MgO Cubes Research Articles

Low-energy-deposited Au clusters investigated by high-resolution electron microscopy and molecular dynamics simulations

We investigated the cluster-surface interaction of Au clusters deposited on MgO cubes and on amorphous carbon, and its influence on the morphology of the Au cluster. Au clusters, produced in a laser vaporization source, are deposited with low energy on carbon-coated microscope grids on which MgO cubes are first deposited as substrates. Clusters on the amorphous carbon as well as clusters on the MgO cubes are studied by high-resolution electron microscopy (HREM). The clusters have different morphologies for the two different surfaces, and a dilation of the Au lattice is also measured for the clusters deposited on the crystalline surface of MgO to perfectly accommodate the MgO lattice. Classical molecular dynamics (MD) is applied to model this behavior. Good agreement is found between experimental cross-section HREM images and theoretical images simulated with the multislice technique using the model calculated by MD.

Atomic Scale Characterization of Supported and Assembled Nanoparticles

ABSTRACTDifferent structural configurations of nanoparticles have been investigated by advanced transmission electron microscopy (TEM) techniques. The cluster-surface interaction of Au clusters, produced in a laser vaporization source and deposited with low energy on MgO cubes, is investigated by high-resolution transmission electron microscopy (HRTEM). A dilation of the Au lattice that perfectly accommodates the misfit with the MgO lattice is measured and modeled with Molecular Dynamics (MD). To study the chemical ordering in bimetallic clusters, Au-Cu alloy clusters with different stoichiometries are produced and deposited in the same way as the Au clusters. Electron diffraction (ED) ring patterns obtained from these alloy clusters lying on amorphous carbon, are indexed as face-centered cubic, which is confirmed by HRTEM. This indicates that clusters of Au-Cu alloys are solid solutions, i.e. no ordering takes place in these clusters. Assembled nanoclusters of Ni3Al, produced by the inert gas condensation (IGC) technique and pressed under high pressure (2 GPa), are investigated by HRTEM. These studies indicate that nanocrystalline Ni3Al consists of small crystallites of random crystallographic orientations separated by grain boundaries, with the presence of several nanoscaled voids. From the selected area electron diffraction (SAED) ring pattern, an incomplete L12 ordering is concluded.

Valence-electron energy loss near edges, truncated slabs, and junctions

Valence losses in electron microscopy can be conveniently computed using classical dielectric excitation theory. In many practical situations, the presence of complex structures gives rise to the possibility of coupling between interface modes, surface modes, and edge modes. In the nonrelativistic case, the losses in such geometries can be calculated by taking a distribution of surface and interface points, each of which is associated with an interface charge. These charges interact self-consistently with each other as well as with the charge of the incident electron. Here, we have applied this boundary charge method to provide scanning transmission electron microscopy energy loss spectra near-edged structures such as truncated slabs and junctions formed by several media. We find the dominant modes associated with such systems and study their contribution to the characteristic energy loss functions. The significance of these excitations compared with the planar interface peaks is discussed in terms of the sample geometry with illustrations for the cases of an MgO cube and an Si/SiO2 thin-film interface. @S0163-1829~99!00939-X#

Fresnel effect in high-resolution TEM imaging of small particles

It is established that analysis of Fresnel contrast visible at the edges of nanoparticles in the high resolution electron microscope provides a valuable tool for determining the size of the particles with near atomic spacing accuracy.Methods based on the Fresnel effect are enjoying increased use in the study of flat precipitates, multilayers, and grain boundaries. In profile imaging studies of solid surfaces, Fresnel effects have been observed by Marks, and by O'Keefe et al. who showed that image structure within Fresnel fringes from MgO cubes can produce “ghost” atoms far outside the specimen surface. However, consequences of the Fresnel effect appear not to have been included in the study of small particles at atomic resolution. In this work, we have used HRTEM image simulation to explore the changes in images of a nanoparticle under various imaging conditions, in particular to relate the model particle size and its apparent size as derived from the HRTEM image.

