Category Archives: electronics

Better (safer, cheaper) battery invented for wearable tech

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A June 5, 2024 news item on phys.org announces new research into ‘aqueous’ wearable batteries,

Researchers have developed a safer, cheaper, better performing and more flexible battery option for wearable devices. A paper describing the “recipe” for their new battery type was published in the journal Nano Research Energy on June 3 [2024].

Fitness trackers. Smart watches. Virtual-reality headsets. Even smart clothing and implants. Wearable smart devices are everywhere these days. But for greater comfort, reliability and longevity, these devices will require greater levels of flexibility and miniaturization of their energy storage mechanisms, which are often frustratingly bulky, heavy and fragile. On top of this, any improvements cannot come at the expense of safety.

As a result, in recent years, a great deal of battery research has focused on the development of “micro” flexible energy storage devices, or MFESDs. A range of different structures and electrochemical foundations have been explored, and among them, aqueous micro batteries offer many distinct advantages.

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A June 5, 2024 Tsinghua University press release on EurekAlert, which originated the news item, provides more detail,

Aqueous batteries—those that use a water-based solution as an electrolyte (the medium that allows transport of ions in the battery and thus creating an electric circuit) are nothing new. They have been around since the late 19th century. However, their energy density—or the amount of energy contained in the battery per unit of volume—is too low for use in things like electric vehicles as they would take up too much space. Lithium-ion batteries are far more appropriate for such uses.

At the same time, aqueous batteries are much less flammable, and thus safer, than lithium-ion batteries. They are also much cheaper. As a result of this more robust safety and low cost, aqueous options have increasingly been explored as one of the better options for MFESDs. These are termed aqueous micro batteries, or just AMBs.

“Up till now, sadly, AMBs have not lived up to their potential,” said Ke Niu, a materials scientist with the Guangxi Key Laboratory of Optical and Electronic Materials and Devices at the Guilin University of Technology—one of the lead researchers on the team. “To be able to be used in a wearable device, they need to withstand a certain degree of real-world bending and twisting. But most of those explored so far fail in the face of such stress.”

To overcome this, any fractures or failure points in an AMB would need to be self-healing following such stress. Unfortunately, the self-healing AMBs that have been developed so far have tended to depend on metallic compounds as the carriers of charge in the battery’s electric circuit. This has the undesirable side-effect of strong reaction between the metal’s ions and the materials that the electrodes (the battery’s positive and negative electrical conductors) are made out of. This in turn reduces the battery’s reaction rate (the speed at which the electrochemical reactions at the heart of any battery take place), drastically limiting performance.

“So we started investigating the possibility of non-metallic charge carriers, as these would not suffer from the same difficulties from interaction with the electrodes,” added Junjie Shi, another leading member of the team and a researcher with the School of Physics and Center zfor Nanoscale Characterization & Devices (CNCD) at the Huazhong University of Science and Technology in Wuhan.

The research team alighted upon ammonium ions, derived from abundantly available ammonium salts, as the optimal charge carriers. They are far less corrosive than other options and have a wide electrochemical stability window.

“But ammonium ions are not the only ingredient in the recipe needed to make our batteries self-healing,” said Long Zhang, the third leading member of the research team, also at CNCD.

For that, the team incorporated the ammonium salts into a hydrogel—a polymer material that can absorb and retain a large amount of water without disturbing its structure. This gives hydrogels impressive flexibility—delivering precisely the sort of self-healing character needed. Gelatin is probably the most well-known hydrogel, although the researchers in this case opted for a polyvinyl alcohol hydrogel (PVA) for its great strength and low cost.

To optimize compatibility with the ammonium electrolyte, titanium carbide—a ‘2D’ nanomaterial with only a single layer of atoms—was chosen for the anode (the negative electrode) material for its excellent conductivity. Meanwhile manganese dioxide, already commonly used in dry cell batteries, was woven into a carbon nanotube matrix (again to improve conductivity) for the cathode (the positive electrode).

Testing of the prototype self-healing battery showed it exhibited excellent energy density, power density, cycle life, flexibility, and self-healing even after ten self-healing cycles.

The team now aims to further develop and optimise their prototype in preparation for commercial production.

About Nano Research Energy

Nano Research Energy is launched by Tsinghua University Press and exclusively available via SciOpen, aiming at being an international, open-access and interdisciplinary journal. We will publish research on cutting-edge advanced nanomaterials and nanotechnology for energy. It is dedicated to exploring various aspects of energy-related research that utilizes nanomaterials and nanotechnology, including but not limited to energy generation, conversion, storage, conservation, clean energy, etc. Nano Research Energy will publish four types of manuscripts, that is, Communications, Research Articles, Reviews, and Perspectives in an open-access form.

About SciOpen

SciOpen is a professional open access resource for discovery of scientific and technical content published by the Tsinghua University Press and its publishing partners, providing the scholarly publishing community with innovative technology and market-leading capabilities. SciOpen provides end-to-end services across manuscript submission, peer review, content hosting, analytics, and identity management and expert advice to ensure each journal’s development by offering a range of options across all functions as Journal Layout, Production Services, Editorial Services, Marketing and Promotions, Online Functionality, etc. By digitalizing the publishing process, SciOpen widens the reach, deepens the impact, and accelerates the exchange of ideas.

Here’s a link to and a citation for the paper,

A self-healing aqueous ammonium-ion micro batteries based on PVA-NH4Cl hydrogel electrolyte and MXene-integrated perylene anode by Ke Niu, Junjie Shi, Long Zhang, Yang Yue, Mengjie Wang, Qixiang Zhang, Yanan Ma, Shuyi Mo, Shaofei Li, Wenbiao Li, Li Wen, Yixin Hou, Fei Long, Yihua Gao. Nano Research Energy (2024)DOI: https://doi.org/10.26599/NRE.2024.9120127 Published: 03 June 2024

This paper is open access by means of a “Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, distribution and reproduction in any medium, provided the original work is properly cited.”

Layer of tin could prevent short-circuiting in lithium-ion batteries

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Lithium-ion batteries are everywhere; they can be found in cell phones, laptops, e-scooters, e-bikes, and more. There are also some well documented problems with the batteries including the danger of fire. With the proliferating use of lithium-ion batteries, it seems fires are becoming more frequent as Samantha Murphy Kelly documents in her Mach 9, 2023 article for CNN news online, Note: Links have been removed,

Lithium-ion batteries, found in many popular consumer products, are under scrutiny again following a massive fire this week in New York City thought to be caused by the battery that powered an electric scooter.

At least seven people have been injured in a five-alarm fire in the Bronx which required the attention of 200 firefighters. Officials believe the incident stemmed from a lithium-ion battery of a scooter found on the roof of an apartment building. In 2022, the the New York City Fire Department responded to more than 200 e-scooter and e-bike fires, which resulted in six fatalities.

