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The Department of Mechanical Engineering of The Hong Kong Polytechnic University (PolyU) has developed a novel technology of embedding highly conductive nanostructure into semiconductor nanofiber. The novel composite so produced has superb charge conductivity, and can therefore be widely applied, especially in environmental arena.

The innovation was awarded the Gold Medal with Congratulations of the Jury at the 45th International Exhibition of Inventions of Geneva, held on 29 March to 2 April this year.

A research team led by Prof. Wallace Leung develops novel semiconductor nanotubes with superb charge conductivity which can be widely used in different applications, especially in environmental arena. (PRNewsfoto/The Hong Kong Polytechnic Univer)

A research team led by Prof. Wallace Leung develops novel semiconductor nanotubes with superb charge conductivity which can be widely used in different applications, especially in environmental arena. (PRNewsfoto/The Hong Kong Polytechnic Univer)

Issues to address

Semiconductor made into nanofiber of diameter as small as 60nm (less than 1/1,000 of a human hair) have been widely used in modern daily life photonic devices (such as solar cells, photocatalyst for cleaning the environment), and non-photonic devices (such as chemical-biological sensor, lithium battery). However, electrons and holes generated by light or energy in semiconductor would readily recombine, thus reduce the current or device effectiveness. Such nature has limited the further development and applications of semiconductor nanofibers.

The novel technology developed by the research team led by Ir. Professor Wallace Leung, Chair Professor of Innovative Products and Technologies of the Department, have overcome such limitation. Applying electrospinning, the team succeeds in inserting highly conductive nano-structure (such as carbon nanotubes, graphene) into semiconductor nanofiber (such as Titanium Dioxide (TiO2 ). The novel nano-composite so produced thus provides a dedicated super-highway for electron transport, eliminating the problem of electron-hole recombination.

Amidst the potentially wide applications of the innovation in many spectrum, Professor Leung’s team has initially embarked on research of applying the novel nano-composite in two environmental aspects: solar cells, and photocatalysts for cleaning air.

Enhanced solar cell efficiency

The latest generation of solar cells (e.g. dye sensitized solar cell (DSSC), perovskite solar cell) are promising clean and renewable energy sources. Yet, for more wide applications, there are still much room for further enhancing their power conversion efficiency and producing in more cost-efficient ways.

By applying PolyU’s novel technology, carbon nanotube/graphene is embedded into the TiO2 component of DSSC and perovskite solar cell, boosting an increase of energy conversion from 40-66%. Compared to commercially available multi-crystalline silicon solar cell common in the market, with current price at US$0.25 (HK$1.94)/kWh, the cost of DSSC with carbon nanotube embedded is 12-32% higher (HK$2.18 – 2.56); while perovskite solar cell embedded with graphene is 28-40% lower (HK$1.17 – 1.40).

Given the superb charge conductivity of the novel semiconductor nanofiber, there is great potential for prompt development of more efficient solar cells, and at lower cost, than the silicon cells.

Enhanced photocatalyst performance in cleaning the air

TiO2 is the most commonly used photocatalyst material in commercially available air-purifying or disinfection devices in the market. However, TiO2 can only be activated by ultraviolet light (i.e. about 6% of solar energy), thus limiting its wider application as it is less effective in indoor environment. It is also relatively ineffective in converting nitric oxide (NO) into nitrogen dioxide (NO2), at a rate of less than 5%.

By applying PolyU’s novel technology, graphene roll is embedded into TZB composite (which mainly compose of TiO2). The novel semiconductor nanofiber so produced has superb conductivity, which provides a graphene superhighway for electrons to transport more quickly to oxide the absorbed pollutants. The technology also significantly increases the novel nano-fiber’s surface exposed for light absorption and trapping harmful molecules.

Such novel semiconductor nanofiber can convert about 90% of NO to NO2, a 35% increase compared to composite without graphene. If compared to high-standard TiO2 nano-particles commonly available in the market, the conversion rate is even 10 times more, yet 10 times more cost-efficient.

Readily available for wide applications

Given the wide uses of semiconductor nanofiber now and in the future, the PolyU groundbreaking technology that develops semiconductor nanofiber with superb charge conductivity has great potential for further development for different applications.

Besides in solar cells and photocatalysts, other obvious examples of making use of such novel technology include the development of biological-chemical sensors with enhanced sensitivity and sensing speed, and lithium batteries with lower impedance and increased storage.

