But for practical applications, such two-dimensional materials must at some point connect with the ordinary world of 3D materials. An international team led by MIT researchers has now come up with a way of imaging what goes on at these interfaces, down to the level of individual atoms, and of correlating the moiré patterns at the 2D-3D boundary with the resulting changes in the material’s properties.

The new findings are described today in the journal Nature Communications, in a paper by MIT graduate students Kate Reidy and Georgios Varnavides, professors of materials science and engineering Frances Ross, Jim LeBeau, and Polina Anikeeva, and five others at MIT, Harvard University, and the University of Victoria in Canada.

Pairs of two-dimensional materials such as graphene or hexagonal boron nitride can exhibit amazing variations in their behavior when the two sheets are just slightly twisted relative to each other. That causes the chicken-wire-like atomic lattices to form moiré patterns, the kinds of odd bands and blobs that sometimes appear when taking a picture of a printed image, or through a window screen. In the case of 2D materials, “it seems like anything, every interesting materials property you can think of, you can somehow modulate or change by twisting the 2D materials with respect to each other,” says Ross, who is the Ellen Swallow Richards Professor at MIT.

While these 2D pairings have attracted scientific attention worldwide, she says, little has been known about what happens where 2D materials meet regular 3D solids. “What got us interested in this topic,” Ross says, was “what happens when a 2D material and a 3D material are put together. Firstly, how do you measure the atomic positions at, and near, the interface? Secondly, what are the differences between a 3D-2D and a 2D-2D interface? And thirdly, how you might control it — is there a way to deliberately design the interfacial structure” to produce desired properties?

Figuring out exactly what happens at such 2D-3D interfaces was a daunting challenge because electron microscopes produce an image of the sample in projection, and they’re limited in their ability to extract depth information needed to analyze details of the interface structure. But the team figured out a set of algorithms that allowed them to extrapolate back from images of the sample, which look somewhat like a set of overlapping shadows, to figure out which configuration of stacked layers would yield that complex “shadow.”

The team made use of two unique transmission electron microscopes at MIT that enable a combination of capabilities that is unrivalled in the world. In one of these instruments, a microscope is connected directly to a fabrication system so that samples can be produced onsite by deposition processes and immediately fed straight into the imaging system. This is one of only a few such facilities worldwide, which use an ultrahigh vacuum system that prevents even the tiniest of impurities from contaminating the sample as the 2D-3D interface is being prepared. The second instrument is a scanning transmission electron microscope located in MIT’s new research facility, MIT.nano. This microscope has outstanding stability for high-resolution imaging, as well as multiple imaging modes for collecting information about the sample.

Unlike stacked 2D materials, whose orientations can be relatively easily changed by simply picking up one layer, twisting it slightly, and placing it down again, the bonds holding 3D materials together are much stronger, so the team had to develop new ways of obtaining aligned layers. To do this, they added the 3D material onto the 2D material in ultrahigh vacuum, choosing growth conditions where the layers self-assembled in a reproducible orientation with specific degrees of twist. “We had to grow a structure that was going to be aligned in a certain way,” Reidy says.

Having grown the materials, they then had to figure out how to reveal the atomic configurations and orientations of the different layers. A scanning transmission electron microscope actually produces more information than is apparent in a flat image; in fact, every point in the image contains details of the paths along which the electrons arrived and departed (the process of diffraction), as well as any energy that the electrons lost in the process. All these data can be separated out so that the information at all points in an image can be used to decode the actual solid structure. This process is only possible for state-of-the-art microscopes, such as that in MIT.nano, which generates a probe of electrons that is unusually narrow and precise.

The researchers used a combination of techniques called 4D STEM and integrated differential phase contrast to achieve that process of extracting the full structure at the interface from the image. Then, Varnavides says, they asked, “Now that we can image the full structure at the interface, what does this mean for our understanding of the properties of this interface?” The researchers showed through modeling that electronic properties are expected to be modified in a way that can only be understood if the full structure of the interface is included in the physical theory. “What we found is that indeed this stacking, the way the atoms are stacked out-of-plane, does modulate the electronic and charge density properties,” he says.

