Globs of an inflammation protein beckon an Alzheimer’s protein and cause it to accumulate in the brain, a study in mice finds. The results, described in the Dec. 21/28 Nature, add new details to the relationship between brain inflammation and Alzheimer’s disease.
Researchers suspect that this inflammatory cycle is an early step in the disease, which raises the prospect of being able to prevent the buildup of amyloid-beta, the sticky protein found in brains of people with Alzheimer’s disease. “It is a provocative paper,” says immunologist Marco Colonna of Washington University School of Medicine in St. Louis. Finding an inflammatory protein that can prompt A-beta to clump around it is “a big deal,” he says.
Researchers led by Michael Heneka of the University of Bonn in Germany started by studying specks made of a protein called ASC that’s produced as part of the inflammatory response. (A-beta itself is known to kick-start this inflammatory process.) Despite being called specks, these are large globs of protein that are created by and then ejected from brain immune cells called microglia when inflammation sets in. A-beta then accumulates around these ejected ASC specks in the space between cells, Haneke and colleagues now propose. A-beta can directly latch on to ASC specks, experiments in lab dishes revealed. The two proteins were also caught in close contact in brain tissue taken from people with Alzheimer’s disease. Researchers didn’t see any ASC specks mingling with A-beta in the brains of people without the disease. Mice engineered to produce lots of A-beta had telltale signs of its accumulation in their brains at 8 and 12 months of age, roughly comparable to middle age in people. But in mice that also lacked the ability to produce ASC specks, this A-beta brain load was much lighter, and these mice performed better on a memory test. Similar reductions in A-beta loads came when researchers used an antibody to prevent A-beta from sticking to ASC specks, results that suggest the specks are needed for A-beta to clump up.
The details show “a quite new and specific mechanism” that’s worth exploring for potential treatments, says Richard Ransohoff, a neuroinflammation biologist at Third Rock Ventures, a venture capital firm in Boston.
To be effective as a treatment, an antibody like the one in the study that kept A-beta from sticking to ASC would need to be able to enter the brain and persist at high levels — a big challenge, Ransohoff says. Still, the results are promising, he says. “I like the data. I like the line of experimentation.”
Many questions remain. The results are mainly from mice, and it’s not clear whether ASC specks and A-beta have similar interactions in human brains. Nor is it obvious how to stop the A-beta from accumulating around the specks without affecting the immune system more generally.
What’s more, the role of the microglia immune cells that release ASC specks is complex, Colonna says. In some cases, microglia serve as brain protectors by surrounding and sequestering sticky A-beta plaques in the brain (SN: 11/30/13, p. 22). But the current results suggest that by releasing ASC specks, the same cells can also make A-beta accumulation worse. The dueling roles of the cells — protective in some cases and potentially harmful in others — make it challenging to figure out how to tweak their behavior therapeutically, Colonna says.
Our Top 10 stories of 2017 cover the science that was earthshaking, field-advancing or otherwise important. But choosing our favorite stories requires some different metrics.
Here are some of our staff’s favorites from 2017, selected for their intrigue, their power, their element of surprise — or because they were just really, really fun.
Stories that moved us “The eclipse the eclipse the eclipse omg the eclipse.”
Astronomy writer Lisa Grossman didn’t hesitate in her e-mail reply when I asked for everyone’s personal favorites of the year. For the Great American Eclipse, Lisa wrote a 10-part preview of questions scientists would pursue during totality. She then traveled to Wyoming for the eclipse itself, reporting from a Baptist summer camp–turned-observatory. The whole experience was surprisingly emotional, Lisa says, and one that has stuck with her. “I keep looking at the sun now and thinking about how all that beautiful gossamer structure is there, all the time, and we just can’t see it. And how lucky we are that the moon is just the size and distance it is, so that we can experience this.” The Cassini spacecraft’s journey to Saturn also struck an emotional chord with the SN staff. “Cassini crashing into Saturn wins the award for ‘2017 science event that made me cry the most,’” says staff writer Laurel Hamers. After traveling 4.9 billion miles over nearly 20 years, the spacecraft dove into Saturn’s atmosphere and vaporized. “It was a very human drama about a machine,” says audience engagement editor Mike Denison. “It was the sort of science story even a layman like me can get very invested in.”
