Atomic-scale Characterization Research Articles

Atomic Scale Study on the Thermal Evolution of Local Structure and Magnetic Properties in Oleic Acid Coated Iron Oxide Nanoparticles

The physics of the binding of nanoparticles of goethite and cubic iron oxide as capped by oleic acid has been elucidated in the present study based on the thermal evolution of local structure and magnetic properties using Mössbauer spectroscopy. It is observed that the goethite nanoparticles predominantly undergo topotactic transformation to hemeatite while the assembly of identical bare (not oleic acid capped) nanoparticles composed of goethite and magnetite are subjected to annealing beyond 520 K. In contrast to the results obtained on the bare nanoparticles, accelerated reduction of oleic acid coated nanoparticles of goethite to fully stoichiometric magnetite and subsequently to wustite and α-Fe is observed in the 750–875 K annealing interval. Atomic scale characterization of core–shell structures of different iron oxide phases obtained during the evolution of oleic acid coated nanoparticles of iron oxides has also been provided. The understanding of the present study could be utilized to obtain desired core–shell structures of iron oxides, providing the scope for studying the exchange interaction between the core and shell structures, which exhibit different magnetic ordering.

Solid-source growth and atomic-scale characterization of graphene on Ag(111)

Silver is a desirable platform for graphene growth because of the potential for hybrid graphene plasmonics and its emerging role as a preferred growth substrate for other two-dimensional materials, such as silicene. Here we demonstrate the direct growth of monolayer graphene on a single-crystal Ag(111) substrate. The inert nature of Ag has made it difficult to use for graphene synthesis using standard chemical vapour deposition techniques, which we have overcome by using an elemental carbon source. Atomic-scale scanning tunnelling microscopy reveals that the atomically clean graphene–silver substrate is free of organic residue and other contaminants. The dendritic graphene possesses a variety of edge terminations, many of which give rise to quantum interferences previously seen only on insulating substrates. This scattering supports spectroscopic evidence that the graphene electronic structure is minimally perturbed by the underlying silver, providing a new system in which graphene is decoupled from its growth substrate.

Atomic Scale Verification of Oxide-Ion Vacancy Distribution near a Single Grain Boundary in YSZ

This study presents atomic scale characterization of grain boundary defect structure in a functional oxide with implications for a wide range of electrochemical and electronic behavior. Indeed, grain boundary engineering can alter transport and kinetic properties by several orders of magnitude. Here we report experimental observation and determination of oxide-ion vacancy concentration near the Σ13 (510)/[001] symmetric tilt grain-boundary of YSZ bicrystal using aberration-corrected TEM operated under negative spherical aberration coefficient imaging condition. We show significant oxygen deficiency due to segregation of oxide-ion vacancies near the grain-boundary core with half-width < 0.6 nm. Electron energy loss spectroscopy measurements with scanning TEM indicated increased oxide-ion vacancy concentration at the grain boundary core. Oxide-ion density distribution near a grain boundary simulated by molecular dynamics corroborated well with experimental results. Such column-by-column quantification of defect concentration in functional materials can provide new insights that may lead to engineered grain boundaries designed for specific functionalities.

Atomic-Scale Characterization of II–VI Compound Semiconductors

Alloys of II–VI compound semiconductors with suitable band gap selection potentially provide broad coverage of wavelengths for photodetector applications. Achievement of high-quality epitaxial growth is, however, essential for successful development of integrated photonic and optoelectronic devices. Atomic-scale characterization of structural defects in II–VI heterostructures using electron microscopy plays an invaluable role in accomplishing this goal. This paper reviews some recent high-resolution studies of II–VI compound semiconductors with zincblende crystal structure, as grown epitaxially on commonly used substrates. Exploratory studies using aberration-corrected electron microscopes are also briefly considered.

Low temperature study of structural phase transitions in niobium hydrides

Niobium (Nb) and its hydrides have been the focus of many studies due to applications as a hydrogen storage material, as a dielectric coating in semiconductor devices and in superconducting radio-frequency cavities. In this paper, we will present the atomic-scale characterization of Nb hydrides using scanning transmission electron microscopy and electron energy loss spectroscopy (EELS) at room and liquid nitrogen temperatures. Although such cavities are formed from ultrahigh purity Nb, using electron beam diffraction, we found that at LN2 temperature, the grains near the surface of cold-worked Nb sheets contain regions exhibiting three different superlattice features, which are identified as β, ε, and ζ-NbHx phases. Z-contrast imaging and EELS at LN2 temperature are utilized to qualify their atomic and electronic structures.

