustin was supported by São Paulo Research Foundation – FAPESP via a doctoral scholarship and a scholarship for a research internship abroad.

“Gustin and Cabral explain theoretically the unique optical and transport properties resulting from interaction between a molybdenum disulfide monolayer [inorganic substance MoS2] and a substrate of azobenzene [organic substance C12H10N2],” Lopez Richard told.

Illumination makes the azobenzene molecule switch isomerization and transition from a stable trans spatial configuration to a metastable cis form, producing effects on the electron cloud in the molybdenum disulfide monolayer. These effects, which are reversible, had previously been investigated experimentally by Emanuela Margapoti in postdoctoral research conducted at UFSCar and supported by FAPESP.

Gustin and Cabral developed a model to emulate the process theoretically. “They performed ab initio simulations [computational simulations using only established science] and calculations based on density functional theory [a quantum mechanical method used to investigate the dynamics of many-body systems]. They also modeled the transport properties of the molybdenum disulfide monolayer when disturbed by variations in the azobenzene substrate,” Richard explained.

While the published paper does not address technological applications, the deployment of the effect to build a light-activated two-dimensional transistor is on the researchers’ horizon.

“The quasi two-dimensional structure makes molybdenum disulfide as attractive as graphene in terms of space reduction and malleability, but it has virtues that potentially make it even better. It’s a semiconductor with similar electrical conductivity properties to graphene’s and it’s more versatile optically because it emits light in the wavelength range from infrared to the visible region,” Richard said.

The hybrid molybdenum-disulfide-azobenzene structure is considered a highly promising material, but a great deal of research and development will be required if it is to be effectively deployed in useful devices.

Researchers at Tokyo Institute of Technology (Tokyo Tech) report a unipolar n-type transistor with a world-leading electron mobility performance of up to 7.16 cm2 V-1 s-1. This achievement heralds an exciting future for organic electronics, including the development of innovative flexible displays and wearable technologies.

Researchers worldwide are on the hunt for novel materials that can improve the performance of basic components required to develop organic electronics.

Now, a research team at Tokyo Tech’s Department of Materials Science and Engineering including Tsuyoshi Michinobu and Yang Wang report a way of increasing the electron mobility of semiconducting polymers, which have previously proven difficult to optimize. Their high-performance material achieves an electron mobility of 7.16 cm2 V-1 s-1, representing more than a 40 percent increase over previous comparable results.

In their study published in the Journal of the American Chemical Society, they focused on enhancing the performance of materials known as n-type semiconducting polymers. These n-type (negative) materials are electron dominant, in contrast to p-type (positive) materials that are hole dominant. “As negatively-charged radicals are intrinsically unstable compared to those that are positively charged, producing stable n-type semiconducting polymers has been a major challenge in organic electronics,” Michinobu explains.

The research therefore addresses both a fundamental challenge and a practical need. Wang notes that many organic solar cells, for example, are made from p-type semiconducting polymers and n-type fullerene derivatives. The drawback is that the latter are costly, difficult to synthesize and incompatible with flexible devices. “To overcome these disadvantages,” he says, “high-performance n-type semiconducting polymers are highly desired to advance research on all-polymer solar cells.”

The team’s method involved using a series of new poly(benzothiadiazole-naphthalenediimide) derivatives and fine-tuning the material’s backbone conformation. This was made possible by the introduction of vinylene bridges[1] capable of forming hydrogen bonds with neighboring fluorine and oxygen atoms. Introducing these vinylene bridges required a technical feat so as to optimize the reaction conditions.

Overall, the resultant material had an improved molecular packaging order and greater strength, which contributed to the increased electron mobility.

Using techniques such as grazing-incidence wide-angle X-ray scattering (GIWAXS), the researchers confirmed that they achieved an extremely short π-π stacking distance[2] of only 3.40 angstrom. “This value is among the shortest for high mobility organic semiconducting polymers,” says Michinobu.