Ross says the findings could help lead to improved kinds of junctions in some microchips, for example. “Every 2D material that’s used in a device has to exist in the 3D world, and so it has to have a junction somehow with three-dimensional materials,” she says. So, with this better understanding of those interfaces, and new ways to study them in action, “we’re in good shape for making structures with desirable properties in a kind of planned rather than ad hoc way.”

“The methodology used has the potential to calculate from the acquired local diffraction patterns the modulation of the local electron momentum,” he says, adding that “the methodology and research shown here has an outstanding future and high interest for the materials science community.”

“Our study focuses on light propagation in cold atomic gases, in which atoms hardly move. On their way out of the gas, photons undergo multiple scattering by the atoms,” Bellando says. “Roughly speaking, the greater the number of these scattering events? — ?the longer it takes the photons to leave the gas, and thus the smaller their escape rates. This classical description fits the so-called radiation trapping, which occurs, for example, when light undergoes a random walk in a glass of milk.”

When taking into account interference and quantum mechanical effects, two mechanisms affect these escape rates: Anderson localisation arising from interference effects in the presence of disorder, and Dicke’s superradiance? — ??cooperative effects stemming from light-mediated interactions between the atoms.

Numerically studying photon escape rates from a three-dimensional cloud of cold atoms allowed the team to consider if there were any marked differences between the behaviour in the simple scalar case? — ?giving a single value to every point in a region — and the more complex vector case that assigns magnitude and direction to every point in a given area.

One of the biggest surprises encountered by the researchers as they collected their results was how well vector field observations agreed with scalar field tests. “Surprisingly, we found no significant difference between the scalar and vectorial models, and in both cases, the dominant mechanism was cooperativity,” says Bellando. “Now we know that the scalar model constitutes an excellent approximation when considering photon escape rates from atomic gases.”

Because the scalar model is much simpler than the vectorial one, the similarity between the two means that in the case of photon escape rates models can use scalar fields rather than vector fields without the risk of losing substantial information.

“Light-matter interaction is an exciting field of research, both theoretically and experimentally,” Bellando concludes. “Advances in this area may have a significant impact on other emerging fields, such as quantum computing.”

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The technology, which uses organic light-emitting diodes (OLEDs), is applied in the same way as water transfer tattoos. That is, the OLEDs are fabricated on to temporary tattoo paper and transferred to a new surface by being pressed on to it and dabbed with water.

The researchers, who described the process in a new paper in the journal Advanced Electronic Materials, say it could be combined with other tattoo electronics to, for instance emit light when an athlete is dehydrated, or when we need to get out of the sun to avoid sunburn. OLEDs could be tattooed on packaging or fruit to signal when a product has passed its expiry date or will soon become inedible, or used for fashion in the form of glowing tattoos.

Professor Franco Cacialli (UCL Physics & Astronomy), senior author of the paper, said: “The tattooable OLEDs that we have demonstrated for the first time can be made at scale and very cheaply. They can be combined with other forms of tattoo electronics for a very wide range of possible uses. These could be for fashion — for instance, providing glowing tattoos and light-emitting fingernails. In sports, they could be combined with a sweat sensor to signal dehydration.

“In healthcare they could emit light when there is a change in a patient’s condition — or, if the tattoo was turned the other way into the skin, they could potentially be combined with light-sensitive therapies to target cancer cells, for instance.

“Our proof-of-concept study is the first step. Future challenges will include encapsulating the OLEDs as much as possible to stop them from degrading quickly through contact with air, as well as integrating the device with a battery or supercapacitor.”

The OLED device the researchers developed is 2.3 micrometres thick in total (less than one 400th of a millimetre) — about a third of the length of a single red blood cell. It consists of an electroluminescent polymer (a polymer that emits light when an electric field is applied) in between electrodes. An insulating layer is placed in between the electrodes and the commercial tattoo paper.

The light-emitting polymer is 76 nanometres thick (a nanometre is a millionth of a millimetre) and was created using a technique called spin coating, where the polymer is applied to a substrate which is spun at high speed, producing an extremely thin and even layer.

Once they had built the technology, the team applied the tattooable OLEDs, which emitted green light, on to a pane of glass, a plastic bottle, an orange, and paper packaging.

