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  • Source Shot Noise Mitigation in Focused Ion Beam Microscopy by Time-Resolved Measurement
    Ultramicroscopy (IF 2.644) Pub Date : 2020-01-27
    Minxu Peng; John Murray-Bruce; Karl K. Berggren; Vivek K Goyal

    Focused ion beam microscopy suffers from source shot noise – random variation in the number of incident ions in any fixed dwell time – along with random variation in the number of detected secondary electrons per incident ion. This multiplicity of sources of randomness increases the variance of the measurements and thus worsens the trade-off between incident ion dose and image accuracy. Repeated measurement with low dwell time, without changing the total ion dose, is a way to introduce time resolution to this form of microscopy. Through theoretical analyses and Monte Carlo simulations, we show that three ways to process time-resolved measurements result in mean-squared error (MSE) improvements compared to the conventional method of having no time resolution. In particular, maximum likelihood estimation provides reduction in MSE or reduction in required dose by a multiplicative factor approximately equal to the secondary electron yield. This improvement factor is similar to complete mitigation of source shot noise. Experiments with a helium ion microscope are consistent with the analyses and suggest accuracy improvement for a fixed source dose by a factor of about 4.

  • Nanoimmunosensor based on atomic force spectroscopy to detect anti-myelin basic protein related to early-stage multiple sclerosis
    Ultramicroscopy (IF 2.644) Pub Date : 2020-01-24
    Pâmela Soto Garcia; Doralina Guimarães Brum; Osvaldo N. Oliveira; Akemi Martins Higa; Jéssica Cristiane Magalhães Ierich; Ariana de Souza Moraes; Flávio Makoto Shimizu; Nancy M. Okuda-Shinagawa; Luís Antonio Peroni; Paulo Diniz da Gama; M. Terêsa Machini; Fabio Lima Leite

    Multiple Sclerosis (MS) is a chronic inflammatory disorder in the central nervous system for which biomarkers for diagnosis still remain unknown. One potential biomarker is the myelin basic protein. Here, a nanoimmunosensor based on atomic force spectroscopy (AFS) successfully detected autoantibodies against the MBP85-99 peptide from myelin basic protein. The nanoimmunosensor consisted of an atomic force microscope tip functionalization with MBP85-99 peptide, which was made to interact with a mica surface coated either with a layer of anti-MBP85-99 (positive control) or samples of cerebrospinal fluid (CSF) from five multiple sclerosis (MS) patients at different stages of the disease and five non-MS subjects. The adhesion forces obtained from AFS pointed to a high concentration of anti-MBP85-99 for the two patients at early stages of relapsing-remitting multiple sclerosis (RRMS), which were indistinguishable from the positive control. In contrast, considerably lower adhesion forces were measured for all the other eight subjects, including three MS patients with longer history of the disease and under treatment, without episodes of acute MS activity. We have also shown that the average adhesion force between MBP85-99 and anti-MBP85-99 is compatible with the value estimated using steered molecular dynamics. Though further tests will be required with a larger cohort of patients, the present results indicate that the nanoimmunosensor may be a simple tool to detect early-stage MS patients and be useful to understand the molecular mechanisms behind MS.

  • Discriminating β, α and α″ phases in metastable β titanium alloys via segmentation: A combined electron backscattering diffraction and energy-dispersive X-ray spectroscopy approach
    Ultramicroscopy (IF 2.644) Pub Date : 2020-01-22
    Frank Niessen; Azdiar A. Gazder

    Annealed metastable β titanium (Ti) alloys comprise body-centred-cubic β and hexagonal-close-packed α phases and possibly, orthorhombic α″ martensite that forms on quenching or deformation. Electron backscattering diffraction is amongst the most popular methods for characterising such multi-phase microstructures. However, the crystallographic similarity between α and α″ martensite renders unambiguous discrimination of these phases via electron backscattering patterns (EBSPs) virtually impossible; thereby limiting the use of EBSD in characterising β-Ti alloys. In this study, we demonstrate that α and α″ martensite are primarily misindexed due to an indiscernible difference between these phases along their [1¯10]α and [010]α″ zone axes. Furthermore, the slight compositional difference between α and α″ is insufficient to discriminate these phases using on-the-fly energy-dispersive X-ray spectroscopy (EDS) spectrum matching. Consequently, a segmentation method was developed that relies on a combination of reindexed EBSPs and grain-median EDS elemental data to unambiguously discriminate β, α and α″ martensite in metastable β Ti alloys. All steps are implemented in an open-source and freely available computer program called phaseSegmenter that makes use of the MTEX toolbox in MATLAB. The program is readily applicable to Ti alloys containing α′, α″ or massively transformed α as well as other phase transforming alloy systems with similar phase discrimination issues.

  • A new approach for repeated tip-sample relocation for AFM imaging of nano and micro sized particles and cells in liquid environment
    Ultramicroscopy (IF 2.644) Pub Date : 2020-01-22
    Abd Alaziz Abu Quba; Gabriele E. Schaumann; Mariam Karagulyan; Dörte Diehl

    The ability to repeatedly find exact the same nano region-of-interest (nROI) is essential for atomic force microscopy (AFM) studies of heterogeneous environmental samples. The large variety of methods makes it difficult to find the most suitable one for a specific research question. We thus conducted a literature research for nROI relocation methods and organized the found references in order to give an overview over relocation methods including the advantages, limitations and documented applications. This survey of nROI relocation methods and their key information facilitates the selection of appropriate methods with respect to a specific research question. Based on this survey, we developed a new AFM relocation approach urgently needed for the study of nano and micro sized particles and cells in air and aqueous environment. This approach uses commercially available TEM grids fully embedded in a semitransparent resin as a glue body on top of which particles and cells are fixed. Relocation of nROI within one grid is based on easily recognizable sample features in micro and nanometer scale. The stable sticking of the studied mineral particles and bacterial cells allows repeated measurements of the same nROI with differently functionalized tips in air as well as in water. Our simple, fast, and cost-effective method allows relocation with an accuracy of 10-40 nm and enables the implementation of AFM/ESEM correlative microscopy.

  • The performance evaluation of direct detection electron energy-loss spectroscopy at 200 kV and 80 kV accelerating voltages
    Ultramicroscopy (IF 2.644) Pub Date : 2020-01-22
    Shaobo Cheng; Alexandre Pofelski; Paolo Longo; Ray D. Twesten; Yimei Zhu; Gianluigi A. Botton

    Direct electron detectors (DeDs) have been widely used for imaging studies because of their higher beam sensitivity, lower noise, improved pixel resolution, etc. However, there have been limited studies related to the performance in spectroscopic applications for the direct electron detection. Hereby, taking the advantage of the DeD installed on a high-performance electron energy-loss spectrometer we systematically studied the performance of a DeD (Gatan's K2 IS) fitted on an aberration-corrected transmission electron microscope (TEM) equipped with an electron monochromator. Using SrTiO3 as the model system, the point spread function in the zero-loss region of the spectrum and the performance for core loss spectroscopy have been investigated under both 200 kV and 80 kV operating conditions. We demonstrate that the K2 detector can achieve an overall better performance at 200 kV than a charge coupled device (CCD) detector. At 80 kV, the K2 DeD is still better than a CCD, except for the relative broad tails of the zero-loss peak. The signal-to-noise ratio is very close for DeD and CCD under 80 kV. Based on our data obtained at different operating voltages, it is clear that DeD will benefit the microscopy community and boost the development of cutting-edge materials science studies by pushing the frontiers in electron energy-loss spectroscopy.

  • A simple program for fast tilting electron-beam sensitive crystals to zone axes
    Ultramicroscopy (IF 2.644) Pub Date : 2020-01-21
    Y. Zhang; R. Yan; T. Sun; Y. Ma

    Tilting crystals to proper zone axes is a necessary but tedious work in taking selected area electron diffraction patterns (SAED) and high-resolution images using transmission electron microscope (TEM). This process not only costs a lot of time but also limits the application of TEM in electron-beam sensitive materials. Therefore, it is desirable to develop a simple method for tilting crystals from random orientations to a specific zone axis quickly. Herein, we describe a novel program, Zones, which can index the electron diffraction pattern and calculate the tilting angles of a double-tilt sample holder from the current orientation to a desired zone axis. It can also bring crystals that are slightly deviated from a zone axis to the exact zone with the help of Laue ring in the diffraction pattern. This program has been successfully applied to studies of zeolites and metal-organic frameworks (MOFs), known as being electron-beam sensitive. The program shows its power not only in saving the operator's time but also in preventing the crystals from quick beam damages.

