Method Developed for Asymmetric Patterning of Nanospheres
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that lead to the formation of chiroptical rosette nanotubes. At millimolar concentrations and in the absence of a promoter, 1 self-assembles into a racemic mixture of helical rosette nanotubes. An instantaneous transition from racemic to homochiral helical nanotubes accompanies the addition of a promoter. At micromolar concentrations, 1 is unassembled but addition of a promoter triggers selfassembly into rosette nanotubes with a helicity predefined by the promoter. The supramolecular reactions involved must first involve molecular recognition between 1 and the promoter. Fenniri and co-workers believe that when viewed as a model process by which predefined molecular properties are expressed at the macromolecular level through programmable stereospecific self-assembly. STEVEN TROHALAKI
Acoustical Memory Observed in LiNbO3 Michael McPherson and colleagues from the National Center for Physical Acoustics at the University of Mississippi have observed acoustical memory in LiNbO3 crystals. An acoustical tone burst was stored inside the crystal and reemitted at a later time. They report in the September issue of Physical Review Letters that the strength of the memory signal is directly related to the dielectrical properties of the ferroelectric material, and is sensitive to both temperature and frequency. Acoustical memory may be useful for investigating energy exchange processes in ferroelectrics, they said. Cylinders of bulk LiNbO 3 ranging from 2.4 cm to 3.9 cm in length and from 2.5 cm to 4.5 cm in diameter were used to demonstrate the effect, where the acoustical longitudinal mode propagation along the [001] axis corresponds with the piezoelectrically active direction. A 39° rotated Y-cut 30-MHz transducer was mounted to the top of the sample and a capacitive receiver was mounted to the bottom to capture the resulting signal. A synthesizer function generator produced sinusoidal tone bursts approximately 5 µs in duration. To verify the results, the experiment was repeated in which the configuration involved the removal of the capacitive receiver, causing the transducer to act as both emitter and receiver. In another configuration, a secondary transducer was mounted to the opposite face of the sample, as a standard two-transducer setup. The samples were tested at various temperatures ranging from 24°C to 46°C and within a signal frequency range of 16–26 MHz. The researchers found that the 942
acoustical memory effect was sensitive to both temperature and frequency, and closely related to the dielectrical properties of the sample, possibly due to the pinning and relaxation of domain walls within the crystal. Within the frequency range, extrema appear in the acoustical memory amplitude, implying a memory center size. The researchers estimate these memory centers to be 140–230 µm for the observed peaks, which corresponds to the size of ferroelectric domains in lithium niobate. CHRISTINE RUSSELL
Removal Rates in Chemical– Mechanical Planarization of Copper and Silica Surfaces Increase with Particle Concentration When Usi
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