An STM study of the potassium-induced removal of the Ni(100)(2x2)p4g-N reconstruction

In previous work we have shown using surface X-ray diffraction (SXRD) and low-energy electron diffraction (LEED) that the Ni(100)p4g(2 x 2)-N reconstruction is removed by coadsorption of greater than or equal to 0.16 ML of potassium. Here we report scanning tunnelling microscopy (STM) observations of this coadsorbate system at 300 K and 120 K. At low coverages (theta(K)less than or equal to 0.10 ML) the potassium atoms decorate step edges where they are imaged as large protrusions. Once the steps are saturated, further adsorption takes place randomly on the terraces. The potassium atoms on the terraces interact strongly with nitrogen atoms, their diffusion is inhibited, and bias-dependent imaging reveals the disruption of the nitrogen overlayer underneath the potassium-induced protrusions. At higher coverages (theta(K)greater than or equal to 0.16 ML), where it is known that the p4g reconstruction is removed, the protrusions appear to coalesce and the surface [although exhibiting c(2 x 2) periodicity] looks disordered. Lowering the temperature of the system after adsorption does not reveal any new behaviour. The results indicate that a local chemical interaction produces the removal of the reconstruction, and provide a plausible explanation for the coverage dependence of the LEED data. (C) 1999 Elsevier Science B.V. All rights reserved.