SEMI, the global industry association representing the electronics manufacturing supply chain, today announced that the global semiconductor materials market increased 2.4 percent in 2016 compared to 2015 while worldwide semiconductor revenues increased 1.1 percent.

According to the SEMI Material Market Data Subscription, total wafer fabrication materials and packaging materials were $24.7 billion and $19.6 billion, respectively. Comparable revenues for these segments in 2015 were $24.0 billion for wafer fabrication materials and $19.3 billion for packaging materials. The wafer fabrication materials segment increased 3.1 percent year-over-year, while the packaging materials segment increased 1.4 percent.

For the seventh consecutive year, Taiwan was the largest consumer of semiconductor materials due to its large foundry and advanced packaging base, totaling $9.8 billion. Korea and Japan maintained the second and third places, respectively, while China rose in the rankings to claim the fourth spot during the same time. Annual revenue growth was the strongest in the China, Taiwan, and Japan markets. The materials market in Europe, Rest of World (ROW) and South Korea experienced nominal growth, while the materials market in North America contracted. (The ROW region is defined as Singapore, Malaysia, Philippines, other areas of Southeast Asia and smaller global markets.)

2015 and 2016 Regional Semiconductor Materials Markets (US$ Billions)

Region 2015* 2016 % Change
Taiwan

9.42

9.79

3.9%
South Korea

7.09

7.11

0.2%
Japan

6.56

6.74

2.8%
China

6.08

6.53

7.3%
Rest of World

6.09

6.12

0.6%
North America

4.97

4.90

-1.4%
Europe

3.07

3.12

1.5%
Total

43.29

44.32

2.4%

Source: SEMI, April 2017 Note: Figures may not add due to rounding.
* 2015 data have been updated based on SEMI’s data collection programs

A new way to grow narrow ribbons of graphene, a lightweight and strong structure of single-atom-thick carbon atoms linked into hexagons, may address a shortcoming that has prevented the material from achieving its full potential in electronic applications. Graphene nanoribbons, mere billionths of a meter wide, exhibit different electronic properties than two-dimensional sheets of the material.

This graphene nanoribbon was made bottom-up from a molecular precursor. Nanoribbon width and edge effects influence electronic behavior. Credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; scanning tunneling microscopy by Chuanxu Ma and An-Ping Li

This graphene nanoribbon was made bottom-up from a molecular precursor. Nanoribbon width and edge effects influence electronic behavior. Credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; scanning tunneling microscopy by Chuanxu Ma and An-Ping Li

“Confinement changes graphene’s behavior,” said An-Ping Li, a physicist at the Department of Energy’s Oak Ridge National Laboratory. Graphene in sheets is an excellent electrical conductor, but narrowing graphene can turn the material into a semiconductor if the ribbons are made with a specific edge shape.

Previous efforts to make graphene nanoribbons employed a metal substrate that hindered the ribbons’ useful electronic properties.

Now, scientists at ORNL and North Carolina State University report in the journal Nature Communications that they are the first to grow graphene nanoribbons without a metal substrate. Instead, they injected charge carriers that promote a chemical reaction that converts a polymer precursor into a graphene nanoribbon. At selected sites, this new technique can create interfaces between materials with different electronic properties. Such interfaces are the basis of semiconductor electronic devices from integrated circuits and transistors to light-emitting diodes and solar cells.

“Graphene is wonderful, but it has limits,” said Li. “In wide sheets, it doesn’t have an energy gap–an energy range in a solid where no electronic states can exist. That means you cannot turn it on or off.”

When a voltage is applied to a sheet of graphene in a device, electrons flow freely as they do in metals, severely limiting graphene’s application in digital electronics.

“When graphene becomes very narrow, it creates an energy gap,” Li said. “The narrower the ribbon is, the wider is the energy gap.”

In very narrow graphene nanoribbons, with a width of a nanometer or even less, how structures terminate at the edge of the ribbon is important too. For example, cutting graphene along the side of a hexagon creates an edge that resembles an armchair; this material can act like a semiconductor. Excising triangles from graphene creates a zigzag edge–and a material with metallic behavior.

To grow graphene nanoribbons with controlled width and edge structure from polymer precursors, previous researchers had used a metal substrate to catalyze a chemical reaction. However, the metal substrate suppresses useful edge states and shrinks the desired band gap.

