UHV Scanning Transmission Electron Microscope Research Articles

Structure, chemistry and bonding at grain boundaries in Ni 3Al—II. The structure of small angle boundaries, Ni-enrichment and its influence on bonding, structure, energy and properties

Small angle [001] twist boundaries and [001] (110) tilt boundaries in B-free and B-doped Ni-rich Ni 3Al (76 at. % Ni) were examined using conventional electron microscopy techniques as well as Energy Dispersive X-ray Spectroscopy (EDS), annular dark field (ADF) imaging and spatially resolved electron energy loss spectroscopy (EELS), in an UHV scanning transmission electron microscope. The interface structure consists of periodically spaced pairs of a 2 〈110〉 partial dislocations, linked by an antiphase boundary (APB). An analysis of the separation of the partials gives APB energies which are lower than in bulk Ni 3Al and which decrease with increasing misorientation angle. EDS, EELS and ADF imaging demonstrate that the APBs are Ni-rich. The observations on the APB chemistry and energy taken together lead to the conclusion that Ni-enrichment lowers grain boundary energy by decreasing the number of high energy AlAl interactions across the APB. These results on small angle boundaries lead to the suggestion that Ni-enrichment also decreases the number of high energy AlAl interactions across the interface at large angle boundaries.

Structure, chemistry and bonding at grain boundaries in Ni 3Al—I. The role of boron in ductilizing grain boundaries

Boron segregation is known to change the fracture mode from intergranular to transgranular in Ni-rich Ni 3Al. This paper addresses the question of why boron segregation at grain boundaries improves their resistance to fracture. Grain boundaries in B-free and B-doped Ni-rich Ni 3Al (76 at. % Ni) were examined using spatially resolved electron energy loss spectroscopy (EELS), energy dispersive X-ray spectroscopy (EDS) and annular dark field (ADF) imaging in an UHV scanning transmission electron microscope, as well as conventional electron microscopy techniques. Ni-enrichment was seen in a 0.5 – 1.0 nm wide region at large angle boundaries, both in the absence and presence of B. Using EELS, B segregation to the boundary was observed to vary along the interface. EELS of the Ni L 2,3 edge showed that the B-rich regions have a bonding similar to that in bulk Ni 3Al, while the B-free regions have a bonding similar to the more Ni-like character of undoped boundaries. These results demonstrate that boron segregation increases the cohesive strength of grain boundaries in Ni 3Al by making the bonding at the boundary similar to that in the bulk and, in this manner, increases their fracture resistance.

The structure, bonding and chemistry of grain boundaries in Ni 3Al

Grain boundaries in B-free and B-doped Ni-rich Ni 3Al (76 at.% Ni) were examined using electron energy loss spectroscopy (EELS), X-ray fluorescence analysis (XRF), and annular dark field (ADF) imaging in a UHV scanning transmission electron microscope, as well as conventional electron microscopy techniques. Ni enrichment is seen in a 0.5–1.0 nm wide region at large angle boundaries, both in the absence and in the presence of B. EELS shows that B segregation varies along the interface, and examination of the Ni L 2.3 edge shows that the B-rich regions have a bonding similar to that in bulk Ni 3Al. These results demonstrate that B segregation increases the cohesive strength of the boundary by making the bonding at the boundary similar to that in bulk Ni 3Al. Small-angle boundaries were examined to address the question of why Ni enrichment at grain boundaries occurs. The interface structure in [001] twist and [001](110) tilt boundaries consists of periodically spaced pairs of a/2〈110〉 partial dislocations, linked by an antiphase boundary (APB). An analysis of the separation of the partials gives APB energies which are lower than in bulk Ni 3Al. EELS and ADF imaging demonstrate that the APBs are Ni-rich. The observations on the APB chemistry and energy lead to the conclusion that Ni enrichment occurs to lower boundary energy by decreasing the number of high-energy bonds across the APB. Ni enrichment at large-angle boundaries plays a similar role.

Observation of the evolution of 55-atom icosahedral Ag clusters by coherent electron nanodiffraction in a UHV stem

The morphology of metallic nano particles fluctuates and rapidly changes under the influence of electron beam irradiation. Structural transformations of small Au particles and disordering of small Pt clusters have been reported. Although the existence of a quasimolten state in nano particles has been experimentally observed there are still many unanswered questions concerning the onset of this unstable state. The previous experiments were carried out in non-UHV microscopes and the particles were prepared ex situ. We report preliminary observations of the evolution of clean Ag nano particles in a UHV scanning transmission electron microscope (STEM).The experiments were performed on a UHV STEM (VG HB-501S), known by the acronym MIDAS (Microscope for Imaging, Diffraction and Analysis of Surfaces). The probe size used for nanodiffraction was approximately 1 nm in diameter with a beam current density of 104 A/cm2. Silver nano particles were formed by evaporating Ag onto a pre-cleaned thin (< 5 nm in thickness) carbon film inside the UHV specimen preparation chamber attached to the MIDAS column.x

Electron Microscopy Studies of The Chemistry And Microstructure of BaF2 / YBa2Cu3O7-x Thin Films

