By Hamish Johnston
If you are a fan of astronomy and the comedian David Mitchell, the Open University has a treat for you. Mitchell and the OU have made a series of 12 short animated videos about the physics of the cosmos.
What is a thermopower wave?
In less than 100 seconds, Michael Strano explains how thermopower waves could be exploited to create smaller and more-efficient batteries.
Scientists delve deeper into carbon nanotubes
The outer walls of both double- and triple-walled carbon nanotubes (CNTs) protect the innermost tubes from interacting with their environment. That is the key finding of a study by researchers in the US, Germany and Japan, who have made the first detailed examination of triple-walled CNTs using resonant Raman spectroscopy. The protection afforded by the outer layer allows the tiny tubes to be studied in more detail than ever before, which could be a boon to those using CNTs to create new technologies.
A single-walled carbon nanotube (SWCNT) resembles a tiny drinking straw with a wall that is just one carbon atom thick. A double-walled carbon nanotube (DWCNT) consists of two concentric SWCNTs coupled together by weak Van der Waals interactions. The inner and outer tubes can either be semiconducting or metallic. However, because the outer tube is in direct contact with its environment, it can be difficult to obtain accurate information about its fundamental physical properties.
Third wall protects the second
To gain a better understanding of the outer tube in a DWCNT, Thomas Hirschmann and Paulo Araujo at the Massachusetts Institute of Technology and colleagues studied individual and bundled triple-walled carbon nanotubes (TWCNTs). A TWCNT can be thought of as a DWCNT wrapped around a SWCNT. The researchers found that the extra outer tube protects the two inner ones from interacting with their environment, thus allowing them to be studied more accurately. An unrolled TWCNT can be thought of as a trilayer graphene ribbon, and has all the outstanding electronic and mechanical properties that this carbon material boasts.
The team was led by MIT’s Mildred Dresselhaus and included scientists from the University of Hamburg, the Nagaoka University of Technology and Shinshu University. The researchers used a very fast yet sensitive Raman spectrometer, which allowed them to detect and characterize the same individual TWCNT with different laser lines under identical experimental conditions. “Only a few groups in the world are equipped with such an instrument capable of characterizing individual CNTs in this way,” said Hirschmann.
Wall-to-wall measurements
“The analyses allowed us to study fundamental properties such as intertube mechanical coupling, wall-to-wall (WtW) distance, metallicity and curvature-dependent intertube interactions,” he explained. “Such knowledge will be of fundamental importance for technological applications that exploit these nanostructures.”
The researchers characterized five individual TWCNTs in detail and found that the WtW distance between the inner two tubes in all the samples ranges from 0.323 to 0.337 nm. These values are larger than the WtW distance observed in previously studied DWCNTs (0.284–0.323 nm). The distances are also closer to the interlayer distance in graphene (0.335 nm).
“We also found that the intertube interactions affect innermost nanotubes differently, according to which metallicity they have, and that the elusive mechanical coupling between the ‘radial breathing mode’, or RBM, of concentric nanotubes does not exist, even for relatively short WtW distances of 0.323 nm,” added Hirschmann. “This is an important finding and shows that, although the TWCNTs are hybrid systems, the tubes themselves are mostly independent of one another.”
Wealth of information
The RBM is the most important spectroscopic signature of a CNT, the frequency of vibration of which is known to be inversely proportional to the tube diameter, he explained. These so-called first-order Raman features provide a wealth of information on the electronic and vibrational structure of these nanomaterials.
“Our analyses also shed more light on the Van der Waals forces mediating the interactions in concentric ordered CNTs, such as DWCNTs and TWCNTs,” said Araujo. “These low-energy interactions are important for technology applications because they affect the electronic and vibrational properties of the tubes.”
The team is now busy analysing shielding phenomena and intertube interaction effects in multi-walled carbon-nanotube systems. Here, intertube interactions not only affect the measured RBMs but also other Raman features. “One of our main goals is to find better conditions in which to grow CNTs by controlling interactions between nanotubes walls,” said Hirschmann. “To this end, we are working closely with Yoong Ahm Kim and colleagues at Shinshu University, who are experts when it comes to synthesizing these nanomaterials.”
The research is described in ACS Nano.
Of mice and ‘little green men’
There’s nothing quite like mentioning extraterrestrials or aliens to get us “Earthlings” all excited or riled up! Late last week, a paper popped up on arXiv, by astronomer Alan Penny from the University of St Andrews. He outlines an incident where, for a short while, the possibility of alien contact was seriously considered. He was talking about what was ultimately the discovery of the first pulsar; but at the time the researchers couldn’t help but wonder if they had come across the first “artificial signal” from outer space.
