55 research outputs found
Detecting charge transfer at defects in 2D materials with electron ptychography
Charge transfer between atoms is fundamental to chemical bonding but has
remained very challenging to detect directly in real space. Atomic-resolution
imaging of charge density is not sufficient by itself, as the change in the
density due to bonding is subtle compared to the total local charge density.
Both sufficiently high sensitivity and accuracy are required, which we
demonstrate here for the detection of charge transfer at defects in
two-dimensional WS\textsubscript{2} via high-speed electron ptychography and
its ability to correct errors due to residual lens aberrations
Smart Alignâa new tool for robust non-rigid registration of scanning microscope data
AbstractMany microscopic investigations of materials may benefit from the recording of multiple successive images. This can include techniques common to several types of microscopy such as frame averaging to improve signal-to-noise ratios (SNR) or time series to study dynamic processes or more specific applications. In the scanning transmission electron microscope, this might include focal series for optical sectioning or aberration measurement, beam damage studies or camera-length series to study the effects of strain; whilst in the scanning tunnelling microscope, this might include bias-voltage series to probe local electronic structure. Whatever the application, such investigations must begin with the careful alignment of these data stacks, an operation that is not always trivial. In addition, the presence of low-frequency scanning distortions can introduce intra-image shifts to the data. Here, we describe an improved automated method of performing non-rigid registration customised for the challenges unique to scanned microscope data specifically addressing the issues of low-SNR data, images containing a large proportion of crystalline material and/or local features of interest such as dislocations or edges. Careful attention has been paid to artefact testing of the non-rigid registration method used, and the importance of this registration for the quantitative interpretation of feature intensities and positions is evaluated.</jats:p
Efficient first principles simulation of electron scattering factors for transmission electron microscopy
Electron microscopy is a powerful tool for studying the properties of
materials down to their atomic structure. In many cases, the quantitative
interpretation of images requires simulations based on atomistic structure
models. These typically use the independent atom approximation that neglects
bonding effects, which may, however, be measurable and of physical interest.
Since all electrons and the nuclear cores contribute to the scattering
potential, simulations that go beyond this approximation have relied on
computationally highly demanding all-electron calculations. Here, we describe a
new method to generate ab initio electrostatic potentials when describing the
core electrons by projector functions. Combined with an interface to
quantitative image simulations, this implementation enables an easy and fast
means to model electron microscopy images. We compare simulated transmission
electron microscopy images and diffraction patterns to experimental data,
showing an accuracy equivalent to earlier all-electron calculations at a much
lower computational cost.Comment: 10 pages, 5 figures, 2 table
High On/Off ratio memristive switching of manganite/cuprate bilayer by interfacial magnetoelectricity
Memristive switching serves as the basis for a new generation of electronic devices. Memristors are two-terminal devices in which the current is turned on and off by redistributing point defects, e.g., vacancies, which is difficult to control. Memristors based on alternative mechanisms have been explored, but achieving both the high On/Off ratio and the low switching energy desirable for use in electronics remains a challenge. Here we report memristive switching in a La_(0.7)Ca_(0.3)MnO_(3)/PrBa_(2)Cu_(3)O_(7) bilayer with an On/Off ratio greater than 103 and demonstrate that the phenomenon originates from a new type of interfacial magnetoelectricity. Using results from firstprinciples calculations, we show that an external electric-field induces subtle displacements of the interfacial Mn ions, which switches on/off an interfacial magnetic âdeadâ layer, resulting in memristive behavior for spin-polarized electron transport across the bilayer. The interfacial nature of the switching entails low energy cost about of a tenth of atto Joule for write/erase a âbitâ. Our results indicate new opportunities for manganite/cuprate systems and other transition-metal-oxide junctions in memristive applications
Direct Imaging of Charge Redistribution due to Bonding at Atomic Resolution via Electron Ptychography
Phase imaging in electron microscopy is sensitive to the local potential
including charge redistribution from bonding. We demonstrate that electron
ptychography provides the necessary sensitivity to detect this subtle effect by
directly imaging the charge redistribution in single layer boron nitride.
Residual aberrations can be measured and corrected post-collection, and
simultaneous atomic number contrast imaging provides unambiguous sub-lattice
identification. Density functional theory calculations confirm the detection of
charge redistribution
Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles
The effect of maghemization on the magnetic properties of magnetite (Fe3O4) grains in the pseudo-single-domain (PSD) size range is investigated as a function of annealing temperature. X-ray diffraction and transmission electron microscopy confirm the precursor grains as Fe3O4 ranging from âŒ150 to âŒ250 nm in diameter, whilst Mössbauer spectrometry suggests the grains are initially near-stoichiometric. The Fe3O4 grains are heated to increasing reaction temperatures of 120â220°C to investigate their oxidation to maghemite (Îł-Fe2O3). High-angle annular dark field imaging and localized electron-energy loss spectroscopy reveal slightly oxidized Fe3O4 grains, heated to 140°C, exhibit higher oxygen content at the surface. Off-axis electron holography allows for construction of magnetic induction maps of individual Fe3O4 and Îł-Fe2O3 grains, revealing their PSD (vortex) nature, which is supported by magnetic hysteresis measurements, including first-order reversal curve analysis. The coercivity of the grains is shown to increase with reaction temperature up to 180°C, but subsequently decreases after heating above 200°C; this magnetic behavior is attributed to the growth of a Îł-Fe2O3 shell with magnetic properties distinct from the Fe3O4 core. It is suggested there is exchange coupling between these separate components that results in a vortex state with reduced vorticity. Once fully oxidized to Îł-Fe2O3, the domain states revert back to vortices with slightly reduced coercivity. It is argued that due to a core/shell coupling mechanism during maghemization, the directional magnetic information will still be correct; however, the intensity information will not be retained
- âŠ