At its core, Cassini’s mission was basic exploration — the same drive that made the moon landing so captivating. “It’s amazing that there is still so much of our solar system we haven’t explored directly, and the goodies from that mission and the final dive will be reported for years to come,” writes acting editor in chief Beth Quill. “Plus, I love the narrative potential of a spacecraft that sacrifices itself.”
Physics writer Emily Conover’s personal favorite was also our No. 1 story, the detection of two neutron stars colliding — a finding that she had predicted. “I’m patting myself on the back a little bit for that,” she says.
“It was a lot of fun to think about how we’ve detected something completely new and confirmed that some of the tangible stuff around us, like the gold in my wedding ring, came from collisions like that,” she adds. “It’s one of those stories that if you think about it hard enough, it makes you feel like a very small part in a giant, wonderful, fascinating universe.”
Stories that surprised us We spend our days devouring science, combing scientific journals, interviewing scientists, attending meetings and reading science news in other publications. You’d think very little would surprise us. Not true. Maria Temming’s story on the discovery of a mysterious void in the Great Pyramid of Giza was one of our most-read stories of the year . By placing detectors throughout the pyramid to measure subatomic particles called muons, researchers discovered a previously unknown cavity inside the pyramid. “The topic was this beautiful juxtaposition of modern, cutting-edge technology — as in the muon detectors — with the ancient technology of pyramid construction,” says Maria, SN ’s technology writer. “It’s also kind of hilarious to think that the Great Mysterious Thing in this story is not the high-energy particles from outer space — that’s the thing we’ve got a handle on!” Science News for Students managing editor and Wild Things blogger Sarah Zielinski has a keen eye for amazing animal stories, so her pick for a favorite story surprised me: It was our May story and infographic on how an asteroid impact would kill you. “You assume that you know what an asteroid impact would do,” Sarah says, “but it turns out that your assumptions are completely wrong.”
Senior writer Tina Hesman Saey has been covering molecular and developmental biology for more than a decade, but was surprised when a new study overturned the idea that female is the default sex in developing mammals, and that only male tissues have to be actively built. A study she reported on this year found that male structures must be demolished to set off female development. “I was amazed that no one knew a basic of developmental biology: that development of female reproductive organs is an active process,” Tina says. A story about circulation in sea spiders takes the surprise prize for biology writer Susan Milius. “I had never written a story about them, so they were on my taxonomic bucket list,” she says. It turns out that oxygen-rich blood circulates up and down the animal’s legs as contractions move bits of food through the digestive tract in the legs. “It’s circulation by gut lump!” Susan says. “This still blows me away.” The story of how the house mouse came to live with people was a favorite for Science News for Students writer and Scicurious blogger Bethany Brookshire. “It was something I’d never thought of before and it was interesting to find how just how much we were affecting the species around us, even the littlest ones!”
Graphic designer Tracee Tibbitts highlighted two more, well, animalistic animal stories: One about a coconut crab attacking a bird, and one about gulls eating hookworms from seals’ feces — directly from the source. “We see a lot of cute animal stories online that give us warm fuzzies or a ‘they’re just like us!’ reaction,” Tracee says. “But both of these stories remind us that — NOPE. Animals are still wild and out there fighting each other for food and resources and survival.”
Stories that intrigued, for better or worse The gene-editing technology CRISPR/Cas9 caught Beth’s attention this year, “though I would say that last year and would say it again next year,” she says. “It is especially interesting to me to watch a technology from its infancy and understand the twists and turns it takes, all the ways it’s used and the ethical implications that arise.”
CRISPR made our Top 10 list this year as Tina had predicted in 2016. “I was right that CRISPR would still be a thing,” Tina says.
