J. Bentley

Detection of Single Atoms and Buried Defects in Three Dimensions by Aberration-Corrected Electron Microscope with 0.5-Å Information Limit

The ability of electron microscopes to analyze all the atoms in individual nanostructures is limited by lens aberrations. However, recent advances in aberration-correcting electron optics have led to greatly enhanced instrument performance and new techniques of electron microscopy. The development of an ultrastable electron microscope with aberration-correcting optics and a monochromated high-brightness source has significantly improved instrument resolution and contrast. In the present work, we report information transfer beyond 50 pm and show images of single gold atoms with a signal-to-noise ratio as large as 10. The instruments new capabilities were exploited to detect a buried Sigma3 {112} grain boundary and observe the dynamic arrangements of single atoms and atom pairs with sub-angstrom resolution. These results mark an important step toward meeting the challenge of determining the three-dimensional atomic-scale structure of nanomaterials.

Surface/Interface-Related Conductivity in Nanometer Thick YSZ Films

Results of the electrical conductivity study of highly textured, ultrathin (15 nm) cubic yttria-stabilized zirconia (YSZ) thin films are presented for the first time. A nanoscale effect that results in exceptionally high ionic conductivity at moderate temperatures is detected in films less than 60 nm thick. The conductivity increases continuously below this level and reaches 0.6 S/cm at 800°C for a 15 nm thick film, which represents the highest reported value for the YSZ system. The observed behavior is attributed to an increasingly significant contribution of the surface/interface conductivity with decreasing film thickness. These observations can have important implications for the development of nanostructured electrochemical devices with enhanced performance.

Soft anisotropic high magnetization Cu/FeCo films

A remarkable reduction in the coercivity Hc of sputtered Fe65Co35 films from 9.6 to 0.7 kA/m was observed using a Cu underlayer as thin as 2.5 nm. The FeCo without Cu exhibited a wide distribution of anisotropy fields up to >80 kA/m while the FeCo with Cu showed a well-defined in-plane uniaxial anisotropy field of 2.3 kA/m up to FeCo thicknesses of at least 1 μm. The saturation magnetostriction was (4.7±0.4)×10−5, independent of Cu thickness while the in-plane tensile stress gradually decreased from 2 to 0.2 GPa as the Cu thickness increased to 10 nm. The Cu changed the preferred orientation of the FeCo from (100) to (110) but more significantly reduced the average grain size from ∼50 to ∼9 nm. This alone is sufficient to explain quantitatively the reduction in Hc using Hoffmann’s ripple theory.

Irradiation Induced Precipitation in Tungsten Based, W-Re Alloys

Tungsten-base alloys containing 5, 11, and 25 pct Re were irradiated in the EBR-II reactor. Irradiation temperatures ranged from 600 to 1500 °C. All compositions were irradiated to fluences in the range 4.3 to 6.1 X 1025 n/m2 (E > 0.1 MeV), and three 25 pct Re samples were also irradiated to 3.7 X 1026 n/m2 at temperatures 700 to 900 °C. Postirradiation examination included measurement of electrical resistivity at room temperature and lower temperatures, X-ray diffraction, optical metallography, microprobe analysis, and transmission electron microscopy. Irradiation induced resistivitydecreases observed in most of the samples suggested second-phase precipitation. Complete results confirmed the precipitate formation in all samples, in disagreement with existing phase diagrams for the W-Re system. Electron diffraction showed the precipitates to be consistent with the cubic, Re-richX-phase and inconsistent with the σ-phase. Large variations in precipitate morphology and distribution were observed between the different compositions and irradiation conditions. For the 5 and 11 pct Re-alloys, spherically symmetric strain fields surrounded the equiaxed precipitate particles, and were observed even where no particles were visible. These strain fields are believed to arise from local Re enrichment. Thermoelectric data show that the precipitation can lead to decalibration of W/Re thermocouples.

Valence state mapping of cobalt and manganese using near-edge fine structures

The properties of transition metal oxides are related to the presence of elements with mixed valences. The spectroscopy analysis of the valence states is feasible experimentally, but a spatial mapping of valence states of transition metal elements is a challenge to existing microscopy techniques. In this paper, with the use of valence state information provided by the white lines and near-edge fine structures observed using the electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM), a novel experimental approach is demonstrated to map the valence state distributions of Mn and Co using the ratio of white lines in the energy-filtered TEM. The valence state map is almost independent of specimen thickness in the thickness range adequate for quantitative EELS microanalysis. An optimum spatial resolution of approximately 2 nm has been achieved for a two-phase Co oxides.

