Electron microscope

Question 1

A greater wavelength gives _____ diffraction

Answer 1

more

Question 2

Why are electron microscopes > optical microscopes

Answer 2

the tiny wavelength of electrons means less diffraction compared to visible light meaning a finer detail can be resolved achieving much greater resolution and magnification.

Question 3

How are electrons provided to the microscopes

Answer 3

by being accelerated towards a sample using a positive electron potential – an electron gun

Question 4

To resolve detail around the size of an atom, the electron wavelength needs to be similar to be…or smaller meaning an anode voltage of…

Answer 4

similar to the diameter of an atom (~0.1nm) or smaller meaning an anode voltage of: > 150V

Question 5

What limits the resolving power of a TEM:

Answer 5

• Sample thickness: scattered electrons suffer slight loss of speed due to sample’s thickness which increases de Broglie wavelength reducing resolving power - Lens aberrations: thermionic emission produces electrons of slightly different speeds so slightly different forces in magnetic field so focused differently blurring the image + causes different de Broglie wavelengths reducing resolution

Question 6

How does a transmission electron microscope work

Answer 6

• evacuated tube in which a beam of electrons is directed at a thin sample using an electron gun - some electrons scattered by structures in sample as they pass through - Magnetic lenses focus scattered electrons onto screen to form magnified image - Condenser lens directs electrons at the sample - Objective lens forms intermediate magnified image - Magnifier/Projector lens magnifies image further to form enlarged image on screen - Sometimes first lens isn’t included

Question 7

Resolving power =

Answer 7

least separation between two objects in the image that can just be seen apart

Question 8

Determinants of TEM resolving power

Answer 8

• Amount of diffraction when scattered electrons pass through lens - Increasing anode pd increases electron’s speed reducing de Broglie wavelength reducing diffraction improving image detail - Also enlarged the image on the screen so a larger, more detailed image is seen - Image is also brighter as more electrons reach the screen per second - Smaller wavelength = more detailed image e.g. blue better than red light

Question 9

Pro and Con of TEM

Answer 9

Pro: electrons give better resolving power and more detail than visible light Con: requires a very thin sample

Question 10

How does a scanning tunnelling microscope:

Answer 10

• Probe moved over surface of sample - Small voltage applied between probe and surface to ensure electrons cross gap from –ve to +ve only - Electrons tunnel from probe to surface resulting in a weak electrical current - The narrower the gap, the greater the current - Probe scanned over surface, current measured against time, image mapped

Question 11

How does quantum tunnelling work

Answer 11

just like waves can cross narrow gaps, the wave nature of electrons means there is a finite probability they can transfer across the gap between the sample surface and tip probe or pass through an apparent barrier such as an air gap if it is narrow enough ~ similar to de Broglie wavelength. If the gap is too wide, the probability of transfer is neglible. A pd must exist across the gap so that the electrons can transfer from –ve to +ve only so a current flows otherwise there would be an equal transfer of electrons in each direction so a current wouldn’t flow.

Question 12

Two modes of STM:

Answer 12

Constant height: height of probe kept constant. As surface of material changes, the gap width changes so the probability of electron transfer changes altering the current. A decrease of current means the gap widens and so the current due to electron transfer is measured and the variation of current with time is used to map the surface Constant current mode: current due to electron transfer is measured and feedback used to keep it constant by changing the height of the prove tip to keep gap width constant by raising the tip height if the surface rises and vice versa so the same amount of electrons transfer each second allowing the variation of the height of the probe tip with time to be used to map the surface of the sample.

Question 13

The diagram shows the tip of a scanning tunnelling microscope (STM) above a conducting surface. The tip is at a potential of –1.0 V relative to the surface. If the tip is sufficiently close to the surface, electrons transfer from the tip to the surface. The tip is made to scan the surface along a horizontal line. Describe and explain the effect on the current between the tip and the surface if the tip moves across a pit in the surface:

Answer 13

• current would fall (1) • then rise again (1) • probability of transfer decreases with increased gap width (1) • gap width widens then reduces as tip moves across pit (1)

Question 14

What is the function of the three lenses

Answer 14

• the condenser lens = to make a wide parallel beam of electrons (to direct electrons straight at the sample/ensure the beam is uniform) • the objective lens = to form a magnified image of the sample • the third magnetic lens = to magnify the image further, to form the image on a screen

Question 15

State and explain the effect on the resolving power of the T.E.M. if the anode voltage of the electron gun is increased:

Answer 15

• resolving power increases with the increase of accelerating pd • electron wavelength becomes smaller the greater the pd • resolving power is greater the smaller the wavelength

Question 16

In practice, the resolving power of a T.E.M. is limited. State and explain one factor that limits the resolving power:

Answer 16

• lens aberrations/defects caused by electrons having a range of speeds • sample thickness which causes loss of electron speed

Question 17

State and explain how the image of an object observed using this transmission electron microscope in part (a) would change when the anode voltage was increased:

Answer 17

• image would be brighter because more electrons reach the screen per sec (1) • image would be more detailed because de Broglie wavelength would be reduced (1) • and because speed of the electrons is increased (1)

Question 18

In the Scanning Tunnelling Microscope (STM), electrons cross a gap between a sharp metal tip and a conducting surface when the gap is small and a potential difference exists across it. (i) Explain, in terms of wave particle duality, why an electron can cross this small gap: (ii) Explain, why it is necessary for a potential difference to exist across the gap?

