Reaction-induced morphological changes of field emitter tips: NO on Rh and Pt

Abstract

Field ion microscopy (FIM) was employed to reveal morphological changes of Rh and Pt tips occurring during the reaction with nitric oxide (NO). The studies were performed by exposing clean specimina of both metals to variable amounts of NO gas and heating them to temperatures as high as 550 K either in the absence or in the presence of an electric field. After NO removal the surfaces were imaged in Ne at cryogenic temperatures and at relatively low electric fields (≤ 28 V/nm) in order to avoid damage of the surface structures by field evaporation. The field-free reaction of NO with Rh at 520 K led to considerable restructuring of the tip. The processes were thermally activated and failed to occur below 460 K. The central (001) pole of the reconstructed form was seen to be enlarged and separated from outer planes by multiple steps. Planes of the 〈100〉 zone were seemingly unstable in the presence of adsorbing (and dissociating) NO. For example, areas of thesz012thesz symmetry appeared to be dissolved due to the growth of vicinal thesz135thesz. The 〈110〉 zone lines were dominated by large thesz113thesz and thesz115thesz planes. As a whole, the number density of kinks was greatly reduced in the resulting morphological shape. The presence of an electric field of up to 28 V/nm had no effect on the reconstruction. The field-free reaction of NO with Pt at 520 K was unable to cause overall morphological changes of the tip. In the presence of an electric field of 8.5 V/nm, however, a pyramidal structure with large thesz111thesz slopes was found to develop. A few layers of (001) orientation were seen to form the truncated top of the pyramid. High-index planes were absent in this structure but were recovered by continuous field evaporation.