HB501 Scanning Transmission Electron Microscope Research Articles

Characterization of Hydrogen-Encapsulated Type-I Silicon Clathrate

This study describes a new stable inorganic clathrate compound encapsulating hydrogen in the Si cage structure. In this work, the characterization of the new type-I Si clathrate, Na6(H2)2Si46, is illustrated. Na6(H2)2Si46 was prepared from NaSi and NH4X (X = Cl and Br) under dynamic vacuum at 300 oC. Rietveld refinement of powder X-ray diffraction data was consistent with the clathrate type-I structure. Solid-state 1 H, 29 Si and 23 Na MAS NMR confirmed the presence of both hydrogen and sodium in the clatharte cages. 23 Na NMR indicated that the guest sites are not fully occupied by Na. In order to validate the structure and investigate the Na content, the synthesized particles were examined using spherical aberration (Cs) corrected VG HB501 scanning transmission electron microscope (STEM) operated at 100 kV. Fig. 2 shows a high-angle annular dark-field (HAADF) image taken along [100] with the convergence semi-angle of 20 mrad and collection semi-angle of 70-210 mrad. Image calculation was made with Kirkland’s code [3] with the thickness of 21 nm and defocus value of 22.5 nm. The calculated image inserted in Fig. 2 is in good agreement with the experimental one. The ring pattern and the center of the ring correspond to the Si24 cage and the 6d sites, respectively. When observed along [100], the 6d sites are aligned in an atomic column which is apart from the nearest columns by 0.23 nm so that the Na occupancy in the 6d sites can be determined by comparing the experimental intensity profiles with calculated ones. Fig. 3 shows an averaged experimental image using 2dx software [4] and calculated images with different Na occupancies in the 6d sites as well as intensity profiles along the line X-Y. The peak intensity at the

A Quantitative Nanodiffraction System for Ultrahigh Vacuum Scanning Transmission Electron Microscopy

Of all the long-lived particles available as probes of condensed matter, and of all the signals available on a modern electron microscope, electron nanodiffraction patterns provide the strongest signal from the smallest volume. The technique is therefore perfectly suited to nanostructural investigations in inorganic chemistry and materials science. The Vacuum Generators HB501S, an ultrahigh vacuum (UHV) variant of the HB501 scanning transmission electron microscope (STEM), with side-entry double-tilt stage, specimen preparation and analysis chamber, three postspecimen lenses, and cold field-emission tip with integral magnetic gun lens, has therefore been modified to optimize nanodiffraction and quantitative convergent beam electron diffraction (QCBED) performance. A one-micrometer grain-size phosphor screen lying on a fiber-optic faceplate atop the instrument is fiber-optically coupled to a 2048 x 2048 charge-coupled device (CCD), 16-bit camera. This arrangement promises to provide much greater sensitivity, larger dynamic range, and a better modulation transfer function (MTF) than conventional single crystal scintillator (YAG) CCD systems, with noticeable absence of cross talk between pixels. The design of the nanodiffraction detector system is discussed, the gain of the detector is measured, the spherical aberration constant of the objective lens is measured by the Ronchigram method, and preliminary results from the modified instrument are shown.

Detecting single atoms of calcium and iron in biological structures by electron energy-loss spectrum-imaging.

As techniques for electron energy-loss spectroscopy (EELS) reach a higher degree of optimization, experimental detection limits for analysing biological structures are approaching values predicted by the physics of the electron scattering. Theory indicates that it should be possible to detect a single atom of certain elements like calcium and iron contained in a macromolecular assembly using a finely focused probe in the scanning transmission electron microscope (STEM). To test this prediction, EELS elemental maps have been recorded with the spectrum-imaging technique in a VG Microscopes HB501 STEM coupled to a Gatan Enfina spectrometer, which is equipped with an efficient charge-coupled device (CCD) array detector. By recording spectrum-images of haemoglobin adsorbed onto a thin carbon film, it is shown that the four heme groups in a single molecule can be detected with a signal-to-noise ratio of approximately 10 : 1. Other measurements demonstrate that calcium adsorbed onto a thin carbon film can be imaged at single atom sensitivity with a signal-to-noise ratio of approximately 5 : 1. Despite radiation damage due to the necessarily high electron dose, it is anticipated that mapping single atoms of metals and other bound elements will find useful applications in characterizing large protein assemblies.