Li and colleagues set out to get rid of this troublesome metal substrate. At the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL, they used the tip of a scanning tunneling microscope to inject either negative charge carriers (electrons) or positive charge carriers (“holes”) to try to trigger the key chemical reaction. They discovered that only holes triggered it. They were subsequently able to make a ribbon that was only seven carbon atoms wide–less than one nanometer wide–with edges in the armchair conformation.

“We figured out the fundamental mechanism, that is, how charge injection can lower the reaction barrier to promote this chemical reaction,” Li said. Moving the tip along the polymer chain, the researchers could select where they triggered this reaction and convert one hexagon of the graphene lattice at a time.

Next, the researchers will make heterojunctions with different precursor molecules and explore functionalities. They are also eager to see how long electrons can travel in these ribbons before scattering, and will compare it with a graphene nanoribbon made another way and known to conduct electrons extremely well. Using electrons like photons could provide the basis for a new electronic device that could carry current with virtually no resistance, even at room temperature.

“It’s a way to tailor physical properties for energy applications,” Li said. “This is an excellent example of direct writing. You can direct the transformation process at the molecular or atomic level.” Plus, the process could be scaled up and automated.

Researchers at North Carolina State University have developed a technique for converting positively charged (p-type) reduced graphene oxide (rGO) into negatively charged (n-type) rGO, creating a layered material that can be used to develop rGO-based transistors for use in electronic devices.

“Graphene is extremely conductive, but is not a semiconductor; graphene oxide has a bandgap like a semiconductor, but does not conduct well at all — so we created rGO,” says Jay Narayan, the John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and corresponding author of a paper describing the work. “But rGO is p-type, and we needed to find a way to make n-type rGO. And now we have it for next-generation, two-dimensional electronic devices.”

Specifically, Narayan and Anagh Bhaumik — a Ph.D. student in his lab — demonstrated two things in this study. First, they were able to integrate rGO onto sapphire and silicon wafers — across the entire wafer.

Second, the researchers used high-powered laser pulses to disrupt chemical groups at regular intervals across the wafer. This disruption moved electrons from one group to another, effectively converting p-type rGO to n-type rGO. The entire process is done at room temperature and pressure using high-power nanosecond laser pulses, and is completed in less than one-fifth of a microsecond. The laser radiation annealing provides a high degree of spatial and depth control for creating the n-type regions needed to create p-n junction-based two-dimensional electronic devices.

The end result is a wafer with a layer of n-type rGO on the surface and a layer of p-type rGO underneath.

This is critical, because the p-n junction, where the two types meet, is what makes the material useful for transistor applications.

A new class of carbon nanotubes could be the next-generation clean-up crew for toxic sludge and contaminated water, say researchers at Rochester Institute of Technology.

Single-walled carbon nanotubes filter dirty water in experiments at RIT. Credit: John-David Rocha and Reginald Rogers

Single-walled carbon nanotubes filter dirty water in experiments at RIT. Credit: John-David Rocha and Reginald Rogers

Enhanced single-walled carbon nanotubes offer a more effective and sustainable approach to water treatment and remediation than the standard industry materials–silicon gels and activated carbon–according to a paper published in the March issue of Environmental Science Water: Research and Technology.

RIT researchers John-David Rocha and Reginald Rogers, authors of the study, demonstrate the potential of this emerging technology to clean polluted water. Their work applies carbon nanotubes to environmental problems in a specific new way that builds on a nearly two decades of nanomaterial research. Nanotubes are more commonly associated with fuel-cell research.

“This aspect is new–taking knowledge of carbon nanotubes and their properties and realizing, with new processing and characterization techniques, the advantages nanotubes can provide for removing contaminants for water,” said Rocha, assistant professor in the School of Chemistry and Materials Science in RIT’s College of Science.

Rocha and Rogers are advancing nanotube technology for environmental remediation and water filtration for home use.

“We have shown that we can regenerate these materials,” said Rogers, assistant professor of chemical engineering in RIT’s Kate Gleason College of Engineering. “In the future, when your water filter finally gets saturated, put it in the microwave for about five minutes and the impurities will get evaporated off.”

Carbon nanotubes are storage units measuring about 50,000 times smaller than the width of a human hair. Carbon reduced to the nanoscale defies the rules of physics and operates in a world of quantum mechanics in which small materials become mighty.