  • Advancing characterisation with statistics from correlative electron diffraction and X-ray spectroscopy, in the scanning electron microscope
    Ultramicroscopy (IF 2.644) Pub Date : 2020-01-21
    T.P. McAuliffe; A. Foden; C. Bilsland; D. Daskalaki-Mountanou; D. Dye; T.B. Britton
  • Beam displacement and blur caused by fast electron beam deflection
    Ultramicroscopy (IF 2.644) Pub Date : 2020-01-15
    Lixin Zhang; Mathijs W.H. Garming; Jacob P. Hoogenboom; Pieter Kruit

    Electrostatic beam blankers are an alternative to photo-emission sources for generating pulsed electron beams for Time-resolved Cathodoluminescence and Ultrafast Electron Microscopy. While the properties of beam blankers have been extensively investigated in the past for applications in lithography, characteristics such as the influence of blanking on imaging resolution have not been fully addressed. We derive general analytical expressions for the spot displacement and loss in resolution induced by deflecting the electron beam in a blanker. In particular, we analyze the sensitivity of both measures to how precise the conjugate focus is aligned in between the deflector plates. We then work out the specific case of a beam blanker driven by a linear voltage ramp as was used in recent studies by others and by us. The result shows that the spot displacement and focus blur can be reduced to the same order as the electron beam probe size, even when using a beam blanker of millimeter or larger scale dimensions. An interesting result is that, by the right choice of the focus position in the deflector, either the spot displacement from the stationary position can be minimized, or the blur can be made zero but not both at the same time. Our results can be used both to characterize existing beam blanker setups and to design novel blankers. This can further develop the field of time-resolved electron microscopy by making it easier to generate pulses with a typical duration of tens of picoseconds in a regular scanning electron microscope at high spatial resolution.

  • Application of electron channeling contrast imaging to 3D semiconductor structures through proper detector configurations
    Ultramicroscopy (IF 2.644) Pub Date : 2019-12-30
    Han Han; Thomas Hantschel; Libor Strakos; Tomas Vystavel; Marina Baryshnikova; Yves Mols; Bernardette Kunert; Robert Langer; Wilfried Vandervorst; Matty Caymax

    Nowadays electron channeling contrast imaging (ECCI) is widely used to characterize crystalline defects on blanket semiconductors. Its further application in the semiconductor industry is however challenged by the emerging rise of nanoscale 3D heterostructures. In this study, an angular multi-segment detector is utilized in backscatter geometry to investigate the application of ECCI to the defect analysis of 3D semiconductor structures such as III/V nano-ridges. We show that a low beam energy of 5 keV is more favorable and that the dimension of 3D structures characterized by ECCI can be scaled down to ∼ 28 nm. Furthermore, the impact of device edges on the collected ECCI image is investigated and correlated with tool parameters and cross-section profiles of the 3D structures. It is found that backscattered electrons (BSE) emitted from the device edge sidewalls and generating the bright edges (edge effects), share a similar angular distribution to those emitted from the surface. We show that the collection of low angle BSEs can suppressed the edge effects, however, at the cost of losing the defect contrast. A positive stage bias suppresses edge effects by removing the inelastically backscattered electrons from the sidewalls, but low loss BSEs from the sidewalls still contribute to the ECCI micrographs. On the other hand, if segments of an angular backscatter (ABS) detector are properly aligned with the nano-ridges, BSEs emitted from the sidewall and the surface can be separated, thus leading to the completely absence of one bright edge on the surface without compromise of the defect contrast. The merging of two such ECCI images reveals the nano-ridge surface without edge effects.

  • High order aberrations calculations of Wien filters using differential algebra methods
    Ultramicroscopy (IF 2.644) Pub Date : 2019-12-30
    Yongfeng Kang; Hangfeng Hu; Jingyi Zhao

    In this paper, a novel scheme based on the differential algebraic (DA) method is proposed to analyze the electron optics properties of the Wien filters. A new software package is then developed, to compute the geometrical and chromatic aberrations up to the fifth order of the Wien filters. For examples, the aberrations of single filter and the double filters are calculated. The calculated geometrical aberration coefficients are comparable with the counterpart calculated by Tang's theory. However, both theories can only match with each other in the calculation of the chromatic aberration coefficient when the dispersion ray is taken into consideration in Tang's theory.

  • Direct measurement of electrostatic potentials at the atomic scale: A conceptual comparison between electron holography and scanning transmission electron microscopy
    Ultramicroscopy (IF 2.644) Pub Date : 2019-12-30
    Florian Winkler; Juri Barthel; Rafal E. Dunin-Borkowski; Knut Müller-Caspary

    Off-axis electron holography and first moment STEM are sensitive to electromagnetic potentials or fields, respectively. In this work, we investigate in what sense the results obtained from both techniques are equivalent and work out the major differences. The analysis is focused on electrostatic (Coulomb) potentials at atomic spatial resolution. It is shown that the probe-forming/objective aperture strongly affects the reconstructed electrostatic potentials and that, as a result of the different illumination setups, dynamical diffraction effects show a specific response with increasing specimen thickness. It is shown that thermal diffuse scattering is negligible for a wide range of specimen thicknesses, when evaluating the first moment of diffraction patterns.

  • Correlating results from high resolution EBSD with TEM- and ECCI-based dislocation microscopy: Approaching single dislocation sensitivity via noise reduction
    Ultramicroscopy (IF 2.644) Pub Date : 2019-12-30
    T.J. Ruggles; Y.S.J. Yoo; B.E. Dunlap; M.A. Crimp; J. Kacher

    High resolution electron backscatter diffraction (HREBSD), an SEM-based diffraction technique, may be used to measure the lattice distortion of a crystalline material and to infer the geometrically necessary dislocation content. Uncertainty in the image correlation process used to compare diffraction patterns leads to an uneven distribution of measurement noise in terms of the lattice distortion, which results in erroneous identification of dislocation type and density. This work presents a method of reducing noise in HREBSD dislocation measurements by removing the effect of the most problematic components of the measured distortion. The method is then validated by comparing with TEM analysis of dislocation pile-ups near a twin boundary in austenitic stainless steel and with ECCI analysis near a nano-indentation on a tantalum oligocrystal. The HREBSD dislocation microscopy technique is able to resolve individual dislocations visible in TEM and ECCI and correctly identify their Burgers vectors.

  • Microscopic investigations of morphology and thermal properties of ZnO thin films grown by atomic layer deposition method
    Ultramicroscopy (IF 2.644) Pub Date : 2019-12-25
    Anna Kaźmierczak-Bałata; Jerzy Bodzenta; Marek Guziewicz

    This work presents the study of morphology and thermal properties of thin ZnO films fabricated by atomic layer deposition. The layers were deposited on n-Si(100) wafers at 200 °C. X-ray diffraction measurements showed the polycrystalline structure of the thin films with preferred (100) orientation. The thinner ZnO layers were fine grained, while the thicker films were formed with larger, elongated grains. Surface roughness and the thermal conductivity were obtained from microscopic measurements. Thermal properties correlated with surface morphology of the ZnO thin films. Variations in thermal conductivity followed the changes in morphology of the layers. The mean surface roughness depended on the number of deposition cycles and varied from 1.1–2.6 nm. Thermal conductivity varied from 0.28 to 4.29 Wm−1K−1 and increased also with an increase of average crystallite size. The possible correlations between electrical conductivity and thermal conductivity were also analyzed. The phonon contribution to total thermal conductivity dominates over the electron thermal conductivity.

  • Enhancing the defect contrast in ECCI through angular filtering of BSEs
    Ultramicroscopy (IF 2.644) Pub Date : 2019-12-24
    Han Han; Thomas Hantschel; Andreas Schulze; Libor Strakos; Tomas Vystavel; Roger Loo; Bernardette Kunert; Robert Langer; Wilfried Vandervorst; Matty Caymax

    In this study, an annular multi-segment backscattered electron (BSE) detector is used in back scatter geometry to investigate the influence of the angular distribution of BSE on the crystalline defect contrast in electron channeling contrast imaging (ECCI). The study is carried out on GaAs and Ge layers epitaxially grown on top of silicon (Si) substrates, respectively. The influence of the BSE detection angle and landing energy are studied to identify the optimal ECCI conditions. It is demonstrated that the angular selection of BSEs exhibits strong effects on defect contrast formation with variation of beam energies. In our study, maximum defect contrast can be obtained at BSE detection angles 53°-65° for the investigated energies 5, 10 and 20 keV. In addition, it is found that higher beam energy is favorable to reveal defects with stronger contrast whereas lower energy ( ≤ 5 keV) is favorable for revealing crystalline defects as well as with topographic features on the surface. Our study provides optimal ECCI conditions, and therefore enables a precise and fast detection of threading dislocations in lowly defective materials and nanoscale 3D semiconductor structures where signal to noise ratio is especially important. A comparison of ECCI with BSE and secondary electron imaging further demonstrates the strength of ECCI in term of simultaneous detection of defects and morphology features such as terraces with atomic step heights.

