Spatially-resolved Electron Energy-loss Spectroscopy Research Articles

Mapping the Structure of a Hydrated Polymer Blend Using Energy-Loss Spectroscopy in the Cryo-STEM

ABSTRACTWe use electron energy-loss spectroscopy (EELS) in the cryo-STEM to determine the spatial distribution of water in a model frozen-hydrated two-phase polymer blend composed of hydrophilic poly(vinylpyrrolidone) (PVP) and hydrophobic poly(styrene) (PS). We demonstrate that it is possible to directly correlate the water spatial distribution with variations in the underlying polymer morphology. HAADF-STEM imaging of both dry and frozen-hydrated specimens shows weak contrast between the polymer phases but gives no information regarding the composition of these phases and no indication of where water might be localized. Spatially-resolved EELS spectra collected at 100 nm pixel size show that this system is composed of discrete PVP-rich domains dispersed in a continuous PS-rich matrix. The PVP-rich domains were found to be hydrated up to a level of ∼ 23 wt%. We have made our compositional maps fully quantitative, given as mass-fraction maps, by measuring the total inelastic scattering cross-sections per unit mass of water, PVP and PS. This is an important quantity which we have determined for an incident beam energy of 200 keV. Hydrated PVP gives rise to hydrogen evolution when irradiated above an electron dose of 1500 e/nm2 as evidenced from changes in the 13 eV region, and this effect gives rise to dose-limited resolution in these experiments.

Time-resolved acquisition technique for spatially-resolved electron energy-loss spectroscopy by energy-filtering TEM.

A time-resolved acquisition technique has been investigated for spatially-resolved electron energy-loss spectroscopy by energy-fitering transmission electron microscopy. Plural (10) spectral images were acquired separately at an energy (E) and a position (Y) coordinate. After correction of both specimen and energy drifts by comparing with the original starting spectrum, the 10 spectral images were added to form a new processed spectral image. This time-resolved acquisition technique was found to improve the signal-to-noise ratio to about the square root of ten for 200 s total exposure time without increasing the image blurring problem.

Properties of GeSi Nanocrystals Embedded in Hexagonal SiC

In this paper high-resolution electron microscopy investigations and molecular dynamics simulations of GeSi nanocrystals buried in 4H-SiC are performed, showing that the experimentally observed shapes of the GeSi nanocrystals are strongly correlated with their orientational relationships. Measurements of the lattice spacing suggest that the nanocrystals are strained. Quantum confinement in selected nanocrystals has been detected using spatially-resolved electron energy loss spectroscopy performed in conjunction with atomic resolution annular dark-field scanning TEM.

Spatially-resolved EELS analysis of multilayer using EFTEM and STEM

Spatially-resolved electron energy-loss spectroscopy (EELS) is applied to the multilayer (SiO2/Si3N4/SiOxNy/Si), a common semiconductor device structure. To observe depth profile at sub-nanometre spatial resolution, scanning transmission electron microscopy (STEM) and energy-filtering transmission electron microscopy (EFTEM)-based techniques are compared in practical application. STEM-based EELS is useful in simultaneously analysing plural elements. EFTEM-based EELS is suitable for analysing the chemical shift and core loss intensity of single element. The practical spatial resolution is of the same order for each technique, both involving beam damage, which can be reduced by decreasing current density in EFTEM and avoiding beam overlapping in STEM.

Fourier images feature of lattice fringes formed by low-loss electrons as observed using spatially-resolved EELS technique.

The lattice fringes of inelastically scattered electrons were investigated using spatially-resolved electron energy-loss spectroscopy (EELS) technique. The intensity profiles of lattice fringes formed by low-loss electrons were observed, and their position and contrast were elucidated with high spatial resolution. It was found that the conventional transmission electron microscope image includes low-loss images, which are the integrated image of through-focus images weighted by the intensity of the EEL spectrum. The modulation feature in the lattice fringes of inelastically scattered electrons is similar to the Fourier images. It means that the exit wave field of elastically scattered electrons is almost preserved in that of low-loss electrons.

Epoxy Infiltration into Nanoporous Aluminum Oxide

This research uses spatially-resolved electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) to study epoxy infiltration into a nanoporous aluminum surface oxide. Imaging by scanning electron microscopy (SEM) shows that the oxide surface of an as-anodized aluminum wire consists of columnar nanopores with diameters ranging from approximately 5 -150 nm. Anodized wires were embedded in a 100 g: 28 g mixture of DGEBA (diglycidyl ether of bisphenol-A) resin and PACM20 (bis(p-aminocyclohexyl)methane) curing agent followed by a two-step cure. Electron-transparent sections were cut by ultramicrotomy. Spatially-resolved carbon and oxygen EELS profiles from the oxide are anti-correlated indicating that oxide pore walls are separated by pore interiors containing epoxy. Spatially-resolved low-loss spectral data are transformed into a measure of apparent specimen thickness. Comparisons of such data with simulations based on experimentally derived oxide topologies indicate that the pores are fully filled.

Structural and Chemical Widths of General Grain Boundaries: Modification of Local Structure and Bonding by Boron-Doping in β-Silicon Carbide

Using a hot-isostatic-pressed boron-doped silicon carbide (SiC) material as an example, we demonstrate that the structural width and the chemical width of general boundaries may be quite different. The high-resolution electron microscopy (HREM) observation did not detect the existence of ∼1 nm thick amorphous film at such grain boundaries (GB). There is only a core structure of 1–2 atomic planes at GB. The chemical width of GB, obtained by the spatially-resolved electron energy-loss spectroscopy (EELS) analysis, is visibly wider than the core region. Furthermore, the spatially-resolved energy-loss near-edge structures (ELNES) analysis not only revealed the chemical bonding between boron and carbon, oxygen and silicon, but also distinguished an extended GB region with chemical bonding modified from that of the grain interiors. Such ELNES analysis defines an ELNES width that is even wider than the chemical width. The three GB widths of different scale construct a comprehensive picture of general boundaries that is remarkably different from general boundaries with amorphous film. Instead of a film confined by the two atomically sharp grain surfaces, there is only one interface, the rough GB core having most of the B–C and Si–O bonds, and the extended grain surface layers, to form such general GB in B-doped SiC.