There are several remaining challenges. “We need to further optimize the backbone structure,” he continues. “At the same time, side chain groups also play a significant role in determining the crystallinity and packing orientation of semiconducting polymers. We still have room for improvement.”

Wang points out that the lowest unoccupied molecular orbital (LUMO) levels were located at -3.8 to -3.9 eV for the reported polymers. “As deeper LUMO levels lead to faster and more stable electron transport, further designs that introduce sp2-N, fluorine and chlorine atoms, for example, could help achieve even deeper LUMO levels,” he says.

In future, the researchers will also aim to improve the air stability of n-channel transistors — a crucial issue for realizing practical applications that would include complementary metal-oxide-semiconductor (CMOS)-like logic circuits, all-polymer solar cells, organic photodetectors and organic thermoelectrics.

Materials that are hybrid constructions (combining organic and inorganic precursors) and quasi-two-dimensional (with malleable and highly compactable molecular structures) are on the rise in several technological applications, such as the fabrication of ever-smaller optoelectronic devices.

An article published in the journal Physical Review B describes a study in this field resulting from the doctoral research of Diana Meneses Gustin and Luís Cabral, both supervised by Victor Lopez Richard, a professor at the Federal University of São Carlos (UFSCar) in Brazil. Cabral was co-supervised by Juarez Lopes Ferreira da Silva, a professor at the University of São Paulo’s São Carlos Chemistry Institute (IQSC-USP). Gustin was supported by São Paulo Research Foundation – FAPESP via a doctoral scholarship and a scholarship for a research internship abroad.

“Gustin and Cabral explain theoretically the unique optical and transport properties resulting from interaction between a molybdenum disulfide monolayer [inorganic substance MoS2] and a substrate of azobenzene [organic substance C12H10N2],” Lopez Richard told.

Illumination makes the azobenzene molecule switch isomerization and transition from a stable trans spatial configuration to a metastable cis form, producing effects on the electron cloud in the molybdenum disulfide monolayer. These effects, which are reversible, had previously been investigated experimentally by Emanuela Margapoti in postdoctoral research conducted at UFSCar and supported by FAPESP.

Gustin and Cabral developed a model to emulate the process theoretically. “They performed ab initio simulations [computational simulations using only established science] and calculations based on density functional theory [a quantum mechanical method used to investigate the dynamics of many-body systems]. They also modeled the transport properties of the molybdenum disulfide monolayer when disturbed by variations in the azobenzene substrate,” Richard explained.

While the published paper does not address technological applications, the deployment of the effect to build a light-activated two-dimensional transistor is on the researchers’ horizon.

“The quasi two-dimensional structure makes molybdenum disulfide as attractive as graphene in terms of space reduction and malleability, but it has virtues that potentially make it even better. It’s a semiconductor with similar electrical conductivity properties to graphene’s and it’s more versatile optically because it emits light in the wavelength range from infrared to the visible region,” Richard said.

The hybrid molybdenum-disulfide-azobenzene structure is considered a highly promising material, but a great deal of research and development will be required if it is to be effectively deployed in useful devices.

Future technologies based on the principles of quantum mechanics could revolutionize information technology. But to realize the devices of tomorrow, today’s physicists must develop precise and reliable platforms to trap and manipulate quantum-mechanical particles.

In a paper published Feb. 25 in the journal Nature, a team of physicists from the University of Washington, the University of Hong Kong, the Oak Ridge National Laboratory and the University of Tennessee, report that they have developed a new system to trap individual excitons. These are bound pairs of electrons and their associated positive charges, known as holes, which can be produced when semiconductors absorb light. Excitons are promising candidates for developing new quantum technologies that could revolutionize the computation and communications fields.