  • Atomic Resolution Convergent Beam Electron Diffraction Analysis Using Convolutional Neural Networks
    Ultramicroscopy (IF 2.644) Pub Date : 2019-12-23
    Chenyu Zhang; Jie Feng; Luis Rangel DaCosta; Paul. M. Voyles

    Two types of convolutional neural network (CNN) models, a discrete classification network and a continuous regression network, were trained to determine local sample thickness from convergent beam diffraction (CBED) patterns of SrTiO3 collected in a scanning transmission electron microscope (STEM) at atomic column resolution. Acquisition of atomic resolution CBED patterns for this purpose requires careful balancing of CBED feature size in pixels, acquisition speed, and detector dynamic range. The training datasets were derived from multislice simulations, which must be convolved with incoherent source broadening. Sample thicknesses were also determined using quantitative high-angle annular dark-field (HAADF) STEM images acquired simultaneously. The regression CNN performed well on sample thinner than 35 nm, with 70% of the CNN results within 1 nm of HAADF thickness, and 1.0 nm overall root mean square error between the two measurements. The classification CNN was trained for a thicknesses up to 100 nm and yielded 66% of CNN results within one classification increment of 2 nm of HAADF thickness. Our approach depends on methods from computer vision including transfer learning and image augmentation.

  • Sub-100 nanosecond temporally resolved imaging with the Medipix3 direct electron detector
    Ultramicroscopy (IF 2.644) Pub Date : 2019-12-07
    Gary W. Paterson, Raymond J. Lamb, Rafael Ballabriga, Dima Maneuski, Val O’Shea, Damien McGrouther

    Detector developments are currently enabling new capabilities in the field of transmission electron microscopy (TEM). We have investigated the limits of a hybrid pixel detector, Medipix3, to record dynamic, time varying, electron signals. Operating with an energy of 60 keV, we have utilised electrostatic deflection to oscillate electron beam position on the detector. Adopting a pump-probe imaging strategy, we have demonstrated that temporal resolutions three orders of magnitude smaller than are available for typically used TEM imaging detectors are possible. Our experiments have shown that energy deposition of the primary electrons in the hybrid pixel detector limits the overall temporal resolution. Through adjustment of user specifiable thresholds or the use of charge summing mode, we have obtained images composed from summing 10,000s frames containing single electron events to achieve temporal resolution less than 100 ns. We propose that this capability can be directly applied to studying repeatable material dynamic processes but also to implement low-dose imaging schemes in scanning transmission electron microscopy.

  • Ultra-High Spatial Resolution Selected Area Electron Channeling Patterns
    Ultramicroscopy (IF 2.644) Pub Date : 2019-12-05
    R.D. Kerns, S. Balachandran, A.H. Hunter, M.A. Crimp

    An approach for producing ultrahigh spatial resolution selected area electron channeling patterns (UHR-SACPs) using the FEI/Thermo Elstar electron column is presented. The approach uses free lens control to directly assign lens and deflector values to rock the beam about precise points on the sample surface and generate the UHR-SACPs. Modification of the lens parameters is done using a service application that is preinstalled on the microscope or using the iFast scripting interface to run a short program to assign lens and deflector currents. Using the approach outlined here, the UHR-SACPs are collected at normal instrument scanning rates and pixel densities, resulting in rapid collection times and sharp patterns with simple push button changes in instrument mode. UHR-SACPs with spatial resolutions of 300 nm with angular ranges of 20o are demonstrated, as are patterns approaching 125 nm spatial resolution with angular ranges of 4o. Such spatial resolution/angular range combinations are significantly better than any reported previously. This approach for rapidly collecting high accuracy crystallographic information greatly enhances the ability to carry out electron channeling contrast imaging (ECCI) for a broad range of materials applications.

  • Quartz Tuning Fork Based Three-Dimensional Topography Imaging for Sidewall with Blind Features
    Ultramicroscopy (IF 2.644) Pub Date : 2019-12-02
    Imtisal Akhtar, Malik Abdul Rehman, Woosuk Choi, Joonho Bae, Yongho Seo

    Atomic force microscopy has a tremendous number of applications in a wide variety of fields, particularly in the semiconductor area for the 3D-stacked device. Imaging three-dimensional (3D) structures with blind features has progressively become a critical technique. Recently, a 3D-atomic force microscopy (AFM) technique has been proposed to image 3D features, especially those having sharp apices, like silicon pillars. However, the scanning strategy has drawbacks, such as long scanning time, and unstable operation, based on the premature algorithm. Herein, an improved 3D-AFM algorithm is reported that overcomes the aforementioned problems by an intelligent 3D scanning algorithm that incorporates sidewall history tracking, troubleshooting for sharp sidewall and sticking, and reactive direction adjustment. The proposed algorithm enables the 3D imagery of ZnO nano-rods and silicon nano-pillars to be achieved by using a high aspect-ratio multiwall carbon nanotube-based AFM probe, without time-consuming disorientation. This study establishes a method to construct a 3D image of arbitrary shape in reduced scanning time.

  • Quantitative analysis of spectroscopic Low Energy Electron Microscopy data: High-dynamic range imaging, drift correction and cluster analysis
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-23
    T.A. de Jong, D.N.L. Kok, A.J.H. van der Torren, H. Schopmans, R.M. Tromp, S.J. van der Molen, J. Jobst

    For many complex materials systems, low-energy electron microscopy (LEEM) offers detailed insights into morphology and crystallography by naturally combining real-space and reciprocal-space information. Its unique strength, however, is that all measurements can easily be performed energy-dependently. Consequently, one should treat LEEM measurements as multi-dimensional, spectroscopic datasets rather than as images to fully harvest this potential. Here we describe a measurement and data analysis approach to obtain such quantitative spectroscopic LEEM datasets with high lateral resolution. The employed detector correction and adjustment techniques enable measurement of true reflectivity values over four orders of magnitudes of intensity. Moreover, we show a drift correction algorithm, tailored for LEEM datasets with inverting contrast, that yields sub-pixel accuracy without special computational demands. Finally, we apply dimension reduction techniques to summarize the key spectroscopic features of datasets with hundreds of images into two single images that can easily be presented and interpreted intuitively. We use cluster analysis to automatically identify different materials within the field of view and to calculate average spectra per material. We demonstrate these methods by analyzing bright-field and dark-field datasets of few-layer graphene grown on silicon carbide and provide a high-performance Python implementation.

  • Quantifying oxygen distortions in lithium-rich transition-metal-oxide cathodes using ABF STEM
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-23
    E. Liberti, J.G. Lozano, M. Perez Osorio, M.R. Roberts, P.G. Bruce, A.I. Kirkland

    Lithium-rich cathodes can store excess charge beyond the transition metal redox capacity by participation of oxygen in reversible anionic redox reactions. Although these processes are crucial for achieving high energy densities, their structural origins are not yet fully understood. Here, we explore the use of annular bright-field (ABF) imaging in scanning transmission electron microscopy (STEM) to measure oxygen distortions in charged Li1.2Ni0.2Mn0.6O2. We show that ABF STEM data can provide positional accuracies below 20 pm but this is restricted to cases where no specimen mistilt is present, and only for a range of thicknesses above 3.5 nm. The reliability of these measurements is compromised even when the experimental and post-processing designs are optimised for accuracy and precision, indicating that extreme care must be taken when attempting to quantify distortions in these materials.

