Showing papers by "Malcolm L. H. Green published in 2016"

Simultaneous atomic-resolution electron ptychography and Z-contrast imaging of light and heavy elements in complex nanostructures.

Abstract: The aberration-corrected scanning transmission electron microscope (STEM) has emerged as a key tool for atomic resolution characterization of materials, allowing the use of imaging modes such as Z-contrast and spectroscopic mapping. The STEM has not been regarded as optimal for the phase-contrast imaging necessary for efficient imaging of light materials. Here, recent developments in fast electron detectors and data processing capability is shown to enable electron ptychography, to extend the capability of the STEM by allowing quantitative phase images to be formed simultaneously with incoherent signals. We demonstrate this capability as a practical tool for imaging complex structures containing light and heavy elements, and use it to solve the structure of a beam-sensitive carbon nanostructure. The contrast of the phase image contrast is maximized through the post-acquisition correction of lens aberrations. The compensation of defocus aberrations is also used for the measurement of three-dimensional sample information through post-acquisition optical sectioning. The use of ptychography with electrons has been limited. Here, Yang et al. demonstrate that the combination of Z-contrast and phase imaging reveals the structure of complex nanomaterials. This practical tool can be used to solve the structure of a beam-sensitive carbon nanostructure at atomic-resolution.

The classification and representation of main group element compounds that feature three-center four-electron interactions

Abstract: This article provides a means to classify and represent compounds that feature 3-center 4-electron (3c–4e) interactions in terms of the number of electrons that each atom contributes to the interaction. Specifically, Class I 3c–4e interactions are classified as those in which two atoms provide one electron each and the third atom provides a pair of electrons (i.e. LX2), while Class II 3c–4e interactions are classified as those in which two atoms each provide a pair of electrons and the third atom contributes none (i.e. L2Z). These classes can be subcategorized according to the nature of the central atom. Thus, Class I interactions can be categorized according to whether the central atom provides one (i.e. μ–X) or two (i.e. μ–L) electrons, while Class II interactions can be categorized according to whether the central atom provides none (i.e. μ–Z) or two (i.e. μ–L) electrons. The use of appropriate structure-bonding representations for these various interactions provides a means to determine the covalent bond classification of the element of interest.

The Covalent Bond Classification Method and Its Application to Compounds That Feature 3-Center 2-Electron Bonds

Abstract: This article provides a means to classify and represent compounds that feature 3-center 2-electron (3c–2e) interactions according to whether (1) the two electrons are provided by one or by two atoms; (2) the central bridging atom provides two, one, or zero electrons; and (3) the interaction is open or closed. Class I 3c–2e bonds are defined as those in which two atoms each contribute one electron to the 3-center orbital, while Class II 3c–2e bonds are defined as systems in which the pair of electrons are provided by a single atom. The use of appropriate structure-bonding representations enables the [ML l X x Z z ] covalent bond classification of the element of interest to be evaluated. This approach is of considerable benefit in predicting metal–metal bond orders that are in accord with theory for dimetallic compounds that feature bridging hydride and carbonyl ligands.