Correlation between morphological transition and preferred thickness of Pb and Ag islands on Si(111)$7\times 7$

It is known that a quantum size effect can induce a morphological transition of Pb nanostructure, from three-dimensional clusters to multilayer two-dimensional islands, grown on the Si(111)$7\times 7$ at low temperature. We use scanning tunneling microscopy to in situ observe the formation of an individual island to figure out the transition process. Our results reveal that every island differing in thickness can be correlated with a unique transition pathway. A similar behavior is also observed in the growth of flat Ag islands on the Si(111)$7\times 7$ substrate at room temperature.

Figure 1

(Color) In situ observations for the growth of two 3D clusters as marked by the numbers in (a). Line profiles at the right side of (a)–(c) represent the morphology evolution of cluster 1 and 2 along arrows in (a). The image size is $52\times 52{\mathrm{nm}}^2$.

Figure 2

(a) A statistic result of the height as a function of the base diameter for clusters without undergoing the growth transition. It can be fitted by a linear line to show the 3D growth of clusters. (b)–(e) Clusters experiencing the transition are separated in terms of the subsequent island thickness, which is also represented with the height as a function of the diameter for both clusters (cross) and islands (open circle). (f) The transition probability of clusters transforming into islands of different thickness.

Figure 3

(a) The growth of flat Ag islands with a preferred thickness of 2 atomic layers on Si(111)$7\times 7$ surface at room temperature at the coverage of 1 ML. (b) At the coverage of 0.6 ML, not only 2-layer islands but also 1-layer islands, as marked by circles, are formed on the surface. (c) Average sizes of 1-layer and 2-layer islands as a function of coverage. Both image size in (a) and (b) are $75\times 75{\mathrm{nm}}^2$.

Figure 4

Scaled size distributions of 1-layer and 2-layer Ag islands. The dash lines connecting data points of 1-layer islands are drawn for the purpose of guiding the eye. The solid curve is the theoretical scaling function of one-atom critical size.