Runzhe Tao

Atomic-scale observation of lithiation reaction front in nanoscale SnO2 materials.

In the present work, taking advantage of aberration-corrected scanning transmission electron microscopy, we show that the dynamic lithiation process of anode materials can be revealed in an unprecedented resolution. Atomically resolved imaging of the lithiation process in SnO2 nanowires illustrated that the movement, reaction, and generation of b = [1[overline]1[overline]1] mixed dislocations leading the lithiated stripes effectively facilitated lithium-ion insertion into the crystalline interior. The geometric phase analysis and density functional theory simulations indicated that lithium ions initial preference to diffuse along the [001] direction in the {200} planes of SnO2 nanowires introduced the lattice expansion and such dislocation behaviors. At the later stages of lithiation, the Li-induced amorphization of rutile SnO2 and the formation of crystalline Sn and LixSn particles in the Li2O matrix were observed.

Direct atomic-scale imaging of hydrogen and oxygen interstitials in pure niobium using atom-probe tomography and aberration-corrected scanning transmission electron microscopy.

Imaging the three-dimensional atomic-scale structure of complex interfaces has been the goal of many recent studies, due to its importance to technologically relevant areas. Combining atom-probe tomography and aberration-corrected scanning transmission electron microscopy (STEM), we present an atomic-scale study of ultrathin (~5 nm) native oxide layers on niobium (Nb) and the formation of ordered niobium hydride phases near the oxide/Nb interface. Nb, an elemental type-II superconductor with the highest critical temperature (T(c) = 9.2 K), is the preferred material for superconducting radio frequency (SRF) cavities in next-generation particle accelerators. Nb exhibits high solubilities for oxygen and hydrogen, especially within the RF-field penetration depth, which is believed to result in SRF quality factor losses. STEM imaging and electron energy-loss spectroscopy followed by ultraviolet laser-assisted local-electrode atom-probe tomography on the same needle-like sample reveals the NbO(2), Nb(2)O(5), NbO, Nb stacking sequence; annular bright-field imaging is used to visualize directly hydrogen atoms in bulk β-NbH.

Selective Adsorption of Manganese onto Rhodium for Optimized Mn/Rh/SiO2 Alcohol Synthesis Catalysts

Using supported rhodium‐based catalysts to produce alcohols from syngas provides an alternative route to conventional fermentation methods. If left unpromoted, Rh catalysts have a strong selectivity towards methane. However, promotion with early transition metal elements has been shown to be effective to increase alcohol selectivity. Therefore, a key design objective is to increase the promoter–metal interaction to maximize their effectiveness. This can be achieved by the use of the strong electrostatic adsorption (SEA) method, which utilizes pH control to steer the promoter precursor (in this case MnO4−) onto Rh oxide supported on SiO2. Mn‐promoted catalysts were synthesized by both SEA and traditional incipient wetness impregnation (IWI) and subsequently characterized by STEM and extended X‐ray absorption fine structure methods. Using STEM–electron energy loss spectroscopy mapping, catalysts prepared by SEA were shown to have a higher degree of interaction between the promoter and the active metal. The reduction behavior of the catalysts obtained by X‐ray absorption near‐edge spectroscopy and temperature‐programmed reduction demonstrated a minimal change in Rh if promoted by SEA. However, catalytic results for CO hydrogenation revealed that a significant improvement of ethanol selectivity is achieved if the promoter was prepared by SEA in comparison with the promoter prepared by IWI. These results suggest that intimate interaction between the promoter and the metal is a critical factor for improving selectivity to higher alcohols.

Electron energy-loss spectroscopy study of metallic Nb and Nb oxides

We present a series of electron energy-loss spectroscopy (EELS) studies on niobium (Nb) and its oxides (NbO, NbO2, and Nb2O5) to develop a reliable method for quantifying the oxidation state in mixed niobium oxide thin films. Our approach utilizes a combination of transmission electron microscopy and EELS experiments with density functional theory calculations to distinguish between metallic niobium and the different niobium oxides. More specifically, the differences in the near-edge fine-structure of the Nb M-edge and O K-edge provide sufficient information to determine the valence state of niobium. Based on these observed changes in the core-loss edges, we propose a linear relationship that correlates the peak positions in the Nb M- and O K-edges with the Nb valence state. The methods developed in this paper are also applied to ultrathin niobium oxide films to examine the effects of low-temperature baking on the films’ oxidation states.

