Understanding STM: The Role of Tunneling Current in Atom Manipulation

[Fermat]

Hello everyone!
I have a little question about STM.
Why can we move atoms with STM by increasing the tunneling current?

 

[kyle_soule]

The tip is moved down towards the atom and this increases the attractive force by increasing the tunneling current. The tip so close overcomes the energy barrier usually present that would prevent the atom from moving. The native atom is then dragged across the foreign surface that was initially used to adsorb.

 

[marcus]

Originally posted by kyle_soule
The tip is moved down towards the atom and this increases the attractive force by increasing the tunneling current...

Please refresh my memory, what is the cause of this attractive
force between the atom and the tip?
does it actually depend on having the tunneling current flow or is
it just a sort of atomic force stickiness? or is it an electrostatic
attraction depending on voltage difference rather than current? sorry if question is
naive---i want to imagine it in very concrete terms if possible---thx

 

[kyle_soule]

Originally posted by marcus
Please refresh my memory, what is the cause of this attractive
force between the atom and the tip?
does it actually depend on having the tunneling current flow or is
it just a sort of atomic force stickiness? or is it an electrostatic
attraction depending on voltage difference rather than current? sorry if question is
naive---i want to imagine it in very concrete terms if possible---thx

I'm pretty sure its electrostatic attraction.

There are two methods of oxidation, AFM-tip induced oxidation and current-induced oxidation. A search on google for AFM tips would yield informative links, also providing explanations of STM/AFM.

 

[Fermat]

Originally posted by kyle_soule
increases the attractive force by increasing the tunneling current.

Why can increasing tunneling current increase the attractive force?

 

[kyle_soule]

Originally posted by Fermat
Why can increasing tunneling current increase the attractive force?

Scanning Tunneling microscopy (STM) provides a way to get the molecular resolution down to 0.2 nm, keeping in mind the layer is thinner than the electron tunneling distance. STM works well with flat lying molecules, because the limit for non-destructive imaging is limited to the the order of 1 V, which corresponds to the typical tunneling distances of the order of 1 to 2 nm [for thicker layers SFM (scanning force microscopy) is used, which yields a smaller lateral resolution, to the order of 1 nm.

STM uses a sharp metallic tip, which is lowered parallel to the surface, and is controlled by an atomic length scale. There are two requirements of the system under investigation: enough electrical conductivity and limited mobility. STM does not require an electronic conductivity of the system, although, if the conductivity is sufficient there is a greater degree of control, resulting in very specific and controlled probing, concerning the energy of the tunneling electrons and density.

The basic concept of STM is as follows: if the electronic orbitals of the outermost atoms of two solids overlap spatially, a tunneling current will flow, given an electric potential difference is applied between the two. A feedback loop maintains the constant current between the tip and the surface. The tunneling current is basically proportional to the local density of states of the surface at the center of curvature of the tip and at the Fermi energy.

This should fill in the gaps in the past posts. I hope