Scanning Tunnelling Microscopy Flashcards
(10 cards)
An electron moving in a finite potential well has…
non-zero probability to be found outside the well.
Describe tunnelling when there is no bias voltage.
When the tip is sufficiently close, e- tunnel to equilibrate the Fermi levels. At equilibrium, only thermally activated e- tunnel, so no net current. (no unoccupied states)
What are two imaging modes of the STM?
constant current
constant height
Describe the constant current imaging mode.
feedback loop is used to maintain a constant tunnelling current - vary tip-sample distance.
Describe the constant height imaging mode.
feedback loop is not operative, the tip is moved across the surface at fixed vertical position. The recorded variation in tunnelling current is due to sample topography.
What is interesting about STM imaging of graphite?
Graphite surface is made up of alpha atoms (second atom directly below) and beta atoms (no atom below). Free e- from alpha tend to be trapped between alpha atoms, e- from beta atoms free to tunnel. so beta atoms, not alpha atoms, are detected.
Why is a high vacuum required in STM imaging?
the presence of a contamination layer on the tip or sample reduces the potential barrier - lower work function measurement.
A negative tip investigates…
occupied states in the sample.
A positive tip investigates…
unoccupied states in the sample.
For a small bias voltage, DOS tip and transmission probability ~ const. This means that DOS sample is proportional to…
dI/dV.
