Forensic Physics
Scanning Electron Microscope

Scanning Electron Microscope

Scanning Electron Microscope (SEM) is a type of electron microscope that produces a three-dimensional image of a microscopic object. As the name suggests a microscope forms images by scanning the surface of the object with the help of a focused beam of electrons.

SEM was first developed in 1937 with the contribution of Manfred von Ardenne. However, the first commercial instrument was launched in 1965. 

SEM image reveals information like the morphology, crystalline structure, chemical composition, and orientation of materials/components of the sample.

SEM requires electrons that have low kinetic energy because the interaction only with the surface up to a few nanometers of depth is required rather than penetrating deep inside the sample.

Principle of Scanning Electron Microscope

SEM works on the principle that “When the accelerated electrons with specific amounts of kinetic energy interact with the surface of the sample, the energy is dissipated in the form of various signals produced by electron-sample interactions”.

The various kinds of signals produced in SEM result from the production of secondary electrons, back-scattered electrons, diffracted scattered electrons, x-ray photons, visible radiations, and heat.

Secondary Electrons

When the accelerated electrons interact with the sample, they transfer their energies to the outer shell electrons of the sample due to inelastic collisions. The outer shell electrons are weakly bound to the nucleus due to which they are easily ejected out. These ejected electrons are called secondary electrons which elucidate the morphology of the sample.

Back-Scattered Electrons

Some of the accelerated electrons interact elastically with the sample which results in the reflection of these electrons with high kinetic energy which helps in elucidating the crystalline structure of the sample. These electrons are called back-scattered electrons.

Diffracted Electrons

Sometimes the accelerated electrons pass very close to the sample atoms which may transfer the energy to the inner shell electrons. These inner shell electrons are ejected from the shell and are called diffracted electrons if the incident electrons have high kinetic energy. They determine the orientation of the materials in the sample.

Characteristic X-rays

The ejection of inner shell electrons produces vacant shells that lead to the emission of characteristic x-rays which helps in elemental analysis.

Visible Radiations

Usually, the incident accelerated electrons when interacting with the sample atoms which excites the lower energy valence shell electrons to the higher energy valence level. Then these electrons return to their ground state which emits luminescence in the form of UV, visible or infrared radiation. It is also called cathode-luminescence which is a characteristic property of a sample.

Instrumentation and Working of SEM

The instrumentation of the SEM is composed of the following components:

  • Electron gun
  • Magnetic lens
  • Condenser lens and Objective Lens
  • Specimen stage
  • Vacuum chamber
  • Detector
  • Recorder 

1. Electron Gun

An Electron gun produces the thermal electrons required for interaction with the sample. The gun is composed of a cathode made up of a thin tungsten filament and an anode made up of a metal plate. The cathode produces thermal electrons when it is heated at a high temperature due to the voltage applied.

The electrons are captured by the anode which passes the beam of electrons through the whole present in the middle of the anode plate. Another electrode called the Wehnelt electrode is placed between the anode and cathode which helps in focussing the current of the electron beam.

2. Magnetic Lens

The magnetic lens is used in SEM. When a direct current is passed through a coiled wire, a rotationally-symmetric magnetic field is formed and a lens action is produced on the electron beam. The strength of the magnetic lens can be changed by changing the electric current passing through the wire.

3. Condenser Lens and Objective Lens

Below the electron gun, a two-stage lens system is placed which is composed of a condenser lens and an objective lens. In between the two lenses, an aperture made up of a metal plate with a hole in its middle is placed.

When the electron beam passes the condenser lens, the aperture focuses the beam to the objective lens. The Condenser lens helps in adjusting the number of electrons that reach the objective lens. An objective lens is used for focusing and producing an effective electron probe.

4. Specimen Stage

The specimen stage is the platform at which the sample is placed to be analyzed. It is a movable stage that can move in horizontal and vertical axes. It can be tilted and rotated according to the needs.

5. Vacuum Chamber

SEM is a very sensitive instrument, the electron beam generated gets easily affected by the air particles,  therefore the optical system and the sample stage need to be evacuated properly. This is done by a high vacuum of 10⁻³ to 10⁻⁴ pascals. 

6. Detector

In SEM, there are different types of signals produced, therefore different detectors are required. For example:

  • Secondary electrons are detected by scintillated photomultiplier detector.
  • Back-scattered electrons are detected by scintillated or semiconductor detectors.
  • X-rays are detected by an x-ray spectroscope.
  • Cathode-luminescence is detected by the cathode-luminescence detectors having cathode ray tubes.

7. Recorder

The signals produced by the detectors are amplified and then passed to the display unit. Earlier the signal used to be displayed by the cathode-ray tube but now with the advancement of technology the signal is displayed in the digital format.

Advantages of Scanning Electron Microscope

  • High magnification of 10,000x
  • High resolution of 0.4 nanometers
  • Highly sensitive
  • It produces a 3D high-quality image of the object
  • Produced signals in digital format which is easily portable
  • Usually easy to operate with user-friendly “intuitive” interfaces.

Disadvantages of Scanning Electron Microscope

  • The instrument is expensive and requires high maintenance
  • The size of the instrument is large therefore huge spacing is required
  • The interference of an electron beam with an air particle may disturb the signals.
  • Only solid samples can be analyzed
  • Sample preparation is a laborious procedure
  • High voltage is required for the operation of SEM

Applications of Scanning Electron Microscope

  • Biological Industry– SEM is a widely used technique in cancer research, microbiology and virology field, and material sciences.
  • Chemical Industry– SEM is used in semiconductor research, nanotechnology, pollution control, the crystalline structure of minerals in mineralogy, etc.
  • Forensic Science – In forensic science, SEM is used for the analysis of paints, fibers, hair, gunshot residue, firearm identification, diatom examination, handwriting examination, gemstones, jewelry examination, etc.

Conclusion

Scanning Electron Microscope is a highly sensitive qualitative and quantitative analytical instrument that has a high magnification power of 10,000x and high resolution. It is a huge instrument that gives a three-dimensional image of the analyte. However, the image is not colored but has high quality. 

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