Andrey Liyu

Implementing an Accurate and Rapid Sparse Sampling Approach for Low-Dose Atomic Resolution STEM Imaging

While aberration correction for scanning transmission electron microscopes (STEMs) dramatically increased the spatial resolution obtainable in the images of materials that are stable under the electron beam, the practical resolution of many STEM images is now limited by the sample stability rather than the microscope. To extract physical information from the images of beam sensitive materials, it is becoming clear that there is a critical dose/dose-rate below which the images can be interpreted as representative of the pristine material, while above it the observation is dominated by beam effects. Here, we describe an experimental approach for sparse sampling in the STEM and in-painting image reconstruction in order to reduce the electron dose/dose-rate to the sample during imaging. By characterizing the induction limited rise-time and hysteresis in the scan coils, we show that a sparse line-hopping approach to scan randomization can be implemented that optimizes both the speed of the scan and the amount of the sample that needs to be illuminated by the beam. The dose and acquisition time for the sparse sampling is shown to be effectively decreased by at least a factor of 5× relative to conventional acquisition, permitting imaging of beam sensitive materials to be obtained without changing the microscope operating parameters. The use of sparse line-hopping scan to acquire STEM images is demonstrated with atomic resolution aberration corrected the Z-contrast images of CaCO3, a material that is traditionally difficult to image by TEM/STEM because of dosage issues.

Self-corrected sensors based on atomic absorption spectroscopy for atom flux measurements in molecular beam epitaxy

A high sensitivity atom flux sensor based on atomic absorption spectroscopy has been designed and implemented to control electron beam evaporators and effusion cells in a molecular beam epitaxy system. Using a high-resolution spectrometer and a two-dimensional charge coupled device detector in a double-beam configuration, we employ either a non-resonant line or a resonant line with low cross section from the same hollow cathode lamp as the reference for nearly perfect background correction and baseline drift removal. This setup also significantly shortens the warm-up time needed compared to other sensor technologies and drastically reduces the noise coming from the surrounding environment. In addition, the high-resolution spectrometer allows the most sensitive resonant line to be isolated and used to provide excellent signal-to-noise ratio.

A sub-sampled approach to extremely low-dose STEM

The inpainting of deliberately and randomly sub-sampled images offers a potential means to image specimens at a high resolution and under extremely low-dose conditions ( ≤1 e−/A2) using a scanning transmission electron microscope. We show that deliberate sub-sampling acquires images at least an order of magnitude faster than conventional low-dose methods for an equivalent electron dose. More importantly, when adaptive sub-sampling is implemented to acquire the images, there is a significant increase in the resolution and sensitivity which accompanies the increase in imaging speed. We demonstrate the potential of this method for beam sensitive materials and in-situ observations by experimentally imaging the node distribution in a metal-organic framework.

A method for measuring the local gas pressure within a gas-flow stage in situ in the transmission electron microscope.

Environmental transmission electron microscopy (TEM) has enabled in situ experiments in a gaseous environment with high resolution imaging and spectroscopy. Addressing scientific challenges in areas such as catalysis, corrosion, and geochemistry can require pressures much higher than the ∼20 mbar achievable with a differentially pumped environmental TEM. Gas flow stages, in which the environment is contained between two semi-transparent thin membrane windows, have been demonstrated at pressures of several atmospheres. However, the relationship between the pressure at the sample and the pressure drop across the system is not clear for some geometries. We demonstrate a method for measuring the gas pressure at the sample by measuring the ratio of elastic to inelastic scattering and the defocus of the pair of thin windows. This method requires two energy filtered high-resolution TEM images that can be performed during an ongoing experiment, at the region of interest. The approach is demonstrated to measure greater than atmosphere pressures of N2 gas using a commercially available gas-flow stage. This technique provides a means to ensure reproducible sample pressures between different experiments, and even between very differently designed gas-flow stages.

Controlling the Reaction Process in Operando STEM by Pixel Sub-Sampling

Recently there has been an increase in the number of experiments making use of either in-situ gas or liquid stages, or using dedicated environmental (scanning) transmission electron microscopes (S/TEM) to study dynamic materials processes. While in-situ observations have traditionally been performed in TEM mode, allowing the intrinsic increase in temporal resolution of the projection image to be utilized, there are a number of key advantages of using the STEM imaging mode for these experiments. Namely, the same physics that makes high angle annular dark field (HAADF) or Z-contrast imaging optimum for quantifying small metal/oxide catalyst particles on a support also makes it optimum for imaging particle dynamics in liquids (now the liquid is the background in the image rather than the support). In addition, the incoherence of the Z-contrast image (i.e. decreased sensitivity to thickness effects) makes it the ideal method to image through the ~100-500 nm thick in-situ liquid cells that are typically used. Somewhat counter intuitively, the STEM imaging process is also optimized for controlling and reducing beam damage – the dose is controlled by the beam size and dwell time of the scan, while the probe only illuminates a small area, thereby reducing heating and depletion effects [1].

Implementing Sub-sampling Methods for Low-Dose (Scanning) Transmission Electron Microscopy (S/TEM)

In many practical applications of high resolution (scanning) transmission electron microscopy, the resolution obtainable in an image is determined solely by the stability of the sample. This is particularly true in the era of aberration corrected S/TEM where the dose on the sample for the highest resolution images can easily exceed 10 electrons/Å during routine operation. This dose sensitivity presents extreme constraints on applications such as 3-D imaging, spectroscopic imaging and in-situ experiments, where prolonged exposure to the beam can result in damage occurring before the experiment can be completed. To ensure that observations are free from damage artifacts, it is therefore essential that images are acquired under conditions where the maximum information can be extracted from the minimum amount of electron dose.

Increasing the Speed of EELS/EDS Mapping Through Dynamic/Adaptive Sampling Methodologies

1. Department of Materials Science & Engineering, Northwestern University, Evanston, IL. 2. OptimalSensing, Southlake, TX. 3. Department of Electrical and Computer Engineering, Duke University, Durham, NC. 4. Pacific Northwest National Laboratory, Richland, WA. 5. Department of Materials Engineering, University of Liverpool, Liverpool, United Kingdom 6. Electron Probe Instrumentation Center (EPIC) Facility, NUANCE Center, Northwestern University, Evanston, IL.

Design and Development of Coded Aperture Compressive Sensing Acquisition for High Frame Rate TEM Imaging

Transmission Electron Microscopy (TEM) enables to probe the dynamics of materials with spatial and temporal resolution that can vary over several orders of magnitude. In several research fields, it is critical to capture the materials behavior at high temporal resolution but due to a limited detector readout rates, the temporal resolution is currently limited mostly to milliseconds. To overcome these limitations, novel concepts of recording high frame video in TEM have been recently explored. Among those concepts, tiling of images on a detector using fast electrostatic deflectors has demonstrated the opportunity to acquire small video series at nanosecond resolution [1,2]. In this work we investigate an alternative approach of acquiring high frame-rate video using a coded aperture acquisition and compressive sensing (CS) reconstruction [3]. In this presentation, we will discuss the development of coded aperture imaging system and present initial results.

Imaging Electrochemical Processes in Li Batteries by Operando STEM

1. Joint Center for Energy Storage Research (JCESR), Pacific Northwest National Laboratory (PNNL), Richland, WA, USA 2. Physical and Computational Science Directorate, PNNL, Richland, WA, USA 3. Materials Science and Engineering, University of Washington, Seattle, WA, USA 4. National Security Directorate, PNNL, Richland, WA, USA 5. Energy and Environmental Directorate, PNNL, Richland, WA, USA 6. Environmental Molecular Sciences Laboratory, PNNL, Richland, WA, USA