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Quantum graphene
Materials Today Volume 17, Number 7 September 2014
and control the transmittance of light in certain areas, could also find uses in anticounterfeit technology, and for optical communication devices that control the amplitude, phase, polarization, propagation direction of light. The study, which was reported in Nano Letters [Wang, et al., Nano Lett. (2014), demonstrated doi:10.1021/nl501302s], how ferrimagnetic inorganic nanorods could be used to construct liquid crystals with optical properties that can be instantly and reversibly controlled simply by altering the direction of an external magnetic field. This approach overcame the usual problem of using of a magnetic field for this purpose, where the low magnetic susceptibility of molecular species requires the use of extremely strong magnetic fields. The team used magnetic nanorods rather than the commercial non-magnetic rod-like molecules, which work in a similar way in optical terms, but have the advantage of responding rapidly to external magnetic
NEWS
A polarization-modulated pattern changes darkness/brightness by rotating the direction of the cross polarizers.
fields. When a magnetic field is applied, the nanorods spontaneously rotate and realign themselves parallel to the field direction, thereby influencing the transmittance of polarized light. The materials involved are made of iron oxide and silica, which are also much cheaper and more environmentally friendly than the commercial organic molecules-based liquid crystals. As study leader Yadong Yin said, the nanorods ‘‘can effectively form liquid crystals and respond strongly to even very weak magnetic fields – even a fridge magnet can operate our liquid crystals.’’ As the crystals
can be operated remotely by an external magnetic field, electrodes are unnecessary, and as the magnetic nanorods are much bigger than those used in commercial liquid crystals their orientation can be manipulated by solidifying the dispersing matrix. The team will now look to reduce the optical absorption of the nanorods, either through modification or by replacing them with other transparent magnetic nanorods. They also hope to explore using the materials to optimize the technology to fit specific application needs. Laurie Donaldson
Roll-to-roll synthesis of CNT supercapacitor electrodes In the last decade, there has been a considerable growth in the widespread use of carbon nanomaterials across a range of industries. But the most common bottleneck to any further development is the scalability of their production. Although CNTs can be synthesized in large quantities, present processes for the growth of vertically-aligned CNTS – particularly of interest to the electronics market – are limited to a small range of substrate materials. But a group of researchers from Clemson University in the US have developed a relatively low-cost roll-to-roll method – their system can grow vertically-aligned CNTs (VACNTs) directly onto aluminium foil ribbons that are continuously draw through a reactor. Their process produces high density, high capacity (50 F/g) forests of aligned CNTs that outperform commercial CNTs. The team also used these ribbons of aligned
CNTs as the electrodes in a range of highperformance supercapacitor cells. Today’s supercapacitors tend to use carbon materials in their electrodes, with their performance related to the electrode’s surface area. So, considerable research effort has focused on using CNTs as supercapacitor electrodes. But issues of substrate preparation and high operating temperatures have rendered the system complex and inefficient. What the Clemson team have done is develop a system that negates these issues – by adapting a standard Chemical Vapor Deposition (CVD) system, they have managed to decrease the growth temperature to 600 8C, which is below the melting temperature of aluminium. This means that it can be used to directly synthesize VACNTs onto a current collector substrate – in this case, aluminium foil ribbons.
The work, recently published in Nano Energy [M.R. Arcila-Velez, et al. Nano Energy 8 (2014) 9–16], also reports on the direct assembly of these VACNT ribbons into supercapacitors. When compared with capacitors made with buckypaper and CNT forests from a stationary CVD set-up, the roll-to-roll electrodes performed well, with a charge capacity of 24.8 mAh/g. But their discharge time (630 ms), energy density (11.5 Wh/kg) and power density (1270 W/kg) all vastly outperformed the other electrodes. The roll-to-roll devices also showed excellent cycle stability, with no loss of performance over more than a thousand cycles. These results demonstrate the real potential for this technique, and the team believe that it offers a viable process for the production of supercapacitor electrodes. Laurie Winkless
Researchers at Columbia University, New York, have spent the last few years studying the fractional quantum Hall effect whereby electrons confined to a thin layer of material and exposed to a large magnetic
field display collective behavior. In 2009, they observed the effect in a single graphene layer and then showed in 2011 that they could measure this effect over large ranges of electron density. However, bilayer
Quantum graphene The fractional quantum Hall effect has been observed in bilayer graphene and shown to be tunable with an electric field, which might allow this material to be used in components of a quantum computer.