The team, led by Xiaodong Xu, the UW’s Boeing Distinguished Professor of both physics and materials science and engineering, worked with two single-layered 2D semiconductors, molybdenum diselenide and tungsten diselenide, which have similar honeycomb-like arrangements of atoms in a single plane. When the researchers placed these 2D materials together, a small twist between the two layers created a “superlattice” structure known as a moiré pattern — a periodic geometric pattern when viewed from above. The researchers found that, at temperatures just a few degrees above absolute zero, this moiré pattern created a nanoscale-level textured landscape, similar to the dimples on the surface of a golf ball, which can trap excitons in place like eggs in an egg carton. Their system could form the basis of a novel experimental platform for monitoring excitons with precision and potentially developing new quantum technologies, said Xu, who is also a faculty researcher with the UW’s Clean Energy Institute.

Excitons are exciting candidates for communication and computer technologies because they interact with photons — single packets, or quanta, of light — in ways that change both exciton and photon properties. An exciton can be produced when a semiconductor absorbs a photon. The exciton also can later transform back into a photon. But when an exciton is first produced, it can inherit some specific properties from the individual photon, such as spin. These properties can then be manipulated by researchers, such as changing the spin direction with a magnetic field. When the exciton again becomes a photon, the photon retains information about how the exciton properties changed over its short life — typically, about a hundred nanoseconds for these excitons — in the semiconductor.

In order to utilize individual excitons’ “information-recording” properties in any technological application, researchers need a system to trap single excitons. The moiré pattern achieves this requirement. Without it, the tiny excitons, which are thought to be less than 2 nanometers in diameter, could diffuse anywhere in the sample — making it impossible to track individual excitons and the information they possess. While scientists had previously developed complex and sensitive approaches to trap several excitons close to one another, the moiré pattern developed by the UW-led team is essentially a naturally formed 2D array that can trap hundreds of excitons, if not more, with each acting as a quantum dot, a first in quantum physics.

A unique and groundbreaking feature of this system is that the properties of these traps, and thus the excitons, can be controlled by a twist. When the researchers changed the rotation angle between the two different 2D semiconductors, they observed different optical properties in excitons. For example, excitons in samples with twist angles of zero and 60 degrees displayed strikingly different magnetic moments, as well as different helicities of polarized light emission. After examining multiple samples, the researchers were able to identify these twist angle variations as “fingerprints” of excitons trapped in a moiré pattern.

In the future, the researchers hope to systematically study the effects of small twist angle variations, which can finely tune the spacing between the exciton traps — the egg carton dimples. Scientists could set the moiré pattern wavelength large enough to probe excitons in isolation or small enough that excitons are placed closely together and could “talk” to one another. This first-of-its-kind level of precision may let scientists probe the quantum-mechanical properties of excitons as they interact, which could foster the development of groundbreaking technologies, said Xu.

“In principle, these moiré potentials could function as arrays of homogenous quantum dots,” said Xu. “This artificial quantum platform is a very exciting system for exerting precision control over excitons — with engineered interaction effects and possible topological properties, which could lead to new types of devices based on the new physics.”

“The future is very rosy,” Xu added.

Researchers at The University of Manchester in the UK, led by Dr Artem Mishchenko, Prof Volodya Fal’ko and Prof Andre Geim, have discovered the quantum Hall effect in bulk graphite – a layered crystal consisting of stacked graphene layers. This is an unexpected result because the quantum Hall effect is possible only in so-called two-dimensional (2D) systems where electrons’ motion is restricted to a plane and must be disallowed in the perpendicular direction. They have also found that the material behaves differently depending on whether it contains odd or even number of graphene layers – even when the number of layers in the crystal exceeds hundreds. The work is an important step to the understanding of the fundamental properties of graphite, which have often been misunderstood, esepcially in recent years.

In their work, published in Nature Physics, Mishchenko and colleagues studied devices made from cleaved graphite crystals, which essentially contain no defects. The researchers preserved the high quality of the material also by encapsulating it in another high-quality layered material – hexagonal boron nitride. They shaped their devices in a Hall bar geometry, which allowed them to measure electron transport in the thin graphite.

