Sample Tilt Research Articles

Unsupervised machine learning and cepstral analysis with 4D-STEM for characterizing complex microstructures of metallic alloys

Four-dimensional scanning transmission electron microscopy, coupled with a wide array of data analytics, has unveiled new insights into complex materials. Here, we introduce a straightforward unsupervised machine learning approach that entails dimensionality reduction and clustering with minimal hyperparameter tuning to semi-automatically identify unique coexisting structures in metallic alloys. Applying cepstral transformation to the original diffraction dataset improves this process by effectively isolating phase information from potential signal ambiguity caused by sample tilt and thickness variations, commonly observed in electron diffraction patterns. In a case study of a NiTiHfAl shape memory alloy, conventional scanning transmission electron microscopy imaging struggles to accurately identify a low-contrast precipitate at lower magnifications, posing challenges for microscale analyses. We find that our method efficiently separates multiple coherent structures while using objective means of determining hyperparameters. Furthermore, we demonstrate how the clustering result facilitates more robust strain mapping to provide immediate and quantitative structural insights.

Effective removal of global tilt from atomically-resolved topography images of vicinal surfaces with narrow terraces

The main feature of vicinal surfaces of crystals characterized by the Miller indices (hhm) is rather small width (less than 10 nm) and substantially large length (more than 200 nm) of atomically-flat terraces. This makes difficult to apply standard methods of image processing and correct visualization of crystalline lattices at the terraces and multiatomic steps. Here we consider two procedures allowing us to minimize effects of both small-scale noise and global tilt of sample: (i) analysis of the difference of two Gaussian blurred images, and (ii) subtraction of the plane, whose parameters are determined by optimization of the histogram of the visible heights, from raw topography image. It is shown that both methods provide nondistorted images demonstrating atomic structures on vicinal Si(556) and Si(557) surfaces.

Three-dimensional drift correction of localised non-raster scanning on atomic force microscopy

Abstract Non-raster scanning can increase the scanning frame rate and measurement speed of atomic force microscopes (AFMs). It is also possible to correct the 3D drift during the non-raster scanning. However, the algorithm for the drift correction depends upon the properties of each scan pattern. While localised non-raster scanning using a rosette scan may be faster than the frequently used Lissajous scanning patterns, the drift correction is more challenging because the scan has crossing points only in local neighbouring segments where there are short temporal and spatial separations of the crossing paths. This design note presents a novel solution that successfully overcomes this problem and extends a drift correction method previously developed for Lissajous scans to the 3D drift correction of localised non-raster scanning using a rosette scan trajectory. The drift in the X, Y and Z axes can be determined using the crossing points and locally repeated scans of the same features. The general procedure is presented together with experiments using rosette scans of a two-dimensional lateral calibration standard. Experimental results have demonstrated that the method can effectively correct both the drift in the three axes and sample tilt, leading to significantly improved images. The method requires only localised crossing points in the scan and does not need additional scans to determine the three-dimensional drift based on cross-correlation and least squares techniques, and it can be used with any AFMs capable of rosette scanning.

Maskless photolithography based on ultraviolet micro-LEDs and direct writing method for improving pattern resolution

Ultraviolet micro-LEDs show great potential as a light source for maskless photolithography. However, there are few reports on micro-LED based maskless photolithography systems, and the studies on the effects of system parameters on exposure patterns are still lacking. Hence, we developed a maskless photolithography system that employs micro-LEDs with peak wavelength 375 nm to produce micrometer-sized exposure patterns in photoresists. We also systematically explored the effects of exposure time and current density of micro-LED on static direct writing patterns, as well as the effects of stage velocity and current pulse width on dynamic direct writing patterns. Furthermore, reducing the size of micro-LED pixels enables obtaining high-resolution exposure patterns, but this approach will bring technical challenges and high costs. Therefore, this paper proposes an oblique direct writing method that, instead of reducing the micro-LED pixel size, improves the pattern resolution by changing the tilt angle of the sample. The experimental results show that the linewidths of the exposed lines decreased by 4.0% and 15.2%, respectively, as the sample tilt angle increased from 0° to 15° and 30°, which confirms the feasibility of the proposed method to improve the pattern resolution. This method is also expected to correct the exposure pattern error caused by optical distortion of the lens in the photolithography system. The system and method reported can be applied in various fields such as PCBs, photovoltaics, solar cells, and MEMS.