Propagation Loss of Long-Range Surface Plasmon Polariton Gold Stripe Waveguides in the Thin-Film Limit

Propagation loss experienced by long-range plasmon polaritons in ultrathin gold stripe waveguides embedded in different polymer cladding materials was studied and correlated with atomic-scale characterization of the gold film structure. We identify the main sources of experimentally observed propagation loss which deviates from ideal values in the thin-film limit. Increased loss can be translated to an increased effective thickness of the ultrathin films due to incomplete surface coverage and the presence of diffuse interfaces, both of which depend significantly on the choice of cladding material. The results illustrate the importance of atomic-scale dynamics of metal film formation for the selection of optimum substrate materials for surface plasmon polariton waveguides, resonant transmission structures, and semitransparent electrical contacts.

ChemInform Abstract: The Surface Science of Graphene: Metal Interfaces, CVD Synthesis, Nanoribbons, Chemical Modifications, and Defects

AbstractReview: 266 refs.

The influence of precipitation on plastic deformation of Al–Cu–Li alloys

The plastic behaviour of two recently developed Al–Cu–Li alloys with different Cu/Li ratios, AA2198 and AA2196, was studied as a function of precipitation state by tensile tests, strain hardening rate analysis, Bauschinger tests, slip line imaging and atomic scale characterization of precipitate shearing by advanced electron microscopy. For early ageing times where δ′ and/or solute clusters are present, plasticity occurs heterogeneously, the strain hardening rate is high, and the Bauschinger effect is moderate. When T1 precipitates are present, plasticity becomes more homogeneous, and the strain hardening rate shows a slower decrease compared with other aluminium alloys containing shearable precipitates. In addition, these ageing conditions show a high reversibility of plastic strain, and it is observed that precipitates are sheared only by single steps. The possibility of a specific shearing mechanism of the T1 phase is discussed in light of these results.

Local atomic structure of a near-sigma 5 tilt grain boundary in MgO

Clear understanding of the structure–property relation in polycrystalline materials requires atomic-resolution imaging of grain boundary (GB) structures especially for the GBs with misalignments. In this study, we apply a bicrystal technique to fabricate the Σ5 and near-Σ5 GBs of MgO with tunable bonding angle and demonstrate that the misalignments in GBs could be compensated by a periodic displacement shift complete dislocation array. The atomic-resolution images reveal that the near-Σ5 GB is composed of an array of normal Σ5 GB structural units intercalated by deformed Σ17 GB units. The atomic-scale characterization presented is applicable to other GB species in a range of polycrystalline materials.

Grain boundaries in graphene grown by chemical vapor deposition

The scientific literature on grain boundaries (GBs) in graphene was reviewed. The review focuses mainly on the experimental findings on graphene grown by chemical vapor deposition (CVD) under a very wide range of experimental conditions (temperature, pressure hydrogen/hydrocarbon ratio, gas flow velocity and substrates). Differences were found in the GBs depending on the origin of graphene: in micro-mechanically cleaved graphene (produced using graphite originating from high-temperature, high-pressure synthesis), rows of non-hexagonal rings separating two perfect graphene crystallites are found more frequently, while in graphene produced by CVD—despite the very wide range of growth conditions used in different laboratories—GBs with more pronounced disorder are more frequent. In connection with the observed disorder, the stability of two-dimensional amorphous carbon is discussed and the growth conditions that may impact on the structure of the GBs are reviewed. The most frequently used methods for the atomic scale characterization of the GB structures, their possibilities and limitations and the alterations of the GBs in CVD graphene during the investigation (e.g. under e-beam irradiation) are discussed. The effects of GB disorder on electric and thermal transport are reviewed and the relatively scarce data available on the chemical properties of the GBs are summarized. GBs are complex enough nanoobjects so that it may be unlikely that two experimentally produced GBs of several microns in length could be completely identical in all of their atomic scale details. Despite this, certain generalized conclusions may be formulated, which may be helpful for experimentalists in interpreting the results and in planning new experiments, leading to a more systematic picture of GBs in CVD graphene.

