To enable nanolaser operation at room temperature using single molecular layer and thin silicon beam

 

For the first time in human history, researchers have built a nanolaser that uses only a single molecular layer. This is placed on a thin silicon beam, which operates at room temperature. The new device, developed by a team of researchers from Arizona State University and Tsinghua University, Beijing, China, could potentially be used to send information between different points on a single computer chip. The lasers also may be useful for other sensing applications in a compact, integrated format.

"This is the first demonstration of room-temperature operation of a nanolaser made of the single-layer material," said Cun-Zheng Ning, an ASU electrical engineering professor who led the research team. Details of the new laser are published in the July online edition of Nature Nanotechnology.

In addition to Ning, key authors of the article, "Room-temperature Continuous-wave Lasing from Monolayer Molybdenum Ditelluride Integrated with a Silicon Nanobeam Cavity," include Yongzhuo Li, Jianxing Zhang, Dandan Huang from Tsinghua University.

Ning said pivotal to the new development is use of materials that can be laid down in single layers and efficiently amplify light (lasing action). Single layer nanolasers have been developed before, but they all had to be cooled to low temperatures using a cryogen like liquid nitrogen or liquid helium. Being able to operate at room temperatures (~77 F) opens up many possibilities for uses of these new lasers," Ning said.

The joint ASU-Tsinghua research team used a monolayer of molybdenum ditelluride integrated with a silicon nanobeam cavity for their device. By combining molybdenum ditelluride with silicon, which is the bedrock in semiconductor manufacturing and one of the best waveguide materials, the researchers were able to achieve lasing action without cooling, Ning said.

A laser needs two key pieces – a gain medium that produces and amplifies photons, and a cavity that confines or traps photons. While such materials choices are easy for large lasers, they become more difficult at nanometer scales for nanolasers. Nanolasers are smaller than 100th of the thickness of the human hair and are expected to play important roles in future computer chips and a variety of light detection and sensing devices.

The choice of two-dimensional materials and the silicon waveguide enabled the researchers to achieve room temperature operation. Excitons in molybdenum telluride emit in a wavelength that is transparent to silicon, making silicon possible as a waveguide or cavity material. Precise fabrication of the nanobeam cavity with an array of holes etched and the integration of two-dimensional monolayer materials was also key to the project. Excitons in such monolayer materials are 100 times stronger than those in conventional semiconductors, allowing efficient light emission at room temperature.

Because silicon is already used in electronics, especially in computer chips, its use in this application is significant in future applications.

"A laser technology that can also be made on Silicon has been a dream for researchers for decades," said Ning. "This technology will eventually allow people to put both electronics and photonics on the same silicon platform, greatly simplifying manufacture."

Silicon does not emit light efficiently and therefore must be combined with other light emitting materials. Currently, other semiconductors are used, such as Indium phosphide or Indium Garlium Arsenide which are hundreds of times thicker, to bond with silicon for such applications.

The new monolayer materials combined with Silicon eliminate challenges encountered when combining with thicker, dissimilar materials. And, because this non-silicon material is only a single layer thick, it is flexible and less likely to crack under stress, according to Ning.

Looking forward, the team is working on powering their laser with electrical voltage to make the system more compact and easy to use, especially for its intended use on computer chips.

The strength of real hair inspires new materials for body armor

Researchers at the University of California San Diego investigate why hair is incredibly strong and resistant to breaking. Credit: iStock.com/natevplas

In a new study, researchers at the University of California San Diego investigate why hair is incredibly strong and resistant to breaking. The findings could lead to the development of new materials for body armor and help cosmetic manufacturers create better hair care products.

