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Physicists at the Institute for Quantum Information and Matter at Caltech have discovered the first three-dimensional quantum liquid crystal — a new state of matter that may have applications in ultrafast quantum computers of the future.

These images show light patterns generated by a rhenium-based crystal using a laser method called optical second-harmonic rotational anisotropy. At left, the pattern comes from the atomic lattice of the crystal. At right, the crystal has become a 3-D quantum liquid crystal, showing a drastic departure from the pattern due to the atomic lattice alone. Credit: Hsieh Lab/Caltech

These images show light patterns generated by a rhenium-based crystal using a laser method called optical second-harmonic rotational anisotropy. At left, the pattern comes from the atomic lattice of the crystal. At right, the crystal has become a 3-D quantum liquid crystal, showing a drastic departure from the pattern due to the atomic lattice alone. Credit: Hsieh Lab/Caltech

“We have detected the existence of a fundamentally new state of matter that can be regarded as a quantum analog of a liquid crystal,” says Caltech assistant professor of physics David Hsieh, principal investigator on a new study describing the findings in the April 21 issue of Science. “There are numerous classes of such quantum liquid crystals that can, in principle, exist; therefore, our finding is likely the tip of an iceberg.”

Liquid crystals fall somewhere in between a liquid and a solid: they are made up of molecules that flow around freely as if they were a liquid but are all oriented in the same direction, as in a solid. Liquid crystals can be found in nature, such as in biological cell membranes. Alternatively, they can be made artificially — such as those found in the liquid crystal displays commonly used in watches, smartphones, televisions, and other items that have display screens.

In a “quantum” liquid crystal, electrons behave like the molecules in classical liquid crystals. That is, the electrons move around freely yet have a preferred direction of flow. The first-ever quantum liquid crystal was discovered in 1999 by Caltech’s Jim Eisenstein, the Frank J. Roshek Professor of Physics and Applied Physics. Eisenstein’s quantum liquid crystal was two-dimensional, meaning that it was confined to a single plane inside the host material — an artificially grown gallium-arsenide-based metal. Such 2-D quantum liquid crystals have since been found in several more materials including high-temperature superconductors — materials that conduct electricity with zero resistance at around -150 degrees Celsius, which is warmer than operating temperatures for traditional superconductors.

John Harter, a postdoctoral scholar in the Hsieh lab and lead author of the new study, explains that 2-D quantum liquid crystals behave in strange ways. “Electrons living in this flatland collectively decide to flow preferentially along the x-axis rather than the y-axis even though there’s nothing to distinguish one direction from the other,” he says.

Now Harter, Hsieh, and their colleagues at Oak Ridge National Laboratory and the University of Tennessee have discovered the first 3-D quantum liquid crystal. Compared to a 2-D quantum liquid crystal, the 3-D version is even more bizarre. Here, the electrons not only make a distinction between the x, y, and z axes, but they also have different magnetic properties depending on whether they flow forward or backward on a given axis.

“Running an electrical current through these materials transforms them from nonmagnets into magnets, which is highly unusual,” says Hsieh. “What’s more, in every direction that you can flow current, the magnetic strength and magnetic orientation changes. Physicists say that the electrons ‘break the symmetry’ of the lattice.”

Harter actually hit upon the discovery serendipitously. He was originally interested in studying the atomic structure of a metal compound based on the element rhenium. In particular, he was trying to characterize the structure of the crystal’s atomic lattice using a technique called optical second-harmonic rotational anisotropy. In these experiments, laser light is fired at a material, and light with twice the frequency is reflected back out. The pattern of emitted light contains information about the symmetry of the crystal. The patterns measured from the rhenium-based metal were very strange–and could not be explained by the known atomic structure of the compound.

“At first, we didn’t know what was going on,” Harter says. The researchers then learned about the concept of 3-D quantum liquid crystals, developed by Liang Fu, a physics professor at MIT. “It explained the patterns perfectly. Everything suddenly made sense,” Harter says.