“The measurements were quite simple.” explains Dr Jun Yin, the first author of the paper. “We passed a small current along the Hall bar, applied strong magnetic field perpendicular to the Hall bar plane and then measured voltages generated along and across the device to extract longitudinal resistivity and Hall resistance.

Dimensional reduction

Fal’ko who led the theory part said: “We were quite surprised when we saw the quantum Hall effect (QHE) – a sequence of quantized plateaux in the Hall resistance – accompanied by zero longitudinal resistivity in our samples. These are thick enough to behave just as a normal bulk semimetal in which QHE should be forbidden.”

The researchers say that the QHE comes from the fact that the applied magnetic field forces the electrons in graphite to move in a reduced dimension, with conductivity only allowed in the direction parallel to the field. In thin enough samples, however, this one-dimensional motion can become quantized thanks to the formation of standing electron waves. The material thus goes from being a 3D electron system to a 2D one with discrete energy levels.

Even/odd number of graphene layers is important

Another big surprise is that this QHE is very sensitive to even/odd number of graphene layers. The electrons in graphite are similar to those in graphene and come in two “flavours” (called valleys). The standing waves formed from electrons of two different flavours sit on either even – or odd – numbered layers in graphite. In films with even number of layers, the number of even and odd layers is the same, so the energies of the standing waves of different flavours coincide.

The situation is different in films with odd numbers of layers, however, because the number of even and odd layers is different, that is, there is always an extra odd layer. This results in the energy levels of the standing waves of different flavours shifting with respect to each other and means that these samples have reduced QHE energy gaps. The phenomenon even persists for graphite hundreds of layers thick.

Observations of the fractional QHE

The unexpected discoveries did not end there: the researchers say they also observed the fractional QHE in thin graphite below 0.5 K. The FQHE is different from normal QHE and is a result of strong interactions between electrons. These interactions, which can often lead to important collective phenomena such as superconductivity, magnetism and superfluidity, make the charge carriers in a FQHE material behave as quasiparticles with charge that is a fraction of that of an electron.

“Most of the results we have observed can be explained using a simple single-electron model but seeing the FQHE tells us that the picture is not so simple,” says Mishchenko. “There are plenty of electron-electron interactions in our graphite samples at high magnetic fields and low temperatures, which shows that many-body physics is important in this material.”

Coming back to graphite

Graphene has been in the limelight these last 15 years, and with reason, and graphite was pushed back a little by its one-layer-thick offspring, Mishchenko adds. “We have now come back to this old material. Knowledge gained from graphene research, improved experimental techniques (such as van der Waals assembly technology) and a better theoretical understanding (again from graphene physics), has already allowed us to discover this novel type of the QHE in graphite devices we made.

“Our work is a new stepping stone to further studies on this material, including many-body physics, like density waves, excitonic condensation or Wigner crystallization.”

The graphite studied here has natural (Bernal) stacking, but there is another stable allotrope of graphite – rhombohedral. There are no reported transport measurements on this material so far, only lots of theoretical predictions, including high-temperature superconductivity and ferromagnetism. The Manchester researchers say they thus now plan to explore this allotrope too.

“For decades graphite was used by researchers as a kind of ‘philosopher’s stone’ that can deliver all probable and improbable phenomena including room-temperature superconductivity,” Geim adds with a smile. “Our work shows what is, in principle, possible in this material, at least when it is in its purest form.”

The connection from fridge magnets to cutting edge materials science is shorter than what one might expect. The reason why a magnet sticks to your fridge is that electronic spins or magnetic moments in the magnetic material spontaneously align or order in one direction, which enables it to exert an attractive force to the steel door of your fridge and reminds you to buy milk.

Magnets are one type of materials with such built-in order. A ‘topological defect’ in such a material occurs as a discontinuity in this order, i.e. a boundary region where the order does not seamlessly transition from one area to another. These topological structures form naturally or can be highly engineered in advanced functional materials.

