Parallel Detection EELS Research Articles

Analysis of Biological Structures by Electron Energy Loss Spectroscopy and Imaging

As techniques for electron energy-loss spectroscopy (EELS) reach a higher degree of optimization, detection limits for analyzing biological structures are approaching those predicted by theory. in favorable specimens, single atom detection is predicted for elemental maps acquired by means of the scanning transmission electron microscope (STEM) equipped with a field emission source, paralleldetection EELS and a spectrum-imaging system. to obtain such results, the electron detector should have a detective quantum efficiency close to unity and a well behaved point-spread function; such design features are now available with a cooled charge-couple device (CCD) array. The energy-filtering transmission electron microscope (EFTEM) provides a complementary approach to mapping elements occurring at higher concentrations but distributed over larger regions of the specimen. Use of an optimized CCD detector in the EFTEM now enables accurate quantitation in addition to high analytical sensitivity, albeit not at the single atom level.

Shock-compression of C–N precursors for possible synthesis of β-C 3N 4

We report on the possible synthesis of the theoretically predicted β-phase of carbon nitride (C 3N 4) by shock compression of sodium dicyanamide mixed with sodium azide and carbon tetra-iodide. Shock-compression experiments were performed on starting precursor blended with ∼95 wt.% Cu powder, statically pressed in steel capsules. The capsules were impacted under conditions of constant shock amplitude, but varying shock-pulse duration. TEM analysis of the recovered shock-compressed residue showed crystallites of a cubic C–N compound dispersed in an amorphous matrix, with overall yield of the crystalline phase being a function of the shock-pulse duration. Parallel-detection electron energy loss spectroscopy of the nitrogen-containing crystallites revealed diamond-like sp 3 bonding. Infra-red spectroscopy indicated absorption lines in regions calculated to be appropriate for β-C 3N 4.

Comparison of STEM EELS Spectrum Imaging vs EFTEM Spectrum Imaging

Abstract Electron energy-loss spectroscopy (EELS) in the transmission electron microscope (TEM) analyzes the energy distribution of the probe electrons after they have lost energy within the sample. The resultant energy-losses are characteristic of elemental, chemical, and dielectric properties and are typically measured in one of two ways. Parallel-detection EELS spectrometers (PEELS) acquire large energy ranges of the energy-loss spectrum simultaneously for rapid acquisition of spectral data at a single area. In contrast, the energy filtering TEM (EFTEM) acquires only a single energy band at once, but does so for thousands or even millions of image pixels simultaneously. Spectrum imaging (SI) involves acquisition of detailed spectroscopic data sufficient for rigorous analysis at each pixel in a digital image. (Fig. la) A STEM EELS spectrum image “data cube” can be acquired by stepping a focused electron probe to each pixel and filling the spectrum image one spectrum at a time.

EFTEM and STEM EELS Spectrum Imaging

Abstract Electron energy-loss spectroscopy (EELS) in the transmission electron microscope (TEM) is a powerful technique that analyzes the inelastic scattering distribution of the fast TEM electrons after they have lost energy within the sample. The resultant energy-losses are characteristic of elemental, chemical, and dielectric properties and are typically measured in one of two ways. Parallel-detection EELS spectrometers (PEELS) acquire spectral data over a large range of energy-loss simultaneously for rapid acquisition of spectral data at a single point. In contrast, the energy filtering TEM (EFTEM) acquires only a single energy band at once, but does so for thousands or even millions of image pixels simultaneously. Spectrum-imaging concerns the acquisition of spectroscopic data of sufficient detail for rigorous analysis at each pixel in a digital image. (Fig. 1) A STEM EELS spectrum image “data cube” can be acquired by stepping a focused electron probe to each pixel and filling the spectrum image one spectrum at a time.

