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The new photovoltaic on the block
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The NIST lens has a focal length of about half a micrometer, a focusing scale that is almost impossible to achieve with conventional glass lenses. Moreover, the active behavior of the meta lens can be switched off by using higher frequency light as a switch, so that the lens can also act like a camera shutter but without the moving parts.
Materials Today Volume 16, Number 6 June 2013
‘Our lens will offer other researchers greater flexibility for manipulating UV light at small length scales,’ explains Lezec. ‘With its high photon energies, UV light has a myriad of applications, including photochemistry, fluorescence microscopy and semiconductor manufacturing. That, and the fact that our lens is so easy to make,
should encourage other researchers to explore its possibilities.’ The NIST team worked with colleagues at the Maryland NanoCenter at the University of Maryland, College Park, Syracuse University and the University of British Columbia, Kelowna, Canada. David Bradley
that of graphite – the most commonly used electrode material in today’s Li-ion batteries [J. Phys. Chem. Lett. (2013) doi:10.1021/ jz400491b]. The work, published in the Journal of Physical Chemistry Letters, confirmed experimental results suggesting that the weak binding between lithium and carbon was limiting the storage capacity of graphene. The Rice researchers went on to theoretically investigate the effects of modifying graphene for better lithium storage, by mechanically stressing it and chemically doping it. While mechanical modification did not increase graphene’s charge capacity, the addition of boron had a large effect. A stack of graphene layers in which a quarter of the carbon atoms were replaced by boron provided the optimum lithium storage capacity. The researchers found that C3B had a
theoretical capacity of 857 mAh/g – over twice as large as graphite’s 372 mAh/g. The team explain that in this arrangement, the boron attracts lithium ions into the matrix, but the bond is not so strong that it inhibits lithium movement in the presence of a more attractive cathode – a key consideration for its use within batteries. These stacks of C3B were also determined to have a comparable power density to graphite and very little volume variation during discharge/charge cycles. The Rice team say that their results help to ‘‘. . .clarify the fundamentals of lithium storage in low-dimensional materials’’. If a method to synthesize the boron–carbon compound in large quantities can be found, it is hoped that this theoretical study can lead to a practical outcome, in the form of high-capacity lithium-C3B batteries. Laurie Winkless
Better batteries with boron A mix of boron and graphene has been offered as a potential lithium storage solution by theorists at Rice University, leading to suggestions that high-capacity graphene batteries may not be too far off. Graphene has been a buzzword in materials research since it was first isolated in 2004, thanks to its impressive mechanical, thermal and electrical properties. The huge surface area of these one-atom-thick sheets of carbon (C) also highlighted graphene as a potential storage material for use in lithium ion batteries. But, in practice, it was found that lithium ions did not form strong enough bonds with pristine sheets of graphene to consider as a storage medium. However, work from a group of theoreticians at Rice University suggests that including specific boron (B) defects in the graphene lattice can hugely increase its charge capacity, making it comparable to
The new photovoltaic on the block A new class of photovoltaic solar cell based on self-assembling block copolymers is being developed by collaborators at Pennsylvania State and Rice universities. The organic materials being developed by chemical engineer Enrique Gomez and his team at PSU and colleague Rafael Verduzco of Rice arrange themselves into distinct layers that could outperform other polymer-based solar cells [Verduzco et al., Nano Lett. (2013) doi:10.1021/nl401420s]. Commercial, silicon-based solar cells are approximately 20% efficient at absorbing and converting incident sunlight into electricity. Experimental advances may have taken that to 25%. However, such devices are costly and fragile. Polymer-based photovoltaics have reached peaks of just 10% efficiency but are far cheaper to make and much tougher; there has thus been a good deal of research aimed at making plastic solar panels to bring solar energy more into 210
Materials Today Volume 16, Number 6 June 2013
the mainstream as well as making it viable in the developing world. ‘You need two components in a solar cell: one to carry electrons, the other to carry positive charges,’ Verduzco explains. It is the imbalance between the two created by energy from sunlight that generates a useful current. The Rice team has now made a block copolymer – P3HT-b-PFTBT – that separates into bands that are about 16 nanometres wide; these bands align perpendicular to the glass substrate on which they are produced with an indium tin oxide (ITO) top layer. The PSU team used the block copo-
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lymer to construct a photovoltaic device in which the polymer bands span top to bottom electrodes and so offer a clear path for electron flow. ‘On paper, block copolymers are excellent candidates for organic solar cells, but no one has been able to get very good photovoltaic performance using them,’ Verduzco adds. ‘We didn’t give up on the idea because there’s really only been a handful of these types of solar cells previously tested. We thought getting good performance was possible if we designed the right materials and fabricated the solar cells under the right conditions.’
