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Abstract: Magnetic two-dimensional materials are a promising platform for novel nano-electronic device architectures. One such layered crystal is the ferromagnetic semiconductor chromium germanium telluride (Cr2Ge2Te6) which recently attracted interest due to its potential for spintronics and memory applications. Here we investigate its properties from the structural standpoint using atomic resolution Scanning Transmission Electron Microscopy (STEM) and present the first atomic resolution images down to its monolayer limit. We develop a novel technique that allows one to map the local tilt with unprecedented spatial resolution using only high-resolution images, enabling mapping of the topography and morphological variation of atomically thin crystals. Using it, we show that the Cr2Ge2Te6 monolayer has an unusually large out-of-plane rippling, with local tilt variation reaching 20° over few nm length scales. We hypothesize that such a strongly buckled structure originates from both point and extended lattice defects which are more prevalent in monolayer crystals. In addition, we correlate the structural observations with the band structure measurements using Angle-Resolved Photoemission Spectroscopy (ARPES). We believe that both the atomic scale insights we have gained on Cr2Ge2Te6 and our novel approach to nanoscale topography mapping will benefit the development of van der Waals heterostructures in both fundamental and applied research.

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Abstract: A novel heterostructured hexagonal-boron nitride (h-BN) flake-coating on multi-wall carbon nanotubes (MWCNT/BN) is reported and synthesized by chemical vapor deposition (CVD). Comprehensive characterization using X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM), combined with electron energy loss spectroscopy (EELS), revealed the atomic structure and growth mechanism, which is further validated by molecular dynamics simulations. The resulting MWCNT/BN structure comprises three distinct layers: an inner carbon nanotube (CNT) core, coaxial BN nanotubes (BNNTs) surrounding the CNT core, and outer BN flakes extending from the BNNTs. We propose that BN layers first form coaxial BNNTs on the CNT surface; as deposition proceeds, BN accumulation generate in-plane and out-of-plane compressive stresses in the h-BN layers. When these stresses exceed a critical threshold, local buckling or cracking occurs, BN flakes emerge and grow further. This work elucidates, for the first time, the formation mechanism of BN nanoflakes on MWCNTs and confirms that the structure is a van der Waals heterostructure. The approach also offers a new route for synthesizing coaxial MWCNT@BN with only a few h-BN layers. Notably, the BN flake coatings provide efficient phonon transport pathways and a large surface area, making this heterostructure highly promising for applications in thermal dissipation.

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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.