57 research outputs found
Commensurate Stripes and Phase Coherence in Manganites Revealed with Cryogenic Scanning Transmission Electron Microscopy
Incommensurate charge order in hole-doped oxides is intertwined with exotic
phenomena such as colossal magnetoresistance, high-temperature
superconductivity, and electronic nematicity. Here, we map at atomic resolution
the nature of incommensurate order in a manganite using scanning transmission
electron microscopy at room temperature and cryogenic temperature ( 93K).
In diffraction, the ordering wavevector changes upon cooling, a behavior
typically associated with incommensurate order. However, using real space
measurements, we discover that the underlying ordered state is
lattice-commensurate at both temperatures. The cations undergo picometer-scale
(6-11 pm) transverse displacements, which suggests that charge-lattice
coupling is strong and hence favors lattice-locked modulations. We further
unearth phase inhomogeneity in the periodic lattice displacements at room
temperature, and emergent phase coherence at 93K. Such local phase variations
not only govern the long range correlations of the charge-ordered state, but
also results in apparent shifts in the ordering wavevector. These
atomically-resolved observations underscore the importance of lattice coupling
and provide a microscopic explanation for putative "incommensurate" order in
hole-doped oxides
Bending and Breaking of Stripes in a Charge-Ordered Manganite
In complex electronic materials, coupling between electrons and the atomic
lattice gives rise to remarkable phenomena, including colossal
magnetoresistance and metal-insulator transitions. Charge-ordered phases are a
prototypical manifestation of charge-lattice coupling, in which the atomic
lattice undergoes periodic lattice displacements (PLDs). Here we directly map
the picometer scale PLDs at individual atomic columns in the room temperature
charge-ordered manganite BiSrCaMnO using
aberration corrected scanning transmission electron microscopy (STEM). We
measure transverse, displacive lattice modulations of the cations, distinct
from existing manganite charge-order models. We reveal locally unidirectional
striped PLD domains as small as 5 nm, despite apparent bidirectionality
over larger length scales. Further, we observe a direct link between disorder
in one lattice modulation, in the form of dislocations and shear deformations,
and nascent order in the perpendicular modulation. By examining the defects and
symmetries of PLDs near the charge-ordering phase transition, we directly
visualize the local competition underpinning spatial heterogeneity in a complex
oxide.Comment: Main text: 20 pages, 4 figures. Supplemental Information: 27 pages,
14 figure
Automated Crystal Orientation Mapping in py4DSTEM using Sparse Correlation Matching
Crystalline materials used in technological applications are often complex
assemblies composed of multiple phases and differently oriented grains. Robust
identification of the phases and orientation relationships from these samples
is crucial, but the information extracted from the diffraction condition probed
by an electron beam is often incomplete. We therefore have developed an
automated crystal orientation mapping (ACOM) procedure which uses a converged
electron probe to collect diffraction patterns from multiple locations across a
complex sample. We provide an algorithm to determine the orientation of each
diffraction pattern based on a fast sparse correlation method. We test the
speed and accuracy of our method by indexing diffraction patterns generated
using both kinematical and dynamical simulations. We have also measured
orientation maps from an experimental dataset consisting of a complex
polycrystalline twisted helical AuAgPd nanowire. From these maps we identify
twin planes between adjacent grains, which may be responsible for the twisted
helical structure. All of our methods are made freely available as open source
code, including tutorials which can be easily adapted to perform ACOM
measurements on diffraction pattern datasets.Comment: 14 pages, 7 figure
Multibeam Electron Diffraction
One of the primary uses for transmission electron microscopy (TEM) is to
measure diffraction pattern images in order to determine a crystal structure
and orientation. In nanobeam electron diffraction (NBED) we scan a moderately
converged electron probe over the sample to acquire thousands or even millions
of sequential diffraction images, a technique that is especially appropriate
for polycrystalline samples. However, due to the large Ewald sphere of TEM,
excitation of Bragg peaks can be extremely sensitive to sample tilt, varying
strongly for even a few degrees of sample tilt for crystalline samples. In this
paper, we present multibeam electron diffraction (MBED), where multiple probe
forming apertures are used to create mutiple STEM probes, all of which interact
with the sample simultaneously. We detail designs for MBED experiments, and a
method for using a focused ion beam (FIB) to produce MBED apertures. We show
the efficacy of the MBED technique for crystalline orientation mapping using
both simulations and proof-of-principle experiments. We also show how the
angular information in MBED can be used to perform 3D tomographic
reconstruction of samples without needing to tilt or scan the sample multiple
times. Finally, we also discuss future opportunities for the MBED method.Comment: 14 pages, 6 figure
Iterative Phase Retrieval Algorithms for Scanning Transmission Electron Microscopy
Scanning transmission electron microscopy (STEM) has been extensively used
for imaging complex materials down to atomic resolution. The most commonly
employed STEM imaging modality of annular dark field produces
easily-interpretable contrast, but is dose-inefficient and produces little to
no contrast for light elements and weakly-scattering samples. An alternative is
to use phase contrast STEM imaging, enabled by high speed detectors able to
record full images of a diffracted STEM probe over a grid of scan positions.
