Surface Electronic Structures and Field Emission Currents at Sodium Overlayers on Low-Index Tungsten Surfaces

TL;DR

This study investigates how sodium overlayers alter the electronic structures and emission currents of tungsten surfaces, providing detailed theoretical insights that align with experimental observations of peak shifts in field emission.

Contribution

The paper introduces a numerical method extending the full-potential linear augmented plane wave approach to analyze surface emission and electronic structure modifications due to Na overlayers on tungsten.

Findings

Na overlayers cause significant shifts in surface state peaks in TEDs.

Hybridization of Na 3s states stabilizes certain tungsten surface states.

Na coverage influences surface density of states, matching experimental trends.

Abstract

The total energy distributions (TEDs) of the emission currents in field emission and surface photofield emission and the overlayer-induced modifications in the surface electronic structures from the technologically important W surfaces with the commensurate W(100)/Na c(2x2), W(110)/Na (2x2) and W(111)/Na (1x1) overlayers are calculated. The TEDs obtained by our recent numerical method that extends the full-potential linear augmented plane wave method for the electronic structures to the study of field and photofield emission are used to interpret the shifts of the peaks in the experimental TEDs in field emission and photofield emission from the W(100) and W(110) surfaces at sub-monolayer and monolayer Na coverage. Hybridization of the 3s Na states with the pairs of dz2-like surface states of the strong Swanson hump in clean W(100) and surface resonances in clean W(111) below the Fermi energy shifts these W states by about -1.2 eV and -1.0 eV, thus stabilizing these states, to yield new strong peaks in the TEDs in field emission and photofield emission from W(100)/Na c(2x2) and W(111)/Na (1x1) respectively. The effect of Na intralayer interactions are discussed and are shown to shift the strong s- and p-like peaks in the surface density of states of W(110) below and above the Fermi energy respectively to lower energy with increased Na coverage, in agreement with experiments.