How does zirconium-oxide coating reduce work function in SEM tips?

[teaforone]

Hi Folks,
I have been studying some scanning electron microscopy and read that the SEM tip is made from tungsten but is coated with a mono-layer of zirconium-oxide to reduce the work function from 4.5ev to 2.7ev, to make it easier to emit electrons (this is a field emission SEM).

Does anyone have any idea how this reduction in work function occurs? What is the physical mechanism and what is occurring at the interface between the two substances to lower the work function? Any references or links to help me comprehend this would be greatly appreciated.

Thanks.

 

[DrDu]

Work function and it's dependency on surface effects is well explained in Ashcroft Mermin, Solid state physics.
Basically there are two important factors: 1. The Fermi energy of the metal, 2. The dipole potential at the surface.
The latter one can be influenced by a surface layer.

 

[teaforone]

Thank you DrDu, I'll try to check that out.