“In all of these fires, these lithium-ion fires, it is not a slow burn; there’s not a small amount of fire, it literally explodes,” FDNY [Fire Dept. New York] Commissioner Laura Kavanagh told reporters. “It’s a tremendous volume of fire as soon as it happens, and it’s very difficult to extinguish and so it’s particularly dangerous.”

A residential fire earlier this week in Carlsbad, California, was suspected to be caused by an e-scooter lithium battery. On Tuesday [March 7, 2023], an alarming video surfaced of a Canadian homeowner running downstairs to find his electric bike battery exploding into flames. [emphasis mine] A fire at a multi-family home in Massachusetts last month is also under investigation for similar issues.

These incidents are becoming more common for a number of reasons. For starters, lithium-ion batteries are now in numerous consumer tech products,powering laptops, cameras, smartphones and more. They allow companies to squeeze hours of battery life into increasingly slim devices. But a combination of manufacturer issues, misuse and aging batteries can heighten the risk from the batteries, which use flammable materials.

“Lithium batteries are generally safe and unlikely to fail, but only so long as there are no defects and the batteries are not damaged or mistreated,” said Steve Kerber, vice president and executive director of Underwriters Laboratory’s (UL) Fire Safety Research Institute (FSRI). “The more batteries that surround us the more incidents we will see.”

In 2016, Samsung issued a global recall of the Galaxy Note 7 in 2016, citing “battery cell issues” that caused the device to catch fire and at times explode. [emphasis mine] HP and Sony later recalled lithium computer batteries for fire hazards, and about 500,000 hoverboards were recalled due to a risk of “catching fire and/or exploding,” according to the U.S. Consumer Product Safety Commission.

In 2020, the Federal Aviation Administration [emphasis mine] banned uninstalled lithium-ion metal batteries from being checked in luggage and said they must remain with a passenger in their carry-on baggage, if approved by the airline and between 101-160 watt hours. “Smoke and fire incidents involving lithium batteries can be mitigated by the cabin crew and passengers inside the aircraft cabin,” the FAA said.

Despite the concerns, lithium-ion batteries continue to be prevalent in many of today’s most popular gadgets. Some tech companies point to their abilities to charge faster, last longer and pack more power into a lighter package.

But not all lithium batteries are the same.

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Kelly’s Mach 9, 2023 article describes the problems (e.g., a short circuit) that may cause fires and includes some recommendations for better safety and for what to do in the event of a lithium-ion battery fire.Her mention of Samsung and the fires brought back memories; it was mentioned here briefly in a December 21, 2016 post titled, “The volatile lithium-ion battery,” which mostly featured then recent research into the batteries and fires.

More recently, I’ve got an update of sorts on lithium-ion batteries and fires on airplanes, from the May/June 2024 posting of the National Business Aviation Association (NBAA) Insider,

A smoke, fire or extreme heat incident involving lithium ion batteries takes place aboard an aircraft more than once per week [emphases mine] on average in the U.S., making it imperative for operators to fully understand these dangerous events and to prepare crews with safety training.

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At any given time, there could be more than 1,000 Li-ion powered devices on board an airliner, while an international business jet might easily be flying with a few dozen. Despite their popularity, few people realize the dangers posed by Li-ion batteries.

Hazards run the gamut, from overheating, to emitting smoke, to bursting into flames or even exploding – spewing bits of white hot gel in all directions. In fact, a Li-ion fire can begin as a seemingly harmless overheat and erupt into a serious hazard in a matter of seconds.

FAA [US Federal Aviation Administration] data shows the scope of the threat: In 2023, more than one Li-ion incident occurred aboard an aircraft each week. Specifically, the agency said there were 208 issues with lithium ion battery packs, 111 with e-cigarettes and vaping devices, 68 with cell phones and 60 with laptop computers. (The FAA doesn’t offer incident data by aircraft type.

Thankfully, the data shows the chances of encountering an unstable mobile device aboard a business aircraft are small. But so is the possibility of a passenger experiencing a heart attack – yet many business aircraft carry defibrillators.

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The threat with lithium ion batteries is known as thermal runaway. When a Li-ion battery overheats due to some previous damage that creates a short circuit [emphasis mine], the unit continues a catastrophic internal chain reaction until it melts or catches fire.

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Short circuits, lithium ion batteries, and the University of Alberta

A July 31, 2024 Canadian Light Source (CLS) news release (also received via email) by Greg Basky announces the University of Alberta research,

Lithium-ion batteries have a lot of advantages. They charge quickly, have a high energy density, and can be repeatedly charged and discharged.

They do have one significant shortcoming, however: they’re prone to short-circuiting.  This occurs when a connection forms between the two electrodes inside the cell. A short circuit can result in a sudden loss of voltage or the rapid discharge of high current, both causing the battery to fail. In extreme cases, a short circuit can cause a cell to overheat, start on fire, or even explode. Video: Thin layer of tin prevents short-circuiting in lithium-ion batteries

A leading cause of short circuits are rough, tree-like crystal structures called dendrites that can form on the surface of one of the electrodes. When dendrites grow all the way across the cell and make contact with the other electrode, a short circuit can occur.

Using the Canadian Light Source (CLS) at the University of Saskatchewan (USask), researchers from the University of Alberta (UAlberta) have come up with a promising approach to prevent formation of dendrites in solid-state lithium-ion batteries. They found that adding a tin-rich layer between the electrode and the electrolyte helps spread the lithium around when it’s being deposited on the battery, creating a smooth surface that suppresses the formation of dendrites. The results are published in the journal ACS Applied Materials and Interfaces [ACS is American Chemical Society]. The team also found that the cell modified with the tin-rich structure can operate at a much higher current and withstand many more charging-discharging cycles than a regular cell.

Researcher Lingzi Sang, an assistant professor in UAlberta’s Faculty of Science (Chemistry), says the CLS played a key role in the research. “The HXMA beamline enabled us to see at a material’s structural level what was happening on the surface of the lithium in an operating battery,” says Sang. “As a chemist, what I find the most intriguing is we were able to access the exact tin structure that we introduced to the interface which can suppress dendrites and fix this short-circuiting problem.” In a related paper the team published earlier this year, they showed that adding a protective layer of tin also suppressed the formation of dendrites in liquid-electrolyte-based lithium-ion batteries.

This novel approach holds considerable potential for industrial applications, according to Sand. “Our next step is to try to find a sustainable, cost-effective approach to applying the protective layer in battery production.”

Here’s a link to and a citation for the latest paper,

Dual-Component Interlayer Enables Uniform Lithium Deposition and Dendrite Suppression for Solid-State Batteries by Xiang You, Ning Chen, Geng Xie, Shihong Xu, Sayed Youssef Sayed, and Lingzi Sang. ACS Appl. Mater. Interfaces 2024, 16, 27, 35761–35770 DOI: https://doi.org/10.1021/acsami.4c05227 Published June 21, 2024 Copyright © 2024 American Chemical Society

This paper is behind a paywall.

