In less than 100 seconds, James Verdon explains the principle of trapping carbon dioxide in rocks.
Coated quantum dots make superior solar cells
Researchers at the University of Toronto in Canada and KAUST in Saudi Arabia have made a solar cell out of colloidal quantum dot (CQD) films that has a record-breaking efficiency of 7%. This is almost 40% more efficient than the best previous devices based on CQDs.
CQDs are semiconductor particles only a few nanometres in size. They can be synthesized in solution, which means that films of the particles can be deposited quickly and without fuss on a range of flexible or rigid substrates – just as paint or ink can be.
CQDs could be used as the light-absorbing component in cheap, highly efficient inorganic solar cells. In a solar cell, high-energy photons hitting the photovoltaic material can produce excited electrons and holes (charge carriers) that have energies at least equal to or greater than the band gaps of the material. The advantage of using CQDs as the photovoltaic material is that they absorb light over a spectrum of wavelengths. This is possible because the band gap of a CQD can be tuned over a large energy range by simply changing the size of the nanoparticles.
Trapped electrons
There is a snag, however – the high surface-area-to-volume ratio of nanoparticles results in bare surfaces that can became “traps” in which electrons invariably get stuck. This means that electrons and holes have time to recombine instead of being whisked apart to produce useful current. The result is a reduction in the efficiency of devices made from CQD films. A team led by Edward Sargent at Toronto may now have come up with a solution to this annoying problem. The researchers have succeeded in passivating the surface of CQD films by completely covering all exposed surfaces using a chlorine solution that they added to the quantum-dot solution immediately after it was synthesized. “We employed chlorine atoms because they are small enough to penetrate all of the nooks and crannies previously responsible for the poor surface quality of the CQD films,” explains Sargent.
The team then spin cast the CQD solution onto a glass substrate that was covered with a transparent conductor. Next, an organic linker was used to bind the quantum dots together. This final step in the process results in a very dense film of nanoparticles that absorbs a much greater amount of sunlight.
Boosting absorption
“Our hybrid passivation scheme employs chlorine atoms to reduce the number of traps for electrons associated with poor CQD film-surface quality while simultaneously ensuring that the films are dense and highly absorbing thanks to the organic linkers,” Sargent says.
Electronic-spectroscopy measurements confirmed that the films contained hardly any electron traps at all, he adds. Synchrotron X-ray scattering measurements at sub-nanometre resolution performed by the scientists at KAUST corroborated the fact that the films were highly dense and contained closely packed nanoparticles. “Most solar cells on the market today are made of heavy crystalline materials,” explains Sargent, “but our work shows that light and versatile materials such as CQDs could potentially become cost-competitive with these traditional technologies. Our results also pave the way for low-cost photovoltaics that could be fabricated on flexible substrates, for example using roll-to-roll manufacturing (in the same way that newspapers are printed in mass quantities).”
Exploring new materials
The team is now looking at further reducing electron traps in CQD films for even higher efficiency. “It turns out that there are many organic and inorganic materials out there that might well be used in such hybrid passivation schemes,” adds Sargent, “so finding out how to reduce electron traps to a minimum would be good.”
The researchers say that they are also interested in using layers of different-sized quantum dots to make a multi-junction solar cell that could absorb over an even broader range of light wavelengths.
The research is described in Nature Nanotechnology.
Geologist claims to have found plate tectonics on Mars
A geologist in the US claims to have found the first strong evidence for plate tectonics on Mars by studying satellite images of a huge trough in the Martian surface. It had been thought, until now, that tectonic movements were only present on Earth.
An Yin, professor of geology at the University of California, Los Angeles, spotted the tectonic activity in Valles Marineris – a 4000-km-long canyon system named after the Mariner 9 Mars orbiter that discovered the system in the 1970s. Valles Marineris stretches one-fifth of the way round the Martian surface and reaches depths of up to 7 km. The Earth’s 1.6-km-deep Grand Canyon is a mere surface scratch in comparison.
The formation of Valles Marineris is still not understood despite four decades of research. The most widely accepted theory is that spreading apart of the Martian surface created the system, similar to how rift valleys form on Earth, with the resulting crack being deepened by erosion. But Yin has now found evidence for a completely different process.
