training for advanced research


Glossary terms about Electron

Electrons are elementary particles, found in all atoms, that are grouped in shells around the nuclei of the atoms.

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52 pages mention Electron

Acceleration voltage vs. specimen type
Reduction in structural details of the specimen surface in SE mode Increased electron build up in insulating samples, causing  
Applications and practical uses - what the SEM can do
Scanning electron microscopy is a remarkably versatile technique. 
Background information - What is scanning electron microscopy?
A Scanning electron Microscope (SEM) is a tool for seeing otherwise invisible worlds of microspace (1 micron = 10-6m) and nanospace (1 nanometer = 10-9m). 
Concepts - introduction
The most important concept in scanning electron microscopy is the use of electrons. 
Conventional (high vacuum) scanning electron microscopy (SEM)
This type of machine is used for routine imaging, using either secondary electrons (SE) or backscattered electrons (BSE). 
Cryo-scanning electron microscope (Cryo-SEM)
A cryo-scanning electron microscope is a conventional SEM that has been fitted with specific equipment that allows samples to be viewed in the frozen state. 
Due to the low energies of secondary electrons (SE) (~2 to 50 eV) they are ejected only from near-surface layers. 
Electromagnetic lenses, apertures and beam size
A series of electromagnetic lenses and apertures are used to reduce the diameter of the source of electrons and to place a small, focused beam of electrons (or spot) onto the specimen. 
Electron column
The electron column focuses and illuminates the specimen using the electron beam generated by the electron gun. 
Electron gun
Thermionic gun Field emission gun Emission Thermionic Field Emission W LaB6 FE Size (nm) 1 x 105 2 x 104 0. 
Electron-beam lithography (E-beam lithography or EBL)
Upon irradiation of focused electron beam, electron-sensitive resists undergo chain-scission or crosslinking, resulting in solubility switch of materials during the subsequent development process (remove/retain exposed material in development depending on the tone of the resist). 
Electron-matter interactions
When the electron beam hits a sample it interacts with the atoms in that sample. 
EM Laboratory Hazards
Some samples may also be a chemical hazard. Electrical: High voltage is present in electron microscopes and other electrical equipment. 
Environmental scanning electron microscope (ESEM)
Example of machine operation using a JEOL instrument
Secondary electron (SE)  
Focused ion beam (FIB) technology
The sample can be sectioned or shaped with the ion beam while it is being monitored by scanning electron microscopy (SEM). 
Generating an image
For routine scanning electron microscope images, secondary electrons (SE) form the usual image of the surface. 
High vacuum mode and pump system
A high vacuum minimises scattering of the electron beam before reaching the specimen. 
In 1923, Busch demonstrated that a beam of electrons could be focused by magnetic or electric fields. 
Images from electrons
electron images from the SEM can be used to achieve different information, for example for topographical, morphological, compositional, or crystallographical studies. 
Introduction - aims and learning outcomes
Calcium carbonate (CaCO3) Calcium carbonate (commonly found in chalk) is present in egg shells, snail shells, and the shells of many small marine organisms. Practice scanning electron microscopy using a virtual SEM  
The SEM uses a beam of high energy electrons generated by an electron gun, processed by magnetic lenses, focused at the specimen surface and systematically scanned (rastered) across the surface of a specimen. 
Magnetic lens system
The condenser lens controls the intensity of the electron beam reaching the specimen. 
In a scanning electron microscope this is achieved by scanning a smaller area. 
Occupational health and safety in an EM lab
Occupational Health and Safety (OH&S) is an important aspect of electron microscopy laboratory and any workplace. 
Perfecting an image - signal processing
The SEM image is a constructed (virtual) intensity map (either digital or analogue) of numbers of electrons ejected from the sample material. 
Practical uses for the SEM
Scanning electron microscopy is used in all scientific fields, in engineering, archaeology and even in art. 
Principles of SEM operation
A scanning electron microscope is a machine comprised of an electron generating component called the gun, a column through which the electron beam travels, a series of lenses to shape the electron beam, the sample chamber at the base, and a series of pumps to keep the system under vacuum. 
Recommended reading
(ed). electron microscopy and microanalysis of crystalline materials. 
Risk assessment example: using a SEM
Notes: There may be leads and vacuum hoses on the floor behind the scanning electron microscope that are required for its operation and which are unable to be relocated. 
Role of sample height
This can be offset by using a smaller objective aperture and putting up with the reduction in electrons that comes with this choice (grainier image). 
Sample insertion and beam activation
While you are waiting for the machine to reach its correct vacuum you can select the electron beam voltage that you will need to use (but don’t turn it on yet). 
Sample preparation
Read the appropriate pages of this module that cover the occupational health and safety issues surrounding the handling of samples for electronmicroscopy. 
Saturating the filament
The more current that is put through the filament, the greater the emission of electrons from the tip region. 
Scan rate and signal to noise ratios
The slower scan rate allows more electrons to be collected at each point along the line of the beam scan. 
Scanning electron microscopy in practice
SEM challenge
This is a secondary electron image taken at 25kV. 
Specimen chamber
The specimen chamber is maintained at high vacuum that minimises scattering of the electron beam before reaching the specimen. 
Spot size
The size (cross sectional diameter) that the cone of the beam makes on the surface of the sample affects 1) the resolution of the image and 2) the number of electrons generated (therefore the graininess of the image). 
The electron gun
The electron gun refers to the top region of the SEM that generates a beam of electrons. 
Troubleshooting: edge effect, charging, sample damage
At high kV the beam penetration and diffusion become larger and result in signal (electrons coming out of the sample) being generated from deeper within the specimen. 
Types of SEM
The different types of scanning electron microscopes in detail:  
Useful links en. 
Variable Pressure or Low Vacuum scanning electron microscopy (LVSEM)
This type of machine is basically like a conventional SEM but has the advantage in low vacuum (LV) mode that the pressure can be adjusted in the sample chamber until the artefact of "electron charging" is removed from images. 
Virtual SEM - bone
Interactive simulation of a scanning electron microscope (SEM) imaging a sample of bone. 
Virtual SEM - rock
Interactive simulation of a scanning electron microscope (SEM) imaging a sample of rock. 
Virtual SEM - slag
Interactive simulation of a scanning electron microscope (SEM) imaging welding slag. 
Virtual SEM - sparkler
Interactive simulation of a scanning electron microscope (SEM) imaging a children's sparkler. 
What is an SEM (Parts of the machine)?
the computer that drives the microscope, with the additional bench controls ancillary equipment that, for example, analyses composition. 
What is astigmatism?
To allow accurate imaging, the electron beam (probe) should be circular in cross section when it reaches the specimen. 
What is resolution?
The smallest distance we can see between points in a light microscope (LM) is about 200 nm [There are 1000000 nm (= nanometers) in 1 mm] whereas a typical scanning electron microscope (SEM) can distinguish gaps smaller than 10 nm. 
What the SEM can't do
Note: Some SEMs can collect true colour images via a wavelength selective cathodolumenence (CL) detector. SEM cannot image through water. Note: An ESEM using a wet Scanning Transmission electron Microscope (STEM) detector can be used to image through thin water films.