Aberration-Corrected STEM: the Present and the Future

Surprising as it may seem, aberration correction for the scanning transmission electron microscope (STEM) is now a practical proposition. The first-ever commercial spherical aberration corrector for a STEM was delivered by Nion to IBM Research Center in June 2000, and other deliveries have taken place since or are imminent. At the same time, the development of corrector hardware and software is still proceeding at full speed, and our understanding of what are the most important factors for the successful operation of a corrector is deepening continuously.Fig. 1 shows two high-angle dark field (HADF) images of [110] Si obtained with the IBM VG HB501 STEM operating at 120 kV, about 2 weeks after we fitted a quadrupole-octupole corrector into it. Fig. 1(a) shows the best HADF image that could be obtained with the corrector's quadrupoles on but its octupoles off. Sample structures were captured down to about 2.5 Å detail, easily possible in a STEM with a high resolution objective lens with a spherical aberration coefficient (Cs) of 1.3 mm. Fig. 1(b) shows a HADF image obtained after the Cs-correcting octupoles were turned on and the corrector tuned up. The resolution has now improved to 1.36 Å. This is sufficient to resolve the correct separation of the closely-spaced Si columns.

Nano-Sized Catalytic Materials Investigated by a STEM-Based Mass Spectroscopic Technique

Knowledge of catalysts' sizes and shapes on their support material is crucial in understanding catalytic properties. With increasing interest in nanosized catalytic materials, it is vital to obtain structural information at the nanometer level in order to understand their catalytic behavior. We have recently demonstrated that very high angle (˜100mrad) annular dark-field (HAADF) images in a dedicated scanning transmission electron microscope (STEM) can be used to quantitatively measure the number of atoms of individual nano-sized clusters on a support material We are presently applying this technique to a bimetallic catalyst, PtRu5, where our data suggest that the shape of the PtRu5 particle is, surprisingly, oblate on the carbon substrate.PtRu5 is of interest for methanol oxidation for applications in batteries. PtRu5 compounds were produced by a molecular precursor method. Imaging was performed on a Field Emission Gun (FEG) Vacuum Generators HB501 STEM operated at 100kV.

A high efficiency annular dark field detector for STEM

A new high efficiency annular dark field (ADF) detector for an HB501 STEM (Scanning Transmission Electron Microscope) has been constructed and tested. This detector uses a single crystal YAP scintillator and a solid quartz light pipe extending from the scintillator (inside the vacuum) to the photomultiplier tube (outside the vacuum). A factor of approximately 100 improvement in signal relative to the original detector has been obtained. This has substantially improved the signal to noise ratio in the recorded high resolution ADF-STEM images.

Electron diffraction from gratings fabricated by electron beam nanolithography

The ability to produce nanometre-scale features in certain inorganic films offers the possibility of phase manipulation of electron waves. Gratings consisting of a square array of 100×100 holes with a spacing as low as 10 nm were drilled in thin films of AlF 3 using the 0.5 nm electron probe of a VG HB501 scanning transmission electron microscope (STEM). The sample thickness was chosen so that its inner potential would change the phase of a 200 keV transmitted electron wave by approximately 2 π with respect to a reference wave through a complete hole. Both constant phase and graded phase gratings have been produced. Electron diffraction patterns from these nanostructures have been observed in a JEOL 2000EX TEM at 200 keV. Simple computer simulations were used to aid the interpretation of the diffraction patterns. The graded phase gratings demonstrate conclusively that the phase of the diffracted electron wave can be manipulated by exploiting the effect of the inner potential.

On the Determination of Partial RDFs for Amorphous Materials

ABSTRACTRadial distribution functions (RDFs) for vitreous silica (V-SiO2) have been obtained from energy-filtered electron diffraction data obtained in the HB5 scanning transmission electron Microscope. Results have been compared with those obtained from high-resolution neutron diffraction experiments, and are in good agreement within experimental errors. It was found to be impractical to obtain partial RDFs for this material from combined neutron, X-ray and electron diffraction data, because the similarities in characteristics of X-ray and electron scattering cause indeter-Minacies. A criterion equation has been given to determine feasibility.

