Scanning Electron Microscopy and Energy Dispersive x-ray Spectrometer (SEM-EDS)
|Analytical technique description:||
Scanning electron microscope (SEM) operates as a normal optical microscope having as light source a "light" characterized by an extremely low wavelength: the electrons. The electrons produced by a tungsten wire and finely focused by the action of Wehnelt electrode have an accelerating voltage variable between 0.3 and 30 KeV . Two-stage lenses, which combine the condenser and objective lenses, are located below the electron gun. The electron beam is focused by the two-stage lenses, and a small electron probe is produced. The microscope operates in high vacuum mode (HV - 10-2/10-3 Pa): in these conditions high magnification images can be obtained (up to 150/200 thousand), with a resolving power of about 1 nm. It can also operate in low vacuum mode ( LV ), the pressure can be set to 1Pa to several hundreds Pa. This condition allows observing and analysing non-conductive samples without any coating, even if the quality of images is lower.
SEM allows morphological studies of surfaces and of crystallographic phases, of organic and inorganic treatments and their penetration into the material. SEM, coupled with energy dispersive spectrometry (EDS), allows elemental analyses of the decay phenomena and mechanisms, of finishing and polychrome surfaces, of new-formed crystals.
When electrons enter the specimen, they are scatterered within the specimen and gradually lose their energy, then they are absorbed in the material. The scattering range of the electrons depends on the electron energy, the atomic number of the elements making up the specimen and the density of the constituent atoms. Various signals are emitted from the specimen: secondary electrons are generated at the top of the specimen and emitted outside the material. So they are very sensitive to the surface and are used to observe the morphology of the specimen surface.
Backscattered electrons are scattered backward and emitted outside the specimen, when the incident electrons are scattered in the specimen; since they have higher energy than secondary electrons, backscattered electrons provide information from a relatively deep region and are sensitive to the composition of the specimen.
X rays are characteristics of each element constituent the specimen. The energy dispersive spectrometer measures the energies of the X-rays emitted from the analyzed material returning spectra and distribution maps of the elements. The detection limit is around 1500-2000 ppm.
SEM: Jeol 5910 LV; EDS: IXRF-2000 (Milan Unit).