- HITACHI S4300-CFE SEM with FEG source with a Bruker (SDD) EDX detector
- JEOL 2000 FX-II TEM with La6B/W source with an Oxford Si(Li) EDX detector
Scanning electron microscope (SEM)
In a scanning electron microscope (SEM), a fine beam of electrons is scanned across the surface of a specimen synchronously with an electron beam on the display screen. A detector monitors the intensity of a chosen signal from the specimen and the brightness of the display spots are modulated according to the detected signal.
It is important to know that the image formed in an SEM is not necessarily that of the surface. As the electron beam penetrates the sample, several signals appear (secondary, backscattered, and Auger electrons; characteristic and Bremsstrahlung X-rays; and even visible light). It is to control the depth to which the electrons penetrate by choosing the appropriate electron energy, and the type of emitted signal used to form the image.
Energy Dispersive Spectroscopy (EDS)
When the high energy incident electron beam bounces through the sample, it leaves thousands of the sample in an excited state atoms with holes in the inner electron shells. The atoms are not in a stable state, thus in order to stabilize the atoms, electrons from outer shells will drop into the inner shells. Since the outer shells are at a higher energy state, the atom must lose some energy. It does this in the form of X-rays. The X-rays emitted from the sample atoms are characteristic in energy and wavelength too. Essentially, each element has characteristic X-ray line(s) that allow a sample’s elemental composition to be identified by a nondestructive technique. Since the X-rays are formed by the electron beam interaction with the sample surface, what ever area of the sample being imaged is analyzed. This allows the SEM to perform elemental analysis in very selected areas as small as 1/2 a micron in size. EDX analysis can also quantify the elements it detects. A quantitative analysis can be performed by a standards or standardless analysis.
Restrictions on samples:
The sample material must be able to withstand a high vacuum environment without outgassing. It must be clean. If the sample is nonconductive (plastic, fiber, polymer), it may be coated with a 20 – 100 nm layer of gold. The instrument can accommodate relatively large samples (specimens up to 160 mm diameter examined) on a five axis, motorized eucentric stage.
Transmission electron microscope (TEM)
In a transmission electron microscope (TEM) the emitted electrons are accelerated through an electromagnetic field. The beam is then formed by condensor lenses and passed through the sample material. The sample is a very thin (less than 100nm) slice of material. The electrons that pass through the sample enter into the objective lense and the image composed by the lens is magnified by several projector lenses. The electrons then hit a phosphor screen, CCD or film and produce an image. On those places where the sample has less density, more electrons get through and the image is brighter. A darker image is produced in areas where the sample is more dense and therefore less electrons pass through.
Restrictions on samples:
The sample material must be able to withstand a high vacuum environment without outgassing. It must be clean. The specimen has to be thin. If it is not, it has to be thinned. In case of solid, bulk material mechanical thinning and polishing is a common methode down to 50-30 micrometer (depending on the sample material). Further thinning is made by ion-beam milling. If the sample is nonconductive, it may be coated with conductive material like carbon.
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Contact: Dr. Csaba Cserháti