7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

1/15

Transmission Electron Microscope

(TEM)

Presented ByPrashant KumarM.Tech.1st Year

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

2/15

Introduction

TEM was first built by Max Knoll and Ernst Ruska in 1931.

1000X more magnification than light microscope.

Used to reveal ultra structure of plant and animal cells aswell as viruses and macromolecules.

Electrons pass through a (very thin) sample (i.e. are

transmitted) to form an image.

Simplistically, In its operation a TEM can be thought of

as analogous to a slide projector.

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

3/15

1. Electron Gun

2. The condenser

system

3. The sample

4. Image formation5. Projection

System

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

4/15

Electron Guns

There are 2 types :-

1. Thermionic Electron Gun

2. Field Emission Gun (FEG)

Thermionic sources produce electrons when heated.

Field emission sources produce electrons when exposed to

an intense electric field.

FEGs give much more brightness than thermionic systems.

FEGs give a more monochromatic electron source and

finer probe (i.e. better resolution).

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

5/15

The Condenser System

The Wehnelt (or 2nd anode in a

FEG) focuses the beam to a

crossover which is accelerated

down the column.

The first condenser de-magnifies

the crossover to give a smaller

point source this is referred to as

C1 or spot size.

The second condenser lens C2 is

used to either converge or spread

the beam of illumination on thesam le intensit or bri htness .

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

6/15

The Condenser System

A condenser aperture is placed in the beam path to remove

electrons far from the optic axis which would reduce

resolution.

The smaller the aperture the better the resolution, but

there is an associated decrease in brightness need to

compromise.

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

7/15

Sample preparation

For a metallic sample:

Cut or slice a section of

material less than 1 mm thick.

Produce 3mm diameter blanksby either Punching, or spark

erosion.

Grind and polish blanks to

less than 80m thick and

0.25m or better finish.

For a non metallic sample:

1. Cut or slice a section of material less

than 1mm thick.

2. Mount on glass slide and Grind to lessthan 80m thick with a 0.25m or

better finish.

3. Mount on support grids if necessary.

4. Dimple to leave ~10 m of material

remaining.

5. Ion beam mill to perforation.

6. Some samples require coating to

prevent charging effects.

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

8/15

For biological samples

Biological samples require fixing and embedding before

being stained with heavy metals (e.g. OsO4) for contrast

prior to ultra microtome sectioning. Very time

consuming.

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

9/15

Image Formation

Objective

lens

Sample

All rays from a point in theobject are gathered by the lens

and converge to a point in the

image.

All parallel rays are focused in

the focal plane.

The back focal plane of the

objective lens contains

groupings of rays that have left

the object at the same angle.

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

10/15

The back focal plane contains the diffraction

pattern of the sample.

Diffraction pattern and image are both formed inthe imaging process

The intermediate lens is then focused on either the

image plane (for the image), or the back focal plane

(for the diffraction pattern).

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

11/15

Projection - Magnification

A series of projector lenses areused to magnify the imageformed by the intermediatelens onto a viewing screen.

Electron micro lenses areelectromagnetic in nature.

They consists of cylindrical softmetal core (pole piece) with ahole drilled through it (bore)

wound with copper wire.

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

12/15

When a current is passed through the coil amagnetic field is created in the bore.

Changing current in the windings changes themagnetic field and effectively changes the focallength of the lens.

Increase the current and focal length f of the lensdecreases so weaker lens f1 gives a highermagnification than stronger lens f2 as imagedistance v increases but the, object distance isunchanged.

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

13/15

Applications of TEM

Nanotechnology

Medical Sciences

Metallurgy Material Sciences

Life Sciences

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

14/15

References1. www.google.com

2. Introduction to nanotechnology By Charles Poole

3. http://www.tcd.ie/CMA/

http://www.google.com/http://www.google.com/

7/27/2019 TRANSMISSION ELECTRON MICROSCOPE.pptx

15/15