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MIT engineers build advanced microprocessor out of carbon nanotubes

MIT engineers build advanced microprocessor out of carbon nanotubes

After years of tackling numerous design and manufacturing challenges, MIT researchers have built a modern microprocessor from carbon nanotube transistors, which are widely seen as a faster, greener alternative to their traditional silicon counterparts.

The microprocessor, described today in the journal Nature, can be built using traditional silicon-chip fabrication processes, representing a major step toward making carbon nanotube microprocessors more practical.

Silicon transistors — critical microprocessor components that switch between 1 and 0 bits to carry out computations — have carried the computer industry for decades. As predicted by Moore’s Law, industry has been able to shrink down and cram more transistors onto chips every couple of years to help carry out increasingly complex computations. But experts now foresee a time when silicon transistors will stop shrinking, and become increasingly inefficient.

Making carbon nanotube field-effect transistors (CNFET) has become a major goal for building next-generation computers. Research indicates CNFETs have properties that promise around 10 times the energy efficiency and far greater speeds compared to silicon. But when fabricated at scale, the transistors often come with many defects that affect performance, so they remain impractical.

The MIT researchers have invented new techniques to dramatically limit defects and enable full functional control in fabricating CNFETs, using processes in traditional silicon chip foundries. They demonstrated a 16-bit microprocessor with more than 14,000 CNFETs that performs the same tasks as commercial microprocessors. The Nature paper describes the microprocessor design and includes more than 70 pages detailing the manufacturing methodology.

The microprocessor is based on the RISC-V open-source chip architecture that has a set of instructions that a microprocessor can execute. The researchers’ microprocessor was able to execute the full set of instructions accurately. It also executed a modified version of the classic “Hello, World!” program, printing out, “Hello, World! I am RV16XNano, made from CNTs.”

“This is by far the most advanced chip made from any emerging nanotechnology that is promising for high-performance and energy-efficient computing,” says co-author Max M. Shulaker, the Emanuel E Landsman Career Development Assistant Professor of Electrical Engineering and Computer Science (EECS) and a member of the Microsystems Technology Laboratories. “There are limits to silicon. If we want to continue to have gains in computing, carbon nanotubes represent one of the most promising ways to overcome those limits. [The paper] completely re-invents how we build chips with carbon nanotubes.”

Joining Shulaker on the paper are: first author and postdoc Gage Hills, graduate students Christian Lau, Andrew Wright, Mindy D. Bishop, Tathagata Srimani, Pritpal Kanhaiya, Rebecca Ho, and Aya Amer, all of EECS; Arvind, the Johnson Professor of Computer Science and Engineering and a researcher in the Computer Science and Artificial Intelligence Laboratory; Anantha Chandrakasan, the dean of the School of Engineering and the Vannevar Bush Professor of Electrical Engineering and Computer Science; and Samuel Fuller, Yosi Stein, and Denis Murphy, all of Analog Devices.

Fighting the “bane” of CNFETs

The microprocessor builds on a previous iteration designed by Shulaker and other researchers six years ago that had only 178 CNFETs and ran on a single bit of data. Since then, Shulaker and his MIT colleagues have tackled three specific challenges in producing the devices: material defects, manufacturing defects, and functional issues. Hills did the bulk of the microprocessor design, while Lau handled most of the manufacturing.

For years, the defects intrinsic to carbon nanotubes have been a “bane of the field,” Shulaker says. Ideally, CNFETs need semiconducting properties to switch their conductivity on an off, corresponding to the bits 1 and 0. But unavoidably, a small portion of carbon nanotubes will be metallic, and will slow or stop the transistor from switching. To be robust to those failures, advanced circuits will need carbon nanotubes at around 99.999999 percent purity, which is virtually impossible to produce today.

The researchers came up with a technique called DREAM (an acronym for “designing resiliency against metallic CNTs”), which positions metallic CNFETs in a way that they won’t disrupt computing. In doing so, they relaxed that stringent purity requirement by around four orders of magnitude — or 10,000 times — meaning they only need carbon nanotubes at about 99.99 percent purity, which is currently possible.

Designing circuits basically requires a library of different logic gates attached to transistors that can be combined to, say, create adders and multipliers — like combining letters in the alphabet to create words. The researchers realized that the metallic carbon nanotubes impacted different pairings of these gates differently. A single metallic carbon nanotube in gate A, for instance, may break the connection between A and B. But several metallic carbon nanotubes in gates B may not impact any of its connections.

In chip design, there are many ways to implement code onto a circuit. The researchers ran simulations to find all the different gate combinations that would be robust and wouldn’t be robust to any metallic carbon nanotubes. They then customized a chip-design program to automatically learn the combinations least likely to be affected by metallic carbon nanotubes. When designing a new chip, the program will only utilize the robust combinations and ignore the vulnerable combinations.

“The ‘DREAM’ pun is very much intended, because it’s the dream solution,” Shulaker says. “This allows us to buy carbon nanotubes off the shelf, drop them onto a wafer, and just build our circuit like normal, without doing anything else special.”