Sound-suppressing silk

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I keep telling a friend that noise will be the ‘new smoking’; i.e., there will be more rules and people will demand enforcement. She doesn’t agree, vociferously so. With the mounting research into the effects that noise has on health and on longevity, it doesn’t matter if I win the ‘argument’, I’m just happy to see research dedicated to mitigating noise levels. From a May 7, 2024 news item on ScienceDaily,

We are living in a very noisy world. From the hum of traffic outside your window to the next-door neighbor’s blaring TV to sounds from a co-worker’s cubicle, unwanted noise remains a resounding problem. [nice bit of wordplay]

Caption: The fabric can suppress sound by generating sound waves that interfere with an unwanted noise to cancel it out (as seen in figure C) or by being held still to suppress vibrations that are key to the transmission of sound (as seen in figure D). Credit: Courtesy of Yoel Fink and Grace (Noel) Yang and Massachusetts Institute of Technology (MIT)

A May 7, 2024 Massachusetts Institute of Technology (MIT) news release (also on EurekAlert), which originated the news item, describes how a surprising material, silk, can be used for suppressing sound, Note: Links have been removed,

To cut through the din, an interdisciplinary collaboration of researchers from MIT and elsewhere developed a sound-suppressing silk fabric that could be used to create quiet spaces. 

The fabric, which is barely thicker than a human hair, contains a special fiber that vibrates when a voltage is applied to it. The researchers leveraged those vibrations to suppress sound in two different ways.

In one, the vibrating fabric generates sound waves that interfere with an unwanted noise to cancel it out, similar to noise-canceling headphones, which work well in a small space like your ears but do not work in large enclosures like rooms or planes. 

In the other, more surprising technique, the fabric is held still to suppress vibrations that are key to the transmission of sound. This prevents noise from being transmitted through the fabric and quiets the volume beyond. This second approach allows for noise reduction in much larger spaces like rooms or cars.

By using common materials like silk, canvas, and muslin, the researchers created noise-suppressing fabrics which would be practical to implement in real-world spaces. For instance, one could use such a fabric to make dividers in open workspaces or thin fabric walls that prevent sound from getting through. 

“Noise is a lot easier to create than quiet. In fact, to keep noise out we dedicate a lot of space to thick walls. [First author] Grace’s work provides a new mechanism for creating quiet spaces with a thin sheet of fabric,” says Yoel Fink, a professor in the departments of Materials Science and Engineering and Electrical Engineering and Computer Science, a Research Laboratory of Electronics principal investigator, and senior author of a paper on the fabric.

The study’s lead author is Grace (Noel) Yang SM ’21, PhD ’24. Co-authors include MIT graduate students Taigyu Joo, Hyunhee Lee, Henry Cheung, and Yongyi Zhao; Zachary Smith, the Robert N. Noyce Career Development Professor of Chemical Engineering at MIT; graduate student Guanchun Rui and professor Lei Zhu of Case Western [Reserve] University; graduate student Jinuan Lin and Assistant Professor Chu Ma of the University of Wisconsin at Madison; and Latika Balachander, a graduate student at the Rhode Island School of Design. The an open-access paper about the research appeared recently in Advanced Materials.

Silky silence

The sound-suppressing silk builds off the group’s prior work to create fabric microphones.

In that research, they sewed a single strand of piezoelectric fiber into fabric. Piezoelectric materials produce an electrical signal when squeezed or bent. When a nearby noise causes the fabric to vibrate, the piezoelectric fiber converts those vibrations into an electrical signal, which can capture the sound. 

In the new work, the researchers flipped that idea to create a fabric loudspeaker that can be used to cancel out soundwaves. 

“While we can use fabric to create sound, there is already so much noise in our world. We thought creating silence could be even more valuable,” Yang says.

Applying an electrical signal to the piezoelectric fiber causes it to vibrate, which generates sound. The researchers demonstrated this by playing Bach’s “Air” using a 130-micrometer sheet of silk mounted on a circular frame.

To enable direct sound suppression, the researchers use a silk fabric loudspeaker to emit sound waves that destructively interfere with unwanted sound waves. They control the vibrations of the piezoelectric fiber so that sound waves emitted by the fabric are opposite of unwanted sound waves that strike the fabric, which can cancel out the noise.

However, this technique is only effective over a small area. So, the researchers built off this idea to develop a technique that uses fabric vibrations to suppress sound in much larger areas, like a bedroom.

Let’s say your next-door neighbors are playing foosball in the middle of the night. You hear noise in your bedroom because the sound in their apartment causes your shared wall to vibrate, which forms sound waves on your side.

To suppress that sound, the researchers could place the silk fabric onto your side of the shared wall, controlling the vibrations in the fiber to force the fabric to remain still. This vibration-mediated suppression prevents sound from being transmitted through the fabric.

“If we can control those vibrations and stop them from happening, we can stop the noise that is generated, as well,” Yang says.

A mirror for sound

Surprisingly, the researchers found that holding the fabric still causes sound to be reflected by the fabric, resulting in a thin piece of silk that reflects sound like a mirror does with light. 

Their experiments also revealed that both the mechanical properties of a fabric and the size of its pores affect the efficiency of sound generation. While silk and muslin have similar mechanical properties, the smaller pore sizes of silk make it a better fabric loudspeaker. 

But the effective pore size also depends on the frequency of sound waves. If the frequency is low enough, even a fabric with relatively large pores could function effectively, Yang says.

When they tested the silk fabric in direct suppression mode, the researchers found that it could significantly reduce the volume of sounds up to 65 decibels (about as loud as enthusiastic human conversation). In vibration-mediated suppression mode, the fabric could reduce sound transmission up to 75 percent.

These results were only possible due to a robust group of collaborators, Fink says. Graduate students at the Rhode Island School of Design helped the researchers understand the details of constructing fabrics; scientists at the University of Wisconsin at Madison conducted simulations; researchers at Case Western Reserve University characterized materials; and chemical engineers in the Smith Group at MIT used their expertise in gas membrane separation to measure airflow through the fabric.

Moving forward, the researchers want to explore the use of their fabric to block sound of multiple frequencies. This would likely require complex signal processing and additional electronics. 

In addition, they want to further study the architecture of the fabric to see how changing things like the number of piezoelectric fibers, the direction in which they are sewn, or the applied voltages could improve performance.

“There are a lot of knobs we can turn to make this sound-suppressing fabric really effective. We want to get people thinking about controlling structural vibrations to suppress sound. This is just the beginning,” says Yang.

