In Situ Observation of Nanoprotrusion Growth on a Carbon Coated Tungsten Nanotip under Field

Nano-protrusions (NPs) on metal surfaces and their contamination layers can be precursors of vacuum breakdown, detrimental to many devices. We run Field Emission (FE) experiments in-situ a Transmission Electron Microscope (TEM) on an amorphous-carbon (a-C) coated tungsten (W) nanotip. In various cases we observe the FE current-voltage (I-V) curves switching abruptly into an increased current state. Accompanying FE simulations show that the enhanced-current I-Vs are consistent with the growth of an NP at the tip apex. The tip growth hypothesis is confirmed by the direct observation of such an NP and its continued growth during successive FE experiments. We attribute this phenomenon to field-induced biased diffusion of atoms on the surface of the a-C coating.