The controllable packing of functional nanoparticles (NPs) into crystalline lattices is of interest in the development of NP-based materials. Here we demonstrate that the size, morphology and symmetry of such supercrystals can be tailored by adjusting the surface dynamics of their constituent NPs. In the presence of excess tetraethylammonium cations, atomically precise [Au₂₅(SR)₁₈]⁻ NPs (where SR is a thiolate ligand) can be crystallized into micrometre-sized hexagonal rod-like supercrystals, rather than as face-centred-cubic superlattices otherwise. Experimental characterization supported by theoretical modelling shows that the rod-like crystals consist of polymeric chains in which Au₂₅ NPs are held together by a linear SR–[Au(I)–SR]₄ interparticle linker. This linker is formed by conjugation of two dynamically detached SR–[Au(I)–SR]₂ protecting motifs from adjacent Au₂₅ particles, and is stabilized by a combination of CH⋯π and ion-pairing interactions between tetraethylammonium cations and SR ligands. The symmetry, morphology and size of the resulting supercrystals can be systematically tuned by changing the concentration and type of the tetraalkylammonium cations.

In the study of framework materials, probing interactions between frameworks and organic molecules is one of the most important tasks, which offers us a fundamental understanding of host–guest interactions in gas sorption, separation, catalysis, and framework structure formation. Single-crystal X-ray diffraction (SCXRD) is a conventional method to locate organic species and study such interactions. However, SCXRD demands large crystals whose quality is often vulnerable to, e.g., cracking on the crystals by introducing organic molecules, and this is a major challenge to use SCXRD for structural analysis. With the development of three-dimensional electron diffraction (3D ED), single-crystal structural analysis can be performed on very tiny crystals with sizes on the nanometer scale. Here, we analyze two framework materials, SU-8 and SU-68, with organic molecules inside their inorganic crystal structures. By applying 3D ED, with fast data collection and an ultralow electron dose (0.8–2.6 e– Å–2), we demonstrate for the first time that each nonhydrogen atom from the organic molecules can be ab initio located from structure solution, and they are shown as distinct and well-separated peaks in the difference electrostatic potential maps showing high accuracy and reliability. As a result, two different spatial configurations are identified for the same guest molecule in SU-8. We find that the organic molecules interact with the framework through strong hydrogen bonding, which is the key to immobilizing them at well-defined positions. In addition, we demonstrate that host–guest systems can be studied at room temperature. Providing high accuracy and reliability, we believe that 3D ED can be used as a powerful tool to study host–guest interactions, especially for nanocrystals. 

Many framework materials such as metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) are synthesized as polycrystalline powders, which are too small for structure determination by single crystal X-ray diffraction (SCXRD). Here, we show that a three-dimensional (3D) electron diffraction method, namely continuous rotation electron diffraction (cRED), can be used for ab initio structure determination of such materials. As an example, we present the complete structural analysis of a biocomposite, denoted BSA@ZIF-CO₃-1, in which Bovine Serum Albumin (BSA) was encapsulated in a zeolitic imidazolate framework (ZIF). Low electron dose was combined with ultrafast cRED data collection to minimize electron beam damage to the sample. We demonstrate that the atomic structure obtained by cRED is as reliable and accurate as that obtained by single crystal X-ray diffraction. The high accuracy and fast data collection open new opportunities for investigation of cooperative phenomena in framework structures at the atomic level.

Metal-organic frameworks (MOFs) are known for their versatile combination of inorganic building units and organic linkers, which offers immense opportunities in a wide range of applications. However, many MOFs are typically synthesized as multiphasic polycrystalline powders, which are challenging for studies by X-ray diffraction. Therefore, developing new structural characterization techniques is highly desired in order to accelerate discoveries of new materials. Here, we report a high-throughput approach for structural analysis of MOF nano- and sub-microcrystals by three-dimensional electron diffraction (3DED). A new zeolitic-imidazolate framework (ZIF), denoted ZIF-EC1, was first discovered in a trace amount during the study of a known ZIF-CO3-1 material by 3DED. The structures of both ZIFs were solved and refined using 3DED data. ZIF-EC1 has a dense 3D framework structure, which is built by linking mono- and bi-nuclear Zn clusters and 2-methylimidazolates (mIm(-)). With a composition of Zn-3(mIm)(5)(OH), ZIF-EC1 exhibits high N and Zn densities. We show that the N-doped carbon material derived from ZIF-EC1 is a promising electrocatalyst for oxygen reduction reaction (ORR). The discovery of this new MOF and its conversion to an efficient electrocatalyst highlights the power of 3DED in developing new materials and their applications.