The next steps are to control architecture and improve performance. ‘Block copolymers still perform poorly compared with polymer/fullerene blends, but there’s a lot of room for improvement. Since block copolymers have two polymeric absorbers, we can really broaden the absorption spectrum by choosing polymers with complementary absorbance. Also, block copolymers should be more thermally stable, and we don’t have to worry about large-scale phase separation, which is an issue in polymer-blend OPVs.’ David Bradley
Nanoscale encounters of the third kind With the ability to minimize friction so crucial in many areas of life – from components for the body (such as hip replacements) or for machines (such as in vehicle transmissions) – it is crucial that parts can move against each other with a minimum of friction to avoid energy loss and wear and tear. The frictions involved are usually that of sticking and sliding. However, scientists in Germany have discovered and characterized a previously unknown kind of friction that occurs on the nanoscale, which they termed ‘desorption stick’. Based on the increasing demand for lowfriction parts for ever-small components, this study into the friction behavior of nanosystems could offer insight into previously unexplainable friction phenomena. Led by Thorsten Hugel and Alexander Holleitner, work by experimental physicists ¨ nchen from the Technische Universita¨t Mu (TUM) was reported in the international
edition of the journal Angewandte Chemie [Balzer et al., Angew. Chem. Int. Ed. (2013) doi:10.1002/anie.201301255]. They explored the reasons for single polymer molecules in a range of solvents either sliding over or sticking to some surfaces. To achieve this they connected the end of a polymer chain to the tip of an atomic force microscope (AFM) before measuring the relevant forces when the polymer molecule was dragged over a range of test surfaces. Apart from the expected mechanisms of slipping and sticking, they were surprised to discover a third nanoscale friction mechanism for specific combinations of polymer, solvent and surface. To their surprise, desorption stick was found not to depend on the speed of movement, the support surface or the adhesive strength of the polymer, but rather the quality of the solvent as well as the chemical nature of the surface involved. Thorsten
Hugel said that ‘although the polymer sticks to the surface, the polymer strand can be pulled from its coiled conformation into the surrounding solution without significant force to be exerted. The cause is probably a very low internal friction within the polymer coil.’ The findings could help in the future development of durable, low-friction surface coatings in polymer-based nanotechnology. The team now intend to characterize desorption stick on more complex surfaces (particularly polymer coatings) as well as working towards a better understanding of how macroscopic friction behavior can be predicted from their nanoscopic measurements. Alexander Holleitner points out that ‘with targeted preparation of polymers, new surfaces could be developed specifically for the nano- and micrometer range.’ Laurie Donaldson
Glassy model The transition from the melt to the solid leads to the accumulation of internal tensile stresses begin to build up. The exploding glass teardrops known as ‘Prince Rupert’s Drops’ or ‘Dutch tears’ are a case in point. Their bulbous end resists impact, but the tiniest tweak to the thin end will lead to the explosive release of that internal tension. Similarly, the properties of safety glass and so-called Gorilla glass re determined largely by internal tensile stresses. Now, a simple model has been developed by an international team to explain the fundamental differences between glass in the
molten and solid state [M. Ballauff et al., Phys. Rev. Lett. 110 (2013) 215701]. ¨ rger of the HelmholtzMiriam Siebenbu ¨ r Materialien und Energie Zentrum Berlin fu in Berlin, Germany and colleagues elsewhere in Germany and in Crete, have modeled a molten glassy system using spherical polymer nanoparticles suspended in aqueous solution. The 150 nm particles have a thermosensitive shell the thickness of which varies with temperature allowing the team to transform the colloidal suspension from a fluid state to a more densely packed ‘glassy’ state. 211
The NIST lens has a focal length of about half a micrometer, a focusing scale that is almost impossible to achieve with conventional glass lenses. Moreover, the active behavior of the meta lens can be switched off by using higher frequency light as a switch, so that the lens can also act like a camera shutter but without the moving parts.
