136 research outputs found
On-column 2p bound state with topological charge \pm1 excited by an atomic-size vortex beam in an aberration-corrected scanning transmission electron microscope
Atomic-size vortex beams have great potential in probing materials' magnetic
moment at atomic scales. However, the limited depth of field of vortex beams
constrains the probing depth in which the helical phase front is preserved. On
the other hand, electron channeling in crystals can counteract beam divergence
and extend the vortex beam without disrupting its topological charge.
Specifically, in this paper, we report atomic vortex beams with topological
charge \pm1 can be coupled to the 2p columnar bound states and propagate for
more 50 nm without being dispersed and losing its helical phase front. We gave
numerical solutions to the 2p columnar orbitals and tabulated the
characteristic size of the 2p states of two typical elements, Co and Dy, for
various incident beam energies and various atomic densities. The tabulated
numbers allow estimates of the optimal convergence angle for maximal coupling
to 2p columnar orbital. We also have developed analytic formulae for beam
energy, convergence-angle, and hologram dependent scaling for various
characteristic sizes. These length scales are useful for the design of
pitch-fork apertures and operations of microscopes in the vortex-beam imaging
mode.Comment: 30 pages, 7 figures, Microscopy and Microanalysis, in pres
Recommended from our members
Valence-programmable nanoparticle architectures.
Nanoparticle-based clusters permit the harvesting of collective and emergent properties, with applications ranging from optics and sensing to information processing and catalysis. However, existing approaches to create such architectures are typically system-specific, which limits designability and fabrication. Our work addresses this challenge by demonstrating that cluster architectures can be rationally formed using components with programmable valence. We realize cluster assemblies by employing a three-dimensional (3D) DNA meshframe with high spatial symmetry as a site-programmable scaffold, which can be prescribed with desired valence modes and affinity types. Thus, this meshframe serves as a versatile platform for coordination of nanoparticles into desired cluster architectures. Using the same underlying frame, we show the realization of a variety of preprogrammed designed valence modes, which allows for assembling 3D clusters with complex architectures. The structures of assembled 3D clusters are verified by electron microcopy imaging, cryo-EM tomography and in-situ X-ray scattering methods. We also find a close agreement between structural and optical properties of designed chiral architectures
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
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
- …