Differential Phase Contrast Technique Research Articles

Atomic-scale revealing defects in ion irradiated 4H-SiC

Here we identified atomic-scale defects in ion irradiated 4H-SiC, by integrated differential phase contrast technique equipped on aberration-corrected transmission electron microscopy and Raman scattering spectrometer. In 21 MeV Ni+ ions irradiated 4H-SiC, in which electronic energy loss is dominant almost at the whole penetration depth, Si- and C-related vacancies and interstitials are successfully imaged. In 900 keV Si+ ions irradiated 4H-SiC, however, the electronic energy loss plays the key role near the surface, and the nuclear and electronic energy loss are comparable at the depth of the peak damaged zone. As a result, 900 keV Si+ ions produce more point defects, atomic-scale in-plane and out-of-plane lattice distortions, and plenty of dislocations at the depth of damage peak. Meantime, these imaged point defects may provide direct detailed information for the vibration modes of defects in the Raman spectrum of the damaged 4H-SiC. This study illustrates the compelling intrinsic nature of point defects induced by energetic ions, providing important fundamental information for nuclear and electronic applications.

Differential phase contrast for quantitative imaging and spectro-microscopy at a nanoprobe beamline.

The interaction of a focused X-ray beam with a sample in a scanning probe experiment can provide a variety of information about the interaction volume. In many scanning probe experiments X-ray fluorescence (XRF) is supplemented with measurements of the transmitted or scattered intensity using a pixelated detector. The automated extraction of different signals from an area pixelated detector is described, in particular the methodology for extracting differential phase contrast (DPC) is demonstrated and different processing methods are compared across a range of samples. The phase shift of the transmitted X-ray beam by the sample, extracted from DPC, is also compared with ptychography measurements to provide a qualitative and quantitative comparison. While ptychography produces a superior image, DPC can offer a simple, flexible method for phase contrast imaging which can provide fast results and feedback during an experiment; furthermore, for many science problems, such as registration of XRF in a lighter matrix, DPC can provide sufficient information to meet the experimental aims. As the DPC technique is a quantitative measurement, it can be expanded to spectroscopic studies and a demonstration of DPC for spectro-microscopy measurements is presented. Where ptychography can separate the absorption and phase shifts by the sample, quantitative interpretation of a DPC image or spectro-microscopy signal can only be performed directly when absorption is negligible or where the absorption contribution is known and the contributions can be fitted.

Visualizing magnetic fields at the atomic scale

In the recent work of Kohno et al., magnetic spin orientation changes were measured at the atomic scale for the first time via the differential phase contrast technique in scanning transmission electron microscopy using a novel magnetic-field-free objective lens system.

Bloch Lines Constituting Antiskyrmions Captured via Differential Phase Contrast.

Much scientific capital has been directed toward exotic magnetic spin textures called Bloch lines, that is, Néel-type line boundaries within domain walls, because their geometry promises high-density magnetic storage. While predicted to arise in high-anisotropy magnets, bulk soft magnets, and thin films with in-plane magnetization, Bloch lines also constitute magnetic antiskyrmions, that is, topological antiparticles of skyrmions. Most domain walls occur as Bloch-type or Néel-type, in which the magnetization rotates parallel or perpendicular to the domain wall across its profile, respectively. The Bloch lines' Néel-type rotation and their minute size make them difficult to directly measure. This work utilizes differential phase contrast (DPC) scanning transmission electron microscopy (STEM) to measure the in-plane magnetization of Bloch lines within antiskyrmions emergent in a non-centrosymmetric Heusler magnet with D2d symmetry, Mn1.4 Pt0.9 Pd0.1 Sn, in addition to Bloch-type skyrmions in an FeGe magnet with B20-type crystal structure to benchmark the DPC technique. Both in-focus measurement and identification of Bloch lines at the antiskyrmion's corners are provided.