Exfoliating and tuning

CNFET fabrication starts with depositing carbon nanotubes in a solution onto a wafer with predesigned transistor architectures. However, some carbon nanotubes inevitably stick randomly together to form big bundles — like strands of spaghetti formed into little balls — that form big particle contamination on the chip.

To cleanse that contamination, the researchers created RINSE (for “removal of incubated nanotubes through selective exfoliation”). The wafer gets pretreated with an agent that promotes carbon nanotube adhesion. Then, the wafer is coated with a certain polymer and dipped in a special solvent. That washes away the polymer, which only carries away the big bundles, while the single carbon nanotubes remain stuck to the wafer. The technique leads to about a 250-times reduction in particle density on the chip compared to similar methods.

Lastly, the researchers tackled common functional issues with CNFETs. Binary computing requires two types of transistors: “N” types, which turn on with a 1 bit and off with a 0 bit, and “P” types, which do the opposite. Traditionally, making the two types out of carbon nanotubes has been challenging, often yielding transistors that vary in performance. For this solution, the researchers developed a technique called MIXED (for “metal interface engineering crossed with electrostatic doping”), which precisely tunes transistors for function and optimization.

In this technique, they attach certain metals to each transistor — platinum or titanium — which allows them to fix that transistor as P or N. Then, they coat the CNFETs in an oxide compound through atomic-layer deposition, which allows them to tune the transistors’ characteristics for specific applications. Servers, for instance, often require transistors that act very fast but use up energy and power. Wearables and medical implants, on the other hand, may use slower, low-power transistors.

The main goal is to get the chips out into the real world. To that end, the researchers have now started implementing their manufacturing techniques into a silicon chip foundry through a program by Defense Advanced Research Projects Agency, which supported the research. Although no one can say when chips made entirely from carbon nanotubes will hit the shelves, Shulaker says it could be fewer than five years. “We think it’s no longer a question of if, but when,” he says.

After years of tackling numerous design and manufacturing challenges, MIT researchers have built a modern microprocessor from carbon nanotube transistors, which are widely seen as a faster, greener alternative to their traditional silicon counterparts.

The microprocessor, described today in the journal Nature, can be built using traditional silicon-chip fabrication processes, representing a major step toward making carbon nanotube microprocessors more practical.

Silicon transistors — critical microprocessor components that switch between 1 and 0 bits to carry out computations — have carried the computer industry for decades. As predicted by Moore’s Law, industry has been able to shrink down and cram more transistors onto chips every couple of years to help carry out increasingly complex computations. But experts now foresee a time when silicon transistors will stop shrinking, and become increasingly inefficient.

Making carbon nanotube field-effect transistors (CNFET) has become a major goal for building next-generation computers. Research indicates CNFETs have properties that promise around 10 times the energy efficiency and far greater speeds compared to silicon. But when fabricated at scale, the transistors often come with many defects that affect performance, so they remain impractical.

The MIT researchers have invented new techniques to dramatically limit defects and enable full functional control in fabricating CNFETs, using processes in traditional silicon chip foundries. They demonstrated a 16-bit microprocessor with more than 14,000 CNFETs that performs the same tasks as commercial microprocessors. The Nature paper describes the microprocessor design and includes more than 70 pages detailing the manufacturing methodology.

The microprocessor is based on the RISC-V open-source chip architecture that has a set of instructions that a microprocessor can execute. The researchers’ microprocessor was able to execute the full set of instructions accurately. It also executed a modified version of the classic “Hello, World!” program, printing out, “Hello, World! I am RV16XNano, made from CNTs.”

“This is by far the most advanced chip made from any emerging nanotechnology that is promising for high-performance and energy-efficient computing,” says co-author Max M. Shulaker, the Emanuel E Landsman Career Development Assistant Professor of Electrical Engineering and Computer Science (EECS) and a member of the Microsystems Technology Laboratories. “There are limits to silicon. If we want to continue to have gains in computing, carbon nanotubes represent one of the most promising ways to overcome those limits. [The paper] completely re-invents how we build chips with carbon nanotubes.”

Joining Shulaker on the paper are: first author and postdoc Gage Hills, graduate students Christian Lau, Andrew Wright, Mindy D. Bishop, Tathagata Srimani, Pritpal Kanhaiya, Rebecca Ho, and Aya Amer, all of EECS; Arvind, the Johnson Professor of Computer Science and Engineering and a researcher in the Computer Science and Artificial Intelligence Laboratory; Anantha Chandrakasan, the dean of the School of Engineering and the Vannevar Bush Professor of Electrical Engineering and Computer Science; and Samuel Fuller, Yosi Stein, and Denis Murphy, all of Analog Devices.

Fighting the “bane” of CNFETs

The microprocessor builds on a previous iteration designed by Shulaker and other researchers six years ago that had only 178 CNFETs and ran on a single bit of data. Since then, Shulaker and his MIT colleagues have tackled three specific challenges in producing the devices: material defects, manufacturing defects, and functional issues. Hills did the bulk of the microprocessor design, while Lau handled most of the manufacturing.

