What is scanning electron microscopy? Basic features of conventional SEM Limitations of conventional SEM Looking at “wet” samples in the high vacuum SEM Cryo-SEM and Environmental SEM
EM Workshops 2013
SEM & TEM
Electron gun Electron Electron gun gun Condenser Condenser lens lens Objective Objective lens lens Specimen Specimen Projector Projector lens lens
Fluorescent Fluorescent screen screen Digital Digital camera camera
Specimen
Scanning electron microscope
Transmission electron microscope Workshop: November 13
EM Workshops 2013
Electron sources for SEM
Schottky Field Emission Source
Tungsten thermionic source
Low brightness
Energy spread ~ 1-2eV
EM Workshops 2013
High brightness
Energy spread < 0.5 eV
Interaction of high-energy electrons with specimen
EM Workshops 2013
Interaction of electrons with specimen
EM Workshops 2013
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S = secondaries
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B = backscattered
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(A are auger)
Secondary and backscattered electrons
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What are secondary electrons?
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They are electrons dislodged from the specimen itself.
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What are backscattered electrons?
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They are primary electrons that have entered the sample and then escaped back out:
EM Workshops 2013
Detectors for SEM
Secondary electrons Backscattered electrons
Everhart Thornley or solid state detector (silicon diode). Usually positioned around the final lens or inside.
Session on X-ray analysis: November 27 EM Workshops 2013
Origin of topographic contrast
Everhart Thornley detector
EM Workshops 2013
Early history of SEM
1935, M. Knoll, Germany – proposed concept of a scanning electron microscope.
1942, Zworykin et al., RCA Laboratories, USA - first working SEM.
1950s, C.W. Oatley et al., Engineering Department, Cambridge - major improvements in electron optics and system, and secondary electron detection (Everhart - Thornley detector). First backscattered detector.
1965: first commercial SEM, Cambridge Instrument Company, Stereoscan Mark 1.
EM Workshops 2013
EM Lab’s high vacuum SEM
EM Workshops 2013
Conventional SEM images
Compact disc
Polypropylene spherulites
Integrated circuit
EM Workshops 2013
Problems with conventional SEM
Can only operate at low pressure (10-5 Torr) No wet samples Surface must be electrically conducting Non-conducting samples have to be coated
EM Workshops 2013
Looking at “wet” samples in the high vacuum SEM
Animal and plant tissues up to 98% water. Options for examining these materials in the high vacuum SEM:
Chemical fixation, drying
Freeze drying
Critical point drying
EM Workshops 2013
Critical point drying •
Evaporative drying of specimens can cause collapse of structures, mainly due to effects of surface tension.
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Effect can be reduced by substitution of water with a liquid with a lower surface tension
Specimen prepared by CPD: Hyphae and spores in Stilton cheese
EM Workshops 2013
Cryo - SEM
Workshop: November 20
EM Workshops 2013
Cryo – SEM of food
Raw potato
EM Workshops 2013
Probiotic yoghurt with bacteria. Scale bar 5μm
Environmental SEM
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Special pumping system allows water vapour to be introduced into chamber.
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Cooled specimen to retain moisture.
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New (backscattered) electron detector.
Cooled specimen
Workshop: November 20
EM Workshops 2013
Introducing water vapour into the chamber enables uncoated samples to be examined
Secondary electrons from sample strike water molecules.
EM Workshops 2013
Positively charged water molecules are attracted to negatively charged sample.
Negative charge at sample surface is neutralized..
EM Lab’s FEI Quanta 600 ESEM
Gaseous secondary electron detector – fits on objective lens. Pressures up to 20 Torr - samples typically cooled to 5oC.
