Structural changes of Ag crystals during reaction with nitric oxide

Field ion microscopy (FIM) was used to study the reaction of nitric oxide ( NO) with silver (Ag) field emitter tips. The clean and almost hemispherical shape of the Ag tip apex was found to undergo restructuring during field-f ree interaction with 150 L NO (1 L=1.3 x 10(-4) Pa.s) at 400 K. Imaging at 57 K revealed an enlarged central (111) pole and a smaller number of layers leading to peripheral planes. Thus, crystal coarsening occurred during rea ction with NO gas. Moreover, planes lying along the [100] zone lines appear ed dark and could only be recovered by field evaporation of a large number of surface layers. Areas of the former {012} planes appeared enlarged. Thes e structural changes are interpreted as being caused by oxygen atoms releas ed during NO decomposition. Cleaning the oxygen-covered tip by field evapor ation and heating it to 400 K reproduced this reconstruction form. This fin ding is explained by the formation of subsurface oxygen and its resegregati on to the surface. However, if the sample, either clean or oxygen-covered, was sputter-cleaned and subsequently heated to 400 K, only slight changes o f the surface structure were found. These changes involved increased local disorder and somewhat enlarged low-index planes as compared to the field-ev aporated clean Ag specimen. (C) 2000 Elsevier Science B.V. All rights reser ved.