Faceting and steady-state shape of Rh- and Pd-covered W field emitter tips

Field emission microscopy (FEM) has been used to investigate the faceting o f tungsten emitter tips of approximately 2000 Angstrom in radius, due to th in overlayer films of Rh or Pd. Both overlayer metals are found to induce t he faceting of the nearly spherical emitter tip upon annealing at elevated temperatures, as demonstrated by the increase in size of {211} facets as we ll as the appearance of new facet orientations, such as {321} or {431}. The faceting transition was observed only above a critical coverage and a thre shold temperature of similar to 800 K. The question of whether the observed faceted morphology represented a steady-state tip shape has also been inve stigated by approaching the final end form starting from two different init ial geometries: a nearly spherical thermally annealed tip, and a nearly pol yhedral built-up tip. When subjected to similar Rh coverages and annealing conditions. both initial geometries converged to the same steady-state tip shape. A similar convergence using Pd was nor observable. Similarities and differences between Rh/W and Pd/W results are discussed and compared to the faceting of the metal-overlayer-covered W(111) single crystal surface. Als o discussed are the observed faceted steady-state tip shape and its relatio n to the equilibrium crystal shape in the light of kinematic limitations in surface diffusion. (C) 1999 Elsevier Science B.V. All rights reserved.