Page 1224 – Physics World

Home » Page 1224

Electronic whiskers could help robots navigate

Researchers at the University of California, Berkeley have made highly sensitive, lightweight “electronic whiskers” that can detect the lightest of touches or a gentle breeze. Made from a mixture of carbon nanotubes and silver nanoparticles, the whiskers could be used to create “skin” for robots and in interfaces between humans and machines, says the team.

Animals use their whiskers to gauge the wind and to navigate around obstacles. The new electronic whiskers, or e-whiskers, made by Ali Javey’s team could be the next best thing to their natural counterparts in terms of size and weight. The researchers made the whiskers by painting composite films of carbon nanotubes and silver nanoparticles onto thin elastic fibres made of the polymer PDMA. The carbon-nanotube “paste” forms a conductive matrix that can be bent and unbent at will without suffering any damage. The silver nanoparticles further increase the conductivity of the composite and also make it highly sensitive to strain.

“The strain sensitivity and electrical resistivity of our composite film is readily tuned by changing the composition ratio of the carbon nanotubes and the silver nanoparticles,” explains Javey.

When the e-whiskers experience a light touch or a gentle breeze, they bend and their resistance changes dramatically. The structures are sensitive to changes in pressure of just 8% – the highest value reported to date for such tactile sensors.

Better-balanced robots

If mounted into arrays, the whiskers could be placed on robotic e-skin so that the machines are better able to navigate. They could also be used in human–machine interfaces, says team member Zhibin Yu. “They might even be ideal for some medical applications, for example in devices that monitor heartbeat and blood pressure,” he says.

The Berkeley team says that it is now looking to make the devices using different printing processes and produce them on a larger scale.

Structures such as these whiskers that mimic biological systems could help in the development of so-called smart and user-interactive electronics, explains Yu. Researchers have already made rudimentary e-skin and electronic eyes on thin, flexible substrates. Such devices are capable of “feeling” and “seeing” their local environment. “Electronic whiskers are another important class of sensor, capable of monitoring surrounding air flow and touch,” he says. “They can also spatially map nearby objects (just like naturally occurring whiskers) – a property that might help improve balance in robots of the future.”

Javey and colleagues describe their research in PNAS.

This article first appeared on nanotechweb.org

Intelligent life on a doughnut, how cats and skiers spin, a marriage made at CERN and more

By Hamish Johnston

There’s definitely an educational vibe to this week’s picks from the Red Folder, which begins with Tanner Higgin’s selection of “Five apps that test your physics skills“. Writing on Mind/Shift, a website based in California and dedicated to learning, Higgin highlights Crayon Physics Deluxe, which allows users to draw physical objects and then let gravity and other physical effects take over. Also featured is Amazing Alex, which allows users to combine more than 30 different household objects to create fantastical Heath Robinson/Rube Goldberg contraptions.

(more…)

Pyramid powers polarizing light source using quantum dots

A new way of generating linearly polarized photons using quantum dots has been developed by an international team of researchers. The photons are emitted from a quantum dot at the top of a semiconductor “micropyramid”, creating an efficient source of polarized light that could be used to develop energy-saving computer and mobile-phone screens, as well as wiretap-proof communications.

A quantum dot (QD) is a semiconductor nanocrystal that displays quantum-mechanical properties thanks to its minute size. The electronic characteristics of QDs are dependent on the size and shape of the individual crystal. As their size and band gap are inversely related, the excitation and photon-emission properties of QDs are highly tuneable.

Predefined polarization

Researchers have long studied QDs and their many possible applications, which include medical imaging and disease detection, LEDs, solar cells and photovoltaics. QDs could also prove to be useful in quantum computing and quantum encryption because they are capable of emitting single photons. But getting the QDs to emit photons that have a predefined polarization has proved difficult and this is an essential requirement for many applications such as LCD screens.

Most devices that create polarized light do so by sending unpolarized light through a polarizing filter. However, at least half of the light (and so an equal amount of energy as heat) is lost in the process, thus making it rather inefficient. To avoid these losses, a better method would involve directly generating polarized light from the source itself. While researchers have known that this is possible with QDs, until now the polarization has either been weak or hard to control and the methods to do it are complex and cumbersome.