A study of the effect of iron island morphology and interface oxidation on the magnetic hysteresis of Fe-MgO (001) thin film composites

Fe-MgO tunnel junctions have received much attention for their use in hard drive read heads and other spintronic applications. The system is particularly interesting because of its magnetoresistive behavior and the abundance and low cost of its constituent elements. However, many questions remain about how the structure and chemistry of the Fe-MgO interface mediates magnetic behavior. In this study, we report on transmission electron microscopy, electron energy loss spectroscopy, and magnetic characterization of Fe-MgO composite films with various morphologies. We explore relationships between film morphology, intermixing, and the resulting effects on magnetic structure. We find the presence of oxidation at the Fe-MgO interface, with a detrimental impact on the saturation magnetization of the composite. We also observe changes in coercivity and magnetocrystalline anisotropy with film morphology and thickness. These results will inform the design of MgO-based tunnel junctions and improve our understanding ...

Low temperature study of structural phase transitions in niobium hydrides

Niobium (Nb) and its hydrides have been the focus of many studies due to applications as a hydrogen storage material, as a dielectric coating in semiconductor devices and in superconducting radio-frequency cavities. In this paper, we will present the atomic-scale characterization of Nb hydrides using scanning transmission electron microscopy and electron energy loss spectroscopy (EELS) at room and liquid nitrogen temperatures. Although such cavities are formed from ultrahigh purity Nb, using electron beam diffraction, we found that at LN2 temperature, the grains near the surface of cold-worked Nb sheets contain regions exhibiting three different superlattice features, which are identified as β, e, and ζ-NbHx phases. Z-contrast imaging and EELS at LN2 temperature are utilized to qualify their atomic and electronic structures.

Giant two-phonon Raman scattering from nanoscale NbC precipitates in Nb

High-purity niobium (Nb), subjected to the processing methods used in the fabrication of superconducting rf cavities, displays micrometer-sized surface patches containing excess carbon. High-resolution transmission electronmicroscopyandelectronenergy-lossspectroscopymeasurementsarepresentedwhichrevealthepresence of nanoscale NbC coherent precipitates in such regions. Raman backscatter spectroscopy on similar surface regions exhibit spectra consistent with the literature results on bulk NbC but with significantly enhanced twophononscattering.Theunprecedentedstrengthandsharpnessofthetwo-phononsignalhaspromptedatheoretical analysis, using density functional theory (DFT), of phonon modes in NbC for two different interface models of the coherent precipitate. One model leads to overall compressive strain and a comparison to ab initio calculations of phonon dispersion curves under uniform compression of the NbC shows that the measured two-phonon peaks are linked directly to phonon anomalies arising from strong electron-phonon interaction. Another model of the extended interface between Nb and NbC, studied by DFT, gives insight into the frequency shifts of the acoustic and optical mode density of states measured by first-order Raman spectroscopy. The exact origin of the stronger two-phononresponseisnotknown atpresentbutitsuggeststhepossibilityof enhanced electron-phonon coupling in transition-metal carbides under strain found either in the bulk NbC inclusions or at their interfaces with Nb metal. Preliminary tunneling studies using a point contact method show some energy gaps larger than expected for bulk NbC.

In-situ study of Nb hydride for SRF cavity applications using aberration-corrected STEM and electron energy loss spectroscopy

Niobium is a type-II superconducting material with the highest critical temperature (Tc = 9.2 K) and generally used to fabricate superconducting radio-frequency (SRF) cavities for linear particle accelerators. We present an atomic-resolution study of the effects that a 48 hour bake at 120 °C in vacuum has on the high-field properties of Nb-based SRF cavities. This bake results in a significant increase in high-field quality factor Q. However, an 800 °C bake for 2 hour reduces the Hc3/Hc2-ratio of cavities. Several mechanisms have been proposed to explain this contrary behavior, including an increased NbOx surface layer thickness and the precipitation of NbHy.[1]