| Summary: | We have performed density functional VASP calculations of a pure and of a carbon-covered (100) tungsten surface under the presence of an electric field <b>E</b> directed away from the surface. Our aim is to answer the question of an increased penetrability of electrons at the collector side of a nanometric tunnel diode when covered by carbon atoms, a purely quantum mechanical effect related to the value of the workfunction Φ. To obtain Φ at a non-zero electric field we have extrapolated back to the electrical surface the straight line representing the linear increase in the potential energy with distance outside the metal-vacuum interface. We have found that under the presence of <b>E</b> the workfunction Φ = E<sub>vac</sub> − E<sub>F</sub> of the (100) pure tungsten surface has a minor dependence on <b>E</b>. However, the carbon-covered tungsten (100) surface workfunction Φ(C − W) has a stronger <b>E</b> dependence. Φ(C − W) decreases continuously with the electric field. This decrease is ΔΦ = 0.08 eV when <b>E</b> = 1 V/nm. This ΔΦ is explained by our calculated changes with electric field of the electronic density of both pure and carbon-covered tungsten. The observed phenomena may be relevant to other surfaces of carbon-covered tungsten and may explain the reported collector dependence of current in Scanning Field Emission Microscopy.
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