Characterization of irradiation-induced dislocation loops and helium bubbles via electron channeling contrast imaging

Characterizing irradiation-induced defects is essential to understand damage mechanisms and develop irradiation-resistant materials. Prevailing strategies rely on transmission electron microscopy (TEM) observations which require rigorous prerequisite to prepare high-quality foils, making it difficult for rapid screening of irradiation-resistant materials. Here, we propose that the electron channeling contrast imaging (ECCI) is expected to provide a novel strategy for characterizing surface irradiation defects of ion-irradiated materials. The comparison between ECCI and conventional cross-sectional TEM for the observation of dislocation loops and helium (He) bubbles is analyzed in a common FeCoNiCr high-entropy alloy. The results indicate that the ECCI images of irradiation-induced defects are optimal at the extra-high tension of 20 kV, with a limit resolution of ∼1.5 nm for visible defects. The irradiation-induced dislocation loops show sharper contrast than bubbles in ECCI images, and the acceptable misorientation was within 4° of sample tilt. These findings offer a promising solution to the challenges for rapid assessment of irradiation damage in ion-irradiated materials.

Deformation of confined liquid interfaces by inhomogeneous electric fields and localized particle forces

HypothesisOil-water interfaces that are created by confining a certain amount of oil in a square shaped pixel (∼200 x 200 μm2 with a height of ∼10 μm) topped by a layer of water, have a curvature that depends on the amount of oil that happens to be present in the confining area. Under the application of an electric field normal to the interface, the interface will deform due to inhomogeneities in the electric field. These inhomogeneities are expected to arise from the initial curvature of the meniscus, from fringe fields that emerge at the confining pixel walls and, if applicable, from interfacially adsorbed particles. Modeling and ExperimentsWe model the shape of the confined oil-water interface invoking capillarity and electrostatics. Furthermore, we measure the initial curvature by tracking the position of interfacially adsorbed particles depending on sample tilt. FindingsWe found that the pixels exhibited meniscus curvature radii ranging from 0.6-7 mm. The corresponding model based minimum oil film thicknesses range between 0.7 and 9 μm. Furthermore, the model shows that the initial meniscus curvature can increase up to 76 percent relative to the initial curvature by the electric field before the oil film becomes unstable. The pixel wall and particles are shown to have minimal impact on the interface deformation.

Statistical 3D morphology characterization of vaterite microspheres produced by engineered Escherichia coli

Hollow vaterite microspheres are important materials for biomedical applications such as drug delivery and regenerative medicine owing to their biocompatibility, high specific surface area, and ability to encapsulate a large number of bioactive molecules and compounds. We demonstrated that hollow vaterite microspheres are produced by an Escherichia coli strain engineered with a urease gene cluster from the ureolytic bacteria Sporosarcina pasteurii in the presence of bovine serum albumin. We characterized the 3D nanoscale morphology of five biogenic hollow vaterite microspheres using 3D high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) tomography. Using automated high-throughput HAADF-STEM imaging across several sample tilt orientations, we show that the microspheres evolved from a smaller more ellipsoidal shape to a larger more spherical shape while the internal hollow core increased in size and remained relatively spherical, indicating that the microspheres produced by this engineered strain likely do not contain the bacteria. The statistical 3D morphology information demonstrates the potential for using biogenic calcium carbonate mineralization to produce hollow vaterite microspheres with controlled morphologies. Statement of significanceThe nanoscale 3D structures of biomaterials determine their physical, chemical, and biological properties, however significant efforts are required to obtain a statistical understanding of the internal 3D morphology of materials without damaging the structures. In this study, we developed a non-destructive, automated technique that allows us to understand the nanoscale 3D morphology of many unique hollow vaterite microspheres beyond the spectroscopy methods that lack local information and microscopy methods that cannot interrogate the full 3D structure. The method allowed us to quantitatively correlate the external diameters and aspect ratios of vaterite microspheres with their hollow internal structures at the nanoscale. This work demonstrates the opportunity to use automated transmission electron microscopy to characterize nanoscale 3D morphologies of many biomaterials and validate the chemical and biological functionality of these materials.

