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A material composed of two one-atom-thick layers of carbon has grabbed the attention of physicists worldwide for its intriguing — and potentially exploitable — conductive properties.
Dr. Fan Zhang, assistant professor of physics in the School of Natural Sciences and Mathematics at The University of Texas at Dallas, and physics doctoral student Qiyue Wang published an article in June with Dr. Fengnian Xia’s group at Yale University in Nature Photonics that describes how the ability of twisted bilayer graphene to conduct electrical current changes in response to mid-infrared light.
From One to Two Layers
Graphene is a single layer of carbon atoms arranged in a flat honeycomb pattern, where each hexagon is formed by six carbon atoms at its vertices. Since graphene’s first isolation in 2004, its unique properties have been intensely studied by scientists for potential use in advanced computers, materials and devices.
If two sheets of graphene are stacked on top of one another, and one layer is rotated so that the layers are slightly out of alignment, the resulting physical configuration, called twisted bilayer graphene, yields electronic properties that differ significantly from those exhibited by a single layer alone or by two aligned layers.
“Graphene has been of interest for about 15 years,” Zhang said. “A single layer is interesting to study, but if we have two layers, their interaction should render much richer and more interesting physics. This is why we want to study bilayer graphene systems.”
A New Field Emerges
When the graphene layers are misaligned, a new periodic design in the mesh emerges, called a moiré pattern. The moiré pattern is also a hexagon, but it can be made up of more than 10,000 carbon atoms.
“The angle at which the two layers of graphene are misaligned — the twist angle — is critically important to the material’s electronic properties,” Wang said. “The smaller the twist angle, the larger the moiré periodicity.”
The unusual effects of specific twist angles on electron behavior were first proposed in a 2011 article by Dr. Allan MacDonald, professor of physics at UT Austin, and Dr. Rafi Bistritzer. Zhang witnessed the birth of this field as a doctoral student in MacDonald’s group.
“At that time, others really paid no attention to the theory, but now it has become arguably the hottest topic in physics,” Zhang said.
In that 2011 research MacDonald and Bistritzer predicted that electrons’ kinetic energy can vanish in a graphene bilayer misaligned by the so-called “magic angle” of 1.1 degrees. In 2018, researchers at the Massachusetts Institute of Technology proved this theory, finding that offsetting two graphene layers by 1.1 degrees produced a two-dimensional superconductor, a material that conducts electrical current with no resistance and no energy loss.
In a 2019 article in Science Advances, Zhang and Wang, together with Dr. Jeanie Lau’s group at The Ohio State University, showed that when offset by 0.93 degrees, twisted bilayer graphene exhibits both superconducting and insulating states, thereby widening the magic angle significantly.
“In our previous work, we saw superconductivity as well as insulation. That’s what’s making the study of twisted bilayer graphene such a hot field — superconductivity. The fact that you can manipulate pure carbon to superconduct is amazing and unprecedented,” Wang said.
New UT Dallas Findings
In his most recent research in Nature Photonics, Zhang and his collaborators at Yale investigated whether and how twisted bilayer graphene interacts with mid-infrared light, which humans can’t see but can detect as heat. “Interactions between light and matter are useful in many devices — for example, converting sunlight into electrical power,” Wang said. “Almost every object emits infrared light, including people, and this light can be detected with devices.”
Zhang is a theoretical physicist, so he and Wang set out to determine how mid-infrared light might affect the conductance of electrons in twisted bilayer graphene. Their work involved calculating the light absorption based on the moiré pattern’s band structure, a concept that determines how electrons move in a material quantum mechanically.
“There are standard ways to calculate the band structure and light absorption in a regular crystal, but this is an artificial crystal, so we had to come up with a new method,” Wang said. Using resources of the Texas Advanced Computing Center, a supercomputer facility on the UT Austin campus, Wang calculated the band structure and showed how the material absorbs light.
The Yale group fabricated devices and ran experiments showing that the mid-infrared photoresponse — the increase in conductance due to the light shining — was unusually strong and largest at the twist angle of 1.8 degrees. The strong photoresponse vanished for a twist angle less than 0.5 degrees.
“Our theoretical results not only matched well with the experimental findings, but also pointed to a mechanism that is fundamentally connected to the period of moiré pattern, which itself is connected to the twist angle between the two graphene layers,” Zhang said.
“The twist angle is clearly very important in determining the properties of twisted bilayer graphene,” Zhang added. “The question arises: Can we apply this to tune other two-dimensional materials to get unprecedented features? Also, can we combine the photoresponse and the superconductivity in twisted bilayer graphene? For example, can shining a light induce or somehow modulate superconductivity? That will be very interesting to study.”
“This new breakthrough will potentially enable a new class of infrared detectors based on graphene with high sensitivity,” said Dr. Joe Qiu, program manager for solid-state electronics and electromagnetics at the U.S. Army Research Office (ARO), an element of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory. “These new detectors will potentially impact applications such as night vision, which is of critical importance for the U.S. Army.”
In addition to the Yale researchers, other authors included scientists from the National Institute for Materials Science in Japan. The ARO, the National Science Foundation and the Office of Naval Research supported the study.