The exciting happenings began in August 1967, when Jocelyn Bell Burnell (then a graduate student working with Antony Hewish – controversially, only Hewish won the Nobel prize for the pulsar discovery in 1974) at the University of Cambridge, noticed a particular source that had a “flickering pattern” that, over a few weeks, she realized showed up regularly each day at the same sidereal time. That December Bell pinpointed the specific position of the source in the sky using another telescope and the discovery was confirmed. In the coming months, three more similar patterns were found and the researchers agreed on “pulsating stars” or pulsars being the source. But during those winter months, the possibility that they had encountered the first alien signal loomed large. In fact, Brunell and colleagues dubbed the first pulsar LGM-1 or “Little Green Men”; although it was changed to CP 1919, and is now known as PSR B1919+21.
In a later article, Brunell said that she and the rest of the team “did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission”. She continues, “It is an interesting problem – if one thinks one may have detected life elsewhere in the universe, how does one announce the results responsibly?”
This is the main subject that Penny tackles in his paper. He talks of researchers put into a position where they had to seriously consider how to tell the world about what might have been an alien signal. His paper contains some first-hand recollections of the people involved in the field at the time, including himself – he was a final-year undergraduate student at Cambridge in 1967.
Penny points out that a lot of the discussion the team had on how to confirm and release their potential finding ultimately agreed with the international guidelines that now exist in the form of SETI’s Detection Protocol – first agreed upon in the 1990s. But Penny also considers the fact that a Reply Protocol – an answer or response to an artificial signal – although proposed, was never agreed upon.
In the light of the numerous exoplanets being discovered by Kepler and other missions, it might just be a good (if slightly too optimistic?) idea to take a new look at these protocols. If nothing else, it might make BBC executives feel more confident in allowing Brian Cox to point a telescope in the general vicinity of an exoplanet.
What exactly will be upgraded at the LHC?
By Hamish Johnston
It’s been quite a rollercoaster ride for physicists working on the Large Hadron Collider (LHC) at CERN. When the collider was first switched on in 2008 it suffered a major explosion when a superconducting connector failed – and was shut down for over a year for repairs. Then in 2010 the LHC began taking data and the excitement about the imminent discovery of the Higgs boson grew and grew – and then on 4 July last year, CERN physicists announced the discovery of a Higgs-like particle.
Physicists propose ‘wireless’ solar cells
A new type of solar cell that relies on a surprising property of certain insulators has been proposed by physicists in Austria, the US and Germany. The design relies on the discovery a decade ago that the interface between two insulating oxides can become metallic, which could eliminate the need for metal wires in solar cells. If the cost of producing layered structures of the oxides can be reduced, the research could lead to a new type of highly efficient photovoltaic cell.
In 2004 Harold Hwang and Akira Ohtomo made the remarkable discovery that when a layer of the insulator lanthanum titanate was grown on the insulator strontium titanate, a 2D electron gas forms at the interface causing it to become metallic. The phenomenon is caused by the accumulation of charge at the edge of a polar oxide as it meets a non-polar oxide. It has since been seen in other oxide interfaces and has been investigated by multiple research groups trying to develop new and improved electronic devices.
Conducting interfaces
Now, an independent team of researchers at the Vienna University of Technology, the Oak Ridge National Laboratory and the University of Würzburg has done calculations that suggest that the effect could be used to create a new type of solar cell – one in which the generated current is extracted via conducting interfaces rather than with metal wires.
Solar cells rely on the photoelectric effect, where a photon striking an electron in the valence band of a material promotes it to the conduction band, leaving a positively charged “hole”. The electrons and holes must be removed from the photovoltaic material without recombining or dissipating their energy into lattice vibrations.
Polar oxides such as lanthanum titanate contain an internal electric field and positively and negatively charged planes of atoms. Oak Ridge’s Satoshi Okamoto and colleagues reasoned that this polarization would help separate the electrons and the holes before they could recombine. If such a polar oxide is paired with the appropriate non-polar oxide, the interfaces would be metallic. As a result, electrons and holes could be extracted from either side of the device without covering the surface with wires, which block some of the light from reaching the active region of the cell.
Maximizing absorption
The researchers first needed a polar oxide that would absorb as much solar energy as possible. A material’s band gap is the energy difference between its valence and conduction bands. Photons with energies less than the band gap cannot create electron–hole pairs, whereas photons with energies greater than the band gap will create pairs. In the latter case, however, energy in excess of the band gap is lost as heat. As a result, the band gap should therefore be low enough to absorb plenty of solar photons, but high enough to extract as much energy as possible from the photons absorbed.
The researchers settled on lanthanum vanadate, which has an band gap of 1.1 eV – visible light is in the 1.5–3.5 eV energy range. They used density functional theory to model the behaviour of a solar cell constructed of a layer of lanthanum vanadate grown on a strontium titanate substrate. While they were unable to make precise predictions about device efficiency based on their results, the researchers suggest that the inherent advantages of the design deserve further investigation.
