Quantitative Measurement of the Surface Self-Diffusion on Au Nanoparticles by Aberration-Corrected Transmission Electron Microscopy

We have developed a spherical aberration corrected transmission electron microscopy(Cs-corrected TEM) technique that allows us to obtain clearer images in real space than everbefore. We applied this technique to titanium oxide, in which light elements such as oxygenare difficult to observe using TEM because of its small cross section and electronicdamage. In the present study, we successfully observed oxygen atoms in rutileTiO2. In addition, this direct observation of oxygen atoms enabled us to study the Magnéli structure(TinO2n−1), which is caused by oxygen vacancies. These vacancies caused an atomic relaxation of thetitanium and oxygen atoms. The relaxed atoms formed a characteristic shear structureof rutile titanium dioxide phase. This shear structure of the Magnéli structure(TinO2n−1) was visualized with a spatial resolution of 0.119 nm. At the same time, the selected areadiffraction (SAD) pattern of the defect structure was obtained. Additional spots wereshown inside the rutile [110] spot. We made structural models of the shear structureand simulated the diffraction pattern and images using a multi-slice simulation.Additional spots in the simulated diffraction patterns accurately reconstructed theexperimental data. We also considered the possibility of the real-space analysis of localstructures using spherical aberration corrected transmission electron microscopy.

Structural and Electronic Properties of BaO/MgO(001)-type Interface Studied via Aberration-corrected Transmission Electron Microscopy and First-principles Calculations

Since the invention of the transmission electron microscope (TEM) in 1931 by Max Knoll and Ernst Ruska, modern TEMs underwent remarkable developments in its overall stability and electron optics. In particular, correction of higher-order lens aberrations has facilitated structural characterization of single molecules at atomic resolution. This improvement in resolution has greatly assisted the design and development of new nano-objects in the field of materials and biological sciences. However, taking full advantage of these developments requires high chemical purity in the sample preparation technique and an electron transparent sample support that not only ensures the reliability in structural characterization, but also boosts the signal-to-noise ratio of TEM images. Moreover, the electron beam in TEMs, in addition to image formation, can be used for structural transformation due to the strong interaction of electrons with the investigated material. In this thesis, the structure of mass-selected molecular species, deposited on graphene via electro-spray ion beam deposition (ES-IBD), is investigated by aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) at acceleration voltages between 60 and 80 kV. Due to its impressive electronic and mechanical properties, single- layer graphene (SLG) is used as TEM substrate throughout this work. The transfer methodology of the SLG, obtained by mechanical exfoliation or chemical vapor deposition (CVD), to the TEM grids is presented, followed by thorough structural characterization to analyze the morphology and crystallinity of the transferred SLG using HRTEM. First, beam-sensitive organic molecules such as cytochrome-class hemeprotein (cytochrome-c), chaperonin complex protein molecule (GroEL), copper-pthalocyanine (CuPc) and com- plex molecular magnetic systems in the form of single molecule magnets (SMMs) were investigated. The results showed that despite the presence of the highly conductive substrate and the use of low electron acceleration voltages, small organic molecules did not survive. However, large globular proteins showed some level of electron resistance before suffering radiation damage. Next, the molecular structure of single phosphotungstic acid (PTA) anion molecules deposited by either ES-IBD or the drop-cast method was analyzed and compared. The results showed homogeneous morphology of the samples prepared by ES-IBD. Further- more, by varying coverage and landing energy, it was possible to modify the required morphology. This allowed better control of deposition as compared to the drop cast ap- proach. A main advantage of ES-IBD is the chemical purity during deposition. This is not guaranteed in the drop-cast technique which is performed under ambient conditions. The main part of this thesis deals with the electron-beam-induced transformation of alkali-iodide molecules (CsI, RbI, KI and NaI). The deposited alkali iodide nanoparticles were intentionally exposed to the electron beam to disintegrate them into single ions. It is found that during electron exposure these ions form regular clusters consisting of 4 to 7 atoms, or 2D-crystals. Notably, in the case of CsI, 2D-crystals form only on BLG, whereas on SLG only atomic clusters were formed. HRTEM in combination with electron energy-loss spectroscopy (EELS) is employed to identify the atomic species of the 2D crystals on the atomic scale. In the case of NaI, 2D-crystals form on SLG. Their crystal structure corresponds to the CsCl-type, which has not been reported before. The etching behavior of graphene in the presence of alkali-iodide species is observed in HRTEM experiments for all the deposited species (CsI, RbI, KI, and NaI). By mea- suring the increase in the perimeter of etched holes in SLG, the rate of etching has been calculated. We found that etching was faster in the presence of both alkali and halide ions, as compared to the presence of only one ionic species. First-principles DFT simulation studies were performed to understand the interaction of alkali-iodide dimers on SLG. These calculations yielded quantitative estimates on the charge transfer between the dimer species and the underlying SLG. It was found that adsorption of alkali-iodide on SLG reduces the bonding electron density between the carbon atoms, thus reducing the C-C binding energy. This effect makes the SLG susceptible to damage when exposed to 80-keV electrons, which establishes the catalytic behavior of alkali-iodide species as etchants of SLG.

