No abstract

Porous metal-organic frameworks (MOFs) have shown wide applications in catalysis, gas storage and separation due to their highly tunable porosity, connectivity and local structures. However, the electron-beam sensitivity of MOFs makes it difficult to achieve the atomic imaging of their bulk and local structures under (scanning) transmission electron microscopy ((S)TEM) to study their structure-property relations. Here, we report the low-dose imaging of a beam-sensitive MOF, MIL-101, under a Cs-corrected STEM based on the integrated differential phase contrast (iDPC) technique. The images resolve the coordination of Cr nodes and organic linkers inside the frameworks with an information transfer of ~1.8Å. The local structures in MIL-101 are also revealed under iDPC-STEM, including the surfaces, interfaces and defects. These results provide an extensible method to image various beam-sensitive materials with ultrahigh resolution, and unravel the whole framework architectures for further defect and surface engineering of MOFs towards tailored functions.

Carbon nanotubes (CNTs) are promising candidates for smart electronic devices. However, it is challenging to mediate their bandgap or chirality from a vapor-liquid-solid growth process. Here, we demonstrate rate-selected semiconducting CNT arrays based on interlocking between the atomic assembly rate and bandgap of CNTs. Rate analysis confirms the Schulz-Flory distribution which leads to various decay rates as length increases in metallic and semiconducting CNTs. Quantitatively, a nearly ten-fold faster decay rate of metallic CNTs leads to a spontaneous purification of the predicted 99.9999% semiconducting CNTs at a length of 154 mm, and the longest CNT can be 650 mm through an optimized reactor. Transistors fabricated on them deliver a high current of 14 μA μm−1 with on/off ratio around 108 and mobility over 4000 cm2 V−1 s−1. Our rate-selected strategy offers more freedom to control the CNT purity in-situ and offers a robust methodology to synthesize perfectly assembled nanotubes over a long scale.

Fatigue resistance is a key property of the service lifetime of structural materials. Carbon nanotubes (CNTs) are one of the strongest materials ever discovered, but measuring their fatigue resistance is a challenge because of their size and the lack of effective measurement methods for such small samples. We developed a noncontact acoustic resonance test system for investigating the fatigue behavior of centimeter-long individual CNTs. We found that CNTs have excellent fatigue resistance, which is dependent on temperature, and that the time to fatigue fracture of CNTs is dominated by the time to creation of the first defect.

Identifying the atomic structures of porous materials in spatial and temporal dimensions by (scanning) transmission electron microscope ((S)TEM) is significant for their wide applications in catalysis, separation and energy storage. However, the sensitivity of materials to electron beams made it difficult to reduce the electron damage to specimens while maintaining the resolution and signal‐to‐noise ratio. It is therefore still challenging to capture multiple images of the same area in one crystal to image the temporal changes of lattices. Usings integrated differential phase contrast (iDPC) STEM, atomic‐resolution imaging of beam‐sensitive zeolite frameworks is achieved with an ultralow dose of 40 e− Å−2, 2–3 orders of magnitude lower than that of conventional STEM. Based on the iDPC technique, not only the atomic 3D architecture of ZSM‐5 crystals but also the changes of frameworks are observed during in situ experiments. Local structures and light‐element aromatics in ZSM‐5 crystals can also be revealed directly under iDPC‐STEM. These results provided not only an efficient tool to image beam‐sensitive materials with ultralow beam current but also a new strategy to observe and investigate the hydrocarbon pools in zeolite catalysts at the single‐molecule scale.