  We report large negative electron affinity (NEA) on diamond (100) using
magnesium adsorption on a previously oxygen-terminated surface. The measured
NEA is up to $(-2.01\pm0.05)$ eV, the largest reported negative electron
affinity to date. Despite the expected close relationship between the surface
chemistry of Mg and Li species on oxygen-terminated diamond, we observe
differences in the adsorption properties between the two. Most importantly, a
high-temperature annealing step is not required to activate the Mg-adsorbed
surface to a state of negative electron affinity. Diamond surfaces prepared by
this procedure continue to possess negative electron affinity after exposure to
high temperatures, air and even immersion in water.
