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Nothing more sophisticated than a lump of graphite, a roll of sticky tape, and a wafer thin sliver of silica are needed to inflate ideas about nanochemistry. Raman spectroscopy and other techniques have been used to reveal the details of the DIY construction of a balloon-like membrane of graphene. The behaviour and properties of the carbon balloon, just a single atom thick, is reported in the journal Nano Letters. The material could have applications in biochemical imaging and in molecular transport studies. Scott Bunch, postdoc Scott Verbridge, graduate students Jonathan Alden and Arend van der Zande, and Jeevak Parpia and Harold Craighead, and Paul McEuen of the Cornell Center for Materials Research, at Cornell University, in Ithaca, New York, explain that, "Membranes are fundamental components of a wide variety of physical, chemical, and biological systems, used in everything from cellular compartmentalization to mechanical pressure sensing." Put simply, a membrane divides space into two regions, and the space either side of the membrane can thus have different physical and/or chemical properties. An example from childhood parties is the balloon. The elevated air pressure within the membrane is counterbalanced by the surface tension of the membrane and compartmentalizes the interior and the exterior. It was the notion of a molecular balloon that had Bunch and colleagues turning to graphene, the relatively novel single-molecule layer form of graphite. Graphene is a unique form of carbon. It resembles a rolled out sheet of hexagonal chicken wire in which vertex of the hexagons is a single carbon atom. Although at the lower limit of how thin any material might be, graphene is nevertheless considered to be the strongest material in the world. The tight covalent bonds spanning just two dimensions hold the carbon atoms together particularly well. Andre Geim and colleagues at the University of Manchester, England, first discovered that isolating graphene was as simple as sticking a piece of sticky tape to pure graphite, then peeling it back and re-sticking it to a silicon dioxide wafer. Peeled back from the wafer, the tape leaves a residue of graphite anywhere from one to a dozen layers thick - and from there researchers can easily identify areas of single-layer-thick graphene. Bunch, who is now an assistant professor at the University of Colorado, worked with Cornell physics professor McEuen and colleagues and colleagues, to follow this procedure, but took it a step further in order to test their graphene's elasticity by modifying the silicon dioxide wafer. They deposited graphene on a wafer etched with holes, trapping gas inside graphene-sealed microchambers. They then created a pressure differential between the gas inside and outside the microchamber. By using tapping atomic force microscope, the researchers could observe the graphene as it bulged in or out in response to pressure changes - like a balloon being pumped up and then the air allowed to escape repeatedly. They were able to inflate the graphene balloons up to several atmospheres without their popping. The researchers also turned the membrane into a tiny drum, measuring its oscillation frequency at different pressures. They found that helium, the smallest gaseous element remained trapped behind a wall of graphene even under several atmospheres of pressure. "When you work the numbers, you would expect that nothing would go through, so it's not a scientific surprise," explains McEuen, "But it does tell you that the membrane is perfect" - since even an atom-sized hole would allow the helium to escape easily. The researchers suggest that such molecular balloons might help in biological imaging of materials in solution. The thin layer would allow no materials to pass through it but would still allow researchers to peer into the solution using microscopy without getting their microscope wet. The same balloons might also be used in studies of the movement of atoms or ions through microscopic holes. "This could serve as sort of an artificial analogue of an ion channel in biology," McEuen adds, or as a way to measure the properties of an atom by observing its effect on the membrane. "You're tying a macroscopic system to the properties of a single atom," he says, "and that gives opportunities for all kinds of single atom sensors." Reference:
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Laying out graphene |