The researchers say that 3-D quantum liquid crystals could play a role in a field called spintronics, in which the direction that electrons spin may be exploited to create more efficient computer chips. The discovery could also help with some of the challenges of building a quantum computer, which seeks to take advantage of the quantum nature of particles to make even faster calculations, such as those needed to decrypt codes. One of the difficulties in building such a computer is that quantum properties are extremely fragile and can easily be destroyed through interactions with their surrounding environment. A technique called topological quantum computing–developed by Caltech’s Alexei Kitaev, the Ronald and Maxine Linde Professor of Theoretical Physics and Mathematics–can solve this problem with the help of a special kind of superconductor dubbed a topological superconductor.

“In the same way that 2-D quantum liquid crystals have been proposed to be a precursor to high-temperature superconductors, 3-D quantum liquid crystals could be the precursors to the topological superconductors we’ve been looking for,” says Hsieh.

“Rather than rely on serendipity to find topological superconductors, we may now have a route to rationally creating them using 3-D quantum liquid crystals” says Harter. “That is next on our agenda.”

ClassOne Technology (classone.com), manufacturer of budget-friendly Solstice plating systems, announced it’s new CopperMax chamber — a design that is demonstrating major copper plating cost reductions for users of ≤200mm wafers.

ClassOne cited actual performance data from a CopperMax pilot installation on a Solstice tool at a Fortune 100 customer. Over a six-month period the customer tracked their actual production operating costs while using the new chamber for copper TSV, Damascene and high-rate copper plating. For the three processes with CopperMax they reported that operating costs were reduced between 95.8% and 98.4% compared with previously used conventional plating chambers.

“Many of our emerging market customers are starting to do copper plating,” said Kevin Witt, President of ClassOne Technology. “So we’ve spent a lot of time on the process, working to reduce customer costs and also increase performance. And the new CopperMax chamber is proving to do both.”

ClassOne pointed out that consumables are the largest cost factor in copper plating. Optimizing copper plating generally requires the use of expensive organic additives — which are consumed very rapidly and need to be replenished frequently.

CopperMax chamber

“We learned, however, that over 97% of those expensive additives were not being consumed by the actual plating process,” said Witt. “Most were being used up simply by contact with the anode throughout the process! So, we designed our new copper chamber specifically to keep additives away from the anode — and the results are pretty dramatic. Significant savings can be realized by high- and medium-volume users with high throughputs as well as by lower-volume and R&D users that have long idle times.”

The company explained that the CopperMax chamber employs a cation-exchange semipermeable membrane to divide the copper bath into two sections. The upper section contains all of the additives, and it actively plates the wafer. The lower section of the bath contains the anode that supplies elemental copper — which is able to travel through the membrane and into the upper section to ultimately plate the wafer. However, the membrane prevents additives from traveling down to the anode, where they would break down and form process-damaging waste products.

As a result, the CopperMax bath remains much cleaner, and bath life is extended by over 20x. This increases uptime, enables higher-quality, higher-rate Cu plating, and it reduces cost of ownership very substantially.

For example, a customer using a Solstice system with six CopperMax chambers and running TSV and high-rate copper plating will save over $300,000 per year just from additive use reductions.

In addition, the CopperMax also reduces Cu anode expenses. The chamber is designed to use inexpensive bulk anode pellets instead of solid machined Cu material, which cuts anode costs by over 50%. And since the pellets have 10x greater surface area they also increase the allowable plating rates.

“Like the rest of our equipment, this new chamber aims to serve all those smaller wafer users who have limited budgets,” said Witt. “Simply stated, CopperMax is going to give them a lot more copper plating performance for a lot less.”

Solstice plating system

In 2016, annual global semiconductor sales reached their highest-ever point, at $339 billion worldwide. In that same year, the semiconductor industry spent about $7.2 billion worldwide on wafers that serve as the substrates for microelectronics components, which can be turned into transistors, light-emitting diodes, and other electronic and photonic devices.

A new technique developed by MIT engineers may vastly reduce the overall cost of wafer technology and enable devices made from more exotic, higher-performing semiconductor materials than conventional silicon.

The new method, reported today in Nature, uses graphene — single-atom-thin sheets of graphite — as a sort of “copy machine” to transfer intricate crystalline patterns from an underlying semiconductor wafer to a top layer of identical material.

The engineers worked out carefully controlled procedures to place single sheets of graphene onto an expensive wafer. They then grew semiconducting material over the graphene layer. They found that graphene is thin enough to appear electrically invisible, allowing the top layer to see through the graphene to the underlying crystalline wafer, imprinting its patterns without being influenced by the graphene.

