This paper could be a starting point:

http://www.sciencedirect.com/science/article/pii/S0039602801010366

The work function of HOPG is (4.6 ev )

this is a very good question and open point (see my doubts in

doi:10.3390/ma11060951). I actually think that AU can be a better

reference sample for SKPM, with less uncertainty, around 5.0 eV.

However, the best thing is to have, in my opinion, the WF measured

ON THE VERY SAME referfence sample that one will use in SKPM,

by means of eg UPS.

Take care.