But now, Per Olof Holtz of Linköping University in Sweden and colleagues, along with other researchers in Thailand, have come up with an alternative method where asymmetrical quantum dots of indium gallium nitride (InGaN) are grown on top of elongated microscopic six-sided gallium nitride (GaN) pyramids. It is the predefined elongation of the micropyramid that determines the polarization direction of the emitted photons. “We’re demonstrating a new way to generate polarized light directly, with a predetermined polarization vector and with a degree of polarization substantially higher than with the methods previously used,” says Holtz.

Top of the pyramid

The researchers constructed arrays of GaN micropyramids, one atom layer at a time, and coated them with layers of indium just a few nanometres thick. Asymmetrical InGaN quantum dots form at the apex of each pyramid. The team then used a continuous-wave ultraviolet laser operating at 266 nm to excite the QDs and found that photons are emitted with a well-defined wavelength and that the light showed a high degree of linear polarization, on average 84%.

Holtz told physicsworld.com that nitride-based semiconductors are favourable for the generation of polarized light because of their valence-band structure. “We have identified the split-off energy as the key parameter, which will determine the degree of polarization for a given asymmetry,” he says. “In principle, the material with the smallest split-off energy will exhibit the highest degree of polarization, because of enhanced band mixing.” He also points out that it is possible to control the direction of the polarization thanks to the elongated pyramidal dots – something that previous methods could not do.

Many advantages

While the QDs that the team used emit violet light with a wavelength of 415 nm, the photons can, in principle, take on any colour within the visible spectrum by simply varying the amount of the indium used. The researchers’ theoretical calculations also point to the fact that an increased amount of indium in the QDs further improves the degree of polarization. The team says that its method should allow the fabrication of ultracompact arrays of photon emitters, with a controlled polarization direction for each individual emitter.

The researchers point out that their method also has other advantages: their QDs can be used at high temperatures; a higher degree of polarization can be achieved using InGaN dots compared with other semiconductor materials; and the method is compatible with current electronic-processing techniques. Their results could help with the building of more energy-effective polarized LEDs in the light source for LCD screens, as well as in quantum-encryption technologies.

The research is published in the Nature periodical Light: Science & Applications.

Heat cloaks hide objects in 3D

“Heat cloaks” that hide objects from thermal energy and are made from readily available materials such as polystyrene and copper have been unveiled by two independent groups in Singapore. Whether these devices will have practical applications remains unclear, but one possible use for such heat cloaks could be to manage heat in electronic circuits and the batteries used in mobile devices.

Cloaking was first proposed in 2006 by Ulf Leonhardt at the University of St Andrews and by John Pendry of Imperial College London and collaborators at Duke University in North Carolina, all of whom pioneered the field of “transformation optics”. They showed that incident electromagnetic waves could be redirected around an object by metamaterials with tailored refractive indices. As far as an observer is concerned, the object is rendered invisible when surrounded by such a cloak – at least in principle, because practical cloaks are difficult to construct. In 2013 Sebastien Guenneau of the Fresnel Institute in Marseille, France, and colleagues at the Karlsruhe Institute of Technology in Germany applied these ideas to heat by building a 2D cloak based on “transformation thermodynamics”. However, 3D heat cloaks based on this concept are difficult to make because they require highly anisotropic materials of comparable thickness to the object to be cloaked.

Now, researchers in Singapore have sought inspiration from an alternative concept for an electromagnetic cloak that was first proposed by Andrea Alú of the University of Texas at Austin in 2009 and constructed in 2013. An object is wrapped in a thin “mantle” that scatters radiation with the opposite phase of the radiation scattered by the object. This makes it appear as if the object scatters no radiation and is therefore invisible. Such a cloak is necessarily imperfect for light, because the metamaterial can only cancel the scattering from dipolar polarized plane waves. More complex wavefronts such as the radial fronts emanating from a point source have higher-order polarization components including quadrupolar, octupolar and beyond that are not cancelled.

No polarization involved

Both Singapore teams realized that such considerations do not matter for a thermal cloak because heat is a scalar quantity, so a thermal front does not carry polarization. Therefore, a simple bilayer construction can preserve the cloaked object from heat while leaving the thermal propagation in the surroundings perfectly unperturbed.