But Susan hadn’t expected CRISPR to creep into her beat as well. “What I missed by light-years was how fast CRISPR would cease to be just Tina’s business and become a matter that someone writing about conservation, ecology and real outdoor evolution has to watch,” Susan says. In an in-depth story on ticks, Susan described preliminary work to engineer mice using CRISPR gene editing to curb the spread of the Lyme disease parasite. “It might happen in six or seven years, and at the current speed, gene editing for wild, free-roaming organisms may, for better or worse — or both — be a real thing,” Susan says. “I certainly see the need for caution, but wow, are the possibilities changing fast.”
Stories scientists tell Part of the fun of many of the stories we cover is talking to the researchers who do the work. “I had so much fun interviewing scientists for [the neutron star collision] story,” Emily says. “Some of the members of LIGO were practically losing their minds about how amazing the detection was. It was so easy to get caught up in the excitement.” Tina got a chance to talk to planetary scientists and astrochemists — not her usual crowd — for a news story on a molecule on Saturn’s moon Titan that could be a key building block for any strange life-forms that might exist in the moon’s frigid methane lakes. In 2016, Tina had written a feature story on what alien life might look like , and in it described computer simulations of a molecule that could form bubblelike structures that resemble cell membranes. The new work showed that the molecule actually does exist on Titan. “It was a thrill to see that one prediction about truly alien life might come true,” Tina says. Laurel enjoyed talking to scientists trying to create better surgical adhesives inspired by slugs, worms and other critters. “People who study weird slime-making animals give the best interviews,” she says.
Associate editor Cassie Martin had a challenging time getting in touch with a scientist for a piece on the cholera epidemic in Yemen. “Finding a scientist and health worker in the war-torn country without actually traveling there took a lot of time and determination,” Cassie says. Once she did, though, she learned more than she expected. “I learned so much about what was happening not only with the epidemic, but about how war affects the scientific enterprise.”
For a video story on the anniversary of the detection of supernova 1987A, web producer Helen Thompson talked to Ian Shelton, who discovered the stellar explosion. The video told the story of the night of the discovery and reviewed all the insights the explosion has given to astronomy. The video also featured Shelton’s voice — and his likeness, in Claymation form. “I got to do a video that combined Claymation and glitter, which are my two favorite things,” Helen says.
Story continues below video The story of the moment You know when someone asks you what your favorite TV show is, and the show that springs to mind is the one you’re binge-watching right now? That often happens with our favorite science stories. Behavioral sciences writer Bruce Bower is putting the finishing touches on a feature story, due out early next year, on fantasy and reality in children’s play. Today, it’s his favorite. “It brings together psychology, anthropology/ethnography and archaeology, an interdisciplinary service that journalists can provide because scientists rarely do,” Bruce says.
For biomedical writer Aimee Cunningham, it’s all of the stories. “The majority of the stories I’ve written this year have met my criteria for why I do this work: to talk to interesting people, learn cool science and share what I find out.”
And that’s what we love about the work that we do. Here’s to 2018 and all the moving, surprising, intriguing, fascinating stories it will bring.
Immune reactions against proteins commonly used as molecular scissors might make CRISPR/Cas9 gene editing ineffective in people, a new study suggests.
About 79 percent of 34 blood donors tested had antibodies against the Cas9 protein from Staphylococcus aureus bacteria, Stanford University researchers report January 5 at bioRxiv.org. About 65 percent of donors had antibodies against the Cas9 protein from Streptococcus pyogenes.
Nearly half of 13 blood donors also had T cells that seek and destroy cells that make S. aureus Cas9 protein. The researchers did not detect any T cells that attack S. pyogenes Cas9, but the methods used to detect the cells may not be sensitive enough to find them, says study coauthor Kenneth Weinberg. Cas9 is the DNA-cutting enzyme that enables researchers to make precise edits in genes. Antibodies and T cells against the protein could cause the immune system to attack cells carrying it, making gene therapy ineffective.
The immune reactions may be a technical glitch that researchers will need to work around, but probably aren’t a safety concern as long as cells are edited in lab dishes rather than in the body, says Weinberg, a stem cell biologist and immunologist.
“We think we need to address this now … as we move toward clinical trials,” he says, but “this is probably going to turn out to be more of a hiccup than a brick wall.”
A new version of the periodic table showcases the predicted properties of 2-D metals, an obscure class of synthetic materials.
