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Scanning electron microscopy (SEM) is a powerful technique for the observation of surfaces which uses a beam of electrons with specific energy to bombard the surface of the material to be analysed, making it possible to obtain images at very high magnifications. The interaction between the beam and the surface material provides a wide range of information which, once processed, turns into images, spectra and chemical analysis maps and images of phase composition, among others. When associated with a dispersive energy x-ray spectrometer (EDS) or wavelength detection (WDS), a quick and effective chemical characterisation can be performed in the regions observed with great geometric precision, using multiple software applications to combine this information. With additional devices it is also possible to use SEM to work in transmission electron microscopy mode (STEM), in which case the beam goes through the material in order to reveal other material characteristics.
SEM allows a high range of magnifications to be obtained and is particularly suitable for studying sample surfaces (when it is necessary to observe the inside of samples they can be cut or fractured). This technique facilitates the knowledge of some characteristics of solid materials, such as morphology, microstructure and surface topography, and can be applied to various materials: metallic, ceramic, polymeric, biological, construction materials or even archeological artifacts.
The application of this technique normally requires that materials are electrically conductive. If they are not, the process used in the preparation of samples is the sputtering on the surface of the sample of a very thin layer of gold (Au), carbon (C) or copper (Cu), in order to achieve the necessary conductivity of the surface. Nowadays, there are however devices that allow the observation of non-conducting materials without modifying the surface, for example the local charge compensation system (Charge Compensation) that introduces a gas at the observation point, facilitating the flow of electrons and avoiding the electrostatic charging of the sample. EDS and WDS systems allow the chemical composition of very small samples to be determined, enabling a timely analysis. Thus, while the SEM allows the user to view images, EDS and WDS allow for the immediate identification of their composition.
High-resolution Merlin microscope by ZEISS (FEG-SEM) with Charge Compensation system, external and In-Lens detectors for both secondary and backscattered electrons. Detector for STEM mode operation.
Chemical analysis systems for EDS and WDS from Oxford Instruments.
JEOL JSM-5310 microscope with tungsten filament and EDS from Oxford Instruments
As a result of electron beam interaction with the surface of the sample, different types of radiation are emitted in electronic scanning microscopy, namely secondary electrons, backscattered electrons, characteristic x-rays and Auger electrons. The volume of electron-matter interaction determines the resolution of the signal used in the characterisation of the sample. Images are formed point-to-point, which enables them to be viewed on screens and the acquisition of images in digital format.
When the electron beam hits a material using the EDS and WDS systems, the electrons are stimulated to higher energy levels. To return to the initial level position, these electrons emit energy in the specific wavelengths of the X-ray spectra of each material. Multiple detectors installed in the SEM chamber measure the energy associated with these electrons identifying them through their characteristic energy. This makes it possible to determine which chemical elements exist at the beam incidence point.