Focused ion beams from GaBiLi liquid metal alloy ion sources for nanofabrication and ion imaging

In this work, we present an overview of nanopatterning and imaging applications using newly developed workflows with focused ion beams (FIBs) produced with a GaBiLi liquid metal alloy ion source. The primary beam of this source type contains gallium, bismuth, and lithium as well as cluster ions which can be separated quickly using a Wien filter. Lithium ion milling has been applied to generate heptamer-arranged nanohole arrays in gold films with high resolution. Workflows for two-step bowtie nanofabrication using lithium and bismuth ions from the same source have been established. Furthermore, we present ion beam imaging results that were obtained with lithium ions on various sample materials. Combining the large sputter yield and high depth resolution of heavy bismuth ions with the high lateral imaging resolution of light lithium ions enables 3D nanoscale tomography using different ion species generated from the same source. Sample tilt is not required due to the top-down geometry of the FIB.

Reliable phase quantification in focused probe electron ptychography of thin materials

Electron ptychography provides highly sensitive, dose efficient phase images which can be corrected for aberrations after the data has been acquired. This is crucial when very precise quantification is required, such as with sensitivity to charge transfer due to bonding. Drift can now be essentially eliminated as a major impediment to focused probe ptychography, which benefits from the availability of easily interpretable simultaneous Z-contrast imaging. However challenges have remained when quantifying the ptychographic phases of atomic sites. The phase response of a single atom has a negative halo which can cause atoms to reduce in phase when brought closer together. When unaccounted for, as in integrating methods of quantification, this effect can completely obscure the effects of charge transfer. Here we provide a new method of quantification that overcomes this challenge, at least for 2D materials, and is robust to experimental parameters such as noise, sample tilt.

High Temperature Characterization of the Stiffness Matrix of Different Steels

The aim of this work is to acquire material data at elevated temperatures for different chemical compositions and phases as basis for numerical simulations incorporating the effect of crystalline texture. Several samples; AISI 304, interstitially free, low carbon, martensite, and pearlite, were measured in a Gleeble® Thermal-Mechanical Simulator 3800 in conjunction with LUS (GLUS®) during annealing from room temperature up to a maximum temperature in the range between 675 and 1000 °C. The laser ultrasonic measurements contain both pressure (P) and shear (S) waves. The arrival time of at least the first S-wave echo and two or more P-wave echoes were used. From the measurements it was observed that there was a slight tilt of the sample and misalignment of the centres of the generation and detection laser beams. An inversion scheme was implemented estimating the nine stiffness coefficients as well as the sample tilt and alignment error. The estimated stiffness coefficients as a function of temperature were successfully obtained from the measurements. Most of the coefficients decrease with increasing temperature, whereas C12, C13, and C23 remain relatively constant. The implemented inversion scheme successfully compensates for sample tilt and laser misalignment.

ToF-SIMS Depth Profiling to Measure Nanoparticle and Polymer Diffusion in Polymer Melts

We apply time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and cross-sectioned trilayer samples to separately measure nanoparticle (NP) and polymer diffusion on micrometer length scales in polymer melts. We fabricate polymer/diffusing medium/polymer trilayer samples and measure the cross section to extract the NP or deuterated polymer distribution in 3D using ToF-SIMS. After correcting the data for sample tilt, deconvoluting the beam resolution, and integrating the data to extract 1D concentration profiles, we fit the data to extract the diffusion coefficient. These results from cross-sectional ToF-SIMS are in excellent agreement with earlier studies using well-established ion beam methods. This work establishes ToF-SIMS as a reliable tool for measuring NP and polymer diffusion coefficients and opens the door to investigating diffusion in more complex polymer systems and across longer time and length scales.

Systematic Errors of Electric Field Measurements in Ferroelectrics by Unit Cell Averaged Momentum Transfers in STEM.

