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discovered_01_2015

FOCUS// THE HZDR RESEARCH MAGAZINE WWW.HZDR.DE 10 11 with much thicker membranes, this chance decreases as the thickness of the membrane increases. At one million layers, the ions are sure to miss a gap and thus cannot pass through unimpeded any more. Since researchers are unable to detect at which precise layer a single ion is decelerated, they only register the ions once they exit the membrane. With normal materials, they usually find virtually all of them strongly decelerated. Only with ultra-thin membranes, such as the one from Bielefeld, can a portion of the ions manage to fly through gaps in all three of the atomic layers they pass. If ions do not find a gap in all three of the layers, they are not only decelerated, but also pick up lots of electrons from the membrane. Since each xenon ion carries a high positive charge and thus absorbs many electrons, a small spot in the membrane will suddenly be missing a large amount of electrons. This obviously destabilizes the membrane, hurling out many hundreds or even a couple of thousand atoms. The spots in the membrane where an ion was decelerated are thus perforated with tiny nanopores. The ion beam turns the membrane into a sort of molecular sieve. It is as yet unclear how exactly these holes are generated. ‘I could imagine that very many electrons are released at the spot of impact on just a few square nanometers, and that a very high positive electric charge is thus concentrated on a tiny area’, Richard Wilhelm ponders. This highly concentrated charge could in turn cause what is called a ‘Coulomb explosion’ where atoms are ejected out of this very small area, creating tiny holes with a diameter of just a few nanometers. Another theory assumes that the intense energy contained in the extremely highly-charged ions heats up the small area to the point that the material evaporates, which would also leave a nano-hole. Regardless of the exact mechanism that generates the tiny pores, another observation is of interest: the higher the positive charge of the xenon ions, the larger the amount of ions that get stuck and tear holes into the membrane. A high ion charge is thus more effective for perforating the membrane with pores of a well-determined nano-size. Since Richard Wilhelm and his colleagues are able to control the charge of the xenon ions, their ion beams are very well suited to producing tailor-made nanosieves. Thus the researchers now have a pretty practical, high-precision recipe for producing molecular sieves. PUBLICATIONS: R. Wilhelm, E. Gruber et al.: ‘Charge exchange and energy loss of slow highly charged ions in 1 nm thick carbon nano- membranes’, in Physical Review Letters 2014 (DOI: 10.1103/ PhysRevLett.112.153201) R. Ritter, R. A. Wilhelm et al.: ‘Fabrication of nanopores in 1 nm thick carbon nanomembranes with slow highly- charged ions’, in Applied Physics Letters 2013 (DOI: 10.1063/1.4792511) CONTACT Institute of Ion Beam Physics and Materials Research at HZDR Dr. Richard A. Wilhelm r.wilhelm@hzdr.de NANO-STRUCTURES: This facility will make it possible to implant individual ions into a surface of mere square nanometers like precision projectiles. Richard Wilhelm (left) and René Heller calibrating the ion beam. Photo: Frank Bierstedt

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