KP Fundamentals

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
• C = ε A/[D0 + D1sin(ωt)]
• 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

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