Overview of Kelvin Probe Method
- Background
- Kelvin method invented by Lord Kelvin over a century ago
- Work Function - energy required to remove an electron from material
- indicates surface conditions such as
- absorbed, evaporated layers, surface reconstruction, surface charging, oxide layer imperfections, surface and bulk recombinations
- Use a reference surface to study a sample electrode
- the two are conductors that form a parallel plate capacitor at the tip
- Capacitance - C = ε A/D
- ε is the permittivity of the material in between plates, A is area connected as capacitor, D is distance between two conductors
- C = Q/V
- C is capacitance, Q is charge, v is voltage
- Q = V ε A / D
- As long as it is possible to determine voltage difference charge can be calculated, ε = permittivity of free-space if instrument is in vacuum
- Null method - apply external voltage V1 to probe. If V1 is equal to Voltage on probe then V is 0, so no current is flowing to or from the probe
- Current Nullification method
- Probe is vibrated in sinusoid and is perpendicular to the surface
- Distance between plates D = D0 + D1sin(ωt)
- D1 is the amplitude of oscillation
- Find new capacitance
- Find Current generated
- I = V dC/dt
- I = V d (ε A/[D0 + D1sin(ωt)]) /dt
- I = V ε A [D1ωsin(ωt)]/[D0 + D1sin(ωt)]2
- Then we nullify this current by bringing Voltage to 0
- This is the solution to noise difficulties that come from null field method
- Distance D is maintained due to parallel movement of probe
- Capacitance remains constant as permittivity and area doesn’t change
- Therefore we are looking for the variations in surface potential/charge
- Current I is generated I = (dV/dt) C
- Drawbacks
- This results in an averaged potential measurement
- Given some potential function, the instrument will measure the enclosed charge averaged over the size of the measuring tip
- Resultant Error
- If diameter of measured potential > diameter of tip error results = 2 times the size of the tip
- If diameter of measured potential = diameter of tip error results = 2 times the size of the measured potential
- If diameter of measured potential < diameter of tip error results = 2 times the size of the measured potential
- To get accurate measurements the probe must be both close to the surface and small in comparison to measured charge to minimize edge effects