Materials Today Volume 16, Number 6 June 2013
‘Our lens will offer other researchers greater flexibility for manipulating UV light at small length scales,’ explains Lezec. ‘With its high photon energies, UV light has a myriad of applications, including photochemistry, fluorescence microscopy and semiconductor manufacturing. That, and the fact that our lens is so easy to make,
should encourage other researchers to explore its possibilities.’ The NIST team worked with colleagues at the Maryland NanoCenter at the University of Maryland, College Park, Syracuse University and the University of British Columbia, Kelowna, Canada. David Bradley
that of graphite – the most commonly used electrode material in today’s Li-ion batteries [J. Phys. Chem. Lett. (2013) doi:10.1021/ jz400491b]. The work, published in the Journal of Physical Chemistry Letters, confirmed experimental results suggesting that the weak binding between lithium and carbon was limiting the storage capacity of graphene. The Rice researchers went on to theoretically investigate the effects of modifying graphene for better lithium storage, by mechanically stressing it and chemically doping it. While mechanical modification did not increase graphene’s charge capacity, the addition of boron had a large effect. A stack of graphene layers in which a quarter of the carbon atoms were replaced by boron provided the optimum lithium storage capacity. The researchers found that C3B had a
theoretical capacity of 857 mAh/g – over twice as large as graphite’s 372 mAh/g. The team explain that in this arrangement, the boron attracts lithium ions into the matrix, but the bond is not so strong that it inhibits lithium movement in the presence of a more attractive cathode – a key consideration for its use within batteries. These stacks of C3B were also determined to have a comparable power density to graphite and very little volume variation during discharge/charge cycles. The Rice team say that their results help to ‘‘. . .clarify the fundamentals of lithium storage in low-dimensional materials’’. If a method to synthesize the boron–carbon compound in large quantities can be found, it is hoped that this theoretical study can lead to a practical outcome, in the form of high-capacity lithium-C3B batteries. Laurie Winkless
Better batteries with boron A mix of boron and graphene has been offered as a potential lithium storage solution by theorists at Rice University, leading to suggestions that high-capacity graphene batteries may not be too far off. Graphene has been a buzzword in materials research since it was first isolated in 2004, thanks to its impressive mechanical, thermal and electrical properties. The huge surface area of these one-atom-thick sheets of carbon (C) also highlighted graphene as a potential storage material for use in lithium ion batteries. But, in practice, it was found that lithium ions did not form strong enough bonds with pristine sheets of graphene to consider as a storage medium. However, work from a group of theoreticians at Rice University suggests that including specific boron (B) defects in the graphene lattice can hugely increase its charge capacity, making it comparable to
The new photovoltaic on the block A new class of photovoltaic solar cell based on self-assembling block copolymers is being developed by collaborators at Pennsylvania State and Rice universities. The organic materials being developed by chemical engineer Enrique Gomez and his team at PSU and colleague Rafael Verduzco of Rice arrange themselves into distinct layers that could outperform other polymer-based solar cells [Verduzco et al., Nano Lett. (2013) doi:10.1021/nl401420s]. Commercial, silicon-based solar cells are approximately 20% efficient at absorbing and converting incident sunlight into electricity. Experimental advances may have taken that to 25%. However, such devices are costly and fragile. Polymer-based photovoltaics have reached peaks of just 10% efficiency but are far cheaper to make and much tougher; there has thus been a good deal of research aimed at making plastic solar panels to bring solar energy more into 210
Materials Today Volume 16, Number 6 June 2013
the mainstream as well as making it viable in the developing world. ‘You need two components in a solar cell: one to carry electrons, the other to carry positive charges,’ Verduzco explains. It is the imbalance between the two created by energy from sunlight that generates a useful current. The Rice team has now made a block copolymer – P3HT-b-PFTBT – that separates into bands that are about 16 nanometres wide; these bands align perpendicular to the glass substrate on which they are produced with an indium tin oxide (ITO) top layer. The PSU team used the block copo-
NEWS
lymer to construct a photovoltaic device in which the polymer bands span top to bottom electrodes and so offer a clear path for electron flow. ‘On paper, block copolymers are excellent candidates for organic solar cells, but no one has been able to get very good photovoltaic performance using them,’ Verduzco adds. ‘We didn’t give up on the idea because there’s really only been a handful of these types of solar cells previously tested. We thought getting good performance was possible if we designed the right materials and fabricated the solar cells under the right conditions.’