  • Spatial and spectral dynamics in STEM hyperspectral imaging using random scan patterns
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-23
    Alberto Zobelli, Steffi Y. Woo, Anna Tararan, Luiz H.G. Tizei, Nathalie Brun, Xiaoyan Li, Odile Stéphan, Mathieu Kociak, Marcel Tencé

    The evolution of the scanning modules for scanning transmission electron microscopes (STEM) allows now to generate arbitrary scan pathways, an approach currently explored to improve acquisition speed and to reduce electron dose effects. In this work, we present the implementation of a random scan operating mode in STEM achieved at the hardware level via a custom scan control module. A pre-defined pattern with fully shuffled raster order is used to sample the entire region of interest. Subsampled random sparse images can then be extracted at successive time frames, to which suitable image reconstruction techniques can be applied. With respect to the conventional raster scan mode, this method permits to limit dose accumulation effects, but also to decouple the spatial and temporal information in hyperspectral images. We provide some proofs of concept of the flexibility of the random scan operating mode, presenting examples of its applications in different spectro-microscopy contexts: atomically-resolved elemental maps with electron energy loss spectroscopy and nanoscale-cathodoluminescence spectrum images. By employing adapted post-processing tools, it is demonstrated that the method allows to precisely track and correct for sample instabilities and to follow spectral diffusion with a high spatial resolution.

  • Voltage contrast imaging with energy filtered signal in a field-emission scanning electron microscope
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-12
    Yoichiro Hashimoto, Shuichi Takeuchi, Takeshi Sunaoshi, Yu Yamazawa

    A new band-pass energy filter (BPF) technique of secondary electron (SE) detection using scanning electron microscope (SEM) was developed to enhance voltage contrast (VC) in SEM images. The energy filtering condition was optimized to enhance VC of dopant distribution using Si p-n structure. The relation between VC and SE energy was investigated by BPF as well as a conventional high-pass filter (HPF). Whereas the p-type regions were always brighter than the n-type region in the case of HPF, the contrast reversal between p region and n region occurred at the low SE energy range in the case of BPF. The variation of signal intensity of BPF against specimen bias voltage can be considered as SE spectrum analysis, and the peak split of the spectra between n-type and p-type regions was obtained. The peak split can be explained with a model with metal-semiconductor contact. This peak split causes the contrast reversal.

  • Patterned Probes for High Precision 4D-STEM Bragg Measurements
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-12
    Steven E Zeltmann, Alexander Müller, Karen C Bustillo, Benjamin Savitzky, Lauren Hughes, Andrew M Minor, Colin Ophus

    Nanoscale strain mapping by four-dimensional scanning transmission electron microscopy (4D-STEM) relies on determining the precise locations of Bragg-scattered electrons in a sequence of diffraction patterns, a task which is complicated by dynamical scattering, inelastic scattering, and shot noise. These features hinder accurate automated computational detection and position measurement of the diffracted disks, limiting the precision of measurements of local deformation. Here, we investigate the use of patterned probes to improve the precision of strain mapping. We imprint a “bullseye” pattern onto the probe, by using a binary mask in the probe-forming aperture, to improve the robustness of the peak finding algorithm to intensity modulations inside the diffracted disks. We show that this imprinting leads to substantially improved strain-mapping precision at the expense of a slight decrease in spatial resolution. In experiments on an unstrained silicon reference sample, we observe an improvement in strain measurement precision from 2.7% of the reciprocal lattice vectors with standard probes to 0.3% using bullseye probes for a thin sample, and an improvement from 4.7% to 0.8% for a thick sample. We also use multislice simulations to explore how sample thickness and electron dose limit the attainable accuracy and precision for 4D-STEM strain measurements.

  • Miniaturized Magnetic Bead-Actuators for Force-Clamp Spectroscopy-Based Single-Molecule Measurements
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-11
    L. Feng, H. Torun

    Force-clamp spectroscopy can mimic the physiological conditions for the proteins under investigation. In addition, it is a direct way of observing the relationship between bond lifetime and molecular forces. However, traditional force-clamp methods rely on active feedback controllers that can introduce artefacts. In this work, we introduce a new method to enable force-clamp spectroscopy without a need for an active feedback. The method is based on miniaturized magnetic beads offering improved stability. As a case study, we performed force-clamp experiments using biotin-streptavidin molecule pairs with and without active feedback. Our results demonstrate the feasibility of force-clamp experiments without feedback and illustrate the advantages of our method.

  • Development of analytical ultrafast transmission electron microscopy based on laser-driven Schottky field emission
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-10
    Chunhui Zhu, Dingguo Zheng, Hong Wang, Ming Zhang, Zhongwen Li, Shuaishuai Sun, Peng Xu, Huanfang Tian, Zian Li, Huaixin Yang, Jianqi Li

    A new design scheme for ultrafast transmission electron microscopy (UTEM) has been developed based on a Schottky-type field emission gun (FEG) at the Institute of Physics, Chinese Academy of Sciences (IOP CAS). In this UTEM setup, electron pulse emission is achieved by integrating a laser port between the electron gun and the column and the resulting microscope can operate in either continuous or pulsed mode. In pulsed mode, the optimized electron beam properties are an energy width of ∼0.65 eV, micrometer-scale coherence lengths and sub-picosecond pulse durations. The potential applications of this UTEM, which include electron diffraction, high-resolution imaging, electron energy loss spectroscopy, and photon-induced near-field electron microscopy, are demonstrated using ultrafast electron pulses. Furthermore, we use a nanosecond laser (∼10 ns) to show that the laser-driven FEG can support high-quality TEM imaging and electron holography when using a stroboscopic configuration. Our results also indicate that FEG-based ultrafast electron sources may enable high-performance analytical UTEM.

  • 更新日期:2019-11-11
  • Toward the Quantitative the Interpretation of Hole-Free Phase Plate Images in a Transmission Electron Microscope.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-06
    Ken Harada, Marek Malac, Misa Hayashida, Koudai Niitsu, Keiko Shimada, Darren Homeniuk, Marco Beleggia

    We present progress toward the quantitative interpretation of phase contrast images obtained using a hole-free phase plate (HFPP) in a transmission electron microscope (TEM). We consider a sinusoidal phase grating test object composed of ∼5 nm deep groves in a ∼13 nm thick amorphous silicon membrane. The periodic grating splits the beam current into direct beam and diffracted side beams in the focal plane of the imaging lens, where the HFPP is located. The physical separation between the beams allows for a detailed study of the HFPP phase shift evolution and its effect on image contrast. The residual phase shift of the electron beam footprint on the phase plate was measured by electron holography and used as input to image simulations that were compared to experimental data. Our results confirm that phase contrast is established by the phase difference between the direct and side beams, which we can estimate by fitting the image contrast evolution in time with an analytical formula describing the image intensity of a sinusoidal strong phase object. We also observed contrast reversal and frequency doubling of the grating image with time, which we interpret as the phase contrast arising from the interference between side beams becoming dominant. Another observation is the lateral displacement of the image fringes, which can be accounted for by a phase difference between the side beams.

  • Nanoscale temperature measurement during temperature controlled in situ TEM using Al plasmon nanothermometry
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-06
    A. Chmielewski, C. Ricolleau, D. Alloyeau, G. Wang, J. Nelayah

    Over recent years, the advent of microelectromechanical system (MEMS)-type microheaters has pushed the limits of temperature controlled in situ transmission electron microscopy (TEM). In particular, by enabling the observation of the structure of materials in their application environments, temperature controlled TEM provides unprecedented insights into the link between the properties of materials and their structure in real-world problems, a clear knowledge of which is necessary for rational development of functional materials with new or improved properties. While temperature is the key parameter in such experiments, accessing the precise temperature of the sample at the nanoscale during observations still remains challenging. In the present work, we have applied aluminium plasmon nanothermometry technique that monitors the temperature dependence of the volume plasmon of Al nanospheres using electron energy loss spectroscopy for in situ local temperature determination over a MEMS microheater. With access to local temperatures between room temperature to 550 ∘C, we have assessed the spatial and temporal stabilities of the microheater when it operates at different setpoint temperatures both under vacuum and in the presence of a static H2 gas environment. Temperature comparisons performed under the two environments show discrepancies between local and setpoint temperatures.

  • Insight into long-period pattern by depth sectioning using aberration-corrected scanning transmission electron microscope
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-05
    Chenzhi Song, Jianlin Wang, Jianping Sun, Yu Liu, Pan Chen, Xiaomin Li, Hongquan Liu, Binghui Ge, Xuedong Bai

    Long-period patterns (LPPs) are widely observed by transmission electron microscopy (TEM) in the study of nanoscale materials. Identifying the origin of LPPs is of significant importance when interpreting TEM images, and for an in-depth understanding of material characteristics. However, the two most common LPP categories, modulated structure and moiré patterns, are not easily differentiated by conventional TEM (CTEM). In this work, an LPP was observed in Cu2-xSe nanoplates by CTEM. And then the depth sectioning with an aberration-corrected scanning transmission electron microscope (AC STEM) has been performed to determine the LPP type. Two misorientated layers were recognized from the depth-series of atomic resolution images of an LPP region, confirming the LPP is a moiré pattern caused by two twisted stacked crystal flakes which commonly exists in nanosized materials. This depth sectioning method is generally applicable for structural characterization of layered systems, and is a powerful approach for the in-situ structural probe of nanomaterials. It is promising to be extended to fast three-dimensional (3D) reconstruction.

