Parallel-detection Electron Energy Loss Research Articles

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.

Synthesis, HRTEM and electron diffraction studies of B/N-doped C and BN nanotubes

Large-scale synthesis of multi-walled BN and single-walled B/N-doped C nanotubes (NT) from C nanotube templates was carried out. The NTs were produced through heating of C templates with B2O3 in a flowing N2 atmosphere at 1503-1773 K. The NTs were analyzed by means of a JEM-3000F high-resolution field emission transmission electron microscope operated at 300 kV and equipped with a parallel detection electron energy loss spectrometer. Particular attention was given to the effects of C template morphology, synthesis temperature and metal oxide promoters on the yield and chemical composition of NTs. Ropes consisting of tens of multi-walled BN NTs were synthesized at 1773 K using MoO3 and PbO promoters. Ropes of single-walled B/N-doped C NTs were produced at high yields by synthesis at 1553 K. Packing of NTs in the ropes and nanotube helicities were studied by electron diffraction. B/N-doped C and BN NT morphologies, formation mechanism and atomic structures are discussed in this paper.

High spatial resolution PEELS characterization of FeAl nanograins prepared by mechanical alloying

We investigate the nanograin “chemical” structure in a nanostructured material of possible industrial application (Fe–Al system) prepared by conventional mechanical alloying via ball milling in argon atmosphere. We restrict ourselves to the structural and nanochemical behaviour of ball-milled nanocrystalline Fe–Al powders with atomic composition Fe 3Al, corresponding to a well-known intermetallic compound of the Fe–Al system. Scanning transmission electron microscopy (STEM) equipped with a parallel detection electron energy loss spectrometer (PEELS) has provided an insight on the “chemical” structure of both nanograins and their surface at a spatial resolution of better than 1 nm. The energy loss near edge structure of the Al L loss reveals that the Al coordination is similar to a B2 compound and the oxidation of the powder during processing may play a significant role in the stabilization of the intermetallic phases. Conventional transmission electron microscopy (TEM) was used for the structural characterization of the material after the ball milling; powder X-ray diffraction (XRD) aided the investigation.

A 200kV Comprehensive Analytical Electron Microscope

With the use of a high-brightness electron source, the performance of the analytical electron microscope (AEM) has significantly improved. It features a variety of sophisticated analytical functions, such as X- ray spot analysis, area analysis, electron beam energy analysis, electron diffraction, and phase analysis using a biprism for holography. Here, we report the results of our analysis using a 200kV comprehensive AEM which has an integrated ZrO/W emitter as its optic source. The AEM (JEM- 2010F) is equipped with an X-ray energy dispersive spectrometer (EDS), scanning image observation device (ASID), parallel detection electron energy loss spectrometer (PEELS), and TV camera.(1) X-ray mapping using EDS and ASID With FEG, the probe current in a 1 nm probe is increased by two-order orders of magnitude, and the background noise generated near the sample and in the illumination system is minimized. As a result, EDS analysis features high sensitivity at a minimum detection of 1% weight fraction from an ultra-micro area of lnm diameter.

High-resolution TEM and AEM study in (Au, Tin) thin film on (100) silicon

The cross-sectional microstructure of a (Au, TiN) thin film deposited on a (100) Si substrate without further heat treatment was studied by CTEM, HRTEM, and AEM. HTREM was performed in a Topcon 002B microscope with interpretable resolution limit of 0.18 nm, and high spatial AEM was done in a Philips 400ST field emission gun microscope at 100 kV using a Gatan 666 parallel-detection electron energy loss spectrometer. Cross-section specimens of the interface were prepared by traditional polishing and ion milling. Temperatures used in the whole process of TEM specimen preparation were below 100°C and a liquid-nitrogen-cooled cold stage was used in ion milling to prevent interaction between Au and Si.Ti was co-deposited with Au by sputtering in a nitrogen atmosphere to increase the hardness of the thin Au contact film. Fig. 1a shows the microstructure of the (Au, Ti, N)/Si contact. The features of this microstructure are very similar to the those of pure gold thin films on Si substrates annealed below the Au-Si eutectic temperature (363°C). The thin, white layer was considered to be the original Au/Si interface by Chang et al.

