SCANNING ELECTRON MICROSCOPY (SEM)
ENERGY DISPERSIVE SPECTROSCOPY (EDS)
Scanning Electron Microscopy (SEM) Energy Dispersive Spectroscopy (EDS) is a combined analytical methodology used to visually capture and represent a detailed image of the substance while qualitatively identifying the elemental make-up of the sample. Some of the information gathered by using SEM-EDS includes qualitative and semi-quantitative chemical analysis, external topography/morphology, chemical composition, crystalline structure and 2-D/3-D image generation.
SEM-EDS instrumentation consists primarily of:
- Electron Source
- Electron Lenses
- Sample Stage
- Solid-state Detector
- Display Monitor
The SEM can provide magnification up to 500,000 times, which is about 250 times better than the best standard microscope. This technology uses electron displacement to produce signals related to the topography of the material and convert these into an image. From pollen grains to blood cells this instrumentation is able to capture a precise picture of a sample and combined with EDS can be used to evaluate the exact composition of a sample.
SEM of pollen grains magnified x500 from the Dartmouth Electron Microscope Facility
EDS Spectral Graph
The EDS works by detecting x-rays characteristic of different elements and arranging them in an energy spectrum which EDS software then analyzes to define and semi-quantify the composition. This is effective for sample sizes as small as a few microns. The electromagnetic waves of the x-rays are extremely difficult to detect as they are only about 0.01 to 100 Angstroms (one Angstrom is a ten-billionth of a meter 1/10,000,000,000).
SEM-EDS is a particularly useful combination when studying unknown materials as imaging reveals some primary characteristics and the signal analysis provides other elemental specifics. Routinely this is used to create high-resolution images of items and to show variations in chemical compositions. EDS provides elemental maps of the sample. Applications for this instrumentation are broad but SEM-EDS is commonly used in geological studies as well as elemental analysis of unknown samples, or contamination testing and failure analysis of substances such as adhesives and coatings. This technology’s value is also in that it produces data quickly, with many images taking less than 4 min to complete.
How does SEM-EDS work?
SEM-EDS works by capturing data revealed by various atomic interactions. The SEM electron beam penetrates the sample on an atomic level and causes various electron displacements to occur, each type of displacement emits electrons or x-rays depending upon the displacement which in return are separated and analyzed to produce the image and elemental map. The three types of electrons that SEM-EDS is primarily concerned with are:
Secondary Electrons – inelastic electrons of low energy that are ricocheted from the sample. These electrons escape their shells and those that are not absorbed stay near the surface and are what provide the topographical information for the 2-D and 3-D images.
Backscattered Electrons – elastic electrons that have collided with the primary electron beam and retained energy. Emitted from the atom at a larger angle than those from inelastic scattering. Inform as to the structure of the topological, crystallographical and compositional surfaces.
Auger Electrons – elastic electrons retaining energy from a collision with the electron beam. Kicked out of inner electron shell creating vacancy. Provide elemental molecular characteristic information.
Characteristic x-rays are emitted when an electron moves to a vacancy in a lower electron shell. The released electromagnetic radiation is elementally specific and thereby can be used to characterize the molecular make-up of the sample under scrutiny.
The EDS detector is what collects and separates these characteristic X-rays in order to identify and semi-quantify the elements in the sample. The photons of electromagnetic radiation are detected by the EDS and converted into a voltage signal proportionate to the original x-ray. Though this sounds simple, such a conversion involves three separate steps. The first step in converting the x-ray energy into the final signal is to transform the energy into a charge through the ionization of atoms in the semiconductor. The semiconductor absorbs the incoming x-ray’s energy and becomes a conduit for the charge. After the x-ray absorbed in such a way as to create this charge bias, it must then be transferred to voltage via the field effect transistor preamplifier. Finally, the electric pulse is processed by the pulse processor to measure the element characteristic signals and plot the voltage ‘ramp’ for an analyst to use in identifying the compounds’ composition.
SEM-EDS is considered to be an extremely valuable analytical methodology as the process is “non-destructive”, allowing for the same sample material to be analyzed repeatedly. In addition to there being no loss of sample volume, the SEM-EDS provides a fairly comprehensive understanding of the material under analysis. This specificity is an invaluable resource to chemical analysis especially as the data provides both image rendering and chemical characterization.