What is the Human Cortex?

The cerebral cortex is the thin surface layer of the brain found in vertebrate animals that has evolved most recently, showing the greatest variation in size among different mammals (it is especially large in humans). Each part of the cerebral cortex is six layered (e.g., L2), with different kinds of nerve cells (e.g., spiny stellate) in each layer. The cerebral cortex plays a crucial role in most higher level cognitive functions, such as thinking, memory, planning, perception, language, and attention. Although there has been some progress in understanding the macroscopic organization of this very complicated tissue, its organization at the level of individual nerve cells and their interconnecting synapses is largely unknown.

Petabyte connectomic reconstruction of a volume of human neocortex. Left: Small subvolume of the dataset. Right: A subgraph of 5000 neurons and excitatory (green) and inhibitory (red) connections in the dataset. The full graph (connectome) would be far too dense to visualize.

Mapping the structure of the brain at the resolution of individual synapses requires high-resolution microscopy techniques that can image biochemically stabilized (fixed) tissue. We collaborated with brain surgeons at Massachusetts General Hospital in Boston (MGH) who sometimes remove pieces of normal human cerebral cortex when performing a surgery to cure epilepsy in order to gain access to a site in the deeper brain where an epileptic seizure is being initiated. Patients anonymously donated this tissue, which is normally discarded, to our colleagues in the Lichtman lab. The Harvard researchers cut the tissue into ~5300 individual 30 nanometer sections using an automated tape collecting ultra-microtome, mounted those sections onto silicon wafers, and then imaged the brain tissue at 4 nm resolution in a customized 61-beam parallelized scanning electron microscope for rapid image acquisition.

Imaging the ~5300 physical sections produced 225 million individual 2D images. The team then computationally stitched and aligned this data to produce a single 3D volume. While the quality of the data was generally excellent, these alignment pipelines had to robustly handle a number of challenges, including imaging artifacts, missing sections, variation in microscope parameters, and physical stretching and compression of the tissue. Once aligned, a multiscale flood-filling network pipeline was applied (using thousands of Google Cloud TPUs) to produce a 3D segmentation of each individual cell in the tissue. Additional machine learning pipelines were applied to identify and characterize 130 million synapses, classify each 3D fragment into various “subcompartments” (e.g., axon, dendrite, or cell body), and identify other structures of interest such as myelin and cilia. Automated reconstruction results were imperfect, so manual efforts were used to “proofread” roughly one hundred cells in the data. Over time, the scientists expect to add additional cells to this verified set through additional manual efforts and further advances in automation.

Source: https://ai.googleblog.com/

Transparent Solar Cells To Boost Personalized Energy

Today, the imminent climate change crisis demands a shift from conventionally used fossil fuels to efficient sources of green energy. This has led to researchers looking into the concept of “personalized energy,” which would make on-site energy generation possible. For example, solar cells could possibly be integrated into windows, vehicles, cellphone screens, and other everyday products. But for this, it is important for the solar panels to be handy and transparent. To this end, scientists have recently developed “transparent photovoltaic” (TPV) devices–transparent versions of the traditional solar cell. Unlike the conventionally dark, opaque solar cells (which absorb visible light), TPVs make use of the “invisible light that falls in the ultraviolet (UV) range.

Conventional solar cells can be either “wet type” (solution based) or “dry type” (made up of metal-oxide semiconductors). Of these, dry-type solar cells have a slight edge over the wet-type ones: they are more reliable, eco-friendly, and cost-effective. Moreover, metal-oxides are well-suited to make use of the UV light. Despite all this, however, the potential of metal-oxide TPVs has not been fully explored until now. To this end, researchers from Incheon National University, Republic of Korea, came up with an innovative design for a metal-oxide-based TPV device. They inserted an ultra-thin layer of silicon (Si) between two transparent metal-oxide semiconductors with the goal of developing an efficient TPV device.