PtRu-modified Au nanoparticles on a carbon support were prepared using a polyol reduction process with various Pt/Ru ratios. The prepared nanoparticles were characterized using transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, cyclic voltammetry, and chronoamperometry. The surfaces of most Au nanoparticles were covered with bimetallic PtRu overlayers with a thickness of 1–2 monolayers. The thin PtRu overlayers represented distinct CO stripping characteristics, which may be attributable to the unique surface structures of the PtRu overlayers on the Au nanoparticles. PtRu utilization was enhanced by as much as two times compared to that of PtRu/C, which can be attributed to the PtRu overlayers that were deposited only on the surface of the Au nanoparticles. The distinct CO stripping characteristics and the enhanced PtRu utilization affected the electrocatalytic activities of methanol oxidation. overlayers exhibited the highest CO tolerance and the highest methanol oxidation activity. The unique electrocatalytic characteristics of the PtRu overlayer structures on the Au nanoparticles are expected to provide methods for reducing the use of active elements.

We present an easy procedure for the synthesis of single crystalline gold nanoparticles with a mean diameter of 4 nm using a DC-sputtering in an argon-oxygen gas mixture. Morphology population statistics have been determined to quantify the influence of oxygen. It is found that the particles undergo a structural transition from predominantly icosahedral to single crystalline particles with increasing amount of oxygen. Aberration-corrected high-resolution transmission electron microscopy investigation proves that likewise prepared single crystalline nanoparticles are defect and oxygen free. In contrast, the icosahedral particles prepared with pure argon show the presence of edge dislocations pointing to an energetic disfavoring already at these relatively small particle sizes. This morphology control of clean and uncovered Au nanoparticles provides a high application potential, e.g., for studying the influence of the particle morphology on plasmonic and catalytic properties.

The preparation of inorganic nanomaterials with a desired structure and specific properties requires the ability to strictly control their size, shape and composition. A series of chemical reactions with platinum compounds carried out within the 1.5 nm wide channel of single-walled carbon nanotubes (SWNTs) have demonstrated the ability of SWNTs to act as both a very effective reaction vessel and a template for the formation of nanocrystals of platinum di-iodide and platinum di-sulphide, materials that are difficult to synthesise in the form of nanoparticles by traditional synthetic methods. The stepwise synthesis inside nanotubes has enabled the formation of Pt compounds to be monitored at each step of the reaction by aberration-corrected high resolution transmission electron microscopy (AC-HRTEM), verifying the atomic structures of the products, and by an innovative combination of fluorescence-detected X-ray absorption spectroscopy (FD-XAS) and Raman spectroscopy, monitoring the oxidation states of the platinum guest-compounds within the nanotube and the vibrational properties of the host-SWNT, respectively. This coupling of complementary spectroscopies reveals that electron transfer between the guest-compound and the host-SWNT can occur in either direction depending on the composition and structure of the guest. A new approach for nanoscale synthesis in nanotubes developed in this study utilises the versatile coordination chemistry of Pt which has enabled the insertion of the required chemical elements (e.g. metal and halogens or chalcogens) into the nanoreactor in the correct proportions for the controlled formation of PtI2 and PtS2 with the correct stoichiometry.