For years, the defects intrinsic to carbon nanotubes have been a “bane of the field,” Shulaker says. Ideally, CNFETs need semiconducting properties to switch their conductivity on an off, corresponding to the bits 1 and 0. But unavoidably, a small portion of carbon nanotubes will be metallic, and will slow or stop the transistor from switching. To be robust to those failures, advanced circuits will need carbon nanotubes at around 99.999999 percent purity, which is virtually impossible to produce today.

The researchers came up with a technique called DREAM (an acronym for “designing resiliency against metallic CNTs”), which positions metallic CNFETs in a way that they won’t disrupt computing. In doing so, they relaxed that stringent purity requirement by around four orders of magnitude — or 10,000 times — meaning they only need carbon nanotubes at about 99.99 percent purity, which is currently possible.

Designing circuits basically requires a library of different logic gates attached to transistors that can be combined to, say, create adders and multipliers — like combining letters in the alphabet to create words. The researchers realized that the metallic carbon nanotubes impacted different pairings of these gates differently. A single metallic carbon nanotube in gate A, for instance, may break the connection between A and B. But several metallic carbon nanotubes in gates B may not impact any of its connections.

In chip design, there are many ways to implement code onto a circuit. The researchers ran simulations to find all the different gate combinations that would be robust and wouldn’t be robust to any metallic carbon nanotubes. They then customized a chip-design program to automatically learn the combinations least likely to be affected by metallic carbon nanotubes. When designing a new chip, the program will only utilize the robust combinations and ignore the vulnerable combinations.

“The ‘DREAM’ pun is very much intended, because it’s the dream solution,” Shulaker says. “This allows us to buy carbon nanotubes off the shelf, drop them onto a wafer, and just build our circuit like normal, without doing anything else special.”

Exfoliating and tuning

CNFET fabrication starts with depositing carbon nanotubes in a solution onto a wafer with predesigned transistor architectures. However, some carbon nanotubes inevitably stick randomly together to form big bundles — like strands of spaghetti formed into little balls — that form big particle contamination on the chip.

To cleanse that contamination, the researchers created RINSE (for “removal of incubated nanotubes through selective exfoliation”). The wafer gets pretreated with an agent that promotes carbon nanotube adhesion. Then, the wafer is coated with a certain polymer and dipped in a special solvent. That washes away the polymer, which only carries away the big bundles, while the single carbon nanotubes remain stuck to the wafer. The technique leads to about a 250-times reduction in particle density on the chip compared to similar methods.

Lastly, the researchers tackled common functional issues with CNFETs. Binary computing requires two types of transistors: “N” types, which turn on with a 1 bit and off with a 0 bit, and “P” types, which do the opposite. Traditionally, making the two types out of carbon nanotubes has been challenging, often yielding transistors that vary in performance. For this solution, the researchers developed a technique called MIXED (for “metal interface engineering crossed with electrostatic doping”), which precisely tunes transistors for function and optimization.

In this technique, they attach certain metals to each transistor — platinum or titanium — which allows them to fix that transistor as P or N. Then, they coat the CNFETs in an oxide compound through atomic-layer deposition, which allows them to tune the transistors’ characteristics for specific applications. Servers, for instance, often require transistors that act very fast but use up energy and power. Wearables and medical implants, on the other hand, may use slower, low-power transistors.

The main goal is to get the chips out into the real world. To that end, the researchers have now started implementing their manufacturing techniques into a silicon chip foundry through a program by Defense Advanced Research Projects Agency, which supported the research. Although no one can say when chips made entirely from carbon nanotubes will hit the shelves, Shulaker says it could be fewer than five years. “We think it’s no longer a question of if, but when,” he says.

Materials provided by Massachusetts Institute of Technology

water transport plants

Researchers develop antigravity water transport system inspired by plants

For hundreds of millions of years, trees have mastered the efficient movement of water upward against gravity. A recent study from researchers shows that they have set up Tree-based water transport system that uses capillary forces to pull out dirty water upward through a pyramidal structure aerogel and thus converting into steam by solar energy to produce fresh, clean water.

The researchers, directed by Aiping Liu at Zhejiang Sci-Tech University and Hao Bai at Zhejiang University, have published a paper on the new water transport and solar steam generation method in the journal ACS Nano. Efficient water transport methods could be used in water purification and desalination in near future.

Liu said that their production method is global and can be industrialized. Their resources have the best qualities, stability and can be recycled. This establishes the opportunity for extensive desalination and sewage treatment in near future.

The mechanism includes two major components. A long, porous, lightweight aerogel for transporting water and a carbon nanotube layer to absorb sunlight and convert water into steam covered in a glass vessel. Capillary forces generated by adhesion between the water molecules and the interior wall of the pores causes water to rise upward in the aerogel. Solar-heated carbon nanotube layer turns the water at the top into steam releasing impurities back. The water droplets formed by condensed steam on the walls of container flow down into a storage for collection.