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Attention, AI nerds! Xilinx and Hackster are challenging developers to combine the power of Xilinx adaptive computing platforms with the Vitis development environment and Vitis AI to solve real-world problems.
There are some heavy-duty hardware platforms involved in this one, so you can apply for one of 60 loaner development kits. Win a prize, and you keep the kit – AND take home a cash award!
// Follow-up note: "ADAS" stands for vehicular "Advanced Driver Assistance Systems". 🙂
With increasing demand for miniaturization of optoelectronics, microlens array has attracted significant attention and become an important micro-optics device widely used in compact imaging, sensing, optical communication and others. Typically, microlens array consists of multiple micron-sized lenses with optical surface smoothness and superior uniformity, which increases the requirement for machining precision.
Despite the tremendous progress made in manufacturing techniques during the past decades, some limitations, such as high time consumption, high process complexity, lack of fabrication flexibility, and difficulty in consistency control for the existing techniques, still exist.
Recently, researchers from the Singapore University of Technology and Design (SUTD) and Southern University of Science and Technology (SUSTech) in Shenzhen, China proposed an approach which integrated oscillation-assisted digital light processing (DLP) 3D printing with grayscale UV exposure to render an ultrafast and flexible fabrication of microlens arrays with optical surface smoothness.
“3D printing of small geometries with optical surface smoothness is a big challenge.” said the project leader, Associate Prof Qi Ge from SUSTech, “In our approach, the computationally designed grayscale patterns are employed to realize microlens profiles upon one single UV exposure which removes the staircase effect existing in the traditional layer-by-layer 3D printing method, and the projection lens oscillation is applied to further eliminate the jagged surface formed due to the gaps between discrete pixels.”
Detailed morphology characterizations including scanning electron microscopy (SEM) and atomic force microscopy (AFM) prove that the integration of projection lens oscillation considerably smoothens the lens surface and reduces the surface roughness from 200 nm to about 1 nm.
“In addition to surface roughness, lens profile also plays a key role in optical performance.” said Chao Yuan, the co-first author of the paper and a postdoctoral research fellow from SUTD, “In order to better assist the grayscale design for microlens array fabrication, we developed a theoretical model to describe the photopolymerization process and predict the lens profile.”
“The DLP based 3D printing affords remarkable flexibility to the fabrication of microlens arrays. Microlenses with different sizes, geometries and profiles are printable upon one single UV exposure with different grayscale patterns.” said Kavin Kowsari, the other co-first author of the paper and a postdoctoral research fellow from SUTD.
“Relative to the other fabrication method, our oscillation assisted DLP based printing method is energy- and time-efficient without degradation of optical performance, which is convenient for commercialization and deployment into mass production.” said Prof Ge, “Also, this approach provides instructive inspirations for other manufacturing fields with high demands for ultra-smooth surfaces.”
This work was funded by SUTD’s Digital Manufacturing and Design (DManD) Centre which is supported by the Singapore National Research Foundation (NRF). The research was published in ACS Applied Materials & Interfaces.
Materials provided by Singapore University of Technology and Design. Note: Content may be edited for style and length.
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In recent years, the seemingly inevitable “internet of things” has attracted considerable attention: the idea that in the future, everything in the physical world — machines, objects, people — will be connected to the internet. Drawing on lessons learned from studies on a variety of marine animals outfitted with sensors, researchers in a new perspective article in ACS Sensors describe how an “internet of health” could revolutionize human medicine.
Imagine a world where you could present your doctor with an entire year’s worth of data on your eating habits, heart rate, sleep-wake cycles, and biomarkers of health, all obtained non-obtrusively from wearable sensors attached to skin or clothing or contained in cell phones. These data could be correlated with information about the environment, such as airborne pollutants and geographical location, to evaluate risks for illnesses or perhaps even prevent them. Michael Strano and colleagues have decades of collective experience with biologging — tagging marine animals with sensors to gain ecological insights, ranging from feeding behaviors to migration. In this perspective article, they share insights from these experiences that could someday help scientists develop an “internet of health.”
Surprisingly, a sensor attached to an organism can potentially uncover new information about seemingly disconnected behaviors, the researchers say. For example, a jaw-motion sensor attached to a sea turtle’s mouth can provide data on the animal’s specific anatomy, but it might also reveal detailed information on the type of food and the duration of feeding, as well as how the turtle captured the prey and ate it. In a study referenced in this article, the researchers used sensors to track the motions of 23 species of marine animals for a decade. The animals’ movements revealed migratory patterns, which the researchers correlated with data on seawater temperature and photosynthetic and human activities to predict how habitats could shift because of climate change. The team emphasizes that each sensor forms a partial but incomplete picture of an organism’s physical state, necessitating the use of multiple sensors. As with animal studies, a challenge for human medicine will be developing comfortable sensors that don’t impact people’s behaviors, the researchers say.
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I don’t generally pay much attention to Intel-based boards. There have been the odd exceptions, but usually run too hot, and are just too expensive, to be particularly interesting. But it looks like there might soon be one more board added to the short stack of exceptions, the new Rock Pi X.
