Microscopes Flashcards
How does a TEM work?
-electron gun emits electrons through thermionic emission
-Then accelerated to high speeds (and therefore short wavelengths) by a large potential difference
-The function of the condenser lens:
The condenser lens’ magnetic field deflects the electrons into a wide beam travelling parallel to the axis of the microscope (straight down)
-This parallel beam is uniformly incident on the sample
-The function of the objective lens:
This lens forms an image of the sample
It deflects the outer electrons in the beam towards the central axis, much like a convex optical lens does for light
-Electrons travelling along the microscope’s axis are not deflected, again similarly to light in a convex lens
-The function of the projector lens (final lens):
This lens causes the beams from the objective lens to spread out, magnifying the image created by the objective lens
This magnified image is directed onto a fluorescent screen, emitting light where electrons are incident
What’s the drawbacks of TEM
-Velocity, WL, Power
-sample
-none same speed
-
-The level of detail available in an image depends on the resolving power
In an electron microscope, electrons need to travel as fast as possible to have the shortest wl and therefore highest resolving power.
-In the TEM the electrons must pass through the sample which reduces the speed of electrons, increasing wavelength and reducing resolving power so electron waves are unable to resolve as much detail as their short wavelength would allow
-Not all electrons emitted by thermionic emission have the same speed, and not all electrons are slowed by the sample to the same degree
This means electrons in the beam have a range of speeds
-Electrons travelling at different velocities through a magnetic field are deflected by different amounts
This means electrons passing through a single point in the sample are projected onto a range of locations on the fluorescent screen instead, forming a blurrier image
STM’s resolution
-able to resolve objects 0.001nm apart
Role of Piezoelectric transducers for STM
-The probe is moved by pieces of equipment called piezoelectric transducers which keeps the probe with a fine tip (few atoms across) held a few nm above the surface of the object
-These are able to move the tip in any direction by tiny increments of 0.001 nm
Why’s there tunnelling current in STM
-The probe is held at a constant potential difference with the surface of the sample
-Some electrons are able to jump, or tunnel, to the tip due to small gap producing tunnelling current
To be able to scan this surface to produce an image of it:
When the tip reaches a raised atom, the distance from the tip to the surface decreases, and more electrons tunnel so the tunnelling current increases
Likewise, when the tip reaches a dip in the surface, the tunnelling current decreases because the gap is larger and fewer electrons tunnel
These changes in tunnelling current are used to produce a map of the surface of a sample
What are the 2 modes STM operate in
-Constant height and Constant current
What happens in constant height mode?
-The tip does not move vertically up or down while scanning areas
-This means that changes in the surface increase or decrease the gap size
-The tunnelling current therefore varies
-This is used to produce an image of the surface
What happens in constant height mode?
-The tip does not move vertically up or down while scanning areas
-This means that changes in the surface increase or decrease the gap size
-The tunnelling current therefore varies
-This is used to produce an image of the surface
What happens in constant current mode?
-When a change in tunnelling current is detected, the tip moves up or down to keep the current constant
This means the gap size is always the same
The vertical motion of the probe is used to map an image of the surface
Explain Quantum tunnelling in STM
-Quantum tunnelling is a result of the wave-like behaviour of particles such as electrons
-The gap between the surface and the tip acts as a barrier for electrons
The amplitude of the matter-wave of the electrons is decreased by this barrier, but on the other side of the gap, this amplitude is non-zero
This effect only occurs if the barrier is weak enough (i.e. the distance is small enough)
This is like thin surfaces not being opaque to visible light because some of the wave can pass through
This results in a current passing from sample to probe
This current is very sensitive to changes in gap distance, which allows the STM to have a great resolving power
