The electric transport of high-temperature superconductors, such as YBa2Cu307-x (YBCO), can be strongly restricted by the presence of high-angle grain boundaries (GB). This weak-link behavior is governed by the macroscopic GB geometry and the microscopic grain boundary structure and composition at the atomic level. Whereas grain boundaries present a considerable impediment to high current applications of high Tc materials, there is considerable commercial interest in exploiting the weak-link-nature of grain boundaries for the design of microelectronic devices, such as superconducting quantum interference devices (SQUIDs). The Josephson junctions which form the basis of this technology can also be formed by introducing artifical barriers into the superconductor. We have examined both types of Josephson junctions by EM techniques in an effort to understand the connection between microstructure/chemistry and electrical transport properties. This knowledge is a valuable resource for the design and production of improved devices.

High electron mobility structures have been built for several years now using strained silicon layers grown on SixGe(1-x) with x in the 25-40% range. In these structures, a thin layer of silicon is grown between layers of unstrained GeSi alloy. Matching of the two lattices in the plane of growth produces a bi-axial strain in the silicon, splitting the conduction band and providing light electron levels for enhanced mobility. If the silicon channel becomes too thick, strain relaxation can occur by injection of misfit dislocations at the growth interface between the silicon and GeSi alloy. The strain field of these dislocations then gives rise to a local potential variation that limits electron mobility in the strained Si channel. This study seeks to verify this mechanism by measuring the absolute conduction band shifts which track the local potential near the misfit dislocations.

Electrical phenomena such as positive temperature coefficient of resistance (PTCR), grain boundary layer capacitance, and varistors are controlled by Schottky barriers to current transport formed in grain boundary regions. These grain boundary potential barriers are a result of charged defect accumulation, either through equilibrium segregation or kinetic diffusion processes. Recent work on Nb and Mn doped BaTiO3 highlights the importance of substitutional defects which activate grain boundary trap states and give rise to the PTCR effect. The dynamic interaction of charge carriers with such trap states also produces the rapid electrical breakdown characteristic of varistors. This study explores the issues of defect species, space charge, and electrical transport in donor doped SrTi03 by electron holography and electron energy loss spectroscopy.

The specific defect species and mechanisms of current transport through donor doped SrTiO3 remain unresolved because of the difficulty in connecting defect distributions (structural information) and resultant Schottky barrier effects (chemical information).

There is much current interest in the magnetotransport properties of systems consisting of two (or more) magnetic metal layers separated by thin insulating layers. Traditional magnetic tunnel junctions (MTJs) are comprised of simple ferromagnet-insulator-ferromagnet trilayer structures and the conductance depends on the relative alignments of the magnetizations in the two ferromagnets. In the case of discontinuous metal/insulator multilayers, negative magnetoresistance (MR) results from spindependent tunneling. In both types of systems, the tunneling phenomena are strongly influenced by the microstructure of the films, particularly the metal/insulator interfaces and the nature and uniformity of the thin oxide barrier layers. In this study, we have used cross-sectional HREM to characterize a variety of magnetic tunnel junctions and discontinuous multilayers.

The MTJs were prepared by rf and dc magnetron sputtering onto thermally oxidized (100) silicon wafers at room temperature. The magnetic layers consisted of thin films of Co, Fe and/or CoFe alloys with thicknesses ∼ 30-50nm, and the barriers included MgO, HfO2, CoO, SiO2 as well as Al2O3 (thicknesses in the range 2-10nm).

Three aspects of radiation damage are of concern to electron microscopists: changes in crystallographic or molecular structure, mass loss and change in chemical composition. Structural change can be monitored from the fading of diffraction patterns or from loss of fine structure in an energy-loss spectrum. Total mass loss, in the form of a reduction in inelastic-scattering power, can be observed from the low-loss spectrum. Mass loss can also be monitored from energy-loss ionization edges, with the advantage that the loss of particular elements can be studied separately. It is possible to assign a characteristic dose De for the disappearance of a particular element.

At room temperature, the amount of damage usually depends on the accumulated dose (exposure) but not on the dose rate (current density). However, cooling the specimen tends to reduce mass loss, probably because of the reduced diffusion coefficients.

1. Introduction

The definition of resolution as introduced by Lord Rayleigh [1] is related to the width of the point spread function of the imaging device. In this definition, noise has not been taken into account. Another definition of resolution has been introduced by Rose [2] in the field of radar and TV. Here the resolution is defined in terms of the dose (D) (i.e. number of imaging particles per unit area) and the signal to noise ratio SNR (i.e. the minimal contrast) A third definition of resolution is based on the idea that the microscope is a communication channel between the object and the observer. The resolution can then be rephrased as the amount of information that is transmitted by the channel in the sense as defined by Shannon [3] as a number of bits per unit area. This definition however does not describe how this information can be deduced and what its precision is.

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.

