Dominance of Fermi-surface holes in p-type tunneling

In-plane uniaxial stress is used to tune continuously the mixing between the heavy-hole (HH) and light-hole (LH) states in a p-type double-barrier structure. The LH1 and HH2 resonant tunneling peaks shift at almost the same rate with stress, in contrast to the corresponding exciton peaks observed by photoreflectance, which exhibit a strong Fano-related anticrossing. Comparison between the observed shifts and a four-band k x p calculation of the state energies in the well provides the first experimental proof that the flow of holes through off-zone center states dominates the resonant tunneling current in p-type structures.     

17O magic angle spinning NMR studies of Brønsted acid sites in zeolites HY and HZSM-5

High-resolution 17O/1H double resonance NMR spectra were obtained for two zeolites, one with a low Si/Al ratio (zeolite HY) and one with a high Si/Al ratio (HZSM-5), to investigate their local structure and Br?nsted acidity. Two different oxygen signals, corresponding to Br?nsted acid sites in supercages and sodalite cages of zeolite HY were readily resolved in the two-dimensional (2-D) 1H-17O heteronuclear correlation (HETCOR) NMR spectra allowing the 17O isotropic chemical shift (deltaCS) and quadrupolar coupling parameters (quadrupolar coupling constant, QCC, and asymmetry parameter, eta) for the two oxygen atoms to be extracted. Similar experiments for HZSM-5 showed that the sites in this system are associated with a much larger distribution in NMR parameters than found in HY. 17O-1H rotational echo double resonance (REDOR) NMR was applied to probe the O-H distances in zeolites HY and HZSM-5. Weaker 17O-1H dephasing was observed for zeolite HZSM-5 in comparison to that of HY, consistent with longer O-H bonds and/or increased proton mobility.     

Synthesis of Trideuteriomethyl 5-Deuterium-β-D-Mannopyranoside

Abstract

The title compound was prepared by first converting trideuteriomethyl 2,3,4-tri-O-benzyl-β-D-mannopyranoside to a 6-bromo-6-deoxy derivative which on elimination by using DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) or DBN (1,5-diazabi-cyclo[4.3.0]non-5-ene) gave a hex-5-enopyranoside derivative. The deuteroboration of the hex-5-enopyranoside followed by oxidation and subsequent deblocking produced trideuteriomethyl 5-deuterium-β-D-mannopyranoside.     

Conversion reaction mechanisms in lithium ion batteries: study of the binary metal fluoride electrodes

Materials that undergo a conversion reaction with lithium (e.g., metal fluorides MF(2): M = Fe, Cu, ...) often accommodate more than one Li atom per transition-metal cation, and are promising candidates for high-capacity cathodes for lithium ion batteries. However, little is known about the mechanisms involved in the conversion process, the origins of the large polarization during electrochemical cycling, and why some materials are reversible (e.g., FeF(2)) while others are not (e.g., CuF(2)). In this study, we investigated the conversion reaction of binary metal fluorides, FeF(2) and CuF(2), using a series of local and bulk probes to better understand the mechanisms underlying their contrasting electrochemical behavior. X-ray pair-distribution-function and magnetization measurements were used to determine changes in short-range ordering, particle size and microstructure, while high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) were used to measure the atomic-level structure of individual particles and map the phase distribution in the initial and fully lithiated electrodes. Both FeF(2) and CuF(2) react with lithium via a direct conversion process with no intercalation step, but there are differences in the conversion process and final phase distribution. During the reaction of Li(+) with FeF(2), small metallic iron nanoparticles (<5 nm in diameter) nucleate in close proximity to the converted LiF phase, as a result of the low diffusivity of iron. The iron nanoparticles are interconnected and form a bicontinuous network, which provides a pathway for local electron transport through the insulating LiF phase. In addition, the massive interface formed between nanoscale solid phases provides a pathway for ionic transport during the conversion process. These results offer the first experimental evidence explaining the origins of the high lithium reversibility in FeF(2). In contrast to FeF(2), no continuous Cu network was observed in the lithiated CuF(2); rather, the converted Cu segregates to large particles (5-12 nm in diameter) during the first discharge, which may be partially responsible for the lack of reversibility in the CuF(2) electrode.