  • Orientation mapping with Kikuchi patterns generated from a focused STEM probe and indexing with commercially available EDAX software
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-04
    George L. Burton, Stuart Wright, Adam Stokes, David R. Diercks, Amy Clarke, Brian P. Gorman

    Relating a crystal's microscopic structure—such as orientation and size—to a material's macroscopic properties is of great importance in materials science. Although most crystal orientation microscopy is performed in the scanning electron microscope (SEM), transmission electron microscopy (TEM)-based methods have a number of benefits, including higher spatial resolution. Current TEM orientation methods have either specific hardware requirements or use software that has limited scope, utility, or availability. In this article, a technique is described for orientation mapping using Kikuchi diffraction patterns generated from a focused STEM probe. One key advantage is that indexing and analysis of the patterns and maps occurs in the robust OIM Analysis software, currently widely used for electron backscatter diffraction (EBSD) and transmission kikuchi diffraction (TKD) analysis. It was found that with minimal to no image processing and by changing only a few software parameters, reliable indexing of Kikuchi patterns is possible. Three samples, a deformed β-Titanium (Ti), a medium carbon heat-treated steel, and BaCe0.8Y0.2O3-δ were tested to determine the effectiveness of the approach. In all three measurements the algorithms effectively and reliably determined the phases and the crystal orientations of the features measured. For the two orientation maps produced, less than 5% of the patterns were misindexed including boundary areas where overlapping patterns existed. An angular resolution of 0.15° was achieved while features <25 nm were able to be spatially resolved.

  • Study and design of a lens-type retarding field energy analyzer without a grid electrode
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-04
    Junhyeok Hwang, Kwang-Il Kim, Takashi Ogawa, Boklae Cho, Dong-Hyun Kim, In-Yong Park

    A retarding field energy analyzer (RFEA) for measuring the energy distribution of charged particles offers the advantages of a simple structure and suitability for simultaneous observations of beam patterns in two dimensions. In this study, lens-based RFEAs without a grid electrode were theoretically investigated with regard to the geometry and lens condition to achieve high performance. The simulation results show that the proposed RFEA can achieve a resolution of 2.6 meV at an energy level of 500 eV. In addition, performance, which is the ratio of the resolution to the beam energy, reached 5.2×10−6. These results indicate that the RFEA designed in this study is capable of high-performance outcomes. The findings here demonstrate that the most important factors when attempting to realize a high-resolution RFEA design are to reduce the sagging effect of the electron beam through the focusing lens and ensure that V″(z) in the retarding electrode is close to zero. The design of the lens-based RFEAs is described in detail.

  • Low-energy in-line electron holographic imaging of vitreous ice-embedded small biomolecules using a modified scanning electron microscope
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-04
    Martin Cheung, Hidehito Adaniya, Cathal Cassidy, Masao Yamashita, Tsumoru Shintake

    Cryo-electron microscopy (cryo-EM) has become the method of choice in the field of structural biology, owing to its unique ability to deduce structures of vitreous ice-embedded, hydrated biomolecules over a wide range of structural resolutions. As cryo-transmission electron microscopes (cryo-TEM) become increasingly specialised for high, near-atomic resolution studies, operational complexity and associated costs serve as significant barriers to widespread usability and adoptability. To facilitate the expansion and accessibility of the cryo-EM method, an efficient, user-friendly means of imaging vitreous ice-embedded biomolecules has been called for. In this study, we present a solution to this issue by integrating cryo-EM capabilities into a commercial scanning electron microscope (SEM). Utilising the principle of low-energy in-line electron holography, our newly developed hybrid microscope permits low-to-moderate resolution imaging of vitreous ice-embedded biomolecules without the need for any form of sample staining or chemical fixation. Operating at 20 kV, the microscope takes advantage of the ease-of-use of SEM-based imaging and phase contrast imaging of low-energy electron holography. This study represents the first reported successful application of low-energy in-line electron holographic imaging to vitreous ice-embedded small biomolecules, the effectiveness of which is demonstrated here with three morphologically distinct specimens.

  • Investigating local oxidation processes in Fe thin films in a water vapor environment by in situ Liquid Cell TEM
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-01
    Jordan W. Ke, Shixiang Zhu, Christopher M. Rouleau, Raymond R. Unocic, Yao Xie, Josh Kacher

    Automated image recognition and analysis techniques were combined with liquid cell transmission electron microscopy to explore the oxidation kinetics of nanocrystalline Fe thin films in a water vapor environment. From in situ microscopy experiments, localized oxidation was observed to initiate in the film then propagate in an unsteady fashion, alternatingly arresting and progressing. The oxidation front propagation occurred via new oxidation sites initiating 10’s of nm ahead of the existing front rather than through a continuous expansion mechanism. The oxidation rate was seen to be highly dependent on electron dose rate, with increasing electron dose rate accelerating the oxidation front propagation and increasing the density of oxidation initiation sites. The in situ experiments were also performed in diffraction space where it was seen that Fe2O3 was formed during oxidation. Coupling in situ microscopy with automated image analysis creates new opportunities for studying the early stages of localized corrosion by providing direct observation of oxidation propagation as well as quantification of the oxidation rates and rapid identification of byproducts.

  • Assessment of Biophysical Properties of Haemonchus contortus from Different Life Cycle Stages with Atomic Force Microscopy
    Ultramicroscopy (IF 2.644) Pub Date : 2019-10-31
    Livio M. Costa-Junior, Carolina R. Silva, Alexandra M.S. Soares, Alan S. de Menezes, Maria R.L. Silva, Alessandro F.T. Amarante, Erivelton F. Costa, Luciana M.R. Alencar
  • Multi-modal and multi-scale non-local means method to analyze spectroscopic datasets
    Ultramicroscopy (IF 2.644) Pub Date : 2019-10-30
    Niklas Mevenkamp, Katherine MacArthur, Vasiliki Tileli, Philipp Ebert, Leslie J. Allen, Benjamin Berkels, Martial Duchamp

    A multi-modal and multi-scale non-local means (M3S-NLM) method is proposed to extract atomically resolved spectroscopic maps from low signal-to-noise (SNR) datasets recorded with a transmission electron microscope. This method improves upon previously tested denoising techniques as it takes into account the correlation between the dark-field signal recorded simultaneously with the spectroscopic dataset without compromising on the spatial resolution. The M3S-NLM method was applied to electron energy dispersive X-ray and electron-energy-loss spectroscopy (EELS) datasets. We illustrate the retrieval of the atomic scale diffusion process in an Al1-xInxN alloy grown on GaN and the surface oxidation state of perovskite nanocatalysts. The improved SNR of the EELS dataset also allows the retrieval of atomically resolved oxidation maps considering the fine structure absorption edge of LaMnO3 nanoparticles.

  • 更新日期:2019-11-01
  • Corrigendum to "Sample cartridge with built -in miniature molecule evaporator for in-situ measurement with a photoemission electron microscope" [Ultramicroscopy 200 (2019) 1-5].
    Ultramicroscopy (IF 2.644) Pub Date : 2019-11-12
    Mohamad-Assaad Mawass,Jens Niederhausen,Michael Hengge,Katherine A Mazzio,Simone Raoux,Florian Kronast

  • Design and development of thermionic emission microscope for the characterization of a multi-beam cathode Ultramicroscopy 202 (2019) 140-147.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-08-14
    Sushil Kumar Shukla,Asish Kumar Singh,Sunil Kumar Pradhan,Ranjan Kumar Barik

  • Conventional vs. model-based measurement of patterned line widths from scanning electron microscopy profiles.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-08-20
    Francesc Salvat-Pujol,John S Villarrubia

    Scanning electron microscopy (SEM) is a practical tool to determine the dimensions of nanometer-scale features. Conventional width measurements use arbitrary criteria, e.g., a 50 % threshold crossing, to assign feature boundaries in the measured SEM intensity profile. To estimate the errors associated with such a procedure, we have simulated secondary electron signals from a suite of line shapes consisting of 30 nm tall silicon lines with varying width, sidewall angle, and corner rounding. Four different inelastic scattering models were employed in Monte Carlo simulations of electron transport to compute secondary electron image intensity profiles for each of the shapes. The 4 models were combinations of dielectric function theory with either the single-pole approximation (SPA) or the full Penn algorithm (FPA), and either with or without Auger electron emission. Feature widths were determined either by the conventional threshold method or by the model-based library (MBL) method, which is a fit of the simulated profiles to the reference model (FPA + Auger). On the basis of these comparisons we estimate the error in the measured width of such features by the conventional procedure to be as much as several nanometers. A 1 nm difference in the size of, e.g., a nominally 10 nm transistor gate would substantially alter its electronic properties. Thus, the conventional measurements do not meet the contemporary requirements of the semiconductor industry. In contrast, MBL measurements employing models with varying accuracy differed one from another by less than 1 nm. Thus, a MBL measurement is preferable in the nanoscale domain.

