A field emission study of silver on rhenium

Abstract

Field emission measurements of the change in average work function ḡf of rhenium with adsorbed silver indicate that a rhenium-silver dipole forms with silver positive, of moment μ 0=5.2±1.5 ×10 −30 C m and polarizability α=29±12 A ̊ 3 . Measurement of the rate of thermal desorption yields a mean binding energy of 2.31 ± 0.04 eV for sub-monolayer silver and 2.69±0.04 eV for a 2.5 monolayer deposit. Changes in work function induced by adsorption of silver on low-index rhenium plane surfaces are characterised by the formation of well-defined states and in this, silver resembles gold. These states are thought to result from a relatively large difference between the binding energy of adatoms on the low-index planes and on the surrounding surfaces, and this differnce is maintained when the surfaces are covered with silver. At the lowest coverage, silver is believed to be absent from all four observed planes and the measured rise in work function is thought to be apparent and to result from a decrease in field strength on the plane due to extension of the plane area by surrounding adsorbed silver. The structures adopted by silver overlayers are not known, but it is argued that on (101̄0) and (101̄1̄) the final state at high coverage has the Ag(111) surface structure. On (112̄0) and (112̄2̄) the silver layer at high coverage is thought to have either Ag(110) or Ag(100) surface structure. The structures of intermediate states found on all four low-index planes remain unkown. Field emission spectroscopy shows that emission from clean (101̄0) is free-electron like and confirms earlier observations that emission from (202̄1) is not. Spectroscopy also reveals a feature in the spectrum from silver on (101̄0) which may be identified with a known surface state on Ag(111), thus providing some support for the assignment of Ag(111) to the surface structure of thick silver layers (> 3 monolayers) on (101̄0).