Our aim was to devise a high-power-producing transparent solar cell, by embedding an ultra-thin film of amorphous Si between zinc oxide and nickel oxide,” explains Prof Joondong Kim, who led the study.

This novel design consisting of the Si film had three major advantages. First, it allowed for the utilization of longer-wavelength light (as opposed to bare TPVs). Second, it resulted in efficient photon collection. Third, it allowed for the faster transport of charged particles to the electrodes. Moreover, the design can potentially generate electricity even under low-light situations (for instance, on cloudy or rainy days). The scientists further confirmed the power-generating ability of the device by using it to operate the DC motor of a fan.

These findings has been published in a study in Nano Energy.

Source: http://www.inu.ac.kr/

The Rise Of Perovskite, The Next-generation Solar Cell

Light-weight, cheap and ultra-thin, perovskite crystals have promised to shake-up renewable energy for some time. Research by Professor Anita Ho-Baillie means they are ready to take the next steps towards commercialisation. Australian scientists have for the first time produced a new generation of experimental solar energy cells that pass strict International Electrotechnical Commission testing standards for heat and humidity. The research findings, an important step towards commercial viability of perovskite solar cells, are published today in the journal Science.

Solar energy systems are now widespread in both industry and domestic housing. Most current systems rely on silicon to convert sunlight into useful energy. However, the energy conversion rate of silicon in solar panels is close to reaching its natural limits. So, scientists have been exploring new materials that can be stacked on top of silicon in order to improve energy conversion rates. One of the most promising materials to date is a metal halide perovskite, which may even outperform silicon on its own.

Perovskites are a really promising prospect for solar energy systems,” said Professor Anita Ho-Baillie, the inaugural John Hooke Chair of Nanoscience at the University of Sydney. “They are a very inexpensive, 500 times thinner than silicon and are therefore flexible and ultra-lightweight. They also have tremendous energy enabling properties and high solar conversion rates.”

In experimental form, the past 10 years has seen the performance of perovskites cells improve from low levels to being able to convert 25.2 percent of energy from the Sun into electricity. It took about 40 years for scientists to develop silicon-cell conversion rates of 26.7 percent. However, unprotected perovskite cells do not have the durability of silicon-based cells, so they are not yet commercially viable.

Perovskite cells will need to stack up against the current commercial standards. That’s what is so exciting about our research. We have shown that we can drastically improve their thermal stability,” Professor Ho-Baillie said.

The scientists did this by suppressing the decomposition of the perovskite cells using a simple, low-cost polymer-glass blanket. The work was led by Professor Ho-Baillie who joined the University of Sydney Nano Institute this year. Lead author, Dr Lei Shi, conducted the experimental work in Ho-Baillie’s research group in the School of Photovoltaic and Renewable Energy Engineering at the University of New South Wales, where Professor Ho-Baillie remains an adjunct professor.  Under continual exposure to the Sun and other elements, solar panels experience extremes of heat and humidity. Experiments have shown that under such stress, unprotected perovskite cells become unstable, releasing gas from within their structures.

Understanding this process, called ‘outgassing’, is a central part of our work to develop this technology and to improve its durability,” Professor Ho-Baillie said. “I have always been interested in exploring how perovskite solar cells could be incorporated into thermal insulated windows, such as vacuum glazing. So, we need to know the outgassing properties of these materials.

Source: https://science.sciencemag.org/

Photonic Chips

Emitting light from silicon has been the ‘Holy Grail’ in the microelectronics industry for decades. Solving this puzzle would revolutionize computing, as chips will become faster than ever. Researchers from Eindhoven University of Technology  (TU-e) now succeeded: they have developed an alloy with silicon that can emit light. The team will now start creating a silicon laser to be integrated into current chips.

