Work Function:

The minimum energy which must be supplied to extract an electron from a solid; symbol , units J(joule), or more often eV (elecronvolt). It is a measure of how tightly electrons are bound to a material.

The work function of several metals, along specific crystalline direction is given below:

Materials	Work function (eV)
Sodium	2.75
Silicon (111)	4.6
Silicon (100)	4.85
Graphite	5.0
Gold (111)	5.31
Gold (110)	5.37
Gold (100)	5.47