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Abstract: Transition metal diiodides such as FeI2, NiI2, and CoI2 are an emerging class of 2D magnets exhibiting rich and diverse magnetic behavior, but their study at the monolayer limit has been severely hindered by fabrication challenges due to their air-sensitivity. Here, we introduce a polymer-free method for clean, rapid, and high-yield assembly of hermetically encapsulated suspended samples of air-sensitive monolayers. Applied to diiodides, it enables atomic resolution characterization of thin samples down to the monolayer limit using transmission electron microscopy. Our imaging, combined with complementary first-principles calculations, reveals an unusually small energy barrier between alternate stable stacking polytypes in few-layer films, enabling extrinsic control of the stacking phase. We also observe stable isolated iodine vacancies that do not aggregate to form extended structures and identify and verify the stability of the various edge configurations of thin samples. These results establish the structural characteristics of these materials in the thin limit and more broadly demonstrate the utility of our transfer platform for creating atomically clean suspended van der Waals heterostructures.

Open Access

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Abstract: Tuning the electronic properties of nanocatalysts via doping with monodispersed hetero-metal atoms is an effective method used to enhance catalytic properties. Doping CuO nanoparticles with monodispersed Co atoms using different reductants affords catalysts (CoBCu/Al2O3 and CoHCu/Al2O3) with strikingly different electronic structures. Compared to CoHCu/Al2O3, the CuO nanoparticles in CoBCu/Al2O3 have longer and weaker Cu-O bonds, with a lower 1s → 4pz antibonding transition and higher 4p → 1s bonding transition (as demonstrated from HERFD-XANES and valence-to-core X-ray emission spectroscopy). The weaker Cu-O bonds in CoBCu/Al2O3 lead to superior redox activity of the CuO nanoparticles, evidenced from operando XAFS and in-situ near ambient pressure-near edge X-ray absorption fine structures studies. Such superior redox properties of CuO in CoBCu/Al2O3 result in a much reduced activation energy of CoBCu/Al2O3 compared to CoHCu/Al2O3 (40.0 vs. 63.5 kJ/mol), thus leading to an enhancement in catalytic performance in the selective catalytic oxidation of NH3 to N2.

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Abstract: Electron ptychography provides a promising avenue towards dose-efficient, high-resolution materials characterisation. Prior work demonstrates the feasibility of this approach, but an overarching view on the reliability of ptychographic images in low-dose scenarios is required. Here, we address this limitation with a systematic study of image clarity across dose, thickness and convergence semi-angle, on a range of materials science specimens. With the now widespread adoption of 4D-STEM and ptychographic imaging, the establishment of the practical parameter space in which one can anticipate a reliably interpretable phase image is urgently needed. In some cases, our parameter space exploration confirms high-resolution imaging at doses of 200 Å .

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Abstract: The electron beam for scanning transmission electron microscopy (STEM) provides rich information about the atomic structure and chemical composition of materials from micron to atomic scale. However, the electron probe can also damage the materials of interest, as the high-energy electrons are often focused on very small sample regions. These effects limit the quality of information which can be extracted from experiments on beam-sensitive materials, such as Li-ion battery materials and metal halide perovskites used in solar cell devices. However, with the increasing interest in these materials to address environmental and societal concerns, a detailed understanding of their microstructure and chemical composition at high spatial resolution is needed to improve their performance and stability. For these materials, the correlation between processing and nanoscale structure-property relationships has been difficult to firmly establish. As shown in Fig. 1a-1c, phase change or amorphisation in beam-sensitive materials can be easily caused by a focused electron probe. Fortunately, this problem can be solved through combined scanning electron nano-diffraction (SEND) and energy dispersive X-ray spectroscopy (EDX) with low electron dose conditions, providing nanoscale crystallographic and chemical information from the specimen. However, the signal-to-noise (SNR) of the EDX data is very poor - with just a few counts in any individual scan prohibiting comprehensive materials characterisation (Fig. 1d). To address this, we perform automated SEND-EDX data acquisition under low dose conditions utilising our automated data analysis workflow. By communicating with two different modalities, i.e., Aztec®; Oxford Instruments and MerlinEM; Quantum Detectors, and using our Python-based software, many SEND-EDX data pairs were simultaneously acquired from a metal halide perovskite. The radially flattened diffraction datasets were then be segmented into distinct phases by using an unsupervised learning approach, non-negative matrix factorisation, and the EDX spectra from identical phases classified earlier were summed across all datasets to enable chemical identification with a much higher SNR than one EDX spectrum image (Fig. 1d) as shown in Fig. 2. In this way we can determine the chemical and crystallographic structure of small phase domains in a highly beam-sensitive multi-phase metal halide perovskite. This research will both demonstrate a novel multi-modal, data-fusion based approach to imaging beam-sensitive materials and shed light on the processing and structure-property relationships of these materials on the nanometre length scale to improve their long-term operational stability.

Open Access

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Abstract: Rigorous comparisons between single site- and nanoparticle (NP)-dispersed catalysts featuring the same composition, in terms of activity, selectivity, and reaction mechanism, are limited. This limitation is partly due to the tendency of single metal atoms to sinter into aggregated NPs at high loadings and elevated temperatures, driven by a decrease in metal surface free energy. Here, we have developed a unique two-step method for the synthesis of single Cu sites on ZSM-5 (termed CuS/ZSM-5) with high thermal stability. The atomic-level dispersion of single Cu sites was confirmed through scanning transmission electron microscopy, X-ray absorption fine structure (XAFS), and electron paramagnetic resonance spectroscopy. The CuS/ZSM-5 catalyst was compared to a CuO NP-based catalyst (termed CuN/ZSM-5) in the oxidation of NH3 to N2, with the former exhibiting superior activity and selectivity. Furthermore, operando XAFS and diffuse reflectance infrared Fourier transform spectroscopy studies were conducted to simultaneously assess the fate of the Cu and the surface adsorbates, providing a comprehensive understanding of the mechanism of the two catalysts. The study shows that the facile redox behavior exhibited by single Cu sites correlates with the enhanced activity observed for the CuS/ZSM-5 catalyst.