One group is based at the National University of Singapore and is led by Cheng-Wei Qiu. This team made a cylindrical bilayer cloak using an outer layer of metal with a high thermal conductivity and an inner layer of thermally insulating expanded polystyrene. This was set in a block with a thermal conductivity intermediate between the two. The researchers derived a formula to calculate the thicknesses and conductivities of the various layers for a cloak of any radius. Using this formula, they constructed a cylindrical cloak inside the block. They heated one side to a temperature of 60 °C and cooled the other to 0 °C. Infrared photography showed that, as the heat diffused through the block, an aluminium cylinder placed inside the cloak stayed at a stable temperature; while outside the cloak, the heat flow was unperturbed. The researchers calculated that if the experiment could be made to work for a cylinder, it would also work for a sphere.

The other team is at Nanyang Technical University and is led by Baile Zhang. These researchers built their cloak by encasing a spherical pocket of air in a stainless-steel block by drilling two hemispherical holes in two identical steel blocks and placing them together. The team lined the air pocket with an ultrathin layer of highly conductive copper by placing a copper disc on top of each hemisphere and punching it into the hole with a moulding rod. This cancelled out the distortion to the heat flow from the air pocket and allowed the heat to pass through the block as though the air space were not present. An arbitrary object could be placed inside the air space, effectively replicating the bilayer set-up of the first team with air as the inner insulator. The Nanyang team demonstrated the effectiveness of its cloaking mechanism for a sphere by separating the two hemispheres at various times and measuring the temperature distributions in the steel blocks.

Mobile-phone applications

Both groups are keen to develop practical applications of their cloaks, particularly for heat management in electric circuits. Zhang suggests that such a cloak might be useful in mobile phones. “The battery is very sensitive to temperature and sometimes it can cause [an] explosion,” he says. “It’s very useful to protect such key components from the thermal flux without affecting other components.”

Alú agrees the work could be useful in complex, temperature-sensitive environments, but he believes the more important point of both this work and that of Guenneau is the extension of cloaking to cover diffusion problems. “So far, cloaking has been mostly focused on waves,” he says. “This work proves that cloaking can also be useful in the transient domain for diffusion problems. It would be interesting to see if we can bring it to other types of diffusion problems such as static currents and tomography problems, to see if cloaks may be useful there.”

Both papers are published in Physical Review Letters.

Ballistic electrons go further in nanoribbons

An international group of researchers has shown that electrons can travel more than 10 μm in graphene nanoribbons without scattering – which is much further than predicted by theory. Even more surprising, the team spotted a large jump in the electrical resistance of sections of nanoribbon that are longer than about 16 μm. These puzzling results could suggest that graphene harbours a new type of electronic transport mechanism hitherto unknown to physicists.

Graphene is a sheet of carbon just one atom thick that has shot to fame over the past decade after physicists worked out how to create freestanding flakes of the material. It is a semimetal that is an extremely good conductor of electricity because its electrons can travel very close to the speed of light. However, the presence of imperfections and impurities mean that an electron can travel only about 10 nm in a freestanding flake of graphene before scattering, which increases the electrical resistance of the material.

A promising way forward is to make devices using epitaxial graphene, which is a single layer of carbon grown on a substrate such as silicon carbide. As well as being amenable to commercial fabrication techniques, devices based on epitaxial graphene can be made with great precision and with small numbers of impurities and imperfections.

Ballistic electrons

Now, Walt de Heer and colleagues at the Georgia Institute of Technology – along with researchers in Germany, France and the US – have created pristine ribbons of epitaxial graphene just 40 nm wide. They have shown that electrons can travel along the edge of the ribbon without scattering. This behaviour is much like photons travelling along an optical fibre, or bullets fired from a gun – which is why the electrons are called “ballistic”.

The nanoribbons were created on a silicon-carbide wafer in a process that involves creating trenches in the wafers and then heating the wafer to a temperature of more than 1000 °C. This causes silicon atoms near the surface to evaporate and carbon atoms to migrate to the sloped walls of the trenches, where they form nanoribbons along the lengths of the trenches (see figure).