The confined interior region of carbon nanotubes has proved to be an effective "nano-test-tube" to conduct chemical reactions in a restricted volume. It also benefits from being thin and relatively transparent to electrons, enabling structural characterization using high-resolution transmission electron microscopy. This permits real-time monitoring of chemical reactions with atomic resolution. Here, we have studied the dynamics of single Pr atoms released from Pr(2)@C(72) metallofullerenes. We show that the Pr atoms form small nanoclusters that subsequently coalesce to ordered, stable nanocrystals within the confines of a carbon nanotube. This process has been tracked in situ with atomic-resolution using low-voltage aberration-corrected high-resolution transmission electron microscopy. We reveal that nanocrystal formation within a nanotube does not generally occur by the addition of single atoms to one pre-existing cluster but rather through aggregation of several smaller clusters. These results provide some of the deepest insights into the dynamics of single-atom behavior in the solid state.

With impressive improvements in instrumental resolution and a simultaneous minimisation of image delocalisation, high-resolution transmission electron microscopy is presently enjoying increased popularity in the atomic-scale imaging of lattice imperfections in a variety of solids. In the present overview, recent progress in spherical aberration corrected imaging performed in troika with the ultra-precise measurement of residual wave aberrations and the numerical retrieval of the exit plane wavefunction from focal series of micrographs is illustrated by highlighting their combined use for the atomic-scale measurement of common lattice imperfections observed in compound semiconductors and high-temperature superconductors.

The controlled etching of graphite and graphene by catalytic hydrogenation is potentially a key engineering route for the fabrication of graphene nanoribbons with atomic precision. The hydrogenation mechanism, though, remains poorly understood. In this study we exploit the benefits of aberration-corrected high-resolution transmission electron microscopy to gain insight to the hydrogenation reaction. The etch tracks are found to be commensurate with the graphite lattice. Catalyst particles at the head of an etch channel are shown to be faceted and the angles between facets are multiples of 30°. Thus, the angles between facets are also commensurate with the graphite lattice. In addition, the results of a post-annealing step suggest that all catalyst particles—even if they are not involved in etching—are actively forming methane during the hydrogenation reaction. Furthermore, the data point against carbon dissolution being a key mechanism during the hydrogenation process.

The smallest carbon nanotubes with the chiral index (3,3), (4,3), or (5,1) were unambiguously identified for the first time. They were grown inside single-wall carbon nanotubes with the diameter of 1.0-1.2 nm, and the chiral indices were experimentally assigned beyond a doubt by an aberration-corrected high-resolution transmission electron microscopy (HR-TEM). In contrast to a theoretical prediction, the (3,3) nanotube is rather unstable and extremely sensitive to the electron beam and, therefore, may not survive alone without the protection of outer nanotube. The cap structure of (3,3) nanotube is also well-explained by a half-dome of C20 fullerene, which consists of six pentagons only.

Antimony trisulfide (Sb2S3) is a promising candidate for cell absorbers due to its appropriate band gap, abundant constituents, non-toxicity, simple composition, and long-term stability. In this study, highly oriented Sb2S3 microbars were prepared utilizing an easy, low-cost solvothermal approach. The effects of the solvents used on the morphological and the crystal structure and optical properties of the Sb2S3 films were inspected using X-ray powder direction (XRD), UV/Vis spectroscopy, and field emission scanning electron microscopy (FESEM). The XRD analysis demonstrated that the Sb2S3 crystals had an orthorhombic phase. The Sb2S3 nanorods/bars mostly grew along the (010) direction. Energy-dispersive X-ray analysis (EDX) peaks had an atomic ratio of 2:3 for Sb:S. UV/Vis absorption spectroscopy showed that the optical energy gap of the Sb2S3 microbars was 1.5 with ethylene glycol as the solvent. Aberration-corrected high-resolution transmission electron microscopy (HRTEM) micrographs demonstrated that the Sb2S3 was dendrite-like consisting of microbars with a standard a length of 8–15 µm and width of 250–400 nm.

