Scanning Tunnelling Microscope (STM)

[EIRE2003]

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

What does this actually mean?

 

[f95toli]

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]

''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.

 

[EIRE2003]

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]

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]

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?

Er... DOS is dN/dE, where N is the number of states per unit volume.

You can't really consider the "energy levels in an atom" when you're doing a STM on a solid. I'm sure I've mentioned this many times. A solid doesn't behave the same way as isolated atoms. That's why "solid state physics" isn't the same field of study as "atomic/molecular physics". An atom's "DOS" is meaningless because an atom has discrete energy levels, and thus a dN/dE operation would be strange. The DOS you get from an STM of a solid, such as a superconductor, gives a continuous result!

Zz.