Juan-Carlos Idrobo

Observation of coherent oxide precipitates in polycrystalline MgB2

Here we describe the results of an atomic resolution study of oxygen incorporation into bulk MgB2. We find that ∼20–100u2009nm sized precipitates are formed by ordered substitution of oxygen atoms onto boron lattice sites, while the basic bulk MgB2 crystal structure and orientation is preserved. The periodicity of the oxygen ordering is dictated by the oxygen concentration in the precipitates and primarily occurs in the (010) plane. The presence of these precipitates correlates well with an improved critical current density and superconducting transition behavior, implying that they act as pinning centers.

Facet-Dependent Disorder in Pristine High-Voltage Lithium–Manganese-Rich Cathode Material

Defects and surface reconstructions are thought to be crucial for the long-term stability of high-voltage lithium-manganese-rich cathodes. Unfortunately, many of these defects arise only after electrochemical cycling which occurs under harsh conditions, making it difficult to fully comprehend the role they play in degrading material performance. Recently, it has been observed that defects are present even in the pristine material. This study, therefore, focuses on examining the nature of the disorder observed in pristine Li1.2Ni0.175Mn0.525Co0.1O2 (LNMCO) particles. Using atomic-resolution Z-contrast imaging and electron energy loss spectroscopy measurements, we show that there is indeed a significant amount of antisite defects present in this material, with transition metals substituting on Li metal sites. Furthermore, we find a strong segregation tendency of these types of defects toward open facets (surfaces perpendicular to the layered arrangement of atoms) rather than closed facets (surfaces parallel to the layered arrangement of atoms). First-principles calculations identify antisite defect pairs of Ni swapping with Li ions as the predominant defect in the material. Furthermore, energetically favorable swapping of Ni on the Mn sites was observed to lead to Mn depletion at open facets. Relatively, low Ni migration barriers also support the notion that Ni is the predominant cause of disorder. These insights suggest that certain facets of the LNMCO particles may be more useful for inhibiting surface reconstruction and improving the stability of these materials through careful consideration of the exposed surface.

Measuring the hole-state anisotropy in MgB2 by electron energy-loss spectroscopy

We have examined polycrystalline MgB 2 by electron energy-loss spectroscopy (EELS) and density of states calculations. In particular, we have studied two different crystal orientations, [110] and [001], with respect to the incident electron beam direction, and found significant changes in the near-edge fine structure of the B K-edge. Density-functional theory suggests that the pre-peak of the B K-edge core loss is composed of a mixture of p x y - and p z -hole states and we will show that these contributions can be distinguished only with an experimental energy resolution better than 0.5 eV. For conventional transmission electron microscope/scanning transmission electron microscope instruments with an energy resolution of ∼1.0 eV the pre-peak still contains valuable information about the local charge-carrier concentration that can be probed by core-loss EELS. By considering the scattering momentum transfer for different crystal orientations, it is possible to analytically separate p x y and p z components from the experimental spectra. With careful experiments and analysis, EELS can be a unique tool measuring the superconducting properties of MgB 2 , doped with various elements for improved transport properties on a subnanometer scale.

Water and Solute Transport Governed by Tunable Pore Size Distributions in Nanoporous Graphene Membranes

Nanoporous graphene has the potential to advance membrane separations by offering high selectivity with minimal resistance to flow, but how mass transport depends on the structure of pores in this atomically thin membrane is poorly understood. Here, we investigate the relationship between tunable pore creation using ion bombardment and oxygen plasma etching, the resulting pore size distributions, and the consequent water and solute transport. Through tuning of the pore creation process, we demonstrate nanofiltration membranes that reject small molecules but offer high permeance to water or monovalent ions. Theoretical multiscale modeling of transport across the membranes reveals a disproportionate contribution of large pores to osmotic water flux and diffusive solute transport and captures the observed trends in transport measurements except for the smallest pores. This work provides insights into the effects of graphene pore size distribution and support layer on transport and presents a framework for designing atomically thin membranes.

Identification and lattice location of oxygen impurities in α-Si3N4

For over 40 years impurities have been believed to stabilize the ceramic α-Si3N4 but there is no direct evidence for their identity or lattice location. In bulk materials electron microscopy can generally image heavy impurities. Here we report direct imaging of N columns in α-Si3N4 that suggests the presence of excess light elements in specific N columns. First-principles calculations rule out Si or N interstitials and suggest O impurities, which are then confirmed by atomically resolved electron-energy-loss spectroscopy. The result provides a possible explanation for the stability of α-Si3N4 with implications for the design of next-generation structural ceramics.

Atomic-Resolution Study of the Interfacial Bonding at Si3N4/CeO2−δ Grain Boundaries

Using a combination of atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope, we examine the atomic and electronic structures at the interface between Si3N4 (101¯0) and CeO2−d intergranular film (IGF). Ce atoms are observed to segregate to the interface in a two-layer periodic arrangement, which is significantly different from the structure observed in a previous study. Our EELS experiments show (i) oxygen in direct contact with the terminating Si3N4 open-ring structures, (ii) a change in the Ce valence from a nominal oxidation state of +3 to almost +4 moving from the interface into the IGF, and (iii) a uniform concentration of Si in the film.

Low-Loss Imaging of Defect Structures in Two Dimensional Materials Using Aberration Corrected Scanning Transmission Electron Microscopy

Aberration correction of electron optics has provided not only higher resolution, but also higher contrast. This is particularly true for scanning transmission electron microscopes (STEMs) operating at lower accelerating voltages, which has allowed atomic resolution of 2-dimensional materials using both annular dark field (ADF) and electron energy loss spectroscopy (EELS) based on core-loss excitations [1,2]. It has until recently been assumed that atomic resolution images based on low-loss or valance EELS (VEELS) would not be possible, due to the assumed delocalization of the inelastic interaction involved. This has recently been shown not to be the case, with atomic resolution VEELS maps obtained from graphene using aberration corrected STEM and explained using first principles theory [3]. In this presentation we discuss the influence of defects and impurities in 2-dimensional materials on experimental VEELS maps, and how the underlying contrast mechanisms can be explained using a combination of density functional theory (DFT) and dynamical electron scattering theory [4].

Single Atom Imaging and Spectroscopy of Impurities in 2D Materials

1. Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 2. Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA 3. Department of Chemistry, Department of Materials Science and NanoEngineering, Rice University, Houston, TX 77005, USA 4. School of Materials Science & Engineering, Nanyang Technological University, 639798, Singapore 5. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 6. Department of Materials Science & Engineering, National University of Singapore, 117575, Singapore