Every year we use and produce significantly more data. But our current technology, based on electronic chips, is reaching its ceiling. The limiting factor is heat, resulting from the resistance that the electrons experience when traveling through the copper lines connecting the many transistors on a chip. If we want to continue transferring more and more data every year, we need a new technique that does not produce heat. Bring in photonics, which uses photons (light particles) to transfer data. In contrast to electrons, photons do not experience resistance. As they have no mass or charge, they will scatter less within the material they travel through, and therefore no heat is produced. The energy consumption will therefore be reduced. Moreover, by replacing electrical communication within a chip by optical communication, the speed of on-chip and chip-to-chip communication can be increased by a factor 1000. Data centers would benefit most, with faster data transfer and less energy usage for their cooling system. But these photonic chips will also bring new applications within reach. Think of laser-based radar for self-driving cars and chemical sensors for medical diagnosis or for measuring air and food quality.

To use light in chips, you will need a light source; an integrated laser. The main semiconductor material that computer chips are made of is silicon. But bulk silicon is extremely inefficient at emitting light, and so was long thought to play no role in photonics. Thus, scientists turned to more complex semiconductors, such as gallium arsenide and indium phosphide. These are good at emitting light but are more expensive than silicon and are hard to integrate into existing silicon microchips.

To create a silicon compatible laser, scientists needed to produce a form of silicon that can emit light. That’s exactly what researchers from Eindhoven University of Technology (TU/e) now succeeded in. Together with researchers from the universities of Jena, Linz and Munich, they combined silicon and germanium in a hexagonal structure that is able to emit light. A breakthrough after 50 years of work.

Nanowires with hexagonal silicon-germanium shells

The crux is in the nature of the so-called band gap of a semiconductor,” says lead researcher Erik Bakkers from TU/e. “If an electron ‘drops’ from the conduction band to the valence band, a semiconductor emits a photon: light.” But if the conduction band and valence band are displaced with respect to each other, which is called an indirect band gap, no photons can be emitted – as is the case in silicon. “A 50-year old theory showed however that silicon, alloyed with germanium, shaped in a hexagonal structure does have a direct band gap, and therefore potentially could emit light,” explains Bakkers.

Shaping silicon in a hexagonal structure, however, is not easy. As Bakkers and his team master the technique of growing nanowires, they were able to create hexagonal silicon in 2015. They realized pure hexagonal silicon by first growing nanowires made from another material, with a hexagonal crystal structure. Then they grew a silicon-germanium shell on this template. Elham Fadaly, shared first author of the study: “We were able to do this such that the silicon atoms are built on the hexagonal template, and by this forced the silicon atoms to grow in the hexagonal structure.” But they could not yet make them to emit light, until now. Bakkers team managed to increase the quality of the hexagonal silicon-germanium shells by reducing the number of impurities and crystal defects. When exciting the nanowire with a laser, they could measure the efficiency of the new material. Alain Dijkstra, also shared first author of the study and responsible for measuring the light emission: “Our experiments showed that the material has the right structure, and that it is free of defects. It emits light very efficiently.”

The findings have been published in the journal Nature.

Source: https://www.tue.nl/

On Mars or Earth, biohybrid can turn CO2 into new products

If humans ever hope to colonize Mars, the settlers will need to manufacture on-planet a huge range of organic compounds, from fuels to drugs, that are too expensive to ship from Earth. University of California, Berkeley, and Lawrence Berkeley National Laboratory (Berkeley Lab) chemists have a plan for that.

For the past eight years, the researchers have been working on a hybrid system combining bacteria and nanowires that can capture the energy of sunlight to convert carbon dioxide and water into building blocks for organic molecules. Nanowires are thin silicon wires about one-hundredth the width of a human hair, used as electronic components, and also as sensors and solar cells.

A device to capture carbon dioxide from the air and convert it to useful organic products. On left is the chamber containing the nanowire/bacteria hybrid that reduces CO2 to form acetate. On the right is the chamber where oxygen is produced

On Mars, about 96% of the atmosphere is CO2. Basically, all you need is these silicon semiconductor nanowires to take in the solar energy and pass it on to these bugs to do the chemistry for you,” said project leader Peidong Yang, professor of chemistry and Energy at UC Berkeley. “For a deep space mission, you care about the payload weight, and biological systems have the advantage that they self-reproduce: You don’t need to send a lot. That’s why our biohybrid version is highly attractive.”

