Structural and electronic properties of the Ti/W(111) adsorption system

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

“…The resulting field strength F at the emitting facet immediately follows from the linear dependence F ¼ bU, where the (facet-specific) proportionality constant b can be determined from the slope of the corresponding FowlerNordheim plot of the FE current once the local work function F of the emitting surface is known [10]. Taking F ¼ 4:4 eV for the clean W(1 1 1), we derive b ¼ 5791 cm À1 [9]. Knowing F enables us to locate the Schottky barrier top V max , as in the classical image potential approximation…”

“…Needless to say, the tungsten tip has been carefully cleaned in advance in a repeated process of combined annealing and flashing [9]. The sample cleanliness has been additionally checked by monitoring the magnitude of the FE current, which falls by only 2% per hour.…”

“…The above procedure assumes the same value of the local bÀfactor (relating the electric field strength at the emitting facet F to the tip-to-screen voltage U via F ¼ bU) for the clean as well as Ticovered W(1 1 1) surfaces. This is a reasonable assumption because ultra-thin overlayers of Ti do not significantly affect the geometry of the (1 1 1) facet at the top of the tip [1,9].…”

“…An energy cut-off of 375 eV has been accepted and a rather fine (39 Â 39 Â 1) GÀcentered grid has been used for the Brillouin zone sampling, as required to reproduce DOS accurately. For other computational details, see [3,9].…”

“…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. Such a procedure has already proven successful in recognizing that Ti atoms indeed adsorb in tungsten-lattice-continuation sites for submonolayer coverages [9]. Here, we apply it to thicker films of titanium with the aim of identifying the actual growth sequence of Ti adlayers on the W(1 1 1) substrate.…”