Trace analysis of transition elements and rare earths by parallel EELS

It has been realized for some years that electron energy loss spectroscopy (EELS) offers the potential for a highly sensitive microanalytical technique. Here, we present measurements from some NIST glass standard reference materials showing that EELS provides both high spatial resolution and trace sensitivity for a wide range of elements including the 3-d transition metals and the lanthanides.Specimens were prepared by grinding the glass and depositing the powder on to thin carbon support films. The energy loss spectra were collected by means of a Gatan 666 parallel-detection spectrometer attached to a VG Microscopes HB501 scanning transmission electron microscope (STEM) operating at 100 keV beam energy. This arrangement gave an energy resolution of approximately 0.4 eV limited by the spread of the field-emission source. Large spectral intensities were obtained by setting the probe current to about 8 nA and the collection semi-angle to 20 mrad. In order to eliminate possible hydrocarbon contamination the specimens were cooled to -160 C.

Analysis of directly frozen macromolecules and tissues in the field‐emission STEM

A VG Microscopes HB501 field-emission high-resolution scanning transmission electron microscope (STEM) was used to image and analyse rapidly frozen, isolated macromolecules and small organelles in tissue cryosections. Dark-field images were obtained from frozen-hydrated microtubules demonstrating that sufficient contrast is available to reveal structural information. The samples were subsequently freeze-dried in the STEM and low-dose (approximately 10(3) e/nm2) dark-field mass maps were recorded with single electron sensitivity. Elemental analysis of individual macromolecules was achievable at high dose using parallel-detection electron energy-loss spectroscopy, albeit with some structural degradation. Detection of copper (320 atoms) in di-decameric haemocyanin molecules was easily within the limits of sensitivity. Elemental analysis of hydrated cryosections is limited by radiation damage to a resolution of approximately 1 micron2. For freeze-dried sections, however, the high probe current and stable cold stage of the HB501 STEM allow energy-dispersive X-ray (EDX) microanalysis of low elemental concentrations in highly localized subcellular volumes. EDX spectra from cryosections of cerebellar cortex show that a 100-s analysis time is sufficient to quantify the calcium content of 400-nm2 regions within Purkinje cell dendrites with an uncertainty of +/- 2 mmol/kg dry weight, equivalent to +/- 12 atoms.

X‐ray depth distribution function, ϕ(ρz), for thin spheres

AbstractSub‐micron spheres of pure materials prepared by electrohydrodynamic atomization were used to determine experimentally the x‐ray distribution function, ϕ(ρz), for spheres using a dedicated HB501 scanning transmission electron microscope at 100 keV. The materials chosen were W, Pd, Ni, V and Al thin spheres. ϕ(ρz) was measured experimentally using a tracer technique. The thin film tracers used were Ir, Ag, Cr, Cu and SiO2, respectively. ϕ(ρz) for the above materials was also calculated using a Monte Carlo trajectory simulation modeled for the tracer technique. The experimental tracer ϕ(ρz) results show a pronounced scatter, especially for large‐diameter spheres. Local variations in the tracer film thicknesses appear to be the reason for such scatter. The trends in the experimental and Monte Carlo results, however, are simple and show a strong dependence on the atomic number of the target material. A general expression for ϕ(ρz) for thin spheres is presented which fits the experimental tracer results. The Monte Carlo results agree well with the fitted data for elements of low atomic number and/or small‐size spheres. For elements of higher atomic number (Pd and W), however, the Monte Carlo ϕ(ρz) curves peak too early with mass thickness. This problem may be due to inadequacies of the scattering cross‐section models used in the analysis.

Digital X-ray mapping on an HB501 STEM; a new approach for the analysis of interfaces

A scanning transmission electron microscope (STEM) has been modified to enable characteristic X-ray peaks to be stored as digital images along with pulse-counted bright-field and dark-field images. The system has been developed for X-ray microanalysis of two-dimensional structures (such as grain boundaries and interfaces). Line averages can be extracted from the images using a standard image-processing package to obtain composition profiles. This system is inherently suited to the analysis of electron-beam-sensitive materials.

Chemical Analysis of Layered Structures by Digital X-ray and EELS Mapping on an HB501 STEM

The development of materials formed from multilayer structures (MLS), with compositional modulation on the nanometre scale, such as semiconductor multiple quantum-well structures (MQW), used as lasers in fibre-optic communications, and metallic MLS such as Co/Cr and Co/W, for use as perpendicular magnetic recording media has led to a need for high spatial resolution chemical analysis techniques. The properties of such materials depends critically on the composition profile across the layers and interfaces. Techniques which can be applied to analysis of MLF in cross-section are energy dispersive X-ray (EDS) or electron energy loss (EELS) analysis using a small probe scanning transmission electron microscope (STEM).A VG HB501 STEM at Oxford, fitted with a windowless EDS detector and an EELS detector has been modified to enable pulse counted digital X-ray and EELS maps (plus bright-field (BF) or annular dark-field (ADF) images) to be collected and stored for post-processing in the Semper image-processing package.

