Surface electronic structures of Ba overlayers on W(100), W(110), and W(111)

Abstract

The total energy distributions ͑TEDs͒ in field emission ͑FE͒ and photofield emission ͑PFE͒ and the work functions have been measured at room temperature for Ba adsorbed on W͑100͒, W͑110͒, and W͑111͒ in the range of coverage from 0 to 1 ML ͑monolayer͒. We observe two initial state peaks and three final state peaks on W͑100͒ / Ba, six initial state peaks and one final state peak on W͑110͒ / Ba, and two initial state peaks and two final state peaks on W͑111͒ / Ba. We extend the full-potential linear augmented plane wave method for the electronic structures of periodic lattices to calculate the emission current in FE and surface PFE at a metaladsorbate-vacuum interface. Our calculations account for the energies of all of the initial state features observed experimentally in FE and surface PFE from clean W͑100͒, in PFE and angle-resolved inverse photoemission spectroscopy from W͑100͒ / Ba at 1 ML, and for all of the peaks observed in FE, surface PFE, and photoemission from W͑110͒ / Ba at 0.6 ML and from W͑111͒ /Ba at 1 ML. The dz 2 -like surface states of the Swanson hump ͓L. W. Swanson and L. C. Crouser, Phys. Rev. Lett. 16, 389 ͑1966͔͒ of clean W͑100͒ hybridize with s-like states of the c͑2 ϫ 2͒ overlayer and are shifted by −1.60 eV to yield a prominent peak in PFE. An isolated Ba c͑2 ϫ 2͒ layer is found to be weakly metallic; the metallicity is greatly enhanced when it is adsorbed on a W͑100͒ substrate. The TEDs in PFE from the atomically less dense overlayer W͑110͒ /Ba ͑2 ϫ 2͒ are dominated by substrate-overlayer interactions, while those from the atomically denser overlayer W͑111͒ /Ba ͑1 ϫ 1͒ are dominated by interactions within the overlayer. Our results yield evidence that above 1 / 3 ML Ba coverage on W͑111͒, which corresponds to a commensurate ͑3 1/2 ϫ 3 1/2 ͒R30°overlayer, the interstitial sites fill in randomly to form a commensurate ͑1 ϫ 1͒ overlayer at 1 ML coverage.

Surface electronic structure of Ti-covered W(111) by photofield emission

“…These two states, predominantly confined to the triplet of outermost W planes, have also been predicted (within a different theoretical approach and at slightly lower energies) by other authors, but never resolved experimentally as a double-peak feature before [7,18]. This indicates that our PFE measurements are in fact capable of revealing fine details of the surface DOS of the investigated sample.…”

“…Although this technique does not give any direct information on atomic positions, it is capable of revealing fine details of the electronic density of states (DOS) in the very surface region of the investigated sample (see, e.g., [4][5][6][7][8] and references therein). Thus, by contrasting the measured PFE spectra with the surface DOS distributions computed for various stable structural models, the latter can either be substantiated or invalidated.…”

A Review of Sc-containing "Scandate" Thermionic Cathodes

“…They found that the arrangement of Ba and O was Ba atop O adsorbed on W, with one Ba atom attached to one O atom in B-type cathodes and one Ba atom to two O atoms in M-type cathodes. Complementary computational work conducted by Ibrahim and Lee [93] also reported that the atomic arrangement of Ba on top of O adsorbed on W is more stable than O atop Ba and could yield work function close to those reported for dispenser cathodes.…”

Work function reduction by BaO: Growth of crystalline barium oxide on Ag(001) and Ag(111) surfaces

“…This system can be used as a low-temperature thermionic cathode material [15]. It has been shown that Ba metal adsorbed on the (100), (110), and (111) faces of W results in a lowering of the work function and an increase in thermionic emission [16]. However, these surfaces are highly susceptible to oxidation and despite a slight increase in the work function of the system upon initial oxidation, the formation of a BaO film results in a further lowering of the work function.…”