Graphene is also rather “slippery” and does not tend to stick to other materials easily, enabling the engineers to simply peel the top semiconducting layer from the wafer after its structures have been imprinted.

Jeehwan Kim, the Class of 1947 Career Development Assistant Professor in the departments of Mechanical Engineering and Materials Science and Engineering, says that in conventional semiconductor manufacturing, the wafer, once its crystalline pattern is transferred, is so strongly bonded to the semiconductor that it is almost impossible to separate without damaging both layers.

“You end up having to sacrifice the wafer — it becomes part of the device,” Kim says.

With the group’s new technique, Kim says manufacturers can now use graphene as an intermediate layer, allowing them to copy and paste the wafer, separate a copied film from the wafer, and reuse the wafer many times over. In addition to saving on the cost of wafers, Kim says this opens opportunities for exploring more exotic semiconductor materials.

“The industry has been stuck on silicon, and even though we’ve known about better performing semiconductors, we haven’t been able to use them, because of their cost,” Kim says. “This gives the industry freedom in choosing semiconductor materials by performance and not cost.”

Kim’s research team discovered this new technique at MIT’s Research Laboratory of Electronics. Kim’s MIT co-authors are first author and graduate student Yunjo Kim; graduate students Samuel Cruz, Babatunde Alawonde, Chris Heidelberger, Yi Song, and Kuan Qiao; postdocs Kyusang Lee, Shinhyun Choi, and Wei Kong; visiting research scholar Chanyeol Choi; Merton C. Flemings-SMA Professor of Materials Science and Engineering Eugene Fitzgerald; professor of electrical engineering and computer science Jing Kong; and assistant professor of mechanical engineering Alexie Kolpak; along with Jared Johnson and Jinwoo Hwang from Ohio State University, and Ibraheem Almansouri of Masdar Institute of Science and Technology.

Graphene shift

Since graphene’s discovery in 2004, researchers have been investigating its exceptional electrical properties in hopes of improving the performance and cost of electronic devices. Graphene is an extremely good conductor of electricity, as electrons flow through graphene with virtually no friction. Researchers, therefore, have been intent on finding ways to adapt graphene as a cheap, high-performance semiconducting material.

“People were so hopeful that we might make really fast electronic devices from graphene,” Kim says. “But it turns out it’s really hard to make a good graphene transistor.”

In order for a transistor to work, it must be able to turn a flow of electrons on and off, to generate a pattern of ones and zeros, instructing a device on how to carry out a set of computations. As it happens, it is very hard to stop the flow of electrons through graphene, making it an excellent conductor but a poor semiconductor.

Kim’s group took an entirely new approach to using graphene in semiconductors. Instead of focusing on graphene’s electrical properties, the researchers looked at the material’s mechanical features.

“We’ve had a strong belief in graphene, because it is a very robust, ultrathin, material and forms very strong covalent bonding between its atoms in the horizontal direction,” Kim says. “Interestingly, it has very weak Van der Waals forces, meaning it doesn’t react with anything vertically, which makes graphene’s surface very slippery.”

Copy and peel

The team now reports that graphene, with its ultrathin, Teflon-like properties, can be sandwiched between a wafer and its semiconducting layer, providing a barely perceptible, nonstick surface through which the semiconducting material’s atoms can still rearrange in the pattern of the wafer’s crystals. The material, once imprinted, can simply be peeled off from the graphene surface, allowing manufacturers to reuse the original wafer.

The team found that its technique, which they term “remote epitaxy,” was successful in copying and peeling off layers of semiconductors from the same semiconductor wafers. The researchers had success in applying their technique to exotic wafer and semiconducting materials, including indium phosphide, gallium arsenenide, and gallium phosphide — materials that are 50 to 100 times more expensive than silicon.

Kim says that this new technique makes it possible for manufacturers to reuse wafers — of silicon and higher-performing materials — “conceptually, ad infinitum.”

An exotic future

The group’s graphene-based peel-off technique may also advance the field of flexible electronics. In general, wafers are very rigid, making the devices they are fused to similarly inflexible. Kim says now, semiconductor devices such as LEDs and solar cells can be made to bend and twist. In fact, the group demonstrated this possibility by fabricating a flexible LED display, patterned in the MIT logo, using their technique.