Senior author Professor Virgilio Mattoli, researcher at Italian Institute of Technology said: “Tattoo electronics is a fast-growing field of research. At the Italian Institute of Technology we have previously pioneered electrodes that we have tattooed onto people’s skin that can be used to perform diagnostic tests such as electrocardiograms. The advantage of this technology is that it is low-cost, easy to apply and use, and washes off easily with soap and water.”

OLEDs were first used in a flatscreen TV 20 years ago. Among the advantages of the technology are that they can be used on flexible, bendy surfaces, and that they can be made from liquid solvents. This means they are printable, providing a cheap way to create bespoke new OLED designs.

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The Rice lab of mechanical engineer Peter Lillehoj has developed a stamp-sized microfluidic chip that measures the concentration of SARS-CoV-2 nucleocapsid (N) protein in blood serum from a standard finger prick. The nanobeads bind to SARS-CoV-2 N protein, a biomarker for COVID-19, in the chip and transport it to an electrochemical sensor that detects minute amounts of the biomarker.

The researchers argued their process simplifies sample handling compared to swab-based PCR tests that are widely used to diagnose COVID-19 and need to be analyzed in a laboratory.

“What’s great about this device is that doesn’t require a laboratory,” Lillehoj said. “You can perform the entire test and generate the results at the collection site, health clinic or even a pharmacy. The entire system is easily transportable and easy to use.”

The research appears in the American Chemical Society journal ACS Sensors.

Lillehoj and Rice graduate student and lead author Jiran Li took advantage of existing biosensing tools and combined them with their own experience in developing simple diagnostics, like a microneedle patch introduced last year to diagnose malaria.

The new tool relies on a slightly more complex detection scheme but delivers accurate, quantitative results in a short amount of time. To test the device, the lab relied on donated serum samples from people who were healthy and others who were COVID-19-positive.

Lillehoj said a longer incubation yields more accurate results when using whole serum. The lab found that 55 minutes was an optimum amount of time for the microchip to sense SARS-CoV-2 N protein at concentrations as low as 50 picograms (billionths of a gram) per milliliter in whole serum. The microchip could detect N protein in even lower concentrations, at 10 picograms per milliliter, in only 25 minutes by diluting the serum fivefold.

Paired with a Google Pixel 2 phone and a plug-in potentiostat, it was able to deliver a positive diagnosis with a concentration as low as 230 picograms for whole serum.

“There are standard procedures to modify the beads with an antibody that targets a particular biomarker,” Lillehoj said. “When you combine them with a sample containing the biomarker, in this case SARS-CoV-2 N protein, they bond together.”

A capillary tube is used to deliver the sample to the chip, which is then placed on a magnet that pulls the beads toward an electrochemical sensor coated with capture antibodies. The beads bind to the capture antibodies and generate a current proportional to the concentration of biomarker in the sample.

The potentiostat reads that current and sends a signal to its phone app. If there are no COVID-19 biomarkers, the beads do not bind to the sensor and get washed away inside the chip.

Lillehoj said it would not be difficult for industry to manufacture the microfluidic chips or to adapt them to new COVID-19 strains if and when that becomes necessary.

The National Institutes of Health, the National Science Foundation and the Rice University COVID-19 Research Fund supported the research.

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The unexpected visitor belongs to a class of icy bodies found in space between Jupiter and Neptune. Called “Centaurs,” they become active for the first time when heated as they approach the Sun, and dynamically transition into becoming more comet-like.

Visible-light snapshots by NASA’s Hubble Space Telescope reveal that the vagabond object shows signs of comet activity, such as a tail, outgassing in the form of jets, and an enshrouding coma of dust and gas. Earlier observations by NASA’s Spitzer Space Telescope gave clues to the composition of the comet-like object and the gasses driving its activity.

“Only Hubble could detect active comet-like features this far away at such high detail, and the images clearly show these features, such as a roughly 400,000-mile-long broad tail and high-resolution features near the nucleus due to a coma and jets,” said lead Hubble researcher Bryce Bolin of Caltech in Pasadena, California.

Describing the Centaur’s capture as a rare event, Bolin added, “The visitor had to have come into the orbit of Jupiter at just the right trajectory to have this kind of configuration that gives it the appearance of sharing its orbit with the planet. We’re investigating how it was captured by Jupiter and landed among the Trojans. But we think it could be related to the fact that it had a somewhat close encounter with Jupiter.”