315
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graphene has much greater potential where two metal gate electrodes (above and below) should allow independent tuning of the charge density in each layer. This opens up the possibility of manipulating the fractional states in new ways, perhaps even leading to exotic ‘non-abelian’ states that could be used for quantum computation. ‘‘We knew that we could fabricate very clean bilayer graphene structures, but we suffered from our inability to make good electrical contact since bilayer graphene develops an electronic ‘band-gap’ under the high magnetic fields and low temperatures required for our experiments,’’ explains team member Cory Dean. The breakthrough came when the team came up with a new design that allowed them to tune the charge density of the contact regions independently from the rest of the device. ‘‘Once we had this new device structure the results were spectacular,’’ he adds [Dean, et al., Science (2014), doi:10.1126/ science.1252875].
Materials Today Volume 17, Number 7 September 2014
In bilayer graphene the question of spin states among the collections of electrons in each layer is rather complicated by the numerous degrees of symmetry at play. Moreover, polarization effects can arise spontaneously in one layer relative to the other. This complexity could be exploited in devices but makes the results all the more impressive and, the team says, provides an interesting new phase space to explore for new and unusual effects.
The team has now shown for the first time that tweaking an applied electric field triggers a phase transition although the exact characteristics of the different phases involves is not yet known. Their findings support the theoretical expectation that the ground state order is tunable. The next stage in their research will attempt to pin down the exact nature of this ordering in the bilayer. ‘‘The implications for this result could be far reaching,’’ Dean adds, ‘‘While we do not yet see any evidence of non-abelian states, the fact that we are able to modify the nature of the fractional quantum Hall effect by electric fields is a really exciting first step.’’ ‘‘We are now working on applying these techniques to pursue the existence of nonabelian quasi-particles,’’ Dean told Materials Today. ‘‘Pushing the device technology to yet cleaner limits, and working with the National High Magnetic Field laboratory to explore these materials at even higher magnetic fields will be the crucial next steps.’’ David Bradley
Aerogels for insulation: it’s all about particle size We’ve all seen images of the ghostly-looking material aerogel. Famously, in 2006, panels of it were used on NASA’s Stardust mission to capture tiny samples of interstellar dust. But here on Earth, its low density and thermal conductivity have attracted the interest of a much more ‘urban’ research effort – in the development of insulating windows. Windows have a huge impact on a building’s energy efficiency, with some figures suggesting that 50% of the total energy loss from a standard office building happens through its windows. As global efforts to produce ‘green’ buildings become ever more ambitious, we’re seeing a growth in research programs on windows. So far, there have been several window innovations which have shown potential to meet the requirement of energy efficient buildings – multilayered, vacuum, and silica aerogel windows. Arild Gustavsen and his team at the Norwegian University of Science and Technology are focused on the use of silica aerogel gran-
Photograph of Aerogel-AB. Scale bar: 10 mm.
ules as the ‘‘filler’’ in double-glazed windows [T. Gao, et al. Appl. Energy 128 (2014) 27–34]. Because aerogel is mechanically very weak, much of the current research on aerogel glazing units (AGUs) focuses on the synthesis of the aerogel. But Gustavsen and his team specifically looked at the effect that aerogel granule size and layer thickness have on the thermal and optical properties of standard double-glazings. Both AGUs show improved thermal insulation performance when compared to
double glazings – AGUs containing ‘large’ aerogel granules (diameter 3–5 mm) showed a 58% reduction in heat loss. Smaller particles (<0.5 mm) had an even larger effect on the thermal conductivity of the window unit – there, the team saw a 63% reduction in heat losses. However, the introduction of these granules did have an effect on the optical transmittance of the windows – Gustavsen showed that the smaller the particle, the more diffuse the transmitted light. The team believe that this property may be useful in situations where glare and/or privacy need to be considered. Highly insulating glazing units are defined as those with U-values of about 0.5–0.7 W/(m2 K) – so far, results on these AGUs fall short. But this work has opened the debate on how to optimize not only the aerogel, but the design of the final glazing units for a range of building applications. Laurie Winkless
Nanocrystalline titania for smart windows Titanium dioxide (or titania, TiO2) is used in a wide range of applications – in everything from paint pigment to ceramics. It can be found in five main mineral forms, 316
the most common three being rutile, brookite and anatase. The unique catalytic properties of the anatase form have been studied for decades, and in the last few years, has
seen a renewed interest, alongside an ongoing debate. Anatase is generally more photocatalytically-active than the other two forms of the material, but as yet, little
and control the transmittance of light in certain areas, could also find uses in anticounterfeit technology, and for optical communication devices that control the amplitude, phase, polarization, propagation direction of light. The study, which was reported in Nano Letters [Wang, et al., Nano Lett. (2014), demonstrated doi:10.1021/nl501302s], how ferrimagnetic inorganic nanorods could be used to construct liquid crystals with optical properties that can be instantly and reversibly controlled simply by altering the direction of an external magnetic field. This approach overcame the usual problem of using of a magnetic field for this purpose, where the low magnetic susceptibility of molecular species requires the use of extremely strong magnetic fields. The team used magnetic nanorods rather than the commercial non-magnetic rod-like molecules, which work in a similar way in optical terms, but have the advantage of responding rapidly to external magnetic
NEWS
A polarization-modulated pattern changes darkness/brightness by rotating the direction of the cross polarizers.
