NEWS Materials Today Volume 16, Number 12 December 2013 Lego-inspires more efficient solar cells Solar panels with Lego-like aluminium studs produce a quarter more electrical current from sunlight than their flat counterparts, find a team of international researchers Most solar cells have thick layers of materials able to absorb sunlight But these absorbing materials are expensive, making up to half the cost of a solar panel ‘To drive down prices new technologies have focused on using thin layers of the material, but they absorb less light and are not yet efficient enough for the market,’ explains team member Nicholas Hylton from Imperial College London, UK ‘Novel ideas are required to increase the amount of light absorbed by thin-film solar cells.’ Tiny gold or silver studs placed on the surface of solar cells have previously been shown to slightly increase efficiency The studs bend and trap light for longer inside the absorbing material The more time the light beams spend inside the material, the more energy is extracted At a microscopic level these studs look like the interlocking children’s building blocks called Lego The efficiency boost was however limited by the type of metal used for the studs ‘The [previous] metal structures absorbed some of the light from the visible part of the solar spectrum before it was scattered into the solar cell beneath,’ Hylton told Materials Today ‘That parasitic effect limited increases in efficiency.’ The study, published in Scientific Reports [Hylton, et al., Sci Rep (2013), doi:10.1038/srep02874], showed that aluminium is a better material for the studs ‘The advantage of aluminium is that this parasitic absorption occurs in the ultraviolet part of the spectrum,’ explains Hylton This means that the aluminium studs scatter more light in the visible region compared to silver and gold ‘As this is where we are able to convert most energy from sunlight, we can get a boost in the electrical current produced by the device by as much as 22% compared to a device without nanostructures.’ An additional advantage of aluminium is that it is cheaper and more abundant that silver and gold The next step is to attempt combining the Lego-inspired material with more conventional anti-reflection coatings, says Hylton Anti-reflection coatings stop light bouncing off solar panels before it can be absorbed ‘If we can this successfully then it may be possible to make thin, highly efficient solar cells at a competitive price.’ Nina Notman Battery boost comes from X-ray studies The mobile connectivity of the modern world hinges is powered by the lithiumion battery, which is stable, relatively quick to charge, quick to discharge However, the current electrodes based on intercalation materials not have the high energy-density of experimental materials, which can be many times higher and so provide more power or the same power for longer Unfortunately, those materials come with their own drawbacks and have not yet reached a marketable state of development Fundamentally, these experimental electrode materials expand to up to three times their original volume and then contract with each charge and discharge cycle as they alloy and de-alloy with lithium ions These volume changes cause microscopic fractures ultimately limiting how fully the battery can be recharged within just two or three cycles The expansion and contraction of such electrodes have now been studied for the first time in detail by scientists from ETH and Paul Scherrer Institute with a view to understanding and so hopefully preventing the processes involved The team has quantified the effect using highresolution X-ray tomography at the Swiss Light Source [Wood, et al., Science (2013), doi:10.1126/science.1241882] The team obtained high-resolution X-ray images of a working tin oxide model 464 electrode and then used a computer to assemble each into a three-dimensional movie of the changes taking place They made measurements over a charge-discharge lasting more than fifteen hours The movies show how the influx of lithium ions causes tin oxide particles to expand through a ‘core–shell’ process, progressing uniformly from the particle surface to its core Volume expands linearly with stored charge and irreversible cracks form as evidenced by the X-ray images ‘This crack-formation is not random,’ Ebner says ‘Cracks grow at locations where the crystal lattice contains pre-existing defects During discharge, the particle volume decreases; however, the material does not reach its original state again; the process is therefore not completely reversible.’ The result is that although the electrode might expand from 50 to 120 Materials Today Volume 16, Number 12 December 2013 micrometres on charging on discharge it only partially contracts resulting to 80 mm However, the tin oxide is held together in a permeable polymer binder If this could be optimized so that the electrode were held together more effectively and so that deformation would not lead to cracking, it might be possible to make the contraction reversible so that recharging repeatedly would be possible Moreover, tin oxide is itself not the optimum material but was used simply to demonstrate how much information about deformation might be extracted using tomography Researchers are working NEWS on other materials less susceptible to volume changes, this study should accelerate their development too Particles of a tin oxide electrode Credit: Martin Ebner, Laboratory for Nanoelectronics, ETH Zurich David Bradley Artificial cell membranes could bring improved biosensors An approach to writing artificial cell membranes on graphene using a nanoscaled tip has been developed in a new study With our bodies containing around 100 trillion cells, each enclosed in a cell membrane that holds numerous proteins, ion channels and other biomolecules that carry out vital functions, these biomimetic membranes could lead to a range of medical and biotechnology applications, including in biosensors and drug delivery and screening The research team, led by Aravind Vijayaraghavan from the University of Manchester and Michael Hirtz from Karlsruhe Institute of Technology, whose study was reported in the journal Nature Communications [Hirtz, et al., Nat Commun (2013), doi:10.1038/ ncomms3591], showed how to write tailored patches of phospholipid membrane directly onto a graphene substrate to produce these membranes that simulate biological structures By examining how lipids spread and self-assemble in this way, they achieved a better understanding of both their structure and control As cell membranes cannot be examined directly easily, model membranes were applied to special surfaces The biomimetic membranes were produced using lipid dippen nanolithography (L-DPN), which has a very sharp tip – extremely accurately operated by machine – and an apex of only a few nanometers, in a process that did not damage the graphene With parallel arrays of the tips, the team showed how different lipid mixtures can be written in parallel, allowing for patterns of variable chemical composition with a size smaller than an individual cell When the lipids are positioned onto graphene, they spread out uniformly, forming high-quality membranes When the lipids contain the corresponding binding sites, the membranes actively bind streptavidin, a protein produced by some bacteria Once the lipids are charged, the charge is transferred from the lipids into the graphene, altering its conductivity, offering potential uses as a detection signal in biosensors The study also has implications in areas such as the engineering of better interfaces between graphene and cells and for making graphene bio-compatible The team now hopes to gain a better understanding of the assembly process, with the aim of applying their biomimetic membranes to the development of new biosensors based on graphene and lipids, as well as sensors that react to the binding of proteins by a change of conductivity and in detecting the function of ion channels in membranes Protein sensors could find applications in medical diagnostics, while controlling the function of ion channels is advantageous in drug research Laurie Donaldson The thinnest, strongest carbon wire While graphene is the wonder material that perhaps usurped the fullerenes and even the nanotubes from the pinnacle of technological interest, there is a new pretender vying for the crown – carbyne Carbynes are carbon chains held together by repeating double bonds or alternating single and triple bonds They are true onedimensional material lacking the 2D sheetlike nature of graphene and the 3D structure of hollow carbon nanotubes Now, Boris Yakobson and his group at Rice University, Texas, USA, have calculated that hypothetical carbyne nanorods could be twice as strong as graphene weight for weight [Yakobson, et al., ACS Nano (2013), doi:10.1021/nn404177r] The team’s first-principles calculations also suggest that stretching carbyne as little as 10 percent alters its electronic band gap Nanoropes or nanorods of carbyne might find uses in electronics Credit: Vasilii Artyukhov/Rice University significantly A 90-degree end-to-end rotation, they predict, makes it a magnetic semiconductor Moreover, adding side chains can allow this twist to be controlled 465 ... has a very sharp tip – extremely accurately operated by machine – and an apex of only a few nanometers, in a process that did not damage the graphene With parallel arrays of the tips, the team... delivery and screening The research team, led by Aravind Vijayaraghavan from the University of Manchester and Michael Hirtz from Karlsruhe Institute of Technology, whose study was reported in the... structures By examining how lipids spread and self-assemble in this way, they achieved a better understanding of both their structure and control As cell membranes cannot be examined directly