Answer 18

(i) • wave-like nature allows an electron (to transfer) (1) • a wave can penetrate thin barriers (or gaps) (1) • (probability of) transfer of an electron (or tunnelling effect) • negligible if gap is too wide (ii) • with a p.d., electrons transfer from – to + only so a current can flow • with zero p.d., equal transfer in either direction

Question 19

The diagram below shows a Transmission Electron Microscope. Electrons from the electron gun pass through a thin sample and then through two magnetic lenses A and B on to a fluorescent screen. An enlarged image of the sample is formed on the screen. State the function of magnetic lens A and the function of magnetic lens B:

Answer 19

A) magnifies (or forms an intermediate image before B) B) magnifies and focuses (or forms an enlarged image on the screen)

Question 20

Explain why greater image detail is seen when the anode voltage is increased:

Answer 20

• increase of voltage causes increase of speed (of the electrons) (1) • hence a reduced de Broglie wavelength (1) • less diffraction for reduced wavelength (1) • better resolution if less diffraction (1)

Question 21

The diagram below shows the probe tip of a scanning tunnelling microscope (STM) above a metal surface. The probe tip is at a constant negative potential relative to the metal surface. Explain why electrons can cross the gap between the probe tip and the surface, provided the gap is sufficiently narrow:

Answer 21

• electrons can behave as waves [or electrons can tunnel across gap] (1) • waves can cross narrow gaps [or non-zero probability of crossing gap] (1) • electron waves would be attenuated too much by large gap • [or probability of transfer negligible if gap too wide][or the narrower the gap, the greater the probability] (1) • electron transfer is from – to + (1)

Question 22

Describe one way in which an STM is used to investigate a surface:

Answer 22

Constant height mode: • tip height constant (1) 3 • current varies as gap width changes (1) • image built up as tip moves across surface [or as tip moves across, a decrease (or increase) of current means the gap widens (or narrows)] (1) Or constant current mode: • tip height altered (1) • to keep current constant (1) • image built up as above or as tip moves across, the tip height rises (or falls) if the surface rises or falls (1)]

Question 23

In a transmission electron microscope, electrons from a heated filament are accelerated through a certain potential difference and then directed in a beam through a thin sample. The electrons scattered by the sample are focused by magnetic lenses onto a fluorescent screen where an image of the sample is formed, as shown in the figure below. State and explain one reason why it is important that the electrons in the beam have the same speed.

Answer 23

• force on an electron in a magnetic field depends on speed (1) • electrons at different speeds would be focussed differently so image would be blurred (1) • [or electrons at different speeds would have different (de Broglie) wavelengths therefore resolution would be reduced]

Question 24

When the potential difference is increased, a more detailed image is seen. Explain why this change happens:

Answer 24

• increase in pd increases speed (1) • increase in speed/momentum/Ek causes reduction of (de Broglie) wavelength (1) • reduced (de Broglie) wavelength gives better resolution (1)

Question 25

In a scanning tunnelling microscope (STM), a metal probe with a sharp tip is scanned across a surface, as shown in the figure below. Explain why electrons transfer between the tip of the probe and the surface when the gap between the tip and the surface is very narrow and a pd is applied across it:

Answer 25

• electrons have a wave-like nature (1) • there is a (small) probability that an electron can cross the gap [or an electron can tunnel across the gap] (1) • transfer is from - to + only (1)

Question 26

Describe how an STM is used to obtain an image of a surface:

Answer 26

Constant height mode: • gap width varies as tip scans across at constant height (1) • current due to electron transfer is measured (1) • current decreases as gap width increases (or vice versa) (1) • variation of current with time is used to map surface (1) Or constant current mode: • current due to electron transfer is measured (1) • feedback used to keep current constant by changing height of probe tip (1) • height of probe tip changed to keep gap width constant (1) • variation of height of probe tip with time used to map surface (1)]

Question 27

STM diagram

Answer 27

probe tip up electrons (1nm gap). Electrons tunnel across the gap in the direction of the probe tip. Cell and ammeter connected between electrons and probe tip on the right hand side. Positive terminal facing probe tip followed by ammeter.

Question 28

TEM diagram

Answer 28

Question 29

In a TEM, state and explain how the image of a thin sample would change if the sample was removed so that the beam passed through a thicker part of the sample

Answer 29

Increasing the thickness of the sample means the electron pasiing through may suffer a loss of speed due to its thickness which increases the de Broglie wavelength = h/mv and so reduces the resolving power meaning a less detailed image

Question 30

In a TEM, state and explain how the image of a thin sample would change in the anode pd was increased

Answer 30

Resolving power increases with the increase of accelerating pd

Electron wavelength becomes smaller as inversely related to voltage

Resolving power is greater the smaller the wavelength

Better quality image as more detailed because wavelength decreases and speed of electrons increase (which also causes a brighter image as more electrons reach the screen per second)

Question 31

What is meant by the term matter waves

Answer 31

Hypothetical waves associated with the motion of a particle that describes wave-like attributes. For example, particles have a de Broglie wavelength and also diffract when accelerated through graphite crystals allowing for applications such as electron microscopes

Question 32

Explain how a graph of current against tip position is used in STM

Answer 32

The image of a surface can be obtained using constant height mode

The voltage and heigh are held constant while the current changes to keep the voltage from changing leading to an image made of current changes over the surface

The gap width varies as the tip scans across at a constant height.

The current due to electron transfer is measuredd

The variation of current is used to map out the surface