“We know carbon as graphite for our pencils, as diamonds, as soot,” Rocha said. “We can transform that soot or graphite into a nanometer-type material known as graphene.”

A single-walled carbon nanotube is created when a sheet of graphene is rolled up. The physical change alters the material’s chemical structure and determines how it behaves. The result is “one of the most heat conductive and electrically conductive materials in the world,” Rocha said. “These are properties that only come into play because they are at the nanometer scale.”

The RIT researchers created new techniques for manipulating the tiny materials. Rocha developed a method for isolating high-quality, single-walled carbon nanotubes and for sorting them according to their semiconductive or metallic properties. Rogers redistributed the pure carbon nanotubes into thin papers akin to carbon-copy paper.

“Once the papers are formed, now we have the adsorbent–what we use to pull the contaminants out of water,” Rogers said.

The filtration process works because “carbon nanotubes dislike water,” he added. Only the organic contaminants in the water stick to the nanotube, not the water molecules.

“This type of application has not been done before,” Rogers said. “Nanotubes used in this respect is new.”

Metamaterials don’t exist in nature, but their ability to make ultra-thin lenses and ultra-efficient cell phone antennas, bend light to keep satellites cooler and let photovoltaics absorb more energy mean they offer a world of possibilities.

Formed by nanostructures that act as “atoms,” arranged on a substrate to alter light’s path in ways no ordinary material can achieve, these surrogate substances can manipulate an incoming light beam to enable the creation of more efficient versions of ubiquitous, valuable devices — optical filters, lasers, frequency converters and devices that steer beams, for example.

But extensive commercial use of metamaterials has been restrained by the limitations imposed by the materials comprising them. Metal-based metamaterials are “lossy” (lose energy) at shorter wavelengths and can operate effectively only at low frequencies, such as the radio frequencies used by radar, before being overwhelmed by their own absorption. Silicon doesn’t emit light and can transmit it only in a limited wavelength range because of its narrow working range (bandgap). So neither class of material can create a metamaterial that will operate in the infrared and optical ranges, where most military and commercial applications would take place.

This three-resonator-thick III-V metasurface of cylindrical resonators illustrates three possible uses: The left light beam changes color as it passes through the metasurfaces, signifying that nonlinear harmonic generation is taking place that converts the light beam to a shorter wavelength. The blue trace in the middle shows a train of pulses passing through the surface. As they pass, the pulse width decreases due to pulse compression, which requires that the phase of the transmitted optical wave vary with the wavelength. The multilayer metasurfaces are able to achieve the correct phase variation -- something not possible with single layer metasurfaces. The beam on the right signifies that these metasurfaces can act as efficient emitters of light. Click on the thumbnail for a high-resolution image. Credit: (Illustration courtesy of Sandia National Laboratories)

This three-resonator-thick III-V metasurface of cylindrical resonators illustrates three possible uses: The left light beam changes color as it passes through the metasurfaces, signifying that nonlinear harmonic generation is taking place that converts the light beam to a shorter wavelength. The blue trace in the middle shows a train of pulses passing through the surface. As they pass, the pulse width decreases due to pulse compression, which requires that the phase of the transmitted optical wave vary with the wavelength. The multilayer metasurfaces are able to achieve the correct phase variation — something not possible with single layer metasurfaces. The beam on the right signifies that these metasurfaces can act as efficient emitters of light. Click on the thumbnail for a high-resolution image. Credit: (Illustration courtesy of Sandia National Laboratories)

Optical metamaterials enter the arena

Sandia National Laboratories researchers are helping lead the way to the use of III-V semiconductors as the building blocks of metamaterials. (III-V refers to elements in those columns in the periodic table.) Sandia researchers have published technical papers, including three in the past year, on work featuring materials like gallium-arsenide and aluminum-arsenide, which are more efficient than metals for optical metamaterial applications, with wider bandgap ranges than silicon. The work is promising enough to have been featured on the covers of two technical journals.

“There is very little work worldwide on all-dielectric metamaterials using III-V semiconductors,” said Sandia researcher Igal Brener, who leads the Sandia work with researchers Mike Sinclair and Sheng Liu. “Our advantage is Sandia’s vast access to III-V technology, both in growth and processing, so we can move pretty fast.”