Effects of Fast Secondary Electrons on Spatiallyresolved Low-Loss Eels of Polystyrene

Abstract A good understanding of the effect of electron irradiation on polymers is necessary in order to optimally utilize the spectroscopic information and resolution of spatially-resolved electron energy-loss spectroscopy (EELS). This investigation studies the effect of electron irradiation on the low-loss spectroscopic signal and spatial resolution obtainable from polystyrene (PS) homopolymer. Because of the conjugated valence electron distribution associated with its pendant phenyl ring, polystyrene is relatively stable under electron irradiation and has well characterized spectroscopic fingerprints including a notable π—π* transition circa 7eV (1). In addition, polystyrene is used as a positive photoresist because it can cross-link effectively when exposed to an electron irradiation (2). The critical dose characterizing degradation of aromatic polymers is of the order 1-10 C/cm2 (3). In practice, the dose delivered to a specimen is determined both by electron probe size and probe current.

Profiling Pvp/Ps Homopolymer Interfaces Using Core-Loss Electron Energy-Loss Spectroscopy

Abstract Despite the physical and technological significance of polymer interfaces (1), they have not been extensively studied using techniques based on electron scattering. Spontak et al. (2) used digital analysis of bright-field images to study Os-stained polystyrene-polydiene block copolymer interfaces, but work in this area has been lacking largely because of difficulties with staining or uncertainties in the distribution of stain at the nanometer length scales relevant to interfaces. Studies of polymer interfaces are now largely done by neutron scattering (3) This research explores the application of spatially-resolved electron energyloss spectroscopy to measure compositional widths of polymer-polymer interfaces. These first studies focus on blends of poly(2-vinyl pyridine) (PVP) dispersed in poly(styrene) (PS) to establish the various electron-optical, electron-scattering, and specimen-dependent limitations (4) to resolving such an interfacial width. The uncompatibilized PS/PVP interface should be relatively sharp. Blends of PS (M.W. 190,000) and PVP ( M.W. 200,000) were made using a 5wt% solution of polymer in THF with a PS/PVP ratio of 7/3. The solution was cast into methanol. The resulting solid precipitate was removed and annealed under vacuum at 120°C for 36 hours.

Measuring Polymer Microstructure using Spatially-Resolved Eels in the Stem

ABSTRACTImage contrast for the examination of multiphase polymers in the transmission electron microscope (TEM) usually requires differential staining by a heavy element (Os, Ru, U). Staining methods have provided a wealth of microstructural information in polymers, but there are situations, particularly where high resolution is needed, where staining is undesirable or impossible. This research has collected microstructural information from multiphase polymers without heavy-element stains using spatially-resolved electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). The technique is known as spectrum imaging. The principal problems facing spectrum imaging in polymer applications are: (1) the identification of spectral fingerprints distinguishing different polymer phases; and (2) the extraction of meaningful microstructural data from large data sets where the signal is weak due to instrumentation and materials constraints. This paper describes applications of spectrum imaging to PE/PS and HDPE/Nylon 6 blends. The aim is to identify adequate spectral features and establish data acquisition and extraction procedures for applications to general, unstained, multiphase polymers.

A quantitative approach for spatially-resolved electron energy-loss spectroscopy of grain boundaries and planar defects on a subnanometer scale

A quantitative approach for spatially-resolved electron energy-loss spectroscopy (SREELS) is demonstrated by investigating grain boundaries and planar faults in ceramics. This approach combines spatially-resolved energy-loss near-edge structure (ELNES), EELS quantification and associated spatial information on a subnanometer scale, and is based on an improved “spatial difference” method. This is a quantitative “spatial difference” which analyses elements present at defects as well as in the bulk, and which is performed with a systematic procedure to subtract completely the signal of the bulk based on the knowledge of ELNES for reference systems. Criteria to prevent artefacts are highlighted. The processed spectrum is dedicated to a defect, and may include signals from more than one element. Spatial information associated to the defect, such as the chemical width of a grain boundary, is obtained from quantification of the spectrum. Applying this approach to linescans (“Spectrum-Line”) not only achieves very high spatial resolution, but also provides an effective probe size. A spectrum for a planar fault of 0.22 nm width was obtained.

Nanoscale Modification Of Amorphous Diamond-Like Carbon Film Surfaces

ABSTRACTScanning Tunneling and Atomic Force Microscopy (STM, AFM) of amorphous tetrahedral carbon films (a-tC) film surfaces grown by pulsed laser deposition indicate extreme flatness and uniform electronic properties.1 This is consistent with the observed predominance of sp3 (diamond-like) bonding in these materials.2 Potential applications of a-tC films may be enhanced with the ability to modify their surfaces on the nanoscale. Exploiting the metastable nature of the sp3 hybridized state of carbon, the high flux electron beam from an STM tip was used to modify ˜100 nm regions; processing in air or in vacuum produces similar results. STM and AFM maps of the modified regions indicate stable morphologic and electronic structures consistent with a local transformation to sp2 (graphitic) hybridization. Surface potentiometric mapping of these regions indicates a correlation between electron dose and a lowering of the surface potential. In addition, spatially-resolved electron energy loss spectroscopy of the modified areas performed with a Scanning Auger Microscope shows plasmon energy shifts that confirm an elevated sp2 content in the modified regions.3