Phase contrast imaging in STEM is highly dose-efficient, able to measure the
structure of beam-sensitive materials and even biological samples. Here, we
comprehensively describe the theoretical background, algorithmic implementation
details, and perform both simulated and experimental tests for three iterative
phase retrieval STEM methods: focused-probe differential phase contrast,
defocused-probe parallax imaging, and a generalized ptychographic gradient
descent method implemented in two and three dimensions. We discuss the
strengths and weaknesses of each of these approaches using a consistent
framework to allow for easier comparison. This presentation of STEM phase
retrieval methods will make these methods more approachable, reproducible and
more readily adoptable for many classes of samples.Comment: 25 pages, 11 figures, 1 tabl
py4DSTEM: a software package for multimodal analysis of four-dimensional scanning transmission electron microscopy datasets
Scanning transmission electron microscopy (STEM) allows for imaging,
diffraction, and spectroscopy of materials on length scales ranging from
microns to atoms. By using a high-speed, direct electron detector, it is now
possible to record a full 2D image of the diffracted electron beam at each
probe position, typically a 2D grid of probe positions. These 4D-STEM datasets
are rich in information, including signatures of the local structure,
orientation, deformation, electromagnetic fields and other sample-dependent
properties. However, extracting this information requires complex analysis
pipelines, from data wrangling to calibration to analysis to visualization, all
while maintaining robustness against imaging distortions and artifacts. In this
paper, we present py4DSTEM, an analysis toolkit for measuring material
properties from 4D-STEM datasets, written in the Python language and released
with an open source license. We describe the algorithmic steps for dataset
calibration and various 4D-STEM property measurements in detail, and present
results from several experimental datasets. We have also implemented a simple
and universal file format appropriate for electron microscopy data in py4DSTEM,
which uses the open source HDF5 standard. We hope this tool will benefit the
research community, helps to move the developing standards for data and
computational methods in electron microscopy, and invite the community to
contribute to this ongoing, fully open-source project
Explaining the Unusual Photoluminescence of Semiconductor Nanocrystals Doped Via Cation Exchange
Aliovalent doping of CdSe nanocrystals (NCs) via cation exchange processes has resulted in interesting and novel observations for the optical and electronic properties of the NCs. However, despite over a decade of study, these observations have largely gone unexplained, partially due to an inability to precisely characterize the physical properties of the doped NCs. Here, electrostatic force microscopy was used to determine the static charge on individual, cation-doped CdSe NCs in order to investigate their net charge as a function of added cations. While the NC charge was relatively insensitive to the relative amount of doped cation per NC, there was a remarkable and unexpected correlation between the average NC charge and PL intensity, for all dopant cations introduced. We conclude that the changes in PL intensity, as tracked also by changes in NC charge, are likely a consequence of changes in the NC radiative rate caused by symmetry breaking of the electronic states of the nominally spherical NC due to the Coulombic potential introduced by ionized cations
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