59 research outputs found
3-D Tracking and Visualization of Hundreds of Pt-Co Fuel Cell Nanocatalysts During Electrochemical Aging
We present an electron tomography method that allows for the identification
of hundreds of electrocatalyst nanoparticles with one-to-one correspondence
before and after electrochemical aging. This method allows us to track, in
three-dimensions (3-D), the trajectories and morphologies of each Pt-Co
nanocatalyst on a fuel cell carbon support. The use of atomic-scale electron
energy loss spectroscopic imaging enables the correlation of performance
degradation of the catalyst with changes in particle/inter-particle
morphologies, particle-support interactions and the near-surface chemical
composition. We found that, aging of the catalysts under normal fuel cell
operating conditions (potential scans from +0.6 V to +1.0 V for 30,000 cycles)
gives rise to coarsening of the nanoparticles, mainly through coalescence,
which in turn leads to the loss of performance. The observed coalescence events
were found to be the result of nanoparticle migration on the carbon support
during potential cycling. This method provides detailed insights into how
nanocatalyst degradation occurs in proton exchange membrane fuel cells
(PEMFCs), and suggests that minimization of particle movement can potentially
slow down the coarsening of the particles, and the corresponding performance
degradation.Comment: Nano Letters, accepte
Epitaxy of hexagonal ABO quantum materials
Hexagonal O oxides (, = cation) are a rich materials class for
realizing novel quantum phenomena. Their hexagonal symmetry, oxygen trigonal
bipyramid coordination and quasi-two dimensional layering give rise to
properties distinct from those of the cubic O perovskites. As bulk
materials, most of the focus in this materials class has been on the rare earth
manganites, MnO ( = rare earth); these materials display coupled
ferroelectricity and antiferromagnetic order. In this review, we focus on the
thin film manifestations of the hexagonal O oxides. We cover the
stability of the hexagonal oxides and substrates which can be used to template
the hexagonal structure. We show how the thin film geometry not only allows for
further tuning of the bulk-stable manganites but also the realization of
metastable hexagonal oxides such as the FeO that combine
ferroelectricity with weak ferromagnetic order. The thin film geometry is a
promising platform to stabilize additional metastable hexagonal oxides to
search for predicted high-temperature superconductivity and topological phases
in this materials class.Comment: The following article has been accepted by Applied Physics Review
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The role of subjectivity in mitigating incentive contracting risks
We examine how subjectivity in performance measurement and reward systems (PMRS) is used to mitigate incentive contracting risks. Drawing on data from 38 interviews with supervisory and subordinate managers in four firms, we provide a more comprehensive explanation of the role of subjectivity in risk mitigation than is evident in the prior literature. We provide empirical evidence of the importance firms place on the use of subjectivity to mitigate the risk of incentive misalignment and employee sorting errors relative to its well-documented role in mitigating employee compensation risk. We find that incentive misalignment arising from unanticipated behavioral responses to performance measures is a particularly important risk, managed through subjective performance assessments. The extent of subjectivity we observe poses a significant risk of errors and bias. We observe that both vertical and horizontal information gathering and review by calibration panels are key strategies to mitigate the downside risk of subjectivity
Data Processing For Atomic Resolution EELS
The high beam current and sub-angstrom resolution of aberration-corrected
scanning transmission electron microscopes has enabled electron energy loss
spectroscopic (EELS) mapping with atomic resolution. These spectral maps are
often dose-limited and spatially oversampled, leading to low counts/channel and
are thus highly sensitive to errors in background estimation. However, by
taking advantage of redundancy in the dataset map one can improve background
estimation and increase chemical sensitivity. We consider two such approaches-
linear combination of power laws and local background averaging-that reduce
background error and improve signal extraction. Principal components analysis
(PCA) can also be used to analyze spectrum images, but the poor
peak-to-background ratio in EELS can lead to serious artifacts if raw EELS data
is PCA filtered. We identify common artifacts and discuss alternative
approaches. These algorithms are implemented within the Cornell Spectrum
Imager, an open source software package for spectroscopic analysis
Hetero-epitaxial EuO Interfaces Studied by Analytic Electron Microscopy
With nearly complete spin polarization, the ferromagnetic semiconductor
europium monoxide could enable next-generation spintronic devices by providing
efficient ohmic spin injection into silicon. Spin injection is greatly affected
by the quality of the interface between the injector and silicon. Here, we use
atomic-resolution scanning transmission electron microscopy in conjunction with
electron energy loss spectroscopy to directly image and chemically characterize
a series of EuO|Si and EuO|YAlO3 interfaces fabricated using different growth
conditions. We identify the presence of europium silicides and regions of
disorder at the EuO|Si interfaces, imperfections that could significantly
reduce spin injection efficiencies via spin-flip scattering
High-quality EuO thin films the easy way via topotactic transformation
Epitaxy is widely employed to create highly oriented crystalline films. A less appreciated, butnonetheless powerful means of creating such films is via topotactic transformation, in which achemical reaction transforms a single crystal of one phase into a single crystal of a differentphase, which inherits its orientation from the original crystal. Topotactic reactions may beapplied to epitactic films to substitute, add or remove ions to yield epitactic films of differentphases. Here we exploit a topotactic reduction reaction to provide a non-ultra-high vacuum(UHV) means of growing highly oriented single crystalline thin films of the easily overoxidizedhalf-metallic semiconductor europium monoxide (EuO) with a perfection rivallingthat of the best films of the same material grown by molecular-beam epitaxy or UHV pulsedlaserdeposition. As the technique only requires high-vacuum deposition equipment, it hasthe potential to drastically improve the accessibility of high-quality single crystalline films ofEuO as well as other difficult-to-synthesize compounds
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