Bumping into the probe

Ballistic transport was confirmed by studying the electrical properties of the nanoribbons using a traditional four-probe measuring scheme. When a probe is attached to a nanoribbon, the ballistic electrons bump into it, which results in a huge increase in the electrical resistance of the nanoribbon. By contrast, a collision with the probe by a conventional conduction electron has a minimal effect on the resistance because these electrons will have already undergone many collisions as they travel along the nanoribbon. When a second probe is placed on the nanoribbons, the resistance increases dramatically again, thus confirming that the current flowing through the nanoribbon is being carried by ballistic electrons.

An important parameter describing ballistic transport is the average distance an electron travels before it collides with the atoms in a nanoribbon. Ballistic electrons encounter very little electrical resistance; but as soon as they scatter, the resistance increases. The team determined the “mean free path” travelled before a collision by measuring the increase in resistance of sections of a nanoribbon that varied in length from 1–5 μm.

Big surprise

This study revealed a small change in resistance over the distances studied and this allowed the team to extrapolate a mean free path of about 100 μm. However, when the researchers measured resistances over longer distances, they got a big surprise. At distances greater than 16 μm the resistance increased rapidly with length and it became clear that the mean free path was much shorter than the extrapolated value.

While the reason for this rapid increase remains a mystery, De Heer speculates that it could have something to do with the fundamental nature of the ballistic charge carriers. Instead of being elementary particles, he believes that the charge carriers could be quasiparticles – particle-like entities that arise out of interactions between electrons and the surrounding graphene lattice. A classic example is the Cooper pair of electrons that transports charge in some superconductors. De Heer suggests that charge-carrying quasiparticles in graphene could have a finite lifetime, which only allows them to travel about 16 μm in a nanoribbon before they decay.

The idea that new physics could be lurking in graphene is also backed up by the fact that the measured resistance of the team’s nanoribbons is much lower than that predicted by theory.

The next step for the researchers is to try to gain a better understanding of exactly how charge is carried in graphene. The team has already made a nanoribbon in which the current is split along two paths before being recombined. By studying the quantum interference that occurs in such devices, the researchers hope to determine important properties of the charge carriers and work out if these are indeed quasiparticles.

The research is described in Nature.

CERN kicks off plans for LHC successor

The CERN particle-physics lab near Geneva is putting plans in place to build a successor to its Large Hadron Collider (LHC). At a meeting to be held at the University of Geneva next week, some 300 physicists and engineers – including current CERN boss Rolf-Dieter Heuer – will discuss a range of options for a possible future collider. This includes plans for a massive next-generation circular collider – with a circumference of 80–100 km – that would accelerate protons to energies of about 100 TeV.

While the 27 km-circumference LHC has been colliding protons at energies of up to 7 TeV in the hunt for new particles since it first switched on in 2008, for more than 30 years physicists have been carrying out R&D on linear colliders that could one day be the LHC’s successor. One leading design effort is the International Linear Collider (ILC), which would accelerate electrons and positrons to about 250 GeV and smash them together at a rate of five times per second. Funding for the $8bn, 31 km-long collider has yet to be found, but Japanese particle physicists are already making moves to host this next-generation particle smasher.

Meanwhile, a design for a higher-energy machine – the Compact Linear Collider (CLIC) – that could operate at 3 TeV is being developed by a team at CERN. Construction of the ILC and CLIC could begin in the coming decade and they would, if built, study the Higgs boson in great detail through the “clean” collisions that can be made from colliding electron and positrons rather than smashing protons together.

Yet it remains unclear whether these machines will be built and physicists have recently been coming up with other proposals that involve circular colliders similar to the LHC. Such colliders do have some advantages, not least that physicists have a lot of experience in building them. In particular, from 1989 to 2000 CERN operated the Large Electron–Positron Collider (LEP), which was located in the same tunnel that now houses the LHC and was used to study the Z and W bosons in detail. “We need to keep our options open about what the next particle collider will be,” says John Ellis of Kings College London, who has been involved in designs for particle colliders beyond the LHC and will be speaking at next week’s meeting. “A bigger, more ambitious machine could offer us more capabilities.”

