These materials could be stacked together like Lego bricks to form a range of devices with different functions, including operating as semiconductors. In this way, they could be used to create ultra-thin, flexible, transparent and wearable electronic devices.

However, separating a bulk crystal material into 2-D flakes for use in electronics has proven difficult to do on a commercial scale.

The existing process, in which individual flakes are split off from the bulk crystals by repeatedly stamping the crystals onto an adhesive tape, is unreliable and time-consuming, requiring many hours to harvest enough material and form a device.

Now researchers in the Department of Mechanical Engineering at MIT have developed a technique to harvest 2-inch diameter wafers of 2-D material within just a few minutes. They can then be stacked together to form an electronic device within an hour.

The technique, which they describe in a paper published in the journal Science, could open up the possibility of commercializing electronic devices based on a variety of 2-D materials, according to Jeehwan Kim, an associate professor in the Department of Mechanical Engineering, who led the research.

The paper’s co-first authors were Sanghoon Bae, who was involved in flexible device fabrication, and Jaewoo Shim, who worked on the stacking of the 2-D material monolayers. Both are postdocs in Kim’s group.

The paper’s co-authors also included students and postdocs from within Kim’s group, as well as collaborators at Georgia Tech, the University of Texas, Yonsei University in South Korea, and the University of Virginia. Sang-Hoon Bae, Jaewoo Shim, Wei Kong, and Doyoon Lee in Kim’s research group equally contributed to this work.

“We have shown that we can do monolayer-by-monolayer isolation of 2-D materials at the wafer scale,” Kim says. “Secondly, we have demonstrated a way to easily stack up these wafer-scale monolayers of 2-D material.”

The researchers first grew a thick stack of 2-D material on top of a sapphire wafer. They then applied a 600-nanometer-thick nickel film to the top of the stack.

Since 2-D materials adhere much more strongly to nickel than to sapphire, lifting off this film allowed the researchers to separate the entire stack from the wafer.

What’s more, the adhesion between the nickel and the individual layers of 2-D material is also greater than that between each of the layers themselves.

As a result, when a second nickel film was then added to the bottom of the stack, the researchers were able to peel off individual, single-atom thick monolayers of 2-D material.

That is because peeling off the first nickel film generates cracks in the material that propagate right through to the bottom of the stack, Kim says.

Once the first monolayer collected by the nickel film has been transferred to a substrate, the process can be repeated for each layer.

“We use very simple mechanics, and by using this controlled crack propagation concept we are able to isolate monolayer 2-D material at the wafer scale,” he says.

The universal technique can be used with a range of different 2-D materials, including hexagonal boron nitride, tungsten disulfide, and molybdenum disulfide.

In this way it can be used to produce different types of monolayer 2-D materials, such as semiconductors, metals, and insulators, which can then be stacked together to form the 2-D heterostructures needed for an electronic device.

“If you fabricate electronic and photonic devices using 2-D materials, the devices will be just a few monolayers thick,” Kim says. “They will be extremely flexible, and can be stamped on to anything,” he says.

The process is fast and low-cost, making it suitable for commercial operations, he adds.

The researchers have also demonstrated the technique by successfully fabricating arrays of field-effect transistors at the wafer scale, with a thickness of just a few atoms.

“The work has a lot of potential to bring 2-D materials and their heterostructures towards real-world applications,” says Philip Kim, a professor of physics at Harvard University, who was not involved in the research.

The researchers are now planning to apply the technique to develop a range of electronic devices, including a nonvolatile memory array and flexible devices that can be worn on the skin.

They are also interested in applying the technique to develop devices for use in the “internet of things,” Kim says.

“All you need to do is grow these thick 2-D materials, then isolate them in monolayers and stack them up. So it is extremely cheap — much cheaper than the existing semiconductor process. This means it will bring laboratory-level 2-D materials into manufacturing for commercialization,” Kim says.

“That makes it perfect for IoT networks, because if you were to use conventional semiconductors for the sensing systems it would be expensive.”

The fossil, likely to be that of a newborn, does not have the recognizable long snout typically seen in the species. The lack of this snout initially perplexed researchers, who struggled to identify which group of mosasaurs it belonged to.

After examining and comparing the fossil to young specimens of closely-related species, such as T. nepaeolicus and T. proriger which already had identifiable noses, researchers finally deemed it to be a young Tylosaurus.

