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 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

Scale 3

Wavelength 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]

Developed by ERNST RUSKA and MAX KNOLL in 1931 in germany It was the first type of electron microscope to be invented

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

TEM 1. ELECTRON GUN 2. ELECTROMAGNETI C LENSES 3. VACUUM PUMPS 4. 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 keV. The extraction of electrons is of two types 1. Thermionic emission using thermal energy 2. Field emission by applying very large electric field 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  Magnification is 1,000,000 times greater than the size of the object  Information about crystal structure and chemical composition can be collected simultaneously

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.

 uses 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 24

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

CONSTRUCTION PARTS of SEM Electron gunAnodeElectromagnetic lensScanning coilsSpecimen HolderDetectorsCRO Tube

SEM 1. electron gun 2. electromagneti c lenses 3. vacuum pumps 4. opening to insert specimen 5. operation panel 6. screen for display 7. cryo – unit for cryo sem 8. electronic instruments

 Samples 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

SPUTTERING

SEM 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

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 magnification from x25 to x250,000

The Resolution of image is poor preparation of sample is difficult and tedious some samples can loose their structural property due to their interaction with the electrons

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.

Comparison SEM TEM

SEM AND TEM PHOTOS SEM 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

art/183561/110970/Scanning-electron-microscope