Flashcards in Transmission Electron Microscopy Deck (53):
Why use an electron microscope?
what is Abbe's equation?
Dmin=wavelength / 2 NA
what is wavelength of visible light?
400 – 700nm
what is resolution /Dmin of light microscope
0.2um or 200nm
what is wavelength of electrons?
what is resolution /Dmin of electron microscope?
what are the two types of electron microscope?
1. transmission electron microscope (TEM)
2. scanning electron microscope (SEM)
what are the similarities of TEM &SEM?
1. both use a beam of electrons
2. electron production
3. condenser lens system
5. high vacuum
How are electrons produced in TEM &SEM?
from a filament
what is filament in TEM &SEM
electron source at the top of the column
what happen before electrons entry into the condenser lens system in TEM &SEM?
negative electrons accelerated towards uncharged anode into condenser lens system
why are the condensers special in TEM &SEM
condensers focus electrons onto the specimen
what does high vacuum do in TEM &SEM?
allow passage of electrons down column
what are the unique feature of TEM?
1. stationary beam
2. thin specimen
4. final image
thin specimen in TEM?
electrons pass through 50nm thick specimen
internal details in TEM?
image of internal detail of specimen formed by image forming magnetic lenses
image forming magnetic lenses
final image in TEM?
final image on a fluorescent screen or photographed
what re the unique feature of SEM?
1. moving beam
2. scan surface of bulk specimen
3. emit electrons
4. image formation
where is moving beam produced in SEM?
scan coils between condenser and specimen
what surface in SEM scanned
natural or created
emit electrons of various energies (X rays)
where are images formed in SEM?
by detects and amplifiers to TV screen
Not by magnetic lenses
what does anode with opening do?
it is earth zero charge
and attract electrons
what does Wehnelt cylinder do?
It is very negative
and repel electrons
what is accelerating voltage if TEM?
potential difference between the filament and the anode
set by operator
what does large accelerating voltage mean?
faster electron with high energy and shorter wavelength
what are the features with fast electrons
1. penetrate specimen more readily
2. lose energy less easily
3. scatter less, less contrast
what is principle of magnetic fields?
1. magnetic field formed as long as the current passing
2. In electron microscope, produced by hollow electro magnet
what is magnetic field in electron microscope?
What are magnetic lenses like
optically like a biconvex glass lens
how to change the strength of the magnetic field
with a switch
what is relation of focal length in magnetic lens?
stronger field short focal length
when is magnetic filed stable ?
1. current not vary
2. no other magnetic field
3. temp not vary
4. avoid mechanical disturbances
what is limited in lenses?
1. limiting aperture to
improve image quality
what does condenser lenses do?
focus beams of electrons onto specimen
two condenser lenses?
1. desirable to achieve small beam in double condenser system
2. not too intense or bright to damage ultrathin specimen
where is specimen chamber?
between illumination system lenses and image forming lenses
How to maintain vacuum in specimen chamber
what are roles of objective lens?
1. resolve and magnifies image of specimen
2. be particularly well engineered
what are other image forming lenses?
intermediated and projector lenses
How to view image?
focused onto fluorescent screen or digital camera
What does electronic focus control?
Coarse and fine
How does contrast in Tem form?
result of scattering of electrons by some parts of the specimen
How does contrast form in light microcopy?
result of colour differences &/or refractive index differences
what electrons are participated in final image formation
scattered through a small angle or not scattered
How will membranes enhanced in TEM?
enhanced by staining with heavy metal stains
what aberrations can be formed in TEM?
similar to LM
astigmatism (particularly important)
How is vacuum in the TEM created?
by a series of pumps
a series of pumps
rotary & diffusion pumps &/or others
How is vacuum in the TEM maintained?
by special seals & airlocks
What does vacuum do?
Electrons have to travel from gun to fluorescent screen or camera. Air in column will prevent this.