Hydrogen-Adsorption Induced Atomic Rearrangement of a Pb Monolayer on Si(111)

We have observed interesting H-atom adsorption induced atomic rearrangements of a Pb monolayer on the Si(111) with a scanning tunneling microscope. A hexagonal ringlike pattern is formed around the point defect. The interactions among nearby H-adsorbed defects can even produce interferencelike superstructures. Phase boundaries are found to either enhance or suppress the formation of the interference pattern. These phenomena are produced by an intricate interplay between electronic and atomic interactions as perturbed by the adsorbed hydrogen atoms.

Figure 1

STM images of the same region with point defects in the original $(1\times 1)$-Pb phase taken at a sample bias of (a) $-0.3\mathrm{V}$, (b) $+0.57\mathrm{V}$, (c) $+1.0\mathrm{V}$, and (d) $+2.0\mathrm{V}$. Around the point defects, a small region of trimer domain is formed. Domains of two types of trimers with different orientations can be seen. An interference superlattice at the right-hand side of the dark point defects can be seen in (c) and (d). We note that no other dark point defects are present to the right of these dark point defects. The pattern observed here is very stable and does not show any change within our observation time of 60 min. The atomic model for the outlined region in (a) is illustrated in (e). We note that the lateral displacement of Pb atoms is exaggerated.

Figure 2

STM images of the same region (a) before exposure and (b) after exposure to atomic hydrogen. The sample bias is $+2.0\mathrm{V}$. The maximum corrugation of the hexagonal ringlike pattern is only $\sim 0.3\AA$. (c) A superlattice of hexagonal cells is formed due to the interaction of two H-adsorbed defects on the upper part of the image. The hexagonal cell between the two defects appears to be a mirror image of the two cells around the defects. Distortion of the hexagonal cells away from the two defects is due to perturbations by other defects (including H-adsorbed defects) around this region. (d) Atomic model for a perfect hexagonal cell. The green spots indicate the local electron concentration in the empty state from 0 to 2 V. The hexagonal cells are outlined with red lines.

Figure 3

STM images of the same region (a) before exposure and (b) after exposure to atomic hydrogen. The sample bias is $+2.0\mathrm{V}$. (a) Some corrugations with a local order in the monolayer Pb-covered region can be seen at the phase boundaries with Si(111)-($7\times 7$). Most of the protrusions can be resolved as Pb trimers or dimers in low biases (not shown), similar to the case shown in Fig. 1. (b) The hexagonal ringlike pattern and the interference pattern in the Pb monolayer appear after exposure to atomic hydrogen.

Figure 4

STM images of a $(1\times 1)$-Pb region (a) before exposure and (b) after exposure to atomic hydrogen. The sample bias is $+2.0\mathrm{V}$. A $1/3\mathrm{M}\mathrm{L}$ $\sqrt{3}\times \sqrt{3}$ phase (indicated with a white arrow) is present at the upper right-hand side of the $1\times 1$ region. Three H-adsorbed defects inside the $1\times 1$ region and some at the phase boundary can be seen, and there is no hexagonal ringlike pattern around them. Note that the boundary between the $1\times 1$ and $\sqrt{3}\times \sqrt{3}$ regions has changed after the hydrogen exposure.