What are the two variants of electron microscopy?
Scanning electron microscopy (SEM)
Transmission electron microscopy (TEM)
What are the differences between electron and optical microscopy?
Electron microscopes operated in vacuum#
short mean free path
electron have higher resolution
electron can perform chemical mapping and spectroscopy
radiation damage can limit the image quality
what are the main advantages of scanning electron microscopy?
electrons have much better resolution
they can study the surface roughness
What is the main application of transmission microscopy?
deals with internal structure near surface region
How can the wavelength of electrons be calculated?
What are the changeable parameters in electron microscopy?
electron column
vacuum conditions
tilting of sample
more detectors
screen
What is the process of a SEM image creation?
the focussed electron beam interacts with the surface and is guided by deflection coils, which guide the beam in 2 directions
The deflected electrons are collected by a detector
What are electron sources?
thermionic electron gun
field emission gun
What is the normal energy of a electron beam?
0.5-40 keV
Label the marked components:
What is requiered of a sample in SEM?
stable against high vacuum
electrically conductive
What is understood under ESEM (enviromental SEM)?
SEM for non conductive materials without coating.
Main difference that there is no vacuum.
How are non conductive samples prepared for the SEM?
Sputtering: Application of thin conductive layer
Give the labeled compounds:
How does the SEM Process work?
Electron beam (primary electrons) is guided over surface, while the collected signal from the electrons that interacted with the surface (secondary electrons) is converted into a grey scale. The magnification is the ratio between the scanned surface and the size of the screen.
What are secondary electrons (SE) in SEM?
<50eV, from the first nm of the surface (inelastic scattering)
contain information about the topografy of the surface and are the producer of the image contrast
deeper produced SE do not have enough energy to reach the surface
What is a geometrical way to increase the secondary electron output.
Tilting the sample.
What are backscattered electrons (BSE)?
>50 eV
>90° scattering
heavier elements create more backscattering than lighter ones and are therefore darker, which gives information about chem. composition
Which image is from secondary electrons, which image from backscatter electrons?
From which region is which radiation?
How are X-Rays generated in SEM?
An electron hits a bound electron out of its orbital close to the core. an outer electron relaxates into this orbital. The released energy is emmitted as a x-ray photon, which is material specific.
On what depends the number of yielded fluorescence x-rays?
The atomic number Z
What are the advantages and disadvantages of Energy Dispersive X-ray Spectroscopy?
very fast process
elemental maps possible
no high accuracy quantitative analysis
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