Nanosized Light Source Offers Possibilities in Bio-Imaging
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between fullerene and water was modeled by a 60-site Lennard-Jones potential. The TIP5P model was used to account for interactions between water molecules. Using the NPT ensemble (the number of particles, the pressure, and the temperature were kept constant), the researchers generated molecular conformations using a standard Monte Carlo technique—a stochastic approach that relies on random number generation. Density functional theory calculations were performed on the fullerene and the first hydration shell for only statistically significant conformations. The bandgap of the hydrated system—calculated as the energy difference between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) averaged over all statistically significant conformations—is 1.1 eV. For the unsolvated system, the HOMO-LUMO gap was found to be 1.9 eV, in good agreement with the experimental value measured in vacuum. The computational method therefore correctly predicts the red shift in the bandgap resulting from hydration, according to the researchers. An analysis of the density of states for the isolated fullerene at 0 K and for the hydrated fullerene at 298 K showed that the red shift of 0.8 eV in the bandgap is caused mainly by structural fluctuations of the hydrating water, not by dielectric screening, which is not expected to substantially redistribute charge on the fullerene surface, said the researchers. The energy difference between the HOMO and LUMO of C60 in the presence of water gives a small average shift of ca. 0.1 eV as a possible solvent effect on the bandgap of the solute. The researchers said that their “procedure avoids the lack of dispersion energy inherent to ab initio molecular dynamics simulations based on DFT,” and that their study “is the first theoretical investigation of the finite temperature impact on the electronic structure of the hydrated C60 fullerene using first principles DFT calculations.” STEVEN TROHALAKI
Biaxially Stretchable “Wavy” Silicon Nanomembranes on Elastomeric Supports Fabricated Y. Huang and J.A. Rogers from University of Illinois at Urbana-Champaign, H. Jiang from Arizona State University, and their colleagues have reported twodimensionally buckled, or “wavy,” single crystalline silicon nanomembranes on elastomeric supports with full biaxial stretchability. As described in the June issue of Nano Letters (p. 1655; DOI: 10.1021/ nl0706244), the fabrication protocols start with silicon-on-insulator (SOI) wafers. 606
A square array of holes (~2.5 μm diameter, and 25 μm pitch) was photolithographically patterned in the top silicon of the SOI wafer. The exposed silicon was removed by reactive ion etching. Immersion of the etched samples in concentrated hydrofluoric acid removed the buried silicon dioxide layer followed by an acetone wash to remove the photoresist. Separately, prepolymers of poly(dimethyl siloxane) (PDMS) were cast and cured against polished silicon wafers to generate flat, elastomeric substrates. Ozone exposure transformed the hydrophobic PDMS surface to a hydr
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