Arrayed in 1-atom-thick sheets, most of these 2-D metals have yet to be seen in the real world. So Janne Nevalaita and Pekka Koskinen, physicists at the University of Jyväskylä in Finland, simulated 2-D materials of 45 metallic elements, ranging from lithium to bismuth. For each sheet, the researchers measured the average chemical bond length, bond strength and the material’s compressibility, how difficult it is to squeeze the atoms closer together. The team then charted those features in the new periodic table. The new work, described in the Jan. 15 Physical Review B, could help researchers identify which 2-D metals are most promising for various applications, like spurring chemical reactions or sensing gases.
These metals are similar to previously studied 2-D materials, such as the supermaterial graphene (SN: 10/3/15, p. 7) and its cousin diamondene (SN: 9/2/17, p. 12). But whereas those materials were made up of covalent bonds — in which pairs of atoms share electrons — these 2-D metals are composed of metallic bonds, where electrons flow more freely among atoms. “It’s a whole new type of family of nanostructures,” Koskinen says. “Sky’s the limit, for what the applications could be.”
Like other superflat materials, some potential 2-D metals might exhibit exotic quantum qualities, such as 2-D magnetism or superconductivity, the ability to transmit electricity without resistance. Such properties may make those materials useful for quantum computing, says Joshua Robinson, a materials scientist at Penn State not involved in the work.
Nevalaita and Koskinen created three periodic tables that chart the properties of 2-D metals with atoms in triangular, square or honeycomb configurations. Using their trio of tables, the researchers discovered that the properties of 2-D metals were related to those of their 3-D counterparts. For instance, atoms of any given metal arranged in a triangular lattice typically had about 70 percent the bond strength of atoms in the 3-D version of that metal. Square and honeycomb lattices generally showed about 66 percent and 54 percent the bond strength of 3-D metals, respectively. The periodic tables revealed similar relationships between 2-D and 3-D metals in bond length and compressibility. These findings could allow researchers to get a quick profile of a 2-D metal that has never been created in the lab or in a computer simulation, just based on the well-known characteristics of its 3-D analog.
Nevalaita and Koskinen also compared the stability of 2-D metals whose atoms were arranged in the three different configurations. The researchers found that many 2-D metals were stable in triangular and honeycomb patterns, but not in squares. Future computer simulations could examine the electric and magnetic properties of these materials, Koskinen says. Knowing the stability and property profiles of 2-D metals could inform which materials scientists fabricate in the lab.
“This is the tip of the iceberg in the area of 2-D metals,” says Mauricio Terrones, a chemical physicist at Penn State not involved in the work.
Closing out a series in the NBA Playoffs is never easy, but the task may have gotten more difficult for the Lakers.
In the fourth quarter of a potential closeout game against the Warriors, Anthony Davis was forced to exit the game after taking an inadvertent shot to the head from Warriors center Kevon Looney.
What does this mean for Davis and the Lakers? Here is what we know about his injury and status moving forward. Anthony Davis injury update While battling for position on the inside, Davis took an inadvertent elbow from Looney to the side of his head. Davis was taken out of the game, brought back for evaluation, and eventually ruled out for Game 5 with what the Lakers ruled as a head injury.
Here's the play where Davis was injured as well as the aftermath that includes him receiving attention on the bench before being taken to the locker room. Turner Sports' Chris Haynes reported that Davis was being evaluated at Chase Center, with the possibility of a concussion looming as a potential diagnosis. Haynes added that Davis was brought to the locker room in a wheelchair following his evaluation.
In a later report, Haynes added that Davis "appears to have avoided a concussion and is doing better now," a positive sign for Davis and the Lakers.
Lakers head coach Darvin Ham told reporters that he spoke with Davis, who he said "seems to be doing really good already." Davis exited Game 5 with 21 points (on 10-of-18 shooting), nine rebounds and three assists in 32 minutes.
Will Anthony Davis play in Game 6? Davis' availability largely depends on the diagnosis of his injury and whether or not he is dealing with a concussion. If Davis is not diagnosed with a concussion, there is a clear path for him to return to the floor for Game 6.