Hair has a strength to weight ratio comparable to steel. It can be stretched up to one and a half times its original length before breaking. "We wanted to understand the mechanism behind this extraordinary property," said Yang (Daniel) Yu, a nano-engineering Ph.D. student at UC San Diego and the first author of the study.
"Nature creates a variety of interesting materials and architectures in very ingenious ways. We're interested in understanding the correlation between the structure and the properties of biological materials to develop synthetic materials and designs—based on nature—that have better performance than existing ones," said Marc Meyers, a professor of mechanical engineering at the UC San Diego Jacobs School of Engineering and the lead author of the study.
In a study published online in Dec. in the journal Materials Science and Engineering C, researchers examined at the nano-scale level how a strand of human hair behaves when it is deformed, or stretched. The team found that hair behaves differently depending on how fast or slow it is stretched. The faster hair is stretched, the stronger it is. "Think of a highly viscous substance like honey," Meyers explained. "If you deform it fast it becomes stiff, but if you deform it slowly it readily pours."
Hair consists of two main parts—the cortex, which is made up of parallel fibrils, and the matrix, which has an amorphous (random) structure. The matrix is sensitive to the speed at which hair is deformed, while the cortex is not. The combination of these two components, Yu explained, is what gives hair the ability to withstand high stress and strain.
And as hair is stretched, its structure changes in a particular way. At the nano-scale, the cortex fibrils in hair are each made up of thousands of coiled spiral-shaped chains of molecules called alpha helix chains. As hair is deformed, the alpha helix chains uncoil and become pleated sheet structures known as beta sheets. This structural change allows hair to handle up a large amount deformation without breaking.
This structural transformation is partially reversible. When hair is stretched under a small amount of strain, it can recover its original shape. Stretch it further, the structural transformation becomes irreversible. "This is the first time evidence for this transformation has been discovered," Yu said.
"Hair is such a common material with many fascinating properties," said Bin Wang, a UC San Diego PhD alumna and co-author on the paper. Wang is now at the Shenzhen Institutes of Advanced Technology in China continuing research on hair.
The team also conducted stretching tests on hair at different humidity levels and temperatures. At higher humidity levels, hair can withstand up to 70 to 80 percent deformation before breaking. Water essentially "softens" hair—it enters the matrix and breaks the sulfur bonds connecting the filaments inside a strand of hair. Researchers also found that hair starts to undergo permanent damage at 60 degrees Celsius (140 degrees Fahrenheit). Beyond this temperature, hair breaks faster at lower stress and strain.
"Since I was a child I always wondered why hair is so strong. Now I know why," said Wen Yang, a former postdoctoral researcher in Meyers' research group and co-author on the paper.
The team is currently conducting further studies on the effects of water on the properties of human hair. Moving forward, the team is investigating the detailed mechanism of how washing hair causes it to return to its original shape.

Use drones and insect biobots to map disaster areas

Credit: North Carolina State University 

 

Researchers at North Carolina State University have developed a combination of software and hardware that will allow them to use unmanned aerial vehicles (UAVs) and insect cyborgs, or biobots, to map large, unfamiliar areas – such as collapsed buildings after a disaster.

"The idea would be to release a swarm of sensor-equipped biobots – such as remotely controlled cockroaches – into a collapsed building or other dangerous, unmapped area," says Edgar Lobaton, an assistant professor of electrical and computer engineering at NC State and co-author of two papers describing the work.
"Using remote-control technology, we would restrict the movement of the biobots to a defined area," Lobaton says. "That area would be defined by proximity to a beacon on a UAV. For example, the biobots may be prevented from going more than 20 meters from the UAV."
The biobots would be allowed to move freely within a defined area and would signal researchers via radio waves whenever they got close to each other. Custom software would then use an algorithm to translate the biobot sensor data into a rough map of the unknown environment.
Once the program receives enough data to map the defined area, the UAV moves forward to hover over an adjacent, unexplored section. The biobots move with it, and the mapping process is repeated. The software program then stitches the new map to the previous one. This can be repeated until the entire region or structure has been mapped; that map could then be used by first responders or other authorities.
"This has utility for areas – like collapsed buildings – where GPS can't be used," Lobaton says. "A strong radio signal from the UAV could penetrate to a certain extent into a collapsed building, keeping the biobot swarm contained. And as long as we can get a signal from any part of the swarm, we are able to retrieve data on what the rest of the swarm is doing. Based on our experimental data, we know you're going to lose track of a few individuals, but that shouldn't prevent you from collecting enough data for mapping."
Co-lead author Alper Bozkurt, an associate professor of electrical and computer engineering at NC State, has previously developed functional cockroach biobots. However, to test their new mapping technology, the research team relied on inch-and-a-half-long robots that simulate cockroach behavior.

In their experiment, researchers released these robots into a maze-like space, with the effect of the UAV beacon emulated using an overhead camera and a physical boundary attached to a moving cart. The cart was moved as the robots mapped the area.

"We had previously developed proof-of-concept software that allowed us to map small areas with biobots, but this work allows us to map much larger areas and to stitch those maps together into a comprehensive overview," Lobaton says. "It would be of much more practical use for helping to locate survivors after a disaster, finding a safe way to reach survivors, or for helping responders determine how structurally safe a building may be.
"The next step is to replicate these experiments using biobots, which we're excited about."
An article on the framework for developing local maps and stitching them together, "A Framework for Mapping with Biobotic Insect Networks: From Local to Global Maps," is published in Robotics and Autonomous Systems. An article on the theory of mapping based on the proximity of mobile sensors to each other, "Geometric Learning and Topological Inference with Biobotic Networks," is published in IEEE Transactions on Signal and Information Processing over Networks.


credit;   Matt Shipman

X-Ray microscopy technique reveals nanoscale information on rechargeable batteries

Better batteries that charge quickly and last a long time are a brass ring for engineers. But despite decades of research and innovation, a fundamental understanding of exactly how batteries work at the smallest of scales has remained elusive.