Interaction between single noble metal atoms and graphene edges has been investigated via aberration-corrected and monochromated transmission electron microscopy. A collective motion of the Au atom and the nearby carbon atoms is observed in transition between energy-favorable configurations. Most trapping and detrapping processes are assisted by the dangling carbon atoms, which are more susceptible to knock-on displacements by electron irradiation. Thermal energy is lower than the activation barriers in transition among different energy-favorable configurations, which suggests electron-beam irradiation can be an efficient way of engineering the graphene edge with metal atoms.

SiO2/Si(100) interfaces were for the first time observed by a spherical aberration-corrected high-resolution transmission electron microscope in a cross-sectional mode. As the Fresnel fringes were not contrasted at the interfaces, the interfacial structures were clearly observed without the need for artificial image contrast. Atomic steps and defects on the Si(100) surfaces were accurately identified. Also, image simulations with the target imaging performance revealed oxygen atomic columns between silicon-silicon bonds. The present instrument is of potential use for semiconductor science and technology, even for the analysis of oxygen atoms at interfaces.

For ABO3 perovskites, octahedral rotations and distortions couple strongly to the functional properties. However, in short period perovskite superlattices, the characterization of the octahedral behavior remains challenging due to the local structural variations of the BO6 octahedra. By aberration-corrected high-resolution transmission electron microscopy, we investigated the local octahedral rotations in a [(4 unit cell (u.c.)//4 u.c.) × 8] LaNiO3/LaGaO3 superlattice grown on a (001) SrTiO3 substrate. The octahedral behavior varies along the growth direction even though the superlattice is coherently strained. Near the substrate, octahedral rotations about [100] and [010] axes in the superlattice are suppressed due to the octahedral connectivity—rotational magnitudes and patterns—between the NiO6 and TiO6 octahedra. Away from the substrate, the magnitudes of [100] and [010] rotations are enhanced as a response to substrate-induced tensile strain. Near the surface of the superlattice, the [100] and [010] rotational magnitudes of NiO6 and GaO6 relax to the bulk values of LaNiO3 and LaGaO3, respectively. Our results indicate that the response of octahedral rotations to epitaxial strain in superlattices is significantly different from that in thin films.

Vanadium carbide MXenes have demonstrated both one of the highest theoretical capacities for Li and Mg storage among all members of the MXene family and a high performance as cathodes for high-energy-density Zn-ion batteries (ZIBs). However, V2C research has been limited because of the instability of delaminated V2C (d-V2C) in water suspension. Herein, we propose improvements of the HF-free synthesis of V2C MXenes via selective etching of the V2AlC MAX phase by a HCl and LiF mixture. A combination of etching in the closed environment of the hydrothermal autoclave under continuous stirring and thermal isolation of a reaction vessel and improved washing techniques result in high-quality d-V2C MXene nanostructures. High crystallinity and chemical purity of the as-synthesized materials is confirmed by X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy, and aberration-corrected high-resolution transmission electron microscopy. A high concentration of Li cations in the etching mixture results in significantly improved stability of d-V2C in water suspension. In addition, cyclic voltammetry analysis is conducted to gauge the electrochemical characteristics of fabricated d-V2C MXenes. Electrochemical tests confirm that fabricated MXenes are highly promising as cathode materials for aqueous ZIBs. Moreover, a charge storage mechanism of V2C MXenes can be tuned through the implementation of different synthetic methods as well as electrochemical activation protocols.