Flexopiezoelectricity at ferroelastic domain walls in WO3 films

The emergence of a domain wall property that is forbidden by symmetry in bulk can offer unforeseen opportunities for nanoscale low-dimensional functionalities in ferroic materials. Here, we report that the piezoelectric response is greatly enhanced in the ferroelastic domain walls of centrosymmetric tungsten trioxide thin films due to a large strain gradient of 106 m−1, which exists over a rather wide width (~20 nm) of the wall. The interrelationship between the strain gradient, electric polarity, and the electromechanical property is scrutinized by detecting of the lattice distortion using atomic scale strain analysis, and also by detecting the depolarized electric field using differential phase contrast technique. We further demonstrate that the domain walls can be manipulated and aligned in specific directions deterministically using a scanning tip, which produces a surficial strain gradient. Our findings provide the comprehensive observation of a flexopiezoelectric phenomenon that is artificially controlled by externally induced strain gradients.

Influence of lens aberrations, specimen thickness and tilt on differential phase contrast STEM images

Differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM) allows for measuring electric and magnetic fields in solids on scales ranging from picometres to micrometres. The DPC technique mainly uses the direct beam, which is deflected by the electric and magnetic fields of the specimen and measured with a beam position sensitive detector. The beam deflection and thus the DPC signal is strongly influenced by specimen thickness, specimen tilt and lens aberrations. Understanding these influences is critical for a solid interpretation and quantification of contrasts in DPC images. To this end, the present study employs DPC-STEM image simulations of SrTiO3 [001] at atomic resolution to analyse the influence of lens aberrations, specimen tilt and thickness and also to give a guideline for the detection of parameters affecting the contrast by performing an analysis of associated scattergrams. Simulations are obtained using the multislice algorithm implemented in the Dr. Probe software with conditions corresponding to a JEOL ARM200F microscope equipped with an octa-segmented annular detector, but results should be similar for other microscopes. Simulations show that due to a non-rigid shift of the detected intensity distribution correct values of projected potentials of specimens thicker than one unit-cell cannot be determined. Regarding the impact of residual lens aberrations, it is found that the shape of the lens aberration phase function determines the symmetry and features in the DPC image. Specimen tilt leads to an elongation of features perpendicular to the tilt axis. The results are confirmed by comparing simulated with experimental DPC images of Si [110] yielding good agreement. Overall, a high sensitivity of DPC-STEM imaging to lens aberrations, specimen tilt and diffraction effects is evidenced.

Quantum mechanical treatment of atomic-resolution differential phase contrast imaging of magnetic materials

Utilizing the Pauli equation based multislice method, introduced in Phys. Rev. Lett. 116, 127203 (2016), we study the atomic resolution differential phase contrast (DPC) imaging on an example of a hard magnet FePt with in-plane magnetization. Simulated center of mass pattern in a scanning transmission electron microscopy (STEM) experiment carries information about both electric and magnetic fields. The momentum transfer remains curl-free, which has consequences for interpretation of the integrated DPC technique. The extracted magnetic component of the pattern is compared to the expected projected microscopic magnetic field as obtained by density functional theory calculation. Qualitative agreement is obtained for low sample thicknesses and a suitable range of collection angles.

Magnetic scanning gate microscopy of CoFeB lateral spin valve

Devices comprised of CoFeB nanostructures with perpendicular magnetic anisotropy and non-magnetic Ta channel were operated in thermal lateral spin valve (LSV) mode and studied by magnetotransport measurements and magnetic scanning gate microscopy (SGM). Due to the short spin diffusion length of Ta, the spin diffusion signal was suppressed, allowing the study of the contribution from the anomalous Nernst (ANE) and anomalous Hall effects (AHE). The magnetotransport measurements identified the switching fields of the CoFeB nanostructures and demonstrated a combination of AHE and ANE when the devices were operated in thermally-driven spin-injection mode. Modified scanning probe microscopy probes were fabricated by placing a NdFeB magnetic bead (MB) on the apex of a commercial Si probe. The dipole magnetic field distribution around the MB was characterized by using differential phase contrast technique and direct measurement of the switching field induced by the bead in the CoFeB nanodevices. Using SGM we demonstrate the influence of localized magnetic field on the CoFeB nanostructures near the non-magnetic channel. This approach provides a promising route towards the study of thermal and spin diffusion effects using local magnetic fields.