An article published this week in the leading journal Nature Materials by FLEET CI Prof Jan Seidel outlines emerging research into different types of ‘defective’ order, i.e. topological structures in materials, and their potential highly interesting applications in nanotechnology and nanoelectronics.

Seidel was invited by the journal editor to review current and discuss future research on domain walls and related topological structures.

Although known for a long time, domain walls as one type of topological structure have only been intensively studied in detail over recent years. It is only with recent developments in high-resolution electron microscopy (HREM) and scanning probe microscopy (SPM) that it has been shown that they can significantly affect macroscopic materials properties, and even more interestingly, that they can exhibit intrinsic properties of their own. Research in this field pioneered in part by Prof Seidel has grown extensively in the last few years and now has entire conferences dedicated to it, such as the annual International Workshop on Topological Structures in Ferroic Materials (TOPO), for which the first meeting was held in 2015 in Sydney.

Nanoelectronics based on topological structures was published in Nature Materials on 20 February 2019. Prof Seidel acknowledges funding support by the Australian Research Council (ARC) through Discovery Grants and the ARC Centre of Excellence in Future Low Energy Electronics Technologies (FLEET).

Prof Jan Seidel is a Professor at the School of Materials Science and Engineering at UNSW Sydney. Contact [email protected]

FLEET is an ARC-funded research centre bringing together over a hundred Australian and international experts to develop a new generation of ultra-low energy electronics, motivated by the need to reduce the energy consumed by computing.

Nanowires have the potential to revolutionize the technology around us. Measuring just 5-100 nanometers in diameter (a nanometer is a millionth of a millimeter), these tiny, needle-shaped crystalline structures can alter how electricity or light passes through them.

EPFL researchers have found a way to control and standardize the production of nanowires on silicon surfaces. This discovery could make it possible to grow nanowires on electronic platforms, with potential applications including the integration of nanolasers into electronic chips and improved energy conversion in solar panels. Credit: Jamani Caillet / EPFL

They can emit, concentrate and absorb light and could therefore be used to add optical functionalities to electronic chips. They could, for example, make it possible to generate lasers directly on silicon chips and to integrate single-photon emitters for coding purposes. They could even be applied in solar panels to improve how sunlight is converted into electrical energy.

Up until now, it was impossible to reproduce the process of growing nanowires on silicon semiconductors – there was no way to repeatedly produce homogeneous nanowires in specific positions. But researchers from EPFL’s Laboratory of Semiconductor Materials, run by Anna Fontcuberta i Morral, together with colleagues from MIT and the IOFFE Institute, have come up with a way of growing nanowire networks in a highly controlled and fully reproducible manner. The key was to understand what happens at the onset of nanowire growth, which goes against currently accepted theories. Their work has been published in Nature Communications.

“We think that this discovery will make it possible to realistically integrate a series of nanowires on silicon substrates,” says Fontcuberta i Morral. “Up to now, these nanowires had to be grown individually, and the process couldn’t be reproduced.”

Getting the right ratio

The standard process for producing nanowires is to make tiny holes in silicon monoxide and fill them with a nanodrop of liquid gallium. This substance then solidifies when it comes into contact with arsenic. But with this process, the substance tends to harden at the corners of the nanoholes, which means that the angle at which the nanowires will grow can’t be predicted. The search was on for a way to produce homogeneous nanowires and control their position.

Research aimed at controlling the production process has tended to focus on the diameter of the hole, but this approach has not paid off. Now EPFL researchers have shown that by altering the diameter-to-height ratio of the hole, they can perfectly control how the nanowires grow. At the right ratio, the substance will solidify in a ring around the edge of the hole, which prevents the nanowires from growing at a non-perpendicular angle. And the researchers’ process should work for all types of nanowires.

“It’s kind of like growing a plant. They need water and sunlight, but you have to get the quantities right,” says Fontcuberta i Morral.

This new production technique will be a boon for nanowire research, and further samples should soon be developed.

