Direct High-Resolution Imaging of Covalent Organic Framework Defects

Covalent organic frameworks (COFs) are crystalline porous materials assembled from covalently linked organic building blocks, attracting broad interest for adsorption, separation, and catalysis [1,2]. Rationally designing and realizing these functions depends critically on resolving their atomic structures. Current structural determination relies mainly on X-ray and electron diffraction techniques, which typically require large crystals and yield spatially averaged information, obscuring local features such as surfaces, interfaces, and defects that strongly influence performance. Transmission electron microscopy (TEM) offers a pathway to direct atomic-scale imaging, yet the extreme beam sensitivity of COFs severely limits achievable resolution on these fragile materials at low doses [3]. Recent advances in four-dimensional scanning transmission electron microscopy (4D-STEM) [4], enabled by fast pixelated detectors and advanced reconstruction algorithms, allow structural information to be extracted at electron doses below typical damage thresholds [5,6]. Owing to the experimental difficulty of fine defocus tuning on COFs—since even short-time focusing adjustments can readily induce beam damage—most datasets were acquired under out-of-focus conditions. As shown in Fig. 1a-1d, even under a favorable in-focus beam, conventional imaging modes, including incoherent bright-field (IBF), annular bright-field (ABF), annular dark-filed (ADF), and integrated center-of-mass (iCOM) images transfer only limited structural information at low dose (∼80 e−/Ų) required to prevent beam damage. Although the dose-efficient iCOM image reveals faint channel-like features, overall structural contrast remains weak. In the more typical out-of-focus acquisitions (Fig. 1e-1h), virtually no meaningful structural information is preserved. Remarkably, tilt-corrected bright-field (tc-BF) image (Fig. 1i) recovers substantially more structural detail by compensating for defocus through correlating specimen depth (defocus) with tilted diffraction pixels in the convergent-beam electron diffraction patterns inherent to 4D-STEM datasets [7]. With direct high-resolution imaging, local defects around the framework lines (marked by red squares) become clearly visible. In addition, we applied a recently developed imaging approach from our group, namely aberration-corrected tc-BF (ac-BF) [8] and achieved a further pronounced improvement (Fig. 1j). The enlarged views (Fig. 1k and Fig. 1l) highlight the enhanced contrast and structural fidelity of ac-BF relative to tc-BF, as indicated by the arrows. Besides low-dose ac-BF imaging, we are also exploring multislice electron ptychographic reconstructions [9], which have already yielded promising preliminary results. Together, these advances represent an important step toward robust direct atomic-resolution imaging of beam-sensitive COFs under very low-dose conditions [10]. Virtual STEM images from 4D-STEM datasets acquired on COFs at ∼80 e-/Å2. In-focus (a-d) and out-of-focus (e-h) incoherent bright-field (IBF), annular bright-field (ABF), annular dark-field (ADF), and integrated center-of-mass (iCOM) images, respectively. (i) Tilt-corrected bright-field (tc-BF) and (j) aberration-corrected tc-BF (ac-BF) images reconstructed from the same dataset as in e-h. (k, l) Enlarged images from the dash square-marked areas in i and j, respectively.