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The quasi-one-dimensional linear chain compound HfTe3 is experimentally and theoretically explored in the few- to single-chain limit. Confining the material within the hollow core of carbon nanotubes allows isolation of the chains and prevents the rapid oxidation which plagues even bulk HfTe3. High-resolution transmission electron microscopy combined with density functional theory calculations reveals that, once the triple-chain limit is reached, the normally parallel chains spiral about each other, and simultaneously a short-wavelength trigonal antiprismatic rocking distortion occurs that opens a significant energy gap. This results in a size-driven metal-insulator transition.

Inorganic nanocomposites synthesized by combination of colloidal nanocrystals (NCs) and inorganic clusters have recently emerged as new materials with novel and unique functionalities. Much of the demonstrated promise of nanocomposites derives from the unique interactions between NC and matrix components - this generates new material properties, which direct unique transport behavior in the overall solid or nanocomposite - be it mass, charge, or heat. While measured empirically, it has remained largely impossible to take an a priori look at material properties and use those as a guideline to design desired transport behavior. Fundamentally, this is because the structural and electronic changes manifest at those interfaces have remained hidden from examination. Here, we provide experimental evidence that transport behavior in nanocrystal-in-matrix (NIM) composites is dictated primarily by interfacial charge transfer associated with electronic and structural reconstructions as the composite forms. Our approach building continuous composite superlattices serves as a starting point for systematic probing of the nanointerface of NIM composites via ultrathin films. A combination of field effect transistor device characterization and photoemission spectroscopy reveals the systematic dependence of the polarity of charge transfer on the selection of matrix materials in NIM composites. We use this insight to combine, by design, different components to tune the carrier type in NIM composites.

It is now established that the 3D structure of homogeneous nanocrystals can be recovered from in-line hologram of single projections. The method builds on a quantitative contrast interpretation of electron exit wave functions. Since simulated exit wave functions of single and bilayers of graphene reveal the atomic structure of carbon-based materials with sufficient resolution, we explore theoretically how the approach can be expanded beyond periodic carbon-based materials to include non-periodic molecular structures. We show here theoretically that the 3D atomic structure of randomly oriented oleic acid molecules can be recovered from a single projection.

Ternary InxGa1−xN alloys became technologically attractive when p-doping was achieved to produce blue and green light emitting diodes (LED)s. Starting in the mid 1990th, investigations of their chemical homogeneity were driven by the need to understand carrier recombination mechanisms in optical device structures to optimize their performance. Transmission electron microscopy (TEM) is the technique of choice to complement optical data evaluations, which suggests the coexistence of local carrier recombination mechanisms based on piezoelectric field effects and on indium clustering in the quantum wells of LEDs. We summarize the historic context of homogeneity investigations using electron microscopy techniques that can principally resolve the question of indium segregation and clustering in InxGa1−xN alloys if optimal sample preparation and electron dose-controlled imaging techniques are employed together with advanced data evaluation.

This article summarizes core aspects of beam-sample interactions in research that aims at exploiting the ability to detect single atoms at atomic resolution by mid-voltage transmission electron microscopy. Investigating the atomic structure of catalytic Co3O4 nanocrystals underscores how indispensable it is to rigorously control electron dose rates and total doses to understand native material properties on this scale. We apply in-line holography with variable dose rates to achieve this goal. Genuine object structures can be maintained if dose rates below ~100 e/Å2s are used and the contrast required for detection of single atoms is generated by capturing large image series. Threshold doses for the detection of single atoms are estimated. An increase of electron dose rates and total doses to common values for high resolution imaging of solids stimulates object excitations that restructure surfaces, interfaces, and defects and cause grain reorientation or growth. We observe a variety of previously unknown atom configurations in surface proximity of the Co3O4 spinel structure. These are hidden behind broadened diffraction patterns in reciprocal space but become visible in real space by solving the phase problem. An exposure of the Co3O4 spinel structure to water vapor or other gases induces drastic structure alterations that can be captured in this manner.