Looking for clues
Yin made use of high-resolution images taken by several Mars orbiters, including NASA’s Mars Odyssey and Mars Reconnaissance Orbiter. He focused particularly on the southern region of Valles Marineris, where a 2400-km-long trough connects three large canyons: the Ius, Melas and Coprates Chasmata. He painstakingly trawled through these images to look for “kinematic indicators” on the Martian surface – marks that reveal how the crust has moved. He discovered faults in the Ius Melas Coprates trough with a consistent, slanted orientation, which indicates a horizontal, shearing motion. He also noticed “headless” landslips at the bottom of the trough – that is, landslips without any traceable source, possibly caused by a horizontal movement of the crust since the landslips occurred.
Furthermore, Valles Marineris is exceptionally long and straight. “On Earth, there is only one kind of fault that can make a very straight and linear trace,” says Yin, “and that’s a ‘strike-slip’ fault – a fault that’s moving horizontally over a very large distance.” He also adds that the rocks on both sides of Valles Marineris are extremely flat, whereas rocks near a rift tend to be tilted.
California on Mars
Yin studied the offsets of three surface features around the fault zone to estimate the magnitude of the slip. All three measurements gave roughly the same value – 150 km – for the total distance moved by the fault. By comparison, the San Andreas Fault in California has moved around 300 km, meaning that, when scaled by the planets’ radii, the two faults are similar (the radius of Earth is around twice that of Mars).
All of Yin’s evidence points to a strike-slip system at a plate boundary, otherwise known as a transform fault. “If you have rigid blocks on the lithosphere of a planet that move horizontally over a large distance, then that’s plate tectonics,” says Yin. He names the two plates “Valles Marineris North” and “Valles Marineris South”.
“Clearly, if the reconstruction is right, this is a large transform fault,” says Norm Sleep, professor of geophysics at Stanford University. Sleep also comments that the fault should have “a net subduction effect at one end and a net spreading effect at the other”.
“The eastern end is a ‘spreading centre’ without eruption of volcanic rocks,” Yin confirms, “whereas the western end is an extensional zone filled with volcanic rocks.”
Primitive tectonics
Yin believes that the Valles Marineris fault zone is still active today but that tremors – or “Marsquakes” – are likely to be rare occurrences. “If our history of Mars is correct, everything has evolved very slowly, tectonically,” he says, “so the fault found in Valles Marineris may wake up once every million years.”
This slow geological pace may explain why the red planet is at a primitive stage of plate tectonics compared with the Earth. Yin notes that the plate-tectonic activity on Mars is localized, covering only around 20–25% of the Martian surface – the rest of Mars reveals no signs of tectonic activity.
So why do Earth and Mars have plate tectonics but not Mercury and Venus? Yin thinks that this is related to the density of a planet’s crust during its early formation, which would determine whether fractured pieces of crust could subduct into the underlying mantle. He hopes to publish this hypothesis in a future paper.
The research is described in Lithosphere.
What is a black hole?
In less than 100 seconds, Andy Young explains why not even light can escape from black holes.
New form of carbon is so hard it can indent diamond
A new form of carbon that is hard enough to indent diamond has been created by a team of researchers from the US and China. The new material, known as ordered amorphous carbon clusters (OACC), is structurally unique in having both crystalline and disordered elements. Created by a team led by Lin Wang of the Carnegie Institution for Science in the US, the material was made by subjecting solvated carbon-60 molecules to phenomenal pressures more than 300,000 times that of the atmosphere.
Carbon comes in many guises, including graphite, diamond, nanotubes, graphene and charcoal. But until now, all have been classified as either crystalline – built from repeating atomic units – or amorphous, that is, lacking the long-range structural order seen in crystals. As a crystalline material composed of amorphous clusters, OACC is the first hybridized carbon structure ever seen that is part amorphous and part crystalline.
Collapsing carbon cages
To make the new form of carbon, the researchers started with molecules of carbon-60 – highly organized spherical cages that resemble footballs because they are built from pentagonal and hexagonal rings of carbon. Wang and colleagues then inserted molecules of an organic solvent, m-xylene, in-between the balls, before compressing the material to immense pressures of more than 32 GPa. On inspection, the spherical carbon cages were found to have broken and collapsed in on themselves to become amorphous carbon clusters that remained locked in their places in a lattice by the solvent molecules.