In a paper published this week in the journal Science, a team led by William Chueh, an assistant professor of materials science and engineering at Stanford and a faculty scientist at the Department of Energy's SLAC National Accelerator Laboratory, has devised a way to peer as never before into the electrochemical reaction that fuels the most common rechargeable cell in use today: the lithium-ion battery.

By visualizing the fundamental building blocks of batteries - small particles typically measuring less than 1/100th of a human hair in size - the team members have illuminated a process that is far more complex than once thought. Both the method they developed to observe the battery in real time and their improved understanding of the electrochemistry could have far-reaching implications for battery design, management and beyond.

"It gives us fundamental insights into how batteries work," said Jongwoo Lim, a co-lead author of the paper and post-doctoral researcher at the Stanford Institute for Materials & Energy Sciences at SLAC. "Previously, most studies investigated the average behavior of the whole battery. Now, we can see and understand how individual battery particles charge and discharge."

The heart of a battery

At the heart of every lithium-ion battery is a simple chemical reaction in which positively charged lithium ions nestle in the lattice-like structure of a crystal electrode as the battery is discharging, receiving negatively charged electrons in the process. In reversing the reaction by removing electrons, the ions are freed and the battery is charged.

These basic processes - known as lithiation (discharge) and delithiation (charge) - are hampered by an electrochemical Achilles heel. Rarely do the ions insert uniformly across the surface of the particles. Instead, certain areas take on more ions, and others fewer. These inconsistencies eventually lead to mechanical stress as areas of the crystal lattice become overburdened with ions and develop tiny fractures, sapping battery

"Lithiation and delithiation should be homogenous and uniform," said Yiyang Li, a doctoral candidate in Chueh's lab and co-lead author of the paper. "In reality, however, they're very non-uniform. In our better understanding of the process, this paper lays out a path toward suppressing the phenomenon."

For researchers hoping to improve batteries, like Chueh and his team, counteracting these detrimental forces could lead to batteries that charge faster and more fully, lasting much longer than today's models.

This study visualizes the charge/discharge reaction in real-time - something scientists refer to as operando - at fine detail and scale. The team utilized brilliant X-rays and cutting-edge microscopes at Lawrence Berkeley National Laboratory's Advanced Light Source.

"The phenomenon revealed by this technique, I thought would never be visualized in my lifetime. It's quite game-changing in the battery field," said Martin Bazant, a professor of chemical engineering and of mathematics at MIT who led the theoretical aspect of the study.

Chueh and his team fashioned a transparent battery using the same active materials as ones found in smartphones and electric vehicles. It was designed and fabricated in collaboration with Hummingbird Scientific. It consists of two very thin, transparent silicon nitride "windows." The battery electrode, made of a single layer of lithium iron phosphate nanoparticles, sits on the membrane inside the gap between the two windows. A salty fluid, known as an electrolyte, flows in the gap to deliver the lithium ions to the nanoparticles.

"This was a very, very small battery, holding ten billion times less charge than a smartphone battery," Chueh said. "But it allows us a clear view of what's happening at the nanoscale."

Significant advances

In their study, the researchers discovered that the charging process (delithiation) is significantly less uniform than discharge (lithiation). Intriguingly, the researchers also found that faster charging improves uniformity, which could lead to new and better battery designs and power management strategies.

"The improved uniformity lowers the damaging mechanical stress on the electrodes and improves battery cyclability," Chueh said. "Beyond batteries, this work could have far-reaching impact on many other electrochemical materials." He pointed to catalysts, memory devices, and so-called smart glass, which transitions from translucent to transparent when electrically charged.

In addition to the scientific knowledge gained, the other significant advancement from the study is the X-ray microscopy technique itself, which was developed in collaboration with Berkeley Lab Advanced Light Source scientists Young-sang Yu, David Shapiro, and Tolek Tyliszczak. The microscope, which is housed at the Advanced Light Source, could affect energy research across the board by revealing never-before-seen dynamics at the nanoscale.

"What we've learned here is not just how to make a better battery, but offers us a profound new window on the science of electrochemical reactions at the nanoscale," Bazant said.