Shinier than gold

The new Sandia dielectric materials — a kind of electrical insulator — offer more than just efficiency. They lose little incoming energy and can even be fabricated in multiple layers to form complex, three-dimensional meta-atoms that reflect more light than shiny gold surfaces, usually considered the ultimate in infrared reflectivity. The III-V materials also emit photons when excited — something that silicon, which can reflect, transmit and absorb — can’t do.

Another advantage is their highly variable outputs, across the color spectrum so they might be used to extend the wavelength range of lasers or for generating “entangled photons” for quantum computing.

Sandia’s approach also is attractive for its relatively simple method of forming the artificial atoms, known as resonators, that are the guts of the metamaterial.

Created under the supervision of Liu, the meta-atoms are a few hundred nanometers in diameter and made of many actual atoms. One of Liu’s improvements was to oxidize these tiny groupings around their perimeters to create layered coatings with a low index of refraction, rather than use a more expensive, time-consuming “flip-chip” bonding process. The complexity of previous methods was an obstacle to cost- and time-efficiency. Other Sandia researchers had used a variant of his simplification previously to make lasers, but not metamaterials, he said.

The oxidized, low-index surface surrounds the high-index core “like in wintertime, you have a coat surrounding you,” Liu said. “To confine light, you need a high refractive-index contrast.” Put another way, interior light bumping into the low-indexed oxide surface is herded back by the refractive difference so it travels along the high-index core.

Liu’s Sandia colleague Gordon Keeler achieved controlled oxidation simply by putting III-V materials in a hot oven and flowing water vapor over the sample. “It will oxidize at a certain rate,” Liu says. “The more material, the longer it takes.”

The man-made meta-atoms are sculpted in place during a lithographic process that permits researchers to make any pattern they chose for the placement of the metamaterial components. “We use simulations to direct us,” Liu said. Spacing is determined to some extent by the size of the manmade atoms.

Fractured cubic nanostructures store unusually large amounts of energy

The researchers experimented with cylindrical and cubic nanostructures, reducing the symmetry of the latter to achieve even better properties.

“Cylinders are much easier to fabricate and typically can be used for conventional metasurfaces,” said Brener. “But broken-symmetry cubes are crucial to obtain very sharp resonances. That’s the key issue of the paper.”

The idea of intentionally reducing the symmetry of a cubic resonator nanostructure originated five or six years ago, said Sinclair, with a serendipitous design that happened to break the intentionally symmetrical shape of the meta-atoms when the team tried to mimic a particular manufacturing flaw.

“During a Laboratory Directed Research and Development [LDRD] Metamaterials Grand Challenge, when we were first fabricating cubic resonators in our effort to see if we could get beyond microwaves into infrared and optical metamaterials, we were playing with the shape of resonators to try to simulate the effect of lithography errors. In one simulation, we happened to cut a corner of the cube and all of a sudden very sharp reflection bands appeared,” Sinclair said.

Prior to that discovery, dielectric resonator metamaterials only showed broad bands that didn’t trap much energy. The researchers found the new sharp resonances allowed greater energy storage — beneficial for efficient frequency conversion, and perhaps even for light emission and lasing.

Exploration of the crimped resonator had to wait for a later project, sponsored by the Department of Energy’s Office of Science. Salvatore Campione, building on previous work by Lorena Basilio, Larry Warne and William Langston — all of Sandia — used electromagnetic simulations to unravel precisely how the cubes trap light. Sandia’s Willie Luk measured the cubes’ reflective properties. Another LDRD grant currently supports research into metamaterial lasing.

“We feel we’ve created a pretty flexible platform for a lot of different kinds of devices,” Sinclair said.

The ongoing work is aided by Sandia’s John Reno, nationally known for growing extremely precise crystalline structures, who contributed the III-V wafers.

Three patents on aspects of the work have been submitted.

The field of metamaterials, an intersection of materials science, physics, nanotechnology and electrical engineering, aims to produce structures with unusual electromagnetic properties. Through the careful combination of multiple materials in a precise periodic arrangement, the resulting metamaterials exhibit properties that otherwise couldn’t exist, such as a negative index of refraction. Some metamaterials can even channel electromagnetic waves around their surfaces, rendering them invisible for certain wavelengths of light.

The precision needed for arranging a metamaterial’s constitutive parts, also known as inclusions, has been a challenging step in their development and application.

Now, University of Pennsylvania engineers have shown a way to make metamaterials with a single inclusion, providing easier fabrication, among other useful features.