A TEM study of neutron-irradiated iron☆

Abstract The results of a transmission electron microscopy study of the defect structure in iron neutron-irradiated to low fluences (⩽ 1 dpa) at temperatures of 455–1013 K are presented. The dislocation microstructures coarsen with increasing irradiation temperature from decorated dislocations, through clusters of dislocation loops, to near-edge, interstitial dislocation loops with b = a 〈100〉, and network segments. Significant cavity formation occurred only at 548–723 K, with homogeneous distributions found only at 623 and 673 K. The maximum swelling of 0.07% occurred at 673 K. Large cavities had a truncated octahedral shape with {111} facets and {100} truncations. Damage halos were observed around boron-containing precipitates. The effects of interstitial impurities on microstructural development and the differences in the observed microstructures compared to those in refractory bcc metals are discussed.

Site-distributions of Fe alloying additions to B2-ordered NiAl

Abstract The site-distributions of Fe alloying additions to B2-ordered NiAl alloys have been measured by ALCHEMI ( a tom l ocation by ch anneling- e nhanced m icroanalysis). Twelve alloy compositions were examined, of stoichiometries Ni50−xAl50Fex (Ni-deficient), Ni50Al50−xFex (Al-deficient), and Ni50−x/2Al50−x/2Fex (intermediate), with four different alloying levels, x=0.25, 2, 5, and 10. The data indicate that Fe tends to act as a buffer between the two sublattices, preferentially occupying the site of the stoichiometrically deficient host element. However, the Fe alloying addition was found to partition between the two sublattices to some extent in all alloys, so that ‘Ni’-site vacancies or Ni anti-site defects are required to maintain the site balance. The observed trends in the site-distributions suggest that the range of compositions reported by Darolia et al. (Darolia R, Lahrman DF, Field RD. Scripta Metall Mater 1992;26:1007). to exhibit enhanced ductility may coincide with an inversion in the site-substitution behavior, where more Ni than Fe occupies the aluminum sublattice.

High-resolution elemental profiles of the silicon dioxide∕4H-silicon carbide interface

High-resolution elemental profiles were obtained from SiO2(N)∕4H‐SiC structures by spatially resolved electron energy-loss spectroscopy (EELS) performed in the scanning transmission electron microscopy mode. The results show that following annealing in NO, N was exclusively incorporated within ∼1nm of the SiO2(N)∕4H‐SiC interface. Mean interfacial nitrogen areal densities measured by EELS were ∼(1.0±0.2)×1015cm−2 in carbon-face samples and (0.35±0.13)×1015cm−2 in Si-face samples; these results are consistent with nuclear reaction analysis measurements. Some of the interface regions in the C-face samples also showed excess carbon that was not removed by the NO annealing process, in contrast with previous results on Si-face samples.

Structural characterization of cobalt catalysts on a silica support

Abstract In situ X-ray diffraction was employed to characterize the structure of several cobalt catalysts on a silica support. The catalysts were reduced in flowing hydrogen at 350 °C for about 16 h and X-ray diffraction patterns were collected. After reduction metallic cobalt was found to be present in the hcp and fcc forms in the ratio of 7:3 and 17% of the hexagonal close-packed planes were found to be faulted. If surface atoms adjacent to stacking faults are sources of active sites, the density of active sites was estimated to be 4.0 × 10 19 per gram of cobalt.

Electrical, structural, and chemical analysis of silicon carbide-based metal-oxide-semiconductor field-effect-transistors

In this study we investigated the morphology and interfacial chemistry of (0001) 4H-SiC-based metal-oxide-semiconductor field-effect transistors (MOSFETs) as a function of post-oxidation annealing in nitric oxide (NO) following wet oxidation. Energy-filtered transmission electron microscopy analyses showed enhanced C/Si concentrations (up to 13%) at distinct locations along the SiO2/SiC interface in the MOSFETs that were not annealed in NO. In contrast, regions of enhanced C/Si concentration were not detected in the MOSFETs that were annealed in NO; instead, these samples showed a trace amount of interfacial N. The introduction of N may therefore be associated with a reduction of C in these samples and may contribute to the higher channel mobility (∼38 cm2/V s) in the samples annealed in NO relative to the samples that were not annealed in NO (∼9 cm2/V s). Rough SiO2/4H-SiC interfaces and nonuniform oxide thickness were observed on both the NO- and the non-NO-annealed samples. The rough interfaces shown i...