Electron Microscope

Comparison b/w light and electron microscopes

LIGHT MICROSCOPE ELECTRON MICROSCOPE

Magnification can be done upto 2000 times

Resolving power is less

Photons are involved

Magnification can be done upto 2 million times

Have much greater resolving power than ordinary microscope

Electrons are involved

3

Scale

Reason for greater resolution and magnification of electron

microscopeWavelength of an electron[de Broglie] is very much

smaller than that of a light photon

Wavelength of an electron = λ=h/(√2mE)Wavelength of a light photon = λ=h/E

First Electron Microscope

Invented by Ernst Ruska

Year-1933

He was awarded the Nobel Prize for physics for his invention in 1986

Construction Of An Electron

Microscope

Construction of EM [TEM]

THE ELECTRON GUN

WORKING OF AN ELECTRON MICROSCOPE

elec mic intro.avi

Types of ELECTRON MICROSCOPE

Scanning EM

Transmission EM

Reflection EM

Scanning Transmission EM

TRANSMISSION ELECTRON

MICROSCOPE

Developed by ERNST RUSKA and MAX KNOLL in 1931 in germany

It was the first type of electron microscope to be invented

TEM

When a beam of electrons is passed through a specimen , a part of it is transmitted and this part when projected on fluorescent screen , its image can be seen by the observer

PRINCIPLE

Construction and working

TEM1. ELECTRON GUN2. ELECTROMAGNE

TIC LENSES3. VACUUM PUMPS4. OPENING TO

INSERT SAMPLES

5. OPERATION PANEL

6. DISPLAY SCREEN 7. WATER SUPPLY

TO COOL THE INSTRUMENT

ELECTRON GUN

The electron gun produces a stream of monochromatic electrons of

energy 100-400keV .

The extraction of electrons is of two types

1. Thermionic emission using thermal energy

2. Field emission by applying very large electric field 1010 A/m

FE gun is more expensive and must be used in high vacuum

conditions.

FIELD EMISSION GUN

The beam of electrons is focused using condenser lenses [1&2]

The beam is restricted by the condensor aperture

Then it strikes the specimen and transmitted

The transmitted portion is focused by Objective Lens into an

image

Intermediate and projector lenses enlarge the image

Image is formed on phosphor screen

Darker area – few electrons – thick region

Lighter area – more electrons – thin region.

Transmission electron microscopy

Advantages

versatile technique for the characterisation

of materials

very high resolution

Resolving power is

Magnifi cation is 1,000,000 times greater

than the size of the object

Information about crystal structure and

chemical composition can be collected

simultaneously

OA21

Disadvantages

No 3-D image

Aberrations due to lenses

Absorption of electrons heats up the sample and

changes its characteristics

Larger current density[j] and hence more current I=jA

{A- Illuminated area}

Specimen must be thin because due to strong

absorption of electrons , the penetration depth is small

In nano science , to find the internal structure of nanomaterials

To get 2-D Image of biological cells , virus , bacteria etc.

In fields such as thin film technology , metallurgy , microbiology etc.

In studying the compositions of paints , alloys etc.

APPLICATIONS

Scanning Electron

Microscope

24

The Scanning Electron Microscopeuses electrons reflected from the surface of a specimen to create image

It captures the images of the specimen surface by scanning it with a high-energy beam of electrons , in a scan pattern

produces a 3-dimensional image of specimen’s surface features

PRINCIPLE

The electrons interact with the atoms that make up the specimen, producing signals that contain information about its surface topography , composition and other properties such as electrical conductivity

CONSTRUCTIONPARTS of SEM

Electron gun

Anode

Electromagnetic lens

Scanning coils

Specimen Holder

Detectors

CRO Tube

SEM1. e lec t ron gun2. e lec t romagnet

i c l enses3. vacuum

pumps4. open ing to

i nse r t spec imen

5. opera t ion pane l

6. sc reen fo r d i sp lay

7. c ryo – un i t f o r c ryo sem

8. e lec t ron i c i n s t ruments

PREPARATION OF SAMPLESSamples must be electrically conductive

If a non conductive material has to be viewed , then it has to be coated by a thin layer of electrically conductive material

This coating is done using a sputter coater

GOLD COATED INSECT WHICH IS TO BE VIEWED THROUGH SEM

SPUTTERING

WORKINGSEM produces signals in the form of secondary electrons , backscattered electrons, characteristic x-rays , light , specimen current, and transmitted electrons

These signals are formed by the interaction of the electron beam with the surface of the specimen and require specialized detectors for their detection

Depth of the specimen can be expressed in the image

Back scattering of electrons help to detect the distribution of elements in the specimen

CHARACTERISTIC X-RAY SIGNAL

Characteristic x-ray signals are formed when the electron beam removes an inner shell electron of the sample causing a high energy electron to fill the space and release energy

Help to identify the composition of elements in sample

Scanning Electron Microscope

ADVANTAGES OF SEM

used to examine specimens of large thickness

Image can be directly viewed

3-Dimensional image can be obtained

has large depth of focus

very high magnifi cation from x25 to x250,000

DISADVATAGES OF SEM• The Resolution of image is poor

• preparation of sample is diff icult and tedious

• some samples can loose their structural property due to their interaction with the electrons

APPLICATIONS Specimens of large thickness can be examined

wide application in medical , science and engineering fields

To find the structural composition of paper pulps, ceramic materials , polymers etc.

Used to get 3-D image of biological cells, DNA , Bacteria etc.

ComparisonSEM TEM

SEM AND TEM PHOTOSSEM TEM

Comparison of images

SEM of the compound eye of a fly!

TEM of bacteria

Some more images obtained using SEM

SEM of Yersinia pestis – which causes plague

SEM image of Streptococcus pyogenes , which causes scarlet fever

EM Image of the chloroplast of spinach

EM Image of RBC

SEM Image of a mesh

RBC OF MAN

NEURON CELL

MOSQUITO

CULTURED CELLS OF HUMAN BEING

HUMAN CELLS

HEAD OF A BLACK ANT

Websites for further info:www.nobelprize.org

www.wikipedia.com

www.pbrc.hawaii.edu/microangela

www.mos.org/sln/SEM/

www.britannica.com/EBchecked/topic-art/183561/110970/Scanning-electron-microscope

Presentation done by…

SHANTHA KUMAR . TEEE

RMD ENGG COLLEGE

THANK YOU!