Posts Tagged ‘carbon’

by Davide Castelvecchi

Long after most chemists had given up trying, a team of researchers has synthesized the first ring-shaped molecule of pure carbon — a circle of 18 atoms.

The chemists started with a triangular molecule of carbon and oxygen, which they manipulated with electric currents to create the carbon-18 ring. Initial studies of the properties of the molecule, called a cyclocarbon, suggest that it acts as a semiconductor, which could make similar straight carbon chains useful as molecular-scale electronic components.

It is an “absolutely stunning work” that opens up a new field of investigation, says Yoshito Tobe, a chemist at Osaka University in Japan. “Many scientists, including myself, have tried to capture cyclocarbons and determine their molecular structures, but in vain,” Tobe says. The results appear in Science1 on 15 August.

Pure carbon comes in several different forms, including diamond, graphite and ‘nanotubes’. Atoms of the element can form chemical bonds with themselves in various configurations: for example, each atom can bind to four neighbours in a pyramid-shaped pattern, as in diamond; or to three, as in the hexagonal patterns that make up the single-atom-thick sheets of graphene. (Such a three-bond pattern is also found in bulk graphite as well as in carbon nanotubes and in the globular molecules called fullerenes.)

But carbon can also form bonds with just two nearby atoms. Nobel-prizewinning chemist Roald Hoffmann at Cornell University in Ithaca, New York, and others have long theorized that this would lead to pure chains of carbon atoms. Each atom might form either a double bond on each side — meaning the adjacent atoms share two electrons — or a triple bond on one side and a single bond on the other. Various teams have attempted to synthesize rings or chains based on this pattern.

But because this type of structure is more chemically reactive than graphene or diamond, it is less stable, especially when bent, says chemist Przemyslaw Gawel of the University of Oxford, UK. Synthesizing stable chains and rings has usually required the inclusion of elements other than carbon. Some experiments have hinted at the creation of all-carbon rings in a gas cloud, but they have not able to find conclusive proof.

One ring
Gawel and his collaborators have now created and imaged the long-sought ring molecule carbon-18. Using standard ‘wet’ chemistry, his collaborator Lorel Scriven, an Oxford chemist, first synthesized molecules that included four-carbon squares coming off the ring with oxygen atoms attached to squares. The team then sent their samples to IBM laboratories in Zurich, Switzerland, where collaborators put the oxygen–carbon molecules on a layer of sodium chloride, inside a high-vacuum chamber. They manipulated the rings one at a time with electric currents (using an atomic-force microscope that can also act as a scanning-tunelling microscope), to remove the extraneous, oxygen-containing parts. After much trial-and-error, micrograph scans revealed the 18-carbon structure. “I never thought I would see this,” says Scriven.

The IBM researchers showed that the 18-carbon rings had alternating triple and single bonds. Theoretical results had disagreed over whether carbon-18 would have this kind of structure, or one made entirely of double bonds.

Alternating bond types are interesting because they are supposed to give carbon chains and rings the properties of semiconductors. The results suggest that long, straight carbon chains might be semiconductors, too, Gawel says, which could make them useful as components of future molecular-sized transistors.

For now, the researchers are going to study the basic properties of carbon-18, which they have been able to make one molecule at a time only. They are also going to keep trying alternative techniques that might yield greater quantities. “This is so far very fundamental research,” Gawel says.

“The work is beautiful,” says Hoffmann, although he adds that it remains to be seen whether carbon-18 is stable when lifted off the salt surface, and whether it can be synthesized more efficiently than one molecule at a time.

doi: 10.1038/d41586-019-02473-z
References
1. Kaiser, K. et al. Science https://doi.org/10.1126/science.aay1914 (2019).

https://www.nature.com/articles/d41586-019-02473-z?utm_source=Nature+Briefing&utm_campaign=8838a84803-briefing-dy-20190819&utm_medium=email&utm_term=0_c9dfd39373-8838a84803-44039353

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A Soviet cow-fattening complex pictured in 1982.Credit: Nikolai Akimov/TASS

by Quirin Schiermeier

The collapse of the Soviet Union in 1991 led to a huge drop in greenhouse-gas emissions because the resulting economic crisis meant many people stopped eating meat.

Meat from domestic livestock farming was a main food staple during communist rule in the region. In 1990, Soviet citizens each consumed an average 32 kilograms of beef a year — 27% more than Western Europeans and four times more than the global average at the time.

But meat demand and livestock production in the region fell drastically when the prices of everyday consumer products soared and the purchasing power of the rouble dwindled in the post-communist economic crisis. An estimated one-third of late-Soviet cropland has been abandoned since.

These changes in the food and agriculture system in the former Soviet nations resulted in a net reduction of 7.6 billion tonnes of greenhouse gases in carbon dioxide equivalent from 1992 to 2011, researchers find from an analysis of data on livestock consumption and international trade1 (see ‘Soviet shocks’). The drop is equivalent to one-quarter of CO2 emissions from Amazon deforestation over the same period. Russia currently emits about 2.5 billion tonnes of greenhouse gases (CO2 equivalent) per year.

