Part II of Surface Primer. Spectroscopic Techniques for Probing Solid Surfaces. Characterization of a Surface. To fully characterize a surface, we need to know: What types of atoms are present at a surface and what is their surface concentration?
To fully characterize a surface, we need to know:
What types of atoms are present at a surface and what is their surface concentration?
Where are the atoms/molecules located on a surface?
How strong is the bonding of adsorbate atoms and how does the nature of the surface bond influence surface reactivity?
Si(111) 7 X 7 LEED Pattern
Atomically clean single crystal surfaces of differing geometric structure have different work functions.
A surface does not present an infinite potential energy barrier to the e-’s within a solid.
The e- wavefunctions may have a non-zero amplitude ‘just outside’ (within 10 Å) of the surface.
Adsorption induces changes in the work function
modifications of the surface dipolar layer
particularly if significant charge transfer occurs between the adsorbate and surface
measurements of ΔΦ yield critical information on the degree of charge reorganization upon adsorption
ΔФ = Фadsorbate covered - Фclean
I(W) = Cexp(-WΦ^(1/2))
The tunnelling current will increase in areas where protrusions exist because of a lowering in the gap distance.
Constant current mode: the value of W (tip gap) is fixed by movement of the tip in the z-direction, while scanning in the xy-plane. A plot of the z-piezo electric voltage vs. lateral position is created.
Good for rough surfaces to avoid tip-surface collisions
ν = 1/2π√K/m
Non-contact (tapping) mode: The tip vibrates close to its resonance frequency. Variations in the sample-tip forces alter the resonance frequency, and the shift is used to measure the magnitude of the forces in action.
Topographic images of surface force vs. lateral position
Good for delicate samples