Knowing the atomic crystal structures of ordered porous solids is essential in understanding their behaviors and properties, developing new applications, and designing new porous materials. Electrons have a much shorter wavelength and much stronger interaction with atoms in a crystal compared with X-ray. Therefore, electron crystallography can effectively determine the structures of nano- and micro-sized crystals. Three-dimensional electron diffraction (3D ED) methods have been developed for the structure determination of various types of complex crystal structures. Continuous rotation electron diffraction (cRED) has unique aspects in both fast data collection and accurate structure determination. 

This thesis focused on the accuracy of crystal structure analysis and the potential of unraveling structural details by cRED. The cRED method was first applied for the ab initio structure determination of a beam-sensitive biocomposite metal-organic framework (MOF), BSA@ZIF-CO₃-1. The atomic structure of BSA@ZIF-CO₃-1 obtained by cRED was the same compared to that obtained by single crystal X-ray diffraction (SCXRD). Accurate atomic structures could be obtained by cRED. The sample of BSA@ZIF-CO₃-1 was initially regarded as a pure new phase, however, during the cRED data collection and processing procedure, two distinct crystal systems and unit cells were revealed. BSA@ZIF-CO₃-1  was identified as the major phase in the sample, and a new MOF, denoted ZIF-EC1, as the minor phase. ZIF-EC1 has a dense 3D framework with high N and Zn densities, which is a promising candidate for electrocatalysis. The discovery of ZIF-EC1 was followed by investigating the effects of improving 3D ED data completeness on the structural analysis. I successfully solved the structures of ZIF-EC1 from each individual dataset with the lowest completeness of 44.5% and refined to a high precession (better than 0.04 Å). Then I merged ten datasets to obtain a high data completeness, the structural model is improved, peaks appear more spherical in the electrostatic potential maps. 

The next part of this thesis was focused on unraveling structural details. By applying cRED, each non-Hydrogen atom from guest molecules can be separately localized from the difference Fourier map for two open framework germanates, SU-8 and SU-68. The atomic structure of both the framework and the guest molecules obtained by cRED is as reliable and accurate as that obtained by SCXRD. In the last part, the application of cRED into determining structures for new materials are highlighted. The structure of two new MOFs, Cd-MOF and Pb-MOF are successfully determined by cRED.

Three-dimensional electron diffraction (3DED) has been proven as an effective and accurate method for structure determination of nano-sized crystals. In the past decade, the crystal structures of various new complex metal-organic frameworks (MOFs) have been revealed by 3DED, which has been the key to understand their properties. However, due to the design of transmission electron microscopes (TEMs), one drawback of 3DED experiments is the limited tilt range of goniometers, which often leads to incomplete 3DED data, particularly when the crystal symmetry is low. This drawback can be overcome by high throughput data collection using continuous rotation electron diffraction (cRED), where data from a large number of crystals can be collected and merged. Here, we investigate the effects of improving completeness on structural analysis of MOFs. We use ZIF-EC1, a zeolitic imidazolate framework (ZIF), as an example. ZIF-EC1 crystallizes in a monoclinic system with a plate-like morphology. cRED data of ZIF-EC1 with different completeness and resolution were analyzed. The data completeness increased to 92.0% by merging ten datasets. Although the structures could be solved from individual datasets with a completeness as low as 44.5% and refined to a high precision (better than 0.04 angstrom), we demonstrate that a high data completeness could improve the structural model, especially on the electrostatic potential map. We further discuss the strategy adopted during data merging. We also show that ZIF-EC1 doped with cobalt can act as an efficient electrocatalyst for oxygen reduction reactions.