Like carbon solar steam generator, plants pull water from the ground across branches and leaves by the help of tiny xylem vessels. Solar radiation causes leaves water to evaporate through tiny pores.

Compared to previous trials of tree-like water transport system showing suboptimal performance, reduced transport speeds and lesser transport distances, the new aerogel design showed optimisation in these fields obtaining upward flow performance of 10 cm and 25 cm in first 5 minutes and 3 hours respectively attaining a high energy conversion efficiency of up to 85%.

To enhance the set-up, researchers have build up the material by pouring aerogel ingredients into a copper tube keeping cold end at -90 degree Celsius causing ice crystal to grow. Tiny structures like pyramid with radially aligned channels, micro-sized pores, wrinkled inner surfaces, and molecular meshes obtained after freeze-drying the tube helped in aerogel best performance.

Liu said that they further want to enhance the speed and length of water transmission and efficiency of water storage to take forward practical implementation and achieve mass production.

Breaching a “carbon threshold” could lead to mass extinction

Breaching a “carbon threshold” could lead to mass extinction

In the brain, when neurons fire off electrical signals to their neighbours, this happens through an “all-or-none” response. The signal only happens once conditions in the cell breach a certain threshold.

Now an MIT researcher has observed a similar phenomenon in a completely different system: Earth’s carbon cycle.

Daniel Rothman, professor of geophysics and co-director of the Lorenz Center in MIT’s Department of Earth, Atmospheric and Planetary Sciences, has found that when the rate at which carbon dioxide enters the oceans pushes past a certain threshold — whether as the result of a sudden burst or a slow, steady influx — the Earth may respond with a runaway cascade of chemical feedbacks, leading to extreme ocean acidification that dramatically amplifies the effects of the original trigger.

This global reflex causes huge changes in the amount of carbon contained in the Earth’s oceans, and geologists can see evidence of these changes in layers of sediments preserved over hundreds of millions of years.

Rothman looked through these geologic records and observed that over the last 540 million years, the ocean’s store of carbon changed abruptly, then recovered, dozens of times in a fashion similar to the abrupt nature of a neuron spike. This “excitation” of the carbon cycle occurred most dramatically near the time of four of the five great mass extinctions in Earth’s history.

Scientists have attributed various triggers to these events, and they have assumed that the changes in ocean carbon that followed were proportional to the initial trigger — for instance, the smaller the trigger, the smaller the environmental fallout.

But Rothman says that’s not the case. It didn’t matter what initially caused the events; for roughly half the disruptions in his database, once they were set in motion, the rate at which carbon increased was essentially the same.  Their characteristic rate is likely a property of the carbon cycle itself — not the triggers, because different triggers would operate at different rates.

What does this all have to do with our modern-day climate? Today’s oceans are absorbing carbon about an order of magnitude faster than the worst case in the geologic record — the end-Permian extinction. But humans have only been pumping carbon dioxide into the atmosphere for hundreds of years, versus the tens of thousands of years or more that it took for volcanic eruptions or other disturbances to trigger the great environmental disruptions of the past. Might the modern increase of carbon be too brief to excite a major disruption?

According to Rothman, today we are “at the precipice of excitation,” and if it occurs, the resulting spike — as evidenced through ocean acidification, species die-offs, and more — is likely to be similar to past global catastrophes.

“Once we’re over the threshold, how we got there may not matter,” says Rothman, who is publishing his results this week in the Proceedings of the National Academy of Sciences.“Once you get over it, you’re dealing with how the Earth works, and it goes on its own ride.”

A carbon feedback

In 2017, Rothman made a dire prediction: By the end of this century, the planet is likely to reach a critical threshold, based on the rapid rate at which humans are adding carbon dioxide to the atmosphere. When we cross that threshold, we are likely to set in motion a freight train of consequences, potentially culminating in the Earth’s sixth mass extinction.

Rothman has since sought to better understand this prediction, and more generally, the way in which the carbon cycle responds once it’s pushed past a critical threshold. In the new paper, he has developed a simple mathematical model to represent the carbon cycle in the Earth’s upper ocean and how it might behave when this threshold is crossed.

Scientists know that when carbon dioxide from the atmosphere dissolves in seawater, it not only makes the oceans more acidic, but it also decreases the concentration of carbonate ions. When the carbonate ion concentration falls below a threshold, shells made of calcium carbonate dissolve. Organisms that make them fare poorly in such harsh conditions.

Shells, in addition to protecting marine life, provide a “ballast effect,” weighing organisms down and enabling them to sink to the ocean floor along with detrital organic carbon, effectively removing carbon dioxide from the upper ocean. But in a world of increasing carbon dioxide, fewer calcifying organisms should mean less carbon dioxide is removed.

“It’s a positive feedback,” Rothman says. “More carbon dioxide leads to more carbon dioxide. The question from a mathematical point of view is, is such a feedback enough to render the system unstable?”