This work is funded, in part, by the National Science Foundation (NSF), the Army Research Office (ARO), the Defense Threat Reduction Agency (DTRA), and the Wisconsin Alumni Research Foundation.

Here’s a link to and a citation for the paper,

Single Layer Silk and Cotton Woven Fabrics for Acoustic Emission and Active Sound Suppression by Grace H. Yang, Jinuan Lin, Henry Cheung, Guanchun Rui, Yongyi Zhao, Latika Balachander, Taigyu Joo, Hyunhee Lee, Zachary P. Smith, Lei Zhu, Chu Ma, Yoel Fink. Advanced Materials DOI: https://doi.org/10.1002/adma.202313328 First published: 01 April 2024

This paper is open access.

Aerogels that are 3D printed from nanocellulose

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The one on the far right looks a bit like a frog (to me),

Caption: Complexity and lightness: Empa researchers have developed a 3D printing process for biodegradable cellulose aerogel. Credit: Empa

An April 4, 2024 Swiss Federal Laboratories for Materials Science and Technology (EMPA) press release (also on EurekAlert) describes some interesting possibilities for nanocellulose,

At first glance, biodegradable materials, inks for 3D printing and aerogels don’t seem to have much in common. All three have great potential for the future, however: “green” materials do not pollute the environment, 3D printing can produce complex structures without waste, and ultra-light aerogels are excellent heat insulators. Empa researchers have now succeeded in combining all these advantages in a single material. And their cellulose-based, 3D-printable aerogel can do even more.

The miracle material was created under the leadership of Deeptanshu Sivaraman, Wim Malfait and Shanyu Zhao from Empa’s Building Energy Materials and Components laboratory, in collaboration with the Cellulose & Wood Materials and Advanced Analytical Technologies laboratories as well as the Center for X-ray Analytics. Together with other researchers, Zhao and Malfait had already developed a process for printing silica aerogels in 2020. No trivial task: Silica aerogels are foam-like materials, highly open porous and brittle. Before the Empa development, shaping them into complex forms had been pretty much impossible. “It was the logical next step to apply our printing technology to mechanically more robust bio-based aerogels,” says Zhao.

The researchers chose the most common biopolymer on Earth as their starting material: cellulose. Various nanoparticles can be obtained from this plant-based material using simple processing steps. Doctoral student Deeptanshu Sivaraman used two types of such nanoparticles – cellulose nanocrystals and cellulose nanofibers – to produce the “ink” for printing the bio-aerogel.

Over 80 percent water

The flow characteristics of the ink are crucial in 3D printing: Tt must be viscous enough in order to hold a three-dimensional shape before solidification. At the same time, however, it should liquefy under pressure so that it can flow through the nozzle. With the combination of nanocrystals and nanofibers, Sivaraman succeeded in doing just that: The long nanofibers give the ink a high viscosity, while the rather short crystals ensure that it has shear thinning effect so that it flows more easily during extrusion.

In total, the ink contains around twelve percent cellulose – and 88 percent water. “We were able to achieve the required properties with cellulose alone, without any additives or fillers,” says Sivaraman. This is not only good news for the biodegradability of the final aerogel products, but also for its heat-insulating properties. To turn the ink into an aerogel after printing, the researchers replace the pore solvent water first with ethanol and then with air, all while maintaining shape fidelity. “The less solid matter the ink contains, the more porous the resulting aerogel,” explains Zhao.

This high porosity and the small size of the pores make all aerogels extremely effective heat insulators. However, the researchers have identified a unique property in the printed cellulose aerogel: It is anisotropic. This means its strength and thermal conductivity are direction-dependent. “The anisotropy is partly due to the orientation of the nanocellulose fibers and partly due to the printing process itself,” says Malfait. This allows the researchers to control in which axis the printed aerogel piece should be particularly stable or particularly insulating. Such precisely crafted insulating components could be used in microelectronics, where heat should only be conducted in a certain direction.

A lot of potential applications in medicine

Although the original research project, which was funded by the Swiss National Science Foundation (SNSF), was primarily interested in thermal insulation, the researchers quickly saw another area of application for their printable bio-aerogel: medicine. As it consists of pure cellulose, the new aerogel is biocompatible with living tissues and cells. Its porous structure is able to absorb drugs and then release them into the body over a long period of time. And 3D printing offers the possibility of producing precise shapes that could, for instance, serve as scaffolds for cell growth or as implants.

A particular advantage is that the printed aerogel can be rehydrated and re-dried several times after the initial drying process without losing its shape or porous structure. In practical applications, this would make the material easier to handle: It could be stored and transported in dry form and only be soaked in water shortly before use. When dry, it is not only light and convenient to handle, but also less susceptible to bacteria – and does not have to be elaborately protected from drying out. “If you want to add active ingredients to the aerogel, this can be done in the final rehydration step immediately before use,” says Sivaraman. “Then you don’t run the risk of the medication losing its effectiveness over time or if it is stored incorrectly.”

The researchers are also working on drug delivery from aerogels in a follow-up project – with less focus on 3D printing for now. Shanyu Zhao is collaborating with researchers from Germany and Spain on aerogels made from other biopolymers, such as alginate and chitosan, derived from algae and chitin respectively. Meanwhile, Wim Malfait wants to further improve the thermal insulation of cellulose aerogels. And Deeptanshu Sivaraman has completed his doctorate and has since joined the Empa spin-off Siloxene AG, which creates new hybrid molecules based on silicon.

Fascinating work and here’s a link to and a citation for the paper,

Additive Manufacturing of Nanocellulose Aerogels with Structure-Oriented Thermal, Mechanical, and Biological Properties by Deeptanshu Sivaraman, Yannick Nagel, Gilberto Siqueira, Parth Chansoria, Jonathan Avaro, Antonia Neels, Gustav Nyström, Zhaoxia Sun, Jing Wang, Zhengyuan Pan, Ana Iglesias-Mejuto, Inés Ardao, Carlos A. García-González, Mengmeng Li, Tingting Wu, Marco Lattuada, Wim J. Malfait, Shanyu Zhao. Advanced Science DOI: https://doi.org/10.1002/advs.202307921 First published: 13 March 2024

This paper is open access.

You can find Siloxene AG here.

Tissue-like bioelectronic mesh system capable of growing with cardiac tissues

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Graphene, heralded for its biocompatibility, features in a March 22, 2024 news item on ScienceDaily about research about biosensing and a mesh system that can grow,

A team of engineers has recently built a tissue-like bioelectronic mesh system integrated with an array of atom-thin graphene sensors that can simultaneously measure both the electrical signal and the physical movement of cells in lab-grown human cardiac tissue. This tissue-like mesh can grow along with the cardiac cells, allowing researchers to observe how the heart’s mechanical and electrical functions change during the developmental process. The new device is a boon for those studying cardiac disease as well as those studying the potentially toxic side-effects of many common drug therapies.

