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

Nanoprotrusion (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 ($a\text{−}\mathrm{C}$) coated tungsten nanotip at various nanoscale vacuum gap distances. We find that under certain conditions, the FE current-voltage ($I\text{−}V$) 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 $I\text{−}V$ 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 nanoprotrusion and its continued growth during successive FE measurements in TEM. We tentatively attribute this phenomenon to field-induced biased diffusion of surface $a\text{−}\mathrm{C}$ atoms, after performing a finite element analysis that excludes the alternative possibility of field-induced plastic deformation.

Figure 1

(a) Schematic diagram of the in situ morphology characterization and field emission measurement system. (b) TEM image of the amorphous carbon-coated tungsten nanotip and the gold plate anode. (c) TEM images of $a\text{−}\mathrm{C}$ coated tungsten nanotip after the FE measurements in $d_3$ gap and the corresponding nanoprotrusion growth. (d) TEM image of the nanotip and anode in contact. Inset in (d): $I\text{−}V$ curves during short circuit with the corresponding resistivity of the coating being $\sim 3.28\times {10}^6\mathrm{\Omega }\mathrm{nm}$.

Figure 2

Measured field emission $I\text{−}V$ curves (dotted lines) from the $a\text{−}\mathrm{C}$ coated tungsten nanotip under different nanogaps. The corresponding simulated curves are given in solid lines, with the fitted gap distances found to be $d_1=50\mathrm{nm}$, $d_2=37\mathrm{nm}$, $d_3=41.5\mathrm{nm}$, and $d_4=17\mathrm{nm}$. The field emission current ($d_{1,2}$) exhibits four states: initial stable low current state 1, low-high current transitional state 2, stable high current state 3, and final stable low current state 4. In the $d_3$ case, the field emission current shows two states: low-high current transitional state 2 with an intensively fluctuating $I\text{−}V$, and stable high current state 3. In the $d_4$ case, only the stable high-current state 3 is observed, together with a permanent NP outgrowth visible in TEM.

Figure 3

Schematic representation of the tip evolution. (a) the initial nanotip before FE measurements. (b),(c) the simulated NP (colored lines) for $d_{1,2}$. The color coding gives the calculated surface electric field distribution, with the NP exhibiting a significant local field enhancement. (d),(e) the stable NP outgrowth visible in TEM after the FE experiments in $d_3$ and $d_4$.

Figure 4

Results of the electrostatic-elastoplastic model for the $a\text{−}\mathrm{C}$ coating under Maxwell stress.