Graphene allows ultrashort pulse generation in solid-state laser
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Octapodal nanocrystals self-assemble into micrometer superstructures
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esearchers have created octapodal nanoparticles that self-assemble on a number of levels to ultimately generate micrometer-sized superstructures. Their work opens the door to fast and reversible cation exchange systems, the possibility of building three-dimensional (3D) ion sensors and porous electrodes, and other applications arising from the ability to establish complex geometries of dielectric and conductive materials. K. Miszta from the Instituto Italiano di Tecnologia in Genova, Italy, J. de Graaf and R. van Roij from Utrecht University, the Netherlands, and their colleagues reported their discovery of this phenomenon in the November issue of Nature Materials (DOI: 10.1038/nmat3121; p. 872). By growing eight CdS pods out of a CdSe core, the researchers were able to fabricate monodisperse, colloidally predictable octapods that approached 100 nm in diameter. These octapods self-assembled into linear chains of interlocked octapods up to 400 nm in length in a tolu-
Graphene allows ultrashort pulse generation in solid-state laser
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raphene is a point-bandgap semiconductor with a linear dispersion of electrons with low energy that can be described by the Dirac equation involving relativistic effects. It also possesses large polarizability when illuminated by light, that is, large optical nonlinearity. These unique characteristics, in combi-
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MRS BULLETIN
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VOLUME 36 • DECEMBER 2011
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the two monomers and induces a bandgap. The researchers said, “Our work suggests that a tunable bandgap can be induced in thicker graphene samples
with ABC (rhombohedral) stacking order, thus providing a still broader class of materials with a tunable bandgap.” Steven Trohalaki
ene solvent. After aging b a the toluene solution for 24 hours, the addition of acetonitrile to the toluene caused the chains to precipitate out into 3D ordered superstructures 2 μm in length, composed entirely of self-assemd c bled chains. To create these structures, the researchers modified a previous procedure which allowed for unprecedented homogeneity and monodispersiFormation of octapod chains in toluene: (a) three-dimensional ty of particles. The team (3D) reconstruction from scanning transmission electron coated the particles with microscopy (STEM) projections; (b) STEM image after 24 h of ageing; (c) cluster size distribution curves at different ageing hydrophobic surfactant times, D is the size of the aggregates, % is the percentage molecules to improve of frequency; and (d) 3D reconstruction of a single chain, interactions before imdemonstrating the interlocking sequence along of the cube in (a), with the octapod centers well aligned on the chain mersing them in toluene. axis, as shown by the cut-through of the inset. Reprinted with The toluene octapod sopermission from Nature Mater. (DOI: 10.1038/nmat3121). © 2011 Macmillan Publishers Ltd. lution (250 μL) was aged for 12–24 hours, and then mixed with 1 mL of acetonitrile. monitored by dynamic light-scattering Two to
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