  • Design for a high resolution electron energy loss microscope.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-10-14
    Marian Mankos,Khashayar Shadman,Raphaël Hahn,Yan J Picard,Daniel Comparat,Olena Fedchenko,Gerd Schönhense,Lionel Amiaud,Anne Lafosse,Nick Barrett

    An electron optical column has been designed for High Resolution Electron Energy Loss Microscopy (HREELM). The column is composed of electron lenses and a beam separator that are placed between an electron source based on a laser excited cesium atom beam and a time-of-flight (ToF) spectrometer or a hemispherical analyzer (HSA). The instrument will be able to perform full field low energy electron imaging of surfaces with sub-micron spatial resolution and meV energy resolution necessary for the analysis of local vibrational spectra. Thus, non-contact, real space mapping of microscopic variations in vibrational levels will be made possible. A second imaging mode will allow for the mapping of the phonon dispersion relations from microscopic regions defined by an appropriate field aperture.

  • Indexing electron backscatter diffraction patterns with a refined template matching approach.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-10-07
    A Foden,D M Collins,A J Wilkinson,T B Britton

    Electron backscatter diffraction (EBSD) is a well-established method of characterisation for crystalline materials. Using this technique, we can rapidly acquire and index diffraction patterns to provide phase and orientation information about the crystals on the material surface. The conventional analysis method uses signal processing based on a Hough/Radon transform to index each diffraction pattern. This method is limited to the analysis of simple geometric features and ignores subtle characteristics of diffraction patterns, such as variations in relative band intensities. A second method, developed to address the shortcomings of the Hough/Radon transform, is based on template matching of a test experimental pattern with a large library of potential patterns. In the present work, the template matching approach has been refined with a new cross correlation function that allows for a smaller library and enables a dramatic speed up in pattern indexing. Refinement of the indexed orientation is performed with a follow-up step to allow for small alterations to the best match from the library search. The refined template matching approach is shown to be comparable in accuracy, precision and sensitivity to the Hough based method, even exceeding it in some cases, via the use of simulations and experimental data collected from a silicon single crystal and a deformed α-iron sample. The speed up and pattern refinement approaches should increase the widespread utility of pattern matching approaches.

  • A new method for measuring nanoparticle diameter from a set of SEM images using a remarkable point.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-10-07
    Loïc Crouzier,Alexandra Delvallée,Sébastien Ducourtieux,Laurent Devoille,Christophe Tromas,Nicolas Feltin

    Scanning Electron Microscopy (SEM) is considered as a reference technique for the determination of nanoparticle (NP) dimensional properties. Nevertheless, the image analysis is a critical step of SEM measuring process and the initial segmentation phase consisting in determining the contour of each nano-object to be measured must be correctly carried out in order to identify all pixels belonging to it. Several techniques can be applied to extract NP from SEM images and evaluate their diameter like thresholding or watershed. However, due to the lack of reference nanomaterials, few papers deals with the uncertainty associated with these segmentation methods. This article proposes a novel approach to extract the NP boundaries from SEM images using a remarkable point. The method is based on the observation that, by varying the electron beam size, the secondary electron profiles crosses each other at this point. First, a theoretical study has been performed using Monte Carlo simulation on silica NP to evaluate the robustness of the method compared with more conventional segmentation techniques (Active Contour or binarization at Full Width at Half-Maximum, FWHM). The simulation results show especially a systematic discrepancy between the NP real size and the measurements performed with both conventional methods. Moreover, generated errors are NP size-dependent. By contrast, it has been demonstrated that a very good agreement between measured and simulated diameters has been obtained with this new technique. As an example, this method of the remarkable point has been applied on SEM images of silica particles. The quality of the segmentation has been shown on silica reference nanoparticles by measuring the modal equivalent projected area diameter and comparing with calibration certificate. The results show that the NP contour can be very accurately delimited with using this point. The measurement uncertainty has been also reduced from 4.3 nm (k = 2) with conventional methods to 2.6 nm (k = 2) using the remarkable point.

  • Understanding amorphization mechanisms using ion irradiation in situ a TEM and 3D damage reconstruction.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-10-05
    Osmane Camara,Matheus A Tunes,Graeme Greaves,Anamul H Mir,Stephen Donnelly,Jonathan A Hinks

    In this work, ion irradiations in-situ of a transmission electron microscope are performed on single-crystal germanium specimens with either xenon, krypton, argon, neon or helium. Using analysis of selected area diffraction patterns and a custom implementation of the Stopping and Range of Ions in Matter (SRIM) within MATLAB (which allows both the 3D reconstruction of the collision cascades and the calculation of the density of vacancies) the mechanisms behind amorphization are revealed. An intriguing finding regarding the threshold displacements per atom (dpa) required for amorphization results from this study: even though the heavier ions generate more displacements than lighter ions, it is observed that the threshold dpa for amorphization is lower for the krypton-irradiated specimens than for the xenon-irradiated ones. The 3D reconstructions of the collision cascades show that this counter-intuitive observation is the consequence of a heterogeneous amorphization mechanism. Furthermore, it is also shown that such a heterogeneous process occurs even for helium ions, which, on average induce only three recoils per ion in the specimen. It is revealed that at relatively high dpa, the stochastic nature of the collision cascade ensures complete amorphization via the accumulation of large clusters of defects and even amorphous zones generated by single-helium-ion strikes.

  • Ni-base superalloy single crystal (SX) mosaicity characterized by the Rotation Vector Base Line Electron Back Scatter Diffraction (RVB-EBSD) method.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-24
    P Thome,S Medghalchi,J Frenzel,J Schreuer,G Eggeler

    In the present work we present the Rotation Vector Base Line Electron Back Scatter Diffraction (RVB-EBSD) method, a new correlative orientation imaging method for scanning electron microscopy (OIM/SEM). The RVB-EBSD method was developed to study crystal mosaicity in as-cast Ni-base superalloy single crystals (SX). The technique allows to quantify small crystallographic deviation angles between individual dendrites and to interpret associated accommodation processes in terms of geometrically necessary dislocations (GNDs). The RVB-EBSD method was inspired by previous seminal approaches which use cross correlation EBSD procedures. It applies Gaussian band pass filtering to improve the quality of more than 500 000 experimental patterns. A rotation vector approximation and a correction procedure, which relies on a base line function, are used. The method moreover features a novel way of intuitive color coding which allows to easily appreciate essential features of crystal mosaicity. The present work describes the key elements of the method and shows examples which demonstrate its potential.

  • Differential electron scattering cross-sections at low electron energies: The influence of screening parameter.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-24
    M Čalkovský,E Müller,M Hugenschmidt,D Gerthsen

    For quantitative electron microscopy the comparison of measured and simulated data is essential. Monte Carlo (MC) simulations are well established to calculate the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) intensities on a non-atomic scale. In this work we focus on the importance of the screening parameter in differential screened Rutherford cross-sections for MC simulations and on the contribution of the screening parameter to the atomic-number dependence of the HAADF-STEM intensity at electron energies ≤ 30 keV. Materials investigated were chosen to cover a wide range of atomic numbers Z to study the Z dependence of the screening parameter. Comparison of measured and simulated HAADF-STEM intensities with different screening parameters known from the literature were tested and failed to generally describe the experimental data. Hence, the screening parameter was adapted to obtain the best match between experimental and MC-simulated HAADF-STEM intensities. The Z dependence of the HAADF-STEM intensity was derived.