Towards TLEP

Delegates at next week’s Geneva meeting will discuss the technologies needed to create these future machines. One leading design for a next-generation circular collider is “TLEP”, which would be housed in an enormous new 80–100 km-circumference tunnel that would most likely be built in Geneva. It could initially collide electrons and positrons (as would both the ILC and CLIC) at energies of about 350–500 GeV. Most of the cost of such a machine would be in excavating the tunnel, with the accelerator itself only accounting for about one-third of the total.

Schematic of a proposed tunnel in Geneva — Blueprint for the future: An outline plan for a future tunnel in Geneva that could house a 80–100 km particle collider. (Courtesy: CERN)

Yet that same 100 km tunnel could then be used well into the future, eventually housing a proton–proton machine that could operate at an energy of up to 100 TeV, much in the same way as the LHC has used the LEP tunnel. This could then look for new particles – such as supersymmetric particles – that the LHC may yet discover. Researchers are planning to complete a conceptual design study for TLEP by 2017 as an input to the next review of the European strategy for particle physics.

Cost concerns

Although Ellis admits that the 100 km tunnel would involve an “enormous investment”, he thinks that the advantages would outweigh such concerns in the long run. “LEP was first approved in 1981 with the original tunnel designed to include the LHC in the future, so that it would be an infrastructure that could serve the community for at least 50 years,” says Ellis. “That is the same for the new tunnel: use it as an electron–positron machine and then later as a hadron collider.”

Indeed, the 100 km tunnel housing the collider could even be built so that it could allow two machines – one electron–positron and one proton–proton – to operate simultaneously, if needed. Ellis says that a preliminary engineering report has already been done on the 100 km tunnel. He claims it threw up no “major show-stoppers”, even if parts of it would be built underneath Lake Geneva. “The geology in the region is quite good for digging,” adds Ellis.

Yet Lyn Evans, who masterminded the construction of the LHC and is now responsible for overseeing the development of the ILC and CLIC, says that, for the moment, the top priority for CERN is the full exploitation of the LHC and its upgrade programme that will include boosting the luminosity and energy of the collider. “A machine of [TLEP’s] size will have a very high cost, so there must be a very strong scientific justification and international support,” he told physicsworld.com.

NSA keys into quantum computing

The US National Security Agency (NSA) has a classified programme to build a quantum computer that can break modern Internet security, according to documents leaked by the former NSA contractor Edward Snowden. The documents, which were published last month in redacted form by the Washington Post, have surprised few physicists working in the field. However, they have led to speculation about the status of NSA research and a renewed debate on the risks of developing quantum computers.

Quantum computers are devices that rely on quantum phenomena such as superposition, in which a system exists in multiple states at once, and entanglement, in which the states of two systems become inextricably linked. Unlike classical computers, which store bits of information in definite values of 0 or 1, quantum computers store information in quantum bits, or qubits, which are a superposition of both. When qubits are entangled, any change in one immediately effects changes in the others. Qubits can therefore work in unison and solve certain complex problems much faster than their classical counterparts.

Some of these problems are purely scientific in nature, such as simulating molecules inside biological cells, which could allow researchers to develop more effective drugs. But one problem quantum computers are expected to be most proficient at is factorizing large numbers. If successful, this would allow supposedly secure information on the Internet to be deciphered, including banking transactions, private messages and government files. Although in principle classical computers could perform the same deciphering, the process would usually take so long as to be unfeasible.

In 2006 the NSA openly announced the creation of a joint institute with the University of Maryland in College Park and the National Institute of Standards and Technology in Gaithersburg, both in the US, to develop quantum technology, including quantum computing. But the new documents reveal an additional classified effort at Maryland with the express purpose of breaking data encryption. They state that the NSA wants to build “a cryptologically useful quantum computer” as part of a programme titled “Penetrating Hard Targets”, which the Post claims has a budget of $79.7m (£48m).

Many physicists working in the field of quantum information believe quantum computing is exactly the sort of technology one would expect the NSA to develop. “If you put my level of surprise on a scale from zero to 10, where 10 is very, very surprised, my answer would be zero,” says Raymond Laflamme, a leading quantum-information theorist who is based at the University of Waterloo in Canada. “If they were not doing it, they would not be doing their job.” Even so, the news has confirmed for many others how important it is to find other ways to make digital information secure.