Towards quantitative analysis of nanometer-sized helium bubbles in irradiated steels using parallel-detection EELS in a STEM

The stress rupture properties of high nickel austenitic alloys and austenitic stainless steels used in the nuclear industry are strongly affected under irradiation by the formation of helium from boron by means of an n-α reaction. As the congregation and growth of helium bubbles (typically 20-50nm diameter) at grain boundaries can cause embrittlement in stressed steels, it is important to gain an understanding of embrittlement mechanisms in the material. Consequently, an accurate measurement of quantities such as bubble pressure and metal surface energy is highly desirable. However, this can only be achieved if the number of helium atoms per unit volume is known. In this work, we report on the use of parallel-detection electron energy loss spectroscopy (PEELS) on a VG HB501 scanning transmission electron microscope (STEM) as a powerful technique for characterising helium bubbles in irradiated steels. In addition, we suggest a simple analytical process that could be used to quantify the amount of helium present in each bubble.

Fine structure of inner shell electron energy loss edges of manganese oxides

Rask et al. showed that electron energy loss spectroscopy (EELS) may be used to identify oxidation states of polyvalent cations. For the case of manganese, differences in oxidation state are reflected in the shape and position of the edges in the EELS spectrum due to inelastic scattering of incident electrons by the inner shells of manganese and oxygen. We have used a scanning transmission electron microscope (STEM) with a cold field emission gun (FEG) and a parallel-detection EELS system to re-investigate the variations in energy loss fine structure of EELS spectra of various manganese oxides with a significantly improved energy resolution.Spectra were recorded at 100 kV using a VG HB501 STEM with post-specimen lenses, and a Galan Model 666 parallel-detection electron energy loss spectrometer. The operation and performance of this system are discussed elsewhere in these proceedings. For the inner shell edges, the energy dispersion produced by the spectrometer was set to give 0.05 eV per channel. so that a complete spectrum of 1024 channels sampled a range of energy loss of 50 eV. The energy resolution, of the order of 0.4 eV, was therefore not limited by the spatial resolution of the photodiode array in the spectrometer. The oxygen K and manganese L2,3 edges were recorded separately.

Mass determination of the nuclear pore complex and its sub-complexesl by high-resolution Scanning Transmission Electron Microscopy (Stem)

The nuclear pore complex (NPC) is an elaborate membrane-bound molecular machine residing in the nuclear envelope of all eukaryotic cells and acting as a passageway for molecular transport between the nucleus and the cytoplasm. Complementary to transmission electron microscopic studies of negatively stained and frozen hydrated NPCs yielding detailed information about their overall size, shape and substructure, the high-resolution scanning transmission electron microscope (STEM) was used to measure the mass, to map the radial mass distribution, and to evaluate the rotational symmetry of unstained native NPCs and of several distinct sub-complexes. “Native” NPCs were prepared for STEM as follows: the nucleus of Xenopus laevis oocytes was placed on a 4 nm carbon film supported by a holey film mounted on a copper mesh grid, and its nuclear envelope ruptured and spread out using fine glass needles. The grid was washed with low salt buffer to remove macroscopic contaminants, and excess liquid was withdrawn with filter paper. Next, tobacco mosaic virus (TMV; serves as internal mass standard) was co-adsorbed, before the grid was washed for 10 sec each on 3 drops of bi-destilled H2O. To disintegrate the NPC into distinct sub-complexes, nuclei were placed on a 4-nm thick carbon film supported by a thick holey carbon foil on a copper grid, ruptured and spread as above, and then detergent extracted. After washing, the samples were frozen in liquid N2, transferred to a VG HB-5 STEM and freeze-dried at 153 K. Low-dose annular dark field (AD) images (50 to 300 e/nm2) were acquired digitally, corrected for nonlinearities in the AD signal due to high mass thickness, and stored on tape for further processing.Native NPCs, with and without plug, are displayed in Fig. 1. As shown in Fig. 2, two types of rings can be distinguished after detergent extraction: bright rings (“outer” rings; thick arrow) and dimmer rings (“inner” rings; thin arrow). In the same preparations particle “pairs” consisting of a ring (Fig. 3, thin arrow) and a plug-spoke complex (Fig. 3, thick arrow) can often be depicted. From these data averaged and 8-fold symmetrized mass maps of the native NPC with plug (Fig. 4; ˜50 NPCs) and without plug (Fig. 5; ˜50 NPCs), the outer ring (Fig. 6; ˜50 rings) and the inner ring (Fig. 7; ˜15 rings) were computed. Corresponding radial mass distributions are displayed at the same scale for comparison (Figs. 8 to 11). The average masses of the different structures are summarized in Table 1 together with a tentative model. These measurements bring speculations about the mass of the NPC to an end which have stated values ranging from 20 MDa to over 100 MDa, and they suggest that the NPC could easily be made of the order of 100 different polypeptides.