  • Influence of pixelization on height measurement in atomic force microscopy.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-24
    Anna P Tolstova,Evgeniy V Dubrovin

    Though AFM is capable of obtaining sub-angstrom resolution in z-direction, the accurate height measurement of protruding particles is hindered by raster nature of this technique. In this work using Monte Carlo simulations we have quantified the influence of pixelization on the mean AFM apparent height (hmean) of spheres and cylinders. We have demonstrated that for a zero size AFM probe hmean may be increasing, decreasing function of a pixel size, or has more complex character depending on the standard deviation of a particle size. Therefore, AFM pixelization effects may induce both under- and overestimation of the true diameter. The observed complex behavior of hmean is explained by interplay of two opposing factors: the mismatch of the position of the "highest" pixel to the real topographical maximum and higher registration probabilities of larger particles. Consideration of the AFM probe size results in even bigger pixelization induced drops of hmean, which may amount to ∼50% of the true value. The obtained results contribute to AFM data interpretation and methodological aspects of AFM operation in many fields of nanoscience. In particular, they may be used for estimation of true height of nanoparticles from their AFM images obtained with different (even low) pixel resolution.

  • Dynamical diffraction effects on the geometric phase of inhomogeneous strain fields.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-23
    L Meißner,T Niermann,D Berger,M Lehmann

    In specimens with an inhomogeneous displacement field in electron beam direction dynamical diffraction effects lead to complex non-linear properties of the diffracted electron wave. Consequently, the diffracted beam's phase contains information about the inhomogeneous displacement field. These phases are experimentally and theoretically investigated under different excitation errors and specimen thicknesses as well as for different depths of the displacement field. An inclined InGaAs layer with a larger lattice constant than the surrounding GaAs matrix serves as controlled displacement field, which is inhomogeneous in electron beam direction with a continuously changing depth. The phase and amplitude of the diffracted beam are measured by dark-field electron holography. The measurements agree with calculations performed by numerical propagation of the electron wave using the Darwin-Howie-Whelan equations. A strong dependency on the excitation conditions is found showing that the interplay between dynamical effects and the strain field must be considered in the interpretation of the geometric phase.

  • Lattice strain mapping using circular Hough transform for electron diffraction disk detection.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-21
    Renliang Yuan,Jiong Zhang,Jian-Min Zuo

    Scanning Electron NanoDiffraction (SEND) is a powerful and versatile technique for lattice strain mapping in nano-devices and nano-materials. The measurement is based on Bragg diffraction from a local crystal volume. However, the resolution and precision of SEND are fundamentally limited by the uncertainty principle and scattering that govern electron diffraction. Here, we propose to measure lattice strain using a focused probe and circular Hough transform to locate the position of non-uniform diffraction disks. Methods for fitting a 2D lattice to the detected disks for strain calculation are described, including error analysis. We demonstrate our technique on a FinFET device for strain mapping at the spatial resolution of 1 nm and strain precision of ∼3×10-4. Using this and simulations, the experimental parameters involved in data acquisition and analysis are thoroughly investigated to construct an optimum strain mapping strategy using SEND.

  • Gazing at crystal balls: Electron backscatter diffraction pattern analysis and cross correlation on the sphere.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-21
    Ralf Hielscher,Felix Bartel,Thomas Benjamin Britton

    We present spherical analysis of electron backscatter diffraction (EBSD) patterns with two new algorithms: (1) band localisation and band profile analysis using the spherical Radon transform; (2) orientation determination using spherical cross correlation. These new approaches are formally introduced and their accuracies are determined using dynamically simulated patterns. We demonstrate their utility with an experimental dataset obtained from ferritic iron. Our results indicate that the analysis of EBSD patterns on the sphere provides an elegant method of revealing information from these rich sources of crystallographic data.

  • Design for a 10 keV multi-pass transmission electron microscope.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-15
    Stewart A Koppell,Marian Mankos,Adam J Bowman,Yonatan Israel,Thomas Juffmann,Brannon B Klopfer,Mark A Kasevich

    Multi-pass transmission electron microscopy (MPTEM) has been proposed as a way to reduce damage to radiation-sensitive materials. For the field of cryo-electron microscopy (cryo-EM), this would significantly reduce the number of projections needed to create a 3D model and would allow the imaging of lower-contrast, more heterogeneous samples. We have designed a 10 keV proof-of-concept MPTEM. The column features fast-switching gated electron mirrors which cause each electron to interrogate the sample multiple times. A linear approximation for the multi-pass contrast transfer function (CTF) is developed to explain how the resolution depends on the number of passes through the sample.

  • Plan-view sample preparation of a buried nanodots array by FIB with accurate EDS positioning in thickness direction.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-11
    Chaorong Zhong,Lina Lin,Ruijuan Qi,Yan Cheng,Xingsen Gao,Rong Huang

    Recently, there are growing demands on focus ion beam (FIB) sample preparation technique in plan-view geometry because it can provide the in-plane microstructure information of thin film and allows direct correlations of the atomic structure via transmission electron microscopy with micrometer-scale property measurements. However, one main technical difficulty is to position the buried thin film accurately in a sandwich structure. In this paper, an on-line positioning method based on the thickness monitoring by EDS is introduced, where the intensities of the characteristic X-ray peaks from different layers are proportional to the relative thickness of them at the same acquisition conditions. A high density array of ∼100 nm squares BiFeO3 nanodots with ∼ 25 nm thickness grown on a 20 nm thick SrRuO3 bottom electrode and (001)-oriented SrTiO3 substrate is selected for demonstration. By monitoring the intensities of Pt-M, Sr-L, Ti-K, Ru-L, Fe-K and Bi-M peaks, the relative thickness of Pt protection layer, the BiFeO3, SrRuO3 and SrTiO3 can be obtained, which provide accurate position of the BFO nanodots array in the thickness direction. With these information, the cutting parameters are optimized and a high quality plan-view specimen of BFO nanodots array is prepared, which is confirmed by high resolution transmission electron microscopy. This positioning method should have a wide application for material science.

  • A spherical harmonic transform approach to the indexing of electron back-scattered diffraction patterns.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-11
    W C Lenthe,S Singh,M De Graef

    A new approach is proposed for the indexing of electron back-scattered diffraction (EBSD) patterns. The algorithm employs a spherical master EBSD pattern and computes its cross-correlation with a back-projected experimental pattern using the spherical harmonic transform (SHT). This approach is significantly faster than the recent dictionary indexing algorithm, but shares the latter's robustness against noise. The underlying theory is presented, followed by example applications, one on a series of Ni EBSD data sets recorded with decreasing signal-to-noise ratio, the other on a large shot-peened Al data set. The dependence of indexing speed and memory usage on the SHT bandwidth is explored. The speed gains of the new algorithm are achieved by executing real-valued Fast Fourier Transforms, explicitly incorporating crystallographic symmetry in the cross-correlation computation, and using efficient loop ordering to improve the caching behavior. The algorithm produces a cross-correlation array in the zyz Euler space; an orientation refinement procedure is proposed based on analytical derivatives of the Wigner d functions. The new approach can be applied to any diffraction modality for which the scattered intensity can be represented on a spherical surface.

  • Significance of electrostatic interactions due to surface potential in piezoresponse force microscopy.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-09
    Daehee Seol,Seunghun Kang,Changhyo Sun,Yunseok Kim

    Piezoresponse force microscopy (PFM) has gradually becomes indispensable tool to investigate local piezoelectric and ferroelectric properties in diverse material systems. However, numerous reports have shown that the PFM signal can originate from several non-piezoelectric origins. Among them, because the electrostatic interaction between the AFM tip/cantilever and sample surface can be readily involved, it can be the most important factor during PFM measurement. In particular, in materials with relatively low piezoelectricity, the situation can be more significant because the PFM signals from weak piezoelectricity can be hidden or buried by the electrostatic interactions. Herein, we examined the significance of the electrostatic interactions induced by the surface potential in PFM. Using piezoelectric and non-piezoelectric materials, we examined how the surface potential-dependent electrostatic interactions can significantly affect the PFM signal. We observed that the electrostatically induced PFM amplitude have a linear relationship with the magnitude of surface potential when the instrumental noise floor is properly considered. Our results demonstrate that electrostatic interactions can be significant and their recognition and minimization are essential during PFM and other AFM-based measurements.