Unbreakable codes?

Encrypted information on the Internet exists on pages whose URL begins with “https://” as opposed to “http://”. It is based on public-key cryptography, which allows someone to send information to someone else by encoding it with a publicly available key. Although anyone on the Internet could intercept and read the message in its encrypted form, only the receiver, who holds a special, private key, can decipher it.

The most common type of public-key encryption is RSA, which was invented by the cryptographers Ron Rivest, Adi Shamir and Len Adleman in the late 1970s. In RSA encryption, both the public and private keys are derived from a pair of large prime numbers, the product of which anyone can find out. If you know the formula, you can in theory work backwards, factorizing the product until you discover the primes – but it is only realistically solvable if your computer is powerful enough.

Quantum computers could do that kind of factorization – and as Laflamme points out, it does not matter that they have not been properly realized yet. Information on the Internet can easily be stored, which indeed the NSA – as well as the UK Government Communications Headquarters (GCHQ), other intelligence agencies and private cloud-computing companies – is doing routinely anyway. That means information encrypted today could be deciphered in 10 years’ time – or whenever quantum computers are finally in use.

How much of a problem that poses depends on the sort of information you are encrypting, explains Laflamme, who gives the example of someone using a computer to buy something with a credit card. The development of a quantum computer is not a threat because in 10 years you will have changed your credit card, and unless you are buying something illegal, you will not care that the NSA knows. “But what if you’re sending the explanation of a new type of classified technology, one you want to keep secret for 20 years?” asks Laflamme. “Well, then it’s problematic.”

There are methods to future-proof the transfer of secret information. One is to create a communication network independent of the Internet through which users can share secret keys, which can then be used to encrypt and decipher messages on the Internet. The security of such networks can be improved further with quantum key distribution (QKD), which in theory guarantees the security of the key transfer – although the latest documents also reveal that the NSA is attempting to exploit practical loopholes in this, too, under a programme known as “Owning the Net”.

Vadim Makarov, who himself studies flaws in practical QKD systems at the University of Waterloo, says that cryptographers are also looking into classical “quantum-safe” encryption algorithms for use on the Internet. Like quantum computing, however, quantum-safe encryption and foolproof QKD systems, which cannot be cracked at all, are also taking time to develop and implement. “I just hope we won’t be too late,” he says.

Secret race

The NSA has not publicly responded to the leaks, but another question raised by the NSA documents is whether the agency could be further ahead in the development of quantum computing than major labs. The main reason functional quantum computers are expected to be many years away is that it is still very difficult to control qubits while protecting them from external interference that can all too easily destroy them. Moreover, no-one is yet sure what type of qubits are most likely to be practical, with physicists exploring types made from trapped ions, photons and superconducting circuits, to name but three.

According to the documents, the NSA expected its scientists to have demonstrated “dynamical decoupling and complete quantum control on two semiconductor qubits” by the end of September 2013. Purely on numbers, the agency would appear to be lagging behind major labs such as the Institute for Experimental Physics at the University of Innsbruck in Austria, which demonstrated entanglement of 14 atomic qubits as far back as 2010. On the other hand, control of qubits made of the semiconductor silicon is less advanced, with only single silicon qubits having been openly demonstrated since 2012. If the NSA has already succeeded in achieving control of two silicon qubits, then it may be ahead in that particular race.

The semiconductor mentioned in the documents could also refer to types of semiconductor that turn superconducting in certain regimes. But experimental quantum physicist Jonathan Home of ETH Zurich in Switzerland believes the NSA is indeed pursuing a regular semiconductor such as silicon, because the “dynamical decoupling” also mentioned – a type of noise mitigation – is not usually applied to superconducting qubits. If the agency is pursuing silicon, that might be because it is easy to build large arrays of silicon devices, Home says. But he adds that it is not so easy with silicon to implement the error correction that would make any devices function like proper qubits. “If I were an NSA manager, maybe I know the solid-state can be scaled up, so I pick that. But maybe I haven’t thought so hard about actual quantum computing,” he says.