The figure considers emissions that result from domestic production of livestock and imported livestock, as well as carbon locked in soils and plants on abandoned Soviet cropland.

“There was a large drop in industrial production and emissions after the collapse of the Soviet Union, so it should be no surprise the same happened with food consumption and production,” says Glen Peters, a carbon-budget specialist at the Center for International Climate Research in Oslo, who was not involved in the analysis. “The study highlights the potential for carbon uptake in the former Soviet Union but also the risks to that carbon being released if agricultural production returns.”

Today, animal agriculture is responsible for 14.5% of human-caused greenhouse-gas emissions globally. Beef is the most emissions-intensive food because pastures are often created by clearing forests and savannahs.

Meat consumption — especially beef — and land-use changes in Russia and central Asia are a widely overlooked factor in calculations of greenhouse-gas emissions from land around the globe, says study author Florian Schierhorn at the Leibniz Institute of Agricultural Development in Transition Economies in Halle, Germany.

Trends in international trade suggest that emissions associated with meat consumption are on the rise again: Russia has over the past decade become a top destination for beef exported mainly from South America.

doi: 10.1038/d41586-019-02024-6

References
1. Schierhorn, F. et al. Environ. Res. Lett. 14, 065009 (2019).

https://www.nature.com/articles/d41586-019-02024-6?utm_source=Nature+Briefing&utm_campaign=34225bcef1-briefing-dy-20190701&utm_medium=email&utm_term=0_c9dfd39373-34225bcef1-44039353

Researchers from North Carolina State University have discovered a new phase of solid carbon, called Q-carbon, which is distinct from the known phases of graphite and diamond. They have also developed a technique for using Q-carbon to make diamond-related structures at room temperature and at ambient atmospheric pressure in air.

Phases are distinct forms of the same material. Graphite is one of the solid phases of carbon; diamond is another.

“We’ve now created a third solid phase of carbon,” says Jay Narayan, the John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and lead author of three papers describing the work. “The only place it may be found in the natural world would be possibly in the core of some planets.”

Q-carbon has some unusual characteristics. For one thing, it is ferromagnetic – which other solid forms of carbon are not.

“We didn’t even think that was possible,” Narayan says.

In addition, Q-carbon is harder than diamond, and glows when exposed to even low levels of energy.
“Q-carbon’s strength and low work-function – its willingness to release electrons – make it very promising for developing new electronic display technologies,” Narayan says.

But Q-carbon can also be used to create a variety of single-crystal diamond objects. To understand that, you have to understand the process for creating Q-carbon.

Researchers start with a substrate, such as sapphire, glass or a plastic polymer. The substrate is then coated with amorphous carbon – elemental carbon that, unlike graphite or diamond, does not have a regular, well-defined crystalline structure. The carbon is then hit with a single laser pulse lasting approximately 200 nanoseconds. During this pulse, the temperature of the carbon is raised to 4,000 Kelvin (or around 3,727 degrees Celsius) and then rapidly cooled. This operation takes place at one atmosphere – the same pressure as the surrounding air.

The end result is a film of Q-carbon, and researchers can control the process to make films between 20 nanometers and 500 nanometers thick.

By using different substrates and changing the duration of the laser pulse, the researchers can also control how quickly the carbon cools. By changing the rate of cooling, they are able to create diamond structures within the Q-carbon.

“We can create diamond nanoneedles or microneedles, nanodots, or large-area diamond films, with applications for drug delivery, industrial processes and for creating high-temperature switches and power electronics,” Narayan says. “These diamond objects have a single-crystalline structure, making them stronger than polycrystalline materials. And it is all done at room temperature and at ambient atmosphere – we’re basically using a laser like the ones used for laser eye surgery. So, not only does this allow us to develop new applications, but the process itself is relatively inexpensive.”
And, if researchers want to convert more of the Q-carbon to diamond, they can simply repeat the laser-pulse/cooling process.

If Q-carbon is harder than diamond, why would someone want to make diamond nanodots instead of Q-carbon ones? Because we still have a lot to learn about this new material.

“We can make Q-carbon films, and we’re learning its properties, but we are still in the early stages of understanding how to manipulate it,” Narayan says. “We know a lot about diamond, so we can make diamond nanodots. We don’t yet know how to make Q-carbon nanodots or microneedles. That’s something we’re working on.”

NC State has filed two provisional patents on the Q-carbon and diamond creation techniques.
The work is described in two papers, both of which were co-authored by NC State Ph.D. student Anagh Bhaumik. “Novel Phase of Carbon, Ferromagnetism and Conversion into Diamond” will be published online Nov. 30 in the Journal of Applied Physics. “Direct conversion of amorphous carbon into diamond at ambient pressures and temperatures in air” was published Oct. 7 in the journal APL Materials.