  We report the field emission (FE) properties of Cu coated vertically aligned
carbon nanotubes (VACNTs) before and after oxidation. The current density was
found to be the maximum (20 mA/cm$^2$) for 3 nm thick Cu coated VACNTs after
oxidation. The variation in conventionally monitored parameters like work
function and field enhancement factor does not explain the experimentally
determined FE current density. A critical analysis of the electronic structure
reveals the importance of presence of Cu$_2$O at the interface of CuO and
VACNTs, which in turn controls the current density of these films. The highly
enhanced FE current density of 3 nm Cu coated VACNTs after oxidation suggests
its potential as a next generation electron source in vacuum microelectronic
devices.
