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Applications tions such as medical imaging and security screening (Nature Nanotechnology 10.1038/ nnano.2011.146). Terahertz radiation is useful for detect ing items such as concealed weapons and explosives because it passes through cloth ing and packaging but is strongly absorbed by metals and other inorganic substances. Feng Wang and colleagues say that they have made the “beginnings of a toolset” for experiments in this wavelength range. The team has come up with a prototype device that consists of an array of graphene nano ribbons with a response to terahertz radia tion that can be tuned by varying the width of the ribbons and the number of charge car riers (electrons and holes) in the structures. In graphene, the concentration of charge carriers can easily be increased or decreased by applying a strong electric field – a tech nique known as electrostatic doping. Build better electronics Graphene could be ideal for use in future electronics applications because electrons whizz through the material at extremely high speeds (thanks to the fact that they behave like relativistic particles with no rest mass). Recently, a new method to increase the amount of current that can be carried by graphene has been unveiled by research ers at the University of California, River side (UCR) and the Argonne National Lab (Nano Lett. 10.1021/nl204545q). The technique involves growing or trans ferring graphene on synthetic diamond or ultrananocrystalline diamond rather than on a conventional silicondioxide substrate. Diamond conducts heat better than silicon or silicon dioxide, removing more heat away from the graphene, which in turn means that the wonder material can sustain even higher current densities. Alexander Balandin and Anirudha Sumant, working together with electrical engineering graduate students in Bal andin’s lab at UCR, have shown that the currentcarrying capacity of graphene can be increased to as high as around 20 μA/nm2 by replacing the silicon diox ide with synthetic diamond or inexpensive ultrananocrystalline diamond. The work could help to develop high frequency transistors, transparent elec trodes and interconnects for replacing copper on silicon dioxide. Ramp up the performance of supercapacitors and batteries A new and simple “dipping” technique that significantly improves the specific capacitance and rate capability of metal oxidebased supercapacitors has been demonstrated by researchers at Stanford University in the US (Nano Lett. 10.1021/ nl2026635). 12 High-performance coating Graphene/manganese electrodes dipped into a carbon-nanotube solution. The technique, developed by Zhenan Bao, Yi Cui and colleagues, involves dip ping a composite electrode made of gra phene/manganeseoxide into a solution containing either carbon nanotubes (CNTs) or a conductive polymer. The CNTs or poly mer coat the electrode and greatly improve its electrical conductivity, so enhancing its specific capacitance (or its ability to store charge) by more than 20% for the CNT coating and 45% for the polymer. Dubbed “conductive wrapping”, the method could be applied to a range of highdensity but insulating electrode mate rials. It may even be used to improve next generation lithiumion battery electrodes made from sulphur, lithium manganese phosphate and silicon. As well as having high specific capacitance, the hybrid electrodes also show good rate capability. They can be used over more than 3000 charge–discharge cycles while retain ing more than 95% of their capacitance. Design new types of batteries Researchers at Hong Kong Polytechnic University claim to have invented a new kind of graphenebased “battery” that runs solely on ambient heat. The device is said to capture the thermal energy of ions in a solution and convert it into elec tricity. The results are in the process of being peer reviewed, but, if confirmed, such a device might find use in a range of applications, including powering artificial organs from body heat, generating renew able energy and running electronic devices (arXiv:1203.0161). Zihan Xu and colleagues made their bat tery by attaching silver and gold electrodes to a strip of graphene. In their experiments, the researchers showed that six of these devices in series placed in a solution of copperchloride ions produced a voltage of more than 2 V – enough to drive a commer cial red lightemitting diode. Kill E. coli Graphene could be used to make antibacte rial paper, according to work by scientists at the Chinese Academy of Sciences in Shang Physics World Focus on: Nanotechnology physicsworld.com hai, who have found that sheets of the mate rial effectively stop the growth of E. coli bacteria without being toxic to human cells. “Ultimately, we would like to develop new antibacterial materials from graphene that could be directly applied onto skin to aid in wound healing,” says Chunhai Fan (ACS Nano 10.1021/nn101097v). Print electronic devices Researchers at the University of Cambridge in the UK have invented a new ink based on graphene, which they have used to print highperformance, transparent, thinfilm transistors and interconnects. The work could lead to graphenebased flexible displays, solar cells and electronic paper (arXiv:1111.4970). To make the ink, the scientists begin by treating graphite flakes in a sonic bath con taining the solvent Nmethylpyrrolidone for several hours. The flakes are then left to settle for a few minutes after sonication. Next, the team decants the dispersions and centrifuges the samples for an hour to filter out any flakes bigger than 1 μm across that might clog the printer nozzle. The ink suits a variety of substrates, including silicon dioxide and quartz. Soak up arsenic A composite material made from reduced graphene oxide (RGO) and magnetite could effectively remove arsenic from drinking water, according to work done in South Korea (ACS Nano 10.1021/nn1008897). The purification process is initiated by dispersing the magnetite–RGO composite in water, where the material soaks up arse nic. Thanks to the presence of the mag netite, the composite can be quickly and efficiently extracted from the water using a permanent magnet. The contribution of the graphene flakes is to increase the number of arsenic adsorption sites. Improve electron sources Fewlayer graphene (FLG) has exceptional physical and chemical properties and is con sidered as a type of fieldemission material thanks to its thin edges. However, to achieve a large fieldenhancement factor, the gra phene sheets must be grown vertical to the substrate rather than in the horizontal con figuration that is typical of most synthesis methods (Nanotechnology 23 015202). One approach, as demonstrated by scien tists in China, is to use microwave plasma enhanced chemical vapour deposition (MPECVD). The team from Sun Yatsen University has synthesized FLG in a vertical growth direction, and shaped the material by adjusting the growth time and ratio of hydro carbon gas. Potential applications include highpower vacuum electron sources. June 2012 PWNANOJun12Graphene.indd 12 15/05/2012 10:15 G Yu, Stanford UniversityPDF Image | Graphene for Solid State Physics II
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