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Caption: A bioelectronic mesh, studded with graphene sensors (red), can measure the electrical signal and movement of cardiac tissue (purple and green) at the same time. Credit: Gao et al., 10.1038/s41467-024-46636-7

A March 21, 2024 University of Massachusetts at Amherst news release (also on EurekAlert), which originated the news item, delves further into the topics of heart disease and biosensor implants, Note: Links have been removed,

Cardiac disease is the leading cause of human morbidity and mortality across the world. The heart is also very sensitive to therapeutic drugs, and the pharmaceutical industry spends millions of dollars in testing to make sure that its products are safe. However, ways to effectively monitor living cardiac tissue are extremely limited.

In part, this is because it is very risky to implant sensors in a living heart, but also because the heart is a complex kind of muscle with more than one thing that needs monitoring. “Cardiac tissue is very special,” says Jun Yao, associate professor of electrical and computer engineering in UMass Amherst’s College of Engineering and the paper’s senior author. “It has a mechanical activity—the contractions and relaxations that pump blood through our body—coupled to an electrical signal that controls that activity.”

But today’s sensors can typically only measure one characteristic at a time, and a two-sensor device that could measure both charge and movement would be so bulky as to impede the cardiac tissue’s function. Until now, there was no single sensor capable of measuring the heart’s dual properties without interfering with its functioning.

The new device is built of two critical components, explains lead author Hongyan Gao, who is pursuing his Ph.D. in electrical engineering at UMass Amherst. The first is a three-dimensional cardiac microtissue (CMT), grown in a lab from human stem cells under the guidance of co-author Yubing Sun, associate professor of mechanical and industrial engineering at UMass Amherst. CMT has become the preferred model for in vitro testing because it is the closest analog yet to a full-size, living human heart. However, because CMT is grown in a test tube, it has to mature, a process that takes time and can be easily disrupted by a clumsy sensor.

The second critical component involves graphene—a pure-carbon substance only one atom thick. Graphene has a few surprising quirks to its nature that make it perfect for a cardiac sensor. Graphene is electrically conductive, and so it can sense the electrical charges shooting through cardiac tissue. It is also piezoresistive, which means that as it is stretched—say, by the beating of a heart—its electrical resistance increases. And because graphene is impossibly thin, it can register even the tiniest flutter of muscle contraction or relaxation and can do so without impeding the heart’s function, all through the maturation process. Co-author Jing Kong, professor of electrical engineering at MIT, and her group supplied this critical graphene material.

“Although there have already been many applications for graphene, it is wonderful to see that it can be used in this critical need, which takes advantage of graphene’s different characteristics,” says Kong.

Gao, Yao and their colleagues then embedded a series of graphene sensors in a soft, stretchable porous mesh scaffold they developed that has close structural and mechanical properties to human tissue and which can be applied non-invasively to cardiac tissue.

“No one has ever done this before,” says Gao. “Graphene can survive in a biological environment without degrading for a very long time and not lose its conductivity, so we can monitor the CMT across its entire maturation process.”

“This is crucial for a number of reasons,” adds Yao. “Our sensor can give real-time feedback to scientists and drug researchers, and it can do so in a cost-effective way. We take pride in using the insights of electrical engineering to help build tools that can be useful to a wide range of researchers.”

In the future, Gao says, he hopes to be able to adapt his sensor to grander scales, even to in vivo monitoring, which would provide the best-possible data to help solve cardiac disease.

This research was supported by the Army Research Office, the National Institutes of Health, the U.S. National Science Foundation, the Semiconductor Research Corporation, and the Link Foundation, as well as the Institute for Applied Life Sciences at UMass Amherst.

Here’s a link to and a citation for the paper,

Graphene-integrated mesh electronics with converged multifunctionality for tracking multimodal excitation-contraction dynamics in cardiac microtissues by Hongyan Gao, Zhien Wang, Feiyu Yang, Xiaoyu Wang, Siqi Wang, Quan Zhang, Xiaomeng Liu, Yubing Sun, Jing Kong & Jun Yao. Nature Communications volume 15, Article number: 2321 (2024) DOI: https://doi.org/10.1038/s41467-024-46636-7 Published: 14 March 2024

This paper is open access.

Graphene-like materials for first smart contact lenses with AR (augmented reality) vision, health monitoring, & content surfing?

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A March 6, 2024 XPANCEO news release on EurekAlert (also posted March 11, 2024 on the Graphene Council blog) and distributed by Mindset Consulting announced smart contact lenses devised with graphene-like materials,

XPANCEO, a deep tech company developing the first smart contact lenses with XR vision, health monitoring, and content surfing features, in collaboration with the Nobel laureate Konstantin S. Novoselov (National University of Singapore, University of Manchester) and professor Luis Martin-Moreno (Instituto de Nanociencia y Materiales de Aragon), has announced in Nature Communications a groundbreaking discovery of new properties of rhenium diselenide and rhenium disulfide, enabling novel mode of light-matter interaction with huge potential for integrated photonics, healthcare, and AR. Rhenium disulfide and rhenium diselenide are layered materials belonging to the family of graphene-like materials. Absorption and refraction in these materials have different principal directions, implying six degrees of freedom instead of a maximum of three in classical materials. As a result, rhenium disulfide and rhenium diselenide by themselves allow controlling the light propagation direction without any technological steps required for traditional materials like silicon and titanium dioxide.

The origin of such surprising light-matter interaction of ReS2 and ReSe2 with light is due to the specific symmetry breaking observed in these materials. Symmetry plays a huge role in nature, human life, and material science. For example, almost all living things are built symmetrically. Therefore, in ancient times symmetry was also called harmony, as it was associated with beauty. Physical laws are also closely related to symmetry, such as the laws of conservation of energy and momentum. Violation of symmetry leads to the appearance of new physical effects and radical changes in the properties of materials. In particular, the water-ice phase transition is a consequence of a decrease in the degree of symmetry. In the case of ReS2 and ReSe2, the crystal lattice has the lowest possible degree of symmetry, which leads to the rotation of optical axes – directions of symmetry of optical properties of the material, which was previously observed only for organic materials. As a result, these materials make possible to control the direction of light by changing the wavelength, which opens a unique way for light manipulation in next-generation devices and applications. 

“The discovery of unique properties in anisotropic materials is revolutionizing the fields of nanophotonics and optoelectronics, presenting exciting possibilities. These materials serve as a versatile platform for the advancement of optical devices, such as wavelength-switchable metamaterials, metasurfaces, and waveguides. Among the promising applications is the development of highly efficient biochemical sensors. These sensors have the potential to outperform existing analogs in terms of both sensitivity and cost efficiency. For example, they are anticipated to significantly reduce the expenses associated with hospital blood testing equipment, which is currently quite costly, potentially by several orders of magnitude. This will also allow the detection of dangerous diseases and viruses, such as cancer or COVID, at earlier stages,” says Dr. Valentyn S. Volkov, co-founder and scientific partner at XPANCEO, a scientist with an h-Index of 38 and over 8000 citations in leading international publications.