Delocalization effect in energy-filtered electron images of surfaces and interfaces

Accurate structure determination is feasible using zero energy-loss images, since image simulations can be done accurately based on elastic scattering theory [1]. Two factors can be assessed from the energy-filtered images: one is the contribution of inelastically scattered electrons to HREM images; and two is the effect of absorption by inelastic scattering on images obtained using 0-loss electrons. In this paper, MgO(l00) surface and Al/Ti interface are used to demonstrate the delocalization effect of valence-excitation in energy-filtered surface and interface images, respectively.Figure 1 shows a pair of MgO lattice images recorded using all the transmitted (Figure la) and just the elastically transmitted (Figure lb) electrons, respectively. The MgO cube is oriented along [110], so that the contrast variation as a continuous function of the specimen thickness is revealed by a single image. When the specimen is thinner than about 20 nm, no significant differences are seen between the filtered and unfiltered images.

{10}<110> edge dislocations in YSZ films grown on (100)MgO by PLD

Yttria-stabilized zirconia (YSZ) is currently used in a variety of applications including oxygen sensors, fuel cells, coatings for semiconductor lasers, and buffer layers for high-temperature superconducting films. Thin films of YSZ have been grown by metal-organic chemical vapor deposition, electrochemical vapor deposition, pulse-laser deposition (PLD), electron-beam evaporation, and sputtering. In this investigation, PLD was used to grow thin films of YSZ on (100) MgO substrates. This system proves to be an interesting example of relationships between interfaces and extrinsic dislocations in thin films of YSZ.In this experiment, a freshly cleaved (100) MgO substrate surface was prepared for deposition by cleaving a lmm-thick slice from a single-crystal MgO cube. The YSZ target material which contained 10mol% yttria was prepared from powders and sintered to 85% of theoretical density. The laser system used for the depositions was a Lambda Physik 210i excimer laser operating with KrF (λ=248nm, 1Hz repetition rate, average energy per pulse of 100mJ).

Measuring the height of steps on MgO cubes using Fresnel contrast in a scanning transmission electron microscope

The possibility of using Fresnel (or phase) contrast in a scanning transmission electron microscope as a means of measuring the heights of steps on the surface of MgO smoke cubes is examined. It is unknown that once the microscope parameters are known it is possible to match the intensities of the Fresnel fringes quantitatively with multislice simulations for step heights of the order of a few monolayers on a cube of thickness 44 nm.

Microstructure of dense MgO penetrated by a silicate liquid

Grain boundaries control most of the critical properties of ceramic materials and devices. The presence of an intergranular amorphous phase in many polycrystalline ceramic compacts has been reported. Grain boundaries may also involve regions of ordered structure and amorphous (glassy) phase. Dense polycrystalline compacts are processed by liquid-phase sintering routes. The addition of sintering aids such as CaO, MgO and SiO2 influence the densification process substantially. During sintering the grain boundaries are wet by liquid phases, and amorphous films are often found at the grain boundaries and triple junctions in the sintered product. The presence of such films not only influences the sintering behavior but it also affects the final properties in the sintered product. These films have therefore been the subject of intense investigation. Silicate-based compositions are the typical glass phases observed in these materials.In the present study, polycrystalline, dense MgO cubes have been penetrated by liquid monlicellite (CaMgSiO4) at 1700°C.

Electron Microscopy Characterization Of Epitaxial Growth Of Ag Deposited On Mgo Microcubes

AbstractThe crystalline structure, the morphology and the chemistry of small Ag particles vacuum-deposited on preheated MgO microcubes were characterized by in situ and ex situ electron microscopy and surface analysis techniques. Observations were made of the nucleation and growth of Ag clusters on MgO surfaces and along surface steps. It is found that Ag particles deposited onto clean MgO crystals have an epitaxial orientation relationship of (100)Agll(100)MgO with [100]Agll[00]MgO. Prior to air exposure Ag particles are stable under the electron beam but they become unstable upon exposure to air. During electron irradiation on air-exposed samples terraces are formed on the surfaces of MgO cubes, preferentially near Ag particles and at their interfaces with the substrate. The stability of vacuum-deposited Ag particles and the growth of the terraces on MgO microcubes seem to depend on air exposure of the sample.

Electron-beam-induced transformations of gold particles epitaxially grown on MgO microcubes

Different epitaxial orientations (100), (110) and (111) of gold particles crystallized on clean MgO cubes were observed in HRTEM. During the electron irradiation, terraces (presumably MgO) are formed on the surfaces of the cubes, preferentially near the gold particles and at their interface with the substrate. During the growth of the terraces, the particles in (110) or (111) orientations rotate on the substrate and stay in the so-called “quasimolten state” until they get the stable (100) orientation. The origin of the growth of the terraces is attributed to the formation of Mg(OH) 2 on the surface of the MgO during the air exposure, which dehydrates in MgO in the electron beam.

