A beautifully crafted presentation describing the history, components and specimen interactions that take place in during a general observation

1. Electron Microscopy:
Scanning and Transmission
By
Dhruvil Hapani
Part V - Engineering Physics

2. History
● In the early 1930’s, there was a scientific desire to see the fine details of the
interior structures of organic cells (nucleus, mitochondria etc).
● Light microscopes being used were limited by the physics of light, the best
ones able to produce 1000X magnification and a resolution of 0.2
micrometers.
● The transmission electron microscope was the first type of electron
microscope to be developed.
● It was developed by Max Knoll and Ernst Ruska in Germany in 1931.

4. ● Electron microscopes are scientific instruments that use a beam of
energetic electron to examine objects on a very fine scale
● It yields information about the topography, morphology, composition,
and crystallographic information of the specimen
● It can achieve magnification upto 10,000,000x and resolution of upto
50 pm
● They use a focused beam of electrons instead of light for analysing the
specimen
Introduction

5. 1. Topography : The surface features of an object, it’s texture; direct relation
between these features and material properties
2. Morphology: The shape and size of the particles making up the object
3. Composition: The elements and compounds that the object is composed of and
the relative amounts of them
4. Crystallographic information : How the atoms are arranged in the object
Characteristics observed

6. Electron gun
● The first and basic part of the microscopes is the source of
electrons. It is usually a V-shaped filament made of LaB6 or
W that is wreathed with Wehenlt electrode.
● Due to negative potential of the electrode, the electrons are
emitted from a small area of the filament (point source)
Condenser lens
● This lens converges the cone of the electron beam to a spot
below it.
● It formulates the beam shape and size
Components of SEM

7. Scanning coils
● After the beam is focused, scanning coils are are used to
scan the electron beam across the specimen surface
● Two scanning coils produce deflections in horizontal and
vertical directions
● This allows it to scan in a raster fashion over a rectangular
area of the sample surface
Objective lens
● This is the last lens in the series that creates the electron
beam that scans the specimen
● The lens focuses the scanning beam to a very small spot on
the specimen
Components of SEM

8. ● Secondary emissions
● X-ray emission
● Backscattering
● Auger emission
● Visible light
● Transmission
● Diffraction
● Absorption
● Heat
Specimen Interactions

9. Secondary electrons
They are produced when an incident electron excited an electron in the
sample by losing most of its energy. The excited electron moves towards the
surface of the sample undergoing collisions until it reaches the surface,
where it can escape if it still has sufficient energy.
Because of collision losses, only secondary electrons that are very near the
surface ( < 10 nm) can exit the sample and be examined.
This feature helps to produce very fine topographical images and examine
the surface features of our sample
Specimen Interactions

10. Backscattered electrons
These electrons consist of high-energy electrons originating in the
electron beam, that are reflected out of the specimen interaction volume.
The production of backscattered electrons varies directly with atomic
number.
This differing production rates causes higher atomic number elements to
appear brighter than lower atomic number elements.
This interaction is utilized to differentiate parts of the specimen that have
different average atomic number.
Specimen Interactions

11. Inelastic interactions
These interactions produces the fingerprint of each element.
1. X-Ray emission 2. Auger emission
Specimen Interactions

12. ● TEM is a technique in which a beam of electrons is transmitted through
a specimen to form an image. The specimen is most often an ultrathin
section less than 100 nm thick.
● An image is formed from the interaction of the electrons with the
sample as the beam passes through the specimen.
● The image is then magnified and focused onto an imaging device, a
layer of photographic film or some sensor.
● It is a major analytical method in the physical, chemical and biological
sciences.
Transmission electron microscopy

13. Magnetic lensing
Glass lenses, of course would impede electrons.
Electron microscope lenses are electromagnetic converging
lenses. A tightly wound copper wire forms a solenoid. Electrons
move through the centre hole in helical path depending on the
lens current.
Condenser lens
This lens gather the electrons and focus them onto the specimen
to illuminate only the area being examined.
Components of TEM

14. Objective lens
The Objective lens is used primarily to focus and initially magnify
the image. An objective aperture is used to enhance specimen
contrast.
Intermediate lens
Intermediate lenses magnify the image coming from the objective
lens. Finally, projector lenses further magnify the image coming
from the intermediate lens and projects it on to the
phosphorescent screen.
Screen
The final image is viewed by projection onto a phosphorescent
screen which gives off photons when irradiated by the electron
beam.
Components of TEM

15. Transmitted beam
When incident electrons are transmitted through the thin specimen
without any interaction occurring inside, then the beam of electrons is
called transmitted.
The transmission of unscattered electrons is inversely proportional to
the specimen thickness. Areas of specimen that are thicker will appear
darker, while thinner areas will appear lighter
Specimen Interactions

16. Diffracted beam
Some electrons are scattered by the atoms in the specimen in an
elastic fashion (no loss of energy). These scattered electrons are then
transmitted through the remaining portion of the specimen.
All electrons follow Bragg’s law and thus are scattered according to
n λ=2 d sin(θ)
These electrons yield information about the orientation, atomic
arrangements and phases present in the area being examined.
Specimen Interactions

17. Beautiful SEM images

18. Thank you for your time!