Measurement of atomic electric fields and charge densities from average momentum transfers using scanning transmission electron microscopy

This study sheds light on the prerequisites, possibilities, limitations and interpretation of high-resolution differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM). We draw particular attention to the well-established DPC technique based on segmented annular detectors and its relation to recent developments based on pixelated detectors. These employ the expectation value of the momentum transfer as a reliable measure of the angular deflection of the STEM beam induced by an electric field in the specimen. The influence of scattering and propagation of electrons within the specimen is initially discussed separately and then treated in terms of a two-state channeling theory.A detailed simulation study of GaN is presented as a function of specimen thickness and bonding. It is found that bonding effects are rather detectable implicitly, e.g., by characteristics of the momentum flux in areas between the atoms than by directly mapping electric fields and charge densities. For strontium titanate, experimental charge densities are compared with simulations and discussed with respect to experimental artifacts such as scan noise. Finally, we consider practical issues such as figures of merit for spatial and momentum resolution, minimum electron dose, and the mapping of larger-scale, built-in electric fields by virtue of data averaged over a crystal unit cell. We find that the latter is possible for crystals with an inversion center. Concerning the optimal detector design, this study indicates that a sampling of 5mrad per pixel is sufficient in typical applications, corresponding to approximately 10×10 available pixels.

Image fusion scheme for differential phase contrast mammography

Latest progresses in breast imaging using differential phase contrast technique pose the question how to fuse multiple information (yielded by the absorption, differential phase, and scattering signals) into a single, but more informative image for clinical diagnosis and evaluation. In this work, we propose an image fusion scheme based on the multiple-resolution (MR) framework. The three signals are first transformed into multiple bands presenting information at different frequency and then a two-step processing follows: section 3.2 an intra-band processing enhances the local signal-to-noise ratio using a novel noise estimation method and context modeling; section 3.3 an inter-band processing weights each band by considering their characteristics and contributions, as well as suppressing the global noise level. The fused image, which looks similar to conventional mammogram but with significantly enhanced detail features, is then reconstructed by inverse transform. This fused image is compatible with clinical settings and enables the radiologists to use their years of diagnosis experiences in mammography.

Optical encryption using pseudorandom complex spatial modulation

In this paper we propose a new (to our knowledge) complex spatial modulation method to encode data pages applicable in double random phase encryption (DRPE) to make the system more resistant to brute-force attack. The proposed modulation method uses data page pixels with random phase and amplitude values with the condition that the intensity of the interference of light from two adjacent pixels should correspond to the encoded information. A differential phase contrast technique is applied to recover the data page at the output of the system. We show that the proposed modulation method can enhance the robustness of the DRPE technique using point spread function analysis. Key space expansion is determined by numeric model calculations.

In situ TEM observation of magnetic materials

Magnetic nanostructures and thin films display novel magnetization reversal behavior as a function of size and shape, which makes them appropriate for a range of technological applications. The spatial resolution of in situ transmission electron microscopy techniques such as Lorentz TEM (LTEM) and off-axis electron holography are well suited to analysis of the magnetic domain structure and magnetization behavior of these magnetic nanostructures and thin films. In this article the various techniques that are applicable are described, including the qualitative LTEM imaging modes and the differential phase contrast technique. In addition, quantitative methods for mapping the magnetic induction via phase reconstruction are discussed. In each case the advantages and limitations are presented. Application of the techniques to various types of magnetic structures is then presented, and the article ends with a short summary and a discussion of future developments in this field.

