Electronic structures of tungsten surfaces with barium overlayers by field emission and photofield emission.

摘要

The total energy distributions (TEDs) in field emission (FE) and photofield emission (PFE) and the work functions have been measured at room temperature for the (100), (110) and (111) W facets with Ba overlayers in the range of coverage from 0 to 1 monolayer. In order to interpret the experimental data, the full-potential linear augmented plane wave method for calculating the electronic structures of periodic lattices within the LDA has been extended to obtain the TEDs in FE and PFE from W/vacuum and W/Ba/vacuum interfaces. A prominent peak observed experimentally at -1.90 eV in PFE from W(100) with a c(2x2) Ba overlayer is attributed, in contrast to previous work, to hybridization of dz2 -like surface states of clean W(100) with s -like states of the overlayer. It is suggested that a prominent asymmetrical peak observed at -0.65 eV in FE from W(111) is due to two bands of dz 2 -like surface resonances, and a prominent peak observed at about -2.0 eV in PFE from W(111) with a (1x1) Ba overlayer is attributed to hybridization of these same resonances with s -like states of the overlayer. It is shown that several of the peaks observed in PFE are induced by the reduced symmetry of the overlayer. It is found that when an isolated (31/2x3 1/2) Ba layer is adsorbed on W(111) it undergoes a nonmetal-to-metal transition and the surface electronic structure is dominated by inter-layer W-Ba interactions. The atomically-denser isolated (1x1) Ba layer is metallic, and when it is adsorbed on W(111) the surface electronic structure is dominated by intra-layer Ba-Ba interactions. These properties are also discussed for Ba overlayers on W(100) and W(110). A c(2x2) Ba overlayer on W(100) induces a strong electric dipole layer between the substrate and the overlayer and a weak oppositely-directed dipole layer outside the surface, which together account quantitatively for the observed reduction in work function. In view of the success of the present method in interpreting the TEDs in FE and PFE at W/Ba interfaces, it is also applied to calculate the surface electronic structures of these interfaces and the results are discussed.