A Scanning Electron Microscope (SEM) is a powerful tool used in various fields such as biology, materials science, and nanotechnology to obtain high-resolution, 3D images of the surface of specimens. It works by scanning a focused electron beam over the surface of a sample and detecting secondary electrons emitted from the sample.

Structure of SEM

1. Electron Gun (Source):

• Function: Generates and emits the electron beam. It typically uses a thermionic emitter (such as tungsten) or a field emission gun (FEG) for higher resolution.
• Components: 

The cathode (which heats up to release electrons).

The anode (which accelerates the electrons).

Electrostatic lenses (to focus and direct the beam).

2. Beam Focusing System:

• The electron beam is focused into a very fine spot using condenser lenses and objective lenses. This ensures high resolution by keeping the electron beam sharp and well-focused.

3. Scanning Coils:

• Function: Control the movement of the electron beam across the specimen surface in a raster scan pattern (left to right, top to bottom).
• Components: Magnetic coils that deflect the electron beam in both horizontal and vertical directions.

4. Specimen Stage:

• Function: Holds and moves the specimen under the electron beam. It is usually adjustable in multiple directions (X, Y, Z axes) and may be tilted to obtain different viewing angles.
• Special features: Some stages can handle temperature control or allow for analysis in different environments (e.g., environmental SEM for wet samples).

5. Detectors:

• Secondary Electron Detector (SED): Detects low-energy secondary electrons emitted from the surface of the sample, which provides high-resolution surface topography.
• Backscattered Electron Detector (BSE): Detects high-energy backscattered electrons, giving information about the sample’s composition and atomic number contrast.
• X-ray Detector: Used for Energy Dispersive X-ray Spectroscopy (EDS), providing elemental composition of the sample.
• Others: Various detectors can be attached depending on the specific type of analysis (e.g., Electron Backscatter Diffraction (EBSD)).

6. Vacuum System:

• Function: SEM operates in a high vacuum environment to prevent the scattering of electrons by air molecules.
• Components: Includes a primary and secondary vacuum chamber and a pumping system (e.g., turbo molecular pumps, rotary pumps).

7. Computer System:

• Function: Controls the microscope, processes the data from the detectors, and generates the images. The system allows operators to adjust parameters like magnification, contrast, and scan speed.

Function of SEM

1. Electron Beam Interaction:

When the electron beam strikes the specimen, several interactions occur, including: 

• Secondary Electron Emission: Low-energy electrons are ejected from the sample’s surface, which helps in imaging the topography.
• Backscattering of Electrons: High-energy electrons are deflected back from the sample’s surface, providing information on the atomic composition.
• X-ray Emission: High-energy electrons interact with atoms in the sample and cause them to emit X-rays, which are characteristic of the elements present.

2. Imaging:

• The secondary and backscattered electrons are collected by the detectors, and a signal is sent to a computer system that processes and generates an image.
• The resulting image is displayed in black and white or false color, with the brightness and contrast representing the number of emitted electrons or their energies.
• The SEM produces highly detailed, high-resolution images of the sample's surface morphology and texture.

3. Resolution:

• SEM offers high magnification (typically between 1,000x to 1,000,000x) and excellent depth of field, providing a three-dimensional view of the sample's surface.
• The resolution depends on the electron beam size, the quality of lenses, and the sample's properties, with modern SEMs reaching resolutions below 1 nanometer.

4. Elemental Analysis:

• Using Energy Dispersive X-ray Spectroscopy (EDS) or Wavelength Dispersive X-ray Spectroscopy (WDS), SEM can provide elemental analysis by detecting characteristic X-rays emitted from the sample when bombarded by electrons.
• This allows for qualitative and quantitative elemental composition information.

5. Surface Characterization:

• SEM is particularly useful for studying surface features such as cracks, pores, textures, and morphology at the nanoscale.
• It is commonly used to examine biological specimens, metal alloys, semiconductors, and nanomaterials.