Switching magnetic domains in magnetic memories requires normally magnetic fields which are generated by electrical currents, hence requiring large amounts of electrical power. Now, teams from France, Spain and Germany have demonstrated the feasibility of another approach at the nanoscale: “We can induce magnetic order on a small region of our sample by employing a small electric field instead of using magnetic fields”, Dr. Sergio Valencia, HZB, points out.

The cones represents the magnetization of the nanoparticles. In the absence of electric field (strain-free state) the size and separation between particles leads to a random orientation of their magnetization, known as superparamagnetism. Credit: HZB

The samples consist of a wedge-shaped polycrystalline iron thin film deposited on top of a BaTiO3 substrate. BaTiO3 is a well-known ferroelectric and ferroelastic material: An electric field is able to distort the BaTiO3 lattice and induce mechanical strain. Analysis by electron microscopy revealed that the iron film consists of tiny nanograins (diameter 2,5 nm). At its thin end, the iron film is less than 0,5 nm thick, allowing for “low dimensionality” of the nanograins. Given their small size, the magnetic moments of the iron nanograins are disordered with respect to each other, this state is known as superparamagnetism.

At the X-PEEM-Beamline at BESSY II, the scientists analysed what happens with the magnetic order of this nanograins under a small electric field. “With X-PEEM we can map the magnetic order of the iron grains on a microscopic level and observe how their orientation changes while in-situ applying an electric field”, Dr. Ashima Arora explains, who did most of the experiments during her PhD Thesis. Their results show: the electrical field induced a strain on BaTiO3, this strain was transmitted to the iron nanograins on top of it and formerly superparamagnetic regions of the sample switched to a new state. In this new state the magnetic moments of the iron grains are all aligned along the same direction, i.e. a collective long-range ferromagnetic order known as superferromagnetism.

The experiments were performed at a temperature slightly above room temperature. “This lets us hope that the phenomenon can be used for the design of new composite materials (consisting of ferroelectric and magnetic nanoparticles) for low-power spin-based storage and logic architectures operating at ambient conditions”, Valencia says.

Controlling nanoscale magnetic bits in magnetic random access memory devices by electric field induced strain alone, is known also as straintronics. It could offer a new, scalable, fast and energy efficient alternative to nowadays magnetic memories.

Organic semiconductors are lightweight, flexible and easy to manufacture. But they often fail to meet expectations regarding efficiency and stability. Researchers at the Technical University of Munich (TUM) are now deploying data mining approaches to identify promising organic compounds for the electronics of the future.

Producing traditional solar cells made of silicon is very energy intensive. On top of that, they are rigid and brittle. Organic semiconductor materials, on the other hand, are flexible and lightweight. They would be a promising alternative, if only their efficiency and stability were on par with traditional cells.

Together with his team, Karsten Reuter, Professor of Theoretical Chemistry at the Technical University of Munich, is looking for novel substances for photovoltaics applications, as well as for displays and light-emitting diodes – OLEDs. The researchers have set their sights on organic compounds that build on frameworks of carbon atoms.

Contenders for the electronics of tomorrow

Depending on their structure and composition, these molecules, and the materials formed from them, display a wide variety of physical properties, providing a host of promising candidates for the electronics of the future.

“To date, a major problem has been tracking them down: It takes weeks to months to synthesize, test and optimize new materials in the laboratory,” says Reuter. “Using computational screening, we can accelerate this process immensely.”

Computers instead of test tubes

The researcher needs neither test tubes nor Bunsen burners to search for promising organic semiconductors. Using a powerful computer, he and his team analyze existing databases. This virtual search for relationships and patterns is known as data mining.

“Knowing what you are looking for is crucial in data mining,” says PD Dr. Harald Oberhofer, who heads the project. “In our case, it is electrical conductivity. High conductivity ensures, for example, that a lot of current flows in photovoltaic cells when sunlight excites the molecules.”

Algorithms identify key parameters

Using his algorithms, he can search for very specific physical parameters: An important one is, for example, the “coupling parameter.” The larger it is, the faster electrons move from one molecule to the next.