Capturing higher-energy photons
The researchers also suggest that the efficiency of the solar cell could be increased further by incorporating a layer of lanthanum ferrate on top of the lanthanum vanadate. Lanthanum ferrate has a band gap of 2.2 eV, so higher-energy photons could be captured in this layer, leaving the lower-energy photons to be captured by the lanthanum vanadate. A project to produce prototype solar cells is under way at the University of Würzburg.
Optimistic but cautious
Okamoto is cautiously optimistic about whether the solar cells could ever be efficient enough to make them economically viable. “They could become competitive, but it will take quite a long time,” he says. “Currently, only a limited number of facilities can grow this kind of heterostructure using very advanced thin-film growth methods. I hope that when people fully understand how best to grow these solar cells the cost will come down.”
Neil Greenham, who works on novel solar cells at the University of Cambridge, describes the research, published in Physical Review Letters, as “an interesting theory paper” but emphasizes it will be impossible to assess whether or not the solar cells will have any practical advantages over current designs until working prototypes are produced. He also questions the assertion that simply incorporating two epitaxial layers could allow electron–hole pairs to be collected with two different energies, suggesting that, unless the electron–hole pairs could be extracted separately, any extra energy captured by an electron in lanthanum ferrate would be dumped into the lattice as it passed through the lanthanum vanadate.
Okamoto responded that photogenerated electrons and holes should cross a thin film of lanthanum vanadate in “a few femtoseconds”, which is less than the time it would take to lose energy to the lattice.
Hunting the Higgs
By Michael Banks in Boston
“It looks like a Standard Model Higgs,” remarks Christopher Hill from Ohio State University. “Everything we have measured has strengthened that position.”
Last year, researchers working at the Large Hadron Collider (LHC) at CERN reported they had found a Higgs-like particle with an energy of around 126 GeV.
Yet while the Higgs looks like that predicted by the Standard Model of particle physics, further measurements were needed before researchers could be sure.
Why are carbon nanotubes so strong?
In less than 100 seconds, Michael Strano explains how carbon nanotubes get their strength.
Shining a light on dark energy
Dust is annoying, particularly when you want to obtain a precise measurement of the expansion of the universe.
Today, Robert Kirshner from Harvard University gave a plenary lecture at the 2013 AAAS meeting in Boston giving participants a tour of the latest in dark-energy research.
Kirshner is a member of the High-Z team that some 15 years ago used observations of supernovae to discover that the expansion of the universe is accelerating.
Indeed, his former students – Brian Schmidt and Adam Riess – shared the 2011 Nobel Prize for Physics together with Saul Perlmutter for this discovery.
Riess, a graduate student at the time, played an important part in figuring out how to account for dust when measuring supernovae distances. This dust surrounding a supernovae is annoying as it absorbs light, which introduces uncertainties in deducing how far away supernovae are.
Reiss managed to account for this well enough to measure the brightness of supernovae to a reasonable precision that could then be used to deduce the accelerating expansion of the universe; but now Kirshner’s team is planning to go a few steps further by doing better measurements.
This will be done by using the Hawaii-based Pan-STARRS telescope to find candidate supernovae and then follow up the measurement in the infra-red to “see” through the dust cloud and get a better measure of the supernova’s brightness.
Kirshner’s team has just started to use the Hubble Space Telescope to do this kind of measurement in the infra-red.
Dark energy is thought to account for around 73% of the total mass-energy of the universe, but Kirshner thinks this new technique will give researchers a more precise measure.
Further ahead, Kirshner is looking forward to more enhanced infra-red observations of the universe by using the James Webb Space Telescope as well as the ground-based Giant Magellan Telescope, which he is involved in constructing.
Japan 101
By Michael Banks in Boston
There is certainly a big presence from Japanese research bodies at the 2013 AAAS meeting in Boston.
In the exhibitors’ hall, the World Premier Institutes (WPI), RIKEN and the Okinawa Institute for Science and Technology all share a large central stall plugging their research and facilities.
Indeed, this presence may well be part of Japan’s drive to increase the number of foreign researchers and students in the country by actively highlighting its top research and facilities, a topic Physics World touched upon in a special report published last September.
After a brief chat at the WPI’s stall, I was handed a book called The Challenging Daily Life, which is published by the WPI’s International Center for Material Nanoarchitechtonics (MANA).
The 136-page book, featuring Japanese-style cartoons, introduces problems that foreigners in Japan come across in their everyday lives, and gives information and hints about how these problems can be solved.
The book features 34 different “episodes” – all based on real experiences of MANA staff – such as the “nightmarish bad weather” in Japan or how to deal with an invitation to a wedding.
My favorite case study, though, is “don’t overeat”, which seems to basically tell people that if you eat too much you will get fat and that while Japanese food is healthy, eating too much will result in weight gain.
I guess as long as it plays a role in making researchers from abroad more comfortable in Japan then it will have done its job.
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