The only other requirement, besides sunlight, is water, which on Mars is relatively abundant in the polar ice caps and likely lies frozen underground over most of the planet, said Yang, who is a senior faculty scientist at Berkeley Lab and director of the Kavli Energy Nanoscience Institute.

The biohybrid can also pull carbon dioxide from the air on Earth to make organic compounds and simultaneously address climate change, which is caused by an excess of human-produced CO2 in the atmosphere.

In a new paper published in the journal Joule, the researchers report a milestone in packing these bacteria (Sporomusa ovata) into a “forest of nanowires” to achieve a record efficiency: 3.6% of the incoming solar energy is converted and stored in carbon bonds, in the form of a two-carbon molecule called acetate: essentially acetic acid, or vinegar.

Source: https://news.berkeley.edu/

Nano-Transistor From DNA-like Material

Computer chips use billions of tiny switches, called transistors, to process information. The more transistors on a chip, the faster the computer. A material shaped like a one-dimensional DNA helix might further push the limits on a transistor’s size. The material comes from a rare earth element called tellurium.

Researchers found that the material, encapsulated in a nanotube made of boron nitride, helps build a field-effect transistor with a diameter of two nanometers. Transistors on the market are made of bulkier silicon and range between 10 and 20 nanometers in scale.  Engineers at Purdue University performed the work in collaboration with Michigan Technological University, Washington University in St. Louis, and the University of Texas at Dallas.

Over the past few years, transistors have been built as small as a few nanometers in lab settings. The goal is to build transistors the size of atomsPeide Ye’s lab at Purdue is one of many research groups seeking to exploit materials much thinner than silicon to achieve both smaller and higher-performing transistors.

These silver, wiggling lines are strings of atoms in tellurium behaving like DNA. Researchers have not seen this behavior in any other material.

This tellurium material is really unique. It builds a functional transistor with the potential to be the smallest in the world,” said Ye, Purdue’s Richard J. and Mary Jo Schwartz Professor of Electrical and Computer Engineering.

The research is published in the journal Nature Electronics.

Source: https://www.purdue.edu/

Hologram operates in forward and backward directions

Hologram techniques are already used in our everyday life. A hologram sticker to prevent from counterfeiting money, Augmented Reality navigation projected in front mirror of a car to guide directions, and Virtual Reality game that allows a user to play in a virtual world with a feeling of live are just a few examples to mention. Recently, thinner and lighter meta-hologram operating in forward and backward directions has been developed. As seen in the movie, Black Panther, people from Wakanda Kingdom communicate to each other through the hologram and, this specific movie scene seems to become reality soon that we can exchange different information with people from different locations.

Junsuk Rho, professor of POSTECH Mechanical Engineering and Chemical Engineering Department in Korea, with his student, Inki Kim developed a multifunctional meta-hologram from a monolayer meta-holographic optical device that can create different hologram images depending on a direction of light incident on the device. Their research accomplishment has been introduced as a cover story in the January 2020 issue of Nanoscale Horizons.

Televisions and beam projectors can only transmit intensity of lights but holographic techniques can save light intensity and its phase information to play movies in three-dimensional spaces. At this time, if metamaterials are used, a user can change nano structures, size, and shapes as desired and can control light intensity and phase at the same time. Meta-hologram has pixel sizes as small as 300 to 400 nanometers but can display very high resolution of holographic images with larger field of view compared to existing hologram projector such as spatial light modulator.

However, the conventional meta-holograms can display images when incident light is in one direction and cannot when light is in the other direction.

To solve such a problem, the research team used two different types of metasurfaces.* One metasurface was designed to have phase information when incident light was in the forward direction and the other one to operate when light was in backward direction. As a result, they confirmed that these could display different images in real-time depending on the directions of light.

