Editing Work function (section)

=== Kelvin probe method ===
{{see also|Volta potential|Kelvin probe force microscope|Scanning Kelvin probe}}

[[File:Kelvin probe setup at flat vacuum.svg|thumb|300 px|Kelvin probe energy diagram at flat vacuum configuration, used for measuring work function difference between sample and probe.]]

The Kelvin probe technique relies on the detection of an electric field (gradient in ''ϕ'') between a sample material and probe material.
The electric field can be varied by the voltage Δ''V''<sub>sp</sub> that is applied to the probe relative to the sample.
If the voltage is chosen such that the electric field is eliminated (the flat vacuum condition), then
:<math>e\Delta V_{\rm sp} = W_{\rm s} - W_{\rm p}, \quad \text{when}~\phi~\text{is flat}.</math>
Since the experimenter controls and knows Δ''V''<sub>sp</sub>, then finding the flat vacuum condition gives directly the work function difference between the two materials.
The only question is, how to detect the flat vacuum condition?
Typically, the electric field is detected by varying the distance between the sample and probe. When the distance is changed but Δ''V''<sub>sp</sub> is held constant, a current will flow due to the change in [[capacitance]]. This current is proportional to the vacuum electric field, and so when the electric field is neutralized no current will flow.

Although the Kelvin probe technique only measures a work function difference, it is possible to obtain an absolute work function by first calibrating the probe against a reference material (with known work function) and then using the same probe to measure a desired sample.<ref name="calib"/>
The Kelvin probe technique can be used to obtain work function maps of a surface with extremely high spatial resolution, by using a sharp tip for the probe (see [[Kelvin probe force microscope]]).