An inexorable rise

Rothman captured this positive feedback in his new model, which comprises two differential equations that describe interactions between the various chemical constituents in the upper ocean. He then observed how the model responded as he pumped additional carbon dioxide into the system, at different rates and amounts.

He found that no matter the rate at which he added carbon dioxide to an already stable system, the carbon cycle in the upper ocean remained stable. In response to modest perturbations, the carbon cycle would go temporarily out of whack and experience a brief period of mild ocean acidification, but it would always return to its original state rather than oscillating into a new equilibrium.

When he introduced carbon dioxide at greater rates, he found that once the levels crossed a critical threshold, the carbon cycle reacted with a cascade of positive feedbacks that magnified the original trigger, causing the entire system to spike, in the form of severe ocean acidification. The system did, eventually, return to equilibrium, after tens of thousands of years in today’s oceans — an indication that, despite a violent reaction, the carbon cycle will resume its steady state.

This pattern matches the geological record, Rothman found. The characteristic rate exhibited by half his database results from excitations above, but near, the threshold. Environmental disruptions associated with mass extinction are outliers — they represent excitations well beyond the threshold. At least three of those cases may be related to sustained massive volcanism.

“When you go past a threshold, you get a free kick from the system responding by itself,” Rothman explains. “The system is on an inexorable rise. This is what excitability is, and how a neuron works too.”

Although carbon is entering the oceans today at an unprecedented rate, it is doing so over a geologically brief time. Rothman’s model predicts that the two effects cancel: Faster rates bring us closer to the threshold, but shorter durations move us away. Insofar as the threshold is concerned, the modern world is in roughly the same place it was during longer periods of massive volcanism.

In other words, if today’s human-induced emissions cross the threshold and continue beyond it, as Rothman predicts they soon will, the consequences may be just as severe as what the Earth experienced during its previous mass extinctions.

“It’s difficult to know how things will end up given what’s happening today,” Rothman says. “But we’re probably close to a critical threshold. Any spike would reach its maximum after about 10,000 years. Hopefully, that would give us time to find a solution.”

“We already know that our CO2-emitting actions will have consequences for many millennia,” says Timothy Lenton, professor of climate change and earth systems science at the University of Exeter. “This study suggests those consequences could be much more dramatic than previously expected. If we push the Earth system too far, then it takes over and determines its own response — past that point there will be little we can do about it.”

Materials provided by Massachusetts Institute of Technology

 

Carbon Escape

An escape route for carbon

As many of us may recall from grade school science class, the Earth’s carbon cycle goes something like this: As plants take up carbon dioxide and convert it into organic carbon, they release oxygen back into the air. Complex life forms such as ourselves breathe in this oxygen and respire carbon dioxide. When microbes eat away at decaying plants, they also consume the carbon within, which they convert and release back into the atmosphere as carbon dioxide. And so the cycle continues.

The vast majority of the planet’s carbon loops perpetually through this cycle, driven by photosynthesis and respiration. There is, however, a tiny fraction of organic carbon that is continually escaping through a “leak” in the cycle, the cause of which is largely unknown. Scientists do know that, through this leak, some minute amount of carbon is constantly locked away and preserved in the form of rock for hundreds of millions of years.

Now, researchers from MIT and elsewhere have found evidence for what may be responsible for carbon’s slow and steady escape route.

In a paper published today in the journal Nature, the team reports that organic carbon is leaking out of the carbon cycle mainly due to a mechanism they call “mineral protection.” In this process, carbon, in the form of decomposed bits of plant and phytoplankton material, gloms onto particles of clay and other minerals, for instance at the bottom of a river or ocean, and is preserved in the form of sediments and, ultimately, rock.

Mineral protection may also explain why there is oxygen on Earth in the first place: If something causes carbon to leak out of the carbon cycle, this leaves more oxygen to accumulate in the atmosphere.

“Fundamentally, this tiny leak is one reason why we exist,” says Daniel Rothman, professor of geophysics in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “It’s what allows oxygen to accumulate over geologic time, and it’s why aerobic organisms evolved, and it has everything to do with the history of life on the planet.”

Rothman’s co-authors on the paper include Jordon Hemingway, who led the work as a graduate student at MIT and the Woods Hole Oceanographic Institution and is now a postdoc at Harvard University, along with Katherine Grant, Sarah Rosengard, Timothy Eglinton, Louis Derry, and Valier Galy.

Burning dirt

Scientists have entertained two main possibilities for how carbon has been leaking out of the Earth’s carbon cycle. The first has to do with “selectivity,” the idea that some types of organic matter, due to their molecular makeup, may be harder to break down than others. Based on this idea, the carbon that is not consumed, and therefore leaks out, has been “selected” to do so, based on the initial organic matter’s molecular structure.

The second possibility involves “accessibility,” the notion that some organic matter leaks out of the carbon cycle because it has been made inaccessible for consumption via some secondary process. Some scientists believe that secondary process could be mineral protection — interactions between organic carbon and clay-based minerals that bind the two together in an inaccessible, unconsumable form.

