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SCANNING ELECTRON

MICROSCOPE

ANGEL ANNA LAL1st YEAR MSc Biotechnology

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INTRODUCTION-TYPES

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Scanning Electron Microscope

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SCANNING ELECTRON MICROSCOPE (SEM) Invented by Max Knoll in 1935. Uses a focused beam of high-energy electrons to

generate images of a sample. 3-Dimentional images are obtained. Magnification ranging from 20X to

approximately 30,000X. Provides 250 times larger image than light microscope. Used in high or low vacuum in wet conditions and

even at wide range elevated temperatures.

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PRINCIPLE The basic principle is that a beam of

electron is generated by a suitable source, typically a tungsten filament or a field emission gun.

The electron beam is accelerated through a high voltage[20 kV] and pass through a system of aperture and electromagnetic lenses to produce thin beam of electrons.

Then beam scans the surface of specimen.

Electrons are emitted from specimen by action of scanning beam and collected by suitably positioned detector.

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SCHEMATIC OF AN SEM

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SAMPLE PREPARATION

Appropriate size & should be dry. Specimens should be electrically

conductive. Coated with ultrathin layer of

electrically conducting method. Eg: Gold, Gold\Palladium

alloy,Platinum,Osmium,Tungsten,Graphite etc.

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A SPIDER SPUTTER-COATED IN GOLD, HAVING BEEN PREPARED FOR VIEWING WITH AN SEM.

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IMAGE FORMATIONElectron gun fitted with tungsten filament

Electron beam focused by one or two condenser, Passes through scanning coils\deflector plates

Primary electron beam interacts with sample-repeated random scattering.

Beam current absorbed by specimen is detected & electronic amplifier amplifies signal & image is displayed.

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Signals:Secondary electrons (SE): mainly

topography Low energy electrons, high resolution Surface signal dependent on curvature

Backscattered electrons (BSE): mainly chemistry High energy electrons “Bulk” signal dependent on atomic number.

Sample

Secondary electrons Backscattered electrons

Incoming electrons

X-rays

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Backscattered electron detector:

Secondary electron detector

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POLLEN OBSERVED THROUGH SEM

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ADVANTAGES Most SEM's are comparatively easy to

operate, with user-friendly interfaces. Many applications require minimal sample

preparation. For many applications, data acquisition is

rapid [less than 5 minutes/image for SEI, BSE.]

Modern SEMs generate data in digital formats, which are highly portable.

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LIMITATIONS Samples must be solid and they must fit

into the microscope chamber. Maximum size in horizontal dimensions

is usually on the order of 10 cm; vertical dimensions are generally much more limited and rarely exceed 40 mm.

For most instruments samples must be stable in a vacuum on the order of 10-5 - 10-6 torr.

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