Here is an excerpt from the NBA's Concussion Policy: There is just a one-day layoff between Games 5 and 6 of the Western Conference Semifinals series between the Lakers and Warriors.
Lakers vs. Warriors schedule Here is the schedule for the second-round series between Los Angeles and Golden State, with the final two games airing on the ESPN family of networks.
Fans in the U.S. can watch the NBA Playoffs on Sling TV, which is now offering HALF OFF your first month! Stream Sling Orange for $20 in your first month to catch all the games on TNT, ESPN & ABC. For games on NBA TV, subscribe to Sling Orange & Sports Extra for $27.50 in your first month. Local regional blackout restrictions apply. SIGN UP FOR SLING: English | Spanish
Date Game Time (ET) TV channel May 2 Lakers 117, Warriors 112 10 p.m. TNT May 4 Warriors 127, Lakers 100 9 p.m. ESPN May 6 Lakers 127, Warriors 97 8:30 p.m. ABC May 8 Lakers 104, Warriors 101 10 p.m. TNT May 10 Warriors 121, Lakers 106 10 p.m. TNT May 12 Game 6 TBD ESPN May 14 Game 7* TBD ABC
On July 31, 2012, Maggie de Grauw looked out the window of her flight back to New Zealand after a holiday in Samoa and glimpsed a mysterious mass floating below. That mass turned out to be a raft of lightweight pumice rock, the product of an erupting underwater volcano called Havre. The 2012 eruption turned out to be the largest of its kind in the last 100 years. And now, the pumice raft has become a crucial clue in revealing the eruption’s surprisingly complex nature. Although underwater eruptions happen all the time, scientists have only recorded such events since the 1990s, and pumice rafts can often float under the radar. Typically, researchers use depth sensors aboard ships to examine the crime scene of an underwater eruption.
But “what we found on the seafloor was almost entirely different from what we expected,” says Rebecca Carey, a volcanologist at the University of Tasmania in Australia. Havre challenges the reliability of the geologic record when it comes to big deep-sea eruptions.
In 2015, Carey and her colleagues set out to get a more detailed view of Havre’s big outburst than what ship-based sensors could reveal. The researchers deployed a robot to measure the depth of the 4-kilometer-wide caldera. Another robot, operated remotely from a ship, allowed the team to get a closer look at specific features in and around the caldera, and to take rock and water samples. A bit of satellite-image detective work revealed the size and path of the pumice raft, which formed no more than 21 1/2 hours after the eruption ended.
The robotic diving duo provided a high-resolution topographic map of the underwater posteruption landscape. The map shows a massive rupture, lava from 14 different vents ranging from 900 to 1,220 meters below the surface, chunks of pumice, landslide deposits and a blanket of ash. This diversity of volcanic material was unexpected, the researchers write January 10 in Science Advances. Although the Havre event was larger than the 1980 eruption of Mount St. Helens, a similar type of volcano that shot a huge column of debris into the air, the seafloor data weren’t indicative of such a large eruption. “When you shoot a lot of material up into water, there’s resistance,” Carey says. “So you expect to see a lot of it deposited on the seafloor.” But using an old seafloor map of Havre and satellite data, Carey and her colleagues calculated that more than 75 percent of the material produced by Havre ended up in the 400-square-kilometer pumice raft. That raft eventually broke apart and washed up on Australian and other South Pacific beaches. Volcanic gases might have pushed debris to the surface, Carey speculates, but it’s impossible to pinpoint a cause.
Many submarine eruptions go unnoticed, and few have been mapped in this manner. Frequently, researchers rely only on clues on the seafloor surface to determine an eruption’s size. And, if Carey’s team had just done that, the researchers would have never known the true size and nature of the eruption.
“That is a real eye-opener from this study,” says Bill Chadwick, a volcanologist at the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Newport, Ore. “What they found tells us a lot about how submarine eruptions behave differently than those on land.”
And if the Havre data are any guide, previous estimates of underwater eruption size may be off. “Now we know that the geological rock record is unfaithful to these very large magnitude powerful events,” Carey says.
