Ferroelectric Domain Inversion Demonstrated in BaMgF 4
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near the tips of the pyramids. They attributed the location of the nucleation sites to the unique strain gradients as a direct result of the template geometry. They said that superlattice instability and compressive strain, due to dissimilar coefficients of thermal expansion, could also favor nucleation. This feature, the researchers suggest, could be used in the fabrication of large arrays of individually addressable silicon quantum dots. The researchers were able to show that Si nanocrystals could be placed in specific locations on a Si wafer by using standard Si processing techniques. CALIN MICLAUS
Theoretical Study Predicts Amide Formation is Preferred on Armchair Single-Walled Carbon Nanotubes Using a theoretical approach to study the reactivity of carboxylic groups toward amines in the direct amidation of singlewalled carbon nanotubes (SWNTs) and its dependence on the nanotube structure, some aspects of the chemical derivatization of these nanotubes have become further understood and their application areas expanded. Application areas of nanotubes could be significantly expanded by chemical derivatization. As reported in the November issue of Nano Letters, researchers Vladimir Basiuk, Elena Basiuk, and José-Manuel Saniger-Blesa of the National Autonomous University of Mexico obtained theoretical evidence that the direct formation of amides proceeds preferentially on armchair SWNT tips as compared with zigzag SWNT tips, according to the scheme SWNT-COOH + HNR1R2 → SWNT-CO-NR1R2 + H2O, where R1
= H or Alk, and R2
(1)
= Alk. The reaction mechanisms were studied using a two-level so-called ONIOM (own n-layered integrated molecular orbital and molecular mechanics) approach, where the molecular system being studied is divided into two layers that are treated with different model chemistries. The results are then automatically combined into the final predicted results. Monocarboxylated short fragments of (10,0) zigzag SWNTs and (5,5) armchair SWNTs were used to interact with methylamine as model reaction systems. The transition states of the molecular systems were also treated and followed using ONIOM refinement. The researchers reported that all geometric optimizations follow the default convergence criteria set in the Gaussian 98W program. According to the report, analysis of total optimized geometries for the reaction complexes shows that there is a weak hydro-
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gen bond between the amino and carboxylic groups in the armchair SWNTs, whereas the corresponding distance in the zigzag nanotubes is far too long to consider it a H bond. The transition states in both models do not differ significantly in geometry, but the energy of the armchair reaction complex is 10.5 kcal mol-1 higher than that of its zigzag counterpart, implying, the researchers said, that there is a stronger methylamine adsorption on the zigzag SWNTs. Also, the armchair amidation products achieve a lower energy compared with the zigzag SWNT tips; therefore, the latter reaction is less energetically favorable, according to the researchers. W
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