Capabilities and Limitations of Biological Electron Energy-Loss Spectroscopy

Abstract Perhaps the ultimate aim of analytical electron microscopy in biology is to detect single atoms or ions bound to isolated macromolecular assemblies and small cellular organelles rapidly frozen in their native state. Although the meaningful spatial resolution in such analyses is limited to ∽10 nm or more by radiation damage, the high intrinsic sensitivity of electron energy-loss spectroscopy (EELS) coupled with recent developments seems to make this rather ambitious goal—originally proposed some twenty years ago—within reach. Here we describe examples where EELS has been able to detect surprisingly small numbers of atoms in biological specimens and discuss some fundamental limits that are encountered as well as some possible strategies for circumventing these difficulties. In our laboratory we have used a scanning transmission electron microscope (STEM) equipped with a field-emission source and parallel-detection EELS because the probe size and collection efficiency of this instrument are optimized to give the lowest detectable number of atoms and lowest detectable atomic fraction.

Irradiation-Induced Microstructural Changes in Alloy X-750

Abstract Alloy X-750 is a γ´-strengthened Ni-base alloy that is often used in nuclear power systems for its excellent corrosion resistance and good mechanical properties. However, radiation-induced segregation (RIS) and irradiation-assisted stress corrosion cracking (IASCC) can occur under neutron irradiation. The present study examines the microstructure and composition profiles in a heat of Alloy X-750 before and after neutron irradiation. The material was in the HTH commercial heat treatment (lh at 1121°C + air cooling + 20h at 704°C). Specimens were prepared from both control material and material irradiated at 360°C to a fluence of 2.3 × 1020 n/cm2 (E > 1 MeV), which was estimated to produce -0.4 displacements per atom. High spatial resolution analytical electron microscopy was performed in a Philips EM400T/FEG operated in the STEM mode with a probe diameter of ∼1.4 nm (FWHM). Composition profiles were acquired in two different ways: all energy dispersive X-ray spectrometry (EDS) and some parallel-detection electron energy-loss spectrometry (PEELS) profiles were acquired as individual spot analyses, whereas the majority of the PEELS profiles were acquired as spectrum lines with ∼10 s acquisition times per point. The quoted compositions are as measured (wt%); no deconvolution for the effect of the excited volume was performed.

Quantitative structure determination of interfaces through Z-contrast imaging and electron energy loss spectroscopy

The simultaneous use of Z-contrast imaging with parallel detection EELS in the STEM provides a powerful means for determining the atomic structure of grain boundaries. The incoherent Z-contrast image of the high atomic number columns can be directly inverted to their real space arrangement, without the use of preconceived structure models. Positions and intensities may be accurately quantified through a maximum entropy analysis. Light elements that are not visible in the Z-contrast image can be studied through EELS; their coordination polyhedra determined from the spectral fine structure. It even appears feasible to contemplate 3D structure refinement through multiple scattering calculations.The power of this approach is illustrated by the recent study of a series of SrTiC>3 bicrystals, which has provided significant insight into some of the basic issues of grain boundaries in ceramics. Figure 1 shows the structural units deduced from a set of 24°, 36° and 65° symmetric boundaries, and 24° and 45° asymmetric boundaries. It can be seen that apart from unit cells and fragments from the perfect crystal, only three units are needed to construct any arbitrary tilt boundary. For symmetric boundaries, only two units are required, each having the same Burgers, vector of a<100>. Both units are pentagons, on either the Sr or Ti sublattice, and both contain two columns of the other sublattice, imaging in positions too close for the atoms in each column to be coplanar. Each column was therefore assumed to be half full, with the pair forming a single zig-zag column. For asymmetric boundaries, crystal geometry requires two types of dislocations; the additional unit was found to have a Burgers’ vector of a<110>. Such a unit is a larger source of strain, and is especially important to the transport characteristics of cuprate superconductors. These zig-zag columns avoid the problem of like-ion repulsion; they have also been seen in TiO2 and YBa2Cu3O7-x and may be a general feature of ionic materials.

