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

摘要

The total energy distributions (TEDs) of the emission currents in fieldemission and surface photofield emission and the overlayer-inducedmodifications in the surface electronic structures from the technologicallyimportant 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 recentnumerical method that extends the full-potential linear augmented plane wavemethod for the electronic structures to the study of field and photofieldemission are used to interpret the shifts of the peaks in the experimental TEDsin field emission and photofield emission from the W(100) and W(110) surfacesat sub-monolayer and monolayer Na coverage. Hybridization of the 3s Na stateswith the pairs of dz2-like surface states of the strong Swanson hump in cleanW(100) and surface resonances in clean W(111) below the Fermi energy shiftsthese W states by about -1.2 eV and -1.0 eV, thus stabilizing these states, toyield new strong peaks in the TEDs in field emission and photofield emissionfrom W(100)/Na c(2x2) and W(111)/Na (1x1) respectively. The effect of Naintralayer interactions are discussed and are shown to shift the strong s- andp-like peaks in the surface density of states of W(110) below and above theFermi energy respectively to lower energy with increased Na coverage, inagreement with experiments.