Physical experiments showed that the location of the dielectric rod and the shape of the ENZ material did not effect the properties of the resulting metamaterial. Credit: University of Pennsylvania

Physical experiments showed that the location of the dielectric rod and the shape of the ENZ material did not effect the properties of the resulting metamaterial. Credit: University of Pennsylvania

Analogous to electronic “doping,” where adding a small amount of atomic impurities to a “pure” material gives it electronic properties necessary for many computational and sensing devices, this “photonic doping” would allow for new ways of sculpting and tailoring light-matter interactions, with future impact on optical technology, such as flexible photonics.

The study, published in the journal Science, was led by Nader Engheta, H. Nedwill Ramsey Professor of Electrical and Systems Engineering, together with members of his group, Iñigo Liberal, Ahmed M. Mahmoud, Yue Li and Brian Edwards.

“Just as in electronic doping, when adding a set of foreign atoms in an otherwise pure material can significantly alter the electronic and optical properties of the host,” Engheta said, “‘photonic doping’ means adding a foreign photonic object in a specialized photonic host structure can change the optical scattering of the original structure in a major way.”

The phenomenon works with a specific class of materials that have permittivity, a parameter that has to do with the electric response of the material, mathematically represented by the Greek letter epsilon, that is nearly zero.

The key quality of these epsilon-near-zero, or ENZ, materials is that the wave’s magnetic field is distributed uniformly throughout the two-dimensional ENZ hosts, regardless of their cross-sectional shape. Such ENZ materials occur either naturally or can be made by traditional metamaterial means.

Rather than engineer complicated periodic structures that significantly alter the optical and magnetic properties of such materials, Engheta and his group devised a way for a single inclusion in a 2-D ENZ structure to accomplish the same task: changing which wavelengths of light that will reflect or pass through, or altering the magnetic response of the structure

“If I want to change the way a piece of material interacts with light, I normally have to change all of it,” Engheta said, “Not here. If I place a single dielectric rod anywhere within this ENZ material, the entire structure will look different from the perspective of an external wave.”

The dielectric rod is a cylindrical structure made out of an insulating material that can be polarized. When inserted in a 2-D ENZ host, it can affect the magnetic field within this host and consequently can notably change the optical properties of the host ENZ material.

Because the wave’s magnetic field in the 2-D ENZ host has a uniform spatial distribution, the dielectric rod can be placed anywhere within the material. Incoming waves thus behave as if the host material has a significantly different set of optical properties. Since the rod does not need to be placed at a precise location, construction of such photonically doped structures may be achieved with relative ease.

Applying these metamaterial concepts via “photonic doping” has implications for information processing systems and applications within telecommunications.

“When we’re working with a wave, this photonic doping can be a new way for us to determine the path this wave takes from A to B within a device,” Engheta said. “With a relatively small change in the dielectric rod, we can make waves ‘go this way’ and ‘don’t go that way.’ That we only need to make a change to the rod, which is a tiny part of the host material, should help with the speed of the device, and, because the effect is the same for the ENZ host with arbitrary shape while keeping its cross-sectional area fixed, this property may be very useful for flexible photonics.”

Further research demonstrates more complicated ways of applying photonic doping to ENZ materials, such as adding multiple rods with different diameters.

“The dielectric property of the rod can be responsive to thermal, optical or electrical changes,” Engheta said. “That means we could use the host ENZ material as the read-out of a sensor, as it would transmit or reflect light due to changes in that rod. Adding more rods would allow for even finer tuning of the material’s response.”

Russian physicists, with their colleagues from Europe through changing the light parameters, learned to generate quasiparticles – excitons, which were fully controllable and also helped to record information at room temperature. These particles act as a transitional form between photons and electrons so the researchers believe that with excitons, they will be able to create compact optoelectronic devices for rapid recording and processing an optical signal. The proposed method is based on use of a special class of materials called metal-organic frameworks. The study appeared in Advanced Materials.

The way of how the light with different wavelengths influences on a MOF crystal: different types of excitons are showed in red and blue (left). Image of crystals (right). Credit: ITMO University

The way of how the light with different wavelengths influences on a MOF crystal: different types of excitons are showed in red and blue (left). Image of crystals (right). Credit: ITMO University

To simplify the description of complex effects in quantum mechanics, scientists have introduced a concept of quasiparticles. One of them which is called exciton is an “electron – hole” pair, which provides energy transfer between photons and electrons. According to the scientific community, this mediation of quasiparticles will help to combine optics with electronics to create a fundamentally new class of equipment – more compact and energy efficient. However, all exciton demo devices either operate only at low temperature, or are difficult to manufacture which inhibits their mass adoption.