In addition, the team applied dual magnetic resonances*2 and antiferromagnetic resonances*3, which are phenomena occurring in silicon nanopillars, to nanostructure design to overcome low efficiency of the conventional meta-hologram. This newly made meta-hologram demonstrated diffraction efficiency higher than 60% (over 70% in simulation) and high-quality and clear images were observed. Furthermore, the new meta-hologram uses silicon and it can be easily produced by following through the conventional semiconductor manufacturing process. The meta-hologram operating in both directions, forward and backward, is expected to set a new hologram platform that can transmit various information to multiple users from different locations, overcoming the limits of the conventional ones which could only transmit one image to a limited location.

Microscopic, ultrathin, ultralightweight flat optical devices based on a metasurface is an impressive technique with great potentials as it can not only perform the functions of the conventional optical devices but also demonstrate multiple functions depending on how its metasurface is designed. Especially, we developed a meta-hologram optical device that operated in forward and backward directions and it could transmit various visual information to multiple users from different locations simultaneously. We anticipate that this new development can be employed in multiple applications such as holograms for performances, entertainment, exhibitions,  automobiles and more,”, said Junsuk Rho who is leading research on metamaterials.

Source: http://postech.ac.kr/

Super Body Armor

According to ancient lore, Genghis Khan instructed his horsemen to wear silk vests underneath their armor to better protect themselves against an onslaught of arrows during battle. Since the time of Khan, body armor has significantly evolved — silk has given way to ultra-hard materials that act like impenetrable walls against most ammunition. However, even this armor can fail, particularly if it is hit by high-speed ammunition or other fast-moving objects.

Researchers at Texas A&M University have formulated a new recipe that can prevent weaknesses in modern-day armor. By adding a tiny amount of the element silicon to boron carbide, a material commonly used for making body armor, they discovered that bullet-resistant gear could be made substantially more resilient to high-speed impacts.

For the past 12 years, researchers have been looking for ways to reduce the damage caused by the impact of high-speed bullets on armor made with boron carbide,” said Kelvin Xie, assistant professor in the Department of Materials Science and Engineering. “Our work finally addresses this unmet need and is a step forward in designing superior body armor that will safeguard against even more powerful firearms during combat.”

Boron carbide, dubbed “black diamond,” is a man-made material, which ranks second below another synthetic material called cubic boron nitride for hardness. Unlike cubic boron nitride, however, boron carbide is easier to produce on a large scale. Also, boron carbide is harder and lighter than other armor materials like silicon carbide, making it an ideal choice for protective gear, particularly ballistic vests.

Despite boron carbide’s many desirable qualities, its main shortfall is that it can damage very quickly upon high-velocity impact.

Boron carbide is really good at stopping bullets traveling below 900 meters per second, and so it can block bullets from most handguns quite effectively,” Xie said. “But above this critical speed, boron carbide suddenly loses its ballistic performance and is not as effective.”

Scientists know high-speed jolts cause boron carbide to have phase transformations — a phenomenon where a material changes its internal structure such that it is in two or more physical states, like liquid and solid, at the same time. The bullet’s impact thus converts boron carbide from a crystalline state where atoms are systematically ordered to a glass-like state where atoms are haphazardly arranged. This glass-like state weakens the material’s integrity at the site of contact between the bullet and boron carbide.

When boron carbide undergoes phase transformation, the glassy phase creates a highway for cracks to propagate,” Xie said. “So, any local damage caused by the impact of a bullet easily travels throughout the material and causes progressively more damage.”

Previous work using computer simulations predicted that adding a small quantity of another element, such as silicon, had the potential to make boron carbide less brittle. Xie and his group investigated if adding a tiny quantity of silicon also reduced phase transformation.

Xie and his collaborators found that even with tiny quantities of silicon, the extent of phase transformation went down by 30%, noticeably reducing the damage from the indentation.

