Electron microscope
Overview.
Electron microscope principles.
Electron microscope mechanism.
Scope.
Overview.
When the optical microscope was invented, it opened up an entire new world.
We were able to see, what humans never saw before.
For example, we did not know that bacteria exists.
The micro world, that the optical telescope opened up,
led to many scientific discoveries.
We have now discovered the electron microscope.
Using this, we can now see matter at the molecular scale.
We can now scientifically observe matter in nano scale.
A nanometer is one billionth of a meter.
This has opened the doors for a whole new world of scientific research.
Many discoveries have already been made.
Many more are likely to be made, in the near future.
Electron microscope principles.
Atoms in all elements exhibit unique arrangement of electrons orbiting the nucleus.
When electrons jump from a higher energy orbit to a lower orbit,
they emit a fixed quantum of energy.
Similarly, they absorb fixed quantum of energy, for electrons to jump to a higher orbit.
This spectral property is unique for each set of orbits, in a given element.
Each element has its own unique spectral properties.
This is discussed in the module, Spectral lines.
The electron microscope makes use of this principle,
to image matter at a nano scale.
Visible light has wave lengths in the range of 400 to 700 nanometers.
Optical microscopes use visible light, to magnify objects.
They extent of magnification is limited by the range of visible light.
Electrons have a much shorter wave length.
Typically they have wave lengths in the range of 2 to 12 picometers.
Higher the energy, the shorter is the wave length.
A picometer, is one trillionth of a meter, or 10 to the power of minus 12 meters.
Using this range, we can discriminate matter at a molecular level.
Electron microscope mechanism.
It has a vacuum column about two meter high.
On top of this column, there is a thermionic cathode.
The cathode emits a stream of electrons.
An anode placed along the path of the electron stream.
A strong electrical field between the cathode and the anode,
accelerates the electrons.
The electron stream passes through a small aperture, to become a focused beam of electrons.
This setup is sometimes called as the electron gun.
The beam is maintained along the vacuum tube, by electro magnetic lenses.
These coils focus the beam to the centre of the vacuum tube.
The specimen is placed, at the other end of the vacuum tube.
The electron beam, now encounters the specimen.
When electrons encounter the specimen, three possible things can happen.
The electrons can pass through the specimen.
The electrons can be absorbed by the specimen.
The electrons can be scattered by the specimen.
The basic function of the electron microscope, is to track the electrons.
This gives us an insight, into the nature of the specimen.
Different regions of the specimen, can have variously transparent properties, to the electrons.
The change in the energy level, at the point of contact,
help us to define the properties of the specimen.
At the end of the tube, there is a fluorescent screen, or photographic film.
This can detect the electrons, that passes through these specimen.
The beam can be directed on different points of the specimen.
This is what a scanning electron microscope does.
The specimen is scanned point by point,
and then row by row, till the entire specimen is scanned.
The result creates an image on the screen.
The pattern of the image, can be interpreted, to understand the specimen.
Scanning more points in the specimen, increases the resolution.
Depending on the microscope, and the objective, the resolution can be increased,
to the desired level.
Some of the electrons, can be reflected or scattered.
Another detector, called the secondary electron detector,
maps these electrons.
This gives us one more dimension to the image.
The scattering pattern can be analysed and interpreted.
Another type of detector, that is used is an x-ray detector.
An electron can knock out an electron, in a particular shell of an element.
For example, an electron in the K shell, can be knocked out.
This causes an electron, from the L shell, to jump to the K shell.
This causes a photon to be emitted.
This photon will have a predetermined well defined quantum, of energy.
The x-ray detector, detects this energy.
From this we can derive the property of this source, in the specimen.
Different elements have well defined energy levels.
They emit and absorb photons, in fixed quantum of energy.
This quantum of energy is directly dependent on the orbitals,
that the electrons jump, from and to.
By measuring this quantum energy, we can for example say,
that an element in a particular point in the specimen is gold.
X-ray detectors can determine the chemical composition of the specimen.
Using a combination of detecting technologies,
we can determine intricate properties and patterns of matter.
Scope.
Electron microscopes can be used for a wide variety, of scientific investigation.
For example:
Intricate crystals structures, of metals has been analysed.
The complex molecular structure of proteins, has been mapped.
The electron microscope has been used for making many other scientific discoveries.
Electron microscope are getting better and better, with higher and higher resolutions.
This is enabling us to see, matter in the pico scale.
Currently, the worlds best electron microscope uses 1.2 mega volts of energy.
It has a resolution of 43 picometers.
This by itself offers exciting possibilities, for scientific analysis.
This will no doubt get even better, opening up an entire new exciting world for us.
Already new branches of nano science are springing up, to explore this possibilities.