Variations in Secondary Electron Emission from MgO Characterized by Secondary Electron Spectroscopy

The recent developments of the in-lens field emission scanning electron microscopes make it possible to image specimen surfaces with subnanometer resolution by collecting high resolution type I secondary electron (SE) signals. SE images with a resolutiori better than 1 nm have also been obtained in scanning transmission electron microscope (STEM) instruments. A problem related to the correct interpretation of these high resolution images is the lack of knowledge on the energies of the collected electrons contributing to the images. On the other hand secondary electron spectroscopy (SES) on clean surfaces has been developed for several years and biased SE imaging of several deposits on metallic and semiconductor crystal surfaces has been demonstrated. The combination of ultra-high resolution SE imaging with SES and Auger electron spectroscopy (AES) can be realized in pur UHV STEM instrument (MIDAS). The description of the performance of the MIDAS is reported elsewhere. We report here some recent results on SE imaging of MgO smoke crystals, in the MIDAS, with energy analysis of the collected secondary electrons.Strong variations of SE signals from different MgO crystals have been observed and the intensity of the collected secondary electrons from some MgO cubes increases under electron beam irradiation. Figure 1 shows a SE image of several MgO cubes, revealing that some of the crystals (indicated by A) have much higher intensity than others (indicated by B) and bright diffuse patches on the dark crystal can be seen (indicated by D). We have used SES to study the variations in SE emission under electron irradiation. The fact that the velocity distribution of the emitted secondary electrons may depend strongly on the surface morphology of the samples studied makes the interpretation of these SE spectra complicated. In order to avoid this problem and other possible complications arising from the detector geometry we acquired SE spectra from a fixed flat area of a single crystal MgO shown in figure 2. The sample was biased at -30 V and the resolution of the SE spectrum is about 1 eV. Figure 3a shows the SE spectra, taken from the exit surface of the crystal. Spectrum 1 was obtained from a poorly emitting MgO cube as that shown in figure 2 (all the spectra were obtained from the circled area). Spectra 2, 3 and 4 were obtained after about 10, 20 and 40 minutes electron irradiation respectively. In order to study spectral shape variations the spectra were normalized so that they all had the same peak intensity. The normalized data were shown in figure 3b.

Structure and morphology of small palladium particles (2–6 nm) supported on MgO micro-cubes

Small MgO cubes were prepared in a controlled atmosphere and protected against air contamination. Their clean surfaces were used as supports for Pd particles (2–6 nm) vacuum-deposited from a Knudsen cell in the same chamber. The Pd particles epitaxially oriented on the substrate were observed by HRTEM, microdiffraction and convergent-beam electron diffraction. An isotropic lattice expansion was determined by microdiffraction on isolated particles, increasing between 2% and 8% for particle sizes decreasing from 6 to 2 nm. From the HRTEM images of particles observed in two directions, parallel and perpendicular to the interface with the MgO substrate, the particles were characterized as single crystals with fcc structure and octahedral shape. The octahedra are truncated by (001) planes at the top and at the interface.

Phase transformations and kinetics in small particles

Small particles of metals supported on various substrates are important heterogeneous catalysts. The rate of catalysis depends critically on the surface morphology of these catalysts and can vary significantly due to strong metal substrate interactions. On the other hand the structure of an isolated small particle is ill-defined and has been shown to fluctuate between various forms, existing in a quasi-molten state. We present here results which shows the transformation of a small particle sitting in a deep potential well on a substrate with a stable shape, into a free floating state which is structurally unstable.The experimental procedure consisted of dispersing gold clusters produced by the reduction of organometallic compounds, onto magnesium oxide smoke particles which were collected onto a microscope grid placed over burning magnesium in air. The specimens were then imaged in a H- 9000 high resolution electron microscope operating at an accelerating voltage of 300 KV. The distribution of small gold panicles on the low energy (100) face of the MgO cube particles is shown in Fig.1

Electron-beam induced nucleation and redispersion of ruthenium on MgO supports

Catalysts prepared aqueously from RUCl3.3H2O on MgO supports generally have poor dispersions. This occurs because the precursor reacts with surface hydroxyl groups and tends to precipitate on the surface. The chloride precursor is not stable in basic solutions such as those encountered in dispersions of MgO powder. Tauster et al. pointed out that the use of non-aqueous preparation minimizes this problem and yields catalysts having higher dispersions. However, MgO powder that is exposed to the atmosphere can readily pick up moisture and hence the surface hydroxyl concentration is not well controlled. In order to develop a better understanding of the nucleation and growth of Ru, we have prepared catalysts supported on MgO that was dehydroxylated by calcining in air at 1200°C overnight and used immediately. All catalysts were prepared by non-aqueous impregnation using low surface area MgO cubes.