To test which of these mechanisms better explains Earth’s carbon leak, Hemingway analyzed sediment samples collected from around the world, each containing organic matter and minerals from a range of river and coastal environments. If mineral preservation is indeed responsible for locking away and preserving carbon over geologic timescales, Hemingway hypothesized that organic carbon bound with clay minerals should last longer in the environment compared with unbound carbon, resisting degradation by foraging microbes, or even other forces such as extreme heat.

The researchers tested this idea by burning each sediment sample and measuring the amount and type of organic carbon that remained as they heated the sample at progressively higher temperatures. They did so using a device that Hemingway developed as part of his PhD thesis.

“It’s been hypothesized that organic matter that sticks to mineral surfaces will stick around longer in the environment,” Hemingway says. “But there was never a tool to directly quantify that.”

“Beating up a natural process”

In the end, they found the organic matter that lasted the longest, and withstood the highest temperatures, was bound to clay minerals. Importantly, in a finding that went against the idea of selectivity, it didn’t matter what the molecular structure of that organic matter was — as long as it was bound to clay, it was preserved.

The results point to accessibility, and mineral preservation in particular, as the main mechanism for Earth’s carbon leak. In other words, all around the world, clay minerals are slowly and steadily drawing down tiny amounts of carbon, and storing it away for thousands of years.

“It’s this clay-bound protection that seems to be the mechanism, and it seems to be a globally coherent phenomenon,” Hemingways says. “It’s a slow leak happening all the time, everywhere. And when you integrate that over geologic timescales, it becomes a really important sink of carbon.”

The researchers believe mineral protection has made it possible for vast reservoirs of carbon to be buried and stored in the Earth, some of which has been pressed and heated into petroleum over millions of years. At the Earth’s geologic pace, this carbon preserved in rocks eventually resurfaces through mountain uplift and gradually erodes, releasing carbon dioxide back into the atmosphere ever so slowly.

“What we do today with fossil fuel burning is speeding up this natural process,” Rothman says. “We’re getting it out of the ground and burning it right away, and we’re changing the rate at which the carbon that was leaked out is being returned to the system, by a couple orders of magnitude.”

Could mineral preservation somehow be harnessed to sequester even more carbon, in an effort to mitigate fossil-fuel-induced climate change?

“If we magically had the ability to take a fraction of organic matter in rivers or oceans and attach it to a mineral to hold onto it for 1,000 years, it could have some advantages,” Rothman says. “That’s not the focus of this study. But the longer soils can lock up organic matter, the slower their return to the atmosphere. You can imagine if you could slow that return process down just a little bit, it could make a big difference over 10 to 100 years.”

Materials provided by Massachusetts Institute of Technology

MIT Household

Pantry ingredients can help grow carbon nanotubes

Baking soda, table salt, and detergent are surprisingly effective ingredients for cooking up carbon nanotubes, researchers at MIT have found.

In a study published this week in the journal Angewandte Chemie, the team reports that sodium-containing compounds found in common household ingredients are able to catalyze the growth of carbon nanotubes, or CNTs, at much lower temperatures than traditional catalysts require.

The researchers say that sodium may make it possible for carbon nanotubes to be grown on a host of lower-temperature materials, such as polymers, which normally melt under the high temperatures needed for traditional CNT growth.

“In aerospace composites, there are a lot of polymers that hold carbon fibers together, and now we may be able to directly grow CNTs on polymer materials, to make stronger, tougher, stiffer composites,” says Richard Li, the study’s lead author and a graduate student in MIT’s Department of Aeronautics and Astronautics. “Using sodium as a catalyst really unlocks the kinds of surfaces you can grow nanotubes on.”

Li’s MIT co-authors are postdocs Erica Antunes, Estelle Kalfon-Cohen, Luiz Acauan, and Kehang Cui; alumni Akira Kudo PhD ’16, Andrew Liotta ’16, and Ananth Govind Rajan SM ’16, PhD ’19; professor of chemical engineering Michael Strano, and professor of aeronautics and astronautics Brian Wardle, along with collaborators at the National Institute of Standards and Technology and Harvard University.

Peeling onions

Under a microscope, carbon nanotubes resemble hollow cylinders of chicken wire. Each tube is made from a rolled up lattice of hexagonally arranged carbon atoms. The bond between carbon atoms is extraordinarily strong, and when patterned into a lattice, such as graphene, or as a tube, such as a CNT, such structures can have exceptional stiffness and strength, as well as unique electrical and chemical properties. As such, researchers have explored coating various surfaces with CNTs to produce stronger, stiffer, tougher materials.

Researchers typically grow CNTs on various materials through a process called chemical vapor deposition. A material of interest, such as carbon fibers, is coated in a catalyst — usually an iron-based compound — and placed in a furnace, through which carbon dioxide and other carbon-containing gases flow. At temperatures of up to 800 degrees Celsius, the iron starts to draw carbon atoms out of the gas, which glom onto the iron atoms and to each other, eventually forming vertical tubes of carbon atoms around individual carbon fibers. Researchers then use various techniques to dissolve the catalyst, leaving behind pure carbon nanotubes.

