Work Functions of Elements
Work function measurements are very sensitive to surface cleanliness as well as crystal orientation. Materials are often measured “as received” and then after “ion etching” to remove surface contamination. Cleanliness of an ion etched surface depends on the vacuum level. If using low vacuum levels, then you will find that the work function changes with time because the surface is re-oxidizing and/or collecting organic gases that reside in the vacuum.
After ion etching the crystal structure within the top few nanometers might have been disturbed. With time, the surface will restructure itself to the original structure, but it will also become slightly contaminated. Heating might be useful. A cluster ion gun might be a useful solution to cleaning the surface without degrading the crystal structure.
Work functions can be measure by XPS, but are more often measured by UPS. Single crystals have different crystal structure on different faces which affects work function by ~10-15%. When cleaving a single crystal you need to determine what crystal structure exists on the new surface just exposed.
Assembled from various reference sources.