Lead author Professor Takuya Konishi, of the Department of Biological Sciences at the University of Cincinnati said, “Having looked at the specimen in 2004 for the first time myself, it too took me nearly 10 years to think out of that box and realize what it really was — a baby Tylosaurus yet to develop such a snout.

For those 10 years or so, I had believed too that this was a neonate of Platecarpus, a medium-sized (5-6m) and short-snouted mosasaur, not Tylosaurus, a giant (up to 13m) mosasaur with a significantly protruding snout.”

The lack of snout in the baby specimen found suggests to researchers that the development of this feature happens extremely quickly, between birth and juvenile stage — something that previous studies on the species had failed to notice.

Konishi further commented, “Yet again, we were challenged to fill our knowledge gap by testing our preconceived notion, which in this case was that Tylosaurus must have a pointy snout, a so-called ‘common knowledge.’

As individual development and evolutionary history are generally linked, the new revelation hints at the possibility that Tylosaurus adults from much older rock units may have been similarly short-snouted, something we can test with future discoveries.”

The fragments found include a partial snout with teeth and tooth bases, partial braincase, and a section of upper jaw with tooth bases. From this, they can estimate the entire baby skull to have been around 30cm (1ft) in total.

Tylosaurus belong to one of the largest-known groups of mosasaurs, up to 13m long, the front 1.8 m of that body being its head. The baby, therefore, was about 1/6 the size of such an adult.

Michael J. Everhart, a Kansas native and a special curator of paleontology at the Sternberg Museum of Natural History, Hays, Kansas, found the tiny specimens in 1991 in the lower Santonian portion of the Niobrara Chalk, in Kansas, which are now housed at the museum. The paper was co-authored by Paulina Jiménez-Huidobro and Michael W. Caldwell of the University of Alberta, Canada.

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Many studies have shown that species evolve in response to human activity at a pace that exceeds natural rates: fish species often become smaller as humans selectively harvest the biggest fish; weeds and insect pests are becoming resistant to pesticides, and pathogens are becoming more resistant to antibiotics.

Rapid evolution in response to human activities could be caused by a strengthening of natural selection, a key influence on the pace of evolution. To better understand the extent to which this occurs, Vincent Fugère and Andrew Hendry from McGill’s Department of Biology reviewed thousands of scientific papers on the topic. Of these, they retained 40 that provide estimates of “selection strength” — the extent to which a particular trait is linked to survival or reproductive success (‘Darwinian fitness’) — in both human-disturbed and natural environments. These cases included, for instance, a weed species found in grasslands sprayed or not sprayed with herbicides; another involved a shark population before and after the construction of a beach resort that induced altered the local mangrove habitat.

From the 40 selected studies, the authors compiled 1,366 estimates covering 102 traits in 37 different species. They then used statistical models to test if there was an overall trend of stronger selection in human-disturbed conditions. While some disturbances caused very strong human-induced selection, others weakened selection, leading to no net effect on average when pooling all available studies.

“We are not arguing that human disturbances do not cause evolution; in fact, I certainly believe the opposite,” says Fugère, lead author of the new paper published in Proceedings of the National Academy of Sciences.

Rather, the finding, which is likely to surprise some evolutionary biologists, highlights the challenges of quantifying evolutionary impacts of humans on the biosphere.

For example, most species in the authors’ database performed well (had higher ‘fitness’) in the disturbed environment, which can weaken estimates of selection strength for statistical reasons. “Biologists know that many species who do poorly in human-impacted environments often go extinct in these environments, a phenomenon known as local extinction, says Fugère. Rates of local extinction have never been higher, yet no species included in our database became locally extinct. So part of the explanation for our main conclusion is that researchers might be studying species that fare relatively better in such environments than the average species on Earth because one can only study selection in species that survive in impacted environments.”

Fugère and Hendry hope that their unexpected result will guide future studies of natural selection and help us better understand evolutionary impacts of environmental changes caused by humans.

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Trying to transfer to other types of plants.

More specifically, in their new studies, the researchers found that the chitinase CHIT5 present in legume roots is required for Nod factor hydrolysis and functional symbiosis. Their work reveals a new and crucial role of the legume host in modulating levels of specific rhizobial Nod factor morphogens during cortical infection to ensure functional symbiosis.

