Scientific publications

Graphene liquid cells (GLCs) present the thinnest possible sample enclosures for liquid phase electron microscopy. However, the actual presence of liquid within a GLC is not always guaranteed. Of key importance is to reliably test the presence of the liquid, which is most frequently water or saline. Here, the commonly used methods for verifying the presence of water were evaluated. It is shown that depending on the type of sample, applying a single criterion does not always conclusively verify the presence of water. Testing liquid filling for a specific GLC sample preparation protocol should thus be considered critically. The most reliable method is direct observation of the water exciton peak using electron energy loss spectroscopy (EELS). But if this method cannot be carried out, water filling of the GLC can be verified from a combination of higher contrast in the image, the presence of bubbles, and an oxygen signal in the EEL spectrum, which can be accomplished at a high electron dose in spot mode. Nanoparticle movement does not always occur in a GLC.

Abstract The design of nanostructured catalysts based on earth-abundant metals that mediate important reactions efficiently, selectively and with a broad scope is highly desirable. Unfortunately, the synthesis of such catalysts is poorly understood. We report here on highly active Ni catalysts for the reductive amination of ketones by ammonia employing hydrogen as a reducing agent. The key functions of the Ni-salen precursor complex during catalyst synthesis have been identified: (1) Ni-salen complexes sublime during catalyst synthesis, which allows molecular dispersion of the metal precursor on the support material. (2) The salen ligand forms a nitrogen-doped carbon shell by decomposition, which embeds and stabilizes the Ni nanoparticles on the γ-Al2O3 support. (3) Parameters, such as flow rate of the pyrolysis gas, determine the carbon supply for the embedding process of Ni nanoparticles.

Understanding the deformation mechanisms of rutile titanium dioxide (TiO2) nanowires (NWs) helps to improve the working reliability of TiO2-based nanoelectrical–mechanical systems and better apply strain engineering to these materials. This work investigated the bending deformation mechanisms at the atomic scale using in situ transmission electron microscopy (TEM) near room temperature. Large bending strains of 3.0% to 5.1% could be observed on individual TiO2 NWs near room temperature. The large bending deformation was attributed to the formation of rich stacking faults (SFs) lying on (101̅) planes and the nucleation and glide of extended dislocations belonging to the {101̅}⟨101⟩ slip system.

To ensure reliability and facilitate the strain engineering of zinc oxide (ZnO) nanowires (NWs), it is significant to understand their flexibility thoroughly. In this study, single-crystalline ZnO NWs with rich axial pyramidal I (π1) and prismatic stacking faults (SFs) are synthesized by a metal oxidation method. Bending properties of the as-synthesized ZnO NWs are investigated at the atomic scale using an in situ high-resolution transmission electron microscopy (HRTEM) technique. It is revealed that the SF-rich structures can foster multiple inelastic deformation mechanisms near room temperature, including active axial SFs’ migration, deformation twinning and detwinning process in the NWs with growth π1 SFs, and prevalent nucleation and slip of perfect dislocations with a continuous increased bending strain, leading to tremendous bending strains up to 20% of the NWs. Our results record ultralarge bending deformations and provide insights into the deformation mechanisms of single-crystalline ZnO NWs with rich axial SFs.

Scanning transmission electron microscopy (STEM) provides structural analysis with sub-angstrom resolution. But the pixel-by-pixel scanning process is a limiting factor in acquiring high-speed data. Different strategies have been implemented to increase scanning speeds while at the same time minimizing beam damage via optimizing the scanning strategy. Here, we achieve the highest possible scanning speed by eliminating the image acquisition dead time induced by the beam flyback time combined with reducing the amount of scanning pixels via sparse imaging. A calibration procedure was developed to compensate for the hysteresis of the magnetic scan coils. A combination of sparse and serpentine scanning routines was tested for a crystalline thin film, gold nanoparticles, and in an in-situ liquid phase STEM experiment. Frame rates of 92, 23 and 5.8 s-1 were achieved for images of a width of 128, 256, and 512 pixels, respectively. The methods described here can be applied to single-particle tracking and analysis of radiation sensitive materials.