BaF2 thin films are being investigated as candidates for use in YBa2Cu3O7-x (YBCO) / BaF2 thin film multilayer systems, given the favorable dielectric properties of BaF2. In this study, the microstructural and chemical compatibility of BaF2 thin films with YBCO thin films is examined using transmission electron microscopy and microanalysis. The specimen was prepared by using laser ablation to first deposit an approximately 2500 Å thick (0 0 1) YBCO thin film onto a (0 0 1) MgO substrate. An approximately 7500 Å thick (0 0 1) BaF2 thin film was subsequendy thermally evaporated onto the YBCO film.Images from a VG HB501A UHV scanning transmission electron microscope (STEM) operating at 100 kV show that the thickness of the BaF2 film is rather uniform, with the BaF2/YBCO interface being quite flat. Relatively few intrinsic defects, such as hillocks and depressions, were evident in the BaF2 film. Moreover, the hillocks and depressions appear to be faceted along {111} planes, suggesting that the surface is smooth and well-ordered on an atomic scale and that an island growth mechanism is involved in the evolution of the BaF2 film.

Ni-Enrichment and its Influence on the Structure Chemistry and Bonding of Grain Boundaries in Ni3Al

AbstractSmall angle [001] twist boundaries and [001[(110) tilt boundaries in B-free and B-doped Ni-rich Ni3AI (76 at. pet. Ni) were examined using conventional electron microscopy techniques as well as annular dark field (ADF) imaging, X-ray Fluorescence (XRF) and spatially resolved electron energy loss spectroscopy (EELS) in an UHV scanning transmission electron microscope. The interface structure consists of periodically spaced pairs of a/2<10> partial dislocations, linked by an antiphase boundary (APB). An analysis of the separation of the partials gives APB energies which are lower than in bulk Ni3Al and which decrease with increasing misorientation angle. EELS, XRF and ADF imaging demonstrate that the APBs are Ni-rich. The observations on the APB chemistry and energy taken together lead to the conclusion that Ni-enrichment occurs to lower boundary energy by decreasing the number of high energy Al-Al bonds across the APB. These results on small angle boundaries lead to the suggestion that Ni-enrichment also occurs at large angle boundaries to decrease the number of high energy bonds across the interface.

Nanometer-resolution surface analysis with Auger electrons

High spatial resolution Auger electron spectra and images have been obtained in a UHV scanning transmission electron microscope. An edge resolution of ≈ 1 nm has been achieved in Auger electron images of thin specimens. The attenuation lengths of the collected Auger electrons and the escape depth of the high energy background secondary electrons have been estimated from these nanometer-resolution images. Surface steps are clearly visible in Auger electron images with high contrast and high resolution. These techniques have been applied to study a model catalyst system. A method has been developed to estimate the number of atoms in metal particles which are smaller than the probe size. The most recent results show that Ag clusters containing less than 10 atoms can be readily detected when supported on thin substrates.

The origins of high-resolution secondary-electron microscopy

The production or generation of SE by focused, fast electron beams is a multistage process which includes excitation of target electrons by the energetic incident beam, subsequent decay yielding hot SE, transport to the surface, and transmission over the surface potential barrier. The ultimate spatial resolution of a SE image formed by scanning a well focussed probe across a surface is limited by the excitation events' spatial derealization through the transverse momentum transferred to the specimen.The accepted model for SE production is not well characterized since it is extremely difficult to separate experimentally the generation, transport and transmission processes during a given SE creation event. Here, we examine the SE generation pathway by correlating SE of a given energy produced by an initial inelastic excitation using time coincidence detection. This technique can be used, for example, to isolate the role of plasmon decay in the SE generation process. The experiments were performed in a Vacuum Generators HB501-S UHV scanning transmission electron microscope (STEM), operating at base pressures of 5 × 10−11 torr.

Nanometer-resolution Auger electron spectroscopy and microscopy of small particles

Small metal particles have peculiar physical and chemical properties and they are especially important in catalysis. A detailed understanding of catalytic processes requires knowledge of both the microstructure and the microchemistry of the catalyst system. Although nanometer scale surface topography of supported catalysts can be studied with secondary electron (SE) signals, a chemically specific and surface sensitive signal such as Auger electrons must be used to extract compositional information about the surface species. Auger electron (AE) spectroscopy (AES) and scanning Auger microscopy (SAM) have been widely used for surface and catalyst characterization. In order to observe the active phases (small particles) of supported catalysts, it is necessary, however, to have nanometer scale resolution in SAM images.With significant improvement in detection efficiency of low energy secondary electrons by employing magnetic parallelizers in our UHV scanning transmission electron microscope (STEM), known as MIDAS (a Microscope for Imaging, Diffraction and Analysis of Surfaces), a spatial resolution of approximately 3 nm in SAM images has been achieved on bulk samples.