Atomic-scale characterization of germanium isotopic multilayers by atom probe tomography

We report comparison of the interfacial sharpness characterization of germanium (Ge) isotopic multilayers between laser-assisted atom probe tomography (APT) and secondary ion mass spectrometry (SIMS). An alternating stack of 8-nm-thick naturally available Ge layers and 8-nm-thick isotopically enriched 70Ge layers was prepared on a Ge(100) substrate by molecular beam epitaxy. The APT mass spectra consist of clearly resolved peaks of five stable Ge isotopes (70Ge, 72Ge, 73Ge, 74Ge, and 76Ge). The degree of intermixing at the interfaces between adjacent layers was determined by APT to be around 0.8 ± 0.1 nm which was much sharper than that obtained by SIMS.

An atomistic view of the interfacial structures of AuRh and AuPd nanorods

In this work we address the challenge of furthering our understanding of the driving forces responsible for the metal-metal interactions in industrially relevant bimetallic nanocatalysts, by taking a comparative approach to the atomic scale characterization of two core-shell nanorod systems (AuPd and AuRh). Using aberration-corrected scanning transmission electron microscopy, we show the existence of a randomly mixed alloy layer some 4-5 atomic layers thick between completely bulk immiscible Au and Rh, which facilitates fully epitaxial overgrowth for the first few atomic layers. In marked contrast in AuPd nanorods, we find atomically sharp segregation resulting in a quasi-epitaxial, strained interface between bulk miscible metals. By comparing the two systems, including molecular dynamics simulations, we are able to gain insights into the factors that may have influenced their structure and chemical ordering, which cannot be explained by the key structural and energetic parameters of either system in isolation, thus demonstrating the advantage of taking a comparative approach to the characterization of complex binary systems. This work highlights the importance of achieving a fundamental understanding of reaction kinetics in realizing the atomically controlled synthesis of bimetallic nanocatalysts.

STEMによる材料界面の原子レベル観察

Aberration-corrected scanning transmission electron microscopy (STEM) is becoming a very powerful tool to directly characterize defect structures in materials and devices at atomic dimensions. Here, we review our recent researches on material interface characterization using STEM. First example is that the direct imaging of individual dopant atoms within a buried alumina interface. The second example is the atomic-scale characterization of Au nanoislands on TiO2 substrates. The ability to directly probe individual atoms within buried interfaces at very high-resolution will significantly assists our understanding of interface structures and related properties in many advanced materials and devices.

The interfacial dynamics of water sandwiched between graphene sheets are governed by the slit width

Atomic scale characterization and fluxional properties of water molecules in the vicinity of the graphene interface is carried out using molecular dynamics (MD) simulations. The structural properties of proximal water molecules near the graphene interface are strongly correlated to their vibrational densities of states while being studied as a function of the slit width of the graphene sheets. Our simulations indicate that the local orientation, ordering and solvation dynamics of interfacial water molecules are a strong function of the graphene slit width. Systematic trends in libration, bending, and stretching bands are correlated with local ordering of water molecules and hydrogen-bonding network. Smaller blue shifts in the intermolecular O…O…O bending mode and larger blue shifts in the O…O intermolecular stretching modes of water molecules are observed for strongly confined water molecules in comparison to bulk water, which is attributed to the interfacial proximity effects resulting in the restricted transverse oscillations of confined water. The O―H stretching band is red-shifted for confined water in comparison to bulk water whereas the libration and bending bands for interfacial water are blue shifted with respect to bulk water. The observed frequency shifts are a consequence of the distortion of the tetrahedral order in confined water caused by lateral diffusion being reduced and also by changes in the distribution of hydrogen bonds. These simulations suggest that the extent of the shifts of confined water in comparison to bulk water are due to the proximity from the hydrophobic surface, their local confinement and hydrogen bonding status.