Source: https://today.tamu.edu/

How To Create Electricity From Snowfall

Researchers from University of California at Los Angeles (UCLA) and colleagues have designed a new device that creates electricity from falling snow. The first of its kind, this device is inexpensive, small, thin and flexible like a sheet of plastic.

The device can work in remote areas because it provides its own power and does not need batteries,” said senior author Richard Kaner, who holds UCLA’s Dr. Myung Ki Hong Endowed Chair in Materials Innovation. “It’s a very clever device — a weather station that can tell you how much snow is falling, the direction the snow is falling, and the direction and speed of the wind.”

The researchers call it a snow-based triboelectric nanogenerator, or snow TENG. A triboelectric nanogenerator, which generates charge through static electricity, produces energy from the exchange of electrons.

Static electricity occurs from the interaction of one material that captures electrons and another that gives up electrons,” said Kaner, who is also a distinguished professor of chemistry and biochemistry, and of materials science and engineering, and a member of the California NanoSystems Institute at UCLA. “You separate the charges and create electricity out of essentially nothing.”

Snow is positively charged and gives up electrons. Silicone — a synthetic rubber-like material that is composed of silicon atoms and oxygen atoms, combined with carbon, hydrogen and other elements — is negatively charged. When falling snow contacts the surface of silicone, that produces a charge that the device captures, creating electricity.

Snow is already charged, so we thought, why not bring another material with the opposite charge and extract the charge to create electricity?” said co-author Maher El-Kady, a UCLA assistant researcher of chemistry and biochemistry.

While snow likes to give up electrons, the performance of the device depends on the efficiency of the other material at extracting these electrons,” he added. “After testing a large number of materials including aluminum foils and Teflon, we found that silicone produces more charge than any other material.”

Findings about the device are published in the journal Nano Energy.

Source: https://newsroom.ucla.edu/

New Perovskite Solar Cells Increase Efficiency By 17%

Researchers have layered different mineral forms of titanium oxide on top of one another to improve perovskite-type solar cell efficiency by one-sixth. The layered titanium oxide layer was better able to transport electrons from the center of the cell to its electrodes. This novel approach could be used to fabricate even more efficient perovskite-type solar cells in future. While most solar cells are made of silicon, such cells are difficult to manufacture, requiring vacuum chambers and temperatures above 1000 °C. Research efforts have therefore recently focused on a new type of solar cell, based on metal halide perovskites. Perovskite solutions can be inexpensively printed to create more efficient, inexpensive solar cells.

In solar cells perovskites can turn light into electricity—but they have to be sandwiched between a negative and positive electrode. One of these electrodes has to be transparent, however, to allow the sun’s light to reach the perovskites. Not only that, any other materials used to help charges flow from the perovskites to the electrode must also be transparent. Researchers have previously found that thin layers of titanium oxide are both transparent and able to transport electrons to the electrode.

Now, a Japan-based research team centered at Kanazawa University has carried out a more detailed study into perovskite solar cells using electron transport layers made of anatase and brookite, which are different mineral forms of titanium oxide. They compared the impact of using either pure anatase or brookite or combination layers (anatase on top of brookite or brookite on top of anatase). The anatase layers were fabricated by spraying solutions onto glass coated with a transparent electrode that was heated to 450 °C. Meanwhile, the researchers used water-soluble brookite nanoparticles to create the brookite layers, as water-soluble inks are more environmentally friendly than conventional inks. These nanoparticles have been yielded poor results in the past; however, the team predicted that combination layers would solve the issues previously encountered when using the nanoparticles.

By layering brookite on top of anatase we were able to improve solar cell efficiency by up to 16.82%,” study coauthor Koji Tomita says.

These results open up a new way to optimize perovskite solar cells, namely via the controlled stacking and manipulation of the different mineral forms of titanium oxide.

The team’s study was recently published in the ACS journal Nano Letters.

Source: https://www.kanazawa-u.ac.jp/