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Register a new userLattice strain measurement of core@shell electrocatalysts with 4D-STEM nanobeam electron diffraction
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Added by
Debangshu Mukherjee
Publication date
2020
fields
Physics
and research's language is
English
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Strain engineering enables the direct modification of the atomic bonding and is currently an active area of research aimed at improving the electrocatalytic activity. However, directly measuring the lattice strain of individual catalyst nanoparticles is challenging, especially at the scale of a single unit cell. Here, we quantitatively map the strain present in rhodium@platinum (core@shell) nanocube electrocatalysts using conventional aberration-corrected scanning transmission electron microscopy (STEM) and the recently developed technique of 4D-STEM nanobeam electron diffraction. We demonstrate that 4D-STEM combined with data pre-conditioning allows for quantitative lattice strain mapping with sub-picometer precision without the influence of scan distortions. When combined with multivariate curve resolution, 4D-STEM allows us to distinguish the nanocube core from the shell and to quantify the unit cell size as a function of distance from the core-shell interface. Our results demonstrate that 4D-STEM has significant precision and accuracy advantages in strain metrology of catalyst materials compared to aberration-corrected STEM imaging and is beneficial for extracting information about the evolution of strain in catalyst nanoparticles.
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Scanning nanobeam electron diffraction (NBED) with fast pixelated detectors is a valuable technique for rapid, spatially resolved mapping of lattice structure over a wide range of length scales. However, intensity variations caused by dynamical diffraction and sample mistilts can hinder the measurement of diffracted disk centers as necessary for quantification. Robust data processing techniques are needed to provide accurate and precise measurements for complex samples and non-ideal conditions. Here we present an approach to address these challenges using a transform, called the exit wave power cepstrum (EWPC), inspired by cepstral analysis in audio signal processing. The EWPC transforms NBED patterns into real-space patterns with sharp peaks corresponding to inter-atomic spacings. We describe a simple analytical model for interpretation of these patterns that cleanly decouples lattice information from the intensity variations in NBED patterns caused by tilt and thickness. By tracking the inter-atomic spacing peaks in EWPC patterns, strain mapping is demonstrated for two practical applications: mapping of ferroelectric domains in epitaxially strained PbTiO3 films and mapping of strain profiles in arbitrarily oriented core-shell Pt-Co nanoparticle fuel-cell catalysts. The EWPC transform enables lattice structure measurement at sub-pm precision and sub-nm resolution that is robust to small sample mistilts and random orientations.
The arrival of direct electron detectors (DED) with high frame-rates in the field of scanning transmission electron microscopy has enabled many experimental techniques that require collection of a full diffraction pattern at each scan position, a field which is subsumed under the name four dimensional-scanning transmission electron microscopy (4D-STEM). DED frame rates approaching 100 kHz require data transmission rates and data storage capabilities that exceed commonly available computing infrastructure. Current commercial DEDs allow the user to make compromises in pixel bit depth, detector binning or windowing to reduce the per-frame file size and allow higher frame rates. This change in detector specifications requires decisions to be made before data acquisition that may reduce or lose information that could have been advantageous during data analysis. The 4D Camera, a DED with 87 kHz frame-rate developed at Lawrence Berkeley National Laboratory, reduces the raw data to a linear-index encoded electron event representation (EER). Here we show with experimental data from the 4D Camera that linear-index encoded EER and its direct use in 4D-STEM phase contrast imaging methods enables real-time, interactive phase-contrast from large-area 4D-STEM datasets. We detail the computational complexity advantages of the EER and the necessary computational steps to achieve real-time interactive ptychography and center-of-mass differential phase contrast using commonly available hardware accelerators.
GaAs nanowires and GaAs-Fe3Si core-shell nanowire structures were grown by molecular-beam epitaxy on oxidized Si(111) substrates and characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Ga droplets were formed on the oxide surface, and the semiconducting GaAs nanowires grew epitaxially via the vapor-liquid-solid mechanism as single-crystals from holes in the oxide film. We observed two stages of growth of the GaAs nanowires, first the regular growth and second the residual growth after the Ga supply was finished. The magnetic Fe3Si shells were deposited in an As-free chamber. They completely cover the GaAs cores although they consist of small grains. High-resolution TEM micrographs depict the differently oriented grains in the Fe3Si shells. Selected area diffraction of electrons and XRD gave further evidence that the shells are textured and not single crystals. Facetting of the shells was observed, which lead to thickness inhomogeneities of the shells.
The combination of core/shell geometry and band gap engineering in nanowire heterostructures can be employed to realize systems with novel transport and optical properties. Here, we report on the growth of InAs/InP/GaAsSb core-dual-shell nanowires by catalyst-free chemical beam epitaxy on Si(111) substrates. Detailed morphological, structural, and compositional analyses of the nanowires as a function of growth parameters were carried out by scanning and transmission electron microscopy and by energy-dispersive X-ray spectroscopy. Furthermore, by combining the scanning transmission electron microscopy-Moire technique with geometric phase analysis, we studied the residual strain and the relaxation mechanisms in this system. We found that InP shell facets are well-developed along all the crystallographic directions only when the nominal thickness is above 1 nm, suggesting an island-growth mode. Moreover, the crystallographic analysis indicates that both InP and GaAsSb shells grow almost coherently to the InAs core along the 112 direction and elastically compressed along the 110 direction. For InP shell thickness above 8 nm, some dislocations and roughening occur at the interfaces. This study provides useful general guidelines for the fabrication of high-quality devices based on these core-dual-shell nanowires.
GaAs/Fe$_{3}$Si core/shell nanowire structures were fabricated by molecular-beam epitaxy on oxidized Si(111) substrates and investigated by synchrotron x-ray diffraction. The surfaces of the Fe$_3$Si shells exhibit nanofacets. These facets consist of well pronounced Fe$_3$Si{111} planes. Density functional theory reveals that the Si-terminated Fe$_3$Si{111} surface has the lowest energy in agreement with the experimental findings. We can analyze the x-ray diffuse scattering and diffraction of the ensemble of nanowires avoiding the signal of the substrate and poly-crystalline films located between the wires. Fe$_3$Si nanofacets cause streaks in the x-ray reciprocal space map rotated by an azimuthal angle of 30{deg} compared with those of bare GaAs nanowires. In the corresponding TEM micrograph the facets are revealed only if the incident electron beam is oriented along [1$overline{1}$0] in accordance with the x-ray results. Additional maxima in the x-ray scans indicate the onset of chemical reactions between Fe$_{3}$Si shells and GaAs cores occurring at increased growth temperatures.
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