# Scanning Tunnelling Microscope (STM)

## Main Question or Discussion Point

''The Scanning Tunnelling Microscope probes the density of states of a material''....

What does this actually mean?

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f95toli
Gold Member
The signal from an STM is proportional to the tunnelling current between the tip of the probe and the surface that is being scanned.
Now, tunnelling current is of course proportional to the tunnelling propability which in turn is determined by -among other things- the number of states that are allowed to tunnell for a specfic bias, i.e. it is proportional to the density of states.

ZapperZ
Staff Emeritus
''The Scanning Tunnelling Microscope probes the density of states of a material''....

What does this actually mean?
It means that the STM is doing "spectroscopy" rather than imaging. It scans something resembling an "I vs. V", at a particular location of a material, where I is the current, and V is the bias potential between the STM tip and the material. The density of states (assuming you know what this is), is proportional to the the derivative of this curve, i.e. dI/dV vs. V.

Zz.

Ok, am I right in suggesting that the 'density of states' is, how closely packed the energy levels in an atom are, or is it how closely packed the energy levels in the material as a whole? That is, the energy gap between the valence band and the conduction band?

G01
Homework Helper
Gold Member
Your first definition of density of states is the more correct one. The density of states determines how closely packed (in energy space) the energy levels in a quantum mechanical system are.

ZapperZ
Staff Emeritus