The team’s paper appears in the February 11, 2021 issue of The Astronomical Journal.

The research team’s computer simulations show that the icy object, called P/2019 LD2 (LD2), probably swung close to Jupiter about two years ago. The planet then gravitationally punted the wayward visitor to the Trojan asteroid group’s co-orbital location, leading Jupiter by about 437 million miles.

Bucket Brigade

The nomadic object was discovered in early June 2019 by the University of Hawaii’s Asteroid Terrestrial-impact Last Alert System (ATLAS) telescopes located on the extinct volcanoes, one on Mauna Kea and one on Haleakala. Japanese amateur astronomer Seiichi Yoshida tipped off the Hubble team to possible comet activity. The astronomers then scanned archival data from the Zwicky Transient Facility, a wide-field survey conducted at Palomar Observatory in California, and realized that the object was clearly active in images from April 2019.

They followed up with observations from the Apache Point Observatory in New Mexico, which also hinted at the activity. The team observed the comet using Spitzer just days before the observatory’s retirement in January 2020, and identified gas and dust around the comet nucleus. These observations convinced the team to use Hubble to take a closer look. Aided by Hubble’s sharp vision, the researchers identified the tail, coma structure and the size of the dust particles and their ejection velocity. These images helped them confirm that the features are due to relatively new comet-like activity.

Although LD2’s location is surprising, Bolin wonders whether this pit stop could be a common pull-off for some sunward-bound comets. “This could be part of the pathway from our solar system through the Jupiter Trojans to the inner solar system,” he said.

The unexpected guest probably will not stay among the asteroids for very long. Computer simulations show that it will have another close encounter with Jupiter in about another two years. The hefty planet will boot the comet from the system, and it will continue its journey to the inner solar system.

“The cool thing is that you’re actually catching Jupiter flinging this object around and changing its orbital behavior and bringing it into the inner system,” said team member Carey Lisse of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. “Jupiter controls what’s going on with comets once they get into the inner system by altering their orbits.”

The icy interloper is most likely one of the latest members of the so-called “bucket brigade” of comets to get kicked out of its frigid home in the Kuiper belt and into the giant planet region through interactions with another Kuiper belt object. Located beyond Neptune’s orbit, the Kuiper belt is a haven of icy, leftover debris from our planets’ construction 4.6 billion years ago, containing millions of objects, and occasionally these objects have near misses or collisions that drastically alter their orbits from the Kuiper belt inward into the giant planet region.

The bucket brigade of icy relics endure a bumpy ride during their journey sunward. They bounce gravitationally from one outer planet to the next in a game of celestial pinball before reaching the inner solar system, warming up as they come closer to the Sun. The researchers say the objects spend as much or even more time around the giant planets, gravitationally pulling on them — about 5 million years — than they do crossing into the inner system where we live.

“Inner system, ‘short-period’ comets break up about once a century,” Lisse explained. “So, in order to maintain the number of local comets we see today, we think the bucket brigade has to deliver a new short-period comet about once every 100 years.”

An Early Bloomer

Seeing outgassing activity on a comet 465 million miles away from the Sun (where the intensity of sunlight is 1/25th as strong as on Earth) surprised the researchers. “We were intrigued to see that the comet had just started to become active for the first time so far away from the Sun at distances where water ice is barely starting to sublimate,” said Bolin.

Water remains frozen on a comet until it reaches about 200 million miles from the Sun, where heat from sunlight converts water ice to gas that escapes from the nucleus in the form of jets. So the activity signals that the tail might not be made of water. In fact, observations by Spitzer indicated the presence of carbon monoxide and carbon dioxide gas, which could be driving the creation of the tail and jets seen on the Jupiter-orbiting comet. These volatiles do not need much sunlight to heat their frozen form and convert them to gas.

Once the comet gets kicked out of Jupiter’s orbit and continues its journey, it may meet up with the giant planet again. “Short-period comets like LD2 meet their fate by being thrown into the Sun and totally disintegrating, hitting a planet, or venturing too close to Jupiter once again and getting thrown out of the solar system, which is the usual fate,” Lisse said. “Simulations show that in about 500,000 years, there’s a 90% probability that this object will be ejected from the solar system and become an interstellar comet.”

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