Interfacial composition analysis of silicon nitride whisker reinforced oxynitride glass systems

Microstructural and compositional analysis in many structural ceramics requires sophisticated analytical electron microscopy because of the light elements (O, N, C and B) present in the materials and the nano-scale amorphous phases at the grain boundaries. In the present study, scanning transmission electron microscopy (STEM) combined with energy-dispersive x-ray spectroscopy (EDS) and parallel-detection electron energy-loss spectroscopy (PEELS) was used to analyze the composition at interfaces in β-Si3N4 whisker reinforced oxynitride glass systems. Composition profiles were acquired by either manually stepping the probe across the interface or by sequential integration during slow-rate line-scanning with a < 2.0 nm probe in the STEM mode of a Philips EM400T/FEG microscope.Silicon nitride, which has been recognized as one of the most promising candidates for high temperature applications, is typically densified by employing various sintering aids such as yttria and alumina. Reaction of the sintering aids and the oxide surface layer on Si3N4 particles forms secondary intergranular phases that are primarily silicon metal oxynitride glasses.

Spatially resolved microanalysis of thin films in the transmission electron microscope

Electron energy-loss spectrometry (EELS) is an important technique for microanalysis of thin films in the transmission electron microscope. Absolute quantification of elements on a scale of less than 10 nm is feasible when the chromatic aberration of the post-specimen electron lenses is properly corrected. The accuracy of the quantification is influenced by several effects. We present a method for the compensation of the chromatic error using a defocus series and a correction of the chromatic image shift. Experiments using specimens with an edge-like concentration function (planar interface, abrupt thickness variation) were performed on a parallel-detection EELS system (GATAN 666 PEELS) and on an energy-selecting imaging filter. Comparison of these results suggests that for accurate quantification on selected positions of the specimen the PEELS should be used, whereas the imaging filter is better suited for a fast overview of the elemental distribution in a larger area. The influence of Bragg scattering on the results of the quantification for both systems is discussed.

High spatial resolution extended energy loss fine structure investigations of silicon dioxide compounds

The near edge fine structure of the silicon L 2,3 edge from Si, SiO 2 and SiSiO 2 interfaces and the extended energy loss fine structure of the oxygen K-edge from SiO 2 and SiSiO 2 are studied by means of parallel detection EELS with high spatial resolution scanning transmission electron microscopy. The “fingerprint” technique using the specific characteristics of the Si L 2,3 edges in Si, SiO 2 and SiSiO 2 interfaces is used to distinguish between spectra obtained from different areas. The EXELFS analysis of the oxygen K-edge implies that the OSi bond length is slightly larger in the SiSiO 2 interface than in SiO 2. The comparison of the EXELFS amplitudes provides a means to observe the differences in coordination numbers and disorder parameters. The results are discussed and compared with data obtained using other techniques.

Atomic-resolution characterization of an SrTiO3 grain boundary in the STEM

High-resolution Z-contrast imaging in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition across an interface can be interpreted intuitively without the need for preconcieved atomic structure models. Since the Z-contrast image is formed by electrons scattered through high angles, parallel detection electron energy loss spectroscopy (PEELS) can be used simultaneously to provide complementary chemical information on an atomic scale. The fine structure in the PEEL spectra can be used to investigate the local electronic structure and the nature of the bonding across the interface. In this paper we use the complimentary techniques of high resolution Z-contrast imaging and PEELS to investigate the atomic structure and chemistry of a 25 degree symmetric tilt boundary in a bicrystal of the electroceramic SrTiO3.Figure 1(a) shows a Z-contrast image of a symmetric region of the tilt boundary. The brightest spots in the image correspond to the increased scattering power of the Sr atomic columns (Z=38) with theless bright spots corresponding to the Ti atomic columns (Z=22). The lighter O atomic columns are notvisible in a Z-contrast image.

Atomic Scale Structure and Chemistry of Interfaces by Z-Contrast Imaging and Electron Energy Loss Spectroscopy in the Stem

AbstractThe macroscopic properties of many materials are controlled by the structure and chemistry at grain boundaries. A basic understanding of the structure-property relationship requires a technique which probes both composition and chemical bonding on an atomic scale. High-resolution Z-contrast imaging in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition across an interface can be interpreted directly without the need for preconceived atomic structure models (1). Since the Z-contrast image is formed by electrons scattered through high angles, parallel detection electron energy loss spectroscopy (PEELS) can be used simultaneously to provide complementary chemical information on an atomic scale (2). The fine structure in the PEEL spectra can be used to investigate the local electronic structure and the nature of the bonding across the interface (3). In this paper we use the complimentary techniques of high resolution Zcontrast imaging and PEELS to investigate the atomic structure and chemistry of a 25° symmetric tilt boundary in a bicrystal of the electroceramic SrTiO3.