Parallel recording for an electron spectrometer on a Scanning Transmission Electron Microscope

A parallel recording system is being built for the electron-energy-loss spectrometer of a VG Microscopes HB501 scanning transmission electron microscope (STEM). A two-dimensional charge-coupled device (CCD) lens-coupled to an ytrrium aluminium garnet (YAG) scintillator is the electron detector. The operating mode for the CCD is described and the expected performance compared with that of a detector using a self-scanned photodiode array. Provision is being made for the bright-field and energy-loss images to be produced.

Implementation of differential phase contrast Lorentz microscopy on a conventional transmission electron microscope

Differential phase contrast (DPC) Lorentz microscopy is a powerful technique for investigating magnetization distributions within magnetic thin films. Previously this imaging technique has only been used on microscopes with nonstandard electron optical configurations because of two conflicting requirements: (1) The electron beam must be focused to a very fine probe on the specimen to obtain a high spatial resolution. (2) The specimen cannot be in the magnetic field of the objective lens, as this would destroy the magnetic domain structures of interest. This paper will describe the implementation of the technique on a conventional microscope, a JEOL JEM 2000 FX fitted with an ASID 20 scanning attachment, which satisfies these two requirements. We have modified the controls for the lenses and deflection coils in the microscope to allow an electron probe of less than 40 nm to be formed using the three condenser lenses, with the objective lens off. A position sensitive quadrant detector, required for DPC imaging, is mounted on a sliding O-ring below the projector lens and can be moved in for DPC work or out for standard microscope operation. Attached to the microscope is a digital image acquisition system based on an IBM PC AT. This system is used to acquire signals from the four quadrants of the detector simultaneously. The signals are then mixed digitally to produce the DPC images and an incoherent bright-field image. These images can be further processed to produce vector maps of the in-plane magnetization distributions within the sample. The spatial resolution of 40 nm is less than that of the VG HB501 STEM vacuum generators (VG) HB501 scanning transmission electron microscope (STEM) previously used for DPC imaging. This is due to the larger emitting area of the LaB6 filament compared to the field emission gun of the VG. The LaB6 has a much lower brightness than the FEG and this necessitates long image acquisition times. DPC images and the corresponding vector map from a NiFe film will be shown to demonstrate the technique.

Microdiffraction studies of small crystallites

Identification of nano-crystallites (<5 nm) on a high surface area support such as a catalyst is critical in the development of improved catalysts. Bright field imaging of small particles can be obscured by the phase contrast of the support. A common approach is to utilize annular dark field microscopy; however, wide angle scattering from the high surface area support can easily appear in the annular dark field image. Often one utilizes the energy dispersive detector to identify conclusively the nanosized metal crystallite, but this approach is troublesome when crystallites are very thin since counting times for imaging can become excessive. One technique for avoiding some of these difficulties is to utilize microdiffraction to identify the crystallite and then to image the crystallite by axial dark field microscopy.In the work reported here, the imaging was performed with a dedicated VG HB-5 scanning transmission electron microscope.

Grain boundary composition in silicon nitride

Since one of the major factors for controlling creep properties in silicon nitride components is the addition of sintering aids, determination of the composition of the resulting intergranular second phases becomes very important. High resolution electron microscopy has been used to investigate the crystalline or non-crystalline character of the grain boundary while analytical electron microscopy studies have focused on the composition of the grain boundary pockets. As sintering aid additions and processing become more complex, multiple second phases are produced. Ascertaining the composition of the grain boundaries and grain boundary pockets is necessary for correlating the microstructure with the creep properties.A dedicated VG HB-5 scanning transmission electron microscope was used to measure the composition of grain boundaries and grain boundary pockets of silicon nitride.