This article summarizes core aspects of beam-sample interactions in research that aims at exploiting the ability to detect single atoms at atomic resolution by mid-voltage transmission electron microscopy. Investigating the atomic structure of catalytic Co3O4 nanocrystals underscores how indispensable it is to rigorously control electron dose rates and total doses to understand native material properties on this scale. We apply in-line holography with variable dose rates to achieve this goal. Genuine object structures can be maintained if dose rates below ~100 e/Å2s are used and the contrast required for detection of single atoms is generated by capturing large image series. Threshold doses for the detection of single atoms are estimated. An increase of electron dose rates and total doses to common values for high resolution imaging of solids stimulates object excitations that restructure surfaces, interfaces, and defects and cause grain reorientation or growth. We observe a variety of previously unknown atom configurations in surface proximity of the Co3O4 spinel structure. These are hidden behind broadened diffraction patterns in reciprocal space but become visible in real space by solving the phase problem. An exposure of the Co3O4 spinel structure to water vapor or other gases induces drastic structure alterations that can be captured in this manner.

Low-temperature synthesis of crystalline silicon and silicon-containing nanowires remains a challenge in synthetic chemistry due to the lack of sufficiently reactive Si precursors. We report that colloidal Si nanowires can be grown using tris(trimethylsilyl)silane or trisilane as the Si precursor by a Ga-mediated solution-liquid-solid (SLS) approach at temperatures of about 200 °C, which is more than 200 °C lower than that reported in the previous literature. We further demonstrate that the new Si chemistry can be adopted to incorporate Si atoms into III-V semiconductor lattices, which holds promise to produce a new Si-containing alloy semiconductor nanowire. This development represents an important step toward low-temperature fabrication of Si nanowire-based devices for broad applications.

Resolution and sensitivity of the latest generation aberration-corrected transmission electron microscopes allow the vast majority of single atoms to be imaged with sub-Ångstrom resolution and their locations determined in an image plane with a precision that exceeds the 1.9-pm wavelength of 300 kV electrons. Such unprecedented performance allows expansion of electron microscopic investigations with atomic resolution into the third dimension. Here we report a general tomographic method to recover the three-dimensional shape of a crystalline particle from high-resolution images of a single projection without the need for sample rotation. The method is compatible with low dose rate electron microscopy, which improves on signal quality, while minimizing electron beam-induced structure modifications even for small particles or surfaces. We apply it to germanium, gold and magnesium oxide particles, and achieve a depth resolution of 1-2 Å, which is smaller than inter-atomic distances.

Making connections between performance and structure in bimetallic catalysts requires knowledge of how the two elements are spatially associated. Elemental maps obtained by analytical TEM methods are an invaluable tool for identifying the location of different elements, but for many samples, visual inspection of elemental maps is insufficient for assessing the degree of element spatial correlation. This is particularly true for beam-sensitive materials where short mapping acquisition times lead to images with high noise and low color depth. In these situations, statistical analysis of elemental maps can be used to identify spatial correlations among the elements in a sample. In this work, the relationship between catalyst performance and bimetallic spatial association was explored using Mn-promoted Co-based Fischer-Tropsch synthesis catalysts prepared by different pretreatment methods. Mn was used as a catalyst additive to suppress methane formation. Catalysts that underwent calcination before reduction produced more methane and fewer long-chain hydrocarbons than catalysts that were directly reduced. The extent to which Co and Mn were spatially associated was assessed using correlation metrics, colocation plots, and histograms generated using data from STEM-EDS maps. Although both catalysts yielded visually similar elemental maps, the results of statistical analysis suggested that the calcined catalyst exhibited greater spatial segregation between the Co and Mn. These findings support the hypothesis that having Mn in close proximity to the Co is essential for the manifestation of Mn promotion effects in Co-based FTS catalysts.