High resolution Auger electron spectroscopy and microscopy of a supported metal catalyst

High spatial resolution Auger electron spectra and scanning Auger microscope (SAM) images of supported metal catalysts have been obtained in a UHV scanning transmission electron microscope. Ag/α-Al 2O 3 was used as a model catalyst system, where silver was evaporated, in situ, onto polycrystalline alumina carriers. Silver particles, as small as 2 nm in diameter, were clearly revealed in SAM images with high contrast. On large islands, an edge resolution < 3 nm was achieved. Information about surface and bulk properties of supported catalysts can be extracted from images formed with different signals generated from the same area which are obtained simultaneously.

High resolution Auger electron imaging of supported metal particles

High spatial resolution Auger electron spectra and images of supported metal particles have been obtained in a UHV scanning transmission electron microscope. An edge resolution < 3 nm has been achieved. The number of atoms in a small particle can be estimated from the integrated intensity of the Auger electrons. This method is very useful for detecting and measuring particles with sizes smaller than the incident probe size. Ag clusters containing less than 20 atoms have been detected when supported on a thin carbon film.

Imaging with high-angle scattered electrons and secondary electrons in the STEM

High-resolution images can be obtained by collecting high-angle scattered electrons with an annular detector in a scanning transmission electron microscope (STEM). For a crystal, with incident electron beam along a principle axis, the high-angle annular dark-field (HAADF) signal may be mainly generated by quasi-elastic (thermal diffuse) scattering (TDS). The angular distribution of TDS is determined by the Fourier transform of the differential of the atomic potential. The collected signal is thus dependent on the atomic number Z, the shape of the atom, the amplitude of the thermal vibration and the total number of atoms encountered by the electron probe. This imaging mode reveals high Z sensitivity with atomic resolution. Angular filtered ADF imaging is also possible by selecting the desired angular window to extract information about thick specimens in transmission geometry or information about bulk specimens in reflection geometry. Subnanometer-resolution secondary electron (SE) images can be obtained simultaneously with HAADF images, providing topographic and material contrast of surfaces of the materials studied. In the UHV STEM MIDAS (microscope for imaging, diffraction and analysis of surfaces) both the entrance and the exit surfaces of the sample can be imaged with secondary electrons. Surface steps can be observed with high contrast. Applications of HAADF and high-resolution SE imaging to the study of several kinds of materials are discussed.

Secondary Electron Imaging of Crystal Surface Steps

High resolution secondary electron (SE) images can be obtained in a scanning transmission electron microscope (STEM) by collecting the type I secondary electrons generated directly by the incident electron beam. The high resolution SE signal is, in general, localized to about 1 nm in the generation process. The escape depth of the secondary electrons, however, varies from about 1 nm for most metals to about 10 nm for some insulating materials. Surface steps can be imaged by collecting high resolution secondary electrons Among other possible contrast mechanisms for SE imaging of surface steps, topographic contrast plays an important role. The topographic contrast of surface steps depends on: (1) the incident electron probe size; (2) step height to SE escape depth ratio; (3) localization of the generated SE signals and (4) the incident beam angle relative to the specimen surface.In our UHV STEM (MIDAS) instrument, both the entrance and exit surfaces of the sample can be imaged by collecting secondary electrons. For thin samples, the resolution is &lt; lnm for SE images of entrance and exit surfaces but for thicker samples the resolution of the SE image obtained from the exit surface will be degraded by the beam broadening effects. For studying steps on thick or bulk crystal surfaces, secondary electrons emitted from the entrance surfaces are, in general, collected to form images with high contrast and high resolution. In this report, all the SE images are obtained, in MIDAS, by collecting secondary electrons emitted from the entrance surfaces of the samples. Figure 1 is a SE image of a MgO smoke crystal revealing the surface growth steps with high contrast. The parallel straight steps, along &lt;100&gt; directions, constitute the edges of unfinished atomic planes on the MgO {100} crystal surfaces. This image indicates that the {100} surfaces of MgO smoke crystals are flat surfaces. Figure 2 shows a SE image of a MoO3 crystal with surface growth steps spiraling from an end-on screw dislocation (the black dots in the SE image represent electron beam induced reduction products). The dark lines (indicated by D) represent up-steps and the bright lines (indicated by B) represent down-steps. It is interesting to note that the two-dimensional crystal growth around the dislocation is anisotropic, forming terraces with long edges parallel to [010] direction.

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.

The Nucleation and Epitaxial Growth of Au and Ag on Thin Silicon Studied With a Scanning Transmission Electron Microscope

ABSTRACTAn UHV Scanning Transmission Electron Microscope (STEM) has been used to study the nucleation and epitaxial growth of Au and Ag deposited at room temperature on H-passivated thin silicon (111) and (100) substrates. Direct observations of the initial stages of overlayer growth were made using bright field (BF) and annular dark field (ADF) imaging modes. Gold was found to grow on silicon in a quasi-continuous layer mode, while 3D island growth with density as high as 4×1012/cm2 was observed in Ag/Si systems. Electron diffraction studies reveal a good epitaxial relationship for Ag films grown both on silicon (111) and (100) substrates. Diffraction also indicates that Au (111) grows on the silicon (111) surface but the grains may be azimuthally rotated.