In the new study, the scientists from ITMO University in Saint Petersburg, Leipzig University in Germany and Eindhoven University of Technology in the Netherlands could generate excitons at room temperature by changing the light parameters. The authors also managed to control the quasiparticles with ultra-high sensitivity of about hundreds of femtoseconds (10-13 s). Finally, they developed an easy method for data recording with excitons. This all became possible through the use of an individual class of materials called metal-organic frameworks.

Metal-organic frameworks (MOF) synthesized at ITMO University, have a layered structure. Between the layers, there is a physical attraction called van der Waals force. To prevent the plates from uncontrollably coming together, the interlayer space is filled with an organic liquid, which fixes the framework to be three-dimensional.

In such crystals, the researchers learned to bring two types of excitons individually: intralayer and interlayer. The first arise when a photon absorbed by the crystal turns into an electron-hole pair inside a layer, but the second appear when an electron and a hole belong to neighboring layers. In some time, both kinds of quasiparticles disintegrate, re-radiating the energy as a photon. But excitons can move around the crystal while they exist.

The life time of intralayer excitons is relatively short, but their high density and agility allow one to use these quasiparticles to generate light in LEDs and lasers, for instance. Interlayer excitons are more stable, but slow-moving, so the researchers propose them to be used for the data recording. Both types of excitons fit processing of an optical signal, according to the physicists.

The innovative approach for information recording concerns the changing a distance between crystal layers to switch “on” and “off” the interlayer excitons. Valentin Milichko, the first author of the paper, associate professor of Department of Nanophotonics and Metamaterials at ITMO University, comments: “We locally heated the crystal with a laser. In the place of exposure, the layers stuck together and the luminescence of excitons disappeared while the rest of the crystal continued shining. This could mean that we recorded 1 bit of information, and the record, in the form of a dark spot, was kept for many days. To delete the data, it was enough to put the MOF into the same organic liquid that supports layers. In this case, the crystal itself is not affected, but the recorded information (the dark spot) disappears.”

The authors believe that in the future the new material will help to bring processing of an optical signal to the usual pattern of zeros and ones: “In fact, we can influence the exciton behavior in the crystal, changing the light intensity. At weak irradiation, excitons are accumulated (in ‘1’ state), but if the laser power increases, the concentration of quasiparticles grows so much that they can instantly disintegrate (in ‘0’ state),” says Valentin Milichko.

Typically, excitons occur in dielectric and semiconductor crystals, but the scientists could create these quasiparticles and get control over them in a completely different class of materials, which never was used for this. The MOF crystal combines organic components with inorganic that gives it additional properties not available for materials of a single nature. Thus, the organic term allows one to generate excitons at room temperature, but inorganic provides their efficient transfer around the crystal.

Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.

Similar to carbon, silicon forms two dimensional networks that are only one atomic layer thick. Like graphene, for whose discovery Andre Geim and Konstantin Novoselov received the Nobel Prize in 2010, these layers possess extraordinary optoelectrical properties. Silicon nanosheets might thus find application in nanoelectronics, for example in flexible displays, field-effect transistors and photodetectors. With its ability to store lithium ions, it is also under consideration as an anode material in rechargeable lithium batteries.

“Silicon nanosheets are particularly interesting because today’s information technology builds on silicon and, unlike with graphene, the basic material does not need to be exchanged,” explains Tobias Helbich from the WACKER Chair for Macromolecular Chemistry at TUM. “However, the nanosheets themselves are very delicate and quickly disintegrate when exposed to UV light, which has significantly limited their application thus far.”

Polymer and nanosheets – the best of both worlds in one

Now Helbich, in collaboration with Professor Bernhard Rieger, Chair of Macromolecular Chemistry, has for the first time successfully embedded the silicon nanosheets into a polymer, protecting them from decay. At the same time, the nanosheets are protected against oxidation. This is the first nanocomposite based on silicon nanosheets.

“What makes our nanocomposite special is that it combines the positive properties of both of its components,” explains Tobias Helbich. “The polymer matrix absorbs light in the UV domain, stabilizes the nanosheets and gives the material the properties of the polymer, while at the same time maintaining the remarkable optoelectronic properties of the nanosheets.”