Li and his colleagues were experimenting with ways to grow CNTs on various surfaces by coating them with different solutions of iron-containing compounds, when the team noticed the resulting carbon nanotubes looked different from what they expected.

“The tubes looked a little funny, and Rich and the team carefully peeled the onion back, as it were, and it turns out a small quantity of sodium, which we suspected was inactive, was actually causing all the growth,” Wardle says.

Tuning sodium’s knobs

For the most part, iron has been the traditional catalyst for growing CNTs. Wardle says this is the first time that researchers have seen sodium have a similar effect.

“Sodium and other alkali metals have not been explored for CNT catalysis,” Wardle says. “This work has led us to a different part of the periodic table.”

To make sure their initial observation wasn’t just a fluke, the team tested a range of sodium-containing compounds. They initially experimented with commercial-grade sodium, in the form of baking soda, table salt, and detergent pellets, which they obtained from the campus convenience store. Eventually, however, they upgraded to purified versions of those compounds, which they dissolved in water. They then immersed a carbon fiber in each compound’s solution, coating the entire surface in sodium. Finally, they placed the material in a furnace and carried out the typical steps involved in the chemical vapor deposition process to grow CNTs.

In general, they found that, while iron catalysts form carbon nanotubes at around 800 degrees Celsius, the sodium catalysts were able to form short, dense forests of CNTs at much lower temperatures, of around 480 C. What’s more, after surfaces spent about 15 to 30 minutes in the furnace, the sodium simply vaporized away, leaving behind hollow carbon nanotubes.

“A large part of CNT research is not on growing them, but on cleaning them —getting the different metals used to grow them out of the product,” Wardle says. “The neat thing with sodium is, we can just heat it and get rid of it, and get pure CNT as product, which you can’t do with traditional catalysts.”

Li says future work may focus on improving the quality of CNTs that are grown using sodium catalysts. The researchers observed that while sodium was able to generate forests of carbon nanotubes, the walls of the tubes were not perfectly aligned in perfectly hexagonal patterns — crystal-like configurations that give CNTs their characteristic strength. Li plans to “tune various knobs” in the CVD process, changing the timing, temperature, and environmental conditions, to improve the quality of sodium-grown CNTs.

“There are so many variables you can still play with, and sodium can still compete pretty well with traditional catalysts,” Li says. “We anticipate with sodium, it is possible to get high quality tubes in the future. And we have pretty high confidence that, even if you were to use regular Arm and Hammer baking soda, it should work.”

For Shigeo Maruyama, professor of mechanical engineering at the University of Tokyo, the ability to cook up CNTs from such a commonplace ingredient as sodium should reveal new insights into the way the exceptionally strong materials grow.

“It is a surprise that we can grow carbon nanotubes from table salt!” says Maruyama, who was not involved in the research. “Even though chemical vapor deposition (CVD) growth of carbon nanotubes has been studied for more than 20 years, nobody has tried to use alkali group metal as catalyst. This will be a great hint for the fully new understanding of growth mechanism of carbon nanotubes.”

This research was supported, in part, by Airbus, Boeing, Embraer, Lockheed Martin, Saab AB, ANSYS, Saertex, and TohoTenax through MIT’s Nano-Engineered Composite aerospace STructures (NECST) Consortium.

Materials provided by Massachusetts Institute of Technology

Abludomelita obtusata

Presence of radioactive carbon found in crustaceans in oceanic trenches

The latest study published in the Journal of American Geophysical UnionGeophysical Research Letters provides evidence regarding the presence of radioactive carbon from the nuclear tests in the muscular tissues of the crustaceans that reside in the ocean trenches.

Organisms which reside in the oceanic trenches including Mariana Trench, the deepest point in the ocean incorporated this carbon in the molecules which comprise their bodies from as early as the 1950s. The study finds that the crustaceans inhabiting the deep oceanic trenches have been feeding on the body of these organisms after they fall to the ocean floor. This is a very alarming situation wherein the effects of human activities can be traced back even to deep ocean floors.

Ning Wang, lead author and geochemist in Chinese Academy of Sciences, Guangzhou, China remarked that even though the circulation of oceans takes several hundreds of years to bring the water containing carbon from the nuclear explosions to the ocean trenches, it can travel much faster through the food chain.

Weidong Sun, co-author of the study also pointed out that there is a presence of very strong interaction between the surface of the ocean and bottom. Thus the human activities can influence biosystems even till 11,000 metres. Thus we need to be careful about our activities.

These results have helped scientists in understanding how organisms have adjusted to the nutrient deficient conditions of the deep ocean. Scientists have observed that the crustaceans have adapted to the harsh conditions by having a slow metabolism which enables them to live for a long period of time.

