Confinement of surface state electrons on Cu and Au field emitters

Field-emission images of clean Cu and Au emitters show a peculiar halo-like ring centered at the {1 1 1} pole. Typical diameter and width of the ring are ∼10 nm and ∼2 nm, respectively. Since we found no geometrical features around the {1 1 1} pole that gives rise to such a ring pattern, we interpreted the ring pattern as representing enhanced emission from an annular terrace that resonantly confines surface electrons. A simple analysis shows that the observed ring pattern appears at a terrace whose width nearly matches the confinement condition.