Transmission Electron Microscopy Flashcards Preview

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Flashcards in Transmission Electron Microscopy Deck (53):
1

Why use an electron microscope?

improve resolution

2

what is Abbe's equation?

Dmin=wavelength / 2 NA

3

what is wavelength of visible light?

400 – 700nm

4

what is resolution /Dmin of light microscope

0.2um or 200nm

5

what is wavelength of electrons?

0.005nm

6

what is resolution /Dmin of electron microscope?

5nm

7

what are the two types of electron microscope?

1. transmission electron microscope (TEM)
2. scanning electron microscope (SEM)

8

what are the similarities of TEM &SEM?

1. both use a beam of electrons
2. electron production
3. condenser lens system
4. condenser
5. high vacuum

9

How are electrons produced in TEM &SEM?

from a filament

10

what is filament in TEM &SEM

electron source at the top of the column

11

what happen before electrons entry into the condenser lens system in TEM &SEM?

negative electrons accelerated towards uncharged anode into condenser lens system

12

why are the condensers special in TEM &SEM

condensers focus electrons onto the specimen

13

what does high vacuum do in TEM &SEM?

allow passage of electrons down column

14

what are the unique feature of TEM?

1. stationary beam
2. thin specimen
3.internal details
4. final image

15

thin specimen in TEM?

electrons pass through 50nm thick specimen

16

internal details in TEM?

image of internal detail of specimen formed by image forming magnetic lenses

17

image forming magnetic lenses

internal detail

18

final image in TEM?

final image on a fluorescent screen or photographed

19

what re the unique feature of SEM?

1. moving beam
2. scan surface of bulk specimen
3. emit electrons
4. image formation

20

where is moving beam produced in SEM?

scan coils between condenser and specimen

21

what surface in SEM scanned

natural or created

22

electron emission?

emit electrons of various energies (X rays)

23

where are images formed in SEM?

by detects and amplifiers to TV screen
Not by magnetic lenses

24

what does anode with opening do?

it is earth zero charge
and attract electrons

25

what does Wehnelt cylinder do?

It is very negative
and repel electrons

26

what is accelerating voltage if TEM?

potential difference between the filament and the anode
set by operator

27

what does large accelerating voltage mean?

faster electron with high energy and shorter wavelength

28

what are the features with fast electrons

1. penetrate specimen more readily
2. lose energy less easily
3. scatter less, less contrast

29

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

30

what is magnetic field in electron microscope?

the lens

31

What are magnetic lenses like

optically like a biconvex glass lens

32

how to change the strength of the magnetic field

with a switch

33

what is relation of focal length in magnetic lens?

stronger field short focal length

34

when is magnetic filed stable ?

1. current not vary
2. no other magnetic field
3. temp not vary
4. avoid mechanical disturbances

35

what is limited in lenses?

1. limiting aperture to
improve image quality
reduce aberrations

36

what does condenser lenses do?

focus beams of electrons onto specimen

37

two condenser lenses?

1. desirable to achieve small beam in double condenser system
2. not too intense or bright to damage ultrathin specimen

38

where is specimen chamber?

between illumination system lenses and image forming lenses

39

How to maintain vacuum in specimen chamber

an airlock

40

what are roles of objective lens?

1. resolve and magnifies image of specimen
2. be particularly well engineered

41

what are other image forming lenses?

intermediated and projector lenses

42

How to view image?

focused onto fluorescent screen or digital camera

43

What does electronic focus control?

Coarse and fine

44

How does contrast in Tem form?

result of scattering of electrons by some parts of the specimen

45

How does contrast form in light microcopy?

result of colour differences &/or refractive index differences

46

what electrons are participated in final image formation

scattered through a small angle or not scattered

47

How will membranes enhanced in TEM?

enhanced by staining with heavy metal stains

48

what aberrations can be formed in TEM?

similar to LM
Chromatic
spherical
astigmatism (particularly important)

49

How is vacuum in the TEM created?

by a series of pumps

50

a series of pumps

rotary & diffusion pumps &/or others

51

How is vacuum in the TEM maintained?

by special seals & airlocks

52

What does vacuum do?

Electrons have to travel from gun to fluorescent screen or camera. Air in column will prevent this.

53

how to view the final image?

1. fluorescent screen in viewing chamber
2. TV screen
3. photographed (as ultrathin fragile)