Ready for sketch comedy she’s not. But a 14-year-old killer whale named Wikie has shown promise in mimicking strange sounds, such as a human “hello” — plus some rude noises.
Scientists recorded Wikie at her home in Marineland Aquarium in Antibes, France, imitating another killer whale’s loud “raspberry” sounds, as well as a trumpeting elephant and humans saying such words as “one, two, three.”
The orca’s efforts were overall “recognizable” as attempted copies, comparative psychologist José Zamorano Abramson of Complutense University of Madrid and colleagues report January 31 in Proceedings of the Royal Society B. Just how close Wikie’s imitations come to the originals depends on whether you’re emphasizing the rhythm or other aspects of sound, Abramson says.
Six people judged Wikie’s mimicry ability, and a computer program also rated her skills. She did better at some sounds, like blowing raspberries and saying “hello-hello,” than others, including saying “bye-bye.” Imitating human speech is especially challenging for killer whales. Instead of vocalizing by passing air through their throats, they sound off by forcing air through passageways in the upper parts of their heads. It’s “like speaking with the nose,” Abramson says.
The research supports the idea that imitation plays a role in how killer whales develop their elaborate dialects of bleating pulses. Cetaceans are rare among mammals in that, like humans, they learn how to make the sounds their species uses to communicate.
When it comes to the dimensions of spacetime, what you see may be what you get.
Using observations from the collision of two neutron stars that made headlines in 2017 (SN: 11/11/17, p. 6), scientists found no evidence of gravity leaking into hidden dimensions. The number of observed large spatial dimensions — kilometer-scale or bigger — is still limited to the three we know and love, the researchers report January 24 at arXiv.org.
Just as insects floating on a pond may be unaware of what’s above or below the water’s surface, our 3-D world might be part of a higher-dimensional universe that we can’t directly observe. However, says astrophysicist David Spergel of Princeton University, a coauthor of the new study, “gravity might be able to explore those other dimensions.” Such extra dimensions might explain some conundrums in physics, such as the existence of dark matter (an as-yet-unidentified source of mass in the universe) and dark energy (which causes the universe’s expansion rate to accelerate), says coauthor Daniel Holz, an astrophysicist at the University of Chicago. “That’s why people get excited about these modifications.”
To look for any hint of leaking gravity, scientists turned to the light and gravitational waves emitted in the neutron star smashup detected on August 17, 2017. The light allowed scientists to find the galaxy where the neutron stars merged. Spergel, Holz and colleagues showed that, given the galaxy’s distance from Earth, the strength of the gravitational waves was as expected. Extra dimensions weren’t stealing, and thus weakening, the observed ripples.
A variety of theories predict extra dimensions of spacetime into which gravity could leak, but the new result applies only to large extra dimensions, Spergel says. That’s because the gravitational waves detected from the neutron star collision have wavelengths of thousands of kilometers. Tiny extra dimensions, smaller than a fraction of a millimeter across, have also been proposed, but they wouldn’t affect such extended ripples. One theory, proposed in 2000 by a group of theoretical physicists including Georgi Dvali, predicts a type of large extra dimension. The effects of gravity leaking into such dimensions would be visible only over long distances — explaining why gravity on smaller scales, such as the size of the solar system, behaves as if there are three spatial dimensions.
Because the gravitational waves don’t seem to weaken on their trek to Earth, they must travel more than about 65 million light-years before leaking into any potential additional dimension, the researchers concluded in the new study.
But other theories of extra dimensions are unaffected by the result. String theory, which posits that particles are made up of infinitesimal vibrating strings, predicts tiny extra dimensions that are curled up on themselves. “We’re not in any way ruling out string theory,” Spergel says. Another variety of extra spacetime dimension, of potentially infinite size, was proposed by physicists Lisa Randall and Raman Sundrum in 1999 (SN: 9/26/09, p. 22). But such theories also would not be ruled out, because gravity can’t penetrate very far into that type of extra dimension.