EXELFS as a tool for quantifying phase distributions in materials

A method has been developed to quantify phase distributions in multiphase materials by using the information contained in the EXtended Energy Loss Fine Structure (EXELFS) of a probe atom in an Electron Energy Loss Spectroscopy (EELS) spectrum. The EELS and EXELFS determine, in addition to the atomic composition, the near-neighbor atomic environment about each atom type separately. The method is applicable to electron-transparent foils observed in a Transmission Electron Microscope (TEM) equipped with a parallel-detection electron energy loss spectrometer. The relative amounts of phases that have a common element can be determined even in the presence of additional phases which do not share that common element. The method is particularly applicable to layered materials and can be used, for example, to determine the local relative thickness of a metal and its oxide layer. The Mg MgO system was used to demonstrate the method, and it was found that the accuracy in relative thickness determination is about 5%. The near-edge fine structure information was consistent with the results of the EXELFS analysis.

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.

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.

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.

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.

Microscopy in solid state science

The Microstructural Physics group at the Cavendish Laboratory is actively involved in a considerable number of research projects which cover a broad range of materials science. In this paper, we describe briefly several such projects, with particular emphasis given to the application of parallel-detection electron energy loss spectroscopy (PEELS) on a scanning transmission electron microscope (STEM) to the analysis of materials such as stainless steels, catalysts, and high temperature superconductors. In addition, we describe a number of related projects that are currently being carried out in the group, particularly those which utilise and develop novel STEM imaging and analytical techniques.

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. The high-resolution Z-contrast imaging technique in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition can be interpreted intuitively. This direct image allows the electron probe to be positioned over individual atomic columns for parallel detection electron energy loss spectroscopy (EELS) at a spatial resolution approaching 0.22nm. In this paper we have combined the structural information available in the Z-contrast images with the bonding information obtained from the fine structure within the EELS edges to determine the grain boundary structure in a SrTiO3 bicrystal.

Determination of the local oxygen stoichiometry in YBa 2Cu 3O 7−δ by electron energy loss spectroscopy in the scanning transmission electron microscope

By using parallel detection electron energy loss spectroscopy (PEELS) in the scanning transmission electron microscope (STEM), the oxygen K-absorption edge spectrum is used to determine δ in YBa 2Cu 3O 7−δ to an accuracy of < 5%. The instability of YBa 2Cu 3O 7−δ to the electron beam enables oxygen reduction experiments to be performed in situ in the microscope. Results show a “two plateau” behaviour similar to that observed in transition temperature versus oxygen stoichiometry measurements, and can be interpreted in terms of oxygen vacancy ordering and charge distribution between the CuO chains and CuO 2 planes. In addition, the sub-nanometer probe of the STEM enables changes in oxygen deficiency to be observed with high spatial resolution. The high spatial resolution has been used to observe fluctuations in oxygen stoichiometry in the region of grain boundaries in bulk sintered samples.

Application of analytical electron microscopy to studies of equilibrium and non-equilibrium segregation in materials

Segregation of solute atoms to grain boundaries, dislocations, and other extended defects can occur under thermal equilibrium or non-equilibrium conditions, such as quenching, irradiation, or precipitation. Generally, equilibrium segregation is narrow (near monolayer coverage at planar defects), whereas non-equilibrium segregation exhibits profiles of larger spatial extent, associated with diffusion of point defects or solute atoms. Analytical electron microscopy provides tools both to measure the segregation and to characterize the defect at which the segregation occurs. This is especially true of instruments that can achieve fine (&lt;2 nm width), high current probes and as such, provide high spatial resolution analysis and characterization capability. Analysis was performed in a Philips EM400T/FEG operated in the scanning transmission mode with a probe diameter of &lt;2 nm (FWTM). The instrument is equipped with EDAX 9100/70 energy dispersive X-ray spectrometry (EDXS) and Gatan 666 parallel detection electron energy loss spectrometry (PEELS) systems. A double-tilt, liquid-nitrogen-cooled specimen holder was employed for microanalysis in order to minimize contamination under the focussed spot.

Segregation in a neutron-irradiated Type 316 stainless steel

Radiation-induced segregation (RIS) and associated irradiation-assisted stress corrosion cracking (IASCC) of austenitic alloys may be a major factor in limiting component lifetimes in water-cooled nuclear reactors. There are some similarities between radiation-induced sensitization/IASCC and thermally-induced sensitization/intergranular stress corrosion cracking. Both processes are associated with chromium depletion at grain boundaries. Segregation to boundaries in a neutron irradiated type 316 stainless steel has been investigated with both energy-dispersive X-ray spectrometry (EDXS) and parallel detection electron energy loss spectrometry (PEELS).All specimens were from the same heat of cold-worked type 316 stainless steel. Both unirradiated control material and material irradiated at ∼300°C to a range of fluences 0.3 - 5 × 1026 neutrons/m2 (E&gt;0.1 MeV) were available. The mass of irradiated material was minimized by mechanically polishing 3-mm-diam. disks to ∼75 μm thickness prior to electropolishing. However, the specific radioactivity of the specimens, which increased with neutron fluence, limited the application of EDXS to the unirradiated and the lowest fluence irradiated materials.