Beyond the healthcare industry, these novel properties of graphene-like materials can find applications in artificial intelligence and machine learning, facilitating the development of photonic circuits to create a fast and powerful computer suitable for machine learning tasks. A computer based on photonic circuits is a superior solution, transmitting more information per unit of time, and unlike electric currents, photons (light beams) flow across one another without interacting. Furthermore, the new material properties can be utilized in producing smart optics, such as contact lenses or glasses, specifically for advancing AR [augmented reality] features. Leveraging these properties will enhance image coloration and adapt images for individuals with impaired color perception, enabling them to see the full spectrum of colors.

Here’s a link to and a citation for the paper,

Wandering principal optical axes in van der Waals triclinic materials by Georgy A. Ermolaev, Kirill V. Voronin, Adilet N. Toksumakov, Dmitriy V. Grudinin, Ilia M. Fradkin, Arslan Mazitov, Aleksandr S. Slavich, Mikhail K. Tatmyshevskiy, Dmitry I. Yakubovsky, Valentin R. Solovey, Roman V. Kirtaev, Sergey M. Novikov, Elena S. Zhukova, Ivan Kruglov, Andrey A. Vyshnevyy, Denis G. Baranov, Davit A. Ghazaryan, Aleksey V. Arsenin, Luis Martin-Moreno, Valentyn S. Volkov & Kostya S. Novoselov. Nature Communications volume 15, Article number: 1552 (2024) DOI: https://doi.org/10.1038/s41467-024-45266-3 Published: 06 March 2024

This paper is open access.

Deriving gold from electronic waste

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Caption: The gold nugget obtained from computer motherboards in three parts. The largest of these parts is around five millimetres wide. Credit: (Photograph: ETH Zurich / Alan Kovacevic)

A March 1, 2024 ETH Zurich press release (also on EurekAlert but published February 29, 2024) by Fabio Bergamin describes research into reclaiming gold from electronic waste, Note: A link has been removed.

In brief

  • Protein fibril sponges made by ETH Zurich researchers are hugely effective at recovering gold from electronic waste.
  • From 20 old computer motherboards, the researchers retrieved a 22-​carat gold nugget weighing 450 milligrams.
  • Because the method utilises various waste and industry byproducts, it is not only sustainable but cost effective as well.

Transforming base materials into gold was one of the elusive goals of the alchemists of yore. Now Professor Raffaele Mezzenga from the Department of Health Sciences and Technology at ETH Zurich has accomplished something in that vein. He has not of course transformed another chemical element into gold, as the alchemists sought to do. But he has managed to recover gold from electronic waste using a byproduct of the cheesemaking process.

Electronic waste contains a variety of valuable metals, including copper, cobalt, and even significant amounts of gold. Recovering this gold from disused smartphones and computers is an attractive proposition in view of the rising demand for the precious metal. However, the recovery methods devised to date are energy-​intensive and often require the use of highly toxic chemicals. Now, a group led by ETH Professor Mezzenga has come up with a very efficient, cost-​effective, and above all far more sustainable method: with a sponge made from a protein matrix, the researchers have successfully extracted gold from electronic waste.

Selective gold adsorption

To manufacture the sponge, Mohammad Peydayesh, a senior scientist in Mezzenga’s Group, and his colleagues denatured whey proteins under acidic conditions and high temperatures, so that they aggregated into protein nanofibrils in a gel. The scientists then dried the gel, creating a sponge out of these protein fibrils.

To recover gold in the laboratory experiment, the team salvaged the electronic motherboards from 20 old computer motherboards and extracted the metal parts. They dissolved these parts in an acid bath so as to ionise the metals.

When they placed the protein fibre sponge in the metal ion solution, the gold ions adhered to the protein fibres. Other metal ions can also adhere to the fibres, but gold ions do so much more efficiently. The researchers demonstrated this in their paper, which they have published in the journal Advanced Materials.

As the next step, the researchers heated the sponge. This reduced the gold ions into flakes, which the scientists subsequently melted down into a gold nugget. In this way, they obtained a nugget of around 450 milligrams out of the 20 computer motherboards. The nugget was 91 percent gold (the remainder being copper), which corresponds to 22 carats.

Economically viable

The new technology is commercially viable, as Mezzenga’s calculations show: procurement costs for the source materials added to the energy costs for the entire process are 50 times lower than the value of the gold that can be recovered.

Next, the researchers want to develop the technology to ready it for the market. Although electronic waste is the most promising starting product from which they want to extract gold, there are other possible sources. These include industrial waste from microchip manufacturing or from gold-​plating processes. In addition, the scientists plan to investigate whether they can manufacture the protein fibril sponges out of other protein-​rich byproducts or waste products from the food industry.

“The fact I love the most is that we’re using a food industry byproduct to obtain gold from electronic waste,” Mezzenga says. In a very real sense, he observes, the method transforms two waste products into gold. “You can’t get much more sustainable than that!”

If you have a problem accessing either of the two previously provided links to the press release, you can try this February 29, 2024 news item on ScienceDaily.

Here’s a link to and a citation for the paper,

Gold Recovery from E-Waste by Food-Waste Amyloid Aerogels by Mohammad Peydayesh, Enrico Boschi, Felix Donat, Raffaele Mezzenga. DOI: https://doi.org/10.1002/adma.202310642 First published online: 23 January 2024

This paper is open access.

Proof-of-concept for implantable batteries that run on body’s own oxygen

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Bioenergy harvesting may be here. Well maybe not yet but we are one step closer according to a March 27, 2024 news item on ScienceDaily,

From pacemakers to neurostimulators, implantable medical devices rely on batteries to keep the heart on beat and dampen pain. But batteries eventually run low and require invasive surgeries to replace. To address these challenges, researchers have devised an implantable battery that runs on oxygen in the body. The study shows in rats that the proof-of-concept design can deliver stable power and is compatible with the biological system.

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This is a dynamic image illustrating the device in action,

Caption: Implantable and bio-compatible Na-O2 battery. Credit: Chem/Lv et al.

A March 27, 2024 Cell Press news release on EurekAlert, which originated the news item, provides more detail about the -proof-of-concept device,

“When you think about it, oxygen is the source of our life,” says corresponding author Xizheng Liu, who specializes in energy materials and devices at Tianjin University of Technology. “If we can leverage the continuous supply of oxygen in the body, battery life won’t be limited by the finite materials within conventional batteries.”

