These 2D materials — such as graphene and borophene — are a class of single-layer, crystalline materials with widespread potential as semiconductors in advanced ultra-thin, flexible electronics. Yet due to their thin nature, the materials are highly sensitive to external environments, and have struggled to demonstrate long-term stability and reliability when utilized in electronic devices.

“Atomically thin 2D materials offer the potential to dramatically scale down electronic devices, making them an attractive option to power future wearable and flexible electronics,” said Vinayak Dravid, Abraham Harris Professor of Materials Science and Engineering at the McCormick School of Engineering.

The study, titled “Direct Visualization of Electric Field induced Structural Dynamics in Monolayer Transition Metal Dichalcogenides,” was published on February 11 in the journal ACS Nano. Dravid is the corresponding author on the paper. Chris Wolverton, the Jerome B. Cohen Professor of Materials Science and Engineering, also contributed to the research.

“Unfortunately, electronic devices now operate as a kind of ‘black box.’ Although device metrics can be measured, the motion of single atoms within the materials responsible for these properties is unknown, which greatly limits efforts to improve performance,” added Dravid, who serves as director of the Northwestern University Atomic and Nanoscale Characterization (NUANCE) Center. The research allows a way to move past that limitation with a new understanding of the structural dynamics at play within 2D materials receiving electrical voltage.

Building upon a previous study in which the researchers used a nanoscale imaging technique to observe failure in 2D materials caused by heat, the team used a high-resolution, atomic-scale imaging method called electron microscopy to observe the movement of atoms in molybdenum disulfide (MoS2), a well-studied material originally used as a dry lubricant in greases and friction materials that has recently gained interest for its electronic and optical properties. When the researchers applied an electric current to the material, they observed its highly mobile sulfur atoms move continuously to vacant areas in the crystalline material, a phenomenon they dubbed, “atomic dance.”

That movement, in turn, caused the MoS2’s grain boundaries — a natural defect created in the space where two crystallites within the material meet — to separate, forming narrow channels for the current to travel through.

“As these grain boundaries separate, you are left with only a couple narrow channels, causing the density of the electrical current through these channels to increase,” said Akshay Murthy, a PhD student in Dravid’s group and the lead author on the study. “This leads to higher power densities and higher temperatures in those regions, which ultimately leads to failure in the material.”

“It’s powerful to be able to see exactly what’s happening on this scale,” Murthy continued. “Using traditional techniques, we could apply an electric field to a sample and see changes in the material, but we couldn’t see what was causing those changes. If you don’t know the cause, it’s difficult to eliminate failure mechanisms or prevent the behavior going forward.”

With this new way to study 2D materials at the atomic level, the team believes researchers could use this imaging approach to synthesize materials that are less susceptible to failure in electronic devices. In memory devices, for example, researchers could observe how regions where information is stored evolve as electric current is applied and adapt how those materials are designed for better performance.

The technique could also help improve a host of other technologies, from transistors in bioelectronics to light emitting diodes (LEDs) in consumer electronics to photovoltaic cells that comprise solar panels.

“We believe the methodology we have developed to monitor how 2D materials behave under these conditions will help researchers overcome ongoing challenges related to device stability,” Murthy said. “This advance brings us one step closer to moving these technologies from the lab to the marketplace.”

“Not only did we lose a lot of the West Antarctic Ice Sheet, but this happened very early during the Last Interglacial,” says Chris Turney, Professor in Earth and Climate Science at UNSW Sydney and lead author of the study.

Fine layers of ancient volcanic ash in the ice helped the team pinpoint when the mass melting took place. Alarmingly, the results indicated that most ice loss occurred within the first millennia, showing how sensitive the Antarctic is to higher temperatures.

“The melting was likely caused by less than 2°C ocean warming — and that’s something that has major implications for the future, given the ocean temperature increase and West Antarctic melting that’s happening today,” Professor Turney says.

During the Last Interglacial, polar ocean temperatures were likely less than 2°C warmer than today, making it a useful period to study how future global warming might affect ice dynamics and sea levels.

“This study shows that we would lose most of the West Antarctic Ice Sheet in a warmer world,” says Professor Turney.

In contrast to the East Antarctic Ice Sheet — which mostly sits on high ground — the West Antarctic sheet rests on the seabed. It’s fringed by large areas of floating ice, called ice shelves, that protect the central part of the sheet.

As warmer ocean water travels into cavities beneath the ice shelves, ice melts from below, thinning the shelves and making the central ice sheet highly vulnerable to warming ocean temperatures.