A consistent definition of probe size and spatial resolution in the analytical electron microscope

A technique for measuring the extremely small (<2 nm) probe sizes generated in a field emission gun analytical electron microscope is described. The technique involves scanning the fine probe across the edge of small oriented MgO cubes. A mathematical description of the intensity profiles is developed and it is shown how this relates to various definitions of the probe size. The experimentally measured probe sizes are then combined in a consistent manner with both beam broadening calculations and experimental measurements. This approach results in the first self‐consistent definition of the spatial resolution of X‐ray microanalysis in thin foils.

The probability of MgO surface excitations with fast electrons

A scanning Transmission Electron Microscope has been used to investigate the probability of exciting surface modes in MgO cubes when fast electrons pass a certain distance outside the specimen. The experimental results give reasonable agreement with the classical theory of dielectric excitation by a moving charge providing relativistic effects are included.

The alignment of gold particles on MgO crystal faces

Gold, evaporated indirectly on the faces of magnesium oxide smoke crystals, is shown by scanning transmission electron microscopy to form well dispersed, well epitaxed crystals of diameter about 2 nm, apparently located randomly on the surface. Scanning reflection electron microscopy reveals that these crystallites tend to be aligned, presumably along straight-line steps on the MgO surfaces. While many of these linear arrays are parallel to the [100] edges of the MgO cubes, many are not but make small angles (1–5°) with such edges and with each other. There is some evidence of preferred angles between these lines and the cube edges, suggesting that jogs in the MgO steps tend to occur regularly with periodicities of 2 to 7 nm.

Thermal-emission spectra of coagulated small MgO crystals

The thermal-emission spectra of small MgO cubes (about 0.1 μm) were measured in the frequency region of their surface phonon modes. From the partial derivative of emissivity with respect to particle density, the emissivity of an MgO coagulation has been determined as a function of frequency. With an assumption of spherical coagulation, it is concluded that the dielectric function of small MgO particles and its temperature dependence are nearly the same as those of a bulk crystal.

Three-dimensional contrast in dark-field images

Abstract It is shown that, under suitable conditions of N-beam interaction, distinctive contrast is obtained from crystal surface detail in the dark-field image which allows edges or steps on the upper (beam-incident) surface to be distinguished from those on the lower (beam-emergent) surface. The ability to distinguish between the orientations of apex up, and apex down, for a crystal of triangular cross-section is shown to derive from a break-down of Friedel's law which results from the imaging process. An interpretation is given in terms of an extended charge-density approximation’ using the theory developed by Moodie for interpretation of aperiodic features in the bright-field image. Illustrations are from [111] orientations of MgO cubes, although this technique is expected to be of more general application.

Theory of the optical properties of ionic crystal cubes

A theory is developed for the optical properties of particles of arbitrary shape, composed of a homogeneous isotropic material with a dielectric constant $\ensuremath{\epsilon}(\ensuremath{\omega})$. The particles are so small that retardation can be neglected. An expression is obtained for the average dielectric constant of a medium containing a small fractional volume of particles. Calculations for a cube show that six resonances contribute to the optical absorption. They span a frequency range such that ${\ensuremath{\epsilon}}^{\ensuremath{'}}(\ensuremath{\omega})$, the real part of the dielectric constant, lies between -3.68 and -0.42, as contrasted with the single resonance for a sphere at ${\ensuremath{\epsilon}}^{\ensuremath{'}}(\ensuremath{\omega})=\ensuremath{-}2$. A comparison of the theory with experiments on the optical absorption of NaCl and MgO cubes shows that the width of the absorption peak can be explained by the frequency range of the cube resonances. Previous theories which assumed spherical particles required an unphysically high damping in $\ensuremath{\epsilon}(\ensuremath{\omega})$ to account for the width.