Neutron star mergers are “a completely new laboratory of testing gravity,” says Dvali, of Ludwig-Maximilians-Universität in Munich, who was not involved with the research. “This is absolutely fascinating and fantastic.” But, Dvali notes, the type of extra dimension he proposed back in 2000 already seems unlikely on these scales. “I would say there is already an extremely strong constraint on leakage coming from cosmology.” No matter how far we peer out into space, the universe seems to follow the normal laws of gravity in three dimensions.
For now, the dimensions of space remain as simple as 1, 2, 3.
Human activities are driving phosphorus levels in the world’s lakes, rivers and other freshwater bodies to a critical point. The freshwater bodies on 38 percent of Earth’s land area (not including Antarctica) are overly enriched with phosphorus, leading to potentially toxic algal blooms and less available drinking water, researchers report January 24 in Water Resources Research.
Sewage, agriculture and other human sources add about 1.5 teragrams of phosphorus to freshwaters each year, the study estimates. That’s roughly equivalent to about four times the weight of the Empire State Building. The scientists tracked human phosphorus inputs from 2002 to 2010 from domestic, industrial and agricultural sources. Phosphorus in human waste was responsible for about 54 percent of the global load, while agricultural fertilizer use contributed about 38 percent. By country, China contributed 30 percent of the global total, India 8 percent and the United States 7 percent.
High-energy particle beams can reveal how 2-D thin sheets behave when the heat is cranked up.
Researchers have devised a way to track how these materials, such as the supermaterial graphene, expand or contract as temperatures rise (SN: 10/3/15, p. 7). This technique, described in the Feb. 2 Physical Review Letters, showed that 2-D semiconductors arranged in single-atom-thick sheets expand more like plastics than metals when heated. Better understanding the high-temp behaviors of these and other 2-D materials could help engineers design sturdy nano-sized electronics. Commonly used silicon-based electronics are “hitting a brick wall,” regarding how much smaller they can get, says Zlatan Aksamija, an electrical engineer at the University of Massachusetts Amherst not involved in the work. Materials made of ultrathin, 2-D films could be ideal for building the next generation of tinier devices.
But electronics warm up as electric current courses through them. If 2-D materials in a nanodevice expand or shrink at different rates from each other when heated, that could change the device’s electronic properties — such as how well it conducts electricity, says Antoine Reserbat-Plantey, a physicist at the Institute of Photonic Sciences in Barcelona not involved in the research. It’s crucial to know how the thin films react to higher temps.
The new method uses a scanning transmission electron microscope to bombard a film with a beam of high-energy particles. That particle beam stirs up electrons in the 2-D sheet, making the electrons swish back and forth through the material. The collective oscillation, called a plasmon, occurs at a frequency that depends on the material’s density, explains Matthew Mecklenburg, a physicist at the University of Southern California in Los Angeles who was not involved in the work.
The plasmon frequency affects how much energy the particles of the microscope beam lose as they streak through the 2-D material: the higher the frequency, the denser the material, and the more energy that is sapped from the beam. By using another instrument to measure the energies of beam particles after they’ve passed through the 2-D material, researchers can discern the material’s density — and track how that density changes as they turn up the heat. Robert Klie, a physicist at the University of Illinois at Chicago, and colleagues used this technique on samples of graphene, which is made of carbon atoms, and four 2-D semiconductors made of transition metal and chalcogen atoms. (Chalcogen elements are found in group 16 on the periodic table and include sulfur and selenium). These materials were arranged in sheets from a single atom to a few atoms thick. The team measured the density of each sample at eight temperatures between about 100° and 450° Celsius. That allowed the scientists to calculate how much each material expanded or contracted per degree of temperature increase.
These measurements revealed that the thinnest structures undergo more significant size changes than thicker sheets: A single layer of graphene, which contracts when heated, shrinks more than materials composed of a few graphene layers. The 2-D semiconductors expand at higher temps, but those made of one-atom-thick sheets swell more than semiconductors a few atoms thick. In fact, the heat response of the single-atom-thick semiconductors is “more like [that] of a plastic than a metal,” Mecklenburg says.
This finding may indicate that, like plastics, some 2-D semiconductors have low melting temperatures, which could affect how or whether they’re used in future electronics.
