Transmission electron microscope is a technique of choice for analysis of internal micro structure. TEM is also used for evaluation of nano structures like fibres, particles or micro structure of cell. This type of electron microscope is invented by Max knoll and Ernst rusker in 1931 at the University of Berlin. It magnifies upto 50 to 50 million times.
PRINCIPLE OF TRANSMISSION ELECTRON MICROSCOPE
In Transmission electron microscope, a beam passes through an ultra thin sample that interacts with sample surface because it passes. The beam having accelerated electrons with higher energy levels (a few hundred keV) are focused on a sample or specimen then the electrons are dispersed or back dispersar elastically or inelastic or produce many interactions, providing different signals sources such as X-rays, Auger electrons or light and some of these are used in transmission electron microscope.
The interaction of the electrons transmitted through the sample forms an image which is 2D, black and white and enlarged and focused on an imaging device, such as a fluorescent screen or on a layer of photographic film or detected by a sensor such as a CCD camera.
INSTRUMENTATION OF TEM
The components of Transmission electron microscope imaging system are listed as follows
The illumination system consists of an electron source, a thermionic gun which emits monochromatic beam of electrons. This beam of electrons possesses k.E. high enough to transverse through the skinny areas of the specimen. Electron source, inside the gun is a pointed hair-pin shaped tungsten filament or a spike shaped lanthanum hexaboride source housed in an electron-accelerating chamber.
The filament is connected to a high voltage supply (about 200-300 kV) which heats it up to about 2700 K, the temperature sufficient for thermionic emission of electrons from tungsten. Immediately after emission from the cathode, electrons are accelerated to their final kinetic energy by applying an electric field parallel to the optic axis. Most of the accelerated electrons are absorbed by the anode plate and only 1% pass through the fine hole, therefore the beam current in a TEM is typically 1% of the emission current from the cathode.
ii) The Condenser
The electron gun is followed by a condenser-lens system that converge the illumination and produces an almost parallel electron beam which is incident on the specimen in form of a spot. In order to realize good contrast and wide range of magnification for viewing very small to larger objects within the specimen, most of the electrons that pass through the specimen should fall within a particular diameter like the screen. In TEM, numbers of condenser lenses are much higher as compared to SEM.
iii) The Stage
The specimen stage holds the specimen perpendicular to the optic axis. The specimen is placed on a hoop shaped grid and inserted through an airlock into the chamber which is evacuated before the specimen enters the TEM column. The sample is held stationary during imaging, although it can be displaced in three planes and tilted to an angle upto defined distance or degrees, with the help of holder adjustments. This helps to align a specific region of specimen into the path of the electron beam in order that all possible regions of the specimen are often visualized.
Electrons beam from the condenser aperture finally strikes the sample and this interaction takes place in three alternative ways by: (i) unscattered electrons (transmitted beam), (ii) elastically scattered electrons (diffracted beam) and (iii) inelastically scattered electrons.
It is an electromagnetic lens made of coils which encounter high excitation current and produce the magnetic field to focus the electrons. The focusing power of lens depends on its excitation and is susceptible to even slight variations in the current; therefore highly stabilized current supply is required for accurate focusing of specimen. The coils of lens utilize high voltage and generate large amount of heat, thus are kept under insulation and are provided with thermal distributors.
v) Intermediate lens and Projector lens
The intermediate lens produces pattern in the back focal plane of the objective lens which is used by the projector lens to form image on the screen. The projector lens produces a large image of several centimeters diameter, across the fluorescent screen. The projector lens has a focal length of a few mm which serves to minimize the image distortion occurring due to high angle scattering of electrons.
vi) TEM screen, Detectors and Image Recorder
A phosphor screen converts the diffraction pattern of electrons into visible light signals. The screen is formed of fluorescing material Zinc Sulfide (ZnS). As it receives the electrons focused by projector lens, it emits light in visible range.
Transmission electron microscope is coupled with detectors which generate electrical signals proportional to the photon intensity. These signals are transformed into image which may be seen directly on the monitor or are often captured during a camera.
WORKING OF TRANSMISSION ELECTRON MICROSCOPE
The working mechanism of sequential transmission electron microscope instruments is as follows:
- High voltage electricity supply power the cathode that generates the beam of electron
- A heated illumination system or tungsten filament in the electron gun generates electron that get focused on the specimen or sample by the condenser lenses.
- The second lens (objective lens) focused the beam of electron onto a certain part of the specimen that is placed on the stage.
- The projector ( the third lens) magnifies or enlarges the image across the fluorescent screen
APPLICATIONS OF TRANSMISSION ELECTRON MICROSCOPE
There are various applications of transmission electron microscope in many fields such as biology, metallurgy, forensic science, medical and many more.
- To visualize and study the micro structure of bacteria, fungi, viruses and many microscopic cell organelles.
- To study and differentiate the structure of plant and animal cells.
- Investigation of gemstones and jewellery
ADVANATGES OF TEM
TEMs offer very powerful magnification and resolutions
Transmission electron microscopes provide information on molecule and compound structure
Images are high quality and detailed
Limitations of TEM
There are a few limitations of transmission electron microscope.
- Extensive sample preparation is required in many materials in order to get a thin sample which is enough to be electron transparent, this makes TEM process a time consuming process with a low throughput of samples.
- Sometimes the original structure of the sample may also changes while the sample preparation process.
- Since the field of view is very less which may raise the possibility that the region which is being analysed may not be depicting the characteristic of the sample.
- Electrons beam may sometimes destroy the sample surface especially in biological samples.
- Images are black and white
For scanning electron microscope go through our blog.