Ce-rich mixed metal oxides comprise a recently discovered class of electrocatalysts for the oxygen evolution reaction (OER). In particular, at current densities below 10 mA cm-2, Ni0.3Fe0.07Co0.2Ce0.43Ox exhibits superior activity compared to the corresponding transition metal oxides, despite the relative inactivity of ceria. To elucidate the enhanced activity and underlying catalytic mechanism, detailed structural characterization of this quinary oxide electrocatalyst is reported. Transmission electron microscopy imaging of cross-section films as-prepared and after electrochemical testing reveals a stable two-phase nanostructure composed of 3-5 nm diameter crystallites of fluorite CeO2 intimately mixed with 3-5 nm crystallites of transition metal oxides alloyed in the rock salt NiO structure. Dosing experiments demonstrate that an electron flux greater than ≈1000 e Å-2 s-1 causes the inherently crystalline material to become amorphous. A very low dose rate of 130 e Å-2 s-1 is employed for atomic resolution imaging using inline holography techniques to reveal a nanostructure in which the transition metal oxide nanocrystals form atomically sharp boundaries with the ceria nanocrystals, and these results are corroborated with extensive synchrotron X-ray absorption spectroscopy measurements. Ceria is a well-studied cocatalyst for other heterogeneous and electrochemical reactions, and our discovery introduces biphasic cocatalysis as a design concept for improved OER electrocatalysts. The unique electrochemical behavior of the Ni0.3Fe0.07Co0.2Ce0.43Oxoxygen evolution electrocatalyst motivates detailed structural characterization to elucidate the underlying catalytic mechanism. Atomic resolution transmission electron microscopy imaging using inline holography techniques reveals a nanostructure in which transition metal oxide alloys form atomically sharp boundaries with ceria nanocrystals. Synchrotron X-ray absorption spectroscopy measurements confirm this unprecedented observation of a multiphase, nanostructured oxygen evolution electrocatalyst.

We observe, using phase maps obtained from a focal series of images, the evolution in time of agold-vacuum interface. What is seen is the reconfiguration and removal of whole columns of goldatoms (typically containing three to nine atoms) at the interface. These structural changes arediscussed with reference to the variation in binding energy along the interface.

High Resolution Transmission Electron Microscopy (HR-TEM) has been used as the ultimate method of thickness measurement for thin films. The appearance of phase contrast interference patterns in HR-TEM images has long been confused as the appearance of a crystal lattice by non-specialists. Relatively easy to interpret crystal lattice images are now directly observed with the introduction of annular dark field detectors for scanning TEM (STEM). With the recent development of reliable lattice image processing software that creates crystal structure images from phase contrast data, HR-TEM can also provide crystal lattice images. The resolution of both methods was steadily improved reaching now into the sub Angstrom region. Improvements in electron lens and image analysis software are increasing the spatial resolution of both methods. Optimum resolution for STEM requires that the probe beam be highly localized. In STEM, beam localization is enhanced by selection of the correct aperture. When STEM measurement is done using a highly localized probe beam, HR-TEM and STEM measurement of the thickness of silicon oxynitride films agree within experimental error. In this paper, the optimum conditions for HR-TEM and STEM measurement are discussed along with a method for repeatable film thickness determination. The impact of sample thickness is also discussed. The key result in this paper is the proposal of a reproducible method for film thickness determination.

We an investigation of a *13 grain boundary in a SrTi03 with a misorientation of 67?4 by HREM, Exit Wave Reconstruction and Z-contrast microscopy. The complementary character of phase contrast imaging and High Angular Dark Field detection is pointed out.

A quantitative comparison is reported between first principle calculations and experiments imaging the column positions around the core of partial dislocations in GaAs. The influence of residual lens aberrations on the determination of column positions is investigated.

The microstructural evolution and structural characteristics and transitions in the thin grain-boundary oxide films in a silicon nitride ceramic, specifically between two adjacent grains and not the triple junctions, are investigated to find their effect on the macroscopic fracture properties. It is found that by heat treating a model Si3N4-2wt percent Y2O3 ceramic for ~;200 hr at 1400 degrees C in air, the fracture toughness can be increased by ~;100 percent, coincident with a change in fracture mechanism from transgranular to intergranular.