fields. When a magnetic field is applied, the nanorods spontaneously rotate and realign themselves parallel to the field direction, thereby influencing the transmittance of polarized light. The materials involved are made of iron oxide and silica, which are also much cheaper and more environmentally friendly than the commercial organic molecules-based liquid crystals. As study leader Yadong Yin said, the nanorods ‘‘can effectively form liquid crystals and respond strongly to even very weak magnetic fields – even a fridge magnet can operate our liquid crystals.’’ As the crystals
can be operated remotely by an external magnetic field, electrodes are unnecessary, and as the magnetic nanorods are much bigger than those used in commercial liquid crystals their orientation can be manipulated by solidifying the dispersing matrix. The team will now look to reduce the optical absorption of the nanorods, either through modification or by replacing them with other transparent magnetic nanorods. They also hope to explore using the materials to optimize the technology to fit specific application needs. Laurie Donaldson
Roll-to-roll synthesis of CNT supercapacitor electrodes In the last decade, there has been a considerable growth in the widespread use of carbon nanomaterials across a range of industries. But the most common bottleneck to any further development is the scalability of their production. Although CNTs can be synthesized in large quantities, present processes for the growth of vertically-aligned CNTS – particularly of interest to the electronics market – are limited to a small range of substrate materials. But a group of researchers from Clemson University in the US have developed a relatively low-cost roll-to-roll method – their system can grow vertically-aligned CNTs (VACNTs) directly onto aluminium foil ribbons that are continuously draw through a reactor. Their process produces high density, high capacity (50 F/g) forests of aligned CNTs that outperform commercial CNTs. The team also used these ribbons of aligned
CNTs as the electrodes in a range of highperformance supercapacitor cells. Today’s supercapacitors tend to use carbon materials in their electrodes, with their performance related to the electrode’s surface area. So, considerable research effort has focused on using CNTs as supercapacitor electrodes. But issues of substrate preparation and high operating temperatures have rendered the system complex and inefficient. What the Clemson team have done is develop a system that negates these issues – by adapting a standard Chemical Vapor Deposition (CVD) system, they have managed to decrease the growth temperature to 600 8C, which is below the melting temperature of aluminium. This means that it can be used to directly synthesize VACNTs onto a current collector substrate – in this case, aluminium foil ribbons.
The work, recently published in Nano Energy [M.R. Arcila-Velez, et al. Nano Energy 8 (2014) 9–16], also reports on the direct assembly of these VACNT ribbons into supercapacitors. When compared with capacitors made with buckypaper and CNT forests from a stationary CVD set-up, the roll-to-roll electrodes performed well, with a charge capacity of 24.8 mAh/g. But their discharge time (630 ms), energy density (11.5 Wh/kg) and power density (1270 W/kg) all vastly outperformed the other electrodes. The roll-to-roll devices also showed excellent cycle stability, with no loss of performance over more than a thousand cycles. These results demonstrate the real potential for this technique, and the team believe that it offers a viable process for the production of supercapacitor electrodes. Laurie Winkless
Researchers at Columbia University, New York, have spent the last few years studying the fractional quantum Hall effect whereby electrons confined to a thin layer of material and exposed to a large magnetic
field display collective behavior. In 2009, they observed the effect in a single graphene layer and then showed in 2011 that they could measure this effect over large ranges of electron density. However, bilayer
Quantum graphene The fractional quantum Hall effect has been observed in bilayer graphene and shown to be tunable with an electric field, which might allow this material to be used in components of a quantum computer.