When using the unit cell average of first moment data from four-dimensional scanning transmission electron microscopy (4D-STEM) to characterize ferroelectric materials, a variety of sources of systematic errors needs to be taken into account. In particular, these are the magnitude of the acceleration voltage, STEM probe semi-convergence angle, sample thickness, and sample tilt out of zone axis. Simulations show that a systematic error of calculated electric fields using the unit cell averaged momentum transfer originates from violation of point symmetry within the unit cells. Thus, values can easily exceed those of potential polarization-induced electric fields in ferroelectrics. Importantly, this systematic error produces deflection gradients between different domains seemingly representing measured fields. However, it could be shown that for PbZr0.2Ti0.8O3, many adjacent domains exhibit a relative crystallographic mistilt and in-plane rotation. The experimental results show that the method gives qualitative domain contrast. Comparison of the calculated electric field with the systematic error showed that the domain contrast of the unit cell averaged electric fields is mainly caused by dynamical scattering effects and the electric field plays only a minor role, if present at all.

Nondestructive Thickness Measurement of Thermal Barrier Coatings for Turbine Blades by Terahertz Time Domain Spectroscopy

Owing to its high penetrability with dielectric materials, terahertz time domain spectroscopy (THz-TDS) is a promising nondestructive measurement technology. The coating thickness deviation and defect of thermal barrier coatings (TBC) will affect its thermal insulation performance and lifetime. In this work, THz-TDS was applied to measure the coating thickness distribution of TBC. The refractive index was obtained by THz-TDS transmission mode. To avoid the normal incidence THz signal loss, the THz signal was reflected from the TBC with a 10° incident angle, which also made the measurement result insensitive to the unevenness and tilt of the TBC sample. In the experiment, the yttria-stabilized zirconia (YSZ) TBC was measured by THz-TDS to estimate the thickness distribution. To validate the thickness measurements, metallography was introduced to correlate the TBC thickness result. The measurement deviation was within 12.1 µm, i.e., 3.45% for the THz-TDS and metallography result. A piece of turbine blade was measured by THz-TDS and a eddy current test. The maximum deviation was 8.48 µm, i.e., 2.36% of these two methods. Unlike the eddy current test, the THz-TDS thickness result was not affected by the cooling holes. The effectiveness of the nondestructive thickness measurement of TBC for turbine blades by THz-TDS was verified.

Optimizing experimental parameters of integrated differential phase contrast (iDPC) for atomic resolution imaging

Integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) technique has been well developed for studying atomic structures at sub-Å resolution with the capability of simultaneously imaging heavy and light atoms even at an extremely low electron dose. As a direct phase contrast imaging technique, atomic resolution iDPC-STEM is sensitive to the imaging conditions. Although great achievements have been made both in aspect of theory and experiments, the influence of experimental parameters on the contrast of atomic resolution iDPC-STEM images has not been systematically investigated. Here, we perform the iDPC-STEM simulations on the prototypical example of SrTiO3 with respect to the routine experimental factors, including the defocus, specimen thickness, accelerating voltage, convergence angle, collection angle, sample tilt and electron dose. Through the evaluation of image contrast and atom column intensity, the parameters are discussed to improve the image contrast and the visibility of light elements. Moreover, the dose-dependent simulations demonstrate the advantage of low dose iDPC-STEM imaging over other conventional STEM modes. Our results provide a practical guideline to experimentally obtain accessible atomic resolution iDPC-STEM images.

AN ANALYSIS ON IMPROVEMENT OF X-RAY DIFFRACTOMETER RESULTS BY CONTROLLING AND CALIBRATION OF PARAMETERS

The X-ray diffractometer in the laboratory is a crucial instrument for analyzing materials in science. It can be used on almost any crystal material, and if the machine parameters are appropriately controlled, it can offer a lot of information about the sample’s characteristics. Nevertheless, the data obtained from these machines are complicated by an aberration function that can be resolved through calibration. In this study, a powder comprising of Barium Sulfate (BaSO4), Zinc Oxide (ZnO) and Aluminum (Al) was used as the first sample and a single crystal sample comprised of Gallium Nitride (GaN) and Aluminum Oxide (Al2O3). The required calibration parameters of the X-ray diffractometer namely: Straight Beam Alignment, Beam Cut Alignment and Sample Tilt Alignment for two samples were analyzed and carried out. Using the results of the X-ray spectrum, important parameters such as corresponding planes for peak positions, d-spacing of planes, intensities, smallest crystallite sizes and lattice parameters, and a comparison with the reference data were all carried out. As another result, the out-of-plane alignment and Full-Width- at Half-Maximum (FWHM) value for GaN could be determined using the rocking curve.