  • 3D surface topography imaging in SEM with improved backscattered electron detector: Arrangement and reconstruction algorithm.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-09
    A A Borzunov,V Y Karaulov,N A Koshev,D V Lukyanenko,E I Rau,A G Yagola,S V Zaitsev

    We propose a new SFS (shape from shading) technique for improved 3D surface reconstruction and imaging of relatively smooth surface topography using the scanning electron microscope (SEM). The new arrangement of backscattered electrons detector plates allows decreasing the initial energy of the electron probe, which makes this SEM technique to be suitable for usage on radiation-sensitive samples like biological tissues. Experiments show high effectiveness of the method, which improves both the gradient sensitivity of the signal and the signal to noise ratio.

  • Electron spill-out effects in plasmon excitations by fast electrons.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-09
    Alberto Rivacoba

    A new non-retarded hydrodynamic approach to the interaction between a fast electron and a diffuse metal-vacuum interface is presented. The metal is characterized by the parameters of a dispersive bulk dielectric function which slowly fade at the interface. The response of the medium is described by the induced charge density, which is self-consistently calculated. This formalism is applied to the study of the energy loss spectrum (EELS) experienced by a fast electron passing by a metal-vacuum interface. In the case of a sharp interface analytical expressions for the loss probability, fully equivalent to that of the Specular Reflection Model (SRM), are found. In an Al interface the effects of the electron density spill-out (modeled according to Lang-Kohn density) on both the longitudinal (EELS) and transverse components of the momentum transfer are studied. The influence of the interface profile on the surface plasmon dispersion in EELS is also discussed, showing that in agreement with previous theoretical and experimental works the dispersion of surface plasmon turns out to be much weaker than the one calculated in the SRM. A possible extension of the theory to study interfaces between transition metals and insulators is also discussed.

  • A method for constrained optimisation of the design of a scanning helium microscope.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-09
    M Bergin,D J Ward,J Ellis,A P Jardine

    We describe a method for obtaining the optimal design of a normal incidence Scanning Helium Microscope (SHeM). Scanning helium microscopy is a recently developed technique that uses low energy neutral helium atoms as a probe to image the surface of a sample without causing damage. After estimating the variation of source brightness with nozzle size and pressure, we perform a constrained optimisation to determine the optimal geometry of the instrument (i.e. the geometry that maximises intensity) for a given target resolution. For an instrument using a pinhole to form the helium microprobe, the source and atom optics are separable and Lagrange multipliers are used to obtain an analytic expression for the optimal parameters. For an instrument using a zone plate as the focal element, the whole optical system must be considered and a numerical approach has been applied. Unlike previous numerical methods for optimisation, our approach provides insight into the effect and significance of each instrumental parameter, enabling an intuitive understanding of effect of the SHeM geometry. We show that for an instrument with a working distance of 1 mm, a zone plate with a minimum feature size of 25 nm becomes the advantageous focussing element if the desired beam standard deviation is below about 300 nm.

  • Analysis of depth-sectioning STEM for thick samples and 3D imaging.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-07
    Eric G T Bosch,Ivan Lazić

    We derive a model that describes 3D volume imaging in depth-sectioning STEM that is valid for all STEM techniques under three well-defined conditions: linearity, undisturbed probe and elastic scattering. The resulting undisturbed probe model generalizes the widely used idea that the undisturbed probe intensity in three dimensions can be used as the point spread function for depth-sectioning ADF-STEM to all STEM techniques including (A)BF- and iDPC-STEM. The model provides closed expressions for depth-sectioning STEM, which follow directly from the 2D expressions for thin samples, and thereby enables analysis of the 3D resolution. Using the model we explore the consequences of the resulting 3D contrast transfer function (CTF) for the z-resolution at different length scales and illustrate this with experiments. We investigate the validity and limitations of the model using multi-slice simulations showing that it is valid and quantitatively accurate for relatively thick amorphous samples but not for crystalline samples in zone-axis due to channeling. We compare depth-sectioning in iDPC- and ADF-STEM and show that iDPC-STEM can extract information from deeper into the sample, all the way till the bottom of the sample, thereby effectively allowing a thickness measurement. Also the difference in optimal focus conditions between iDPC- and ADF-STEM is explained. Finally, we propose practical criteria for deciding whether a sample is thin or thick.

  • Tunable electron beam pulser for picoseconds stroboscopic microscopy in transmission electron microscopes.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-03
    Chunguang Jing,Yimei Zhu,Ao Liu,Karl Schliep,Xuewen Fu,Yubin Zhao,Eric Montgomery,Wade Rush,Alexei Kanareykin,Michael Katz,June Lau

    For two decades, time-resolved transmission electron microscopes (TEM) have relied on pulsed-laser photoemission to generate electron bunches to explore sub-microsecond to sub-picosecond dynamics. Despite the vast successes of photoemission time-resolved TEMs, laser-based systems are inherently complex, thus tend not to be turn-key. In this paper, we report on the successful retrofit of a commercial 200 keV TEM, without an external laser, capable of producing continuously tunable pulsed electron beams with repetition rates from 0.1 GHz up to 12 GHz and a tunable bunch length from tens of nanoseconds down to 10 ps. This innovation enables temporal access into previously inaccessible regimes: i.e., high repetition rate stroboscopic experiments. Combination of a pair of RF-driven traveling wave stripline elements, quadrupole magnets, and a variable beam aperture enables operation of the instrument in (1) continuous waveform (CW) mode as though the instrument was never modified (i.e. convention TEM operation mode, where the electrons from the emission cathode randomly arrive at the sample without resolvable time information), (2) stroboscopic (pump-probe) mode, and (3) pulsed beam mode for dose rate sensitive materials. To assess the effect of a pulsed beam on image quality, we examined Au nanoparticles using bright field, high-resolution TEM imaging and selected area diffraction in both continuous and pulsed-beam mode. In comparison of conventional TEMs, the add-on beam pulser enables the observation of ultrafast dynamic behavior in materials that are reversible under synchronized excitation.

  • Subsurface imaging of rigid particles buried in a polymer matrix based on atomic force microscopy mechanical sensing.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-09-02
    Weijie Zhang,Yuhang Chen,Yaoping Hou,Wenting Wang,Huarong Liu,Lei Zheng

    Several subsurface imaging methods based on atomic force microscopy (AFM) linear nanomechanical mapping, namely contact resonance (CR), bimodal and harmonic AFMs, are investigated and compared. Their respective subsurface detection capability is estimated and evaluated on a model specimen, which is prepared by embedding SiO2 microparticles in a PDMS elastomer. The measured CR frequency, bimodal and harmonic amplitudes are related to local mechanical properties by analyzing cantilever dynamics and further linked to subsurface depths of the particles by finite element analysis. The maximum detectable depths are obtained from the apparent particle diameters in subsurface image channels via employing a simple geometrical model. Under common experimental settings, results demonstrate that the depth limits reach up to about 812 nm, 212 nm and 127 nm for CR, bimodal and harmonic AFM modes, respectively. The depth sensitivity can be tuned and optimized by using either different cantilever eigenmodes in CR-AFM or spectroscopy analysis in bimodal and harmonic AFMs. The three imaging methods have their own suitable application situations. The comparisons can advance a further step into understanding the subsurface image contrast via AFM mechanical sensing.

  • On the resolution of EBSD across atomic density and accelerating voltage with a particular focus on the light metal magnesium.
    Ultramicroscopy (IF 2.644) Pub Date : 2019-08-31
    Abhishek Tripathi,Stefan Zaefferer

    We measured the physical lateral resolution of the electron backscatter diffraction (EBSD) technique for the case of pure magnesium and tungsten and compared these data with other values from literature. Spatial resolution, among other parameters, depends significantly on the accelerating voltage and the atomic number of the material. For the case of lighter metals, it is supposed to be lower than in the case of heavier metals for a given accelerating voltage. In the present work, lateral resolution was measured in dependence of accelerating voltage on a straight high angle grain boundary which was positioned parallel (horizontal boundary) and perpendicular (vertical boundary) to the tilt axis of the specimen. For magnesium the best lateral resolution of 240 nm was obtained at an accelerating voltage of 5 kV. The resolution dramatically worsened to values as high as 3500 nm as the voltage was increased from 15 kV to 30 kV. The aspect ratio of horizontal and vertical lateral resolution tended to 1.0 at the accelerating voltage of 5 kV and to 2.5 at the accelerating voltage of 30 kV. These values as function of accelerating voltages were compared with those obtained on the high atomic number metal tungsten. Here resolution at 5 kV was about a quarter of that of magnesium. With increasing voltage, the value almost didn't change. For all voltages the resolution aspect ratio stayed close to 1.0.

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上海纽约大学William Glover