“Let’s say you want to install solar cells on your car, which is not completely flat — the body has curves,” Kim says. “Can you coat your semiconductor on top of it? It’s impossible now, because it sticks to the thick wafer. Now, we can peel off, bend, and you can do conformal coating on cars, and even clothing.”

Going forward, the researchers plan to design a reusable “mother wafer” with regions made from different exotic materials. Using graphene as an intermediary, they hope to create multifunctional, high-performance devices. They are also investigating mixing and matching various semiconductors and stacking them up as a multimaterial structure.

“Now, exotic materials can be popular to use,” Kim says. “You don’t have to worry about the cost of the wafer. Let us give you the copy machine. You can grow your semiconductor device, peel it off, and reuse the wafer.”

Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have developed a proof-of-principle photoelectrochemical cell capable of capturing excess photon energy normally lost to generating heat.

Using quantum dots (QD) and a process called Multiple Exciton Generation (MEG), the NREL researchers were able to push the peak external quantum efficiency for hydrogen generation to 114 percent. The advancement could significantly boost the production of hydrogen from sunlight by using the cell to split water at a higher efficiency and lower cost than current photoelectrochemical approaches.

Details of the research are outlined in the Nature Energy paper Multiple exciton generation for photoelectrochemical hydrogen evolution reactions with quantum yields exceeding 100%, co-authored by Matthew Beard, Yong Yan, Ryan Crisp, Jing Gu, Boris Chernomordik, Gregory Pach, Ashley Marshall, and John Turner. All are from NREL; Crisp also is affiliated with the Colorado School of Mines, and Pach and Marshall are affiliated with the University of Colorado, Boulder.

Beard and other NREL scientists in 2011 published a paper in Science that showed for the first time how MEG allowed a solar cell to exceed 100 percent quantum efficiency by producing more electrons in the electrical current than the amount of photons entering the solar cell.

“The major difference here is that we captured that MEG enhancement in a chemical bond rather than just in the electrical current,” Beard said. “We demonstrated that the same process that produces extra current in a solar cell can also be applied to produce extra chemical reactions or stored energy in chemical bonds.”

The maximum theoretical efficiency of a solar cell is limited by how much photon energy can be converted into usable electrical energy, with photon energy in excess of the semiconductor absorption bandedge lost to heat. The MEG process takes advantages of the additional photon energy to generate more electrons and thus additional chemical or electrical potential, rather than generating heat. QDs, which are spherical semiconductor nanocrystals (2-10 nm in diameter), enhance the MEG process.

In current report, the multiple electrons, or charge carriers, that are generated through the MEG process within the QDs are captured and stored within the chemical bonds of a H2 molecule.

NREL researchers devised a cell based upon a lead sulfide (PbS) QD photoanode. The photoanode involves a layer of PbS quantum dots deposited on top of a titanium dioxide/fluorine-doped tin oxide dielectric stack. The chemical reaction driven by the extra electrons demonstrated a new direction in exploring high-efficiency approaches for solar fuels.

3D-Micromac AG, a developer of laser micromachining and roll-to-roll laser systems for the photovoltaic, medical device and electronics markets, is presenting its highly productive microCELL systems for laser processing of crystalline solar cells at the SNEC 2017 International Photovoltaic Power Generation Conference & Exhibition, to be held April 17-21 in Shanghai, China.

Second-generation microCELL OTF laser system from 3D-Micromac for laser contact opening (LCO) of high-efficiency PERC solar cells.

Second-generation microCELL OTF laser system from 3D-Micromac for laser contact opening (LCO) of high-efficiency PERC solar cells.

In addition to showcasing the microCELL TLS, a production solution for half-cell cutting with Thermal Laser Separation (TLS), 3D-Micromac will introduce its second-generation microCELL OTF system for Laser Contact Opening (LCO) of high-efficiency Passivated Emitter Rear Contact (PERC) solar cells.

The industry-proven microCELL OTF systems produce a selective opening on backside-passivated multi- and monocrystalline solar cells to allow more light to be absorbed by the solar cell. The newly introduced second-generation system provides outstanding productivity with a throughput of more than 8,000 wafers per hour–double the throughput of the previous-generation microCELL OTF system and well above that of competing solutions. This is facilitated by dual-lane wafer handling and on-the-fly laser processing.