Going back in time

To undertake their research, Professor Turney and his team travelled to the Patriot Hills Blue Ice Area, a site located at the periphery of the West Antarctic Ice Sheet, with support from Antarctic Logistics and Expeditions (or ALE).

Blue ice areas are the perfect laboratory for scientists due to their unique topography — they are created by fierce, high-density katabatic winds. When these winds blow over mountains, they remove the top layer of snow and erode the exposed ice. As the ice is removed, ancient ice flows up to the surface, offering an insight into the ice sheet’s history.

While most Antarctic researchers drill down into the ice core to extract their samples, this team used a different method — horizontal ice core analysis.

“Instead of drilling kilometres into the ice, we can simply walk across a blue ice area and travel back through millennia. By taking samples of ice from the surface we are able to reconstruct what happened to this precious environment in the past,” Professor Turney says.

Through isotope measurements, the team discovered a gap in the ice sheet record immediately prior to the Last Interglacial. This period of missing ice coincides with the extreme sea level increase, suggesting rapid ice loss from the West Antarctic Ice Sheet. The volcanic ash, trace gas samples and ancient DNA from bacteria trapped in the ice all support this finding.

Learning from the Last Interglacial

Ice age cycles occur approximately every 100,000 years due to subtle changes in Earth’s orbit around the Sun. These ice ages are separated by warm interglacial periods. The Last Interglacial is the most recent warm period to our current interglacial period, the Holocene.

While human contribution to global warming makes the Holocene unique, the Last Interglacial remains a useful research point to understand how the planet responds to extreme change.

“The future is heading far beyond the range of anything we’ve seen observed in the scientific instrumental record of the last 150 years,” says Professor Turney. “We have to look further into the past if we’re going to manage future changes.”

During the Last Interglacial, global mean sea levels were between 6m and 9m higher than present day, although some scientists suspect this could have reached 11m.

The sea level rise in the Last Interglacial can’t be fully explained by the Greenland Ice Sheet melt, which accounted for a 2m increase, or ocean expansion from warmer temperatures and melting mountain glaciers, which are thought to have caused less than a 1m increase.

“We now have some of the first major evidence that West Antarctica melted and drove a large part of this sea level rise,” says Professor Turney.

An urgent need to minimise future warming

The severity of the ice loss suggests that the West Antarctic Ice Sheet is highly sensitive to future ocean warming.

“The West Antarctic Ice Sheet is sitting in water, and today this water is getting warmer and warmer,” says Professor Turney, who is also a Chief Investigator of the ARC Centre of Excellence for Australian Biodiversity and Heritage (CABAH).

Using data gained from their fieldwork, the team ran model simulations to investigate how warming might affect the floating ice shelves. These shelves currently buttress the ice sheets and help slow the flow of ice off the continent.

The results suggest a 3.8m sea level rise during the first thousand years of a 2°C warmer ocean. Most of the modelled sea level rise occurred after the loss of the ice shelves, which collapsed within the first two hundred years of higher temperatures.

The researchers are concerned that persistent high sea surface temperatures would prompt the East Antarctic Ice Sheet to melt, driving global sea levels even higher.

“The positive feedbacks between a warming ocean, ice shelf collapse, and ice sheet melt suggests that the West Antarctic may be vulnerable to passing a tipping point,” stressed Dr Zoë Thomas, co-author and ARC Discovery Early Career Research Award (DECRA) Fellow at UNSW.

“As it reaches the tipping point, only a small increase in temperature could trigger abrupt ice sheet melt and a multi-metre rise in global sea level.”

At present, the consensus of the Intergovernmental Panel on Climate Change (IPCC) 2013 report suggests that global sea level will rise between 40cm and 80cm over the next century, with Antarctica only contributing around 5cm of this.

The researchers are concerned that Antarctica’s contribution could be much greater than this.

“Recent projections suggest that the Antarctic contribution may be up to ten times higher than the IPCC forecast, which is deeply worrying,” says Professor Christopher Fogwill, co-author and Director of The Institute for Sustainable Futures at the UK University of Keele.

“Our study highlights that the Antarctic Ice Sheet may lie close to a tipping point, which once passed may commit us to rapid sea level rise for millennia to come. This underlines the urgent need to reduce and control greenhouse gas emissions that are driving warming today.”

Notably, the researchers warn that this tipping point may be closer than we think.

“The Paris Climate Agreement commits to restricting global warming to 2°C, ideally 1.5°C, this century,” says Professor Turney.

“Our findings show that we don’t want to get close to 2°C warming.”

Professor Turney and his team hope to expand the research to confirm just how quickly the West Antarctic Ice Sheet responded to warming and which areas were first affected.

