Materials Science


by Elaine F. Schumacher


Microscopy for Materials Characterization: Illuminating Structures With Light and Electrons


E


ngineered materials abound in products that we use every day. They can range from a bulk material like the glass in a window to a disper- sion of titanium dioxide nanoparticles in a sunscreen formulation. Every material, from the plastic in the sole of a running shoe to the semi- conductor chip in a laptop, has undergone a development process that includes an iterative cycle of characterization and performance testing. By gaining insight into the structure and composition of a material and link- ing that knowledge to properties that govern performance, designers can build better products while shortening development time.


Demands for improved product performance continually evolve. We want things that are smaller, lighter, faster, stronger, and greener. As performance specifications tighten, the need to understand and design materials moves from the bulk to the atomic level. Microscopy in all of its forms can provide insights into key material characteristics, elucidating size, texture, morphol- ogy, crystallinity (or lack thereof), elemental composition, and electronic state, at scales ranging from several micrometers to single atoms. Perhaps surprisingly, microscopes at two ends of the resolution scale, the light microscope and the transmission electron microscope, are very similar in function and in the types of information provided. A comparison of some key features will illustrate this similarity, and will also demonstrate why microscopy tools and techniques are central to materials characterization.


Comparing image formation in light and


electron microscopes Figure 1 shows schematic cross-sections of imaging modes for three types of microscopes regularly used for materials characterization: the light microscope (left), the transmission electron microscope (TEM, center), and the scanning electron microscope (SEM, right). Common elements include an illumination source (a source of visible light or an electron gun) and a condenser lens above the specimen to focus the light or electron beam. After the condenser lens, the configuration of the SEM diverges from that of the light microscope and the TEM. In the latter two, the objective lens that forms the image is positioned below the specimen, and a static, near- parallel beam of illumination is projected onto a viewing point or onto a camera for recording. Though the TEM incorporates some additional lenses, the overall process by which the image is created and projected for viewing and recording is identical to that of the light microscope.


The SEM differs significantly in that the objective lens sits above the speci- men, and the beam is formed into a fine, focused probe that is rastered across the specimen using scanning coils, building up an image point by point. The image can be recorded on various detectors positioned above the specimen.


These three types of microscopes provide complementary information over different size ranges. The unaided human eye can resolve features down to


Light


Microscope (LM)


Transmission Electron Microscope (TEM)


Light source (lamp) Condenser lens Condenser lens


Specimen Objective lens


Objective lens aperture


Objective lens


Intermediate lens


Specimen


Projector lens


Eyepiece Naked eye


Detector


Scanning circuit


Image amplifier


Scanning coil


Scanning Electron Microscope (SEM)


Electron source (electron gun)


Fluorescent screen


CRT © JEOL, Ltd.


Figure 1 – Schematic cross-sections of imaging modes in the light micro- scope, the TEM and the SEM (JEOL).


about 100 μm in size, and the best light microscopes allow us to see fea- tures as small as 1 μm. Because SEMs and TEMs utilize electrons, which have higher energies and shorter wavelengths than visible light, much smaller objects can be resolved. The highest-resolution SEMs can now distinguish features a few nanometers in size, while aberration-corrected TEMs allow for imaging and analysis at the atomic level. All three types of microscope can be used to provide fundamental information about size, morphology, and texture of materials, as shown in Figure 2. Though the SEM and TEM are typically grouped together as electron optical instruments, the TEM and the light microscope share a common mechanism for image formation.


Crystallographic analysis Interaction of energetic wave sources such as light, X-rays, or electrons with


the regularly spaced planes of atoms in a crystalline specimen generate patterns containing useful structural information. The type of pattern and the amount of detail provided about the crystal structure will vary with the technique used; some examples are shown in Figure 3. In an SEM equipped with a detector for electron backscatter diffraction (EBSD), a Kikuchi pattern that shows crystallographic orientation can be obtained; one common ap- plication is mapping of different orientations in a polycrystalline material. When coupled with elemental analysis by energy dispersive X-ray spec- trometry (EDS) in the SEM, a Kikuchi pattern can be indexed to identify the crystal system of a material. The diffraction mechanism, Bragg scattering, is the same as in the TEM, but the pattern is formed by electrons backscat- tered from near the surface of the sample. The pattern is captured on the fluorescent screen of the EBSD detector.


AMERICAN LABORATORY • 24 • SEPTEMBER 2014


Deflection coil


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