Extending the limits of molecular microanalysis

Modern polymer blends are frequently composed of domains and interfacial phases having submicron dimensions. Previously, die characterization of specific unknown submicron polymer phases has been limited to selective staining methods that help to classify the phase but rarely lead to chemical identification. The present procedure uses parallel detection electron energy loss spectroscopy (PEELS) to perform submicron molecular microanalysis on beam sensitive materials. Polymer domains are first differentiated by their elemental composition and then by their characteristic carbon core loss edge structure. These spectra are compared to spectra recorded from polymers of known composition.A polymer film composed of alternating 0.5 μm layers of polycarbonate (PC) and polymethylmethacrylate (PMMA) was used as a test specimen (Fig. 1). Ultra-thin sections (<50 nm) were prepared by microtomy, collected on unsupported 600 mesh copper grids and examined at −160°C usinga VG HB601UX dedicated STEM fitted with a Gatan 666 UHV PEELS. The combination of beam blanking, simplecontrol of electron dose, UHV, low energy spread FEG, stage stability and the ability to produce a high contrast image at a very low electron dose makes this instrument ideally suited for this experiment.

Trace Analysis of Nanoscale Materials by Analytical Electron Microscopy

ABSTRACTTrace analysis of nanometer-scale objects can be performed with parallel-detection electron energy loss spectrometry in the analytical electron microscope. Spectra are collected in the second difference mode with the beam current chosen to maximize the spectral count rate. Numerous elements can be detected at trace levels below 100 parts per million atomic, including transition metal, alkali metal, alkaline earth, and rare earth elements, provided they have a “white line” resonance structure at the ionization edge. Trace nanoanalysis by AEM/PEELS permits direct examination of the microscopic distribution of trace constituents.

Determination of interface structure and bonding at atomic resolution in the STEM

Theo scanning transmission electron microscope can now routinely produce a probe of atomic dimensions, 2.2Å at 100 kV and 1.3Å at 300 kV. This capability opens up a new strategy for determining the structure and bonding of materials at the atomic scale, which is particularly advantageous for complex interfaces such as grain boundaries. The Z-contrast image allows the high-Z column locations to be determined by direct inspection, and quantified through maximum entropy methods to a positional accuracy of currently ~0.2Å. With the same electron probe, located to sub-Ångstrom precision with reference to the Z-contrast image, parallel detection EELS can determine light element coordination from core edge fine structure, in principle around a single atomic column.The attractive features of this approach are the increased resolution associated with incoherent imaging, 0.44 Cs1/4 λ3/4 compared with 0.66 Cs1/4 λ3/4 for phase contrast imaging, and the highly local nature of the image, which makes possible a greatly extended regieme of intuitive image interpretation, and also permits column-by-column spectroscopy without beam broadening.

Interface and grain boundary structures in YBa2Cu3O7-x and YBa2Cu4O8 materials

Y2BaCuO5YBa2Cu3O7-x (Y211/Y123) interfaces in melt-processed YBa2Cu3O7-x were studied by high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Yttrium enrichment and barium depletion were observed locally at the Y211/Y123 interfaces where Y123 (001) facets were present. This effect may be interpreted as the result of lattice substitution of Ba by Y near these interfaces. Cation nonstoichiometry was found near Y211/Y123 interfaces where liquid phases (Cu-Ba-O) were present. This chemical disorder introduces numerous point defects in the Y123, and these defects may act as additional pinning sites alongwith stacking faults. A comparison of grain boundary (GB) chemical composition in polycrystalline YBa2Cu3O7-x and YBa2Cu4O8(Y124), studied using nanoprobe parallel-detection electron energy-loss spectroscopy (EELS), is presented. The studies of Y124 show that stoichiometric grain boundaries can also form weak links between superconducting grains. It is suggested that weak-link behavior is determined largely by misorientation at grain boundaries.

