EXAFS Offers Evaluation of Intrinsic Template Binding Site of Molecularly Imprinted Polymers
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well-preserved in the channel waveguides they fabricated, and believe that O3+ ion implanted channel waveguides in Nd:YAG ceramic are suitable as active integrated devices. JOAN J. CARVAJAL
Ab Initio Calculations Demonstrate Impermeability of Graphene with Defects to Helium Atoms Applications for graphene as ultrathin barrier membranes were envisaged after recent experiments showed that graphene sheets are impenetrable to standard gases, including He. It was also recently suggested that graphene needs to be free of defects— even single-atom vacancies—in order to maintain its impermeability. Recently, however, O. Leenaerts, B. Partoens, and F.M. Peeters from the University of Antwerp in Belgium used ab initio calculations to show that graphene with common defects is still impermeable, and that large defects are required to make graphene permeable. As reported in the November 10, 2008 issue of Applied Physics Letters (DOI: 10.1063/1.3021413; #193107), Leenaerts, Partoens, and Peeters used density functional theory (DFT) to simulate defects in a 4 × 4 × 4 graphene supercell (one defect requires a larger supercell) with a distance of 16 Å between adjacent layers. The researchers selected six defects that preserve the sp2 hybridization of the 6-12 C atoms that form the periphery of the defect. The researchers first examined the approach of a He atom toward the center of a carbon hexagon in a graphene monolayer, keeping, as a first approximation, the C atoms fixed, and calculated the potential energy barrier to be so large that penetration at any temperature is impossible. The barrier to He penetration was barely decreased when the graphene was allowed to relax, as shown with molecular dynamics simulations performed within DFT formalism. The researchers showed that, at the turning point of the He atom, graphene relaxation is very small because it occurs on a much larger time scale than that for the motion of the much lighter He; almost all graphene relaxation occurs after deflection of the He atom. Graphene relaxation was therefore ignored to a very good approximation when calculating potential energy barriers to He penetration for graphene with defects, although the researchers said that this approximation may not hold for heavier penetrants. For the six defects studied, the lowest penetration barrier was always found in the center of the defect, as expected intuitively. The researchers also found that the penetration barrier decreases exponentially 76
with the size of the defect, as measured by the number of C atoms that form the defect; extrapolation shows that large defects are required for He penetration at room temperature. The researchers concluded that “even defective graphene is a suitable candidate for making impermeable nanomembranes for future applications and therefore can be considered the thinnest possible material for constructing a micro- or nanoballoon.” STEVEN TROHALAKI
Ho3+-Doped Nanophase Glass Ceramics Enhance Efficiency of Si Solar Cells In the December 15, 2008, issue of Optics Letters (DOI: 10
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