Transmission Electron Microscopy evolves rapidly as a primary tool to investigate nano structures on a truly atomic level. Its resolution reaches into the sub Angstrom region by now. Together with a better correction of lens aberrations, sensitivities are drastically enhanced. Utilizing advanced electron microscopes, it is feasible to promote experiments that aim to detect single atoms. This enables local investigations of non-stoichiometry. This paper reviews the current state-of-the-art.

The physical thickness of silicon oxynitride gate dielectric materials currently in development have dimensions in the range of 15-20 Angstrom (~;;6-8 oxygen atoms), while approaching the dielectric constant equivalent oxide thickness (EOT) of 12 Angstrom silicon dioxide. These structures present serious challenges in meeting stringent requirements within the semiconductor industry for precise determination of thickness, interfacial roughness and chemical distribution. Limitations in conventional HRTEM must be removed that would minimize errors in such measurements. Our approach was to use the National Center for Electron Microscopy (NCEM) One Angstrom Microscope (O Angstrom M), together with focal series acquisition (FSA) and exit wave reconstruction (EWR) techniques to obtain

In recent years it became possible to extend the resolution of field emission microscopes into the sub Angstrom region by reconstruction of the electron exit wave from a focal series of lattice images. Thereby, it is now feasible to investigate defects and interfaces on a truly atomic scale in many materials systems. On the other hand, progress in theory enables scientists to calculate the total energy configuration of systems that are comparable in size with what is typically investigated by high-resolution transmission electron microscopy. Therefore, a quantitative investigation of the agreement between theoretical calculations and experiments is feasible. In this contribution we investigate the core structure of a 30-degree partial dislocation in low temperature grown GaAs:Be. Ab initio electronic structure total energy calculations are used to compute the expected structure for both the Ga-centered and the As-centered 30 degree partial dislocation cores in GaAs. NCEM's One Angstrom Microscope with an information limit of ~ 0.08 nm is used to image this structure. It is shown that it is possible to discriminate between Ga and As columns in phase contrast microscopy if wedge shaped samples are utilized. The agreement between theory and experiment is tested by a determination of the column position in - and around the dislocation core with pico meter precision. The agreement between experiment and theory is remarkable. Currently, strain relaxation and column bending in the ~ 5nm thin TEM foil are major sources of difference between calculations and experiments and they are accessed quantitatively.

Transmission Electron Microscopy developed from an imaging tool into a quantitative electron beam characterization tool that locally accesses structure, chemistry, and bonding in materials with sub Angstrom resolution. Experiments utilize coherently and incoherently scattered electrons. In this contribution, the interface between gallium nitride and sapphire as well as thin silicon gate oxides are studied to understand underlying physical processes and the strength of the different microscopy techniques. An investigation of the GaN/sapphire interface benefits largely from the application of phase contrast microscopy that makes it possible to visualize dislocation core structures and single columns of oxygen and nitrogen at a closest spacing of 85 pm. In contrast, it is adequate to investigate Si/SiOxNy/poly-Si interfaces with incoherently scattered electrons and electron spectroscopy because amorphous and poly crystalline materials are involved. Here, it is demonstrated that the SiOxNy/poly-Si interface is rougher than the Si/SiOx interface, that desirable nitrogen diffusion gradients can be introduced into the gate oxide, and that a nitridation coupled with annealing increases its physical width while reducing the equivalent electrical oxide thickness to values approaching 1.2 nm. Therefore, an amorphous SiNxOy gate dielectric seems to be a suitable substitute for traditional gate oxides to further increase device speed by reducing dimensions in Si technology.

It is reported that lattice imaging with a 300 kV field emission microscope in combination with numerical reconstruction procedures can be used to reach an interpretable resolution of about 80 pm for the first time. A retrieval of the electron exit wave from focal series allows for the resolution of single atomic columns of the light elements carbon, nitrogen, and oxygen at a projected nearest neighbor spacing down to 85 pm. Lens aberrations are corrected on-line during the experiment and by hardware such that resulting image distortions are below 80 pm. Consequently, the imaging can be aberration-free to this extent. The resolution enhancement results from increased electrical and mechanical stability's of the instrument coupled with a low spherical aberration coefficient of 0.595 + 0.005 mm.