Atomic Scale Characterization and Magneto-Transport Properties of Mechanically Milled Cu–Co Type Alloys

We have prepared Cu80Co20 powders by high-energy ball milling under ambient atmosphere. The evolutions of both the microstructure and the magnetic properties have been investigated as a function of milling time. 3D-focused ion beam was used for microstructural characterization for low milling time (1.5 h). For higher milling time (20 h) atom probe tomography was used to investigate at the atomic scale the elemental distribution of chemical species. In the first steps of milling, the powder presents a ferromagnetic signal due to the presence of micrometric cobalt particles. After 20 h of milling, the powder is composed of CoO particles with diameter around 10–50 nm surrounded by a copper rich matrix (Cu-84 ± 2%, Co-16 ± 2%) and a few cobalt-rich nanoclusters. For this milling time, a typical giant magnetoresistive effect is observed. A positive magnetoresistive effect is also observed at 5 K under low magnetic fields, which is related to the presence of Co oxides clusters. After a heat treatment at 450 °C ...

Atomic-scale characterization of the nucleation and growth of SnO2 particles in oxidized CuSn alloys

The internal oxidation of Sn was investigated to understand the oxidation kinetics of monophase CuSn alloys. SnO 2 particles were characterized by analytical transmission electron microscopy . The orientation relationship between SnO 2 and Cu was determined with a special emphasis on the atomic-scale structure of Cu/SnO 2 interfaces (misfit dislocations and chemical structure). Habit planes with a pure oxygen plane terminating the SnO 2 phase are greatly favored and large misfits promote the growth of plate-shaped precipitates.

In Situ TEM Electromechanical Testing of Nanowires and Nanotubes

The emergence of one-dimensional nanostructures as fundamental constituents of advanced materials and next-generation electronic and electromechanical devices has increased the need for their atomic-scale characterization. Given its spatial and temporal resolution, coupled with analytical capabilities, transmission electron microscopy (TEM) has been the technique of choice in performing atomic structure and defect characterization. A number of approaches have been recently developed to combine these capabilities with in-situ mechanical deformation and electrical characterization in the emerging field of in-situ TEM electromechanical testing. This has enabled researchers to establish unambiguous synthesis-structure-property relations for one-dimensional nanostructures. In this article, the development and latest advances of several in-situ TEM techniques to carry out mechanical and electromechanical testing of nanowires and nanotubes are reviewed. Through discussion of specific examples, it is shown how the merging of several microsystems and TEM has led to significant insights into the behavior of nanowires and nanotubes, underscoring the significant role in-situ techniques play in the development of novel nanoscale systems and materials.

Atomic-scale characterization of nitrogen-doped graphite: Effects of dopant nitrogen on the local electronic structure of the surrounding carbon atoms

We report the local atomic and electronic structure of a nitrogen-doped graphite surface by scanning tunnelling microscopy, scanning tunnelling spectroscopy, X-ray photoelectron spectroscopy, and first-principles calculations. The nitrogen-doped graphite was prepared by nitrogen ion bombardment followed by thermal annealing. Two types of nitrogen species were identified at the atomic level: pyridinic-N (N bonded to two C nearest neighbours) and graphitic-N (N bonded to three C nearest neighbours). Distinct electronic states of localized {\pi} states were found to appear in the occupied and unoccupied regions near the Fermi level at the carbon atoms around pyridinic-N and graphitic-N species, respectively. The origin of these states is discussed based on the experimental results and theoretical simulations.

ChemInform Abstract: Building Bridges in Catalysis Science. Monodispersed Metallic Nanoparticles for Homogeneous Catalysis and Atomic Scale Characterization of Catalysts under Reaction Conditions

AbstractReview: 36 refs.

Building Bridges in Catalysis Science. Monodispersed Metallic Nanoparticles for Homogeneous Catalysis and Atomic Scale Characterization of Catalysts Under Reaction Conditions

The reactivity of small (<1.5 nm), highly oxidized metallic nanoparticles and the utilization of Sum Frequency Generation spectroscopy (SFG) and Scanning Tunneling Microscopy (STM) for investigations of catalysts under reaction conditions are discussed in this review paper. Batch and flow reactor studies were carried out using highly oxidized 40 atom clusters (Pt, Pd and Rh) to measure reaction rate and product distribution of electrophilic reactions, using toluene as a solvent. These heterogeneous catalysts show reactivity which is similar and sometimes even higher than the homogeneous catalysts. The combination of an in situ SFG and STM measurements facilitate a detection of the surface structure and reaction intermediates under reaction conditions. While the STM detects the surface reconstruction and the mobility of products and reactants molecules, the SFG can correlate the reactivity and more importantly the selectivity, to the active surface intermediates. The recent developments in these two research areas are detailed in this review paper.