The new tool generation meets customers’ requirements for inline integration into two- or three-line metallization machinery since the throughput of the single laser process step now matches that of the other process steps in the production line–ensuring that the laser process is not the bottleneck in material flow.

Besides PERC, the tool can also be used for laser-doped selective emitter processes.

A team of Columbia Engineering researchers, led by Applied Physics Assistant Professor Nanfang Yu, has invented a method to control light propagating in confined pathways, or waveguides, with high efficiency by using nano-antennas. To demonstrate this technique, they built photonic integrated devices that not only had record-small footprints but were also able to maintain optimal performance over an unprecedented broad wavelength range.

Artistic illustration of a photonic integrated device that in one arm an incident fundamental waveguide mode (with one lobe in the waveguide cross-section) is converted into the second-order mode (with two lobes in the waveguide cross-section), and in the other arm the incident fundamental waveguide mode is converted into strong surface waves, which could be used for on-chip chemical and biological sensing. Credit: Nanfang Yu/Columbia Engineering

Artistic illustration of a photonic integrated device that in one arm an incident fundamental waveguide mode (with one lobe in the waveguide cross-section) is converted into the second-order mode (with two lobes in the waveguide cross-section), and in the other arm the incident fundamental waveguide mode is converted into strong surface waves, which could be used for on-chip chemical and biological sensing. Credit: Nanfang Yu/Columbia Engineering

Photonic integrated circuits (ICs) are based on light propagating in optical waveguides, and controlling such light propagation is a central issue in building these chips, which use light instead of electrons to transport data. Yu’s method could lead to faster, more powerful, and more efficient optical chips, which in turn could transform optical communications and optical signal processing. The study is published online in Nature Nanotechnology April 17.

“We have built integrated nanophotonic devices with the smallest footprint and largest operating bandwidth ever,” Yu says. “The degree to which we can now reduce the size of photonic integrated devices with the help of nano-antennas is similar to what happened in the 1950s when large vacuum tubes were replaced by much smaller semiconductor transistors. This work provides a revolutionary solution to a fundamental scientific problem: How to control light propagating in waveguides in the most efficient way?”

The optical power of light waves propagating along waveguides is confined within the core of the waveguide: researchers can only access the guided waves via the small evanescent “tails” that exist near the waveguide surface. These elusive guided waves are particularly hard to manipulate and so photonic integrated devices are often large in size, taking up space and thus limiting the device integration density of a chip. Shrinking photonic integrated devices represents a primary challenge researchers aim to overcome, mirroring the historical progression of electronics that follows Moore’s law, that the number of transistors in electronic ICs doubles approximately every two years.

Yu’s team found that the most efficient way to control light in waveguides is to “decorate” the waveguides with optical nano-antennas: these miniature antennas pull light from inside the waveguide core, modify the light’s properties, and release light back into the waveguides. The accumulative effect of a densely packed array of nano-antennas is so strong that they could achieve functions such as waveguide mode conversion within a propagation distance no more than twice the wavelength.

“This is a breakthrough considering that conventional approaches to realize waveguide mode conversion require devices with a length that is tens of hundreds of times the wavelength,” Yu says. “We’ve been able to reduce the size of the device by a factor of 10 to 100.”

Yu’s teams created waveguide mode converters that can convert a certain waveguide mode to another waveguide mode; these are key enablers of a technology called “mode-division multiplexing” (MDM). An optical waveguide can support a fundamental waveguide mode and a set of higher-order modes, the same way a guitar string can support one fundamental tone and its harmonics. MDM is a strategy to substantially augment an optical chip’s information processing power: one could use the same color of light but several different waveguide modes to transport several independent channels of information simultaneously, all through the same waveguide. “This effect is like, for example, the George Washington Bridge magically having the capability to handle a few times more traffic volume,” Yu explains. “Our waveguide mode converters could enable the creation of much more capacitive information pathways.”

He plans next to incorporate actively tunable optical materials into the photonic integrated devices to enable active control of light propagating in waveguides. Such active devices will be the basic building blocks of augmented reality (AR) glasses–goggles that first determine the eye aberrations of the wearer and then project aberration-corrected images into the eyes–that he and his Columbia Engineering colleagues, Professors Michal Lipson, Alex Gaeta, Demetri Basov, Jim Hone, and Harish Krishnaswamy are working on now. Yu is also exploring converting waves propagating in waveguides into strong surface waves, which could eventually be used for on-chip chemical and biological sensing.