315
NEWS
graphene has much greater potential where two metal gate electrodes (above and below) should allow independent tuning of the charge density in each layer. This opens up the possibility of manipulating the fractional states in new ways, perhaps even leading to exotic ‘non-abelian’ states that could be used for quantum computation. ‘‘We knew that we could fabricate very clean bilayer graphene structures, but we suffered from our inability to make good electrical contact since bilayer graphene develops an electronic ‘band-gap’ under the high magnetic fields and low temperatures required for our experiments,’’ explains team member Cory Dean. The breakthrough came when the team came up with a new design that allowed them to tune the charge density of the contact regions independently from the rest of the device. ‘‘Once we had this new device structure the results were spectacular,’’ he adds [Dean, et al., Science (2014), doi:10.1126/ science.1252875].
Materials Today Volume 17, Number 7 September 2014
In bilayer graphene the question of spin states among the collections of electrons in each layer is rather complicated by the numerous degrees of symmetry at play. Moreover, polarization effects can arise spontaneously in one layer relative to the other. This complexity could be exploited in devices but makes the results all the more impressive and, the team says, provides an interesting new phase space to explore for new and unusual effects.
The team has now shown for the first time that tweaking an applied electric field triggers a phase transition although the exact characteristics of the different phases involves is not yet known. Their findings support the theoretical expectation that the ground state order is tunable. The next stage in their research will attempt to pin down the exact nature of this ordering in the bilayer. ‘‘The implications for this result could be far reaching,’’ Dean adds, ‘‘While we do not yet see any evidence of non-abelian states, the fact that we are able to modify the nature of the fractional quantum Hall effect by electric fields is a really exciting first step.’’ ‘‘We are now working on applying these techniques to pursue the existence of nonabelian quasi-particles,’’ Dean told Materials Today. ‘‘Pushing the device technology to yet cleaner limits, and working with the National High Magnetic Field laboratory to explore these materials at even higher magnetic fields will be the crucial next steps.’’ David Bradley
Aerogels for insulation: it’s all about particle size We’ve all seen images of the ghostly-looking material aerogel. Famously, in 2006, panels of it were used on NASA’s Stardust mission to capture tiny samples of interstellar dust. But here on Earth, its low density and thermal conductivity have attracted the interest of a much more ‘urban’ research effort – in the development of insulating windows. Windows have a huge impact on a building’s energy efficiency, with some figures suggesting that 50% of the total energy loss from a standard office building happens through its windows. As global efforts to produce ‘green’ buildings become ever more ambitious, we’re seeing a growth in research programs on windows. So far, there have been several window innovations which have shown potential to meet the requirement of energy efficient buildings – multilayered, vacuum, and silica aerogel windows. Arild Gustavsen and his team at the Norwegian University of Science and Technology are focused on the use of silica aerogel gran-
Photograph of Aerogel-AB. Scale bar: 10 mm.
ules as the ‘‘filler’’ in double-glazed windows [T. Gao, et al. Appl. Energy 128 (2014) 27–34]. Because aerogel is mechanically very weak, much of the current research on aerogel glazing units (AGUs) focuses on the synthesis of the aerogel. But Gustavsen and his team specifically looked at the effect that aerogel granule size and layer thickness have on the thermal and optical properties of standard double-glazings. Both AGUs show improved thermal insulation performance when compared to
double glazings – AGUs containing ‘large’ aerogel granules (diameter 3–5 mm) showed a 58% reduction in heat loss. Smaller particles (<0.5 mm) had an even larger effect on the thermal conductivity of the window unit – there, the team saw a 63% reduction in heat losses. However, the introduction of these granules did have an effect on the optical transmittance of the windows – Gustavsen showed that the smaller the particle, the more diffuse the transmitted light. The team believe that this property may be useful in situations where glare and/or privacy need to be considered. Highly insulating glazing units are defined as those with U-values of about 0.5–0.7 W/(m2 K) – so far, results on these AGUs fall short. But this work has opened the debate on how to optimize not only the aerogel, but the design of the final glazing units for a range of building applications. Laurie Winkless
Nanocrystalline titania for smart windows Titanium dioxide (or titania, TiO2) is used in a wide range of applications – in everything from paint pigment to ceramics. It can be found in five main mineral forms, 316
the most common three being rutile, brookite and anatase. The unique catalytic properties of the anatase form have been studied for decades, and in the last few years, has
seen a renewed interest, alongside an ongoing debate. Anatase is generally more photocatalytically-active than the other two forms of the material, but as yet, little