Low-Dose Electron Microscopy Imaging of Electron Beam-Sensitive Crystalline Materials

Abstract

Conspectus As one of the most widely used characterization tools in materials science, (scanning) transmission electron microscopy ((S)­TEM) has the unique ability to directly image specimens with atomic resolution. Compared to diffraction-based techniques, the main advantage of (S)­TEM imaging is that in addition to the periodic average structures of crystalline materials, it can be used to probe nonperiodic local structures such as surfaces, interfaces, dopants, and defects, which have crucial impacts on material properties. However, many crystalline materials are extremely sensitive to electron beam irradiation, which can only withstand dozens (or even fewer) of electrons per square angstrom before they undergo structural damage. Although using electron doses lower than the thresholds can in principle preserve their structures, the thus acquired images are too noisy to be useful. Consequently, high-resolution imaging of the inherent structures of such electron beam-sensitive materials using (S)­TEM is a long-standing challenge. In recent years, the advances in electron detectors and image-acquisition methods have enabled high-resolution (S)­TEM with ultralow electron doses, largely overcoming this challenge. A series of highly electron beam-sensitive materials that are traditionally considered impossible to be imaged with (S)­TEM, including metal organic frameworks (MOFs), covalent organic frameworks (COFs), organic–inorganic hybrid halide perovskites, and supramolecular crystals, have been successfully imaged at atomic resolutions. This technological advance has greatly expanded the application range of electron microscopy. This Account focuses on our recent works pertaining to the high-resolution imaging of electron beam-sensitive materials using very low electron doses. We first explain that the use of direct-detection electron-counting (DDEC) cameras provides the hardware basis for successful low-dose high-resolution TEM (HRTEM). Subsequently, we introduce a suite of methods to address the challenges peculiar to low-dose HRTEM, including rapid search for crystal zone axes, precise alignment of the image stack, and accurate determination of the defocus value. These methods, combined with the use of a DDEC camera, ensure efficient imaging of electron beam-sensitive crystalline materials in the TEM mode. Moreover, we demonstrate that integrated differential phase contrast STEM (iDPC-STEM) is an effective method for acquiring directly interpretable atomic-resolution images under low-dose conditions. In addition, we share our views on the great potential of four-dimensional STEM (4D-STEM) in imaging highly electron beam-sensitive materials and provide preliminary simulation results to demonstrate its feasibility. Finally, we discuss the significance of developing (S)­TEM specimen preparation techniques applicable for sensitive materials and the advantages of using the cryogenic focused ion beam (cryo-FIB) technique for this purpose.

Morphological Anisotropy in Metal–Organic Framework Micro/Nanostructures

“…In situ TEM techniques are advanced characterization tools for monitoring the growth process of anisotropic crystals [55] . Considering that most MOFs are highly sensitive to electron beam irradiation, low‐dose TEM [27] with little structural damage of MOFs crystals is recommended for in situ investigation on their structural evolution.…”

Revolutionizing the structural design and determination of covalent–organic frameworks: principles, methods, and techniques

“…10 3 –10 4 e − Å −2 ) and even ranging from a few to a few tens of electrons per Å 2 . 265–267 These critical dose values are so low that under such an accumulated dose the EM image resolution is no longer determined by the electron optics but the electron dose used for imaging, due to the poor SNR. The dose-limited resolution (DLR, δ ) can be defined by the following equation: 264 where “SNR” stands for signal-to-noise ratio, which measures the signals with a desired level of uncertainty; the “Rose criterion” sets a minimum SNR of 5 to distinguish features with 100% accuracy; 268 “detective quantum efficiency” (DQE) refers to the quantum detection efficiency of electron detectors; “ C ” denotes the contrast between resolution elements, which is related to the peak-to-background ratio (PBR); 264 “ F ” represents the signal efficiency, which is the fraction of primary electrons that reach the detector and are used for image formation; and “ D C ” is the critical electron dose that is applied to the specimen.…”

Applications of Transmission Electron Microscopy in Phase Engineering of Nanomaterials

“…Due to its low electron utilization efficiency, HAADF-STEM necessitates a high electron dose, typically on the order of 10 5 –10 6 electrons per square angstrom (e – /Å 2 ), to produce images with an acceptable signal-to-noise ratio (SNR). This high electron dose level exceeds the electron tolerance limit of numerous electron-beam-sensitive materials that are the objects of investigation in PEN, such as crystalline–amorphous composites, metallic nanosheets, and MOFs. − Therefore, HAADF-STEM is not a suitable method for imaging these materials.…”