Long-term goal of nanoelectronics – In leaps and bounds to industrial application

Its flexibility and durability against external influences also makes the newly developed material amenable to standard polymer technology for industrial processing. This puts actual applications within an arm’s reach.

The composites are particularly well suited for application in the up and coming field of nanoelectronics. Here, “classical” electronic components like circuits and transistors are implemented on scales of less than 100 nanometers. This allows whole new technologies to be realized – for faster computer processors, for example.

Nanoelectronic photodetector

The first successful application of the nanocomposite constructed by Helbich was only recently presented in the context of the ATUMS Graduate Program (Alberta / TUM International Graduate School for Functional Hybrid Materials): Alina Lyuleeva and Prof. Paolo Lugli from the Institute of Nanoelectronics at TU Munich, in collaboration with Helbich and Rieger, succeeded in building a photodetector based on these silicon nanosheets.

To this end, they mounted the polymer embedded silicon nanosheets onto a silicon dioxide surface coated with gold contacts. Because of its Lilliputian dimensions, this kind of nanoelectronic detector saves a lot of both space and energy.

The research is part of the ATUMS Graduate Program (Alberta / TUM International Graduate School for Functional Hybrid Materials (ATUMS; IRTG 2022)) in which German and Canadian scientists in the fields of chemistry, electrical engineering and physics collaborate closely. Their goal is not only to create novel functions based on nanoparticles and polymer materials, but, at the same time, to develop first applications. The work is funded by the German Research Council (DFG) and the Natural Science and Engineering Research Council of Canada (NSERC).

Technavio analysts forecast the global carbon nanotube (CNT) market to grow at a staggering CAGR of almost 22% during the forecast period, according to their latest report.

The research study covers the present scenario and growth prospects of the global CNT market for 2017-2021. To calculate the market size, the report considers the revenue generated from the sales of CNTs worldwide.

The production capacities of CNTs will expand due to their growing demand. Factors such as the need to enhance the efficiency of electronic and semiconductor products, high use of CNTs in the aerospace and defense sectors, and the need to increase the efficiency of energy-sector-related devices are driving the market.

Technavio’s sample reports are free of charge and contain multiple sections of the report including the market size and forecast, drivers, challenges, trends, and more.

Technavio hardware and semiconductor analysts highlight the following three factors that are contributing to the growth of the global CNT market:

  • Advantages due to physical properties
  • Potential to replace other materials
  • Rise in production capacities

Advantages due to physical properties

The structure of CNTs is closely related to graphite, which is traditionally made by stacking sheets of carbon on top of another. These sheets can easily slide over each other. CNTs are made by rolling these sheets into a cylinder, with their edges joined. This structure offers extraordinary electrical, mechanical, optical, thermal, and chemical properties to CNTs.

Sunil Kumar Singh, a lead embedded systems analyst at Technavio, says, “Being a carbon-based product, CNTs are not vulnerable to environmental or physical degradation issues. Due to this advantage, CNTs are in high demand and are used in multiple applications such as medicine, aerospace and defense, electronics, automotive, energy, construction, and sports.”

Potential to replace other materials

CNTs have the potential to replace the key materials in some industries such as semiconductor and energy. Research centers are developing CNTs that can be used in solar cells as an alternative to silicon, which is the key material used in producing electricity from solar energy. By using CNTs instead of silicon, the conversion efficiency of solar cells can be enhanced.

“CNTs have the potential to replace indium-tin-coated films, which are fragile and expensive. These films are used in liquid crystal displays, solar cells, organic light-emitting diodes, touchscreens, and high-strength materials like bulletproof vests and hydrogen fuel cells used to power cars,” adds Sunil.

Rise in production capacities

Production capacity for CNTS for 2015 was 4,567 metric tons globally. MWCNT dominates this market space due to its low production cost and high-scalability. Whereas, SWCNT still has issues with scaling up the volume produced and reduced the cost. Techniques available for CNT production such as substrate-free growth and substrate-bound growth while deploying vapor-solid-solid (VSS) and vapor-liquid-solid (VLS) are widely adopted for catalyst-based synthesis.

Many CNT vendors are investing heavily in new production facilities to meet the growing demand from sectors such as consumer goods, electrical and electronics, energy, healthcare, automobile, and aerospace and defense. Among countries, China has increased the production of CNTs backed by high government funding for nanomaterials.