Scanning Electron Microscopy (SEM)

Principle of Scanning Electron Microscopy (SEM)

• SEM scans a focused electron beam across the specimen’s surface. Secondary electrons emitted from the specimen surface are collected to form an image.
• SEM provides high-resolution, three-dimensional images that reveal the specimen’s surface topology.

Procedure for Scanning Electron Microscopy (SEM)

• Specimen Preparation:

  • Fixation: The specimen is fixed to preserve its structure, similar to TEM.
  • Dehydration: Critical point drying is used to prevent the collapse of structures caused by surface tension.
  • Mounting: The specimen is attached to an aluminum stub using conductive adhesives.
  • Coating: A thin layer of conductive metal (e.g., gold or platinum) is sputter-coated on the specimen to prevent charging and improve signal quality.

• Operation of the SEM:

  • Vacuum System: The specimen chamber is evacuated to reduce electron scattering.
  • Electron Source: An electron beam is generated using a thermionic or field emission gun.
  • Beam Focusing: Electromagnetic lenses are used to focus the electron beam into a fine spot.
  • Scanning: The electron beam is raster-scanned over the specimen’s surface.

• Detection and Imaging:

  • Secondary Electron Detector: Secondary electrons emitted from the specimen are collected.
  • Image Formation: The intensity of these electrons is used to form a grayscale image displayed on a monitor.

• Image Processing:

  • Adjustments: Brightness, contrast, and focus are modified for optimal image quality.
  • Data Capture: Images are saved digitally for further analysis.

Applications

• Surface Morphology: Studying the texture and topography of materials.
• Biology: Examining surface structures of cells and tissues.
• Nanotechnology: Visualizing nanoparticles and nanostructures.

Advantages of Scanning Electron Microscopy:

• High-resolution imaging of surfaces.
• Depth of field allows for 3D-like visualization.
• No need for ultra-thin specimens.

Limitations of Scanning Electron Microscopy:

• Internal structures cannot be viewed.
• Non-conductive specimens require a conductive coating.
• Specimens must withstand vacuum conditions.

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