Monoatomic pits

Fig. 1. STM image of a chemically polished Ag(l 11) electrode in 0.01 M HCIO4, showing stepped terrace domains with monoatomic steps, a monoatomic island, and a monoatomic pit [4].

Monoatomic islands and monoatomic pits are observed regularly, with typical average... 

Fig.2. Schematic presentation of the local progress of Pb underpotential deposition at monoatomic pits, monoatomic islands, and stepped terrace domains of non-ideal chemically polished Ag(l 11) electrodes [4]. For fiarther explanation see the text.

In the monoatomic pits, the adsorbate coverage grows inwards from the decorated pit boundaries, leading to a hexagonally close-packed (hep) monolayer that covers the pit completely. 

In the monoatomic pits, that are already covered in peak A2 by a complete adsorbate coverage, no further reaction occurs. 

The progress of adsorbate formation in the monoatomic pits and at monoatomic islands has not been investigated yet. In contrast to the system Pb/Ag(lll), a higher-periodicity superstructure imagii the adsorbate-substrate registry has been resolved only feintly [18]. 

A very similar transformation of the original hep adlayer to a surface alloy coverage with the same Tl-Tl interatomic distances and [V3 x V3]R30° symmetry has been observed in the system Tl/Ag(lll) during extended polarization of the incompletely formed first T1 adsorbate layer. As in the system Pb/Ag(lll), there is strong evidence that the transformations proceed from the boundaries of the peripheral adsorbate-free domains inwards on the terraces. However, in contrast to the system Pb/Ag(lll), the transformed coverages include both ordered and disordered domains, and then-desorption results in the formation of monoatomic pits in the substrate with widths of ca. 3 to 10 nm [3]. These pits diminish and finally vanish within a few minutes by coalescence and lateral displacement, at a rate that can be increased markedly by positive shift of the substrate potential. 

The early UPD lead deposition on Au(lll) was reported by Green et lei.iez yjjggg studies indicated that a deposit developed a monoatomic high adlayer associated probably with Pb atoms in a (1x1) structure, and that during removal of this layer a series of surface pits were formed. Later on, Tao et al. showed that an (1x1) layer is fonned, indeed, in the UPD process showing images of atomic resolution. ... 

This last point requires some comments. Small pits (on a smooth face) can be represented by a combination of monoatomic steps and kinks (see Section III.4). Therefore their behavior, if they are large enough, is close to that of the roughest face on an atomic scale. Also their interference will be more visible, on the C E) curves, for an atomically smooth face than for an atomically rough one. For instance, their effect is more visible in the fee system, for the (111) face than for the (210) face. On the contrary, for the fee system, a small facet, (111) oriented will alter the C E) curve of a rough face more than that of a smooth face. 

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