Volkan Ortalan

Macromolecular structural dynamics visualized by pulsed dose control in 4D electron microscopy

Macromolecular conformation dynamics, which span a wide range of time scales, are fundamental to the understanding of properties and functions of their structures. Here, we report direct imaging of structural dynamics of helical macromolecules over the time scales of conformational dynamics (ns to subsecond) by means of four-dimensional (4D) electron microscopy in the single-pulse and stroboscopic modes. With temporally controlled electron dosage, both diffraction and real-space images are obtained without irreversible radiation damage. In this way, the order-disorder transition is revealed for the organic chain polymer. Through a series of equilibrium-temperature and temperature-jump dependencies, it is shown that the metastable structures and entropy of conformations can be mapped in the nonequilibrium region of a “funnel-like” free-energy landscape. The T-jump is introduced through a substrate (a “hot plate” type arrangement) because only the substrate is made to absorb the pulsed energy. These results illustrate the promise of ultrafast 4D imaging for other applications in the study of polymer physics as well as in the visualization of biological phenomena.

4D Scanning Transmission Ultrafast Electron Microscopy: Single-Particle Imaging and Spectroscopy

We report the development of 4D scanning transmission ultrafast electron microscopy (ST-UEM). The method was demonstrated in the imaging of silver nanowires and gold nanoparticles. For the wire, the mechanical motion and shape morphological dynamics were imaged, and from the images we obtained the resonance frequency and the dephasing time of the motion. Moreover, we demonstrate here the simultaneous acquisition of dark-field images and electron energy loss spectra from a single gold nanoparticle, which is not possible with conventional methods. The local probing capabilities of ST-UEM open new avenues for probing dynamic processes, from single isolated to embedded nanostructures, without being affected by the heterogeneous processes of ensemble-averaged dynamics. Such methodology promises to have wide-ranging applications in materials science and in single-particle biological imaging.

Highly Stable Bimetallic AuIr/TiO2 Catalyst: Physical Origins of the Intrinsic High Stability against Sintering

It has been a long-lived research topic in the field of heterogeneous catalysts to find a way of stabilizing supported gold catalyst against sintering. Herein, we report highly stable AuIr bimetallic nanoparticles on TiO2 synthesized by sequential deposition-precipitation. To reveal the physical origin of the high stability of AuIr/TiO2, we used aberration-corrected scanning transmission electron microscopy (STEM), STEM-tomography, and density functional theory (DFT) calculations. Three-dimensional structures of AuIr/TiO2 obtained by STEM-tomography indicate that AuIr nanoparticles on TiO2 have intrinsically lower free energy and less driving force for sintering than Au nanoparticles. DFT calculations on segregation behavior of AuIr slabs on TiO2 showed that the presence of Ir near the TiO2 surface increases the adhesion energy of the bimetallic slabs to the TiO2 and the attractive interactions between Ir and TiO2 lead to higher stability of AuIr nanoparticles as compared to Au nanoparticles.

Quantitative Z‐Contrast Imaging of Supported Metal Complexes and Clusters—A Gateway to Understanding Catalysis on the Atomic Scale

Z‐contrast imaging in an aberration‐corrected scanning transmission electron microscope can be used to observe and quantify the sizes, shapes, and compositions of the metal frames in supported mono‐, bi‐, and multimetallic metal clusters and can even detect the metal atoms in single‐metal‐atom complexes, as well as providing direct structural information characterizing the metal–support interface. Herein, we assess the major experimental challenges associated with obtaining atomic resolution Z‐contrast images of the materials that are highly beam‐sensitive, that is, the clusters readily migrate and sinter on support surfaces, and the support itself can drastically change in structure if the experiment is not properly controlled. Calibrated and quantified Z‐contrast images are used in conjunction with ex situ analytical measurements and larger‐scale characterization methods such as extended X‐ray absorption fine structure spectroscopy to generate an atomic‐scale understanding of supported catalysts and their function. Examples of the application of these methods include the characterization of a wide range of sizes and compositions of supported clusters, primarily those incorporating Ir, Os, and Au, on highly crystalline supports (zeolites and MgO).

Core–Shell Type Ionic Liquid/Metal Organic Framework Composite: An Exceptionally High CO2/CH4 Selectivity

Here, we present a new concept of a core-shell type ionic liquid/metal organic framework (IL/MOF) composite. A hydrophilic IL, 1-(2-hydroxyethyl)-3-methylimidazolium dicyanamide, [HEMIM][DCA], was deposited on a hydrophobic zeolitic imidazolate framework, ZIF-8. The composite exhibited approximately 5.7 times higher CO2 uptake and 45 times higher CO2/CH4 selectivity at 1 mbar and 25 °C compared to the parent MOF. Characterization showed that IL molecules deposited on the external surface of the MOF, forming a core (MOF)-shell (IL) type material, in which IL acts as a smart gate for the guest molecules.