To build a safe and efficient battery, the researchers made its electrodes out of a sodium-based alloy and nanoporous gold, a material with pores thousands of times smaller than a hair’s width. Gold has been known for its compatibility with living systems, and sodium is an essential and ubiquitous element in the human body. The electrodes undergo chemical reactions with oxygen in the body to produce electricity. To protect the battery, the researchers encased it within a porous polymer film that is soft and flexible.

The researchers then implanted the battery under the skin on the backs of rats and measured its electricity output. Two weeks later, they found that the battery can produce stable voltages between 1.3 V and 1.4 V, with a maximum power density of 2.6 µW/cm2. Although the output is insufficient to power medical devices, the design shows that harnessing oxygen in the body for energy is possible.

The team also evaluated inflammatory reactions, metabolic changes, and tissue regeneration around the battery. The rats showed no apparent inflammation. Byproducts from the battery’s chemical reactions, including sodium ions, hydroxide ions, and low levels of hydrogen peroxide, were easily metabolized by the body and did not affect the kidneys and liver. The rats healed well after implantation, with the hair on their back completely regrown after four weeks. To the researchers’ surprise, blood vessels also regenerated around the battery.

“We were puzzled by the unstable electricity output right after implantation,” says Liu. “It turned out we had to give the wound time to heal, for blood vessels to regenerate around the battery and supply oxygen, before the battery could provide stable electricity. This is a surprising and interesting finding because it means that the battery can help monitor wound healing.”

Next, the team plans to up the battery’s energy delivery by exploring more efficient materials for the electrodes and optimizing the battery structure and design. Liu also noted that the battery is easy to scale up in production and choosing cost-effective materials can further lower the price. The team’s battery may also find other purposes beyond powering medical devices.

“Because tumor cells are sensitive to oxygen levels, implanting this oxygen-consuming battery around it may help starve cancers. It’s also possible to convert the battery energy to heat to kill cancer cells,” says Liu. “From a new energy source to potential biotherapies, the prospects for this battery are exciting.”

Here’s a link to and a citation for the paper,

Implantable and bio-compatible Na-O2 battery by Yang Lv, Xizheng Liu, Jiucong Liu, Pingli Wu, Yonggang Wang, Yi Ding. Chem DOI: https://doi.org/10.1016/j.chempr.2024.02.012 In press, corrected proof Published online: March 27, 2024 Copyright © 2024 Elsevier Inc.

The paper appears to be open access.

University of Waterloo researchers get one step closer to secure quantum communication on a global scale

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A March 25, 2024 news item on phys.org announcds Canadian research into quantum communication, Note: Links have been removed,

Researchers at the University of Waterloo’s Institute for Quantum Computing (IQC) have brought together two Nobel prize-winning research concepts to advance the field of quantum communication.

Scientists can now efficiently produce nearly perfect entangled photon pairs from quantum dot sources. The research, “Oscillating photonic Bell state from a semiconductor quantum dot for quantum key distribution,” was published in Communications Physics

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A March 25, 2024 University of Waterloo news release (also on EurekAlert), which originated the news item, delves further into the topic of quantum physics and communication,

Entangled photons are particles of light that remain connected, even across large distances, and the 2022 Nobel Prize in Physics recognized experiments on this topic. Combining entanglement with quantum dots, a technology recognized with the Nobel Prize in Chemistry in 2023, the IQC research team aimed to optimize the process for creating entangled photons, which have a wide variety of applications, including secure communications.

“The combination of a high degree of entanglement and high efficiency is needed for exciting applications such as quantum key distribution or quantum repeaters, which are envisioned to extend the distance of secure quantum communication to a global scale or link remote quantum computers,” said Dr. Michael Reimer, professor at IQC and Waterloo’s Department of Electrical and Computer Engineering. “Previous experiments only measured either near-perfect entanglement or high efficiency, but we’re the first to achieve both requirements with a quantum dot.”

By embedding semiconductor quantum dots into a nanowire, the researchers created a source that creates near-perfect entangled photons 65 times more efficiently than previous work. This new source, developed in collaboration with the National Research Council of Canada in Ottawa, can be excited with lasers to generate entangled pairs on command. The researchers then used high-resolution single photon detectors provided by Single Quantum in The Netherlands to boost the degree of entanglement.

“Historically, quantum dot systems were plagued with a problem called fine structure splitting, which causes an entangled state to oscillate over time. This meant that measurements taken with a slow detection system would prevent the entanglement from being measured,” said Matteo Pennacchietti, a PhD student at IQC and Waterloo’s Department of Electrical and Computer Engineering. “We overcame this by combining our quantum dots with a very fast and precise detection system. We can basically take a timestamp of what the entangled state looks like at each point during the oscillations, and that’s where we have the perfect entanglement.”

To showcase future communications applications, Reimer and Pennacchietti worked with Dr. Norbert Lütkenhaus and Dr. Thomas Jennewein, both IQC faculty members and professors in Waterloo’s Department of Physics and Astronomy, and their teams. Using their new quantum dot entanglement source, the researchers simulated a secure communications method known as quantum key distribution, proving that the quantum dot source holds significant promise in the future of secure quantum communications.

Here’s a link to and a citation for the paper,

Oscillating photonic Bell state from a semiconductor quantum dot for quantum key distribution by Matteo Pennacchietti, Brady Cunard, Shlok Nahar, Mohd Zeeshan, Sayan Gangopadhyay, Philip J. Poole, Dan Dalacu, Andreas Fognini, Klaus D. Jöns, Val Zwiller, Thomas Jennewein, Norbert Lütkenhaus & Michael E. Reimer. Communications Physics volume 7, Article number: 62 (2024)
DOI: https://doi.org/10.1038/s42005-024-01547-3 Published: 24 February 2024

This paper is open access.

A graphene joke (of sorts): What did the electron ‘say’ to the phonon in the graphene sandwich?

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Unfortunately, there isn’t a punch line but I appreciate the effort to inject a little lightness into the description of a fairly technical achievement, from a February 12, 2024 news item on Nanowerk, Note: A link has been removed,

Electrons carry electrical energy, while vibrational energy is carried by phonons. Understanding how they interact with each other in certain materials, like in a sandwich of two graphene layers, will have implications for future optoelectronic devices.

Key Takeaways

Twisted graphene layers exhibit unique electrical properties.

Electron-phonon interactions crucial for energy loss in graphene.

Discovery of a new physical process involving electron-phonon Umklapp scattering.

Potential implications for ultrafast optoelectronics and quantum applications.

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A February 9, 2024 Eindhoven University of Technology (TU/e; Netherlands) press release, which originated the news item, is reproduced here in its entirety, Note: Links have been removed,

Electrons carry electrical energy, while vibrational energy is carried by phonons. Understanding how they interact with each other in certain materials, like in a sandwich of two graphene layers, will have implications for future optoelectronic devices. Recent work has revealed that graphene layers twisted relative to each other by a small ‘magic angle’ can act as perfect insulator or superconductor. But the physics of the electron-phonon interactions are a mystery. As part of a worldwide international collaboration, TU/e researcher Klaas-Jan Tielrooij has led a study on electron-phonon interactions in graphene layers. And they have made a startling discovery.