Federated Learning Based on Data Divergence and Differential Privacy in燜inancial Risk Control Research

In the financial sector, data are highly confidential and sensitive, and ensuring data privacy is critical. Sample fusion is the basis of horizontal federation learning, but it is suitable only for scenarios where customers have the same format but different targets, namely for scenarios with strong feature overlapping and weak user overlapping. To solve this limitation, this paper proposes a federated learning-based model with local data sharing and differential privacy. The indexing mechanism of differential privacy is used to obtain different degrees of privacy budgets, which are applied to the gradient according to the contribution degree to ensure privacy without affecting accuracy. In addition, data sharing is performed to improve the utility of the global model. Further, the distributed prediction model is used to predict customers’ loan propensity on the premise of protecting user privacy. Using an aggregation mechanism based on federated learning can help to train the model on distributed data without exposing local data. The proposed method is verified by experiments, and experimental results show that for non-iid data, the proposed method can effectively improve data accuracy and reduce the impact of sample tilt. The proposed method can be extended to edge computing, blockchain, and the Industrial Internet of Things (IIoT) fields. The theoretical analysis and experimental results show that the proposed method can ensure the privacy and accuracy of the federated learning process and can also improve the model utility for non-iid data by 7% compared to the federated averaging method (FedAvg).

Optimising large-area crystal orientation mapping of nanoscale β phase in α + β titanium alloys using EBSD

α + β titanium alloys, such as the commercially important alloy Ti-6Al-4 V (Ti64), develop complex textures during thermomechanical processing due to the allotropic phase transformation between the β and α phases. These phases are crystallographically related through the Burgers orientation relationship (BOR) and are often characterised by electron backscatter diffraction (EBSD) in the scanning electron microscope (SEM). However, the BOR can be destroyed for the primary α in wrought Ti64, and techniques that utilise the BOR to reconstruct the β phase from the room temperature α-phase data cannot be used. Instead, the β texture must be measured from the residual, nanoscale β ligaments in the room temperature microstructure, which are challenging to index because of orientation and phase overlap in the EBSD Kikuchi patterns. In this work, the SEM-EBSD acquisition and processing parameters were systematically varied to determine how best to index the residual β in Ti64, and an experimental methodology was thus developed to measure the β-phase texture efficiently and reliably after thermomechanical processing. The best compromise for maximising indexing of the residual β was achieved with a low current (~1 nA in this case), and additional indexing was achieved by increasing the sample stage tilt (to 75° in this case) and by limited the scanning frame size. It was shown that the β-phase texture could be reliably measured with the optimised beam and tilt settings using a relatively coarse step size (3 μm), but this approach does not yield any morphological or spatially relevant microstructure data. Thus, it was proposed that numerous multi-scale scans be performed with different settings to characterise the residual β phase in T64, each optimised to acquire either bulk texture analysis or microstructure spatial and morphological detail. • Residual β in Ti64 can be detected with EBSD using carefully selected parameters. • A ~ 1 nA current was the best compromise for maximising β indexing and EBSP intensity. • Additional β indexing was also achieved by increasing the sample tilt to 75°. • Dictionary Indexing can use biased pattern solutions to boost β-phase indexing.

(Digital Presentation) Characterization of Anisotropic Strain in Anelastic Material By Raman Spectroscopy