Trace elemental analysis at nanometer spatial resolution by parallel-detection electron energy loss spectroscopy

Parallel-detection electron energy loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM) and a field emission source provides an unprecedented sensitivity for elemental microanalysis. By deflecting the energy loss spectrum across a parallel detector and computing the difference spectrum from sequentially collected energy-shifted spectra, the effects due to detector pattern noise are nearly eliminated so that signals less than 0.1% of the background can be readily detected. Measurements on a series of glass standard reference materials show that EELS provides both high spatial resolution and trace sensitivity at the 10 atomic ppm level for a wide range of elements including the alkaline earths, 3-d transition metals, and the lanthanides. For analytical volumes with dimensions of the order of 10 nm, this translates into near-single atom detectability.

Detection limits for Analytical Electron Microscopy with Electron Energy Loss Spectrometry and energy-dispersive x-ray spectrometry

The measurement of trace level constituents, arbitrarily defined for this study as concentration levels below 1 atom percent, has always been considered problematic for analytical electron microscopy (AEM) with energy dispersive x-ray spectrometry (EDS) and electron energy loss spectrometry (EELS). In a landmark study of various microanalysis techniques, Wittry evaluated the influence of various instrumental factors (source brightness, detection efficiency, accumulation time) and physical factors (cross section, peak-to-background) upon detection limits. Although the ionization cross section, fluorescence yield, and collection efficiency favor EELS over EDS, the peak-to-background ratio of EELS spectra is much lower than that of EDS spectra, leading Wittry to suggest that the limit of detection should be 0.1 percent for EDS and 1 percent for EELS for practical measurement conditions. Recent developments in parallel detection EELS (PEELS) indicate that a re-evaluation of the situation for trace constituent determination is needed for those elements characterized by "white line" resonance structures at the ionization edge.

EELS of laser-irradiated kapton polymide

It has recently been demonstrated that the electrical resistivity of a high temperature polymer, polyimide, can be changed permanently from 1017 Ωcm to 10-1 Ωcm by KrF (248 nm) excimer laser irradiation. Further, using a holographic technique, an array of 500-nm-wide electrically conducting wires has been produced on polyimide by direct laser ablation. There is obvious interest in exploiting this phenomenon to create integrated microelectronic devices with conventional polymers as both dielectrics and conductors. The imaging of such patterned nanostructures by scanning and transmission electron microscopy (SEM and TEM) is reported elsewhere. Knowledge of compositional changes that accompany the structural changes is necessary for understanding formation mechanisms, but analytical electron microscopy (AEM) of polymers is usually severely limited by electron beam damage effects, especially when high spatial resolution microanalysis is attempted with focussed probes. However, parallel-detection electron energy-loss spectrometry (PEELS) has been successfully used to investigate laser-induced compositional changes in polyimide (Kapton™, E.I. Dupont) with no apparent signs of beam damage.

Elemental mapping with an energy-selecting imaging filter

Imaging filters produce energy-selected images in a few seconds, and chemical maps formed by processing of several images taken at different energy losses in typically less than one minute. On the other hand, imaging filters do not provide detailed spectra from each specimen point, and are vulnerable to artifacts due to variations in specimen thickness, and other effects influencing EELS background extrapolation and subtraction. These include diffraction contrast arising particularly in crystalline samples, edge overlap, and extended fine structures (EXELFS) in the pre-edge region caused by major edges at lower energies. We have therefore been exploring the practical usefulness of imaging filters on a range of specimens from materials science and biology. The results suggest that the imaging capability combined with full paralleldetection EELS performance delivers a very powerful experimental set-up.Figure 1 shows an energy-filtered bright field image of a steel sample containing about 1 % Cu, obtained at 120 keV with the Gatan Imaging Filter (GIF) attached to a Philips CM12ST microscope.