“We only tested one location, so we don’t know whether it was the first sector of Antarctica that melted, or whether it melted relatively late. How these changes in Antarctica impacted the rest of the world remains a huge unknown as the planet warms into the future” he says.

“Testing other locations will give us a better idea for the areas we really need to monitor as the planet continues to warm.”

This work was support by Laureate and Future Fellowship and Linkage funding by the Australian Research Council.

The study, publishing today in the Proceedings of the National Academy of Sciences, indicates that the byproducts of burning coal in Europe in the late 18th century made their way to the Dasuopu glacier in the central Himalayas, some 6,400 miles as the crow flies from London, the birthplace of the Industrial Revolution.

“The Industrial Revolution was a revolution in the use of energy,” said Paolo Gabrielli, lead author of the study and a principal investigator and research scientist at The Ohio State University Byrd Polar and Climate Research Center and the School of Earth Sciences.

“And so the use of coal combustion also started to cause emissions that we think were transported by winds up to the Himalayas.”

The research team that published this study was part of a larger international team that traveled to Dasuopu in 1997 to drill ice cores from the glacier. The cores provide a record of snowfall, atmospheric circulation and other environmental changes over time; the Byrd Center has one of the largest collections of ice cores in the world.

Dasuopu — at 7,200 meters or 23,600 feet above sea level — is the highest-altitude site in the world where scientists have obtained a climate record from an ice core. Dasuopu is located on Shishapangma, one of the world’s 14 tallest mountains, which are all located in the Himalayas.

For this study, the research team analyzed one core taken from Dasuopu in 1997 for 23 trace metals.

The ice cores operate as a sort of timeline, and show new ice forming in layers on the glacier over time. It is possible for researchers to tell almost the precise year a layer of the glacier formed because of environmental clues like snowfall or other known natural or human-made disasters. The ice the researchers evaluated formed between 1499 and 1992, the team determined. Their goal was to see whether human activity had affected the ice in any way, and, if so, when the effects had begun.

Their analysis showed it had: The team found higher-than-natural levels of a number of toxic metals, including cadmium, chromium, nickel and zinc, in the ice starting at around 1780 — the very start of the Industrial Revolution in the United Kingdom. Those metals are all byproducts of burning coal, a key part of industry at the end of the 18th century and throughout the 19th and 20th centuries.

The researchers found that those metals were likely transported by winter winds, which travel around the globe from west to east.

They also believe it is possible that some of the metals, most notably zinc, came from large-scale forest fires, including those used in the 1800s and 1900s to clear trees to make way for farms.

“What happens is at that time, in addition to the Industrial Revolution, the human population exploded and expanded,” Gabrielli said. “And so there was a greater need for agricultural fields — and, typically, the way they got new fields was to burn forests.”

Burning trees adds metals, primarily zinc, to the atmosphere. Gabrielli said it is difficult to tell whether the glacial contamination comes from human-made or natural forest fires. And there are few high-altitude records of large-scale fires from around the world, making that contamination more difficult to track.

The contamination in the ice core records was most intense from about 1810 to 1880, the scientists’ analysis found. Gabrielli said that is likely because winters were wetter than normal in Dasuopu during that time period, meaning more ice and snow formed. That ice and snow, he said, would have been contaminated by fly ash from the burning of coal or trees that made its way into the westerly winds — and greater quantities of contaminated ice and snow means more contamination on the glacier.

Noteworthy to Gabrielli was that the contamination appeared long before humans scaled the mountains around Dasuopu. The first mountain climbers reached the summit of Mount Everest, at 29,029 feet the world’s highest peak above sea level, in 1953. Shishapangma, at 26,335 feet the 14th-highest peak in the world, was first climbed in 1964. The Dasuopo glacier drilling site is about 2,700 feet below the summit.

Gabrielli said it is also important to note the difference between “contamination” and “pollution.”

“The levels of metals we found were higher than what would exist naturally, but were not high enough to be acutely toxic or poisonous,” he said. “However, in the future, bioaccumulation may concentrate metals from meltwater at dangerous toxic levels in the tissues of organisms that live in ecosystems below the glacier.”

This study shows that human activity altered the atmosphere in the Himalayas starting at the end of the 18th century. But a previous study Byrd Polar Center researchers published in 2015 showed that in other parts of the world, namely Peru, human mining for silver contaminated the air in South America as many as 240 years before the Industrial Revolution.

“What is emerging from our studies, both in Peru and in the Himalayas, is that the impact of humans started at different times in different parts of the planet,” Gabrielli said.