Characterization of iron-implanted silicon carbide

As part of extensive fundamental studies of ion-implanted ceramics, analytical electron microscopy (AEM) has been used to characterize [0001] single crystal 6H α-silicon carbide implanted at room temperature with 160 keV 57Fe ions to fluences of 1,3, and 6 x 1016 ions/cm2. AEM was performed at 100 kV with a Philips EM400T equipped with a field emission gun, a 6585 STEM unit, an EDAX 9100/70 energy dispersive X-ray spectrometer (EDS), and a Gatan 666 parallel-detection electron energy loss spectrometer (PEELS).A micrograph of a cross-sectioned specimen implanted with 6 × 1016 ions/cm2 is shown in Fig. 1. The implanted region extending to 195 nm from the surface is amorphous, as confirmed by convergent beam electron diffraction (CBED) patterns (insets), and contains iron at levels easily detectable by EDS (Fig. 2). The 15 nm lattice fringes of the edge-on 6H basal planes in the substrate provide an unambiguous depth calibration scale. The transition layer at the end of the ion range exhibits the expected strain contrast. The depth profile of the implanted iron was measured by EDS line scans perpendicular to the implant surface; the mean of three of these scans is shown by the filled symbols in Fig. 3. The integrated iron profile agrees with the expected 6.0 × 1016 ions/cm2 only if the density of the amorphous SiC is equal to that of its crystalline forms, rather than the 20% lower density reported earlier.

Separating the surface and volume plasmon energy loss intensity distributions in MgO and MgAl2O4

Broadening on the low energy side of the MgAl2O4 (spinel) volume plasmon, which increases with decreasing sample thickness (Fig. 1a), affected multiple least squares (MLS) fitting when spectra such as these were used in quantitative analysis. The inverse dependency of broadening on thickness is indicative of a surface effect. Potential artifacts from sample preparation (argon milling) were investigated. Spinel stock was ground in methanol on a grinding disk of 15μm-diameter diamond, and the tailings were placed onto a holey-carbon film. Electron-transparent regions of spinel that extended beyond holes in the film were used to obtain the energy loss single scattering distributions (SSD) shown in Fig. 1b. Parallel-detection electron energy loss spectroscopy (PEELS) was performed at 100 kV with a Philips EM400T/FEG analytical electron microscope. These SSD were obtained using the procedures described earlier, and show, relative to those in Fig. 1a, identical broadening on the left side of the plasmon with decreasing specimen thickness. This similarity indicates the broadening effect is not due to milling but to surface plasmon excitation.

Quantification of Nitrogen in Solution in Stainless Steels Using Parallel EELS

The purpose of this work is to detect and quantify nitrogen which is dissolved in steels using parallel detection Electron Energy Loss Spectroscopy (PEELS). PEELS in the Transmission Electron Microscope (TEM) has a high detection efficiency and is potentially powerful for studying the behaviour of light elements in commercial steels with a high spatial resolution

Interfacial boundaries in Si3N4-based ceramic composites: Constraints from matrix effects and stability of microstructure

Abstract The stability of the microstructure of Si3N4-based composites with discontinuous SiC reinforcement components (whiskers, platelets) is addressed, focusing on the role of interfacial boundaries. This approach basically explores four issues: 1. (1) The evaluation of structural configuration and interfacial chemistry of internal interfaces by means of HREM imaging and parallel detection electron energy loss spectroscopy (PEELS), considering the effects of different oxygen sources in the SiC/Si3N4 system. 2. (2) The influence of whisker-related impurities, such as excess carbon and oxygen, on the liquid-phase formation process and microstructural development of the Si3N4 baseline material. 3. (3) The variety of local microstructures along interfacial boundaries in Si3N4-based composites employing SiC-whiskers,-platelets and -fibers due to thermodynamic constraints which emphasize the dynamic nature of internal interfaces in ceramic matrix composites. 4. (4) The fundamental problem of correlating microscopic characteristics from grain and phase boundaries with the fracture resistance behavior of composite materials, considering that macroscopic, matrix-related parameters contribute to the mechanical response.