Faceting of Pt-covered W, Pt-covered Ir, and Pd-covered Mo field emitter tips

The adsorption of ultrathin films of Pt on W, Pt on Ir, and Pd on Mo field emitters has been studied with an emphasis on faceting processes, by means of field electron microscopy (FEM) under UHV conditions. The film thickness es ranged from fraction; of an atomic monolayer to several monolayers. From the analysis of field emission patterns and current-voltage measurements w e have found that the steady-state shape of the emitter microcrystal (emitt er tip) can be changed by the presence of the metallic adsorbate film (in g eneral a transition occurs from a nearly hemispherical shape to a nearly po lyhedron shape). The transition is connected with the faceting of the subst rate, which is manifested by changes in size and in electron emission of so me planes. The effect is particularity pronounced for the Pt/W and Pd/Mo sy stems at adsorbate coverages Theta > 1 ML in the substrate temperature rang e, roughly, 700-1400 K depending on the adsorption system. Under such condi tions, planes of the zone type (1 1 0)-(1 0 1) of the W and Mo crystals (bc c structure) appear as facets, e.g. {1 1 2}, {1 2 3} and {1 7 8}, whereas f or Ir (fcc), planes of the (0 0 1)-(1 1 1) zone appear, e.g. {1 1 6} or {1 1 5}. (C) 2001 Elsevier Science Ltd. All rights reserved.