To construct magnetic memories, elements with two stable magnetization states are needed. Promising candidate for such magnetic elements are tiny rings, typically of the order of few micrometers, with clockwise or counterclockwise magnetization as the two states. Unfortunately, switching between those two states directly requires a circular magnetic field which is not easy to achieve.

A magnetic field pulse switches the initial vortex state to "onion state" with two walls. In the subsequent magnetic snapshots the domain wall motion is shown. After 58 ns both walls meet and annihilate, thus completing the switching process into the opposite sense of rotation. Credit: HZB

A magnetic field pulse switches the initial vortex state to “onion state” with two walls. In the subsequent magnetic snapshots the domain wall motion is shown. After 58 ns both walls meet and annihilate, thus completing the switching process into the opposite sense of rotation. Credit: HZB

Switching in asymmetric nanorings

But this problem can be solved, as demonstrated by a team of scientists from several institutions in Germany including Helmholtz-Zentrum Berlin: If the hole in the ring is slightly displaced, thus making the ring thinner on one side, a simple, uniaxial magnetic field pulse of some nanoseconds duration can switches between the two possible “vortex states” used for data storage (clockwise and counterclockwise).

Short magnetic field pulse is sufficient

The scientists recorded the time evolution of the magnetization dynamics of the device at the Maxymus-Beamline at BESSY II employing time-resolved x-ray microscopy during and after the short magnetic field pulse was applied. They observed how the magnetic field pulse leads in a first step to an intermediate “onion state” in the ring. This state is characterized by two domain walls, where different magnetization zones meet each other. After the external field pulse has vanished, these domain walls move towards each other and annihilate, which results in a stable opposite magnetization of the ring “vortex state”.

Very fast process for spintronics

“Our measurements show domain wall automotion with an average velocity of about 60 m/s. This is very fast for spintronic devices at zero applied field,” Dr. Mohamad-Assaad Mawass, lead author of the publication in Physical Review Applied, points out. Mawass has worked on these experiments already for his PhD at Johannes Gutenberg University Mainz (group of Prof. Kläui) in a joined project with Max Planck Institute for intelligent system at Stuttgart (Schütz-Department). He then continued his research as a postdoc research Scientist at X-PEEM beamline at HZB.

Details of domain wall motion observed

Another observation concerns the effect of the detailed topological nature of the walls in the annihilation process. According to the results, this effect influence the dynamics only on a local scale where walls experience an attractive or repulsive interaction once they get very close to each other without inhibiting the annihilation of walls through automotion. “The domain wall inertia and the stored energy, in the system, allows the walls to overcome both the local extrinsic pinning and the topological repulsion between DWs carrying the same winding number,” said Mawass. “We believe to have identified a robust and reliable switching process by domain wall automotion in ferromagnetic rings,” Mawass states. “This could pave the way for further optimization of these devices.”

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.

A new method to improve semiconductor fiber optics may lead to a material structure that might one day revolutionize the global transmission of data, according to an interdisciplinary team of researchers.

Researchers are working with semiconductor optical fibers, which hold significant advantages over silica-based fiber optics, the current technology used for transmitting nearly all digital data. Silica — glass — fibers can only transmit electronic data converted to light data. This requires external electronic devices that are expensive and consume enormous amounts of electricity. Semiconductor fibers, however, can transmit both light and electronic data and might also be able to complete the conversion from electrical to optical data on the fly during transmission, improving delivery speed.

Amorphous silicon core is inside a 1.7-micron inner-diameter glass capillary. Credit: Penn State

Amorphous silicon core is inside a 1.7-micron inner-diameter glass capillary. Credit: Penn State

Think of these conversions as exit ramps on the information superhighway, said Venkatraman Gopalan, professor of materials science and engineering, Penn State. The fewer the exits the data takes, the faster the information travels. Call it “fly-by optoelectronics,” he said.

In 2006, researchers, led by John Badding, professor of chemistry, physics, and materials science and engineering, first developed silicon fibers by embedding silicon and other semiconductor materials into silica-fiber capillaries. The fibers, comprised of a series of crystals, were limited in their ability to transmit data because imperfections, such as grain boundaries at the surfaces where the many crystals within the fiber core bonded together, forced portions of the light to scatter, disrupting the transmission.

