2.2 Electron Microscopic Methods
A microscope is any device that allows details detection that it is impossible for the human eye. The main difference between optical and electron microscopes is their resolution. In 1931, the first electron microscope was developed. Its operation was based on the classic optical microscopy, but instead of visible light as source, it uses electron beams. Electron microscopy techniques have found diverse applications in adsorbent study for real-space imaging and structural analyses (by electron diffraction techniques). Scanning Electron Microscopy and Transmission Electron Microscopy are described:
2.2.1 Transmission Electron Microscopy (TEM)
A photo of the TEM JEOL 2010 microscope in the University of Bristol.
Transmission Electron Microscopy is one of most powerful techniques in materials science, which is widely used in the characterization of adsorbent. It has ability to examine the constitutional characteristics of the adsorbent such as shape and size, crystallinity and chemical variations at a resolution down to the nanometer scale. With advanced design, modern TEM enables lattice defects, atoms and even their movements to be seen.
In terms of its construction, a general TEM usually consists of six basic components, as follows:
1) Source providing illumination
3) An optical system
4) A sample chamber
6) Vacuum system.
The analysis capacity of TEM has been significantly enhanced by integration of several advanced techniques into the instrument. These techniques include spectrometers, such as energy-dispersive X-ray analysis (EDX) and electron energy loss spectroscopy (EELS).
To examine materials by TEM requires a sample that normally should be less than 3 mm in diameter with the area of interest sufficiently thin to allow electrons to penetrate it.
Schematic diagram of a Transmission Electron Microscope
2.2.2 Scanning Electron Microscopy (SEM)
A photo of FESEM JEOL 6300 FEG microscope in University of Bristol.
In adsorbent study, SEM is a tool that is commonly used to analyse adsorbents’ morphology. If compared to TEM techniques, SEM has less resolution. However, SEM has good enough resolution for analysing adsorbents (up to 0.1 µm) and provides three-dimensional images.
The operating principle of a scanning electron microscope is similar to TEM, the main differences lay in how electron beams are used to hit the sample and how reflected particles are converted into images. SEM operation is based on a high-energy beam of electrons ranging from 5-50 kV that impact a solid bulk, i.e. the sample. The beam runs through lenses system where it is condensed (focused); according to the principles of electronic microscopy, the smaller the beam the better resolution because the energy is concentrated in a smaller surface. Also, a coil arrangement magnetically deflect the incident beam, responsible for scanning the electrons on the specimen, line by line and point by point. In this case, the electrons that are not absorbed by the sample, but are bounced in different directions and with different characteristics as such backscattered electrons and X-rays, Auger electrons, secondary electrons among others. The image of the sample is usually generated by secondary and backscattered electrons, meanwhile, the X-rays are used for elemental identification.
These bounced particles, produce different type of signals due to their different nature and properties like density and chemical identity. Finally, signals are collected, amplified and transformed into visual images on a cathode ray tube, similar to that of televisions.
The requirement for samples being analyzed under electron microscopes, is that they can conduct electricity, for non-conductive materials, they are usually coated with thin films of conductive metals such as gold. The figure below shows principal constituents of SE microscope.
Diagram of main constituents of the Scanning Electron Microscope
In summary, an SEM consists of three distinct parts: an electron column; a detection system; and a viewing system. Two electron beams are controlled simultaneously by the same scan generator: one is the incident electron beam; the other is for the cathode ray tube (CRT) screen. The incident beam is scanned across the sample, line by line, and the signal from the resulting secondary electrons is collected, detected, amplified and used to control the intensity of the second electron beam. Thus a map of intensity of secondary electron emission from the scanned area of the sample will be shown on the CRT screen as variations in brightness, reflecting the surface morphologies of the specimen. Given this mechanism, the magnification of the SEM image can be adjusted simply by changing the dimensions of the area scanned on the sample surface.
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