180 research outputs found
Nanoscale assembly processes revealed in the nacroprismatic transition zone of Pinna nobilis mollusc shells
Intricate biomineralization processes in molluscs engineer hierarchical
structures with meso-, nano-, and atomic architectures that give the final
composite material exceptional mechanical strength and optical iridescence on
the macroscale. This multiscale biological assembly inspires new synthetic
routes to complex materials. Our investigation of the prism-nacre interface
reveals nanoscale details governing the onset of nacre formation using
high-resolution scanning transmission electron microscopy. A wedge polishing
technique provides unprecedented, large-area specimens required to span the
entire interface. Within this region, we find a transition from nanofibrillar
aggregation to irregular early-nacre layers, to well-ordered mature nacre
suggesting the assembly process is driven by aggregation of nanoparticles
(~50-80 nm) within an organic matrix that arrange in fiber-like polycrystalline
configurations. The particle number increases successively and, when critical
packing is reached, they merge into early-nacre platelets. These results give
new insights into nacre formation and particle-accretion mechanisms that may be
common to many calcareous biominerals.Comment: 5 Figure
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
Extended Depth of Field for High Resolution Scanning Transmission Electron Microscopy
Aberration-corrected scanning transmission electron microscopes (STEM)
provide sub-angstrom lateral resolution; however, the large convergence angle
greatly reduces the depth of field. For microscopes with a small depth of
field, information outside of the focal plane quickly becomes blurred and less
defined. It may not be possible to image some samples entirely in focus.
Extended depth-of-field techniques, however, allow a single image, with all
areas in-focus, to be extracted from a series of images focused at a range of
depths. In recent years, a variety of algorithmic approaches have been employed
for bright field optical microscopy. Here, we demonstrate that some established
optical microscopy methods can also be applied to extend the ~6 nm depth of
focus of a 100 kV 5th-order aberration-corrected STEM (alpha_max = 33 mrad) to
image Pt-Co nanoparticles on a thick vulcanized carbon support. These
techniques allow us to automatically obtain a single image with all the
particles in focus as well as a complimentary topography map.Comment: Accepted, Microscopy and Microanalysi
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
Breaking the Crowther Limit: Combining Depth-Sectioning and Tilt Tomography for High-Resolution, Wide-Field 3D Reconstructions
To date, high-resolution (< 1 nm) imaging of extended objects in
three-dimensions (3D) has not been possible. A restriction known as the
Crowther criterion forces a tradeoff between object size and resolution for 3D
reconstructions by tomography. Further, the sub-Angstrom resolution of
aberration-corrected electron microscopes is accompanied by a greatly
diminished depth of field, causing regions of larger specimens (> 6 nm) to
appear blurred or missing. Here we demonstrate a three-dimensional imaging
method that overcomes both these limits by combining through-focal depth
sectioning and traditional tilt-series tomography to reconstruct extended
objects, with high-resolution, in all three dimensions. The large convergence
angle in aberration corrected instruments now becomes a benefit and not a
hindrance to higher quality reconstructions. A through-focal reconstruction
over a 390 nm 3D carbon support containing over one hundred dealloyed and
nanoporous PtCu catalyst particles revealed with sub-nanometer detail the
extensive and connected interior pore structure that is created by the
dealloying instability
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