Raman spectroscopy has recently been applied for non-destructive characterization of strain in crystalline thin films. It makes use of the numerical value of the mode Grüneisen parameter γ , which equals (minus) the ratio of the relative change in the frequency of a Raman-active vibrational mode and the strain-induced relative change in the unit cell volume. In the presence of in-plane, compressive biaxial strain, aliovalent doped CeO2-films exhibit values of γ which are ~30% smaller than the literature values for the bulk ceramics under isostatic stress. This discrepancy is due in large part to the negative contribution from the anelastic (time-dependent) mechanical properties of oxygen-deficient ceria, thereby raising questions concerning the relationship between Raman mode frequency and anelastic strain. Here we propose a way to "separate" anelastic and elastic contributions to the Grüneisen parameter.500-650nm thick films , x=0.1-0.2, were deposited by RF magnetron sputtering on (100) p-Si wafers (1Ω-cm, tSi=280mm). The films (S-films) were in the fluorite phase with pronounced (111) texture and exhibited biaxial , compressive strain. The in- and out-of-plane lattice parameters (aS || , aS ⊥ ) of the S-films were determined by XRD at different sample tilt angles. The in-plane stress (σbi) in the S-films was determined by profilometry from the change in Si-wafer curvature following film deposition. From σbi and the film biaxial modulus, the elastic component of the in-plane compressive strain (ue ) was calculated (~ -0.2%).Strain-free (relaxed) R-films were obtained by partial substrate removal, forming 2 mm diameter self-supported membranes. Pronounced membrane buckling indicated that residual strain was negligible (aR || ≈ aR ^ ). From the increase in area upon substrate removal, measured with optical profilometry, the total S-film in-plane strain (ut ) was calculated from aS ||/aR-1 ≈ -0.5%. The anelastic strain component may be calculated as ua= ut-ue . The small, anisotropic strain-induced blue shift of the frequency of the F2g -phonon mode for S-films suggests that the F2g mode responds primarily to the elastic component of the strain.As a control, 430nm thick films of mechanically elastic Y2O3 were deposited by sputtering on a Si wafer covered with a 1.5 µm thick PECVD deposited SiO2 layer. This layer supported the Y2O3-films upon membrane formation, but prevented complete strain relaxation. The Y2O3-films were in the double fluorite phase with strong (222) texture and, as with the S-films, were under in-plane compressive strain. The Y2O3 S-film lattice parameters were determined by acquiring XRD-patterns at different sample tilt angles, while membrane in- and out-of-plane lattice parameters were determined with XRD in the transmission and reflection modes, respectively. The values of γ calculated from the Raman frequency of the [Fg +Ag ] peak position were close to the literature values for bulk yttria under isostatic stress.This work should serve as a sample protocol for strain characterization of doped ceria thin films. It supports the concept that the discrepancy between the behavior of Raman modes of isostatically and anisotropically strained doped ceria is related to anelasticity. Our results shed light upon strain characterization by Raman spectroscopy and may lay the foundation for future studies to elucidate selective sensitivity to various strain components.

Random conical tilt reconstruction without particle picking in cryo-electron microscopy.

A method is proposed to reconstruct the 3D molecular structure from micrographs collected at just one sample tilt angle in the random conical tilt scheme in cryo-electron microscopy. The method uses autocorrelation analysis on the micrographs to estimate features of the molecule which are invariant under certain nuisance parameters such as the positions of molecular projections in the micrographs. This enables the molecular structure to be reconstructed directly from micrographs, completely circumventing the need for particle picking. Reconstructions are demonstrated with simulated data and the effect of the missing-cone region is investigated. These results show promise to reduce the size limit for single-particle reconstruction in cryo-electron microscopy.

Femtosecond pulsed laser ablation for paint removal at oblique illumination: Effect of the incidence angle

This research studies how the angle of incidence affects the ablation process when a pulsed femtosecond laser with a wavelength of 1040 nm and a high repetition rate is applied to spray paints used for graffiti. Flat test samples were painted using four different spray paints (red, blue, black and silver). All paints had an organic base but different physicochemical properties. For each paint, the ablation process was characterized using confocal microscopy, determining the threshold fluence and the penetration depth of the laser radiation at normal incidence. For different sample tilt angles in the range of 0°–60°, the effect of oblique incidence was analysed in terms of its effect on the ablation rate and the geometric properties (depth and width) of the ablation grooves on each paint. The results show a clear decrease in the ablation rate as the angle of incidence moves away from normal incidence. Moreover, it can also be observed that for a given angle, groove depth is the property most affected by beam tilt. These results represent a necessary step in addressing both cleaning and other laser ablation processes on non-flat surfaces in a controlled and reproducible manner.