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The Surface Work Function

When a group of atoms or molecules are
brought together to form a solid the highest
occupied energy level, or fermi-level is termed
the work function. The work function is continuous
across the interior of the solid, however at the
surface the electron energy is influenced by the
exact state of the surface, e.g. type orientation and
direction of the outer atoms and molecules. Thus different crystallographic orientations of the same solid may have
different work functions. The work function of a surface
can be modified by adsorbed layers, for instance in corrosion or oxidation of an iron surface, or by modification of composition or controlled contamination as in the case of polymer or inorganic semiconductors.

When two or more materials are brought together the Fermi-levels equalize by a flow of electrons from the lower work function to the high. Detecting these electrons this in essence is the way all Kelvin Probe systems work.

The work function is often described in introductory textbooks as 'the energy required to remove an electron from a surface atom to infinity or equivalently the vacuum level'. Although this description appears easy, in surface analysis one has to ask further questions such as which type of electron, where is it taken from and where does it end up?

For example as an electron is removed a short distance 'd' from a conducting solid there is a so-called attractive image force (due to an imaginary positive charge a distance 'd' inside the solid). The force on the electron diminishes as 'd' gets larger and by approximately 30-50 nm it is exceedingly small. However some work function detection systems actually operate within this region thus the electron is not removed to infinity. This effect, coupled with the local electric fields in the vicinity of the tip, distort the work function data recorded.

In all KP Technology systems the Kelvin Probe tip always operates at sufficiently large distances to produce the 'True Work Function' and further the mean spacing of the tip electrode above the sample is tightly controlled producing high- stability, repeatable measurements and allowing automatic system set-up.

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