Microscopy is use of a microscope to magnify objects and to study those objects which are small enough to be visualised by naked eyes. The word microscopy came from Greek word micros meaning small and skopein meaning to look or to see. There are in general, three types of microscopy. They are-
Today, we will be discussing about electron microscopy. To be more specifically about Scanning Electron Microscopy (SEM). As the name itself indicates it is a type of electron microscopy so more or less electrons will be in use for studying objects with SEM.
The scanning electron microscopy technique uses a focused beam of high energy electrons to produce images of a sample. It does so by scanning the surface of the sample by the electron-sample interaction. The electrons interact with the atoms in the particular sample and produces various signals at the surface of solid specimen. These signals contains information about the surface topography and composition of the sample. The high energy beam of electron is scanned in a way called as raster scan pattern and and image is formed by the combined position of the beam and intensity of the detected signal. In most of the SEM, a secondary electron detector is used to detect electrons excited by electron beam. SEM was developed due to inability of efficiency of the wavelength of light microscope. Electron microscopes have comparatively very small wavelengths than that of conventional light microscope which enables better resolution power. The revolving power of an microscope is the ability of the lens to separate or distinguish small objects that are in close proximity.
Working Principle: The high energy beam of electrons in a SEM carry significant amount of kinetic energy which dissipates as variety of signals produced by the electron-sample interaction. An electron source or electron gun is necessary to produce high energy beam of electrons. The electron beam formed by the electron gun is accelerated towards the specimen by using a positive electrical potential. A metal aperture is used to confined and focused the beam of electrons. The beam is turned into a thin, focused and monochromatic beam with the help of magnetic lenses attached to the SEM instrument. The electrons in the beam interacts with the atoms of the specimen and produces signals than contain information about its surface topography, composition and other electrical properties. These interactions and effects are then transformed into an image. The SEM image includes secondary electron, backscattered electrons and diffracted backscattered electrons, photons, visible light and heat. Mainly secondary electrons and backscattered electrons are used to imaging samples. Secondary electrons are used to study the information about surface topography and morphology whereas backscattered electrons are used to provide a contrast among multiphase structures present in a sample. SEM analysis is considered to be ‘non-destructive’ in nature i.e., volume loss of the sample is insignificant when x-rays generated by the electrons interact with the sample.
Instrumentation: The components of a SEM includes following parts-
Electron gun: Electrons are produced with the help of electron gun or electron source at a voltage of 1-40kv. Three basic types of electron sources that are used in SEM viz. Tungsten filament, Lanthanum hexaboride, and Field emission gun (FEG).
Electron lenses: SEM has several condenser lenses that focuses the beam of electrons to a narrow, thin and monochromatic beam of electrons that form a spot called a spot size.
Scanning coil: A scanning coil is used to deflect the beam over the surface of specimen.
Detector: Detector is made up of various detectors that are able to separate secondary electrons, backscattered electrons and diffracted backscattered electrons. The voltage speed and density of the specimen highly influences the functioning of the detector.
Sample stage: A sample stage is used to place the sample or specimen to be observed.
Display device: The display device is used to visualise the data obtain from scanning the surface of the specimen.
Application of SEM: SEM has enormous applications in the field of chemistry, medicines, biology, industry etc.
B I B L I O G R A P H Y
Scanning electron microscopy by SERC
The applications and practical use of SEM
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PS The thumbnail image is being created by me using canva.com by taking template image from LabX