The radioactive carbon, C-14 is created due to the interaction of cosmic rays with nitrogen present in the atmosphere. It is quite less abundant than non-radioactive carbon but can be detected in almost all organisms. Besides this, it is also used for determining the ages of fossils, archaeological samples.

The amount of carbon in the atmosphere has been doubled due to the nuclear weapon tests which were carried out in the 1950s. However, the levels dropped to some extent when these tests were stopped. Hence the levels in 1990 reduced to almost 20 percent of the pre-test levels.

In this study, researchers analysed the amphipods which were collected in 2017 from several locations such as  Mariana, New Britain Trenches in the Pacific Ocean which ranges as far as 11 kilometres below the surface. Amphipods are a group of small crustacean which gets their food by feeding on the dead organisms on the ocean floor.

The C-14 levels in amphipods were much more than that of the organic matter on the ocean floor. This helps in understanding how organisms adapt to the environment and increase their longevity.

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Graphene

Researchers demonstrate working of quantum computers with help of graphene

A new material consisting of only one sheet of carbon atoms can give rise to new and unique designs of optical quantum computing devices. Researchers from the University of Vienna and Institute of Photonic Sciences, Barcelona have proved that tailored structures of graphene lead to the interaction of singular photons. The study has been published in the npj Quantum Information.

Photons interact with the environment to a very less degree, which makes it quite suitable for the storage and transmission of quantum information. However this same property makes it very difficult to interpret the information which has been stored in them.

For building a quantum photonic computer, it is essential for a photon to alter the state of second. This is called a quantum logic gate and a quantum computer requires millions of these. This can be achieved with the help of a ‘non-linear material’, in which there is interaction of two photons. But the standard non-linear materials are not efficient to construct a quantum logic gate.

However it has been recently understood that the nonlinear interactions can be highly improved with the help of plasmons. Plasmons make the light bind to the electrons which are located at the surface. Then these electrons facilitate a very strong interaction between the photons. In presence of these positives, a drawback is that the plasmons decay in the standard materials before the actual quantum effects can occur.

Philip Walther, from University of Vienna who led the team of researchers made a proposal to manufacture plasmons in the graphene material. Graphene has been only discovered in 2004 by Andre Geim and Konstantin Novoselov at University of Manchester. Though it was observed way back in 1962, it had not been independently isolated and studied then. For their work, the duo was awarded the Nobel Prize in Physics in 2010.

The unique arrangement of electrons in graphene leads to strong nonlinear interactions, which allows the plasmons to remain for a long duration. In the graphene quantum logic gate, scientists have demonstrated that if singular plasmons in nanoribbon are made from graphene, then it allows for the interaction of electrical fields of two plasmons in different nanoribbons. This makes way for quantum computation if each of the plasmons remain in their ribbons, since many gates can be applied to them.

Irati Alonso Calafell, who is the first author on this paper remarked that strong non linear interaction in graphene does not allow two plasmons to be in the same ribbon.

Extragalactic Space Balls

Hubble Space Telescope confirms presence of buckyballs in interstellar matter

The Hubble Space Telescope has detected proof of buckminster fullerene in the ionised state in the interstellar medium. Buckminsterfullerene is the carbon compound which is commonly known as buckyball.

The interstellar medium comprises of the matter and the radiations which are present in the space between the stars of the galaxy. It mainly comprises of gases in ionic, atomic form as well as dust and other rays. The energy which is present in this region in the form of electromagnetic radiations is called interstellar radiation field.

In a study led by Martin Cordiner, an astrophysicist at NASA Goddard Space Flight Centre, it has been confirmed through observations of the Hubble Telescope in 11 star systems that Buckminsterfullerene is present. It contains 60 atoms of carbon arranged in a fused ring structure which is quite similar to a soccer ball shape. First synthesized in 1984, it is the most commonly found fullerene. It is found in soot.

It has been detected in the nebula and in gas around a star. Previously, the largest known elements which were detected in the diffused interstellar medium contained a maximum of three atoms heavier than hydrogen. Hence this makes the discovery of buckyballs a steep rise in the restriction of size.

It is quite difficult to study the individual elements present in the interstellar medium since there is a combination of many factors. Besides this, the environment in which they are created is also unknown making it pretty challenging.

Scientists used a different scanning method to get a very high signal-to-noise spectra of seven stars. They have been reddened to a great extent by the interstellar medium. Four stars remained unreddened and they were probed for absorption signals at four specific wavelengths. These wavelengths are 9348, 9365, 9428, and 9577 Å.

Scientists found reliable detections of three of the strongest absorption lines in the spectra of the seven stars which were reddened. No sign of absorption was found in the four stars that were not reddened. Out of the absorptions, 9348 Å was not detected. But as this has been predicted to a very weak feature, it is not much surprising. These results were in line with the predictions made by the laboratory.

The confirmation of buckyballs in the interstellar medium will help us to know about the several other characteristics and components of the diffused ISM. It will also help us to understand the situations in which the molecules exist in the extreme conditions of the space.