Diffracting aperture based differential phase contrast for scanning X-ray microscopy

It is demonstrated that in a zone plate based scanning X-ray microscope, used to image low absorbing, heterogeneous matter at a mesoscopic scale, differential phase contrast (DPC) can be implemented without adding any additional optical component to the normal scheme of the microscope. The DPC mode is simply generated by an appropriate positioning and alignment of microscope apertures. Diffraction from the apertures produces a wave front with a non-uniform intensity. The signal recorded by a pinhole photo diode located in the intensity gradient is highly sensitive to phase changes introduced by the specimen to be recorded. The feasibility of this novel DPC technique was proven with the scanning X-ray microscope at the ID21 beamline of the European Synchrotron Radiation facility (ESRF) operated at 6 keV photon energy. We observe a differential phase contrast, similar to Nomarski's differential interference contrast for the light microscope, which results in a tremendous increase in image contrast of up to 20 % when imaging low absorbing specimen.

En face imaging of single cell layers by differential phase-contrast optical coherence microscopy

Optical coherence microscopy (OCM) is capable of imaging the backscattering potential of a sample with high transversal and axial resolution. We report on a combination of OCM with a differential phase-contrast technique that permits imaging of the subwavelength optical path differences that occur between a narrow beam probing a sample and its surrounding. This technique allows small transversal refractive-index variations close to a selected interface to be seen. We report on the method and present first images of a test sample and a single cell layer. The cells act as phase objects; imaging the phase properties improves the contrast compared with that of intensity images.

User Access to Lorentz Microscopy at the NCEM, LBNL, Berkeley.

A standard Philips CM200FEG electron microscope, without the special Lorentz lens, has been optimized for Lorentz imaging. The necessary field-free sample region is obtained by switching off the objective lens in the free lens mode. The limited range of magnification is compensated for by a post-column Gatan image filter (GIF) which magnifies by a factor of _ 20. Fresnel imaging is performed by defocusing with the diffraction lens. The use of low angle diffraction, in combination with the apertures located at the selected area aperture plane, allow Foucault imaging. The TEM analog of differential phase contrast (DPC) imaging has been implemented. This method makes it possible to obtain quantitave induction maps of the in-plane magnetization. TEM DPC is based on a series of Foucault images, recorded with different incremental beam tilts, which are processed to yield images equivalent to the quadrant signals obtained by the STEM DPC technique.

TEM imaging and evalution of magnetic structures in Co/Cu multilayers

In this paper, a method is described which uses electron microscopic imaging of magnetic domains (“Lorentz microscopy”) to measure the degree of ferromagnetic coupling between magnetic layers of a multilayer system. Additionally, the in situ observation of magnetisation reversal processes during a hysteresis cycle allows insights into the micromagnetic structural changes during the cycle, which are useful to explain the macroscopic magnetic properties of the material under investigation. This application of electron microscopic imaging of magnetic structures is given for a system of multilayered material which exhibits the giant magnetoresistance effect (GMR effect). This effect shows a dependency of the electrical conductivity on the strength and direction of an applied external magnetic field, which makes it an interesting effect for magnetic sensor applications. For a complete undestanding of the physical causes of this effect it is not sufficient to perform macroscopic magnetic measurements only, such as measurements of the macroscopic anisotropies or the macroscopic hysteresis loop. Instead, a microscopic investigation is necessary to prevent speculative misinterpretations of the macroscopic magnetic behaviour. These microscopic investigations can easily be done with an electron microscope, using techniques for the imaging of magnetic structures, such as Fresnel and Foucault imaging, or the differential phase contrast technique (DPC). The investigations presented in this paper deal with the micromagnetic domain structures of a multilayered [Co/Cu] N system that exhibits the GMR effect, and their structural reasons as well as the behaviour of the magnetic structures under the influence of an external magnetic field. The observations lead to a clearer understanding of the physical reasons that contribute to the GMR effect. The results include the measurement of the volume fraction of ferromagnetically or antiferromagnetically coupled regions in the samples, the structural reasons for an imperfect coupling of adjacent layers, the observation of local micromagnetic changes through a hysteresis cycle and the formation of 360° walls in the samples. The combination of these results yield a more complete understanding of the factors that contribute to the GMR effect.