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Physics World Focus on: Nanotechnology physicsworld.com Focus light A tiny bubble of graphene could be used to make an optical lens with an adjustable focal length. That is the claim of physicists at the University of Manchester in the UK, who have shown that the curvature of such bubbles can be controlled by applying an external voltage. Devices based on the dis- covery could find use in adaptive-focus sys- tems that try to mimic how the human eye works (Appl. Phys. Lett. 99 093103). It turns out that graphene can be stretched by up to 20%, which means that bubbles of various shapes can be “blown” from the material. This property, combined with the fact that graphene is transparent to light yet impermeable to most liquids and gases, could make the material ideal for creating adaptive-focus optical lenses. Such lenses are used in mobile-phone cameras, webcams and auto-focusing eye glasses, and are usually made of transpar- ent liquid crystals or fluids. Although such devices work well, they are relatively dif- ficult and expensive to make. In principle, graphene-based adaptive optics could be fabricated using much simpler methods than those used for existing devices. They could also become cheaper to produce if industrial-scale processes to manufacture graphene devices become available. Make high-performance modulators A modulator containing a double layer of graphene has been unveiled by researchers at the University of California, Berkeley and the Lawrence Berkeley National Labo- ratory in the US. The high-performance device, which operates at 1 GHz, has many advantages over silicon photonics, includ- ing a small footprint, low power consump- tion and low optical loss. Applications include telecommunications and on-chip data communication (Nano Lett. 10.1021/ nl204202k). “Compared with silicon-based optical modulators, this double-layer graphene device has separate electrical and optical control modules,” says team member Ming Liu. “This is a first and allows us to opti- mize both the electrical and optical design separately, and avoid the trade-off between speed and optical losses.” Store hydrogen Vehicles and other systems powered by hydrogen have the advantage of emitting only water as a waste product. An impor- tant challenge, however, is storing enough hydrogen onboard a car so that it can travel as far as a vehicle powered by fossil fuels. If hydrogen is stored as a compressed gas, it takes up far too much space – and liquefying hydrogen is expensive in terms of both cost and energy. One solution to this problem is to exploit June 2012 Applications PWNANOJun12Graphene.indd 13 15/05/2012 10:16 Magic membranes Flakes of graphene oxide could be used to separate water from other liquids. the fact that many solid materials will absorb large amounts of hydrogen, and researchers have identified stacked layers of oxidized graphene as a promising can- didate. Scientists from the NIST Center for Neutron Research in the US have made graphene-oxide frameworks that can hold roughly 1% of their weight in hydrogen. This value is 100 times more than graphene oxide and compares well with MOF-5 (the most studied metal-organic framework to date for hydrogen storage), which absorbs about 1.3 wt% (Angew. Chem. Int. Ed. Engl. 49 8902). Remove water from a mixture Scientists have reported that membranes made from graphene oxide appear to be highly permeable to water while being impermeable to all other liquids and gases. The membranes consist of millions of small flakes of graphene oxide with nanometre-sized empty channels (or cap- illaries) between the flakes that favour the passage of monolayers of water and resist other substances (Science 335 442). Graphene oxide is similar to ordinary gra- phene but is covered with molecules, such as hydroxyl groups (OH). Remove unwanted heat from electronics University of California, Riverside scien- tists say that they have made a new thermal interface material (TIM) that could effi- ciently remove unwanted heat from elec- tronic components such as computer chips and light-emitting diodes. The material is a composite made of graphene and multilayer graphene (Nano Lett. 10.1021/nl203906r). Unwanted heat is a big problem in mod- ern electronics based on conventional sili- con circuits – and the issue is getting worse as devices become ever smaller and more sophisticated. Graphene could be ideal as a filler material in TIMs to carry away heat because pure graphene has a large intrinsic room-temperature thermal conductivity that lies in the 2000–5000 W m–1 K–1 range. These values are higher than those of dia- mond, the best bulk-crystal heat conductor that is known. Alexander Balandin and colleagues have now succeeded in increasing the ther- mal conductivity of a routinely employed industrial epoxy-resin-based TIM, or “grease” as it is better known in the indus- try, from around 5.8 W m–1 K–1 to a record 14 W m–1 K–1. The filler particles in this case consist of an optimized mixture of graphene and few-layer graphene, with the volume fraction of the carbon-based mate- rial in the epoxy being very low at just 2%. Form transparent electrodes for displays Tae-Woo Lee of Pohang University of Sci- ence and Technology in South Korea and colleagues have developed a way to increase the work function of graphene films and lower the sheet resistance so that the ultrathin material can be made into an effi- cient anode for organic light-emitting diode applications (Nature Photonics 10.1038/ nphoton.2011.318). “The graphene anode demonstrated excellent bending stability with a bending radius of 0.75 cm and a strain of 1.25%,” says Lee. “And we observed that the gra- phene devices maintained almost the same current density even after being bent and straightened 1000 times.” Make rare-element-free magnets Graphene can be made magnetic by form- ing honeycomb-like arrays of hydrogen-ter- minated nanopores on it. So say researchers in Japan, based at Aoyama Gakuin Uni- 13 University of ManchesterPDF Image | Graphene for Solid State Physics II
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