Two-component additive manufacturing of nanothermite structures via reactive inkjet printing

With an eye towards improving the safety of the deposition of energetic materials while broadening the scope of materials compatible with inkjet printing, this work demonstrates the use of combinatorial inkjet printing for the deposition of energetic materials. Two largely inert colloidal suspensions of nanoaluminum and nanocopper (II) oxide in dimethylformamide with polyvinylpyrrolidone were sequentially deposited on a substrate using piezoelectric inkjet printing. The materials were deposited in such a way that the aluminum and copper (II) oxide droplets were adjacent, and overlapping, to allow for in situ mixing of the components. The alternating deposition was repeated to create a sample with multiple layers of energetic materials. Energetic performance was subsequently tested on samples printed with 3, 5, and 7 layers of materials using a spark igniter. This ignition event was observed with a high speed camera and compared to representative samples printed with pre-mixed nanothermite. High speed ther...

Participation of interfacial hydroxyl groups in the water-gas shift reaction over Au/MgO catalysts

Au/MgO and Au/Mg(OH)2 catalysts were prepared and used as model systems to study the participation of the Au–support interface in the water-gas shift reaction (WGS). Au/MgO and Au/Mg(OH)2 showed similar WGS kinetics, consistent with a similar WGS reaction mechanism. However, Au/MgO had a lower apparent reaction order with respect to H2O and was identified as having a higher specific WGS rate compared with Au/Mg(OH)2 at the same average Au particle size. The focus of the work is on Au/MgO, where we observed a correlation between the hydroxyl group coverage and WGS rate. The measured kinetic isotope effect, DFT calculations, and operando FTIR for that catalyst are all consistent with surface carboxyl formation as the rate-determining step. Comparisons of hydroxyl group coverage with and without Au suggest that the formation of OH groups is strongly influenced by the presence of Au and likely to be highest at the Au–MgO interface, as supported by theoretical calculations. Temperature programmed reaction shows that Au is necessary to catalyze reaction of the surface OH groups with CO. This work confirms the importance of the metal support interface in WGS catalysis and suggests that the unique chemistry at the interface offers both an explanation of catalyst behaviour and a new opportunity to design materials with improved function for additional catalytic applications.

Implementation of Sparse Image Acquisition in a Conventional Scanning Transmission Electron Microscope

STEM is one of the foremost methods for nanoscale characterization of materials, but high current densities can cause damage in electron-beam sensitive materials. Here, we report our recent approach to implement a sparse acquisition in STEM mode executed by a random sparse-scan and a signal processing algorithm called model-based iterative reconstruction (MBIR) [1]. In this method, a small portion (such as 5%) of randomly chosen unit sampling areas (i.e. electron probe positions), which corresponds to pixels of a STEM image of the specimen are scanned with an electron probe to obtain a sparse image. Sparse images are then reconstructed using the MBIR inpainting algorithm to produce an image of the specimen at the original resolution that is consistent with an image obtained using conventional scanning methods. Experimental results for down to 5% sampling show consistency with the full STEM image acquired by the conventional scanning method.

Investigation of Nanoparticle Reactions with Laser Heating by In situ TEM

Nanoscale materials have significantly different and enhanced properties compared to their micro or macroscopic counterparts. As a result, they have been investigated in many studies [1]. In particular, Al nanoparticles has attracted significant attention, especially in the study of propellants, explosives and pyrotechnics, due to their high energy density [2], non-toxicity, commercial availability and low cost [3]. Fluorine, which is often called as material of extremes because of its high reactivity, is generally preferred as an additive in Al nanoparticle systems. Their reaction produces one of the strongest bonds ever determined and improve the general performance of the propellants [4]. The structural changes in fluorine-based polymers during their reactions with nano-aluminum particles can be observed via transmission electron microscopes (TEM) [5].

Dynamic Investigation of Metal-support Interactions in Heterodimer Nanoparticles by in situ Transmission Electron Microscopy

Catalysts are critical components of many current technologies and essential components of sustainable energy systems ranging from fuel cells and batteries to turning biomass into useful chemicals or fuels by enabling reactions to be guided quickly and efficiently along desirable pathways rather than those that are inefficient or lead to unwanted byproducts. Fundamental investigations of catalyst structures and the mechanisms of catalytic reactions requires characterization imaging of surfaces at the atomic scale and probing the structures and energetics of the reacting molecules as they function under reaction condition on varying time and length scales. High-angle annular dark field (HAADF) STEM is an indispensable technique for analyzing heterogeneous catalysts, in particular those comprising high atomic number (Z) metallic nanoparticles (NPs) dispersed on low-Z supports. Readily interpretable atomic-scale Z-contrast imaging with high spatial resolution spectroscopy, including X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS), allows researchers to investigate structural information such as dimensions, morphologies, and size distribution of catalytic NPs as well as their material chemistry (e.g., chemical composition, bonding of catalytic particles with supports at interface, etc.).