Semicrystalline block copolymers containing one crystalline block, such as PCL, show more complicated morphologies than amorphous block copolymers. This is because the microstructure of semicrystalline block copolymers is determined by the interplay between the microphase-separation temperature, crystallization temperature, and glass transition temperatures of the respective blocks. This paper describes initial results of thermodynamic and kinetic effects associated with microphase separation and crystallization in as-cast semicrystalline (PS)0.62(PCL)0.38, (PB)0.2(PCL)0.8 and (PS)0.35(PB)0.15(PCL)0.5 block copolymers, where the subscripts denote the mass fraction of each block.

Semicrystalline block copolymers (PS-b-PCL with Mn=209,000 g/mol, PB-b-PCL with Mn=146,000 g/mol and PS-b-PB-b-PCL with Mn= 150,000 g/mol) were prepared by anionic polymerization. Films were cast from 5% (mass fraction) solutions in toluene into ∼lmm thick films. A relatively slow solvent evaporation over approximately a 10 d period was achieved by partially covering the dishes containing polymer solutions.

Phase segregation is important in determining the properties of many complex polymers, including polyurethanes. Achieving a better understanding of the links between formulation, chemical nature of segregated phases, and physical properties, has the potential to aid development of improved polymers. However, the sub-micron size of segregated features precludes detailed chemical analysis by most existing methods. Zone-plate based, scanning transmission X-ray microscopes (STXM) at NSLS and ALS provide quantitative chemical analysis (speciation) of segregated polymer phases at ∼50 nm spatial resolution. Image sequences acquire much more data with less radiation damage, than spot spectra. After alignment, they provide high quality near edge spectra, and thus quantitative analysis, at full spatial resolution.

Fig. 1 shows an image and spectra acquired with the NSLS STXM of a macro-phase segregated TDI polyurethane. Spectral decomposition using model polymer spectra is used to measure the local urea, urethane and polyether content.

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.

Determination of internal structure of complex polymeric materials often requires a variety of analytical techniques. One technique that we have developed and recently employed for the first time for quantitative analysis is x-ray linear dichroism microscopy. With sensitivity to the presence and orientation of specific chemical functionalities within polymer molecules, x-ray linear dichroism microscopy provides a powerful tool to probe the internal structure of such molecules. We have used this ability to quantitatively analyze the lateral orientational order (tangential and radial orientation of transverse sections) within different grades of poly(p-phenylene terephthalamide) (Kevlar) fibers.

Kevlar fibers are highly crystalline and possess a complicated internal structure that exhibits lateral radial orientation. Figure 1 shows x-ray micrographs of (a) Kevlar 149, (b) Kevlar 49 and (c) Kevlar 29 fibers imaged at a photon energy specific to the aromatic functionality.

We are using the term "Industrial Polymers" to refer to polymers [plastics] that are produced by the ton or (in the case of films) by the mile. For example, in descending order of world-wide use (tonnage), the top eight of these polymers are polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), styrene polymers (including polystyrene - PS, and acrylonitrile-butadienestyrene/ styrene-acrylonitrile - ABS/SAN), polyesters (PETP), polyurethane (PU), phenolics and aminoplastics.

Industrial polymers, which have been produced by the millions of tons for the last five decades and are of obvious social and economic importance, have been exhaustively characterized. Structural features which affect physical properties and indicate process variables have been studied by many techniques other than microscopy (x-ray diffraction, thermal analysis, rheology, chromatographies, etc.). Microscopy techniques for polymer characterization have been well documented. Our motivation to apply field emission (high resolution) scanning electron microscopy to the study of polymers is: (1) The application of low voltage, high resolution SEM to biological materials is well characterized.

Superabsorbent polymers are often designed with increased density of crosslinking in the outer layer of the particles in order to improve liquid retention under load. For efficient product design, it is desirable to directly measure the spatial variation in crosslink density. Typically employed techniques (such as solvent uptake or measuring the changes in various mechanical properties such as the modulus) do not provide spatially resolved crosslink density information. We have applied Scanning Transmission X-ray Microscopy to examine the swelling of inhomogenously crosslinked superabsorbent polymers in deionized water and salt water solution.

Scanning Transmission X-ray Microscopy (STXM) is an effective way to study the chemical and morphological character of polymers on a sub micron spatial scale.1 STXM image contrast is based on core electron excitation by x-ray absorption; an interaction that has remarkable chemical sensitivity. Beam damage is less than in TEM microscopes and samples can be examined in wet and in ambient conditions.

Diblock copolymers consist of two chemically different polymer chains (or blocks) joined by a covalent bond. Because of the incompatibility between the two blocks and connectivity constraints, diblock copolymers spontaneously self-assemble into microphase-separated nanometer size domains that exhibit ordered morphologies at equilibrium. Commonly observed microdomain morphologies in bulk samples are periodic arrangements of lamellae, cylinders, and spheres. Recent studies show that the thin films of such block copolymers are of great potential for many nanoapplications including nanolithography and template synthesis for inorganic materials. Here, we report TEM studies of osmylated and ozonated thin diblock copolymer films.

