  Nano-protrusion (NP) on metal surface and its inevitable contamination layer
under high electric field is often considered as the primary precursor that
leads to vacuum breakdown, which plays an extremely detrimental effect for high
energy physics equipment and many other devices. Yet, the NP growth has never
been experimentally observed. Here, we conduct field emission (FE) measurements
along with in-situ Transmission Electron Microscopy (TEM) imaging of an
amorphous-carbon (a-C) coated tungsten nanotip at various nanoscale vacuum gap
distances. We find that under certain conditions, the FE current-voltage (I-V)
curves switch abruptly into an enhanced-current state, implying the growth of
an NP. We then run field emission simulations, demonstrating that the temporary
enhanced-current I-V is perfectly consistent with the hypothesis that a NP has
grown at the apex of the tip. This hypothesis is also confirmed by the
repeatable in-situ observation of such a nano-protrusion and its continued
growth during successive FE measurements in TEM. We tentatively attribute this
phenomenon to field-induced biased diffusion of surface a-C atoms, after
performing a finite element analysis that excludes the alternative possibility
of field-induced plastic deformation.