A method designed by Xiaoyu Ji, doctoral candidate in materials science and engineering, improves on the polycrystalline core of the fiber by melting a high-purity amorphous silicon core deposited inside a 1.7-micron inner-diameter glass capillary using a scanning laser, allowing for formation of silicon single crystals that were more than 2,000 times as long as they were thick. This method transforms the core from a polycrystal with many imperfections to a single crystal with few imperfections that transmits light much more efficiently.

That process, detailed in a trio of articles published in ACS Photonics, Advanced Optical Materials, and Applied Physics Letters early this year, demonstrates a new methodology to improve data transfer by eliminating imperfections in the fiber core that can be made of various materials. Gopalan said equipment constraints kept the crystals from being longer.

Because of the ultra-small core, Ji was able to melt and refine the crystal structure of the core material at temperatures of about 750 to 930 degrees Fahrenheit, lower than a typical fiber-drawing process for silicon core fibers. The lower temperatures and the short heating time that can be controlled by the laser power and the laser scanning speed also prevented the silica capillary, which has different thermal properties, from softening and contaminating the core.

“High purity is fundamentally important for high performance when dealing with materials designated for optical or electrical use,” said Ji.

The important takeaway, said Gopalan, is that this new method lays out the methodology for how a host of materials can be embedded into fiber optics and how voids and imperfections can be reduced to increase light-transfer efficiency, necessary steps to advancing the science from its infancy.

“Glass technology has taken us this far,” said Gopalan. “The ambitious idea that Badding and my group had about 10 years ago was that glass is great, but can we do more by using the numerous electronically and optically active materials other than plain glass. That’s when we began trying to embed semiconductors into glass fiber.”

Like fiber-optic cable, which took decades to become a reliable data-delivery device, decades of work likely remains to create commercially viable, semiconductor fiber networks. It took 10 years for researchers to reach polycrystalline fibers to specifications that are far better, but are still not competitive with traditional fiber-optic cable.

“Xiaoyu has been able to start from nicely deposited amorphous silicon and germanium core and use a laser to crystallize them, so that the whole semiconductor fiber core is one nice single crystal with no boundaries,” said Gopalan. “This improved light and electronic transfer. Now we can make some real devices, not just for communications, but also for endoscopy, imaging, fiber lasers and many more.”

Gopalan said he is not only in the business of creating commercially viable materials. He is interested in dreaming big and taking the long view on new technologies. Perhaps one day, every new home constructed might have a semiconductor fiber, bringing faster internet to it.

“This is why we got into this in the first place,” said Gopalan. “Badding’s group was able to figure out how to put silicon and germanium and metals and other semiconductors into the fiber, and this method improves on that.”

Despite the many advances in portable electronic devices, one thing remains constant: the need to plug them into a wall socket to recharge. Now researchers, reporting in the journal ACS Nano, have developed a light-weight, paper-based device inspired by the Chinese and Japanese arts of paper-cutting that can harvest and store energy from body movements.

paper cutting

Researchers have developed a paper-based device inspired by the Chinese and Japanese arts of paper-cutting that can harvest and store energy from body movements. Credit: American Chemical Society

Portable electronic devices, such as watches, hearing aids and heart monitors, often require only a little energy. They usually get that power from conventional rechargeable batteries. But Zhong Lin Wang, Chenguo Hu and colleagues wanted to see if they could untether our small energy needs from the wall socket by harvesting energy from a user’s body movements. Wang and others have been working on this approach in recent years, creating triboelectric nanogenerators (TENGs) that can harness the mechanical energy all around us, such as that created by our footsteps, and then use it to power portable electronics. But most TENG devices take several hours to charge small electronics, such as a sensor, and they’re made of acrylic, which is heavy.

So the researchers turned to an ultra-light, rhombic paper-cut design a few inches long and covered it with different materials to turn it into a power unit. The four outer sides, made of gold- and graphite-coated sand paper, comprised the device’s energy-storing supercapacitor element. The inner surfaces, made of paper and coated in gold and a fluorinated ethylene propylene film, comprised the TENG energy harvester. Pressing and releasing it over just a few minutes charged the device to 1 volt, which was enough to power a remote control, temperature sensor or a watch.