A further parameter is the “reorganization energy”: It defines how costly it is for a molecule to adapt its structure to the new charge following a charge transfer – the less energy required, the better the conductivity.

The research team analyzed the structural data of 64,000 organic compounds using the algorithms and grouped them into clusters. The result: Both the carbon-based molecular frameworks and the “functional groups”, i.e. the compounds attached laterally to the central framework, decisively influence the conductivity.

Identifying molecules using artificial intelligence

The clusters highlight structural frameworks and functional groups that facilitate favorable charge transport, making them particularly suitable for the development of electronic components.

“We can now use this to not only predict the properties of a molecule, but using artificial intelligence we can also design new compounds in which both the structural framework and the functional groups promise very good conductivity,” explains Reuter.

Publication:

Finding the Right Bricks for Molecular Lego: A Data Mining Approach to Organic Semiconductor Design
Christian Kunkel, Christoph Schober, Johannes T. Margraf, Karsten Reuter, Harald Oberhofer
Chem. Mater. 2019, 31, 3, 969-978 – DOI: 10.1021/acs.chemmater.8b04436
https://pubs.acs.org/doi/10.1021/acs.chemmater.8b04436

Optical circuits are set to revolutionize the performance of many devices. Not only are they 10-100 times faster than electronic circuits, but they also consume a lot less power. Within these circuits, light waves are controlled by extremely thin surfaces called metasurfaces that concentrate the waves and guide them as needed. The metasurfaces contain regularly spaced nanoparticles that can modulate electromagnetic waves over sub-micrometer wavelength scales.

The new method employs a natural process already used in fluid mechanics: dewetting. CREDIT © Vytautas Navikas / 2019 EPFL

Metasurfaces could enable engineers to make flexible photonic circuits and ultra-thin optics for a host of applications, ranging from flexible tablet computers to solar panels with enhanced light-absorption characteristics. They could also be used to create flexible sensors to be placed directly on a patient’s skin, for example, in order to measure things like pulse and blood pressure or to detect specific chemical compounds.

The catch is that creating metasurfaces using the conventional method, lithography, is a fastidious, several-hour-long process that must be done in a clean room. But EPFL engineers from the Laboratory of Photonic Materials and Fiber Devices (FIMAP) have now developed a simple method for making them in just a few minutes at low temperatures – or sometimes even at room temperature – with no need for a clean room. The EPFL’s School of Engineering method produces dielectric glass metasurfaces that can be either rigid or flexible. The results of their research appear in Nature Nanotechnology.

Turning a weakness into a strength

The new method employs a natural process already used in fluid mechanics: dewetting. This occurs when a thin film of material is deposited on a substrate and then heated. The heat causes the film to retract and break apart into tiny nanoparticles. “Dewetting is seen as a problem in manufacturing – but we decided to use it to our advantage,” says Fabien Sorin, the study’s lead author and the head of FIMAP.

With their method, the engineers were able to create dielectric glass metasurfaces – rather than metallic metasurfaces – for the first time. The advantage of dielectric metasurfaces is that they absorb very little light and have a high refractive index, making it possible to effectively modulate the light that propagates through them.

To construct these metasurfaces, the engineers first created a substrate textured with the desired architecture. Then they deposited a material – in this case, chalcogenide glass – in thin films just tens of nanometers thick. The substrate was subsequently heated for a couple of minutes until the glass became more fluid and nanoparticles began to form in the sizes and positions dictated by the substrate’s texture.

The engineers’ method is so efficient that it can produce highly sophisticated metasurfaces with several levels of nanoparticles or with arrays of nanoparticles spaced 10 nm apart. That makes the metasurfaces highly sensitive to changes in ambient conditions – such as to detect the presence of even very low concentrations of bioparticles. “This is the first time dewetting has been used to create glass metasurfaces. The advantage is that our metasurfaces are smooth and regular, and can be easily produced on large surfaces and flexible substrates,” says Sorin.