Deep sub-Ångstrom imaging of 2D materials with a high dynamic range detector (1801.04630v1)

Abstract: Aberration-corrected optics have made electron microscopy at atomic-resolution a widespread and often essential tool for nanocharacterization. Image resolution is dominated by beam energy and the numerical aperture of the lens ({\alpha}), with state-of-the-art reaching ~0.47 {\AA} at 300 keV. Two-dimensional materials are imaged at lower beam energies to avoid knock-on damage, limiting spatial resolution to ~1 {\AA}. Here, by combining a new electron microscope pixel array detector with the dynamic range to record the complete distribution of transmitted electrons and full-field ptychography to recover phase information from the full phase space, we increased the spatial resolution well beyond the traditional lens limitations. At 80 keV beam energy, our ptychographic reconstructions significantly improved image contrast of single-atom defects in MoS2, reaching an information limit close to 5{\alpha}, corresponding to a 0.39 {\AA} Abbe resolution, at the same dose and imaging conditions where conventional imaging modes reach only 0.98 {\AA}.

• The paper introduces a novel EMPAD-based ptychographic technique that achieves 0.39 Å resolution at 80 keV, surpassing conventional limits.
• It employs full-field ptychography to recover precise phase information, enabling high-resolution imaging with minimal electron dose.
• The approach enhances visualization of atomic defects in 2D materials, providing actionable insights for advanced electron microscopy applications.

Deep Sub-Ångstrom Imaging of 2D Materials Using EMPAD and Ptychography

This paper presents an advancement in electron microscopy using an innovative electron microscope pixel array detector (EMPAD) and full-field ptychography, achieving deep sub-ångstrom resolution for two-dimensional (2D) materials. The central challenge in imaging 2D materials lies in achieving atomic resolution without inflicting damage on the sample due to high-energy electron beams. Traditionally, aberration-corrected optics in electron microscopy have enabled atomic-resolution imaging; however, these operate optimally at high beam energies (~300 keV), which are unsuitable for fragile 2D materials that require lower beam energies to minimize knock-on damage. Conventional imaging techniques in this low-energy regime, such as ADF-STEM, face limitations due to reduced resolution and image contrast.

The research introduces a novel EMPAD capable of capturing the entire spectrum of transmitted electrons with a high dynamic range (1,000,000:1), enabling the recovery of phase information through ptychography at low beam energies (80 keV). The paper effectively extends the limits of traditional lens-based resolution by exploiting interference patterns from overlapping scattered beams. The ptychographic approach significantly improves spatial resolution, achieving an Abbe resolution of 0.39 Å, compared to the 0.98 Å limit of conventional modes under similar conditions.

Key Findings and Numerical Analysis

1. Improved Spatial Resolution: The introduction of a full-field ptychographic reconstruction at 80 keV enhanced the image contrast of single-atom defects in MoS2, achieving a spatial resolution of 0.39 Å. This surpasses the typical 0.98 Å threshold reached by conventional methods, indicating a substantial leap in imaging capability without compromising sample integrity.
2. Detector Capabilities: The EMPAD ensures single-electron detection sensitivity and maintains a signal-to-noise ratio of 140 at 200 keV. The device functions efficiently across a wide energy range (20-300 keV), facilitating near-complete electron utilization (99.95% of the incident beam) and allowing a comprehensive collection of diffraction data within a minute.
3. Ptychographic Reconstruction Algorithm: Employing the extended ptychographic iterative engine (ePIE) algorithm enables robust phase retrieval while accommodating aberrations and noise, further establishing ptychography as a potent tool for high-resolution imaging.
4. Resolution Benchmarking: The experimental results juxtaposed coherent BF, iCoM, ADF, and ptychographic images, illustrating the substantial gains in phase reconstruction and information limit (up to 5α) offered by the ptychographic method. Additionally, spatial resolution tests using a twisted bilayer MoS2 setup demonstrated atom separation clearly visible at distances as small as 0.60 Å, verifying the technique's ability to resolve atomic features well within the Sparrow limit.

Implications and Future Directions

The implementation of EMPAD with full-field ptychography sets a new standard for electron microscopy, particularly for the imaging of 2D and dose-sensitive materials. The ability to maintain high resolution at substantially reduced electron doses has profound implications for materials science, allowing for the characterization of minute structural features such as lattice displacements, defect structures, and dopant distribution. The technique holds potential for further exploitation in ultra-low voltage microscopy and could enhance 3D tomography investigations. Future research may focus on refining the reconstruction algorithms to account for challenges such as scan distortions or thick specimen imaging, thereby broadening the applications of this technology.

In conclusion, the paper delineates a significant improvement in sub-ångstrom imaging capabilities, demonstrating that when equipped with advanced, high-dynamic-range detectors like EMPAD, electron ptychography offers a compelling, dose-efficient alternative to traditional imaging methods in electron microscopy.