“The solvent molecules play a crucial role,” explains Wang. “For pure carbon-60, when the carbon cages collapse, the entire crystal turns amorphous. But in this material, because there are some solvent molecules there, even when the carbon cages collapse, they don’t move around.”
Making an impression
But what really astounded Wang and colleagues was when they squeezed the new form of carbon to pressures of up to 60 GPa between the tips of a pair of diamonds in a diamond-anvil cell. When they released the pressure and then inspected their kit, they found that the diamonds – the hardest material in nature – had actually been indented by their sample.
The hardness of diamonds can be attributed to the fact that each carbon atom is joined to its neighbours by a quartet of strong covalent bonds. Simon Parsons, a molecular crystallographer at the University of Edinburgh in the UK who was not involved with the study, is intrigued that a molecular structure can approach this sort of hardness. “I would expect diamond with its fully 3D extended structural framework to be harder than something with occluded solvent molecules,” he says. “But that is not the case here.”
Molecular simulations backed up what the researchers found in the lab, namely that when pressures of up to 30 GPa are applied and removed, the carbon cages bounce back to their original crystalline shape. But with pressures of more than 32 GPa, the material undergoes a permanent transformation, with the bonds in the carbon-60 cages breaking and reforming. When the pressure is removed, the new superhard structure can be recovered to ambient conditions still intact. When the researchers heated the OACC to drive off the solvent, its long-range order disappeared and it was reduced to its collapsed disordered building blocks, thus further confirming the crucial role of the solvent in providing OACC’s periodicity.
The hard line
One potential advantage of the new material is that it is made at room temperature. But whether it could be economically competitive with synthetic diamond, which is created at temperatures of about 1500°C, remains to be seen. Nevertheless, the researchers believe that OACC could have a range of potential mechanical, electronic and electrochemical uses.
“We know that when we dope carbon-60 with some alkali metal, it can transform into a superconductor,” says Wang. “We do not yet know, but we can expect that [the new material] should have some interesting electronic properties too.”
For now, Wang and his team are continuing to test OACC’s properties and probe the atomic structure of its collapsed carbon clusters, as well as investigating what materials can be fabricated with alternative solvent molecules at high pressure.
It is this aspect that Parsons is most excited about. “The thing that stands out for me about this work”, he says, “is that carbon-60 will crystallize with various different solvents, and those solvates will each have a different periodicity, which enables you to engineer different crystal structures by changing the solvent.”
The research appears in Science.
What is the most important criterion when choosing a postdoc position?
By Tushna Commissariat
We’re taking a slightly different tack with our Facebook polls over the next few weeks, with a series of polls focused on careers. More specifically, we’re after your views on postdocs – that crucial stage of an academic physicist’s career that lies between earning a PhD and finding (or, in many cases, not finding) a permanent academic post.
This is a topic that’s been in the news a lot lately, with a growing number of voices arguing that there is something seriously wrong with an academic career path that supports large numbers of PhDs and postdocs but produces very few permanent or tenure-track academic jobs for them to move into. We’ll look into that a bit more over the next few weeks, and if you have a personal experience of postdoc-hood – good or bad – that you’d like to share, please get in touch via pwld@iop.org. For this week’s Facebook poll, however, we’ve got a comparatively simple question that anyone – not just postdocs – can answer.
What is the most important criterion when choosing a postdoc position?
Location
Institutional resources
Prestige of supervisor
Prestige of institution
Have your say by visiting our Facebook page, and please feel free to explain your response or give us more suggestions by posting a comment below the poll.
Last week we asked you which planet from our solar system, apart from the Earth, you find the most captivating. Unsurprisingly, Mars came out on top with about 38% of votes, followed by 20% for Jupiter, 14% for Venus, and 12% for Saturn, while 10% of you felt that exoplanets are a much more interesting option. Sadly, Uranus got only four votes, Neptune three votes and Mercury just the one vote. So it looks like Mars is still captivating Earth.
Thank you to everyone who took part and we look forward to hearing from you again in this week’s poll.
Giving physics some soul
The Congregation performing live (Hooper is fourth from right). (Courtesy: D T Kindler)
By Michael Banks
It seems as if Fermilab physicist Dan Hooper has finally hit the big time. Not for his latest theory on the Higgs boson or dark matter but rather through his involvement in the soul band The Congregation.
Guitarist Hooper formed the band about three years ago and it now consists of a drummer, bass player, singer, horn player and keyboard player.