If the NSA is developing quantum computers, does that mean that other intelligence agencies such as GCHQ are too? Physicists contacted by Physics World were not sure whether an agency outside the US would have the resources. But one point is obvious: over every development in quantum computing in the coming years, the spies will be watching.

Does boron-based graphene exist?

Two international teams of researchers have reached different conclusions about whether or not boron can form planar sheets similar to graphene. One team worked in the lab to identify hexagonal clusters of 36 boron atoms with a central hole. If joined together, the researchers believe that such structures could form a perfectly flat, atom-thick sheet that they have dubbed “borophene” – however, they have yet to see sheets. The second team used electronic-structure theory to calculate that such boron monolayers would spontaneously break down into bilayer structures. One of these bilayers would, the calculations suggest, have electronic properties similar to graphene, despite having a completely different atomic structure.

Graphene is a sheet of carbon just one atom thick that has attracted great interest because of its remarkable electronic and mechanical properties. Since graphene was first isolated in 2004, researchers have sought similar 2D materials, particularly those with different or complementary properties. One atom of interest is boron, which is the only non-metal in group three of the periodic table. Boron–boron bonds are not truly ionic, covalent or metallic. Instead, the bonds are both strong and highly delocalized. Like carbon, calculations suggest that boron should be able to form giant planar structures and fullerenes – structures such as buckyballs and nanotubes that have atom-thick skins. Indeed, boron nanotubes have been observed – although their precise structure remains uncertain. In 2007 Hui Tang and Sohrab Ismail-Beigi of Yale University predicted a stable planar boron structure containing regular hexagonal holes, which they called the α-sheet.

Frozen boron

Now, Lai-Sheng Wang and colleagues at Brown University in the US and Tsinghua University in China have tried to create graphene-like boron by using a laser to vaporize atoms from the surface of bulk boron. The vapour is then frozen rapidly with a jet of helium gas to form clusters of atoms. Mass spectrometry is used to separate out the negatively charged clusters and identify their masses, and therefore how many atoms they contain. Finally, the tiny particles are analysed using photoelectron spectroscopy, which measures how much energy is needed to liberate an electron from a nanoparticle.

The team focused on nanoparticles containing 36 atoms (B₃₆^–) and found that the electrons were strongly bound, implying that the clusters were very stable. To find out their structure, the researchers turned to computational chemistry. The most stable isomer they found was a perfect hexagonal structure with one atom missing at its centre. A similar structure with two missing atoms was slightly less stable. They calculated the predicted photoelectron spectrum of the most stable cluster and compared it with the spectra that they had obtained in the experiment. They found generally good agreement, which suggests that the most stable isomer was the one they had in fact seen. Minor deviations between the theoretical and observed spectra could be explained by the presence of some two-hole isomer structures in the samples.

The isolated B₃₆^– nanoparticle is not perfectly flat. However, if the atoms at the corners were removed to create further holes along the joins, this molecule would then become a section of the α-sheet that was predicted in 2007. Wang and colleagues have dubbed this material borophene. Tantalizingly, the delocalized nature of boron–boron bonding suggests that it could be fully metallic, which might make it an even better electrical conductor than graphene. Wang emphasizes, however, that their computer models cannot predict whether or not it would have the distinctive property that gives graphene such extraordinary electron mobility: the transport of electrons as though they were massless particles. Nor can the researchers tell chemists how to produce the structure.

Cannot exist in nature

Not everyone agrees with the team’s conclusion regarding the α-sheet. The crystallographer Artem Oganov points out that calculations done by his group at Stonybrook University in the US and colleagues in China suggest that the α-sheet cannot exist in nature. In a upcoming paper in Physical Review Letters, the team uses a new, highly successful algorithm for structural prediction developed by Oganov’s group to show that the α-sheet structure is “so unstable it will spontaneously do something to increase its thickness by putting more atoms on top of itself”.

Ball-and-stick illustration of a buckled boron bilayer — Ball-and-stick illustration of how boron atoms would be arranged in a buckled bilayer. The red and blue balls are boron atoms in non-equivalent positions. (Courtesy: Artem Oganov)

Among the more stable phases, the team calculated, would be a buckled bilayer structure called Pmmn-boron. Oganov and colleagues also calculate that electrons would travel in this material according to the Dirac equation, effectively making them massless particles. If confirmed, such a material would be the first planar “Dirac semimetal” not to have a graphene-like atomic structure, and the first in which the electronic conductivity within the plane was dependent on direction.