The Rock Pi X from Radxa occupies the relatively sparsely populated low-end of the x86 single-board computer market, powered by an Intel Atom x5-Z8300 Cherry Trail processor it will be priced starting from $39.
The board will ship with 1, 2, or 4GB of RAM, and in two separate models, the Model A and Model B. The cheaper Model A lacking both Wi-Fi and Bluetooth support. Both, however, will have a microSD card slot, eMMC flash storage, an HDMI 1.4 port, a headphone jack, a single USB 3 port, and two USB 2 ports, as well as a USB Type-C OTG port, and a Raspberry Pi-like 40-pin connector with two ADC, two PWM, and two I2C connectors.
The board also shares the same footprint as the Raspberry Pi, and apart from the Intel processor shares a lot of similarities with Radxa’s Rock Pi 4, based around the Rockchip RK3399, which they released at the tail end of last year.
Interestingly of course, because of the Intel processor, the new Rock Pi X board will be able run Windows 10. Although if you opt for the cheapest model, with only 1GB of RAM, you’ll be restricted to running Windows in 32-bit mode as the 64-bit edition requires a minimum of 2GB of RAM.
There isn’t any news on availability quite yet, but the Rock Pi X Model A will ship priced at $39 for the 1GB model, $49 for the 2GB model, or $65 for the 4GB model. While the Model B, with Wi-Fi and Bluetooth support, will ship at $49 for the 1GB model, $59 for the 2GB model, or $75 for the 4GB model which represents the top end specification available.
[h/t: CNX Software]
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Author: Alasdair Allan
The nanomaterial graphene has received significant attention for its potential uses in everything from solar cells to tennis rackets. But a new study by Brown University researchers finds a surprising new use for the material: preventing mosquito bites.
In a paper published in Proceedings of the National Academy of Sciences, researchers showed that multilayer graphene can provide a two-fold defense against mosquito bites. The ultra-thin yet strong material acts as a barrier that mosquitoes are unable to bite through. At the same time, experiments showed that graphene also blocks chemical signals mosquitoes use to sense that a blood meal is near, blunting their urge to bite in the first place. The findings suggest that clothing with a graphene lining could be an effective mosquito barrier, the researchers say.
“Mosquitoes are important vectors for disease all over the world, and there’s a lot of interest in non-chemical mosquito bite protection,” said Robert Hurt, a professor in Brown’s School of Engineering and senior author of the paper. “We had been working on fabrics that incorporate graphene as a barrier against toxic chemicals, and we started thinking about what else the approach might be good for. We thought maybe graphene could provide mosquito bite protection as well.”
To find out if it would work, the researchers recruited some brave participants willing to get a few mosquito bites in the name of science. The participants placed their arms in a mosquito-filled enclosure so that only a small patch of their skin was available to the mosquitoes for biting. The mosquitoes were bred in the lab so they could be confirmed to be disease-free.
The researchers compared the number of bites participants received on their bare skin, on skin covered in cheesecloth and on skin covered by a graphene oxide (GO) films sheathed in cheesecloth. GO is a graphene derivative that can be made into films large enough for macro-scale applications.
It was readily apparent that graphene was a bite deterrent, the researchers found. When skin was covered by dry GO films, participants didn’t get a single bite, while bare and cheesecloth-covered skin was readily feasted upon. What was surprising, the researchers said, was that the mosquitoes completely changed their behavior in the presence of the graphene-covered arm.
“With the graphene, the mosquitoes weren’t even landing on the skin patch — they just didn’t seem to care,” said Cintia Castilho, a Ph.D. student at Brown and the study’s lead author. “We had assumed that graphene would be a physical barrier to biting, through puncture resistance, but when we saw these experiments we started to think that it was also a chemical barrier that prevents mosquitoes from sensing that someone is there.”
To confirm the chemical barrier idea, the researchers dabbed some human sweat onto the outside of a graphene barrier. With the chemical ques on the other side of the graphene, the mosquitoes flocked to the patch in much the same way they flocked to bare skin.
Other experiments showed that GO can also provide puncture resistance — but not all the time. Using a tiny needle as a stand-in for a mosquito’s proboscis, as well as computer simulations of the bite process, the researchers showed that mosquitoes simply can’t generate enough force to puncture GO. But that only applied when the GO is dry. The simulations found that GO would be vulnerable to puncture when it was saturated with water. And sure enough, experiments showed that mosquitoes could bite through wet GO. However, another form of GO with reduced oxygen content (called rGO) was shown to provide a bite barrier when both wet and dry.
A next step for the research would be to find a way to stabilize the GO so that it’s tougher when wet, Hurt says. That’s because GO has a distinct advantage over rGO when it comes to wearable technology.
“GO is breathable, meaning you can sweat through it, while rGO isn’t,” Hurt said. “So our preferred embodiment of this technology would be to find a way to stabilize GO mechanically so that is remains strong when wet. This next step would give us the full benefits of breathability and bite protection.”
All told, the researchers say, the study suggests that properly engineered graphene linings could be used to make mosquito protective clothing.
Other co-authors on the study were Dong Li, Muchun Liu, Yue Liu and Huajian Gao. The study was funded by the National Science Foundation (CMMI-1634492)
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