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www.reading.ac.uk/EMLab
Scanning Electron Microscopy
Amanpreet Kaur
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EM Workshops 2017
Scanning Electron Microscopy
What is scanning electron microscopy?
Looking at “wet” samples in the high vacuum SEM
Basic features of conventional SEM
Limitations of conventional SEM
Cryo-SEM and Environmental SEM
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EM Workshops 2017
SEM & TEM
Scanning electron microscope
Electron gun
Specimen
Electron gun
Condenser lens
Objective lens
Specimen
Projector lens
Fluorescent screen
Electron gun
Digital camera
Condenser lens
Objective lens
Specimen
Projector lens
Fluorescent screen
Electron gun
Digital camera
Transmission electron microscope
Workshop: 31st October
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EM Workshops 2017
SEM
Scanning electron microscope
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EM Workshops 2017
Electron sources for SEM
Tungsten thermionic source
Schottky Field Emission Source
Low brightness
Energy spread ~ 1-2eV
High brightness
Energy spread < 0.5 eV
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EM Workshops 2017
SEM
Scanning electron microscope
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Interaction of high-energy electrons with specimen
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EM Workshops 2017
Interaction of electrons with specimen
• S = secondary
• B = backscattered
• (A are auger)
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EM Workshops 2017
Secondary and backscattered electrons
• What are backscattered electrons?
• They are primary electrons that have entered the sample and then escaped back out.
• What are secondary electrons?
• They are electrons dislodged from the specimen itself.
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EM Workshops 2017
Detectors for SEM
Everhart Thornley detector (ETD): Scintillator – photomultiplier system.
Secondary electrons
Everhart Thornley or solid state detector (silicon diode).
Usually positioned around the final lens or inside.
Backscattered electrons
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EM Workshops 2017
◊ Secondary electrons Topographic
information
◊ Backscattered electrons
◊ X-rays
Compositional
information
- Spectra
- Maps
Session on X-ray analysis and Elemental Mapping: 21st November
Information given by different signals
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EM Workshops 2017
Origin of topographic contrast
Everhart
Thornley
detector
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EM Workshops 2017
Detector efficiency contrast
pseudo-octahedral crystal of the compound [NC6H8][GaGe3S8]
a mixed Germanium and Gallium Sulphide framework material
50%
collected – somewhat
bright
100% collected -
Bright
10% collected -
Dark
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EM Workshops 2017
Early history of SEM
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.
1935, M. Knoll, Germany – proposed concept of a scanning electron microscope.
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EM Workshops 2017
EM Lab’s high vacuum SEM
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EM Workshops 2017
Conventional SEM images
Compact
disc
Polypropylene spherulites
Integrated circuit
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EM Workshops 2017
Problems with conventional SEM
No wet samples
Surface must be electrically conducting
Non-conducting samples have to be coated
Can only operate at low pressure (10-5 Torr)
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EM Workshops 2017
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:
Freeze drying
Chemical fixation, critical point drying
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EM Workshops 2017
Critical point drying
Specimen prepared by CPD: Hyphae and spores in Stilton cheese
Evaporative drying of specimens can cause collapse of structures, mainly due to effects of surface tension.
Effect can be reduced by substitution of water with a liquid with a lower surface tension
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EM Workshops 2017
Sample prepared by fixation & critical point drying
Euphorbia seed. Amal Al Hasan, SBS.
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Cryo - SEM
Workshop: 7th November
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EM Workshops 2017
Cryo – SEM of food
Probiotic yoghurt with bacteria. Scale bar 5μm
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Potato showing starch grains Scale bar 100 μm
EM Workshops 2017
Environmental SEM
Workshop: 7th November
Cooled specimen
• Special pumping system allows water vapour to be introduced into chamber.
• Cooled specimen to retain moisture.
• New (backscattered) electron detector.
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EM Workshops 2017
Introducing water vapour into the chamber enables uncoated samples to be examined
Positively charged water molecules are attracted to negatively charged sample.
Negative charge at sample surface is neutralized..
Secondary electrons from sample strike water molecules.
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EM Lab’s FEI Quanta 600 ESEM
Pressures up to 20 Torr - samples typically cooled to 5oC.
Gaseous secondary electron detector – fits on objective lens.
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EM Workshops 2017
Environmental SEM
• Raspberry buds : Hiroyuki Imanishi, Plant Sciences
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EM Workshops 2017
Becoming an EMLab user
Sherrie Foo
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EM Workshops 2017
Booking system
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EM Workshops 2017
Remaining workshops
Date
Topic
31 October 2017 Transmission Electron
Microscopy
7 November 2017 Environmental & Cryo
SEM
14 November 2017 Biological specimen
preparation for SEM
21 November 2017 X-ray Analysis and
Elemental Mapping
28 November 2017 Biological specimen
preparation for TEM
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EM Workshops 2017
► Questions?
Electron Microscopy Laboratory
► Demo
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