What did the electron say to the phonon between two layers of graphene?

This might sound like the start of a physics meme with a hilarious punchline to follow. But that’s not the case according to Klaas-Jan Tielrooij. He’s an associate professor at the Department of Applied Physics and Science Education at TU/e and the research lead of the new work published in Science Advances.

“We sought to understand how electrons and phonons ‘talk’ to each other within two twisted graphene layers,” says Tielrooij.

Electrons are the well-known charge and energy carriers associated with electricity, while a phonon is linked to the emergence of vibrations between atoms in an atomic crystal.

“Phonons aren’t particles like electrons though, they’re a quasiparticle. Yet, their interaction with electrons in certain materials and how they affect energy loss in electrons has been a mystery for some time,” notes Tielrooij.

But why would it be interesting to learn more about electron-phonon interactions? “These interactions can have a major effect on the electronic and optoelectronic properties of devices, made from materials like graphene, which we are going to see more of in the future.”

Twistronics: Breakthrough of the Year 2018

Tielrooij and his collaborators, who are based around the world in Spain, Germany, Japan, and the US, decided to study electron-phonon interactions in a very particular case – within two layers of graphene where the layers are ever-so-slightly misaligned.

Graphene is a two-dimensional layer of carbon atoms arranged in a honeycomb lattice that has several impressive properties such as high electrical conductivity, high flexibility, and high thermal conductivity, and it is also nearly transparent.

Back in 2018, the Physics World Breakthrough of the Year award went to Pablo Jarillo-Herrero and colleagues at MIT [Massachusetts Institute of Technology] for their pioneering work on twistronics, where adjacent layers of graphene are rotated very slightly relative to each other to change the electronic properties of the graphene.

Twist and astound!

“Depending on how the layers of graphene are rotated and doped with electrons, contrasting outcomes are possible. For certain dopings, the layers act as an insulator, which prevents the movement of electrons. For other doping, the material behaves as a superconductor – a material with zero resistance that allows the dissipation-less movement of electrons,” says Tielrooij.

Better known as twisted bilayer graphene, these outcomes occur at the so-called magic angle of misalignment, which is just over one degree of rotation. “The misalignment between the layers is tiny, but the possibility for a superconductor or an insulator is an astounding result.”

How electrons lose energy

For their study, Tielrooij and the team wanted to learn more about how electrons lose energy in magic-angle twisted bilayer graphene, or MATBG for short.

To achieve this, they used a material consisting of two sheets of monolayer graphene (each 0.3 nanometers thick), placed on top of each other, and misaligned relative to each other by about one degree.

Then using two optoelectronic measurement techniques, the researchers were able to probe the electron-phonon interactions in detail, and they made some staggering discoveries.

“We observed that the energy vanishes very quickly in the MATBG – it occurs on the picosecond timescale, which is one-millionth of one-millionth of a second!” says Tielrooij.

This observation is much faster than for the case of a single layer of graphene, especially at ultracold temperatures (specifically below -73 degrees Celsius). “At these temperatures, it’s very difficult for electrons to lose energy to phonons, yet it happens in the MATBG.”

Why electrons lose energy

So, why are the electrons losing the energy so quickly through interaction with the phonons? Well, it turns out the researchers have uncovered a whole new physical process.

“The strong electron-phonon interaction is a completely new physical process and involves so-called electron-phonon Umklapp scattering,” adds Hiroaki Ishizuka from Tokyo Institute of Technology in Japan, who developed the theoretical understanding of this process together with Leonid Levitov from Massachusetts Institute of Technology in the US.

Umklapp scattering between phonons is a process that often affects heat transfer in materials, because it enables relatively large amounts of momentum to be transferred between phonons.

“We see the effects of phonon-phonon Umklapp scattering all the time as it affects the ability for (non-metallic) materials at room temperature to conduct heat. Just think of an insulating material on the handle of a pot for example,” says Ishizuka. “However, electron-phonon Umklapp scattering is rare. Here though we have observed for the first time how electrons and phonons interact via Umklapp scattering to dissipate electron energy.”

Challenges solved together

Tielrooij and collaborators may have completed most of the work while he was based in Spain at the Catalan Institute of Nanoscience and Nanotechnology (ICN2), but as Tielrooij notes. “The international collaboration proved pivotal to making this discovery.”

So, how did all the collaborators contribute to the research? Tielrooij: “First, we needed advanced fabrication techniques to make the MATBG samples. But we also needed a deep theoretical understanding of what’s happening in the samples. Added to that, ultrafast optoelectronic measurement setups were required to measure what’s happening in the samples too.”

Tielrooij and the team received the magic-angle twisted samples from Dmitri Efetov’s group at Ludwig-Maximilians-Universität in Munich, who were the first group in Europe able to make such samples and who also performed photomixing measurements, while theoretical work at MIT in the US and at Tokyo Institute of Technology in Japan proved crucial to the success of the research.

At ICN2, Tielrooij and his team members Jake Mehew and Alexander Block used cutting-edge equipment particularly time-resolved photovoltage microscopy to perform their measurements of electron-phonon dynamics in the samples.

The future

So, what does the future look like for these materials then? According to Tielrooij, don’t expect anything too soon.

“As the material is only being studied for a few years, we’re still some way from seeing magic-angle twisted bilayer graphene having an impact on society.”

But there is a great deal to be explored about energy loss in the material.

“Future discoveries could have implications for charge transport dynamics, which could have implications for future ultrafast optoelectronics devices,” says Tielrooij. “In particular, they would be very useful at low temperatures, so that makes the material suitable for space and quantum applications.”

The research from Tielrooij and the international team is a real breakthrough when it comes to how electrons and phonons interact with each other.

But we’ll have to wait a little longer to fully understand the consequences of what the electron said to the phonon in the graphene sandwich.

Illustration showing the control of energy relaxation with twist angle. Image: Authors

Here’s a link to and a citation for the paper,

Ultrafast Umklapp-assisted electron-phonon cooling in magic-angle twisted bilayer graphene by Jake Dudley Mehew, Rafael Luque Merino, Hiroaki Ishizuka, Alexander Block, Jaime Díez Mérida, Andrés Díez Carlón, Kenji Watanabe, Takashi Taniguchi, Leonid S. Levitov, Dmitri K. Efetov, and Klaas-Jan Tielrooij. Science Advances 9 Feb 2024 Vol 10, Issue 6 DOI: 10.1126/sciadv.adj1361

This paper is open access.