On 9 August the “60s-era soul band” opened a joint gig by the US rock bands Garbage and the Flaming Lips in Madison, Wisconsin. “The show went great – although we did get some rain,” Hooper told physicsworld.com. “We were well received, and had a great time.”
Not resting on their laurels, the band is getting ready to release its latest album on 28 September. Right Now Everything will be available to buy on the band’s website.
Hooper, who goes by the stage name Charlie Wayne and who also writes the band’s lyrics, says that the band steers clear of anything physics related, as well as any rock-band antics. “We don’t do a lot of smashing guitars and such anymore,” says Hooper.
So will the band’s success force Hooper to give up his physics career? “I can’t imagine doing that,” he says. “Doing physics is the best job someone like me could have – even compared with playing rock and roll for a living.”
First room-temperature maser developed
British researchers have, for the first time, built a prototype solid-state maser that works at room temperature, with no permanent applied magnetic field. Masers, which do the same thing with microwave radiation that lasers do with visible light, have not become widely used thanks to their difficult operating conditions – some require cryogenic refrigeration or vacuum chambers and sometimes strong magnetic fields. The researchers claim that their device could have a range of applications in the future – from the detection of explosives to detecting the atomic states of atoms in quantum computing.
Extreme conditions
There are two basic types of masers. Atomic and molecular masers were the first type to be invented back in 1958. They require bulky vacuum chambers and can only emit very low powers. The second and more useful type – solid state masers – exploit transitions between spin states of paramagnetic ions in a solid crystal. They are far more powerful and can produce perhaps the most sensitive, low-noise detectors of faint microwave signals yet developed. Unfortunately, to sustain the necessary population inversion in a conventional solid-state maser requires liquid-helium refrigeration, usually accompanied by a strong DC magnetic field.
The need for these extreme conditions has meant that, while NASA has been willing to invest in maintaining solid-state masers to receive the faint signals transmitted by the Voyager space probes, more everyday applications have been ruled out. “For example, you could use a maser to improve the accuracy of an airport body scanner,” says lead author Mark Oxborrow of the National Physical Laboratory in Teddington, UK, “but that would increase the cost of the device considerably. So I think there are many applications that have just been rendered impracticable by the requirement of cryogenics.”
New operating mechanisms
Oxborrow and colleagues at Imperial College London produced their maser by substituting a soft polymer – p-terphenyl doped with pentacene – for the usual crystalline ruby as the gain medium. In addition, instead of pumping it with a microwave source, as is traditional for a solid-state maser, they used a 585 nm medical laser designed for the treatment of vascular lesions. These changes allowed them to utilize a phenomenon known as “spin-selective intersystem crossing”, which had never been used in a maser and is still not completely understood, to sustain the population inversion in the absence of cryogenic temperatures or a strong, permanent magnetic field. “It is not just that we have taken the traditional technology and just improved things in various directions to get it to work at room temperature.” explains Oxborrow. “The operating mechanism of our room-temperature maser is completely different from the conventional solid-state maser.”
Impressive but potentially problematic?
Aharon Blank, a chemist at the Technion-Israel Institute of Technology in Haifa, Israel, who was part of a previous, unsuccessful project 10 years ago to develop a room-temperature solid-state maser, is impressed by the research. However, he points out a number of aspects of the design that could potentially prove problematic. First, although the device can operate at zero field, a magnetic field is required to tune the magnetic field at which it operates. While inconvenient, he does not believe this would prove fatal for a commercial device based on the technology. “There are commercial devices in use today that use a static magnetic field to vary the frequency,” he says, “so that is not a major problem.”
One problem, however, is serious. At present, like the first lasers, the device is only capable of operating in pulsed rather than continuous mode. Masers are used mainly to detect and amplify very faint incoming microwave radiation, and the uses of a detector that cannot stay continuously on are limited. On the flip side, Oxborrow suggests that it could be used to listen for radar echoes, for example. The team are currently experimenting further with their device to ascertain whether or not it can be made to operate in continuous form and, if so, how this can be achieved.
The research is published in Nature.
What is antimatter?
In less than 100 seconds, Helen Heath explains why some particles have equal but opposite partners.
How do you recognize a penguin in a crowd?
In less than 100 seconds, Peter Barham explains how penguins possess unique coats.
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