Correlation between structure models and mass transport - an in situ LEEM study on the formation of the (4 × 4)O...

The formation of the (4 × 4)O structure on the Ag(111) surface was investigated by low-energy electron microscopy (LEEM). The (4 × 4)O structure has a lower density of Ag atoms than the close-packed (111) surface, so that the formation is correlated with mass transport and a changing surface morphology. By tuning the imaging conditions, LEEM enabled detection of step morphology changes over a several μm-wide region of the Ag crystal containing a sufficiently high number of large terraces separated by monoatomic steps. To form the (4 × 4)O structure, oxygen was adsorbed by decomposing NO₂ at increased temperatures. The data show that (4 × 4)O islands nucleate at the steps and that atomic terraces increase on a mesoscopic length scale as the islands grow. The observations are explained by a mechanism in which O atoms sink into the close-packed surface and Ag atoms are ejected. The released Ag atoms become attached to the descending atomic steps, where they are used to overgrow the step by the forming (4 × 4)O structure. The large field of view, combined with the ability to visualize monoatomic steps, enabled us to quantify the increasing terrace areas both on a large scale covering several steps and in the area of a single growing island. The amount of mass transport is consistent with the stacking fault reconstruction model of the (4 × 4)O structure, which contains 12 surface Ag atoms per unit cell. A recent proposal that the (4 × 4)O structure is a mixture of several structures, some of which have lower Ag surface densities, is not consistent with these results.