Morphogens are key signals for organized development in multicellular organisms. The crucial role of self-produced morphogens like retinoic acid or transformation growth factor-beta during embryo development has been long demonstrated. Molecules of microbial nature have also been shown to induce a morphogenetic response in symbiotic eukaryotic hosts, and recently gained an increased attention following the increased focus on microbiome studies. The nitrogen-fixing root nodules are lateral organs induced by a microbial morphogen. In their new studies, the researchers reveal for the first time that a developmental switch in the legume plant is ensured by the host-controlled modulation of microbial morphogen, Nod factor.

Deepening our understanding of Nod factor signalling

Nod factors have been known for decades as signalling molecules produced by rhizobia to trigger and enable dinitrogen-fixing symbiosis. In addition to their signalling capacity, Nod factors have been considered morphogens based on their effect on host developmental programmes; nodule organogenesis and infection thread formation. The researchers have found that the model legume Lotus japonicus modulates the levels of the Nod factor morphogen via CHIT5 to control colonisation of nodule primordia.

chit5 mutants display an unbalanced symbiotic signalling reminiscent of what is seen in developmental arrest at an early primordia stage. We believe that our work provides the basis for a novel layer of symbiosis research, enabling a deeper understanding of Nod factor signalling during cortical infection with direct consequences for the switch to nitrogen-fixing status. This knowledge becomes especially essential in the context of the research group’s current ambitious engineering projects aiming at transferring the nitrogen fixation ability to non-legumes, which could potentially have great agricultural importance.

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Imagine now controlling quantum matter in this same way. Rather than forming a normal liquid, like a melted slushy under the sun for too long, quantum matter can form a superfluid. This mysterious and counterintuitive form of matter was first observed in liquid helium at very low temperatures, less than 2 Kelvin above absolute zero. The helium atoms have a strong tendency to form a crystal, like the water molecules in a slushy, and this restricts the superfluid state of helium to very low temperatures and low pressures.

But what if you could turn on the blades in your slush machine for quantum matter? What if you could disrupt the crystalline order so that the superfluid could flow freely even at temperatures and pressures where it usually does not? This is indeed the idea that was demonstrated by a team of scientists led by Ludwig Mathey and Andreas Hemmerich from the University of Hamburg. They have disrupted crystalline order in a quantum system in a controlled manner by shining light on it that oscillates in time at a specific frequency. Physicists use the term “driving” to describe this kind of periodic change applied to the system — an action performed by the churning blades in a slushy machine. Their work, published in Physical Review Letters, identified a fundamental mechanism for how a typical system with competing phases respond to an external periodic driving.

The researchers studied a gas of cold atoms placed between two highly reflecting mirrors. The mirrors form a cavity, which serves as a resonator for photons as the atoms scatter them multiple times before being detected in experiments. To provide a source of photons, an external pump laser beam is directed at the cloud of atoms.

Similar to how water can change its phase from liquid to ice, this light-matter system also exhibits a phase transition, a quantum one. Atoms from an initially homogeneous gas spontaneously organize themselves in a checkerboard pattern when the intensity of the pump beam gets sufficiently strong. The self-organization comes at the expense of the superfluid which is suppressed by the crystalline order. This is one of the many examples in nature of competition where one phase wins over the other. The researchers show that with a little bit of “drive,” you can tip the balance in favor of the underdog, in this example, the superfluid phase. “We observe from our computer simulations that a periodic modulation of the pump intensity can destabilize the dominant self-organized phase,” explains lead author Jayson Cosme. “This allows the previously unstable homogeneous phase to reemerge and this restores the superfluid. It’s light induced superfluidity.”

The same team of scientists then indeed observed their prediction in an experiment conducted in the group of Andreas Hemmerich. ´Intuitively one might expect that if we shake the system, all it does is heat up. It was intriguing to see a clear signature of the quantum fluid reemerging’, explains Andreas Hemmerich.

The enhancement or suppression of a phase due to an external driving force has also been suggested in other physical systems. For instance, in high-temperature superconductors, laser pulses can melt the equilibrium dominant striped order paving the way for superconductivity to emerge — a phenomenon called light-induced superconductivity. The fundamental mechanism that can help explain this process is still a subject of debate. ´We proposed this type of light control of superfluidity to demonstrate the principle that has been hypothesized for light induced superconductivity´, explains Ludwig Mathey. With this finding, cold atom physics demonstrates a general, counterintuitive mechanism of controlling phase transitions in many-body systems. It opens a new chapter of solid state physics in which scientists not only measure equilibrium properties of matter, but rather design a non-equilibrium state with desired properties via light control.

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