Principle
Scanning Electron Microscopy 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 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
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.