Principle:

Transmission Electron Microscope was introduced by Max Knoll and Ernst Ruska in the year 1931. Commercial transmission electron microscope developed in the year 1939.Transmission electron microscope is a technique in which electrons passes through the specimen slide. The transmitted electrons can be magnified when electrons interact. The magnified image can be seen via fluorescent screen or with the help of sensors like CCD (Charge Coupled Device) camera. The smaller TEM image magnification is due to electron absorption, material thickness and composition. Higher TEM image magnification is due to wave interactions.

Optics:

TEM converge the image with the help of lens. There are three stages of lens namely

• Condenser lens: Forms a primary beam
• Objective lens: Focus the beam on sample
• Projector lens: Magnify the beam on imaging device

 Display of TEM Image:

It has a phosphor screen made out of zinc sulphide of 10 – 100 µm in size. If required it records the image as film or doped with yttrium aluminum garnet screen with CCD.

Components of TEM:

TEM has following components

• Vacuum system
• Specimen holder
• Electron gun has filament, Wehnelt cap, biasis circuit and anode
• Electromagnetic lenses
• Electrostatic plates or apertures

Vacuum system: vacuum is created to reduce the frequency of collision of electrons with other gas atoms and to permit difference in cathode voltage and ground voltage to avoid the arc formation. Hence TEM is evacuated at very low pressure with the help of diaphragm pump or rotary vane pump. Diffusion pump increases vacuum level for an effective operation. It is referred as mean free path. There should be re-evacuation of TEM as the specimen holders are replaced on a continuous basis.

Specimen holder: Specimen stage is designed with airlocks. Grid diameter size of specimen is 3.05mm and mesh size is maximum 100 µm. Grid is made of Cu, Mo, platinum or gold.

Electrogun: It has several basic components. When filament is connected to a power supply of negative charge, electrons come out of electron gun. It passes through anaode plate and column. Wehnelt cylinder has a more negative charge compared to filament converges the beam.

Electromagnetic lens: It can be electrostatic or magnetic in operation. Coils generate magnetic fields within the lens. Electromagnetic coils generated beam via convex lens. High voltage of electromagnetic coil has a variable current. Hence for this purpose it is insulated to avoid shock or short circuit. Lenses are made of iron, cobalt and nickel alloys.

Apertures: They are annular structured plates made of metal. This allows axial diffusion of electrons. Intensity of the beam is reduced for the beam to capture the electrons and it also removes the highly scattered electrons. This reduces diffraction and interaction of other beams with the sample. Fixed aperture is used in condenser lens.

Sample preparation:

It is complex procedure. The sample should be few nanometers dense. A sample of high quality has a thickness equal to the mean free path of electrons comparatively. Sample preparation is specific for each material. Preparing extremely thin for the electron transport is tedious and hence it requires hands on experience. Proper care should be taken so the sample structure should not be disturbed during the sample preparation. Some material requires thin section for analysis. Biological samples are fixed by staining techniques. Electron beam can possibly endanger the biological sample materials. The ways by which samples can be prepared are as follows.

• Sectioning of tissue
• Staining of samples : Negative staining and Positive staining
• Mechanical milling
• Chemical etching
• Ion etching

Advancement in TEM:

By the addition of detectors, specimen stages TEM is modified in recent days. Scanning Transmission Electron Microscope is designed in such a way that it rasters the electrostatic beam to pass through the sample to get an image. The beam is reflected by a scanning coil and gathered together using a suitable detector. CryoTEM is a type of TEM specially designed with a specimen stage of liquid nitrogen or helium temperatures. LVEM is a combo design of TEM, STEM and SEM in single equipment. It requires very low voltage of 5 kV for better image quality. It is designed specially for the biological samples.