The next steps are to control architecture and improve performance. ‘Block copolymers still perform poorly compared with polymer/fullerene blends, but there’s a lot of room for improvement. Since block copolymers have two polymeric absorbers, we can really broaden the absorption spectrum by choosing polymers with complementary absorbance. Also, block copolymers should be more thermally stable, and we don’t have to worry about large-scale phase separation, which is an issue in polymer-blend OPVs.’ David Bradley
Nanoscale encounters of the third kind With the ability to minimize friction so crucial in many areas of life – from components for the body (such as hip replacements) or for machines (such as in vehicle transmissions) – it is crucial that parts can move against each other with a minimum of friction to avoid energy loss and wear and tear. The frictions involved are usually that of sticking and sliding. However, scientists in Germany have discovered and characterized a previously unknown kind of friction that occurs on the nanoscale, which they termed ‘desorption stick’. Based on the increasing demand for lowfriction parts for ever-small components, this study into the friction behavior of nanosystems could offer insight into previously unexplainable friction phenomena. Led by Thorsten Hugel and Alexander Holleitner, work by experimental physicists ¨ nchen from the Technische Universita¨t Mu (TUM) was reported in the international
edition of the journal Angewandte Chemie [Balzer et al., Angew. Chem. Int. Ed. (2013) doi:10.1002/anie.201301255]. They explored the reasons for single polymer molecules in a range of solvents either sliding over or sticking to some surfaces. To achieve this they connected the end of a polymer chain to the tip of an atomic force microscope (AFM) before measuring the relevant forces when the polymer molecule was dragged over a range of test surfaces. Apart from the expected mechanisms of slipping and sticking, they were surprised to discover a third nanoscale friction mechanism for specific combinations of polymer, solvent and surface. To their surprise, desorption stick was found not to depend on the speed of movement, the support surface or the adhesive strength of the polymer, but rather the quality of the solvent as well as the chemical nature of the surface involved. Thorsten
Hugel said that ‘although the polymer sticks to the surface, the polymer strand can be pulled from its coiled conformation into the surrounding solution without significant force to be exerted. The cause is probably a very low internal friction within the polymer coil.’ The findings could help in the future development of durable, low-friction surface coatings in polymer-based nanotechnology. The team now intend to characterize desorption stick on more complex surfaces (particularly polymer coatings) as well as working towards a better understanding of how macroscopic friction behavior can be predicted from their nanoscopic measurements. Alexander Holleitner points out that ‘with targeted preparation of polymers, new surfaces could be developed specifically for the nano- and micrometer range.’ Laurie Donaldson
Glassy model The transition from the melt to the solid leads to the accumulation of internal tensile stresses begin to build up. The exploding glass teardrops known as ‘Prince Rupert’s Drops’ or ‘Dutch tears’ are a case in point. Their bulbous end resists impact, but the tiniest tweak to the thin end will lead to the explosive release of that internal tension. Similarly, the properties of safety glass and so-called Gorilla glass re determined largely by internal tensile stresses. Now, a simple model has been developed by an international team to explain the fundamental differences between glass in the
molten and solid state [M. Ballauff et al., Phys. Rev. Lett. 110 (2013) 215701]. ¨ rger of the HelmholtzMiriam Siebenbu ¨ r Materialien und Energie Zentrum Berlin fu in Berlin, Germany and colleagues elsewhere in Germany and in Crete, have modeled a molten glassy system using spherical polymer nanoparticles suspended in aqueous solution. The 150 nm particles have a thermosensitive shell the thickness of which varies with temperature allowing the team to transform the colloidal suspension from a fluid state to a more densely packed ‘glassy’ state. 211