An asymmetric block copolymer was synthesized with a composition of 36 kg/mol for the poly(styrene)(PS) block and 11 kg/mole for the poly (butadiene)(PB) block such that the microphase separation produces PB cylinders in a matrix of PS. The films were spin-coated from solution onto thin silicon nitride windows and annealed above the glass transition temperature in vacuum.

Polymer electrolytes are a topic of interest because of their potential applications in electrochemical devices. However, very few systems exhibit both high conductivities and mechanical strength. Composite polymer electrolytes, composed of conventional polymer electrolytes and inert fillers, provide an avenue to enhance mechanical strength while maintaining ionic conductivity, if the particle sizes of these inert fillers are sufficiently small (< 1 μm in diameter). The determination of the morphology for these inert fillers in polymer matrixes was undertaken to further our understanding of ion transport phenomena in these composite systems. This paper presents an in situobservation of inert fillers in liquid media using environmental TEM.

Surface-modified fume silica was prepared by a two-step process. (1) SiCl4 was used to activate surface silanol (SiOH) groups and generate reactive SiCl groups on the surface of fume silica particles (Aldrich, 7 nm in diameter).

Atomic force microscopy (AFM) was introduced in 1986 by Binnig, Quate and Gerber. In this method, a sample is scanned beneath a small, sharp silicon or silicon nitride probe attached to the apex of a flexible cantilever. Cantilever deflection is measured to give height information corresponding to the sample topography. Since AFM relies on tip-sample force interaction, the technique can be applied to insulators as well as to conducting and semiconducting materials. AFM therefore extends local probe studies to an important class of materials which can be difficult to investigate by electron microscopy and spectroscopy techniques owing to problems with sample charging. Among other materials, AFM has been used extensively to characterize the morphology, roughness, nanostructure, chain packing and conformation of polymer surfaces at the nanometer scale.

Early AFM studies of polymers were conducted in the contact mode and included the investigation of polymerized monolayers of n-(2-aminoethyl)-10,12-tricosadiynamide (AE-TDA) and poly(octadecylacrylate) (PODA) at submonolayer coverage.

Scanned-probe microscopy (SPM) has found application to a variety of polymer systems, primarily for the examination of surface morphology and the assessment of surface properties. In fact, SPM is unique in its ability to probe both phase morphology and mechanical properties simultaneously. The examination of industrial elastomers and their blends by SPM techniques can provide morphological information historically obtained only by transmission-electron microscopy (TEM). Since contrast with the SPM can be generated by differences in mechanical response of the constituent phases, polymer blends which are difficult to etch or stain for TEM can be successfully examined. In many cases measurements of polymer morphology can be made more easily with SPM, since time-consuming steps associated with thin-section production and staining are not required. Contact and tapping-mode operation along with force modulation and phase measurement can be used to examine phase microstructure, phase morphology, filler type, filler distribution, and viscoelastic response in elastomer systems.

Carboxylated latex has become very important in the formation of polymer films. In order to study the film and its properties it is important to know the morphology of the latex that is forming the film. The latex for this study has been examined by transmission electron microscopy(TEM) using positive preferential stains such as ruthenium tetroxide (RuO4) and cesium hydroxide(CsOH); and uranyl acetate(UAc) as a negative stain.

The polybutyl acrylate(PBA)/ polymethylmethacrylate(PMMA) composite latex particles consist of a soft core phase and a hard second phase with varying amounts of acrylic acid in the core, the shell and in both core and shell. The latexes were examined before and after cleaning. The cleaning was necessary in order to remove the surfactant and any small particles that might have formed during the emulsion polymerization. Prior to cleaning, the small particles adhered to the surfaces of the particles making it difficult to determine the final morphology of the latex particles.

The study of wetting and dewetting in thin polymer films has many implications for technological applications of polymer films, including dielectric films to control conductivity and colloidal paint systems. We have started to use a novel approach, Near Edge X-ray Absorption Fine Structure (NEXAFS) microscopy, to characterize dewetting in thin polymer films. We have investigated a polystyrene/brominated polystyrene (PS/PBrS) bilayer model system with the Scanning Transmission X-ray Microscope (STXM) at the National Synchrotron Light Source. Our method offers quantitative thickness mapping of all constituent components in these thin polymer films.

NEXAFS imaging of polymers has advantages over other forms of microscopy primarily because of its low damage, good chemical sensitivity, and the possibility for quantitative analysis. Figure 1 shows four micrographs of one PS/PBrS bilayer annealed for one week at 170 C.

In this work we apply the Hayashi-Nishi method for quantitatively analyzing the state of mixing of a secondary component in a polymeric matrix. We (1) evaluate the method using computer-generated morphologies and (2) compare the state of mixing of a core-shell impact modifier in polymers having differing affinity for the impact modifier. In the Hayashi-Nishi method two numbers, related to dispersion and distribution, are required to quantify the state of mixing of a secondary phase in a polymeric matrix. The 5-parameter in the Hayashi-Nishi method is defined as

The Ai's are the areas of each individual secondary phase particle or aggregate and nis the number of such particles in the image. S is related to the size distribution of these aggregate areas and is a measure of dispersion of the secondary phase in that particular image.