Oganov points out that his group’s own results do not directly contradict those of Wang’s team, as the latter claims only to have detected isolated B₃₆^– nanoparticles, not the complete α-sheet. “I have not looked at boron nanoparticles,” Oganov says, “but now I will, because I’m curious.”

Wang’s paper is published in Nature Communications. A preprint of Oganov’s paper is available on arXiv.

Room-temperature quantum dots emit single photons

Gallium-nitride quantum dots can emit single photons at room temperature, according to new experimental observations made by researchers in Japan. The findings prove once and for all that such gallium-nitride quantum dots, which are wide-band-gap semiconductors, can be employed as room-temperature single-photon sources. The structures might prove to be ideal for on-chip communications in quantum-information processors and could also be a source of “flying” qubits for the quantum computing of the future.

Quantum dots (QDs) made from gallium-nitride materials could have many potential applications thanks to their unique properties, which include the fact that they are very stable (both chemically and at high temperatures) and have a large breakdown voltage. They can also emit photons over a wide range of wavelengths from the ultraviolet to infrared parts of the electromagnetic spectrum.

Unfortunately, until now, no-one had ever seen single-photon emission from these materials at room temperature because sample quality was poor. “Although researchers have observed room-temperature single-photon emission from other nanostructures, such as the colour centres in diamond, this is the first time that the photons have been seen emanating from a structure in which the quantum emitter has been fabricated at a pre-defined location,” says team member Mark Holmes of the University of Tokyo. “Indeed, previous studies relied on structures that had formed at random locations on a substrate.”

Growing quantum dots

The Tokyo team, led by QD pioneer Yasuhiko Arakawa, fabricated its devices in a clean room using a process called “selective-area metal-organic” chemical vapour deposition. The researchers grew the quantum dots on sapphire substrates covered with a 25 nm layer of aluminium nitride. This process included sputtering a 25 nm deep silicon-dioxide layer onto the substrate surface, and then creating arrays of apertures (25 nm in diameter) using electron-beam lithography and reactive-ion etching. These apertures then house gallium nitride nanowires and quantum dots, which were grown separately. The researchers’ process allows them to control where each QD ends up on the substrate, since the in-plane position of every dot depends on where each nanowire is located in the first place and its distance from the substrate is defined by the nanowire height (which is about 700 nm). Arakawa and colleagues then measured the light-emission properties of their quantum dots by exciting them with a pulsed laser beam and detecting the light that came out.

Single check

In theory, a single-photon quantum emitter should emit a single photon per excitation light pulse. To test this, the researchers split the light emitted into two paths and, using two separate detectors, measured the time elapsed between the light pulses recorded at each detector. “For a pure single-photon source, we should not see photons at both detectors simultaneously – something that we verified in an experiment for the first time for this kind of site controlled gallium nitride quantum dot,” says Holmes.

More importantly, the observations hold up even when the quantum dots are at room temperature. Holmes explains that this is because the team is using small gallium-nitride dots the positions of which the researchers can control accurately. Such dots are less contaminated spectrally, which means that they can still be detected at such high temperatures.

Single-photon emitters are often touted as being ideal for quantum-cryptography applications, but the researchers believe that the devices they have made will be more suited to on-chip communications for quantum-information processors. “We are now busy looking at ways to measure the operating speeds of our devices,” says Holmes. “We are also trying to make them work by injecting current into them rather than exciting them with a laser.”

The current work is detailed in Nano Letters.

This article first appeared on nanotechweb.org

Giving the public access to research

By Michael Banks

Library users in the UK now have access to hundreds of thousands of journal articles following a new initiative called Access to Research, which was rolled out yesterday.

The two-year pilot programme will allow public-library users in the UK to freely access 8000 journals from 17 publishers including IOP Publishing, which publishes Physics World, as well as Elsevier, Nature Publishing Group and Wiley.

Last year, about 250 libraries from 10 local authorities, the majority of which are in southern England, were involved in testing the programme, with the initiative now being launched nationwide.

(more…)

Browse articles by content type